Synthesis And Lithium Storage Properties Of Molybdenum Based Anode Materials In Lithium Ion Batteries

Lithium-ion batteries?LIBs? have been widely used to power new energy cars, electronics and portable devices due to its high energy density and long cycle life. The current anode in LIBs is made of graphite, however, it suffers from low theroretical energy density?362 mAhg-1? and poor rate performance. Molybdenum?Mo? base compounds are promising anode materials in lithium-ion battery due to their high oxidation state, variable valence and rich morphology. However, the Mo-based anode materials generally have low electrical conductivity and poor structure stability. Nanostructural and nanocomposite anode material have been demonstrated to be a pratical route to improve electrochemical properties. In this thesis, we have designed and fabricated two Mo-based anode materials by combining MoO₂ and MoS₂ with carbon materials to improve their lithium-storage performance respectively. The free-standing MoO₂@RGO paper electrodes and hierarchical MoS₂/PPy exhibited excellent lithium-storage performance and the morphology, structure-depenent electrochemical properties were revealed. The main contents are as follows:?1? We prepared one-dimensional MoO3 nanobelts?NBs? via a hydrothermal route using Mo and H2O2 as precusors. Free-standing core/shell nanostructured MoO3@GO films were produced by a electrostatic self-assembly method and following vacuum filtration. The as-synthesized MoO3 NBs were dispersed in poly?diallyldimethylammonium chloride??PDDA? solution under vigorous stirring to form positively charged MoO3–PDDA by the absorption of PDDA cations. The positively-charged MoO3–PDDA was then dispersed in a negatively-charged GO aqueous solution to form self assembled MoO3@GO nanocomposites. MoO₂@RGO paper electrode was prepared by thermal treatment of as-obtained free-standing MoO3@GO film under Ar, which could be directely used as electrode of lithium-ion battery without adhesive and conductive material. The MoO3 was reduced into MoO₂ and GO was transformed into RGO by thermal treatment. In such electrode configuration, RGO nanosheets provided fast charge transfer channels and improved the utilization rate of active materials and rate performance. At the same time, RGO coating offered space buffer volume of MoO₂ during charging and discharging processes. Thus, the cycle stability of MoO₂@RGO could be improved. The free-standing MoO₂@RGO paper electrode could deliver a high capacity of 802 mAhg-1 at 500 mAg-1 after 100 cycles and the capacity of 349 mAhg-1 was retained when the current density was increased to 2000 mAg-1, suggesting good rate performance. The method in this experiment could be extended to prepare free-standing paper electrodes based on other composite materials for enhanced lithium-storage performance and flexible energy storage devices.?2? Hierarchical MoS₂/PPy nanotubes were fabricated by hydrothermal treatment of PPy coated MoO3 nanobelts in thiourea solution. The thin MoS₂ nanosheets were directly grown on the outer surface of PPy nanotubes, forming hierarchical MoS₂/PPy nanostructure. Conductive polypyrrole tube provided fast charge and ion transfer channels, and thin MoS₂ nanosheets offered more active sites for lithium ions, resulting in improved specific capacity and rate performance. The discharge specific capacity for the 1st cycle of MoS₂/PPy could reach 1152 mAhg-1 and the first cycle columbic efficiency was 80%. When the current density was 200 mAg-1, the specific capacity kept 900 mAhg-1 over 60 cycles. Even at the current density as high as 2 Ag-1, the specific capacity also reached 380 mAhg-1 over 60 cycles.