Improvement in the Energy Density of Na 3 V 2 (PO 4 ) 3 by Mg Substitution

Na 3 V 2 (PO 4 ) 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 Na 3+ x V 2‐ x Mg x (PO 4 ) 3 ( x =0.1 to 0.7) for use as a cathode material in sodium‐ion batteries. In this work, Na 3.2 V 1.8 Mg 0.2 (PO 4 ) 3 was found to exhibit a larger reversible capacity than the theoretical capacity of undoped Na 3 V 2 (PO 4 ) 3 , as a result of the larger number of Na + in the initial composition, as well as access to the V 4+ /V 5+ redox couple. In contrast, although Mg‐rich samples such as Na 3.5 V 1.5 Mg 0.5 (PO 4 ) 3 showed a relatively clear plateau for the V 4+ /V 5+ redox couple, the total discharge capacities were lower than that of the undoped Na 3 V 2 (PO 4 ) 3 because of the irreversibility in the V 4+ /V 5+ redox region. ICP data clearly indicated that Mg 2+ are stable within the NASICON structure during redox cycling and that Na + is the charge carriers in this cathode.