Crystal Structure of Na 2 V 2 (PO 4 ) 3 , an Intriguing Phase Spotted in the Na 3 V 2 (PO 4 ) 3 –Na 1 V 2 (PO 4 ) 3 System

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