Investigation of LiMn2O4 Thin-Film Electrode Using Redox Reaction of Ferrocene

Abstract

Introduction Lithium-ion batteries (LIBs) have been expected as the power supplies of electric vehicles (EVs). LiMn2O4 is widely desired as the positive active materials for EVs-use, However, LiMn2O4 suffer from severe degradation at elevated temperature, leading to poor cycleability of LIBs. One of the factors of degradation is formation of surface film. The surface film might bring passivation of the electrode, however, detailed properties of the surface film were not fully understood. In this study, we investigated passivation behavior of LiMn2O4using redox reaction of ferrocene. Experimental LiMn2O4 thin-film electrode was prepared by pulsed laser deposition. Three-electrode cell was used for electrochemical measurements of LiMn2O4 thin-film electrode. Lithium-ion de-intercalation/intercalation was conducted in 1 mol dm-3 LiPF6/propylene carbonate (PC) by cyclic voltammetry (0.1 mV s-1, 3.5 – 4.2 V vs. Li+/Li.). Before and after the lithium-ion de-intercalation/intercalation process, redox reaction of ferrocene was measured in 1 mol dm-3 LiClO4/PC containing 1 mmol dm-3 ferrocene by cyclic voltammetry (10 mV s-1, 3.0 – 3.6 V vs. Li+/Li.). Results and discussion Figure 1 shows cyclic voltammograms (CVs) of LiMn2O4 thin-film electrode at room temperature. Around 4.0 and 4.2 V, reversible peaks of lithium-ion de-intercalation/intercalation, which are characteristic of LiMn2O4, were observed. As the cycle number increased, peak currents gradually decreased. It indicated degradation of LiMn2O4 proceeded during cycling test. Figure 2 shows the CVs of redox reaction of ferrocene on LiMn2O4 thin-film electrode before and after the cycling process of lithium-ion de-intercalation/intercalation. Reversible redox peaks of ferrocene were observed before the initial cycle. However, these peaks disappeared after the initial cycle, indicating passivation of LiMn2O4thin-film electrode. X-ray photoelectron spectroscopy revealed that LiF was formed on the surface, and it should be the cause of the passivation. The influence of electrolyte, elevated temperature and additive on the passivation behavior will be discussed in the meeting. Acknowledgment This work was partially supported by CREST, JST. Figure 1