Electrode Resistance Test Parameter Selection Basis

The evaluation of the coating uniformity of the positive and negative electrodes is an important monitoring item in the battery production process. If the uniformity of the electrode sheets after coating or rolling is poor, it will significantly affect the performance consistency of the back-end cells. At present, the commonly used method to evaluate the coating uniformity of the pole piece is mainly to monitor the thickness, quality and resistance, among the selection of test parameters for electrode sheet resistance (membrane resistance), such as test pressure and pressure holding time, is extremely important to the stability of the measurement results, this paper proposes a more appropriate test parameter based on the specific test data of the electrode sheet resistance, so as to ensure the accuracy and stability of the test data.

Figure 1. Schematic diagram of pole piece resistance test

1. Laboratory Equipment

Equipment Model: BER2500 (IEST), electrode diameter 14mm. The equipment is shown in Figures 2(a) and 2(b).

2. Data Analysis

We carried out in-situ thickness and resistance tests on the positive and negative electrodes after rolling under different pressures (5-60MPa), as shown in Figure 3. It can be seen from the figure that with the increase of the test pressure, the resistance of the positive and negative electrodes decreases continuously, and the maximum thickness of the positive electrode changes by about 4 μm, and the maximum thickness of the negative electrode changes by about 25 μm. During the test, it is necessary not to affect the state of the sample to be tested as much as possible, and at the same time, it is necessary to ensure that the test electrode is in close contact with the sample. Therefore, we recommend that the test pressure of the positive electrode piece be 25MPa, and the negative electrode piece be selected as 5MPa.

Figure 3. Test results of resistance and thickness of positive and negative electrodes under different pressure conditions

Next, we further explored the appropriate pressure holding time, and tested the resistance and thickness data of the positive and negative electrodes under the same pressure for 100 seconds continuously, as shown in Figure 4. It can be seen from the figure that as the holding time increases, the resistance of the positive and negative electrodes decreases continuously, and the maximum thickness of the positive electrode varies by no more than 1 μm, and the maximum thickness of the negative electrode varies by about 1.5 μm. During the test, it is necessary to shorten the test time as much as possible, improve the test efficiency, and at the same time ensure the stability of the test data. Therefore, we recommend that the pressure holding time of the positive and negative electrodes be selected as 15s.

Figure 4. Test results of resistance and thickness of positive and negative electrodes under different holding time conditions

Using the above test conditions, we selected 10 positive pole pieces to conduct MSA analysis on the equipment. Since each position of the pole piece cannot be reused after testing, it is a destructive test. We chose nested GR&R research, and the results are shown in Figure 5, which shows that the repeatability and distinguishability of the equipment are at an excellent level.

Figure 5. GR&R Analysis of Electrode Sheet Resistance Meter

3.Summary

Through the exploration of the test parameters (pressure and holding time) of the electrode resistance, we recommend 25MPa for the positive electrode, 5MPa for the negative electrode, and a holding time of 15s for both, the repeatability and distinguishability of the resistance data obtained from the test under this condition reached an excellent level.

4.Reference Materials

1.Serena W. Peterson and Dean R. Wheeler, Direct Measurements of Effective Electronic Transport in Porous Li-Ion Electrodes. Journal of The Electrochemical Society, 161 (14) A2175-A2181 (2014).

2.Hiroki Kondo et al. Influence of the Active Material on the Electronic Conductivity of the Positive Electrode in Lithium-Ion Batteries. Journal of The Electrochemical Society, 2019,166 (8) A1285-A1290

3.B.G. Westphal et al. Influence of high intensive dry mixing and calendering on relative electrode resistivity determined via an advanced two point approach. Journal of Energy Storage 2017, 11, 76–85