Preparation And Study Of Lithium Storage Performance Of Silicon-containing Biomass-based Doping C/SiO_x Composites

In order to reduce environmental pollution,achieve the national"carbon peak"and"carbon neutrality"strategic goals,and accelerate the development of green and intelligent ships,it is urgent to develop ship energy storage systems with high energy density.As an efficient green drive system,lithium-ion batteries have the advantages of high energy density,voltage stability,long service life and so on.It is widely used in the field of vehicles,ships and other transport tools.It is an important way to improve the energy density of lithium-ion battery by using silicon-based anode materials instead of traditional graphite anode materials.However,the poor conductivity,low initial coulombic efficiency（ICE）,and large volume expansion of silicon-based anode materials lead to insufficient cycle stability,which seriously restricts their wide application.In this thesis,a variety of heteroatom doped biomass-based C/SiO_x composites were designed and prepared applying silicon-containing agricultural waste rice husks（RHs）as raw material by heteroatom doping modification and simple high-temperature carbonization methods.Effects of the composition,structure and morphology of composites on lithium storage properties were systematically studied.The purpose of this research is to realize the high-value application of waste RHs and comprehensively improve the electrochemical properties of biomass-based C/SiO_x composites.The main research work is as follows:（1）Single N-doped RHs-derived porous C/SiO_x composites（N-C/SiO_x）were fabricated using RHs as raw material and urea as N source via the simple heating treatment.Silicon-based materials have high theoretical specific capacity.The C/SiO_x composites synthesized by direct carbonization of RHs have limited their electrochemical performance due to their small porosity and poor conductivity.N-doping into RHs-based C/SiO_x composites through urea not only increases the specific surface area,but also introduces different types of C-N bonds,improving the conductivity and lithium storage properties of composites.Different types of C-N bonds have different effects on the electrochemical properties of composites.When the mass ratio of RHs to urea is 7:3（sample S7-3）,the graphite-N content of composites is higher than that of other samples（3.75 at%）,and S7-3 shows the best conductivity among all samples.When the mass ratio of RHs to urea is 8:2（sample S8-2）,the total content of pyrrole-N,pyridine-N and N-O in composites is higher than that of other samples.S8-2 possesses the highest low valence Si content and reversible specific capacity in all samples.S8-2 electrode delivers a high reversible capacity of 1018.5 m Ah/g with enhanced ICE of 72.2%at 0.1 A/g.Even at 1.0 A/g,a stable cycling capacity of 623.3 m Ah/g still can be obtained after 1000 cycles.The N-doped porous C matrix effectively alleviates the volume expansion of SiO_x particles.（2）Based on the single element doping,double S and N co-doped porous C/SiO_xcomposites（SN-C/SiO_x）were synthesized using thiourea as additive and adjusting the content of functional groups via a carbonization method.The double S and N co-doping enlarges interlayer spacing,elevates specific surface area,introduces C-S and C-N bonds,raises degree of defects,alleviates volume expansion,improves electrical conductivity and increases reversible specific capacity.Moreover,S and N binary-doping reduces the content of C-O and C=O functional groups in composites,decreases the formation of irreversible Li⁺,reduces the thickness of solid electrolyte interphase（SEI）film,and improves the ICE.The optimized S7-3 retains the cycling capacity of 1150 m Ah/g at 0.1 A/g with ameliorated ICE of 70.4%and stable cycling capacity of 632 m Ah/g at 1.0 A/g after 1200 cycles.（3）The introduction of new functional groups on the surface of materials can effectively improve electrochemical performance.P and N binary-doped porous C/SiO_x composites（PN-C/SiO_x）were prepared by soaking RHs in（NH₄）₃PO₄solution and solid phase calcination.PN-C/SiO_xhas the hierarchical porous structure.P,N atoms and-O-PO₃H₂ groups are introduced into the C skeleton after（NH₄）₃PO₄ assisted carbonization procedure,which increases the specific surface area and defect degree of composites,and raises the ion transport ability and specific capacity.The introduced-O-PO₃H₂groups on the surface of C skeleton react with electrolyte to form a-C-O-PO₃Li₂layer in the first cycling.The-C-O-PO₃Li₂layer acts as a preformed SEI to inhibit the growth of SEI in the later stage and reduces the consumption of Li⁺,thus enhancing the ICE.The optimized PN/2 presents an excellent reversible specific capacity of 1078 m Ah/g at 0.1 A/g and improved ICE of 73.6%.Even at a higher current density of 1.0 A/g,a steady specific capacity of 622 m Ah/g still could be achieved after 1000 cycles.The Li⁺storage of PN/2 is mainly dominated by the surface pseudocapacitive process.（4）In order to further improve the electrochemical properties of RHs-derived C/SiO_xcomposites,double B and N co-doped porous C/SiO_x composites（BN-C/SiO_x）were successfully synthesized applying RHs as raw material and NH₄HB₄O₇ as the porogen reagent and heteroatom source via a high-temperature solid state route.B and N co-doping reduces the diffusion batters on the C skeleton,thus enhancing the ion transport ability and electrochemical dynamic.The increased specific surface area and introduced structural defects raise the Li⁺storage active sites,which elevates the reversible specific capacity of composites.The optimized BN-25 anode retains the cycling capacity of 1165 m Ah/g at 0.1A/g,which is more than 2 times higher than that of undoped sample.The ICE also increases from 45.6%to 74.3%.Even at a large current density of 1.0 A/g,the reversible capacity of BN-25 electrode can be remained at 650 m Ah/g up to 1200 cycles.The SEI film formed on the surface of BN-25 electrode is more stable than that of undoped C/SiO_xelectrode.