In this work, we study the effects of three salt-type additives, particularly lithium difluoro(oxalato)borate (LiDFOB), lithium difluoro(bisoxalato)phosphate (LiDFBP), and lithium difluorophosphate (LiPO2F2) on the charging capacity and elevated temperature performance of high-voltage LiNi0.55Co0.15Mn0.3O2 and graphite. These salt-type additives possess different functional groups and thus they perform differently; therefore, it is especially important to investigate the impact of functional group on the electrochemical properties. The experimental results show that the three additives modify the interface of positive cathode or anode. Among them, a study on chemical composition of cathode surface film demonstrates that LiPO2F2 can produce a LiF-rich interface film and LiDFOB can produce borate on the cathodes. Therefore, they bring a notable improvement in elevating the cycling performance and storage performance of a high-voltage LiNi0.55Co0.15Mn0.3O2 electrode. Additionally, a study on chemical composition of anode surface film shows that the three salt-type additives produce a high concentration of P-O, which can greatly reduce charge transfer impedance of the anode electrode, and improve the room temperature cycling and rate performance.
Keywords: LiNi0.55Co0.15Mn0.3O2; functional groups; high voltage; salt-type additives; surface film.