Voltage limitations and energy density constraints in Zn-ion hybrid supercapacitors: A theoretical and experimental study

Zn-ion hybrid supercapacitors (ZIHSs) have recently attracted considerable interest due to their use of low-cost, environmentally benign materials and their potential for high energy storage density, enabled by the high specific capacity of Zn anodes and relatively high operating voltages. In this work, we investigate the theoretical and experimental limitations of ZIHSs, with a focus on specific energy. A theoretical model for specific energy based on the active materials involved in charge transfer is derived. The analysis reveals that, in addition to the specific capacitances of both electrodes and the electrolyte salt concentration, the maximum operating cell voltage exerts the most significant influence on the achievable specific energy. Experimentally, ZIHSs were subjected to galvanostatic cycling followed by voltage holding for 100 h. Coulombic efficiency, gas evolution, and gas composition were analyzed. The results show that the stable working voltage is below 1.5 V, notably lower than expected, primarily due to side reactions and Zn dendrite growth. The formation of dendrites during cycling leads to performance degradation and limits the cycle life, highlighting key challenges that must be addressed for the practical deployment of ZIHSs.