Dual-functions of the carbon-confined oxygen on the capacitance and cycle stability enhancements of Zn-ion capacitors

Zinc-ion capacitors (ZICs) are promising energy storage devices due to their balance between the energy and power densities inherited from Zn-ion batteries and supercapacitors, respectively. However, the low specific capacitance of carbon cathode materials and the dendrite growth on Zn anode have set fatal drawbacks to their energy density and cycle stability. Herein, we demonstrate that, in 1 M Zn(CF₃SO₃)₂/DMF (N, N-dimethylformamide) electrolyte, confining oxygen in carbon cathode materials via high-energy ball milling can synergistically introduce additional pseudocapacitance on the cathode side while suppressing the dendrite growth on Zn anode side, which jointly lead to high energy density (94 Wh kg⁻¹ at 448 W kg⁻¹) and long cycle stability of ZICs. The hydroxyl group in carbon cathode can be transformed to C—O—Zn together with the release of protons during the initial discharge, which in turn stimulates the defluorination of $\mathrm{C}{\mathrm{F}}_3{\text{SO}}_3^{-}$$\mathrm{C}{\mathrm{F}}_3{\text{SO}}_3^{-}$ anions and formation of ZnF₂ on both cathode and anode. The ZnF₂ formed on the surface of the Zn anode suppresses the dendrite growth by regulating the Zn²⁺ deposition/stripping in a reticular structure, resulting in the excellent cycle stability. This work provides a facile strategy to rationally design and construct high energy and stable ZICs through engineering the oxygen-bearing functional groups in carbon cathode materials.