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, Na3.2V1.8 Mg-0.2(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.