Study on Preparation and Modification Electrochemical Performance of Si/C Anode Materials for Libs

Author:Tang Xiao Fu

Supervisor:song yan wen guang wu


Degree Year:2018





Currently,though lithium-ion batteries(LIBs)are promising energy storage source,their energy densities are still not sufficient to meet the increasing energy density requirements of electric vehicles.Silicon-based materials possess many advantages,such as abundant reserves,high safety,relatively low lithium-uptake voltage(below 0.5V),and high theoretical specific capacity(4200 mAhg-1).However,Si electrodes commonly suffer from rapid capacity fade.This stems from huge volum expansion and contraction(>300%)due to Li insertion and extraction during charge/discharge cycles,leading to its structural collapse and instability of the solid-electrolyte inter-phase.Moreover,Si also suffers from the low intrinsic electric conductivity,leading to poor rate capability.These shortcomings hinder their practical implementation.Therefore,it is very meaningful to develop Si-based anode materials with outstanding electrochemical performance.The major aim of this paper is to develope Si-based materials with high electrochemical performance adopting diverse synthetic methods with reasonable design.Porous silicon is prepared through the magnesiothermic reduction of diatomite and then porous Si-based anode materials with good cycle stability are synthesized with the porous silicon.The specific contents and results are as follows:(1)Preparation of Si-based anode material with N-doped graphene and CNTs as conductive matrices.Si-based composites composed of silicon nanoparticles coated with N-doped carbon and conductive matrices of N-doped graphene and CNTs are fabricated by a facile method.Furthermore,the agglomeration phenomenon of silicon nanoparticles is improved thourgh electrostatic modification,raising the dispersity of silicon nanoparticles.When evaluated as an anode material for LIBs,the Si-rGO-CNT-C anode shows good electrochemical performance.Under current density of 0.1 Ag-1,it exhibits a discharge capacity of 609.6 mAhg-1 after 100 cycles.Moreover,the P@Si-rGO-CNT-C composite has better electrochemical performance than that of Si-rGO-CNT-C composite after electrostatic modification.When tested at a current density of 1 Ag-1,the composite anode can still deliver a reversible capacity of 748.1 mAhg-1,as well as excellent electrochemical reversibility.(2)Preparation of porous graphene/silicon composite with electrostatic self-assembly.Silicon nanoparticles are easy to oxidize to form a layer of silicon oxide on their surface and endow with a negative charge.Graphene oxide shows negative charge owing to ionization of the carboxylic acid and phenolic hydroxyl groups existing on the GO.Thus,silicon nanoparticles and GO have poor binding force in aqueous solution due to electrostatic repulsion.In view of the negatively charged Si nanoparticles and GO,we intentionally use the positively ch arged PDDA to realize the self-assembly of Si nanoparticles and GO.Besides,we present melamine formaldehyde resin as the bonding material to crosslink GO sheets to prevent the restacking driven by interlayer van der Waal force orπ-πconjugation between the graphene nanosheets.As a result,the silicon nanoparticles are well covered with N-doped graphene in the composite.Under a current density of 5 Ag-1,the composite anode exhibits a discharge capacity of 1132 mAhg-1 after 100 cycles.The discharge capacity of PSi-3DrGO composite is 1017 mAhg-1 after 200 cycles,exhibiting outstanding capacity retention of 75%compared with the 6th cycle.Besides,it also exhibits good rate performance.After 10 cycles at a very high current of 5 Ag-1,the composite anode can recover to approximately starting level when the cycle rate current density is reduced to 0.1 Ag-1.The excellent electrochemical performance is put down to the well covered of silicon nanoparticles with N-doped graphene,which helps buffering the volume change of silicon during the charge and discharge processes and improves the instrinsic electrical conductivity of the composite.The simple and effective synthetic method provides a new strategy for preparation of other high capacity anode materials.(3)Porous silicon is prepared by combining mechanical ball milling and a modified magnesiothermic reduction method,employing NaCl as an effective heat scavenger from cheap diatomite.About 5nm carbon layer is covered outside the porous silicon with polydopamine through subsequent heat treatment.pSi-rGO-CNT-C composite composed of porous silicon coated with N-doped carbon and conductive matrices of N-doped graphene and CNTs is also fabricated.The rich voids in the porous silicon can buffer the stress stemming from the volume change due to Li insertion and extraction during charge/discharge cycles and also shorten the diffusion and transmission path of the electrons and lithium ions.When evaluated as anode materials for LIBs,the C@pSi-I anodes show a reversible capacity of 1116.7 mAhg-1 after 200 cycles and excellent cyclability with 72%capacity retention compared with the 2nd cycle after 150 cycles at a current density of 200 mAg-1.The pSi-rGO-CNT-C exhibits better discharge capacity than that of Si-rGO-CNT-C under current density of 200 mAg-1.The porous silicon,prepared by combining mechanical ball milling and a modified magnesiothermic reduction method,exhibits excellent electrochemical performance and is expected to be a promising anode material for the next generation lithium-ion batteries.