Molecular Design of Fluorinated Ester Electrolytes via Skeleton Regulation Enables Aggressive Cathode Chemistries in 5V‐Class Fast‐Charging Lithium‐Ion Batteries

Abstract

Lithium‐ion batteries can deliver higher energy densities when using high‐voltage cathodes such as LiNi 0.5 Mn 1.5 O 4 (LNMO), yet their practical application is hindered by the catalytic nature of high‐voltage cathode materials which usually cause electrolytes decomposition. Although the utilization of fluorinated carbonates as electrolyte solvent considerably inhibited the parasitic reaction at cathode side, the irrational fluorination often leads to the sluggish Li + transport which finally causes poor rate performance. Here, we design a family of trifluoromethyl‐substituted carboxylate esters as electrolyte solvents. We found that the spatial arrangement between the ‐CF 3 and the carbonyl group in these fluorinated esters is critical for regulating the electrolyte solvation structures. A short distance (α‐substitution) severely weakens the solvating power of the solvent, resulting in a substantial decrease in ionic conductivity, whereas an extended separation (γ‐substitution) compromises the oxidation resistance against high‐voltage cathodes. Only the trifluoromethylation at the β‐position yields optimal solvent (methyl 3,3,3‐trifluoropropionate, MTFP) that can simultaneously meets high‐voltage and fast‐charging requirements of LNMO||Gr cells when pairing with fluoroethylene carbonate and 1 M LiPF 6 . In such electrolyte, the Ah‐level LNMO||Gr pouch cells deliver 78% capacity retention at 6C charge/discharge rate (13.2 mA cm −2 ), and stably operate over 300 cycles at the high rate of 3C.