Study On The Preparation Of Rice Husk-derived Silicon/carbon Composites Modified By Imidazole-based Ionic Liquids And Their Application For Lithium-ion Battery Anodes

Silicon remained one of the most promising anode materials for lithium-ion batteries（LIB）with high performance,owing to its excellent specific capacity(4200 m Ah g^-1),which is much higher compared to the widely commercialized graphite anodes(372 m Ah g^-1).However,the application of Si in LIBs is limited by structural fragmentation due to volume expansion during lithiation/delithiation processes,combined with its poor electrical conductivity.Thankfully,the synthesis of carbon-coated Si can inhibit the volume expansion of Si as well as significantly enhance its conductivity.Although this process can improve the battery performance,the high cost of silicon resources and complexity of the synthesis process remained major bottlenecks.Rice husk（RH）is an abundant agricultural waste with a large annual output（200 million t/a）and a high Si content（15-20 wt%）,hence it could be used as a precursor for low-cost silicon/carbon（Si/C）composites for application as LIB anode material.Meanwhile,ionic liquids（ILs）also emerged as new“green solvents”for biomass conversion with the potential to form graphite carbon at high temperatures.To further improve the electrochemical performance of the anode material for Li-ion batteries,ILs are used to improve the graphitization of the material and coupling the silicon-carbon interface.This paper uses RH and ionic liquids as raw materials to ensure high specific capacity,improve life cycle,and also simplify the preparation process of Si/C based LIB anodes.Firstly,core-shell structured Si/C composite was prepared,followed by the preparation of a core-shell structure-based Si/C composite from RH-derived silicon nanoparticles（Si NPs）and pitch,as well as the preparation of stable core-shell Si/C composites using RH as a Si source and carbon source,respectively,for application as anode materials for lithium-ion batteries.The main contents and conclusions of this paper are as follows:（1）The rice husk was dissolved in 1-butyl-3-methylimidazolium acetate（BMIMAc O）,and Si NPs were prepared by one-step carbonization at 400°C/800°C.The preparation of core-shell Si/C composites（Si@Si O_X@C）as anode materials for LIB.Laser raman spectrometer（Raman）,X-ray diffractometer（XRD）,X-ray photoelectron spectroscopy（XPS）,and Brunner-Emmet-Teller measurements（BET）structural tests show that BMIMAc O can promote the degree of graphitization of biomass,provide N atom doping,and form chemical bonds such as Si-N bonds between the carbon layer and Si Ox layer.The structural morphologies studied by scanning electron microscope（SEM）and transmission electron microscope（TEM）indicate that BMIMAc O dissolves RHs and synthesizes biochar to coat the outer layer of Si NPs,forming a three-layered core-shell structure of Si@Si Ox@C.Cyclic voltammetry（CV）,galvanostatic intermittent titration technique（GITT）,impedance,pseudocapacitance,and charge-discharge tests have verified that this single-silicon-coated form significantly enhances the structural stability of the electrode,suppresses the expansion of the core Si,and improves the overall conductivity of the material.Therefore,the specific capacity and stability of the electrode are significantly improved.When Si@Si O_X@C-800 was applied to the anode of LIB,it exhibited excellent reversible specific capacity and outstanding cycling stability(The specific capacity at 1 A g^-1 current density is 460m Ah g^-1,and the capacity retention rate after 1000 cycles is 105.7%),which is higher than most of the Si/C materials reported in the literature.The Si/C composite of core-shell structure synthesized by an environmentally friendly way provides a new idea for enhancing the stability of Si/C material.（2）The mixture formed by dissolving the pitch through BMIMAc O was carbonized with the Si NPs thermally reduced by RH magnesium to form a three-layer core-shell Si/C composite（Si/Si Ox@C）with a pitch/Si Ox coating of RH-derived Si NPs.Electron microscope morphology tested by SEM and TEM and structural tested by XRD,Raman,and XPS show that BMIMAc O dissolves the pitch to make it uniformly cover the Si NPs to form chemical bonds such as Si-C,Si-O-C,and Si-N bonding interface,while the oxygen-containing functional groups of pitch and BMIMAc O enable Si Ox layers to grow on the surface of Si NPs to buffer the volume expansion of Si.When used as anode of LIB,electrochemical characterizations such as CV,impedance,and charge-discharge tests were carried out.The RH-Si/Si Ox@C electrode maintains good long-term cycle stability.At a current density of 2 A g^-1,the reversible specific capacity is 518 m Ah g^-1,and the capacity retention rate is 72.31%after 1000 cycles.Compared with other similar literature,the electrochemical performance has been greatly improved.This facile and effective method provides a new direction for the preparation of other biomass high-performance LIB anode materials.（3）The RH-based Si NPs prepared by magnesium thermal reduction with BMIMAc O and RH was successfully prepared by a one-step carbonization method,and a core-shell Si/C composite（RH-Si@Si Ox@C）was successfully prepared for anode material for LIB.With BMIMAc O as solvent and coupling agent,morphology and structural characterization indicate that the bonding of chemical bonds such as Si-N bond,Si-C bond,and C-N bond between the interfaces has been completed,providing a fast channel for the transport of lithium-ions and electrons.The volume change of Si NPs during charge and discharge process is greatly buffered.Electrochemical tests show that the Si@3C material exhibits good cycling stability as an anode material for LIB(220 m Ah g^-1 after 2000 long cycles at a large current density of 2 A g^-1 high capacity and capacity retention rate as high as 81.4%)and electrical conductivity(diffusion coefficient is 2.58×10^-10 cm² s^-1),suitable for commercialization as a fast-charging material for promotion.The research in the above three chapters proved that can build a stable coupling interface between the Si/C layers can be introduced through ionic liquids to form a Si/C composite with a core-shell structure.This material has excellent electrochemical performance,and its specific capacity and cycle stability is much higher than other similar reports.The process explored in this paper has the advantages of low cost and simple process,which provides a new way for the utilization of all components of RH.