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, FeSe_2-xS_x 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 FeSe_2-xS_x. The carbon-free nature of the FeSe_2-xS_x 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 FeSe_2-xS_x 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 FeSe_2-xS_x//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 FeSe_2-xS_x microspheres.