Compatibility of N-Methyl-N-propylpyrrolidinium Cation Room-Temperature Ionic Liquid Electrolytes and Graphite Electrodes

To realize high-safety, high-performance lithium ion secondary batteries using room-temperature ionic liquids, we examined [graphite electrodelroom-temperature ionic liquid-lithium salt binary electrolytellithium metal electrode] cells by varying the cation structure of the room-temperature ionic liquid. In particular, a bis(fluorosulfonyl)imide anion based room-temperature ionic liquid cell showed high charge/discharge capacity and reversibility. By application of electrochemical calorimetric measurements during charge/discharge operation, the formation of a suitable interfacial layer, such as a solid electrolyte interphase between the graphite electrode and the room-temperature ionic liquid-lithium salt binary electrolyte was indicated. Moreover, we found that not only the anion structure but also the cation structure was important for the suitable charge/discharge of the graphite electrode.