Particle Size Dictates Transport Mechanism and Performance in Li6PS5Cl Conversion Cathodes for All-Solid-State Batteries.

An emerging paradigm in all-solid-state batteries (ASSBs) is the integration of electrolyte and active material within a single material, a concept exemplified by Li6PS5Cl (LPSCl), a common sulfide solid electrolyte that can become electrochemically active through conversion reactions. Yet whether this chemistry enables high-capacity energy storage or is ultimately limited by detrimental decomposition remains unresolved. Here, we show that particle size governs both accessible capacity and Li transport in LPSCl-carbon cathodes. Unexpectedly, although particle refinement progressively increases the accessible conversion capacity, its effect on transport kinetics is not monotonic: rate capability first deteriorates as the particles become finer, but recovers upon further refinement into the ultrafine regime, accompanied by a similar recovery in cycling stability. Modeling and impedance measurements show that the conventional bulk ionic pathway continues to deteriorate because of decomposition-induced blocking and loss of network connectivity, even as the overall chemical-transport response recovers. We rationalize this divergence using an interfacial job-sharing framework, in which increasing interfacial fraction and connectivity provide an additional coupled ion/electron transport contribution. The optimized cathode delivers 1096 Wh kg-1 and retains approximately 100% capacity after 100 cycles.