Studies On High Capacity Silicon/Carbon Composite Anodes For Lithium-ion Batteries

Silicon is the most promising candidate as next generation anode for high energy density lithium ion batteries?LIBs?,ascribed to its high theoretical capacity(4200 mAh g^-1),modest voltage platform?0.4 V?,and high safety.However,a large volume changes during the lithiation/delithiation lead to pulverization of the Si,mechanical integrity damage of the electrode,and rapid capacity fading upon cycling,which impede its industrial applications.To overcome these issues,many efforts have been devoted to design rational Si nanostructures and/or coupling with carbon buffers ascribed its good electrical conductivity and mechanical properties.This thesis focuses on designing rational active materials nanostructures and electrode macrostructures to improving the electrochemistry performance of Si/C anodes.The research contents and results could be seen as follows:?1?The Si-nanosheets/carbon composites.In this work,silicon nanosheets with a multilayer structure and an appropriate degree of oxidation are synthesized by a topochemical reaction of calcium silicide.The thickness of the Si nanosheets and the interval of the multilayer structure is about 30 nm and 100 nm,respectively.The hierarchical structure and ultrathin thickness of the silicon nanosheets can effectively accommodate the volume changes with the void space.After carbon coating by chemical vapor deposition?CVD?processes,the Si-nanosheets/carbon composites delivers a high reversible capacity of 1200 mAh g^-1 at current density of 150 mA g^-1 and exhibits excellent cycling stability with a capacity retention of 91% after 35 cycles.This work also provides a feasible,moderate and high yielding synthetic method for industrial synthesis Si nanosheets as anodes for lithium ion batteries.?2?Si-graphene 3D porous composites.In this work,silicon-graphene composites with 3D interconnected pores are synthesized by hydrothermal and thermal reduction processes.The Si nanoparticles are encapsulated in the 3D porous graphene matrixes providing enough void space for volume expansion of Si and good electronic conductive path.The Si-graphene 3D porous composites with an appropriate pore size of 500 nm deliver a high reversible capacity(1173 mAh g^-1 at 75 mA g^-1)and exhibit excellent cycling performance?40 cycles with 98.7% capacity retention?,good rate capability(837 mA h g^-1 at 750 mA g^-1).Meanwhile,the Sigraphene 3D porous composites provide new ideas for the synthesis and structure designing of high performance Si/C composites anodes.?3?Sandwich structure Si/C anode.We design a novel sandwich-structured Si/C electrode that is sandwiched between two conductive carbon layers.In this configuration,the bottom carbon layer functions as a buffer layer to increase the adhesion to the Cu foil and to avoid the peeling-off of the active materials,while the top carbon layer on the electrolytes side serves as a barrier layer to prevent the electrode surface from cracking and delaminating.The synergy of the two conductive carbon layers lead to stress relieving,good electrode integrity,and low chargetransfer resistance.Compared to conventional electrodes,the sandwich-structured Si/C electrode delivers a high reversible capacity(1230 mA h g^-1 at 150 mA g^-1)and exhibits excellent cycling stability without obvious capacity decay after 70 cycles,good rate capability(800 mA h g^-1 at 750 mA g^-1),and low charge-transfer resistance?9 ??.This simple and effective design would be a promising approach to obtain high performance and cost effective Si anodes on a large-scale,especially for industrial manufacturing of highenergy-density lithium-ion batteries.