Formation Of Solid Electrolyte Interface Film And Gas Generation On TiNb₂O₇ Anode Material In Lithium Ion Battery

LIBs are widely used in modern portable electronic devices and have also been regarded as promising power sources for hybrid electric vehicles（HEVs） batteries and large scale electricity storage systems（ESSs）, because of their high energy density, high voltage, low self-discharge, wide working temperature range, environmentally friendly properties and long cycle life. However, there are still many challenges in developing LIBs that meets the growing energy demands, in which an essential challenge is improving the safety and electrochemical performance of batteries. As one of the most promising anode material for LIBs, Ti Nb 2O7（TNO） has attracted great interest recently, as a result of its theoretical capacity of 387.6 m Ah /g, as well as excellent electrochemical and cycling behaviors. In addition, another important factor contributing to the overall acceptance of TNO as a suitable anode for LIBs is its high Li+ intercalation potential of 1.6 V, which makes it shun SEI film formation according to previous literature. Such point of view i s based on a widely accepted argument that the electrolyte solution can only be reduced below 1.0 V. Nevertheless, we found the formation of the SEI film on TNO material surface.In this paper, we used the high temperature solid phase synthesis TNO materials with excellent electrochemical and cycling performance. When used homemade electrolyte replace commercial electrolyte, TNO material performance had a great improvement. Compared the different solvents, it was found that the solvents of EC: DEC: DMC = 1:1:1 wth the best performance. In this work, we also found that SEI film, which was mainly composed of Li₂CO₃, Li F, ROLi and ROCO2 Li, was formed on the surface of TNO electrode for the first time. This finding further supports that SEI films could be formed on anodes when cycled between 1.0 and 3.0 V. Besides, our work also firstly provided evidences on interfacial reactions between electrolyte and TNO anodes. The formation and dissolution of SEI films were investigated through changes of Nb 3d XPS peaks during the charge and discharge processes, which made us suspect the stability of TNO in electrolytes in spite of its high redox potential. The formation and evolution of SEI film on the surface of TiNb₂O₇（TNO） electrode was investigated by SEM, TEM, XPS and FT-IR. According to the results of FT-IR and XPS, the SEI layer was formed during the lithium insertion（discharge） process and dissolved during the following lithium extraction（charge） process, SEM images also showed the same results. However, the SEI film was progressively increased during the long term cycles. Besides, soft packed LiFePO₄/TNO LIBs were assembled to investigate the gassing behaviors of TNO anodes. Gas generation and cell swelling phenomena were observed in the charge-discharge test. These facts manifested that TNO electrode/electrolyte interfacial reactions led to SEI film formation and generated gasses like H2, CO2, and CO, which were the main reason for swelling of the cell. In order to improve the cyclic performance of cells, we used electrolyte additive of 0.3 mass. % vinylene carbonate（VC）, which was found to be an effective way to suppress the gassing swelling of LIBs, in a mechanism of accelerating SEI film formation on TNO surface to prevent direct contact between electrolyte and TNO anodes.