Surface chemistry of LiFePO4 cathode material as unraveled by HRTEM and XPS

Abstract

During the first cycle of lithium-ion batteries, a passivation layer is formed on the surface of electrode due to electrolyte decomposition, which is called solid electrolyte interface (SEI). In this study, the evolution of LiFePO4 cathode surface and components of SEI layer are investigated in different states by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The results suggest that SEI layer is mainly formed during the first cycle and remains stable in the following cycles. Furthermore, SEI layer can be divided into two parts, and components of these two parts are different. The outer layer that is near to electrolyte is composed of organic components, and the inner layer that is near to active material is composed of inorganic components. Taken together, the study reveals SEI layer formation process on LiFePO4 cathode surface, which is of significance for understanding the influence of SEI layer on electrochemical performance.