Self-generated phosphate/oxysulfide passivation from dry-processed single-crystal NMC811 stabilizes argyrodite sulfide solid-state battery interfaces

Abstract

Pushing sulfide all solid-state batteries (SSBs) beyond 500 Wh kg⁻¹ requires high voltage, high loading layered cathodes without accelerating sulfide electrolyte decomposition or losing interfacial contact. Polycrystalline NMC811 commonly fails under these conditions due to grain-boundary oxygen release, chemo-mechanical cracking, and growth of resistive cathode–electrolyte interphases that isolate the sulfide conductor. Here we show demonstration that thin-coated single-crystal NMC811 (SC811), fabricated by solvent-free, dry processing into high-loading composite cathodes (4.6 mAh cm⁻²) cathodes and paired with a Si anode and thin, sheet-type argyrodite Li6PS5Cl (LPSCl) solid-state electrolyte (SSE) separator, spontaneously forms an intrinsic phosphate/oxysulfide-rich interphase (PO2⁻/PO3⁻ + S⁻) that suppresses the SSE degradation. Raman, ToF-SIMS, XPS, and XRD show preserved Li₆PS₅Cl signatures after cycling for single-crystal electrodes, while polycrystalline and alternative single-crystal chemistries exhibit pronounced S–O/P₂S₅-type products and structural contraction. Leveraging FIB-SEM microstructures, a pixel-resolved diffusion model reveals 2–3 orders-of-magnitude higher effective Li diffusivity in single-crystal composite cathodes, consistent with sharper H2–H3 redox, minimal DRT-resolved impedance growth, and improved rate capability (190 mAh g⁻¹ at C/3). The single-crystal architecture retains ≥89 % capacity over ≥200 cycles and exhibits very low parasitic current during 4.3 V / 180 h holds. Importantly, this study represents the first systematic investigation of single-crystal versus polycrystalline Ni-rich cathodes in sulfide all-solid-state batteries under calendar-aging conditions. We reveal that SC811 undergoes surface self-passivation during cell operation, forming a chemically stable interphase that mitigates time-dependent interfacial degradation even in device-relevant architectures. These results establish intrinsic, coating-lean self-passivation as a practical design pathway for high-loading sulfide SSB cathodes.