Oxygen-Tuned Na₃V₂(PO₄)₂F_3–2yO_2y (0 ≤ y < 1) as High-Rate Cathode Materials for Rechargeable Sodium Batteries

Abstract

The abundant reserves of sodium resources and its low price make the desire to build energy storage systems on room-temperature rechargeable sodium batteries. The current challenge exists in exploring high-performance key materials, especially cathode materials. Vanadium-based phosphates have attracted extensive attention as a class of promising cathode materials for sodium batteries due to their stable structure, large capacity, and high voltage advantages. In this paper, sodium vanadium oxyfluorophosphate, Na3V2(PO4)2F3–2y O2y (0 ≤ y < 1), is exploited to improve its electrochemical performance through oxygen tuning. The effect of oxygen amount on the structure, morphology, and performance is comprehensively and comparatively investigated. It is found that the optimal electrochemical performance is achieved in Na3V2(PO4)2F2O when y = 0.5. Its specific discharge capacity at 0.1, 0.5, 1, 8, and 20 C is 125, 111, 101, 71, and 58 mA h g–1, respectively. The superior rate performance and good cyclability are ascribed to the low impurity, mixed V3+/V4+ valence state, small particle size, suitable residual carbon coating, low charge transfer resistance, fast Na+ diffusion, and particularly the regulation of charge/discharge potentials as well as polarizations due to proper oxygen tuning.

A novel solid-state synthesis route for high voltage Na3V2(PO4)2F3−2yO2y cathode materials for Na-ion batteries

“…Taken together, these results demonstrate that NVPF 3−2 y O 2 y materials with y > 0 can offer very stable cycling due to their robust structural network. Overall, as previously suggested, 45 NVPF 2 O might perform particularly well within this class of compounds. However, it is important to mention that the actual stability can only be assessed in full cells, which will be object of future work.…”

Thermal stability of Na3-xV2(PO4)2F3-yOy: influence of F- for O2- substitution and degradation mechanisms

“…5 Additionally, NVPF can easily accommodate various chemical substitutions such as O 2-for Fthat allows the preparation of the solid-solution Na3V 3+ 2-yV 4+ y(PO4)2F3-yOy (0 ≤ y ≤ 2). [28][29][30][31][32][33][34] Depending on the synthesis conditions, this substitution can be either finely tuned or not, leading to discrepancies in lattice parameters and electrochemical signatures reported in the literature. 6,16,35 It was previously demonstrated that O 2for Fsubstitution in Na3V 3+ 2-yV 4+ y(PO4)2F3-yOy does not significantly impact the specific capacity since two Na + ions can be reversibly extracted whatever the oxygen content.…”

NaCo7.38(PO4)4Cl4 in a series of homeotype compounds with stable cationic substructures derived from the sulphohalite archetype

“…According to ref. 34, the studied influence of the oxygen amount on the Na 3 V 2 (PO 4 ) 2 F 3−2 y O 2 y (0 ≤ y < 1) structure, morphology, and performance has demonstrated that the optimal electrochemical activity is achieved in Na 3 V 2 (PO 4 ) 2 F 2 O ( y = 0.5). Its specific discharge capacity at 20 °C is 58 mA h g −1 .…”