Carbon-coated WS2 nanosheets supported on carbon nanofibers for high-rate potassium-ion capacitors

Abstract

Potassium-ion capacitors (PICs) have received increasing attention because of their high energy/power densities and the abundance of potassium. However, achieving high-rate battery-type anodes to match the capacitor-type cathodes is still a great challenge. Herein, we design a freestanding anode by supporting carbon-coated WS₂ nanosheets on carbon nanofibers (C-WS₂@CNFs) for PICs. Benefiting from its unique nanostructure and the high purity of WS₂, the C-WS₂@CNFs anode exhibits a high reversible capacity of 319 mA h g⁻¹ at 50 mA g⁻¹ and a high rate performance of 168 mA h g⁻¹ at 10 A g⁻¹. An intercalation–conversion reaction mechanism with an intermediate phase of K₂S₅ is verified by in situ Raman, in situ XRD, and ex situ XRD and TEM. First-principles calculations confirm that the C-WS₂@CNFs anode possesses fast K⁺ diffusion kinetics and a high K-adsorption ability. The potassium ion capacitor, assembled from the anode of C-WS₂@CNFs and the cathode of activated carbon nanofibers, delivers a superior energy density up to 180.4 W h kg⁻¹, a good rate capability, and a long cycle life.