Tuning the FEC‐Related Electrolyte Solvation Structures in Ether Solvents Enables High‐Performance Lithium Metal Anode

Abstract Lithium metal is the most promising high‐energy‐density anode. However, it is incompatible with high‐voltage cathodes in ether solvents due to their narrow electrochemical window. Herein, fluoroethylene carbonate (FEC) co‐solvent is introduced to regulate the Li⁺ solvation structures in ether solvents, including cyclic ether (1,3‐dioxolane [DOL]) and linear glymes with different chain lengths (1,2‐dimethoxyethane [DME], diglyme [G₂] and triglyme [G₃]). The apparently different effects of ether solvents on solvation ability and interaction strength with FEC are revealed. FEC plays a diverse role and function in 1 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)‐ether/FEC electrolyte, thus relevant batteries perform distinct performances due to various ionic dynamics and solid‐electrolyte interphase. The Li⁺‐solvation structures are explored by Raman and nuclear magnetic resonance spectroscopies. Specifically, part of FEC molecules are inserted into the first solvation shell in 1 m LiTFSI‐DOL/FEC because of the weak solvation ability of DOL and strong interaction of DOL‐FEC, leading to few coordinated TFSI⁻ and sluggish interfacial kinetics. In sharp contrast, FEC as a weak coordinated solvent almost exclusively occupies the second solvation sheath in 1 m LiTFSI‐glyme/FEC, favoring TFSI⁻ coordination and rapid de‐solvation dynamics. Ultimately, the LiNi_0.8Co_0.1Mn_0.1O₂/Li battery in G₂/FEC presents the most excellent performance, derived from abundant free‐FEC and rapid ionic kinetics.