Dicarbonyl Electrolyte for High-Voltage Lithium Metal Batteries: Importance of the Entropy of Solvation in Bidentate Solvent

An electrolyte system based on dicarbonyl solvents has been proposed and implemented in lithium metal batteries (LMBs). Among several dicarbonyl solvents explored, dimethyl dicarbonate (DMDC) emerged as the most promising electrolyte candidate. Interestingly, despite the structural similarities between DMDC and its monocarbonyl counterpart DMC, the DMDC electrolyte exhibits higher solvent coordination, even at the same lithium-to-carbonyl ratio. This leads to the formation of a more robust solid–electrolyte interphase and enhances the anodic stability of the electrolyte. Notably, in various electrolyte systems, the solvation energy is typically enthalpy-driven. However, the bidentate nature of DMDC introduces a significant role for the entropy of solvation because the Li+-DMDC complexes release fewer free molecules. With the addition of non-solvating co-solvent 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), which reduces the viscosity of the electrolyte, the DMDC-TTE system enables remarkably stable cycling of the lithium anode by manipulating the entropy of solvation. This introduces a new strategy for tailoring functional electrolyte systems.