In this paper, the electrode resistance method is used to evaluate the effects of the conductive agent content and the compaction density on the electronic conductivity of the electrode, which provides a strong support for researchers to determine the optimal formulation and process parameters.

As an important intermediate product in the production process of lithium-ion batteries, electrodes must be monitored and controlled reliably to ensure excellent performance and stability.1 The advantages and disadvantages of electrode formulations and process parameters can greatly affect the final performance of lithium-ion batteries in terms of cycling, multiplication rate, safety, and so on. Electrode sheet contains active material, conductive agent and binder, in order to improve the energy density of lithium-ion battery, the proportion of active material is higher and higher, but the appropriate proportion of the conductive agent content and the type of conductive agent on the performance of the battery multiplication should not be ignored.2-7. In the electrode sheet rolling process, set the appropriate roller pressure to ensure that the compacted density of the electrode sheet is in the appropriate range, and then achieve the balance of the electrode sheet electronic and ionic conductivity, to the R&D and the development and development of the electronic and ionic conductivity of the electrode sheet, to ensure the stability of the performance of the electrode sheet. The balance of ionic conductance is also a big challenge for researchers.

1. Experimental equipment and test methods

1.1 Experimental equipment: Battery Electrode Resistance Tester BER1300 (IEST), electrode diameter 14mm, can be applied pressure 5~60MPa. the equipment is shown in Fig. 1(a) and 1(b).

Figure 1. (a) BER1300 appearance; (b) BER1300 structure

1.2 Test method: cut the electrode sheet to be tested into a rectangular size of about 5cm×10cm, place it on the sample table, set the parameters of test pressure and holding time on the MRMS software, and start the test, and the software automatically reads the thickness of the electrode sheet, resistance, resistivity, conductivity and other data.

2. Data analysis

2.1 Influence of conductive carbon content in positive electrode sheet

For the positive electrode sheet, due to the poor conductivity of the active material itself, the addition of a certain percentage of conductive agent is undoubtedly to improve the conductivity of the electrode sheet is “snow in the charcoal”. Change the content of conductive carbon in the ternary electrode sheet for 1%, 3%, 5%, 7%, other process parameters remain unchanged, the use of BER1300 to test the resistivity of the electrode sheet, the test pressure is set to 25MPa, the holding time of 25s, the parallel samples tested five times, the results are shown in Figure 2. Using minitab on the four groups of different conductive carbon content of the electrode sheet resistivity analysis of variance, from the test results can be seen P<0.05, indicating that the four groups of electrode sheet resistivity has a significant difference, and by the law of change of the mean value can be seen, with the increase in the conductive carbon content, the ternary electrode sheet resistivity gradually reduced, when the conductive carbon content is greater than 5%, the resistivity reduction is smaller, the research and development personnel can be combined with the requirements for battery R&D personnel can combine the requirements of the energy density of the battery to determine the optimal ratio of conductive carbon.

Fig. 2. Analysis of variance of resistivity of four groups of ternary electrodes with different conductive carbon contents

2.2 Effect of the content of conductive agent in the cathode electrode

In the cathode electrode, since the graphite material itself has good conductivity, adding a conductive agent with better conductivity is “icing on the cake”. The content of carbon nanotubes in the graphite electrode was changed to 2%, 3%, and 4%, respectively, and other process parameters remained unchanged. The electrode resistivity was tested using BER1300, the test pressure was set to 25MPa, the pressure holding time was 25s, and the parallel samples were tested 5 times. The results are shown in Figure 3. Minitab was used to perform variance analysis on the resistivity of the three groups of electrode sheets with different carbon nanotube contents. From the test results, it can be seen that P<0.05, indicating that the resistivity of the three groups of electrode sheets is significantly different, and from the change law of the mean, it can be seen that with the increase of the conductive agent content, the resistivity of the graphite electrode sheet almost shows a linear decrease, indicating that the addition of carbon nanotubes can improve the electronic conductivity of the electrode sheet. R&D personnel can determine the optimal conductive agent ratio based on the requirements for battery energy density.

Fig. 3. Variance analysis of resistivity of graphite electrodes with three groups of different conductive agent contents

2.3 Influence of Compaction Density of Electrode Sheet

Compaction density affects the porosity and tortuosity of the electrode sheet, which in turn affects the electronic conductivity and ionic conductivity of the electrode sheet. For the four kinds of positive electrode sheet roll pressure different pressure, other parameters are the same, can get different compaction density of the electrode sheet, using BER1300 test electrode sheet resistivity, test pressure is set to 5MPa, holding time of 25s, parallel samples test 5 times, the results are shown in Figure 4. From the resistivity trend, the resistivity of the four types of wafers decreased with the increase of compaction density, but the slope of the curve change is different. For lithium cobalt oxide (LCO) electrode sheet, when the compacted density reaches 3.3g/cm3, the resistivity reduction is not too significant, lithium iron phosphate (LFP) electrode sheet to reach a compacted density of 1.84g/cm3 or more, the resistivity gradually tends to stabilize, and for the two kinds of different nickel content of NCM811 and NCM622 ternary electrode, the compacted density of 3.08g/cm3 or more, the resistivity reduction tends to be stabilized, while for two different nickel content NCM811 and NCM622 ternary electrode, compacted density of 3.08g/cm3 or more, the resistivity reduction tends to be stabilized. For the two different nickel contents of NCM811 and NCM622 ternary electrode chips, the resistivity reduction tends to be slow when the compacted density reaches 3.08g/cm3 or above, and the resistivity of the electrode chips decreases with the increase of the nickel content, which indicates that the increase of the nickel content can enhance the electronic conductivity of the electrode chips.

Fig. 4. Variance analysis of resistivity of four groups of anode electrode sheets under different compaction density conditions

3. Summary

In this paper, the electrode resistance method is used to evaluate the effect of conductive agent content and compaction density on the electronic conductivity of lithium-ion battery electrode, with the increase of conductive agent content and compaction density can be increased to a certain extent to enhance the conductivity of the electrode, researchers and developers can be further combined with the requirements of the battery’s energy density and ionic conductivity to determine the optimal formulation and process parameters.

4. Reference

[1] IEST, “A New Method for Monitoring Electrode Stability and Uniformity”, https://mp.weixin.qq.com/s/O3wwYuhkY3XspeDDE5qczQ.

[2] Xu Jieru, Li Hong, et al. Conductivity test and analysis methods for research of lithium batteries[J]. Energy Storage Science and Technology, 2018, 7(5) 926-955.

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

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

[5] Rinaldo Raccichini, Alberto Varzi, Stefano Passerini and Bruno Scrosati，The role of graphene for electrochemical energy storage[J]，Nature Materials, 2015 , 3, 14.

[6] Wu Xiangkun, Zhan Qiushe, Zhang Lan, Zhang Suojiang. Microstructure optimization and controllable preparation technology progress of lithium battery electrode sheet [J]. Applied Chemistry, 35(9): 1076-1092.

[7] Nobuhiro Ogihara，et al．Impedance Spectroscopy Characterization of Porous Electrodes under Different Electrode Thickness Using a Symmetric Cell for High Performance Lithium-Ion Batteries[J]．The Journal of Physical Chemical C, 2015, 119(9):4612-4619.