A Safe, High-Performance, and Long-Cycle Life Zinc-Ion Hybrid Capacitor Based on Three-Dimensional Porous Activated Carbon

The rapid development of electronic products has increased the demand for safe, low-cost, and high-performance energy storage devices. Lithium-ion batteries have been commercialized owing to their high energy density. However, the limited lithium resources and their uneven distribution have triggered the search for alternative energy storage systems. In this context, rechargeable aqueous zinc-ion batteries have gained immense attention owing to their low cost and environmental friendliness. Nevertheless, it is highly challenging to develop zinc-ion battery cathode materials with both high capacity and long cycle life. Hence, in this study, we prepared three-dimensional porous activated carbon (3DAC) with high specific surface area by using ethylenediaminetetraacetic acid (EDTA) tetrasodium salt hydrate as the raw material. We developed a zinc-ion hybrid capacitor (ZIHC) in 1 mol.L-1 ZnSO4 using 3DAC as the cathode and a zinc foil as the anode. The ZIHC stored charge by the reversible deposition/dissolution of Zn2+ on the zinc anode and rapid reversible adsorption/desorption of ions on the 3DAC cathode. Owing to the large specific surface area and highly porous structure of the 3DAC cathode, the assembled ZIHC exhibited excellent electrochemical performance. It worked well over the voltage range of 0.1-1.7 V, providing a high specific capacitance of 213 mAh.g(-1) at the current density of 0.5 A.g(-1) (the highest value reported till date). The ZIHC showed specific capacities of 182, 160, 139, 130, 127, 122, and 116 mAh.g(-1) at the current densities of 1, 2, 4, 6, 8, 10, and 20 A.g(-1), respectively. Meanwhile, it exhibited the highest energy density of 164 Wh.kg(-1) (at a power density of 390 W.kg(-1)) and still delivered the highest power density of 9.3 kW.kg(-1) with a high energy density of 74 Wh.kg(-1). In addition, our ZIHC also exhibited excellent cycling stability. After 20000 cycles at 10 A.g(-1), it retained 90% of its initial capacity and exhibited high Coulombic efficiency (approximate to 100%). In order to investigate the causes of capacity decay, we examined the cycled zinc foil by scanning electron microscopy and X-ray diffraction. The results showed that a large number of Zn4SO4(OH)(6)center dot 3H(2)O disordered dendrites were formed on the surface of the zinc foil. These dendrites inhibited the reversible deposition/dissolution of zinc ions, resulting in the capacity decay of the ZIHC during the cycling process. This study will be helpful for developing next-generation high-performance energy storage devices.