In the complex system of lithium-ion batteries, the electrolyte plays an indispensable role as the “blood” inside the battery, responsible for transferring lithium ions between the positive and negative electrodes, thus realizing the storage and release of electrical energy. The performance of the electrolyte directly affects the overall performance of the battery, including energy density, cycle life, charge/discharge rate, and operating temperature range.

For a battery to achieve excellent rate performance, the electrolyte needs to have a high lithium ion transport capacity, and the speed of lithium ion transport is directly related to the electrolyte performance [1]. As shown in Figure 1(a), lithium-ion batteries in the charging process will go through four processes: (1) lithium ions and solvent molecules to form solvated lithium, solvated lithium in the potential difference and concentration difference driven by liquid diffusion; (2) in the solid electrolyte interface (sold electrolyte interphase, SEI) membrane interface solvated lithium ions will be separated from the solvent molecules that is (2) the solvated lithium ions will be separated from the solvent molecules at the SEI membrane interface, i.e., the desolvation process; (3) the desolvated lithium ions are transported in the SEI membrane; and (4) the lithium ions are transported in the active material body to form an intercalation compound^{[2, 3]}

Fig. 1.(a) Schematic diagram of the charging process of a lithium-ion battery; (b) Schematic diagram of the energy barriers at different stages of the charging process^{[3, 4]}

From the four aspects mentioned above, it can be seen that the diffusion and migration of lithium ions are the main factors affecting the rate performance of the battery, and it is well known that the electrode sheet curvature represents the degree of curvature of the transport path of the porous electrode, which can characterize the difficulty of lithium ions migrating through the electrode sheete coating, and thus reflect the rate performance of the battery. Considering from the perspective of electrode microstructure, the electrode sheet curvature is defined as the square of the ratio between the actual transport distance of lithium ions in the electrode and the vertical distance, i.e., τ=(L’/L)², which is mainly related to the microstructure of the electrode, especially the structure of the pores, such as pore diameter size, pore throat size, pore connectivity, etc. The calculation of curvature is usually done by the method of calculating the porous electrode coating. The usual methods for calculating the curvature include image analysis, which tests the actual connectivity path of the pores, and then calculates it according to the definition; or electrochemical methods to test the effective conductivity or diffusion coefficient, and calculates it according to the effective conductivity (or the effective diffusion coefficient), σ_eff =σ*ε/τ. For example, testing the ionic impedance of symmetric cells, and then calculating the effective conductivity actually measured according to the thickness and area of the electrodes: 1/σ_eff = (Rion*A)/d , so that τ/ε=σ/σ_eff can be calculated.In this paper, by analyzing the correlation between the zigzag degree of the positive and negative electrodes in different electrolytes and the rate performance of the battery, we can preliminarily judge the rate performance of the electrolyte, and improve the research and development efficiency of the electrolyte.

1. Test Conditions & Methods

1.1 Test Equipment

Symmetric battery assembly as well as testing: Electrode Sheet Tortuosity Tester & Separator Ion Conductivity Testerdeveloped(EIC1400) by IEST is used as shown in Fig. 2, which contains four battery assembly fixtures (Fig. 2(b)), and can realize four-channel rapid testing of electrochemical impedance spectra. The pressure range is 0~20Kg and the frequency range is 100KHz~0.01Hz.

Fig. 6. Rate-capacity retention curves of batteries in different electrolytes

Fig. 6 and Table 1 demonstrate the capacity and capacity retention of soft pack batteries assembled with the same electrode sheet and different electrolytes at different multiplicities. From the table, it can be seen that the capacity of the battery decreases with the increase of multiplication rate. When the multiplication rate is equal to 10C, the capacity retention rate of the battery is 90.55% for formulation 1, 93.92% for formulation 2, and 89% for formulation 3. Combined with the data of McMullin’s number, it can be seen that the difficulty of lithium-ion migration in the coating is affected by the electrolyte formulation, and the batteries with large values of McMullin’s number correspond to poorer multiplication rates, which indicates that the results of the electrochemical performance test of the electrolyte and the results of the flexural test can be corresponded to each other. Therefore, we can predict the rate performance of the electrolyte through the curvature test of the electrodes in different electrolytes, which can quickly correlate the electrolyte formulation with the electrochemical performance, accelerate the development of electrolytes, and shorten the evaluation cycle.

3. Summary

This paper assembled symmetrical batteries and soft-pack batteries with different electrolyte formulations, tested the tortuosity of the electrode sheet in different electrolytes and the rate performance of the battery, and found that there is a certain correlation between the two. Therefore, we can preliminarily judge the rate performance of the electrolyte by testing the tortuosity of the electrode sheet. In addition to judging the rate performance of different electrolytes, the tortuosity test of the electrode sheet can also be used to study the effects of electrode formulation, porosity, main material morphology, diaphragm type, etc. on the performance of lithium-ion batteries.

4. References

[1] Li N, Chen Z P, Ren W C, et al. Flexible graphene-based lithium ion batteries with ultrafast charge and discharge rates[J]. Proceedings of the national academy of sciences of the United States of America, 2012, 109 (43): 17360-17365.

[2] Yamada Y, Furukawa K, Sodeyama K, et al. Unusual stability of acetonitrile-based superconcentrated electrolytes for fast-charging lithium-ion batteries [J]. Journal of the American chemical society, 2014, 136(13): 5039-5046.

[3] Caiwl, Yao Y X, Zhu G L, et al. A review on energy chemistry of fast-charging anodes [J]. Chemical society reviews, 2020, 49 (12): 3806-3833.

[4] Yin ZG, Wu NN, Cao MH, et al. Progress of electrolyte for fast-charging lithium-ion batteries[J]. New Energy Progress, 2024, 12(2): 216-226.