Evaluation of the Solid-Electrolyte Interphase Formation in a Bis(fluorosulfonyl)Amide Based Ionic Liquid in the Presence of Lithium Ion Using Different Redox Probes

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Lithium metal has long been considered an attractive anode material for rechargeable lithium batteries due to its high theoretical specific capacity and markedly negative redox potential. However, widespread use of lithium metal anodes in commercial batteries has been hindered by the challenge of controlling lithium deposition. This can lead to the formation of undesirable morphologies, including dendrites and whisker-like structures. These structures have the potential to cause thermal runaway and short circuits. The solid-electrolyte interphase (SEI) formed on the anode is considered to be one of the factors influencing the control of the deposited lithium morphology. The SEI is composed of the reduction products of the electrolytes. Investigation of the nature of the SEI is expected to improve the cyclability of lithium metal anodes. In a bis(fluorosulfonyl)amide (FSA⁻)-based ionic liquid containing high concentrations of LiFSA, suppression of dendrite growth and improvement in the cyclability of lithium anodes have been reported. These improvements indicate that FSA⁻-derived SEI has a positive effect on the deposition and dissolution of lithium[1]. A large number of studies have been conducted to investigate the chemical and physical properties of the SEI. Most of these studies have been performed using ex-situ measurements such as X-ray photoelectron spectroscopy and scanning transmission electron microscopy. In the case of these methods, the SEI may have been altered or damaged during the sample preparation and measurement process. Therefore, it is essential to perform in-situ measurement when investigating the nature of the SEI. The redox probe method is a technique for observing the SEI through in-situ measurement. This method can monitor the initial stage of SEI formation. It has mainly been carried out using the redox reaction of ferrocenium (Fc⁺)/ferrocene (Fc). The redox reaction of Fc⁺/Fc is an outer-sphere electron transfer reaction. The standard rate constant for this type of redox couple is affected by changes in the tunneling distance between the electrode and the redox species. The SEI is considered to be an electronically insulating phase. Therefore, when the SEI is formed on the electrode and the tunneling distance is increased, a shift in the peak on the cyclic voltammogram is observed. Kato et al. reported the use of this method to estimate the potential for SEI formation on a Pt or GC electrode in 0.5 M LiFSA/BMPFSA[2,3]. Okazaki et al. also reported that the SEI was dissolved and/or dispersed in the electrolyte in 2.9 M LiFSA/BMPFSA[4]. In the present study, SEI formation in 0.5 M LiFSA/BMPFSA was investigated using different redox probes. The redox reactions of Fc and nickelocene (Nc) on a Pt electrode were studied by cyclic voltammetry in 0.5 M LiFSA/BMPFSA. The potential was measured against an Ag|Ag(I) reference electrode, which had a potential of 0.39 V vs. Fc|Fc⁺[5]. A shift in the anodic peak potential was observed on a Pt electrode in 0.5 M LiFSA/BMPFSA containing 10 mM Fc and Nc after holding the electrode at –2.0 V for 1 hour. This shift is due to SEI formation on the Pt electrode due to an increase in the tunneling distance and a decrease in the standard rate constant. The peaks shifted differently for Nc and Fc. It was suggested that the detectable thickness of the SEI was dependent on the standard rate constant of the probes. Acknowledgment This study was supported by the Green Technologies of Excellence (GteX, JPMJGX23S0) program of the Japan Science and Technology Agency (JST). References [1] N. Serizawa, R. Yamashita, and Y. Katayama, J. Phys. Chem. C , 127 , 10434 (2023). [2] S. Kato, N. Serizawa, and Y. Katayama, J. Electrochem. Soc. , 169 , 076509 (2022). [3] S. Kato, N. Serizawa, and Y. Katayama, J. Electrochem. Soc. , 170 , 056504 (2023). [4] S. Okazaki, N. Serizawa, and Y. Katayama, Electrochemistry , 92 , 043006 (2024). [5] S. Kato, N. Serizawa, and Y. Katayama, J. Electrochem. Soc. , 170 , 046504 (2023).

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