A method for testing the effect of excess negative electrode size on lithium-ion battery capacity

Abstract

This invention discloses a method for testing the impact of excessive negative electrode size on the capacity of lithium-ion batteries, comprising the following steps: S1: preparing lithium-ion batteries and grouping them into test battery groups and reference battery groups; S2: charging the test battery groups; S3: placing the test battery groups on a shelf for several days, during which time the batteries are float-charged; S4: charging the reference battery groups; S5: performing a first discharge on the test battery groups and the reference battery groups to obtain the discharge capacity Q1; then performing a second discharge on the lithium-ion batteries to obtain the discharge capacity Q2; S6: calculating the discharge capacity difference. This invention proposes a testing method that effectively evaluates the impact of excessive negative electrode size on the actual capacity of lithium-ion batteries, providing important technical reference for battery health status assessment and battery management system design.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery testing, and in particular to a method for testing the effect of excessive negative electrode size on the capacity of lithium-ion batteries. Background Technology

[0002] The inventory development and sales figures for electric vehicles over the past few years indicate a rapid increase in demand for pure electric vehicles and plug-in hybrid electric vehicles, especially as long driving range and battery life are crucial issues for achieving mobile energy storage. Therefore, reliable lifespan prediction is essential for both manufacturers and end consumers, and reversible and irreversible capacity losses in lithium-ion batteries must be considered. For commercial lithium-ion batteries, to improve safety, the negative electrode is typically slightly larger than the positive electrode, with an excess size generally ranging from 1 mm to several centimeters, depending on the battery geometry. It has long been believed that excess negative electrode size has no significant impact on lithium-ion battery performance; however, recent industry research shows that anomalies in lithium-ion batteries, including coulombic efficiencies greater than 1 or increased capacity in the early stages of cycling, are related to excess negative electrode size. This excess size can significantly affect battery capacity over days or even months. Furthermore, the capacity impact of excess negative electrode size can be superimposed on other reversible effects, such as the stripping reaction of lithium plating on the negative electrode, greatly increasing the difficulty of analyzing reversible and irreversible capacity losses and reducing the accuracy of lithium-ion battery lifespan interpretation and prediction.

[0003] Currently, there are very few reports on the negative electrode overcapacity region. For example, a method for preparing a long-life, high-capacity silicon negative electrode lithium-ion battery, disclosed in Chinese patent literature (publication number CN110911732A), involves achieving a super-overcapacity negative electrode, which is 1.2 to 4 times the capacity of the positive electrode. Simultaneously, a polarity-reversal transition lithium storage method is used to replenish lithium in the negative electrode. This involves adding lithium carbonate, lithium oxalate, and lithium hydroxide as lithium source materials during the stirring of the silicon negative electrode material, and adding graphite, silicon, sulfur, and phosphorus as lithium storage materials during the stirring of the positive electrode material. After the battery is filled with electrolyte, the negative electrode is connected to the positive electrode of the charger, and the positive electrode is connected to the negative electrode of the charger for charging. After all the lithium in the lithium source premixed in the silicon anode material has entered the lithium storage material premixed in the cathode material, charging is stopped. Then, the battery positive electrode is connected to the positive electrode of the charger and the battery negative electrode is connected to the negative electrode of the charger to charge and form the battery. Lithium is first embedded in the negative electrode and forms an SEI film on the surface of the negative electrode. Although this invention reduces the consumption of positive electrode lithium, increases battery capacity, and has very little expansion of silicon anode, improving the stability of silicon anode and extending battery life, it does not effectively assess the impact of excessive anode size on the actual capacity of lithium-ion batteries and cannot provide technical reference for battery health status assessment and battery management system design. Summary of the Invention

[0004] This invention aims to overcome the problem that existing technologies lack effective methods for assessing the impact of excessive negative electrode size on the actual capacity of lithium-ion batteries, and cannot provide technical references for battery health status assessment and battery management system design. It provides a testing method for the design of lithium-ion battery negative electrode sheets.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for testing the effect of negative electrode excess size on the capacity of lithium-ion batteries includes the following steps: S1: Prepare several lithium-ion batteries and group them according to the negative electrode excess size, dividing them into test battery groups and reference battery groups; S2: Charge the lithium-ion batteries of the test battery groups under constant voltage conditions with a first constant current; S3: Let the lithium-ion batteries of the test battery groups rest for several days under certain conditions. When the voltage of the lithium-ion batteries drops during the resting process, float charge the batteries with a float charging current; S4: Charge the lithium-ion batteries of the reference battery groups under constant voltage conditions with a first constant current; S5: Discharge the lithium-ion batteries of the test battery groups and the reference battery groups for the first time with a second constant current until the minimum rated voltage is reached, obtaining the discharge capacity Q1 of each group of lithium-ion batteries; then, perform a second constant voltage discharge on the lithium-ion batteries within a certain time range until the current reaches the cutoff current, obtaining the discharge capacity Q2; S6: Calculate the sum of the discharge capacity Q1 and the discharge capacity Q2 to obtain the total discharge Q of the lithium-ion batteries and calculate the discharge capacity difference. The design of the excess size of the negative electrode in a lithium-ion battery has a significant impact on battery performance. The area opposite the positive electrode in a lithium-ion battery is the active negative electrode region, and the portion extending beyond the positive electrode is the excess negative electrode region. When the state of charge (SOC) of the lithium-ion battery is high, lithium ions flow from the active region into the excess negative electrode region; conversely, at a low SOC, lithium ions flow back from the excess negative electrode region into the active region. During charging, the potential difference between the active region and the excess negative electrode region is small, and the time required for lithium ions to flow into the excess negative electrode region is longer than that required for flowing out during the discharge process. Therefore, a longer measurement time is needed. This invention proposes a method for testing the impact of excess negative electrode size on the capacity of lithium-ion batteries. Lithium-ion batteries are grouped and tested according to their excess negative electrode size. By adding a high-rate discharge step, the total discharge volume Q of the lithium-ion battery is obtained, and the discharge capacity difference is calculated, effectively evaluating the impact of excess negative electrode size on the actual capacity of the lithium-ion battery.

[0007] As a preferred embodiment of the present invention, the standard for preparing the lithium-ion battery in S1 is as follows: the positive electrode material used is any one of the commercial lithium-ion battery positive electrode materials; the negative electrode material is graphite or silicon-carbon composite material; the conductive agent is one or more of carbon black, carbon nanotubes, conductive fibers, and conductive graphite; the battery design capacity is in the range of 50-60Ah; the number of positive electrode sheets is 20; the number of negative electrode sheets is 21; the battery length is in the range of 400-500mm; and the battery width is in the range of 90-110mm. The positive electrode material used in the lithium-ion battery prepared by the present invention is any one of the currently commercially available lithium-ion battery positive electrode materials, including lithium iron phosphate, layered transition metal oxides, lithium manganese oxide, and lithium manganese iron phosphate, etc. The initial state of charge of the prepared lithium-ion battery is assumed to be 0% SOC.

[0008] As a preferred embodiment of the present invention, the grouping standard of the test battery pack in S1 is as follows: divided into n groups according to the excess size of the negative electrode, namely A1 group, A2 group, ..., A n Groups, each group corresponding to a1mm, a2mm, ..., a n mm, then divide each group into m groups, namely A n1 Group,A n2 Group,...,A nm The test battery packs consist of n×m groups, where n and m are both positive integers. The grouping standard for the reference battery packs in S1 is as follows: they are divided into n groups according to the excess size of the negative electrode, namely group B1, group B2, ..., group B... n Groups, each group corresponding to a1mm, a2mm, ..., a n mm, then divide each group into m groups, namely B n1 Group B n2 Group, ..., B nm The reference battery pack consists of n×m groups, where n and m are both positive integers. This invention sets up a test battery pack and a reference battery pack. The discharge capacity is obtained and the difference in discharge capacity is calculated by performing corresponding operations on the test and reference battery packs. Unlike the test battery, the reference battery does not require long-term storage (S3). Since the reference battery does not experience the long-term storage of lithium ions, there is no process of lithium flowing from the active region to the excess negative electrode region; that is, there is no loss of lithium in the active region. Therefore, the total discharge capacity difference between the reference and test batteries is the amount of lithium flowing from the active region to the excess negative electrode region, and all other possible effects are canceled out.

[0009] As a preferred embodiment of the present invention, the charging operation of S2 specifically involves: charging A... n1 Group,A n2 Group,...,A nm The lithium-ion batteries in the group were charged to α1% SOC, α2% SOC, ..., αm %SOC, where α m Not greater than 100. In actual testing, the state of charge (SOC) of a lithium-ion battery can be set by the implementer according to the battery material system and actual needs.

[0010] As a preferred embodiment of the present invention, under certain conditions in S3, namely 30-40℃, the magnitude of the float charging current is in the range of 0.02-0.05C. During the resting process, the lithium-ion batteries in the test group will experience a voltage drop. Float charging the batteries with a small current of 0.02-0.05C ensures that the excess area of ​​the negative electrode is filled with lithium to the maximum extent. If the resting temperature of the test battery group is too low, it will hinder lithium-ion flow and prolong the test time. If the temperature is too high, it will easily lead to rapid irreversible self-discharge of the battery. That is, at high temperatures, a series of side reactions will occur inside the lithium-ion battery, leading to an irreversible decrease in the capacity of the positive and negative electrodes, thus causing a high experimental error.

[0011] As a preferred embodiment of the present invention, the charging operation of S4 specifically involves: charging B... n1 Group B n2 Group, ..., B nm The lithium-ion batteries in the group were charged to α1% SOC, α2% SOC, ..., α m %SOC, where α m Not exceeding 100. During long-term storage, lithium ions flow from the active region to the excess region of the negative electrode in the test battery pack. However, the amount of lithium flowing in cannot be directly measured. During subsequent discharge, the lithium embedded in the excess region cannot flow back to the active region in a short time, resulting in a loss of subsequent discharge capacity. To eliminate the influence of the battery's internal relaxation process and internal resistance, a reference battery needs to be added. Unlike the test battery pack, the reference battery pack does not need to be stored for a long time. The reference battery, with an initial SOC of 0%, is charged at a constant current and constant voltage rate of 0.3-0.5C to the same level as the test battery pack. Then, it is immediately subjected to the same discharge operation as the test battery pack. The reference battery pack does not experience the process of lithium ions flowing from the active region to the excess region of the negative electrode after long-term storage, meaning that there is no loss of lithium in the active region. Therefore, the difference in total discharge capacity between the reference battery and the test battery is the amount of lithium flowing from the active region to the excess region of the negative electrode, and all other possible effects are canceled out.

[0012] As a preferred embodiment of the present invention, the magnitude of the first constant current in S2 and S4 is in the range of 0.3-0.5C.

[0013] In a preferred embodiment of the present invention, the magnitude of the second constant current in S5 is within the range of 3-5C, the certain time range is within 80-96h, and the magnitude of the cutoff current is 0.001C. S5 performs discharge operations on the reference battery pack and the test battery pack respectively. The first discharge uses a large current of 3-5C to discharge the battery at a constant current until the minimum rated voltage (0% SOC) is reached, and the resulting discharge capacity is Q1. The second discharge uses a constant voltage discharge within 80-96h, with a discharge cutoff current of 0.001C, and the resulting discharge capacity is Q2. If the time range for the second discharge exceeds 96h, it will prolong the test time and increase the test cost. If it is less than 80h, the lithium flow inside the battery has not reached equilibrium, resulting in a large experimental error. Q1 + Q2 is the total discharge capacity Q of the battery. The difference in the total discharge capacity Q measured between the test battery pack and the reference battery pack shows the influence of the excessive size of the negative electrode on the actual capacity of the lithium-ion battery.

[0014] Therefore, the present invention has the following beneficial effects: it proposes a test method that effectively evaluates the impact of excessive negative electrode size on the actual capacity of lithium-ion batteries, and provides an important technical reference for battery health status assessment and battery management system design. Attached Figure Description

[0015] Figure 1 This is a flowchart of the testing method of the present invention;

[0016] Figure 2 This is a schematic diagram of the negative electrode excess region of the lithium-ion battery of the present invention;

[0017] Among them, 1. negative electrode excess area; 2. positive electrode plate. Detailed Implementation

[0018] The present invention will now be further described with reference to the accompanying drawings and specific embodiments.

[0019] like Figure 1As shown, a method for testing the effect of negative electrode excess size on the capacity of a lithium-ion battery includes the following steps: S1: Prepare several lithium-ion batteries and group them according to the negative electrode excess size, dividing them into test battery groups and reference battery groups; S2: Charge the lithium-ion batteries of the test battery groups under constant voltage conditions with a first constant current; S3: Let the lithium-ion batteries of the test battery groups rest for several days under certain conditions. When the voltage of the lithium-ion batteries drops during the resting process, float charge the batteries with a float charging current; S4: Charge the lithium-ion batteries of the reference battery groups under constant voltage conditions with a first constant current; S5: Discharge the lithium-ion batteries of the test battery groups and the reference battery groups for the first time with a second constant current until the minimum rated voltage is reached, obtaining the discharge capacity Q1 of each group of lithium-ion batteries; then, perform a second constant voltage discharge on the lithium-ion batteries within a certain time range until the current reaches the cutoff current, obtaining the discharge capacity Q2; S6: Calculate the sum of the discharge capacity Q1 and the discharge capacity Q2 to obtain the total discharge Q of the lithium-ion batteries and calculate the discharge capacity difference. The design of the negative electrode excess size in lithium-ion batteries has a significant impact on battery performance. The area opposite the positive electrode in a lithium-ion battery is the negative electrode active region, and the portion extending beyond the positive electrode is the negative electrode excess region 1. When the lithium-ion battery has a high state of charge (SOC), lithium ions flow from the active region into the negative electrode excess region; conversely, at a low SOC, lithium ions flow back into the active region from the negative electrode excess region 1. During charging, the potential difference between the active region and the negative electrode excess region 1 is small, and the time required for lithium ions to flow into the negative electrode excess region 1 is longer than that required for the discharge process. Therefore, a longer measurement time is needed. This paper proposes a method to test the impact of negative electrode excess size on the capacity of lithium-ion batteries. Lithium-ion batteries are grouped and tested according to their negative electrode excess size. By adding a high-rate discharge step, the total discharge volume Q of the lithium-ion battery is obtained, and the discharge capacity difference is calculated, effectively evaluating the impact of negative electrode excess size on the actual capacity of lithium-ion batteries.

[0020] The standard for preparing lithium-ion batteries in S1 is as follows: the positive electrode material is any one of the commercially available lithium-ion battery positive electrode materials; the negative electrode material is graphite or silicon-carbon composite material; the conductive agent is one or more of carbon black, carbon nanotubes, conductive fibers, and conductive graphite; the battery design capacity is in the range of 50-60 Ah; the number of positive electrode sheets is 20; the number of negative electrode sheets is 21; the battery length is in the range of 400-500 mm; and the battery width is in the range of 90-110 mm. The positive electrode material used in the lithium-ion battery prepared in this invention is any one of the currently available commercially available lithium-ion battery positive electrode materials, including lithium iron phosphate, layered transition metal oxides, lithium manganese oxide, and lithium manganese iron phosphate, etc.

[0021] The grouping standard for the battery packs tested in S1 is as follows: they are divided into n groups according to the excess size of the negative electrode, namely A1 group, A2 group, ..., A... nGroups, each group corresponding to a1mm, a2mm, ..., a n mm, then divide each group into m groups, namely A n1 Group,A n2 Group,...,A nm The test battery packs are divided into n×m groups, where n and m are both positive integers; the grouping criteria for the reference battery packs in S1 are as follows: they are divided into n groups according to the excess size of the negative electrode, namely group B1, group B2, ..., group B... n Groups, each group corresponding to a1mm, a2mm, ..., a n mm, then divide each group into m groups, namely B n1 Group B n2 Group, ..., B nm There are n×m groups of reference batteries, where n and m are both positive integers. This invention sets up test battery groups and reference battery groups. The discharge capacity is obtained and the difference in discharge capacity is calculated by performing corresponding operations on the test battery groups and reference battery groups. Unlike the test battery, the reference battery does not need to undergo S3 (long-term storage). The reference battery does not experience the long-term storage of lithium ions, and therefore there is no process of lithium flowing from the active region into the negative electrode excess region 1; that is, there is no loss of lithium in the active region. Therefore, the total discharge capacity difference between the reference battery and the test battery is the amount of lithium flowing from the active region into the negative electrode excess region 1, and all other possible effects are canceled out.

[0022] The charging operation of S2 is as follows: A n1 Group,A n2 Group,...,A nm The lithium-ion batteries in the group were charged to α1% SOC, α2% SOC, ..., α m %SOC, where α m Not greater than 100. In actual testing, the state of charge (SOC) of a lithium-ion battery can be set by the implementer according to the battery material system and actual needs.

[0023] Under certain conditions in S3, specifically at 30-40℃, the float charging current is within the range of 0.02-0.05C. The lithium-ion batteries in the test group exhibit voltage drops during storage. Float charging the batteries with a small current of 0.02-0.05C ensures maximum lithium filling in the excess negative electrode area. Excessively low storage temperatures hinder lithium-ion flow and prolong testing time, while excessively high temperatures can lead to rapid irreversible self-discharge. At high temperatures, a series of side reactions occur inside the lithium-ion battery, causing an irreversible decrease in the capacity of both the positive and negative electrodes, thus resulting in higher experimental errors.

[0024] The charging operation of S4 is as follows: B n1Group B n2 Group, ..., B nm The lithium-ion batteries in the group were charged to α1% SOC, α2% SOC, ..., α m %SOC, α m The value is no greater than 100. During long-term storage, lithium ions flow from the active region of the negative electrode to the excess region 1 of the negative electrode in the test battery pack. However, the amount of lithium flowing in cannot be directly measured. During subsequent discharge, the lithium embedded in the excess region of the negative electrode cannot flow back to the active region in a short time, resulting in a loss of subsequent discharge capacity. To eliminate the influence of the battery's internal relaxation process and internal resistance, a reference battery needs to be added. Unlike the test battery pack, the reference battery pack does not need to be stored for a long time. The reference battery, with an initial SOC of 0%, is charged at a constant current and constant voltage rate of 0.3-0.5C to the same level as the test battery pack. Then, it is immediately subjected to the same discharge operation as the test battery pack. The reference battery pack does not experience the process of lithium ions flowing from the active region to the excess region 1 of the negative electrode after long-term storage, that is, there is no loss of lithium in the active region. Therefore, the difference in total discharge capacity between the reference battery and the test battery is the amount of lithium flowing from the active region to the excess region 1 of the negative electrode. All other possible effects are canceled out.

[0025] The magnitude of the first constant current in S2 and S4 is in the range of 0.3-0.5C.

[0026] The magnitude of the second constant current in S5 is within the range of 3-5C, and the time range is within 80-96 hours, with a cutoff current of 0.001C. S5 performs discharge operations on the reference and test battery packs. The first discharge uses a high current of 3-5C to discharge the battery to the minimum rated voltage (0% SOC), and the resulting discharge capacity is Q1. The second discharge uses a constant voltage discharge within 80-96 hours, with a discharge cutoff current of 0.001C, and the resulting discharge capacity is Q2. If the time range of the second discharge exceeds 96 hours, it will prolong the test time and increase the test cost. If it is less than 80 hours, the lithium flow inside the battery has not reached equilibrium, resulting in a large experimental error. Q1 + Q2 is the total discharge capacity Q of the battery. The difference in the total discharge capacity Q measured between the test and reference battery packs shows the influence of the excess size of the negative electrode on the actual capacity of the lithium-ion battery.

[0027] The present invention will be further described below with reference to a specific embodiment:

[0028] First, a large-size stacked soft-pack lithium-ion power battery was prepared. The positive electrode material was lithium iron phosphate, the negative electrode material was graphite, and the conductive agents were conductive fibers and conductive graphite. The battery design capacity was 50Ah, with 20 positive electrode sheets and 21 negative electrode sheets. The battery length was 500mm and the width was 110mm. While maintaining a consistent positive electrode design, the negative electrode excess dimensions were designed to be 1mm, 3mm, and 5mm. Then, soft-pack lithium-ion batteries were trial-produced according to normal procedures. The fresh batteries after capacity testing were grouped, and the negative electrode excess dimensions of the test battery packs were designed to be 1mm, 3mm, and 5mm. Each type of negative electrode excess dimension lithium-ion battery pack was further divided into 3 groups. The test battery packs were charged at a constant current and constant voltage rate of 0.4C to 30%, 50%, and 100% SOC, respectively. Afterward, the test battery packs were placed in an environment of 30-40℃ for 3-5 days. The grouping of the reference battery packs was the same as that of the test battery packs. If you don't manufacture lithium-ion batteries yourself, but instead choose them directly from the market, you won't know the size of the negative electrode excess region 1, and the design conditions and material systems will not be exactly the same, affecting the test results.

[0029] During the storage process, the battery was kept at a constant state of charge. When the battery voltage dropped during storage, it was float-charged with a small current of 0.03C to ensure maximum lithium filling in the excess negative electrode area. Then, the reference battery pack was charged at a constant current and constant voltage rate of 0.4C to 30%, 50%, and 100% SOC, respectively, corresponding to the test battery pack. Immediately afterward, both the test and reference battery packs were discharged at a high current of 4C to the minimum rated voltage, i.e., 0% SOC. The resulting discharge capacity was Q1. The battery was then subjected to constant voltage discharge for 80-96 hours with a discharge cutoff current of 0.001C, resulting in a discharge capacity of Q2. Q1 + Q2 is the total discharge capacity Q of the battery. The specific principle is as follows: During long-term storage, in experimental batteries with 30%, 50%, and 100% SOC, lithium ions will flow from the active region of the negative electrode to the excess region 1 of the negative electrode. However, the amount of lithium flowing in cannot be directly measured. During subsequent 4C high-current discharge, the lithium embedded in the excess region of the negative electrode cannot flow back to the active region in a short time, resulting in a loss of subsequent discharge capacity. To eliminate the influence of the battery's internal relaxation process and internal resistance, a reference battery needs to be added. Unlike the test battery, the reference battery does not need to be stored for a long time. The reference battery with an initial SOC of 0% is charged at a constant current and constant voltage rate of 0.4C to 30% SOC, and then immediately discharged at a high current rate of 4C to the minimum rated voltage, i.e., 0% SOC. The resulting discharge capacity is Q1. The battery is then subjected to constant voltage discharge within 80-96 hours, with a discharge cutoff current of 0.001C, resulting in a discharge capacity of Q2. Q1 + Q2 is the total discharge capacity Q of the reference battery. The total discharge capacity Q of the reference battery, charged to 50% SOC and 100% SOC, was measured using the same method. The reference battery did not experience the process of lithium ions flowing from the active region to the excess negative electrode region during long-term storage; that is, there was no loss of lithium in the active region. Therefore, the difference in total discharge capacity ΔQ between the reference battery and the experimental battery represents the amount of lithium flowing from the active region to the excess negative electrode region, and all other possible effects are offset.

[0030] Table 1 shows the total discharge capacity difference for different negative electrode excess sizes. It can be seen that as the negative electrode excess size increases from 1 mm to 5 mm, the total discharge capacity loss increases after resting under different states of charge. When the negative electrode excess size is 5 mm, the reversible capacity loss at 100% SOC reaches 4.18%. This value is significant for the battery management system's assessment of the actual discharge capacity of lithium-ion batteries. In the lithium-ion battery system used in this invention, the negative electrode excess size should not exceed 3 mm.

[0031] Table 1. Calculation results of the impact of different negative electrode excess size designs on actual discharge capacity.

[0032]

[0033] The above results demonstrate that the excessive size design of the negative electrode has a significant impact on the actual capacity of lithium-ion batteries. The method proposed in this invention can effectively evaluate this impact, has a wide range of applications, and can be used to evaluate the actual discharge capacity of all types of commercial lithium-ion batteries under different operating conditions (SOC, temperature, etc.), thereby reducing the actual error of the battery management system.

[0034] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions conceived without creative effort should be covered within the scope of protection of the present invention.

Claims

1. A method for testing the effect of excess negative electrode size on the capacity of a lithium-ion battery, characterized in that, Includes the following steps: S1: Prepare several lithium-ion batteries and group them according to the negative electrode excess size, dividing them into test battery groups and reference battery groups; S2: Charge the lithium-ion batteries of the test battery pack under constant voltage conditions with a first constant current, wherein the magnitude of the first constant current is in the range of 0.3-0.5C. S3: The lithium-ion batteries of the test battery pack are left to rest for several days under certain conditions. When the voltage of the lithium-ion batteries drops during the resting process, the batteries are float-charged using a float charging current. S4: Charge the lithium-ion battery of the reference battery pack under constant voltage conditions with a first constant current. S5: The lithium-ion batteries of the test battery pack and the lithium-ion batteries of the reference battery pack are discharged for the first time with a second constant current. The magnitude of the second constant current is in the range of 3-5C. The discharge is carried out to the minimum rated voltage. The first discharge is carried out with a large current of 3-5C to discharge the battery to the minimum rated voltage, i.e., 0% SOC. The discharge capacity Q1 of each lithium-ion battery pack is obtained. Then, the lithium-ion batteries are discharged for the second time with constant voltage within a certain time range until the current reaches the cutoff current. The second discharge is carried out with constant voltage discharge of the battery pack within 80-96 hours. The cutoff current of the discharge is 0.001C. The discharge capacity Q2 is obtained. S6: Calculate the sum of discharge capacity Q1 and discharge capacity Q2 to obtain the total discharge amount Q of the lithium-ion battery and calculate the discharge capacity difference.

2. The method for testing the effect of excess negative electrode size on the capacity of a lithium-ion battery according to claim 1, characterized in that, The standard for preparing lithium-ion batteries in S1 is as follows: the positive electrode material is any one of the commercial lithium-ion battery positive electrode materials, the negative electrode material is graphite or silicon-carbon composite material, the conductive agent is one or more of carbon black, carbon nanotubes, conductive fibers and conductive graphite, the battery design capacity is in the range of 50-60Ah, the number of positive electrode sheets is 20, the number of negative electrode sheets is 21, the battery length is in the range of 400-500mm, and the battery width is in the range of 90-110mm.

3. The method for testing the effect of excess negative electrode size on the capacity of a lithium-ion battery according to claim 1, characterized in that, The grouping standard for the test battery packs in S1 is as follows: they are divided into n groups according to the excess size of the negative electrode, namely A1 group, A2 group, ..., A... n Groups, each group corresponding to a1mm, a2mm, ..., a n mm, then divide each group into m groups, namely A n1 Group,A n2 Group,...,A nm There are n×m groups, where n and m are both positive integers, and the total number of test battery groups is n×m.

4. The method for testing the effect of excess negative electrode size on lithium-ion battery capacity according to claim 1, characterized in that, The charging operation of S2 specifically involves: charging A... n1 Group,A n2 Group,...,A nm The lithium-ion batteries in the group were charged to α1% SOC, α2% SOC, ..., α m %SOC, where α m No more than 100.

5. The method for testing the effect of excess negative electrode size on the capacity of a lithium-ion battery according to claim 1, characterized in that, Under certain conditions in S3, namely 30-40℃, the magnitude of the float charge current is in the range of 0.02-0.05C.

6. The method for testing the effect of excess negative electrode size on the capacity of a lithium-ion battery according to claim 1, characterized in that, The charging operation of S4 specifically involves: charging B... n1 Group B n2 Group, ..., B nm The lithium-ion batteries in the group were charged to α1% SOC, α2% SOC, ..., α m %SOC, where α m No more than 100.

7. The method for testing the effect of excess negative electrode size on the capacity of a lithium-ion battery according to claim 1, characterized in that, The grouping standard for the reference battery pack in S1 is as follows: it is divided into n groups according to the excess size of the negative electrode, namely group B1, group B2, ..., group B... n Groups, each group corresponding to a1mm, a2mm, ..., a n mm, then divide each group into m groups, namely B n1 Group B n2 Group, ..., B nm There are n×m groups, where n and m are both positive integers, and the reference battery packs consist of n×m groups in total.

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