Effect of FEC electrolyte additive on the electrochemical performance of nickel-rich NCM ternary cathode

It has been well established recently that fluorinated electrolyte additives such as Fluoroethylene carbonate (FEC) could promote the formation of LiF-based solid electrolyte interphase that can stabilize lithium metal anode. Meanwhile, the impact of FEC additive on the cathode side, particularly for the high energy density nickel-rich LiNi1-x-yCoxMnyO2 (NCM) ternary cathodes, remains unclear. In this study, we investigated the structural and chemical composition of cathode electrolyte interphase (CEI) and its electrochemical performance to elucidate the effect of FEC additive on the LiNi0.9Co0.05Mn0.05O2 (NCM90) cathode for high energy lithium-ion batteries. It is discovered that the FEC additive in carbonate electrolyte (BE-FEC) can produce LiFbased CEI, which could stabilize the NCM90 surface and improve the cycle performance at low cut-off voltage. While the formation of a thick LiF layer under high cut-off voltage and high rate leads to a higher polarization and slower Li+ transport kinetics, which in turn deteriorates the battery performance. Whereas in carbonate electrolyte (BE), under low voltage, the unstable Li2CO3-based amorphous CEI components form on the NCM90 surface while an intermediate rock salt structure can also be identified, leading to poor cycle life. While under high voltage, the BE sample shows superior electrochemical performance due to the formation of a thin LiF layer from the decomposition of the LiPF6. Our work provides a comprehensive understanding of the role of FEC in the CEI of nickel-rich cathodes, offering practical guidance for the design of electrolytes for high-energy nickel-rich cathodes.