Method for assembling lithium iron phosphate square battery

Abstract

The application provides a method for assembling lithium iron phosphate square batteries, which comprises the following steps: (1) charging and discharging the battery cell, wherein the discharging comprises a first discharging process and a second discharging process, the capacity of the first discharging process is recorded as C1, and the capacity of the second discharging process is recorded as C2; (2) charging the battery cell processed in the step (1) to a specified SOC and placing the battery cell for a time, and then testing the voltage V1 and the internal resistance R1 of the battery cell; (3) aging the battery cell processed in the step (2) and placing the battery cell for a time, and then testing the voltage V2 and the internal resistance R2 of the battery cell; (4) screening the battery cell according to the following conditions: the capacity C1+C2 of the battery cell is less than a preset capacity value, the internal resistances R1 and R2 of the battery cell are greater than a preset resistance value respectively, and the difference between the voltages V1 and V2 is greater than a preset voltage value; and (5) assembling, wherein the assembling comprises arranging N battery cells in a row, and the capacities C1+C2 of the first and last battery cells are greater than the capacities C1+C2 of the middle battery cells. The assembling method can improve the battery capacity.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of batteries, in particular, to a method for grouping lithium iron phosphate square batteries. BACKGROUND

[0002] With the development of new energy technology, lithium ion batteries are widely used in power, electronics, energy storage and other fields. The application proportion of lithium iron phosphate battery in the power field is also increasing due to its long cycle and high safety advantages. However, for lithium iron phosphate batteries, the deintercalation of lithium ions in the olivine structure of the positive material lithium iron phosphate is a two-dimensional track, which limits the transmission of lithium ions, resulting in a large temperature influence on the capacity of lithium iron phosphate batteries. The capacity cannot be normally played at low temperature, which greatly reduces the endurance at low temperature in the application process of power batteries.

[0003] Therefore, in order to improve the performance of lithium iron phosphate battery at low temperature, the material, battery cell system and battery pack design are optimized, such as reducing the particle size of lithium iron phosphate material at the material level, coating conductive carbon, optimizing the electrolyte formula at the battery cell level to improve the ionic conductivity of the electrolyte, and using aerogel felt as a heat preservation layer design to make the self-heat of the battery cell not easy to dissipate, improve the environmental temperature, and achieve the effect of improving the battery capacity. However, these methods are relatively complex and tedious, and the cost is high.

[0004] At present, the battery capacity can also be improved by grouping, but the battery capacity cannot be effectively played in the grouping process. SUMMARY

[0005] The present application aims to at least solve one of the technical problems in the related art to some extent.

[0006] The present application provides a method for grouping lithium iron phosphate square batteries, comprising: (1) charging and discharging the battery cell, the discharging comprising first discharging and second discharging, the capacity of the first discharging being denoted as C1, and the capacity of the second discharging being denoted as C2; (2) charging the battery cell treated in step (1) to a specified SOC and placing it, testing the voltage V1 and the internal resistance R1 of the battery cell; (3) aging the battery cell treated in step (2) and placing it, testing the voltage V2 and the internal resistance R2 of the battery cell; (4) screening the battery cell according to the conditions that the capacity C1+C2 of the battery cell is less than a preset capacity value, the internal resistance R1 and R2 of the battery cell are greater than a preset resistance value respectively, and the difference between V1 and V2 is greater than a preset voltage value; (5) grouping, the grouping comprising arranging a number N of battery cells in a row, and the capacity C1+C2 of the first and last battery cells being greater than the capacity C1+C2 of the middle battery cells.

[0007] The method of the present application can effectively reduce the problem that the module stops discharging due to the fact that the capacity of the first and last battery reaches the lower limit voltage, that is, the battery capacity can be improved by the above method.

[0008] According to the embodiment of the present application, in step (1), the charging process is charging to the cut-off voltage at 0.3-0.5C and then charging to the cut-off current at constant voltage, and the standby time is 30-60 minutes.

[0009] According to the embodiment of the present application, in step (1), the first discharging process is discharging to 2.5V at a rate of 0.5-1.0C, the discharging capacity is recorded as C1, and the standby time is 30-60 minutes.

[0010] According to the embodiment of the present application, in step (1), the second discharging process is discharging to 2.0V at a rate of 0.1-0.3C, the discharging capacity is recorded as C2, and the standby time is 30-60 minutes.

[0011] According to the embodiment of the present application, in step (2), the specified SOC is 5%≤SOC≤30%, and the standby time is 6-8 hours.

[0012] According to the embodiment of the present application, in step (3), the aging temperature is 35-55℃.

[0013] According to the embodiment of the present application, in step (3), the aging time is 72-120 hours.

[0014] According to the embodiment of the present application, in step (3), the standby time of aging is 6-8 hours.

[0015] According to the embodiment of the present application, the initial grouping before the grouping further comprises grouping the battery cells according to the range of the battery cell capacity C1+C2 being 1-5%.

[0016] According to the embodiment of the present application, the initial grouping comprises grouping the battery cells according to the range of the battery cell internal resistance R1 and R2 being 3-10% respectively and independently.

[0017] According to the embodiment of the present application, the initial grouping comprises grouping the battery cells according to the range of the difference between the battery cell voltage V1 and V2 being 10-25%.

[0018] According to the embodiment of the present application, the grouping comprises arranging a number N of battery cells, and the capacity C1+C2 of the first and last battery cells is greater than the capacity C1+C2 of the middle battery cells, and the battery cell internal resistance R1, R2 and the difference between the battery cell voltage V1 and V2 are of the same level. BRIEF DESCRIPTION OF DRAWINGS

[0019] The above and / or additional aspects and advantages of the present application will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings in which:

[0020] Figure 1 is a module battery pack power chart of Example 1.

[0021] Figure 2 is a module battery pack power chart of Comparative Example 1. DETAILED DESCRIPTION

[0022] Embodiments of the present application are described in detail below. The embodiments described below are exemplary only, and are not to be construed as limiting the present application.

[0023] The inventor found that, as long as heat transfer is performed through two large faces in contact with the outside during charging and discharging of a lithium iron phosphate square battery, after the battery is assembled into a module, the two batteries at the head and tail of the module have a slow heat loss due to the two large faces being in contact with each other, and one large face of the two batteries at the head and tail is in contact with the outside for fast heat dissipation, resulting in a lower temperature rise of the two batteries at the head and tail of the module during discharging at low temperature, a larger polarization than other battery cells during discharging, and a first reaching of the lower limit voltage at the end of discharging, which causes the entire module to stop discharging and the battery power to be unable to be effectively utilized.

[0024] Therefore, the present application provides a method for assembling lithium iron phosphate square batteries, comprising: (1) charging and discharging the battery cells, wherein the discharging comprises first discharging and second discharging, the capacity of the first discharging is denoted as C1, and the capacity of the second discharging is denoted as C2; (2) charging the battery cells processed in step (1) to a specified SOC and storing, testing the voltage V1 and the internal resistance R1 of the battery cells; (3) aging the battery cells processed in step (2) and storing, testing the voltage V2 and the internal resistance R2 of the battery cells; (4) screening the battery cells according to the conditions that the capacity C1+C2 of the battery cells is less than a preset capacity value, the internal resistances R1 and R2 of the battery cells are greater than a preset resistance value, respectively, and the difference between V1 and V2 is greater than a preset voltage value; and (5) assembling, wherein the assembling comprises arranging a number N of battery cells in a row, and the capacity C1+C2 of the battery cells at the head and tail is greater than the capacity C1+C2 of the battery cells in the middle.

[0025] The method of the present application can effectively reduce the problem that the battery power reaches the lower limit voltage at the end of discharging, and the module stops discharging, by screening the battery cells according to the preset conditions, and assembling the battery cells with the capacity of the battery cells at the head and tail being higher than that of the battery cells in the middle.

[0026] It can be understood that the preset resistance value refers to the threshold value of the cell resistance, the preset voltage value refers to the self-discharge voltage value, and the preset capacity value refers to the rated capacity.

[0027] As an example, in a lithium iron phosphate battery, the preset resistance value is 0.5 mΩ, the preset voltage value is 20 mv, and the preset capacity value is 75 Ah.

[0028] According to the present application, in step (1), the cell is charged and discharged, and the discharging includes a first discharging process and a second discharging process, the capacity of the first discharging process is denoted as C1, and the capacity of the second discharging process is denoted as C2.

[0029] In some embodiments, in step (1), the charging process is charging to the cut-off voltage at 0.3-0.5C and then constant voltage charging to the cut-off current, and the standby time is 30-60 minutes.

[0030] In some embodiments, in step (1), the first discharging process is discharging to 2.5V at a rate of 0.5-1.0C, the discharging capacity is denoted as C1, and the standby time is 30-60 minutes.

[0031] In some embodiments, in step (1), the second discharging process is discharging to 2.0V at a rate of 0.1-0.3C, the discharging capacity is denoted as C2, and the standby time is 30-60 minutes.

[0032] By discharging the cell in the first discharging process and the second discharging process, the rate of the second discharging process is smaller than that of the first discharging process, which can depolarize at the end of discharging, help to realize the true capacity of the cell during the capacity grading, and reduce the cost.

[0033] According to the present application, the cell after step (1) is charged to a specified SOC and the standby time is tested, the voltage V1 and the internal resistance R1 of the cell are tested. This step, charging the cell to the specified SOC is beneficial to better screening out bad cells according to the voltage difference V1-V2.

[0034] In some embodiments, in step (2), the specified SOC is 5%≤SOC≤30%.

[0035] In some embodiments, in step (2), the standby time is 6-8h, for example, 6h, 7h, 8h, etc.

[0036] According to the present application, the voltage V1 and the internal resistance R1 of the cell after step (2) are tested.

[0037] In some embodiments, in step (3), the temperature of the aging treatment is 35-55℃, for example, 35℃, 40℃, 45℃, 50℃, 55℃, etc.

[0038] In some embodiments, in step (3), the aging treatment is performed for 72-120 hours, such as 72 hours, 80 hours, 100 hours, 120 hours, etc.

[0039] In some embodiments, in step (3), the resting time after the aging treatment is 6-8 hours, such as 6 hours, 7 hours, 8 hours, etc.

[0040] The voltage V2 and the internal resistance R2 of the battery cell after the treatment in step (3) are tested.

[0041] According to the present application, the battery cell is screened according to that the capacity C1+C2 of the battery cell is less than a preset capacity value, the internal resistances R1 and R2 of the battery cell are respectively greater than a preset resistance value, and the difference between V1 and V2 is greater than a preset voltage value. In this step, the qualified battery cell can be preliminarily screened for subsequent grouping.

[0042] In some embodiments, before the subsequent grouping, a primary grouping is further included to classify the screened battery cell. The primary grouping includes grouping the battery cell according to that the range of the capacity C1+C2 is 1-5%, and during the grouping, 1-3 Ah is taken as the reference.

[0043] As an example, the battery cell is grouped according to that the range of the capacity C1+C2 is 2%.

[0044] In some embodiments, the primary grouping includes respectively independently grouping the battery cell according to that the range of the internal resistances R1 and R2 is 3-10%, and during the grouping, 0.1-3 mΩ is taken as the reference.

[0045] As an example, the battery cell is grouped according to that the range of the internal resistance R1 is 5% and the range of the internal resistance R2 is 5%.

[0046] In some embodiments, the primary grouping includes grouping the battery cell according to that the range of the difference between the voltages V1 and V2 is 10-25%, and during the grouping, 1-3 mv is taken as the reference.

[0047] As an example, the battery cell is grouped according to that the range of the difference between the voltages V1 and V2 is 10%.

[0048] According to the present application, the grouping includes arranging a row of N battery cells, and the capacity C1+C2 of the first and last battery cells is greater than that of the middle battery cells, and N is an integer greater than 3. In this step, the first and last battery cells in a row of battery cells are selected as the battery cells with higher capacity, so as to increase the power of the module.

[0049] In some embodiments, the group includes arranging a number N of battery cells, and the capacity C1+C2 of the first and last battery cells is greater than the capacity C1+C2 of the middle battery cells, and the battery cell internal resistance R1, R2 and the battery cell voltage V1 and V2 difference are the same level group.

[0050] It can be understood that the same level group refers to grouping battery cells, according to the difference of the battery cell internal resistance R1, R2, the battery cells are divided into the first group, the second group, the third group, etc. from small to large, and according to the difference of the battery cell voltage V1 and V2, the battery cells are divided into the first group, the second group, the third group, etc. from small to large, and the same level group refers to the first group or the second group or the third group, etc.

[0051] The scheme of the present application will be described below through specific embodiments. It should be noted that the following embodiments are only used to illustrate the present application, and should not be regarded as limiting the scope of the present application.

[0052] Embodiment 1

[0053] After optimizing the first and last battery cells of the module, the battery pack is configured, the battery cell size is 26*220*115 (length* width* height), C is 75Ah, and the battery pack is composed of 4 modules, each module has 27 battery cells.

[0054] The specific implementation scheme is as follows:

[0055] S1: After formation, the battery cell is charged to 3.65V at room temperature with 0.5C, and then constant voltage charging is performed, the cutoff current is 0.05C, and the standby time is 30 minutes;

[0056] S2: Discharge to 2.5V at room temperature with a rate of 0.5C, and the discharge capacity is recorded as C1, and the standby time is 30 minutes;

[0057] S3: Discharge to 2.0V at room temperature with a rate of 0.1C, and the discharge capacity is recorded as C2, and the standby time is 30 minutes;

[0058] S4: Charge to a specified 20% SOC at room temperature with a rate of 0.1C, and the standby time is 6h;

[0059] S5: Measure the battery cell voltage V1 and internal resistance R1 at room temperature;

[0060] S6: Age for 3 days in a high-temperature environment of 45℃, and then leave the high-temperature environment, and the standby time at room temperature is 6h;

[0061] S7: Measure the battery cell voltage V2 and internal resistance R2 at room temperature;

[0062] S8: Select the defective battery cell, the battery cell capacity C1+C2<75Ah, the battery cell internal resistance R1, R2>0.5mΩ, and the battery cell voltage difference V1-V2>20mv;

[0063] S9: Initial grouping. Based on the range of cell capacity C1+C2 being 1.5Ah, the cells are grouped into groups of 75Ah<C1+C2≤76.5Ah, 76.5Ah<C1+C2≤78Ah, and 78Ah<C1+C2.

[0064] Based on the fact that the range of internal resistance R1 is 5% and the range of internal resistance R2 is 0.2mΩ, the cells are grouped into groups of 0.4mΩ≤R1 or R2<0.42mΩ, 0.42mΩ≤R1 or R2<0.44mΩ, 0.44mΩ≤R1 or R2<0.46mΩ, 0.46mΩ≤R1 or R2<0.48mΩ, and 0.48mΩ≤R1 or R2<0.5mΩ.

[0065] Based on the fact that the range of the voltage difference between the internal resistance cells V1 and V2 is 2mV, the cells are grouped into groups as follows: V1-V2<10mV, 10mV≤V1-V2<12mV, 12mV≤V1-V2<14mV, 14mV≤V1-V2<16mV, 16mV≤V1-V2<18mV, and 18mV≤V1-V2<20mV.

[0066] S10: Select a group of 25 cells with a cell capacity of 75Ah < C1 + C2 < 76.5Ah, cell internal resistance of 0.4mΩ < R1, R2 < 0.42mΩ, and cell voltage difference of 10mV < V1 - V2 < 12mV.

[0067] Then select the first and last two cells. The cell capacity is selected as C1+C2>78Ah, the cell internal resistance is 0.4mΩ<R1, R2<0.42mΩ, and the cell voltage difference is 10mV<V1-V2<12mV.

[0068] Comparative Example 1

[0069] Example 1

[0070] The cell dimensions of the module are 26*220*115 (length*width*height), C is 75Ah, and the battery pack consists of 4 modules, each with 27 cells.

[0071] The specific implementation plan is as follows:

[0072] S1: Charge the formed battery cell at 0.5C to 3.65V at room temperature, then switch to constant voltage charging with a cutoff current of 0.05C and a resting time of 30 minutes;

[0073] S2: Discharge to 2.5V at a rate of 0.5C at room temperature, the discharge capacity is recorded as C1, and the resting time is 30 minutes;

[0074] S3: Discharge to 2.0V at a rate of 0.1C at room temperature, the discharge capacity is recorded as C2, and the resting time is 30 minutes;

[0075] S4: Charge to specified 20% SOC at room temperature with a rate of 0.1C, and the standby time is 6h;

[0076] S5: Measure the cell voltage V1 and internal resistance R1 at room temperature;

[0077] S6: Age for 3 days at a high temperature of 45°C, exit the high-temperature environment, and the standby time at room temperature is 6h;

[0078] S7: Measure the cell voltage V2 and internal resistance R2 at room temperature;

[0079] S8: Select the poor cells, the cell capacity C1+C2<75Ah, the cell internal resistance R1 and R2>0.5mΩ, and the cell voltage difference V1-V2>20mv;

[0080] S9: Primary grouping, according to the range of cell capacity C1+C2 of 1.5Ah, the cells are grouped, and the groups are 75Ah

[0081] According to the range of internal resistance R1 of 5%, and the range of internal resistance R2 of 0.2mΩ, the cells are grouped, and the groups are 0.4mΩ≤R1 or R2<0.42mΩ, 0.42mΩ≤R1 or R2<0.44mΩ, 0.44mΩ≤R1 or R2<0.46mΩ, 0.46mΩ≤R1 or R2<0.48mΩ, and 0.48mΩ≤R1 or R2<0.5mΩ;

[0082] According to the range of the difference between the internal resistance cell voltages V1 and V2 of 2mv, the cells are grouped, and the groups are V1-V2<10mv, 10mv≤V1-V2<12mv, 12mv≤V1-V2<14mv, 14mv≤V1-V2<16mv, 16mv≤V1-V2<18mv, and 18mv≤V1-V2<20mv.

[0083] S10: Take 27 cells, the cell capacity is selected from the group of 75Ah

[0084] Test Example

[0085] Discharge test the cells at -7°C and under CLTC (China Light Vehicle Test Cycle) conditions, the test method is as follows: after the battery pack is fully charged, discharge at variable power according to the CLTC discharge flow table for 1800s as one cycle, and repeat the variable power cycle until the discharge voltage is 2.0V.

[0086] Figure 1 The power test graph of the battery module of Example 1.

[0087] Figure 2 The power test graph of the battery module of Comparative Example 1.

[0088] In combination Figure 1 And 2 It can be seen that the discharge capacity of Example is 23.123 kWh, and the discharge capacity of Comparative Example 1 is 21.563 kWh, and the discharge capacity of the optimized configuration is improved by 6.7% compared with the traditional configuration scheme.

[0089] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. All patents and publications identified herein are incorporated herein by reference in their entirety. The terms "comprising" or "including" or "having" are intended to be open-ended terms that specifically permit the inclusion of one or more features, components, materials, or steps, but not exclude the inclusion of other features, components, materials, or steps.

[0090] In the description of the present specification, the description of the terms "one embodiment", "another embodiment", and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment are included in at least one embodiment of the present application. In the present specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Also, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, the person skilled in the art can combine and combine the different embodiments or examples described in the present specification and the features of the different embodiments or examples, without contradiction. In addition, it should be noted that in the present specification, the terms "first", "second" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features.

[0091] Although the embodiments of the present application have been shown and described above, it is understood that the above embodiments are exemplary and cannot be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application.

Claims

1. A method for assembling lithium iron phosphate square batteries, characterized in that, include: (1) The battery cell is charged and discharged. The discharge process includes a first discharge process and a second discharge process. The capacity of the first discharge process is C1 and the capacity of the second discharge process is C2. (2) Charge the battery cell processed in step (1) to the specified SOC and leave it to stand. Test the voltage V1 and internal resistance R1 of the battery cell. (3) The battery cell processed in step (2) is aged and then placed aside. The voltage V2 and internal resistance R2 of the battery cell are tested. (4) Select cells based on the following: the cell capacity C1+C2 is less than the preset capacity value, the cell internal resistance R1 and R2 are respectively greater than the preset resistance value, and the difference between V1 and V2 is greater than the preset voltage value. (5) Grouping, wherein the grouping includes a row of N cells, and the capacity of the first and last cells C1+C2 is greater than the capacity of the middle cells C1+C2, and N is an integer greater than 3.

2. The method according to claim 1, characterized in that, In step (1), the charging process is to charge at 0.3 to 0.5C to the cutoff voltage, then switch to constant voltage charging to the cutoff current, and the resting time is 30 to 60 minutes.

3. The method according to claim 1, characterized in that, In step (1), the first discharge process is to discharge to 2.5V at a rate of 0.5 to 1.0C, the discharge capacity is recorded as C1, and the resting time is 30 to 60 minutes.

4. The method according to claim 1, characterized in that, In step (1), the second discharge process is to discharge to 2.0V at a rate of 0.1 to 0.3C, the discharge capacity is recorded as C2, and the resting time is 30 to 60 minutes.

5. The method according to claim 1, characterized in that, In step (2), the specified SOC is 5% ≤ SOC ≤ 30%, and the resting time is 6 to 8 hours.

6. The method according to claim 1, characterized in that, In step (3), aging satisfies at least one of the following conditions: Temperature 35~55℃; Time: 72–120 hours; The resting time is 6 to 8 hours.

7. The method according to claim 1, characterized in that, The process of grouping cells into groups includes an initial grouping, which involves grouping the cells according to the range of cell capacity C1+C2 being 1-5%.

8. The method according to claim 7, characterized in that, The initial grouping includes independently grouping the cells according to the difference of 3 to 10% between the internal resistances R1 and R2 of the cells.

9. The method according to claim 8, characterized in that, The initial grouping includes grouping the battery cells according to the range of 10-25% of the difference between the cell voltages V1 and V2.

10. The method according to claim 8, characterized in that, The grouping includes a row of N cells, with the capacity of the first and last cells (C1+C2) being greater than that of the middle cells (C1+C2), and the differences between the cell internal resistances R1 and R2 and the cell voltages V1 and V2 being of the same level.

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