Preparation And Electrochemical Performance Of Si/C Composite Anode Materials For Lithium Ion Batteries

Li-ion batteries(LIBs)infulences our daily life by powering our portable electronics,and electric transport.In spite of their eminent potential,the conventional LIBs suffers from low capacity,high cost and limited power output,which is difficult to meet the requirements of vehicle electrification for high energy density,high power density and long cycle life.To realize this object,a variety of electrode materials with high specific capacities have been explores in recent years.As for anode materials of LIB,lithium-alloying materials,transition metal oxides and so on are investigated.Compared with conventional graphite anodes,silicon(Si)showing a ten times larger theoretical specific capacity(4200 mAh/g)than graphite,is regarded as the most promising candidate for LIBs anodes.However,elemental silicon expreiences a tremendous volume change(>400%)during the lithiation and delithiation processes,which induces particle pulverization,loss of electrical contact with the conductive additive or the current collector,and even peeling off from the current collector.The volume charge also lead to fracture and re-formation of the solid electrolyte interphase(SEI)layer,resulting in continuous consumption of the electrolyte,and then increased impedance and rapid capacity fading.Significant efforts have been devoted to alleviating the issues mentioned above.The strategies investigated include Si material design through nanostructures,porous structures and nanocomposites.In this thesis,according to the recent research progress on Si anode material,a simple carbon modifying and porous structure Si designed were designed and investigated to improve the electrochemical performance of the Si anode material.The first part of this work focused on the dual carbon modified structure,which synthesized by double carbonization processes,the low-temperature carbonization on Si particles and polymer gel high-temperature carbonization,respectively.This structure design will provide a strong buffering and high electrial conductivity to Si active materials.The second part of this work was focused on the preparation of porous Si/C composite.The porous silicon can effectively buffer the volume charge of Si.Moreover,the three-dimensional carbon structure can decrease the repeated SEI formation(especially on Si nanomaterials)and promote the transfer of electrons.The detail experimental results and conlusions are as follows:(1)A dual-carbon modified Si composite was prepared by simple two-step carbonization.Compared with single-carbon modified Si composite and elemental Si,the dual-carbon modified Si composite shows a higher electrochemical performance.At the current density of 200 mA/g,the dual-carbon modified Si composite showed a capacity of 681 mAh/g after 150 charge/discharge cycles with the capacity retent ratio of 82% relative to the second cycle.Comparatively,the single carbon modified Si composite showed a capacity of 613 mAh/g after 150 cycles and the cycle performance of elemetal Si is even much worse.The results indicates that the synergistic effect of dual-carbon structure not only mitigate the volume charge of Si,but also maintain the electrode structure's stability.(2)The porous Si prepared by high-temperature diffusion doping and metal-assisted chemical etching was imbeded in a three-dimensional carbon structure.This composite showed favorable cycling stability compared to porous Si without carbon modification.At the current of 200 mA/g,the porous composite showed a capacity of 954 mAh/g after 50 cycles with the capacity retent ratio of 86.4% relative to the second cycle,which was much better than Si/C(PAN)in cycling stability and specific capacity.The excellent properties of PSi/C are mainly attributed to the effective buffering effect and electrode structure retension of porous structure and to mitigate the volume change during charge/discharge cycling.