Electrochemical properties and sodium ion diffusion in Na3V(PO4)2

Abstract

Understanding the material process and the mechanism behind during battery operation is essential to discover or identify new electrode for the sodium ion batteries. Using first-principles calculations, we systematically investigated the fundamental physical properties, electrochemical activity as well as sodium ion diffusion of the new discovered cathode material, Na3V(PO4)2. The results identified that the Na+ ion on Na2 sites is preferentially extracted from the host, which is responsible for the electrochemical redox process, instead of that on Na1 sites, proposed experimentally. The predicted cell voltage delivers two ‘plateaus’ with average voltage profile at ~ 3.1 and ~ 3.9 V vs. Na+/Na, corresponding to the V3+/V4+ and V4+/V5+ redox process, respectively, reproducing the experimental observations. Actually, the redox activity of V ions is accompanied with the formation of small polaron as manifested by the magnetic moments and spin density analysis. Furthermore, the fade of the reversible capacity is attributed to the considerable large volume variation (>5%) during the charging-discharging cycling, due to the flexibility of yavapaiite-type layers. The sodium ion diffusion is accomplished through the step-wise ion-exchange mode, in which the Na+ ions originated from both Na1 and Na2 sites migrate, progressively and independently, along the a axis between the yavapaiite-type layers, creating a zigzag Na2→Na1→Na2 trajectory. Meanwhile, the activation energy is calculated to be reasonable for facile ion diffusion. In addition, the fundamental electronic structures on different charging stages are examined thoughtfully. Our results not only provide a valuable insight on the electronic properties, electrochemical behavior of sodium disintercalation process and diffusion mechanism of Na3V(PO4)2, but also inevitably highlight the clues to suppress the large volume variation upon charge/discharge process, and then to improve the cycling capacity.