Controllable synthesis of layered K0.296Mn0.926O2 to assemble 2.4 V aqueous potassium-ion supercapacitors for double high devices

Abstract

Aqueous potassium-ion hybrid supercapacitors (APHSs) are among the promising technologies for energy storage because it combines the advantages of potassium-ion batteries and supercapacitors, which can provide high energy density and power density. However, the biggest obstacle to APHS is finding suitable positive electrode materials that can facilitate rapid transport for K⁺ intercalation/deintercalation and industrial preparation. Herein, a commercially valuable electrode material, K_0.296Mn_0.926O₂, is developed via a simple method, which can be fabricated on a large scale. The crystal structure and atomic arrangement of the as-prepared K_0.296Mn_0.926O₂ were identified by the Rietveld-refined XRD and high-angle annular dark-field STEM. Importantly, the upper potential of K_0.296Mn_0.926O₂ can extend to 1.2 V with a specific capacitance of 313 F g⁻¹ at 1 A g⁻¹, which is larger than that of commercial MnO₂ electrode material. Density functional theory (DFT) results show that the diffusion barrier of the potassium ion in K_0.296Mn_0.926O₂ (0.06 eV) is lower than that of MnO₂ (0.25 eV), and K_0.296Mn_0.926O₂ exhibits improved interfacial K⁺ adsorption ability. Moreover, an APHS was assembled by using K_0.296Mn_0.926O₂ as the positive electrode and the commercial active carbon (AC) as the negative electrode. The K_0.296Mn_0.926O₂//AC exhibits a “double high device” characteristic with a high energy density of 70 W h kg⁻¹ and a high power density of 6000 W kg⁻¹, and the voltage window extends to 2.4 V. This work represents a significant step forward in the exploitation of K-ion storage and accelerates the development of APHS.