Synthesis And Characterizition Of CoS And Fe₃S₄ As New Cathode Materials For Secondary Magnesium Battery

Secondary magnesium batteries have attracted great attention as promising candidates for energy storage device, owing to the relatively high theoretical specific capacity（3833 m Ah cm-3, 2205 m Ah g-1）, low cost, high safety and environmentally friendly nature of Mg metal.Similar as widely used Li-ion batteries, secondary magnesium batteries are mainly based on the intercalation/de-intercalation mechanism. Mg²⁺ can afford two times the charge of Li+ to be transferred per ion, providing an opportunity for utmost capacity. However, caused by the strong polarization effect of Mg²⁺, magnesium batteries suffer from slow solid-state Mg diffusion in most host. Large amount of work has been devoted to searching for ideal cathode materials with high energy density, long cyclic life and low cost. Until now, only Chevrel type MxMo6T8（M=Metal, T=S, Se） materials exhibit appreciable electrochemical performance. Whereas this kind of materials are far from satisfactory in the view of energy density, the search of practically usable cathode of secondary Mg battery remains a major challenge. In this work, new cathode material Co S and Fe₃S₄ were used in secondary magnesium battery.Nanostructured Co S and Co S/CNT composite were synthesized by solvothermal method and were characterized by XRD, SEM, EDS, TEM and nitrogen adsorption-desorption isotherm measurements. Their electrochemical behavior in secondary Mg battery was explored by the electrochemical data and XRD patterns. The results showed that the specific surface area and pore volume of Co S/CNT composite is higher than bare Co S. The specific discharge capacity of both Co S/CNT composite and bare Co S increased as cycling going on. The highest specific discharge capacity for Co S/CNT composite was 509 m Ah g-1, which is much higher than that of bare Co S（325.4 m Ah g-1）. It was found in this work that the interplanar space of（1 1 0） and（1 0 2） face expanded in charge-discharge cycling, which is benefit for more Mg²⁺ insertion, so the specific capacity increases along with cycling.Nanostructured greigite（Fe₃S₄） prepared by hydrothermal method was firstly used as a cathode material for secondary magnesium battery. The feasibility whether Mg²⁺ can insert into the framework of Fe₃S₄ was explored by chemical intercalation tests. The electrochemical behavior of Fe₃S₄ cathode were characterized by galvanostatic charge-discharge cycling, CV, EIS and XRD. The results revealed that Fe source, S source and reaction temperature had remarkably influence on the phase composition of the hydrothermal product in the synthesis of Fe₃S₄. Mg²⁺ can insert into the framework of Fe₃S₄ without notable structural change of the host. Fe₃S₄ could be reversibly cycled in secondary magnesium battery. The capacity reached to 267 m Ah g-1 on the first discharge process, then gradually dropped to 110 m Ah g-1 after 50 cycles.