Enhanced efficiency and stability of 3.3 V Cu-Li batteries by tuning the cation-anion interaction in the electrolyte

Cu-Li battery with Cu metal cathode and Li metal anode is a candidate for next-generation energy storage system. While self-discharge of the battery can be suppressed with an anion exchange membrane, the voltage polarization depends strongly on the electrolyte. Specifically, when an electrolyte with 3 M LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) in dimethyl carbonate (DMC) is used, overpotential increases with cycling. In this work, we reveal why the voltage polarization changes, and reduce and stabilize it by replacing DMC solvent with a mixed solvent composed of dimethoxyethane (DME) and propylene carbonate (PC). The new electrolyte has higher ionic conductivity and stable solvation structure with more free TFSI− anions upon cycling, which also facilitates uniform plating of metal ions on the metal electrodes. These characteristics enable a stable Cu-Li battery with minimal change in overpotential for more than 1500 cycles at a current density of 2 mA cm−2.