Na3V2(PO4)3 (NVP) is
one of the most widely studied structures as a cathode of sodium-ion
batteries, although the relatively high cost of vanadium and low voltage
need to be further improved compared with the counterpart of cathode
materials of lithium-ion batteries. Herein, vanadium (V) of NVP was
partially substituted by Al to exploit the sodium storage capability
using the valence change of V4+/V5+ in higher
voltage, without loss in the capacity. Since Al is lighter and less
expensive, the theoretical capacity will be slightly increased and
the energy density will be enhanced with the increased voltage. A
family of carbon-coated Na3AlxV2–x(PO4)3 (NAVP@C, x = 0, 1/3, 1/2, 2/3, 3/4, 1) composites
were prepared to comprehensively investigate the effect of Al content
on extraction/intercalation of Na+ in NAVP@C. In every
NAVP that contains Al, a charge/discharge voltage plateau at 3.9–4.1
V along with the common one at 3.4 V was observed, indicating that
the redox of a higher valence than V4+ was made use of.
However, the NAVP could not fully extract/intercalate two Na+ ions with the introduction of Al, owing to the shrinking crystal
size with the Al substitution. Among all these members, Na3AlxV2–x(PO4)3 (x = 1/3) exhibited
the largest reversible charge/discharge capacity around 113.82 mAh
g–1, corresponding to an energy density of 405.99
Wh kg–1, calculated only from the cathode. All family
members of NAVP exhibit noteworthy cycling stability and C-rate performance.