Separator-Programmed Electric-Double-Layer Ordering for Stabilizing Thin Lithium-Metal Batteries

Thin lithium-metal anodes are essential for high-energy-density batteries, but their limited Li inventory makes them vulnerable to nonuniform Li deposition, inactive Li formation, and unstable interphase growth. Here, we propose a saturated lithium bis(fluorosulfonyl)imide (LiFSI)-coated separator as a localized salt reservoir that regulates the electric double layer at the Li metal interface. By comparing pristine polyethylene, boehmite-coated, and LiFSI-coated boehmite separators, we separated the ceramic layer effect from LiFSI-induced interfacial regulation. Li||Li symmetric-cell tests, Li deposition morphology, and finite-element simulations showed that the LiFSI-coated separator homogenizes interfacial ion transport and suppresses nonuniform Li growth. Potential-of-zero-charge analysis and polarization-dependent Raman spectroscopy revealed a compact electric double layer with enhanced charge accumulation and ordered ion orientation. Operando Raman measurements showed that the local salt reservoir mitigates FSI⁻ depletion during cycling. Consequently, LFP||20 μm thin-Li-metal full cells with the LiFSI-coated separator showed reduced Li nucleation overpotential, enhanced Li plating/stripping kinetics, improved rate capability, and stable cycling with 91.9% capacity retention after 250 cycles. This work demonstrates that separator chemistry can program ion orientation and interphase formation, providing a strategy for stabilizing thin lithium-metal batteries.