Optimization of Electrolyte Composition to Suppress Al Corrosion in Localized High-Concentration Electrolytes
Authors: Aoyama Keigo , Nakagawa Hirotaka 
Published: December 2024

Introduction

Electrolytes with LiFSI added as lithium salt have advantages such as high thermal stability[1] and high cycle life performance at high temperatures. However, electrolytes with LiFSI have a problem of Al corrosion of a current collector of positive electrode due to the reaction of Al with FSI– and free solvent molecules[2]. To address this problem, there have been researches of suppressing Al corrosion by high-concentration electrolytes[3]. In high-concentration electrolytes, interactions between Li+, FSI– and solvents are enhanced leading to increasing ionic association and reducing free solvent molecules. On the other hand, poor rate performance with high-concentration electrolytes is true in most cases due to low ionic conductivity and high viscosity. In this study, localized high-concentration electrolytes with lower viscosity and better rate performance were investigated by diluting high-concentration electrolyte with a fluorinated ether solvent. We examined multiple ratios of electrolyte components such as solvents, a diluent, and LiFSI in localized high-concentration
electrolytes and suppress Al corrosion by finding the ratio that enhances interactions between Li+, FSI– and the solvents.

Experimental

The electrolyte system consisted of the blends of 1,2-Dimethoxyethane (DME) and Ethylene carbonate (EC) as solvents, with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) serving as a diluent. The ratio was 17:7:76 wt%, and the concentration of LiFSI ranged from 0.1 M to 1.7 M. Linear sweep voltammetry was performed to analyze the Al corrosion behavior in each electrolyte. Experiments were conducted at a temperature of 25°C, a scanning speed of 1.0 mV/s, and a scanning potential range of 3.0-5.0 V (vs. Li/Li+). An Al plate electrode was used as the working electrode, Li metal as the reference electrode, and Li metal as the counter electrode. Also, raman spectroscopy measurements were performed to analyze the solvation structure of each electrolyte. The measurements were performed at room temperature.

Results and Discussion

Figure 1 shows the results of linear sweep voltammetry. Sharp increases in oxidation current possibly due to Al corrosion at higher than around 4.0 V were observed at 0.5 M and 0.1 M, but not at 1.0 M or higher. To analyze the relationship between the Al corrosion and solvation structure, raman spectroscopy measurements were performed as shown in Figure2. The peaks at 0.5 M and 0.1 M were attributed to free FSI–. However, the peak shifted to a higher frequency region above 1.0 M. This is due to enhanced interactions between Li+, FSI– and the solvents, and it leads to prevention of Al corrosion. From these results, it is indicated that Al corrosion can be suppressed by controlling the solvation structure of FSI by adjusting the salt concentration. At the poster session, we will also report the results of our evaluation of Al corrosion behavior and solvation structure by changing the ratio of DME, EC, and TTE. In addition, the results of the investigation of the ratio of solvents and a diluent for high cycle life performance and low internal resistance, as well as mechanism of performance improvement will be reported by our colleague.

References:

[1] H. B. Han et al., Journal of Power Sources, 196, 3623 (2011).
[2] A. Yamada et al., Journal of The Electrochemical Society, 162 (14) A2406-A2423 (2015).
[3] A. Yamada et al., Nat. Commun., 7, 12032 (2016).

Figure 1