Asymmetric Ultrathin Salt-Rich Composite Polymer Electrolyte Formed by Spin Coating for High-Performance Lithium Metal Batteries

Solid-state lithium metal batteries (SSLMBs) are being intensively explored as next-generation energy storage solutions, combining high energy density with enhanced safety. However, conventional composite polymer electrolytes face critical challenges including low room-temperature ionic conductivity, limited lithium ion (Li + ) transference numbers, and mechanically incompatible interfaces. Herein, we design a spin-coated asymmetric high-salt polymer electrolyte where a polyvinylidene fluoride (PVDF) framework spans the entire membrane while a sulfolane-containing polyethylene oxide (PEO)/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-rich cathode-side layer establishes a high-dielectric microenvironment. This converts ion aggregates into free Li + coordinated with ether oxygens, endowing the cathode side with low interfacial impedance and a robust solid electrolyte interphase (CEI). Meanwhile, the anode side maintains high modulus and homogenized Li + flux to suppress dendrite growth. Dispersed lithium lanthanum titanate (LLTO) fillers further construct an inorganic percolated pathway for Li + transport. This spin-coated asymmetric electrolyte achieves an optimized Li + transference number of 0.78. Full cells paired with lithium iron phosphate (LFP) and Li anodes deliver 82.5% capacity retention after 500 cycles accompanied by nearly 100% Coulombic efficiency and excellent rate-capability recovery. Our results highlight the potential of this asymmetric high-salt polymer electrolyte platform for developing thin, high-energy density SSLMBs.