Asynchronous-to-Synchronous Transition of Li Reactions in Solid-Solution Cathodes

The composition dynamics regulate the accessible capacity and rate performance of rechargeable batteries. Heterogeneous Li reactions can lead to nonuniform electrochemical activities and amplify mechanical damage in the cell. Here, we employ <i>operando</i> optical microscopy as a laboratory tool to map the spatial composition heterogeneity in a solid-solution cathode for Li-ion batteries. The experiments are conducted at slow charging conditions to investigate the thermodynamic origins. We observe that the active particles charge asynchronously with reaction fronts propagating on the particle surfaces during the first charge, while subsequent (dis)charge cycles transition to a synchronous behavior for the same group of particles. Such transition is understood by computational modeling, which incorporates the dependence of Li diffusivity and interfacial reaction rate on the state of charge. The optical experiments and theoretical modeling provide insight into the reaction heterogeneity of porous electrodes and electrochemical conditioning for layered oxide cathodes.