Method for evaluating dynamic performance of positive electrode active material

Abstract

The invention discloses a method for evaluating the dynamic performance of a positive electrode active material, which comprises the following steps: preparing the positive electrode active material into a button cell, and charging the button cell; the button cell is subjected to discharging treatment, the discharging treatment comprises first-rate discharging and second-rate discharging, charging treatment is carried out between the first-rate discharging and the second-rate discharging, the discharging rate of the first-rate discharging ranges from 0.05 C to 0.2 C, and the discharging rate of the second-rate discharging ranges from 1 C to 2 C; testing the discharge capacity Q1 of the button cell at a first rate, and testing the discharge capacity Q2 of the button cell at a second rate; and based on the ratio of Q2 to Q1, evaluating the dynamic performance of the positive electrode active material. Therefore, the dynamic performance of different batches of positive electrode active materials with similar particle sizes can be evaluated through a relatively simple method, and the positive electrode active material meeting the required dynamic performance requirement is obtained through screening.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular, to a method for evaluating the kinetic performance of positive electrode active materials. Background Art

[0002] As an important component of lithium-ion batteries, positive electrode active materials affect key performance such as battery capacity and first-cycle efficiency. Their kinetic performance is crucial to the battery's cycling and rate discharge performance. How to effectively and accurately evaluate the kinetic performance of positive electrode active materials is very important for optimizing the preparation process of positive electrode active materials and improving product yield. In related technologies, positive electrode active materials are usually made into pole pieces, and then the pole pieces are assembled into button batteries to evaluate the gram capacity of positive electrode active materials. However, how to effectively evaluate the kinetic performance of positive electrode active materials at a lower cost, so as to achieve rapid and accurate evaluation of multiple batches of positive electrode active materials, still needs further discussion and research.

[0003] It should be noted that the above statements are only used to provide background technical information related to this application and do not necessarily constitute prior art. Summary of the Invention

[0004] In the first aspect of the present application, a method for evaluating the kinetic performance of a positive electrode active material is proposed, comprising: performing a discharge treatment on a button cell, wherein the discharge treatment includes a first rate discharge and a second rate discharge, and performing a charging treatment between the first rate discharge and the second rate discharge, wherein the discharge rate of the first rate discharge is 0.05C-0.2C and the discharge rate of the second rate discharge is 1C-2C; testing the discharge capacity Q1 of the button cell at the first rate, and testing the discharge capacity Q2 of the button cell at the second rate; and evaluating the kinetic performance of the positive electrode active material based on the ratio of Q2 to Q1. Thus, the kinetic performance of different batches of positive electrode active materials with similar particle sizes can be evaluated by a relatively simple method, and positive electrode active materials that meet the required kinetic performance requirements can be screened.

[0005] In some embodiments, the positive electrode active material includes lithium iron phosphate, and the lithium iron phosphate satisfies the structural formula LiFe x Mn 1-x PO4, wherein x is 0.8-1, the upper limit voltage of the charging process is 3.75 V, and the cutoff voltages of the first rate discharge and the second rate discharge are both 2.5 V. Thus, it can be applied to a variety of lithium iron phosphate materials.

[0006] In some embodiments, the charging process includes a first constant current charge, a second constant current charge, and a third constant current charge performed sequentially, wherein the upper limit voltage of the first constant current charge is 3.2 V, the upper limit voltage of the second constant current charge is 3.4 V-3.75 V, and the upper limit voltage of the third constant current charge is 3.75 V. This can significantly eliminate polarization differences in button batteries and improve the accuracy of test results.

[0007] In some embodiments, the charge rates of the first constant current charge, the second constant current charge, and the third constant current charge are independently 0.01 C-0.5 C. This can shorten the test time and eliminate the polarization difference of the button battery.

[0008] In some embodiments, the button cell is allowed to rest for 10 minutes or more between adjacent constant current charging cycles. For example, the button cell may be allowed to rest for 10 minutes or more between two constant current charging cycles. This can further eliminate polarization differences in the button cell and improve the accuracy of the test results.

[0009] In some embodiments, the third constant current charging process further includes constant voltage charging until the cut-off current is 50 μA. This can further eliminate the polarization difference of the button battery and improve the accuracy of the test results.

[0010] In some embodiments, the discharge process includes sequentially performing the first rate discharge and the second rate discharge, thereby improving the accuracy of the discharge capacity test result.

[0011] In some embodiments, the Dv50 particle size of the lithium iron phosphate is d, the coating area density of the lithium iron phosphate slurry on the aluminum foil surface is p, and d and p meet at least one of the following conditions: d is less than or equal to 1 μm, and p is greater than or equal to 280 mg / 1540.25 mm 2 ; d is 1μm-5μm, p is 200mg / 1540.25mm 2 -280mg / 1540.25mm 2 ; d is 5μm-10μm, p is 180mg / 1540.25mm 2 -250mg / 1540.25mm 2 ; d is 10μm-15μm, p is 150mg / 1540.25mm 2 -220mg / 1540.25mm 2 ; d is 15μm-20μm, p is 120mg / 1540.25mm 2 -200mg / 1540.25mm 2 ; d is greater than or equal to 20μm, p is less than or equal to 180mg / 1540.25mm 2Thus, the influence of particle size on the kinetic performance of lithium iron phosphate materials can be reduced.

[0012] In some embodiments, the Dv50 particle size of the lithium iron phosphate is d, the mass fraction of the conductive agent in the lithium iron phosphate slurry is w, and d and w meet at least one of the following conditions: d is less than or equal to 1 μm, w is less than or equal to 1.5 wt%; d is 1 μm-5 μm, w is 0.8 wt%-2 wt%; d is 5 μm-10 μm, w is 1.8 wt%-2.5 wt%; d is 10 μm-15 μm, w is 1.5 wt%-3 wt%; d is 15 μm-20 μm, w is 1.5 wt%-3.5 wt%; d is greater than or equal to 20 μm, w is 1.8 wt%-3.5 wt%. In this way, the effect of particle size on the dynamic performance of the lithium iron phosphate material can be reduced.

[0013] In some embodiments, the Dv50 particle size of the lithium iron phosphate is d, the compaction density of the positive electrode active material layer on the surface of the positive electrode sheet is D, and d and D meet at least one of the following conditions: d is less than or equal to 1 μm, and D is 2.4 g / cm 3 -2.8g / cm 3 ; d is 1μm-5μm, D is 2.3g / cm 3 -2.8g / cm 3 ; d is 5μm-10μm, D is 2.3g / cm 3 -2.7g / cm 3 ; d is 10μm-15μm, D is 2.2g / cm 3 -2.6g / cm 3 ; d is 15μm-20μm, D is 2.1g / cm 3 -2.5g / cm 3 ; d is greater than or equal to 20 μm, D is less than or equal to 2.4 g / cm 3 Thus, the dynamic performance of lithium iron phosphate itself can be better reflected.

[0014] In some embodiments, the ambient temperature of the charging process and the ambient temperature of the discharging process are independently 25±2° C. Thus, the accuracy of the evaluation result can be further improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0016] Figure 1 This is a flow chart of an evaluation method according to an embodiment of the present application;

[0017] Figure 2This is a schematic diagram of a charging process flow according to an embodiment of the present application;

[0018] Figure 3 This is a kinetic test of different batches of lithium iron phosphate powder with a particle size of 1.6μm-2.6μm. DETAILED DESCRIPTION

[0019] The following describes the embodiments of the present application in detail. Examples of the embodiments are shown in the accompanying drawings, but unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repeated descriptions of substantially identical structures may be omitted. This is to avoid unnecessary lengthiness in the following description and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided to enable those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0020] Unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by technicians in the technical field to which this application belongs; the terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit this application; unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0021] The terms "include" and "have" in the description and claims of this application and any variations thereof are open expressions, that is, including the contents specified in this application but not excluding other contents.

[0022] In the description of this application, regardless of whether the word "about" or "approximately" is used, all numbers disclosed herein are approximate values. The value of each number may vary by less than 10% or by a reasonable difference considered by a person skilled in the art, such as 1%, 2%, 3%, 4% or 5%.

[0023] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0024] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0025] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0026] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly specifying the quantity of the technical features indicated. "First feature" and "second feature" may include one or more of the features.

[0027] In the description of this application, "A and / or B" may include the case of A alone, the case of B alone, or any of the cases of A and B, where A and B are only used for example, and may be any technical feature connected by "and / or" in this application.

[0028] In this application, the writing order of each step does not mean a strict execution order and does not constitute any limitation on the implementation process. The specific execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of this application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially. For example, it is mentioned that the method may also include step (c), indicating that step (c) can be added to the method in any order. For example, the method may include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0029] Differences in positive electrode active materials and manufacturing processes are the main factors affecting battery kinetic performance. During the mass production of batteries, each process step is strictly controlled and has a high degree of consistency. For positive electrode active materials, even the same positive electrode active materials with similar particle sizes will have different kinetic properties. In order to make the battery have better kinetic performance and consistency, it is necessary to evaluate the kinetic performance of the positive electrode active materials in order to improve the consistency of the battery from the root. Compared with soft-pack batteries, button batteries have the advantages of simple preparation process, fewer consumables, short preparation time, and less interference from other components in the battery. When designing batteries, button batteries can be used to test and evaluate the electrical properties of positive electrode active materials.

[0030] During the charging process, button-type batteries can experience excessive internal polarization differences due to factors such as excessive upper voltage limits and charge rates. This internal polarization difference prevents the positive electrode active material from being fully utilized during charging. As a result, even when the button battery is fully charged, some of the positive electrode active material still fails to release lithium ions, and the capacity measured during discharge cannot accurately reflect the gram capacity of the positive electrode active material. During the discharge of button-type batteries, the discharge rate also affects the gram capacity of the positive electrode active material. Generally speaking, the higher the discharge rate, the greater the internal polarization difference of the battery, resulting in lower gram capacity utilization.

[0031] In the present application, the discharge capacities of different batches of positive electrode active materials at the first rate (0.05C-0.2C) are all thermodynamic capacities. The positive electrode active materials can all stably and fully release lithium ions from the negative electrode sheet, and lithium ions can also fully enter the interior of the positive electrode active material at a rate lower than the first rate, so the charge and discharge capacities can be fully exerted. Therefore, the difference in discharge capacity of the positive electrode active material at the first rate is small, and the difference in gram capacity between different batches of positive electrode active materials is a normal fluctuation. For example, there may be a difference of about 6 mAh / g. The discharge capacities of different batches of positive electrode active materials at the second rate (1C-2C) are all kinetic capacities. When discharged at the second rate (1C-2C), due to the differences in morphology, internal lattice defects, coating layer content and coating integrity between different batches of positive electrode active materials, the discharge capacities of different batches of positive electrode active materials at the second rate vary greatly. That is, the discharge difference of the positive electrode active material at the second rate is closely related to the differences in the structure and chemical composition of the positive electrode active material itself.

[0032] By testing the discharge capacity Q1 of the positive electrode active material at the first rate and the discharge capacity Q2 at the second rate in the fully charged state, Q1 can indicate the gram capacity that can be released when the positive electrode active material is fully discharged at a low rate, and Q2 can indicate the gram capacity that can be exerted when the positive electrode active material is discharged at a higher rate. Thus, the ratio of Q2 to Q1 can eliminate the gram capacity difference of the positive electrode active material itself when the capacity is fully exerted when the positive electrode active material is discharged at a low rate. Furthermore, the ratio of Q2 to Q1 can reflect the large gram capacity difference caused by differences in the structure, chemical composition, etc. of different batches of positive electrode active materials at a higher rate, thereby reflecting the difference in kinetic performance, reducing the difference in the first capacity of mass production caused by the poor kinetic performance of certain batches of positive electrode active materials, and improving the accuracy of kinetic evaluation. At the same time, it can also eliminate the difference in kinetic evaluation caused by the capacity difference of the prepared finished batteries.

[0033] Fully charged means that the battery's state of charge is 100%. Of course, fully charged can also refer to the situation where the battery's state of charge is other values, for example, the state of charge is greater than 90%. This application does not limit this.

[0034] In the present application, the kinetic performance of the positive electrode active material refers to the rate at which lithium ions migrate between the positive electrode sheet and the negative electrode sheet, which has a great influence on the rate characteristics of the battery.

[0035] In a first aspect of the present application, the present application proposes a method for evaluating the kinetic performance of a positive electrode active material, so that the kinetic performance of the positive electrode active material can be evaluated by a relatively simple method.

[0036] In some embodiments, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries.

[0037] As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.8 Co 0.15 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0038] The battery's charge and discharge processes involve the intercalation and deintercalation of lithium, leading to different molar Li contents at different discharge states. The molar Li contents listed in this application for the positive electrode active materials refer to the initial state of the material, i.e., the state before addition. When the positive electrode active material is used in a battery system, the molar Li content will change after charge and discharge cycles.

[0039] In the list of positive electrode active materials for lithium-ion batteries in this application, the molar content of O is only a theoretical value. Lattice oxygen release will cause the molar content of oxygen to change, and the actual molar content of O will fluctuate.

[0040] The following describes the method for evaluating the kinetic performance of the positive electrode active material in the present application by taking lithium iron phosphate as an example.

[0041] Specifically, refer to Figure 1 , methods for evaluating the kinetic performance of lithium iron phosphate include:

[0042] S110: Apply lithium iron phosphate slurry on the surface of aluminum foil and dry it

[0043] In some embodiments, in this step, lithium iron phosphate, a conductive agent, and a binder are mixed in a solvent to obtain lithium iron phosphate slurry, and the lithium iron phosphate slurry is coated on the surface of an aluminum foil and dried to form a positive electrode active material layer to obtain a positive electrode sheet.

[0044] In some embodiments, the lithium iron phosphate satisfies the structural formula LiFe x Mn 1-x PO4, where x is 0.8-1.

[0045] As an example, when x is 1, the lithium iron phosphate satisfies the structural formula LiFePO 4 ; when x is 0.8-1, and x is not 1, the lithium iron phosphate is manganese-doped lithium iron phosphate.

[0046] As an example, the lithium iron phosphate material may be screened, and ultrasonic or vibrating screens may be selected for screening, so that the particle size distribution of the lithium iron phosphate material has a smaller fluctuation.

[0047] In some embodiments, when the Dv50 particle size of the lithium iron phosphate is d and the coating area density of the lithium iron phosphate slurry on the aluminum foil surface is p, d and p satisfy at least one of the following conditions:

[0048] d is less than or equal to 1μm, p is greater than or equal to 280mg / 1540.25mm 2 ;

[0049] d is 1μm-5μm, p is 200mg / 1540.25mm 2 -280mg / 1540.25mm 2 ;

[0050] d is 5μm-10μm, p is 180mg / 1540.25mm 2 -250mg / 1540.25mm 2 ;

[0051] d is 10μm-15μm, p is 150mg / 1540.25mm 2 -220mg / 1540.25mm 2 ;

[0052] d is 15μm-20μm, p is 120mg / 1540.25mm 2 -200mg / 1540.25mm 2 ;

[0053] d is greater than or equal to 20 μm, p is less than or equal to 180 mg / 1540.25 mm 2 .

[0054] As the particle size of lithium iron phosphate increases, the length of the lithium ion insertion and extraction path in lithium iron phosphate also tends to increase. In order to reduce the effect of particle size on the lithium ion transmission distance and reduce the effect of lithium ion transmission distance on the kinetic performance of lithium iron phosphate, the coating surface density should be reduced for lithium iron phosphate materials with larger particle sizes, so that the total lithium ion transmission distance exhibited by lithium iron phosphate in the positive electrode active material layer containing lithium iron phosphate of different particle sizes is similar, reducing the deviation in test results caused by excessive lithium ion transmission distance. In particular, different batches of lithium iron phosphate should be tested with similar or identical coating surface densities as much as possible.

[0055] As an example, the coating thickness of the lithium iron phosphate slurry on the surface of the aluminum foil may be 100 μm-400 μm.

[0056] In some embodiments, a conductive agent needs to be added to the lithium iron phosphate slurry to improve the conductivity of the positive electrode active material layer, which is conducive to the sufficient deintercalation of lithium ions in the lithium iron phosphate. When the Dv50 particle size of the lithium iron phosphate is d and the mass fraction of the conductive agent in the lithium iron phosphate slurry is w, d and w meet at least one of the following conditions:

[0057] d is less than or equal to 1 μm, w is less than or equal to 1.5 wt%;

[0058] d is 1 μm-5 μm, w is 0.8 wt%-2 wt%;

[0059] d is 5 μm-10 μm, w is 1.8 wt%-2.5 wt%;

[0060] d is 10 μm-15 μm, w is 1.5 wt%-3 wt%;

[0061] d is 15 μm-20 μm, w is 1.5 wt%-3.5 wt%;

[0062] d is greater than or equal to 20 μm, and w is 1.8 wt% to 3.5 wt%.

[0063] As the particle size of lithium iron phosphate increases, the distance between lithium iron phosphate particles also tends to increase. When the particle size of lithium iron phosphate increases, more conductive agent is required to form electrical connections between lithium iron phosphate particles, thereby reducing the impact of the distance between lithium iron phosphate particles on the dynamic performance of lithium iron phosphate.

[0064] As an example, a total of 100 parts by weight of lithium iron phosphate, a conductive agent, and a binder can be dissolved in a solvent (for example, N-methylpyrrolidone) and stirred evenly to obtain a lithium iron phosphate slurry, wherein the lithium iron phosphate is 80 parts by weight to 90 parts by weight; the weight of the conductive agent can refer to the mass fraction of the conductive agent in the aforementioned lithium iron phosphate slurry, and the rest is the binder. When testing, different batches of lithium iron phosphate should try to use similar or identical conductive agent content and lithium iron phosphate content.

[0065] As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0066] As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0067] It should be noted that when testing different batches of lithium iron phosphate powder, the same conductive agent and binder should be used to prepare the lithium iron phosphate slurry.

[0068] In some embodiments, after forming the positive electrode active material layer, a cold pressing process may be performed to increase the compaction density. When the Dv50 particle size of the lithium iron phosphate is d and the compaction density of the positive electrode active material layer on the surface of the positive electrode sheet is D, d and D satisfy at least one of the following conditions:

[0069] d is less than or equal to 1 μm, D is 2.4 g / cm 3 -2.8g / cm 3 ;

[0070] d is 1μm-5μm, D is 2.3g / cm 3 -2.8g / cm 3 ;

[0071] d is 5μm-10μm, D is 2.3g / cm 3 -2.7g / cm 3 ;

[0072] d is 10μm-15μm, D is 2.2g / cm 3 -2.6g / cm 3 ;

[0073] d is 15μm-20μm, D is 2.1g / cm 3 -2.5g / cm 3 ;

[0074] d is greater than or equal to 20 μm, D is less than or equal to 2.4 g / cm 3 .

[0075] When d and D satisfy the aforementioned relationship, the compaction density of the positive electrode active material layer is larger, which can shorten the total distance of lithium ion transmission in the positive electrode active material layer, reduce damage to the lithium iron phosphate particles, and better reflect the true level of the kinetic performance of the lithium iron phosphate material. Specifically, different batches of lithium iron phosphate should try to use the same or similar compaction density when testing.

[0076] It should be noted that for lithium iron phosphate powders with large differences in particle size, the dynamic performance varies greatly. Even through the optimization of the aforementioned coating surface density, conductive agent content, compaction density, etc., there will still be large differences in the dynamic performance. Therefore, the method in this application is more accurate in evaluating the dynamic performance of lithium iron phosphate materials with similar particle sizes. For example, the particle size of lithium iron phosphate powder can be divided with reference to the above content, and the dynamic performance of different batches of lithium iron phosphate materials within each particle size range can be evaluated.

[0077] S120: Use lithium sheet as negative electrode to assemble button battery

[0078] In some embodiments, 1 mol / L LiPF6 is used as the electrolyte in the electrolyte, a mixed solution of ethylene carbonate: diethyl carbonate: dimethyl carbonate in a volume ratio of 1:1:1 is used as the solvent of the electrolyte, and a lithium sheet is used as the negative electrode. After the electrode is prepared, it is assembled into a button battery in a glove box. After the button battery is assembled, it is left to stand for 12 hours to 48 hours to allow the electrolyte to evenly infiltrate the positive electrode and the negative electrode.

[0079] S200: Charging the button battery

[0080] In some embodiments, after the button cell is prepared, the button cell is charged, and the upper limit voltage of the charging process is 3.75V. The charging process may include a first constant current charge, a second constant current charge, and a third constant current charge performed in sequence, thereby significantly eliminating the polarization difference of the button cell and improving the accuracy of the test results. Specifically, referring to Figure 2 , the charging process may include:

[0081] When the upper limit voltage of the charging process is 3.75V, lithium iron phosphate can release lithium ions more fully and maintain a high structural stability, thereby increasing the number of lithium insertion sites during discharge treatment and improving the accuracy of the discharge capacity test results.

[0082] S210: Performing a first constant current charge on the button battery

[0083] In some embodiments, the upper limit voltage of the first constant current charging is 3.2V, and 3.2V is the voltage platform of lithium iron phosphate in a full battery.

[0084] In some embodiments, the charging rate of the first constant current charging may be 0.01C-0.5C.

[0085] As an example, the rate of the first constant current charging may be 0.01C, 0.05C, 0.1C, 0.2C, 0.3C, 0.4C or 0.5C.

[0086] Performing constant current charging at the aforementioned rate can not only charge to the required voltage quickly and shorten the test time, but also eliminate the polarization difference inside the button battery during the charging process.

[0087] In some embodiments, it takes a short time for lithium iron phosphate to initially reach the voltage platform. At this time, the internal polarization difference of the battery is large. After the first constant current charging, the internal polarization difference of the battery can be eliminated by standing still, and the standing time is greater than or equal to 10 minutes.

[0088] S220: Perform the second constant current charging on the button battery

[0089] In some embodiments, the upper limit voltage of the second constant current charging is 3.4V-3.75V. Therefore, by performing step-by-step constant current charging and allowing the button battery to rest between adjacent constant current charging steps, the internal polarization difference during the button battery charging process can be effectively eliminated, thereby improving the accuracy of the test results.

[0090] As an example, the upper limit voltage of the second constant current charging may be 3.4V, 3.45V, 3.5V, 3.55V, 3.6V, 3.65V, 3.7V or 3.75V.

[0091] As an example, the charging process may include multiple second constant current charges, the upper limit voltage of each second constant current charge should be higher than the previous second constant current charge, and the upper limit voltage of each second constant current charge is between 3.4V-3.75V. In this way, the internal polarization difference of the button battery during the charging process can be further eliminated, and the accuracy of the test results can be improved.

[0092] In some embodiments, the charging rate of the second constant current charging may be 0.01C-0.5C.

[0093] As an example, the rate of the second constant current charging may be 0.01C, 0.05C, 0.1C, 0.2C, 0.3C, 0.4C or 0.5C.

[0094] Performing constant current charging at the aforementioned rate can not only charge to the required voltage quickly and shorten the test time, but also eliminate the polarization difference inside the button battery during the charging process.

[0095] In some embodiments, after the second constant current charging, the battery may be left to stand to eliminate internal polarization differences, and the standing time is greater than or equal to 10 minutes.

[0096] S230: Perform the third constant current charging on the button battery

[0097] In some embodiments, the upper limit voltage of the third constant current charging is 3.75V. The voltage upper limit of lithium iron phosphate generally does not exceed 3.75V. If it exceeds 3.75V, it may cause overcharging, damage to the structure of lithium iron phosphate, and when applied to a full battery, the negative electrode voltage is less than 0V to generate lithium dendrites, resulting in puncture of the diaphragm and other defects.

[0098] In some embodiments, the charging rate of the third constant current charging may be 0.01C-0.5C.

[0099] As an example, the rate of the third constant current charging may be 0.01C, 0.05C, 0.1C, 0.2C, 0.3C, 0.4C or 0.5C.

[0100] Performing constant current charging at the aforementioned rate can not only charge to the required voltage quickly and shorten the test time, but also eliminate the polarization difference inside the button battery during the charging process.

[0101] In some embodiments, after the third constant current charging, the battery may be left to stand to eliminate internal polarization differences, and the standing time is greater than or equal to 10 minutes.

[0102] It should be noted that the time for the first constant current charging, the second constant current charging, and the third constant current charging is not particularly limited, and the judgment is based on whether the voltage at the end of charging reaches the designed value.

[0103] In some embodiments, reference Figure 2 , the third constant current charging process further includes:

[0104] S240: Constant voltage charging of button batteries

[0105] In some embodiments, constant voltage charging is performed until the cutoff current is 50 μA. When the button cell is first charged to 3.75V, there is still a certain polarization difference inside the button cell. By maintaining the voltage for constant voltage charging, the charging current gradually decreases, and the remaining small amount of lithium ions continues to slowly escape, allowing the capacity of the lithium iron phosphate to be fully utilized as much as possible.

[0106] In some embodiments, the ambient temperature of the charging process is 25±2° C. Thus, the accuracy of the evaluation results can be further improved, the charging curve can be recorded more accurately, and the fluctuation caused by the test error can be reduced.

[0107] In some embodiments, after the charging process is completed, the button battery is subjected to a discharge process, and the discharge process includes a first rate discharge and a second rate discharge, the charging process is performed between the first rate discharge and the second rate discharge, and the cutoff voltages of the first rate discharge and the second rate discharge are both 2.5V. When the cutoff voltage of the discharge process is 2.5V, the measured capacity includes the main lithium iron phosphate discharge platform, which can better reflect the kinetic capacity of lithium iron phosphate and fully reflect the kinetic performance of lithium iron phosphate. When the cutoff voltage is lower than 2.5V, for example, when the cutoff voltage is as low as 2.0V, it is reflected as thermodynamic capacity, and the discharge capacity is significantly reduced at this time, and its kinetic performance cannot be reflected.

[0108] Specifically, refer to Figure 1 , discharge treatment includes:

[0109] S310: Discharging the button battery at a first rate

[0110] In some embodiments, the discharge rate of the first rate discharge is 0.05C-0.2C.

[0111] As an example, the discharge rate of the first rate discharge may be 0.05C, 0.1C, 0.15C, or 0.2C.

[0112] When the discharge rate of the first rate discharge is within the aforementioned range, the discharge capacity Q1 of the first rate discharge may indicate the gram capacity that can be released when the lithium iron phosphate material is fully discharged at a low rate.

[0113] It should be noted that after the button battery is discharged at the first rate or the second rate, it is necessary to recharge the button battery to 3.75V before discharging at another rate. The specific charging process can refer to some or all of the steps in the above charging process, and the relevant parameters of the charging process can refer to some or all of the technical features in the above embodiments, which will not be repeated here.

[0114] In some embodiments, the button battery is allowed to stand for more than or equal to 10 minutes between the first rate discharge and the second rate discharge, so that internal polarization differences of the battery can be eliminated by the standing time.

[0115] S320: Discharging the button battery at a second rate

[0116] In some embodiments, the discharge rate of the second rate discharge is 1C-2C.

[0117] As an example, the discharge rate of the first rate discharge may be 1C, 1.1C, 1.2C, 1.3C, 1.4C, 1.5C, 1.6C, 1.7C, 1.8C, 1.9C, or 2C.

[0118] When the discharge rate of the second rate discharge is within the aforementioned range, the discharge capacity Q2 of the second rate discharge can indicate the specific capacity that the lithium iron phosphate material can exert when discharged at a higher rate.

[0119] In some embodiments, the discharge process includes sequentially performing the first rate discharge and the second rate discharge. Since the second rate discharge can easily cause lattice distortion or even structural shattering when lithium ions enter the lithium iron phosphate, while a low rate has almost no effect, the first rate discharge is prioritized before the second rate discharge, thereby improving the accuracy of the test results.

[0120] In some embodiments, the ambient temperature of the discharge process is 25±2° C. Thus, the accuracy of the evaluation results can be further improved, the charge and discharge curves can be recorded more accurately, and the fluctuation caused by the test error can be reduced.

[0121] It should be noted that when performing discharge treatment on different batches of lithium iron phosphate materials, the same discharge rate of the first rate discharge and the same discharge rate of the second rate discharge should be used.

[0122] In some embodiments, the discharge capacity Q1 of the button cell at a first rate is tested, and the discharge capacity Q2 of the button cell at a second rate is tested; and the kinetic performance of the positive electrode active material is evaluated based on the ratio of Q2 to Q1.

[0123] Specifically, when the ratio of Q2 to Q1 is lower than 82%, during the charging process of the positive electrode active material, the lithium ions in the lithium iron phosphate cannot be completely released, causing the charging voltage to reach the upper limit early and the charging to end early, resulting in the battery's discharge capacity being low and unable to meet usage requirements. Moreover, as the usage time increases, the battery capacity is prone to rapid decline.

[0124] As an example, when the discharge rate of the first rate discharge is 0.1C and the discharge rate of the second rate discharge is 1C, when Q2 / Q1 is greater than or equal to 82%, it is considered that the kinetic performance of the batch of lithium iron phosphate materials is good.

[0125] In some embodiments, a high-precision coulomb efficiency meter may be used to record the charge and discharge curves to reduce errors caused by the testing equipment.

[0126] The present invention will be described below by way of specific examples. It should be noted that the following examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. Where specific techniques or conditions are not specified in the examples, they are determined according to the techniques or conditions described in the literature in this area or according to the product specifications. Reagents or instruments not specified by manufacturer are all commercially available conventional products.

[0127] Example 1

[0128] Preparation of button cells: The positive electrode active material (lithium iron phosphate with a particle size range of 1.6 μm-2.6 μm), conductive agent (carbon black) and binder (polyvinylidene fluoride, dissolved in N-methylpyrrolidone, mass fraction of 6 wt%) were mixed in a mass ratio of 96:2:2 and stirred at high speed using a homogenizer for 60 minutes to prepare a slurry with a coating weight of 260 mg / 1540.25 mm 2 The coated electrode is placed on the aluminum foil as the current collector and dried in a drying oven at 80°C for 4 hours. The dried electrode is cut and rolled to a compact density of 2.6 g / cm 3 After weighing, the sample was dried in a vacuum drying oven at 60°C for 12 hours and then transferred to an argon atmosphere glove box to obtain a positive electrode. The prepared positive electrode was used as the positive electrode, the lithium metal sheet as the negative electrode, and the polypropylene porous material Celgard 2300 as the separator. The electrolyte used a 1.0 mol / L solute of LiPF6 and a solvent of ethylene carbonate: diethyl carbonate: dimethyl carbonate (volume ratio of 1:1:1) were used to assemble a button cell model CR2032.

[0129] The button cell prepared above was subjected to the following charging and discharging treatments:

[0130] (1) Let the button stand for 3 hours;

[0131] (2) Charge to 3.2V at a constant current of 0.01C;

[0132] (3) Let stand for 15 minutes;

[0133] (4) 0.5C constant current charging for 60s;

[0134] (5) Let stand for 15 minutes;

[0135] If the voltage of the button battery is ≥3.5V after step (5), proceed to step (7); otherwise, proceed to step (6);

[0136] (6) Repeat steps (4) and (5);

[0137] (7) 0.1C constant current charging for 90s;

[0138] (8) Let stand for 5 minutes;

[0139] If the voltage of the button battery is ≥3.75V after step (8), proceed to step (10); otherwise, proceed to step (9);

[0140] (9) Cycle (7) and (8);

[0141] (10) Constant voltage charging to 3.75V, cut-off current 50μA;

[0142] (11) Let stand for 15 minutes;

[0143] (12) 0.1C DC discharge to 2.5V, record the discharge capacity Q1;

[0144] (13) Let stand for 30 minutes;

[0145] (14) 0.01C constant current charging to 3.2V (reaching the voltage platform);

[0146] (15) Let stand for 15 minutes;

[0147] (16) 0.5C constant current charging for 60s;

[0148] (17) Let stand for 15 minutes;

[0149] If the voltage of the button battery is ≥3.5V after step (17), proceed to step (19); otherwise, proceed to step (18);

[0150] (18) looping steps (16) and (17);

[0151] (19) 0.1C constant current charging for 90s;

[0152] (20) Let stand for 5 minutes;

[0153] If the voltage of the button battery is ≥3.75V after step (20), proceed to step (22); otherwise, proceed to step (21);

[0154] (21) Loop steps 19-20;

[0155] (22) Constant voltage charging to 3.75V, cut-off current 50μA;

[0156] (23) Let stand for 15 minutes;

[0157] (24) 1C DC discharge to 2.5V, record the discharge capacity Q2.

[0158] 250 batches of lithium iron phosphate materials were tested, and the test results are shown in Figure 3 Among them, lithium iron phosphate materials with Q2 / Q1 greater than or equal to 82% have better dynamic performance.

[0159] Five different batches of lithium iron phosphate materials with a Q2 / Q1 ratio greater than 82% were assembled into full cells. The cells were charged at a constant current of 0.01C to 3.75V, then charged at a constant voltage to 3.75V with a cutoff current of 50μA. The cells were then discharged at a DC current of 0.1C to 2.5V, and the discharge capacity (Q3) was recorded. After 30 minutes of rest, the cells were charged at a constant current of 0.01C to 3.75V, then charged at a constant voltage to 3.75V with a cutoff current of 50μA. The cells were then discharged at a DC current of 1C to 2.5V, and the discharge capacity (Q4) was recorded. The test results are shown in Table 1.

[0160] Table 1

[0161] serial number Q2 / Q1 Q3 / Ah Q4 / Ah Q4 / Q3 1 82.53％ 289.97 237.12 81.78％ 2 83.73％ 290.93 242.33 83.29％ 3 85.15％ 291.12 245.02 84.17％ 4 86.54％ 292.42 251.85 86.13％ 5 87.57％ 293.84 258.98 88.14％

[0162] As can be seen from the test results in Table 1, for different batches of positive electrode active materials, the larger the ratio of Q2 / Q1, the higher the discharge capacity of the battery made from the corresponding positive electrode active material at a high rate (such as the second rate), and the better the kinetic performance. Furthermore, from Q2 / Q1 and Q4 / Q3, it can be seen that the evaluation results of the kinetic performance of the positive electrode active material by the method in this application have consistent test results at the battery level. The evaluation method for positive electrode active materials in this application can be relatively simple to evaluate the kinetic performance of positive electrode active materials of similar particle size from different batches, thereby screening out positive electrode active materials that meet the required kinetic performance requirements.

[0163] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A method for evaluating the kinetic performance of a positive electrode active material, characterized in that: include: Performing a discharge process on the button battery, wherein the discharge process includes a first rate discharge and a second rate discharge, and a charging process is performed between the first rate discharge and the second rate discharge, wherein the discharge rate of the first rate discharge is 0.05C-0.2C, and the discharge rate of the second rate discharge is 1C-2C; Testing the discharge capacity Q1 of the button battery at a first rate, and testing the discharge capacity Q2 of the button battery at a second rate; The kinetic performance of the positive electrode active material is evaluated based on the ratio of Q2 to Q1.

2. The method according to claim 1, characterized in that The positive electrode active material includes lithium iron phosphate, and the lithium iron phosphate satisfies the structural formula LiFe x Mn 1-x PO4, wherein x is 0.8-1, the upper limit voltage of the charging process is 3.75V, and the cutoff voltages of the first rate discharge and the second rate discharge are both 2.5V.

3. The method according to claim 2, characterized in that The charging process includes a first constant current charge, a second constant current charge and a third constant current charge performed in sequence, wherein the upper limit voltage of the first constant current charge is 3.2V, the upper limit voltage of the second constant current charge is 3.4V-3.75V, and the upper limit voltage of the third constant current charge is 3.75V.

4. The method according to claim 3, characterized in that The charging rates of the first constant current charging, the second constant current charging and the third constant current charging are independently 0.01C-0.5C.

5. The method according to claim 3 or 4, characterized in that The button battery is allowed to stand for more than or equal to 10 minutes between adjacent constant current charges.

6. The method according to any one of claims 3 to 5, characterized in that: The third constant current charging process also includes constant voltage charging until the cut-off current is 50 μA.

7. The method according to any one of claims 1 to 6, characterized in that The discharge process includes the first rate discharge and the second rate discharge performed sequentially.

8. The method according to any one of claims 2 to 6, characterized in that: The Dv50 particle size of the lithium iron phosphate is d, the coating area density of the lithium iron phosphate slurry on the aluminum foil surface is p, and d and p meet at least one of the following conditions: d is less than or equal to 1μm, p is greater than or equal to 280mg / 1540.25mm 2 ; d is 1μm-5μm, p is 200mg / 1540.25mm 2 -280mg / 1540.25mm 2 ; d is 5μm-10μm, p is 180mg / 1540.25mm 2 -250mg / 1540.25mm 2 ; d is 10μm-15μm, p is 150mg / 1540.25mm 2 -220mg / 1540.25mm 2 ; d is 15μm-20μm, p is 120mg / 1540.25mm 2 -200mg / 1540.25mm 2 ; d is greater than or equal to 20 μm, p is less than or equal to 180 mg / 1540.25 mm 2 .

9. The method according to any one of claims 2 to 6, characterized in that: The Dv50 particle size of the lithium iron phosphate is d, the mass fraction of the conductive agent in the lithium iron phosphate slurry is w, and d and w meet at least one of the following conditions: d is less than or equal to 1 μm, w is less than or equal to 1.5 wt%; d is 1 μm-5 μm, w is 0.8 wt%-2 wt%; d is 5 μm-10 μm, w is 1.8 wt%-2.5 wt%; d is 10 μm-15 μm, w is 1.5 wt%-3 wt%; d is 15 μm-20 μm, w is 1.5 wt%-3.5 wt%; d is greater than or equal to 20 μm, and w is 1.8 wt% to 3.5 wt%.

10. The method according to any one of claims 2 to 9, characterized in that: The Dv50 particle size of the lithium iron phosphate is d, the compaction density of the positive electrode active material layer on the surface of the positive electrode plate is D, and d and D meet at least one of the following conditions: d is less than or equal to 1 μm, D is 2.4 g / cm 3 -2.8g / cm 3 ; d is 1μm-5μm, D is 2.3g / cm 3 -2.8g / cm 3 ; d is 5μm-10μm, D is 2.3g / cm 3 -2.7g / cm 3 ; d is 10μm-15μm, D is 2.2g / cm 3 -2.6g / cm 3 ; d is 15μm-20μm, D is 2.1g / cm 3 -2.5g / cm 3 ; d is greater than or equal to 20 μm, D is less than or equal to 2.4 g / cm 3 .

11. The method according to any one of claims 1 to 10, characterized in that The ambient temperatures of the charging process and the discharging process are independently 25±2° C.

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