Complex chemomechanical interplay exists over a wide range of length scales within the hierarchically structured lithium-ion battery. At the mesoscale, the interdependent structural complexity and chemical heterogeneity collectively govern the local chemistry and, as a result, critically influence the cell level performance. Here we investigate the morphology and state of charge (SOC) inhomogeneity within secondary NCA particles that were cycled in solid polymer batteries. We observe substantial inhomogeneity in the nickel oxidation state (a proxy for SOC) and loss of structural integrity within secondary particles after only 20 cycles due to significant intergranular cracking. The formation of mesoscale cracks causes loss of ionic and electrical contact within cathode particles, triggering increases in local impedance and rearrangement of transport pathways for charge carriers. This can eventually lead to deactivation of subparticle level domains in solid-state lithium-ion batteries. Our findings highlight the importance of proper mesoscale strain and defect management in polymer lithium-ion batteries.