Understanding the Structural Phase Transitions in Na3V2(PO4)3 Symmetrical Sodium-Ion Batteries Using Synchrotron-Based X-Ray Techniques

Sodium-ion batteries (SIBs) hold great potential for use in large-scale grid storage applications owing to their low energy cost compared to lithium analogs. The symmetrical SIBs employing Na3V2(PO4)(3) (NVP) as both the cathode and anode are considered very promising due to negligible volume changes and longer cycle life. However, the structural changes associated with the electrochemical reactions of symmetrical SIBs employing NVP have not been widely studied. Previous studies on symmetrical SIBs employing NVP are believed to undergo one mole of Na+ storage during the electrochemical reaction. However, in this study, it is shown that there are significant differences during the electrochemical reaction of the symmetrical NVP system. The symmetrical sodium-ion cell undergoes approximate to 2 moles of Na+ reaction (intercalation and deintercalation) instead of 1 mole of Na+. A simultaneous formation of Na5V2(PO4)(3) phase in the anode and NaV2(PO4)(3) phase in the cathode is revealed by synchrotron-based X-ray diffraction and X-ray absorption spectroscopy. A symmetrical NVP cell can deliver a stable capacity of approximate to 99 mAh g(-1), (based on the mass of the cathode) by simultaneously utilizing V3+/V2+ redox in anode and V3+/V4+ redox in cathode. The current study provides new insights for the development of high-energy symmetrical NIBs for future use.

Automated summary

This study reveals some key structural changes of symmetrical sodium-ion batteries during electrochemical reactions, and demonstrates through experiments that symmetrical NVP cells can stably provide high energy capacity.