Ionic Liquid Additive Enables Dual-Interfacial Stabilization for High-Voltage Lithium–Metal Batteries with Low-Concentration Electrolytes

Advanced high-voltage lithium-metal batteries require electrolyte designs that balance interfacial stability with efficient ion transport. This study demonstrates that incorporating a trace amount of the ionic liquid additive 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM-OTf) into a low-concentration electrolyte (LCE) effectively modulates the Li-ion solvation structure. This modification enables dual interfacial stabilization of both the Li-metal anode and the high-voltage (NCM811) cathode. Under stringent testing conditions (0.4 M lithium salt, 4.6 V cutoff voltage, 1.0 C rate), NCM811||Li full cells exhibit a 45% enhancement in capacity retention compared to the baseline LCE, maintaining 77.6% of their initial capacity after 200 cycles. Furthermore, the cells demonstrate improved Li plating/stripping Coulombic efficiency, along with a denser, more uniform Li deposition morphology. The EMIM-OTf additive effectively suppresses parasitic reactions at high voltages while preserving the favorable ion-transport properties inherent to the LCE. This trace-additive strategy provides a practical and cost-effective pathway for developing high-energy lithium-metal batteries, thereby expanding the design principles for low-concentration electrolytes.