CN117969742A - Method and device for comparing coating integrity of positive electrode active materials - Google Patents

Abstract

The application provides a method and a device for comparing the coating integrity of a positive electrode active material. The method comprises the following steps: providing a plurality of groups of positive electrode active materials, wherein the positive electrode active materials comprise iron element, and at least part of the surfaces of the positive electrode active materials are provided with coating layers; respectively taking powder samples of multiple groups of positive electrode active materials with the same mass and the same type, mixing the powder samples with the same concentration, the same volume and the same type of acid solution, and reacting the mixed powder samples at the same temperature and the same time, wherein the acid solution comprises a complexing agent; respectively carrying out solid-liquid separation on the mixture after each group of reaction to obtain filtrate and insoluble matters; and respectively detecting the concentration of the metal element Fe in each group of filtrate, and evaluating the coating integrity of the plurality of groups of positive electrode active materials according to the concentration of the metal element Fe. The method and the device improve the accuracy of the iron concentration detection result and the accuracy of the comparison result of the coating integrity of the positive electrode active material.

Description

Method and device for comparing coating integrity of positive electrode active materials

Technical Field

The present application relates to the technical field of positive electrode active material detection, and in particular to a method and device for comparing the coating integrity of positive electrode active materials.

Background technique

The power market and energy storage market have increasingly higher requirements for battery performance. Developing high-performance cathode active materials to improve battery performance is an important measure in this field. Coating modification is the main means to improve the performance of cathode active materials. The integrity of the coating has a significant impact on the production and processing of electrode materials and the electrochemical performance of batteries.

Taking lithium iron phosphate, the active material of the positive electrode, as an example, carbon coating lithium iron phosphate forms a stable interface. The carbon coating can alleviate the decomposition of the electrolyte and prevent hydrofluoric acid attack, metal ion dissolution, gas production and other structural problems. It is an important part of the actual production process. However, there is currently a lack of simple and effective methods to visually detect the coating integrity of the positive electrode active material. Therefore, improvement is urgently needed.

Summary of the invention

The purpose of the present application is to provide a method for comparing the coating integrity of positive electrode active materials, which can control the degree of reaction between the acidic solution and the positive electrode active material in multiple groups of reactions within an appropriate range, improve the accuracy of the concentration detection results of the metal element Fe in the filtrate, and improve the accuracy of the comparison results of the coating integrity of the positive electrode active material; the purpose of the present application is also to provide a device that can implement the method of the present application.

In a first aspect, an embodiment of the present application provides a method for comparing the coating integrity of a positive electrode active material, comprising:

Providing a plurality of groups of positive electrode active materials, wherein the positive electrode active materials contain iron and at least a portion of the surface of the positive electrode active materials has a coating layer;

Respectively taking multiple groups of positive electrode active material powder samples of the same mass and the same type and mixing them with the acidic solution of the same concentration, the same volume and the same type and reacting them at the same temperature and the same time, wherein the acidic solution contains a complexing agent; the time for mixing the powder sample of each group of positive electrode active material with the acidic solution for reacting is 1 min to 90 min respectively;

Separating the mixture after each reaction into solid and liquid to obtain a filtrate and an insoluble substance;

The concentration of the metal element Fe in each group of filtrates is detected respectively, and the coating integrity of the multiple groups of positive electrode active materials is evaluated according to the concentration of the metal element Fe.

According to the method of the embodiment of the present application, the coating integrity will affect the dissolution amount and dissolution rate of the metal elements in the positive electrode active material. Taking _LiFePO4 as an example, the dissolution rate of the lithium element is too fast, resulting in no obvious distinction in the test results, while the dissolution rate of the iron element is moderate. Different metal elements have obvious distinctions for positive electrode active material samples with different coating integrity. Selecting the iron element is conducive to improving the accuracy of the test results.

Taking the coating layer with carbon as the main component as an example, the coating integrity is different, the acid resistance of the positive electrode active material is different, and the ability to maintain the stability of the material structure in an acidic environment is different. By measuring the dissolution rate and amount of metallic iron elements after the positive electrode or material powder is evenly mixed with the acid solution, the coating integrity is evaluated to ensure the accuracy of the iron element dissolution test results and improve the accuracy of the comparison results.

In the method of this embodiment, powder samples of multiple groups of positive electrode active materials of the same mass and type are mixed with acidic solutions of the same concentration, volume and type and reacted at the same temperature and time. The concentration of Fe in the filtrate is detected, and the coating integrity of the multiple groups of positive electrode active materials is evaluated according to the concentration of the metal element Fe, so that the degree of reaction of the acidic solution and the positive electrode active material in the multiple groups of reactions is controlled within an appropriate range, thereby improving the accuracy of the concentration of the metal element Fe in the filtrate after the reaction, and can detect the coating integrity between multiple groups of positive electrode active materials with different parameters, thereby improving the accuracy of the comparison results of the coating integrity of the positive electrode active materials.

In the embodiment of the present application, the powder samples of each group of positive electrode active materials are mixed with the acidic solution for reaction for 1 min to 90 min, respectively. By controlling the appropriate time, the content of Fe dissolved is limited, and the degree of reaction between the acid and the positive electrode active material is controlled, the accuracy of the comparison result is further improved.

As the reaction time progresses, if the reaction time exceeds a certain level, the acid may react with the iron element from the part where the positive electrode active material is coated more completely. At this time, the coating integrity between the positive electrode active materials is evaluated by the dissolved Fe content, which reduces the accuracy.

The method of the present application simplifies the detection process of the coating integrity of the positive electrode active material with a carbon coating layer, improves the accuracy of comparing the coating integrity of the positive electrode active material, and the detection process is low-cost and short-cycle, which is convenient for batch detection of the coating integrity of positive electrode active materials from different batches or manufacturers, improves the accuracy of batch detection of the concentration of dissolved Fe, and thus improves the accuracy of the comparison result.

The positive electrode active material may contain trace amounts of trivalent iron ions, or during the reaction between the positive electrode active material and the acidic solution, trace amounts of oxygen in the acidic solution or oxygen in the air may convert divalent iron ions into trivalent iron ions. Trivalent iron ions are prone to become insoluble, suspended, or partially precipitated in the solution, affecting the detection results of divalent iron ions in the solution.

The acidic solution of the present application contains a chelating agent. The chelating agent may inhibit the conversion of divalent iron ions into trivalent iron ions, or inhibit the trivalent iron ions from becoming insoluble, suspended or partially precipitated in the solution; thereby retaining the content of iron ions dissolved from the positive electrode active material after reacting with the acid to a great extent, thereby improving the accuracy of the comparison result.

In some optional embodiments, the ratio of specific surface areas of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials is 1:(0.6-1.8).

According to the embodiments of the present application, controlling the ratio of the specific surface areas of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials within the above-mentioned range is beneficial to controlling the degree of reaction between the positive electrode active material and the acid and the contact area between the positive electrode active material and the acidic solution, is beneficial to comparing the coating integrity of multiple groups of positive electrode active materials with similar specific surface areas, and is also beneficial to improving the accuracy of the comparison results.

In some optional embodiments, the ratio of the volume particle diameters Dv50 of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials is 1:(0.6-2.2).

According to the embodiments of the present application, controlling the ratio of the volume particle size Dv50 of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials within the above-mentioned range is beneficial to controlling the degree of reaction between the positive electrode active material and the acid and the contact area between the positive electrode active material and the acidic solution, is beneficial to comparing the coating integrity of multiple groups of positive electrode active materials with similar volume particle size Dv50, and is also beneficial to improving the accuracy of the comparison results.

In some optional embodiments, the ratio of the particle size distribution spans of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials is 1:(0.4-1.6).

According to the embodiments of the present application, controlling the ratio of the particle size distribution spans of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials within the above-mentioned range is beneficial to controlling the degree of reaction between the positive electrode active material and the acid and the balance between the positive electrode active material and the acidic solution, is beneficial to comparing the coating integrity of multiple groups of positive electrode active materials with similar particle size distribution spans, and is also beneficial to improving the accuracy of the comparison results.

In some optional embodiments, the ratio of the coating thickness of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials is 1:(0.5-2.0).

According to the embodiment of the present application, controlling the ratio of the coating layer thickness of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials within the above-mentioned range is beneficial to controlling the degree of reaction between the positive electrode active materials and the acid, is beneficial to comparing the coating integrity of multiple groups of positive electrode active materials with similar coating layer thicknesses, and is also beneficial to improving the accuracy of the comparison results.

In some optional embodiments, the mass content of the coating layer in the positive electrode active material is 0.8% to 2.4%.

According to the embodiments of the present application, controlling the mass content of the coating layer of any positive electrode active material in the multiple groups of positive electrode active materials within the above range reflects to a certain extent that the thickness of the coating layer in the positive electrode active material is beneficial to controlling the degree of reaction between the positive electrode active material and the acid, is beneficial to comparing the coating integrity of multiple groups of positive electrode active materials with similar coating layer thicknesses, and is also beneficial to improving the accuracy of the comparison results.

In some optional embodiments, multiple groups of positive electrode active materials have the same general chemical structure formula.

Under the same conditions, since the coating layer coats the positive electrode active material, the iron element in the positive electrode active material is inside, and having the same general chemical structure will cause the iron element to react with the acid at the same reaction rate, and the reaction is more balanced, which is conducive to the iron element dissolving in the acidic solution at the same rate. Therefore, controlling any two groups of positive electrode active materials in multiple groups of positive electrode active materials to have the same general chemical structure is conducive to controlling the degree of reaction between the positive electrode active material and the acid, is conducive to comparing the coating integrity of multiple groups of positive electrode active materials with similar coating layer thickness, and is also conducive to improving the accuracy of the comparison results.

Those skilled in the art may select a positive electrode active material containing iron known in the art. In some optional embodiments, the positive electrode active material includes one or more of a lithium iron phosphate material coated with a coating layer, a lithium manganese iron phosphate material coated with a coating layer, and their respective doping and modification materials.

Those skilled in the art may select materials known in the art as the coating layer of the positive electrode active material. In some optional embodiments, the coating layer comprises more than 90% carbon by mass, and the remainder is one or more of oxides and/or fluorides of metal elements, wherein the metal elements include one or more of iron, manganese, lithium, and aluminum.

In some optional embodiments, multiple groups of positive electrode active material powder samples of the same mass are respectively immersed in acidic solutions of the same concentration and type and reacted at the same temperature and time.

According to the embodiment of the present application, controlling the reaction parameters of the powder samples of multiple groups of positive electrode active materials reacting with acid is beneficial to controlling the degree of reaction within the same time, so as to determine the content of metallic iron in the solution at the same reaction degree, thereby improving the accuracy of comparison of the coating integrity of multiple groups of positive electrode active materials.

In order to control the reaction degree of the positive electrode active material within an appropriate range, the pH change (ΔpH) of the reaction solution before and after the reaction can be controlled to be as small as possible, and at the same time, a more accurate comparison of the coating integrity of the positive electrode active material can be made based on the concentration of the dissolved metal element Fe. The initial concentration of the acidic solution and the consumption of H ⁺ during the reaction can be strictly limited. Control can be carried out in the following ways:

In some optional embodiments, the initial concentration C _acid of the acidic solution is 0.002 mol/L to 1 mol/L.

In some optional embodiments, the volume of the acidic solution is 30 ml to 2000 ml.

In some optional embodiments, the mass of each group of powder samples of the positive electrode active material is 0.2 g to 10 g.

In some optional embodiments, the ratio of the mass of the powder sample of each group of positive electrode active materials to the volume of the acidic solution is 0.0025 g:1 ml to 0.5 g:1 ml.

In some alternative embodiments, the acidic solution comprises a reducing acid.

According to the embodiment of the present application, the acidic solution includes a reducing acid, and thus the acidic solution has reducing properties. The reducing property of the acidic solution can inhibit the conversion of divalent iron ions into trivalent iron ions. The reducing property of the acidic solution can convert the original trivalent iron ions into divalent iron ions, thereby stabilizing the content of iron ions dissolved from the positive electrode active material after the reaction with the acid in the filtrate to a great extent, thereby improving the accuracy of the comparison results.

The acidic solution may be an acid recognized in the art that can react with the positive electrode active material. In some optional embodiments, the acidic solution includes one or more of ascorbic acid, citric acid, acetic acid, hydrochloric acid (HCl), hydrofluoric acid (HF), sulfuric acid (H ₂ SO ₄ ) and phosphoric acid (H ₃ PO ₄ ).

When the acid in the acidic solution is not reducing or in order to improve the reducing property of the acidic solution, in order to stabilize the content of iron ions dissolved from the positive electrode active material after the reaction with the acid in the filtrate, the accuracy of detecting the content of iron in the filtrate is improved. In some optional embodiments, the acidic solution contains a reducing agent, and the reducing agent includes one or more of a soluble fluoride salt and a soluble disalt of ethylenediaminetetraacetic acid.

In order to stabilize the content of iron ions dissolved by the positive electrode active material after the reaction with the acid in the filtrate, reduce or reduce the iron ions in the solution to become insoluble matter, suspended matter or partial precipitation, affecting the accuracy of the content of iron in the filtrate. A complexing agent is added to the acidic solution. In some optional embodiments, the complexing agent includes one or more of ethylenediaminetetraacetic acid or its salt, diisopropyltriaminepentaacetic acid or its salt, dithiodiisopropylaminetetraacetic acid or its salt, thioacetate, and soluble thiocyanide.

In addition, in order to prevent some trivalent iron ions from forming precipitation with excess phosphate ions in the positive electrode active material or in the acidic solution, thereby introducing test errors, a small amount of a complexing agent that can complex with the iron element can be added to the solution.

In order to reduce the influence of the complexing agent on the pH of the acidic solution and reduce the influence of the complexing agent on the subsequent detection of the iron ion concentration, in some optional embodiments, the molar concentration of the complexing agent in the acidic solution is 0.005 mol/L to 1 mol/L.

In some optional embodiments, the powder samples of each group of positive electrode active materials are mixed with the acidic solution for reaction at a temperature of 5° C. to 65° C.

According to the embodiments of the present application, controlling the temperature of the reaction can control the degree of reaction between the acid and the powder sample of the positive electrode active material, which is beneficial to detect the concentration of Fe under relative conditions in each group of reactions, compare the coating integrity of the positive electrode active materials, and improve the accuracy of the detection of the dissolved iron element concentration and the accuracy of the batch comparison of the coating integrity results of the positive electrode active materials.

The solid-liquid separation of the reaction mixture can be carried out in a manner commonly used in the art. In some optional embodiments, the solid-liquid separation of each group of reaction mixtures can be carried out by one or more of centrifugal filtration, suction filtration, and membrane filtration.

The method for detecting the concentration of the metal element Fe can be carried out by using a method known in the art for detecting the concentration of metal elements in a solution. In some optional embodiments, the method for detecting the concentration of the metal element Fe in each group of filtrates is one or more of inductively coupled plasma spectroscopy, ultraviolet spectrometry, electrochemical method, atomic absorption method or atomic emission method.

In a second aspect, the present application provides a method for detecting whether the coating integrity of a positive electrode active material meets the standard, comprising the following steps:

Providing a positive electrode active material sample that meets the standards and a positive electrode active material sample to be tested; the two groups of positive electrode active material samples respectively contain iron elements and the two groups of positive electrode active materials respectively have a coating layer on at least part of their surfaces;

The positive electrode active material samples of the same mass are mixed with the acidic solution of the same concentration, volume and type and reacted at the same temperature and time, wherein the acidic solution contains a complexing agent; the time for mixing the powder samples of each group of positive electrode active materials with the acidic solution to react is 1 min to 90 min respectively;

Separating the reaction mixture into solid and liquid to obtain a filtrate and an insoluble substance;

The concentration of the metal element Fe in the filtrate was detected respectively;

If the concentration of the metal element Fe of the positive electrode active material sample to be tested is less than or equal to the concentration of the metal element Fe of the positive electrode active material sample that meets the standard, the coating integrity of the positive electrode active material sample to be tested meets the standard.

In the method of this embodiment, powder samples of multiple groups of positive electrode active materials of the same mass and type are taken respectively, mixed with acidic solutions of the same concentration, volume and type, and reacted at the same temperature and time. The concentration of Fe in the filtrate is detected, and the coating integrity of the multiple groups of positive electrode active materials is evaluated according to the concentration of the metal element Fe. This not only improves the accuracy of the concentration of the metal element Fe in the filtrate after the reaction, but also controls the degree of reaction between the acidic solution and the positive electrode active material in the multiple groups of reactions within an appropriate range, so that a more accurate comparison of the coating integrity of the positive electrode active material can be made according to the concentration of the dissolved metal element Fe.

The method of the present application simplifies the detection process of the coating integrity of the positive electrode active material with a carbon coating layer, improves the accuracy of comparing the coating integrity of the positive electrode active material, and the detection process is low-cost and short-cycle, which is convenient for batch detection of the coating integrity of positive electrode active materials from different batches or manufacturers, improves the accuracy of batch detection of the concentration of dissolved Fe, and thus improves the accuracy of the comparison result.

The method of the embodiment of the present application can intuitively judge the coating integrity of the positive electrode active material without complicated experimental process, saving the verification cycle of raw materials in the production process and reducing costs.

In a third aspect, the present application also provides a device for comparing the coating integrity of positive electrode active materials, including:

A body storage module, used to contain multiple groups of positive electrode active materials, wherein the positive electrode active materials contain iron elements and at least part of the surface of the positive electrode active materials has a coating layer;

An acidic solution storage module, used for containing an acidic solution, wherein the acidic solution contains a complexing agent;

A mixing reaction module, used to mix the acidic solution with multiple groups of positive electrode active materials respectively, and to make the powder samples of multiple groups of positive electrode active materials of the same mass be mixed with the acidic solution of the same concentration, the same volume and the same type and react at the same temperature and the same time;

A solid-liquid separation module is used to perform solid-liquid separation on the mixture after each group of reactions and obtain a filtrate and an insoluble matter;

A detection module, used to detect the concentration of the metal element Fe in each group of filtrates;

The judgment module is used to evaluate the coating integrity of the plurality of groups of positive electrode active materials according to the concentration of the metal element Fe in each group of positive electrode active materials.

The device of the present application can take powder samples of multiple groups of positive electrode active materials of the same mass and type, mix them with acidic solutions of the same concentration, volume and type, and react them at the same temperature and time, realize closed-loop control of the detection process, automatically adjust the reaction conditions, ensure the same reaction conditions, thereby improving the accuracy of the concentration of the metal element Fe and the accuracy of the comparison results. In addition, the detection process has low cost and short cycle, which is convenient for batch detection of the coating integrity of positive electrode active materials from different batches or manufacturers, and improves the accuracy of the comparison results.

The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present application. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

FIG1 shows a graph showing the results of the dissolution content of the metallic element iron and manganese in the reaction of lithium manganese iron phosphate with acid using different coating processes provided in one embodiment of the present application.

FIG. 2 shows a schematic structural diagram of a device for comparing coating integrity of positive electrode active materials provided in one embodiment of the present application.

Detailed ways

The following embodiments of the technical solution of the present application will be described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

In the description of the embodiments of the present application, the term "and/or" is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and/or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character "/" in this article generally indicates that the associated objects before and after are in an "or" relationship.

In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the embodiments of the present application.

In the description of the embodiments of the present application, unless otherwise clearly specified and limited, technical terms such as "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

The following examples more specifically describe the disclosure of the present application, which are intended for illustrative purposes only, as it will be apparent to those skilled in the art that various modifications and variations may be made within the scope of the disclosure of the present application. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by weight, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing, and the instruments used in the examples are commercially available.

As described in the background art, coating modification is the main means to improve the performance of positive electrode active materials. The integrity of the coating has a significant impact on the production and processing of positive electrode active materials and the electrochemical performance of batteries. The integrity of the coating interface is an important means to measure the performance of positive electrode active materials.

In the related technology, the integrity of the coating interface is usually judged by coating the surface of the positive electrode active material sample with the coating layer, rolling the powder sample into a sheet, and finally testing it. The coating is generally made by atomic layer deposition, chemical vapor deposition, evaporation, molecular beam epitaxy, physical sputtering, etc., which has a high production cost and a complicated operation procedure.

In addition, the process of rolling powder samples into sheets may cause a certain degree of damage to the particle morphology, exposing the uncoated interior and causing measurement errors. The coating thickness is limited by the penetration of X-rays into the coating material. During the test, it is necessary to ensure that the detection depth of the ray just penetrates the coating and acts on the sample surface to ensure the accuracy of the surface atomic ratio test. The conditions are harsh and it is difficult to ensure the same accuracy every time the test is carried out, so the detection accuracy is difficult to achieve.

It is worth mentioning that when testing samples that have been rolled into sheets, the test area is limited, and the calculation of the sample coverage is easily affected by regional interference. At the same time, the calculation formula assumes that the coated atom requires exactly one coating atom to be coated. This calculation logic needs further scrutiny.

Currently, there is a lack of simple and effective methods to visually detect or compare the coating integrity of positive electrode active materials.

In view of this, the present application proposes an acid dissolution method to determine the iron content of multiple groups of positive electrode active materials to compare the coating integrity of multiple groups of positive electrode active materials. Compared with the traditional method, it has the advantages of low cost, low equipment requirements, and simple operation.

Method for comparing coating integrity of positive electrode active materials

In a first aspect, an embodiment of the present application provides a method for comparing the coating integrity of a positive electrode active material, comprising:

Providing a plurality of groups of positive electrode active materials, wherein the positive electrode active materials contain iron and at least a portion of the surface of the positive electrode active materials has a coating layer;

Respectively taking multiple groups of positive electrode active material powder samples of the same mass and the same type and mixing them with the acidic solution of the same concentration, the same volume and the same type and reacting them at the same temperature and the same time, wherein the acidic solution contains a complexing agent; the time for mixing the powder sample of each group of positive electrode active material with the acidic solution for reacting is 1 min to 90 min respectively;

Separating the mixture after each reaction into solid and liquid to obtain a filtrate and an insoluble substance;

The concentration of the metal element Fe in each group of filtrates is detected respectively, and the coating integrity of the multiple groups of positive electrode active materials is evaluated according to the concentration of the metal element Fe.

The concentration C _Fe of the metal element Fe is defined as C _Fe = (mass of the metal element Fe to be detected)/(total mass of the solution to be detected). The concentration can be detected by a method commonly used in the art. Since the concentration C _Fe is usually low, it is usually measured in ppm (parts per million) for convenience.

The concentration of the metal element Fe obtained by the detection reflects the amount of metal element Fe dissolved from the positive electrode active material during the reaction. Under the same parameters and reaction conditions, for the same type of positive electrode active material (for example, for _LiFePO4 positive electrode active material), the higher the concentration of metal element Fe C _Fe , the more metal Fe is dissolved, indicating that the coating integrity of the positive electrode active material is poorer.

Taking LiFePO ₄ , a positive electrode active material with a carbon coating layer, as an example, the dissolution of iron is a complex process, but it is not desired to be limited by theory. The schematic reaction equation of LiFePO ₄ and acidic solution is:

LiFePO ₄ +2H ⁺ →Fe ²⁺ +Li ⁺ +1/3HPO ₄ ^2- +1/3H ₂ PO ₄ ^- +1/3H ₃ PO ₄

According to the method of the embodiment of the present application, the coating integrity will affect the dissolution amount and dissolution rate of the metal elements in the positive electrode active material. Taking _LiFePO4 as an example, the dissolution rate of the lithium element is too fast, resulting in no obvious distinction in the test results, while the dissolution rate of the iron element is moderate, which has obvious distinction for positive electrode active material samples with different coating integrity, which is beneficial to improve the accuracy of the test results.

Taking the coating layer with carbon as the main component as an example, the coating integrity is different, the acid resistance of the positive electrode active material is different, and the ability to maintain the stability of the material structure in an acidic environment is different. By controlling the dissolution rate and amount of metallic iron after the powder of the positive electrode active material is evenly mixed with the acid solution, the coating integrity is evaluated to ensure the accuracy of the iron dissolution test results and improve the accuracy of the comparison results.

In the method of this embodiment, powder samples of multiple groups of positive electrode active materials of the same mass and type are mixed with acidic solutions of the same concentration, volume and type and reacted at the same temperature and time. The concentration of Fe in the filtrate is detected, and the coating integrity of the multiple groups of positive electrode active materials is evaluated according to the concentration of the metal element Fe, so that the degree of reaction between the acidic solution and the positive electrode active material in the multiple groups of reactions is controlled within an appropriate range. This not only improves the accuracy of the concentration of the metal element Fe in the filtrate after the reaction, but also detects the coating integrity between multiple groups of positive electrode active materials with different parameters, so that a more accurate comparison of the coating integrity of the positive electrode active material can be made according to the concentration of the dissolved metal element Fe.

The method of the present application simplifies the detection process of the coating integrity of the positive electrode active material with a carbon coating layer, improves the accuracy of comparing the coating integrity of the positive electrode active material, and the detection process is low-cost and short-cycle, which is convenient for batch detection of the coating integrity of positive electrode active materials from different batches or manufacturers, and improves the accuracy of batch detection of the concentration of dissolved Fe, thereby improving the accuracy of the comparison result.

The positive electrode active material may contain trace amounts of trivalent iron ions, or during the reaction between the positive electrode active material and the acidic solution, trace amounts of oxygen in the acidic solution or oxygen in the air may convert divalent iron ions into trivalent iron ions. Trivalent iron ions are prone to become insoluble, suspended, or partially precipitated in the solution, affecting the detection results of divalent iron ions in the solution.

The acidic solution of the present application contains a chelating agent, which can inhibit the conversion of divalent iron ions into trivalent iron ions, or inhibit the trivalent iron ions from easily becoming insoluble, suspended or partially precipitated in the solution; thereby retaining the content of iron ions dissolved from the positive electrode active material after reacting with the acid to a great extent, thereby improving the accuracy of the comparison result.

In the embodiment of the present application, the powder samples of each group of positive electrode active materials are mixed with the acidic solution for reaction for 1 min to 90 min, respectively. By controlling the appropriate time, the content of Fe dissolved is limited, and the degree of reaction between the acid and the positive electrode active material is controlled, the accuracy of the comparison result is further improved.

As the reaction time progresses, if the reaction time exceeds a certain level, the acid may react with the iron element from the part where the positive electrode active material is coated more completely. At this time, the coating integrity between the positive electrode active materials is evaluated by the dissolved Fe content, which reduces the accuracy.

Taking the positive electrode active material incoming material inspection process as an example, the inspection of the coating integrity of the positive electrode active material may not require the determination of specific values. The coating integrity between multiple groups of positive electrode active materials with different parameters can be inspected. It is only necessary to compare whether the coating integrity of positive electrode active materials from different batches or different manufacturers is at the same level, so that the performance of the positive electrode active materials and the battery can be controlled at the same level, reducing the differences between positive electrode active materials from different batches and different manufacturers, and improving the inspection speed.

The following is an explanation based on the properties of multiple groups of positive electrode active materials themselves:

In some optional embodiments, the ratio of specific surface areas of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials is 1:(0.6-1.8).

Optionally, the ratio of the specific surface areas of any two groups of positive electrode active materials in the multiple groups of positive electrode active materials can be any ratio of 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1.0, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8 or a range of their compositions.

The specific surface area of the positive electrode active material is an important parameter of the material surface characteristics, which directly affects the contact area between the material and the external environment. When the positive electrode active material with a larger specific surface area reacts with acid, the contact area increases, which accelerates the reaction rate. Under the same conditions, since the coating layer covers the positive electrode active material, the iron element in the positive electrode active material is inside. The positive electrode active material with a large specific surface area will cause the iron element to react with the acid faster and dissolve in the acidic solution.

Therefore, controlling the ratio of the specific surface areas of any two groups of positive electrode active materials in the multiple groups of positive electrode active materials within the above range is beneficial to controlling the degree of reaction between the positive electrode active materials and the acid and the contact area between the positive electrode active materials and the acidic solution, and is beneficial to comparing the coating integrity of multiple groups of positive electrode active materials with similar specific surface areas, and is also beneficial to improving the accuracy of the comparison results. In some embodiments, the specific surface area of the multiple groups of positive electrode active materials can be 6 to 21 m²/g.

The specific surface area of the positive electrode active material is a well-known meaning in the art and can be measured by instruments and methods well-known in the art. For example, it can be tested by a nitrogen adsorption specific surface area analysis test method and calculated by the BET (Brunauer Emmett Teller) method, wherein the nitrogen adsorption specific surface area analysis test can be performed by a Tri Star II specific surface and pore analyzer produced by Micromeritics, USA.

In some optional embodiments, the ratio of the volume particle diameters Dv50 of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials is 1:(0.6-2.2).

Optionally, the ratio of the volume particle size Dv50 of any two groups of positive electrode active materials in the multiple groups of positive electrode active materials can be any ratio or a range of their compositions of 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1.0, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2.0, 1:2.1, 1:2.2.

The volume particle size Dv50 of the positive electrode active material is a parameter material of the reaction particle size distribution, which reflects the contact area between the material and the external environment to a certain extent. Materials with smaller volume particle sizes have higher specific surface areas, which means that the contact area with the acid is larger, and smaller particles may promote a more thorough reaction. Materials with smaller volume particle sizes usually lead to faster reactions because the reactants are easier to contact and react.

Under the same conditions, since the coating layer covers the positive electrode active material, the iron element in the positive electrode active material is inside, and the smaller volume particle size will cause the iron element to react with the acid faster and dissolve in the acidic solution.

Therefore, controlling the ratio of the volume particle size Dv50 of any two groups of positive electrode active materials in the multiple groups of positive electrode active materials within the above range is beneficial to controlling the degree of reaction between the positive electrode active material and the acid and the contact area between the positive electrode active material and the acidic solution, and is beneficial to comparing the coating integrity of multiple groups of positive electrode active materials with similar volume particle size Dv50, and is also beneficial to improving the accuracy of the comparison results. In some embodiments, the volume particle size Dv50 of the multiple groups of positive electrode active materials can be 0.6 to 5 μm.

The volume average particle size D _v 50 is a well-known meaning in the art, which indicates the particle size corresponding to when the cumulative volume distribution percentage of the material reaches 50%, and can be measured by instruments and methods well-known in the art. For example, it can be conveniently measured by a laser particle size analyzer with reference to GB/T19077-2016 particle size distribution laser diffraction method, such as the Mastersizer 2000E laser particle size analyzer of Malvern Instruments Ltd., UK.

In some optional embodiments, the ratio of the particle size distribution spans of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials is 1:(0.4-1.6).

Optionally, the ratio of the particle size distribution spans of any two groups of positive electrode active materials in the multiple groups of positive electrode active materials can be any ratio of 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1.0, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6 or a range of their compositions.

The particle size distribution span of the positive electrode active material is an indicator to measure the width of the particle size distribution. The larger the particle size distribution span SPAN value, the wider the particle size distribution; the smaller the particle size distribution span SPAN value, the narrower the particle size distribution. Materials with smaller particle size distribution span SPAN values indicate that the particle size distribution of the positive electrode active material is narrower, which usually enables a more uniform reaction when reacting with acid, and also helps the reaction proceed more thoroughly.

Under the same conditions, since the coating layer covers the positive electrode active material, the iron element in the positive electrode active material is inside, and the smaller SPAN value of the particle size distribution of the positive electrode active material will cause the iron element to react with the acid faster and more evenly and dissolve in the acidic solution.

Therefore, controlling the ratio of the particle size distribution spans of any two groups of positive electrode active materials in the multiple groups of positive electrode active materials within the above range is beneficial to controlling the degree of reaction between the positive electrode active material and the acid and the balance between the positive electrode active material and the acidic solution, and is beneficial to comparing the coating integrity of multiple groups of positive electrode active materials with similar particle size distribution spans, and is also beneficial to improving the accuracy of the comparison results. In some embodiments, the particle size distribution span SPAN of the multiple groups of positive electrode active materials can be 2 to 12.

The particle size distribution span (SPAN) is an indicator to measure the width of the particle size distribution. The commonly used formula for calculating SPAN is (Dv90-Dv10)/Dv50. The larger the SPAN value, the wider the particle size distribution; the smaller the SPAN value, the narrower the particle size distribution.

The volume particle size distribution Dv50, also known as the average particle size or median particle size, represents the particle size corresponding to 50% of the volume distribution of the positive electrode active material particles; the volume particle size distribution Dv90 represents the particle size corresponding to 90% of the volume distribution of the positive electrode active material particles; the volume particle size distribution Dv10 represents the particle size corresponding to 10% of the volume distribution of the positive electrode active material particles. The above particle size distributions can be measured using instruments and methods known in the art. For example, it can be conveniently measured using a laser particle size analyzer, such as the Mastersizer 3000 laser particle size analyzer of Malvern Instruments Ltd., UK.

In some optional embodiments, the ratio of the coating thickness of any two groups of positive electrode active materials among the multiple groups of positive electrode active materials is 1:(0.5-2.0).

Optionally, the ratio of the coating layer thickness of any two groups of positive electrode active materials in the multiple groups of positive electrode active materials can be any ratio or range of their combinations of 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1.0, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2.0.

The thickness of the coating layer of the positive electrode active material is also an important factor affecting the reaction between the iron element in the positive electrode active material and the acid. The thinner the coating layer thickness of the positive electrode active material, the less difficult it is for the acid to react with the iron element in the positive electrode active material, and the faster the reaction rate. This may be because the thinner coating layer reduces the diffusion path between protons or ions and the positive electrode active material, making it easier for the reactants to quickly reach the reaction site.

Under the same conditions, since the coating layer coats the positive electrode active material, the iron element in the positive electrode active material is inside, and the smaller thickness of the coating layer of the positive electrode active material will cause the iron element to react with the acid faster and dissolve in the acidic solution. Therefore, controlling the ratio of the coating layer thickness of any two groups of positive electrode active materials in the multiple groups of positive electrode active materials within the above range is beneficial to controlling the degree of reaction between the positive electrode active material and the acid, and is beneficial to comparing the coating integrity of multiple groups of positive electrode active materials with similar coating layer thicknesses, and is also beneficial to improving the accuracy of the comparison results. In some embodiments, the coating layer thickness of the multiple groups of positive electrode active materials can be 1.5 to 6 nm.

The coating thickness test is mainly carried out by ion beam etching FIB. The specific method may include the following steps: randomly select a number of single particles from the positive electrode active material powder to be tested, cut a thin slice with a thickness of about 100nm from the middle position or near the middle position of the selected particle, and then perform a transmission electron microscope (TEM) test on the thin slice to measure the thickness of the coating layer, measure 3-5 positions, and take the average value. The original image obtained by the above TEM test can also be opened in the DigitalMicrograph software, and the coating layer can be identified through the lattice spacing and angle information, and the thickness of the coating layer is measured. The thickness of the selected particle at three positions is measured and the average value is taken.

In some optional embodiments, the mass content of the coating layer in the positive electrode active material is 0.8% to 2.4%.

Optionally, the mass content of the coating layer in the positive electrode active material may be any value selected from 0.8%, 1.0%, 1.5%, 2.0%, 2.4%, or a range of their compositions.

According to the embodiments of the present application, controlling the mass content of the coating layer of any positive electrode active material in the multiple groups of positive electrode active materials within the above range reflects to a certain extent that the thickness of the coating layer in the positive electrode active material is beneficial to controlling the degree of reaction between the positive electrode active material and the acid, is beneficial to comparing the coating integrity of multiple groups of positive electrode active materials with similar coating layer thicknesses, and is also beneficial to improving the accuracy of the comparison results.

The mass content of the coating layer in the positive electrode active material can be detected by a common method in the art, such as a carbon-sulfur analyzer. The main component of the coating layer is generally carbon. As an example, the mass content of the coating layer in the positive electrode active material can be detected by infrared absorption method.

In some optional embodiments, multiple groups of positive electrode active materials have the same general chemical structure formula.

The different chemical structural formulas of the positive electrode active materials will affect the reaction rate and degree of the positive electrode active materials with the acid. Positive electrode active materials with higher iron content usually increase the reaction rate with the acid, and more iron active sites may be available to participate in the reaction, providing more reaction sites, thereby accelerating the entire reaction process.

Under the same conditions, since the coating layer coats the positive electrode active material, the iron element in the positive electrode active material is inside, and having the same general chemical structure will cause the iron element to react with the acid at the same reaction rate, and the reaction is more balanced, which is conducive to the iron element dissolving in the acidic solution at the same rate. Therefore, controlling any two groups of positive electrode active materials in multiple groups of positive electrode active materials to have the same general chemical structure is conducive to controlling the degree of reaction between the positive electrode active material and the acid, is conducive to comparing the coating integrity of multiple groups of positive electrode active materials with similar coating layer thickness, and is also conducive to improving the accuracy of the comparison results.

Those skilled in the art may select a positive electrode active material containing iron known in the art. In some optional embodiments, the positive electrode active material includes one or more of a lithium iron phosphate material coated with a coating layer, a lithium manganese iron phosphate material coated with a coating layer, and their respective doping and modification materials.

As an example, the structural formula of the lithium iron phosphate material is LiFePO ₄ . As an example, the structural formula of the lithium iron phosphate material is LiMn _x Fe _1-xy M _y PO ₄ , wherein 0<x<1, 0≤y<1, and M is a transition metal element except Mn and Fe.

Those skilled in the art may select materials known in the art as the coating layer of the positive electrode active material. In some optional embodiments, the coating layer comprises more than 90% carbon by mass, and the remainder is one or more of oxides and/or fluorides of metal elements, wherein the metal elements include one or more of iron, manganese, lithium, and aluminum.

Generally speaking, the main component of the coating layer does not react with the acid. The main component can be understood as a component that is more than 90%. In some embodiments, the coating layer includes 90% to 100% carbon element by mass percentage.

The concentration of dissolved metal element Fe is used as an indicator to evaluate the coating integrity of the positive electrode active material. The key and difficulty to the success of this method lies in how to select a suitable acidic environment and reaction parameters, and control the acidic solution to have the same degree of corrosion (or the same degree of reaction) on multiple groups of positive electrode active materials, so as to more accurately determine the content of metal iron in the solution and improve the accuracy of comparison.

The following is an explanation of multiple groups of positive electrode active materials, acidic environments and reaction parameters:

In some optional embodiments, multiple groups of positive electrode active material powder samples of the same mass are respectively immersed in acidic solutions of the same concentration and type and reacted at the same temperature and time.

According to the embodiment of the present application, controlling the reaction parameters of the powder samples of multiple groups of positive electrode active materials reacting with acid is beneficial to controlling the degree of reaction within the same time, so as to determine the content of metallic iron in the solution at the same reaction degree, thereby improving the accuracy of comparison of the coating integrity of multiple groups of positive electrode active materials.

In order to control the reaction degree of the positive electrode active material within an appropriate range, the pH change (ΔpH) of the reaction solution before and after the reaction can be controlled to be as small as possible, and at the same time, a more accurate comparison of the coating integrity of the positive electrode active material can be made based on the concentration of the dissolved metal element Fe. The initial concentration of the acidic solution and the consumption of H ⁺ during the reaction can be strictly limited. Control can be carried out in the following ways.

In some optional embodiments, the initial concentration C _acid of the acidic solution is 0.002 mol/L to 1 mol/L.

Optionally, the initial concentration C of the acidic solution can be any value or a range _{of its composition} in 0.002mol/L, 0.005mol/L, 0.01mol/L, 0.015mol/L, 0.02mol/L, 0.03mol/L, 0.04mol/L, 0.05mol/L, 0.06mol/L, 0.07mol/L, 0.08mol/L, 0.09mol/L, 0.10mol/L, 0.20mol/L, 0.30mol/L, 0.40mol/L, 0.50mol/L, 0.60mol/L, 0.70mol/L, 0.80mol/L, 0.90mol/L, 1.0mol/L. In some optional embodiments, the initial concentration C of _the acidic solution is 0.01mol/L to 1mol/L.

According to the embodiment of the present application, the initial concentrations of the acidic solutions of the groups that react with the positive electrode active material are the same. The initial concentrations of the acidic solutions of the groups can be arbitrarily selected within the above range.

In some optional embodiments, the volume of the acidic solution is 30 ml to 2000 ml.

Optionally, the volume of the acidic solution is any value among 30ml, 50ml, 100ml, 200ml, 300ml, 400ml, 500ml, 600ml, 700ml, 800ml, 900ml, 1000ml, 1500ml, 2000ml or a range thereof.

In some optional embodiments, the mass of each group of powder samples of the positive electrode active material is 0.2 g to 10 g.

Optionally, the mass of each group of powder samples of the positive electrode active material may be any value among 0.2g, 0.5g, 1g, 2g, 3g, 4g, 5g, 6g, 7g, 8g, 9g, 10g or a range thereof.

In some optional embodiments, the ratio of the mass of the powder sample of each group of positive electrode active materials to the volume of the acidic solution is 0.0025 g:1 ml to 0.5 g:1 ml.

Optionally, the ratio of the mass of the powder sample of each group of positive electrode active material to the volume of the acidic solution can be any ratio or range of its composition among 0.0025g:1ml, 0.0050g:1ml, 0.0075g:1ml, 0.010g:1ml, 0.015g:1ml, 0.020g:1ml, 0.025g:1ml, 0.050g:1ml, 0.1g:1ml, 0.2g:1ml, 0.3g:1ml, 0.4g:1ml, and 0.5g:1ml.

According to the embodiment of the present application, the ratio of the mass of the powder sample of the positive electrode active material in each group to the volume of the acidic solution is the same. The ratio of each group can be uniformly selected arbitrarily within the above range.

In some alternative embodiments, the acidic solution comprises a reducing acid.

According to the embodiment of the present application, the acidic solution includes a reducing acid, and thus the acidic solution has reducing properties. The reducing property of the acidic solution can inhibit the conversion of divalent iron ions into trivalent iron ions. The reducing property of the acidic solution can convert the original trivalent iron ions into divalent iron ions, thereby stabilizing the content of iron ions dissolved from the positive electrode active material after the reaction with the acid in the filtrate to a great extent, thereby improving the accuracy of the comparison results.

The acidic solution may be _an acid recognized in the art that can react with the positive _electrode active material. In some optional embodiments, the acidic solution includes one or more of ascorbic acid, citric acid, acetic acid, HCl, HF, _H2SO4 and _H3PO4 .

When the acid in the acidic solution is not reducing or in order to improve the reducing property of the acidic solution, in order to stabilize the content of iron ions dissolved from the positive electrode active material after the reaction with the acid in the filtrate, the accuracy of detecting the content of iron in the filtrate is improved. In some optional embodiments, the acidic solution contains a reducing agent, and the reducing agent includes one or more of a soluble fluoride salt and a soluble ethylenediaminetetraacetic acid disalt. In some embodiments, the reducing agent includes one or more of sodium fluoride, potassium fluoride, disodium ethylenediaminetetraacetate, and dipotassium ethylenediaminetetraacetate. In some optional embodiments, the molar concentration of the reducing agent in the acidic solution is 0.005 mol/L to 1 mol/L.

According to the embodiment of the present application, the type and concentration of the reducing agent in each group of acidic solutions are the same. Further, the type and concentration of the reducing agent can be arbitrarily selected within the above range.

In order to stabilize the content of iron ions dissolved by the positive electrode active material after the acid reaction in the filtrate, reduce or reduce the iron ions in the solution to become insoluble matter, suspended matter or partial precipitation, affecting the accuracy of the content of iron in the filtrate. Add a complexing agent in an acidic solution. In some optional embodiments, the complexing agent includes one or more of ethylenediaminetetraacetic acid or its salt, diisopropyltriaminepentaacetic acid or its salt, dithiodiisopropylaminetetraacetic acid or its salt, thioacetate, and soluble thiocyanide. In some embodiments, the complexing agent includes disodium ethylenediaminetetraacetic acid, dipotassium ethylenediaminetetraacetic acid, diisopropyltriaminepentaacetic acid, sodium diisopropyltriaminepentaacetate, potassium diisopropyltriaminepentaacetate, dithiodiisopropylaminetetraacetic acid, potassium dithiodiisopropylaminetetraacetate, sodium dithiodiisopropylaminetetraacetate, potassium thioacetate, sodium thioacetate, sodium thiocyanide, and potassium thiocyanide.

In addition, in order to prevent some trivalent iron ions from forming precipitation with excess phosphate ions in the positive electrode active material or in the acidic solution, thereby introducing test errors, a small amount of a complexing agent that can complex with the iron element can be added to the solution.

According to the embodiment of the present application, the type of complexing agent in each group of acidic solutions is the same. Further, the type of complexing agent can be arbitrarily selected within the above range.

In order to reduce the influence of the complexing agent on the pH of the acidic solution, and reduce the influence of the complexing agent on the subsequent detection of the iron ion concentration. In some optional embodiments, the molar concentration of the complexing agent in the acidic solution is 0.005 mol/L to 1 mol/L. According to the embodiment of the present application, the molar concentration of the complexing agent in each group of acidic solutions is the same. Further, the concentration of the complexing agent can be arbitrarily selected in the following range.

Optionally, the molar concentration of the complexing agent in the acidic solution can be any value among 0.005mol/L, 0.01mol/L, 0.015mol/L, 0.02mol/L, 0.03mol/L, 0.04mol/L, 0.05mol/L, 0.06mol/L, 0.07mol/L, 0.08mol/L, 0.09mol/L, 0.10mol/L, 0.20mol/L, 0.30mol/L, 0.40mol/L, 0.50mol/L, 0.60mol/L, 0.70mol/L, 0.80mol/L, 0.90mol/L, 1.0mol/L, or a range of their combinations.

In some embodiments, the acidic solution is reducing and contains a complexing agent.

Those skilled in the art can determine the amount of powder of the appropriate positive electrode active material, the choice of acid, the volume and concentration of the acidic solution, etc. according to actual requirements, so as to quickly detect the concentration of Fe or compare the coating completeness of the positive electrode active material. Therefore, according to the contents described in this article, those skilled in the art will consider scaling up or down each parameter system in proportion, such as scaling up or down the amount of powder and the volume of the acidic solution in proportion, to obtain the same result.

In some optional embodiments, the powder samples of each group of positive electrode active materials are mixed with the acidic solution for reaction at a temperature of 5° C. to 65° C.

Optionally, the temperature for mixing the powder samples of each group of positive electrode active materials with the acidic solution for reaction can be any value among 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C or a range of their combinations.

According to the embodiment of the present application, the temperature at which each group of powder samples of positive electrode active materials are mixed with an acidic solution for reaction is the same. Further, the temperature of the reaction can be arbitrarily selected within the following range. Controlling the temperature of the reaction can control the degree of reaction between the acid and the powder sample of the positive electrode active material, which is beneficial for detecting the concentration of Fe in each group of reactions under relative conditions, comparing the coating integrity of the positive electrode active materials, and improving the accuracy of the detection of the dissolved iron element concentration and the accuracy of the results of batch comparison of the coating integrity of the positive electrode active materials.

In some optional embodiments, the time for mixing and reacting the powder samples of each group of positive electrode active materials with the acidic solution is 1 min to 90 min.

Optionally, the time for mixing the powder samples of each group of positive electrode active materials with the acidic solution for reaction can be any value among 1min, 1.5min, 2min, 2.5min, 3min, 5min, 10min, 20min, 30min, 40min, 50min, 60min, 70min, 80min, 90min or a range of their combinations.

According to the embodiment of the present application, the time for each group of powder samples of positive electrode active materials to be mixed with the acidic solution for reaction is the same. Further, the reaction time can be arbitrarily selected within the above range. Controlling the reaction time can control the degree of reaction between the acid and the powder sample of the positive electrode active material, which is beneficial for detecting the concentration of Fe in each group of reactions under relative conditions, comparing the coating integrity of the positive electrode active materials, and improving the accuracy of the detection of the dissolved iron element concentration and the accuracy of the results of batch comparison of the coating integrity of the positive electrode active materials.

The solid-liquid separation of the reaction mixture can be carried out in a manner commonly used in the art. It is worth noting that attention should be paid to the separation time and the degree of contact with the air during separation in order to control the degree of reaction between the powder samples of each group of positive electrode active materials and the acidic solution. In some optional embodiments, the solid-liquid separation of each group of reaction mixtures includes one or more of centrifugal filtration, suction filtration, and membrane filtration. For example, high-speed centrifugation or a microporous needle filter membrane can be used for separation. In some embodiments, a filter membrane with a pore size of 0.2um is most commonly used for filtration.

The method for detecting the concentration of the metal element Fe can be carried out by using a method known in the art for detecting the concentration of metal elements in a solution. In some optional embodiments, the method for detecting the concentration of the metal element Fe in each group of filtrates is one or more of inductively coupled plasma spectroscopy, ultraviolet spectrometry, electrochemical method, atomic absorption method or atomic emission method.

The coating integrity of the LiFePO ₄ positive electrode active material can be qualitatively or quantitatively evaluated according to the concentration of the metal element Fe.

When quantitative evaluation is performed according to the method of the present application, for example, a series of _LiFePO4 positive active materials with known different coating degrees can be used as standard references. Specifically, appropriate and determined experimental parameters and conditions are selected as reference parameters and conditions through experiments, and the concentration of the metal element Fe obtained by each of the series of standard references under the reference parameters and conditions is determined C _Fe standard, while ensuring that the parameters of each reaction are within the preferred range defined above. Then, the coating integrity of each reference is plotted against the concentration C _Fe standard of each metal element Fe and curve fitting is performed. When it is necessary to quickly determine the coating integrity of a certain _LiFePO4 positive active material, the concentration of the metal element Fe in the _LiFePO4 positive active material to be tested is detected using the parameters and conditions used as references, and the concentration C _Fe test of the metal element Fe is obtained, while ensuring that the experimental parameters and conditions selected by the experiment are consistent with the test conditions when the curve is fitted. Then, based on the measured C _Fe test, the coating integrity of the material can be quickly inferred using the above-mentioned fitting curve.

Method for detecting whether the coating integrity of positive electrode active materials meets the standards

In a second aspect, the present application provides a method for detecting whether the coating integrity of a positive electrode active material meets the standard, comprising the following steps:

Providing a positive electrode active material sample that meets the standards and a positive electrode active material sample to be tested; the two groups of positive electrode active material samples respectively contain iron elements and the two groups of positive electrode active materials respectively have a coating layer on at least part of their surfaces;

The positive electrode active material samples of the same mass are mixed with the acidic solution of the same concentration, volume and type and reacted at the same temperature and time, wherein the acidic solution contains a complexing agent; the time for mixing the powder samples of each group of positive electrode active materials with the acidic solution to react is 1 min to 90 min respectively;

Separating the reaction mixture into solid and liquid to obtain a filtrate and an insoluble substance;

The concentration of the metal element Fe in the filtrate was detected respectively;

If the concentration of the metal element Fe of the positive electrode active material sample to be tested is less than or equal to the concentration of the metal element Fe of the positive electrode active material sample that meets the standard, the coating integrity of the positive electrode active material sample to be tested meets the standard.

In the method of this embodiment, multiple groups of powder samples of positive electrode active materials of the same mass and type are respectively mixed with acidic solutions of the same concentration, volume and type and reacted at the same temperature and time. The concentration of Fe in the filtrate is detected, and the coating integrity of the multiple groups of positive electrode active materials is evaluated according to the concentration of the metal element Fe. This can not only improve the accuracy of the concentration of the metal element Fe in the filtrate after the reaction, but also control the degree of reaction between the acidic solution and the positive electrode active material in the multiple groups of reactions within an appropriate range, so that a more accurate comparison of the coating integrity of the positive electrode active material can be made according to the concentration of the dissolved metal element Fe.

The method of the present application simplifies the detection process of the coating integrity of the positive electrode active material with a carbon coating layer, improves the accuracy of comparing the coating integrity of the positive electrode active material, and the detection process is low-cost and short-cycle, which is convenient for batch detection of the coating integrity of positive electrode active materials from different batches or manufacturers, improves the accuracy of batch detection of the concentration of dissolved Fe, and thus improves the accuracy of the comparison result.

The method of the embodiment of the present application can intuitively judge the coating integrity of the positive electrode active material without complicated experimental process, saving the verification cycle of raw materials in the production process and reducing costs.

According to an embodiment of the present application, a method for detecting whether the coating integrity of a positive electrode active material meets the standard can be a qualitative evaluation method. In this method, a _LiFePO4 positive electrode active material with known coating integrity is used as a standard reference. Specifically, appropriate experimental parameters and conditions are selected as reference parameters and conditions through experiments, and the concentration of the metal element Fe obtained by the standard reference under the reference parameters and conditions is determined. The C _Fe standard, while ensuring that the conditions for reacting with the acid are within the preferred range defined above. When it is necessary to quickly determine the coating integrity of a certain _LiFePO4 positive electrode active material, the concentration of the metal element Fe in the _LiFePO4 positive electrode active material to be tested is detected using the parameters and conditions used as references, and the concentration of the metal element Fe is obtained. C _Fe test, while ensuring that the test conditions are consistent with the C _Fe standard test conditions. By comparing the C _Fe standard and the C _Fe test, the coating integrity of the _LiFePO4 positive electrode active material to be tested can be quickly evaluated. Specifically, if C _Fe standard > C _Fe test, the coating integrity of the LiFePO ₄ positive electrode active material used as the standard reference is worse than the coating integrity of the tested positive electrode active material, that is, it is more incomplete; if C _Fe standard < C _Fe test, the coating integrity of the LiFePO ₄ positive electrode active material used as the standard reference is better than the coating integrity of the tested positive electrode active material, that is, it is more complete.

The properties of the multiple groups of positive electrode active materials themselves in any embodiment of the method of the first aspect, such as the coating conditions, the mass of any multiple groups of positive electrode active materials, the concentration and volume of the acidic solution, the type of acid, the reaction temperature and time and other conditions can be independently applied to the method of this embodiment, and will not be elaborated here.

Device for comparing coating integrity of positive electrode active materials

Based on the same inventive concept, the embodiment of the present application also provides a device for comparing the coating integrity of positive electrode active materials.

The device for comparing the coating integrity of positive electrode active materials provided in an embodiment of the present application is described in detail below in conjunction with FIG. 1 .

FIG. 1 shows a device for comparing coating integrity of positive electrode active materials provided by one embodiment of the present application.

As shown in FIG. 1 , the apparatus 100 for comparing the coating integrity of the positive electrode active material may include:

The powder storage module 101 is used to contain multiple groups of positive electrode active materials, wherein the positive electrode active materials contain iron and at least part of the surface of the positive electrode active materials has a coating layer;

The acidic solution storage module 102 is used to contain an acidic solution, wherein the acidic solution contains a complexing agent;

The mixing reaction module 103 is used to mix the acidic solution with multiple groups of positive electrode active materials respectively, and make the powder samples of multiple groups of positive electrode active materials of the same mass be mixed with the acidic solution of the same concentration, the same volume and the same type and react at the same temperature and the same time; the time for the powder samples of each group of positive electrode active materials to be mixed and reacted with the acidic solution is 1 min to 90 min respectively;

The solid-liquid separation module 104 is used to perform solid-liquid separation on the mixture after each reaction and obtain a filtrate and an insoluble matter;

The detection module 105 is used to detect the concentration of the metal element Fe in each group of filtrates;

The judgment module 106 is used to evaluate the coating integrity of the plurality of groups of positive electrode active materials according to the concentration of the metal element Fe in each group of positive electrode active materials.

The device of the present application can take multiple groups of powder samples of positive electrode active materials of the same mass and type, mix them with acidic solutions of the same concentration, volume and type, and react them at the same temperature and time, thereby realizing closed-loop control of the detection process, automatically adjusting the reaction conditions, and ensuring the same reaction conditions, thereby improving the accuracy of the concentration of the metal element Fe and improving the accuracy of the comparison results.

Moreover, the detection process is low-cost and short-cycle, and is convenient for batch testing of the coating integrity of positive electrode active materials from different batches or manufacturers, thereby improving the accuracy of the comparison results.

In some embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the powder storage module 101 may include:

A specific surface area detection module or a specific surface area input module, used to detect or input the specific surface areas of any two groups of positive electrode active materials among a plurality of groups of positive electrode active materials used to compare the coating integrity;

The specific surface area selection module is used to select the positive electrode active materials whose specific surface area ratio between any two groups of positive electrode active materials is 1:(0.6-1.8) for comparison. When the specific surface area ratio between any two groups of positive electrode active materials is not within the range, a prompt is given or subsequent processes are not performed.

In some embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the powder storage module 101 may include:

A volume particle size Dv50 detection module or a volume particle size Dv50 input module, used to detect or input the volume particle sizes Dv50 of any two groups of positive electrode active materials among a plurality of groups of positive electrode active materials used to compare coating integrity;

The volume particle size Dv50 selection module is used to select the positive electrode active materials whose volume particle size Dv50 ratio of any two groups of positive electrode active materials is 1: (0.6-2.2) for comparison. When the volume particle size Dv50 ratio of any two groups of positive electrode active materials is not within the range, a prompt is given or the subsequent process is not performed.

In some embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the powder storage module 101 may include:

A particle size distribution span detection module or a particle size distribution span input module, used to detect or input the particle size distribution spans of any two groups of positive electrode active materials among a plurality of groups of positive electrode active materials used to compare coating integrity;

The particle size distribution span selection module is used to select the positive electrode active materials whose particle size distribution span ratio of any two groups of positive electrode active materials among multiple groups of positive electrode active materials is 1:(0.4-1.6) for comparison. When the ratio of the particle size distribution span of any two groups of positive electrode active materials is not within the range, a prompt is given or the subsequent process is not performed.

In some embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the powder storage module 101 may include:

A coating layer thickness detection module or a coating layer thickness input module, used to detect or input the coating layer thickness of any two groups of positive electrode active materials among a plurality of groups of positive electrode active materials used to compare coating integrity;

The coating thickness selection module is used to select the positive electrode active materials whose coating thickness ratio between any two groups of positive electrode active materials is 1:1:(0.5-2.0) for comparison. When the coating thickness ratio between any two groups of positive electrode active materials is not within the range, a prompt is given or the subsequent process is not performed.

In some embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the powder storage module 101 may include:

A coating layer main component detection module or a coating layer main component input module, used to detect or input the main components of the coating layers of any two groups of positive electrode active materials among a plurality of groups of positive electrode active materials used to compare coating integrity;

The coating layer main component selection module is used to select the coating layer main components of any two groups of positive electrode active materials from multiple groups of positive electrode active materials for comparison. When the coating main components of any two groups of positive electrode active materials are different, a prompt is given or subsequent processes are not performed.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the powder storage module 101 may include:

A positive electrode active material chemical structure formula input module, used to input the chemical structure formulas of any two groups of positive electrode active materials among a plurality of groups of positive electrode active materials used to compare coating integrity;

The chemical structure general formula selection module is used to select the positive electrode active materials of the chemical structure general formula of any two groups of positive electrode active materials from multiple groups of positive electrode active materials for comparison. When the chemical structure general formula of any two groups of positive electrode active materials is different, a prompt is given or subsequent processes are not performed.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the acidic solution storage module 102 may include:

An acid solution initial concentration detection module or an acid solution initial concentration input module, used to detect or input the initial concentration of the acid solution for comparing the coating integrity;

The initial concentration selection module of the acid solution is used to select an acid solution with an initial concentration C _acid in the range of 0.002 mol/L to 1 mol/L for use with multiple groups of positive electrode active materials. When the initial concentration C _acid of the acid solution is not within the range, a prompt is given or subsequent processes are not performed.

In some optional embodiments, the acidic solution includes one or more of ascorbic acid, citric acid, acetic acid, HCl, HF, H ₂ SO ₄ and H ₃ PO ₄ .

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the acidic solution storage module 102 may include:

The reducing agent input module is used to input the type of reducing agent and the concentration of the reducing agent in the acidic solution. In some embodiments, the molar concentration of the reducing agent in the acidic solution is 0.005 mol/L to 1 mol/L.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the acidic solution storage module 102 may include:

The complexing agent input module is used to input the type of complexing agent in the acidic solution and the concentration of the complexing agent. In some embodiments, the complexing agent includes one or more of disodium ethylenediaminetetraacetate, dipotassium ethylenediaminetetraacetate, diisopropyltriaminepentaacetic acid, sodium diisopropyltriaminepentaacetate, potassium diisopropyltriaminepentaacetate, dithiodiisopropylaminetetraacetic acid, potassium dithiodiisopropylaminetetraacetate, sodium dithiodiisopropylaminetetraacetate, potassium thioacetate, sodium thioacetate, sodium thiocyanide, and potassium thiocyanide. In some optional embodiments, the molar concentration of the complexing agent in the acidic solution is 0.005 mol/L to 1 mol/L.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the acidic solution storage module 102 may include:

The module for judging whether the acidic solution is reducible or contains a complexing agent is used to judge whether the acidic solution is reducible or contains a complexing agent. When the acidic solution is neither reducible nor contains a complexing agent, a prompt is given or the subsequent process is not performed.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the mixing reaction module 103 may include:

The volume distribution module of the acidic solution is used to provide and distribute the acidic solution of each group of positive electrode active materials for comparing the coating integrity to ensure that the volume of the acidic solution of each group is the same. In some optional embodiments, the volume of the acidic solution is 30ml to 2000ml.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the mixing reaction module 103 may include:

The powder sample distribution module of the positive electrode active material is used to provide and distribute the mass of each group of powder samples of the positive electrode active material for comparing the coating integrity to ensure the mass of each group of powder samples. In some optional embodiments, the mass of each group of powder samples of the positive electrode active material is 0.2g to 10g.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the mixing reaction module 103 may include:

A first detection submodule, used to detect the ratio of the mass of each group of positive electrode active material powder samples to the volume of the acidic solution;

The selection submodule is used to select a mixture in which the ratio of the mass of the powder sample in the initial state to the volume of the acidic solution is within the range of 0.0025g:1ml to 0.5g:1ml to ensure that it reacts under appropriate conditions. When the ratio of the mass of the powder sample to the volume of the acidic solution is not within the range, a prompt is given or subsequent processes are not performed.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the mixing reaction module 103 may include:

The reaction temperature control module is used to control the temperature of each group of positive electrode active material powder samples mixed with the acid solution to react at the same temperature, and the temperature is selected from 5°C to 65°C.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the mixing reaction module 103 may include:

The reaction temperature detection module is used to detect whether the powder samples of each group of positive electrode active materials are mixed with the acid solution to react at a temperature of 5°C to 65°C. When the temperature of the detected reaction is not within the range, a prompt is given or the subsequent process is not performed.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the mixing reaction module 103 may include:

The reaction time control module is used to ensure that the powder samples of each group of positive electrode active materials are mixed with the acid solution for the same reaction time, and the time can be selected within 1 minute to 90 minutes.

In some optional embodiments, in order to more accurately obtain the concentration of metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the solid-liquid separation module 104 may include any one of the following modules: a centrifugal filtration module, a suction filtration module, and a membrane filtration module.

In some optional embodiments, in order to more accurately obtain the concentration of the metal element Fe and improve the accuracy of the results of comparing the coating integrity of the positive electrode active material, the detection module 105 may include any one of the following modules: an inductively coupled plasma spectroscopy detection module, an ultraviolet spectroscopy detection module, an electrochemical detection module, an atomic absorption detection module, and an atomic emission detection module.

It should be noted that the device can execute the method described in any of the above embodiments, and the specific content is not repeated here.

Although the present application has been described with reference to preferred embodiments, various modifications may be made thereto and parts thereof may be replaced with equivalents without departing from the scope of the present application. In particular, the various technical features mentioned in the various embodiments may be combined in any manner as long as there is no structural conflict. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Application Example 1

This embodiment provides a method for comparing the coating integrity of positive electrode active materials, comprising:

A plurality of groups of positive electrode active materials are provided, wherein the positive electrode active materials contain iron and at least part of the surface of the positive electrode active materials has a coating layer; wherein the structural formula of the positive electrode active materials is LiFePO ₄ , the plurality of groups of positive electrode active materials are respectively divided into three groups of powder materials: 0326P1, 0325P2, and 0326P5; the specific surface areas of the positive electrode active materials of any two groups of the plurality of groups of positive electrode active materials are respectively 12.8m ² /g, 13.0m ² /g, and 13.2m ² /g; the volume particle sizes Dv50 are respectively 0.76μm, 0.8μm, and 0.83μm; the coating layer is carbon, and the mass content of the coating layer in the positive electrode active material is 2.0%, 1.6%, and 2.4%;

Respectively taking multiple groups of positive electrode active material powder samples of the same mass and type and mixing them with the same concentration, volume and type of acidic solution and reacting them at the same temperature and for the same time, wherein the acidic solution is a 0.1 M sulfuric acid solution;

Separating the mixture after each reaction into solid and liquid to obtain a filtrate and an insoluble substance;

The concentration of the metal element Fe in each group of filtrates is detected respectively, and the coating integrity of the multiple groups of positive electrode active materials is evaluated according to the concentration of the metal element Fe.

Specifically: 0.5 g of disodium ethylenediaminetetraacetic acid chelating agent was added to the acidic solution, and then 1 g of three batches of lithium iron phosphate materials with different coating states were mixed with 50 mL of 0.1 M sulfuric acid solution, and acid dissolution reaction was carried out at 25 °C. The reaction time was 30 min, 60 min, and 90 min, respectively.

Application Comparative Example 1

The difference between the comparative example 1 and the example 1 is that 0.5 g of disodium ethylenediaminetetraacetate is not added.

The filtrates of Application Example 1 and Application Comparative Example 1 were tested by inductively coupled plasma spectroscopy. The test results are shown in Table 1.

Table 1

The test results in Table 1 show that in the three groups of powder materials 0326P1, 0325P2, and 0326P5, the chelating agent disodium ethylenediaminetetraacetate is added, and the iron concentration is 0325P2>0326P5>0326P1, and the coating integrity is: 0325P2<0326P5<0326P1. Without the addition of the chelating agent disodium ethylenediaminetetraacetate, the iron concentration is 0325P2>0326P1>0326P5; the coating integrity is: 0325P2<0326P1<0326P5.

The test results show that when no ligand is added, obvious precipitation can be observed to improve the integrity of the coating, and the detected amounts of iron ions in different sample solutions are very close, with no differentiation. When a ligand is added, the ligand is complexed with trivalent iron, and the detection value of iron ions in the solution increases by an order of magnitude, and samples with different carbon coating states have obvious differentiation. In addition, as the reaction time increases, limited by the total content of iron, the amount of iron dissolved in the sample solution will gradually approach the peak value, and the sample differences cannot be effectively identified. Therefore, in order to accurately distinguish the differences in sample coating, a suitable detection time should be selected.

Application Example 2

The difference between the second application example and the first application example is that: due to the different acid concentrations, the multiple groups of positive electrode active materials are divided into six groups of powder materials, namely 0326P1, 0325P2, 0326P5, 0325P6, 0326P6, and 0325P6-E. The specific surface areas of any two groups of positive electrode active materials in the multiple groups of positive electrode active materials are 12.8m2 ^/ g, ^13.0m2/ g, 13.2m2 ^/ g, 12.9m2 ^/ g, 13.1m2 ^/ g, and 13.2m2 ^/ g, respectively, and the volume particle sizes Dv50 are 0.76μm, 0.8μm, 0.83μm, 0.8μm, 0.78μm, and 0.81μm, respectively. The coating layer is carbon, and the mass content of the coating layer in the positive electrode active material is 2.0%, 1.6%, 2.4%, 1.9%, 2.3%, and 2.1%. 1 g of six batches of samples with different coating states were respectively subjected to acid dissolution test in 50 mL of 0.01 M sulfuric acid solution, and 0.5 g of disodium ethylenediaminetetraacetic acid complexing agent was added. The iron dissolution test was repeated three times. The test results are shown in Table 2.

Table 2

The test results in Table 2 show that, compared with the test results of the three groups of powder materials 0326P1, 0325P2, and 0326P5 in Application Example 1, reducing the acid concentration can significantly reduce the amount of iron element dissolution in Application Example 2, and the test results of different coated samples have a high degree of differentiation, indicating that controlling the acid concentration is beneficial to controlling the degree of reaction between the material and the acid, which is beneficial to improving the differentiation of the iron content and improving the accuracy of the test results.

At the same time, the difference in coating integrity of the three samples 0326P1, 0325P2 and 0326P5 was further confirmed; after adding the chelating agent disodium ethylenediaminetetraacetic acid, the concentration of iron was 0325P2>0326P5>0326P1, and the coating integrity was 0325P2<0326P5<0326P1. The acid dissolution test method has high repeatability and can accurately identify the coating integrity differences of different batches of samples. The comparison results are consistent with the results of sampling observation under an electron microscope, and the comparison results are accurate.

The test results of the three groups of powder materials, 325P6, 0326P6 and 0325P6-E, show that the comparison method of the present application can compare the coating integrity of the positive electrode active material by the iron concentration, and the comparison results are consistent with the results of sampling observation under an electron microscope, indicating that the comparison results of the coating integrity of the present application are accurate.

Application Example 3

The difference between the third application example and the first application example is that the types of acids are different, and the ascorbic acid with a certain reducing property has a concentration of 0.05 mol/L and 0.02 mol/L respectively. Six groups of 50 mL ascorbic acid acid solutions with two concentrations were mixed with 1 g of lithium iron manganese phosphate materials, which were products from different batches and had different coating states. Multiple groups of positive electrode active materials lithium iron manganese phosphate were divided into three groups of powder materials, 0203-M3, 0203-M5, and 0203-M6. The acid dissolution reaction was carried out at 25 °C, and the reaction time was 5 min and 10 min. The structural formula of the positive electrode active materials is LiFe _0.4 Mn _0.6 PO ₄ , and the multiple groups of positive electrode active materials are divided into three groups of powder materials, 0203-M3, 0203-M5, and 0203-M6. The specific surface areas of any two groups of positive electrode active materials are within 10.9m ² /g, 12.2m ² /g, and 11.5m ² /g, respectively. The volume particle size Dv50 is 0.8μm, 1.0μm, and 1.1μm, respectively. The span values are 2.2, 2.8, and 3.1, respectively. The coating layer is carbon, and the mass content of the coating layer in the positive electrode active material powder is 1.7%, 2.3%, and 2.0%, respectively. Ascorbic acid can reduce trivalent iron to divalent iron and reduce the formation of precipitation. The test results are shown in Table 3.

table 3

The test results show that the coating integrity of the positive electrode active material of the present application is 0203-M3＜0203-M5＜0203-M6. This shows that the method of the present application can effectively distinguish lithium manganese iron phosphate materials with different coating states, and the comparison results are consistent with the results of sampling observation under an electron microscope, and the comparison results are accurate. The method of the present application has a certain degree of universality.

Application Example 4

This embodiment provides a method for comparing the coating integrity of positive electrode active materials, comprising: respectively subjecting 1 g of three batches of coated LiFePO ₄ samples with different coating states to an acid dissolution test in 50 mL of 0.01 M sulfuric acid solution, and adding 0.5 g of disodium ethylenediaminetetraacetate, reacting at room temperature for a certain period of time at a stirring speed of 300 r/min, using a 1 mL syringe to draw the solution and filtering it through a filter head with a pore size of 0.2 μm, diluting and fixing the volume, and conducting an element content test, and conducting 100 experiments, wherein the test results of iron dissolution in three repeated experiments are shown in Table 2, and the accuracy of the test results of 100 experiments is shown in the following table. . It is known that the coating integrity of three groups of positive electrode active materials is A3>A2>A1. The control group uses a sulfuric acid solution with a concentration of 0.01 M, and no complexing agent disodium ethylenediaminetetraacetate is added to react with the A1 powder. Among them, the specific surface areas of LiFePO ₄ samples range from 9.0m ² /g, 10.2m ² /g, and 11.8m ² /g, respectively; the volume particle size Dv50 ranges from 1.6μm, 2.2μm, and 2.7μm, respectively; and the mass content of carbon in LiFePO ₄ samples is 1.2%, 1.8%, and 2.1%, respectively.

Table 4

The evaluation results of powder materials such as A1, A2, and A3 by adding an acidic solution of disodium ethylenediaminetetraacetate showed that the test results of the same coated samples had a high degree of differentiation. The lower the coating integrity of the three samples, the higher the amount of iron dissolved. The acid dissolution test method has high repeatability and can accurately identify the differences in coating integrity of samples from different batches. The coating integrity of the positive active material of this application is A3>A2>A1. The comparison results are consistent with the known results of sampling observations under an electron microscope, indicating that the comparison results of the coating integrity of this application are accurate.

In other repetitive experiments of the embodiments of the present application, in each group of 100 parallel experiments after removing obvious experimental errors, the evaluation results of powder materials such as A1, A2, and A3 with the addition of an acidic solution of disodium ethylenediaminetetraacetate were higher in the 30min and 60min groups; the evaluation results of powder materials such as A1, A2, and A3 with the addition of an acidic solution of no disodium ethylenediaminetetraacetate were lower in the 30min and 60min groups.

Moreover, when the amount of Fe dissolved in the solution was detected after 2h and 3h of reaction, the evaluation results of the coating integrity of powder materials such as A1, A2, and A3 based on the Fe dissolved amount were significantly lower than those of the 30min and 60min groups.

Therefore, it is shown that the comparison results of multiple groups of positive electrode active material powders using the method of the present application can more quickly improve the accuracy of the comparison results within the reaction time of the present application, thereby improving the reliability of the method of the present application in practical applications.

According to known standard samples, the surface area of lithium iron phosphate material is in the range of 10 m ² /g, the volume particle size Dv50 is 2.4 μm, the span value is in the range of 12, and the mass content of carbon in the LiFePO ₄ sample is 1.8%.

Application Example 5

1 g of four batches of coated LiFe _0.5 Mn _0.5 PO ₄ samples with different coating processes were C1, C2, C3, and C4. 0.5 g of disodium ethylenediaminetetraacetate complexing agent was added to 100 mL of 0.05 M citric acid solution, among which the specific surface areas of LiFe _0.5 Mn _0.5 PO ₄ samples were 12.5 m ² /g, 13.0 ^{m 2} /g, and 13.3 m ² /g, respectively, Dv50 was 0.77 μm, 0.82 μm, and 0.84 μm, respectively, and the volume particle size span values were 7.5, 8.1, and 8.5, respectively. Carbon was coated on the surface of LiFe _0.5 Mn _0.5 PO ₄ sample particles, and the mass content of carbon in LiFe _0.5 Mn _0.5 PO ₄ samples was 1.5%. The citric acid solution was mixed with 1 g of LiFe _0.5 Mn _0.5 PO ₄ sample and reacted at room temperature at a stirring speed of 300 r/min for 5 minutes. The solution was then aspirated with a 1 mL syringe and filtered through a filter head with a pore size of 0.2 μm. The element content was tested after dilution and constant volume. The average results of the iron and manganese dissolution tests of three parallel experiments are shown in Figure 1.

The test results show that the coating integrity is C1＜C2＜C3＜C4. With the improvement of the coating process, the iron and manganese dissolution of samples in groups C1, C2, C3 and C4 gradually decreases, which is consistent with the direction of coating improvement process. The LiFe _0.5 Mn _0.5 PO ₄ sample has better coating integrity.

According to the LiFe _0.5 Mn _0.5 PO ₄ standard sample, the standard value of iron dissolution tested under the same conditions as above is 1800ppm. Only the C3 and C4 group samples meet the test requirements. The samples meet the standards and comply with the feed standards and improvement standards.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and they should all be included in the scope of the claims and specification of the present application. In particular, as long as there is no structural conflict, the various technical features mentioned in the various embodiments can be combined in any way. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims (14)

1. A method of comparing the coating integrity of a positive electrode active material, comprising:

Providing a plurality of groups of positive electrode active materials, wherein the positive electrode active materials comprise iron element, and at least part of the surfaces of the positive electrode active materials are provided with coating layers;

Respectively taking powder samples of multiple groups of positive electrode active materials with the same mass and the same kind, mixing the powder samples with the same concentration, the same volume and the same kind of acid solutions, and reacting the mixed powder samples at the same temperature and the same time, wherein the acid solutions comprise complexing agents; the powder samples of each group of positive electrode active materials are mixed with the acid solution for reaction for 1 min-90 min respectively;

respectively carrying out solid-liquid separation on the mixture after each group of reaction to obtain filtrate and insoluble matters;

And respectively detecting the concentration of metal element Fe in each group of filtrate, and evaluating the coating integrity of the plurality of groups of positive electrode active materials according to the concentration of the metal element Fe.

2. The method of claim 1, wherein the plurality of sets of positive electrode active materials satisfy one or more of the following conditions:

1) The ratio of the specific surface areas of any two groups of positive electrode active materials in the plurality of groups of positive electrode active materials is 1: (0.6-1.8);

2) The ratio of the volume particle diameter Dv50 of any two groups of positive electrode active materials in the plurality of groups of positive electrode active materials is 1: (0.6-2.2);

3) The ratio of the particle size distribution spans of any two groups of positive electrode active materials in the plurality of groups of positive electrode active materials is 1: (0.4-1.6);

4) The ratio of the thicknesses of the coating layers of any two groups of positive electrode active materials in the plurality of groups of positive electrode active materials is 1: (0.5-2.0);

5) The mass content of the coating layer in the positive electrode active material is 0.8-2.4%;

6) The groups of positive electrode active materials have the same chemical structural general formula.

3. The method according to claim 1 or 2, wherein the positive electrode active material comprises one or more of a lithium iron phosphate material covered by a cover layer, a lithium manganese iron phosphate material covered by a cover layer, and respective doping modified materials thereof; the coating layer comprises more than 90% of carbon and the balance of one or more of oxides and/or fluorides of metal elements in percentage by mass, wherein the metal elements comprise one or more of iron element, manganese element, lithium element and aluminum element.

4. The method according to claim 1 or 2, wherein the powder samples of the plurality of sets of positive electrode active materials each having the same mass are immersed in the same concentration and the same kind of acidic solution and reacted at the same temperature and time.

5. The method according to claim 1 or 2, wherein the ratio of the mass of the powder sample of each group of positive electrode active materials to the volume of the acidic solution is 0.0025g:1ml to 0.5g:1ml.

6. The method according to claim 1 or 2, wherein the mass of the powder sample of each group of the positive electrode active material is 0.2g to 10g.

7. The method according to claim 1 or 2, characterized in that the acidic solution fulfils one or several of the following conditions:

1) The acidic solution includes a reducing acid;

2) The acidic solution comprises one or more of ascorbic acid, citric acid, acetic acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and phosphoric acid;

3) The initial concentration C _Acid(s) of the acid solution is 0.002 mol/L-1 mol/L;

4) The volume of the acid solution is 30 ml-2000 ml;

5) The acidic solution contains a reducing agent, wherein the reducing agent comprises one or more of soluble fluoride salt and soluble ethylenediamine tetraacetic acid disodium salt.

8. The method according to claim 1 or 2, characterized in that the initial concentration C _Acid(s) of the acidic solution is 0.01mol/L to 1mol/L.

9. The method according to claim 1 or 2, wherein the complexing agent satisfies one or more of the following conditions:

1) The complexing agent comprises one or more of ethylenediamine tetraacetic acid or salt thereof, diisopropyltriamine pentaacetic acid or salt thereof, dimercaptodiisopropylamine tetraacetic acid or salt thereof, thioacetate and soluble thiocyanide;

2) The molar concentration of the complexing agent in the acidic solution is 0.005 mol/L-1 mol/L.

10. The method according to claim 1 or 2, wherein the powder samples of the positive electrode active materials of the respective groups are mixed with an acidic solution to perform the reaction at a temperature of 5 ℃ to 65 ℃, respectively.

11. The method according to claim 1 or 2, wherein the solid-liquid separation of the mixture after each group of reactions comprises one or more of centrifugal filtration, suction filtration and filtration with a filter membrane.

12. The method according to claim 1 or 2, wherein the method for detecting the concentration of the metal element Fe in each group of filtrates is one or more of inductively coupled plasma spectroscopy, ultraviolet spectroscopy, electrochemical methods, atomic absorption methods, or atomic emission methods.

13. A method of detecting whether the coating integrity of a positive electrode active material meets the standards, comprising:

Providing a standard positive electrode active material sample and a positive electrode active material sample to be detected; the two groups of positive electrode active material samples respectively contain iron element, and at least part of the surfaces of the two groups of positive electrode active materials are respectively provided with a coating layer;

Respectively taking positive electrode active material samples with the same mass, mixing the positive electrode active material samples with the same concentration, the same volume and the same kind of acid solution, and reacting the positive electrode active material samples at the same temperature and the same time, wherein the acid solution comprises a complexing agent; the powder samples of each group of positive electrode active materials are mixed with the acid solution for reaction for 1 min-90 min respectively;

respectively carrying out solid-liquid separation on the mixture after the reaction to obtain filtrate and insoluble matters;

respectively detecting the concentration of metal element Fe in the filtrate;

and if the concentration of the metal element Fe of the positive electrode active material sample to be detected is smaller than or equal to the concentration of the metal element Fe of the positive electrode active material sample reaching the standard, the coating integrity of the positive electrode active material sample to be detected reaches the standard.

14. An apparatus for comparing the coating integrity of a positive electrode active material, comprising:

the powder storage module is used for accommodating a plurality of groups of positive electrode active materials, wherein the positive electrode active materials comprise iron element, and at least part of the surfaces of the positive electrode active materials are provided with coating layers;

an acidic solution storage module for containing an acidic solution, the acidic solution comprising a complexing agent;

The mixing reaction module is used for respectively mixing the acid solution and a plurality of groups of positive electrode active materials, mixing powder samples of a plurality of groups of positive electrode active materials with the same mass with the acid solution with the same concentration, the same volume and the same kind, and reacting at the same temperature and the same time;

the solid-liquid separation module is used for respectively carrying out solid-liquid separation on the mixture after each group of reaction to obtain filtrate and insoluble matters, and the mixing reaction time of the powder sample of each group of positive electrode active materials and the acid solution is 1-90 min respectively;

The detection module is used for respectively detecting the concentration of the metal element Fe in each group of filtrate;

And the judging module is used for evaluating the coating integrity of the plurality of groups of positive electrode active materials according to the concentration of the metal element Fe of each group of positive electrode active materials.