Studies on the Sodium Storage Performances of Na₃Al_xV_2–x(PO₄)₃@C Composites from Calculations and Experimental Analysis

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 Na₃Al_xV_2–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, Na₃Al_xV_2–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.