Although transitional metal dichalcogenides have been regarded as appealing electrodes for sodium/potassium-ion batteries (SIBs/PIBs) owing to their high theoretical capacity, it is a key challenge to realize dichalcogenide anodes with long-period cycling performance and high-rate capability because of their poor conductivity and large volumetric change. Herein, polypyrrole-encapsulated VSe2 nanoplates (VSe2@PPy) were prepared by the selenization of VOOH hollow nanospheres and subsequent in situ polymerization and coating by pyrrole. Benefiting from the inherent metallicity of VSe2, the improvement in the conductivity and the structural protection provided by the PPy layer, the VSe2@PPy nanoplates exhibited enhanced sodium/potassium-storage performances, delivering a superior rate capability with a capacity of 260.0 mA h g–1 at 10 A g–1 in SIBs and 148.6 mA h g–1 at 5 A g–1 in PIBs, as well as revealing an ultrastability in cycling of 324.6 mA h g–1 after 2800 cycles at 4 A g–1 in SIBs. Moreover, the insertion and conversion mechanisms of VSe2@PPy in SIBs with intermediates of Na0.6VSe2, NaVSe2, and VSe were elucidated by in situ/ex situ X-ray diffraction combined with ex situ transmission electron microscopy observation and in situ potentio-electrochemical impedance spectroscopy during the sodiation and desodiation processes. Density functional theory calculations show that the strong coupling between VSe2 and PPy not only causes it to have a stronger total density of states and a built-in electric field, leading to an increased electrical conductivity, but also effectively decreases the ion diffusion barrier.

No abstract

No abstract

Although metallic chalcogenides are deemed as attractive sodium anode materials recently, the electrochemical performance is severely confined by the liability of structural collapse and sluggish ion diffusion kinetics. Herein a composite of carbon-encapsulated bimetallic selenides MoSe2–Sb2Se3 was prepared by a hydrothermal method on the basis of abundant reaction sites, high activity, an extra built-in electric field generated from heterointerfaces, and synergistic effects between the different components. Equally important, the carbon coating is effective to support the structural stability by restraining the vast volumetric variation to achieve the purpose of improving the cycling performance. The density functional theory calculation results indicate that the band gap is narrowed and that the work function is decreased on the interface of the MoSe2–Sb2Se3 heterojunction, leading to an additional driving force stemming from the introduction of the built-in electric field and the formation of the Sb–Se (Se from MoSe2) bond. Therefore, the resultant composite presents increased reaction kinetics and good electrochemical properties by acquiring a capacity of 376.0 mA h g–1 over 580 cycles at 2.0 A g–1 for the half-cell and 276 mA h g–1 over 750 cycles at 2 A g–1 for the full-cell. This work highlights bimetallic selenides with facilitated ion transferability with high performance.

Designing multiphase composition is believed to availably boost the structural integrity and electrochemical properties of sodium-ion battery anodes. Herein, a conceive of nanoflowers, assembled with Bi 2 S 3 nanorods, is demonstrated to construct the multiphase composition involving TiO 2 coating and polypyrrole (PPy) encapsulation. Bi 2 S 3 acted as the dominating active material, in consideration of the low content of TiO 2 , which ensured the high capacity of the composite. The dual-structural restrain of the TiO 2 and PPy coatings can effectively alleviate volume variation based on the pseudo-"zero-strain" effect of TiO 2 and high flexibility of PPy shells. Meanwhile, the heterointerface greatly enhanced the coupling effect between Bi 2 S 3 and TiO 2 and thus improved the electrochemical performance, which was proved by the results of density functional theory calculation and electrochemical tests. Combining the regulation from the Bi 2 S 3 /TiO 2 heterojunction and the dual-structural restrain effect, the Bi 2 S 3 /TiO 2 @PPy electrode exhibited excellent rate performance and superior cycle stability (275.8 mA h g −1 over 500 cycles at 10 A g −1 ). This study indicates that designing multiphase composition can be very promising and provides a structural insight to construct high stability in electrodes for sodium-ion batteries.