Bridging role of ethyl methyl carbonate in fluorinated electrolyte on ionic transport and phase stability for lithium-ion batteries

Abstract

Developing electrolytes with a higher oxidation potential is essential to the performance of lithium-ion batteries (LIBs). Fluorine-containing solvent (1, 1, 2, 2-tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropyl ether (TTE)) is a promising candidate for high-voltage use; however, its weak interaction with lithium hexafluorophosphate (LiPF6) containing electrolyte results in phase separation and causes concerns in practical applications. In this work, ethyl methyl carbonate (EMC) is selected to resolve the phase separation. The solvation structure, oxidation stability, and transport property of 1 M LiPF6 in fluoroethylene carbonate (FEC)/TTE and FEC/TTE/EMC (3:7 and 3:5:2 by vol., respectively) are investigated. The solvation energy of TTE in LiPF6 can be greatly increased by the inclusion of EMC and achieve improved thermodynamic stability. Upon adding EMC, the solvation structure is altered from Li+(FEC)2(PF6−) to Li+(FEC)(EMC)(PF6−) and confirmed by the Raman spectroscopy result. The transference number of Li+ in the improved electrolyte also evidently increases, as confirmed both theoretically and experimentally. Besides, 1 M LiPF6 in EFC/TTE/EMC (3:5: 2 by vol.) exhibits high oxidation potential ~ 5.31 V (vs. Li/Li+) and significantly enhances the transference number of Li+. This work offers a fluorine-containing electrolyte with a stable phase and high oxidation potential for LIB's practical application.