Improvement in the Energy Density of Na3V2(PO4)3 by Mg Substitution

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. Better than predicted: To improve the cathode performance of Na3V2(PO4)3 in Na‐ion batteries, Mg2+ substitutional doping for V3+ was investigated. It allows access to the V4+/V5+ redox couple and to introduce more Na+ in the initial composition. As a result, Na3+xV2‐xMgx(PO4)3 successfully produces a larger energy density than the theoretical value of Na3V2(PO4)3.