High-Voltage-Compatible Dual-Ether Electrolyte for Lithium Metal Batteries

To improve the energy density of rechargeable batteries, lithium metal batteries (LMBs) consisting of high-voltage cathodes and metallic lithium anodes have been actively studied. Ether-based organic electrolytes, particularly 1,2-dimethoxyethane (DME), show good performance for LMB anodes, but due to low oxidation stability, they are difficult to apply in conjunction with high-reaction-voltage cathodes such as lithium cobalt oxide (LCO). Here, a dual-solvent system with fluorinated ether with high oxidative stability added as a co-solvent is used to increase the electrolyte oxidation stability. It is found that tetrafluoroethyl tetrafluoropropyl ether (TTE) is most suitable for use as the co-solvent, due to its non-solvation of Li salts and miscibility with DME arising from its non-polar nature. Introduction of a DME/TTE dual solvent with 1.0 M lithium bis(fluorosulfonyl)imide (LiFSI) increases the concentration of the salt coordinated by DME to an ultra-high electrolyte concentration of 5.0 M LiFSI. The oxidation stability of the electrolyte is improved by increasing the amount of the DME–Li+ complex. The DME/TTE ratio and the Li salt concentration of the dual-ether electrolyte are optimized through physical and electrochemical analyses. Compared to the conventional carbonated electrolyte, the dual-ether electrolyte shows low resistance and improved cycle performance in LMBs with an LCO cathode.