Insights about the spatial dynamics of Li intercalation and the heterogeneous distribution of crystal defect leverage an improved understanding of the rate performance, electrochemical and mechanical degradation mechanisms of solid-state batteries. Here, we investigate the role of secondary particle size and discharge rate on state-of-charge heterogeneities and dislocation activity in single crystal and polycrystalline NMC cathodes. Fig. shows the predicted and experimental voltage-capacity profiles of the polycrystal cathode during galvanostatic discharge tests , where the discharge rate is gradually increased from 0.1 C to 2 C. Furthermore, as shown in Fig. and Supplementary Fig. , for both single crystal and polycrystalline cathodes, the capacity-rate trade-off can be improved by decreasing the secondary particle size of cathodes. The good agreement between simulations and experiments confirms the effectiveness of the developed physics-based chemo-mechanical model. Fig. suggests that both single crystal and polycrystalline cathodes exhibit similar spatial dynamics of lithiation across a broad range of discharge rates, from 0.25 C to 5 C. However, there are significant differences in dislocation activity between single crystalline and polycrystalline cathodes. At a low discharge rate of 0.25 C, we observe a high level of dislocation activity in the polycrystalline particle, while the dislocation activity is relatively low in the single crystal particle. Transitioning a high discharge rate of 5 C, dislocations accumulate at the edge of both polycrystalline and single crystal particles; however, the polycrystalline particle exhibits a much broader defect-rich region. Chemo-mechanical phase-field dislocation modeling and operando synchrotron X-ray diffraction experiments demonstrate that the dimensions and geometries of cathode particles remarkably impact the formation of misfit dislocations in phase-transforming cathodes . Energy-based stability analysis of misfit dislocations reveals that the minimum critical size for dislocation-free LiFePO 4 particles is around 47 nm; below this size, particles are unlikely to host a misfit dislocation at the phase boundary . Synchrotron X-ray diffraction experiments indicate that large misfit strains can be effectively circumvented in electrodes comprising V 2 O 5 nanospheres with diameters of 49 nm . In the current study, the primary particles in polycrystalline cathodes range from 300 nm to 1 µm in diameter, while single crystal particles range from 2 µm to 12 µm. Thus, the size of the cathode particles is above the minimum critical size of dislocation-free particles . Consequently, in this study, plastic shear in the cathodes is mainly driven by anisotropic and heterogeneous compositional strains. The current results suggest that dislocations primarily form as a result of the Li inhomogeneity-induced strain gradient within single crystal particles under high current density conditions. For polycrystalline cathodes, both operating conditions and the random arrangement of primary particles in the secondary particle play a critical role in dislocation heterogeneity. Furthermore, as depicted in Fig. , a positive correlation between the Li concentration and dislocation activity is evident for the single crystal particle, whereas a relatively high dislocation activity is observed in the polycrystalline particle, irrespective of Li content. The particle size-dependent effect on the stability of misfit dislocations should be incorporated into the developed chemo-mechanical model, when the dimension of the electrode particles approaches the critical size .