Dual ions enable vanadium oxide hydration with superior Zn2+ storage for aqueous zinc-ion batteries

Because of many advantages, aqueous rechargeable zinc-ion batteries (ZIBs) have become the best alternative energy storage device to lithium-ion batteries (LIBs). Current studies have shown that the insertion of metal ions can adjust the layer spacing of hydrated vanadium oxide (V2O5·nH2O, denoted as VOH) and improve their electrochemical performance. However, the specific role of different metal ions has not been studied. Here, we have synthesized a bimetallic ion intercalation VOH, in which a one-valent metal K+ and a divalent alkaline earth metal Mg2+ are simultaneously inserted into the VOH interlayer by a one-step hydrothermal method, denoted as KMgVOH. In this material, Mg2+ can increase the layer spacing of VOH, expand the ion transport channel and improve the specific capacity of the battery. Meanwhile, K+ can make the connection between V-O layers closer and stabilize the structure of the material. The interaction of the two ions greatly improves the electrochemical performance of VOH. This KMgVOH cathode delivers an unprecedented high specific capacity of 423 mAh·g−1 at 0.1 A·g−1, and the capacity can still reach 222 mAh·g−1 after a long cycle. This improvement for vanadium-based materials provides a new idea for zinc ion storage.