The performance and mechanism of copper-cobalt sulfide cathode for hybrid Mg2+/Li+ batteries

Abstract

Hybrid Mg2+/Li+ batteries are a potential energy storage device possessing fast ion migration kinetics and high safety. As a ternary transition metal sulfide, the CuCo2S4 with high theoretical capacity is an ideal conversion-type cathode material and it has been successfully synthesized via one step solvothermal method in this work. At room temperature, combined with hybrid Mg2+/Li+ batteries electrolyte prepared by adding LiCl salts to all-phenyl complex (APC) electrolyte (LiCl-APC hybrid electrolyte) as well as Mg metal anode, the CuCo2S4 cathode demonstrates a high discharge capacity of 517.5 mA h g−1 at the current density of 0.1 C. And it shows good long-term cycle performance with the reversible capacities of 172.3 mA h g−1 and 86.6 mA h g−1 even over 100 cycles at 0.1 C and 0.5 C, respectively. In addition, the reaction mechanism is revealed by ex-situ X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) characterizations. The results show that CuCo2S4 is converted to new active materials (CuS, CoS and Co9S8) after the first cycle, and then completely changes into CuS and CoS after 10 cycles due to the transformation of Co9S8 into CoS during the cycling. As a result, CoS and CuS dominate the subsequent reversible cycles. This work confirms that CuCo2S4 is an appropriate cathode material for high performance hybrid Mg2+/Li+ batteries.