Aqueous transition-metal ion batteries: Materials and electrochemistry

The constant pursuit of alternative energy sources stimulates the rapid exploitation of energy storage systems. Compared to alkali metal-ion batteries, aqueous transition-metal ion batteries have captured increasing attention because of their high safety, eco-friendliness, abundant resources, and low cost. More importantly, their multivalent chemistry offers opportunities to realize storage technologies with higher energy. Although these bright prospects have fostered progress in recent years, practical deployment of these batteries has been denied by several scientific and technological issues including sluggish reaction kinetics, poor electrochemical revers-ibility, and low material stability. In this comprehensive overview, we focus on the materials and electro-chemistry of several booming aqueous transition-metal ion batteries such as Zn, Cu, Fe, and Mn-ion systems. State-of-the-art progress accompanied with the solutions to addressing the above-mentioned issues are high-lighted. A particular focus is laid on zinc-ion batteries, which are ready to become commercially available. Finally, the remaining challenges and future directions in the field are also outlined. We anticipate that this review will supply a clear understanding of the current status and meaningful guidelines for researchers in developing aqueous transition-metal ion batteries.