| Summary: | Abstract Recently, all‐solid‐state sodium batteries (Na‐ASSBs) have received increased interest owing to their high safety and potential of high energy density. The potential of Na‐ASSBs based on sodium superionic conductor (NASICON)‐structured Na3V2(PO4)3(Na3VP) cathodes have been proven by their high capacity and a long cycling stability closely related to the microstructural evolution. However, the detailed kinetics of the electrochemical processes in the cathodes is still unclear. In this work, the sodiation/desodiation process of Na3VP is first investigated using in situ high‐resolution transmission electron microscopy (HRTEM). The intermediate Na2V2(PO4)3 (Na2VP) phase with the P21/c space group, which would be inhibited by constant electron beam irradiation, is observed at the atomic scale. With the calculated volume change and the electrode–electrolyte interface after cycling, it can be concluded that the Na2VP phase reduces the lattice mismatch between Na3VP and NaV2(PO4)3 (NaVP), preventing structural collapse. Based on the density functional theory calculation (DFT), the Na+ ion migrates more rapidly in the Na2VP structure, which facilitates the desodiation and sodiation processes. The formation of Na2VP phase lowers the formation energy of NaVP. This study demonstrates the dynamic evolution of the Na3VP structure, paving the way for an in‐depth understanding of electrode materials for energy‐storage applications.
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