Functionalized Vanadium Oxide as the Cathode Material for Rechargeable Aqueous Zinc-ion Batteries (2020)

Battery offers a viable solution for storing intermittent energy supplies associated with renewable energy production. Although lithium-ion batteries take up the most battery market, they are still limited by lithium metal resources, high cost and safety concerns. With this regard, aqueous batteries with mildly acidic electrolytes hold a promise for large-scale energy storage. In particular, zinc, as an attractive alternative to lithium metal, has been employed in aqueous rechargeable batteries due to its low-cost, high safety and environmental friendliness. Layered vanadium oxide (V2O5) as cathode material has gained enhanced interests in the studies of rechargeable aqueous zinc-ion batteries (RAZBs) due to its relatively high capacity. However, commercial V2O5 shows poor stability during cycling since the zinc ion intercalation causes degradation of the cathode and battery components. Therefore, in this project, two strategies involve surface coating and metal-ion doping are utilized to improve the electrochemical performance of vanadium-based electrodes in RAZBs.&#13;\nFirst, we introduce a coating method to fabricate polymer-modified cathode materials for aqueous zinc-ion batteries, which display improved electrochemical performances under both ambient and elevated temperature conditions. A polypyrrole-coated cathode is demonstrated, and the assembled battery deliveries a high capacity of 195.7 mAh·g-1 at the current rate of 5 C (200 mAh·g-1 corresponds 1 C), with only 9.5% capacity decay at room temperature after 200 cycles. At an elevated temperature (60°C), the polymer-coated battery still shows outstanding capacity retention, of 80% vs. 25% for bare V2O5 cathode after 150 cycles. Therefore, coating conductive polymers on the surface of cathode materials stabilizes the structure of the positive electrode at high temperatures and offer a viable approach to realize the thermal stability of such batteries.&#13;\nSecond, two kinds of metal ions (Zn2+ and Na+) are doped simultaneously into the V2O5 interlayer by a molar ratio of Zn:Na = 0.3:0.43 to form a metal-ion doped cathode material Zn0.3Na0.43V2O5 (ZNVO). To enlarge the specific surface area, the commercial V2O5 is optimized into nanobelts by a hydrothermal method. The doped positive electrode in 2M ZnSO4 electrolytic solution reaches over 300 mAh·g-1 initial discharge capacity at 5 C, which is much higher than that of undoped electrode material (V2O5 nanobelts). Besides, in order to prevent the extraction of Na ions from the positive electrode, additional 2M sodium salt is added to the 2M ZnSO4 aqueous solution to prepare a dual-ion electrolyte. This dual-ion system (containing dual ion-doped positive electrode and dual ion electrolyte) offers a long-term cycle life, ~ 89% capacity retention after 4000 cycles, and a relatively high discharge capacity of 190 mAh·g-1 at 5 C during fast charge/discharge process. More importantly, this dual-ion electrolyte effectively suppresses zinc dendrite formation on the anode surface because of the electrostatic shield mechanism, where creating a positively charged shield around the sharp zinc protuberances. Thus, this dual-ion system provides the excellent electrochemical performance of Zn // ZNVO batteries and holds a promise for realizing practical applications of zinc-ion batteries.