Co-design of Active Material and Solid Electrolyte Particulate Phases in Solid-State Battery Composite Electrodes

The performance of solid-state batteries (SSBs) is strongly influenced by the solid-state cathode architecture, particularly the particle sizes of the active material (AM) and solid electrolyte (SE). While smaller SE particles are known to consistently enhance composite-level effective ionic transport, the underpinning role of the AM particle size remains unclear. Although smaller AM particles are often assumed to enhance the rate capability, some experimental observations have shown conflicting trends. This study addresses this ambiguity by uncovering a mechanistic regime in which favorable AM particle sizes are governed by the trade-off between transport limitations and reaction kinetics. We investigate the fundamental question: Is there a mechanistic limit for the AM/SE particulate phase size pair that delivers an optimal electrochemical performance? Our results demonstrate that this regime is strongly coupled with intrinsic material properties such as Li diffusivity within the AM, ionic conductivity of the SE, the cathode loading. Smaller AM particles enhance lithiation/delithiation kinetics but increase ionic transport resistance, while larger AM particles reduce transport resistance but are limited by sluggish Li diffusion within the AM particles. Our study provides design guidelines for tailoring optimal particle sizes to achieve high-performance SSB cathodes, enabling simultaneous improvement in energy and power density.