Ammonium Vanadium Oxide [(NH₄)₂V₄O₉] Sheets for High Capacity Electrodes in Aqueous Zinc Ion Batteries

Aqueous rechargeable Zn-ion batteries (ARZIBs) are being extensively investigated for large scale energy storage applications owing to their high safety, low cost, and environmental friendliness. Increasing attention has been paid to the high capacity cathode materials with stable host structures and fast channels for diffusion of Zn2+ giving rise to high performance. Herein, we report ammonium vanadium oxide [(NH4)2V4O9] sheets as a high capacity cathode material for ARZIBs for the first time. (NH4)2V4O9 sheets are demonstrated to exhibit greatly enhanced overall electrochemical performance, including high capacities of 376, 336, 322, 318, 285, 271, and 259 mA h g–1 at 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, and 1 A g–1, respectively. Also, they show a high rate capacity as well as excellent cycle lifespan with 328 mA h g–1 after 100 cycles at 0.1 A·g–1; 262 mA h g–1 after 100 cycles at 0.2 A·g–1; 249 mA h g–1 after 300 cycles at 0.5 A·g–1; and 125 mA h g–1 after 2000 cycles at 5 A·g–1; respectively. The assembled Zn//(NH4)2V4O9 battery delivers a high energy density of 301 Wh kg–1 at 197 W kg–1 based on the mass of the (NH4)2V4O9, which is superior to some of the state-of-the-art cathode materials for ARZIBs. In parallel with the electrochemical performance demonstrated, we have clarified the key reaction mechanisms involved in the reversible (de)intercalation of Zn2+, studied by multiple analytical methods. This work not only provides a facile route to synthesize (NH4)2V4O9 sheets but also proves that they can be a promising cathode material for ARZIBs.