Role of solution structure in the electrochemical intercalation of Ca2+ into graphite layers

In this study, we investigated the electrochemical intercalation of Ca²⁺ into graphite as an anode material for calcium-ion batteries (CIBs). The electrochemical intercalation of Ca²⁺ into a graphite electrode is possible when γ-butyrolactone (GBL) is utilized as a solvent, resulting in a reversible charge/discharge capacity. The GBL-based electrolyte allows a reversible redox reaction, thereby resulting in the intercalation and deintercalation of Ca²⁺ within the graphite electrode. Conversely, Ca²⁺ cannot be intercalated between the graphite layers in the ethylene carbonate–diethyl carbonate (EC–DEC)–based electrolyte. Analyses of the solution structures of both cases indicated that the interaction between the GBL solvent and Ca²⁺ was weak whereas that between the EC–DEC solvent and Ca²⁺ was strong. As a result of analyzing the surface of the negative electrode after charging and discharging from XPS, it was confirmed that a component that seems to be a solid electrolyte interphase (SEI) was confirmed in the graphite electrode using the GBL-based electrolyte.