Effect of La3+ Modification on the Electrochemical Performance of Na3V2(PO4)2F3

Abstract

La 3+ modification of Na 3 V 2 (PO 4) 2 F 3 was performed by the direct mechanochemically assisted solid-state synthesis of the Na 3 V 2−x La x (PO 4) 2 F 3 compositions, and by the LaPO 4 coating of the as-prepared Na 3 V 2 (PO 4) 2 F 3 via the precipitation method. It has been shown that no noticeable substitution of the V 3+ ions by the La 3+ ions occurs in the Na 3 V 2 (PO 4) 2 F 3 structure under the synthesis conditions; meanwhile, the introduction of the La 3+ ions into the reagent mixture leads to the formation of the LaPO 4 phase, and accordingly, an increase in the NaF/VPO 4 ratio. The latter results in the formation of the Na 3 PO 4 and Na 3 VF 6 surface impurity phases, which possess high ionic and electronic conductivity, respectively, and significantly enhances the electrical conductivity and the cycling performance of the composite cathode material both in Na and Li cells, while simple surface modification of Na 3 V 2 (PO 4) 2 F 3 by LaPO 4 via precipitation does not.

Aluminum substitution for vanadium in the Na3V2(PO4)2F3 and Na3V2(PO4)2FO2 type materials

“…Kosova et al have demonstrated that Lanthanum substitution was in fact a surface modification effect by LaPO4. 15 The group of Ceder has studied gallium and aluminium substitution for vanadium, with the aim to disrupt the charge and Na + orderings within the lattice and thus lower the energy barrier for the reversible extraction of the 3 rd Na. 16,17 Among the different metals, aluminium drew our attention as Al is cheaper, lighter, more environmentally friendly than V and, as a nonelectrochemically active element, could participate to the stabilization of the material upon cycling.…”