CN114527172B - Evaluation method for stability of battery anode material - Google Patents

Abstract

The invention belongs to the technical field of battery material performance test, and particularly relates to an evaluation method of stability of a battery anode material. The evaluation method provided by the invention takes the positive reference potential change rate as an evaluation index, has the characteristics of rapid evaluation and comprehensive evaluation of the positive electrode material, and is different from the existing single evaluation based on a material level or comprehensive long-cycle evaluation based on a battery level. Compared with material level evaluation, the method has comprehensive consideration and higher evaluation accuracy; compared with the evaluation of the battery level, the method can accurately evaluate the difference of the cycle performance of a plurality of positive electrode materials in a short time on the premise of taking the comprehensive evaluation of the materials into consideration and without expensive instruments and equipment.

Description

A method for evaluating the stability of battery positive electrode materials

Technical Field

The invention belongs to the technical field of battery material performance testing, and in particular relates to a method for evaluating the stability of a battery positive electrode material.

Background technique

Lithium-ion batteries are widely used in mobile phones, tablet computers and power equipment due to their high energy density and long cycle life. The cycle performance of lithium-ion batteries is the key to their service life and value retention. The performance of positive electrode materials is the key to the cycle stability of lithium-ion batteries. For example, the crystal structure, doping coating, structural defects of positive electrode materials, residual alkali in the material preparation process, grain distribution and primary particle size of materials will directly affect the performance of positive electrode materials. However, the positive electrode materials currently provided by various companies are of varying quality, so it is necessary to pre-screen positive electrode materials with reliable performance.

The main evaluation methods for battery material performance in the prior art are: first, based on the material or pole piece level, explore certain or certain characteristics of the material, lack the evaluation of the overall performance of the battery, and the comprehensiveness and accuracy of the evaluation are low; second, based on the battery for a long cycle, compare the capacity decay of multiple materials during the cycle. Although this method is relatively comprehensive and has a high evaluation accuracy, it takes a long time to cycle for a long time, the evaluation speed is slow, and the resource consumption is high. For example, the current evaluation of ternary positive electrode materials requires more than 1000 cycles, about 4 months. However, in the context of high-speed iteration of lithium-ion batteries, rapid evaluation of positive electrode material performance means whether the product can seize the market opportunity. Therefore, it is crucial to develop a reliable and rapid positive electrode material evaluation method. For example, there is also a dQ/dV test method in the prior art, but this method requires ultra-high precision coulomb meter (UHPC) for testing. Although the number of cycles can be reduced to less than 200 times, the multiplier used in the cycle is small, the actual time consumption is relatively long, and the equipment is expensive and not popular.

In view of this, it is urgent to develop a method that can evaluate the stability of battery positive electrode materials in cycles at the battery level within short-term cycles without the need for expensive instruments and equipment.

Summary of the invention

Therefore, the technical problem to be solved by the present invention is to overcome the defects of the testing method of positive electrode materials in the prior art, such as long testing time and reliance on expensive instruments and equipment, thereby providing a method for evaluating the stability of battery positive electrode materials.

The testing principle of the present invention is: ideally, in a battery, the lithium ions released from the positive electrode during charging should all be embedded back into the positive electrode during discharge. Due to the irreversible phase change and anisotropic volume expansion of the positive electrode material, the active sites of the positive electrode material are reduced, resulting in the inability of the Li ions embedded in the negative electrode to be embedded back into the positive electrode during discharge, which is manifested as a change in the positive electrode potential.

The present invention utilizes the phenomenon that the positive electrode potential changes with the cycle process, and monitors the change of the potential of the positive electrode to the reference electrode (hereinafter referred to as: positive reference potential) during the cycle process through a three-electrode battery. Generally, the positive reference potential of positive electrode materials with stable structure and good cycle performance has less volatility during the cycle process; positive electrode materials with poor structural stability and poor cycle performance have more structural collapse or irreversible phase changes during the cycle process, resulting in greater changes in the positive reference potential. Therefore, by comparing the rate of change of the positive reference potential under the same number of cycles, the stability of the positive electrode material during the cycle can be determined. Preferably, by comparing the rate of change of the positive reference potential under the fully charged battery state of 100% SoC (battery state of charge) under the same number of cycles, this is because, when fully charged, the positive electrode potential is high and there is a maximum value, which is easy to determine and accurately compare.

To this end, the present invention provides the following technical solutions:

The present invention provides a method for evaluating the stability of a positive electrode material of a battery, comprising the following steps:

S1, preparing the positive electrode material to be tested into a three-electrode battery;

Specifically, the preparation method of the three-electrode battery is a standard method in the field, and reference can be made to patents CN107293778A, CN108987836A, CN108630980A, and CN203562453U. Typically and non-limitingly, the three-electrode battery is made by placing a copper wire between the positive and negative electrodes of the battery and separating them with a diaphragm, leading the copper wire out of the battery as one electrode, and then charging the positive electrode/copper wire electrode pair with a constant current (copper wire as the negative electrode, current 0.1mA or other), until the voltage is 0V and stop (for example: 0.1mA charging for 2h). Then, the negative electrode/copper wire electrode pair is charged with a constant current (copper wire as the negative electrode, current 0.1mA or other), until the voltage is 0V and stop (for example: 0.1mA charging for 2h), and a three-electrode battery is obtained; the copper wire can also be a lithium metal electrode such as a porous lithium foil or a lithium belt (when a lithium metal reference electrode is used, it is not necessary to charge the reference electrode, and the three-electrode battery is obtained after the production is completed).

S2, setting cycle parameters according to the design requirements and evaluation requirements of the three-electrode battery, cycling the three-electrode battery, and monitoring the positive reference potential;

Specifically, the cycle parameter setting is determined according to the evaluated materials and battery design requirements (such as meeting fast charging or long life). Different experimental objectives require different parameters, and those skilled in the art can determine them according to the intended use of the battery to be evaluated.

S3, plotting the positive reference potential as the ordinate and the number of cycles as the abscissa, comparing the rate of change of the positive reference potential of the positive electrode material to be tested compared to the initial state at the same number of cycles, and determining the cycle stability of the positive electrode material.

Specifically, the initial positive reference potential is V ₀ , and the positive reference potential after a certain cycle is V _x , and the change rate (positive reference potential change rate) = (V _x - V ₀ )/V ₀ .

Optionally, the cycle parameters in step S2 include: test temperature, cycle voltage, cycle rate, and cycle number.

Optionally, the test can be a constant temperature test or a variable temperature test, and the test temperature is between -20°C and 60°C. Optionally, the test temperature is between 30°C and 55°C, and preferably, the test temperature is preferably 45°C.

Optionally, the cycle voltage is a discharge cut-off voltage ~ a charge cut-off voltage V; for example, the cycle voltage of a lithium iron phosphate battery is 2.5-3.65V; the cycle voltage of an NCM battery is 2.8-4.35V.

Optionally, the number of cycles is ≤500; optionally, the number of cycles is 100-350.

Optionally, in step S1, the reference electrode of the three-electrode battery is a conventional scrap coin electrode in the art, typically and non-limitingly, the scrap coin electrode can be a copper wire plated with lithium, or a directly implanted porous lithium foil, lithium belt or other lithium metal electrode.

Optionally, the positive electrode material is a binary positive electrode material, a ternary positive electrode material or a lithium iron phosphate positive electrode material. The binary positive electrode material is at least one of nickel-manganese-based materials; the ternary positive electrode material is at least one of nickel-cobalt-manganese-based, nickel-aluminum-cobalt-based, and nickel-magnesium-manganese-based.

The preparation method of the specific three-electrode battery is a conventional method in the art, which is typical and non-limiting, and can refer to patents CN107293778A, CN108987836A, CN108630980A, and CN203562453U. It is sufficient to ensure that the three-electrode battery prepared from the positive electrode material to be tested has the same parameters except for the positive electrode material itself (including but not limited to: slurry, coating, electrolyte, diaphragm, negative electrode, etc.).

Typically, the positive electrode of the three-electrode battery includes a current collector and a positive electrode active material coated on the current collector, and the positive electrode active material is selected from at least one of lithium iron phosphate, lithium manganese iron phosphate, nickel manganese oxide material, nickel oxide material, lithium cobalt oxide material, nickel cobalt oxide material, and nickel manganese cobalt oxide material. The coating process can adopt the existing coating and cold pressing process. Specifically, the positive electrode active material, the conductive agent, and the binder are mixed uniformly in a conventional proportion and added to the solvent to prepare the positive electrode slurry; the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, dried and cold pressed, and then die-cut and stripped to prepare the positive electrode sheet. The solid content of the positive electrode slurry can be 70-75%, the conductive agent can be a conventional conductive agent, such as acetylene black, the binder can be a conventional binder, such as styrene-butadiene rubber or vinylidene fluoride PVDF, and the solvent can be a conventional organic solvent, such as N-methylpyrrolidone NMP.

The negative electrode of the three-electrode battery includes a current collector and a negative electrode active material coated on the current collector, wherein the negative electrode active material is selected from at least one of graphite, hard carbon, soft carbon, and mesophase carbon microspheres. The coating process can adopt the existing coating and cold pressing process. Specifically, the negative electrode active material, the conductive agent, the thickener, and the binder are mixed in a conventional proportion, added to the solvent water, mixed evenly and made into a negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil, dried and cold pressed to make a negative electrode sheet. The solid content of the negative electrode slurry can be 50-55%, the conductive agent can be a conventional conductive agent, such as acetylene black, the binder can be a conventional binder, such as styrene-butadiene rubber or polyvinylidene fluoride PVDF, and the thickener can adopt a conventional thickener, such as sodium hydroxymethyl cellulose.

The reference electrode of the three-electrode battery includes any form of metal lithium electrode such as lithium-plated copper wire, porous lithium foil, lithium ribbon, etc.

The electrolyte of the present invention can be a conventional commercially available lithium ion electrolyte, or it can be made by itself using existing conventional materials. For example, an electrolyte including a solvent, a lithium salt and an additive can be used, wherein the solvent is selected from at least one of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate. The lithium salt is selected from lithium hexafluorophosphate and/or lithium tetrafluoroborate; the additive is selected from at least one of vinylene carbonate, propylene carbonate, vinyl sulfate and lithium difluorophosphate. The molar concentration of the lithium salt is 0.8-1.2 mol/L, and a mixed solution of ethylene carbonate (EC), dimethyl carbonate (DEC) and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1-1:2:2 can be used as a solvent. The volume percentage of the additive can be 0.5-5%. The present invention can use existing traditional diaphragms, such as PE diaphragms, PP diaphragms, PP/PE composite films, or other commercially available diaphragms.

Optionally, in step S3, the rate of change of the positive parameter potential compared to the initial state is cycled at a certain required temperature. When the number of cycles is the same, the smaller the rate of change of the positive parameter potential corresponding to 100% SoC is, the better the stability of the positive electrode material is.

Preferably, the cycle temperature is set at 30°C to 55°C (preferably 45°C), and the number of cycles is 100 to 500 (preferably within 350).

The technical solution of the present invention has the following advantages:

The method for evaluating the stability of a positive electrode material of a battery provided by the present invention comprises the following steps: S1, preparing the positive electrode material to be tested into a three-electrode battery; S2, setting the cycle parameters according to the design requirements and evaluation requirements of the three-electrode battery, cycling the three-electrode battery, and monitoring the positive reference potential; S3, plotting with the positive reference potential as the ordinate and the number of cycles as the abscissa, comparing the rate of change of the positive reference potential of the positive electrode material to be tested compared to the initial state, and determining the cycle stability of the positive electrode material. The present invention has the characteristics of rapid evaluation and comprehensive evaluation of positive electrode materials, which is different from the existing single evaluation based only on the material level, or the comprehensive long cycle evaluation based on the battery level. Compared with the material level evaluation, the present invention has both comprehensive considerations and higher evaluation accuracy; compared with the battery level evaluation, this method can accurately evaluate the differences in the cycle performance of multiple positive electrode materials in a relatively short time under the premise of taking into account the comprehensive evaluation of materials and without the need for expensive instruments and equipment. This is because the attenuation of battery capacity is affected by multiple factors such as the positive electrode, negative electrode, and electrolyte. At a smaller number of cycles, the attenuation of capacity is not necessarily caused by the positive electrode material, which will cause the capacity retention rate to fluctuate within a shorter number of cycles, and it is impossible to distinguish the differences in the cycle performance of various positive electrode materials. However, the positive reference potential directly detects the performance changes of the positive electrode material. The slight voltage changes of the positive electrode will be captured by the positive reference potential. As the changes in the positive electrode structure accumulate during the cycle, the positive reference potential continues to increase, which can easily distinguish the differences in the cycle performance of various positive electrode materials.

The method for evaluating the stability of the positive electrode material of a battery provided by the present invention can accelerate the decay rate of the positive electrode material by further limiting the test temperature, but does not affect the changes in other materials and performance of the battery. Therefore, the cycle period can be greatly shortened, and the difference in the positive parameter potentials of multiple positive electrode materials can be more obviously shown under the same cycle period.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

FIG1 is a graph showing a positive reference potential change of the positive electrode material lithium nickel cobalt manganese oxide (model: NCM613, named material 1) in Example 1 of the present invention;

2 is a graph showing the positive reference potential change of the positive electrode material lithium nickel cobalt manganese oxide (model: NCM613, named material 2) in Example 1 of the present invention;

3 is a graph showing the positive reference potential change of the positive electrode material lithium nickel cobalt manganese oxide (model: NCM613, named material 1) in Example 2 of the present invention;

4 is a graph showing the positive reference potential change of the positive electrode material lithium nickel cobalt manganese oxide (model: NCM613, named material 2) in Example 2 of the present invention;

FIG. 5 is a conventional cycle capacity retention rate curve (25° C.) of the positive electrode material lithium nickel cobalt manganese oxide (model: NCM613, named material 2) in the comparative example of the present invention.

Detailed ways

The following examples are provided for a better understanding of the present invention, but are not intended to limit the best mode of implementation, nor to limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be purchased commercially.

The three-electrode batteries in the embodiments of the present invention are prepared by the following method: The preparation of the three-electrode battery includes:

(1) Preparation of positive electrode sheet: The positive electrode material, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were respectively mixed in a mass ratio of 96:2:2 to obtain a mixture, which was added to a solvent N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry (solid content of 70%). The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil at an area density of 19 mg/ ^cm2 . The thickness of the aluminum foil was 12 μm, dried at 100°C, cold pressed, and then die-cut and slit to obtain a positive electrode sheet for a lithium-ion battery.

(2) Preparation of negative electrode sheet: negative electrode active material graphite, conductive agent acetylene black, thickener sodium carboxymethyl cellulose (CMC), and binder styrene butadiene rubber (SBR) are mixed in a mass ratio of 95:1.5:1.5:2 to obtain a mixture, and the mixture is added into solvent water and mixed evenly to prepare a negative electrode slurry (solid content is 50%); the negative electrode slurry is evenly coated on the negative electrode current collector copper foil at a surface density of 11 mg/ ^cm2 , the thickness of the copper foil is 6 μm, and then dried at 90°C and cold pressed to prepare the negative electrode sheet for the lithium ion battery to be manufactured.

(3) Preparation of reference electrode sheet: A 0.2 μm copper wire was placed between the positive electrode and the negative electrode, separated by a separator, and the copper wire was led out of the battery as one electrode. The positive electrode/copper wire electrode pair was then charged at a constant current until the voltage reached 0 V (0.1 mA charging for 2 h). The negative electrode/copper wire electrode pair was then charged at a constant current until the voltage reached 0 V (0.1 mA charging for 2 h), and a three-electrode battery was obtained.

(4) Preparation of electrolyte: Lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a volume ratio of 5:3:2 to obtain a lithium hexafluorophosphate solution with a concentration of 1.15 mol/L, and 1 vt% of vinylene carbonate, 0.5 vt% of lithium difluorophosphate, and 0.5 vt% of vinyl sulfate DTD were added to obtain a lithium ion battery electrode solution.

(5) The positive electrode sheet, PE separator (purchased from Enjie Company, model: SV12), and negative electrode sheet were assembled in a stacked manner to obtain a battery electrode group, dried in a vacuum drying oven, injected with electrolyte, and sealed to obtain a battery (model 33220102, thickness 33 mm, width 220 mm, height 102 mm, nominal voltage 3.72 V).

The preparation method of the battery in the comparative example of the present invention refers to the preparation method of the above three-electrode battery, except that the reference electrode is not included.

Example 1

This embodiment provides a method for evaluating the stability of a positive electrode material of a battery, comprising the following steps:

Step 1: Prepare two cathode materials to be evaluated. Both materials are lithium nickel cobalt manganese oxide, model NCM613, from different manufacturers. Ensure that except for the cathode materials to be evaluated, other production technical parameters and material parameters are the same. Prepare a three-electrode battery according to the above method;

Step 2: According to the battery design parameters and evaluation requirements, select a suitable temperature T, place the battery in the target temperature environment, cycle the battery n times at a suitable rate within a suitable voltage cycle range, and record the change in the potential of the positive electrode relative to the reference electrode; in this embodiment, the temperature is 45°C, the cycle voltage range is 2.8V~4.35V, constant current and constant voltage (4.35) charging/constant current discharge is adopted, the rate is 1C/1C, and the number of cycles is 350.

Step 3: Plot the positive reference potential against the number of cycles, take the positive reference potential at 100% SoC as the analysis object, compare the positive reference potential change rate of the battery prepared by the target positive electrode material to be tested (compared to the initial state), and determine the cyclic stability of the material.

The calculation method of the positive reference potential change rate is: c=( _Vx - _V0 )/ _V0 , where c is the positive reference potential change rate, _Vx is the positive reference potential after x cycles (100% SoC), and _V0 is the initial positive reference potential (100% SoC).

After the three-electrode battery made of the two positive electrode materials was cycled 350 times at 45°C, the positive parameter change rate of material 1 (Figure 1) with poor performance was 0.502%, and the positive parameter change rate of material 2 (Figure 2) with better performance was 0.235%.

Example 2

This embodiment provides a method for evaluating the stability of a positive electrode material of a battery, comprising the following steps:

Step 1: Prepare two positive electrode materials to be evaluated, both of which are lithium nickel cobalt manganese oxide, both of which are NCM613, and come from different manufacturers (same as Example 1), ensuring that except for the positive electrode materials to be evaluated, other production technical parameters and material parameters are the same, and prepare a three-electrode battery according to the above method;

Step 2: According to the battery design parameters and evaluation requirements, select a suitable temperature T, place the battery in the target temperature environment, cycle the battery n times at a suitable rate within a suitable voltage cycle range, and record the change in the potential of the positive electrode relative to the reference electrode; in this embodiment, the temperature is -15°C, the cycle voltage range is 2.8V~4.35V, constant current and constant voltage charging/constant current discharging is adopted, the rate is 1C/1C, and the number of cycles is 350.

Step 3: Plot the positive reference potential versus the number of cycles, with 100% SoC as the positive reference potential as the analysis object, compare the positive reference potential change rate of the battery prepared with the target positive electrode material to be tested (compared to the initial state), and determine the cyclic stability of the material.

After the three-electrode battery made of the two positive electrode materials was cycled for about 350 cycles at -15°C, the positive parameter change rate of material 1 (Figure 3) with poor performance was 0.04%, and the positive parameter change rate of material 2 (Figure 4) with better performance was 0.02%.

Comparative Example 1

This comparative example provides a method for evaluating the stability of a positive electrode material of a battery, comprising the following steps:

Two positive electrode materials to be evaluated are prepared. Both materials belong to lithium nickel cobalt manganese oxide, and both are NCM613. They come from different manufacturers (same as Example 1). The batteries prepared from materials 1 and 2 are charged at constant current and constant voltage/discharged at constant current at 25°C, with a rate of 1C/1C. The capacity retention rate is shown in Figure 5. It can be seen from the figure that after 1500 cycles, the cycle stability of material 1 is better than that of material 2 through linear prediction. However, after the actual cycle reaches about 2600 times, the disadvantage of the cycle stability of material 1 is revealed. When the capacity retention rate of material 1 decays to 80%, the capacity retention rate of material 2 is still 86%.

From the above test results, it can be seen that the test method provided by the embodiment of the present invention can test the advantages and disadvantages of different positive electrode materials when the cycle is 350 cycles compared with the comparative example, and the test result is consistent with the result of the comparative example cycle of 2600 cycles, which proves the accuracy of the test result. By comparing the data between the embodiments of the present invention, it can be seen that at the preferred test temperature, the decay rate of the positive electrode material can be accelerated, so the cycle period can be greatly shortened, and the difference in the positive reference potential of multiple positive electrode materials can be more obviously shown under the same cycle period.

Obviously, the above embodiments are merely examples for clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims (10)

1. A method for evaluating the stability of a positive electrode material of a battery, characterized in that it comprises the following steps: S1, preparing the positive electrode material to be tested into a three-electrode battery; S2, setting the cycle parameters according to the design and evaluation requirements of the three-electrode battery, cycling the three-electrode battery, the number of cycles is ≤500, the cycle rate is 1C/1C, and the positive reference potential is monitored; S3, plotting the positive reference potential as the ordinate and the number of cycles as the abscissa, comparing the rate of change of the positive reference potential of the positive electrode material to be tested compared to the initial state under the premise of consistent number of cycles, to determine the cycle stability of the positive electrode material. 2. The method for evaluating the stability of a positive electrode material of a battery according to claim 1, wherein the cycle parameters in step S2 include: test temperature, cycle voltage, cycle rate, and number of cycles. 3 . The method for evaluating the stability of a positive electrode material of a battery according to claim 2 , wherein the test temperature is between -20° C. and 60° C. 4. The method for evaluating the stability of a positive electrode material of a battery according to claim 3, characterized in that the test temperature is 30°C to 55°C; Optionally, the test temperature is 45°C. 5. The method for evaluating the stability of a positive electrode material of a battery according to claim 2, characterized in that, in the step S3, a smaller rate of change of the positive reference potential compared to the initial state represents a better stability of the positive electrode material. 6. The method for evaluating the stability of a positive electrode material of a battery according to any one of claims 1 to 5, characterized in that the three-electrode battery uses a copper wire, a lithium foil or a lithium ribbon as a reference electrode. 7. The method for evaluating the stability of a battery positive electrode material according to claim 6, wherein the positive electrode material is at least one of a binary positive electrode material, a ternary positive electrode material or a lithium iron phosphate positive electrode material. 8. The method for evaluating the stability of a positive electrode material of a battery according to claim 6, characterized in that when a copper wire is used as a reference electrode, after the three-electrode battery is assembled, constant current charging is performed on the positive electrode/reference electrode and the negative electrode/reference electrode respectively. 9 . The method for evaluating the stability of a positive electrode material of a battery according to claim 8 , wherein the constant current charging is performed until the voltage reaches 0V. 10. The method for evaluating the stability of a positive electrode material of a battery according to claim 5, wherein a change rate of the positive reference potential after 350 cycles at 45°C compared to the initial state is ≤0.5%, indicating that the positive electrode material has good stability.