Interfacial reactions between graphite electrodes and propylene carbonate-based solutions: Electrolyte-concentration dependence of electrochemical lithium intercalation reaction

摘要

This study examines the electrochemical reactions occurring at graphite negative electrodes of lithium-ion batteries in a propylene carbonate (PC) electrolyte that contains different concentrations of lithium salts such as, LiClO_4, LiPF_6 or LiN(SO_2C_2F_5)_2. The electrode reactions are significantly affected by the electrolyte concentration. In concentrated solutions, lithium ions are reversibly intercalated within the graphite to form stage 1 lithium-graphite intercalation compounds (Li-GICs), regardless of the lithium salt used. On the other hand, electrolyte decomposition and exfoliation of the graphene layers occur continuously in the low-concentration range. In situ analysis with atomic force microscopy reveals that a thin film (thickness of ～8 nm) forms on the graphite surface in a concentrated solution, e.g., 3.27 mol kg~(-1) LiN(SO_2C_2F_5)_2/PC, after the first potential cycle between 2.9 and 0 V versus Li~+/Li. There is no evidence of the co-intercalation of solvent molecules in the concentrated solution.