Preparation And Electrochemical Performances Of Molybdenum Dioxide Composite Materials For Li-ion Batteries

Molybdenum dioxide?MoO₂?is a type of transition metal oxides with properties of low resistivity(8.8×10^-5?·cm,close to that of metals),high melting point and high chemical stability.It shows promises for various applications such as electrically induced color display,catalysts,lithium-ion batteries and super capacitors because of its efficient charge transport properties.However,application of molybdenum dioxide in energy storage was limited by the volume expansion effects generally existing to metal oxides.Thus,in this work,we synthesized molybdenum dioxide materials compositing with bismuth oxide and carbon nanotubes in an attempt to improve their electrochemical performances for lithium ion storage.The main contents are as follows:1.Using ammonium molybdate tetrahydrate?AHM?as a source of molybdenum,and bismuth nitrate pentahydrate?Bi?NO₃?₃·5H₂O?as a source of bismuth,flake shaped Bi/Bi₂O₃/MoO₂ composite materials were synthesized by hydrothermal and thermal reduction methods.The electrochemical test results show that Bi/Bi₂O₃/MoO₂ exhibited reasonable specific capacity but its cycle performance was limited under high charge/discharge rates.In order to resolve this issue,we coated the material with a carbon layer with a thickness of tens of nanometers,and the final product Bi/MoO₂/C composite was obtained.Electrochemical test results showed that Bi/MoO₂/C demonstrated excellent reversible capacity,robust rate capability,and stable cycleability,superior to some other metal oxides.Such superb Li storage capabilities suggest the Bi/MoO₂/C composite materials could serve as potential negative electrodes for rechargeable lithium-ion batteries.2.Sphere shaped Bi₂Mo₃O₁₂ nanoparticles were readily synthesized via a simple hydrothermal method adjusting the ratio of molybdenum and bismuth elements.The final product was sphere shaped nanoparticles,this structured Bi₂Mo₃O₁₂ features large surface area?45.027 m²/g?and rich mesopores?7-13 nm?which can ensure electrolyte infiltration and shorten Li-ion diffusion path in the mesopores.Electrochemical examinations show that the Bi₂Mo₃O₁₂ nanospheres exhibited stable cycling retention performance than the Bi₂Mo₃O₁₂ sheet materials at 250 mA g^-1 charge and discharge rate for 160 cycles.This result suggests that morphology change and particle size reduction are beneficial to the improvement of electrochemical properties of transition metal oxides.3.MoO₂/CNTs composites were readily fabricated via hydrolysis of molybdate tetrahydrate with the addition of surfactant?PVP?through hydrothermal method.The MoO₂ nanoparticles are grown on the carbon nanotubes evenly,leading to the formation80-120 nm diameter nanotubes under the control of experimental conditions.The testing results of MoO₂/CNTs composites show that the existence of CNTs not only improves electrical conductivity of molybdenum dioxide composite materials,but also provides some protection to minimize volume expansion effect of molybdenum dioxide.The electrochemical properties of MoO₂/CNTs have shown obvious enhancement over that of bare MoO₂ for Li-ion storage.