A method for judging optimal liquid injection amount of lithium ion battery

Abstract

The application provides a judgment method for optimal injection amount of lithium ion battery, which comprises the following steps: providing a plurality of un-injected battery cells, injecting electrolyte into the un-injected battery cells according to an injection coefficient to obtain a plurality of injected battery cells; performing at least one step of aging, formation and degassing on the injected battery cells, and performing an alternating current impedance test on the injected battery cells after each step ends, judging a minimum injection coefficient according to the impedance value, and determining the optimal injection amount according to the minimum injection coefficient. The judgment method provided by the application is a non-destructive analysis method, which does not need to disassemble and damage the appearance of the battery cell, the data is intuitive and reliable, the related test equipment is simple, the analysis cost is low, and the method is simple and practical.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of batteries and relates to a method for judging optimal electrolyte injection amount of a lithium ion battery. BACKGROUND

[0002] With the continuous development of society, people have an increasing demand for lithium ion batteries with advantages of high energy density, safety and reliability, and no pollution, and an increasing requirement for battery performance. Electrolyte provides a channel for lithium ion transfer in the battery and is a bridge connecting the positive and negative electrodes. The electrolyte lithium salt in the electrolyte is a source of lithium ions, which ensures that there are enough lithium ions in the positive and negative electrodes during the charging and discharging cycle, so as to realize reversible circulation. Therefore, it is necessary to ensure that there is a sufficient amount of electrolyte in the battery to maintain the operation of the battery.

[0003] In the industrial production of large batteries, the electrolyte injection amount not only affects the wetting rate of the electrode and the separator, but also limits the capacity of the battery and affects the service life of the battery. The electrolyte injection amount is a difficult qualitative parameter, and the influence of the electrolyte injection amount in the actual battery on the cycle performance of the battery is not a linear relationship. Excessive electrolyte consumption will lead to a decrease in the overall energy density of the battery and increase the production cost of the battery invisibly, and the internal space will lead to an increase in the internal pressure of the battery, which has a certain risk of liquid leakage. The influence of insufficient electrolyte on the performance of the battery is fatal, which causes the internal resistance of the battery to be too high, the cycle life to rapidly decay in the later stage, and in severe cases, lithium precipitation will occur with the cycle, which will cause safety risks.

[0004] CN110148793A discloses a method for judging the electrolyte infiltration state of a lithium ion battery, which comprises the following steps: (1) configuring a colored infiltration agent, then adding the colored infiltration agent into the electrolyte, and mixing uniformly to obtain colored electrolyte; and (2) injecting the colored electrolyte into the battery, and disassembling the battery when the battery completes the processes of primary injection, formation and secondary injection, and judging the electrolyte infiltration state by whether the color area distribution on the separator and the electrode is uniform, so as to adjust the primary injection amount, the secondary injection amount and the formation process. This method needs to disassemble the battery, and the test and analysis cost is high, and the electrolyte infiltration state is judged by naked eye observation, so the result is subjective.

[0005] CN109546070B discloses a method for determining the electrolyte injection amount of a lithium battery. The electrolyte injection of the lithium battery is divided into primary injection and secondary injection. First, different primary injection amounts are designed for experiments, the formation gas production amount is determined, the residual electrolyte amount of the disassembled battery is determined, and the appropriate primary injection amount is formulated. Then, different secondary injection amounts are experimented, the residual electrolyte amount of the disassembled battery is determined, the subsequent gas production amount is determined, and the appropriate secondary injection amount is formulated. This method also needs to disassemble the battery, and the test and analysis cost is high.

[0006] CN106159346A discloses a complete set of calculation method of lithium ion battery injection amount, according to the true density of various raw materials used in lithium ion battery, the theoretical porosity is calculated, and then the actual injection amount of electrolyte is obtained according to the density of electrolyte and the calculated theoretical porosity. However, the theoretical injection amount will have a certain deviation from the actual battery design, the fluctuation of positive and negative plate coating amount, and the porosity of different positive and negative main materials and separators have great influence on the calculation result of theoretical value, so the value of theoretical calculation is only for reference.

[0007] Therefore, there is an urgent need for a judgment method of lithium ion battery injection amount with low test cost and strong data reliability. SUMMARY

[0008] In view of the deficiencies in the prior art, the purpose of the present application is to provide a judgment method of optimal injection amount of lithium ion battery. The judgment method provided by the present application is a non-destructive analysis method, which does not need to disassemble and damage the appearance of the battery, the data is intuitive and reliable, the related test equipment is simple, the analysis cost is low, and it is simple and practical.

[0009] To achieve this purpose, the present application adopts the following technical solutions:

[0010] The present application provides a judgment method of optimal injection amount of lithium ion battery, which comprises the following steps:

[0011] (1) providing a plurality of un-injected battery cells, injecting electrolyte into the un-injected battery cells according to the injection coefficient, and obtaining a plurality of injected battery cells;

[0012] (2) performing at least one of aging, formation and degassing on the plurality of injected battery cells, and performing AC impedance test on the injected battery cells after each step, determining the minimum injection coefficient according to the impedance value, and determining the optimal injection amount according to the minimum injection coefficient.

[0013] The present application provides a judgment method of optimal injection amount of lithium ion battery, which performs AC impedance test on the injected battery cells at different stages, and determines the optimal injection amount of the battery cells by the impedance value. The judgment method combines multiple processes to comprehensively determine the injection amount, so as to solve the problem of error in single process determination. Moreover, the judgment method is a non-destructive analysis method, which does not need to disassemble and damage the appearance of the battery, the data is intuitive and reliable, the related test equipment is simple, the analysis cost is low, and it is simple and practical.

[0014] It should be noted that the "several" in the "several un-injected battery cells" of the present application refers to at least 3, for example, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20, etc.

[0015] The optimal liquid injection amount is determined by the high-frequency impedance value obtained by the electrochemical impedance spectroscopy (EIS), and the high-frequency impedance value (HFR) is the intercept of the high-frequency region (>1 kHz) of the EIS curve and the real axis.

[0016] As a preferred technical solution of the present application, the injection amount of the electrolyte = injection coefficient * un-injected cell capacity.

[0017] Preferably, the injection coefficient is distributed in a gradient manner.

[0018] Preferably, the injection coefficient is an arithmetic sequence.

[0019] Preferably, the change amount of each gradient of the injection coefficient is 0.05-0.5 g / Ah, for example, it can be 0.05 g / Ah, 0.1 g / Ah, 0.15 g / Ah, 0.2 g / Ah, 0.25 g / Ah, 0.3 g / Ah, 0.35 g / Ah, 0.4 g / Ah, 0.45 g / Ah or 0.5 g / Ah, etc.

[0020] Preferably, the injection coefficient is controlled in the range of 1-4 g / Ah, for example, it can be 1 g / Ah, 1.2 g / Ah, 1.4 g / Ah, 1.6 g / Ah, 1.8 g / Ah, 2 g / Ah, 2.2 g / Ah, 2.4 g / Ah, 2.6 g / Ah, 2.8 g / Ah, 3 g / Ah, 3.2 g / Ah, 3.4 g / Ah, 3.6 g / Ah, 3.8 g / Ah or 4 g / Ah, etc.

[0021] In the present application, the gradient of the injection coefficient can be adjusted according to the verification requirements, and the gradient range and precision can be freely designed. For the cell that roughly verifies the injection amount, the designed gradient change amount and range can be enlarged, and for the cell that needs to accurately determine the optimal injection amount, the designed gradient change amount and range can be reduced.

[0022] As a preferred technical solution of the present application, in step (2), the several injected cells are sequentially subjected to the aging, formation and degassing steps, and the high-frequency impedance value of the injected cell after the aging step is recorded as HFR w ;

[0023] The high-frequency impedance value of the injected cell after the formation step is recorded as HFR f ;

[0024] The high-frequency impedance value of the injected cell after the degassing step is recorded as HFR d ;

[0025] HFR w , HFR f and HFRd a high frequency resistance average value, denoted as HFR ave , according to HFR ave determines the minimum liquid injection coefficient.

[0026] In the present application, the multi-step impedance average value is calculated to reduce the test error and increase the judgment accuracy.

[0027] As a preferred technical solution of the present application, the determination of the minimum liquid injection coefficient according to HFR ave is performed in the following manner: obtaining the HFR ave of the battery corresponding to each liquid injection coefficient, plotting a graph with the liquid injection coefficient as the horizontal coordinate and the high frequency resistance average value as the vertical coordinate, determining the inflection point with the absolute value of the slope less than 0.05 in the graph, the horizontal coordinate of the inflection point being the minimum liquid injection coefficient, and the liquid injection amount corresponding to the inflection point being the minimum liquid injection amount.

[0028] The plotted graph is not specifically limited in the present application, and can be a line graph or a fitted curve graph.

[0029] In the present application, the inflection point can be determined from the plotted graph, and the impedance sharply decreases before the inflection point and gently decreases after the inflection point. After the determination of the inflection point, the minimum liquid injection coefficient is obtained.

[0030] When the plotted graph is a line graph, the inflection point is one of the designed liquid injection coefficients.

[0031] As a preferred technical solution of the present application, the determination of the optimal liquid injection amount according to the minimum liquid injection coefficient is performed in the following manner:

[0032] The batteries with the liquid injection coefficient greater than or equal to the minimum liquid injection coefficient are screened out for cycling, and after the cycling step is completed, the high frequency resistance value of the battery with the liquid injection is denoted as HFR c , the ratio of the HFR c of the battery corresponding to each liquid injection coefficient to HFR ave is calculated and denoted as K, the liquid injection coefficient corresponding to the minimum K value being the optimal liquid injection coefficient, and the liquid injection amount corresponding to the optimal liquid injection coefficient being the optimal liquid injection amount.

[0033] In the present application, when the horizontal coordinate of the inflection point is one of the designed liquid injection coefficients, the batteries with the liquid injection coefficient greater than or equal to the horizontal coordinate of the inflection point are screened out for cycling; when the horizontal coordinate of the inflection point is not a designed liquid injection coefficient, the batteries with the liquid injection coefficient greater than the horizontal coordinate of the inflection point are screened out for cycling.

[0034] The present application only performs the cycling test on the screened batteries, which can improve the test efficiency and save the test cost.

[0035] In the process of judging the liquid injection amount, a series of liquid injection coefficients with a wide range and a large gradient change amount can be designed, and the high-frequency resistance average value HFR ave The relationship diagram of the liquid injection coefficient is screened, and a series of liquid injection coefficients with a small gradient change amount are designed based on the screened part of the liquid injection coefficient, and analysis and testing are performed. This is conducive to quickly screening the optimal liquid injection amount of the lithium ion battery.

[0036] In order to improve the accuracy, a plurality of battery cells can be tested for each liquid injection coefficient, and the average value is taken to judge.

[0037] Preferably, the test parameters of the cycle are: first constant current charging at 0.1-1.2C, for example, 0.1C, 0.2C, 0.3C, 0.5C, 0.7C, 0.8C, 0.9C, 1C, 1.1C or 1.2C, etc., then constant voltage charging at 0.01-0.08C, for example, 0.01C, 0.02C, 0.03C, 0.04C, 0.05C, 0.06C, 0.07C or 0.08C, etc., and then constant current discharging at 0.1-1.2C after charging is completed, for example, 0.1C, 0.2C, 0.3C, 0.5C, 0.7C, 0.8C, 0.9C, 1C, 1.1C or 1.2C, etc.

[0038] Preferably, the end condition of the cycle is that the capacity retention rate of the liquid injected battery cell decreases to 75-85%, for example, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84% or 85%, etc.

[0039] As a preferred technical solution of the present application, the aging temperature is 40-50℃, for example, 40℃, 41℃, 42℃, 43℃, 44℃, 45℃, 46℃, 47℃, 48℃, 49℃ or 50℃, etc.

[0040] Preferably, the aging time is 22-26h, for example, 22h, 23h, 24h, 25h or 26h, etc.

[0041] As a preferred technical solution of the present application, after the formation step is completed, the liquid injected battery cell is discharged to 2-2.6V, for example, 2V, 2.1V, 2.2V, 2.3V, 2.4V, 2.5V or 2.6V, etc., and then the alternating current impedance test is performed.

[0042] Preferably, an aging step is further provided between the formation and degassing.

[0043] Preferably, the temperature of the aging is 40-50℃, for example, it can be 40℃, 41℃, 42℃, 43℃, 44℃, 45℃, 46℃, 47℃, 48℃, 49℃ or 50℃, etc.

[0044] Preferably, the time of the aging is 12-14h, for example, it can be 12h, 12.5h, 13h, 13.5h or 14h, etc.

[0045] As a preferred technical solution of the present application, the degassing and the circulation are further provided with a step of separate capacity.

[0046] Preferably, the temperature of the circulation is 20-30℃, for example, it can be 20℃, 21℃, 22℃, 23℃, 24℃, 25℃, 26℃, 27℃, 28℃, 29℃ or 30℃, etc.

[0047] Preferably, after the step of the circulation, the liquid injected battery cell is discharged to 2-2.6V, for example, it can be 2V, 2.1V, 2.2V, 2.3V, 2.4V, 2.5V or 2.6V, etc., and then the AC impedance test is performed.

[0048] As a preferred technical solution of the present application, the test frequency of the AC impedance test is 1-10 5 Hz.

[0049] Preferably, the amplitude of the AC signal of the AC impedance test is 8-12mV, for example, it can be 8mV, 9mV, 10mV, 11mV or 12mV, etc.

[0050] As a preferred technical solution of the present application, the judging method specifically comprises the following steps:

[0051] (I) providing a plurality of non-liquid injected battery cells, injecting electrolyte into the non-liquid injected battery cells according to a gradient distribution of liquid injection coefficient to obtain a plurality of liquid injected battery cells;

[0052] (II) the plurality of liquid injected battery cells are sequentially subjected to aging, formation and degassing, and the liquid injected battery cells are subjected to AC impedance test after the end of each step; after the end of the aging step, the high-frequency impedance value of the liquid injected battery cell is recorded as HFR w ; after the end of the formation step, the high-frequency impedance value of the liquid injected battery cell is recorded as HFR f ; after the end of the degassing step, the high-frequency impedance value of the liquid injected battery cell is recorded as HFR d , and the minimum liquid injection coefficient is determined according to the average value of the high-frequency impedance of HFR w , HFR f and HFR d .

[0053] (III) Select batteries with an injection coefficient greater than or equal to the minimum injection coefficient for cycling. After the cycling step is completed, the high-frequency impedance value of the injected battery cell is recorded as HFR. c Calculate the HFR of the battery corresponding to each injection coefficient. c With HFR ave The ratio of is denoted as K. The injection coefficient corresponding to the smallest K value is the optimal injection coefficient, and the injection volume corresponding to the optimal injection coefficient is the optimal injection volume.

[0054] However, this does not apply to all values ​​listed; other unlisted values ​​within this range also apply.

[0055] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0056] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0057] This invention provides a method for determining the optimal electrolyte injection amount in lithium-ion batteries. It involves performing AC impedance testing on electrolyte-filled cells at different stages, and using the impedance values ​​to determine the optimal electrolyte injection amount. This method combines multiple processes to comprehensively determine the electrolyte injection amount, thus addressing the error problem inherent in single-process judgments. Furthermore, this method is a non-destructive analysis method, requiring no disassembly or damage to the cell's appearance. The data is intuitive and reliable, the relevant testing equipment is simple, the analysis cost is low, and it is simple and practical. Attached Figure Description

[0058] Figure 1 A graph showing the relationship between high-frequency impedance value and injection coefficient provided for embodiments of the present invention.

[0059] Figure 2 A graph showing the relationship between the average high-frequency impedance and the injection coefficient provided in an embodiment of the present invention. Detailed Implementation

[0060] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0061] Example

[0062] This embodiment provides a method for determining the optimal electrolyte injection volume for a lithium-ion battery, the method specifically including the following steps:

[0063] S1: Take a 3Ah battery cell that is sealed on three sides, with one side having an injection port and has passed the baking test. Calculate the theoretical injection volume m, where m = injection coefficient * uninjected cell capacity. The 6 sets of injection coefficients are shown in Table 1. Weigh the cell m1 before injection and weigh the cell m2 after injection. Calculate the actual injection volume m' = m2 - m1, so that m' = m. After injection, seal the injection port of the cell. Each set of injection coefficients corresponds to 3 cells that have been injected.

[0064] S2: Place the electrolyte-filled cells in a high-temperature oven at 45±5℃ for 24 hours. Perform AC impedance testing (EIS) on the aged electrolyte-filled cells and record the high-frequency impedance values. Take the average of the high-frequency impedance values ​​of the three cells corresponding to each electrolyte filling coefficient to obtain the HFR corresponding to each electrolyte filling coefficient. w All subsequent steps shall be performed in the same manner;

[0065] S3: Perform a formation step on the filled battery cells. After formation, discharge the cells to 2.5V and perform EIS testing to obtain the HFR corresponding to each group of filling coefficients. f ;

[0066] S4: After aging the electrolyte-filled cells at 45℃ for 12 hours, a degassing step is performed. The degassed cells are then subjected to EIS testing to obtain the HFR corresponding to each group of electrolyte filling coefficients. d The high-frequency impedance values ​​of the electrolyte-filled cells are summarized in Figure 1 middle;

[0067] S5: Calculate the average high-frequency impedance HFR ave =(HFR w +HFR f +HFR d A line graph was plotted with the injection coefficients of the six groups as the x-axis and the average high-frequency impedance as the y-axis, as shown below. Figure 2 As shown, the inflection point in the graph where the absolute value of the slope is less than 0.05 is determined. The x-coordinate of the inflection point is the minimum injection coefficient, which is 2.6 g / Ah. The injection volume corresponding to the inflection point is the minimum injection volume.

[0068] S6: Batteries with a liquid filling coefficient greater than or equal to the minimum liquid filling coefficient were selected. The selected liquid filling coefficients were 2.6g / Ah, 3.0g / Ah, and 3.4g / Ah. These batteries underwent capacity testing. After capacity testing, they were subjected to cycle testing at 25℃. The test conditions were: first, 1C constant current charging, followed by C / 20 constant voltage charging. After charging, they were discharged at 1C constant current. The cycle test was continued until 80% capacity retention was achieved before removal from the grid. Finally, the batteries were discharged to 2.5V for EIS testing to obtain the HFR corresponding to each liquid filling coefficient. c ;

[0069] S7: Calculate HFR of the battery corresponding to 2.6 g / Ah, 3 g / Ah and 3.4 g / Ah c The ratio of HFR ave to the ratio of HFR 5 , denoted as K, the calculated K value is shown in Table 2, the minimum K value corresponds to the optimal injection coefficient, the optimal injection coefficient is 3.0 g / Ah, the injection amount corresponding to the optimal injection coefficient is the optimal injection amount.

[0070] The test conditions of EIS in this embodiment are: test frequency 1-10 5 Hz, AC signal amplitude 10 mV.

[0071] Table 1

[0072] Group Coefficient of injection Injection amount A 1.4 g / Ah 4.2g B 1.8 g / Ah 5.4g C 2.2 g / Ah 6.6g D 2.6 g / Ah 7.8g E 3.0 g / Ah 9.0g F 3.4 g / Ah 10.2g

[0073] Table 2

[0074] Group HFR c ]]> HFR ave ]]> K D 14.8 mΩ 6.6 mΩ 2.242 E 13.9 mΩ 6.5 mΩ 2.138 F 14.6 mΩ 6.3 mΩ 2.317

[0075] Applicants declare that the above description is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and those skilled in the art should understand that any changes or replacements within the technical scope disclosed by the present application can be easily thought of by any person skilled in the art, and all of them fall within the protection scope and disclosure scope of the present application.

Claims

1. A method for determining the optimal electrolyte injection volume for a lithium-ion battery, characterized in that, The judgment method comprises the following steps: (1) providing a plurality of un-injected battery cells, injecting electrolyte into the un-injected battery cells according to injection coefficients to obtain a plurality of injected battery cells; (2) a plurality of liquid injected battery cells are sequentially subjected to aging, formation and degassing, and an alternating current impedance test is performed on the liquid injected battery cells after the end of each step, the high frequency impedance value of the liquid injected battery cell after the end of the aging step is recorded as HFR w , the high frequency impedance value of the liquid injected battery cell after the end of the formation step is recorded as HFR f , the high frequency impedance value of the liquid injected battery cell after the end of the degassing step is recorded as HFR d , the high frequency impedance average value of HFR w , HFR f and HFR d is calculated, the high frequency impedance average value is recorded as HFR ave , the minimum liquid injection coefficient is determined according to HFR ave , and the optimal liquid injection amount is determined according to the minimum liquid injection coefficient; The determination of the optimal injection amount according to the minimum injection coefficient is performed in the following manner: The battery with the injection coefficient greater than or equal to the minimum injection coefficient is subjected to a cycle, and the high-frequency impedance value of the battery after the cycle is recorded as HFR c The ratio of the HFR c of the battery corresponding to each injection coefficient to the HFR ave is calculated and recorded as K, the injection coefficient corresponding to the minimum K value is the optimal injection coefficient, and the injection amount corresponding to the optimal injection coefficient is the optimal injection amount.

2. The judging method according to claim 1, characterized by The temperature of the cycle is 20-30℃; The test parameters of the cycle are: first constant current charging at 0.1-1.2C, then constant voltage charging at 0.01-0.08C, and after the charging is completed, constant current discharging at 0.1-1.2C; The end condition of the cycle is that the capacity retention rate of the injected battery cell decreases to 75-85%.

3. The method of claim 2, wherein After the cycle step is completed, the injected battery cell is discharged to 2-2.6V, and then AC impedance testing is performed; The test frequency of the alternating current impedance test is 1~10 5 Hz; The amplitude of the AC signal of the AC impedance testing is 8-12mV.

4. The judging method according to claim 1, characterized by The injection amount of the electrolyte = injection coefficient * un-injected battery cell capacity; The injection coefficient is in a gradient distribution; the change amount of each gradient is 0.05-0.5g / Ah; The injection coefficient is an arithmetic sequence; the injection coefficient is controlled within the range of 1-4g / Ah.

5. The method of claim 1, wherein The HFR according to the HFR ave The determination of the minimum liquid injection coefficient is performed in the following manner: obtaining the HFR of the battery cell after liquid injection corresponding to each liquid injection coefficient ave Drawing a graph with the liquid injection coefficient as the horizontal coordinate and the average high-frequency impedance as the vertical coordinate, determining the inflection point with the absolute value of the slope less than 0.05 in the graph, the horizontal coordinate of the inflection point being the minimum liquid injection coefficient, and the liquid injection amount corresponding to the inflection point being the minimum liquid injection amount.

6. The method of claim 1, wherein The temperature of the aging is 40-50℃, and the time of the aging is 22-26h.

7. The method of claim 1, wherein After the formation step is completed, the injected battery cell is discharged to 2-2.6V, and then AC impedance testing is performed.

8. The method of claim 1, wherein The aging step is further provided between the formation and the degassing; The temperature of the aging is 40-50℃, and the time of the aging is 12-14h.

9. The method of claim 1, wherein The capacity grading step is further provided between the degassing and the cycle.

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