Bulk-surface engineering of PEO-based electrolytes via controlled [NMP-Li+] coordination for room-temperature quasi-solid-state lithium metal batteries

The practical deployment of poly(ethylene oxide) (PEO)-based solid electrolytes is significantly hampered by their low room-temperature ionic conductivity, stemming from high PEO crystallinity and strong ethylene oxide (EO)-Li⁺ coordination. Herein, we introduce a bulk-surface engineering strategy utilizing controlled N-methylpyrrolidone (NMP) coordination and a simple two-step drying process. This approach creates an asymmetric-structured PEO electrolyte where distinct molecular environments are engineered in the bulk and at the surface. Experimental and theoretical analyses reveal that controlled NMP coordination during the initial drying establishes a bulk [NMP-Li⁺] solvation structure that effectively weakens EO-Li⁺ binding, dramatically enhancing Li⁺ transport kinetics. Crucially, the subsequent drying phase intentionally depletes NMP from the electrolyte surface, forming a unique NMP-deficient phase. This engineered surface eliminates the thermodynamic instability of NMP towards Li metal, fostering a robust solid electrolyte interphase. Consequently, the asymmetric electrolyte (NP-x) achieves a high room-temperature ionic conductivity (0.14 mS cm⁻¹) and Li⁺ transference number (0.41). Symmetrical Li-Li cells demonstrate ultra-stable cycling exceeding 2000 h at 25°C. The obtained solid-state Li-LiFePO₄ cells deliver a high specific capacity (158.4 mAh g⁻¹ at 0.2 C) with 89% capacity retention over 500 cycles, and maintain stable cyclability even at 0 and –15°C. This solvent coordination-mediated bulk-surface decoupling offers a fresh perspective for enhancing the Li⁺ transport and the interface stability of PEO-based electrolyte.