Direct Atomic-Scale Observation of a Li-Inserted Li₄Ti₅O₁₂ Surface in an Ionic Liquid Electrolyte by Electrochemical Atomic Force Microscopy

Abstract

Spinel-type lithium titanate (Li 4 Ti 5 O 12 (LTO)) is known as a negative electrode material for lithium-ion batteries (LIBs). In this study, the surface structure of the LTO(111) electrode in an ionic liquid (IL) electrolyte was investigated by electrochemical atomic force microscopy (EC-AFM). A quartz tuning fork sensor with a sharpened tungsten tip, so-called the qPlus sensor, was used as an AFM force sensor instead of a Si cantilever, and the AFM equipment was embedded in a vacuum chamber to decrease the water concentration included in the IL electrolyte. The in situ analysis of the surface structure before and after the insertion of Li ions into the LTO electrode revealed that the atomic steps on the LTO(111) surface were kept even after the insertion of Li ions. We also succeeded in atomic-resolution topographic imaging on the LTO(111) electrode before and after Li-ion insertion. The hexagonally arranged bright spots with a period of ∼0.3 nm were clearly imaged after Li-ion insertion, which corresponded well to the structure of the ion arrangement of the Li 7 Ti 5 O 12 (111) surface. Namely, after Li-ion insertion, it was revealed that the LTO electrode changed its crystal structure from spinel-type Li 4 Ti 5 O 12 to rocksalt-type Li 7 Ti 5 O 12 while maintaining the surface structure with atomic steps.

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

“…Details of the equipment configuration are described in our previous paper. 23) The vacuum AFM was used for obtaining Fig. 6(b).…”