Dry‐Processed, High‐Loading NMC811 Cathodes for Sulfide Solid‐State Batteries: Catholyte Size Effects

All‐solid‐state batteries (ASSBs) with sulfide electrolytes promise safer, high‐energy storage, yet practical deployment increasingly depends on high‐loading composite cathodes where microstructure‐driven transport limitations and cathode–electrolyte interphase growth dominate performance. Here, we quantify how catholyte particle size, under solvent‐free dry processing, governs cathode architecture and full‐cell behavior in sulfide ASSBs employing a Si anode and NMC811 composite cathode with Li ₆ PS ₅ Cl (LPSCl) solid‐state electrolyte. By systematically tuning LPSCl particle size (d50 ≈ nano–5 µm) while holding cathode composition and mass loading constant, we isolate a fundamental trade‐off between interfacial contact area and microstructural uniformity. A nano catholyte delivers the highest initial capacity and rate capability, but severe catholyte clustering produces a heterogeneous composite that accelerates interfacial impedance growth, suppresses apparent lithium transport at high states of charge, and triggers rapid capacity decay and premature failure. In contrast, an intermediate 2–3 µm catholyte forms a dense, homogeneous microstructure with stable ionic pathways, yielding the most stable impedance evolution and superior durability (92.6% capacity retention over 100 cycles). Coarser 3–5 µm catholyte increases void formation and reduces contact, limiting capacity with only moderate stability. Correlating cross‐sectional focused ion beam–scanning electron microscopy with electrochemical impedance spectroscopy/distribution of relaxation time and galvanostatic intermittent titration technique establishes a mechanistic framework linking catholyte size to packing, transport homogeneity, and interphase growth, providing actionable design rules for durable, high‐energy dry‐processed sulfide ASSBs.