Tuning and understanding the solvent ratios of localized saturated electrolytes for lithium-metal batteries

Abstract

LiPF₆-based localized saturated electrolytes (LSEs) have been shown to greatly stabilize lithium-metal batteries with high-Ni cathodes to attain high energy densities for commercial feasibility. A mixture of fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) has been proven to be a promising solvent, but regulating the solvent ratios in order to understand the changes in the electrolyte properties and the implications for cycling performance warrants investigation. Herein, four different LSEs with varying FEC/EMC solvent ratios are comprehensively screened through several electrochemical and materials characterization techniques. It is found that the ideal FEC/EMC (v/v) ratio of 1/1 results in remarkable Li-metal half-cell cycling with the cobalt-free LiNiO₂ cathode, achieving an exceptional 80.6% retention over 600 cycles with a high cutoff voltage of 4.4 V. The largely improved cycling stability comes from the formation of inorganic-rich and robust cathode–electrolyte interphase and anode solid-electrolyte interphase development along with the best Li plating and stripping efficiency. The formation of beneficial interphase species is further correlated to the unique Li⁺ solvation structure in the electrolyte. Therefore, a careful balance of FEC and EMC solvents is found to be critical for achieving the best overall performance in lithium-metal batteries.