Ether‐Modified Nonflammable Phosphate Enabling Anion‐Rich Electrolyte for High‐Voltage Lithium Metal Batteries

Abstract Phosphate‐based localized high‐concentration electrolytes (LHCE) feature high flame retardant and satisfactory cathodic stability for lithium metal batteries. However, stable cycling of those electrolytes at ultra‐high upper cut‐off voltages for long‐term stability remains challenging. Herein, an ether‐modified phosphate, diethyl (2‐methoxy ethoxy) methylphosphonate (DMEP), is designed for high‐voltage applications. The ether modification enhances the stability of the Li ⁺ ‐DMEP‐FSI ⁻ coordination structure, promoting the formation of cation‐anion aggregates (AGG) dominated solvation structure, which favors the generation of LiF‐rich cathode electrolyte interphase layers compared to triethyl phosphate (TEP)‐based LHCE. Consequently, cathode degradation, including transition‐metal dissolution and electrode cracking, is well‐suppressed. The LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811)||Li full cells using DMEP‐based LHCEs show more than 90.7% capacity retention at an ultrahigh upper cut‐off voltage of 4.7 V after 100 cycles. Notably, DMEP‐LHCE exhibits enhanced safety than that of TEP‐LHCE, suggesting its versatility and potential for next‐generation lithium metal batteries.