Evaluation of Solid-Electrolyte Interphase Formation on a Copper Electrode in 1-Butyl-1-Methylpyrrolidinium Bis(fluorosulfonyl)Amide Ionic Liquid Electrolyte Using a Redox Probe

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The solid-electrolyte interphase (SEI) is a phase formed on an electrode by the decomposition of an electrolyte containing a lithium salt [1]. The SEI is thought to prevent further decomposition of the electrolyte, while the Li+ conductivity of the SEI enables the electrode reaction involving Li+. The Li metal anode has been investigated as an ideal anode for rechargeable Li batteries due to its negative electrode potential and high specific capacity. The cyclability of the Li anode is known to be related to the morphology of the Li metal during charge and discharge processes. The morphology of Li metal has been considered to be influenced by the properties of the SEI formed on Li metal. Therefore, the study of the SEI is important to improve the cyclability of the Li metal anode. We have previously reported that the SEI formation can be evaluated by monitoring the redox reaction of ferrocenium (Fc+)/ferrocene (Fc) as a redox probe [2-4]. The rate constant for Fc+/Fc is affected by the thickness of the SEI formed on an electrode. In the case of Fc, the SEI formation on a Cu electrode cannot be evaluated because the anodic dissolution of Cu occurs at a potential more negative than the redox potential of Fc+/Fc [5]. The redox potential of nickelocenium (Nc+)/nickelocene (Nc) is known to be about 0.5 V more negative than that of Fc+/Fc [6]. Therefore, it is expected to evaluate the SEI formation on a Cu electrode using Nc as a redox probe. In the present study, the evaluation of SEI formation on a Cu electrode was attempted in an ionic liquid, 1-buty-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) containing LiFSA. An anodic and cathodic current peak were observed at –0.8 V vs. Ag|Ag(I) in the cyclic voltammogram of a Cu electrode in 0.5 M LiFSA/BMPFSA containing 10 mM Nc. The redox peaks were unchanged after keeping the electrode at –1.5 V vs. Ag|Ag(I) for 12 h, whereas some decomposition products were identified in the F 1s spectrum of a Cu electrode kept at –1.5 V vs. Ag|Ag(I) for 3 h, suggesting that the redox reaction of Nc+/Nc is so fast that the thickness of the SEI formed at this potential is insufficient to induce a peak shift [7]. On the other hand, a peak shift was observed at –2.0 V due to an increase in the growth rate of the SEI at the more negative potential. Thus, SEI formation on a Cu electrode was successfully observed using Nc as a redox probe. [1] E. Peled and S. Menkin, J. Electrochem. Soc., 164, A1703 (2017). [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] N. Serizawa, T. Hisada, and Y. Katayama, Electrochemistry, 92, 043009 (2024). [6] S. Momose, M. L. Thomas, N. Serizawa, and Y. Katayama, PRiME 2024, L02-3825, Honolulu, Oct. 7 (2024). [7] J. D. Gribble and S. Wherland, Inorg. Chem., 29, 1130 (1990).

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