Ionic Transport in LPSCl/PIB Composite Electrolytes: Effects of Particle Size, Binder Molecular Weight, and Stack Pressure

Slurry‐processed composite solid electrolyte membranes for all‐solid‐state batteries (ASSBs) are governed by particle size and binder molecular weight. Here, we vary Li 6 PS 5 Cl (LPSCl) average particle size ( d p = ~5.6, ~7.5, and ~9.0 µm) and polyisobutylene (PIB) binder molecular weight (M w = ~1050, ~1550, and ~3050 kg mol −1 ) across a factorial design to resolve their effect on ionic conductivity ( σ ion ), ionic tortuosity factor ( τ ), and activation volume ( V * ) under controlled stack pressure ( P s ). Physics‐based electrical equivalent circuit (EEC) models reveal that mechanical consolidation drives a transition from interface‐controlled to constriction‐dominated transport at a formulation‐dependent onset pressure, evidencing geometric and rheological effects as separable yet interacting control parameters. From the EEC parameters, a geometric–rheological decoupling framework has been derived and validated against unseen d p – M w combinations ( R 2 = 0.82 for σ ion and 0.85 for τ and V * ), establishing binder M w as the dominant variable whose penalty cannot be recovered by higher P s . Particle size acts as a secondary modulator: ~7.5 µm particle selection balances contact‐network density against binder kinetics, while at ~9 µm particle‐skeleton dynamics increasingly dominate, defining the framework’s upper quantitative boundary. Dry‐room calendering identifies lower‐ M w , intermediate‐ d p combinations that retain above 75% of original σ ion as a window for scalable processing.