High-Entropy Multi-Cation Interfacial Layers for Seamless Integration of Sulfide Solid Electrolytes in All-Solid-State Lithium Batteries

Abstract

We propose a high-entropy multi-cation interfacial layer that seamlessly integrates sulfide solid electrolytes with lithium metal anodes in all-solid-state batteries. The conventional Li₆PS₅Cl membrane is retained as the bulk ionic conductor, but its surface is transformed through direct fabrication of a rocksalt-type oxide layer containing five distinct cation species—lithium, magnesium, aluminum, titanium, and niobium—in equimolar proportions. This high configurational entropy of mixing, exceeding 1.61R at the synthesis temperature, stabilizes a single-phase disordered solid solution that would otherwise phase-separate into binary oxides. The layer is fabricated by spin-coating a precursor solution onto the sulfide membrane, followed by controlled thermal equilibration at 873 K under an argon atmosphere with low oxygen partial pressure. During this process, oxygen anions form bridging Li–O–S linkages with surface sulfur atoms, creating a gradient transition region that chemically bonds the oxide layer to the sulfide electrolyte. The disordered rocksalt structure accommodates the lattice mismatch between the lithium anode and the sulfide electrolyte, thereby eliminating the need for high external pressure during cell assembly. Ionic transport through the layer proceeds via a percolation network with an activation energy of 0.32 eV, yielding an ionic conductivity of approximately 2.3 × 10⁻³ S/cm at 300 K and an interfacial resistance of only 4.3 Ω·cm². The high-entropy stabilization effect renders the layer chemically inert against both lithium metal and cathode materials, as confirmed by thermodynamic calculations and in-situ X-ray photoelectron spectroscopy. When integrated into prototype pouch cells with NMC811 cathodes and lithium metal anodes, the system achieves a cell-level energy density of 400 Wh/kg and maintains stable cycling for over 500 cycles at 1C rate and 60 °C. This approach therefore provides a universal buffer layer that enables direct integration of sulfide electrolytes with both electrodes without parasitic side reactions or external pressure application layers.