Li₃V₂(PO₄)₃ ­(LVP) and Na₃V₂(PO₄)₃ (NVP) are promising cathode materials for both lithium and sodium ion batteries, respectively. However, they have different structure when they are prepared by synthetic process. LVP prefers monoclinic structure but NVP formed rhombohedral structure and this difference caused different electrochemical behavior for lithium ion and sodium ion. The most distinguishing difference of the materials is available number of ions that participate in electrochemical reaction. Monoclinic LVP utilize whole lithium ions, thus LVP exhibits high theoretical capacity of 197 mAh/g. However, rhombohedral NVP only can use two sodium ions in the structure that limits it theoretical capacity of 118 mAh/g. The limitation mainly contributed to different crystallographic position of sodium ions in the structure that caused different migration energy. Thus, we need to approach to the problem with structural view point to overcome the electrochemical limitation of NVP.

To deduce the electrochemical behavior limitation of NVP with structural viewpoint, we prepared LVP materials with rhombohedral structure to observe the differences between the samples. The materials which have similar rhombohedral type structure LVP could be prepared from rhombohedral NVP by ion exchange method. From the rhombohedral LVP, we observed that more than two lithium ions can be extracted from the structure and new voltage plateau at high voltage which are not observed in NVP case. In this study, we will suggest different electrochemical behavior of LVP and NVP cathode materials based on different cation type and detailed structural studies during electrochemical reaction which will give the clues for understanding the different electrochemical behavior. The derived structural difference would also give new insights for modifying the structural features of NVP to enhance the electrochemical performances.