Preparation And Electrochemical Performance Of Functionalized Silicon Anode Nano Materials

Higher energy densities and long cycle life of lithium-ion battery(LIB)was need due to the popularity and development of portable electronic devices and electric vehicle.Silicon has been considered as the most promising anode materials for LIB as high gravimetric and low operating voltage.But a large volume change during charging and discharging processes,which causes drastic change in thickness of electrode and severe fracture and pulverization of the active materials and rapid capacity fading.Therefore,many great efforts have been made to address the issue.In this dissertation,silicon-carbon composites with different components and structures are designed and synthesized via various synthetic methods.The main results are following:(1)The structure of Si/C particles on graphene sheet was obtained by cost-effective and environment friendly process.First,commercial silicon nanoparticles were coated with polydopamine(Si/PDA)through the self-polymerization of dopamine.Second,the Si/PDA particles reacted with graphene oxide(GO)in aqueous solution,due to the hydrogen bonds between the functional groups in polydopamine and carbonyl groups on GO.Last,the Si/C-G was obtained by carbonization process of Si/PDA-GO.The carbon layer from PDA can be buffer the volume change of Si during the Si-Li alloying reaction.The graphene as barrier can effectively avoid the aggregation of Si/C particles.The Si/C particles can anchor on the surface of graphene and prevent graphene from stacking.The flexibility of graphene as secondary buffer structure can ensure the effective accommodation of huge volume expansion.And the graphene can protect the Si/C particles from contacting with electrolyte directly.This strategy can maintain the stable interface at electrode/electrolyte,which can promote the reaction kinetics.(2)Nanodiamond was introduced into the Si/C-G.The Si/C and nanodiamond were homogeneous dispersed on the graphene layer.The excellent heat stability of nanodiamond can maintain the unconsolidated structure,avoiding the agglomeration of Si/C and the stack of graphene.Nanodiamond has high adsorption energy for Li+that it is easy to adsorb Li+.And the concentration of Li+ on the surface of Si nanoparticles were increased which can accelerate the reaction of alloying between Si and Li+.The negative electrode materials present excellent rate performance,achieving 528.7 mAh g-1 at 35790 mA g-1.(3)The SiO was coated by saccharose and carbonization(Si/SiOx/C).The carbon layer not only can improve the conductivity of Si/SiOx,but also can protect the Si/SiOx from contacting with electrolyte directly.The disproportionation reaction of SiO can generate Si and silica.Abundant of pores were generated by removing the silica,which can provide space for the volume expansion of Si/SiOx.After that,CNT was in-situ formed on the Si/SiOx/C.The CNT built the network structure,which can coat the Si/SiOx/C and keep the electrical contact of Si/SiOx/C during the lithiation/dislithiation.The high electronic conductivity of CNT can further improve the conductivity of Si/SiOx/C.Abundant porosity of CNT network as the secondary buffer can provide space for the volume expansion,realizing good cycle performance.The Si/SiOx/C-CNT was mixed with graphite as anode.The result show good cycle stability and after 200 cycles it is no obvious decrease.To evaluate the feasibility for practical application,the full-cell with pre-lithiation Si/SiOx/C-CNT as negative and commercial LiFePO4 as positive.After 250 cycles,the reversible discharge capacity of 143.1 mA h g-1 was retained,which prove that the anode electrode material has high stability.