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 Na+ 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 mAh·g−1 at a high current density of 5 ​A·g−1 after 2000 cycles and an enhanced rate performance of 305.8 mAh·g−1 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, confirming the potential of applications for the FeSe2-xSx microspheres.