Improving the energy density of Na3V2(PO4)3-based positive electrodes through V/Al substitution

Abstract

The crystal chemistry and the electrochemical properties upon Na⁺ extraction/insertion of NASICON-type Na₃Al_yV_2−y(PO₄)₃ compositions (y = 0.1, 0.25 and 0.5) were investigated. It was found that this family of V/Al substituted NASICON materials undergoes multiple reversible phase transitions between −50 °C and 250 °C upon heating, from monoclinic to rhombohedral symmetry, related to progressive disordering of Na⁺ ions within the framework. Na⁺ insertion/extraction mechanisms were monitored by operando X-ray diffraction. It is shown for the first time that substitution of aluminum for vanadium in Na₃Al_0.5V_1.5(PO₄)₃ increases significantly the theoretical energy density of these promising positive electrodes (425 W h kg⁻¹) due to its lighter molecular weight and the possibility of reversible operation on the V⁴⁺/V⁵⁺ redox couple at 3.95 V vs. Na⁺/Na.