Coupling of Metallic VSe2 and Conductive Polypyrrole for Boosted Sodium-Ion Storage by Reinforced Conductivity Within and Outside

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.