Achieving battery-level energy density in carbon/metal sulfide asymmetric supercapacitors using organic radicals

Asymmetric supercapacitors (ASCs) are promising candidates for high-power output applications; however, their theoretical capacity remains largely unrealized owing to the low specific capacity of carbon negative electrodes. Traditional strategies for enhancing the specific capacity of carbon via structural optimization often compromise the tap density, electrical conductivity, and rate performance of the material. In this study, we address this bottleneck by incorporating 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxyl (4OT) as a redox mediator into the electrolyte to construct ASCs with well-matched capacities and potential windows between the two electrodes. With 50, 100 and 200 mM 4OT added in electrolytes, the activated carbon electrodes achieve specific capacities of 113, 181 and 263 mAh·g⁻¹ at 2 A·g⁻¹. The Ni₃S₂/CoNi₂S₄ positive electrode exhibited a specific capacity of 415 mAh·g⁻¹, benefiting from its superior electrical conductivity, abundant active sites, and enhanced electrochemical activity. Notably, introducing 4OT to the electrolyte effectively balances the capacity and potential window of the two electrodes. Consequently, the as-assembled ASCs deliver a maximum energy density of 55 Wh·kg⁻¹, which surpasses previously reported values. Our work demonstrates that the rational selection and application of redox mediators have great potential for balancing electrode capacity and boosting the energy density of high-performance ASCs.