Confined Assembly of Hydrated Vanadium Oxide into Hollow Mesoporous Carbon Nanospheres for Fast and Stable K + Storage Capability.

The rational structural design of the electrode materials is significant to enhance the electrochemical performance for potassium ion storage, benefiting from the shortened ion diffusion distance, increased conductivity, and pseudo-capacitance promotion. Herein, hydrated vanadium oxide (HVO) nanosheets with enriched oxygen defects are well confined into hollow mesoporous carbon spheres (HMCS), producing O _d -VOH@C nanospheres through one-step hydrothermal reaction. Attributed to the restricted growth in the HMCS, the HVO nanosheets are loosely packed, generating abundant interfacial boundaries and large specific areas. As a result, O _d -VOH@C nanospheres show increased reaction kinetics and well buffer the volume effects for the K ⁺ storage. O _d -VOH@C delivers stable capacities of 138 mAh g ^-1 at 2.0 A g ^-1 over 10 000 cycles in half-cells attributed to the high pseudo-capacitance contribution. The K ⁺ storage mechanism of insertion and conversion reaction is confirmed by ex situ X-ray diffraction, Raman, and X-ray photoelectron spectroscopy analyses. Moreover, the symmetric potassium-ion capacitors of O _d -VOH@C//O _d -VOH@C deliver a high energy density of 139.6 Wh kg ^-1 at the power density of 948.3 W kg ^-1 .
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