Lewis-Acid Fluorides: Unlocking High-Performance Solid-State Polymer Electrolytes
Solid polymer electrolytes (SPEs) are promising for safer, high-energy solid-state lithium batteries, yet they suffer from low ionic conductivity (10^-5–10^-6 S/cm) and low lithium-ion transference numbers (t_Li+ ≈ 0.2) due to sluggish Li+ diffusion and incomplete salt dissociation. Polyethylene oxide (PEO) hosts exhibit high crystallinity, restricting conduction to amorphous domains, and a narrow electrochemical stability window (~3.6–3.8 V), limiting compatibility with high-voltage cathodes. This study introduces nanoscale Lewis-acid fluorides (e.g., AlF3, 5–10 wt%) into PEO–LiTFSI to address these limitations. The additive preferentially binds TFSI− anions, enhancing salt dissociation and transference number; disrupts PEO crystallinity, increasing amorphous content and segmental mobility; and forms a LiF-rich interphase that suppresses dendrites and parasitic reactions. Symmetric Li|Li cells with AlF3 cycled >3600 h without short-circuit, versus ~550 h for neat PEO. The approach extends to other polymers (polycarbonates, PVDF-HFP) and metal systems (Na, Zn, Mg), promising room-temperature conductivities beyond 10^-4 S/cm, near-unity cation transport, and dendrite-free cycling. This work establishes Lewis-acid fluorides as a versatile strategy to transform polymer electrolytes into actively engineered media for high-performance solid-state batteries.