Atomistic structure of a spinel Li4Ti5O12(111) surface elucidated by scanning tunneling microscopy and medium energy ion scattering spectrometry

Spinel lithium titanate (Li4Ti5O12, LTO) is one of the promising anode materials for high-performance lithium-ion batteries (LIBs). It is crucial to investigate atomistic structures of LTO surfaces to understand the phenomena at LTO/electrolyte interfaces such as CO2-gas generation which greatly affects the performance and safety of LIBs. By applying scanning tunneling microscopy (STM) and medium energy ion scattering spectrometry (MEIS) to a LTO(111) film prepared from a TiO2 wafer, we found that there exist two kinds of Li-terminated (111) terraces bounded by steps with different heights. In the major terraces, the top hexagonal Li layer is stacked above the oxygen layer, while the top Li layer is stacked above the Ti–Li layer in the minor terraces. The relative stability between the two surface structures seems to depend on the atmosphere due to different stoichiometry. For the major terraces, the LTO surface should have electronic holes due to oxygen-rich stoichiometry, which is a possible origin of CO2 generation via redox interaction with electrolyte molecules.