Electrode Resistance Method Is Used To Evaluate The Uniformity 0f Coating On The AB Surface Of The Pole Piece

Lithium-ion batteries are a comprehensive system assembled from positive and negative electrodes, separators, and electrolytes. When the battery is working, electrons and ions are transported in the microstructure of the electrodes, and a series of chemical and electrochemical reactions occur. Therefore, The conductivity of the pole piece and the uniformity of the conductive network are important factors affecting the performance of the battery. For the pole piece coating process, most of them are double-sided single-layer coating at present. After the A-side coating is completed, the B-side coating is carried out. 

The method of evaluating the uniformity of coating on both sides mainly uses thickness measurement or surface density, but When the slurry changes to a certain extent due to the difference in coating time, the conductive uniformity of the AB surface may also be different. How to more comprehensively evaluate the uniformity of the double-sided pole piece is the key to the quality monitoring of the pole piece¹. In this paper, the pole sheet resistance method is used to try different test methods to distinguish the difference between the AB surface, and finally the measurement method that can effectively distinguish the difference is clarified, which can be used to evaluate the consistency of the coating and rolling process, and help the cell production process quality control. 

Figure 1. Double-sided Pole Piece Roll

1.Experimental Equipment And Test Methods

1.1 Experimental Equipment: Model BER1300 (IEST), the electrode diameter is 14mm, and the equipment is shown in Figure 2 (a) and 2 (b).

Figure 2. (a) Appearance of BER1300; (b) Structure of BER1300

1.2 Test Method: Place the electrode to be tested between the two electrodes of the electrode resistance meter, set the test pressure and dwell time parameters on the MRMS software, start the test, and the software will automatically read the data such as electrode thickness, resistance, resistivity, and conductivity.Each electrode is randomly selected from different positions for testing, and the coefficient of variation COV (Coefficient of Variation) is calculated according to formula (1). The larger the COV, the worse the uniformity of the electrode.

2.Evaluate the Significance Of AB Surface Coating Uniformity

2.1 Possible Reasons for Differences in AB Topcoat

The coating process stage involves many process parameters, each of which affects the coated pole piece differently. For example, the characteristic parameters of the slurry, the electrode slurry is micron-scale active solid particles, nano-scale conductive agent particles suspended in the binder solution, the solid particles are subjected to gravity, Brownian motion force, buoyancy, etc., there are sedimentation, random Brownian motion, agglomeration-Depolymerization and other movement processes, so the distribution state of the slurry conductive agent, active particles and the interaction between them will inevitably change, which will affect the coating uniformity. Therefore, during the coating process, there may be differences in the length direction of the pole piece and the AB surface, such as inconsistent surface density, different distribution states of conductive agents and binders, etc. 

At the same time, during the drying process of the electrode wet coating, when the solvent drying process migrates to the surface of the electrode , the binder and conductive agent particles dissolved in the solvent may migrate together with the evaporation of the solvent, causing the binder to float and conduct electricity. The distribution of the agent is also uneven, especially after the surface A is coated and dried, the process of coating the B surface is carried out. When the drying parameters of the B surface are the same as those of the A surface, due to the influence of the A surface coating, the wet coating of the B surface pole piece The drying state and rate may be different, which may easily lead to differences in the AB surface, especially the distribution state of the binder and conductive agent, resulting in differences in the bonding strength and conductivity of the Ab surface of the coating.

2.2 Effect of AB Surface Coating Difference on Performance

The difference in AB surface coating will inevitably lead to poor consistency of the battery, especially the difference in AB surface coating may lead to the following problems: (1) The density of the two surfaces is inconsistent, or the utilization rate of the active material is inconsistent due to the difference in conductivity, which leads to the actual AB surface. The capacity ratio N/P of the negative and positive electrodes is different, and there may be a phenomenon that lithium will not be deposited on one side and lithium will be deposited on the other side; (2) Due to the difference in conductivity of the AB side, the degree of lithiation or the state of charge on the two sides of the battery is different. During the long-term cycle, the long-term accumulation of different stresses on the AB surface will cause cracks in the electrode, and the coating will fall off and fail. Therefore, it is of great significance to evaluate the uniformity and difference of AB topcoat.

3.Data Analysis

3.1 Introduction of Five Test Methods

In order to evaluate the coating consistency of the AB side of the double-sided pole piece, we use five test methods as shown in Figure 3, in which methods 1 and 2 are to place the front and back of the electrode in the middle of the upper and lower test electrodes, and methods 3 and 4 The electrode pieces are folded in opposite directions and placed in the middle of the upper and lower test electrodes. Method 5 is based on method 4. Cut the folded line to disconnect the electrode pieces at the corners, and then place them in the middle of the upper and lower test electrodes. Analyze the total resistance measured by the five test methods. 

The current transmission path and calculation formula are shown in Figure 3, including the resistance of the A or B side coating itself, the resistance of the foil itself, the test electrode and the A or B side coating Layer contact resistance, coating-to-foil contact resistance, and coating-to-coating contact resistance. When ignoring the slight difference between the AB surface coating and the surface state of the upper and lower electrodes, it can be theoretically inferred from the calculation formulas of each method that there is no significant difference between Mode 1 and Mode 2, and the difference between Mode 3 and Mode 4 mainly comes from the A or B surface coating Because the applied current flows more in the two test methods, it will choose to penetrate the foil with better conductivity. The total resistance of mode 5 is slightly greater than the sum of mode 1 and mode 2, because this mode has one more contact resistance of coating A and coating B. Next, we further verify the theoretical calculation results through the measured resistance data.

Figure 3. Schematic diagram of five double-sided electrode test methods and theoretical resistance calculation formula

3.2 Electrode Resistance Test in Five Ways

Select the double-sided positive and negative electrodes respectively to test the resistance of the electrode in the above five ways. Use 25MPa to test the pressure, hold the pressure for 15s, and test 5 parallel points in each group. The results are shown in Figure 4. Considering that there may be some differences in the overall uniformity distribution of the electrode, the resistance values measured by the first two ways of placing the electrode in front and back can be considered to have no difference, while the two ways of folding A face up and folding B face up, There is a significant difference in the resistance value, and the difference between the two methods mainly comes from the difference in the A and B coatings themselves. When the folded line is cut, it can be clearly seen from Figure 4(b) and (d) that the total resistance increases more than twice, which is consistent with the theoretical explanation. At this time, the applied current continues after penetrating the foil. Vertically across the underlying coating to reach the lower electrode. Therefore, when evaluating the coating consistency of the positive and negative electrode sheets AB, the resistance of the electrode sheet can be tested by folding the front and back of the electrode sheet.

Figure 4. Electrode resistance comparison chart using five test methods for positive and negative electrodes

4. Conclusion

In this paper, the BER series electrode sheet resistance meter of IEST is used to compare the resistance difference of positive and negative electrode sheets in different test methods. Combining theoretical analysis and actual measurement data, it is found that when the positive and negative electrode sheet is used for testing, it can be used for Distinguishing the difference between the AB side of the double-sided coated pole piece, this method can be used to evaluate the consistency of the coating and rolling process, and help the quality control of the cell production process.

5.References

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