The electrochemical kinetics of battery electrodes at the single-particle scale are measured as a function of state-of-charge, and interpreted with the aid of concurrent transmission X-ray microscopy (TXM) of the evolving particle microstructure. An electrochemical cell operating with near-picoampere current resolution is used to characterize single secondary particles of two widely-used cathode compounds, NMC333 and NCA. Interfacial charge transfer kinetics are found to vary by two orders of magnitude with state-of-charge (SOC) in both materials, but the origin of the SOC dependence differs greatly. NCA behavior is dominated by electrochemically-induced microfracture, although thin binder coatings significantly ameliorate mechanical degradation, while NMC333 demonstrates strongly increasing interfacial reaction rates with SOC for chemical reasons. Micro-PITT is used to separate interfacial and bulk transport rates, and show that for commercially relevant particle sizes, interfacial transport is rate-limiting at low SOC, while mixed-control dominates at higher SOC. These results provide mechanistic insight into the mesoscale kinetics of ion intercalation compounds, which can guide the development of high performance rechargeable batteries.
