Study of Surface Reaction of Spinel Li4Ti5O12 during the First Lithium Insertion and Extraction Processes Using Atomic Force Microscopy and Analytical Transmission Electron Microscopy

Spinel lithium titanate (Li4Ti5O12, LTO) is a promising anode material for a lithium ion battery because of its excellent properties such as high rate charge?discharge capability and life cycle stability, which were understood from the viewpoint of bulk properties such as small lattice volume changes by lithium insertion. However, the detailed surface reaction of lithium insertion and extraction has not yet been studied despite its importance to understand the mechanism of an electrochemical reaction. In this paper, we apply both atomic force microscopy (AFM) and transmission electron microscopy (TEM) to investigate the changes in the atomic and electronic structures of the Li4Ti5O12 surface during the charge?discharged (lithium insertion and extraction) processes. The AFM observation revealed that irreversible structural changes of an atomically flat Li4Ti5O12 surface occurs at the early stage of the first lithium insertion process, which induces the reduction of charge transfer resistance at the electrolyte/Li4Ti5O12 interface. The TEM observation clarified that cubic rock-salt crystal layers with a half lattice size of the original spinel structure are epitaxially formed after the first charge?discharge cycle. Electron energy loss spectroscopy (EELS) observation revealed that the formed surface layer should be α-Li2TiO3. Although the transformation of Li4Ti5O12 to Li7Ti5O12 is well-known as the lithium insertion reaction of the bulk phase, the generation of surface product layers should be inevitable in real charge?discharge processes and may play an effective role in the stable electrode performance as a solid-electrolyte interphase (SEI).