Solvation Structures Tailored by the Steric Effect of Phosphorus‐Based Eutectic Electrolytes for Stable High‐Voltage Lithium‐Metal Batteries

Li metal batteries (LMBs) are plagued by critical challenges, including electrolyte flammability, unstable interfaces, and dendritic growth. Phosphate-based electrolytes are intrinsically inflammable, yet they are incompatible with Li metal anodes. In this study, a phosphorus-based deep eutectic electrolyte (DEE) is engineered by integrating lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), triphenyl phosphate (TPP), and 1,2-dimethoxyethane (DME) in a 1:2:2 molar ratio. This design leverages the ability of DME to enhance the ionic conductivity and Li⁺ transference number (0.61), while exploiting the sterically hindered structure of TPP to disrupt excessive Li⁺-DME coordination and foster anion-rich solvation structures. It supports the formation of a robust LiF/Li_xN-enriched solid-electrolyte interphase that suppresses dendrites and enables exceptional cycling stability. LiFePO₄|DEE|Li cells achieve 80% capacity retention after 2000 cycles at 1C and 1600 cycles at 5C. Notably, the DEE sustains operation for over 300 cycles at 4.45 V in high-voltage LiCoO₂||Li cells at 0.2C and operates across a wide temperature range from -10 to 50 °C. The practical viability of the DEE is applied in LiFePO₄||Li full cells (N/P = 1.75). A molecular-level design strategy is presented for the development of nonflammable electrolytes for LMBs that bolster the interfacial stability and operate in a wide temperature range.