Na3V2(PO4)3, with a NASICON-type structure, is a promising cathode material for use in sodium-ion batteries based on a two-electron reaction and operating at 3.4 V. Herein, we report the synthesis of Na3+xV2-xMgx(PO4)3 (x=0.1 to 0.7) for use as a cathode material in sodium-ion batteries. In this work, Na3.2V1.8Mg0.2(PO4)3 was found to exhibit a larger reversible capacity than the theoretical capacity of undoped Na3V2(PO4)3, as a result of the larger number of Na+ in the initial composition, as well as access to the V4+/V5+ redox couple. In contrast, although Mg-rich samples such as Na3.5V1.5Mg0.5(PO4)3 showed a relatively clear plateau for the V4+/V5+ redox couple, the total discharge capacities were lower than that of the undoped Na3V2(PO4)3 because of the irreversibility in the V4+/V5+ redox region. ICP data clearly indicated that Mg2+ are stable within the NASICON structure during redox cycling and that Na+ is the charge carriers in this cathode.
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