An accelerated evaluation method for lithium-ion battery cycle performance

Abstract

The present invention discloses an accelerated evaluation method for the cycle performance of a lithium-ion battery, comprising: step S1, pre-selecting a fresh battery and a reference battery of the same battery system as the battery to be tested, analyzing and obtaining a characteristic SOC interval in which the battery system to be tested experiences cycle attenuation, and determining an accelerated test SOC interval; step S2, performing an accelerated cycle test on the battery to be tested and the reference battery within the accelerated test SOC interval, respectively, to obtain an accelerated cycle capacity retention curve of the battery; step S3, comparing the accelerated cycle capacity retention curves of the battery to be tested and the reference battery, to determine the superiority or inferiority of the cycle performance of the battery to be tested relative to that of the reference battery. The present invention is suitable for the development of lithium-ion battery products, and can be used for rapid comparative analysis of battery cycle performance during material screening and system optimization, effectively shortening the battery development cycle and improving R&D efficiency. At the same time, by reducing cycle test energy consumption, it can indirectly reduce battery development costs.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery performance testing, and in particular to an accelerated evaluation method for the cycle performance of a lithium-ion battery. Background Art

[0002] At present, lithium-ion batteries have the advantages of high specific energy, many cycles and long storage time. They are not only widely used in portable electronic devices (such as mobile phones, digital cameras and laptops), but also widely used in large and medium-sized electric equipment such as electric vehicles, electric bicycles and electric tools. Therefore, the performance requirements for lithium-ion batteries are getting higher and higher.

[0003] Cycle life is a core indicator that characterizes the performance of lithium-ion batteries. With the improvement of battery performance standards in energy storage systems and electric vehicles, the cycle life of lithium-ion batteries has reached thousands or even tens of thousands of times. Therefore, the lengthy cycle testing process has greatly delayed the development progress of battery products. How to speed up the evaluation of battery cycle performance and find reasonable and effective cycle evaluation methods have become key technical issues that need to be urgently addressed in the lithium-ion battery industry.

[0004] Currently, in the field of accelerated cycle evaluation of lithium-ion batteries, accelerated testing is mainly performed by changing stress conditions such as battery temperature, pressure, voltage, and current. However, changes in these stresses may cause quantitative or even qualitative changes in the complex chemical and electrochemical reactions inside the lithium-ion battery, resulting in inconsistencies between the accelerated test results and the attenuation factors occurring in the actual measured cycle, leading to large deviations in the accelerated test results. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for accelerating the evaluation of the cycle performance of lithium-ion batteries in response to the technical defects of the prior art.

[0006] To this end, the present invention provides a method for accelerating the evaluation of the cycle performance of a lithium-ion battery, comprising the following steps:

[0007] Step S1: For a battery to be tested that needs to be evaluated for cycle performance, a fresh battery and a reference battery of the same battery system as the battery to be tested are pre-selected, and a characteristic SOC interval of cycle attenuation of the battery system to be tested is obtained by analysis, and an accelerated test SOC interval is determined;

[0008] Step S2, performing accelerated cycle tests on the battery under test and the reference battery within the accelerated test SOC range obtained in the first step, and obtaining accelerated cycle capacity retention rate curves of the battery under test and the reference battery;

[0009] Step S3 , by comparing the accelerated cycle capacity retention rate curve of the battery to be tested with the accelerated cycle capacity retention rate curve of the reference battery, the quality of the cycle performance of the battery to be tested relative to the cycle performance of the reference battery is determined.

[0010] It can be seen from the technical solution provided by the present invention that, compared with the prior art, the present invention provides an accelerated evaluation method for the cycle performance of lithium-ion batteries. The method is scientifically designed and suitable for the development of lithium-ion battery products. When used for material screening and system optimization, the rapid comparative analysis of battery cycle performance can effectively shorten the battery development cycle and improve R&D efficiency. At the same time, by reducing the energy consumption of cycle testing, it indirectly reduces the battery development cost, and has good application prospects and promotion value.

[0011] In the present invention, the accelerated test interval is first determined based on the cycle attenuation analysis of the battery system to be tested, the battery is subjected to an accelerated cycle test using the actual cycle format, and the charge and discharge capacity of the battery is measured using the actual cycle format at different stages of the accelerated cycle for calculation of the capacity retention rate. Further, by comparing the capacity retention rate versus cycle number curves of the battery to be tested and the reference battery, the superiority or inferiority of the cycle performance of the battery to be tested relative to the reference battery is judged.

[0012] In the present invention, since the charge and discharge capacities of the battery under test are maintained constant during the accelerated cycling process, if no side reactions occur, the battery state after the cycle remains the same as before the cycle, i.e., the process change rate is 0. Similarly, the higher the degree of side reactions in the battery, the greater the rate of change during the accelerated cycling process. Based on this, by adding analysis of DCIR (direct current resistance) and polarization voltage during each accelerated cycle, the cause of poor battery cycle performance can be determined, providing a reference for battery performance improvement.

[0013] The method provided by this invention, because it performs accelerated cycling analysis within the characteristic decay range, can significantly shorten the battery cycling performance evaluation cycle compared to full SOC cycling testing, thereby improving R&D efficiency. Furthermore, accelerated cycling can include analysis of the battery's DCIR (direct current resistance) and polarization voltage process change rate, providing a reference for battery cycle failure analysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A flowchart of a method for accelerating the evaluation of the cycle performance of a lithium-ion battery provided by the present invention;

[0015] Figure 2This is a schematic diagram of a characteristic SOC interval analysis curve of the cycle decay of a fresh battery (i.e., a fresh battery with a capacity retention rate of 100%) and a cycled battery (i.e., a reference battery with a capacity retention rate of 95%) in the same battery system as the battery to be tested in Example 1.

[0016] Figure 3 A schematic diagram of the accelerated cycle capacity retention curves of the test battery and the reference battery in Example 1 is provided for an accelerated evaluation method for the cycle performance of a lithium-ion battery provided by the present invention;

[0017] Figure 4 A schematic diagram showing a comparison of the DCIR (direct current resistance) of the test battery and the reference battery in Example 1 after the first 500 accelerated cycles of an accelerated evaluation method for the cycling performance of a lithium-ion battery provided by the present invention;

[0018] Figure 5 This is a schematic diagram comparing the actual cycle results (i.e., actual cycle performance curves) of the test battery and the reference battery in Example 1 in an accelerated evaluation method for the cycle performance of a lithium-ion battery provided by the present invention. DETAILED DESCRIPTION

[0019] In order to enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and implementation methods.

[0020] See also Figures 1 to 5 The present invention provides a method for accelerating the evaluation of the cycle performance of a lithium-ion battery, comprising the following steps:

[0021] Step S1: For a battery to be tested whose cycle performance needs to be evaluated, a fresh battery and a reference battery of the same battery system as the battery to be tested are pre-selected, and a characteristic SOC (i.e., capacity retention, also known as state of charge) interval of the battery system to be tested (i.e., the battery system to which the battery to be tested belongs) where cycle attenuation occurs is obtained by analysis, and an accelerated test SOC interval is determined;

[0022] Step S2, performing accelerated cycle tests on the battery under test and the reference battery within the accelerated test SOC range obtained in the first step, and obtaining accelerated cycle capacity retention rate curves of the battery under test and the reference battery;

[0023] Step S3, comparing the accelerated cycle capacity retention curve of the battery under test with the accelerated cycle capacity retention curve of the reference battery to determine the quality of the cycle performance of the battery under test relative to that of the reference battery;

[0024] In steps S2 and S3 , the fresh battery is a battery that has not experienced capacity decay (ie, the capacity retention rate is 100%).

[0025] In the present invention, in specific implementation, the step S1 specifically includes the following operations:

[0026] Step S11, selecting a fresh battery and a reference battery, performing preset charge and discharge cycle operations on each, and collecting the battery voltage V, charge capacity Q1, and discharge capacity Q2 of the fresh battery and the reference battery in real time;

[0027] Among them, fresh batteries are batteries that have not experienced capacity decay;

[0028] A reference battery is a battery whose capacity decay is greater than or equal to a preset ratio (e.g., 5%);

[0029] In the present invention, the fresh battery, the reference battery, and the battery to be tested are batteries belonging to the same battery system;

[0030] It should be noted that for the present invention, two batteries with the same battery system as the battery to be tested (e.g., developed batteries with the same chemical system or model) are used as reference batteries (i.e., the reference battery in product development, which serves as the evaluation basis for the performance comparison and analysis of subsequent batteries produced when different materials and processes are developed). The two reference batteries, one of which is a fresh battery and the other has undergone cycle testing, are defined as the reference battery, and the capacity decay of the reference battery is ≥5%.

[0031] In the present invention, batteries of the same battery system refer to cells of the same specifications and models, that is, cells with the same dimensions and chemical system. For example, all of them are 18650-sized lithium iron phosphate batteries (cylindrical lithium iron phosphate cells). Of course, depending on the user's needs, they can also be identical batteries of the same size and with a similar chemical system (such as lithium cobalt oxide or NCM ternary system).

[0032] In the present invention, the battery to be tested and the reference battery may be of the same model or chemical system, but the positive electrode, negative electrode, electrolyte, and diaphragm materials thereof may be different in one kind (such as the positive electrode or the negative electrode is different) or two kinds (such as the positive electrode and the negative electrode are different) or multiple kinds (such as the positive electrode, the negative electrode, and the electrolyte are all different).

[0033] In specific implementation, the test battery and the reference battery can be two batteries with the same battery components except for one different electrode material (different negative electrode material or different positive electrode material). In other words, they can be two batteries with only one different negative electrode material (negative electrode material or positive electrode material). In other words, they can be batteries of the same system using two different negative electrode materials, or batteries of the same system using two different positive electrode materials. For example, they can be two 21700 cylindrical lithium-ion batteries with different negative electrode materials, or two 21700 cylindrical lithium-ion batteries with different positive electrode materials.

[0034] In a specific implementation, the test battery and the reference battery can also be two batteries with identical components except for different electrolytes, for example, two 21700 cylindrical lithium-ion batteries with different electrolytes.

[0035] In a specific implementation, the test battery and the reference battery can also be two batteries with identical components except for the different separators. For example, they can be two 21700 cylindrical lithium-ion batteries with different separators.

[0036] In the present invention, the test battery and the reference battery may also have the same material components but different manufacturing processes (such as different formation processes, etc.), that is, batteries made of any materials and process conditions that may cause different battery cycle performance.

[0037] In step S1, the battery to be tested is a battery whose cycle performance needs to be evaluated, that is, a newly developed battery to be tested, whose cycle performance is unknown and needs to be compared and analyzed with a reference battery to evaluate whether its cycle performance is better or worse than the reference battery with known performance.

[0038] In step S1, the difference in battery capacity retention between the fresh battery and the reference battery is greater than a preset value (e.g., greater than 5%). In other words, compared to the fresh battery, the reference battery with a capacity decay of ≥5% has experienced significant performance degradation, which may include reduced electrode material activity, loss of active lithium, and increased battery polarization.

[0039] In step S11, the preset charge-discharge cycle operation includes one discharge operation and one charge operation, specifically: first, a preset charge current (0.05C to 0.5C) is used for constant current charging to a preset charge upper limit voltage, and then, a preset discharge current (0.05C to 0.5C) is used for constant current discharging to a preset discharge lower limit voltage;

[0040] It should be noted that for each specification and model of battery product, during development, the upper charging limit voltage and preset lower discharge limit voltage of the developed battery can be determined in combination with customer needs and based on a fixed chemical system design, that is, the product's operating voltage range, and the battery's upper charging limit voltage and lower discharge limit voltage will be clearly given in the battery product specification sheet.

[0041] Step S12, obtaining a characteristic SOC interval of cyclic decay of the battery system under test according to a preset first acquisition method or a preset second acquisition method;

[0042] In step S12, according to a preset first acquisition method, a characteristic SOC interval of the battery system to be tested that undergoes cyclic decay is obtained, which specifically includes the following steps:

[0043] Step S121A: For the fresh battery and the reference battery, the battery voltage V (specifically, the battery charging voltage) is differentiated by the battery charging capacity Q1 to obtain dQ / dV of the fresh battery and the reference battery;

[0044] Step S122A: For the fresh battery and the reference battery, plotting the capacity increment (IC) curve (dQ1 / dV-SOC) of the two on a single graph, using dQ1 / dV as the ordinate and the state of charge (SOC) corresponding to the battery's charge capacity Q1 as the abscissa.

[0045] Step S123A, using the capacity increment (IC) curve of the fresh battery as the reference curve, compare the capacity increment (IC) curve of the reference battery with the reference curve of the fresh battery, determine the operation according to the preset characteristic SOC interval, and determine the characteristic SOC interval of the reference battery system (that is, equal to the battery system to be tested, because the reference battery system is the same as the battery system to be tested) in which cycle decay occurs in the two capacity increment (IC) curves (dQ1 / dV-SOC); the reference battery system is equal to the battery system to be tested (also equal to the fresh battery system); the characteristic SOC interval of the reference battery system in which cycle decay occurs, including the lower limit SOC L and upper limit SOC U ;

[0046] In the present invention, the characteristic SOC interval is the SOC interval corresponding to the starting position and the ending position of the peak where the peak value is significantly reduced, or the peak where the peak value is significantly reduced and the peak position is significantly shifted, by comparing the peaks of the two curves one by one on the curves of the fresh battery and the reference battery (such as the capacity increment IC curve, i.e., the dQ / dV-SOC curve); that is, the characteristic SOC interval must meet the condition that the peak value is significantly reduced, while the peak position shift may or may not occur. If the peak position shift occurs, it is preferably the SOC interval corresponding to the peak where both the peak value is significantly reduced and the peak position shift occurs.

[0047] In the present invention, in step S123A, the characteristic SOC interval determination operation is preset, including the following steps:

[0048] First, on the capacity increment (IC) curves (dQ / dV-SOC curves) of the fresh battery and the reference battery, by comparing the peaks of the two curves one by one, the peak where the peak value of the peak is significantly reduced, or the peak where the peak value is significantly reduced and the peak position is significantly shifted is determined, and then used as the characteristic peak of the cycle attenuation;

[0049] The peak value of the peak is significantly reduced, which means that the peak value reduction ratio of the peak is greater than or equal to the preset peak reduction value;

[0050] The peak position of the wave crest is significantly offset, which means that the peak position of the wave crest corresponds to the SOC offset amplitude greater than or equal to the preset peak position offset value;

[0051] Then, the SOC intervals corresponding to the starting position and the ending position of the characteristic peak of the cyclic decay are taken as characteristic SOC intervals.

[0052] In specific implementation, the preset characteristic SOC interval determination operation includes the following steps:

[0053] When the battery capacity decays by 5%, on the capacity increment (IC) curves (dQ / dV-SOC curves) of the fresh battery and the reference battery, the peaks of the two curves are compared one by one (that is, the 1st, 2nd peaks ... Nth peaks of the fresh battery curve are compared with the 1st peak, 2nd peak ... Nth peaks of the reference battery curve, where N is a natural number greater than 2), and the peak with a peak reduction ratio of ≥10% or the peak with a peak reduction ratio of ≥10% and a peak position corresponding to an SOC offset amplitude of ≥3% is determined as the characteristic peak of the cyclic attenuation, preferably: the peak reduction ratio of ≥10% and the peak position corresponding to an SOC offset amplitude of ≥3% are used as the characteristic peak of the cyclic attenuation;

[0054] The horizontal coordinate corresponding to the starting position of the peak (i.e., the characteristic peak of cyclic attenuation) is the lower limit value SOC of the characteristic SOC interval. L The horizontal coordinate corresponding to the end position of the peak is the upper limit value SOC of the characteristic SOC interval. U This means that in the electrochemical reaction corresponding to this peak, the battery not only experiences attenuation of the active material activity (peak value decreases), but also may experience an increase in polarization (peak position corresponding to voltage or SOC shifts).

[0055] like Figure 2 As shown, the peak height of the first peak of the dQ / dV-SOC curve (i.e., capacity increment curve) of a battery with 100% capacity retention (i.e., a fresh battery) is 11.3Ah / V, and the peak height of the first peak of a battery with 95% capacity retention is 8.5Ah / V. The peak reduction ratio of the first peak reaches (11.3-8.5) / 11.3=24.8%. This first peak is the characteristic peak of the battery's cycle attenuation, and the horizontal axis SOC corresponding to the starting position of the peak is 7%, which is the lower limit SOC L , and the horizontal coordinate SOC corresponding to the end position of the peak is 18%, which is the upper limit SOC U .

[0056] It should be noted that in the present invention, during the characteristic SOC range testing process, the selected batteries are reference batteries and fresh batteries of the same system as the battery to be tested, that is, developed products. During the test, a fresh battery and a reference battery that has undergone at least 5% capacity decay are selected for comparative testing. The test results of the two batteries are compared in a single graph. As the capacity decays, the peak with a peak decrease or a peak decrease and peak position shift is selected as the characteristic peak of the cyclic decay, and the characteristic SOC range is determined based on the starting and ending positions of the peak.

[0057] like Figure 2 In the figure, the battery with 100% capacity retention (fresh battery) and the battery with 95% capacity retention (reference battery) are batteries of the same system, except that the battery with 95% capacity retention has experienced capacity decay after cycle testing. As long as the battery chemistry and model remain unchanged, the characteristic SOC ranges of the fresh battery and the reference battery are universal.

[0058] In step S123A, in specific implementation, according to a preset characteristic SOC interval determination operation, a characteristic SOC interval in which cyclic attenuation occurs in the reference battery system is determined in the two capacity increment (IC) curves. Specifically, in the capacity increment (IC) curves of the fresh battery and the reference battery, the state of charge (SOC) interval corresponding to a significant decrease in the peak value (i.e., the value of the highest point) of the curve and a significant shift in the peak position of the peak is determined as the characteristic SOC interval in which cyclic attenuation occurs in the reference battery system.

[0059] It should be noted that, in the present invention, the peak value of the peak on the capacity increment (IC) curve is the highest value of each peak, and the horizontal coordinates corresponding to the starting position and the ending position of the peak indicate its position, that is, the peak position.

[0060] It should be noted that a decrease in the peak value of the peak indicates a decrease in the reactivity of the active material (positive or negative electrode) in the electrochemical reaction. When the battery capacity decays by 5%, if the peak value on the corresponding dQ / dV-SOC curve (capacity increment IC curve) decreases by 10% or more, the impact of test error can be ruled out and the peak value of the curve can be determined to have significantly decreased.

[0061] It should be noted that for the capacity increment (IC) curves of fresh batteries and reference batteries, the peak positions of the two curves shift, which is usually caused by increased battery polarization. When the battery capacity decays by 5%, the SOC shift corresponding to the starting or ending position of the characteristic peak on the corresponding dQ / dV-SOC curve is ≥3%. The influence of test error can be ruled out and it can be determined that the peak position of the curve has shifted significantly.

[0062] In step S12, according to a preset second acquisition method, a characteristic SOC interval of the battery to be tested that undergoes cycle decay is obtained, which specifically includes the following steps:

[0063] Step S121B: for the fresh battery and the reference battery, the battery voltage V 充 or the battery voltage V during discharge 放 , perform differential processing on the battery's charge capacity Q1 or discharge capacity Q2 to obtain the dV of the fresh battery and the reference battery 充 / dQ1 or dV 放 / dQ2;

[0064] Step S122 B: For the fresh battery and the reference battery, dV 充 / dQ1 or dV 放 / dQ2 as the ordinate, and the state of charge (SOC) corresponding to the battery's charge capacity Q1 or discharge capacity Q2 as the abscissa, and plot the differential voltage (DV) curve (dV / dQ-SOC) between the two on a single graph.

[0065] Step S123 B: Using the differential voltage curve of the fresh battery as a reference curve, the differential voltage of the reference battery is compared with the reference curve of the fresh battery, and an operation is determined based on a preset characteristic SOC interval. From the two differential voltage curves, a characteristic SOC interval in which cyclic attenuation occurs for the reference battery system (i.e., the battery system to be tested, since the reference battery system is the same as the battery system to be tested); the reference battery system is equivalent to the battery system to be tested (and also equivalent to the fresh battery system);

[0066] The characteristic SOC range of the reference battery system where cycle decay occurs, including the lower limit SOC L and upper limit SOC U ;

[0067] In the present invention, the characteristic SOC interval is the SOC interval corresponding to the starting position and the ending position of the peak where the peak value is significantly reduced, or the peak where the peak value is significantly reduced and the peak position is significantly shifted, by comparing the peaks of the two curves one by one on the curves of the fresh battery and the reference battery (for example, the differential voltage curve, i.e., the dV / dQ-SOC curve); that is, the characteristic SOC interval must meet the condition that the peak value is significantly reduced, while the peak position shift may or may not occur. If the peak position shift occurs, it is preferably the SOC interval corresponding to the peak where both the peak value is significantly reduced and the peak position shift occurs.

[0068] In the present invention, in step S123B, the characteristic SOC interval determination operation is preset, including the following steps:

[0069] First, on the differential voltage curve (dV / dQ-SOC curve) of the fresh battery and the reference battery, by comparing the peaks of the two curves one by one, the peak where the peak value of the peak is significantly reduced, or the peak where the peak value is significantly reduced and the peak position is significantly shifted is determined, and then used as the characteristic peak of the cycle decay;

[0070] The peak value of the peak is significantly reduced, which means that the peak value reduction ratio of the peak is greater than or equal to the preset peak reduction value;

[0071] The peak position of the wave crest is significantly offset, which means that the peak position of the wave crest corresponds to the SOC offset amplitude greater than or equal to the preset peak position offset value;

[0072] Then, the SOC intervals corresponding to the starting position and the ending position of the characteristic peak of the cyclic decay are taken as characteristic SOC intervals.

[0073] In specific implementation, the preset characteristic SOC interval determination operation includes the following steps:

[0074] When the battery capacity decays by 5%, on the differential voltage curve (dV / dQ-SOC curve) of the fresh battery and the reference battery, the peaks of the two curves are compared one by one (that is, the 1st, 2nd peak...Nth peak of the fresh battery curve are compared with the 1st peak, 2nd peak...Nth peak of the reference battery curve, where N is a natural number greater than 2), and the peak with a peak reduction ratio of ≥10% or the peak with a peak reduction ratio of ≥10% and a peak position corresponding to an SOC offset amplitude of ≥3% is determined as the characteristic peak of cyclic attenuation, preferably: the peak reduction ratio of ≥10% and the peak position corresponding to an SOC offset amplitude of ≥3% are used as the characteristic peak of cyclic attenuation;

[0075] The horizontal coordinate corresponding to the starting position of the peak (i.e., the characteristic peak of cyclic attenuation) is the lower limit value SOC of the characteristic SOC interval. L The horizontal coordinate corresponding to the end position of the peak is the upper limit value SOC of the characteristic SOC interval. U This means that in the electrochemical reaction corresponding to this peak, the battery not only experiences attenuation of the active material activity (peak value decreases), but also may experience an increase in polarization (peak position corresponding to voltage or SOC shifts).

[0076] It should be noted that in the present invention, during the characteristic SOC range testing process, the selected batteries are reference batteries and fresh batteries of the same system as the battery to be tested, that is, developed products. During the test, a fresh battery and a reference battery that has undergone at least 5% capacity decay are selected for comparative testing. The test results of the two batteries are compared in a single graph. As the capacity decays, the peak with a peak decrease or a peak decrease and peak position shift is selected as the characteristic peak of the cyclic decay, and the characteristic SOC range is determined based on the starting and ending positions of the peak.

[0077] It should be noted that the battery with 100% capacity retention (fresh battery) and the battery with 95% capacity retention (reference battery) are batteries of the same system, except that the battery with 95% capacity retention has experienced capacity decay after cycle testing. As long as the battery chemistry and model remain unchanged, the characteristic SOC ranges of the fresh battery and the reference battery are universal.

[0078] In the present invention, the characteristic SOC interval is specifically obtained by comparatively analyzing the curves of two batteries with different capacity retention rates (ie, a fresh battery and a reference battery with a cycle capacity decay of ≥5%).

[0079] In step S123 B, in specific implementation, according to the preset characteristic SOC interval determination operation, the characteristic SOC interval in which the reference battery system undergoes cyclic attenuation is determined in the two differential voltage curves. Specifically, in the differential voltage curves of the fresh battery and the reference battery, the state of charge (SOC) interval corresponding to a significant decrease in the peak value (i.e., the value of the highest point) of the curve and a significant shift in the peak position of the peak is determined as the characteristic SOC interval in which the reference battery system undergoes cyclic attenuation.

[0080] It should be noted that, in the present invention, the peak value of the peak on the differential voltage curve is the highest value of each peak, and the horizontal coordinates corresponding to the starting position and the ending position of the peak indicate its position, that is, the peak position.

[0081] It should be noted that a decrease in the peak value of the peak indicates a decrease in the reactivity of the active material (positive or negative electrode) in the battery's electrochemical reaction. When the battery capacity decays by 5%, if the peak value on the corresponding differential voltage curve decreases by 10% or more, the impact of test error can be ruled out and the peak value of the curve can be determined to be significantly reduced.

[0082] It should be noted that for the differential voltage curves of the fresh battery and the reference battery, the peak positions on the two curves shift, which is usually caused by increased battery polarization. When the battery capacity decays by 5%, the SOC shift corresponding to the starting or ending position of the characteristic peak on the corresponding dQ / dV-SOC curve is ≥3%. The influence of test error can be ruled out and it can be determined that the peak position of the curve has shifted significantly.

[0083] Step S13, determining the accelerated test SOC interval according to the characteristic SOC interval of the battery system to be tested where cycle decay occurs; the accelerated test SOC interval includes the lower limit SOC CL and upper limit SOC CU ;

[0084] The accelerated test SOC interval includes the characteristic SOC interval in which the entire battery system to be tested undergoes cyclic attenuation, or includes the characteristic SOC interval in which part of the battery system to be tested undergoes cyclic attenuation.

[0085] In step S13, in order to shorten the test period, the lower limit SOC of the test SOC interval is accelerated. CL =SOC L ±10%, the upper limit of the SOC range of the accelerated test CU =SOC U ±10%; in specific implementation, SOC is preferred CL =SOC L ±5%, SOC CU=SOC U ±5%.

[0086] In step S2, the battery to be tested is subjected to an accelerated cycle test within the accelerated test SOC range obtained in the first step, and an accelerated cycle capacity retention curve of the battery to be tested is correspondingly obtained, which specifically includes the following steps:

[0087] Step S21A, performing a preset multiple (e.g., three) charge-discharge cycle operations on the battery under test using the actual cycle mode of the reference battery (each charge-discharge cycle operation includes one discharge operation and one charge operation), and using the battery charge capacity and discharge capacity obtained during the last charge-discharge cycle operation as the initial charge capacity C0 and initial discharge capacity D0 of the battery under test;

[0088] It should be noted that the actual cycle mode refers to the charging and discharging cycle mode developed during battery development based on customer needs for life evaluation, including the upper limit charging voltage, lower limit discharging voltage, charging current, discharging current, and rest time. For example, in the embodiment, for a 52Ah iron-lithium battery, the cycle mode may be: constant current charging at 1C until its upper limit voltage is 3.65V, then switching to constant voltage charging, stopping when the current drops to 0.02C, resting for 15-30 minutes, and discharging at 1C until the lower limit voltage reaches 2.0V, resting for 15-30 minutes, and then continuing the charge and discharge cycle.

[0089] As for the actual cycle format, when developing the battery, the product developers formulate the cycle format used for life evaluation based on customer needs, obtain the customer's consent, and clearly specify the specific format content in the specification.

[0090] It should be noted that in the present invention, because the battery to be tested is an experimental battery (R&D experimental battery) produced by material diversification (replacing the same type of materials due to performance improvement or cost reduction requirements) or process optimization based on the reference battery, the battery to be tested is a successful (i.e., the performance of the battery to be tested is better than or equivalent to the reference battery) or failed (i.e., the performance of the battery to be tested is worse than the reference battery), and is the same type of battery to be used to replace the reference battery. The actual cycle mode of the battery to be tested and the reference battery are the same. The actual cycle mode of the battery to be tested can be obtained from the specification sheet of the reference battery.

[0091] Step S22A, using the actual cycle mode of the reference battery as the accelerated cycle mode, sequentially performing multiple (e.g., n, where n is a natural number greater than 1) stages of accelerated cycle testing on the battery under test (i.e., repeatedly performing multiple stages of accelerated cycle testing), and obtaining the charge capacity and discharge capacity of the battery under test after each stage of accelerated cycle testing, and recording the total number of accelerated cycles corresponding to the end of each stage of accelerated cycle testing (i.e., the total number of accelerated cycles performed from the start of the first stage of accelerated cycle testing to the end of the accelerated cycle testing of that stage);

[0092] In step S22A, the test contents of the multiple stages of the accelerated cycle test are the same;

[0093] Each stage of the accelerated cycle test includes the following operations:

[0094] The first step is to charge the battery to the lower limit of the SOC range of the accelerated test with a predetermined charging current (e.g., a small current of 0.05-0.5C). CL , then let it sit for a preset time (e.g. 10-30 minutes);

[0095] In the second step, the battery is subjected to the same accelerated cycle test operation for a preset number of times (e.g., N times, where N is a natural number greater than 1, e.g., N times is 50 times);

[0096] Each accelerated cycle test operation is specifically as follows: select the charging current Ic and discharging current Id corresponding to the accelerated test SOC range in the actual cycle format of the battery to be tested, and then successively perform a charging operation on the battery to be tested with the charging current Ic for a preset charging time tc and a discharging operation on the battery with the discharging current Id for a preset discharge time td, so that the battery is within the accelerated test SOC range (from the lower limit SOC CL To upper limit SOC CU ) has the same charge and discharge capacity; that is, Ic*tc=Id*td;

[0097] The actual cycle mode of the battery under test is equivalent to the actual cycle mode of the reference battery;

[0098] It should be noted that the charging current Ic and discharging current Id corresponding to the accelerated test SOC range in the actual cycle mode of the battery under test can be obtained from the actual cycle mode specified in the specification sheet of the reference battery belonging to the same battery system. For example, in the embodiment, the actual cycle mode is 1C constant current charging to an upper limit voltage of 3.65V, then switching to constant voltage charging, stopping when the current drops to 0.02C, standing for 15-30 minutes, and then discharging at a constant current of 1C until the lower limit voltage of 2.0V is reached. Therefore, the charging current and discharge current of the battery under test in the accelerated test SOC range are both 1C.

[0099] The third step is to continue to perform a preset number of (e.g., 2 to 5) full charge and discharge cycles on the battery under test using the actual cycle format of the reference battery.

[0100] In the third step, as mentioned above, the actual cycle mode refers to the charge and discharge cycle mode developed during battery development for life evaluation based on customer needs. This includes the upper limit charge voltage, lower limit discharge voltage, charge current, discharge current, and rest time. For example, the battery under test may be charged at a constant current of 1C to an upper limit voltage of 3.65V, then switched to a constant voltage charge of 3.65V. The current is stopped when it drops to 0.02C, and the battery is left to rest for 15-30 minutes. Then, the battery is discharged at a constant current of 1C until the lower limit voltage reaches 2.0V.

[0101] Step 4: The battery charge capacity and discharge capacity obtained during the last full charge and discharge cycle operation are used as the charge capacity and discharge capacity of the battery under test after the accelerated cycle test at each stage;

[0102] It should be noted that in the third step, full charge and discharge, that is, charging and discharging are performed according to the actual cycle format, that is, charging to the upper limit voltage of the battery, and discharging to the lower limit voltage of the battery: for example, it can be: 1C constant current charging to the upper limit voltage of 3.65V, then switching to 3.65V constant voltage charging, stopping when the current drops to 0.02C, standing for 15min~30min, and then discharging at a constant current of 1C until the lower limit voltage reaches 2.0V. Then stand for 15min~30min, and then perform the same charge and discharge cycle again.

[0103] In the present invention, full charge and discharge are relative to accelerated cycle. Accelerated cycle is performed within a certain SOC range, while full charge and discharge is performed within the entire SOC range. The purpose of full charge and discharge here is to evaluate how much charging and discharging capacity of the tested battery is left after a certain period of accelerated cycle according to the actual cycle test format, so as to calculate and evaluate the capacity retention rate.

[0104] It should be noted that when the number of charge and discharge cycles reaches N, the battery is subjected to 2-5 full charge and discharge cycles according to the actual cycle system to be examined, and the charge and discharge capacity of the last cycle is recorded as the charge capacity C of the battery after N accelerated cycles. N and discharge capacity D N At this time, the corresponding battery charging capacity retention rate is C N / C0, discharge capacity retention rate is D N / D0.

[0105] Step S23A, calculating the battery charge capacity retention rate and the battery discharge capacity retention rate of the battery under test in each stage of the accelerated cycle test based on the charge capacity and discharge capacity of the battery under test in each stage of the accelerated cycle test, and the initial charge capacity C0 and initial discharge capacity D0 obtained in step S21A;

[0106] In step S23A, the battery charge capacity retention rate of the battery under test in each stage of the accelerated cycle test is equal to the charge capacity C of the battery under test in each stage of the accelerated cycle test. 待测 Divide by the initial charge capacity C0; that is, C 待测 / C0;

[0107] In step S23A, the battery discharge capacity retention rate of the battery under test in each stage of the accelerated cycle test is equal to the discharge capacity D of the battery under test in each stage of the accelerated cycle test. 待测 Divide by the initial discharge capacity D0; that is, D 待测 / D0.

[0108] Step S24A, plotting an accelerated cycle capacity retention curve for the battery to be tested, with the battery charge capacity retention rate and the battery discharge capacity retention rate in each stage of the accelerated cycle test as the vertical coordinate and the total number of accelerated cycles corresponding to the end of each stage of the accelerated cycle test as the horizontal coordinate.

[0109] In specific implementation, a step may be further included between step S24A and step S23A:

[0110] In step S25A, the battery charging capacity retention rate and the battery discharging capacity retention rate of the battery to be tested in each stage of the accelerated cycle test are compared in real time with the preset battery life end capacity retention rate to determine whether the battery charging capacity retention rate or the battery discharging capacity retention rate is less than the preset battery life end capacity retention rate. If so, continue to step S24A; otherwise, return to step S22A and step S23A.

[0111] In specific implementation, the preset battery life end capacity retention rate is usually 80%, that is, the battery discharge capacity after cycling is reduced to 80% of the initial (fresh battery) discharge capacity.

[0112] It should be noted that, through step S25A, the condition for the end of the accelerated cycle of the battery to be tested in step S2 can be changed from nN times (i.e., n stages, N accelerated cycle test operations are performed in each stage) to the charging capacity retention rate or the discharging capacity retention rate when the battery capacity retention rate reaches the end of the battery life condition.

[0113] In step S2, an accelerated cycle test is performed on the reference battery within the accelerated test SOC range obtained in the first step, and an accelerated cycle capacity retention curve of the reference battery is obtained accordingly, which specifically includes the following steps:

[0114] Step S21B, performing a preset multiple (e.g., three) charge-discharge cycle operations on the reference battery (each charge-discharge cycle operation includes one discharge operation and one charge operation) in accordance with the actual cycle mode of the reference battery, and using the battery charge capacity and discharge capacity obtained during the last charge-discharge cycle operation as the initial charge capacity C1 and initial discharge capacity D1 of the reference battery;

[0115] Step S22B, using the actual cycle mode of the reference battery as the accelerated cycle mode, sequentially performing multiple (e.g., n, where n is a natural number greater than 1) stages of accelerated cycle testing on the reference battery (i.e., repeatedly performing multiple stages of accelerated cycle testing), and obtaining the charge capacity and discharge capacity of the reference battery in each stage of the accelerated cycle testing, and recording the total number of accelerated cycles corresponding to the end of each stage of the accelerated cycle testing (i.e., the total number of accelerated cycles performed from the start of the first stage of the accelerated cycle testing to the end of the accelerated cycle testing of that stage);

[0116] In step S22B, the test contents of the multiple stages of the accelerated cycle test are the same;

[0117] Each stage of the accelerated cycle test includes the following operations:

[0118] The first step is to charge the battery to the lower limit of the SOC range of the accelerated test with a predetermined charging current (e.g., a small current of 0.05-0.5C). CL , then let it sit for a preset time (e.g. 10-30 minutes);

[0119] In the second step, the battery is subjected to a preset number of times (e.g., N times, where N is a natural number greater than 1) of the same accelerated cycle test operation;

[0120] Each accelerated cycle test operation is specifically as follows: select the charging current Ic and discharging current Id corresponding to the accelerated test SOC interval in the actual cycle format of the reference battery, and then successively perform a charging operation on the reference battery with the charging current Ic for a preset charging time tc and a discharging operation on the battery with the discharge current Id for a preset discharge time td, so that the battery is within the accelerated test SOC interval (from the lower limit SOC CL To upper limit SOC CU ) has the same charge and discharge capacity; that is, Ic*tc=Id*td;

[0121] The actual cycle mode of the battery under test is equivalent to the actual cycle mode of the reference battery;

[0122] It should be noted that the charging current Ic and discharging current Id corresponding to the accelerated test SOC interval in the actual cycle mode of the reference battery can be obtained from the actual cycle mode specified in the specification sheet of the reference battery. For example, in the embodiment, the actual cycle mode is 1C constant current charging to an upper limit voltage of 3.65V, then switching to constant voltage charging, stopping when the current drops to 0.02C, standing for 15-30 minutes, and then discharging at a constant current of 1C until the lower limit voltage of 2.0V is reached. Therefore, the charging current and discharging current of the reference battery in the accelerated test SOC interval are both 1C.

[0123] The third step is to continue to perform a preset number of (e.g., 2 to 5) full charge and discharge cycles on the reference battery using the actual cycle format;

[0124] Step 4: The battery charge capacity and discharge capacity obtained during the last full charge and discharge cycle operation are used as the charge capacity and discharge capacity of the reference battery after the accelerated cycle test at each stage;

[0125] It should be noted that in the third step, full charge and discharge, that is, charging and discharging are performed according to the actual cycle format, that is, charging to the upper limit voltage of the battery, and discharging to the lower limit voltage of the battery: for example, it can be: 1C constant current charging to the upper limit voltage of 3.65V, then switching to 3.65V constant voltage charging, stopping when the current drops to 0.02C, standing for 15min~30min, and then discharging at a constant current of 1C until the lower limit voltage reaches 2.0V. Then stand for 15min~30min, and then perform the same charge and discharge cycle again.

[0126] In the present invention, full charge and discharge are relative to accelerated cycle. Accelerated cycle is carried out within a certain SOC range, while full charge and discharge is charging and discharging within the entire SOC range. The purpose of full charge and discharge here is: after evaluating a certain period of accelerated cycle, how much charging and discharging capacity of the reference battery is left when tested according to the actual cycle standard, so as to calculate and evaluate the capacity retention rate.

[0127] Step S23B, calculating the battery charge capacity retention rate and the battery discharge capacity retention rate of the reference battery in each stage of the accelerated cycle test based on the charge capacity and discharge capacity of the reference battery in each stage of the accelerated cycle test, and the initial charge capacity C1 and initial discharge capacity D1 obtained in step S21B;

[0128] In step S23B, the battery charge capacity retention rate of the reference battery in each stage of the accelerated cycle test is equal to the charge capacity C of the reference battery in each stage of the accelerated cycle test. N参比Divide by the initial charge capacity C1; that is, C 参比 / C1;

[0129] In step S23B, the battery discharge capacity retention rate of the reference battery in each stage of the accelerated cycle test is equal to the discharge capacity D of the reference battery in each stage of the accelerated cycle test. N参比 Divide by the initial discharge capacity D1, which is equal to D 参比 / D1.

[0130] Step S24B, using the battery charge capacity retention rate and the battery discharge capacity retention rate of the reference battery in each stage of the accelerated cycle test as the vertical axis and the total number of accelerated cycles corresponding to the end of each stage of the accelerated cycle test as the horizontal axis, to draw an accelerated cycle capacity retention rate curve of the reference battery.

[0131] In specific implementation, a step may be further included between step S24B and step S23B:

[0132] In step S25B, the battery charge capacity retention rate and the battery discharge capacity retention rate of the reference battery in each stage of the accelerated cycle test are compared in real time with the preset battery life end capacity retention rate to determine whether the battery charge capacity retention rate or the battery discharge capacity retention rate is less than the preset battery life end capacity retention rate. If so, continue to step S24B; otherwise, return to step S22B and step S23B.

[0133] In specific implementation, the preset battery life end capacity retention rate is usually 80%, that is, the battery discharge capacity after cycling is reduced to 80% of the initial (fresh battery) discharge capacity.

[0134] It should be noted that, through step S25B, the condition for ending the accelerated cycle of the reference battery in step S2 can be changed from nN times (i.e., n stages, N accelerated cycle test operations in each stage) to the charging capacity retention rate or the discharging capacity retention rate when the battery capacity retention rate reaches the end condition of the battery life.

[0135] In step S3, in a specific implementation, if the accelerated cycle capacity retention rate curve (i.e., the charge and discharge capacity retention rate curve) of the battery to be tested is above the accelerated cycle capacity retention rate curve (i.e., the charge and discharge capacity retention rate curve) of the reference battery, it is determined that the cycle performance of the battery to be tested is better than the cycle performance of the reference battery;

[0136] In step S3, in specific implementation, if the accelerated cycle capacity retention curve (i.e., the charge and discharge capacity retention curve) of the battery to be tested is below the accelerated cycle capacity retention curve (i.e., the charge and discharge capacity retention curve) of the reference battery, it is judged that the cycle performance of the battery to be tested is inferior to the cycle performance of the reference battery.

[0137] In the present invention, in step S3, in a specific implementation, if the accelerated cycle capacity retention rate curve (i.e., the charge and discharge capacity retention rate curve) of the accelerated cycle of the battery to be tested substantially coincides with the accelerated cycle capacity retention rate curve (i.e., the charge and discharge capacity retention rate curve) of the reference battery, it is necessary to repeat the accelerated cycle test of step S2, repeatedly obtain the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery, until the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery are completely separated, and then, based on the relative position (i.e., above or below) of the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery, the cycle performance of the battery to be tested is judged compared with the cycle performance of the reference battery;

[0138] When the degree of overlap between the accelerated cycle capacity retention rate curve of the battery under test and the accelerated cycle capacity retention rate curve of the reference battery is greater than or equal to a preset first ratio (e.g., 85%), it is determined that the two substantially overlap;

[0139] When the degree of overlap between the accelerated cycle capacity retention rate curve of the battery under test and the accelerated cycle capacity retention rate curve of the reference battery is less than or equal to a preset second ratio (e.g., 5%), it is determined that the two are completely separated;

[0140] The preset first ratio is greater than the preset second ratio.

[0141] It should be noted that, when the accelerated cycle test of step S2 is currently being repeated, the total number of accelerated cycles corresponding to the end of the last stage of the accelerated cycle test in the multiple stages of the accelerated cycle test included in step S2 is greater than the total number of accelerated cycles corresponding to the end of the last stage of the accelerated cycle test in the multiple stages of the accelerated cycle test included in step S2 when the accelerated cycle test was previously executed. For example, if the previous total number of accelerated cycles was 200, then the total number of accelerated cycles when repeating step S2 is required to be greater than 200, for example, 300.

[0142] In the present invention, in step S3, the accelerated cycle capacity retention curve of the reference battery can be used as benchmark data to establish a database, and can be directly used as a reference curve for comparative analysis during subsequent screening and evaluation of batteries of the same system.

[0143] Based on the above technical solution, it can be seen that for the present invention, first, the accelerated test interval is determined according to the cycle attenuation analysis of the battery system to be tested, the battery is subjected to an accelerated cycle test in an actual cycle format, and the charge and discharge capacity of the battery is measured in an actual cycle format at different stages of the accelerated cycle for calculation of the capacity retention rate. Further, by comparing the capacity retention rate versus cycle number curves of the battery to be tested and the reference battery, the superiority or inferiority of the cycle performance of the battery to be tested relative to the reference battery is judged.

[0144] Compared with the prior art, the accelerated evaluation method for lithium-ion battery cycle performance provided by the present invention has the following beneficial technical effects:

[0145] 1. The method provided by the present invention first determines the characteristic SOC interval and accelerated test SOC interval in which the battery to be tested undergoes cycle attenuation. Then, an accelerated cycle test is performed within the accelerated test SOC interval. The cycle performance of the electrode material to be tested is judged by analyzing the capacity retention rate after the accelerated cycle. Compared with the traditional full SOC cycle test, the evaluation time can be significantly shortened.

[0146] 2. The present invention first determines the characteristic SOC interval in which the battery to be tested experiences cycle attenuation. Within this SOC interval, the cycle attenuation characteristics of the battery system to be tested are significant. Therefore, it can be used as the accelerated test SOC interval for the cycle to shorten the cycle evaluation time.

[0147] 3. In the present invention, in the selected accelerated test SOC interval, the battery is subjected to an accelerated cycle test according to the charging current Ic and the discharging current Id in the actual cycle format, and the charging and discharging time is cut off to make the battery within the accelerated test SOC interval (from the lower limit SOC CL To upper limit SOC CU ) have the same charge and discharge capacities, i.e., Ic*tc=Id*td. Since the accelerated cycling process maintains the same charge and discharge capacities, if no side reactions occur, the battery state after the cycle remains the same as before the cycle, i.e., the process change rate is 0. Similarly, the higher the degree of side reactions in the battery, the greater the rate of change during the accelerated cycling process.

[0148] Based on this, the present invention can assist in determining the cause of poor battery cycle performance by adding analysis of DCIR (direct current resistance) and polarization voltage during each accelerated cycle, thereby providing a reference for improving battery performance.

[0149] 4. In the present invention, in the rapid evaluation of the cycle performance of the battery to be tested, the measured battery charge and discharge capacity retention rates are plotted as the ordinate and the corresponding total number of accelerated cycles as the abscissa to obtain the accelerated cycle capacity retention rate curves of the test battery and the reference battery respectively; the cycle performance of the electrode material to be tested is evaluated by comparing the positions of the curves. Rather than simply analyzing by comparing the capacity retention rates of one or several times under the same number of accelerated cycles, the present invention ensures the accuracy and comprehensiveness of the analysis results. In particular, when the capacity retention rate curves of the test battery and the reference battery intersect, the results of the curve analysis can maintain accuracy, while if only the capacity retention rate points of the previous few times are analyzed, misjudgment may occur.

[0150] In order to more clearly understand the technical solution of the present invention, the technical solution of the present invention is described below through specific embodiments.

[0151] Example 1.

[0152] The present invention will be described in detail below using the test of a commercial prismatic lithium-ion battery as an example with reference to the accompanying drawings to further illustrate the essential features and significant advancements of the present invention.

[0153] In this embodiment, the test sample is a prismatic lithium iron ion experimental battery with a 1C capacity of 52Ah. The battery under test is of the same model as the reference battery, but uses a different negative electrode.

[0154] The battery testing equipment is a conventional charge and discharge instrument. The equipment used in this embodiment is the Arbin BT2000 charge and discharge test system.

[0155] In Example 1, the accelerated evaluation method for the cycle performance of a lithium-ion battery provided by the present invention comprises the following steps:

[0156] Step 1: Determine the characteristic SOC range and accelerated test SOC range where the battery system under test undergoes cycle attenuation.

[0157] The specific operation is as follows: First, select a fresh battery (i.e. a fresh battery with a capacity retention rate of 100%) and a cycled battery (i.e. a reference battery with a capacity retention rate of 95%) of the same system as the battery to be tested to perform characteristic SOC interval analysis of cycle attenuation. Specifically, the battery is charged and discharged at a low current of 0.1C, and the battery voltage is differentiated by the charging capacity to obtain dQ / dV, with dQ / dV as the vertical coordinate, and the real-time state of charge SOC corresponding to the battery charging capacity is used as the horizontal coordinate to draw the capacity increment IC curve (see Appendix). Figure 2 ). Figure 2It can be seen that in the range of 7% SOC to 18% SOC, as the battery capacity retention rate decreases, the height of the first lithium insertion peak on the two capacity increment IC curves decreases significantly, which is the SOC range where the battery system under test decays most significantly. Therefore, the characteristic SOC range of battery cycle decay in this system is 7% SOC to 18% SOC, which is the lower limit of the characteristic SOC range. L 7%, upper limit SOC U It is 18%.

[0158] The accelerated test SOC interval is determined based on the characteristic SOC interval of the cyclic decay. The accelerated test SOC interval is a characteristic SOC interval that includes or partially includes the cyclic decay. This interval includes the lower limit value SOC CL and upper limit SOC CU In order to fully shorten the test cycle, SOC is generally preferred. CL =SOC L ±10%, SOC CU =SOC U ±10%, more preferably SOC CL =SOC L ±5%, SOC CU =SOC U ±5%.

[0159] In Example 1, considering the test cycle and the impact of large battery polarization in the low state of charge range, the accelerated test range is selected to be 10%-20% SOC, that is, SOC CL 10%, SOC CU is 20%.

[0160] Step 2: Perform accelerated cycle tests on the battery under test and the reference battery in the accelerated test range (10% SOC-20% SOC), and obtain the accelerated cycle capacity retention curves of the battery under test and the reference battery. Specific operations include the following:

[0161] In the first step, take the battery to be tested and the reference battery, and perform three charge and discharge cycles using the actual cycle format to be examined (1C charge and discharge), and record the charge and discharge capacity of the third cycle as the initial charge capacity and initial discharge capacity of the battery.

[0162] The actual cycle mode (1C charge and discharge) investigated is as follows: the battery is charged and discharged at 1C=53A, the constant current charge cut-off voltage is 3.65V, the constant voltage charge is cut off when the current drops to 0.05C=2.65A, and the discharge cut-off voltage is 2.5V. In Table 1, the initial charge capacity C0 and initial discharge capacity D0 of the battery under test at the third cycle are recorded, and the initial charge capacity C0 of the reference battery is recorded. 0S and initial discharge capacity D 0S .

[0163] Step 2: Charge the battery to the lower limit of the accelerated test range (10% SOC-20% SOC), i.e., 10% SOC, at a low current of 0.2C, and let it stand for 10-30 minutes.

[0164] Step 3: Select the charge current Ic and discharge current Id corresponding to the actual cycling conditions of the battery under test within the accelerated test SOC range (10%-20% SOC). Set the charge and discharge times to ensure that the battery's charge and discharge capacities within the characteristic SOC range (10%-20% SOC) are the same, i.e., Ic*tc=Id*td. Ic=53A, tc=360s, Id=53A, td=360s. The battery is charged and discharged 500 times within the accelerated test range (10%-20% SOC).

[0165] Step 4: When the number of charge and discharge cycles reaches 500 (i.e., N is 500), the test battery and the reference battery are subjected to two full charge and discharge cycles using the actual cycle mode of 1C charge and discharge, and the charge and discharge capacity of the last cycle is recorded as the charge capacity C of the battery after 500 accelerated cycles. N and discharge capacity D N At this time, the corresponding battery charging capacity retention rate is C N / C0, discharge capacity retention rate is D N / D0.

[0166] In step 5, the accelerated cycling process from step 2 to step 4 is repeated n times (n is equal to 3) (the accelerated cycling process from step 2 to step 4 is one stage of accelerated cycling test) and ends when the number of accelerated cycles of the battery reaches 1500. The charge capacity retention rate and discharge capacity retention rate of the battery after 500, 1000, and 1500 accelerated cycles are obtained (i.e., the battery charge capacity retention rate and battery discharge capacity retention rate in these three stages are obtained) and recorded in Table 1.

[0167] Table 1: Schematic diagram of the charge and discharge capacity and retention rate of the reference battery and the test battery during accelerated cycling.

[0168]

[0169] Step 6: Use the battery charge and discharge capacity retention rates measured in the above steps as the ordinate and the corresponding total number of accelerated cycles as the abscissa to plot the accelerated cycle capacity retention rate curves of the test battery and the reference battery, respectively, as shown in the attached figure. Figure 3 shown.

[0170] The third step is a rapid evaluation of the cycle performance of the battery to be tested: by comparing the accelerated cycle capacity retention curve of the battery to be tested with the accelerated cycle capacity retention curve of the reference battery, the quality of the cycle performance of the battery to be tested relative to that of the reference battery is judged.

[0171] from Figure 3 It can be seen that during 1500 accelerated cycles, the capacity retention curve of the battery under test is below that of the reference battery, and the distance between the two curves is getting larger and larger, and it is impossible for the two curves to cross again. Therefore, it is determined that the cycle performance of the battery under test is inferior to that of the reference battery.

[0172] In order to further understand the reasons for the poor cycle performance of the battery under test, the DCIR (direct current resistance) of the battery during the first 500 accelerated cycles was analyzed, as shown in the attached figure. Figure 4 .

[0173] It should be noted that the DCIR value is calculated using a method known in the industry. This is calculated by dividing the voltage difference between the discharge time of 0 seconds and 3 seconds by the discharge current. This will not be further elaborated here.

[0174] Depend on Figure 4 It can be seen that the DCIR of the tested battery and its growth rate during the cycle process are higher than those of the reference battery, which may be the reason for its poor cycle performance and also provides a certain reference direction for improving battery performance.

[0175] In this embodiment, by performing accelerated cycle tests on the battery to be tested and the reference battery, it can be found that the cycle performance of the battery to be tested is inferior to that of the reference battery, and is consistent with the actual cycle test results, such as Figure 5 .

[0176] After testing, it was found that the accelerated cycle evaluation method for the cycle performance of lithium-ion batteries provided by the present invention only takes 36 days to evaluate the cycle performance between the battery to be tested and the reference battery.

[0177] In addition, if Figure 4 As shown in the figure, according to the results of 1000 accelerated cycle tests (approximately 24 days), it has been determined that the cycle performance of the battery under test is worse than that of the reference battery. This result is consistent with the actual cycle test results, while the existing conventional cycle test takes 36 days and can only cycle about 250 times. In addition, the existing conventional cycle test still cannot fully and accurately determine the cycle performance of the battery under test compared with the reference battery from the actual cycle curve. Therefore, the use of the accelerated evaluation method of the present invention can effectively improve R&D efficiency and shorten the battery R&D cycle. At the same time, the analysis of DCIR can provide a reference for improving battery cycle performance.

[0178] In summary, compared with the existing technology, the accelerated evaluation method for the cycle performance of lithium-ion batteries provided by the present invention is scientifically designed and suitable for lithium-ion battery product development. When used for material screening and system optimization, the rapid comparative analysis of battery cycle performance can effectively shorten the battery development cycle and improve R&D efficiency. At the same time, by reducing the energy consumption of cycle testing, it indirectly reduces the battery development cost, and has good application prospects and promotion value.

[0179] In the present invention, the accelerated test interval is first determined based on the cycle attenuation analysis of the battery system to be tested, the battery is subjected to an accelerated cycle test using the actual cycle format, and the charge and discharge capacity of the battery is measured using the actual cycle format at different stages of the accelerated cycle for calculation of the capacity retention rate. Further, by comparing the capacity retention rate versus cycle number curves of the battery to be tested and the reference battery, the superiority or inferiority of the cycle performance of the battery to be tested relative to the reference battery is judged.

[0180] In the present invention, since the charge and discharge capacities of the battery under test are maintained constant during the accelerated cycling process, if no side reactions occur, the battery state after the cycle remains the same as before the cycle, i.e., the process change rate is 0. Similarly, the higher the degree of side reactions in the battery, the greater the rate of change during the accelerated cycling process. Based on this, by adding analysis of DCIR (direct current resistance) and polarization voltage during each accelerated cycle, the cause of poor battery cycle performance can be determined, providing a reference for battery performance improvement.

[0181] The method provided by this invention, because it performs accelerated cycling analysis within the characteristic decay range, can significantly shorten the battery cycling performance evaluation cycle compared to full SOC cycling testing, thereby improving R&D efficiency. Furthermore, accelerated cycling can include analysis of the battery's DCIR (direct current resistance) and polarization voltage process change rate, providing a reference for battery cycle failure analysis.

[0182] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for accelerating the evaluation of the cycle performance of a lithium-ion battery, characterized in that: The following steps are involved: Step S1: For a battery to be tested that needs to be evaluated for cycle performance, a fresh battery and a reference battery of the same battery system as the battery to be tested are pre-selected, and a characteristic SOC interval of cycle attenuation of the battery system to be tested is obtained by analysis, and an accelerated test SOC interval is determined; The step S1 includes the following operations: Step S11, selecting a fresh battery and a reference battery, performing preset charge and discharge cycle operations on each, and collecting the battery voltage V, charge capacity Q1, and discharge capacity Q2 of the fresh battery and the reference battery in real time; Step S12, obtaining a characteristic SOC interval of cyclic decay of the battery system under test according to a preset first acquisition method or a preset second acquisition method; Step S13, determining an accelerated test SOC interval according to a characteristic SOC interval of cycle decay of the battery system to be tested; In step S12, according to a preset first acquisition method, a characteristic SOC interval of the battery system to be tested that undergoes cyclic decay is obtained, including the following steps: Step S121A: For the fresh battery and the reference battery, the battery charging capacity Q1 is differentiated by the battery charging voltage V to obtain dQ / dV of the fresh battery and the reference battery; Step S122A: For the fresh battery and the reference battery, plotting the capacity increment curves of the two on a graph using dQ / dV as the ordinate and the state of charge (SOC) corresponding to the battery's charge capacity Q1 as the abscissa, respectively; Step S123A, using the capacity increment curve of the fresh battery as a reference curve, comparing the capacity increment curve of the reference battery with the reference curve of the fresh battery, determining an operation based on a preset characteristic SOC interval, and determining, from the two capacity increment curves, a characteristic SOC interval where cyclic degradation of the reference battery system occurs; In step S123A, the preset characteristic SOC interval determination operation includes the following steps: First, on the capacity increment curves of the fresh battery and the reference battery, by comparing the peaks of the two curves one by one, the peak where the peak value of the peak is significantly reduced, or the peak where the peak value is significantly reduced and the peak position is significantly shifted is determined, and then used as the characteristic peak of the cycle attenuation; The peak value of the peak is significantly reduced, which means that the peak value reduction ratio of the peak is greater than or equal to the preset peak reduction value; The peak position of the wave crest is significantly offset, which means that the peak position of the wave crest corresponds to the SOC offset amplitude greater than or equal to the preset peak position offset value; Then, the SOC interval corresponding to the starting position and the ending position of the characteristic peak of the cyclic decay is taken as the characteristic SOC interval; In step S12, according to a preset second acquisition method, a characteristic SOC interval of the battery to be tested that undergoes cycle decay is obtained, including the steps of: Step S121B: for the fresh battery and the reference battery, the battery voltage V 充 or the battery voltage V during discharge 放 , perform differential processing on the battery's charge capacity Q1 or discharge capacity Q2 to obtain the dV of the fresh battery and the reference battery 充 / dQ1 or dV 放 / dQ2; Step S122 B: For the fresh battery and the reference battery, dV 充 / dQ1 or dV 放 / dQ2 is used as the vertical axis, and the state of charge SOC corresponding to the battery's charging capacity Q1 or discharging capacity Q2 is used as the horizontal axis. The differential voltage curve between the two is plotted in a graph; Step S123 B: Using the differential voltage curve of the fresh battery as a reference curve, the differential voltage of the reference battery is compared with the reference curve of the fresh battery, and the operation is determined according to the preset characteristic SOC interval. The characteristic SOC interval where the reference battery system experiences cyclic decay is determined in the two differential voltage curves; In step S123B, the preset characteristic SOC interval determination operation includes the following steps: First, on the differential voltage curves of the fresh battery and the reference battery, by comparing the peaks of the two curves one by one, the peak where the peak value of the peak is significantly reduced, or the peak where the peak value is significantly reduced and the peak position is significantly shifted is determined, and then used as the characteristic peak of the cycle decay; The peak value of the peak is significantly reduced, which means that the peak value reduction ratio of the peak is greater than or equal to the preset peak reduction value; The peak position of the wave crest is significantly offset, which means that the peak position of the wave crest corresponds to the SOC offset amplitude greater than or equal to the preset peak position offset value; Then, the SOC interval corresponding to the starting position and the ending position of the characteristic peak of the cyclic decay is taken as the characteristic SOC interval ； Step S2, performing accelerated cycle tests on the battery under test and the reference battery within the accelerated test SOC range obtained in the first step, and obtaining accelerated cycle capacity retention rate curves of the battery under test and the reference battery; In step S2, the battery to be tested is subjected to an accelerated cycle test within the accelerated test SOC range obtained in the first step, and an accelerated cycle capacity retention rate curve of the battery to be tested is correspondingly obtained, including the steps of: Step S21A, performing a preset number of charge-discharge cycle operations on the battery under test using the actual cycle mode of the reference battery, and using the battery charge capacity and discharge capacity obtained during the last charge-discharge cycle as the initial charge capacity C0 and initial discharge capacity D0 of the battery under test; Step S22A, using the actual cycle mode of the reference battery as the accelerated cycle mode, sequentially performing multiple stages of accelerated cycle testing on the battery under test, obtaining the charge capacity and discharge capacity of the battery under test after each stage of the accelerated cycle test, and recording the total number of accelerated cycles corresponding to the end of each stage of the accelerated cycle test; Step S23A, calculating the battery charge capacity retention rate and the battery discharge capacity retention rate of the battery under test in each stage of the accelerated cycle test based on the charge capacity and discharge capacity of the battery under test in each stage of the accelerated cycle test, and the initial charge capacity C0 and initial discharge capacity D0 obtained in step S21A; Step S24A, plotting an accelerated cycle capacity retention curve for the battery under test, with the battery charge capacity retention rate and the battery discharge capacity retention rate in each stage of the accelerated cycle test as the ordinate and the total number of accelerated cycles corresponding to the end of each stage of the accelerated cycle test as the abscissa; In step S2, an accelerated cycle test is performed on the reference battery within the accelerated test SOC range obtained in the first step to obtain an accelerated cycle capacity retention curve of the reference battery, including the following steps: Step S21B, performing a preset number of charge-discharge cycle operations on the reference battery using the actual cycle mode of the reference battery, and using the battery charge capacity and discharge capacity obtained during the last charge-discharge cycle operation as the initial charge capacity C1 and initial discharge capacity D1 of the reference battery; Step S22B, using the actual cycle mode of the reference battery as the accelerated cycle mode, sequentially performing multiple stages of accelerated cycle tests on the reference battery, obtaining the charge capacity and discharge capacity of the reference battery in each stage of the accelerated cycle test, and recording the corresponding total number of accelerated cycles at the end of each stage of the accelerated cycle test; Step S23B, calculating the battery charge capacity retention rate and the battery discharge capacity retention rate of the reference battery in each stage of the accelerated cycle test based on the charge capacity and discharge capacity of the reference battery in each stage of the accelerated cycle test, and the initial charge capacity C1 and initial discharge capacity D1 obtained in step S21B; Step S24B, plotting an accelerated cycle capacity retention curve of the reference battery with the battery charge capacity retention rate and the battery discharge capacity retention rate of the reference battery in each stage of the accelerated cycle test as the ordinate and the total number of accelerated cycles corresponding to the end of each stage of the accelerated cycle test as the abscissa; Step S3 , by comparing the accelerated cycle capacity retention rate curve of the battery to be tested with the accelerated cycle capacity retention rate curve of the reference battery, the quality of the cycle performance of the battery to be tested relative to the cycle performance of the reference battery is determined.

2. The method for accelerating the cycle performance evaluation of a lithium-ion battery according to claim 1, wherein: In step S1, the fresh battery is a battery that has not experienced capacity decay; the reference battery is a battery whose capacity decay is greater than or equal to a preset ratio; The SOC range of the accelerated test includes the lower limit SOC CL and upper limit SOC CU ; Among them, the accelerated test SOC interval includes the characteristic SOC interval of the entire battery system to be tested that undergoes cyclic attenuation, or includes the characteristic SOC interval of part of the battery system to be tested that undergoes cyclic attenuation.

3. The accelerated evaluation method for the cycle performance of a lithium-ion battery according to claim 2, wherein: In step S12, the reference battery system is identical to the battery system to be tested; The characteristic SOC range of the reference battery system where cycle decay occurs, including the lower limit SOC L and upper limit SOC U .

4. The accelerated evaluation method for the cycle performance of a lithium-ion battery according to claim 2, wherein: In step S12, the reference battery system is identical to the battery system to be tested; The characteristic SOC range of the reference battery system where cycle decay occurs, including the lower limit SOC L and upper limit SOC U .

5. The method for accelerating the cycle performance evaluation of a lithium-ion battery according to claim 1, wherein: In step S1, the battery to be tested and the reference battery are two batteries having the same components except for the negative electrode material, positive electrode material, electrolyte or separator; In step S1 , the difference between the battery capacity retention rates of the fresh battery and the reference battery is greater than a preset value.

6. The accelerated evaluation method for the cycle performance of a lithium-ion battery according to claim 1, wherein: In step S11, the preset charge-discharge cycle operation includes one discharge operation and one charge operation, specifically: first charging with a preset charging current to a preset upper charge voltage limit, and then discharging with a preset discharge current to a preset lower discharge voltage limit; In step S13, the lower limit value SOC of the acceleration test SOC interval is CL =SOC L ±10%, the upper limit of the SOC range of the accelerated test CU =SOC U ±10%.

7. The accelerated evaluation method for the cycle performance of a lithium-ion battery according to claim 1, wherein: In step S22A, the accelerated cycle test at each stage specifically includes the following operations: The first step is to charge the battery to the lower limit of the SOC range of the accelerated test with a pre-set charging current. CL , then let it sit for a preset time; The second step is to perform a preset number of identical accelerated cycle test operations on the battery; Each accelerated cycle test operation specifically involves selecting the charging current Ic and discharging current Id corresponding to the accelerated test SOC range in the actual cycle format of the battery under test, then charging the battery under test with the charging current Ic for a preset charging time tc and discharging the battery with the discharging current Id for a preset discharge time td, such that the charge and discharge capacities of the battery within the accelerated test SOC range are the same; that is, Ic*tc=Id*td; The actual cycle mode of the battery under test is equivalent to the actual cycle mode of the reference battery; The third step is to continue to perform multiple full charge and discharge cycle operations on the battery under test in the actual cycle mode; Step 4: The battery charge capacity and discharge capacity obtained during the last full charge and discharge cycle operation are used as the charge capacity and discharge capacity of the battery under test after the accelerated cycle test at each stage; In step S22B, the accelerated cycle test at each stage specifically includes the following operations: The first step is to charge the battery to the lower limit of the SOC range of the accelerated test with a pre-set charging current. CL , then let it sit for a preset time; The second step is to perform a preset number of identical accelerated cycle test operations on the battery; Each accelerated cycle test operation is specifically as follows: selecting the charging current Ic and discharging current Id corresponding to the accelerated test SOC range in the actual cycle format of the reference battery, then charging the reference battery with the charging current Ic for a preset charging time tc and discharging the battery with the discharge current Id for a preset discharge time td, so that the charging and discharging capacities of the battery in the accelerated test SOC range are the same; that is, Ic*tc=Id*td; The actual cycle mode of the battery under test is equivalent to the actual cycle mode of the reference battery; The third step is to continue to perform a preset multiple full charge and discharge cycle operation on the reference battery according to the actual cycle system of the reference battery; In the fourth step, the battery charging capacity and discharge capacity obtained during the last full charge and discharge cycle operation are used as the charging capacity and discharge capacity of the reference battery after the accelerated cycle test at each stage.

8. The accelerated evaluation method for the cycle performance of a lithium-ion battery according to claim 1, wherein: Between step S24A and step S23A, there is also a step: Step S25A, comparing the battery charge capacity retention rate and the battery discharge capacity retention rate in each stage of the accelerated cycle test of the battery under test with the preset battery life end capacity retention rate in real time, and determining whether the battery charge capacity retention rate or the battery discharge capacity retention rate is less than the preset battery life end capacity retention rate. If so, continue to step S24A; otherwise, return to step S22A and step S23A; And / or, between step S24B and step S23B, a step is further included: In step S25B, the battery charge capacity retention rate and the battery discharge capacity retention rate of the reference battery in each stage of the accelerated cycle test are compared in real time with the preset battery life end capacity retention rate to determine whether the battery charge capacity retention rate or the battery discharge capacity retention rate is less than the preset battery life end capacity retention rate. If so, continue to step S24B; otherwise, return to step S22B and step S23B.

9. The method for accelerating the cycle performance evaluation of a lithium-ion battery according to claim 1, wherein: In step S3, if the accelerated cycle capacity retention rate curve of the battery to be tested is above the accelerated cycle capacity retention rate curve of the reference battery, it is determined that the cycle performance of the battery to be tested is better than the cycle performance of the reference battery; In step S3, if the accelerated cycle capacity retention curve of the battery to be tested is below the accelerated cycle capacity retention curve of the reference battery, it is determined that the cycle performance of the battery to be tested is inferior to that of the reference battery.

10. The accelerated evaluation method for the cycle performance of a lithium-ion battery according to claim 1, wherein: In step S3, if the accelerated cycle capacity retention rate curve of the battery to be tested is substantially identical to the accelerated cycle capacity retention rate curve of the reference battery, the accelerated cycle test of step S2 needs to be repeated to repeatedly obtain the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery until the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery are completely separated. Then, the cycle performance of the battery to be tested is judged relative to the cycle performance of the reference battery based on the relative positions of the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery. When the degree of overlap between the accelerated cycle capacity retention rate curve of the battery to be tested and the accelerated cycle capacity retention rate curve of the reference battery is greater than or equal to a preset first ratio, it is determined that the two substantially overlap; When the degree of overlap between the accelerated cycle capacity retention rate curve of the battery to be tested and the accelerated cycle capacity retention rate curve of the reference battery is less than or equal to a preset second ratio, it is determined that the two are completely separated; The preset first ratio is greater than the preset second ratio.

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