Study On The Application Of MoO₂in High-power Capacitors And High-capacity Lithium-sulfur Batteries

With the development of science and technology, lithium-ion batterys don’t begin to meet the demand of high energy device requirements of electrochemical storage system. Electrochemical storage system with high performance is necessary. Lithium-sulfur batteries and supercapacitors with high specific capacity and high specific energy begin to receive more and more attention. MoO2with metal-like electrical conductivity, high melting point, high chemical stability and high theoretical specific capacity has become a potential material of electrochemical energy storage.Based on transition metal oxides MoO2as the research object, ordered mesoporous M0O2is prepared using hard template. We study on the electrochemical properties of MoO2in the supercapacitor and lithium-sulfur batteries using the modern test methods. The contents include the following aspects.Ordered mesoporous MoO2is prepared using (NH4)6Mo7O24·4H2O as molybdenum source and silica KIT-6as hard template. As is shown by XRD, BET and SEM, the MoO2with long-range order mesoporous structure and high surface area of68.7m2/g is an inverse replica of KIT-6template. The optimized experimental conditions are that the mixture with1.47g of (NH4)6Mo7O24-4H2O precursor and1.0g of KIT-6template are calcined at650℃for6h under5%H2/Ar atmosphere, which is then etched with2M NaOH solution to remove silica template.The specific capacitance of MoO2is145.75F/g at the scan rate of5mV/s in the three electrode system. When the scan rate increases to100mV/s,78.8%of primary capacitance is preserved, signifying an excellent high rate capability of MoO2. The dominating working ions for the charge storage of MoO2in aqueous LiOH electrolyte are proved to be Li+ions through EQCM measurement. Mesoporous MoO2/S composites are prepared by heating the mesoporous MoO2and elemental sulfur. When the load of sulfur is40%, Mesoporous MoO2/S composites show good electrochemical performance in lithium sulfur batteries. An initial specific capacity of1236.8mAh/g at0.5C and reversible capacity of683.8mAh/g over100cycles are achieved. However, the performance of mesoporous MoO2/S composite do not compare with carbon-sulfur composite. With a metal-like conductive, MoO2can be made of microporous structure. Through the comparison of mesoporous and microporous carbon material in the next chapter, microporous materials may have better storage performance.ZIF-C/S composites are prepared by heating the ZIF-C and elemental sulfur. The sulfur content of the composite is34%. An initial specific capacity of ZIF-C/S composite is1236.8mAh/g at0.5C. The discharge capacities during the100、300、and500th cycles are850、724and618mAh/g, respectively. A stable discharge capacity of ZIF-C/S composite is about793mAh/g at1C,705mAh/g at2C and624mAh/g at3C. ZIF-C/S composite show more excellent cycle performance and rate performance than CMK-3/S composites.