Study of the Hydrate-Melt/Li₄Ti₅O₁₂ Interphase by Scanning Electron Microscopy-Based Spectroscopy

To develop safe and low-cost Li-ion batteries (LIBs), recently, an aqueous-based electrolyte so-called "hydrate-melt" (HDM) electrolyte is proposed. Li₄Ti₅O₁₂ is a promising negative electrode material for a LIB with such a HDM electrolyte because of its unexpected reversible Li insertion and extraction properties without usually inevitable water reduction. The solid-electrolyte interphase formation is one of the reasons for this stable reaction, although a detailed analysis is not yet performed. Here, a Li₄Ti₅O₁₂ electrode surface reacted in a HDM electrolyte is investigated by scanning electron microscopy-based analysis. Surface reaction products are clearly observed on the Li₄Ti₅O₁₂ surface after the Li insertion reaction in a HDM electrolyte. Energy-dispersive X-ray spectroscopy and Auger electron spectroscopy indicated that the products do not contain any components originated from Li salts, whereas anion-derived passivation films seem to cover a bare surface below the products. Further, the surface products are identified as Li₂O by the feature of Li-K-edge reflection electron energy-loss spectrum. The Li₂O formation would be one of the key issues for stable Li insertion and extraction of a Li₄Ti₅O₁₂ electrode in a HDM electrolyte.