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

Spinel-typelithium titanate (Li4Ti5O12 (LTO))is known as a negative electrode material for lithium-ionbatteries (LIBs). In this study, the surface structure of the LTO(111)electrode in an ionic liquid (IL) electrolyte was investigated byelectrochemical atomic force microscopy (EC-AFM). A quartz tuningfork sensor with a sharpened tungsten tip, so-called the qPlus sensor,was used as an AFM force sensor instead of a Si cantilever, and theAFM equipment was embedded in a vacuum chamber to decrease the waterconcentration included in the IL electrolyte. The in situ analysisof the surface structure before and after the insertion of Li ionsinto the LTO electrode revealed that the atomic steps on the LTO(111)surface were kept even after the insertion of Li ions. We also succeededin atomic-resolution topographic imaging on the LTO(111) electrodebefore and after Li-ion insertion. The hexagonally arranged brightspots with a period of & SIM;0.3 nm were clearly imaged after Li-ioninsertion, which corresponded well to the structure of the ion arrangementof the Li7Ti5O12(111) surface. Namely,after Li-ion insertion, it was revealed that the LTO electrode changedits crystal structure from spinel-type Li4Ti5O12 to rocksalt-type Li7Ti5O12 while maintaining the surface structure with atomic steps.