High-Endurance Zinc Ion Supercapacitors with Wide Temperature Tolerance

Metal ion capacitors are hybrid electrochemical cells that bridge the divide between batteries and electrochemical capacitors, enabling them to provide high energy densities at rapid charging or discharging rates. These devices combine a redox metal anode, similar to that found in batteries, to enhance the capacity for charge storage and an electric double-layer cathode that allows fast kinetics and sustains high power density. The first metal ion capacitors were based on lithium and sodium ions, but potential safety hazards associated with these metals prompted the search for other alternatives. In particular, zinc ion supercapacitors (ZICs) emerge as an appealing choice with advantages of environmental safety, a high theoretical capacity as a divalent system, and an abundance of zinc reserves unaffected by geopolitical factors.<br/> <br/>However, zinc ion cells are currently limited by the imbalance in the utilization ratio between the metal anode and the cathode. This imbalance, used to compensate for dendritic loss upon redox cycling, restricts the overall energy density and poses a significant barrier to commercialization. This study presents a new scalable approach to tackle the dendritic issue at the anode and increase the capacity of the cathode by redox polymers, to achieve anode-free supercapacitors with more balanced utilization ratios and a wide temperature tolerance. The resulting zinc ion supercapacitor demonstrated state-of-the-art energy and power densities with 84% capacity retention after 50,000 cycles, and operated down to a low temperature of -60°C. The mechanistic findings in this report provide the design guidelines for zinc ion supercapacitors with high cumulative capacities and extended cycle lifetime to minimize maintenance costs, crucial for high-endurance applications such as in un-interruptible power supplies and energy-harvesting systems.