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

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 LiNi0.8Co0.1Mn0.1O2/Li battery in G2/FEC presents the most excellent performance, derived from abundant free-FEC and rapid ionic kinetics. The co-solvent FEC plays a totally different role and function in various 1 m LiTFSI-ether/FEC electrolytes, including cyclic ether (DOL) and linear glymes (DME, G2, and G3). In G2/FEC, FEC only locates in the second Li+ solvation structure resulting in fast dynamics and efficient SEI. Whereas, partial FEC takes part in the solvation structure in DOL/FEC resulting in poor performance. image