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

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_0.8Co_0.1Mn_0.1O₂ (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.