Highly conductive S-doped FeSe2-xSx microsphere with high tap density for practical sodium storage.

Metal selenides have been explored as promising sodium storage materials owing to their high theoretical capacity. However, sluggish Naþ diffusion and low electronic conductivity of selenides still hinder their practical applications. Herein, FeSe2-xSx microspheres have been prepared via a self-doping solvothermal method using NH4Fe(SO4)2 as both the Fe and S source, followed by gas phase selenization. The density functional theory calculation results reveal that S doping not only improves the Na adsorption, but also lower the diffusion energy barrier of Na atoms at the S doping sites, at the same time enhance the electronic conductivity of FeSe2-xSx. The carbon-free nature of the FeSe2-xSx microspheres results in a low specific surface area and a high tap density, leading to an initial columbic efficiency of 85.6%. Compared with pure FeSe2, such FeSe2-xSx delivers a high reversible capacity of 373.6 mAhg1 at a high current density of 5 A-g-1 after 2000 cycles and an enhanced rate performance of 305.8 mAhg1 at even 50 A-g-1. Finally, the FeSe2-xSx//NVP pouch cells have been assembled, achieving high energy and volumetric energy densities of 118 Wh-kg-1 and 272 mWh-cm-3, respectively, con- firming the potential of applications for the FeSe2-xSx microspheres. [ABSTRACT FROM AUTHOR]