Chemically Active LiH2PO4 Interphase Precursor Enables Self-Limited Interface Stabilization in Sulfide-Based All-Solid-State Batteries

The commercialization of sulfide-based all-solid-state batteries (ASSBs) is severely limited by the interfacial degradation between layered oxide cathodes and sulfide solid electrolytes, resulting in electrolyte decomposition, impedance growth, and rapid capacity fading. Herein, we propose lithium dihydrogen phosphate (LiH2PO4, LDP) as a low-cost polyanionic interphase precursor to regulate the cathode–electrolyte interface. A uniform LDP coating was deposited onto polycrystalline NCM622 particles, producing a continuous 40-nm-thick phosphate layer. Although the LDP-coated electrode exhibited a higher initial interfacial resistance than the bare cathode, electrochemical analyses revealed a capacity retention of 94.1% after 100 cycles, compared with 73.9% for the uncoated electrode. X-ray photoelectron spectroscopy demonstrated that LiH2PO4 is not chemically inert toward Li6PS5Cl but undergoes a controlled initial reaction with sulfur-deficient species to generate a phosphate-rich artificial interphase while suppressing the unstable P–[S]n–P species. This interphase rapidly reaches chemical equilibrium, suppressing sulfur precipitation, SOx formation, and subsequent electrolyte decomposition. Cross-sectional STEM–EDS and focused ion beam analyses revealed that the artificial phosphate interphase inhibited elemental interdiffusion, suppressed chemical mixing, and preserved mechanical contact during prolonged cycling. These findings demonstrate that the electrochemical penalty associated with the initial formation of the LDP interphase represents the necessary cost of constructing a chemically stable interface rather than a degradation process. This study introduces a new interfacial design strategy based on chemically active sacrificial interphase precursors, providing an alternative to conventional inert oxide coatings for realizing long-term stable sulfide-based all-solid-state batteries.