A Design Approach to Lithium-Ion Battery Electrolyte Based on Diluted Solvate Ionic Liquids

An equimolar mixture of lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) and triglyme (G3) or tetraglyme (G4) yields the stable molten complexes, [Li(G3)][TFSA] or [Li(G4)][TFSA], respectively, classified into solvate ionic liquids (SILs). The Li-conducting SIL electrolytes have favorable thermal and electrochemical properties, but their intrinsic high viscosities and low ionic conductivities impede widespread application. In this study, SILs were diluted with organic solvents, such as toluene, hydrofluoroether (HFE) and propylene carbonate (PC), to enhance their ionic conductivity. Subsequently, the performance of a battery consisting of diluted SILs, LiCoO 2 , and graphite electrodes was evaluated. The electrochemical stability and charge/discharge behavior of the LiCoO 2 cathode and graphite anode were greatly influenced by the stability of the complex cations, [Li(G3)] + or [Li(G4)] + , in the diluted SILs. Unfavorable ligand exchange between the glyme and PC occurred in PC-diluted SILs. Oxidative decomposition of the uncoordinated glyme and pitting corrosion of Al current collector deteriorated the battery performance of LiCoO 2 half-cell with PC-diluted SILs. We demonstrate that toluene- and HFE-diluted SILs, which do not contain chemicals such as carbonate solvent and LiPF 6 used in commercialized Li-ion batteries, allow both LiCoO 2 cathode and graphite anode to operate stably.