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

Abstract

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 x V2–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 x V2–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.

The prospect and challenges of sodium‐ion batteries for low‐temperature conditions

“…The operating voltage of Na 3 V 2 (PO 4 ) 3 can be increased through doping with heteroatoms. [100][101][102] For example, a highvoltage cathode material composed of Na 3 V 2 (PO 4 ) 2 O 2 F was proposed by replacing one-third of PO4 3− in Na 3 V 2 (PO 4 ) 3 with highly electronegative F − and O 2− in different proportions, which exhibited two voltage plateaus at about 4.01 and 3.60 V (vs. Na + /Na) with a specific capacity of 96.1 mAh g −1 at 0.2 C under −25°C. [103] Besides, the O in Na 3 V 2 (PO 4 ) 2 O 2 F can be further substituted by F to obtain Na 3 V 2 (PO 4 ) 2 F 3 , with a theoretical energy density of about 507 Wh kg −1 , which is close to the energy density of the current cathode material of LiFePO 4 (530 Wh kg −1 ) in lithium-ion batteries.…”

Partial Modification Strategies of NASICON-Type Na3V2(PO4)3 Materials for Cathodes of Sodium-Ion Batteries: Progress and Perspectives

“…Even at a low temperature of 20 °C and 5C, 98.9% of its initial capacity could be retained after 1000 cycles (Figure 7e). Moreover, Wu et al 107 also indicated that moderate doping of Al 3+ into NVP can improve electrochemical performance. Especially, when the Al 3+ content is 1/3(Na 3 V 5/3 Al 1/3 (PO 4 ) 3 ), the largest practical reversible discharge capacity of 113.82 mAh g −1 at 0.5C was obtained with the formation of V 4.2+ (Figure 7f).…”

“…(f) Rate performance of Na 3 Al x V 2– x (PO 4 ) 3 @C composites ( x = 0, 1/3, 1/2, 2/3, 3/4, 1). Reproduced with permission from ref . Copyright 2021 American Chemical Society.…”

Sodium Ion Storage in Na4MnV(PO4)3@C Free‐Standing Electrode

“…The V 2p core-level spectrum consists of V(III) 2p 3/2 (516.1 eV) and V(III) 2p 1/2 (523.0 eV) components (Figure 3b). [48][49][50] Upon charging to 3.8 V, the main doublet for Mn shifts towards higher binding energies, corresponding to the oxidation of Mn 2+ to Mn 3+ . Similarly, the main doublet for V shifts towards higher binding energy, which can be assigned to the oxidation of V 3+ to V 4+ .…”