Ion occupation and migration pathways are investigated to explore the ion-migration mechanism of Na 3 V 2 (PO 4 ) 3 with the help of first principles calculations. Na 3 V 2 (PO 4 ) 3 with a NASICON framework generates high performances as a cathode material in sodium-ion batteries.

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Excellent C-rate and cycling performance with a high specific capacity of 117.6 mA h g(-1) have been achieved on NASICON-structure Na3V2(PO4)3 sodium-ion batteries. Two different Na sites, namely Na(1) and Na(2), are reported in the open three-dimensional framework, of which the ions at the Na(2) sites should be mainly responsible for the electrochemical properties. It is vitally important and interesting to find that there are two kinds of possible ion occupation of Na ions in Na3V2(PO4)3 and the investigation of ion-extraction number is firstly explored by discussing ion occupations with the help of first-principles calculations. The ion occupation of 0.75 for all Na sites is suitable for the configuration of [Na3V2(PO4)3]2, and the two-step extraction process accompanied by structure reorganization can account for the theoretical capacity of Na3V2(PO4)3.

The electrochemical properties of Na3V2(PO4)2F3 cathode utilized in the sodium ion battery are investigated, and the ion migration mechanisms are proposed as combined via the first principles calculations. Two different Na sites, namely, the Na(1) and Na(2) sites, could cause two sodium ions of Na3V2(PO4)2F3 to be extracted or inserted by a two-step electrochemical process accompanied by structural reorganization that could be responsible for the redox reaction of V(3+/4+). Because the calculated average voltage (V(avg)) of the second charging plateau is 4.04 V for the optimized system but 4.38 V for the unoptimized one, the reorganization of the cathode system can make a stable configuration and lower the extraction energy. Three designed pathways for sodium ions along the x, y, z directions in Na3V2(PO4)2F3, known as a 3D ions transport tunnel, have activation energies (Ea) of 0.449, 0.2, and 0.323 eV, respectively, by using DFT calculations, demonstrating the different feasibilities of the migration directions.