In Situ Atomic-Scale Investigation of Structural Evolution During Sodiation/Desodiation Processes in Na 3 V 2 (PO 4 ) 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 Na 3 V 2 (PO 4 ) 3 (Na 3 VP) 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 Na 3 VP is first investigated using in situ high-resolution transmission electron microscopy (HRTEM). The intermediate Na 2 V 2 (PO 4 ) 3 (Na 2 VP) phase with the P2 1 /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  Na 2 VP phase reduces the lattice mismatch between Na 3 VP and NaV 2 (PO 4 ) 3 (NaVP), preventing structural collapse. Based on the density functional theory calculation (DFT), the Na + ion migrates more rapidly in the Na 2 VP structure, which facilitates the desodiation and sodiation processes. The formation of  Na 2 VP phase lowers the formation energy of NaVP. This study demonstrates the dynamic evolution of the Na 3 VP structure, paving the way for an in-depth understanding of electrode materials for energy-storage applications.