Fabrication Of Molybdenum/Niobium-based Nanostructural Composite Electrode Materials For Electrochemical Energy Storage

With the ever-rising demand for energy and increasing concern about environmental pollution caused by the enormous consumption of fossil fuels,it is urgent to find and develop renewable and clean energy to replace fossil fuels.Developing new-type,efficient and stable electrochemical energy storage devices such as lithium-ion batteries and supercapacitors is an important way to make full use of new energy resources and improve the energy utilization efficiency.Among many factors that affect the electrochemical performance of lithium-ion batteries and supercapacitors,electrode material is a critical part.Therefore,developing electrode materials with high specific capacity,high rate capability,long cycle life as well as safe and reliable properties is the most important research work for lithium-ion batteries and supercapacitors in the future.In this doctoral dissertation,we focus on the design,fabrication and characterization of molybdenum/niobium-based nanostructural composite electrode materials,and explore their electrochemical energy storage properties.The main research work and findings of the dissertation are summarized as follows:(1)The TiC@C/MoO3-x nanocomposite with cactus-like structure has been fabricated by the combination of chemical vapour deposition and thermal evaporation,and their lithium storage properties were studied systematically.By means of characterization analysis,the results show that the as-prepared molybdenum oxide nanowires contain a certain concentration of oxygen vacancies,which improves the conductivity of molybdenum oxide nanowires.In addition,TiC@C with high conductivity can provide a fast electron transport channel for the electron transfer of molybdenum oxide during the electrochemical reaction.Therefore,when evaluated as anode materials for lithium-ion batteries,the TiC@C/MoO3-x nanocomposites exhibit good cycle stability and high rate capability.A discharge capacity of?500 mAh/g can be maintained after 500 cycles at the current density of 1 A/g.Even at the current density as high as 10 A/g,a discharge capacity of 245 mAh/g can be still delivered.(2)Carbon nanofibers(CNFs)flexible films have been synthesized through the combination of electrospinning and heat treatment.Then CNFs/MoS2 nanosheets flexible composite films have been achieved by hydrothermal method using CNFs flexible films as substrate,and their lithium storage properties were studied in detail.In this composite films,ultrathin MoS2 nanosheets can effectively reduce the volume change during lithium-ion intercalation and de-intercalation,and the nanovoids formed by the interconnected MoS2 nanosheets are very favorable for the full contact between active materials and electrolyte,which could greatly shorten the lithium-ion diffusion path.In addition,the CNFs with high conductivity can provide a fast electron transport channel for the electron transfer of MoS2 during the electrochemical reaction.As a result,CNFs/MoS2 nanosheets composite films exhibit good electrochemical lithium storage performance as anode materials for lithium-ion batteries.The discharge capacity of 648 mAh/g can be obtained after 75 cycles at the current density of 100 mA/g.When the current density increases to 1000 mA/g,the discharge specific capacity is still as high as 511 mAh/g.(3)The free-standing flexible hybrid film consisting of alternating stacked mesoporous Mo2N nanobelts(NBs)and rGO nanosheets(MMNNBs/rGO)has been fabricated by annealing the free-standing MoO3/GO flexible film acquired by vacuum filtration in NH3 at high temprature.In such architecture,Mo2N NBs have good electrical conductivity,similar to that of metals,combining with highly conductive rGO,thus the alternating stacked and interconnected networks of MMNNBs/rGO ensure the fast electron transfer during electrochemical reaction.Meanwhile,mesoporous Mo2N NBs are very beneficial to the sufficient contact between active materials and electrolyte,which could greatly shorten the ion transport distance and reduce polarization phenomenon caused by ion concentration difference on the surface of electrodes,so the high rate capability of electrode materials can be achieved.Furthermore,the pseudocapacitance behavior of Mo2N ensures the high capacitive performance of the hybrid film electrodes.As a consequence,the MMNNBs/rGO hybrid film electrode shows very good capacitive performance.The all-solid-state flexible supercapacitors based on MMNNBs/rGO hybrid film electrodes exhibit a large volumetric capacitance of 15.4 F/cm3 at a current density of 100 mA/cm3 and a high energy density of 1.05 mW h/cm3 at a power density of 0.035 W/cm3.(4)The orthorhombic phase Nb2O5 mesoporous nanospheres(T-Nb2O5 MNSs)have been fabricated by the solvothermal method combined with high temperature annealing treatment.Then T-Nb2O5 MNSs/rGO nanocomposite is obtained by further thermally treating the T-Nb2O5 MNSs/GO nanocomposite acquired by compositing T-Nb2O5 MNSs with GO through electrostatic interaction and vacuum filtration method.In this composite,T-Nb2O5 MNSs greatly benefit the sufficient contact between the active materials and electrolyte,which is favorable for the fast lithium-ion diffusion and improves the kinetics of electrochemical reaction.Moreover,the cross-linked conductive networks of rGO increase the electrical conductivity throughout the electrode materials,which is beneficial to the improvement of rate performance of electrode materials.Therefore,T-Nb2O5 MNSs/rGO nanocomposite exhibits better electrochemical lithium storage performance.The lithium-ion hybrid supercapacitors comprising T-Nb2O5 MNSs/rGO anodes and mesoporous carbon-coated rGO(MC/rGO)cathodes with 1 M LiPF6 in a mixture of ethylene carbonate(EC)and dimethyl carbonate(DMC)(1:1,V/V)as the electrolyte deliver a large specific capacitance of 45.9 F/g at a current density of 0.05 A/g in the voltage range of 0.5-3 V and a high energy density of 56 W h/kg at a power density of 88 W/kg.