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.