Preparation And Electrochemical Properties Of Porous Silicon-Carbon Composites For Lithium-ion Batteries

With the depletion of non-renewable energy sources such as coal and oil,as well as the resultant environmental concerns,the research of sustainable renewable energy and efficient energy storage devices is of great importance.While the rechargeable lithiumion batteries,as the most promising energy storage devices,have been widely used in portable electronics and electric vehicles.However,the energy density of the currently commercialized graphite anode is limited due to its relatively low theoretical specific capacity.Therefore,it is imperative to investigate other alternative anode materials with higher theoretical specific capacity and better safety to improve the energy density of lithium-ion batteries.Among numerous prospective candidates,Si anode has garnered extensive interest profited from its ultrahigh theoretical capacity,relatively low discharge potential and abundance in nature.However,its poor electronic conductivity and dramatic volume change during repetitive cycling,seriously limit the cycling and rate performance of electrode materials,which impede its widespread practical application.In this context,the electrochemical performance of silicon anode was improved by the porous structure design and the carbon layer coating method in this thesis.The main research works are as follows:(1)Silica and phenolic resin are chosen as silicon and carbon source,respectively.The porous Si/C composites were synthesized by magnesiothermic reduction,sol-gel coating method and high temperature pyrolysis.The structure characterization and electrochemical performance of the produced electrode material were analyzed in detail.The porous structure of Si provides abundant void space to accommodate volume change,which can alleviate the mechanical stress arise from volume expansion during the lithiation process,but also increase the infiltration of liquid electrolyte to shorten the path distance for Li+transport and effectively improve the kinetics of electrochemical reaction.The carbon coating layer not only improve the electrical conductivity,but also functions as buffering matrices to alleviate the internal mechanical stress and guarantee the structural integrity,thereby improve the electrochemical performance of the Si anode.The designed porous Si/C anode shows excellent cycling stability and deliver a high reversible specific capacity of 1186 mAh g-1 with a large capacity retention of 84.5%after 300 cycles at a current density of 0.5 A g-1.Additionally,the electrode also shows excellent rate capability of 882 mAh g-1 at a high current density of 6 A g-1.(2)The CNTs-intertwined and N-doped porous carbon wrapped silicon composite were synthesized by sol-gel coating method and high temperature pyrolysis,phenolic resin and ZnCo-ZIFs are chosen as carbon source.The structure characterization and electrochemical performance of the prepared anode material were analyzed.The synergistic effect of CNTs and CoSi2/Co decorated N-doped porous carbon layer would construct an interconnected 3D conductive network,which can not only buffer the volume change of Si active particles,but also prominently accelerate the kinetics of electrochemical reaction,thereby improving the electrochemical performance of Si anode obvisouly.After 200 cycles at a current density of 0.5 A g-1,the electrode can still deliver a reversible specific capacity of 1191 mAh g-1,and the excellent rate capability of 930 mAh g-1 at a high current density of 6 A g-1 can be acquired.