Boosting the sodium storage performance of iron selenides by a synergetic effect of vacancy engineering and spatial confinement

The introduction of S induces a large number of Se vacancies in iron selenide, adjusts the electronic structure of iron selenide, reduces the Na+ diffusion barrier, and optimizes the sodium storage performance of iron selenide together with the carbon confinement strategy. [Display omitted] •Se vacancies adjust the electron density and improve the conductivity of material.•Se vacancies promote Na+ migration and enhance the reaction kinetics of material.•The carbon confinement strengthens the structural stability of the electrode.•The material possesses excellent electrochemical properties. Recently, iron selenides have been considered as one of the most promising candidates for the anodes of sodium-ion batteries (SIBs) due to their cost-effectiveness and high theoretical capacity; however, their practical application is limited by poor conductivity, large volume variation and slow reaction kinetics during electrochemical reactions. In this work, spatially dual-carbon-confined VSe-Fe3Se4-xSx/FeSe2-xSx nanohybrids with abundant Se vacancies (VSe-Fe3Se4-xSx/FeSe2-xSx@NSC@rGO) are constructed via anion doping and carbon confinement engineering. The three-dimensional crosslinked carbon network composed of the nitrogen-doped carbon support derived from polyacrylic acid (PAA) and reduced graphene enhances the electronic conductivity, provides abundant channels for ion/electron transfer, ensures the structure integrity, and alleviates the agglomeration, pulverization and volume change of active material during the chemical reactions. Moreover, the introduction of S into iron selenides induces a large number of Se vacancies and regulates the electron density around iron atoms, synergistically improving the conductivity of the material and reducing the Na+ diffusion barrier. Based on the aforementioned features, the as-synthesized VSe-Fe3Se4-xSx/FeSe2-xSx@NSC@rGO electrode possesses excellent electrochemical properties, exhibiting the satisfactory specific capacity of 630.1 mA h g−1 after 160 cycles at 0.5 A/g and the reversible capacity of 319.8 mA h g−1 after 500 cycles at 3 A/g with the low-capacity attenuation of 0.016 % per cycle. This investigation provides a feasible approach to develop high-performance anodes for SIBs via a synergetic strategy of vacancy engineering and carbon confinement.