Synthesis Of Manganese Oxides,Silicon Oxides Electrode Materials And Their Electrochemical Properties

Lithium-ion batteries(LIBs)have been widely used in portable electronic products liking mobile phones,laptops,digital cameras,as well as electric vehicles owing to their high operation voltage,wide temperature range,long cycle life and low toxicity.Meanwhile,due to the increased demand for advanced functional portable electronic products and the pursuit of longer mileage of electric vehicles,it is necessary to develop the superior LIB s with higher energy density.The energy density of LIBs is determined by the specific capacity of anode and cathode materials to a great extent.According to the different lithium storage mechanism,anode materials can be divided into three categories:carbon-based anode,transition metal compounds anode and alloy-type anode materials.As a typical representative of carbon anode,graphite has been widely used in practical production.However,the theoretical specific capacity of graphite anode is only 372 mAh g-1,which can’t meet the high energy density of lithium-ion batteries.For this reason,the higher capacity materials of manganese oxide and silicon oxide,the typical transition metal oxide and alloy material,were selected as the research objects in the theses.However,the poor inherent electrical conductivity and huge volume expansion during the charging and discharging process resulting in their eclipsed cyclic stability and rate performance.Aimed at the issue,a series of systematic research works were carried out and summarized as follows:(1)MnO nanomaterials were prepared by adjusting the calcination temperature,and it is found that the products exhibited the best electrochemical properties when the calcination temperature is 400℃.Furthermore,in order to improve the initial coulombic efficiency of MnO,a flower ball MnO/reduced graphene oxide(rGO)composite was synthesized with the template of glass substract.The MnO/rGO composites exhibit an excellent electrochemical performance as the anode of LIBs.At a current density of 100 mA g-1,the initial discharge/charge specific capacity of the MnO/rGO composites are 1060 mAh g-1 and 740 mAh g-1,respectively.An improved initial coulomb efficiency of 69.8%can be obtained.Besides,the discharge specific capacity of composite is as high as 697 mAh g-1 after 100 cycles.Even at a high current density of 2000 mA g-1,the reversible specific capacity still maintained at 271 mAh-1.The excellent lithium storage performance of MnO/rGO composite is attributed to the porous structure between flower balls,which buffer the volume expansion to improve the cycle stability.Besides,rGO act as a charge conduction path to effectively improve the conductivity of MnO,and thus a superior rate performance was achieved.(2)Graphene oxide(GO)was used as crystal nucleus and capping reagent to synthesis the MnO2.The morphology of MnO2 can be directly controlled by modifying the amount of GO.The morphology of MnO2 range from nanorods to nanoparticles,and the particle size distribution about 19 nm.The richen voids between MnO2 nanoparticles accommodate the volume expansion of the MnO2 during the charge/discharge process.The electrochemical measurements show that the initial coulombic efficiency of MnO2 is as high as 94.5%.And under the current density of 100 mA g-1,the discharge specific capacity retention rate of MnO2 reaches to 70%(813 mAh g-1)after 80 cycles.(3)Developing a hydrothermal method to prepare nitrogen-doped carbon/SiOx(N-C/SiOx)composites,and the influence of the ratio of SiO2 to glucose on electrochemical properties was investigated in detail.The results indicate that an appropriate carbon content in the composites promote a high reversible specific capacity and enhanced cycling stability of the N-C/SiOx composites.After tested as anode electrode material of LIBs,the N-C/SiOx composites shown excellent electrochemical performance.The reversible capacity still steady at 802 mAh g-1 under 200 mA g-1 after 300 cycles.(4)Rice husk-derived SiOx/C composite was synthesized by one-step pyrolysis method.The SiOx/C composite exhibits a superior electrochemical performance as a bifunctional electrode material.On the one hand,when used as the anode of LIBs,the SiOx/C composite show excellent cycling stability.A reversible specific capacity of 582 mAh g-1 can be obtained at a high current density of 500 mA g-1 after 500 cycles.On the other hand,after loaded sulfur powder on the surface of SiOx/C composite,its electrochemical performance of Li-S batteries was studied.The reversible specific capacity composite maintained at 675 mAh g-1 after 100 cycles,the good cycling stability of SiOx/C composite is attribute to the catalytic effect of SiOx.