Study On The Structural Design,composition Control Of SiO_x/Carbon Anode Material And Their Storage Performance Of Lithium Ions

With the large-scale commercialization of lithium-ion batteries and the continuous development of devices with high energy demand,the low specific capacity(372 m Ah g^-1)of commercial graphite anode material for lithium-ion batteries is far from meeting the energy demand.Silicon based anode materials for high energy density lithium-ion batteries have many advantages,such as high specific capacity(4200 m Ah g^-1),high reserves in the earth’s crust,no pollution and so on.However,in practical application,the pulverization problem and extremely low conductivity of silicon-based anode materials in the cycle process limit their electrochemical performance.Therefore,many methods are urgently proposed to solve these problems,such as constructing nanostructured particles and introducing conductive carbon to improve their electrochemical performance.Although the cycle-life of silicon-based anode has been promoted,its large-scale industrialization has encountered many obstacles.In conclusion,this paper is committed to fine structural design and composition control to obtain silicon-based anode materials with long cycle life and high reversible capacity.In this work,a silicon-based material with an average particle size of 1.12μm,suitable SiO₂ content and uniformly encapsulated carbon components inside and outside was prepared as a potential anode material for lithium storage,and the effect of SiO₂ content on the lithium storage performance of SiO_x/C anode was systematically studied.Then,on the basis of the appropriate amount of SiO₂ buffer matrix,by adjusting the carbon component position and SiO_x particle size,the effects of carbon component position and particle size on the electrochemical performance of silicon-based anode materials were studied.Finally,the reaction kinetics and lithium storage mechanism were analyzed.The results are as follows:1.By controlling the amount of hydrofluoric acid,the structure,content and distribution of SiO₂ buffer matrix were well adjusted.Excessive use of hydrofluoric acid leads to the collapse of structure,excessive loss of SiO₂ buffer matrix and the growth of active Si particles.Too little use of hydrofluoric acid leads to the accumulation of SiO₂ inert to lithium.The C-MH-2E-SiO_x samples with 28.2%SiO₂,59.5%Si and 12.3%C show ideal microstructure.Nanocrystalline silicon is uniformly distributed on the SiO₂ buffer matrix,which makes the volume expansion of silicon be alleviated by network,thus showing a low volume expansion rate（36.4%）.2.The uniform mesoporous SiO_x was synthesized by appropriate amount of hydrofluoric acid,and then the carbon phase was uniformly infiltrated into the inside and outside of SiO_xparticles,namely C-MH-2E-SiO_x,by liquid phase coating method.Compared with the MH-C-2E-SiO_x coated with carbon only,the C-MH-2E-SiO_x coated with carbon inside and outside SiO_x particles can greatly reduce the charge transfer resistance and improve the transport rate of lithium ions in active materials.The initial coulombic efficiency is 78.2%and the first reversible capacity is 2298 m Ah g^-1.At the current density of 200 m A g^-1,the reversible capacity is still 716 m Ah g^-1 after 300 cycles.When the current density increases to2000 m A g^-1,the reversible capacity is stable at 786 m Ah g^-1.When the current density returns to 200 m A g^-1,the capacity retention rate is 87.7%.However,MH-C-2E-SiO_x with only outer carbon layer shows relatively poor lithium storage performance,with 66.7%first coulomb efficiency and 367 m Ah g^-1 specific capacity after 100 cycles.3.Finally,the effect of SiO_x/C particle size on its electrochemical properties was studied.The results showed that the 0.83μm SiO_x/C anode at 200 m A g^-1 current density,its capacity retention rate is 87.5%after 70 cycles.Compared with 3.95μm SiO_x/C,its capacity retention of is 55.2%after 70 cycles.But 0.83μm SiO_x/C,its higher surface energy and specific surface area will reduce the initial coulombic efficiency of electrode,which is only 69.6%.And 1.12μm SiO_x/C presented an highest initial coulombic efficiency（78.2%）and an high capacity retention rate（75.2%）after 70 cycles at 200 m A g^-1.Finally,the electrochemical reaction kinetics analysis shows that with the increase of scanning speed,the contribution rate of the capacitance in the capacity of C-MH-2E-SiO_x is increasing,and the final value reaches61%.