A capacity division method to improve capacity attenuation consistency

Abstract

The present invention proposes a capacity division method for improving the consistency of capacity attenuation, which relates to the field of batteries. The method comprises the following steps: S1, constant current charging in the first stage, charging to the upper limit of the cell voltage platform, and standing still; S2, constant current charging in the second stage, charging to the upper limit of the cell voltage platform, and standing still; S3, constant current and constant voltage charging, charging until the current reaches 0.02C, and standing still; S4, constant current discharge, discharging to the lower limit of the cell voltage platform, and standing still; S5, deep constant current discharge, discharging until the cell voltage is lower than the lower limit of the voltage platform of step S4 by 0.2-0.5V, and standing still; S6, constant current charging, and standing still. The capacity division method is used to solve the problem that the capacity difference of the same-level cell capacity is too large after standing still for a period of time, improve the consistency of the module cell, and reduce safety risks.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and in particular to a capacity division method for improving the consistency of capacity attenuation. Background Art

[0002] Lithium-ion batteries are widely used in the field of new energy vehicles due to their high energy density and flexible design. The manufacturing process of lithium-ion batteries has an important impact on the performance of the battery, such as the pre-charge and discharge system, and the capacity-based charge and discharge process.

[0003] The battery capacity grading process is to determine the capacity of the battery cell by charging and discharging the battery under a fixed requirement environment, and then grading the capacity of the battery cell; in the subsequent module matching, in order to improve the consistency of the module battery cell, the battery cells of the same capacity level will be matched. The higher the consistency of the battery cell capacity in the module, the better the performance of the module in the subsequent use process.

[0004] At present, the accuracy of battery capacity testing is relatively high, but for batteries in electric vehicles, a battery pack includes more than a dozen modules, and each module includes more than a dozen or more battery cells. While ensuring the consistency of the initial capacity when the modules are assembled, the consistency of the cell capacity decay must also be controlled. For the cells currently on the market, the formation capacity is mostly based on 500mAh as a gear. When the capacity is divided and the gear is determined, the maximum difference in the capacity of the cells in the same gear is 500mAH, but when the formation is completed and the modules are assembled, the maximum capacity difference in the same gear may reach more than 1000mAH, that is, the consistency after capacity decay is very poor. This leads to poor consistency in the capacity decay of the assembled cells, and there will be safety hazards in long-term use. Summary of the invention

[0005] The purpose of the present invention is to provide a capacity division method for improving the consistency of capacity attenuation, so as to solve the problem that the capacity difference of the same-level battery cells is too large after standing for a period of time, improve the consistency of module battery cells, and reduce safety risks.

[0006] The present invention solves the technical problem by adopting the following technical solutions.

[0007] On the one hand, an embodiment of the present invention provides a capacity division method for improving capacity attenuation consistency, comprising the following steps:

[0008] S1, the first stage of constant current charging, charging to the upper limit of the cell voltage platform, and then leaving it alone;

[0009] S2, the second stage of constant current charging, charging to the upper limit of the cell voltage platform, and then leaving it to stand;

[0010] S3, constant current and constant voltage charging, charging until the current reaches 0.02C, and then standing;

[0011] S4, constant current discharge, discharge to the lower limit of the cell voltage platform, and let it stand;

[0012] S5, deep constant current discharge, discharge until the cell voltage is lower than the voltage platform lower limit of step S4 by 0.2-0.5V, and let stand;

[0013] S6, constant current charging, stand still.

[0014] In step S1, constant current charging is first performed to fill the battery cell. During the charging process, after charging to the upper limit voltage, polarization will occur inside the battery cell, causing the voltage to drop. Therefore, in step S2, constant current charging is performed with a larger current to supplement the battery cell capacity. In step S3, constant voltage charging is performed to supplement the voltage loss caused by the polarization of the battery cell. The constant current discharge in step S4 is discharged to the lower limit voltage of the platform, which is the capacity calibration stage, that is, this discharge capacity is the initial capacity of the battery cell. S5 is the deep discharge stage, that is, based on S4, the voltage is within 0.2-0.5V lower than the lower limit voltage of the S4 platform as the lower limit. Deep discharge is used to shield the influence of polarization of some batteries. That is to say, for batteries that have not undergone deep discharge, there is a deviation between the electrode potential and the equilibrium electrode potential, but for batteries that have undergone deep discharge, the electrode potential is basically consistent with the equilibrium electrode potential.

[0015] In some embodiments of the present invention, the current of the constant current charging in the first stage is 0.05-0.5C, and the current of the constant current charging in the second stage is 0.2-0.8C.

[0016] In some embodiments of the present invention, in steps S1 and S2, the upper limit of the cell voltage platform is 3.65V.

[0017] In some embodiments of the present invention, the current of the constant current constant voltage charging is 0.3-1C. The current of the constant current discharge is 0.2-1C, and the lower limit of the cell voltage platform is 2.5V. The first stage and the second stage adopt step-by-step current charging, which is conducive to the optimal charging capacity.

[0018] In some embodiments of the present invention, the current of the deep constant current discharge is 0.01-0.08C.

[0019] In some embodiments of the present invention, in steps S1-S6, the standing time is ≥5 min.

[0020] Compared with the prior art, the embodiments of the present invention have at least the following advantages or beneficial effects:

[0021] The capacity division method for improving the consistency of capacity attenuation provided by the present invention is to set the capacity in the S1-S4 stage, that is, to calibrate the cell capacity, and then perform a small current deep discharge in S5. After the small current deep discharge, the electrode potential is basically consistent with the equilibrium electrode potential, which can improve the consistency of the cell capacity attenuation, and the small current deep discharge can ensure that the cell structure is not damaged. During the use of the battery, the extreme difference of the capacity of each module battery is small, the service life of the battery is longer, and the safety risks caused by poor consistency are reduced. By adding a small current deep discharge process, the method is simple and low-cost. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0023] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other. The present invention will be described in detail below with reference to specific embodiments.

[0024] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0025] Example 1

[0026] Batch produce 50Ah lithium iron phosphate cells without capacity division. The upper limit voltage of this type of cell is 3.65V and the lower limit voltage is 2.5V. Charge and discharge according to the following capacity division process (Table 1); determine the capacity of each cell.

[0027] Table 1

[0028]

[0029] In Table 1, the constant current discharge capacity of step 8 is the capacity of each battery cell, which is C1. C1 is classified according to the lithium iron phosphate 50Ah capacity classification standard (01 class: 50500mAh-51500mAh; 02 class 51500mAh-52000mAh; 03 class 52000mAh-52500mAh; 04 class 52500mAh-53000mAh).

[0030] 15 cells are randomly selected as a group for each gear, and 4 groups correspond to 4 gears, with a total of 60 cells. The difference between the maximum and minimum values ​​of capacity C1 in each gear group is recorded, that is, the capacity range. The capacity range of gear 01 is 404.28mAh, the capacity range of gear 02 is 474.44mAh, the capacity range of gear 03 is 434.93mAh, and the capacity range of gear 04 is 446.54mAh.

[0031] The above 4 groups of battery cells were transported to a high temperature storage (45°C) and left to stand for 7 days. Their capacities were tested according to the following discharge system (Table 2), i.e., the constant current discharge capacity in process step 2 was the cell capacity C2 after high temperature standing.

[0032] Table 2

[0033]

[0034] The difference between the maximum and minimum values ​​of capacity C2 in each gear group is recorded, that is, the capacity range of the battery cell after high-temperature standing is 663.20mAh for gear 01, 1228.33mAh for gear 02, 824.55mAh for gear 03, and 1036.52mAh for gear 04. It can be seen that after high-temperature standing, the capacity range between each gear changes greatly, with a minimum of 663.20mAh and a maximum of 1228.33mAh, and the capacity attenuation consistency is poor.

[0035] Example 2

[0036] The uncapacitated lithium iron phosphate battery cells 50Ah battery cells produced in the same batch as Example 1 were selected. The upper limit voltage of this type of battery cells was 3.65V and the lower limit voltage was 2.5V. The battery cells were charged and discharged according to the following capacity division process (Table 3); and the capacity of each battery cell was determined.

[0037] Table 3

[0038]

[0039] Similar to Example 1, the constant current discharge capacity of step 8 in Table 3 above is the capacity of each battery cell, recorded as C1. Different from Example 1, a new step 10 deep discharge (i.e., the capacity division method provided by the present invention) is added in the capacity division process of Example 2, and the battery cell is discharged to 2.0V at a discharge current of 0.05C, which is 0.5V lower than the lower limit voltage of 2.5V. A small current deep discharge is performed within this voltage range without damaging the internal structure of the battery cell.

[0040] C1 is classified according to the lithium iron phosphate 50Ah capacity classification standard (01 class: 50500mAh-51500mAh; 02 class: 51500mAh-52000mAh; 03 class: 52000mAh-52500mAh; 04 class: 52500mAh-53000mAh).

[0041] Fifteen cells are randomly selected as a group for each gear, and 60 cells are formed into 4 groups with 4 gears. The difference between the maximum and minimum values ​​of capacity C1 in each gear group is recorded, that is, the capacity range. The capacity range of gear 01 is 433.21mAh, the capacity range of gear 02 is 458.20mAh, the capacity range of gear 03 is 456.15mAh, and the capacity range of gear 04 is 418.93mAh.

[0042] The above 4 groups of battery cells were transported to a high temperature storage (45°C) and left to stand for 7 days. Their capacities were tested according to the following discharge system (Table 4), i.e., the constant current discharge capacity of process step No. 2 was the cell capacity C2 after high temperature standing.

[0043] Table 4

[0044]

[0045] The difference between the maximum and minimum values ​​of capacity C2 in each gear group was recorded, that is, the capacity of the battery cell was extremely poor after high-temperature standing. The capacity range of gear 01 was 435.18mAh, the capacity range of gear 02 was 444.21mAh, the capacity range of gear 03 was 416.24mAh, and the capacity range of gear 04 was 409.42mAh. Compared with before high-temperature standing, the capacity range of each gear did not change much. It can be seen that increasing the deep low current 0.05C discharge to 2.0V can improve the consistency of capacity attenuation.

[0046] The cells processed by the capacity division process in Implementation 1 and Example 2 were selected, and the cells with a pressure difference delatV between 2.5-5.5mV, an IR between 0.83-0.91mΩ, a first efficiency between 94-96%, and a liquid retention volume of about 158g were screened through 7 days of high-temperature static self-discharge, and there was no obvious difference or abnormality in the process.

[0047] Perform the following tests on the battery cells:

[0048] ①Electrical performance test

[0049] The results show that the cells of Example 1 and Example 2 have a 2C charge capacity retention rate of 75% and a 2C discharge capacity retention rate of 97%. After 28 days of storage at room temperature, the capacity retention rate is 98% and the capacity recovery rate is 99%. After 30 days of high-temperature storage, the capacity retention rate is 96% and the capacity recovery rate is 97.7%.

[0050] ②Safety test:

[0051] Short circuit test - no fire or explosion in a full-charge short circuit test;

[0052] Needle puncture test: 70% SOC needle puncture does not cause fire or explosion;

[0053] Heating test - no fire or explosion in a 130℃ hot box;

[0054] Extrusion test - passed the 30% extrusion test without fire or explosion.

[0055] It can be seen that the battery cells of Example 1 and Example 2 have the same performance, that is, the deeply discharged battery cells and the non-deeply discharged battery cells are at the same level, and no obvious difference is observed.

[0056] In summary, the capacity division method for improving the consistency of capacity attenuation provided by the present invention performs a low-current deep discharge after the cell capacity is calibrated. After the low-current deep discharge, the consistency of the cell capacity attenuation can be improved, and the battery capacity difference of each module battery is small during use, and the battery life is longer, reducing the safety risks caused by poor consistency. By adding a low-current deep discharge process, the method is simple and low-cost.

[0057] The embodiments described above are part of the embodiments of the present invention, rather than all of the embodiments. The detailed description of the embodiments of the present invention is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

Claims

1. A capacity division method for improving the consistency of capacity attenuation, characterized in that: The following steps are involved: S1, the first stage of constant current charging, charging to the upper limit of the cell voltage platform, and then leaving it alone; S2, the second stage of constant current charging, charging to the upper limit of the cell voltage platform, and then leaving it to stand; S3, constant current and constant voltage charging, charging until the current reaches 0.02C, and then standing; S4, constant current discharge, discharge to the lower limit of the cell voltage platform, and let it stand; S5, deep constant current discharge, discharge until the cell voltage is lower than the voltage platform lower limit of step S4 by 0.2-0.5V, and let stand; S6, constant current charging, stand still.

2. The capacity division method for improving capacity attenuation consistency according to claim 1, characterized in that: The current of the constant current charging in the first stage is 0.05-0.5C, and the current of the constant current charging in the second stage is 0.2-0.8C.

3. The capacity division method for improving capacity attenuation consistency according to claim 1, characterized in that: In the steps S1 and S2, the upper limit of the cell voltage platform is 2.5-4.2V.

4. The capacity division method for improving capacity attenuation consistency according to claim 1, characterized in that: In step S3, the current of the constant current and constant voltage charging is 0.3-1C.

5. The capacity division method for improving capacity attenuation consistency according to claim 1, characterized in that: In step S4, the constant current discharge current is 0.2-1C, and the lower limit of the cell voltage platform is 2.5V.

6. The capacity division method for improving capacity attenuation consistency according to claim 1, characterized in that: In step S5, the current of the deep constant current discharge is 0.01-0.08C.

7. The capacity division method for improving capacity attenuation consistency according to claim 1, characterized in that: In the steps S1-S6, the standing time is ≥5 min.

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