Elucidating the charge-transfer and Li-ion-migration mechanisms in commercial lithium-ion batteries with advanced electron microscopy

Understanding the charge-transfer and Li-ion-migration mechanisms in complex electrochemical environments is critical to improving the performance of commercial lithium-ion batteries (LIBs). Advanced electron microscopy and the associated characterization techniques have significantly assisted in clarifying the structure–function relationships of commercial LIBs by providing localized nano/atomic-scale information concerning the following aspects: atomic structures of light/heavy elements, spatial distributions of structural phase transitions, Li⁺ occupation, interfacial phase structures, occupation and migration of elements, elemental distribution in the interfacial layer, Li⁺ concentration, and interfacial space charge layer. Besides, the development of various in situ techniques coupled with electron microscopy can enable comprehensive understanding of the structural evolution, growth of lithium dendrites at the anode, as well as the ion transport and charge accumulation at the electrode–electrolyte interface in LIBs during charging and discharging. This review summarizes the recent progress of how advanced electron microscopy contributes to elucidating key structural information and evolution in commercial LIBs. Emphasis is placed on (1) the discussions of transition metal dissolution and charge-transfer mechanisms during charging and discharging of LIB cathodes; (2) the morphologies, structures, and compositions of solid-electrolyte-interphase (SEI)/cathode– electrolyte-interface (CEI) films, along with their influence on battery performance; (3) the effects of crystal structures, internal crystal defects, and interface structure on ion transport. The lithiation and delithiation processes in LIBs are scrutinized, and strategies for optimizing ion migration are proposed. This information has been collated to enable a deeper understanding of the charge-transfer and ion-migration mechanisms in commercial LIBs, and to provide guidance for improving battery performance.