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

Abstract

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 <i>in situ</i> 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 Na_0.6VSe₂, NaVSe₂, and VSe were elucidated by <i>in situ</i>/<i>ex situ</i> X-ray diffraction combined with <i>ex situ</i> transmission electron microscopy observation and <i>in situ</i> 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.