A Solvothermal Route to Synthesis of CuxMo6S8 Phase for Cathode Active Material of Rechargeable Mg Battery

It was reported that the ternary molybdenum chalcogenide compound, Mo6S8, so-called Chevrel phase, is unique for cathode active material of Mg secondary battery due to its allowance of a fast transport of divalent cations. Because it is thermodynamically metastable, the preparation of it was carried out with indirect method of removing Cu from stable Cu2Mo6S8 phase, which was followed by the substitution of Mg for use as the cathode material of the Mg battery. There were several reports on the preparation of Cu2Mo6S8 phase by high temperature solid state reactions. Because the vapor pressure of sulfur is relatively high at the high reaction temperature, the reaction should be strictly controlled. On the other hand, a direct preparation of Mg2Mo6S8 phase by solid state reaction was reported to have an insufficient electrochemical activity due to the formation of MgO oxide film on it. In addition to the high temperature solid state reaction, it was also reported that a low temperature method can be used for the formation of chalcogenide compounds. The solvothermal method has benefits of a simple and direct process at a relatively low temperature and pressure, an easy control of stoichiometry and a low cost. With the method, we reported the synthesis of chalcogenide compounds, CuInSe2 and Cu(InGa)Se2 which is used for light-absorber layer for the compound solar cell. From the research, we realized that liquid phase and solvent play an important role in the formation of the chalcopyrite nanoparticles. Using the same methodology, in this work, we report the first solvothermal route to the synthesis of Cu2Mo6S8 particles. A series of reaction was carried out with the same starting materials including Mo compound and Cu and Se elements as a function of reaction temperature and time.