Obtaining V(PO) by sodium extraction from single-phase NaV(PO) (1 < x < 3) positive electrode materials.

We report on single-phase NaV(PO) compositions (1.5 ≤ x ≤ 2.5) of the Na super ionic conductor type, obtained from a straightforward synthesis route. Typically, chemically prepared c-NaV(PO), obtained by annealing an equimolar mixture of NaV(PO) and NaV(PO), exhibits a specific sodium-ion distribution (occupancy of the Na(1) site of only 0.66(4)), whereas that of the electrochemically obtained e-NaV(PO) (from NaV(PO)) is close to 1. Unlike conventional NaV(PO), when used as positive electrode materials in Na-ion batteries, the NaV(PO) compositions lead to unusual single-phase Na extraction/insertion mechanisms with continuous voltage changes upon Na extraction/insertion. We demonstrate that the average equilibrium operating voltage observed upon Na deintercalation from single-phase NaV(PO) is increased up to an average value of ~3.70 V versus Na/Na (thanks to the activation of the V/V redox couple) compared to 3.37 V versus Na/Na in conventional NaV(PO), thus leading to an increase in the theoretical energy density from 396.3 Wh kg to 458.1 Wh kg. Electrochemical and chemical Na deintercalation from c-NaV(PO) enables complete Na-ion extraction, increasing energy density.