Correlation Between Electrode Tortuosity And Electrochemical Performance

1. Preface

By analyzing the correlation between the electrode tortuosity and the rate performance of the battery, this paper can preliminarily judge the rate performance of the battery at the electrode end and improve the R&D efficiency of materials or electrodes.

In lithium-ion batteries, as the thickness of the positive or negative electrode of the battery increases, the proportion of active materials also increases significantly, which can effectively improve the energy density of the single battery. Therefore, the development of thick electrodes is of great significance to improving the energy density of the battery. However, as the thickness of the electrode increases, the liquid phase lithium ion transmission of the electrode is hindered, resulting in an increase in the internal resistance of the battery, a decrease in the utilization rate of the active materials, and a significant attenuation of the cycle performance and rate performance^[1]. Researchers usually test the rate performance and cycle performance by assembling button cells or soft pack batteries, but the long cycle testing of the battery will lead to low R&D efficiency, so shortening the material evaluation cycle becomes particularly important. The tortuosity of the electrode represents the degree of curvature of the porous electrode transmission path. It is another important parameter related to the transmission characteristics besides porosity^[2].  It can characterize the difficulty of lithium ion migration in the coating, thereby reflecting the rate performance of the battery.

2. Test Conditions & Methods

2.1 Test Equipment

Symmetric battery assembly and testing: The multi-channel ion conductivity test system (EIC1400) developed by IEST is used as shown in Figure 1, which contains four-channel symmetric battery assembly fixtures, electrochemical impedance test system, fitting software and so on, and it can provide a high-purity argon atmosphere to realize multi-channel fast electrochemical impedance spectroscopy test. The frequency range is 1000~0.1HZ.

Half-cell assembly and testing: steel case 2032 is used to assemble the half-cell of electrode-to-lithium cell, and its electrochemical performance is tested by charging and discharging equipment.

Figure 1. Schematic diagram of the multi-channel ion conductivity testing system

2.2 Test samples

Preparation of different thicknesses of anode electrodes: Graphite was selected as the active material to prepare the slurry, and the preparation of different thicknesses of electrodes was realized by controlling the gap of the coating scraper, with the gap sizes of 100 μm, 200 μm, and 400 μm, respectively.

2.3 Testing Process

Tortuosity test: The electrode and diaphragm were stacked in the order of electrode – diaphragm – electrode and put into 4 channels -> Close the door of the silo, evacuate the inner cavity – fill with high purity argon gas to remove the water in the inner cavity -> Quantitatively inject liquid into each channel, and then leave it for 10mins. Test the EIS of the battery after 10min–>Finally, get the curvature of the electrode through the fitting and calculation of the software.

Battery test: Test the charging and discharging performance of the battery under different multiplication rate (0.1C/0.2C/0.5C) respectively.

2.4 McMullin Number Calculation Method

In the formula: τ is the tortuosity; Rion is the ionic resistance; A is the electrode area; ε is the electrode porosity; σ is the electrolyte conductivity; d is the electrode thickness.Since the testing method of electrode porosity is relatively complicated, the ratio of tortuosity and porosity, that is, McMullin number (Nm=τ / ε), is usually used to characterize the tortuosity of the electrode, as shown in Equation (2).

The impedance of the symmetric cell was tested using the electrochemical workstation, and the EIS obtained is shown in Fig. 2. The Nyquist plot of the electrochemical impedance spectrum at this point is characterized by the shape of the intersection of the line segments in the low-frequency region and the line segments in the high-frequency region, which is a typical Nyquist plot for the absence of electrochemical reactions. The low-frequency line segment in the Nyquist diagram is extended until it intersects with the X-axis, and the difference between the intersection point and the intersection point of the high-frequency line segment and the X-axis is three times of the ionic impedance of the coating of the electrode, and the ionic impedance of the electrode can be obtained by substituting the ionic impedance of the fitted Rion into Equation (2) to obtain the MacMillan number of the electrode, which can be analyzed as the degree of the zigzagging of the electrode.

Figure 2. Electrochemical impedance spectrum of symmetrical battery

3. Results Analysis

Figure 3. Impedance spectra of negative electrode plates with different thicknesses: 100 μm (a); 200 μm (b); 400 μm (c) and the corresponding McMullin number (d)

The electrochemical impedance spectra of symmetric cells assembled with negative electrode sheets of different thicknesses were tested, and the results are shown in Figs. 3(a), 3(b), and 3(c). The impedance spectra were fitted to obtain the ionic resistance of each electrode, and then the ionic resistance values were substituted into Eq. (2) to obtain the electrode McMullin number, as shown in Figure 3(d). 100 μm, 200 μm, and 400 μm corresponded to the McMullin numbers of 4.61, 6.15 , and 6.61, respectively, and the trend of the data showed that the McMullin number increased with the increase of the thickness of the electrode.

Due to the complex connectivity between pores within the porous electrode, such as blind and half-through pores, fine throat dimensions, etc., when the thickness of the electrode increases, the ion transport path tends to be more tortuous and the actual transport distance increases exponentially, which leads to higher pore tortuosity. Generally, for porous electrodes, electrochemical testing of pore curvature methods include: (1) Polarization-Interrupt Method (eRDM), a fixed DC current is passed through the cell for a few minutes after polarization to “interrupt” the current. During polarization, Li+ ions are produced at one electrode and deposited at the other electrode to deplete the Li+ ions, creating a concentration gradient in the cell. After the current is interrupted, this concentration gradient is then relaxed or equilibrated. As the cell relaxes, the potential gradually approaches zero. A semi-log plot of cell potential versus time is plotted and the slope of the relaxation curve is used to calculate the MacMullin number and the curvature value (Figure 4a); (2) the electrochemical impedance method (eSCM), which measures the electrode impedance in a non-embedded symmetric cell and fits the resulting impedance curve on a Nyquist plot, thus determining the effective ionic resistance of the electrode (Rion) and calculating the electrode’s MacMullin number and curvature (Figure. 4b), i.e., the methods used in this paper. Using these two methods, assuming the four cases of electrode porosity as shown in Figure 5, the MacMullin number is exactly the same when the first eRDM method is used for simulation, while in fact, due to the complexity of the pore structure, the results of simulation using the second eSCM method show that even if the porosity is the same due to the difference in pore structure, the pore curvature and the MacMullin number are also completely different. The results show that the pore tortuosity and MacMullin number are completely different even for the same porosity due to different pore structures. The test equipment developed by us is more in line with the actual situation and is able to reflect the performance of the electrodes.

Figure 4. Schematic diagram of the electrochemical testing method for electrode tortuosity ^[3]

Figure 5. Comparison of results of two testing methods for electrode tortuosity ^[3]

Figure 6 and Table 1 demonstrate the capacity and capacity retention of negative electrode sheets with different thicknesses at different multiplicities. It can be seen that the capacity of each electrode decreases with the increase of multiplicity, but the capacity retention rate is 100 μm > 200 μm > 400 μm, which indicates that the electrode obtained by 100 μm coating has the best multiplicity performance, and the 400 μm electrode has the worst performance. Combined with the data of McMullin number, it can be seen that with the increase of the thickness of the electrode, the curvature of the electrode becomes larger, and the multiplicity performance of the cell becomes worse subsequently.

Figure 6. Electrochemical properties of electrodes at different rates: capacity-voltage curve (a); rate-capacity retention curve (b)

Table 1.Specific capacity of electrodes at different multiplicities

The electrochemical performance test results and the pore curvature test results can completely correspond to each other, which indicates that we can predict the performance of the electrode through the test of the electrode pore curvature, correlate the electrode structure and performance prediction quickly, and accelerate the design and process development of the electrode.

4. Summary

In this paper, the graphite negative electrode sheets with different thicknesses were assembled into symmetric cells and half-cells to test the curvature of the electrode sheets and the multiplicity performance of the batteries, and it was found that the curvature of the electrode sheets increased with the increase of the thickness, and the multiplicity performance of the batteries decreased with the increase of the thickness, which indicates that there is a certain correlation between the curvature of the electrode sheets and the multiplicity performance of the batteries. Therefore, we can test the curvature of the pole piece to initially determine the battery’s multiplication performance. In addition to determining the multiplication performance of different thicknesses of the pole piece, the test can also be used to study the effects of electrode formula, porosity, main material morphology, electrolyte type, diaphragm type, etc. on the performance of lithium-ion batteries.

5. References

[1] Sun Weibing et al. A low curvature thick electrode and its preparation method and application. CN115312777A. 2022.

[2] Wang Chenyang, Zhang Anbang, Chang Zenghua, et al. Progress in structure design and preparation of porous electrodes for lithium ion batteries[J]. Materials Engineering, 2022, 50 (1): 67-79.

[3] Benjamin Delattre, Ruhul Amin, Jonathan Sander, Joël De Coninck, Antoni P. Tomsia1 and Yet-Ming Chiang. The electrode tortuosity factor: why the conventional tortuosity factor is not well suited for quantifying transport in porous Li-ion battery electrodes and what to use instead [J]. Electrochem. Soc. 2018,165:A388.