Two-dimensional analysis of the interfacial solvation structure of an ionic liquid electrolyte on a hydrogen-terminated Si electrode by atomic force microscopy

Abstract

Ionic liquids (ILs) have been intensively studied as new electrolytes for lithium-ion batteries (LIBs). Structural analysis of interfaces between an IL-based electrolyte and an LIB electrode would provide beneficial information for improving LIBs. In this study, we investigated the interfacial structures between an IL, 1-methyl-1-propyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide, and a H-terminated Si(111) electrode in the presence and absence of Li salt by frequency modulation atomic force microscopy utilizing a quartz tuning fork sensor. Two-dimensional frequency shift mapping imaging of the solvation structure at the interface showed that the layered solvation structure was only observed in the absence of Li salts in the ILs, which was in good agreement with our previous studies performed on IL/lithium titanate interfaces. Combined with electrochemical measurements, the partial disappearance of the layered solvation structure in the Li salt-doped IL was strongly suggested to be due to the Li-ion insertion/extraction at the IL/Si interface.