Study On The Controlled Construction And Properties Of Silicon/Carbon Anode Materials For Lithium Ion Batteries

At present,the commercial anode materials for lithium-ion batteries on the market are mainly graphite,but the theoretical specific capacity of graphite is only 372 m Ah/g,which seriously restricts the further development of lithium-ion battery.Therefore,the modification of traditional graphite or the search for new anode materials for lithium-ion batteries has become the focus of research in the world.The lithium storage mechanism of traditional graphite is lithium storage between graphite layers.Creating new storage spaces such as nanopores and nanocavities in graphite materials is an effective way to increase the lithium storage capacity of graphite.Silicon has become the most potential anode material because of its rich crustal reserves,high theoretical specific capacity(4200 m Ah/g),and safe voltage platform(0.4?0.6 V).However,there is serious volume expansion in the process of charge and discharge of silicon-based materials,and the conductivity of silicon materials is poor.The volume change of silicon-based materials will cause the crushing of silicon particles in different degrees,which leads to the rapid attenuation of specific capacity.Therefore,the key to solve the problem of silicon anode materials is to carry out carbon coating,design reasonable buffer space for silicon materials,and composite with carriers with good electrical conductivity,low cost and easy large-scale commercial production.Based on the above problems,this paper has done a series of studies.The research contents are as follows:(1)Expanding graphite is an effective way to create a new lithium storage space in graphite materials.Conventional expanded graphite has too much expansion volume(>100 m L/g),which leads to low material bulk density.We perform micro-expansion treatment on traditional graphite.Compared with the preparation method of mild expanded graphite that has been reported in the literature,we simplify the preparation process of mild expanded graphite(MEGMs).This experiment uses low-cost spherical graphite SG-17 as a raw material,mild expanded graphite with an expansion volume of less than 10m L/g was prepared by a combination of airtight oxidation and microwave expansion treatment.We explored the influence of the amount of potassium permanganate of different oxidants,the temperature and time of closed oxidation on the expansion volume of graphite,using scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray powder diffraction(XRD)and other characterization methods explored the microstructure under different process parameters,we found that when the oxidizing dose is 0.2 g,oxidized at 75?for 30 min,the expansion volume of the material MEGM-T75t30 after microwave radiation expansion for 10 s is9.73 m L/g,and the layer spacing reaches 0.3409 nm.The electrochemical performance test shows that the material MEGM-T75t30 has a reversible specific capacity of 446.7 m Ah/g after 100 cycles at a current density of 100m A/g,which is 120.1%of the theoretical capacity of graphite.At the same time,the material also has good electrochemical performance at a high current density.At current densities of 800,1600,and 3200 m A/g,the reversible specific capacities are 330,223,and 116 m Ah/g,respectively.Therefore,the combination of airtight oxidation and microwave treatment to prepare mild expanded graphite is expected to become an economical and effective method for preparing high-performance lithium-ion battery anode materials.(2)We choose nano-silicon with a particle size of about 35 nm as the silicon source,silicon dioxide(SiO₂)as the template sacrificial layer,and phenolic resin as the outer carbon source,which are coated and combined with the mild expanded graphite MEGMs prepared in the first work,heat treatment and HF etching process to prepare grape-like yolk-shell Si/C composite Si@void@C-MEGMs.The yolk-shell structured silicon-carbon composite microspheres are hidden in grape leaves(nanosheets of mild expanded graphite)like clusters of grapes.The composite material not only provides a buffer space for the volume expansion of nano-silicon during charge and discharge,but the carrier MEGMs also improves the conductivity of the composite material and enhances the stability of the cycle performance.We characterized the material by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray powder diffraction(XRD),thermogravimetry analysis,nitrogen adsorption and desorption,and the electrochemical properties of the composite material were tested.After 100 cycles at a current density of 100 m A/g,the reversible specific capacity of the composite Si@void@C-MEGMs is 681.3m Ah/g.After 170 cycles,the reversible specific capacity can reach 778.7m Ah/g,and the specific capacity increase rate is 14.3%.Studies have shown that the grape-like yolk-shell Si/C composite Si@void@C-MEGMs provides sufficient buffer space for nano-silicon,and the carrier MEGMs improves the conductivity of the composite.The preparation method in this study is relatively simple and low in cost,which is expected to promote the practical application of Si/C anode materials.(3)Considering that SiO₂ is a sacrificial layer,HF etching is required,which not only increases the workload but also causes certain harm due to the use of HF.On the basis of the second work,we used T-Siwith a thickness of about 90 nm as the core,and made use of the characteristic that melamine resin(MF)has a very low carbon residue(about 10%)under high temperature pyrolysis,and used MF as a sacrificial layer Nitrogen-doped yolk-shell structure silicon-carbon composite microspheres are compounded with mild expanded graphite MEGMs,and heat-treated to obtain composite materials T-Si@void@C-MEGMs.Therefore,the preparation of T-Si@MF material with good dispersion and moderate particle size is the most basic and key step of this experiment.Through scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray powder diffraction(XRD),thermogravimetry and other characterization methods,we can see that the particle size of the material T-Si@MF is about 150 nm,which can relieve the volume expansion of T-Si.It also proved that the nanometer T-Siwas successfully coated on the inner layer of melamine resin,which laid the foundation for the subsequent coating of phenolic resin.However,the current experimentally prepared material Si@MF@RF has a serious agglomeration phenomenon,which in turn affects the electrochemical performance of the subsequent composite material T-Si@void@C-MEGMs,so further adjustments are needed.The design and preparation of a new composite material T-Si@void@C-MEGMs can avoid the harm to human body and production equipment caused by the use of highly toxic HF,and provide a new idea for Si/C anode materials for lithium ion batteries.