A method for determining insufficient electrolyte level in lithium iron phosphate batteries

Abstract

This invention relates to a method for determining whether the electrolyte content of a lithium iron phosphate battery is insufficient, comprising: (1) preparing several lithium iron phosphate batteries with different electrolyte contents; (2) assembling the positive electrode of the battery into a coin cell and testing to obtain the initial charge capacity Cinitial of the fresh positive electrode; (3) performing cycle tests on the lithium iron phosphate batteries until the charge capacity decays to 80% of the initial charge capacity; (4) discharging the batteries after the cycle test, disassembling the batteries, taking the positive electrode, assembling a coin cell, and testing to obtain the initial charge capacity of the positive electrode after the cycle, denoted as Cfinal; (5) calculating the decay ratio of the positive electrode at the end of the cycle = 100% - Cfinal / Cinitial, and judging the calculation result. The method of this invention can not only determine whether the electrolyte content of a lithium iron phosphate battery is insufficient, but also obtain the critical value of the electrolyte content of a lithium iron phosphate battery not being too low by judging whether several different electrolyte contents are insufficient.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, specifically relating to a method for determining insufficient electrolyte in lithium iron phosphate batteries. Background Technology

[0002] Lithium iron phosphate batteries are widely used in power, energy storage and other fields due to their ultra-long cycle life, excellent safety performance, good high-temperature performance and extremely low price.

[0003] The electrolyte volume in lithium iron phosphate batteries needs to be appropriate; too much or too little is undesirable. Too much electrolyte can lead to safety hazards and higher costs, while too little electrolyte can cause performance deviations. While the amount of electrolyte in a lithium iron phosphate battery can be controlled by testing its safety performance, there is currently no accurate or quantifiable method to determine if it is too little.

[0004] Therefore, a relatively accurate method is needed to determine whether the electrolyte content of lithium iron phosphate batteries is too low throughout their entire life cycle. Summary of the Invention

[0005] The purpose of this invention is to provide a method for determining whether the electrolyte level in a lithium iron phosphate battery is insufficient. This method can not only determine whether the electrolyte level in a lithium iron phosphate battery is insufficient, but also obtain a critical value for the electrolyte level in a lithium iron phosphate battery that is not too low by judging whether the electrolyte level is insufficient through several different electrolyte level determinations.

[0006] The technical solution adopted by the present invention to solve the above problems is as follows: a method for determining insufficient electrolyte in lithium iron phosphate batteries, comprising the following steps: (1) Prepare batteries: Prepare several lithium iron phosphate batteries with different amounts of electrolyte. The electrolyte amounts are m1, m2, m3, ... in order of increasing amount. The corresponding lithium iron phosphate batteries are B-1, B-2, B-3, ... in order. (2) Fresh electrode test: Assemble coin cells from several positive electrode sheets of lithium iron phosphate batteries in step (1) and test them to obtain the secondary charging capacity of the fresh positive electrode sheets, which are denoted as Cinitial-1, Cinitial-2, Cinitial-3...; (3) Cyclic test: Cyclic tests were conducted on several lithium iron phosphate batteries with different electrolyte contents from step (1) until the charging capacity decreased to 80% of the initial charging capacity; (4) Disassembly and analysis: The lithium iron phosphate batteries B-1, B-2, B-3... after the cycle test in step (3) were discharged, and then the batteries were disassembled, the positive electrode was taken, and the coin half-cells were assembled and tested to obtain the first charge capacity of the positive electrode after the cycle, which was recorded as C_end-1, C_end-2, C_end-3...; (5) Calculation of positive electrode decay: The decay ratio of the positive electrode of the lithium iron phosphate battery at the end of the cycle in step (4) = 100% - C_end / C_beginning, where C_beginning is C_beginning-1, C_beginning-2, C_beginning-3... and the corresponding C_end is C_end-1, C_end-2, C_end-3...; (6) Judgment: Judge the positive electrode attenuation calculation results in step (5): If |100%-C_end / C_initial-20%|≤5%, then the electrolyte content of the corresponding lithium iron phosphate battery is considered not too low. If |100%-C_end / C_initial-20%|>5%, then the electrolyte content of the corresponding lithium iron phosphate battery is considered to be too low.

[0007] Preferably, the only difference between the plurality of lithium iron phosphate batteries containing different amounts of electrolyte is the amount of electrolyte.

[0008] Preferably, based on the positive electrode attenuation calculation results in step (5), the critical value of insufficient electrolyte can also be obtained.

[0009] Preferably, the range of electrolyte amount in step (1) is: 2.5g / Ah*battery capacity ≤ electrolyte amount ≤ 6g / Ah*battery capacity.

[0010] Preferably, in step (4), the lithium iron phosphate battery is discharged to 2.0V.

[0011] Preferably, the half-cells in steps (2) and (4) require the addition of sufficient electrolyte, i.e., the electrolyte is added until it overflows, to ensure that the electrode is completely wetted by the electrolyte.

[0012] Compared with the prior art, the advantages of the present invention are as follows: The present invention provides a method for determining whether the electrolyte level in a lithium iron phosphate battery is insufficient. This method can not only determine whether the electrolyte level in a lithium iron phosphate battery is insufficient, but also obtain a critical value for the electrolyte level not being too low by judging whether the electrolyte level is insufficient for several different electrolyte levels. This is of great significance for guiding the research and development design of appropriate electrolyte levels. Implementation

[0013] The present invention will be further described in detail below with reference to the embodiments. Example 1

[0014] A method for determining insufficient electrolyte level in a lithium iron phosphate battery includes the following steps: (1) Prepare batteries: Prepare 7 120Ah lithium iron phosphate batteries with different electrolyte amounts. These 7 lithium iron phosphate batteries are completely the same except for the electrolyte amount. The electrolyte amounts from low to high are m1=521g, m2=533g, m3=546g, m4=558g, m5=570g, m6=583g, and m7=595g. The corresponding lithium iron phosphate batteries are labeled B-1, B-2, B-3, B-4, B-5, B-6, and B-7.

[0015] The composition of the lithium iron phosphate battery is as follows: by mass percentage, the positive electrode is 95% lithium iron phosphate (DY-1): 2% carbon black (SP): 3% binder (HSV900); the negative electrode is 95% graphite (FSN-1): 1.5% carbon black (SP): 1.5% dispersant (CMC2200): 2% binder (GD1346L); the separator is a 9+3 (9+3 is the separator model, 9 represents the thickness of the base film, and 3 represents the thickness of the ceramic coating, both in μm) PE coated separator; the electrolyte is Tinci E8087.

[0016] (2) Fresh electrode test: The positive electrode of the 7 lithium iron phosphate batteries in step (1) was assembled into a coin cell and tested. The average value of the secondary charging specific capacity of the fresh positive electrode was Cinitial = 152 mAh / g. It should be noted that the test method of the 7 lithium iron phosphate battery positive electrode coin cells is the same and the materials used are the same. The values ​​of the secondary charging specific capacity are relatively close. Generally, the average value is sufficient.

[0017] (3) Cyclic test: The lithium iron phosphate batteries B-1, B-2, B-3, B-4, B-5, B-6 and B-7 with different electrolyte amounts were subjected to a 1C cycle test at room temperature until the charging capacity decayed to 80% of the initial charging capacity (equivalent to a 20% decay of the battery charging capacity).

[0018] (4) Disassembly and analysis: Taking B-1 battery as an example, the B-1 battery after the cycle test in step (3) was discharged to 2.0V, and then the battery was disassembled, the positive electrode was taken out, the coin cell half cell was assembled, and the test was carried out. The initial charge capacity of the positive electrode after the cycle was C_final = 136 mAh / g.

[0019] (5) Calculation of positive electrode degradation: The degradation ratio of the positive electrode of B-1 battery at the end of the cycle in step (4) = 100% - C_end / C_initial = 10.5%; (6) Judgment: Since |100%-C_final / C_initial-20%|=9.5%>5%, the electrolyte volume m1 is considered to be too low. (7) Repeat steps: Repeat steps (4), (5) and (6) above to obtain the relevant data for batteries B-1, B-2, B-3, B-4, B-5, B-6 and B-7, as shown in the table below:

[0020] Based on the table above, the critical value for ensuring that the electrolyte content of this battery is not too low is 570g. Example 2

[0021] A method for determining insufficient electrolyte level in a lithium iron phosphate battery includes the following steps: (1) Prepare batteries: Prepare four 50Ah lithium iron phosphate batteries with different electrolyte amounts. These four lithium iron phosphate batteries are identical except for the amount of electrolyte. The electrolyte amounts from low to high are m1=145g, m2=155g, m3=165g, and m4=175g. The corresponding lithium iron phosphate batteries are labeled B-1, B-2, B-3, and B-4.

[0022] The composition of the lithium iron phosphate battery is as follows: by mass percentage, the positive electrode is 96.5% lithium iron phosphate (Defang DF-5): 1.5% carbon black (SP): 2% binder (1810); the negative electrode is 96% graphite (Shanshan QG-1): 1% carbon black (SP): 1.2% dispersant (Daicel CMC2200): 1.8% binder (Daoying GD1346L); the separator is a 9+3 (9+3 is the separator model, 9 represents the thickness of the base film, and 3 represents the thickness of the ceramic coating, both in μm) PE coated separator; the electrolyte is Tinci E8087.

[0023] (2) Fresh electrode test: The positive electrode sheets of the four lithium iron phosphate batteries in step (1) were assembled into coin cells and tested to obtain the average secondary charging capacity of the fresh positive electrode sheets, Cinitial = 154 mAh / g. It should be noted that the test methods for the four lithium iron phosphate battery positive electrode sheets and the materials used are the same, so the secondary charging capacity values ​​are relatively close. Generally, the average value can be taken.

[0024] (3) Cyclic test: The lithium iron phosphate batteries B-1, B-2, B-3 and B-4 with different electrolyte amounts were subjected to a 1C cycle test at room temperature until the charging capacity decayed to 80% of the initial charging capacity (equivalent to a 20% decrease in battery charging capacity).

[0025] (4) Disassembly and analysis: Taking B-1 battery as an example, the B-1 battery after the cycle test in step (3) was discharged to 2.0V, and then the battery was disassembled, the positive electrode was taken out, the coin cell half cell was assembled, and the test was carried out. The initial charge capacity of the positive electrode after the cycle was C_final = 135 mAh / g.

[0026] (5) Calculation of positive electrode degradation: The degradation ratio of the positive electrode of battery B-1 at the end of the cycle in step (4) = 100% - C_end / C_initial = 12.3%; (6) Judgment: Since |100%-C_final / C_initial-20%|=7.7%>5%, the electrolyte volume m1 is considered to be too low. (7) Repeat steps: Repeat steps (4), (5) and (6) above to obtain the relevant data for batteries B-1, B-2, B-3 and B-4, as shown in the table below:

[0027] Based on the table above, the critical value for ensuring that the electrolyte content of this battery is not too low is 165g.

[0028] In addition to the above embodiments, the present invention also includes other embodiments. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A method for determining the lack of electrolyte in a lithium iron phosphate battery, characterized by: Includes the following steps: (1) Prepare batteries: Prepare several lithium iron phosphate batteries with different amounts of electrolyte. The electrolyte amounts are m1, m2, m3, ... in order of increasing amount. The corresponding lithium iron phosphate batteries are B-1, B-2, B-3, ... in order. (2) Fresh electrode test: Assemble coin cells from several positive electrode sheets of lithium iron phosphate batteries in step (1) and test them to obtain the secondary charging capacity of the fresh positive electrode sheets, which are denoted as Cinitial-1, Cinitial-2, Cinitial-3...; (3) Cyclic test: Cyclic tests were conducted on several lithium iron phosphate batteries with different electrolyte contents from step (1) until the charging capacity decreased to 80% of the initial charging capacity; (4) Disassembly and analysis: The lithium iron phosphate batteries B-1, B-2, B-3... after the cycle test in step (3) were discharged, and then the batteries were disassembled, the positive electrode was taken out, the coin half-cells were assembled, and the test was carried out to obtain the first charge capacity of the positive electrode after the cycle, which was recorded as C-1, C-2, C-3...; (5) Calculation of positive electrode decay: The decay ratio of the positive electrode of the lithium iron phosphate battery at the end of the cycle in step (4) = 100% - C_end / C_beginning, where C_beginning is C_beginning-1, C_beginning-2, C_beginning-3... and the corresponding C_end is C_end-1, C_end-2, C_end-3...; (6) Judgment: Judge the positive electrode attenuation calculation results in step (5): If |100%-C_end / C_initial-20%|≤5%, then the electrolyte content of the corresponding lithium iron phosphate battery is considered not too low. If |100%-C_end / C_initial-20%|>5%, then the electrolyte content of the corresponding lithium iron phosphate battery is considered to be too low.

2. The method for determining insufficient electrolyte level in a lithium iron phosphate battery according to claim 1, characterized in that: The only difference between the several lithium iron phosphate batteries containing different amounts of electrolyte is the amount of electrolyte.

3. The method for determining insufficient electrolyte level in a lithium iron phosphate battery according to claim 1, characterized in that: Based on the positive electrode decay calculation results in step (5), the critical value for insufficient electrolyte can also be obtained.

4. The method for determining insufficient electrolyte level in a lithium iron phosphate battery according to claim 1, characterized in that: The range of electrolyte volume in step (1) is 2.5g / Ah*battery capacity to 6g / Ah*battery capacity.

5. The method for determining insufficient electrolyte level in a lithium iron phosphate battery according to claim 1, characterized in that: In step (4), the lithium iron phosphate battery is discharged to 2.0V.

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