Evaluation of Electrolyte Wetting Performance At The Electrode Level – Anode Electrode Sheets With Different Compaction Densities

1. Preface

Electrolyte is the core part of lithium-ion battery research and development, which is not only an important medium to ensure ion transmission, but also an important foundation for the battery to obtain high voltage and high specific energy. The relevant parameters of the electrolyte and its wettability on the electrode and diaphragm directly affect the performance of the battery. Among them, the effect of electrolyte wetting in the electrode is closely related to the compaction density, pore size, porosity and other parameters of the electrode itself, and the assessment of electrolyte wetting in the electrode can be used as a key indicator of the optimization of the electrode level process, and can provide a new direction for the research and development of high-performance batteries; likewise, as a lithium-ion battery one of the main materials of the diaphragm, the advantages and disadvantages of its wetting of electrolyte are also key indicators of the impact of the battery’s performance, therefore, the development of a new type of diaphragm is a key indicator of the battery’s performance. As one of the main materials of lithium-ion battery, the wettability of the electrolyte is also a key indicator of the battery performance. Therefore, it is necessary to develop a device that can effectively evaluate the wettability of electrolyte for positive and negative electrodes as well as for the diaphragm.

Figure 1. Schematic diagram of the capillary wetting method and traditional wetting test method

Traditional electrolyte wettability test methods generally include contact angle measurement method, wettability time method, wettability height method, etc., in which the contact angle measurement method is to add drops of electrolyte to the surface of the electrode or diaphragm, and determine the wettability of the electrolyte according to the contact angle of the electrolyte and the electrode or diaphragm, the electrolyte in the electrolyte or diaphragm spread on the surface of the test is faster, and the test is usually required to be equipped with a high-speed camera instrument. The overall test cost and difficulty are high, and it is difficult to examine the complete wetting time and wetting speed of the electrolyte on the electrode or diaphragm; the wetting time method is usually to take a certain amount of electrolyte drops on the surface of the electrode to test the length of time for the complete wetting of the electrolyte, and to evaluate the difference of electrolyte wetting of the electrode in terms of the difference in time; and the wetting height method is to completely immerse the fixed-size electrode in or one end of the electrode in electrolyte, and then determine the electrolyte wetting of the electrode in accordance with the time difference between the two. The immersion height method involves completely immersing a fixed-size electrode or immersing one end in electrolyte, and evaluating the electrolyte wetting performance of the electrode based on the mass of electrolyte immersed in the electrode within a certain period of time.

In order to solve the limitations of the traditional test methods, the R&D team of IEST has developed a set of Electrolyte Electrolyte Wetting Testing System(EWS1100), which can quantitatively evaluate the electrolyte wetting performance, based on the principle of capillary diffusion of the electrolyte in the electrode sheet and diaphragm, and the method of capillary wetting, equipped with a high-precision mechanical control system and a visual acquisition system, which can quantitatively evaluate the difference of electrolyte wetting between different positive and negative electrode sheets and diaphragms, and provide an effective means for electrolyte wetting evaluation. Figure 1 shows the schematic diagram of the principle of the capillary wetting method and the traditional wetting test method. The electrolyte is injected into the capillary tube, and after the capillary glass tube is vertically contacted with the surface of the electrode , the capillary liquid level decreases as the electrolyte continuously infiltrates the coating. The visual recognition system records the capillary liquid level height in real time, the dynamic evolution of the liquid level height is the electrolyte wetting process in real time, the height change is the amount of electrolyte wetting.

This paper is mainly based on the capillary wetting test system, combined with different compaction density of anode electrode sheets for systematic testing, to evaluate the differences in electrolyte wetting of the electrode sheets at different compaction densities.

2. Laboratory Equipment and Test Methods

2.1 Experimental Equipment

Model EWS1100 (IEST), the appearance of the equipment is shown in Figure 2.

Figure 2. Exterior view of the EWS1100 device

2.2 Sample Preparation and Testing

2.2.1 Sample Preparation

After uniformly coating the slurry under the same process formulation conditions, we respectively used different pressures, such as small, medium and large, for roll pressing, and obtained a total of four compacted finished anode electrode sheets, in which the pressure of the electrode sheets roll pressing was 1＜2＜3＜4. The compaction density of the four electrode sheets was calculated by means of cutting-thickness-measuring-weighing, and the size of the compaction density was also presented as 1(1.35 g/cm³) ＜The size of the compacted density also shows 1 (1.35 g/cm³) <2 (1.5 g/cm³) <3 (1.6 g/cm³) <4 (1.65 g/cm³), i.e., with the increase of the roller pressure, the compacted density also shows an increasing trend.

2.2.2 Testing process

Pre-treatment of samples to be tested → sample standardization and fixation → equipment on-line and software parameter design → capillary automation suction → capillary automation downward pressure test → visual recognition system real-time monitoring of capillary liquid surface height → data acquisition and processing.

2.2.3 Electrolyte wettability test of different compaction density electrodes

Combined with EWS1100 equipment, the capillary wettability test of 1/2/3/4 electrodes was conducted respectively to compare the wettability difference between different electrodes.

3. Analysis of the Wettability of Electrode Sheets With Different Compaction Densities

The electrolyte wettability of the electrode is closely related to the porosity of the electrode, through the particle size and distribution of the material, particle morphology and compaction density can effectively adjust the porosity of the material and the distribution of the voids, which in turn directly affects its electrolyte wettability. Research shows that the higher the compaction density, the lower the porosity and the smaller the diameter of the most frequent voids. The size of the compaction density directly affects the capacity of the battery, currently combined with the material end, there are two main ways to improve the battery capacity: one is to improve the capacity per unit mass of active material, and the other is to improve the compaction density per unit volume of material. Compaction density is usually combined with the battery process for comprehensive assessment, the greater the compaction density, the more active material filling, the greater the volume specific capacity of the battery, but the actual battery cell process research and development is not the greater the compaction density of the battery performance is better, research has shown that the compaction density is too large will cause the battery’s ionic conductivity, cycling performance, multiplicative discharge performance deterioration, and so on. Among them, the ionic conductivity is directly related to the electrolyte wettability of the electrode, and this paper mainly combines the capillary wetting method to evaluate the difference in electrolyte wetting performance of electrodes with different compaction density.

Figure 3 shows the electrolyte wettability curves of four different compaction densities of the electrode, from the curve, with the increase of compaction density, the slope of the electrolyte wettability curve of the electrode decreases gradually, i.e., the greater the density of compaction, the worse the wettability, which is consistent with the reported studies; Figure 4 shows the relationship between electrolyte wettability of the electrode and the change of the density of compaction at different points in time, as can be seen in the figure, the electrolyte wettability of the electrode with the compaction density changes in the relationship between the time of wettability and the density of compaction. It can be seen that the wettability of different electrodes at different time points with the increase of compaction density shows a decreasing trend, and the wettability of the electrodes shows a better linear relationship with the increase of compaction density, but the change of linear slope increases with the prolongation of the wettability time. The actual positive and negative electrodes are porous structures, which can also be regarded as capillary structures with different pores, and the wetting process of the electrolyte in the electrode can be understood as the capillary absorption effect. The Lucas – Washburn wetting model is usually used to describe the kinetics of liquid absorption by the capillary effect in the electrode sheet, as shown in Equation (1)-, where h is the liquid absorption height, t is the liquid absorption time, c is the shape coefficient corresponding to the capillary with different gaps, r is the capillary radius, cr is a constant value called the formal radius, σ is the surface tension of the liquid, and η is the liquid viscosity. Washburn’s equation can better describe the relationship between conduction distance and conduction time when the liquid conducts horizontally and vertically, combining the results of Fig. 3 & Fig. 4, it can be further clarified that the wetting of the electrolyte in the electrode sheet also contains the wetting in the vertical direction and the diffusive conduction in the horizontal direction, and from the trend of the slope of the wetting rate with the compaction density at different time points, the longer the wetting time is, the larger is the slope of the change with the compaction density. The longer the wetting time, the greater the slope of change with compaction density. Since the length L and width W of the electrode are much larger than its thickness B, the wetting process is considered to be pure radial absorption. That is, the effect of sample thickness can be neglected and the propagation of the wetting boundary line is uniform throughout the electrode thickness. Electrodes with different compaction densities have different porosities and different amounts of electrolyte that can be absorbed, and the effect of compaction density becomes more significant as the wetting time increases.

Figure 3. Electrolyte wetting curves for different compaction density electrodes

Figure 4. Variation curves of electrolyte wetting with compaction density for electrodes at different time points

Table 1 is a summary of the electrolyte wetting repeatability test of different compacted electrodes at different times, where each sample was tested in 4 groups for repeatability, and the mean, standard deviation and coefficient of variation were calculated. From the deviation value, the standard deviation of the test results of each type of electrode was less than 0.15, and the coefficient of variation COV was controlled within 10%, which further illustrates the accuracy and effectiveness of the test method.

Table 1. Summary of electrolyte wetting results of different electrodes at different times

4. Summary

In this paper, the electrolyte wetting performance assessment of electrodes with different compaction densities has been carried out by using a creative solution, the capillary wetting method, and from the results, it can be seen that the wetting differences of each electrode can be effectively differentiated, and it is consistent with the results of the reported studies, which can be used as an effective means for the assessment of electrode hierarchical wettability. Moreover, the method overcomes the limitations of the traditional wetting equilibrium method and provides a better understanding of electrolyte migration in porous electrodes, thus helping to optimize the composition of the electrolyte and the microstructural design of the electrodes to achieve fast and complete wetting, reduce battery production costs and improve product quality.

5. References

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