CN112363075A

CN112363075A - Lithium ion battery aging evaluation method

Info

Publication number: CN112363075A
Application number: CN201911149141.9A
Authority: CN
Inventors: 宫娇娇; 资小林; 许梦清; 高明霞
Original assignee: Wanxiang Group Corp; Wanxiang A123 Systems Asia Co Ltd
Current assignee: Wanxiang A123 Systems Asia Co Ltd
Priority date: 2019-11-21
Filing date: 2019-11-21
Publication date: 2021-02-12
Anticipated expiration: 2039-11-21
Also published as: CN112363075B

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a method for evaluating aging of a lithium ion battery, which comprises the following steps: dividing a plurality of identical lithium ion batteries with initial SOC of 100% into two groups, and carrying out an aging test at the same temperature; the calendar aging test group is placed for N days, the comprehensive aging test group is charged and discharged periodically, the circulation working condition of each period is designed according to the actual service condition of the battery, and the circulation lasts for N days; performing capacity test every M days; respectively calculating the capacity attenuation rate delta Q of the calendar aging test group and the comprehensive aging test group_CalendarAnd Δ Q_{General assembly}(ii) a By Delta Q_{General assembly}Minus Δ Q_CalendarObtaining the attenuation rate Delta Q of the cyclic aging capacity_{Circulation of}. The evaluation method can separately research the calendar aging and the cyclic aging, and the cyclic aging adopts the cyclic working condition which is more in line with the reality, the analysis result is more in line with the reality, and the evaluation method can be used for estimating the service life of the lithium ion battery and researching the agingMechanism and selection of optimal operating conditions.

Description

Lithium ion battery aging evaluation method

Technical Field

The invention relates to the technical field of lithium ion batteries, in particular to an evaluation method for aging of a lithium ion battery.

Background

The lithium ion battery is the most potential energy source for the development of the hybrid electric vehicle and the pure electric vehicle at present, the performance of the lithium ion battery directly influences the performance of the whole vehicle, and the price and the service life of the lithium ion battery are the most important influencing factors for the economy of the electric vehicle. The lithium ion battery can meet the requirements of the electric automobile on power, energy and capacity, and has obvious advantages compared with other types of batteries in the aspects of safety and environmental protection. However, the contradiction between the limited service life of the lithium ion battery and the high replacement cost becomes a bottleneck problem of the lithium ion battery technology, and further the large-scale popularization of the electric automobile is limited. The main causes of aging of lithium ion batteries are loss of positive and negative electrode materials and loss of active lithium. Wherein, the loss of the anode and cathode materials means that the anode and cathode active materials are isolated or dissolved; causes of active lithium loss include the generation of an interfacial film (SEI), the consumption of recyclable lithium by thickening, and lithium plating induced when a battery is charged under severe conditions such as a high rate or a low temperature. The research on the aging process of the lithium ion battery is beneficial to correctly estimating the service life of the lithium ion battery, selecting the best practical operation condition and is one of the important ways for improving the performance of the power lithium ion battery.

The aging of the lithium ion battery mainly comprises two types: cyclic aging and calendar aging. The cycle aging refers to aging caused when the battery is charged and discharged, and is determined by measuring the capacity (energy) retention rate during continuous charge and discharge cycles; calendar aging refers to the irreversible loss of capacity generated during storage of a battery, and is determined by measuring the capacity (energy) retention rate of the battery at a certain state of charge or at rest of the battery state of charge (SOC). Most of the time when the electric automobile is actually used stays in a parking lot, so the aging of the lithium ion battery is determined by two aging mechanisms. In order to analyze the attenuation of the lithium ion battery more accurately, it is necessary to separately study the superimposed calendar aging and cycle aging, but the current report only analyzes one mechanism alone, cannot meet the actual use requirement of the electric vehicle, and has certain technical limitations. In addition, in the prior art, the lithium ion battery cycle test is to continuously charge and discharge the battery by adopting a fixed current within a certain SOC range, but the method is not in line with the actual use working condition of the electric automobile, for example, the electric automobile can have a relatively long rest after being charged and discharged for a certain time.

For example, chinese patent with application number CN201811131200.5 discloses a method and an apparatus for estimating the life of a lithium ion battery, which are used to determine the cycle life of the lithium ion battery with a specific failure probability value under the same cycle life experimental condition; chinese patent application No. CN201610384214.2 discloses a method for testing calendar life of a power lithium ion battery, which can be used for analyzing the influence of different shelf conditions (charge state, temperature, etc.) on the calendar life of the lithium ion battery. Both patents have studied only one aging mechanism.

Chinese patent application No. CN201710381912.1 discloses a method for predicting the life of a power battery, which comprises: determining a battery capacity attenuation stage according to the service life attenuation rate of the battery monomer; establishing a capacity attenuation model of capacity retention rate, chemical reaction rate and time; combining a battery capacity attenuation stage to obtain a cycle life attenuation model and a calendar life attenuation model; training a cycle life attenuation model and a calendar life attenuation model by using the measurement data of the single battery, and determining parameters in the models; generating a cycle life attenuation curve and a calendar life attenuation curve according to the cycle life attenuation model and the calendar life attenuation model; and superposing the two curves according to a preset proportion to obtain a battery life prediction curve. The cell measurement data includes: testing data obtained by testing the cycle working condition of the battery monomer after the battery monomer is circularly charged and discharged for N times under different charging and discharging currents, temperatures and discharging depths; and (4) obtaining test data obtained by calendar life test after the battery monomer is kept stand for M months at different temperatures and SOC. The battery life prediction method separately studies calendar aging and cycle aging, but the cycle life attenuation condition is data obtained by continuously charging and discharging the battery, and is not in line with the actual use condition of the power battery.

Disclosure of Invention

In order to solve the technical problem, the invention provides an evaluation method for aging of a lithium ion battery. The method realizes effective separation of calendar aging and cycle aging, wherein the cycle aging adopts a cycle working condition which is more in line with the actual service condition of the battery, so that the analysis result is more practical, and important technical support can be provided for estimation of the service life of the lithium ion battery, research of an aging mechanism and selection of the optimal operation condition.

The specific technical scheme of the invention is as follows:

a method for evaluating the aging of a lithium ion battery comprises the following steps:

(1) dividing a plurality of lithium ion batteries with the same specification and initial SOC of 100% into two groups, wherein the number of the batteries in each group is more than or equal to 2, and respectively carrying out calendar aging test and comprehensive aging test at the same experiment temperature;

(2) calendar aging test group: the batteries of the group are placed for N days, wherein N is a natural number greater than 0;

(3) comprehensive aging test group: the battery of the battery pack is periodically charged and discharged, the cycle working condition of each cycle is designed according to the actual use condition of the lithium ion battery, intermittent charging and discharging are carried out in the same cycle, and the cycle lasts for N days, wherein N is a natural number greater than 0;

(4) respectively carrying out capacity test on two groups of batteries every M days, wherein M is a natural number greater than 0, and the specific process of the capacity test is as follows:

(a) fully charging and discharging the battery in a constant temperature environment, measuring the electric quantity discharged by the battery, namely the battery capacity, taking an average value after circulating for multiple times to obtain the capacity of a single battery, and respectively calculating the average value Q of the capacity of each group of batteries_nWherein N is the number of days of testing, and N is more than or equal to 0 and less than or equal to N;

(b) recovering the SOC of the battery to a state before the capacity test;

(5) calculating a capacity fade rate Δ Q:

wherein Q is_nMeasured for the nth dayBattery capacity of (Q)₀Is the initial battery capacity; respectively calculating capacity attenuation rates of the two groups of batteries, and recording the capacity attenuation rates of the calendar aging test group as delta Q_CalendarAnd the capacity attenuation rate of the comprehensive aging test group is recorded as delta Q_{General assembly}；

(6) Calculating net cyclic aging capacity decay Rate DeltaQ_{Circulation of}：ΔQ_{Circulation of}＝ΔQ_{General assembly}-ΔQ_Calendar。

By the evaluation method of the invention, the capacity attenuation rate delta Q caused by calendar aging can be obtained_CalendarCapacity fade rate Δ Q due to cyclic aging_{Circulation of}And actual total capacity decay Rate DeltaQ_{General assembly}. For most batteries, the capacity decline of the batteries has obvious rules along with the change of time, so the battery life can be predicted by analyzing the change relations of calendar aging, cycle aging and actual total capacity decay along with time. By calculating Δ Q_CalendarAnd Δ Q_{Circulation of}At Δ Q_{General assembly}The influence of calendar aging and cyclic aging on the actual capacity attenuation of the lithium ion battery can be respectively evaluated according to the percentage of the total amount of the lithium ion battery, and important technical support is provided for the research of the aging mechanism of the lithium ion battery. By designing different cycle conditions and comparing the attenuation conditions of the battery capacity under different cycle conditions, the influence of different operating conditions (including the discharge depth, the charge-discharge current, the charge-discharge frequency and the like) on the attenuation of the battery capacity in the same time can be evaluated, the optimal operating condition can be selected, and the service life of the battery can be prolonged.

In the prior art, the cyclic aging test is performed by continuously charging and discharging the battery, but the cyclic aging rates of the batteries with different aging conditions are different, and the cyclic aging test result is different from the actual cyclic aging test result due to the fact that the cyclic working condition is not consistent with the actual use condition. In actual use, the lithium ion battery of the electric automobile is charged and discharged discontinuously, the lithium ion battery is in a shelf state for most of time, and the actual total capacity attenuation is the superposition of cyclic aging and calendar aging, so that the evaluation method provided by the invention passes through delta Q_{General assembly}And Δ Q_CalendarSubtracting to obtain Δ Q_{Circulation of}Wherein Δ Q_{General assembly}The test adopts a test which is more consistent with a power batteryCyclic behavior of the actual use case, so calculated Δ Q_{Circulation of}Will better meet the actual situation. In the evaluation method of the present invention, the capacity fading caused by calendar aging during the shelf life and during the charge and discharge was measured as the capacity fading at the shelf life of 100% SOC, i.e., Δ Q_CalendarSince the SOC of the battery is not 100% during charge and discharge and during the interval between charge and discharge, Δ Q_CalendarThere is a certain difference from the actual, but the difference is to Δ Q_{Circulation of}The resulting error is much smaller than in the prior art. In short, the error in the cyclic aging test performed with the prior art results from the effect of the aging conditions on the cyclic aging rate, which the present invention eliminates, and introduces another much smaller error resulting from the effect of the SOC on the calendar aging rate.

Preferably, in the step (1), the experimental temperature is 15-55 ℃.

Temperature is an important stress affecting the capacity fade of lithium ion batteries. The recommended use temperature of the battery is generally 15-35 ℃, and the limit temperature in actual use is 55 ℃; when the experimental temperature is 45-55 ℃, the aging of the battery can be accelerated and the experimental time can be effectively shortened due to the accelerated corrosion of the surface of the anode material, the accelerated consumption of active lithium caused by the instability of an interfacial film (SEI) and the like. The battery life at different temperatures has a certain relationship, so that the battery life at the actual use temperature can be estimated according to the battery life at a certain experimental temperature.

Preferably, in the step (a), the battery is fully charged at a rate of 0.3 to 1.2C and a voltage of 2.6 to 4.2V.

Preferably, in the step (a), the number of the cycles is 2 to 3.

The experimental error can be reduced by taking the average value in multiple cycles, but if the cycle times are too many, the accuracy of the experimental result is influenced because the battery is aged in the charging and discharging process.

Preferably, in the step (b), the SOC of the battery is returned to a state before the capacity test at a rate of 0.3 to 0.5C.

The reason for charging at the rate of 0.3-0.5C is that the high-rate charging is easy to cause lithium plating and negative electrode material loss, so that the increase of the internal resistance and the reduction of the capacity of the battery are accelerated, and the service life of the battery is shortened, so that in order to avoid the influence on the aging of the battery, the charging current for restoring the SOC to the state before the test after the capacity test is finished cannot be too large; too low a charging current will result in too long a charging time.

Preferably, in the step (3), the battery is charged and discharged at a rate of 0.2 to 1.0C.

The charging and discharging current of 0.2-1.0C accords with the actual use condition of the lithium ion battery of the electric automobile.

Preferably, in the step (3), the cycle condition is that the battery is fully charged with a constant voltage and a constant current for 2-3 times in one day when the day is finished, the battery is discharged to 50-70% of SOC for the first time, the battery is discharged to 30-40% of SOC for the second time, and the charging and discharging current is 0.2-0.5C.

Preferably, in the step (3), the cycle condition is that the battery is fully charged with constant voltage and constant current once every day, the battery is discharged once a day until the SOC is 30-70%, and the charging and discharging current is 0.2-0.5C.

Preferably, in the step (3), the cycle condition is that continuous charging and discharging are carried out for 5-8 times in one day, the charging and discharging current is 0.8-1.0C, and the battery is fully charged at a constant voltage and a constant current after each day.

Preferably, in the step (3), the cycle is performed by fully charging the battery once at constant voltage and constant current at the end of each week, discharging the battery 5 times in a week, discharging the battery to 10-30% SOC for the last time, and controlling the charging and discharging current to be 0.2-0.5C.

The four circulation working conditions can better simulate the actual service condition of the battery, so that the circulation aging test result is more practical.

Compared with the prior art, the invention has the following advantages:

(1) the calendar aging and the cyclic aging are effectively separated, and the practical use condition of the lithium ion battery of the electric automobile is met;

(2) the cyclic aging research adopts a cyclic working condition which is more in line with the reality, so that the analysis result is more in line with the reality;

(3) the method can provide important technical support for the estimation of the service life of the lithium ion battery, the research of an aging mechanism and the selection of the optimal operating condition.

Drawings

FIG. 1 is a graph showing the cycle aging capacity fading rate of the batteries of groups #2 to #5 in examples and comparative examples 1 to 3 as a function of the number of cycles;

FIG. 2 is a graph of SOC versus time over a period of a cyclic condition in accordance with the present invention;

FIG. 3 is a graph of SOC versus time over a period of another cyclic condition in accordance with the present invention;

FIG. 4 is a graph of SOC versus time over a period of another cyclic condition of the present invention;

FIG. 5 is a graph of SOC versus time over a period of another cyclic condition of the present invention;

FIG. 6 is a graph of the calendar aged capacity fade rate of the batteries of group #1 as a function of shelf time in the example;

FIG. 7 is a graph showing the actual total capacity fade rate of the cells of groups #2 to #5 according to the number of cycles in the example.

Detailed Description

The present invention will be further described with reference to the following examples.

Examples

(1) 30 lithium ion batteries with the same specification and initial SOC of 100% are divided into two groups, the number of the batteries in the calendar aging test group (marked as group #1) and the number of the batteries in the comprehensive aging test group are respectively 6 and 24, and aging tests are respectively carried out at 55 ℃.

(2) Calendar aging test group: the batteries of the group were left for 210 days.

(3) Comprehensive aging test group: the cells were divided into 4 groups (referred to as groups #2 to #5), each of which had 6 cells. The 4 groups of batteries are periodically charged and discharged according to 4 circulation working conditions shown in figures 2 to 5 and are continuously circulated for 210 days. Wherein, curve 1 in fig. 2 represents that the battery is fully charged with a constant voltage and a constant current at the end of each day, and is discharged for 2 times in one day, the first time is discharged to 60% SOC, the second time is discharged to 40% SOC, and the charging and discharging current is 0.3C; curve 2 in fig. 3 shows that at the end of each day, the battery is fully charged with a constant voltage and a constant current, discharged once a day, and the charging and discharging current is 0.3C, which is different from fig. 2 in that the depth of discharge is small and the battery is discharged only to 70% SOC; curve 3 in fig. 4 represents 6 consecutive charges and discharges with a charge and discharge current of 1.0C, each discharge to 60% SOC, each charge to 90% SOC, full charge at constant voltage and constant current at the end of each day; curve 4 in fig. 5 shows that at the end of each week, the battery was fully charged at a constant voltage and constant current, and was discharged 5 times in a week, and was finally discharged to 30% SOC, with a charge-discharge current of 0.3C.

(4) The capacity test was performed every 2 days (i.e., day 0, day 2, day 4 … …) on the batteries of groups #1 to #4, and every 7 days (i.e., day 0, day 7, day 14 … …) on the battery of group #5, according to the following specific procedures:

(a) fully charging the battery at normal temperature with a multiplying power of 0.6C and a voltage of 3.4V, measuring the electric quantity discharged by the battery, namely the battery capacity, taking an average value after circulating for 2 times, namely the capacity of a single battery, and respectively calculating the average value Q of the capacities of each group of batteries_nWherein n is the number of days of testing, and n is more than or equal to 0 and less than or equal to 210;

(b) the SOC of the battery was recovered to a state before the capacity test at a rate of 0.4C.

(5) Calculating a capacity fade rate Δ Q:

wherein Q is_nBattery capacity, Q measured for the nth day₀Is the initial battery capacity. Respectively calculating the capacity attenuation rate of each group of batteries, and recording the capacity attenuation rate of the calendar aging test group as delta Q_CalendarAnd the capacity attenuation rate of the comprehensive aging test group is recorded as delta Q_{General assembly}。

(6) Respectively calculating net cyclic aging capacity attenuation rate delta Q under each cyclic working condition_{Circulation of}：ΔQ_{Circulation of}＝ΔQ_{General assembly}-ΔQ_CalendarWherein, Δ Q_{General assembly}The capacity attenuation rate of the experimental group under the circulation condition in the comprehensive aging test group is shown.

The capacity fade rate due to calendar aging, the capacity fade rate due to cyclic aging, and the actual total capacity fade rate are respectively Δ Q_Calendar、ΔQ_{Circulation of}And Δ Q_{General assembly}. By the above method, the following conclusions can be reached:

(1) the calendar aging, the cycle aging and the change relation of the actual total capacity attenuation along with the time are respectively shown in fig. 6, fig. 1 and fig. 7, and on the basis, a battery life attenuation model is established, so that important reference data can be provided for estimating the battery life;

(2) compared with Delta Q_CalendarIn other words,. DELTA.Q_{Circulation of}At Δ Q_{General assembly}The percentage of the aging agent is larger, which shows that the influence of the cyclic aging on the actual capacity attenuation of the lithium ion battery is larger, and thus, an important technical support can be provided for the research of the aging mechanism of the lithium ion battery;

(3) as shown in FIG. 7, the actual total capacity fade rate Δ Q of group #3 among groups #2 to #5 was calculated by the same number of days in the cycle_{General assembly}And the minimum value indicates that the battery is aged slowest under the second circulation working condition in the four circulation working conditions adopted in the experiment, and the service life of the battery can be effectively prolonged under the second circulation working condition.

Comparative example 1

The method for testing the cyclic aging capacity decay rate in the prior art comprises the following steps:

(1) taking 6 lithium ion batteries with the same specification as the embodiment, continuously charging and discharging at a multiplying power of 0.3C, discharging to 40% SOC each time, charging to 100% SOC each time, only 5min interval between the charging stage and the discharging stage, and recording each charging and discharging as a cycle.

(2) The capacity test was performed every 2 cycles (i.e., 0 th, 2 nd, 4 th … …), and the specific procedure was as follows:

(a) fully charging the battery at normal temperature with a multiplying power of 0.6C and a voltage of 3.4V, measuring the electric quantity discharged by the battery, namely the battery capacity, taking an average value after circulating for 2 times, namely the capacity of a single battery, and respectively calculating the average value Q of the capacities of each group of batteries_nWherein n is the number of test cycles;

(3) Calculating the cyclic aging capacity attenuation rate Delta Q_{Circulation of}：

Wherein Q is_nCell capacity, Q, measured for the nth cycle₀Is the initial battery capacity.

Comparative example 2

(1) taking 6 lithium ion batteries with the same specification as the embodiment, continuously charging and discharging at a multiplying power of 0.3C, discharging to 70% SOC each time, charging to 100% SOC each time, only 5min interval between the charging stage and the discharging stage, and recording each charging and discharging as a cycle.

Comparative example 3

(1) taking 6 lithium ion batteries with the same specification as the embodiment, carrying out continuous charging and discharging at the multiplying power of 1C, discharging to 60% SOC each time, charging to 100% SOC each time, only 5min interval between the charging stage and the discharging stage, and recording each charging and discharging as a cycle.

Comparative example 4

The aging evaluation of the lithium ion battery is carried out according to the following steps:

(1) 18 lithium ion batteries with the same specification and 30% of initial SOC (state of charge) are divided into two groups, the number of the batteries in the calendar aging test group (marked as group #6) and the number of the batteries in the comprehensive aging test group are respectively 6 and 12, and aging tests are respectively carried out at 55 ℃.

(3) Comprehensive aging test group: the group of cells was divided into 2 groups (identified as group #7 and group #8, respectively), of 6 cells each. The 2 groups of batteries are periodically charged and discharged according to 2 circulation working conditions shown in fig. 2 and fig. 3, and are continuously circulated for 210 days. Wherein, curve 1 in fig. 2 represents that the battery is fully charged with a constant voltage and a constant current at the end of each day, and is discharged for 2 times in one day, the first time is discharged to 60% SOC, the second time is discharged to 40% SOC, and the charging and discharging current is 0.3C; curve 2 in fig. 3 shows that at the end of each day, the battery is fully charged with a constant voltage and a constant current, discharged once a day, and the charge-discharge current is 0.3C, which differs from fig. 2 in that the depth of discharge is small and only the discharge reaches 70% SOC.

(4) The batteries of group #1, group #7 and group #8 were subjected to a capacity test every 2 days (i.e., day 0, day 2, day 4 … …) as follows:

(5) Calculating a capacity fade rate Δ Q:

TABLE 1

Measured Delta Q of comparative examples 1 to 3_{Circulation of}Δ Q measured after the same number of cycles as groups #2 to #4 in examples_{Circulation of}Are compared and based on

The deviation between the two is calculated. As can be seen from FIG. 1 and Table 1, the Δ Q measured in the comparative example after the same number of cycles_{Circulation of}Are all smaller than the corresponding experimental group in the examplesObtained Delta Q_{Circulation of}And the deviation is not less than 28%, which shows that the measured delta Q is obtained by the evaluation method of the present invention_{Circulation of}Measured delta Q compared to prior art solutions_{Circulation of}There is a large difference between them.

TABLE 2

Further, to verify the influence of the error source of the present invention, SOC affecting calendar aging rate, on the evaluation results, the measured Δ Q of group #7 and group #8 in comparative example 4_{Circulation of}Δ Q measured after the same number of cycles as in the example of group #2 and group #3, respectively_{Circulation of}Are compared and based on

The deviation between the two is calculated. As can be seen from Table 2, the Δ Q measured in comparative example 4 after the same number of cycles_{Circulation of}Are all larger than the delta Q measured by the corresponding experimental groups in the examples_{Circulation of}But the deviations are all less than 6.1%. The experimental result shows that SOC can affect the calendar aging rate, thereby causing errors on the test of the cyclic aging in the evaluation method, but SOC can cause the delta Q_{Circulation of}The error caused by the method is very small, and compared with the prior art, the accuracy of the cyclic aging test method provided by the invention is obviously improved.

The raw materials and equipment used in the invention are common raw materials and equipment in the field if not specified; the methods used in the present invention are conventional in the art unless otherwise specified.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and all simple modifications, alterations and equivalents of the above embodiments according to the technical spirit of the present invention are still within the protection scope of the technical solution of the present invention.

Claims

1. A method for evaluating aging of a lithium ion battery is characterized by comprising the following steps:

(b) recovering the SOC of the battery to a state before the capacity test;

(5) calculating a capacity fade rate Δ Q:

wherein Q is_nBattery capacity, Q measured for the nth day₀Is the initial battery capacity; respectively calculating capacity attenuation rates of the two groups of batteries, and recording the capacity attenuation rates of the calendar aging test group as delta Q_CalendarAnd the capacity attenuation rate of the comprehensive aging test group is recorded as delta Q_{General assembly}；

2. The method for evaluating aging of lithium ion battery according to claim 1, wherein in the step (1), the experimental temperature is 15-55 ℃.

3. The method for evaluating aging of lithium ion battery according to claim 1, wherein in the step (a), the battery is fully discharged at a voltage of 2.6-4.2V at a rate of 0.3-1.2C.

4. The method according to claim 1, wherein in the step (a), the number of cycles is 2-3.

5. The method for evaluating aging of lithium ion batteries according to claim 1, wherein in the step (b), the SOC of the batteries is recovered to a state before the capacity test at a rate of 0.3-0.5C.

6. The method for evaluating aging of a lithium ion battery according to claim 1, wherein in the step (3), the battery is charged and discharged at a rate of 0.2 to 1.0C.

7. The method according to claim 1, wherein in the step (3), the cycle is performed by fully charging the lithium ion battery at a constant voltage and a constant current at the end of each day, discharging the battery 2-3 times in a day, discharging the battery to 50-70% SOC for the first time, discharging the battery to 30-40% SOC for the second time, and controlling the charging and discharging current to 0.2-0.5C.

8. The method for evaluating aging of lithium ion battery according to claim 1, wherein in step (3), the cycle condition is that the battery is fully charged with constant voltage and constant current at the end of each day, the battery is discharged once a day to 30-70% SOC, and the charging and discharging current is 0.2-0.5C.

9. The method for evaluating the aging of the lithium ion battery according to claim 1, wherein in the step (3), the cycle condition is that continuous charging and discharging are carried out for 5 to 8 times in a day, the charging and discharging current is 0.8 to 1.0 ℃, and the lithium ion battery is fully charged with a constant voltage and a constant current at the end of each day.

10. The method according to claim 1, wherein in the step (3), the cycle is performed by fully charging at constant voltage and constant current at the end of each week, discharging 5 times in a week, discharging the last time to 10-30% SOC, and controlling the charging and discharging current to 0.2-0.5C.