Study Of Mesoporous SiO_x/Carbon Composite Materials For Lithium Ion Energy Storage System

Silicon is a promising lithium storage anode material with high specific theoretical capacity(4200 mAh g^-1),low working potential（0.4 V vs.Li⁺/Li）,and high crustal abundance（27%）.However,silicon shows poor conductivity(6.7×10^-4 S cm^-1)and massive volume change（>300%）during lithiation/delithiation process which easily result in poor rate and cycle performance.These drawbacks greatly restrict the practical applications of silicon.To tackle these tough issues,two types of mesoporous SiO_x/carbon composites are prepared,and their electrochemical properties as anode materials for lithium ion batteries or lithium ion capacitors are investigated.These materials possess the following advantages:The SiOx shows less volume change with respect to Si;combination with carbon improves the electrical conductivity of the composites and avoids direct contact of SiOx with the electrolyte,suppressing side reactions;the mesoporous structure accommodates the volume change during cycling and serves as fast channel for lithium ion diffusion.These measures work synergistically to improve the electrochemical performance of the silicon/carbon anode materials.The main research contents and results of this thesis are as follows:1.Mesoporous SiO₂ is used as precursor,and mesoporous SiO_x is obtained by magnesiothermic reduction.Then,various metal phthalocyanine are partially deposited into mesoporous channels.After carbonized at high temperature,the SiOx/carbon（SiO_x@C-M,M=Fe,Co,Ni）composite materials with mesoporous structure and nitrogen-doped graphitized carbon coating is obtained.It is found that,when nickel（Ⅱ）phthalocyanine is used as precursor,the obtained composite exhibits superior cycleability owing to the formation of robust carbon coating layer.The SiO_x@C-Ni composites deliver a reversible specific capacity of 1026 mAh g^-1.The capacity retention rate is increased from 36.5%of mesoporous SiOx to 75.8%of SiOx@C-Ni after 100 cycles at 0.2 A g^-1.And the material can still maintain capacity as high as 532mAh g^-1 after 1000 cycles at current density of 1 A g^-1.The SiO_x@C-M composites is coupled with LiFePO₄ cathode to construct a full cell.The SiO_x@C-Ni//LiFePO₄ full battery exhibits a capacity retention rate of 95%after 100 cycles at 1 C rate.2.Mesoporous SiO₂ coated with graphene oxide is synthesized by using P123 as porogn,TEOS as silicon source,and GO as carbon source.After aluminothermic reduction,the reduced graphene oxide-coated mesoporous SiOx（MPSiOx@rGO）composite is obtained.The composites deliver a reversible specific capacity of 818mAh g^-1.The commercial activated carbon（YP-50）with high specific surface area(1694 m² g^-1)is used as positive electrode material,and the pre-lithiated MPSiO_x@rGO electrode is used as negative electrode,assembling lithium ion capacitors.The lithium-ion capacitor shows a reversible specific capacity of 144 mAh g^-1 at 0.2 C,and the first coulomb efficiency is up to 99%.At a high rate of 10 C,the initial specific capacity is maintained at 86 mAh g^-1.Even after 10,000 cycles,the capacity still retained at 46mAh g^-1,with a capacity retention rate of 53.5%.MPSiO_x@rGO//AC lithium ion capacitors show excellent rate capability and cycle stability.