Particle Size Dictates Transport Mechanism and Performance in Li 6 PS 5 Cl Conversion Cathodes for All‐Solid‐State Batteries

ABSTRACT 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 Li 6 PS 5 Cl (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.