Limit current confirmation method for lithium ion battery cell

Abstract

The invention discloses a limiting current confirmation method of a lithium ion battery cell, and belongs to the field of new energy batteries. Comprising the following steps: determining a plurality of platform sections of the lithium ion battery cell by taking SOC as a segmentation standard; at the normal temperature, constant-current charging with different multiplying power and time control is carried out in different platform sections, then constant-voltage charging is carried out until the current is reduced to a preset threshold value, finally constant-current discharging is carried out, and charging and discharging data are collected in the process; according to the charging and discharging data, whether the lithium separation risk exists in the battery cell at the moment is judged so as to confirm the limiting current of the lithium ion battery cell in each platform section at the normal temperature; and calculating the limiting current of the lithium ion battery cell in each platform section at other temperatures according to the limiting current and the constant-current charging temperature rise at the normal temperature. According to the invention, the state of lithium precipitation of the battery cell is judged in multiple directions according to the change rate among the self capacity, voltage and temperature change of the battery cell so as to confirm the limiting current, the reliability is higher, the efficiency is higher, and the precision is higher.

Description

Technical Field

[0001] The present invention belongs to the field of new energy batteries. Specifically, the present invention relates to a method for confirming the limiting current of a lithium-ion battery cell. Background Art

[0002] Currently, there are higher requirements for the charging time in application terminals such as cars, commercial vehicles, and heavy trucks. Especially for vehicle batteries used for operation, the requirements for charging time are more stringent. Therefore, higher challenges are posed to the charging methods of vehicle batteries. The prerequisite for determining a good charging method is to clearly understand the limiting charging current of the battery cell at different temperatures and different SOCs. Generally, the industry defaults the initial lithium plating point as the limiting point. Currently, the common methods for determining the lithium plating critical point of the battery cell are either making three electrodes or disassembling the battery cell, both of which require wasting a large number of battery cells and involve extremely large workloads.

[0003] A lithium-ion battery cell is a secondary battery cell that can be repeatedly charged and discharged. It is mainly composed of anode and cathode electrode sheets, a separator, an electrolyte, mechanical parts, etc. For vehicle battery cells used for operation, the capacity and energy density are generally relatively large, making it inconvenient to make three electrodes. Moreover, the excellent rate of special three electrodes is low and the sample safety is low, seriously affecting the experimental accuracy and safety guarantee. Using the method of disassembling the battery to confirm lithium plating has a large workload and low efficiency. Confirming lithium plating through the internal resistance method, the biggest drawback of this method is its low accuracy. The main reasons are as follows: First, it has high requirements for the connection method between the battery cell and the device. The internal resistance difference caused by improper connection may exceed the internal resistance change of the battery cell itself. Second, it has high requirements for the white noise of the test environment. Third, the internal resistance of large battery cells is small itself, and the internal resistance change caused by lithium plating is also small. Therefore, it is very difficult to directly judge the lithium plating critical point.

[0004] Therefore, the present invention newly proposes a method for confirming the limiting current of a lithium-ion battery cell. Summary of the Invention

[0005] The present invention aims to overcome the deficiencies of the prior art and proposes a method for confirming the limiting current of a lithium-ion battery cell to achieve the following objectives: Using the change rates among the capacity, voltage, and temperature changes of the battery cell itself to comprehensively judge whether the battery cell is in a lithium plating state and then confirm the limiting current, with higher reliability, higher efficiency, and higher accuracy.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: A method for confirming the limiting current of a lithium-ion battery cell, the method comprising:

[0007] Step S1: Confirm multiple platform segments of the lithium-ion battery cell using SOC as the segmentation standard;

[0008] Step S2: At room temperature, perform constant current charging controlled by time at different rates in different platform segments, then switch to constant voltage charging until the current drops to a preset threshold, and finally perform constant current discharging. During the process, collect charge and discharge data, including constant current charging capacity, constant voltage charging capacity, constant current discharging capacity, and constant current charging temperature rise. According to the charge and discharge data, determine whether there is a risk of lithium plating in the battery cell at this time to confirm the limiting current of the lithium-ion battery cell in each platform segment at room temperature.

[0009] Step S3: Based on the limiting current and constant current charging temperature rise at room temperature, estimate the limiting current of the lithium-ion battery cell in each platform segment at other temperatures.

[0010] Preferably, step S1 includes:

[0011] At room temperature, perform multiple cycles of constant current and constant voltage charge and discharge tests on the lithium-ion battery cell;

[0012] During the test process, collect the voltage V, charge Q, and SOC data of the battery cell in real time;

[0013] According to the collected data, plot the dQ / dV curve with dQ / dV as the ordinate and SOC as the abscissa, and confirm multiple platform segments of multiple lithium-ion battery cells according to the trend of the curve. Each peak in the dQ / dV curve represents the starting point or ending point of a platform segment.

[0014] Preferably, during the multiple cycles of constant current and constant voltage charge and discharge tests on the lithium-ion battery cell, the charging current range is between 0.05C and 0.5C.

[0015] Preferably, step S2 includes:

[0016] Step S21: For any platform segment, the starting SOC of the platform segment is A, and the ending SOC is B; perform constant current charging at a charging rate C k The constant current charging time is T1; k represents the natural ordinal number, and the larger k is, the larger the charging rate C k T1 represents the time for the battery cell to be charged from A to B at the charging rate C k under the true capacity of the battery cell;

[0017] Step S22: After T1 time ends, maintain the current voltage for constant voltage charging;

[0018] Step S23: After constant voltage charging until the current drops to the preset current lower limit, discharge the battery to completion at the preset discharge rate;

[0019] Step S24: Repeat steps S21 to S23 multiple times, and collect the charge and discharge data for each cycle, including constant current charging capacity, constant voltage charging capacity, constant current discharging capacity, and constant current charging temperature rise;

[0020] Step S25: According to the charge and discharge data during each cycle, determine whether there is a risk of lithium plating in the battery cell at this time. If there is, use the previous charging rate C k-1 as the limiting current of the battery cell under the current platform segment. If not, use the charging rate C k to replace C k+1 , and then repeat steps S21 to S25 as described above.

[0021] Preferably, the judgment conditions for whether there is a risk of lithium plating in the battery cell include:

[0022] Condition 1: The constant voltage charging capacity increases in n consecutive charge and discharge cycles, and the increase amplitude each time is greater than a preset first amplitude. n represents the preset number of consecutive charge and discharge cycles;

[0023] Condition 2: The constant current discharge capacity decreases in m consecutive charge and discharge cycles, and the decrease amplitude each time is greater than a preset second amplitude. m represents the preset number of consecutive charge and discharge cycles;

[0024] Condition 3: The linear coefficient between lnq and lnI is less than a preset threshold; q represents heat; I represents the charging current;

[0025] If any two of the above Conditions 1 to 3 are satisfied, it is determined that there is a risk of lithium plating in the battery cell at this time.

[0026] Preferably, step S2 further includes: After using the confirmed limiting current of the lithium-ion battery cell in each platform segment to perform multiple constant current and constant voltage charge and discharge tests of step S2 on the battery cell, collect the charge and discharge data and disassemble and verify the battery cell.

[0027] Preferably, step S2 further includes: On the basis of the confirmed limiting current in each platform segment, increase 0.1C respectively, and then perform multiple constant current and constant voltage charge and discharge tests of step S2 on the battery cell again, collect the charge and discharge data and disassemble and verify the battery cell.

[0028] Preferably, in step S3, the calculation formula for the limiting current of the lithium-ion battery cell in each platform segment at other temperatures is expressed as follows:

[0029] I = x * I0 * (T - T 环境 ) / q0;

[0030] I0 represents the limiting current of the lithium-ion battery cell in any platform segment at room temperature; I represents the limiting current of the corresponding platform segment at other temperatures; T - T 环境 represents the constant current charging temperature rise of the battery cell in the corresponding platform segment, which is a preset fixed value during the calculation process. T represents the actual maximum temperature of the battery cell, T 环境Represents the current ambient temperature; q0 represents the constant current charging temperature rise of the lithium-ion battery cell in the corresponding platform segment at room temperature; x represents the coefficient that changes with temperature T.

[0031] Preferably, the method further includes: step S4, repeating step S2 under low temperature and high temperature extreme environments to obtain the limiting current of the lithium-ion battery cells in each platform segment under low temperature and high temperature; and comparing and verifying with the result estimated in step S3.

[0032] The technical effect of the present invention is as follows: the present invention changes the traditional method of confirming the current of the battery cell, and uses the rate of change of the battery cell's own capacity, voltage, and temperature to judge whether the battery cell is in a state of lithium deposition and then confirm the limiting current. Among them, three conditions are set to judge whether the battery cell is in a state of lithium deposition, avoiding misjudgment caused by errors caused by one of the conditions. The present invention also reversely verifies the confirmed limiting current by disassembling. Compared with the traditional disassembly confirmation method, the present invention theoretically only needs to disassemble one battery cell. Generally, based on the principles of repeatability and reproducibility, it may be necessary to disassemble two or three more batteries for re-inspection, thereby improving the reliability and accuracy of the confirmed limiting current and reducing the cost. The present invention obtains the limiting current at other temperatures by extrapolation, which is more efficient. At the same time, reverse verification under low and high temperature environments is set to ensure the reliability and accuracy of the extrapolated limiting current. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 A flow chart of a method for determining a limiting current of a lithium-ion battery cell provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0034] The following is a further detailed description of the specific implementation of the present invention through the description of the embodiments with reference to the accompanying drawings, the purpose of which is to help those skilled in the art have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention, and to facilitate its implementation. It should be noted that the words "first", "second" and the like described in this application are only for the convenience of describing the technical solution to distinguish different components, and are not used to limit this application. In order to make the technical solution of the present invention clearer, the present invention is explained through the following embodiments.

[0035] As mentioned above, the various preset values ​​described in this application can be flexibly selected according to actual conditions during implementation, and this does not limit this application.

[0036] The present invention provides a method for determining the limiting current of a lithium-ion battery cell, such as Figure 1 As shown, the method includes:

[0037] Step S1, confirming multiple platform segments of lithium-ion batteries using SOC as a segmentation standard;

[0038] Step S2, at room temperature, perform time-controlled constant current charging at different rates in different platform segments, then switch to constant voltage charging until the current drops to a preset threshold, and finally perform constant current discharge. During the process, collect charging and discharging data, including constant current charging capacity, constant voltage charging capacity, constant current discharge capacity, and constant current charging temperature rise; based on the charging and discharging data, determine whether the battery cell has a lithium plating risk at this time to confirm the limiting current of the lithium-ion battery cell in each platform segment at room temperature.

[0039] Step S3, based on the limiting current at room temperature and the temperature rise of constant current charging, the limiting current of the lithium-ion battery cells in each platform segment at other temperatures is estimated.

[0040] In addition, the method of the present invention also includes:

[0041] Step S4: Repeat step S2 under low temperature and high temperature extreme environments to obtain the limiting current of the lithium-ion battery cells in each platform segment under low temperature and high temperature, and compare and verify with the result estimated in step S3. That is, a reverse verification link under low temperature and high temperature environments is set to ensure the reliability and accuracy of the estimated limiting current.

[0042] The embodiments of the present invention are described in detail below with reference to specific embodiments.

[0043] First, prepare 50 200Ah lithium iron phosphate system hard-shell battery cells that have been mass-produced. The product is designed to have a maximum continuous current of 1C.

[0044] In step S1 of this embodiment:

[0045] At room temperature, i.e. 25°C, the lithium-ion battery cells were subjected to multiple cycles of constant current and constant voltage charge and discharge tests, i.e. 3 batteries were randomly selected, and charged and discharged to empty at a constant current and constant voltage of 0.2C at room temperature, and the cycle was repeated 3 times. Among them, the charging current range was limited to between 0.05C and 0.5C, because too small a current for too long would be detrimental to the experimental progress control, and too large a current would cover the platform area, especially for the lithium iron phosphate system, which has a long platform and a narrow voltage range, and a large rate cannot be used.

[0046] During the test, the voltage V, charge Q, and SOC data of the battery cell are collected in real time; based on the collected data, a dQ / dV curve is drawn with dQ / dV as the ordinate and SOC as the abscissa, and multiple platform segments of multiple lithium-ion battery cells are confirmed based on the trend of the curve. Each peak in the dQ / dV curve represents the starting point or end point of a platform segment. In this embodiment, the battery cell is divided into the following platform segments based on the trend of the curve, with SOC as the segmentation standard: N1: 0-5%; N2: 5-30%; N3: 30-60%; N4: 60-90%; N5: 90-100%.

[0047] In step S2 of this embodiment:

[0048] Specifically, in step S21, for any platform segment, the starting point of the state of charge (SOC) of the platform segment is A, and the ending point is B; with a charging rate C k constant current charging is performed, and the constant current charging time is T1; k represents the natural ordinal number, and the larger k is, the larger the charging rate C k is; T1 represents the time for the battery cell to be charged from A to B at the charging rate C k under the true capacity of the battery cell, that is:

[0049] T1 = 1 / C * X, where C is the charging rate and X is the percentage of SOC; for example, when charging 5% SOC at 0.5C, the charging time T1 = 1 / 0.5 * 5% * 60 = 6 min. In this embodiment, first, 3 battery cells are randomly selected, and the limit current of the N1 platform segment is tried to be confirmed starting from half of the continuous maximum current of 1C designed for the product, that is, first, constant current charging is performed at 0.5C for 6 min.

[0050] In step S22, after the time T1 ends, constant voltage charging is performed while maintaining the current voltage. From common knowledge, constant voltage charging can alleviate the lithium plating caused by high-rate charging and ensure the service life of the same battery cell experiment.

[0051] In step S23, after constant voltage charging until the current drops to the preset lower current limit, the battery is discharged at the preset discharge rate. In this embodiment, after 6 minutes, constant voltage charging is performed while maintaining the current voltage. After constant voltage charging until the current is 0.05C, the battery is discharged at a discharge rate of 0.5C.

[0052] In step S24, steps S21 to S23 are repeatedly executed multiple times, and the charge and discharge data of each cycle are collected, including constant current charging capacity, constant voltage charging capacity, constant current discharge capacity, and constant current charging temperature rise. In this embodiment, steps S21 to S23 are repeatedly executed 10 times, and the constant current charging capacity, constant voltage charging capacity, constant current discharge capacity, and constant current charging temperature rise of each cycle are collected.

[0053] In step S25, according to the charge and discharge data of each cycle, it is judged whether there is a risk of lithium plating in the battery cell at this time. Among them, the judgment conditions for whether there is a risk of lithium plating in the battery cell set in this application include:

[0054] Condition 1: The constant voltage charging capacity increases in n (set to 3 times in this embodiment) consecutive charge and discharge cycles, and the increase amplitude each time is greater than the preset first amplitude (set to 10% in this embodiment), where n represents the preset number of consecutive charge and discharge cycles. The normal constant voltage charging capacity will first decrease and then become stable with the number of cycles; for the battery cell with lithium plating, the change trend of its constant voltage charging capacity will first decrease and then increase with the number of cycles.

[0055] Condition 2: The constant-current discharge capacity decreases in each of the consecutive m (set to 3 times in this embodiment) charge-discharge cycles, and the decrease amplitude each time is greater than the preset second amplitude (set to 10% in this embodiment). m represents the preset number of consecutive charge-discharge cycles. The normal discharge capacity will decrease slowly, or be stable, or increase, but the lithium plating capacity will decrease rapidly.

[0056] Condition 3: The linear coefficient between lnq and lnI is less than the preset threshold (set to 0.95 in this embodiment); q represents heat; I represents the charging current. According to the heat generation formula q = I 2 rt, under normal circumstances, the internal resistance will not change significantly, and the time is set. Taking the logarithm of the above formula, it is found that there is a proportional relationship between lnq and lnI.

[0057] If any two of the above Conditions 1 to 3 are satisfied, it is determined that the battery cell has a risk of lithium plating at this time. In this embodiment, three conditions are set to judge the lithium plating state of the battery cell, avoiding misjudgment caused by errors in one of the conditions.

[0058] If there is a risk of lithium plating, the previous charging rate C k-1 is used as the limit current of the battery cell in the current platform segment. If there is no risk of lithium plating, the charging rate C k is replaced with C k+1 and then the steps S21 to S25 are repeated. In this embodiment, it is experimentally measured that at a charging rate of 0.5C, none of the three conditions are met. Therefore, the next current span is increased to 0.8C and the test is continued. The steps S21 to S25 are repeated, and it is found that still none of the conditions are met. Then the current span is increased to 1C for testing. After repeating steps S21 to S25, it is found that only the constant-voltage charging capacity meets the judgment conditions. The current is continued to be increased to 1.1C, and it is found that the constant-voltage charging capacity and the constant-current discharge capacity meet Conditions 1 and 2, so it is determined that there is a risk of lithium plating. At this time, the limit current of the N1 segment is directly set to 1C.

[0059] After the limiting current of the N1 section is confirmed, then steps S21 to S25 are cycled to confirm the limiting current of the N2 section. Among them, since both theoretically and based on historical experience, the current borne by the N2 section is larger than that of the N1 section, so the confirmation starts directly from 1C, which improves the confirmation efficiency. First, charge the lower limit SOC of N2 with the maximum current obtained in the previous step, then perform constant current and constant voltage charging, and then discharge at 0.5C constant current until it is emptied. Repeat this 10 times. Measured through experiments, none of the three conditions are met, so directly repeat steps S21 to S25 at 1.2C. Measured through experiments, only the constant voltage capacity meets the judgment conditions, so continue to conduct experiments at 1.3C. Measured through experiments, still only the constant voltage capacity meets the judgment conditions. Continue to increase to 1.4C for experiments. Measured through experiments, the proportion of the constant voltage capacity and the temperature meet the judgment conditions, so directly define 1.3C as the maximum current of N2.

[0060] Next, explore the limiting current under N3. Since both theoretically and based on historical experience, the current borne by the N3 section is equivalent to that of the N2 section, so the experiment starts directly by subtracting one point from 1.3C, that is, starting from 1.2C. First, charge step by step to the lower limit SOC of N3 with the maximum current obtained in the previous step, then perform constant current and constant voltage charging, and then discharge at 0.5C constant current until it is emptied. Repeat this 10 times. Measured through experiments, only one of the three conditions is met. So directly repeat the experimental steps at 1.3C. Measured through experiments, still only one condition is met. So continue to repeat the experimental steps at 1.4C. Measured through experiments, all three conditions are satisfied, so define the maximum current of N3 as 1.3C.

[0061] Next, explore the limiting current under N4. Since both theoretically and based on historical experience, the current borne by the N4 section is equivalent to that of the N3 section, or only a little different, so the experiment starts directly by subtracting two points from 1.3C, that is, starting from 1.1C. First, charge step by step to the lower limit SOC of N4 with the maximum current obtained in the previous step, then perform constant current and constant voltage charging, and then discharge at 0.5C constant current until it is emptied. Repeat this 10 times. Measured through experiments, none of the three conditions are met. So directly repeat the experimental steps at 1.2C. Measured through experiments, still only one condition is met. So continue to repeat the experimental steps at 1.3C. Measured through experiments, all three conditions are satisfied, so define the maximum current of N4 as 1.2C.

[0062] Subsequently, verify the maximum current of N5. Since according to theory or historical data, the internal resistance in the final stage will rise sharply, so the current in this section is generally less than that of the N1 section. For safety reasons, start the verification from 0.5C. Measured through experiments, no judgment conditions are met at 0.5C, and it is directly increased to 0.8C. Measured through experiments, only one condition is met. Then conduct experiments at 0.9C. Measured through experiments, two conditions are met, so the maximum current of N5 uses 0.8C.

[0063] This embodiment confirms the completion of the limiting currents of segments N1 to N5 in sequence: N1: 1C; N2: 1.3C; N3: 1.3C; N4: 1.2C; N5: 0.8C. Then, this embodiment also uses the limiting currents of the lithium-ion cells in each platform segment after confirmation to cycle the cells 10 times in the constant current and constant voltage charge and discharge test of step S2, and disassembles the cells for verification after collecting the charge and discharge data. Reverse verification is performed by disassembly. Compared with the traditional disassembly confirmation method, the present invention theoretically only needs to disassemble one cell. Generally, based on the principles of repeatability and reproducibility, it may be necessary to disassemble two or three more cells for re-inspection, which improves the reliability and accuracy of the confirmed limiting current while reducing costs.

[0064] In order to further improve the reliability and accuracy of the confirmed limiting current, step S2 of this embodiment also includes: adding 0.1C to the limiting current of each platform segment after confirmation, and then performing the constant current and constant voltage charge and discharge test of step S2 on the battery cell multiple times, collecting the charge and discharge data and disassembling the battery cell for verification, so as to verify the confirmed reliability and accuracy as a comparison.

[0065] In step S3 of this embodiment:

[0066] After confirming the limit current and constant current charging temperature rise at room temperature, the limit current of lithium-ion cells in each platform segment at other temperatures is calculated. The calculation formula is as follows:

[0067] I=x*I0*(TT 环境 ) / q0;

[0068] I0 represents the limiting current of the lithium-ion battery cell in any platform segment at room temperature; I represents the limiting current of the corresponding platform segment at other temperatures; TT 环境 represents the constant current charging temperature rise of the battery cell in the corresponding platform segment, which is a preset fixed value (set to 15°C in this embodiment) during the calculation process, T represents the actual maximum temperature of the battery cell, T 环境 Indicates the current ambient temperature; q0 indicates the constant current charging temperature rise of the corresponding platform segment lithium-ion battery at room temperature; x indicates the coefficient that changes with temperature T, which needs to be obtained through experiments during preliminary identification, and the value range of x is between 0.8 and 1.2. Generally, the internal resistance will decrease at high temperatures and increase at low temperatures. Therefore, for the low temperature segment of 0 to 5°C, the range of x is 1 to 1.2; for the high temperature segment of 50 to 55°C, the range of x is 0.8 to 1.

[0069] The present invention obtains the limiting current at other temperatures by inference, which is significantly more efficient than the traditional method of confirming the limiting current through a large number of experiments. At the same time, in order to ensure the reliability and accuracy of the inferred limiting current, this embodiment also sets up reverse verification under low and high temperature environments, namely:

[0070] Step S4: Under the extreme environments of low temperature and high temperature, repeat the above Step S2 to obtain the extreme currents of the lithium-ion battery cells within each plateau segment at low temperature and high temperature; and compare and verify with the results deduced in the above Step S3.

[0071] To prove the reliability of the method of this embodiment, Comparative Examples 1 and 2 are also set in this application. The matrix diagrams of the extreme currents of each plateau segment at each temperature obtained by the method of this embodiment are shown in Table 1.

[0072]

[0073] Table 1

[0074] Comparative Example 1: 200Ah Lithium Iron Phosphate Aluminum Shell Battery Cell (Three-Electrode Method)

[0075] 1. Take 50 semi-finished lithium iron phosphate products of 200Ah in mass production, bare battery cells and corresponding mechanical parts.

[0076] 2. Unfold the negative electrode corresponding to the outermost positive electrode, then paste a copper wire on the separator directly contacted by the negative electrode. The place where it contacts the electrode plate needs to be covered with the separator. The thickness of the copper wire is about 0.6mm. Too thin copper wire is easy to break, and too thick copper wire will pierce the electrode plate.

[0077] 3. Re-drill holes near the edge of the top cover, lead out the copper wire, and seal it with AB glue or the glue used for pack.

[0078] 4. Other steps are assembled according to the normal assembly process of the battery cell. The speed of this assembly process must be pure manual production.

[0079] 5. After pre-lithiation of the battery cell, it is completely sealed. The success rate of making the three electrodes can be counted in this step, only 67%. It is speculated that most of the defective products are due to the displacement or breakage of the copper wire during the assembly process, and some are due to unsuccessful pre-lithiation. Supplementary note, the specially made three electrodes need to be scrapped after the experiment, otherwise there is a safety risk.

[0080] The experimental process is relatively simple. First, directly carry out constant current charging at different rates in segments of SOC under the condition of 25°C. Note that the SOC segments are from small to large and the rates are also from small to large. When the negative electrode voltage is lower than 0V, it is the lithium deposition window. The matrix diagrams of the extreme currents of each plateau segment at each temperature of this method are shown in Table 2.

[0081]

[0082] Table 2

[0083] Then, the limiting currents confirmed by the method of this embodiment and the limiting current confirmed in Comparative Example 1 are respectively used to perform the same charge and discharge tests on the battery cells and then disassembled for confirmation. It is found that the interfaces of the battery cells obtained in Comparative Example 1 are not always lithium deposition. Instead, there are significant color differences in the middle and edge regions, or bubble-like imprints, indicating that inappropriate current does not necessarily directly cause lithium deposition. It may be due to local current stress concentration that breaks the uniformity of lithium intercalation at the interface.

[0084] Then, the limiting currents confirmed by the method of this embodiment are all increased by 0.1C rate, and at the same time, the limiting current confirmed in Comparative Example 1 is also increased by 0.1C rate. Then, on this basis, the same charge and discharge tests on the battery cells are performed and then disassembled for confirmation. It is found that: a small amount of lithium deposition is confirmed in the battery cells disassembled according to Comparative Example 1, but although there is no lithium deposition in the battery cells disassembled according to this embodiment, there are also abnormalities at the interface. Therefore, the current matrix diagram obtained in this embodiment has higher safety and reliability. Supplementary note, only the rates different from those in Comparative Example 1 are selected for the battery cell rates according to this embodiment, and the same parts are not arranged repeatedly.

[0085] The present invention has been described exemplarily in conjunction with the accompanying drawings. Obviously, the specific implementation of the present invention is not limited by the above methods. As long as various non-substantive improvements are made by adopting the method concept and technical solution of the present invention; or without improvement, the above concept and technical solution of the present invention are directly applied to other occasions, they are all within the protection scope of the present invention.

Claims

1. A method for determining the limiting current of a lithium-ion battery cell, characterized in that: The method comprises: Step S1, confirming multiple platform segments of lithium-ion batteries using SOC as a segmentation standard; Step S2, at room temperature, performing time-controlled constant current charging at different rates in different platform segments, then switching to constant voltage charging until the current drops to a preset threshold, and finally performing constant current discharge. During the process, charging and discharging data are collected, including constant current charging capacity, constant voltage charging capacity, constant current discharge capacity, and constant current charging temperature rise; based on the charging and discharging data, it is determined whether the battery cell has a lithium plating risk at this time to confirm the limiting current of the lithium-ion battery cell in each platform segment at room temperature; Step S3, based on the limiting current at room temperature and the temperature rise of constant current charging, the limiting current of the lithium-ion battery cells in each platform segment at other temperatures is estimated.

2. The method for determining the limiting current of a lithium-ion battery cell according to claim 1, characterized in that: The step S1 comprises: At room temperature, the lithium-ion battery is subjected to multiple cycles of constant current and constant voltage charge and discharge tests; During the test, the voltage V, charge Q, and SOC data of the battery cell are collected in real time; Based on the collected data, a dQ / dV curve is drawn with dQ / dV as the vertical axis and SOC as the horizontal axis, and multiple platform segments of multiple lithium-ion cells are confirmed based on the trend of the curve. Each peak in the dQ / dV curve represents the starting point or end point of a platform segment.

3. The method for determining the limiting current of a lithium-ion battery cell according to claim 2, characterized in that: The charging current range of the constant current and constant voltage charge and discharge test for lithium-ion batteries is between 0.05C and 0.5C.

4. The method for determining the limiting current of a lithium-ion battery cell according to claim 1, characterized in that: The step S2 comprises: Step S21: for any platform segment, the SOC starting point of the platform segment is A, and the SOC end point is B; the charging rate is C. k Constant current charging is performed, and the constant current charging time is T1; k represents a natural ordinal number, and the larger k is, the higher the charging rate C k The larger the capacity, the higher the charge rate. k The time to charge from A to B; Step S22: After the T1 time is over, the current voltage is maintained for constant voltage charging; Step S23, constant voltage charging until the current drops to a preset current lower limit, and then discharging the electricity at a preset discharge rate; Step S24, looping through steps S21 to S23 for multiple times, and collecting charge and discharge data during each cycle, including constant current charging capacity, constant voltage charging capacity, constant current discharging capacity, and constant current charging temperature rise; Step S25: According to the charge and discharge data of each cycle, it is determined whether the battery cell has a risk of lithium plating at this time. If so, the last charge rate C is increased. k-1 As the limiting current of the battery cell in the current platform segment, if it does not exist, the charging rate C k Replace with C k+1 Then, repeat steps S21 to S25.

5. The method for determining the limiting current of a lithium-ion battery cell according to claim 4, characterized in that: The criteria for determining whether a battery cell has a risk of lithium plating include: Condition 1: The constant voltage charging capacity increases in n consecutive charge and discharge cycles and the increase amplitude each time is greater than a preset first amplitude, where n represents the preset number of consecutive charge and discharge cycles; Condition 2: the constant current discharge capacity decreases in m consecutive charge and discharge cycles and the amplitude of each decrease is greater than a preset second amplitude, where m represents the preset number of consecutive charge and discharge cycles; Condition 3: The linear coefficient between lnq and lnI is less than a preset threshold; q represents heat; I represents charging current; If any two of the conditions 1 to 3 are met, it is determined that the battery cell has a risk of lithium plating.

6. A method for determining the limiting current of a lithium-ion battery cell according to any one of claims 1 to 5, characterized in that: The step S2 also includes: after performing the constant current and constant voltage charge and discharge test of step S2 on the battery cell multiple times using the confirmed limiting current of each platform segment lithium-ion battery cell, collecting the charge and discharge data and disassembling and verifying the battery cell.

7. The method for determining the limiting current of a lithium-ion battery cell according to claim 6, characterized in that: The step S2 also includes: after confirming that the limiting current of each platform segment is completed, increasing 0.1C respectively, and performing the constant current and constant voltage charge and discharge test of step S2 on the battery cell multiple times, collecting the charge and discharge data and disassembling and verifying the battery cell.

8. The method for determining the limiting current of a lithium-ion battery cell according to claim 1, characterized in that: In step S3, the calculation formula of the limiting current of the lithium-ion battery cell in each platform segment at other temperatures is expressed as follows: I=x*I0*(T-T 环境 ) / q0; I0 represents the limiting current of the lithium-ion battery cell in any platform segment at room temperature; I represents the limiting current of the corresponding platform segment at other temperatures; TT 环境 It indicates the constant current charging temperature rise of the battery cell in the corresponding platform segment. It is a preset fixed value during the calculation process. T indicates the actual maximum temperature of the battery cell, T 环境 Represents the current ambient temperature; q0 represents the constant current charging temperature rise of the lithium-ion battery cell in the corresponding platform segment at room temperature; x represents the coefficient that changes with temperature T.

9. A method for determining the limiting current of a lithium-ion battery cell according to claim 1 or 8, characterized in that: The method further includes: step S4, repeating step S2 under low temperature and high temperature extreme environments to obtain the limiting current of the lithium-ion battery cells in each platform segment under low temperature and high temperature; and comparing and verifying with the result estimated in step S3.

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