Highly Concentrated Carbonate Electrolytes for Stable High-Voltage Lithium Metal Batteries

Lithium metal batteries (LMBs) have been widely studied due to their high energy density; however, the practical implementation of LMBs is limited by issues of uncontrolled dendrite growth, continuous electrolyte decomposition, and poor Coulombic efficiency (CE). Highly concentrated electrolytes (HCEs) have emerged as a promising approach to addressing the above issues. In this work, we propose a new HCE system based on a single carbonate solvent of 2,2,2-trifluoroethyl methyl carbonate (FEMC) with a high concentration of lithium bis(fluorosulfonyl)imide (LiFSI). The resulting electrolytes exhibit enhanced anodic stability and improved compatibility with lithium metal anodes and high-voltage cathodes. The optimized 4 M LiFSI–FEMC HCE achieved the highest CE for Li plating/stripping in Li/Cu cell and lowest overpotential in Li/Li symmetric cells, outperforming both low-concentration FEMC and ethyl methyl carbonate (EMC)-based electrolytes. In full-cell configurations with LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes, the HCE demonstrates stable cycling with minimal capacity fade over 250 cycles. Importantly, the HCE enables stable operation of 4.6 V high-voltage NCM811/Li cells for more than 120 cycles with a high-capacity retention of 88.61%. Post-mortem analysis revealed a more uniform and compact solid electrolyte interphase and a thinner cathode electrolyte interphase, contributing to the enhanced cycling performance.