Molybdenum Oxide-Based Composites Prepared Through PEG Induction And Their Electrochemical Properties For Lithium Ion Batteries

Molybdenum oxides and molybdenum-containing hybrid materials(MCHMs)have been widely used in many fields,such as lithiumion batteries(LIBs),catalysis,photo-chromism/luminescence and gas sensor due to their remarkable optical,electrochemical,catalytic and mechanical properties.Usually,different applications have specific requirements for the morphology,size and composition.At present,the hydrothermal method,chemical precipitation method,sol-gel method and flame method have been employed to synthesis the molybdenum oxides and MCHMs to meet the specific requirements.However,these methods exhibit various problems such as harsh reaction condition,long reaction time,use of toxic organic solvents,and high cost of raw materials.Most of them are unsuitable for large scaleproduction.Therefore,a new preparation method was proposed in this dissertation.The MCHMs with different morphologies were synthesized from molybdenum precursor solution induced by polyethylene glycol(PEG).The influence of various reaction conditions on the phase,morphology,and size of MCHMs was investigated in detail.Moreover,the regulation mechanism of molybdenum oxides in the PEG polymeric system and reaction mechanism of molybdenum precursor with PEG were revealed.MoO3 nanosheet and microribbon,hierarchical porous MoO2/C and MoO2/Mo2C/C microspheres were synthesized after heat treatment,and the related electrochemical properties as anode materials for LIBs were also tested.The main innovative progress is summarized as follows:(1)MCHMs were prepared from peroxomolybenum solution induced by PEG polymer.The effects of various reaction parameters including reaction temperature,reaction time and PEG concentration on the phase,morphology,and size of MCHMs were systemically investigated.The effect of PEG polymeric system on the microstructure of molybdenum oxides and reaction mechanism of molybdenum precursor with PEG were revealed.It was found that PEG concentration has dramatic influence on the MCHMs.When the molecular weight of PEG is 8000,the change of the product with the increase of PEG concentration was as follows.The morphology changed in the sequence:ribbon-needle-fiber-sphere-cube.The phase altered from H2MoO5 to MoO3·H20 and then to amorphous state.The color shifted from yellow to green,blue and then to dark blue.The valence state of the molybdenum converted from +6 to +5.The PEG polymer acted not only as a structure-directing agent to construct MCHMs with different morphologies and sizes,but also as a surfactant to enhance the dispersion of particles.The mild reducibility of PEG polymer also converted a portion of Mo(VI)into Mo(V).(2)MoO3 was prepared by the calcination of MCHMs in air.The influence of PEG concentration,calcination temperature and calcination time on the phase and morphology of MoO3 was investigated in detail and the electrochemical properties of MoO3 as anode materials for LIBs were tested.The accelerated transport of lithium ions and electrons as well as the effective mitigation of the volume change due to the special layered structure contributed to the admirable electrochemical performance of MoO3 nanosheet.At a current density of 100 mA g-1,the MoO3 delivered the discharge specific capacity of 1373 mAh g-1 in the first cycle and 540 mAh g-1 after 100 cycles,respectively.(3)Hierarchical porous MoO2/C microsphere was synthesized through heat treatment of MCHMs in nitrogen at a relatively low temperature.The influence of calcination temperature on the structure and electrochemical performance of MoO2/C microsphere was investigated.The as-prepared microsphere with a diameter of 1.5 ?m had rough surface and good dispersity.There are abundant pores in the microsphere consisting of many primary nanoparticles.With the increase of the calcination temperature,the nanoparticle size slightly grew,the content of carbon showed a reduction trend,and the pore structure changed from microporous to mesoporous.After calcination at 500°C,the MoO2/C microspheres exhibited the discharge specific capacity of 768 mAh g-1 in the first cycle and 800 mAh g-1 after 300 cycles at a current density of 100 mA g-1.Additionally,the microstructure can be preserved well.(4)Hierarchical mesoporous MoO2/Mo2C/C microsphere was prepared by heat treatment of MCHMs in nitrogen at a relatively high temperature.The effect of calcination temperature on the product was studied.In addition,MoO2/Mo2C/C composites obtained at different calcination temperatures were used as anode materials for LIBs and the electrochemical properties were tested.The results showed that MoO2/Mo2C/C composite was obtained at 700 and 800?.The carbon from the carbonization of PEG polymer not only converted Mo(VI)into Mo(IV),but also reacted with a part of MoO2 resulting in the formation of Mo2C.The MoO2/Mo2C/C microspheres obtained from calcination at 700? exhibited high capacity and good cycling stability.At a current density of 100 mA g-1,the MoO2/Mo2C/C microspheres exhibited the discharge specific capacity of 720 mAh g-1 in the first cycle and 665 mAh g-1 after 100 cycles without the destroy of microstructure.