Deciphering Structural Origins of Highly Reversible Lithium Storage in High Entropy Oxides with In Situ Transmission Electron Microscopy

Configurational entropy-stabilized single-phase high-entropy oxides (HEOs) have been considered revolutionary electrode materials with both reversible lithium storage and high specific capacity that are difficult to fulfill simultaneously by conventional electrodes. However, precise understanding of lithium storage mechanisms in such HEOs remains controversial due to complex multi-cationic oxide systems. Here, distinct reaction dynamics and structural evolutions in rocksalt-type HEOs upon cycling are carefully studied by in situ transmission electron microscopy (TEM) including imaging, electron diffraction, and electron energy loss spectroscopy at atomic scale. The mechanisms of composition-dependent conversion/alloying reaction kinetics along with spatiotemporal variations of valence states upon lithiation are revealed, characterized by disappearance of the original rocksalt phase. Unexpectedly, it is found from the first visualization evidence that the post-lithiation polyphase state can be recovered to the original rocksalt-structured HEOs via reversible and symmetrical delithiation reactions, which is unavailable for monometallic oxide systems. Rigorous electrochemical tests coupled with postmortem ex situ TEM and bulk-level phase analyses further validate the crucial role of structural recovery capability in ensuring the reversible high-capacity Li-storage in HEOs. These findings can provide valuable guidelines to design compositionally engineer HEOs for almighty electrodes of next-generation long-life energy storage devices.