CN119092645B

CN119092645B - Lithium ion secondary battery, positive electrode active material and electric device

Info

Publication number: CN119092645B
Application number: CN202411556363.3A
Authority: CN
Inventors: 陈旭东; 别常峰; 刘宏宇; 何思远; 栗晓静; 林迎曦; 贾晓阳; 倪欢; 柳娜
Original assignee: Contemporary Amperex Technology Co Ltd
Current assignee: Contemporary Amperex Technology Co Ltd
Priority date: 2024-11-04
Filing date: 2024-11-04
Publication date: 2025-03-21
Anticipated expiration: 2044-11-04
Also published as: CN119092645A

Abstract

The present application relates to a lithium ion secondary battery, a positive electrode active material, and an electric device. The lithium ion secondary battery comprises a positive electrode plate, wherein the positive electrode plate comprises a positive electrode film layer, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises a mixture of a first granular lithium-containing phosphate material and a second rod-containing lithium-containing phosphate material, the first lithium-containing phosphate material and the second lithium-containing phosphate material have diffraction peaks A between 29 degrees and 30 degrees and diffraction peaks B between 25 degrees and 26 degrees in an X-ray diffraction spectrum, the intensity ratio of the diffraction peaks A to the diffraction peaks B is I _A/I_B, the second lithium-containing phosphate material is 1.1- _A/I_B -0.95, I _A/I_B of the second lithium-containing phosphate material is larger than I _A/I_B of the first lithium-containing phosphate material, the first lithium-containing phosphate material and the second lithium-containing phosphate material both contain carbon coating layers, and the carbon coating amount in the first lithium-containing phosphate material is smaller than the carbon coating amount in the second lithium-containing phosphate material.

Description

Lithium ion secondary battery, positive electrode active material, and electric device

Technical Field

The application relates to the technical field of batteries, in particular to a lithium ion secondary battery, an anode active material and an electric device.

Background

In recent years, as the application range of lithium ion secondary batteries is becoming wider, the lithium ion secondary batteries are widely applied to energy storage power supply systems such as hydraulic power, firepower, wind power and solar power stations, and various fields such as electric tools, electric bicycles, electric motorcycles, electric automobiles, military equipment, aerospace and the like. Among them, lithium-containing phosphate batteries such as lithium iron phosphate have been greatly developed.

As the demand for batteries increases, higher demands are being placed on the energy density, low temperature performance, etc. of the batteries. However, lithium-containing phosphate materials such as lithium iron phosphate materials have a significant decrease in ion diffusion rate at low temperatures due to inherent low electron conductivity, thereby affecting performance at low temperatures, particularly energy density, capacity retention and power performance at low temperatures.

Disclosure of Invention

In order to achieve the above object, a first aspect of the present application provides a lithium ion secondary battery, a positive electrode active material, and an electric device that are superior in energy density, capacity retention, and power performance under low temperature conditions.

According to a first aspect of the application, a lithium ion secondary battery is provided, the lithium ion secondary battery comprises a positive electrode plate, the positive electrode plate comprises a positive electrode film layer, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises a mixture of a first granular lithium-containing phosphate material and a second rod-containing lithium-containing phosphate material, the first lithium-containing phosphate material and the second lithium-containing phosphate material have diffraction peaks A and B between 29 and 30 degrees and between 25 and 26 degrees in an X-ray diffraction spectrum, the intensity ratio of the diffraction peaks A and B is I _A/I_B, the second lithium-containing phosphate material is 1.1- _A/I_B -0.95, I _A/I_B of the second lithium-containing phosphate material is larger than I _A/I_B of the first lithium-containing phosphate material, the first lithium-containing phosphate material and the second lithium-containing phosphate material both contain carbon coating layers, and the carbon coating amount in the first lithium-containing phosphate material is smaller than the carbon coating amount in the second lithium-containing phosphate material.

In addition, the adopted second lithium-containing phosphate material has diffraction peaks at specific positions, the intensity ratio I _A/I_B of the diffraction peaks A and B is limited, and the carbon coating amount difference and the I _A/I_B difference of the first lithium-containing phosphate material and the second lithium-containing phosphate material are controlled, so that the low-temperature performance of the lithium-ion secondary battery is improved, and the good energy density, capacity retention rate and power performance under the low-temperature condition are considered.

In some of these embodiments, in the positive electrode active material, the mass content of the first lithium-containing phosphate material is greater than or equal to the mass content of the second lithium-containing phosphate material.

In some of these embodiments, the mass content of the second lithium-containing phosphate material is 5% -50% of the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material.

In some of these embodiments, the mass content of the second lithium-containing phosphate material is 10% -30% of the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material.

The use of the second lithium-containing phosphate material increases the power performance of the battery, but the higher the use of the second lithium-containing phosphate material affects the energy density, and the control of the mass content of the second lithium-containing phosphate in the above range is beneficial to balancing the low-temperature performance and the volumetric energy density of the lithium ion secondary battery, so that the lithium ion secondary battery has good low-temperature performance and volumetric energy density.

In some of these embodiments, at least one of the following conditions is satisfied:

(1) The carbon coating amount in the first lithium-containing phosphate material is 1% -1.3%;

(2) The carbon coating amount in the second lithium-containing phosphate material is 1.3% -1.8%;

(3) The difference between the carbon coating amount in the second lithium-containing phosphate material and the carbon coating amount in the first lithium-containing phosphate material is 0.1% -0.8%.

The difference of the carbon coating amount is controlled, so that the low-temperature performance of the secondary battery is facilitated, and the energy density, the capacity retention rate and the power performance are better under the low-temperature condition.

(1) The first lithium-containing phosphate material meets the condition that I _A/I_B is more than or equal to 0.92 and more than or equal to 0.8;

(2) The second lithium-containing phosphate material satisfies that I _A/I_B is more than or equal to 1.05 and more than or equal to 0.95.

The second lithium-containing phosphate material has an I _A/I_B within this range, which can further enhance the low-temperature performance of the lithium ion secondary battery.

In some of these embodiments, the primary particle average particle size of the first lithium-containing phosphate material is greater than the primary particle average particle size of the second lithium-containing phosphate material.

The larger carbon coating can inhibit the particle size growth of the lithium-containing phosphate material, so that the particle size of the lithium-containing phosphate material is difficult to obtain, the compaction density of the first lithium-containing phosphate material is reduced, and the gram capacity of the first lithium-containing phosphate material is reduced by increasing the carbon coating amount, so that the coating amount of the first lithium-containing phosphate material with larger particle size is controlled to be smaller, and the first lithium-containing phosphate material mainly plays a role in providing high compaction density so as to improve the energy density of the battery. The average particle size of primary particles of the second lithium-containing phosphate material is smaller, the probability of occurrence of side reactions is larger, therefore, the carbon coating amount of the second lithium-containing phosphate material needs to be increased, the carbon coating integrity is improved, the probability of occurrence of side reactions is reduced, the effective capacity exertion and the electronic conductivity of the second lithium-containing phosphate material are improved, and therefore, the coating amount of the second lithium-containing phosphate material with smaller particle size is larger, and the second lithium-containing phosphate material mainly plays a role in high dynamic performance so as to improve the power performance of the battery. Thus, the first lithium-containing phosphate material with larger particle size and smaller coating amount and the second lithium-containing phosphate material with smaller particle size and larger coating amount are used in a mixing manner, so that the lithium ion secondary battery has better energy density, capacity retention rate and power performance under the low-temperature condition.

In some embodiments, the ratio of the primary particle average particle size of the first lithium-containing phosphate material to the primary particle average particle size of the second lithium-containing phosphate material is 3-40. By controlling the ratio, the second lithium-containing phosphate material can be better filled in gaps of the first lithium-containing phosphate material, a better close packing effect is realized, and meanwhile, the second lithium-containing phosphate material with smaller particle size can better play a role in improving the power performance of the battery.

In some embodiments, the primary particles of the first lithium-containing phosphate material have an average particle size of 700nm to 1500nm.

In some embodiments, the primary particles of the first lithium-containing phosphate material have an average particle size of 700nm to 1200nm.

The average particle size of primary particles of the first lithium-containing phosphate material is controlled within the range, the first lithium-containing phosphate material and the second lithium-containing phosphate material can be better mutually filled, better close packing is realized, and the integral compaction density of the positive electrode film layer is improved, so that good power performance, higher capacity retention rate and energy density are both considered.

In some embodiments, the primary particles of the second lithium-containing phosphate material have an average particle size of 20nm to 260nm.

In some embodiments, the primary particles of the second lithium-containing phosphate material have an average particle size of 80nm to 160nm.

The average particle diameter of the primary particles of the second lithium-containing phosphate material is increased, and since the transmission paths between the particles are prolonged, controlling the average particle diameter of the primary particles of the second lithium-containing phosphate material within the above-described range can provide good migration rate of lithium ions in the positive electrode active material, which is advantageous for the lithium ion secondary battery to give consideration to good capacity retention rate, power performance and energy density under low temperature conditions. Meanwhile, the first lithium-containing phosphate material with larger particle size can be combined, so that the compaction density can be improved, and the energy density of the battery can be improved.

In some embodiments, the second lithium-containing phosphate material has an aspect ratio of (1.1-3.9): 1.

The length-diameter ratio of the second lithium-containing phosphate material is controlled within the range, so that the rod-shaped lithium-containing phosphate material in the second lithium-containing phosphate material is mainly short, a better (010) crystal face exposure effect can be provided, the low-temperature performance can be better exerted, in addition, the length-diameter ratio is controlled within the range, the specific surface area is in a proper range, the side reaction can be favorably controlled, the service life of a battery can be favorably prolonged, in addition, the upper limit of the length-diameter ratio is further controlled, the compaction density of a positive electrode plate is controlled within a proper range, and the energy density of the battery can be favorably improved.

(1) The powder compaction density of the first lithium-containing phosphate material at 3t pressure is 2.38 g/cm ³~2.68 g/cm³;

(2) The powder compaction density of the second lithium-containing phosphate material at 3t pressure is 1.85 g/cm ³~2.35 g/cm³;

(3) The compaction density of the positive electrode film layer on one side of the positive electrode plate is 2.45g/cm ³~2.8g/cm³.

In some of these embodiments, the lithium-containing phosphate in the first lithium-containing phosphate material and the second lithium-containing phosphate material has a chemical formula of Li _βFe_αM_(1-α)PO₄, where 0.2. Ltoreq.α.ltoreq.1, 1. Ltoreq.β.ltoreq.1.1, and M includes at least one of Ti, V, mg, cu, mn, cr, zn, pb, ca, co, ni, sr.

In some embodiments, in the positive electrode film layer, the total mass content of the positive electrode active material is 92% -98%.

In some of these embodiments, the positive electrode film layer further comprises a binder, the positive electrode film layer satisfying at least one of the following characteristics:

(1) In the positive electrode film layer, the mass content of the binder is 1% -4%;

(2) The binder comprises at least one of polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, sodium carboxymethyl cellulose, polyurethane, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.

In some of these embodiments, the positive electrode film layer further includes a conductive agent, the positive electrode film layer satisfying at least one of the following characteristics:

(1) In the positive electrode film layer, the mass content of the conductive agent is 1% -4%;

(2) The conductive agent comprises at least one of carbon nano tube, conductive carbon, graphite, acetylene black, metal fiber, organic conductive polymer, graphene, carbon black, ketjen black, carbon dots and carbon nano fiber.

In some of these embodiments, the lithium ion secondary battery further comprises an electrolyte comprising an additive comprising at least one of 1-methyl-3- [ (3-trimethoxysilyl) -propyl ] imidazole bis trifluoromethanesulfonamide, aluminum oxide grafted 1-methyl-3-propylpyrrolidinbis (trifluoromethanesulfonic acid) imide, lithium difluorodioxaborate, fluoroethylene carbonate, and 1, 3-dioxapentacyclic. The electrolyte contains the additives, so that the low-temperature performance of the battery can be improved.

In a second aspect of the present application, there is provided a positive electrode active material comprising a mixture of a first lithium-containing phosphate material containing particles and a second lithium-containing phosphate material containing rods, the first lithium-containing phosphate material and the second lithium-containing phosphate material having a diffraction peak A between 29 DEG and 30 DEG and a diffraction peak B between 25 DEG and 26 DEG in an X-ray diffraction pattern, the intensity ratio of the diffraction peak A to the diffraction peak B being I _A/I_B, the second lithium-containing phosphate material satisfying that 1.1.gtoreq.I _A/I_B.gtoreq.0.95, the I _A/I_B of the second lithium-containing phosphate material being greater than I _A/I_B of the first lithium-containing phosphate material, the first lithium-containing phosphate material and the second lithium-containing phosphate material both containing a carbon coating, the carbon coating amount in the first lithium-containing phosphate material being less than the carbon coating amount in the second lithium-containing phosphate material.

In some of these embodiments, the positive electrode active material is the positive electrode active material in the lithium ion secondary battery provided in the first aspect of the present application.

In a third aspect of the present application, there is provided an electric device comprising at least one of the lithium ion secondary battery provided in the first aspect of the present application and the positive electrode active material provided in the second aspect of the present application.

The power utilization device comprises the lithium ion secondary battery provided by the application, and therefore has at least the same advantages as the lithium ion secondary battery.

The details of one or more embodiments of the application are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the application will be apparent from the description and drawings, and from the claims.

Drawings

For a better description and illustration of embodiments or examples provided by the present application, reference may be made to one or more of the accompanying drawings. Additional details or examples used to describe the drawings should not be construed as limiting the scope of any of the disclosed applications, the presently described embodiments or examples, and the presently understood best mode of carrying out these applications. Also, like reference numerals are used to designate like parts throughout the accompanying drawings. In the drawings:

fig. 1 is a schematic view of a lithium ion secondary battery according to an embodiment of the present application.

Fig. 2 is an exploded view of the lithium ion secondary battery of the embodiment of the present application shown in fig. 1.

Fig. 3 is a schematic view of a battery module according to an embodiment of the present application.

Fig. 4 is a schematic view of a battery pack according to an embodiment of the present application.

Fig. 5 is an exploded view of the battery pack of the embodiment of the present application shown in fig. 4.

Fig. 6 is a schematic view of an electric device in which a lithium ion secondary battery according to an embodiment of the present application is used as a power source.

Fig. 7 is an SEM image of the first and second lithium-containing phosphate materials of example 1.

Fig. 8 is an XRD pattern of a second lithium-containing phosphate material prepared according to example 1 of the present application.

Fig. 9 is an enlarged view of fig. 8 at diffraction angle 2Theta from 24 deg. to 31 deg..

Reference numerals illustrate:

1. The battery pack comprises a battery pack body, an upper box body, a lower box body, a battery module body, a battery unit body, a battery pack body, a battery module, a battery unit body, a battery shell, an electrode assembly, a battery pack cover plate, a battery module, an electrode assembly, a battery cover plate and an electric device.

Detailed Description

The following description of the embodiments of the present application will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present application, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the application without making any inventive effort, are intended to be within the scope of the application.

The "range" disclosed herein may be defined in terms of lower and upper limits, with a given range being defined by the selection of a lower limit and an upper limit, the selected lower and upper limits defining the boundaries of the particular range. Ranges may be defined in this way as either inclusive or exclusive of the endpoints, any of which may be independently inclusive or exclusive, and any combination may be made, i.e., any lower limit may be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. Furthermore, if minimum range values 1 and 2 are listed, and if maximum range values 3,4, and 5 are also listed, then the following ranges are all contemplated as 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise indicated, the numerical range "a-b" represents a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed throughout, and "0-5" is simply a shorthand representation of a combination of these values. When a certain parameter is expressed as an integer of 2 or more, it is disclosed that the parameter is, for example, an integer of 2, 3,4, 5, 6, 7,8, 9, 10, 11, 12 or the like. For example, when a parameter is expressed as an integer selected from "2-10", the integers 2, 3,4, 5, 6, 7,8, 9 and 10 are listed.

In the present application, "plural", etc., refer to, unless otherwise specified, an index of 2 or more in number. For example, "one or more" means one kind or two or more kinds.

All embodiments of the application and alternative embodiments may be combined with each other to form new solutions, unless otherwise specified.

Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment or implementation of the application. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Those of skill in the art will explicitly and implicitly appreciate that the embodiments described herein may be combined with other embodiments. Reference herein to "embodiments" is intended to have a similar understanding.

It will be appreciated by those skilled in the art that in the methods of the embodiments or examples, the order of writing the steps is not meant to be a strict order of execution and the detailed order of execution of the steps should be determined by their functions and possible inherent logic. All the steps of the present application may be performed sequentially or randomly, preferably sequentially, unless otherwise specified. For example, the method comprises steps (a) and (b), meaning that the method may comprise steps (a) and (b) performed sequentially, or may comprise steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), may include steps (a), (c) and (b), may include steps (c), (a) and (b), and the like.

In the present application, the open technical features or technical solutions described by words such as "contain", "include" and the like are considered to provide both closed features or solutions composed of the listed members and open features or solutions including additional members in addition to the listed members unless otherwise stated. For example, a includes a1, a2, and a3, and may include other members or no additional members, unless otherwise stated, and may be considered as providing features or aspects of "a consists of a1, a2, and a 3" as well as features or aspects of "a includes not only a1, a2, and a3, but also other members".

In the present application, a (e.g., B), where B is one non-limiting example of a, is understood not to be limited to B, unless otherwise stated.

In the present application, "optional" refers to the presence or absence of the possibility, i.e., to any one of the two parallel schemes selected from "with" or "without". If multiple "alternatives" occur in a technical solution, if no particular description exists and there is no contradiction or mutual constraint, then each "alternative" is independent.

As described in the background art, as the demand for batteries increases, higher demands are being placed on the energy density, low temperature performance, etc. of the batteries. However, lithium-containing phosphate materials such as lithium iron phosphate materials have a significant decrease in ion diffusion rate at low temperatures due to inherent low electron conductivity, thereby affecting performance at low temperatures, particularly energy density, capacity retention and power performance at low temperatures.

Therefore, how to improve the energy density, capacity retention and power performance of lithium ion secondary batteries under low temperature conditions is an urgent problem to be solved.

Based on this, an embodiment of the present application provides a lithium ion secondary battery, a positive electrode tab, a positive electrode active material, and an electric device. The positive electrode tab and the positive electrode active material will be described in detail with reference to a lithium ion secondary battery.

An embodiment of the application provides a lithium ion secondary battery, which comprises a positive electrode plate, wherein the positive electrode plate comprises a positive electrode film layer, the positive electrode film layer comprises a positive electrode active material, and the positive electrode active material comprises a mixture of a first granular lithium-containing phosphate material and a second rod-shaped lithium-containing phosphate material. In other words, the first lithium-containing phosphate material comprises a particulate lithium-containing phosphate material and the second lithium-containing phosphate material comprises a rod-shaped lithium-containing phosphate material.

The first lithium-containing phosphate material and the second lithium-containing phosphate material have diffraction peaks A between 29 and 30 degrees and diffraction peaks B between 25 and 26 degrees in an X-ray diffraction pattern, and the intensity ratio of the diffraction peaks A to the diffraction peaks B is I _A/I_B. The second lithium-containing phosphate material satisfies that I _A/I_B is more than or equal to 1.1 and more than or equal to 0.95. The second lithium-containing phosphate material has an I _A/I_B that is greater than an I _A/I_B of the first lithium-containing phosphate material.

The first lithium-containing phosphate material and the second lithium-containing phosphate material each contain a carbon coating layer, and the carbon coating amount in the first lithium-containing phosphate material is smaller than the carbon coating amount in the second lithium-containing phosphate material.

The lithium-containing phosphate material takes lithium iron phosphate material as an example, a diffraction peak A is arranged between 29 degrees and 30 degrees in an X-ray diffraction spectrum, the diffraction peak A corresponds to a (010) crystal face of the lithium iron phosphate material, the A peak and the B peak are two main peaks of the lithium iron phosphate material, and the intensity ratio of the A peak (010) can indirectly represent the exposure proportion of the (010) crystal face. Further, in the present application, the I _A/I_B of the first lithium iron phosphate material containing the particles is 0.8 to 0.92, and as an example, the I _A/I_B of the first lithium iron phosphate material may be 0.8, 0.82, 0.85, 0.86, 0.88, 0.89, 0.9, 0.91, 0.92, or a range formed by any two of the above points as an end value.

The ratio of (010) crystal face exposure of the second lithium-containing phosphate material can be regulated and controlled by controlling the intensity ratio I _A/I_B of the diffraction peak of the second lithium-containing phosphate material in the X-ray diffraction pattern within the range, so that the electrochemical reaction kinetics performance of the second lithium-containing phosphate material can be regulated and controlled. Specifically, the higher the I _A/I_B of the second lithium-containing phosphate material, the higher the proportion of (010) crystal face exposure of the second lithium-containing phosphate material. It is presumed that this is because lithium ions undergo intercalation and deintercalation reactions in the second lithium-containing phosphate material along the b-axis during the electrochemical reaction, and thus the higher the (010) crystal face exposure ratio is, the more advantageous is in enhancing the electrochemical reaction kinetics, so that the second lithium-containing phosphate material exhibits excellent low-temperature properties when applied to a lithium-ion secondary battery.

Meanwhile, the carbon coating amount of the second lithium-containing phosphate material is controlled to be larger, the electronic conductivity of the second lithium-containing phosphate material is further improved, so that the power performance of the second lithium-containing phosphate material is improved, the coating amount of the second lithium-containing phosphate material can be increased, the coating integrity is improved, the side reaction of the second lithium-containing phosphate material is reduced, the consumption of active components of the second lithium-containing phosphate material can be reduced, the capacity retention rate of a battery is improved, I _A/I_B of the first lithium-containing phosphate material is smaller than I _A/I_B of the second lithium-containing phosphate material, the compaction density of the first lithium-containing phosphate material is improved, more carbon has an adverse effect on the compaction density of the first lithium-containing phosphate material, and the gram capacity of the first lithium-containing phosphate material is influenced by more carbon, so that the carbon coating amount of the first lithium-containing phosphate material is smaller than the carbon coating amount of the second lithium-containing phosphate material, and the energy density, the capacity retention rate and the power performance of the lithium ion secondary battery under low-temperature conditions are improved.

In addition, the adopted second lithium-containing phosphate material has the specific intensity ratio I _A/I_B of the diffraction peak A to the diffraction peak B, and controls the difference of carbon coating amounts and the difference of I _A/I_B of the first lithium-containing phosphate material and the second lithium-containing phosphate material, thereby improving the low-temperature performance of the lithium-ion secondary battery and giving consideration to good energy density, capacity retention rate and power performance under the low-temperature condition.

In some embodiments, the difference between the carbon coating amount in the second lithium-containing phosphate material and the carbon coating amount in the first lithium-containing phosphate material is 0.1% -0.8%, and as an example, the difference is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, or a range formed by any two of the above points as an end value, and further may be 0.3% -0.8%, and still further is 0.5% -0.8%. The difference of the carbon coating amount is controlled, so that the low-temperature performance of the secondary battery is facilitated, and the energy density, the capacity retention rate and the power performance are better under the low-temperature condition.

In some embodiments, the carbon coating amount of the first lithium-containing phosphate material is 1% -1.3%, and as an example, the carbon coating amount of the first lithium-containing phosphate material may be 1%, 1.1%, 1.2%, 1.3%, or a range formed by any two of the above points as an end value.

In some embodiments, the carbon coating amount of the second lithium-containing phosphate material is 1.3% -1.8%, and as an example, the carbon coating amount of the second lithium-containing phosphate material may be 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, or a range formed by any two of the above points as end values.

The qualitative and quantitative test of the positive electrode active material in the positive electrode plate of the lithium ion secondary battery can be carried out by adopting the following test method:

1) Battery disassembly

A. fully discharging the battery to a fully discharged state with a voltage of 2.0V;

b. Then disassembling the battery in a glove box, and controlling the humidity in the glove box to be within 2%;

c. Separating out a positive electrode plate in the battery, and then soaking the positive electrode plate in DMC (dimethyl carbonate) and sufficiently washing electrolyte on the positive electrode plate;

d. The positive electrode sheet was removed and placed under a vacuum oven and dried at 80 ℃ for 5h ℃.

2) Positive electrode active material separation

A. separating the positive electrode film layer of the dried positive electrode plate from the positive electrode current collector;

b. grinding the separated positive electrode film layer to powder, dissolving by adopting NMP (N-methyl pyrrolidone) under the heating condition, filtering and collecting filter residues, and repeating the dissolving and filtering steps until the binder is sufficiently removed;

c. the mixture, which was thoroughly washed of the binder, was filtered and dried.

3) Separation of conductive agent

Dispersing the dried mixture powder in a liquid medium (the density of the liquid medium is greater than 1.2 g/cm ³, such as nitrobenzene, bromobenzene or carbon tetrachloride, and the like), stirring to uniformly and basically mix the mixture, standing for a long time, layering the solution, wherein the upper layer is conductive agent such as conductive carbon, and the lower layer is positive electrode active material.

4) Separation of first and second lithium-containing phosphate materials

The principle is that the first lithium-containing phosphate material and the second lithium-containing phosphate material cannot be separated through the density difference because the density difference is smaller, and when the first lithium-containing phosphate material and the second lithium-containing phosphate material are separated, the difference of the surface wettability of the first lithium-containing phosphate material and the second lithium-containing phosphate material needs to be further utilized, wherein the carbon coating amount of the first lithium-containing phosphate material is smaller than that of the second lithium-containing phosphate material, so that the hydrophilicity of the first lithium-containing phosphate material is greater than that of the second lithium-containing phosphate material, the first lithium-containing phosphate material is separated in a hydrophilic liquid phase medium by adopting a bubble flotation method, the first lithium-containing phosphate material is more easily precipitated, and the second lithium-containing phosphate material is less hydrophilic and is subjected to flotation.

The method comprises the following specific steps:

a. Drying the anode active material of the lower layer in the step 3) to obtain mixture powder;

b. Similar to the coal screening process, the separation is performed by air bubble flotation using a suitable hydrophilic liquid medium (e.g., water, glycol or glycerol, etc.) and a suitable flotation agent (e.g., fat 190).

The preparation method comprises the specific steps of taking 1Kg of mixture anode material in a liquid phase medium, adjusting the mixture anode material to a proper solid content, adding a flotation agent, stirring to generate bubbles, collecting floating foam, washing and drying to obtain a second lithium-containing phosphate material, collecting slurry at the bottom, washing and drying to obtain a first lithium-containing phosphate material.

C. And respectively drying the separated first lithium-containing phosphate material and the separated second lithium-containing phosphate material, and respectively carrying out subsequent characterization analysis.

5) The second lithium-containing phosphate material was subjected to the I _A/I_B test method:

Weighing a certain amount of separated second lithium-containing phosphate material, testing the diffraction peak intensity ratio of the second lithium-containing phosphate material through an X-ray, placing the second lithium-containing phosphate material on a testing platform of an X-ray diffractometer (model Shimadzu XRD-7000) by using a copper target X-ray diffractometer, scanning at a starting angle of 10 degrees and an ending angle of 90 degrees, and a step length of 0.013, then starting testing to obtain a diffraction pattern of the second lithium-containing phosphate material within a diffraction angle range of 10 degrees to 90 degrees, and determining the intensity ratio I _A/I_B of the diffraction peak according to the diffraction pattern.

The separated first and second lithium-containing phosphate materials were then subjected to ICP testing with reference to GB/T33822-2017, respectively, to obtain the respective carbon coating amounts.

As an example, the intensity ratio I _A/I_B of the diffraction peak a to the diffraction peak B of the second lithium-containing phosphate material may be 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, 1.02, 1.04, 1.05, 1.06, 1.08, 1.1, or a range in which any two of the above-mentioned point values are constituted as end values. Further, I _A/I_B satisfies that I _A/I_B is not less than 0.96, and further, I _A/I_B is not less than 0.98.

In some of these embodiments, the intensity ratio I _A/I_B of diffraction peaks A and B of the second lithium-containing phosphate material satisfies 1.1.gtoreq.I _A/I_B.gtoreq.0.95, further 1.05.gtoreq.I _A/I_B.gtoreq.0.95, still further 1.05.gtoreq.I _A/I_B.gtoreq.0.96. Within this range, it can further enhance the low temperature performance of the lithium ion secondary battery.

The first lithium-containing phosphate material and the second lithium-containing phosphate material of the present application may be commercially available. The method for preparing the second lithium-containing phosphate material is not particularly limited as long as the object of the present application can be achieved.

In some embodiments, the second lithium-containing phosphate material may be prepared by liquid phase synthesis, exemplified by lithium iron phosphate, comprising the steps of:

and step A, mixing an iron source, a lithium source and a phosphorus source according to a certain proportion, and adding a solvent for mixing to obtain a mixed solution A.

Further, the solvent is selected from alcohol, water or alcohol-water mixed solvent. Further, the alcohol is one selected from ethanol, methanol, ethylene glycol and glycerol. Further, the iron source includes, but is not limited to, at least one of ferrous salts such as ferrous sulfate, ferrous chloride, ferrous acetate, ferrous oxalate, and the like. Further, the source of phosphorus includes, but is not limited to, at least one of phosphoric acid, monoammonium phosphate, (at least one of NH ₄)₂HPO₄、LiH₂PO₄、Li₃PO₄ and (NH ₄)₃PO₄). Further, the source of lithium includes, but is not limited to, at least one of lithium hydroxide, lithium oxide, lithium chloride, lithium nitrite, lithium nitrate, lithium oxalate, lithium carbonate, lithium acetate, lithium phosphate, lithium dihydrogen phosphate and lithium dihydrogen phosphate.

Further, the addition amounts of the iron source, the lithium source and the phosphorus source are based on the molar concentration ratio of the iron atom, the phosphorus atom and the lithium atom of 0.5 mol:0.65 mol:1.5: 1.5 mol.

And B, adding a surfactant into the mixed solution A, and then adding a pH regulator to regulate the pH of the mixed solution A to 7-8 to obtain a mixed solution B.

Wherein the surfactant includes, but is not limited to, at least one of sodium citrate, sodium lactate, sodium malate, and sodium tartrate. Further, as an example, the mass ratio of the surfactant to the mixed liquid a is 1:100.

And C, transferring the mixed solution B into a reaction kettle, sealing and reacting for 1-6 hours at the temperature of 170-250 ℃, cooling, and filtering and washing the precipitate obtained by the reaction to obtain the lithium iron phosphate material.

In the hydrothermal precipitation process, the (010) crystal face of the lithium iron phosphate exposes more iron ions, and the lithium iron phosphate is adsorbed on the (010) surface due to the coordination effect of the surfactant, so that the further growth speed of the (010) surface is limited, the thickness of the (010) crystal direction is reduced, the energy required by the deintercalation reaction of the lithium ions along the (010) crystal direction is minimum, the transmission speed of the lithium ions can be obviously improved, and the solid-phase reaction diffusion resistance is reduced.

And D, drying the lithium iron phosphate material obtained in the step C, uniformly mixing the dried lithium iron phosphate material with a carbon source, and then sintering and carbonizing the dried lithium iron phosphate material in a nitrogen inert atmosphere to obtain the carbon-coated lithium iron phosphate material.

Further, the carbon source includes, but is not limited to, at least one of glucose, sucrose, starch, polyethylene glycol. Further, the sintering carbonization temperature is 750-800 ℃. Further, the mass ratio of the lithium iron phosphate material to the carbon source obtained in the step C is 20:1.

In the application, the intensity ratio I _A/I_B of the diffraction peak can be regulated and controlled by controlling parameters such as the hydrothermal reaction time t in the step C, the concentration C of the hydrothermal reactant in the step C, the concentration Cx of the surfactant added in the hydrothermal reactant in the step C, or the type selection of the surfactant. The inventor discovers that the I _A/I_B value is continuously increased along with the extension of the reaction time, but the I _A/I_B value can show a descending trend after the reaction time is continuously prolonged, and based on the method, the liquid phase reaction time is regulated to be 1-6 h, and the I _A/I_B value is increased along with the extension of the reaction time within the reaction time range. The synthesis process of the granular first lithium-containing phosphate material is different from that of the granular second lithium-containing phosphate material, and the preparation method of the second lithium-containing phosphate material by adopting liquid phase reaction is convenient for better adjusting the crystal face exposure ratio, namely I _A/I_B.

In some of these embodiments, the mass content of the first lithium-containing phosphate material is greater than or equal to the mass content of the second lithium-containing phosphate material in the positive electrode active material.

In some of these embodiments, the mass content of the second lithium-containing phosphate material is 5% -50%, further may be 10% -30%, of the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material. As an example, the mass content may be 5%, 8%, 10%, 12%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range constituted by any two of the above-mentioned point values as end values.

The use of the second lithium-containing phosphate material increases the power performance of the battery, but the higher the use of the second lithium-containing phosphate material affects the energy density, and the control of the mass content of the second lithium-containing phosphate material within the above range is beneficial to balancing the low-temperature performance and the volumetric energy density of the lithium ion secondary battery, so that the lithium ion secondary battery has good low-temperature performance and volumetric energy density.

For the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material in the positive electrode sheet in the battery, the mass content of the second lithium-containing phosphate material can be detected by the following method:

The first lithium-containing phosphate material and the second lithium-containing phosphate material which are separated in the steps 1) to 4) are obtained by adopting the steps for carrying out qualitative and quantitative tests on the positive electrode active material in the positive electrode plate of the lithium ion secondary battery, and the respective masses are obtained by respectively weighing, so that the mass ratio of the first lithium-containing phosphate material to the second lithium-containing phosphate material in the positive electrode plate can be obtained.

The primary average particle diameter refers to the average particle diameter of the primary particles. The primary particles in the application have no obvious agglomeration interface in the particle section, but may have tiny pores and point and line defects. The term "primary average particle diameter" means an average value of primary particle diameters of all particles, and is equal in value to the total particle diameter divided by the total particle number.

Further, the ratio of the average primary particle diameter of the first lithium-containing phosphate material to the average primary particle diameter of the second lithium-containing phosphate material is3 to 40, and may be 3, 3.1, 3.5, 4,5, 6, 7, 8, 8.5, 8.75, 9, 10, 12, 15, 16, 17, 18, 18.75, 19, 20, 25, 30, 35, 40, or may be in a range in which any two of the above values are used as the end values, and may be 5 to 15, and may be further 5 to 10, as an example. By controlling the ratio, the second lithium-containing phosphate material can be better filled in gaps of the first lithium-containing phosphate material, a better close packing effect is realized, and meanwhile, the second lithium-containing phosphate material with smaller particle size can better play a role in improving the power performance of the battery.

In some of these embodiments, the primary particle average particle size of the first lithium-containing phosphate material is 700nm to 1500nm, and as an example, the primary particle average particle size of the first lithium-containing phosphate material is 700nm, 750nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, or a range consisting of any two of the above points as an end value. Further, the primary particles of the first lithium-containing phosphate material may have an average particle diameter of 700nm to 1200nm.

In some embodiments, the primary particle size distribution of the first lithium-containing phosphate material is in the range 600nm to 3000nm. The particle size distribution range refers to the interval from the minimum to the maximum of the particle size of the particles in the material, and this range reflects the overall span of the particle size in the material.

In some embodiments, the primary particles of the second lithium-containing phosphate material have an average particle size of 20nm to 260nm. As an example, the primary particle average particle diameter of the second lithium-containing phosphate material is 20nm、30nm、40nm、50nm、60nm、70nm、80nm、90nm、100nm、110nm、120nm、130nm、140nm、150nm、160nm、180nm、200nm、210nm、220nm、240nm、250nm、260nm, or in a range constituted by any two of the above-mentioned point values as end values. Further, the average primary particle size of the second lithium-containing phosphate material may be 80nm to 160nm.

In some embodiments, the second lithium-containing phosphate material has an aspect ratio of (1.1-3.9): 1. As examples, the aspect ratio may be 1.1:1, 1.2:1, 1.4:1, 1.5:1, 1.8:1, 2:1, 2.5:1, 2.6:1, 3:1, 3.5:1, 3.6:1, 3.9:1, or within a range of any two of the above as endpoints. Further, the aspect ratio of the second lithium-containing phosphate material is (1.5-2.6): 1. The length-diameter ratio of the second lithium-containing phosphate material is controlled within the range, so that the rod-shaped lithium-containing phosphate material in the second lithium-containing phosphate material is mainly short, a better (010) crystal face exposure effect can be provided, the low-temperature performance can be better exerted, in addition, the length-diameter ratio is controlled within the range, the specific surface area is in a proper range, the side reaction can be favorably controlled, the service life of a battery can be favorably prolonged, in addition, the upper limit of the length-diameter ratio is further controlled, the compaction density of a positive electrode plate is controlled within a proper range, and the energy density of the battery can be favorably improved.

It is understood that the particulate includes one or more of spherical and spheroid.

The average primary particle diameter of the first lithium-containing phosphate material and the average primary particle diameter of the second lithium-containing phosphate material in the positive electrode sheet, and the aspect ratio of the second lithium-containing phosphate material can be detected by the following methods.

The positive pole piece is cut by adopting an argon ion beam perpendicular to the large surface of the positive pole piece, a cross section is exposed, a scanning electron microscope is adopted for photographing, a long diameter statistical method is adopted for carrying out statistical analysis on the particle size of the lithium-containing phosphate material (the long diameter of the long diameter is adopted as the primary particle size), and specifically, the total number of primary particles of the lithium-containing phosphate and the sum of the primary particle sizes of the primary particles of the lithium-containing phosphate can be counted in the scanning electron microscope photograph, and the primary average particle size of the particles of the lithium-containing phosphate material = the primary particle size of the total lithium-containing phosphate material/the total number of the lithium-containing phosphate material.

In the process, the total number of the lithium-containing phosphate particles with the primary particle diameter larger than 600nm and the total number of the lithium-containing phosphate particles with the primary particle diameter larger than 600nm in the scanning electron microscope photo are counted independently, namely the total number of the first lithium-containing phosphate particles and the total number of the primary particle diameters, wherein the primary average particle diameter of the first lithium-containing phosphate particles = the total number of the primary particle diameters of the first lithium-containing phosphate materials/the total number of the first lithium-containing phosphate materials.

In the above process, the total number of lithium-containing phosphate particles with primary particle diameters less than or equal to 600nm and the sum of primary particle diameters of lithium-containing phosphate particles with primary particle diameters less than or equal to 600nm are counted separately, namely the total number of second lithium-containing phosphate particles and the sum of primary particle diameters, and the primary average particle diameter of the second lithium-containing phosphate material particles = the sum of primary particle diameters of the second lithium-containing phosphate material/the total number of the second lithium-containing phosphate material.

In the process, when the particle size of the lithium-containing phosphate material is statistically analyzed by adopting a long-diameter statistical method, the long diameter a and the short diameter b of each particle are measured, and the ratio of the long diameter a to the short diameter b is obtained, namely the length-diameter ratio of the particles, so that the first lithium-containing phosphate material and the second lithium-containing phosphate material in the particles can be intuitively detected and distinguished.

In some of these embodiments, the first lithium-containing phosphate material has a powder compaction density of 2.38 g/cm ³~2.68 g/cm³ at a pressure of 3 t. In some of these embodiments, the second lithium-containing phosphate material has a powder compaction density of 1.85 g/cm ³~2.35 g/cm³ at a pressure of 3 t.

In some of these embodiments, the positive electrode film layer has a compacted density of 2.45g/cm ³~2.8g/cm³ on one side of the positive electrode sheet.

In the application, the pole piece compaction density testing method comprises the following steps:

when the pole piece is coated on one side, the compaction density of the film layer on one side of the pole piece=m/(V1-V2), when the pole piece is coated on two sides, the compaction density of the film layer on one side of the pole piece=m/[ 2× (V1-V2) ], m represents the weight of the film layer, V1 represents the volume of the pole piece, and V2 represents the volume of the current collector. m can be obtained by subtracting the weight of the current collector from the weight of the current collector, the product of the surface area of the current collector and the thickness of the current collector is the volume V1 of the current collector, the product of the surface area of the current collector and the thickness of the current collector is the volume V2, and the thickness of the current collector are obtained by measuring the thicknesses of the empty foil and the film area of the lug area respectively through a ten-thousandth ruler.

In some of these embodiments, the lithium-containing phosphate in the first lithium-containing phosphate material and the second lithium-containing phosphate material has a chemical formula of Li _βFe_αM_(1-α)PO₄, where 0.2.ltoreq.α.ltoreq.1, 1.ltoreq.β.ltoreq.1.1, and M is a doped metal element, including but not limited to at least one of Ti, V, mg, cu, mn, cr, zn, pb, ca, co, ni, sr. It will be appreciated that the specific types of lithium-containing phosphate in the first lithium-containing phosphate material and the second lithium-containing phosphate material may be the same or different.

The lithium-containing phosphate includes doped or undoped lithium iron phosphate. As an example, the first and second lithium-containing phosphate materials may include at least one of undoped lithium iron phosphate (LiFePO ₄), lithium manganese iron phosphate, lithium cobalt iron phosphate, lithium nickel iron phosphate.

It is understood that the battery is accompanied by deintercalation and consumption of lithium (Li) during charge and discharge, and the content of Li in the positive electrode active material varies when the battery is discharged to different states. In the present application, the Li content is the initial state of the material unless otherwise specified in the list of the positive electrode active materials. The positive electrode active material is applied to a positive electrode plate in a battery system, and the content of Li in the positive electrode active material contained in the plate is generally changed after charge and discharge cycles. The content of Li may be measured by a molar content, but is not limited thereto. The "Li content is the initial state of the material", which refers to the state before the positive electrode slurry is fed. It is understood that new materials obtained by suitable modification based on the listed positive electrode active materials are also within the category of positive electrode active materials, the foregoing suitable modification being indicative of acceptable modification modes for the positive electrode active materials, non-limiting examples being coating modification.

In the present application, the content of oxygen (O) is only a theoretical state value, and the molar content of oxygen changes due to lattice oxygen release, and the actual O content floats. The content of O may be measured by molar content, but is not limited thereto.

In some embodiments, in the positive electrode film layer, the total mass content of the positive electrode active material is 92% -98%, which may be 92%, 93%, 94%, 95%, 96%, 97%, 98%, or a range formed by any two of the above points as an end value.

Further, the positive electrode film layer further includes a binder. Further, in the positive electrode film layer, the mass content of the binder is 1% to 4%, and may be 1%, 2%, 3%, 4% or a range formed by any two of the above points as an end value, as an example.

Further, the binder comprises at least one of polyvinylidene fluoride (PVDF), polyvinyl alcohol, polytetrafluoroethylene (PTFE), sodium carboxymethyl cellulose, polyurethane, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluoroacrylate resin.

Further, the positive electrode film layer further includes a conductive agent.

Further, the mass content of the conductive agent is 1% to 4%, and may be 1%, 2%, 3%, 4% or a range formed by any two of the above-mentioned point values as an end value, for example.

Further, the conductive agent includes at least one of carbon nanotube, conductive carbon, graphite, acetylene black, metal fiber, organic conductive polymer, graphene, carbon black, ketjen black, carbon dot, and carbon nanofiber. Wherein the carbon nanofibers include, but are not limited to, gas phase carbon nanofibers.

In some embodiments, the positive electrode sheet may be prepared by dispersing the above components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent to form a positive electrode slurry, coating the positive electrode slurry on at least one side surface of the positive electrode current collector, and performing processes such as drying, cold pressing, and the like. The type of solvent may be selected from, but is not limited to, any of the foregoing embodiments, such as N-methylpyrrolidone (NMP). The surface of the positive electrode current collector coated with the positive electrode slurry can be a single surface of the positive electrode current collector or two surfaces of the positive electrode current collector. The solid content of the positive electrode slurry may be 40wt% to 80wt%. The viscosity of the positive electrode slurry at room temperature can be adjusted to 5000 mPas to 25000 mPas. When the positive electrode slurry is coated, the coating unit area density in dry weight (minus solvent) may be 15mg/cm ²~35mg/cm².

It can be appreciated that another embodiment of the present application also provides a method for preparing the above lithium ion secondary battery, which includes a step of forming the above positive electrode sheet.

The positive pole piece comprises the positive pole film layer and a positive pole current collector. The positive electrode film layer is arranged on at least one surface of the positive electrode current collector.

As a non-limiting example, the positive electrode current collector has two surfaces opposing in its own thickness direction, and the positive electrode active material layer is provided on either one or both of the two surfaces opposing the positive electrode current collector.

In some embodiments, the positive current collector may employ a metal foil or a composite current collector. For example, as the metal foil, aluminum foil may be used. The composite current collector may include a polymeric material base layer and a metal layer formed on at least one surface of the polymeric material base layer. The composite current collector may be obtained by forming a metal material on a polymeric material substrate. In the positive electrode current collector, non-limiting examples of the metal material may include one or more of aluminum, aluminum alloy, nickel alloy, titanium alloy, silver alloy, and the like. In the positive electrode current collector, non-limiting examples of the polymer material substrate may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), and the like.

In general, a lithium ion secondary battery includes a positive electrode tab, a negative electrode tab, an electrolyte, and a separator. During the charge and discharge of the battery, active ions are inserted and extracted back and forth between the positive electrode plate and the negative electrode plate. The electrolyte plays a role in ion conduction between the positive electrode plate and the negative electrode plate. The positive pole piece and the negative pole piece are oppositely arranged, and the isolating film is arranged between the positive pole piece and the negative pole piece and mainly plays a role in preventing the positive pole and the negative pole from being short-circuited, and meanwhile ions can pass through the isolating film.

Negative pole piece

The negative pole piece comprises a negative pole current collector. Further, the negative electrode tab may further include a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material.

As a non-limiting example, the anode current collector has two surfaces opposing in its own thickness direction, and the anode active material layer is provided on either or both of the two surfaces opposing the anode current collector.

In some of these embodiments, the negative current collector may be a metal foil or a composite current collector. For example, as the metal foil, copper foil may be used. The composite current collector may include a polymeric material base layer and a metal layer formed on at least one surface of the polymeric material base material. The composite current collector may be obtained by forming a metal material on a polymeric material substrate. In the negative electrode current collector, non-limiting examples of the metal material may include one or more of copper, copper alloy, nickel alloy, titanium alloy, silver alloy, and the like. In the negative electrode current collector, non-limiting examples of the polymer material substrate may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), and the like.

In some of these embodiments, the negative active material may employ a negative active material for a battery, which is well known in the art. As non-limiting examples, the anode active material may include one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, lithium titanate, and the like. The silicon-based material may include one or more of elemental silicon, silicon oxygen compounds, silicon carbon composites, silicon nitrogen composites, and silicon alloys. The tin-based material may include one or more of elemental tin, tin oxide, and tin alloys. However, the present application is not limited to these materials, and other conventional materials that can be used as a battery anode active material may be used. These negative electrode active materials may be used alone or in combination of two or more.

In some of these embodiments, the negative electrode active material layer may further optionally include a binder. The binder may include one or more of Styrene Butadiene Rubber (SBR), polyacrylic acid (PAA), sodium Polyacrylate (PAAs), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium Alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

In some of these embodiments, the anode active material layer may further optionally include a conductive agent. The conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

In some of these embodiments, the anode active material layer may optionally further include other adjuvants, such as a thickener (e.g., sodium carboxymethyl cellulose (CMC-Na)), and the like.

In some of these embodiments, the negative electrode tab may be prepared by dispersing the above-described components for preparing the negative electrode tab, such as a negative electrode active material, a conductive agent, a binder, and any other components, in a solvent (non-limiting example of a solvent, such as deionized water) to form a negative electrode slurry, coating the negative electrode slurry on at least one side surface of a negative electrode current collector, and performing processes such as drying, cold pressing, and the like. The surface of the negative electrode current collector coated with the negative electrode slurry may be a single surface of the negative electrode current collector or may be two surfaces of the negative electrode current collector. The solid content of the negative electrode slurry may be 40wt% to 60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000 to 10000 mPas. When the negative electrode slurry is coated, the coating unit surface density of the negative electrode slurry (excluding the solvent) can be 75-220 g/m ² based on dry weight. The compacted density of the negative electrode sheet may be 1.0 g/cm ³ ~ 1.8 g/cm³.

Electrolyte composition

The electrolyte has the function of conducting ions between the positive pole piece and the negative pole piece. The type of electrolyte is not particularly limited in the present application, and may be selected according to the need. For example, the electrolyte may be liquid, gel, or all solid.

In some embodiments, the electrolyte is an electrolyte. The electrolyte includes an electrolyte salt and a solvent.

In some of these embodiments, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF ₆), lithium tetrafluoroborate (LiBF ₄), lithium perchlorate (LiClO ₄), lithium hexafluoroarsenate (LiAsF ₆), lithium bis-fluorosulfonimide (LiFSI), lithium bis-trifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorophosphate (LiPO ₂F₂), lithium difluorooxalato borate (lidadiob), lithium difluorooxalato phosphate (LiDFOP), and lithium tetrafluorooxalato phosphate (LiTFOP).

In some embodiments, the solvent comprises at least one of an ether solvent, an ester solvent, and a sulfone-based solvent.

As an example, the ether solvent may include at least one of ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TRGDME), tetraethylene glycol dimethyl ether (TEGDME), and 1, 3-Dioxolane (DOL);

as an example, the ester solvent may include at least one of Ethylene Carbonate (EC), propylene Carbonate (PC), butylene Carbonate (BC), vinylene Carbonate (VC), fluoroethylene carbonate (FEC), ethylmethyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), γ -Butyrolactone (BL), 1, 3-propane sultone (1, 3-PS), methyl Propionate (MP), methyl Butyrate (MB), ethyl Acetate (EA), ethyl Propionate (EP), propyl Propionate (PP), and Ethyl Butyrate (EB). Further, the ester solvent may include Ethyl Propionate (EP), and the addition of Ethyl Propionate (EP) to the solvent may have an improving effect on low temperature properties.

As an example, the sulfone-based solvent includes dimethyl sulfoxide (DMSO).

Further, the solvent includes an ester solvent and an ether solvent.

In some of these embodiments, the electrolyte further optionally includes an additive. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives capable of improving certain properties of the battery, such as additives that improve the overcharge performance of the battery, additives that improve the high or low temperature performance of the battery, and the like.

Further, the additive improving low temperature performance may include at least one of an ionic liquid type additive, lithium difluorodioxaborate (LiODFB), fluoroethylene carbonate (FEC), and 1, 3-Dioxapentacyclic (DOL).

Further, the ionic liquid type additive comprises at least one of 1-methyl-3- [ (3-trimethoxysilyl) -propyl ] imidazole bis (trifluoromethanesulfonamide) and aluminum oxide grafted 1-methyl-3-propylpyrrolidine bis (trifluoromethanesulfonic acid) imide (Al ₂O₃ -PY-TFSI), which can reduce the ion transmission activation energy, further reduce the charge transfer impedance at low temperature, can effectively improve the low temperature performance of the lithium ion secondary battery, and can keep the capacity retention rate of the lithium ion secondary battery stable at-20 ℃. The lithium difluoro-oxalato-borate (LiODFB) can reduce the charge transfer resistance and improve the low-temperature performance. Fluoroethylene carbonate (FEC) can reduce the viscosity of the electrolyte, improve the low-temperature conductivity of the electrolyte, and improve the low-temperature capacity and the low-temperature power. The 1, 3-dioxygen pentacyclic (DOL) has the characteristics of low freezing point and low viscosity, can reduce the charge transfer impedance of the electrolyte at low temperature, improves the migration rate of Li ions, and improves the low-temperature performance.

Isolation film

In some of these embodiments, a separator is also included in the lithium ion secondary battery. The type of the separator is not particularly limited, and any known porous separator having good chemical stability and mechanical stability can be used.

In some embodiments, the material of the isolation film may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multilayer composite film, and is not particularly limited. When the separator is a multilayer composite film, the materials of the respective layers may be the same or different, and are not particularly limited.

In some embodiments, the thickness of the separator is 6 μm to 40 μm, optionally 12 μm to 20 μm.

In some of these embodiments, the positive electrode tab, the negative electrode tab, and the separator may be manufactured into an electrode assembly through a winding process or a lamination process.

In some of these embodiments, the lithium ion secondary battery may include an outer package. The outer package may be used to encapsulate the electrode assembly and electrolyte described above.

In some of these embodiments, the outer package of the lithium ion secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the lithium ion secondary battery may also be a pouch type pouch. The material of the soft bag can be plastic, and further, non-limiting examples of the plastic can comprise one or more of polypropylene, polybutylene terephthalate, polybutylene succinate and the like.

In yet another embodiment of the present application, an electrical device is provided. The power utilization device comprises the lithium ion secondary battery.

The lithium ion secondary battery and the electric device according to the present application will be described below with reference to the drawings.

The lithium ion secondary battery comprises at least one battery cell. The lithium ion secondary battery may include 1 or more battery cells.

In the present application, unless otherwise indicated, "battery cell" refers to a basic unit capable of achieving mutual conversion of chemical energy and electric energy, and further, generally includes at least a positive electrode sheet, a negative electrode sheet, and an electrolyte. During the charge and discharge of the battery, active ions are inserted and extracted back and forth between the positive electrode plate and the negative electrode plate. The electrolyte plays a role in conducting active ions between the positive electrode plate and the negative electrode plate.

The shape of the battery cell is not particularly limited in the present application, and may be cylindrical, square or any other shape. For example, the lithium ion secondary battery shown in fig. 1 is one battery cell, which is an example of a battery cell of a square structure as one example.

In some of these embodiments, the lithium ion secondary battery may include an outer package. The outer package may be used to encapsulate the electrode assembly and electrolyte as described above. In some of these embodiments, the outer package of the lithium ion secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the lithium ion secondary battery may also be a pouch type pouch. The material of the soft bag can be plastic, and further, non-limiting examples of the plastic can comprise one or more of polypropylene, polybutylene terephthalate, polybutylene succinate and the like.

In some of these embodiments, referring to fig. 2, the overpack may include a housing 51 and a cover 53. The housing 51 may include a bottom plate and a side plate connected to the bottom plate, where the bottom plate and the side plate enclose a receiving chamber. The housing 51 has an opening communicating with the accommodation chamber, and the cover plate 53 can be provided to cover the opening to close the accommodation chamber. The positive electrode tab, the negative electrode tab, and the separator may be formed into the electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is packaged in the receiving chamber. The electrolyte is impregnated in the electrode assembly 52. The number of the electrode assemblies 52 included in the battery cell 5 may be one or more, and one skilled in the art may select according to actual needs.

In some embodiments, the lithium ion secondary battery may be a battery module or a battery pack. The battery module includes at least one battery cell. The number of battery cells included in the battery module may be one or more, and one skilled in the art may select an appropriate number according to the application and capacity of the battery module.

Fig. 3 is a battery module 4 as an example. Referring to fig. 3, in the battery module 4, a plurality of battery cells 5 may be sequentially arranged in the longitudinal direction of the battery module 4. Of course, the arrangement may be performed in any other way. The plurality of battery cells 5 may be further fixed by fasteners.

Alternatively, the battery module 4 may further include a housing having an accommodating space in which the plurality of battery cells 5 are accommodated.

In some embodiments, the battery modules may be further assembled into a battery pack, and the number of battery modules included in the battery pack may be one or more, and one skilled in the art may select an appropriate number according to the application and capacity of the battery pack.

Fig. 4 and 5 are battery packs 1 as an example. Referring to fig. 4 and 5, a battery case and a plurality of battery modules 4 disposed in the battery case may be included in the battery pack 1. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be covered on the lower box body 3 and forms a closed space for accommodating the battery module 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

In addition, an embodiment of the application also provides an electric device, which comprises the lithium ion secondary battery provided by the application. The lithium ion secondary battery may be used as a power source of an electric device, and may also be used as an energy storage unit of the electric device. The powered devices may include, but are not limited to, mobile devices, electric vehicles, electric trains, boats and ships, and satellites, energy storage systems, and the like. The mobile device may be, for example, a cellular phone, a notebook computer, etc., and the electric vehicle may be, for example, a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc., but is not limited thereto.

As the electric device, a lithium ion secondary battery may be selected according to its use requirement.

Fig. 6 is an electric device 6 as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle or the like. In order to meet the high power and high energy density requirements of the power consumption device for the lithium ion secondary battery, a battery pack or a battery module may be employed.

As another example, the device may be a cell phone, tablet computer, notebook computer, or the like. The device is generally required to be light and thin, and a lithium ion secondary battery can be used as a power source.

In order to make the technical problems, technical schemes and beneficial effects solved by the application more clear, the application will be further described in detail below with reference to the embodiments and the accompanying drawings. It will be apparent that the described embodiments are only some, but not all, embodiments of the application. The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the application, its application, or uses. All other embodiments, which can be made by a person skilled in the art based on the embodiments of the application without any inventive effort, are intended to fall within the scope of the application.

The examples are not to be construed as limiting the specific techniques or conditions described in the literature in this field or as per the specifications of the product. The reagents or apparatus used were conventional products commercially available without the manufacturer's attention.

Example 1

(1) Preparation of positive electrode plate

1. First lithium iron phosphate material (granular)

The first lithium iron phosphate material adopts carbon coated LiFePO ₄ with the average particle size of primary particles of 800nm, and the particle size distribution of the material is in the range of 600 nm-3000 nm. The mass content of carbon is 1.1%, and the powder compaction density of the first lithium iron phosphate material under the pressure of 3t is 2.55 g/cm ³.

2. Preparation of a second lithium iron phosphate material (rod-like):

And step A, mixing ferrous sulfate, lithium hydroxide and phosphoric acid with water according to the molar concentration ratio of iron atoms, phosphorus atoms and lithium atoms of 0.5 mol:0.65 mol:1.5: 1.5 mol to obtain a mixed solution A.

And B, adding a surfactant sodium citrate into the mixed solution A, wherein the mass ratio of the surfactant to the mixed solution A is 1:100, and then adding a pH regulator to regulate the pH of the mixed solution A to 7-8 to obtain a mixed solution B.

And C, transferring the mixed solution B into a reaction kettle, sealing and reacting for 1-6 hours at the temperature of 200 ℃, cooling, and filtering and washing the precipitate obtained by the reaction to obtain the lithium iron phosphate material.

And D, uniformly mixing the dried lithium iron phosphate material obtained in the step C with glucose as a carbon source (the mass ratio of the lithium iron phosphate material to the carbon source is 20:1), and sintering and carbonizing at 800 ℃ in a nitrogen inert atmosphere to obtain carbon-coated LiFePO ₄, namely a second lithium iron phosphate material, wherein the average particle size of primary particles is 80nm, the mass content of the carbon coating is 1.7%, and the powder compaction density of the second lithium iron phosphate material under the pressure of 3t is 2.1g/cm ³.

3. Preparation of positive electrode plate

And mixing the first lithium iron phosphate material and the second lithium iron phosphate material according to the mass ratio of 9:1 to obtain the positive electrode active material.

The positive electrode active material, the conductive agent carbon black (Super P) and the binder polyvinylidene fluoride (PVDF) are uniformly mixed in a proper amount of solvent N-methyl pyrrolidone (NMP) according to the mass ratio of 95:3:2, so that positive electrode slurry with the viscosity of 20000 mPa.s is obtained.

Uniformly coating the anode slurry on an anode current collector aluminum foil with the thickness of 15 mu m at the coating weight of 280 g/m ², drying at 100 ℃ to form an anode film layer, and obtaining an anode plate through the procedures of drying, cold pressing, slitting, cutting and the like, wherein the compacted density of the anode plate is 2.6g/cm ³.

(2) Preparing a negative electrode plate:

Mixing graphite as a cathode active material, conductive carbon black as a conductive agent, styrene-butadiene rubber as a binder and sodium carboxymethylcellulose according to the mass ratio of 93:3:2:2, adding deionized water as a solvent, stirring the mixture under the action of a vacuum stirrer until the system is uniform to obtain cathode slurry with the viscosity of 20000 mPa.s, uniformly coating the prepared cathode slurry on the surface of copper foil with the thickness of 8 mu m according to the coating weight of 200g/m ², drying the copper foil by a 100 ℃ oven, and then carrying out cold pressing, tab forming, slitting and other procedures to obtain the cathode pole piece.

(3) Isolation film

Polyethylene isolation film with thickness of 9 μm is selected.

(4) Preparation of electrolyte

In the environment with the water content less than 10 ppm, mixing non-aqueous organic solvents of ethylene carbonate, methyl ethyl carbonate and diethyl carbonate according to the volume ratio of 1:1:1 to obtain an electrolyte solvent, and then dissolving lithium salt LiPF6 in the mixed solvent to prepare the electrolyte with the lithium salt concentration of 1 mol/L.

(5) Preparation of the battery:

The positive electrode plate, the isolating film and the negative electrode plate are sequentially stacked, and are wound by winding equipment to obtain a bare cell, the electrode assembly is placed in an outer square shell, electrolyte is injected after drying, and the lithium ion secondary battery is finally obtained through the procedures of vacuum packaging, standing, formation, exhaust and the like. The injection coefficient of the electrolyte is 3.6g/Ah.

Comparative examples 1 to 2 and examples 2 to 6

Which is substantially the same as in example 1 except that at least one of the strength ratio I _A/I_B, the primary particle average particle diameter, and the aspect ratio of the second lithium iron phosphate material is different. The strength ratio I _A/I_B, primary particle average particle size, and aspect ratio of the second lithium iron phosphate material in each example can be adjusted by adjusting the hydrothermal reaction time in step C, and specific parameters are shown in table 1.

Comparative example 3

Comparative example 3 is substantially the same as example 1 except that the second lithium iron phosphate material has a strength ratio I _A/I_B, specifically as shown in table 1, which was prepared by solid phase synthesis and was in the form of spherical particles.

Examples 7 to 10

It is substantially the same as in example 1 except that in the preparation step of the positive electrode sheet, the mixing mass ratio of the first lithium iron phosphate and the second lithium iron phosphate is different, and thus the mass content of the second lithium iron phosphate is different in the total amount of both the first lithium iron phosphate and the second lithium iron phosphate, as specifically shown in table 1.

Examples 11 to 12

The difference is that the primary particles of the first lithium iron phosphate material used in the examples 1 and 11 to 12 have average particle diameters different from each other, the particle diameter distribution is 600nm to 3000nm, the mass content of carbon is the same as that in the example 1, and the I _A/I_B of the first lithium iron phosphate material in the examples 1 and 11 to 12 is 0.88, 0.89 and 0.85, respectively.

The parameters are shown in table 1.

The following is a performance test.

(1) Scanning Electron Microscope (SEM) testing.

The first lithium iron phosphate material and the second lithium iron phosphate material prepared in example 1 were subjected to scanning electron microscopy, and the results are shown in fig. 7 (a) and (B), respectively.

(2) XRD test (X-ray test).

XRD detection is carried out on the carbon-coated lithium iron phosphate material obtained in the step D in the embodiment 1, specifically, a substance to be detected is placed on a testing platform of an X-ray diffractometer (model Shimadzu XRD-7000) by using a copper target X-ray diffractometer, the scanning initial angle is 10 degrees, the scanning end angle is 90 degrees, the step size is 0.013, then the testing is started, a diffraction pattern of the second active substance in the range of 10 degrees to 90 degrees is obtained, and the intensity ratio I _A/I_B of a diffraction peak is determined according to the diffraction pattern.

The obtained XRD patterns, as shown in fig. 8 and 9, have diffraction peaks a between 29 ° and 30 ° and diffraction peaks B between 25 ° and 26 ° in the X-ray diffraction pattern, and the intensity ratio I _A/I_B =0.96 of the diffraction peaks a to B.

(3) -20 ℃ Capacity retention test:

The lithium ion secondary batteries prepared in each example and comparative example were subjected to heat preservation at 25 ℃ for 2 hours, then subjected to constant current charging to 3.65V at a rate of 0.33C, subjected to constant voltage charging to 0.05C at 3.65V, subjected to standing at 25 ℃ for 2 hours after the charging is completed, then subjected to direct current discharging to 2.5V at a rate of 0.5C, and recorded as the normal temperature discharge capacity of C ₀;

The lithium ion secondary batteries prepared in each example and comparative example were subjected to heat preservation at 25 ℃ for 2 hours, then subjected to constant current charging to 3.65V at a rate of 0.33C, subjected to constant voltage charging to 0.05C at 3.65V, and after the charging was completed, the batteries were allowed to stand at-20 ℃ for 2 hours, then subjected to direct current discharging to 2.5V at a rate of 0.5C, and the discharge capacity of-20 ℃ was recorded as C ₁. The capacity retention rate of the lithium ion secondary battery at-20 ℃ is (C ₁/C₀) multiplied by 100 percent.

(4) -20 ℃ Power performance test:

The capacity calibration is carried out, namely, after the lithium ion secondary batteries prepared in each example and the comparative example are subjected to heat preservation at 25 ℃ for 2 hours, the lithium ion secondary batteries are charged to 3.65V at a constant current of 0.33 ℃, are charged to 0.05V at a constant voltage of 3.65V, the batteries to be tested are stationary for 2 hours at 25 ℃ after the charging is completed, then are discharged to 2.5V at a direct current of 0.33 ℃, and the normal-temperature discharge capacity of the batteries is recorded to be C ₀;

The SOC (state of charge) is regulated, namely, the lithium ion secondary battery with calibrated capacity is subjected to heat preservation for 2 hours at 25 ℃, and then discharged for 144 minutes at a discharge rate of 1/3C ₀, and the capacity of the lithium ion secondary battery is regulated to 20 percent of SOC;

And (3) performing power test, namely standing the lithium ion secondary battery with the SOC of 20% at the temperature of minus 20 ℃ for 2 hours, discharging for 30 seconds at a discharge rate of 3C ₀ under a pulse current I, recording that the voltage before 3C ₀ is V ₁ and the discharge end voltage of 30 seconds is V ₂, and calculating a value DCR of (V ₁-V₂)/I, wherein the power performance of the battery can be represented by the data. The smaller the DCR value, the greater the power performance of the battery.

(5) -20 ℃ Volumetric energy density E _x calculation:

After the lithium ion secondary batteries prepared in each example and comparative example were incubated at 25 ℃ for 2 hours, they were charged to 3.65V at a constant current of 0.33C, charged at a constant voltage of 3.65V to 0.05C, and after the charging was completed, the batteries were left to stand still at-20 ℃ for 2 hours, and then discharged at a direct current of 0.33C to 2.5V, and the discharge energy E ₀ (Wh), the coating area S (dm ²) of the positive electrode sheet, and the positive electrode sheet thickness h (dm) of the lithium batteries were recorded, then the volume energy density E _x=E₀/(sxh) was expressed in Wh/dm ³.

The partial parameters and performance test results of each example and comparative example are shown in table 1.

TABLE 1

The first LFP and the second LFP in table 1 are a first lithium iron phosphate material and a second lithium iron phosphate material, respectively. The mass content of the second LFP refers to the mass content of the second lithium iron phosphate material in the total amount of both the first lithium iron phosphate material and the second lithium iron phosphate material.

The positive electrode active material of the positive electrode sheet in comparative example 1 uses only the second lithium iron phosphate material, which has good low-temperature capacity retention rate and low-temperature power performance, but has a low energy density due to a low compacted density. The positive electrode active material of the positive electrode sheet in comparative example 2 uses only the first lithium iron phosphate material, and has poor low-temperature capacity retention rate and low-temperature power performance, so that the energy density is reduced although the compacted density of the sheet is high.

Comparative example 3 uses a first lithium iron phosphate material with a lower I _A/I_B, which has improved capacity retention and energy density compared to comparative example 2, but still has a higher DCR value at low temperatures, indicating a smaller improvement in power performance.

Compared with comparative examples 1-3, the positive electrode active material of the positive electrode sheet of each example adopts the first lithium iron phosphate material and the second lithium iron phosphate material to be mixed, and can give consideration to good low-temperature capacity retention rate, low-temperature power performance and energy density.

As can be seen from examples 1 to 3, the I _A/I_B value of the second lithium iron phosphate material increases in the range of 0.95 to 1.05, and the DCR value thereof decreases at low temperature, which means that the low temperature power performance is improved, and the low temperature capacity retention rate and the energy density are also improved. As can be seen from examples 1 and 4 to 6, the average primary particle size of the second lithium iron phosphate material is increased, and the low-temperature DCR value is increased due to the longer transmission path between the particles, so that the length-diameter ratio of the second lithium iron phosphate material is controlled within a proper range, and the rod-like shape of the second lithium iron phosphate material is mainly in a short rod shape, so that a better (010) crystal face exposure effect can be provided, low-temperature performance is improved, and a better capacity retention rate and energy density are obtained. As can be seen from examples 1 and 7-10, as the amount of the second lithium iron phosphate material increases, the low-temperature DCR value decreases, the low-temperature power performance and the capacity retention rate are greatly improved, and the energy density is firstly improved and then reduced, because the amount of the second lithium iron phosphate material is larger, the amount of the first lithium iron phosphate material is smaller, and thus the energy density is reduced. As is clear from examples 1 and 11 to 12, the average primary particle diameter of the first lithium iron phosphate material was further controlled, and a low-temperature DCR value (i.e., a good power performance), a high capacity retention rate, and an energy density were simultaneously achieved.

The technical features of the above-described embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above-described embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.

The above examples illustrate only a few embodiments of the application, which are described in detail and are not to be construed as limiting the scope of the application. It should be noted that it will be apparent to those skilled in the art that several variations and modifications can be made without departing from the spirit of the application, which are all within the scope of the application. Accordingly, the scope of protection of the present application is to be determined by the appended claims.

Claims

1. A lithium ion secondary battery, characterized in that it comprises a positive electrode plate, wherein the positive electrode plate comprises a positive electrode film layer; the positive electrode film layer comprises a positive electrode active material, wherein the positive electrode active material comprises a mixture of a first lithium-containing phosphate material containing particles and a second lithium-containing phosphate material containing rods, wherein the first lithium-containing phosphate material and the second lithium-containing phosphate material have a diffraction peak A between 29° and 30° and a diffraction peak B between 25° and 26° in an X-ray diffraction spectrum, and an intensity ratio of the diffraction peak A to the diffraction peak B is _IA / _IB ; the second lithium-containing phosphate material satisfies: _1.1≥IA / _IB≥0.95 ; the _IA / _IB of the second lithium-containing phosphate material is greater than the _IA / _IB of the first lithium-containing phosphate material; the first lithium-containing phosphate material and the second lithium-containing phosphate material both comprise a carbon coating layer, the carbon coating amount in the first lithium-containing phosphate material is less than the carbon coating amount in the second lithium-containing phosphate material, and the carbon coating amount refers to the mass content of carbon.

2 . The lithium-ion secondary battery according to claim 1 , wherein in the positive electrode active material, the mass content of the first lithium-containing phosphate material is greater than or equal to the mass content of the second lithium-containing phosphate material.

3. The lithium-ion secondary battery according to claim 1, characterized in that the mass content of the second lithium-containing phosphate material is 5% to 50% of the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material.

4 . The lithium-ion secondary battery according to claim 2 , wherein a mass content of the second lithium-containing phosphate material is 10% to 30% of the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material.

5. The lithium ion secondary battery according to claim 1, characterized in that at least one of the following conditions is satisfied:

(1) The carbon coating amount in the first lithium-containing phosphate material is 1% to 1.3%;

(2) The carbon coating amount in the second lithium-containing phosphate material is 1.3% to 1.8%;

(3) The difference between the carbon coating amount in the second lithium-containing phosphate material and the carbon coating amount in the first lithium-containing phosphate material is 0.1% to 0.8%.

6. The lithium ion secondary battery according to claim 1, characterized in that at least one of the following conditions is satisfied:

(1) The first lithium-containing phosphate material satisfies: 0.92 ≥ _{I A} / I _B ≥ 0.8;

(2) The second lithium-containing phosphate material satisfies: 1.05 ≥ _{I A} /I _B ≥ 0.95.

7 . The lithium-ion secondary battery according to claim 1 , wherein an average primary particle size of the first lithium-containing phosphate material is greater than an average primary particle size of the second lithium-containing phosphate material.

8 . The lithium-ion secondary battery according to claim 7 , wherein the ratio of the average primary particle size of the first lithium-containing phosphate material to the average primary particle size of the second lithium-containing phosphate material is 3 to 40.

9 . The lithium-ion secondary battery according to claim 1 , wherein the average primary particle size of the first lithium-containing phosphate material is 700 nm to 1500 nm.

10 . The lithium-ion secondary battery according to claim 8 , wherein the average primary particle size of the first lithium-containing phosphate material is 700 nm to 1200 nm.

11 . The lithium-ion secondary battery according to claim 1 , wherein the average primary particle size of the second lithium-containing phosphate material is 20 nm to 260 nm.

12 . The lithium-ion secondary battery according to claim 11 , wherein the average primary particle size of the second lithium-containing phosphate material is 80 nm to 160 nm.

13 . The lithium-ion secondary battery according to claim 1 , wherein the aspect ratio of the second lithium-containing phosphate material is (1.1-3.9):1.

14. The lithium ion secondary battery according to any one of claims 1 to 10, characterized in that at least one of the following conditions is satisfied:

(1) The powder compaction density of the first lithium-containing phosphate material under a pressure of 3t is 2.38 g/cm ³ to 2.68 g/cm ³ ;

(2) The powder compaction density of the second lithium-containing phosphate material under a pressure of 3t is 1.85 g/cm ³ to 2.35 g/cm ³ ;

(3) The compaction density of the positive electrode film layer on one side of the positive electrode sheet is 2.45 g/cm ³ to 2.8 g/cm ³ .

15. The lithium-ion secondary battery according to any one of claims 1 to 10, characterized in that the chemical formula of the lithium-containing phosphate in the first lithium-containing phosphate material and the second lithium-containing phosphate material is Li _β Fe _α M _(1-α) PO ₄ , wherein 0.2≤α≤1, 1≤β≤1.1, and M includes at least one of Ti, V, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, and Sr.

16 . The lithium-ion secondary battery according to claim 1 , wherein in the positive electrode film layer, the total mass content of the positive electrode active material is 92% to 98%.

17. The lithium-ion secondary battery according to any one of claims 1 to 10, characterized in that the positive electrode film layer further comprises a binder, and the positive electrode film layer satisfies at least one of the following characteristics:

(1) In the positive electrode film layer, the mass content of the binder is 1% to 4%;

(2) The binder includes at least one of polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, sodium carboxymethyl cellulose, polyurethane, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.

18. The lithium-ion secondary battery according to any one of claims 1 to 10, characterized in that the positive electrode film layer further comprises a conductive agent, and the positive electrode film layer satisfies at least one of the following characteristics:

(1) In the positive electrode film layer, the mass content of the conductive agent is 1% to 4%;

(2) The conductive agent includes at least one of conductive carbon, metal fiber and organic conductive polymer.

19 . The lithium ion secondary battery according to claim 18 , wherein the conductive carbon comprises at least one of carbon nanotubes, graphite, graphene, carbon black, Ketjen black, carbon dots and carbon nanofibers, and the carbon black comprises acetylene black.

20. The lithium ion secondary battery according to any one of claims 1 to 10, characterized in that the lithium ion secondary battery further comprises an electrolyte, the electrolyte comprises an additive, and the additive comprises at least one of 1-methyl-3-[(3-trimethoxysilyl)-propyl]imidazole bis(trifluoromethanesulfonamide), 1-methyl-3-propylpyrrolidine bis(trifluoromethanesulfonic acid)imide grafted with aluminum oxide, lithium difluorobisoxalatoborate, fluoroethylene carbonate and 1,3-dioxolane.

21. A positive electrode active material, characterized in that the positive electrode active material comprises a mixture of a first lithium-containing phosphate material containing particles and a second lithium-containing phosphate material containing rods, the first lithium-containing phosphate material and the second lithium-containing phosphate material have a diffraction peak A between 29° and 30° and a diffraction peak B between 25° and 26° in an X-ray diffraction spectrum, and the intensity ratio of the diffraction peak A to the diffraction peak B is _IA / _IB ; the second lithium-containing phosphate material satisfies: 1.1≥IA/ _IB≥0.95 ; the _IA / _IB of the second lithium-containing phosphate material is greater than the _IA / _IB of the first lithium-containing phosphate material; the first _lithium -containing phosphate material and the second lithium-containing phosphate material both contain a carbon coating layer, the carbon coating amount in the first lithium-containing phosphate material is less than the carbon coating amount in the second lithium-containing phosphate material, and the carbon coating amount refers to the mass content of carbon.

22 . The positive electrode active material according to claim 21 , wherein the positive electrode active material is the positive electrode active material according to any one of claims 2 to 20 .

23. An electrical device, characterized in that it comprises the lithium-ion secondary battery according to any one of claims 1 to 20 and at least one of the positive electrode active materials according to claim 21 or 22.