Mechanism of Sodium Ion Storage in Na₇[H₂PV₁₄O₄₂] Anode for Sodium-Ion Batteries

In this work, the authors explore the sodium salt of the 14-vanado(V)phosphate, Na₇[H₂PV₁₄O₄₂], as a potential anode material for sodium-ion batteries (NIBs). The multi-electron redox activity of the polyoxovanadate [H₂PV₁₄O₄₂]^7-leads to high capacity. This polyanion is synthesized by a simple aqueous solution procedure and isolate as a sodium salt with different numbers of crystal waters, Na₇[H₂PV₁₄O₄₂]·nH₂O (n = 15–24). Na₇[H₂PV₁₄O₄₂] as anode in NIBs exhibits a high and reversible capacity of 322 mA h g⁻¹ at 25 mA g⁻¹ with a high cycling stability (with capacity retention of 87% after 120 cycles). Some of the V⁵⁺ ions in [H₂PV₁₄O₄₂]^7- can be reduced to V³⁺ after being discharged to 0.01 V versus Na/Na⁺, resulting in an average oxidation state of V^3.7+, as based on ex situ X-ray photoelectron spectroscopy and in situ synchrotron X-ray absorption near edge structure studies. The crystalline material becomes amorphous during the charge/discharge processes, which can be observed by in situ synchrotron X-ray diffraction, indicating that functionality does not require crystallinity. The authors propose that the charge storage mechanism of Na₇[H₂PV₁₄O₄₂] anodes mainly involves redox reactions of V accompanied by insertion/extraction of Na ions in-between polyoxo-14-vanadate ions and adsorption/desorption of Na ions on the surface of the vanadate material.