Development of <i>Operando</i> Transmission Electron Microscopy for All-solid-state Lithium-ion Batteries

A key challenge in advancing solid-state Li batteries is the accurate probing of internal electrochemical reactions under operating conditions.Operando scanning transmission electron microscopy coupled with electron energy-loss spectroscopy (STEM-EELS) is a powerful technique that enables the real-time, real-space observation of ionic diffusion, phase transitions, and changes in the electronic states of transition metals and oxygen during charging and discharging.In Ni-rich layered cathodes, operando STEM-EELS reveals non-uniform delithiation and phase evolution at the nanoscale, highlighting the critical role of twodimensional Li-ion diffusion and grain architecture in polycrystalline particles.Meanwhile, in lithium titanate anodes, the study uncovers contrasting diffusion behaviors during Li insertion and extraction, including the formation of core-shell structures and preferential surface diffusion, both of which significantly impact the overall rate performance.These findings collectively indicate that battery electrodes undergo highly non-uniform reactions during electrochemical cycling, with intricate gradients in the Li content, transition-metal valences, and oxygen redox states dynamically evolving.These gradients are influenced by factors such as the crystalline orientation, grain boundaries, surfaces, and particle morphology.By providing nanoscale insights into these fundamental processes, operando STEM-EELS provides valuable guidance for optimizing material design, refining electrode architecture, and enhancing the overall battery performance.