Surface atomic layer fluorination of Li4Ti5O12: Investigation of the surface electrode reactivity and the outgassing behavior in LiBs

This paper presents the interest of the surface fluorination of Li4Ti5O12 anode material in Li-ion batteries by decomposition of solid XeF2. Enhanced electrochemical behaviors are observed for the most fluorinated material: both specific capacities (by 12% for 100 cycles) and C-rate capability are improved. The Li4Ti5O12/electrolyte interface interactions conduct usually to side reactions and gas generation. Operando ATR-FTIR and GC-MS analysis carried out show that the fluorination fades the catalytic properties of Ti4+ and thus reduces the CO2 gas generation by 5 times during the first cycle. Indeed, the fluorination of Li4Ti5O12 protects the surface from the formation of a solid electrolyte interface by the formation of Li-F passivating layer. XPS analysis demonstrate that the passivation is effective to protect the surface of the electrodes from carbonates and salt degradation products during the first cycles, which are determinant for batteries life span.