4 years ago

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

Improvement in the Energy Density of Na3V2(PO4)3 by Mg Substitution
Liwei Zhao, Shigeto Okada, Yuto Yoshioka, Atsushi Inoishi, Ayuko Kitajou
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.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/celc.201700540

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