Coupled X-ray diffraction and electrochemical studies of the mixed Ti/V-containing NASICON: Na2TiV(PO4)(3)

The solid state synthesis of NaTiV(PO4)(3) and Na3TiV(PO4)(3) is reported. Soft chemical oxidation of the latter composition led to NASICON Na2TiV(PO4)(3) with intermediate unit cell parameters (s.g.: R (3) over barc, a = 8.610(3) angstrom, c = 21.755(6) angstrom) when compared to NaTiV(PO4)(3) and Na3TiV(PO4)(3). Coupled structural/electrochemical studies were carried out on Na2TiV(PO4)(3) for oxidation and reduction by operando X-ray diffraction. While the oxidation mechanism towards NaTiV(PO4)(3) is quite simple, the reduction mechanisms involve several mono-and bi-phasic mechanisms leading to Na3TiV(PO4)(3) as an intermediate composition and to Na4TiV(PO4)(3) eventually.