Nanosynthesis and Characterization of Cu_1.8Se_0.6S_0.4 as a Potential Cathode for Magnesium Battery Applications

Copper selenide (Cu-Se) and copper sulfide (Cu-S) are promising cathodes for magnesium-ion batteries. However, the low electronic conductivity of Cu-Se system results in a poor rate capability and unsatisfactory cycling performance. Mg-ion batteries based on the Cu-S cathode exhibited large kinetic barriers during the recharging process owing to the presence of polysulfide species. This work attempts to circumvent this dilemma by doping Cu_1.8Se by sulfur, which replaces the selenium in the CuSe lattice to form Cu_1.8Se_0.6S_0.4 nanocrystalline powder. The presence of sulfur will increase the electronic conductivity, and the presence of selenium will mitigate the effect of polysulfide species that hinder the kinetics of Mg²⁺. Herein, a Cu_1.8Se_0.6S_0.4 nanocrystalline powder was synthesized by the solid-state reaction, yielding a highly pure and stoichiometric powder. The crystallographic structure of the nanopowder and the conversion-type storage mechanism have been attested via ex situ X-ray diffraction and energy-dispersive X-ray analysis. The nanocrystalline feature of Cu_1.8Se_0.6S_0.4 was demonstrated by high-resolution transmission electron microscopy. An apparent surface morphology change during the charging/discharging process has been visualized by a field emission scanning electron microscope. Diffuse reflectance spectroscopy has discussed the variation of the band gap during charging and discharging. The full Mg/Cu_1.8Se_0.6S_0.4 cells presented an initial discharge capacity of 387.99 mAh g^-1 at a current density of 0.02 mA cm^-2; moreover, they show moderate diffusion kinetics with $$ ≈ 10^-15 cm^-2 s^-1.