| In recent years,with the rapid development of emerging industries such as electric vehicles and smart grids,there rise higher requirements for the energy density,rate capability,cycle life,and economic cost of lithium-ion batteries.At present,the commercial graphite anode has been unable to meet market demand.Silicon is considered to be the most promising anode material for the next generation lithium-ion batteries due to its ultra-high theoretical capacity,rich natural reserves,and lower voltage plateau.However,the lithium-inserting process is accompanied by a huge volume and the internal generated stress causes the pulverization of particles which leads to the continuously generation of solid electrolyte interphase(SEI)layer.These phenomena result in the low coulombic efficiency and large capacity attenuation of silicon anodes.Besides,the rate performance is restricted by the inherent low electronic conductivity of silicon anodes.These two shortcomings limit the commercial application of silicon anodes in lithium-ion batteries.Considering the above problems,Fe,Fe3O4 and carbon are combined with silicon.Through rational design and optimization,this thesis is aiming to alleviate the huge volume change and improve the electronic conductivity of Si while exerting its high theoretical capacity,in order to enhance the electrochemical performance of siliconbased anodes.The main research contents and conclusions of this thesis are as following:(1)A three-dimensional sponge-like Si/Fe/Fe3O4@graphitic carbon(SFFO@GC)composite material is successfully synthesized by ball-milling and thermal decomposition with the addition of hydrated ferric nitrate,using commercial silicon powder as the silicon source and sodium chloride as the hard template.The effects of the addition of Fe(NO3)3·9H2O and glucose on the phase composition,morphology and electrochemical performance of the composite materials are investigated systematically.It is found that the addition of ferric nitrate could increase the graphitization degree of carbon,and the chemical composition and morphological feature of the SFFO@GC composite obtained are the best when 0.73 g of Fe(NO3)3·9H2O and 0.5 g of glucose are used,showing the best electrochemical performance.At a current density of 0.5 A g-1,the first discharge specific capacity is 1970 mA h g-1,and it can still provide a high capacity of 1191 mA h g-1 after 200 cycles.In addition,the conductive network composed of graphitized carbon and iron nanoparticles significantly improves the overall electronic conductivity,and the designed anode shows excellent rate performance.When the current density is increased to 5.0 A g-1,the reversable specific capacity still maintains 1097 mA h g-1.(2)The optimized SFFO@GC composite material is used as anode and commercial lithium-rich manganese-based oxides(LR-NCM),lithium cobaltite(LCO)and lithium iron phosphate(LFP)are used as cathodes.LR-NCM/SFFO@GC,LCO/SFFO@GC and LFP/SFFO@GC button-type full batteries are assembled.The relationship between capacity and voltage of the full cell system is analyzed from the charge-discharge curves of the cathode and anode in their half-cell systems,which could provide a suggestion for the selection of operating voltage range of full-cell system.Then the effects of different pre-lithiation methods and voltage ranges on the performance of the full cells are assessed.The results show that an ideal way of prelithiation is that SFFO@GC anode experiences two cycles under 0.2 A g-1 in the voltage window of 0.01-2.5 V and the corresponding full-cell shows good cycle stability.By appropriately increasing the lower limit of the voltage,the capacity cycling stability of the full cells is enhanced and the average voltage is higher in a smaller voltage window.Finally,comparing the performance of different full battery systems,it is found that at a current density of 0.5 C,LR-NCM/SFFO@GC shows the highest specific capacity and specific energy but declines fast during cycling,while LFP/SFFO@GC exhibits outstanding cycling stability. |
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