Crystal Structure of Na₂V₂(PO₄)₃, an Intriguing Phase Spotted in the Na₃V₂(PO₄)₃–Na₁V₂(PO₄)₃ System

The Na superionic conductor (NASICON) Na3V2(PO4)3 is an important positive electrode material for Na-ion batteries. Here, we investigate the mechanisms of phase transition in NaxV2(PO4)3 (1 ? x 4) upon nonequilibrium battery cycling. Unlike widely believed two-phase reaction a Na3V2(PO4)3–Na1V2(PO4)3 system, determine, first time, structure recently reported intermediate Na2V2(PO4)3 using operando synchrotron X-ray diffraction. Density functional theory calculations further support existence phase. We propose two possible crystal structures analyzed by Rietveld refinement. models with space groups P21/c or P2/c new show similar unit cell parameters but different atomic arrangements, including vanadium charge ordering. As appearance accompanied symmetry reduction, Na(1) and Na(2) sites split into several positions Na2V2(PO4)3, which one splitting position found to be vacancy, whereas are almost fully filled. reduces lattice mismatch between Na1V2(PO4)3 phases, facilitating fast transition. This work paves way better understanding great rate capabilities Na3V2(PO4)3.