Anion Concentration–Regulated Interface Stabilization Enabling High‐Performance Lithium Metal Batteries

Rechargeable Li metal batteries offer high energy density due to the high capacity and low reduction potential of Li metal anodes, but their practical application is hindered by dendritic growth that induces significant volume expansion, low Coulombic efficiency (CE), and safety risks. To address these challenges, we rationally designed an electrolyte based on a systematic investigation of the effect of anion‐rich environments on interfacial stability. To specifically probe the role of anions, we employed a concentrated electrolyte system. Lithium bis(fluoromethanesulfonyl)imide (LiFSI) was selected as the lithium salt in combination with diethyl ether (DEE), a monodentate ether that reduces steric hindrance and promotes simplified solvation behavior. The optimized 6 m LiFSI‐DEE electrolyte enabled Li symmetric and Li/Cu cells to cycle stably for over 1200 h at 0.5 mA c m −2 and 1 mAh cm −2 , demonstrating superior interfacial stability, a benefit that extended to Li/LFP full cells and Cu/LFP anode‐less cells, both of which showed significantly improved cycling performance. The 6 m electrolyte promotes anion decomposition, forming a LiF‐rich solid electrolyte interphase (SEI) that stabilizes the interface. In addition, the interfacial morphological evolution was directly visualized by operando optical microscopy and SEM, confirming a more uniform and compact Li deposition. These results highlight that anion concentration effectively modulates the Li + solvation environment and SEI chemistry, providing a robust design strategy for next‐generation Li metal battery electrolytes.