Chemical and mechanical properties interplay on the nanometric scale and collectively govern the functionalities of battery materials. Understanding the relationship between the two can inform the design of battery materials with optimal chemomechanical properties for long-life lithium and sodium batteries. We found that chemomechanical properties of cathode materials are associated with phase transformation behaviors and can promote the cathode–anode crosstalk in full cells. In this presentation, we will first discuss an integrative approach of mapping valence states and constructing chemical topographies to investigate the redox phase transformation in layered oxide cathode materials at the secondary particle level under thermal abuse conditions. We discover that, in addition to the three-dimensional heterogeneous phase transformation, there is a mesoscale dynamic evolution of local valence curvatures in valence state topographies. The relative probability of negative and positive local valence curvatures alternates during the phase transformation. Such a behavior is likely associated with local chemical environments (e.g., grain boundaries, compositional variation) and can determine chemomechanical properties and ionic transport in cathode materials. Then we will discuss a mechanism of nanoscale mechanical breakdown in layered oxide cathode materials, originating from oxygen release at high states of charge. We observe that the mechanical breakdown of charged cathode materials proceeds via a two-step pathway involving intergranular and intragranular crack formation. Owing to the oxygen release, sporadic phase transformations from the layered structure to the spinel and/or rocksalt structures introduce local stress, which initiates microcracks along grain boundaries and ultimately leads to the detachment of primary particles; i.e., intergranular crack formation. Furthermore, intragranular cracks (pores and exfoliations) form, likely due to the accumulation of oxygen vacancies and continuous phase transformations at the surfaces of primary particles. Finally, we will highlight some methods that we develop to overcome chemomechanical breakdown.