In Situ Atomic-Scale Investigation of Structural Evolution During Sodiation/Desodiation Processes in Na₃V₂(PO₄)₃-Based All-Solid-State Sodium Batteries

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.