Method for evaluating cycle performance of lithium ion battery

Abstract

The invention relates to a method for evaluating the cycle performance of a lithium ion battery. The method comprises the following steps: performing a cycle test with a set cycle number on the lithium ion battery according to a cycle performance test; obtaining a real part impedance variable coefficient of the lithium ion battery according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test; according to the lithium ion battery after the cycle test, obtaining a steady-state voltage of the lithium ion battery after the cycle test for a set time; according to the voltage differential capacity curve of the lithium ion battery after the cycle test, obtaining the maximum voltage of the lithium ion battery corresponding to the maximum phase change peak of the voltage differential capacity curve; and evaluating the cycle performance of the lithium ion battery according to the real part impedance variable coefficient, the steady-state voltage and the maximum voltage. According to the scheme provided by the invention, the cycle performance of the lithium ion battery can be quickly and accurately evaluated, the evaluation cost is reduced, the evaluation time is shortened, and the evaluation efficiency is improved.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a method for evaluating the cycle performance of lithium-ion batteries. Background Art

[0002] Lithium-ion batteries are widely used in various portable electronic devices, electric vehicles and other fields due to their high energy density and long life. However, lithium-ion batteries will gradually age during use, resulting in performance degradation. Therefore, accurately evaluating the cycle performance of lithium-ion batteries in the early stages of the cycle and quickly screening lithium-ion systems with better positive and negative electrode kinetics and matching are of great significance for improving the efficiency and accuracy of lithium-ion system development.

[0003] The related technology of lithium-ion battery cycle performance testing mainly relies on the analysis of the charge and discharge curve, internal resistance, thickness and disassembly interface of the lithium-ion battery, which often takes a long time to obtain accurate results and requires the battery to be destroyed for destructive testing. Therefore, the related technology of lithium-ion battery cycle performance testing has high testing costs, long testing time and low testing efficiency. Summary of the invention

[0004] In order to solve or partially solve the problems existing in the related art, the present application provides a method for evaluating the cycle performance of a lithium-ion battery, which can quickly and accurately evaluate the cycle performance of a lithium-ion battery, reduce the cost of evaluation, shorten the time of evaluation, and improve the efficiency of evaluation.

[0005] The first aspect of the present application provides a method for evaluating the cycle performance of a lithium ion battery, the method comprising: Perform cycle tests on lithium-ion batteries for a set number of cycles according to cycle performance tests; Obtaining a real impedance variation coefficient of the lithium ion battery according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test; According to the lithium-ion battery after the cycle test, obtaining a steady-state voltage of the lithium-ion battery after the cycle test after being left for a set time; According to the voltage differential capacity curve of the lithium ion battery after the cycle test, obtaining the maximum voltage of the lithium ion battery corresponding to the maximum phase change peak of the voltage differential capacity curve; The cycle performance of the lithium-ion battery is evaluated according to the real impedance variation coefficient, the steady-state voltage, and the maximum voltage.

[0006] Preferably, the method further comprises: obtaining a charge transfer impedance change rate of the lithium ion battery after the cycle test according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test; The cycle performance of the lithium-ion battery is evaluated based on the real impedance variation coefficient, the steady-state voltage, the maximum voltage, and the charge transfer impedance change rate.

[0007] Preferably, obtaining the real impedance variation coefficient of the lithium-ion battery according to the electrochemical impedance spectrum of the lithium-ion battery after the cycle test includes: Performing an electrochemical impedance spectroscopy test at a set electrochemical impedance frequency on the lithium ion battery after the cycle test to obtain an electrochemical impedance spectrum of the lithium ion battery after the cycle test; Obtaining a real impedance mean and a real impedance standard deviation of the lithium ion battery according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test; The real impedance variation coefficient of the lithium ion battery is obtained according to the real impedance mean value and the real impedance standard deviation of the lithium ion battery.

[0008] Preferably, the step of obtaining the steady-state voltage of the lithium-ion battery after the cycle test after being left for a set time comprises: According to the lithium-ion battery after the cycle test, the lithium-ion battery in a fully charged state after the cycle test is left for a set time, and a steady-state voltage of the lithium-ion battery is obtained.

[0009] Preferably, obtaining the maximum voltage of the lithium-ion battery corresponding to the maximum phase change peak of the voltage differential capacity curve of the lithium-ion battery after the cycle test comprises: According to the voltage differential capacity curve of the lithium ion battery after the cycle test, the voltage corresponding to the maximum phase change peak of the lithium ion battery in the voltage differential capacity curve is obtained, and the absolute value of the voltage corresponding to the maximum phase change peak is used as the maximum voltage.

[0010] Preferably, obtaining the charge transfer impedance change rate of the lithium ion battery after the cycle test according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test includes: Obtaining, according to the electrochemical impedance spectrum of the lithium-ion battery after the cycle test, a charge transfer impedance of the lithium-ion battery at a first set amount of electricity after the cycle test; The charge transfer impedance change rate of the lithium ion battery after the cycle test is obtained according to the charge transfer impedance of the lithium ion battery at the second set power before the cycle test and the charge transfer impedance of the lithium ion battery at the first set power after the cycle test.

[0011] Preferably, the set cycle number is n, and n is an integer multiple of 50.

[0012] Preferably, n is any one of 50, 100, 150, 200, 250, and 300.

[0013] Preferably, the test temperature of the cycle performance test includes a plurality of set temperatures, wherein the temperature difference between two adjacent set temperatures among the plurality of set temperatures is greater than or equal to 10°C.

[0014] Preferably, the set time is greater than or equal to 2 hours.

[0015] Preferably, the frequency range of setting the electrochemical impedance frequency is 0.05 Hz to 50000 Hz.

[0016] The technical solution provided by this application may have the following beneficial effects: The technical solution of the present application is simple in method and highly operable. It does not require disassembly of the lithium-ion battery and does not require completion of conventional cycles of cycle performance testing. It can reduce the material cost and time cost of cycle performance testing of the lithium-ion battery. According to the real impedance variation coefficient, steady-state voltage, and maximum voltage of the lithium-ion battery after the cycle test, the cycle kinetic performance and matching of the lithium-ion battery are quantitatively determined from the impedance and voltage dimensions. According to the advantages and disadvantages of the kinetic performance and matching of the lithium-ion battery, the cycle performance of the lithium-ion battery can be quickly and accurately evaluated, thereby reducing the cost of evaluation, shortening the time of evaluation, and improving the efficiency of evaluation, thereby providing effective data support for shortening the system screening time of lithium-ion batteries and improving the development efficiency of lithium-ion batteries.

[0017] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above and other objects, features and advantages of the present application will become more apparent by describing in more detail exemplary embodiments of the present application in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments of the present application.

[0019] Figure 1 It is a schematic diagram of a flow chart of a method for evaluating the cycle performance of a lithium-ion battery shown in an embodiment of the present application; Figure 2 is another schematic flow chart of a method for evaluating the cycle performance of a lithium-ion battery shown in an embodiment of the present application; Figure 3 The EIS test frequency and the real axis Z of the A1 group of lithium ion batteries and the B1 group of lithium ion batteries shown in the embodiment of the present application are w Schematic diagram of the curve; Figure 4is a schematic diagram of voltage curves of the A1 group of lithium-ion batteries and the B1 group of lithium-ion batteries after being left for 3 hours, shown in an embodiment of the present application; Figure 5 is a schematic diagram of dQ / dV curves of the A1 group of lithium-ion batteries and the B1 group of lithium-ion batteries shown in the embodiments of the present application; Figure 6 is a schematic diagram of dQ / dV curves of the A2 group of lithium-ion batteries and the B2 group of lithium-ion batteries shown in the embodiments of the present application; Figure 7 It is a schematic diagram of the thickness expansion rate of the lithium ion battery group A and the lithium ion battery group B in the 25°C cycle performance test shown in the embodiment of the present application; Figure 8 It is a schematic diagram of the capacity retention rate of the lithium ion battery group A and the lithium ion battery group B in the 25°C cycle performance test shown in the embodiment of the present application; Fig. 9 It is a schematic diagram of the thickness expansion rate of the lithium ion battery group A and the lithium ion battery group B in the 45°C cycle performance test shown in the embodiment of the present application; Fig.10 It is a schematic diagram of the capacity retention rate of the lithium ion battery group A and the lithium ion battery group B in the 45°C cycle performance test shown in the embodiment of the present application; Fig.11 The EIS test frequency and the real axis Z of the C1 group of lithium ion batteries and the D1 group of lithium ion batteries shown in the embodiment of the present application are w Schematic diagram of the curve; Fig.12 is a schematic diagram of voltage curves of the C1 group of lithium-ion batteries and the D1 group of lithium-ion batteries after being left for 3 hours, shown in an embodiment of the present application; Fig.13 is a schematic diagram of dQ / dV curves of a C1 group of lithium-ion batteries and a D1 group of lithium-ion batteries shown in an embodiment of the present application; Fig.14 is a schematic diagram of dQ / dV curves of the C2 group of lithium-ion batteries and the D2 group of lithium-ion batteries shown in an embodiment of the present application; Fig.15 It is a schematic diagram of the thickness expansion rate of the lithium ion battery group C and the lithium ion battery group D in the 25°C cycle performance test shown in the embodiment of the present application; Fig.16 25° C. cycle performance test of lithium ion battery group C and lithium ion battery group D shown in the embodiment of the present application; Fig.17 It is a schematic diagram of the thickness expansion rate of the lithium ion battery group C and the lithium ion battery group D in the 45°C cycle performance test shown in the embodiment of the present application; Fig.18Schematic diagram of the capacity retention rate of the lithium ion battery group C and the lithium ion battery group D in the 45° C. cycle performance test shown in the embodiment of the present application. DETAILED DESCRIPTION

[0020] The embodiments of the present application will be described in more detail below with reference to the accompanying drawings. Although the embodiments of the present application are shown in the accompanying drawings, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided to make the present application more thorough and complete, and to fully convey the scope of the present application to those skilled in the art.

[0021] The terms used in this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The singular forms of "a", "said" and "the" used in this application and the appended claims are also intended to include plural forms unless the context clearly indicates other meanings. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0022] It should be understood that although the terms "first", "second", "third", etc. may be used in this application to describe various information, this information should not be limited to these terms. These terms are only used to distinguish the same type of information from each other. For example, without departing from the scope of this application, the first information may also be referred to as the second information, and similarly, the second information may also be referred to as the first information. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "multiple" is two or more, unless otherwise clearly and specifically defined.

[0023] The embodiment of the present application provides a method for evaluating the cycle performance of a lithium-ion battery, which can quickly and accurately evaluate the cycle performance of a lithium-ion battery, reduce the cost of evaluation, shorten the time of evaluation, and improve the efficiency of evaluation.

[0024] The technical solution of the embodiments of the present application is described in detail below with reference to the accompanying drawings.

[0025] Figure 1 It is a schematic flow chart of a method for evaluating the cycle performance of a lithium-ion battery shown in an embodiment of the present application.

[0026] See also Figure 1 , a method for evaluating the cycle performance of a lithium-ion battery, comprising: Step 101 , performing a cycle test of a set number of cycles on a lithium-ion battery according to a cycle performance test.

[0027] In one embodiment, a fresh fully-charged lithium-ion battery can be subjected to a cycle test of a set number of cycles according to a cycle performance test of the lithium-ion battery, without completing a conventional number of cycles of the cycle performance test of the lithium-ion battery. In an early stage of the cycle performance test of the lithium-ion battery, a lithium-ion battery after the cycle test is obtained; wherein the fresh fully-charged lithium-ion battery may be a new fully-charged lithium-ion battery that has not been subjected to a cycle test.

[0028] Step 102, obtaining the real impedance variation coefficient of the lithium-ion battery according to the electrochemical impedance spectrum of the lithium-ion battery after the cycle test.

[0029] In one embodiment, an electrochemical impedance spectroscopy (EIS) test may be performed on the lithium-ion battery after the cycle test to obtain the electrochemical impedance spectrum of the lithium-ion battery after the cycle test; the real impedance of the lithium-ion battery after the cycle test may be obtained based on the electrochemical impedance spectrum of the lithium-ion battery after the cycle test; and the real impedance variation coefficient of the lithium-ion battery may be calculated based on the real impedance of the lithium-ion battery after the cycle test.

[0030] Step 103 , obtaining a steady-state voltage of the lithium-ion battery after the cycle test after being left for a set time according to the lithium-ion battery after the cycle test.

[0031] In one embodiment, the lithium-ion battery after the cycle test can be charged or discharged to obtain the lithium-ion battery with a set SOC (State of Charge) after the cycle test; the lithium-ion battery with a set SOC after the cycle test is placed for a set time to obtain the steady-state voltage of the lithium-ion battery after the cycle test.

[0032] SOC (State of Charge) is used to reflect the remaining capacity of the battery. Its value is defined as the ratio of the remaining capacity to the battery capacity. It is usually expressed as a percentage and its value range is 0% to 100%. When SOC = 0%, it means the battery is fully discharged, and when SOC = 100%, it means the battery is fully charged.

[0033] Step 104 , obtaining a maximum voltage corresponding to a maximum phase change peak of the voltage differential capacity curve of the lithium ion battery according to the voltage differential capacity curve of the lithium ion battery after the cycle test.

[0034] In one embodiment, a voltage differential capacity curve of the lithium ion battery after the cycle test can be drawn based on the lithium ion battery after the cycle test; a maximum phase change peak of the voltage differential capacity curve can be obtained based on the voltage differential capacity curve of the lithium ion battery after the cycle test; a voltage corresponding to the maximum phase change peak can be obtained based on the maximum phase change peak of the voltage differential capacity curve, and the voltage corresponding to the maximum phase change peak can be used as the maximum voltage corresponding to the maximum phase change peak of the voltage differential capacity curve of the lithium ion battery after the cycle test.

[0035] Step 105 , evaluating the cycle performance of the lithium-ion battery according to the real impedance variation coefficient, the steady-state voltage, and the maximum voltage of the lithium-ion battery.

[0036] In one embodiment, the cycle performance of the lithium-ion battery can be evaluated based on the real impedance variation coefficient, steady-state voltage, and maximum voltage of the lithium-ion battery after the cycle test.

[0037] In one embodiment, the smaller the real impedance variation coefficient of the lithium-ion battery after the cycle test, the smaller the impedance data fluctuation, the better the dynamic performance and matching of the lithium-ion battery, and the better the cycle performance of the lithium-ion battery.

[0038] In one embodiment, the greater the steady-state voltage of the lithium-ion battery after the cycle test, the smaller the polarization, the better the kinetic performance and matching of the lithium-ion battery, and the better the cycle performance of the lithium-ion battery.

[0039] In one embodiment, the greater the maximum voltage of the lithium-ion battery after the cycle test, the better the dynamic performance and matching of the lithium-ion battery, and the better the cycle performance of the lithium-ion battery.

[0040] The cycle performance evaluation method of the lithium-ion battery of the embodiment of the present application performs a cycle test on the lithium-ion battery for a set number of cycles according to the cycle performance test; obtains the real impedance variation coefficient of the lithium-ion battery according to the electrochemical impedance spectrum of the lithium-ion battery after the cycle test; obtains the steady-state voltage of the lithium-ion battery after the cycle test after the cycle test for a set time; obtains the maximum voltage corresponding to the maximum phase change peak of the voltage differential capacity curve of the lithium-ion battery after the cycle test according to the voltage differential capacity curve of the lithium-ion battery after the cycle test; evaluates the cycle performance of the lithium-ion battery according to the real impedance variation coefficient, the steady-state voltage, and the maximum voltage; the cycle performance evaluation method of the lithium-ion battery of the embodiment of the present application The method is simple and has strong operability. It does not require disassembly of the lithium-ion battery and does not require completion of conventional cycles of cycle performance testing, and can reduce the material cost and time cost of cycle performance testing of the lithium-ion battery. According to the real impedance variation coefficient, steady-state voltage, and maximum voltage of the lithium-ion battery after the cycle test, the cycle kinetic performance and matching of the lithium-ion battery are quantitatively determined from the impedance and voltage dimensions. According to the advantages and disadvantages of the kinetic performance and matching of the lithium-ion battery, the cycle performance of the lithium-ion battery can be quickly and accurately evaluated, the evaluation cost is reduced, the evaluation time is shortened, and the evaluation efficiency is improved, thereby providing effective data support for shortening the system screening time of lithium-ion batteries and improving the development efficiency of lithium-ion batteries.

[0041] Figure 2 It is another schematic flow chart of the method for evaluating the cycle performance of a lithium-ion battery shown in an embodiment of the present application. Figure 2 Relative to Figure 1 The scheme of the present application is described in more detail.

[0042] See also Figure 2 , a method for evaluating the cycle performance of a lithium-ion battery, comprising: Step 201, according to the cycle performance test, obtaining a lithium ion battery before the cycle test and a lithium ion battery after the cycle test.

[0043] In one embodiment, the test temperature of the cycle performance test may include a plurality of set temperatures, wherein the temperature difference between two adjacent set temperatures among the plurality of set temperatures is greater than or equal to 10°C.

[0044] In a specific embodiment, the test temperature of the cycle performance test may include a first set temperature and a second set temperature, and the temperature difference between the second set temperature and the first set temperature is greater than or equal to 10°C (degrees Celsius); the first set temperature may be a lower temperature, for example, 25°C; the second set temperature may be a higher temperature, for example, 45°C.

[0045] In one embodiment, the conventional cycle number of the cycle performance test is N, that is, when evaluating the cycle performance of two groups of lithium-ion batteries through the cycle performance test, the two groups of lithium-ion batteries need to be cycled N times to clearly distinguish the cycle performance of the two groups of lithium-ion batteries. The set cycle number n of the embodiment of the present application can be less than or equal to 1 / 2, or 1 / 3, or 1 / 4, or 1 / 5, or 1 / 10 of the conventional cycle number N.

[0046] In one embodiment, the number of cycles is set to n, where n is an integer multiple of 50.

[0047] In one embodiment, n may be any one of 50, 100, 150, 200, 250, and 300.

[0048] In one embodiment, a plurality of identical fresh fully-charged lithium-ion batteries may be prepared, wherein the fresh fully-charged lithium-ion batteries may be new fully-charged lithium-ion batteries that have not been subjected to a cycle test; according to a cycle performance test, a cycle test with n cycles and a test temperature of a first set temperature is performed on some of the fresh fully-charged lithium-ion batteries to obtain a first group of lithium-ion batteries after the cycle test; according to the cycle performance test, a cycle test with n cycles and a test temperature of a second set temperature is performed on some of the lithium-ion batteries to obtain a second group of lithium-ion batteries after the cycle test; and the lithium-ion batteries that have not been subjected to the cycle test are marked as a third group of lithium-ion batteries.

[0049] Step 202 , performing an electrochemical impedance spectroscopy test at a set electrochemical impedance frequency on the lithium ion battery that has not been subjected to the cycle test and the lithium ion battery that has been subjected to the cycle test, and obtaining the electrochemical impedance spectrum of the lithium ion battery that has been subjected to the cycle test.

[0050] In one embodiment, the frequency range of the set electrochemical impedance frequency of the electrochemical impedance spectroscopy test is 0.05 Hz to 50000 Hz.

[0051] In one embodiment, an electrochemical impedance spectroscopy test with a set electrochemical impedance frequency can be performed on the first group of lithium-ion batteries to obtain the electrochemical impedance spectrum of the first group of lithium-ion batteries; an electrochemical impedance spectroscopy test with a set electrochemical impedance frequency can be performed on the second group of lithium-ion batteries to obtain the electrochemical impedance spectrum of the second group of lithium-ion batteries; and an electrochemical impedance spectroscopy test with a set electrochemical impedance frequency can be performed on the third group of lithium-ion batteries to obtain the electrochemical impedance spectrum of the third group of lithium-ion batteries.

[0052] Step 203 , obtaining the charge transfer impedance change rate of the lithium ion battery after the cycle test according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test.

[0053] In one embodiment, the charge transfer impedance of the lithium ion battery at a first set charge after the cycle test can be obtained based on the electrochemical impedance spectrum of the lithium ion battery after the cycle test; the charge transfer impedance change rate of the lithium ion battery after the cycle test can be obtained based on the charge transfer impedance of the lithium ion battery at the second set charge that has not been cycled, and the charge transfer impedance of the lithium ion battery at the first set charge after the cycle test.

[0054] In one embodiment, during the early cycle of a lithium-ion battery (a cycle test with a cycle number of n), the SEI (Solid Electrolyte Interface) membrane of the lithium-ion battery is continuously consumed and reorganized, and the charge transfer impedance Rct of the SEI membrane is sei Theoretically, the steady-state charge transfer resistance Rct of the SEI film can be defined as sei Low sensitivity to lithium-ion battery dynamics. Charge transfer impedance Rct describes the resistance of electrochemical reactions on the electrode surface and reflects the difficulty of electron transfer between the electrode and the electrolyte.

[0055] In the embodiments of the present application, the cycle performance of the lithium-ion battery can be evaluated according to the change rule of the charge transfer impedance Rct (charge transfer impedance change rate) of the lithium-ion battery after the cycle test.

[0056] In one embodiment, the first charge transfer impedance Rct of the first group of lithium-ion batteries can be obtained according to the electrochemical impedance spectrum of the first group of lithium-ion batteries. n-1 ; According to the electrochemical impedance spectrum of the third group of lithium-ion batteries that have not been cycled, obtain the third charge transfer impedance Rct0 of the third group of lithium-ion batteries at different SOCs; according to the first charge transfer impedance Rct of the first group of lithium-ion batteries n-1 , a third charge transfer impedance Rct0, and a charge transfer impedance change rate f of the charge transfer impedance of the first group of lithium-ion batteries after the cycle test relative to the charge transfer impedance of the third group of lithium-ion batteries without the cycle test 1-1 , where the charge transfer impedance change rate f of the first group of lithium-ion batteries is 1-1 = Rct n-1 / Rct0.

[0057] In one embodiment, the second charge transfer impedance Rct of the second group of lithium-ion batteries can be obtained according to the electrochemical impedance spectrum of the second group of lithium-ion batteries. n-2 ; According to the electrochemical impedance spectrum of the third group of lithium-ion batteries that have not been cycled tested, the third charge transfer impedance Rct0 of the third group of lithium-ion batteries that have not been cycled tested at different SOCs is obtained; according to the second charge transfer impedance Rct of the second group of lithium-ion batteriesn-2 , the third charge transfer impedance Rct0, and the charge transfer impedance change rate f of the charge transfer impedance of the second group of lithium-ion batteries after the cycle test relative to the charge transfer impedance of the third group of lithium-ion batteries without the cycle test 1-2 , where the charge transfer impedance change rate f of the second group of lithium-ion batteries is 1-2 =Rct n-2 / Rct0.

[0058] In one embodiment, the first charge transfer impedance Rct of the first group of lithium-ion batteries is n-1 It can be the charge transfer impedance of the first set amount of electricity; the second charge transfer impedance Rct of the second group of lithium-ion batteries n-2 It can be the charge transfer impedance of the first set power; the third charge transfer impedance Rct0 of the third group of lithium-ion batteries can be the charge transfer impedance of the second set power; wherein, the first set power and the second set power can be represented by the SOC of the lithium-ion battery, the first set power corresponds to 100% SOC of the lithium-ion battery, and the second set power corresponds to 50% SOC of the lithium-ion battery.

[0059] SOC (State of Charge) is used to reflect the remaining capacity of the battery. Its value is defined as the ratio of the remaining capacity to the battery capacity. It is usually expressed as a percentage and its value range is 0% to 100%. When SOC = 0%, it means the battery is fully discharged, and when SOC = 100%, it means the battery is fully charged.

[0060] Step 204 , obtaining the real impedance variation coefficient of the lithium ion battery according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test. In one embodiment, the real impedance mean and the real impedance standard deviation of the lithium ion battery are obtained according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test; the real impedance variation coefficient of the lithium ion battery is obtained according to the real impedance mean and the real impedance standard deviation of the lithium ion battery.

[0061] In one embodiment, the electrochemical impedance spectroscopy curve of a lithium-ion battery with good positive and negative electrode kinetic performance and good matching is relatively smooth, and the electrochemical impedance values ​​in the high-frequency region and the low-frequency region are relatively close. Therefore, the real impedance Z of the electrochemical impedance spectroscopy can be used. w The coefficient of variation is used to evaluate the kinetic performance and matching of lithium-ion batteries; based on the kinetic performance and matching of lithium-ion batteries, the cycle performance of lithium-ion batteries is evaluated.

[0062] In one embodiment, the real impedance Z of the first group of lithium-ion batteries can be obtained according to the electrochemical impedance spectrum of the first group of lithium-ion batteries. w，n-1 ; According to the real impedance Z w，n-1, calculate the real impedance Z of the first group of lithium-ion batteries w，n-1(Ave.) , real impedance standard deviation Z w，n-1(sigma) ,; According to the real impedance mean Z w，n-1(Ave.) , real impedance standard deviation Z w，n-1(sigma) , calculate the real impedance variation coefficient f of the real impedance of the first group of lithium-ion batteries 2-1 , where f 2-1 = Z w，n-1(Ave.) / Z w，n-1(sigma) .

[0063] In one embodiment, the real impedance Z of the second group of lithium-ion batteries can be obtained according to the electrochemical impedance spectrum of the second group of lithium-ion batteries. w，n-2 ; According to the real impedance Z w，n-2 , calculate the real impedance Z of the second group of lithium-ion batteries w，n-2(Ave.) , real impedance standard deviation Z w，n-2(sigma) ,; According to the real impedance mean Z w，n-2(Ave.) , real impedance standard deviation Z w，n-2(sigma) , calculate the real impedance variation coefficient f of the real impedance of the second group of lithium-ion batteries 2-2 , where f 2-2 = Z w，n-2(Ave.) / Z w，n-2(sigma) .

[0064] Step 205 , according to the lithium-ion battery after the cycle test, the lithium-ion battery in a fully charged state after the cycle test is placed on hold for a set time, and a steady-state voltage of the lithium-ion battery is obtained.

[0065] In one embodiment, information about the electrode reaction kinetics and interface characteristics can be obtained by analyzing the change of voltage over time. That is, based on the basic principle of the relaxation voltage method, after applying a transient disturbance on the electrode surface, the relaxation process of the electrode voltage over time is observed, and the change of voltage over time is analyzed to obtain information about the electrode reaction kinetics and interface characteristics.

[0066] In one embodiment, the polarization characteristics of the lithium-ion battery can be evaluated based on the relaxation process of the voltage over time and the voltage V reaching a steady state after a specific relaxation time t; and the cycle performance of the lithium-ion battery can be evaluated based on the polarization characteristics of the lithium-ion battery.

[0067] In one embodiment, the first group of lithium-ion batteries can be charged and discharged, and the SOC of the first group of lithium-ion batteries can be adjusted so that the SOC of the first group of lithium-ion batteries is 100%, thereby obtaining the first group of lithium-ion batteries in a fully charged state; based on the relaxation voltage method, the first group of lithium-ion batteries in a fully charged state are placed aside for a set time to obtain the relaxation voltage V of the first group of lithium-ion batteries. c，n-1 ; Set the relaxation voltage V c，n-1As the steady-state voltage f of the first group of lithium-ion batteries 3-1 , where f 3-1 =V c，n-1 .

[0068] In one embodiment, the second group of lithium-ion batteries can be charged and discharged, and the SOC of the second group of lithium-ion batteries can be adjusted so that the SOC of the second group of lithium-ion batteries is 100%, thereby obtaining a second group of lithium-ion batteries in a fully charged state; based on the relaxation voltage method, the second group of lithium-ion batteries in a fully charged state are placed aside for a set time to obtain a relaxation voltage V of the second group of lithium-ion batteries. c，n-2 ; Set the relaxation voltage V c，n-2 As the steady-state voltage f of the second group of lithium-ion batteries 3-2 , where f 3-2 =V c，n-2 .

[0069] In one embodiment, the set time may be greater than or equal to 2 hours.

[0070] Step 206 , obtaining the maximum voltage corresponding to the maximum phase change peak of the voltage differential capacity curve of the lithium ion battery according to the voltage differential capacity curve of the lithium ion battery after the cycle test.

[0071] In one embodiment, the voltage corresponding to the maximum phase change peak of the voltage differential capacity curve of the lithium ion battery after the cycle test can be obtained, and the absolute value of the voltage corresponding to the maximum phase change peak is used as the maximum voltage.

[0072] In one embodiment, voltage differential capacity (dQ / dV) refers to the change in capacity Q during the charge and discharge process of a lithium-ion battery within a small voltage change range. The peak area of ​​the voltage differential capacity (dQ / dV) curve is closely related to the capacity change of the battery, and each peak on the dQ / dV curve represents an electrochemical reaction. The peak point represents the phase change point of the material. By analyzing the peak area on the dQ / dV curve, the capacity and efficiency of the lithium-ion battery can be evaluated, and the cycle performance of the lithium-ion battery can be evaluated.

[0073] In one embodiment, the resistance to insertion and extraction of lithium ions increases, the polarization impedance increases, and the discharge peak position shows a trend of shifting to the left or decreasing the peak intensity. According to the voltage differential capacity curve of the lithium ion battery after the cycle test, the peak displacement and peak shape data during the discharge of the lithium ion battery after the cycle test are analyzed. According to the absolute value of the voltage corresponding to the maximum phase change peak of the voltage differential capacity curve of the lithium ion battery after the cycle test (in the voltage differential capacity curve, the voltage during charging is a positive value, and the voltage during discharge is a negative value), the dynamic performance and matching of the lithium ion battery can be evaluated; according to the dynamic performance and matching of the lithium ion battery, the cycle performance of the lithium ion battery is evaluated; the greater the absolute value of the voltage corresponding to the maximum phase change peak of the voltage differential capacity curve of the lithium ion battery after the cycle test, the better the dynamic performance and matching of the lithium ion battery, and the better the cycle performance of the lithium ion battery.

[0074] In one embodiment, the voltage V corresponding to the maximum phase change peak of the dQ / dV curve of the first group of lithium-ion batteries can be obtained according to the dQ / dV curve of the first group of lithium-ion batteries. n-1，(dQ / dV，max) , and the voltage V corresponding to the maximum phase change peak n-1，(dQ / dV，max) The absolute value of the maximum voltage f corresponding to the maximum phase change peak in the voltage differential capacity curve of the first group of lithium-ion batteries 4-1 ,in, .

[0075] In one embodiment, the voltage V corresponding to the maximum phase change peak of the dQ / dV curve of the second group of lithium-ion batteries can be obtained according to the dQ / dV curve of the second group of lithium-ion batteries. n-2，(dQ / dV，max) , and the voltage V corresponding to the maximum phase change peak n-2，(dQ / dV，max) The absolute value of the maximum voltage f corresponding to the maximum phase change peak in the voltage differential capacity curve of the second group of lithium-ion batteries 4-2 ,in, .

[0076] Step 207 , evaluating the cycle performance of the lithium-ion battery according to the real impedance variation coefficient, the steady-state voltage, the maximum voltage, and the charge transfer impedance change rate.

[0077] In one embodiment, the differences in kinetic performance and matching of lithium-ion batteries under different single-factor variables of the lithium-ion battery system can be quantitatively determined from the impedance and voltage dimensions based on four parameters of the real impedance variation coefficient f2, steady-state voltage f3, maximum voltage f4, and charge transfer impedance change rate f1 of the lithium-ion battery after the cycle test, and the cycle performance of the lithium-ion battery under different single-factor variables can be evaluated; wherein different single-factor variables include but are not limited to lithium-ion battery design differences such as different diaphragms, different positive electrodes, different negative electrodes, different electrolytes, different surface densities, and different compaction densities.

[0078] In one embodiment, the charge transfer impedance change rate f of the first group of lithium-ion batteries may be 1-1 , Real impedance variation coefficient f 2-1 , steady-state voltage f 3-1 , maximum voltage f 4-1 , and the charge transfer impedance change rate f of the second group of lithium-ion batteries 1-2 , real impedance variation coefficient f 2-2 , steady-state voltage f 3-2 , maximum voltage f 4-2 , quantitatively determine the kinetic performance and matching of lithium-ion batteries under different single-factor variables of the lithium-ion battery system from the dimensions of impedance and voltage, evaluate the differences in the kinetic performance and matching of lithium-ion batteries under different single-factor variables, and evaluate the cycle performance of lithium-ion batteries under different single-factor variables. In one embodiment, the charge transfer impedance change rate f of the first group of lithium-ion batteries is 1-1 The charge transfer impedance change rate f of the second group of lithium-ion batteries 1-2 The smaller it is, the better the kinetic performance and matching of the lithium-ion battery, and the better the cycle performance of the lithium-ion battery; the charge transfer impedance change rate f of the first group of lithium-ion batteries 1-1 The charge transfer impedance change rate f of the second group of lithium-ion batteries 1-2 The larger it is, the worse the kinetic performance and matching of the lithium-ion battery, and the worse the cycle performance of the lithium-ion battery.

[0079] In one embodiment, the real impedance variation coefficient f of the first group of lithium-ion batteries is 2-1 and the real impedance variation coefficient f of the second group of lithium-ion batteries 2-2 The smaller it is, the smaller the fluctuation of the impedance data of the lithium-ion battery is, the better the kinetic performance and matching are, and the better the cycle performance is; the real impedance variation coefficient f of the first group of lithium-ion batteries 2-1 and the real impedance variation coefficient f of the second group of lithium-ion batteries 2-2 The larger it is, the greater the fluctuation of the impedance data of the lithium-ion battery, the worse the kinetic performance and matching, and the worse the cycle performance.

[0080] In one embodiment, the steady-state voltage f of the first group of lithium-ion batteries is 3-1 and the steady-state voltage f of the second group of lithium-ion batteries 3-2 The larger the value, the smaller the polarization of the lithium-ion battery, the better the kinetic performance and matching, and the better the cycle performance of the lithium-ion battery; the steady-state voltage f of the first group of lithium-ion batteries 3-1 and the steady-state voltage f of the second group of lithium-ion batteries 3-2 The smaller it is, the greater the polarization of the lithium-ion battery, the worse the kinetic performance and matching, and the worse the cycle performance of the lithium-ion battery.

[0081] In one embodiment, the maximum voltage f of the first group of lithium-ion batteries 4-1 and the maximum voltage f of the second group of lithium-ion batteries 4-2 The larger the value, the better the kinetic performance and matching of the lithium-ion battery, and the better the cycle performance of the lithium-ion battery; the maximum voltage f of the first group of lithium-ion batteries 4-1 and the maximum voltage f of the second group of lithium-ion batteries 4-2 The smaller it is, the worse the kinetic performance and matching of the lithium-ion battery, and the worse the cycle performance of the lithium-ion battery.

[0082] In one embodiment, the charge transfer impedance change rate f of the first group of lithium-ion batteries may be 1-1 , Real impedance variation coefficient f 2-1 , steady-state voltage f 3-1 , maximum voltage f 4-1 , and the charge transfer impedance change rate f of the second group of lithium-ion batteries 1-2 , Real impedance variation coefficient f 2-2 , steady-state voltage f 3-2 , maximum voltage f 4-2 For example, the charge transfer impedance change rate f of the first group of lithium-ion batteries can be used to evaluate the cycle performance of the lithium-ion battery. 1-1 , Real impedance variation coefficient f 2-1 , steady-state voltage f 3-1 , maximum voltage f 4-1 , and the charge transfer impedance change rate f of the second group of lithium-ion batteries 1-2 , maximum voltage f 4-2 , evaluate the cycling performance of lithium-ion batteries.

[0083] The cycle performance evaluation method of the lithium-ion battery of the embodiment of the present application performs a cycle test on the lithium-ion battery for a set number of cycles according to the cycle performance test; obtains the real impedance variation coefficient of the lithium-ion battery according to the electrochemical impedance spectrum of the lithium-ion battery after the cycle test; obtains the steady-state voltage of the lithium-ion battery after the cycle test after the cycle test for a set time; obtains the maximum voltage corresponding to the maximum phase change peak of the voltage differential capacity curve of the lithium-ion battery after the cycle test according to the voltage differential capacity curve of the lithium-ion battery after the cycle test; evaluates the cycle performance of the lithium-ion battery according to the real impedance variation coefficient, the steady-state voltage, and the maximum voltage; the cycle performance evaluation method of the lithium-ion battery of the embodiment of the present application The method is simple and has strong operability. It does not require disassembly of the lithium-ion battery and does not require completion of conventional cycles of cycle performance testing, and can reduce the material cost and time cost of cycle performance testing of the lithium-ion battery. According to the real impedance variation coefficient, steady-state voltage, and maximum voltage of the lithium-ion battery after the cycle test, the cycle kinetic performance and matching of the lithium-ion battery are quantitatively determined from the impedance and voltage dimensions. According to the advantages and disadvantages of the kinetic performance and matching of the lithium-ion battery, the cycle performance of the lithium-ion battery can be quickly and accurately evaluated, the evaluation cost is reduced, the evaluation time is shortened, and the evaluation efficiency is improved, thereby providing effective data support for shortening the system screening time of lithium-ion batteries and improving the development efficiency of lithium-ion batteries.

[0084] Furthermore, in the cycle performance evaluation method of the lithium-ion battery in the embodiment of the present application, the cycle number is set to n, where n is an integer multiple of 50, and n is any one of 50, 100, 150, 200, 250, and 300; the set cycle number of the cycle test in the embodiment of the present application is less than the conventional cycle number of the cycle performance test. By reducing the cycle number, the time for the cycle performance evaluation of the lithium-ion battery is reduced. According to the real impedance variation coefficient, steady-state voltage, and maximum voltage of the lithium-ion battery after the cycle test with a cycle number of n (early stage of the cycle performance test), the cycle kinetic performance and matching of the lithium-ion battery are quantitatively determined from the impedance and voltage dimensions. According to the advantages and disadvantages of the kinetic performance and matching of the lithium-ion battery, the cycle performance of the lithium-ion battery can be quickly and accurately evaluated, the time cost of the evaluation is reduced, the time of the evaluation is shortened, and the efficiency of the evaluation is improved.

[0085] The following is a detailed description of the method for evaluating the cycle performance of the lithium-ion battery of the present invention.

[0086] Embodiment 1: In Example 1, a fresh fully-charged lithium-ion battery group A and a fresh fully-charged lithium-ion battery group B of a 4.45V (volt) lithium cobalt oxide / graphite system under different single factor variables (for example, different electrolytes) can be divided. The fresh fully-charged lithium-ion battery group A is divided into a group A1 lithium-ion battery, a group A2 lithium-ion battery, and a group A3 lithium-ion battery, wherein the group A1 lithium-ion battery is a lithium-ion battery group that has undergone a cycle test of 200 cycles and a test temperature of 25°C, the group A2 lithium-ion battery is a lithium-ion battery group that has undergone a cycle test of 200 cycles and a test temperature of 45°C, and the group A3 lithium-ion battery is a lithium-ion battery group that has not undergone a cycle test. The fresh fully-charged lithium-ion battery group B is divided into a group B1 lithium-ion battery, a group B2 lithium-ion battery, and a group B3 lithium-ion battery, wherein the group B1 lithium-ion battery is a lithium-ion battery group that has undergone a cycle test of 200 cycles at a test temperature of 25°C, the group B2 lithium-ion battery is a lithium-ion battery group that has undergone a cycle test of 200 cycles at a test temperature of 45°C, and the group B3 lithium-ion battery is a lithium-ion battery group that has not undergone a cycle test.

[0087] An electrochemical impedance spectroscopy test with an EIS test frequency of 0.05 Hz to 50000 Hz (Hertz) is performed on the A1 group lithium ion batteries, the A2 group lithium ion batteries, the A3 group lithium ion batteries, the B1 group lithium ion batteries, the B2 group lithium ion batteries, and the B3 group lithium ion batteries, respectively, to obtain electrochemical impedance spectra of the A1 group lithium ion batteries, the A2 group lithium ion batteries, the A3 group lithium ion batteries, the B1 group lithium ion batteries, the B2 group lithium ion batteries, and the B3 group lithium ion batteries.

[0088] Figure 3 The EIS test frequency and the real axis Z of the A1 group of lithium ion batteries and the B1 group of lithium ion batteries shown in the embodiment of the present application are w Schematic diagram of the curve. Figure 3 As shown, curve 301 is the EIS test frequency and real axis Z of the A1 group of lithium-ion batteries. w Curve 302 is the EIS test frequency and the real axis Z of the B1 group of lithium ion batteries. w curve.

[0089] Get the Rct of group A1 lithium-ion batteries respectively 200-A1 , Z w，200-A1 、Rct of Group A2 lithium-ion batteries 200-A2 , Z w，200-A2 , Rct of Group A3 lithium-ion battery at SOC 50% 0-A 、Rct of Group B1 lithium-ion batteries 200-B1 , Z w，200-B1 , Rct of Group B2 lithium-ion batteries 200-B2 , Zw，200-B2 , Rct of Group B3 lithium-ion battery at SOC 50% 0-B .

[0090] The relaxation voltage test was performed on the lithium-ion batteries of group A1, group A2, group B1 and group B2 respectively. The relaxation voltage test was performed by leaving the batteries for 3 hours after being fully charged (SOC was 100%). The relaxation voltage V c，200-A1 , the relaxation voltage V of the A2 group of lithium-ion batteries c，200-A2 , relaxation voltage V of group B1 lithium-ion battery c，200-B1 , the relaxation voltage V of the B2 group of lithium-ion batteries c，200-B2 .

[0091] Figure 4 Schematic diagram of voltage curves of the A1 group of lithium-ion batteries and the B1 group of lithium-ion batteries after being left alone for 3 hours, shown in the embodiment of the present application. Figure 4 As shown, curve 401 (solid line) is the voltage curve of the A1 group of lithium-ion batteries after being left for 3 hours, and curve 402 (dashed line) is the voltage curve of the B1 group of lithium-ion batteries after being left for 3 hours.

[0092] The dQ / dV curves of the lithium ion batteries in group A1, group A2, group B1 and group B2 are plotted according to the discharge curves of the lithium ion batteries in group A1, group A2, group B1 and group B2 respectively.

[0093] Figure 5 Schematic diagram of the dQ / dV curves of the A1 group of lithium-ion batteries and the B1 group of lithium-ion batteries shown in the embodiment of the present application. Figure 5 As shown, curve 501 (solid line) is the dQ / dV curve of the A1 group of lithium-ion batteries; curve 502 (dashed line) is the dQ / dV curve of the B1 group of lithium-ion batteries. Figure 6 Schematic diagram of the dQ / dV curves of the A2 group of lithium-ion batteries and the B2 group of lithium-ion batteries shown in the embodiment of the present application. Figure 6 As shown, curve 601 (solid line) is the dQ / dV curve of the A2 group of lithium-ion batteries; curve 602 (dashed line) is the dQ / dV curve of the B2 group of lithium-ion batteries.

[0094] According to the dQ / dV curves of the lithium-ion batteries in group A1, A2, B1, and B2, the relaxation voltage V of the lithium-ion batteries in group A1 is obtained. c，200-A1 , the relaxation voltage V of the A2 group of lithium-ion batteries c，200-A2 , relaxation voltage V of group B1 lithium-ion batteryc，200-B1 、V of Group B2 lithium-ion battery c，200-B2 .

[0095] According to the data of the above-mentioned lithium-ion batteries in group A1, group A2, group A3, group B1, group B2, and group B3, the parameters of lithium-ion batteries in group A after 200 cycles are obtained: the charge transfer impedance change rate f 1-A1 (25℃) and f 1-A2 (45℃), real impedance variation coefficient f 2-A1 (25℃), steady-state voltage f 3-A1 (25℃), maximum voltage f 4-A1 (25℃) and f 4-A2 (45℃); obtain the parameters of lithium-ion battery group B after 200 cycles: charge transfer impedance change rate f 1-B1 (25℃) and f 1-B2 (45℃), real impedance variation coefficient f 2-B1 (25℃), steady-state voltage f 3-B1 (25℃), maximum voltage f 4-B1 (25℃) and f 4-B2 (45°C); the parameters of the lithium-ion battery group A after 200 cycles and the fully charged lithium-ion battery group B are shown in Table 1.

[0096] Table 1:

[0097] In Table 1, f1 (25°C), data of Group A represents the charge transfer impedance change rate f of Group A1 lithium-ion batteries. 1-A1 (25℃); the other data in Table 1 are similar, and represent the parameters of lithium-ion battery group A after 200 cycles: f 1-A2 (45℃), real impedance variation coefficient f 2-A1 (25℃), steady-state voltage f 3-A1 (25℃), maximum voltage f 4-A1 (25℃) and f 4-A2 Parameters of lithium-ion battery group B after 200 cycles at 45°C: Charge transfer impedance change rate f 1-B1 (25℃) and f 1-B2 (45℃), real impedance variation coefficient f 2-B1 (25℃), steady-state voltage f 3-B1 (25℃), maximum voltage f 4-B1 (25℃) and f 4-B2 (45°C). Group A is better than Group B means that the cycle performance of lithium ion battery group A is better than the cycle performance of lithium ion battery group B.

[0098] like Figure 7 As shown, curve 701 (solid line) represents the thickness expansion rate change curve of fresh fully charged lithium ion battery group A at different cycles of cycle performance test at 25°C; curve 702 (dashed line) represents the thickness expansion rate change curve of fresh fully charged lithium ion battery group B at different cycles of cycle performance test at 25°C. Figure 8 As shown, curve 801 (solid line) represents the capacity retention rate change curve of fresh fully charged lithium ion battery group A at different cycles of 25°C cycle performance test; curve 802 (dashed line) represents the capacity retention rate change curve of fresh fully charged lithium ion battery group B at different cycles of 25°C cycle performance test. Fig. 9 As shown, curve 901 (solid line) represents the thickness expansion rate change curve of fresh fully charged lithium ion battery group A at different cycles of 45°C cycle performance test; curve 902 (dashed line) represents the thickness expansion rate change curve of fresh fully charged lithium ion battery group B at different cycles of 45°C cycle performance test. Fig.10 As shown, curve 1001 (solid line) represents the capacity retention rate change curve of fresh fully charged lithium ion battery group A at different cycles of 45°C cycle performance test; curve 1002 (dashed line) represents the capacity retention rate change curve of fresh fully charged lithium ion battery group B at different cycles of 45°C cycle performance test.

[0099] like Figure 7 and Figure 8 As shown in the figure, the difference in capacity retention between lithium-ion battery group A and lithium-ion battery group B can be clearly distinguished after 600 cycles at 25°C. Fig. 9 and Fig.10 As shown in the figure, the difference in capacity retention rate between lithium-ion battery group A and lithium-ion battery group B can only be clearly distinguished after 400 cycles at 45°C. The capacity retention rate and thickness expansion rate of lithium-ion battery group A and lithium-ion battery group B after 200 cycles at 25°C and 45°C respectively did not show obvious differences. To evaluate the difference in cycle performance between lithium-ion battery group A and lithium-ion battery group B, it is necessary to further disassemble lithium-ion battery group A and lithium-ion battery group B or require more cycles (such as 9- Fig.10 As shown, at 45°C and 400 cycles, the capacity retention rates began to differ. The expansion thickness of Group A and Group B met the standards, and there was no abnormal thickness growth. The cycle performance of fresh fully charged lithium-ion batteries Group A and fresh fully charged lithium-ion batteries Group B was determined.

[0100] according to Figure 7-10The curve shows that the cycle performance of lithium-ion battery group A is better than that of lithium-ion battery group B. According to the parameters of lithium-ion battery group A and lithium-ion battery group B in Table 1, the cycle performance of lithium-ion battery group A is better than that of lithium-ion battery group B. Therefore, the cycle performance evaluation method of the lithium-ion battery in the embodiment of the present application can quantitatively determine the cycle kinetic performance and matching of the lithium-ion battery from the impedance and voltage dimensions according to the real impedance variation coefficient, steady-state voltage, and maximum voltage of the lithium-ion battery after the cycle test with a cycle number of n (early cycle performance test) without disassembling the lithium-ion battery in the early stage of the cycle performance test. According to the advantages and disadvantages of the kinetic performance and matching of the lithium-ion battery, the cycle performance of the lithium-ion battery can be quickly and accurately evaluated, the time cost and material cost of the evaluation can be reduced, the time of the evaluation can be shortened, and the efficiency of the evaluation can be improved, which provides effective data support for shortening the system screening time of lithium-ion batteries and improving the development efficiency of lithium-ion batteries.

[0101] Embodiment 2: In Example 2, a fresh fully-charged lithium-ion battery group C and a fresh fully-charged lithium-ion battery group D of a 4.5V lithium cobalt oxide / graphite system under different single factor variables (for example, different separators) can be used. The fresh fully-charged lithium-ion battery group C is divided into a C1 group of lithium-ion batteries, a C2 group of lithium-ion batteries, and a C3 group of lithium-ion batteries, wherein the C1 group of lithium-ion batteries is a lithium-ion battery group that has undergone a cycle test of 200 cycles and a test temperature of 25°C, the C2 group of lithium-ion batteries is a lithium-ion battery group that has undergone a cycle test of 200 cycles and a test temperature of 45°C, and the C3 group of lithium-ion batteries is a lithium-ion battery group that has not undergone a cycle test. The fresh fully-charged lithium-ion battery group D is divided into a D1 group of lithium-ion batteries, a D2 group of lithium-ion batteries, and a D3 group of lithium-ion batteries, wherein the D1 group of lithium-ion batteries is a lithium-ion battery group that has undergone a cycle test of 200 cycles at a test temperature of 25°C, the D2 group of lithium-ion batteries is a lithium-ion battery group that has undergone a cycle test of 200 cycles at a test temperature of 45°C, and the D3 group of lithium-ion batteries is a lithium-ion battery group that has not undergone a cycle test.

[0102] An electrochemical impedance spectroscopy test with an EIS test frequency of 0.05 Hz to 50000 Hz (Hertz) is performed on the C1 group lithium ion batteries, the C2 group lithium ion batteries, the C3 group lithium ion batteries, the D1 group lithium ion batteries, the D2 group lithium ion batteries, and the D3 group lithium ion batteries, respectively, to obtain electrochemical impedance spectra of the C1 group lithium ion batteries, the C2 group lithium ion batteries, the C3 group lithium ion batteries, the D1 group lithium ion batteries, the D2 group lithium ion batteries, and the D3 group lithium ion batteries.

[0103] Fig.11The EIS test frequency and the real axis Z of the C1 group of lithium ion batteries and the D1 group of lithium ion batteries shown in the embodiment of the present application are w Schematic diagram of the curve. Fig.11 As shown, curve 1101 is the EIS test frequency and real axis Z of group C1 lithium-ion batteries. w Curve 1102 is the EIS test frequency and real axis Z of the D1 group of lithium ion batteries w curve.

[0104] Get the Rct of the C1 group of lithium-ion batteries respectively 200-C1 , Z w，200-C1 , Rct of C2 group lithium-ion battery 200-C2 , Z w，200-C2 , Rct of C3 group lithium-ion battery at SOC 50% 0-C , Rct of D1 group lithium-ion battery 200-D1 , Z w，200-D1 , Rct of D2 group lithium-ion battery 200-D2 , Z w，200-D2 , D3 group lithium-ion battery at SOC 50% Rct 0-D .

[0105] The relaxation voltage test was performed on the lithium-ion batteries of group C1, group C2, group D1 and group D2 respectively. The relaxation voltage test was performed by leaving the batteries for 3 hours after being fully charged (SOC was 100%). The relaxation voltage V c，200-C1 , the relaxation voltage V of the C2 group of lithium-ion batteries c，200-C2 , relaxation voltage V of D1 group lithium-ion battery c，200-D1 , relaxation voltage V of the D2 group of lithium-ion batteries c，200-D2 .

[0106] Fig.12 Schematic diagram of voltage curves of the C1 group of lithium-ion batteries and the D1 group of lithium-ion batteries after being left alone for 3 hours, shown in the embodiment of the present application. Fig.12 As shown, curve 1201 (solid line) is the voltage curve of the C1 group of lithium-ion batteries after being left for 3 hours, and curve 1202 (dashed line) is the voltage curve of the D1 group of lithium-ion batteries after being left for 3 hours.

[0107] The dQ / dV curves of the C1 group lithium ion battery, the C2 group lithium ion battery, the D1 group lithium ion battery and the D2 group lithium ion battery are drawn according to the discharge curves of the C1 group lithium ion battery, the C2 group lithium ion battery, the D1 group lithium ion battery and the D2 group lithium ion battery respectively.

[0108] Fig.13Schematic diagram of dQ / dV curves of the C1 group of lithium-ion batteries and the D1 group of lithium-ion batteries shown in the embodiment of the present application. Fig.13 As shown, curve 1301 (solid line) is the dQ / dV curve of the C1 group of lithium-ion batteries; curve 1302 (dashed line) is the dQ / dV curve of the D1 group of lithium-ion batteries. Fig.14 Schematic diagram of dQ / dV curves of the C2 group of lithium-ion batteries and the D2 group of lithium-ion batteries shown in the embodiment of the present application. Fig.14 As shown, curve 1401 (solid line) is the dQ / dV curve of the C2 group of lithium-ion batteries; curve 1402 (dashed line) is the dQ / dV curve of the D2 group of lithium-ion batteries.

[0109] According to the dQ / dV curves of the C1 group lithium-ion battery, the C2 group lithium-ion battery, the D1 group lithium-ion battery, and the D2 group lithium-ion battery, the relaxation voltage V of the C1 group lithium-ion battery is obtained. c，200-C1 , the relaxation voltage V of the C2 group of lithium-ion batteries c，200-C2 , relaxation voltage V of D1 group lithium-ion battery c，200-D1 、V of D2 group lithium-ion battery c，200-D2 .

[0110] According to the data of the above-mentioned lithium-ion batteries of group C1, group C2, group C3, group D1, group D2, and group D3, the parameters of lithium-ion batteries of group C after 200 cycles are obtained respectively: the charge transfer impedance change rate f 1-C1 (25℃) and f 1-C2 (45℃), real impedance variation coefficient f 2-C1 (25℃), steady-state voltage f 3-C1 (25℃), maximum voltage f 4-C1 (25℃) and f 4-C2 (45℃); obtain the parameters of lithium-ion battery group D after 200 cycles: charge transfer impedance change rate f 1-D1 (25℃) and f 1-D2 (45℃), real impedance variation coefficient f 2-D1 (25℃), steady-state voltage f 3-D1 (25℃), maximum voltage f 4-D1 (25℃) and f 4-D2 (45°C); the parameters of the lithium-ion battery group C with 200 cycles and the lithium-ion battery group D with 200 cycles are specifically shown in Table 2.

[0111] Table 2:

[0112] In Table 2, f1 (25°C), data of group C represents the charge transfer impedance change rate f of group C1 lithium-ion batteries. 1-C1 (25℃); the other data in Table 2 are similar, and represent the parameters of lithium-ion battery group C after 200 cycles: f 1-C2 (45℃), real impedance variation coefficient f 2-C1 (25℃), steady-state voltage f 3-C1 (25℃), maximum voltage f 4-C1 (25℃) and f 4-C2 Parameters of lithium-ion battery group D after 200 cycles at 45°C: Charge transfer impedance change rate f 1-D1 (25℃) and f 1-D2 (45℃), real impedance variation coefficient f 2-D1 (25℃), steady-state voltage f 3-D1 (25℃), maximum voltage f 4-D1 (25℃) and f 4-D2 (45°C). Group C is better than Group D, which means that the cycle performance of lithium ion battery group C is better than the cycle performance of lithium ion battery group D.

[0113] like Fig.15 As shown, curve 1501 (solid line) represents the thickness expansion rate change curve of fresh fully charged lithium ion battery group C at different cycles of cycle performance test at 25°C; curve 1502 (dashed line) represents the thickness expansion rate change curve of fresh fully charged lithium ion battery group D at different cycles of cycle performance test at 25°C. Fig.16 As shown, curve 1601 (solid line) represents the capacity retention rate change curve of fresh fully charged lithium ion battery group C at different cycle times in the 25°C cycle performance test; curve 1602 (dashed line) represents the capacity retention rate change curve of fresh fully charged lithium ion battery group D at different cycle times in the 25°C cycle performance test.

[0114] like Fig.17 As shown, curve 1701 (solid line) represents the thickness expansion rate change curve of fresh fully charged lithium ion battery group C at different cycles of 45°C cycle performance test; curve 1702 (dashed line) represents the thickness expansion rate change curve of fresh fully charged lithium ion battery group D at different cycles of 45°C cycle performance test. Fig.18 As shown, curve 1801 (solid line) represents the capacity retention rate change curve of fresh fully charged lithium ion battery group C at different cycle times in the 45°C cycle performance test; curve 1802 (dashed line) represents the capacity retention rate change curve of fresh fully charged lithium ion battery group D at different cycle times in the 45°C cycle performance test.

[0115] like Fig.15 and Fig.16As shown in the figure, the difference in capacity retention between lithium-ion battery group C and lithium-ion battery group D can be clearly distinguished after 600 cycles at 25°C. Fig.17 and Fig.18 As shown in the figure, the difference in capacity retention rate between lithium-ion battery group C and lithium-ion battery group D can only be clearly distinguished after 300 cycles at 45°C. The capacity retention rate and thickness expansion rate of lithium-ion battery group C and lithium-ion battery group D after 200 cycles at 25°C and 45°C, respectively, did not show obvious differences. To evaluate the difference in cycle performance between lithium-ion battery group C and lithium-ion battery group D, it is necessary to further disassemble lithium-ion battery group C and lithium-ion battery group D or require more cycles (such as 17- Fig.18 As shown, at 45°C and 300 cycles, the capacity retention rates began to differ. The expansion thickness of group C and group D met the standard, and there was no abnormal thickness growth. The cycling performance of fresh fully charged lithium-ion battery group C and fresh fully charged lithium-ion battery group D was determined.

[0116] Example 3: Example 3 is different from Example 2 in that the number of cycles n in Example 3 is 50. The parameters of the fresh fully-charged lithium-ion battery group C and the fresh fully-charged lithium-ion battery group D after the cycle test with the number of cycles n=50 are shown in Table 3.

[0117] Table 3:

[0118] Example 4: Example 4 is different from Example 2 in that the number of cycles n in Example 4 is 100. The parameters of the fresh fully-charged lithium-ion battery group C and the fresh fully-charged lithium-ion battery group D after the cycle test with the number of cycles n=100 are shown in Table 4.

[0119] Table 4:

[0120] Example 5: Example 5 is different from Example 2 in that the number of cycles n in Example 5 is 300. The parameters of the fresh fully-charged lithium-ion battery group C and the fresh fully-charged lithium-ion battery group D after the cycle test with the number of cycles n=300 are shown in Table 5.

[0121] Table 5:

[0122] according to Figure 15-18The curve shows that the cycle performance of lithium-ion battery group C is better than that of lithium-ion battery group D. According to the parameters of lithium-ion battery group C and lithium-ion battery group D under different cycle times in Tables 2 to 5, the cycle performance of lithium-ion battery group C is also better than that of lithium-ion battery group D. Therefore, the cycle performance evaluation method of the lithium-ion battery in the embodiment of the present application can quantitatively determine the cycle kinetic performance and matching of the lithium-ion battery from the impedance and voltage dimensions according to the real impedance variation coefficient, steady-state voltage, and maximum voltage of the lithium-ion battery after the cycle test with a cycle number of n (early cycle performance test) without disassembling the lithium-ion battery in the early stage of the cycle performance test. According to the advantages and disadvantages of the kinetic performance and matching of the lithium-ion battery, the cycle performance of the lithium-ion battery can be quickly and accurately evaluated, the time cost and material cost of the evaluation can be reduced, the time of the evaluation can be shortened, and the efficiency of the evaluation can be improved, which provides effective data support for shortening the system screening time of lithium-ion batteries and improving the development efficiency of lithium-ion batteries.

[0123] The embodiments of the present application have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles of the embodiments, practical applications, or improvements to the technology in the market, or to enable other persons of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

1. A method for evaluating the cycle performance of a lithium-ion battery, characterized in that: include: Perform cycle tests on lithium-ion batteries for a set number of cycles according to cycle performance tests; Obtaining a real impedance variation coefficient of the lithium ion battery according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test; According to the lithium-ion battery after the cycle test, obtaining a steady-state voltage of the lithium-ion battery after the cycle test after being left for a set time; According to the voltage differential capacity curve of the lithium ion battery after the cycle test, obtaining the maximum voltage of the lithium ion battery corresponding to the maximum phase change peak of the voltage differential capacity curve; The cycle performance of the lithium-ion battery is evaluated according to the real impedance variation coefficient, the steady-state voltage, and the maximum voltage.

2. The method according to claim 1, characterized in that Also includes: Obtaining a charge transfer impedance change rate of the lithium ion battery after the cycle test according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test; The cycle performance of the lithium-ion battery is evaluated based on the real impedance variation coefficient, the steady-state voltage, the maximum voltage, and the charge transfer impedance change rate.

3. The method according to claim 1 or 2, characterized in that: The step of obtaining the real impedance variation coefficient of the lithium-ion battery according to the electrochemical impedance spectrum of the lithium-ion battery after the cycle test includes: Performing an electrochemical impedance spectroscopy test at a set electrochemical impedance frequency on the lithium ion battery after the cycle test to obtain an electrochemical impedance spectrum of the lithium ion battery after the cycle test; Obtaining a real impedance mean and a real impedance standard deviation of the lithium ion battery according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test; The real impedance variation coefficient of the lithium ion battery is obtained according to the real impedance mean value and the real impedance standard deviation of the lithium ion battery.

4. The method according to claim 1 or 2, characterized in that: The step of obtaining a steady-state voltage of the lithium-ion battery after the cycle test after being left for a set time according to the lithium-ion battery after the cycle test comprises: According to the lithium-ion battery after the cycle test, the lithium-ion battery in a fully charged state after the cycle test is left for a set time, and a steady-state voltage of the lithium-ion battery is obtained.

5. The method according to claim 1 or 2, characterized in that: The step of obtaining the maximum voltage of the lithium-ion battery corresponding to the maximum phase change peak of the voltage differential capacity curve according to the voltage differential capacity curve of the lithium-ion battery after the cycle test comprises: According to the voltage differential capacity curve of the lithium ion battery after the cycle test, the voltage corresponding to the maximum phase change peak of the lithium ion battery in the voltage differential capacity curve is obtained, and the absolute value of the voltage corresponding to the maximum phase change peak is used as the maximum voltage.

6. The method according to claim 2, characterized in that The step of obtaining the charge transfer impedance change rate of the lithium ion battery after the cycle test according to the electrochemical impedance spectrum of the lithium ion battery after the cycle test comprises: Obtaining, according to the electrochemical impedance spectrum of the lithium-ion battery after the cycle test, a charge transfer impedance of the lithium-ion battery at a first set amount of electricity after the cycle test; The charge transfer impedance change rate of the lithium ion battery after the cycle test is obtained according to the charge transfer impedance of the lithium ion battery at the second set power before the cycle test and the charge transfer impedance of the lithium ion battery at the first set power after the cycle test.

7. The method according to claim 1, characterized in that: The set cycle number is n, and n is an integer multiple of 50.

8. The method according to claim 1, characterized in that: The test temperature of the cycle performance test includes a plurality of set temperatures, wherein the temperature difference between two adjacent set temperatures among the plurality of set temperatures is greater than or equal to 10°C.

9. The method according to claim 1, characterized in that: The set time is greater than or equal to 2 hours.

10. The method according to claim 3, characterized in that: The frequency range of the electrochemical impedance spectroscopy frequency is 0.05 Hz to 50000 Hz.

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