Anion-Complementary Soft Solvation Electrolytes Stabilizing Dual Interfaces for High-Voltage Lithium Metal Batteries across Wide Temperatures

Lithium metal batteries (LMBs) face severe interfacial degradation due to uncontrolled lithium dendrite growth and electrolyte decomposition. Conventional electrolytes fail to concurrently stabilize lithium metal anodes and high-voltage cathodes owing to competing parasitic reactions. Here, we report a softly solvating electrolyte composed of 1,3-dioxane (1,3-DX) and dual salts─lithium bis(fluorosulfonyl)imide/lithium hexafluorophosphate (LiFSI/LiPF6)─that leverages anion-complementary coordination to decouple interfacial requirements. The solvent’s steric hindrance softens Li+–solvent interactions, enabling FSI– and PF6– to spatially separate their interfacial functions: FSI– drives LiF-rich solid-electrolyte interphase (SEI) formation on Li metal, while PF6– constructs a thin (<10 nm) cathode–electrolyte interphase (CEI) on LiNi0.5Co0.2Mn0.3O2 (NCM523) cathodes. Raman and 17O NMR spectra confirm suppressed solvent coordination and anion-aggregate dominance. Li||NCM523 cells utilizing this electrolyte achieve over 80% capacity retention after 100 cycles at 4.3 V, with a high average Coulombic efficiency (CE) above 99.0%. Cross-sectional scanning electron microscopy/transmission electron microscopy (SEM/TEM) reveals crack-free cathodes and robust, homogeneous SEI and CEI layers. Remarkably, the Li||NCM523 cells maintain 36.9% capacity retention of their room-temperature performance (54.7 vs 148.3 mAh g–1) at 0.2 C and −30 °C, highlighting the electrolyte’s low-temperature compatibility. This work establishes an anion-synergistic design strategy to reconcile bulk ion transport with dual electrodes.