(301h) Zinc Anode Design for High-Energy Rechargeable Aqueous Zn-Air Batteries

Alternative energy carriers have been sought after due to fossil fuels’ slow regeneration and environmental concerns such as climate change, air and water pollution. When carrying clean electricity from solar or wind, batteries are promising to alleviate the current energy and environmental problems. Lithium-ion batteries are widely used energy storage system because of their outstanding energy density and rechargeability. However, safety is always a concern because the use of flammable organic electrolytes. On the other hand, batteries that use aqueous electrolytes have enhanced safety (Fig. 1a), ion conductivity, and cost-effectiveness. Within the stability window of water, zinc is an attractive anode material because it is the most active metal that is stable with water. By using aqueous electrolyte, zinc-based batteries not only are safer, but also can be manufactured in ambient air rather than dry room, and has much higher tolerance to moisture and air during operation. Having two valence electrons and high density, zinc metal has three times the volumetric capacity of lithium metal. Among various zinc-based batteries, Zn-air has a theoretical volumetric energy density (6134 Wh/L) that is more than four times of conventional Li-ion batteries (1400 Wh/L). Primary Zn-air batteries have already been the battery of choice for hearing aids, which require extremely high energy density and safety. Finally, zinc is abundant, low-cost, and environmentally benign, rendering them suitable for large scale applications.

Although stable cycling of Zn anodes in mild acidic electrolyte has been demonstrated, an alkaline electrolyte is ideal for zinc-air batteries, because oxygen cathode has minimum overpotential in alkaline electrolyte. However, the performance of Zn anodes in alkaline electrolyte is limited by passivation (Fig. 1b), dissolution (Fig. 1c) and hydrogen evolution (Fig. 1d) problems. Through SEM investigation, critical passivation size was found to be ~ 2 µm. Thus, sub-micron-sized Zn anodes, with feature size smaller than the critical passivation size, won’t have passivation problem. As a result, we focus our research on nanoscale. However, Zn dissolution and hydrogen evolution of nanosized anodes will be accelerated because of large electrode-electrolyte surface area. I have designed and fabricated 4 types of zinc anodes to control the interface between the electrode and electrolyte by creating interfaces with ion-sieving and hydrogen evolution suppressive properties. These designs (listed below) have successfully overcome passivation, dissolution, hydrogen evolution problems.

• Graphene oxide-modified zinc anode (Zn@GO, Fig. 1e)
• A lasagna-inspired nanoscale zinc anode (ZnO@GO, Fig. 1f)
• Sealed ZnO nanorod anode (Zn@TiN_xO_y, Fig. 1g)
• A hydrogen-evolution-suppressing anode (Fig. 1h)

All these anodes show superior performance compared with unmodified anodes. These anodes can be paired with air cathodes to make high energy Zn-air batteries. The nanoscale design principles here can potentially be applied to overcome intrinsic limitations of other battery materials. Besides, I will also present my recent research progress on operando optical microscopy analysis of Zn anodes. Dendrite growth and the regulation of Zn metal deposition will be visualized.