Synthesis, structure, and sodium mobility of sodium vanadium nitridophosphate : A zero-strain and safe high voltage cathode material for sodium-ion batteries

Herein, the nitridophosphate Na₃V(PO₃)₃N is synthesized by solid state method. X-ray diffraction (XRD) and Rietveld refinement confirm the cubic symmetry with P2₁3 space group. The material exhibits very good thermal stability and high operating voltage of 4.0 V vs. Na/Na+ due to V$^{3+}$/V$^{4+}$ redox couple. In situ X-ray diffraction studies confirm the two-phase (de-)sodiation process to occur with very low volume changes. The refinement of the sodium occupancies reveal the low accessibility of sodium cations in the Na2 and Na3 sites as the main origin for the lower experimental capacity (0.38 eq. Na⁺, 28 mAh g$^{-1}$) versus the theoretical one (1.0 eq. Na⁺, 74 mAh g$^{-1}$). These observations provide valuable information for the further optimization of this materials class in order to access their theoretical electrochemical performance as a potentially interesting zero-strain and safe high-voltage cathode material for sodium-ion batteries