Effect of Fluoroethylene Carbonate Electrolyte Additives on the Electrochemical Performance of Nickel-Rich NCM Ternary Cathodes

Abstract

It has been well established recently that fluorinated electrolyte additives such as fluoroethylene carbonate (FEC) could promote the formation of LiF-based solid electrolyte interphases that can stabilize lithium metal anodes. Meanwhile, the impact of FEC additives 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 a cathode electrolyte interphase (CEI) and its electrochemical performance to elucidate the effect of FEC additives 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 a LiF-based CEI, which could stabilize the NCM90 surface and improve the cycle performance at low cut-off voltage. The formation of a thick LiF layer under high cut-off voltage and high rate has been observed to result in increased polarization and slower Li+ transport kinetics, ultimately leading to a deterioration in battery performance. On the other hand, in a carbonate electrolyte (BE) and under low voltage, the unstable Li2CO3-based CEI components on the NCM90 surface with an intermediate rock salt phase in between contribute to poor long-term performance and reduced reliability. While under high voltage, the BE sample shows superior electrochemical performance due to the formation of a thin LiF layer from the decomposition of 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 high-voltage nickel-rich cathodes.