Carbon-Encapsulated Ni Se /CoSe Heterostructured Nanospheres: Sodium/Potassium-Ion Storage Anode with Prominent Electrochemical Properties.

Heterogeneous structures are used as energy storage devices because of their ability to accelerate charge transfer, which greatly contributes to the rate capability of devices. However, the construction of heterostructures with conspicuous electrochemical properties remains a huge challenge. In this study, a design of heterostructured Ni Se /CoSe nanospheres encapsulated by a carbon shell (Ni Se /CoSe @C) synthesized through facile hydrothermal and annealing methods is presented. The Ni Se /CoSe @C exhibits excellent cyclic performance with a capacity of 420 mA h g at 0.5 A g after 100 cycles for Na-storage and 330.1 mA h g at 0.1 A g after 200 cycles for K-storage. The excellent cyclic performance can be attributed to the carbon coating that maintains the structural stability and enhances electrical conductivity, and significantly, the heterostructures that promote ion/electron transport. The sodium storage mechanism of the Ni Se /CoSe @C is revealed by ex situ X-ray powder diffraction, ex situ high-resolution transmission electron microscopy, and in situ electrochemical impedance spectra analyses. The first principles density functional theory calculation is performed to prove that the heterostructure on the Ni Se /CoSe interface can induce an electric field and thus improve the electrochemical reaction kinetics. This study provides an effective approach for constructing heterostructured composites for high-performance alkaline batteries.