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

La3+ modification of Na3V2(PO4)2F3 was performed by the direct mechanochemically assisted solid-state synthesis of the Na3V2−xLax(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.