Preparation And Properties Of Mo-based Composites As Anode Materials For Sodium Ion Batteries

Lithium-ion batteries are the most widely used energy storage devices and have been applied to every aspect of people’s daily lives.The development of lithium batteries has been limited due to uneven distribution of lithium resources,limited reserves,and high prices.The sodium element and the lithium element are in the same main group,and the physical and chemical properties are relatively close.In comparison,sodium resources are widely distributed,abundant in reserves,low in price,and low in oxidation potential of sodium ion batteries.Sodium ion batteries are expected to be an ideal substitute for lithium ion batteries.The ionic radius of sodium ions is large,and the kinetics of insertion and extraction are slow,which affects the performance of the battery.The electrode material determines the performance of the battery.It is an important issue to develop high-performance anode materials and improve the reaction kinetics of sodium storage.Molybdenum-based electrode materials,such as molybdenum sulfide and molybdenum oxide have the advantages of large specific capacity,good sodium ion insertion and extraction kinetics,and the most suitable anode materials.However,a single molybdenum-based material has a problem of short life and poor rate performance in the field of sodium storage.In this thesis,MoS₂,MoO₃ and MoP were studied,and they were combined with carbon materials to obtain three-dimensional network structure,hollow structure and irregular granular composite materials,which successfully improved molybdenum-based materials.Cycle stability and rate performance.（1）Using citric acid as carbon source and sodium chloride as hard template,the composite of three-dimensional carbon loaded MoS₂ nanosheets was obtained by freeze-drying and annealing subsequently.Due to its special structure and composition,the composite electrode exhibits excellent electrochemical performance during the charge and discharge at constant current density.The specific capacity of MoS₂/C composite was maintained at 50.5%after 1000 cycles at the current density of 1.0 A g^-1.At the current density of 5.0 A g^-1,the specific capacity of MoS₂/C composite was maintained at 350 mAh g^-1 after 1000 cycles.On the basis of this experiment,we carried out an extended experiment to replace the MoS₂ nanosheets with sulfur source and molybdenum source to form MoS₂ nanorods in situ to obtain three-dimensional composites.The composite also has stable cycle performance,and the discharge capacity of the battery remains at 82.5%after 450 cycles.（2）Using 2-methylimidazole as carbon source and sodium molybdate as molybdenum source,the composite with cubic hollow carbon-coated MoO₃nanoparticles was obtained by freeze-drying and annealing subsequently.In the composite,the MoO₃ nanoparticles are dispersed evenly into the carbon layer.Due to its special structure and ultra-high conductivity,the hollow cubic carbon material exhibits excellent rate performance and cycle performance.At the current density of0.5 A g^-1,the specific capacity is still maintained at 150 mAh g^-1 after 3000 cycles.（3）Using 2-methylimidazole as carbon source,sodium molybdate as molybdenum source and sodium hypophosphite as phosphorus source,the composite with carbon coated MoP nanoparticles was obtained by freeze-drying and annealing subsequently.The composite was used as anode for sodium-ion battery and the sodium-ion battery exhibited excellent rate performance.At the large current density of 5.0 A g^-1,the specific capacity of sodium-ion battery is about 150 mAh g^-1.