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

Abstract

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.: Rc, a = 8.610(3) Å, c = 21.755(6) Å) 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.