Impact of Particle Size on Kinetics and Transport in Polycrystalline Li-Ion Battery Materials: Comparing Thin-Electrode and Single-Particle Measurements

Accurate modeling of lithium transport in polycrystalline cathode active materials (AM) is essential for reliable battery simulations, yet reported solid-state diffusion coefficients ( D s ) often vary widely, partly due to particle-scale effects. Here, we investigate the influence of secondary particle size on reaction kinetics and solid-state transport in NMC622 cathodes by combining particle-size-fractionated model electrodes (PSF-MEL) with single-particle electrochemistry (SPE) from the same AM batch. To our knowledge, this is the first direct experimental comparison of SPE and electrode-level kinetic and transport measurements on identical AM. Both approaches show that exchange current normalized by AM mass and characteristic diffusion time constant are largely independent of secondary particle size in near-equilibrium measurements. These findings contradict the Monolithic Particle Model and support the Cracked Particle Model, where electrolyte penetration into intergranular cracks decouples effective reaction and diffusion lengths from external particle dimensions. Consequently, D s derived under monolithic assumptions systematically overestimate intrinsic lithium diffusivity. Mass-normalized exchange current densities and diffusion time constants may improve AM comparison and battery model parameterization. Particle size distribution model implementations should be interpreted cautiously under near-equilibrium conditions. Finally, PSF-MEL with GITT- and impedance-based analyses offer a robust alternative to SPE, applicable to other material classes to probe particle-scale effects.