铁负载酸活化蒙脱石制备硅/硅化铁纳米复合材料及其储锂性能

Iron Silicide/Silicon Nanomaterials Derived from Iron/Acid-Montmorillonite for High Performance Lithium-Ion Battery Anode

纳米硅的储锂容量高,被认为是最有应用前景的新一代锂电负极材料。然而纳米硅的体积膨胀效应和低电导性不利于其实际应用。将天然蒙脱石酸活化和铁负载改性,再经过镁热还原反应,得到了原位复合的硅/硅化铁纳米复合材料。硅化铁具有良好的导电性和机械稳定性,可显著提升硅负极的循环稳定性和倍率性能。在1 A/g的电流密度下循环500圈后,硅/硅化铁纳米复合材料的比容量高达781 m A·h/g,容量保持率为77%。该方法为制备高性能硅基负极材料提供了新思路。

Introduction Silicon is considered as one of the most promising lithium anode materials due to its high lithium storage capacity.However,the volume expansion effect and semiconductor properties of silicon hinder its practical application.Two effective strategies exist for mitigating this issue,i.e.,reducing the size of silicon to nanoscale;and introducing non-active matrices to improve stability and conductivity.In recent decades,the incorporation of iron silicide(FeSi_(2))with silicon(Si)has attracted much attention due to its high conductivity,mechanical hardness and environmental friendliness.Unfortunately,the Si/FeSi_(2) nanocomposites are mainly prepared by high-energy ball milling,which has some drawbacks such as low purity,large particle size,and uncontrollable morphology,making it difficult to achieve the commercial application.Porous silica derived from clay minerals is extensively investigated for the preparation of silicon nanoparticles by magnesiothermic reduction.In addition,porous silica has a large specific surface area,which facilitates the effective dispersion and loading of iron oxides.The reduction of iron oxide and silica dioxide to Fe and Si can be achieved by magnesium.In this paper,a natural montmorillonite(Mnt)with a high content of silica(65%,in mass)was utilized as a raw material.Silicon/iron silicide(Si/FeSi_(2))nanocomposites were synthesized via modification of Mnt(including acid etching and iron loading)and molten salt-assisted magnesiothermic reduction.Methods A porous silica(SiO_(2))was prepared via leaching Mnt at a high concentration of HCl.Subsequently,a iron oxide was loaded on SiO_(2) via precipitation in different Fe/Si mass ratios(i.e.,0,10%,and 20%).Modified SiO_(2)(Fe-SiO_(2))was mixed with Mg and NaCl in a mass ratio of 1.00:0.85:3.00.Afterwards,the mixture was heated in an inert atmosphere at 650℃for 5 h in a heating rate of 5℃/min.After cooling naturally,the product was washed with 1 mol/L HCl and 1%(in mass)HF in sequence to remove the by-product.Finally,it was rinsed with ultra-pure water until neutral and dried at 80℃.The sample were named as Si,Si/FeSi_(2)-1,and Si-FeSi_(2)-2,corresponding to the Fe/Si mass ratio of 0,10%,and 20%,respectively.Results and discussion The XRD patterns show that Si/FeSi_(2) nanocomposites are consisted of Si andɑ-FeSi_(2).An increase in Fe content in precursors leads to a higherα-FeSi_(2) content in the product.Based on the results of RIR value method and elemental composition analysis,the mass ratio of FeSi_(2) to Si in Si/FeSi_(2)-1 and Si/FeSi_(2)-2 is 5%and 10%,respectively.The average grain sizes of Si,Si/FeSi_(2)-1,and Si/FeSi_(2)-2 are 20,40 nm,and 48 nm,respectively,according to Scherrer's equation,.The grain size of silicon increases with the augmentation of Fe content in the precursor.This phenomenon analyzed by both transmission electron microscopy and scanning electron microscopy images is since the melting temperature of silicon nanoparticle is lower than that of bulk silicon(~1410℃),the addition of iron greatly increases the local temperature(ΔHFe_(2)O_(3)=-309 kJ/mol vs.ΔHSiO_(2)=-130 kJ/mol),and the addition of Fe can effectively reduces the melting point of nanocrystalline grains in Fe/Si eutectic alloys.Moreover,the specific surface areas of Si,Si/FeSi_(2)-1,and Si/FeSi_(2)-2 are 89,42 m2/g,and 33 m2/g,respectively.The gradual decrease in specific surface area coincides with the concurrent increase in grain size.As anode electrodes,Si/FeSi_(2) nanocomposites exhibit a higher initial coulombic efficiency,compared with Si due to the smaller specific surface area.The rate performance show that Si exhibits higher capacity than Si/FeSi_(2)-1 and Si/FeSi_(2)-2 at a low current density of 0.1 A/g.However,when the current density increases gradually,the capacity of Si is lower than that of Si/FeSi_(2)-1 and Si/FeSi_(2)-2.This phenomenon indicates that the incorporation ofɑ-FeSi_(2) enhances both electron conduction and ion diffusion rates.After undergoing 500 cycles at a current of 1 A/g,the capacities of Si,Si/FeSi_(2)-1,and Si/FeSi_(2)-2 are 417,781 mA·h/g,and 561 mA·h/g,respectively.The corresponding capacity retention rates are 35%,77%,and 50%,respectively.The cycling performance demonstrates that Si/FeSi_(2) nanocomposites have a more robust structure,compared to the Si electrode.In addition,the cyclic stability of Si/FeSi_(2)-1 is inferior to that of Si/FeSi_(2)-2,potentially due to the reduced structural stability resulting from a larger particle size.Conclusions The Si/FeSi_(2) nanocomposites were prepared via modification of montmorillonite and molten salt-assisted magnesiothermic reduction.This method utilized magnesium powder to reduce Fe_(2)O_(3) and SiO_(2) for the in-situ formation of Si/FeSi_(2) nanocomposites.The composition of Si and FeSi_(2) nanocomposites could be adjusted via altering the ratio of Si/Fe in the precursor.Moreover,the reduction of Fe_(2)O_(3) resulted in an increase in grain size and a decrease in specific surface area of silicon nanoparticles.As anode materials,Si/FeSi_(2) composites demonstrated the superior rate performance and cycle stability,compared to Si.This phenomenon could be due to the incorporation ofɑ-FeSi_(2),effectively enhancing the kinetics of lithium ions and electrons on the electrode.In addition,as an inert substance,FeSi_(2) could reduce the volume expansion effect of silicon,thereby maintaining the stability of electrode structural.The structural stability of the Si/FeSi_(2)-2 electrode was relatively weaker than that of the Si/FeSi_(2)-1 electrode.Therefore,Fe content in the precursor could be of paramount importance for achieving high-performance silicon-based nanocomposite anodes.