Preface

Lithium-ion (Li-ion) batteries have become one of the main energy storage solutions for the consumer, power and energy storage markets due to their long cycle life and high rate performance. Currently lithium-ion batteries still have challenges in reducing production costs and improving performance and durability, therefore, it is of great importance to have a deep understanding of the impact of the production process on the battery, as well as the assessment of the special needs of the production process, and the future trends and directions depend not only on the changes in lithium-ion battery materials, but also on the production process. As the core component of lithium-ion batteries, the preparation process of positive and negative electrode sheet, such as coating and rolling, is a key process to ensure that lithium electrode wafers achieve the required compaction density and reach the designed capacity. In order to increase battery capacity, improve electronic conductivity and electrochemical performance, cell manufacturers usually target different coating and rolling processes for production. In-depth study and understanding of the evolution of the electrode microstructure during the coating and rolling process, as well as the influence of process parameters on the final structure and performance of the electrode, is conducive to more refined control of the electrode to improve the overall performance, which is of great significance to the design and production control of lithium-ion batteries [1].

Electronic conductivity is one of the key factors that determine the performance of the battery. Taking anode graphite as an example, migrating lithium ions are embedded/de-embedded between the graphite layered structure during charging/discharging. A simple and advanced two-probe method was used to determine the resistivity of the electrode sheet, and the effect of the roll pressing process on the electron transfer characteristics of the entire electrode cross-section was simulated by applying a pressure load to the electrode sheet, and the resistivity of the electrode sheet that was actually pressed to the highest compaction density was reduced to about one-third of that of the un-rolled electrode sheet [2]. As the positive electrode active material shows low conductivity, a large number of conductive agent materials are often introduced in the production process, and the conductive network constructed by the conductive agent materials plays a major role in the conduction of electrons. Rolling after the active particles more compact, active particles at the junction of the conductive agent is compressed is the main reason for the increase in the conductivity of the coating, while the conductivity of the surface of the active material is only affected by a small effect, so this is the pulping, coating process of the uniform distribution of the components of the requirements of a higher level [3].

This paper comprehensively analyzes the conductivity and compaction performance of single and double-sided electrode sheet of different active materials, which can effectively distinguish the performance differences between the electrode sheet in different coating states, and provide an effective means for more refined control of electrodes in the design and manufacturing process of lithium-ion batteries and improve the comprehensive performance, and also further promote the enhancement of the battery’s overall electrical performance.

1.Experimental equipment and test methods

1.1 Experimental equipment

Test equipment model:Battery Electrode Sheet Resistance Tester BER2500 (IEST), electrode diameter 14mm, can be applied to the pressure range of 5 ~ 60MPa. equipment shown in Figure 1 (a) and 1 (b).

Figure 1. (a) BER2500 appearance; (b) BER2500 structure

1.2 Sample preparation and testing

1.2.1 Sample Preparation:Prepare lithium cobalt oxide (LCO) and ternary material (NCM) slurry with the same formula, and at the same time configure graphite (GR) slurry, and coat LCO and NCM slurry on aluminum foil and GR slurry on copper foil, and coat two sheets of each type of electrode sheet. After the electrode sheets were dried, one of them was selected to be coated on the reverse side to make a double-sided electrode sheet. This resulted in single-sided and double-sided electrode sheets of three different active materials.

1.2.2 Sample testing: Combined with the BER2500 equipment independently developed by Yuanneng Technology, single-point mode, variable-voltage mode and steady-state mode were used to test and compare the resistance and compaction performance of the single- and double-sided samples of the three materials. The test procedure is as follows: single-point mode: under the pressure of 25MPa (positive pole)/5MPa (negative pole), 6 points are evenly selected on the electrode sheet for pressure application, and the pressure is held after the pressure reaches the specified pressure, and the holding pressure is held for 15S; variable-pressure and steady-state modes: a point is selected on the electrode sheet for pressure application, and the pressure is held for 15S when the pressure reaches 5MPa, and the pressure is lifted to the next pressure point after the end of the pressure-holding, and the holding-pressure picking point is held for 15S at intervals of 5MPa and 5MPa. After the end of holding pressure, the pressure is raised to the next pressure point, and the interval is 5MPa, the holding pressure is taken to test the resistance of the electrode sheet and the thickness change of the electrode sheet from 5 to 60MPa, and the test ends at 60MPa in the variable pressure mode. At the end of the variable pressure mode, the same pressure interval (5MPa) gradually reduce the pressure and hold pressure at the corresponding pressure point for 15S to read the data, the pressure drops to 5MPa, the end of the steady state mode. During the test, the software will simultaneously record the thickness and real-time resistance of the electrode sheet and output the data file.

2.Data Analysis

The resistivity tests of single- and double-sided electrodes of three different active materials were conducted under different pressures, as shown in Fig. 2 (taking NCM as an example), and the test results show that there is a difference between the resistivity of single-sided NCM and double-sided NCM under a small pressure, and the resistivity of the two tends to be the same with the increase of the test pressure. A similar trend can be observed in the resistivity test results of LCO and GR. Comparing the resistivity of single and double-sided electrodes under the same pressure point, the resistivity of double-sided electrodes of LCO and NCM materials is slightly larger than that of single-sided electrodes under small pressure, and the resistivity is close to the same with the increase of pressure, and this tendency is mainly caused by the difference in the contact resistance of single and double-sided electrodes under test. This trend is mainly due to the different contact resistances of the single and double-sided electrodes tested. The resistivity of GR samples under different pressure points is relatively consistent.

The results obtained from the traditional four-probe method of testing can only describe the resistance of the coating because the direction of current transfer is parallel to the coating, ignoring the interface resistance of the substrate and the coating, and ignoring the coating gradient of the electrode sheet, which is not able to fully characterize the resistance value of the electrode sheet. The electron conduction path in the test method we used is basically the same as that of a real battery electrode sheet, and the Cu foil, Al foil, and probe materials are all highly conductive, thus the bulk phase resistance of the collector and probe accounts for a very small portion of the resistance. However, the contact resistance of the probe and the coating is not negligible. For a single-sided electrode sheet, the probe on one side is in direct contact with the collector, whereas for a two-sided coated electrode sheet, the probes are in contact with the coating, and this difference in contact results in a difference in resistivity. When the test pressure is increased, the contact pressure between the probe and the coating or the fluid collector is also greater, and the percentage of contact resistance decreases, and the resistivity difference between the single and double-sided electrode sheets becomes smaller. Therefore, in the subsequent single-point mode, the LCO and NCM were tested using a pressure of 25 MPa, and the GR was tested using a pressure of 5 MPa.

Figure 2. Resistivity variation curves of three single and double sided electrode sheet

Figures 3(a), 3(b), and 3(c) show the resistance, resistivity, and total thickness data of the three groups of single- and double-sided electrodes with different active materials under the same pressure, and six different positions were selected for testing. The overall results show that the resistivity COVs of single-sided and double-sided electrode sheets of the same material do not differ much from each other at different test positions; however, comparing the three different materials, the resistivity COVs show the trend of GR＞LCO＞NCM, which is related to the nature of the active materials and the microstructure of the electrodes, and on the one hand, the resistivity of the material itself affects the test COVs, and the lower the resistivity, the effect of the contact resistance is The smaller the resistivity, the greater the effect of contact resistance, thus affecting the test results, GR sample resistivity is very low, so the resistivity of the COV is larger. In addition, the softness or hardness of the material itself may also affect the test results. Under pressure, the interfacial contact of a material that is easy to deform may be tighter and the test consistency better. For both LCO and NCM samples, LCO is more difficult to deform (Fig. 4), and the COV of resistivity is larger, and the graphite samples were tested with smaller pressure, which may be a reason for the larger COV; on the other hand, the nature of paste dispersion during paste preparation is relevant, and the more uniformly dispersed the paste is, the smaller the COV of resistivity of the coated single- or double-sided wafers is.

From Fig. 3(a), the resistances of the three double-sided electrodes are approximately equal to twice the resistance of their corresponding single-sided electrodes, while the sample thicknesses of the three double-sided electrodes are all slightly less than twice the thickness of their corresponding single-sided electrodes (Fig. 3(c)), so that the resistivity of the single- and double-sided electrodes (Fig. 3(b)) does not have a great difference as affected by the sample thickness. Moreover, when the test is conducted under a larger pressure, the contact pressure between the probe and the coating or the collector is also larger, and the proportion of the probe contact resistance is also lower for single- and double-sided electrode sheets, and the difference in resistivity between single- and double-sided electrode sheets is relatively small.

Figure 3. Consistency of three single and double sided electrode sheets at different locations

The steady-state mode was used to carry out loading compaction-unloading rebound tests on the electrode sheets under different quantitative pressure conditions, and the thickness changes were recorded. The thickness deformation was normalized and calculated based on the initial pressure point of 5MPa to obtain the stress-strain curves of different electrode sheets (as shown in Figure 4), and their deformation conditions were summarized (as shown in Table 1).

Fig. 4. Stress-strain (compaction performance) curves of three single and double-sided electrode sheets

Table 1.Summary of deformation of three single and double-sided electrode sheets

From the result charts, it can be seen that the maximum deformation, reversible deformation and irreversible deformation trends of the thickness of single and double sided electrode sheets of the three kinds of active materials are the same, and the amount of change in the thickness of the electrode sheets coated on both sides is not much different from that of the single sided electrode sheets, due to the fact that the coating mainly deforms during the compaction process compared with that of the metal material collector, so that this single and double sided difference is just the difference in the proportion of the collector’s thickness in the electrode sheets caused by the different proportion of the collector’s thickness in the electrode sheets. It shows that the compaction performance of the electrode sheet is mainly determined by the nature of the active material.

3.Summary

In this paper, the BER2500 series pole resistor testing equipment is used to test the conductivity and compaction performance of single-sided coated and double-sided coated NCM, LCO and GR poles, which can effectively differentiate the performance differences of poles in different coating states. The choice of the number of coated surfaces in the actual production process should be combined with the specific process formula to make reasonable choices to improve the capacity of the battery, but also effectively enhance the overall electrical performance of the battery. The overall electrical performance of the battery can also be effectively improved. This conductivity test method can be used to quickly study the effect of the process on the resistivity of the electrode sheet, and even complete the test directly in the production line.

4. Reference

[1] Lv Zhaocai, Wang Yuxi, Wang Zhitao, Sun Xiaohui, Li Jingkang. Influence of heated calendering process on cathode film performance of lithium-ion batteries[J]. Energy Storage Science and Technology,

[2] Zhang Caixia. Research on processing performance of artificial graphite[J]. Power Supply Technology,2022,46(11):1256-1260.

[3] SONG Lan, XIONG Ruoyu, SONG Huaxiong, TAN Penghui, ZHANG Yun, ZHOU Huamin. Multiscale nonuniformity of lithium-ion batteries[J]. Energy Storage Science and Technology,2022,11(02):487-502.