Enhancing the cycling stability of Ni-rich LiNi0.83Co0.11Mn0.06O2 cathode at 4.5 V via 2,4-difluorobiphenyl additive

Abstract

Ni-rich LiNixCoyMn1-x-yO2 (Ni-rich NCM; x ≥ 0.8, 0 < y < 1) is the most prominent cathode material to establish a practical high-energy density of lithium-ion batteries for electric vehicle. However, long-term capacity fading limits the commercial applications of Ni-rich NCM, especially at a high cut-off voltage (>4.3 V). Herein, 2,4-difluorobiphenyl (FBP) is proposed as a fluorine-based cathode electrolyte interphase (CEI)-forming additive for Ni-rich LiNi0.83Co0.11Mn0.06O2 (NCM83). The structural characteristics of FBP originate from the overcharge protection of biphenyl, whereas the fluorine atoms are preferable for high-voltage conditions. The addition of 1 wt% FBP to the electrolyte enhances the cycling stability at the 4.5 V cut-off voltage. Electrochemical impedance spectroscopy and rate capability results indicate a fast kinetics at the NCM83 surface with FBP additive upon the formation of a stable CEI. Images from scanning electron microscopy and transmission electron microscopy after 150 cycles of NCM83 show the thin deposit layer of CEI upon introduction of FBP. The X-ray photoelectron spectroscopy results demonstrate the suppression of electrolyte and salt decomposition. This work suggests an opportunity to develop a completely new functional additive by introducing a fluorine component to existing additives.