A rapid evaluation method for fast-charging graphite cycle performance of lithium-ion batteries

Abstract

The present invention relates to a method for quickly evaluating the cycle performance of fast-charged graphite for lithium-ion batteries, specifically: (1) at room temperature, first discharge the standard fast-charged battery and the fast-charged battery to be tested, then fully charge them with a large current, and after fully charging, let them stand until the voltage remains unchanged, and record the voltage drops of the two batteries respectively; (2) perform a high-temperature cycle test on the standard fast-charged battery and the fast-charged battery to be tested; (3) evaluate and judge the cycle performance of the two fast-charged graphites by comparing the voltage drop and high-temperature cycle performance of the standard fast-charged battery and the fast-charged battery to be tested. The method for quickly evaluating the cycle performance of fast-charged graphite for lithium-ion batteries of the present invention can quickly screen and evaluate fast-charged graphite, shorten the evaluation cycle of fast-charged graphite, and the method is simple, easy to operate and implement.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries, and in particular relates to a method for quickly evaluating the fast-charging graphite cycle performance of lithium ion batteries. Background Art

[0002] Lithium-ion batteries have outstanding advantages such as high energy density, no memory effect, long cycle life, fast charging and discharging, and low self-discharge. They are widely used in portable devices, electric vehicles, and energy storage fields, and have huge market demand.

[0003] The current power battery market has higher and higher requirements for battery charging speed, and the frequency modulation energy storage market also requires batteries to have a charging current of more than 2C. There are many factors that affect the battery charging speed: including the chemical system, charging method, etc. Among them, the negative electrode has the greatest impact on the charging speed in the chemical system, so it is necessary to select the appropriate fast-charging graphite.

[0004] The current method for evaluating and screening the cycle performance of fast-charging graphite is to make a full battery and then conduct a cycle life test according to a certain cycle format, mostly full charge and discharge, until 80% of the initial life is reached and the test is terminated. With a 2C charge and discharge format, it takes about 250 days to complete 3,000 cycles of testing. It can be seen that such a long test cycle not only wastes a lot of time, but also increases a lot of costs including manpower, material resources and electricity consumption.

[0005] Given that the traditional method of evaluating the negative electrode cycle performance is too time-consuming, energy-intensive, and costly, it takes up a long time to test the equipment, which invisibly increases the equipment cost. It is urgent to study a method for quickly evaluating the fast-charging graphite cycle performance of lithium-ion batteries. Summary of the invention

[0006] The purpose of the present invention is to provide a method for quickly evaluating the cycle performance of fast-charged graphite for lithium-ion batteries, which can quickly screen and evaluate fast-charged graphite.

[0007] The technical solution adopted by the present invention to solve the above problems is: a method for quickly evaluating the fast-charging graphite cycle performance of a lithium-ion battery, comprising the following steps:

[0008] (1) At room temperature, discharge the standard fast-charging battery B containing fast-charging graphite G at 1C to the discharge cut-off voltage, let it stand for a long time, and then charge the battery B with a large current constant current constant voltage to the charge cut-off voltage V, let it stand for a long time until the voltage remains unchanged, which is recorded as V stable, where the large current is greater than 1C;

[0009] (2) At room temperature, the fast-charging battery B-1 to be tested containing the fast-charging graphite G1 is discharged at 1C to the discharge cut-off voltage, and then fully rested. Then, the battery B-1 is charged with a large current constant current constant voltage to the charge cut-off voltage V1, and fully rested until the voltage remains unchanged, which is recorded as V1 stable, where the large current is greater than 1C;

[0010] Among them, V=V1, the standard fast-charge battery B and the fast-charge battery to be tested B-1 are only different in the fast-charge graphite of the negative electrode, and the rest are exactly the same;

[0011] (3) Calculate the voltage drop of the battery, where the voltage drop of the standard fast-charge battery B is VVstable, and the voltage drop of the fast-charge battery B-1 to be tested is V1- V1stable;

[0012] (4) Perform high temperature cycling on the standard fast-charge battery B and the fast-charge battery to be tested B-1 to compare their cycling performance;

[0013] (5) The results of steps (3) and (4) are analyzed on a case-by-case basis, as follows:

[0014] a. If the voltage drop V1- V1stable ≤ V- Vstable, and the high-temperature cycle performance of the fast-charge battery B-1 to be tested is better than or equal to the standard fast-charge battery B, then it can be judged that the room-temperature high-rate cycle performance of the fast-charge graphite G1 is better than or equal to the fast-charge graphite G;

[0015] b. If the voltage drop V1- V1stable>V- Vstable, the high temperature cycle performance of the fast-charge battery B-1 to be tested is worse than that of the standard fast-charge battery B, then it can be judged that the room temperature high rate cycle performance of the fast-charge graphite G1 is worse than that of the fast-charge graphite G;

[0016] c. If the voltage drop V1- V1stable ≤ V- Vstable, the high temperature cycle performance of the fast-charged battery B-1 to be tested is worse than that of the standard fast-charged battery B, then the maximum temperature Tmax of the surface of the battery cycled at room temperature and high rate is compared. If Tmax≤35°C, it can be judged that the room temperature and high rate cycle performance of the fast-charged graphite G1 is better than or equal to that of the fast-charged graphite G. If Tmax>35°C, it can be judged that the room temperature and high rate cycle performance of the fast-charged graphite G1 is worse than that of the fast-charged graphite G.

[0017] d. If the voltage drop V1- V1stable>V- Vstable, the high-temperature cycle performance of the fast-charged battery B-1 to be tested is better than or equal to that of the standard fast-charged battery B, then the battery that is cycled at a high rate at a normal temperature is fully disassembled to observe whether there is lithium deposition on the surface of the negative electrode. If lithium deposition occurs, it can be determined that the normal temperature high rate cycle performance of the fast-charged graphite G1 is worse than that of the fast-charged graphite G. If no lithium deposition occurs, it can be determined that the normal temperature high rate cycle performance of the fast-charged graphite G1 is better than or equal to that of the fast-charged graphite G.

[0018] Wherein, in step (5), the high rate in the room temperature high rate cycle is greater than or equal to 2°C, and the number of cycles is greater than or equal to 10 times.

[0019] Preferably, the fast-charging battery is any one of a lithium iron phosphate lithium-ion battery, a ternary lithium-ion battery, and a lithium manganese oxide lithium-ion battery.

[0020] Preferably, the discharge cut-off voltage in steps (1) and (2) is 2.5V, 2.75V or 3V, and the charge cut-off voltage in steps (1) and (2) is 3.65V or 4.2V.

[0021] Preferably, the sufficient standing time in steps (1) and (2) is ≥ 60 min.

[0022] Preferably, the specific parameters of the high temperature cycle in step (4) are: temperature ≥ 45°C, and the number of cycles is 300-500 times.

[0023] Compared with the prior art, the advantages of the present invention are:

[0024] The invention provides a method for quickly evaluating the cycle performance of fast-filling graphite, which can quickly screen and evaluate the fast-filling graphite, shortens the evaluation period of the fast-filling graphite, and the method is simple and easy to operate and implement. DETAILED DESCRIPTION

[0025] The present invention is further described in detail below with reference to the embodiments. Example

[0026] A rapid evaluation method for fast-filled graphite comprises the following steps:

[0027] (1) At room temperature, a standard lithium iron phosphate fast-charged battery B (the components of battery B: the positive electrode is 96% Defang DY-1: 2% SP: 2% HSV900; the negative electrode is 95% Keda KD-4: 1.5% SP: 1.5% CMC2200: 2% GD1346L; the diaphragm is a 9+3 (9+3 is the diaphragm model, 9 represents the thickness of the base film, 3 represents the thickness of the coating, both in microns) PE coated ceramic diaphragm; the electrolyte is Tianci E8087; all percentages are mass percentages) was discharged at 1C to a discharge cut-off voltage of 2.5V, and then allowed to stand for 60 minutes. Then, battery B was charged with a high current constant current and constant voltage of 2C to a charge cut-off voltage of 3.65V, and then allowed to stand for 60 minutes until the voltage remained unchanged, recorded as 3.35V.

[0028] (2) At room temperature, the lithium iron phosphate fast-charge battery B-1 (the negative electrode graphite is Shanshan CAG-3MT, and the rest is exactly the same as battery B) is discharged at 1C to a discharge cut-off voltage of 2.5V, and then allowed to stand for 60 minutes. Then, the battery B-1 is charged with a high current constant current and constant voltage of 2C to a charge cut-off voltage of 3.65V, and then allowed to stand for 60 minutes until the voltage remains unchanged, which is recorded as 3.40V.

[0029] The lithium iron phosphate standard fast-charging battery B and the lithium iron phosphate fast-charging battery to be tested B-1 are identical except for the fast-charging graphite in the negative electrode.

[0030] (3) Calculate the voltage drop of the battery. The voltage drop of the lithium iron phosphate standard fast-charge battery B is 3.65-3.35=0.30V, and the voltage drop of the lithium iron phosphate fast-charge battery to be tested B-1 is 3.65-3.40=0.25V.

[0031] (4) The standard lithium iron phosphate fast-charged battery B and the lithium iron phosphate fast-charged battery to be tested B-1 were cycled at 45°C for 400 times at 1C. The results showed that the battery B was 400 (85%) and the battery B-1 was 400 (88%). Therefore, the 45°C 1C cycle of the battery B-1 was better.

[0032] (6) By comparing and analyzing the results of step (3) and step (4), it is found that compared with the standard lithium iron phosphate fast-charged battery B, the lithium iron phosphate fast-charged battery B-1 has a smaller voltage drop and a better 1C cycle at a high temperature of 45°C. It can be judged that the room temperature 2C cycle performance of the fast-charged graphite CAG-3MT is better than or equal to the fast-charged graphite KD-4.

[0033] 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 quickly evaluating the fast-charging graphite cycle performance of a lithium-ion battery, characterized in that: The following steps are involved: (1) At room temperature, discharge the standard fast-charging battery B containing fast-charging graphite G at 1C to the discharge cut-off voltage, let it stand for a long time, and then charge the battery B with a large current constant current constant voltage to the charge cut-off voltage V, let it stand for a long time until the voltage remains unchanged, which is recorded as V stable, where the large current is greater than 1C; (2) At room temperature, the fast-charging battery B-1 to be tested containing the fast-charging graphite G1 is discharged at 1C to the discharge cut-off voltage, and then fully rested. Then, the battery B-1 is charged with a large current constant current constant voltage to the charge cut-off voltage V1, and fully rested until the voltage remains unchanged, which is recorded as V1 stable, where the large current is greater than 1C; Among them, V=V1, the standard fast-charge battery B and the fast-charge battery to be tested B-1 are only different in the fast-charge graphite of the negative electrode, and the rest are exactly the same; (3) Calculate the voltage drop of the battery, where the voltage drop of the standard fast-charge battery B is VVstable, and the voltage drop of the fast-charge battery B-1 to be tested is V1- V1stable; (4) Perform high temperature cycling on the standard fast-charge battery B and the fast-charge battery to be tested B-1 to compare their cycling performance; (5) The results of steps (3) and (4) are analyzed on a case-by-case basis, as follows: a. If the voltage drop V1- V1stable ≤ V- Vstable, and the high-temperature cycle performance of the fast-charge battery B-1 to be tested is better than or equal to the standard fast-charge battery B, then it can be judged that the room-temperature high-rate cycle performance of the fast-charge graphite G1 is better than or equal to the fast-charge graphite G; b. If the voltage drop V1- V1stable>V- Vstable, the high temperature cycle performance of the fast-charge battery B-1 to be tested is worse than that of the standard fast-charge battery B, then it can be judged that the room temperature high rate cycle performance of the fast-charge graphite G1 is worse than that of the fast-charge graphite G; c. If the voltage drop V1- V1stable ≤ V- Vstable, the high temperature cycle performance of the fast-charged battery B-1 to be tested is worse than that of the standard fast-charged battery B, then only the maximum temperature Tmax of the surface of the fast-charged battery B-1 to be tested under normal temperature and high rate cycling is compared. If Tmax≤35°C, it can be judged that the normal temperature and high rate cycle performance of the fast-charged graphite G1 is better than or equal to that of the fast-charged graphite G. If Tmax>35°C, it can be judged that the normal temperature and high rate cycle performance of the fast-charged graphite G1 is worse than that of the fast-charged graphite G. d. If the voltage drop V1- V1stable>V- Vstable, the high temperature cycle performance of the fast-charged battery B-1 to be tested is better than or equal to the standard fast-charged battery B, then only the fast-charged battery B-1 to be tested that is cycled at a high rate at a normal temperature is fully disassembled to observe whether there is lithium deposition on the surface of the negative electrode. If lithium deposition occurs, it can be determined that the normal temperature high rate cycle performance of the fast-charged graphite G1 is worse than that of the fast-charged graphite G. If no lithium deposition occurs, it can be determined that the normal temperature high rate cycle performance of the fast-charged graphite G1 is better than or equal to that of the fast-charged graphite G. Wherein, in step (5), the high rate in the room temperature high rate cycle is greater than or equal to 2°C, and the number of cycles is greater than or equal to 10 times.

2. The rapid evaluation method for fast-charging graphite cycle performance of a lithium-ion battery according to claim 1, characterized in that: The fast-charging battery is any one of a lithium iron phosphate lithium-ion battery, a ternary lithium-ion battery, and a lithium manganese oxide lithium-ion battery.

3. The rapid evaluation method for fast-charging graphite cycle performance of a lithium-ion battery according to claim 1, characterized in that: The discharge cut-off voltage in steps (1) and (2) is 2.5V, 2.75V or 3V, and the charge cut-off voltage in steps (1) and (2) is 3.65V or 4.2V.

4. The rapid evaluation method for fast-charging graphite cycle performance of a lithium-ion battery according to claim 1, characterized in that: The sufficient standing time in steps (1) and (2) is ≥ 60 min.

5. The rapid evaluation method for fast-charging graphite cycle performance of a lithium-ion battery according to claim 1, characterized in that: The specific parameters of the high temperature cycle in step (4) are: temperature ≥ 45°C, and the number of cycles is 300-500 times.