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

Abstract In this work, the authors explore the sodium salt of the 14‐vanado(V)phosphate, Na 7 [H 2 PV 14 O 42 ], as a potential anode material for sodium‐ion batteries (NIBs). The multi‐electron redox activity of the polyoxovanadate [H 2 PV 14 O 42 ] 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, Na 7 [H 2 PV 14 O 42 ]·nH 2 O (n = 15–24). Na 7 [H 2 PV 14 O 42 ] 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 V 5+ ions in [H 2 PV 14 O 42 ] 7‐ can be reduced to V 3+ 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 Na 7 [H 2 PV 14 O 42 ] 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.