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

Resumen

La³⁺ modification of Na₃V₂(PO₄)₂F₃ was performed by the direct mechanochemically assisted solid-state synthesis of the Na₃V_2−xLa_x(PO₄)₂F₃ compositions, and by the LaPO₄ coating of the as-prepared Na₃V₂(PO₄)₂F₃ via the precipitation method. It has been shown that no noticeable substitution of the V³⁺ ions by the La³⁺ ions occurs in the Na₃V₂(PO₄)₂F₃ structure under the synthesis conditions; meanwhile, the introduction of the La³⁺ ions into the reagent mixture leads to the formation of the LaPO₄ phase, and accordingly, an increase in the NaF/VPO₄ ratio. The latter results in the formation of the Na₃PO₄ and Na₃VF₆ 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 Na₃V₂(PO₄)₂F₃ by LaPO₄ via precipitation does not.