A method for determining the capacity loss type of lithium-ion batteries during high-temperature storage

Abstract

The present invention relates to a method for judging the capacity loss type of a lithium-ion battery during high-temperature storage, specifically: (1) taking a fresh lithium iron phosphate battery and conducting a high-temperature storage test; (2) calculating the capacity retention rate and recovery rate of the high-temperature storage test based on the test results; (3) calculating the capacity loss, including large current reversible capacity loss, small current reversible capacity loss, and small current irreversible capacity loss; (4) the correspondence between the capacity loss type and the cause; (5) determining the main capacity loss type based on the capacity loss size relationship of different capacity loss types, finding the corresponding cause, and making targeted improvements to the high-temperature storage based on the cause. The judgment method of the present invention can distinguish the capacity loss types of high-temperature storage, determine the causes corresponding to different capacity loss types, and determine the main capacity loss type, which is conducive to making targeted improvements based on the cause.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a method for determining the capacity loss type of a lithium-ion battery during high-temperature storage. Background Art

[0002] In some application scenarios of lithium-ion batteries, they will be placed in high temperature for a long time, which requires lithium-ion batteries to have good high-temperature storage performance. The actual battery will inevitably have capacity loss after high-temperature storage. The types of capacity loss in high-temperature storage include reversible capacity loss and irreversible capacity loss. The reversible capacity loss is divided into: high-current reversible capacity loss and low-current reversible capacity loss. The irreversible capacity loss refers to the small-current irreversible capacity loss.

[0003] There are many reasons for the loss of high-temperature storage capacity of lithium-ion batteries, generally the following three reasons: (1) micro short circuit; (2) polarization; (3) positive and negative electrode interface reaction; reasons (1) and (2) will lead to reversible capacity loss, of which (1) is reversible for large currents and (2) is reversible for small currents; reason (3) will lead to irreversible capacity loss. Therefore, when improving the high-temperature storage performance of mobile phones, R&D personnel need to find the cause. Only by finding the cause can they prescribe the right remedy.

[0004] In summary, there is an urgent need for a method for determining the capacity loss type of lithium-ion batteries during high-temperature storage. Summary of the invention

[0005] The purpose of the present invention is to provide a method for determining the type of capacity loss of a lithium-ion battery during high temperature storage.

[0006] The technical solution adopted by the present invention to solve the above-mentioned problem is a method for determining the capacity loss type of a lithium-ion battery stored at high temperature, comprising the following steps:

[0007] (1) Take a fresh lithium-ion battery C and perform a high-temperature storage test. The specific steps are as follows:

[0008] a. Charge and discharge battery C three times at room temperature, with a current of I, a voltage range of 2.5-3.65V, and a charge cut-off current of 0.05C. Leave the battery for a while between each charge and discharge, and record the average discharge capacity, which is the initial capacity D1;

[0009] b. Fully charge battery C at room temperature with a current of I, constant current and constant voltage charging, a cut-off voltage of 3.65V, a cut-off current of 0.05C, and then leave it at a temperature of T for a rest time of t;

[0010] c. After standing, discharge battery C to 2.5V with current I, and record the discharge capacity, which is the capacity retention value D2;

[0011] d. Charge and discharge battery C three times, with a current of I, a voltage range of 2.5-3.65V, and a charge cut-off current of 0.05C. Leave it for a while between each charge and discharge, and record the average value of the discharge capacity, which is the large current recovery value D3;

[0012] e. Charge and discharge battery C three times, with a current of I', a voltage range of 2.5-3.65V, and a charge cut-off current of 0.05C. Leave it for a while between each charge and discharge, and record the average value of the discharge capacity, which is the small current recovery value D4;

[0013] Among them, the current I is in the range of 0.5-2C, and the current I' is in the range of 0.1-0.2C;

[0014] (2) Calculate the capacity retention rate and recovery rate of the high-temperature storage test: capacity retention rate = D2 / D1, high current capacity recovery rate = D3 / D1, low current capacity recovery rate = D4 / D1;

[0015] (3) Calculation of capacity loss: large current reversible capacity loss = D3 / D1-D2 / D1; small current reversible capacity loss = D4 / D1- D3 / D1; reversible capacity loss = large current reversible capacity loss + small current reversible capacity loss; irreversible capacity loss = small current irreversible capacity loss = 100%-D4 / D1; capacity loss = reversible capacity loss + irreversible capacity loss;

[0016] (4) Correspondence between capacity loss type and cause: The cause of large current reversible capacity loss is micro short circuit; the cause of small current reversible capacity loss is polarization; the cause of small current irreversible capacity loss is positive and negative electrode interface reaction;

[0017] (5) By comparing and ranking the size relationships of the three types of capacity loss, namely, large current reversible capacity loss, small current reversible capacity loss, and small current irreversible capacity loss, the main type of capacity loss is determined, its corresponding cause is found, and targeted improvements to high-temperature storage are made based on the cause.

[0018] Preferably, the temperature T in step (1) is 25-60° C., and the time t is 7-180 days.

[0019] Preferably, the standing time between each charge and discharge in step (1) is 30 minutes.

[0020] The verification method of the method of the present invention comprises the following specific steps:

[0021] (1) Verification: The reason for the loss of high current reversible capacity is micro short circuit. The detailed process is as follows:

[0022] a. Lithium-ion batteries C1 and C2 were prepared according to the mass production process. The raw materials and processes of the two batteries were exactly the same. The only difference was that when C1 was wound, the dust removal was turned on; when C2 was wound, the dust removal was turned off and some dust was sprayed onto the winding needle.

[0023] b. Perform high temperature storage test on two batteries, the test method is the same as the above invention method, where the large current I=1C, the small current I'=0.1C, and calculate the capacity retention rate and recovery rate, as well as different types of capacity loss. According to the results, if there is a significant difference in the large current reversible capacity loss D3 / D1- D2 / D1, and there is no significant difference in the small current reversible capacity loss D4 / D1-D3 / D1, and there is no significant difference in the small current irreversible capacity loss 100%- D4 / D1, it can be judged that the cause of the large current reversible capacity loss is micro-short circuit.

[0024] (2) Verification: The reason for the loss of reversible capacity at low current is polarization. It is generally recognized in the industry that low current can eliminate polarization and does not require verification.

[0025] (3) Verification: The reason for the irreversible capacity loss at low current is the interface reaction between the positive and negative electrodes. The detailed process is as follows:

[0026] a. Take two batteries prepared for mass production before injection, and inject two different electrolytes A and B. The formula of electrolyte A is: the concentration of lithium salt LiPF6 is 1.0 mol / L, the mass ratio of solvent is EC:EMC=35:65, and the additive is 2.5% VC; the formula of electrolyte B is: the concentration of lithium salt LiPF6 is 1.0 mol / L, the mass ratio of solvent is EC:EMC=35:65, the additive is 2.5% VC, 1% LiODFB, and then lithium iron phosphate batteries CA and CB are prepared according to the mass production process.

[0027] b. Perform high temperature storage test on two batteries, the test method is the same as the above invention method, where the large current I=1C, the small current I'=0.1C, and calculate the capacity retention rate and recovery rate, as well as different types of capacity loss. According to the results, if there is no significant difference in the large current reversible capacity loss D3 / D1-D2 / D1, and there is no significant difference in the small current reversible capacity loss D4 / D1-D3 / D1, and there is a significant difference in the small current irreversible capacity loss 100%-D4 / D1, it can be judged that the cause of the small current irreversible capacity loss is the positive and negative electrode interface reaction.

[0028] Compared with the prior art, the advantages of the present invention are:

[0029] The method for determining the capacity loss type of a lithium-ion battery during high-temperature storage of the present invention can distinguish the capacity loss type of a lithium-ion battery during high-temperature storage, determine the causes corresponding to different capacity loss types, and determine the main capacity loss type, which is conducive to targeted improvements based on the causes. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below with reference to the embodiments.

[0031] The lithium-ion battery of this embodiment is a square aluminum shell lithium-ion battery, the positive electrode is lithium iron phosphate, and the negative electrode is graphite. Example

[0032] A method for determining the capacity loss type of a lithium-ion battery during high-temperature storage comprises the following steps:

[0033] (1) Take a fresh 100Ah lithium iron phosphate battery C, select temperature T = 55°C and time t = 28 days, and perform a high temperature storage test. The specific steps are as follows:

[0034] a. Charge and discharge battery C three times at room temperature, with a current of I=1C, a voltage range of 2.5-3.65V, a charge cut-off current of 0.05C, and a 30-min standstill between charge and discharge. Record the average of the discharge capacity, which is the initial capacity D1=101.5Ah;

[0035] b. Fully charge battery C at room temperature (current I=1C constant current and constant voltage charging, cut-off voltage 3.65V, cut-off current 0.05C), and then leave it at temperature T=55℃ for t=28 days;

[0036] c. After the rest period, discharge the battery to 2.5V with a current of I=1C and record the discharge capacity, which is the capacity retention value D2=93.38Ah;

[0037] d. Charge and discharge the battery three times, with a current of I=1C, a voltage range of 2.5-3.65V, a charge cut-off current of 0.05C, and a 30-minute rest between charge and discharge. Record the average discharge capacity, which is the high current recovery value D3=95.92Ah;

[0038] e. Charge and discharge the battery three times with a current of I'=0.1C, a voltage range of 2.5-3.65V, a charge cut-off current of 0.05C, and a 30-min rest period between charge and discharge. Record the average discharge capacity, which is the small current recovery value D4=96.12Ah.

[0039] (2) Calculate the capacity retention rate and recovery rate of the high-temperature storage test: capacity retention rate = D2 / D1 = 92.0%, high-current capacity recovery rate = D3 / D1 = 94.50%, low-current capacity recovery rate = D4 / D1 = 94.70%.

[0040] (3) Calculation of capacity loss: large current reversible capacity loss = D3 / D1- D2 / D1=2.5%; small current reversible capacity loss = D4 / D1- D3 / D1=0.2%; reversible capacity loss = large current reversible capacity loss + small current reversible capacity loss = 2.7%; irreversible capacity loss = small current irreversible capacity loss = 100%-D4 / D1=5.3%; capacity loss = reversible capacity loss + irreversible capacity loss = 8%.

[0041] (4) Correspondence between capacity loss type and cause: The cause of large current reversible capacity loss is micro short circuit; the cause of small current reversible capacity loss is polarization; the cause of small current irreversible capacity loss is positive and negative electrode interface reaction.

[0042] (5) By comparing and ranking the size relationship of the three types of capacity loss, namely, large current reversible capacity loss of 2.5%, small current reversible capacity loss of 0.2%, and small current irreversible capacity loss of 5.3%, 5.3% is the largest, and it is determined that small current irreversible capacity loss is the main type of capacity loss. The reason is the interface reaction between the positive and negative electrodes. Targeted improvements can be made to high-temperature storage based on the reasons.

[0043] In addition to the above embodiments, the present invention also includes other implementation modes. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the protection scope of the claims of the present invention.

Claims

1. A method for determining the capacity loss type of a lithium-ion battery during high-temperature storage, characterized in that: The steps include: (1) Take a fresh lithium-ion battery C and perform a high-temperature storage test. The specific steps are as follows: a. Charge and discharge battery C three times at room temperature, with a current of I, a voltage range of 2.5-3.65V, and a charge cut-off current of 0.05C. Leave the battery for a while between each charge and discharge, and record the average discharge capacity, which is the initial capacity D1; b. Fully charge battery C at room temperature with a current of I, constant current and constant voltage charging, a cut-off voltage of 3.65V, a cut-off current of 0.05C, and then leave it at a temperature of T for a rest time of t; c. After standing, discharge battery C to 2.5V with current I, and record the discharge capacity, which is the capacity retention value D2; d. Charge and discharge battery C three times, with a current of I, a voltage range of 2.5-3.65V, and a charge cut-off current of 0.05C. Leave it for a while between each charge and discharge, and record the average value of the discharge capacity, which is the large current recovery value D3; e. Charge and discharge battery C three times, with a current of I', a voltage range of 2.5-3.65V, and a charge cut-off current of 0.05C. Leave it for a while between each charge and discharge, and record the average value of the discharge capacity, which is the small current recovery value D4; Among them, the current I is in the range of 0.5-2C, and the current I' is in the range of 0.1-0.2C; (2) Calculate the capacity retention rate and recovery rate of the high-temperature storage test: capacity retention rate = D2 / D1, high current capacity recovery rate = D3 / D1, low current capacity recovery rate = D4 / D1; (3) Calculation of capacity loss: large current reversible capacity loss = D3 / D1-D2 / D1; small current reversible capacity loss = D4 / D1- D3 / D1; reversible capacity loss = large current reversible capacity loss + small current reversible capacity loss; irreversible capacity loss = small current irreversible capacity loss = 100%-D4 / D1; capacity loss = reversible capacity loss + irreversible capacity loss; (4) Correspondence between capacity loss type and cause: The cause of large current reversible capacity loss is micro short circuit; the cause of small current reversible capacity loss is polarization; the cause of small current irreversible capacity loss is positive and negative electrode interface reaction; (5) By comparing and ranking the size relationships of the three types of capacity loss, namely, large current reversible capacity loss, small current reversible capacity loss, and small current irreversible capacity loss, the main type of capacity loss is determined, its corresponding cause is found, and targeted improvements to high-temperature storage are made based on the cause.

2. The method for determining the capacity loss type of a lithium-ion battery during high-temperature storage according to claim 1, characterized in that: The temperature T in step (1) is 55-60° C., and the time t is 7-180 days.

3. The method for determining the capacity loss type of a lithium-ion battery during high-temperature storage according to claim 1, characterized in that: In step (1), the rest period between each charge and discharge is 30 minutes.