Preparation Of Silicon Carbon Anode Materials By Magnesium Thermal Method And Its Electrochemical Properties

Improving the specific energy of lithium-ion batteries is an inevitable requirement for technological development.The theoretical specific capacity of the conventional graphite anode material is 372 mAh g^-1,which limits the energy density of the lithium ion battery,and it is difficult to continue to support the development of the power battery.The theoretical specific capacity of silicon anode materials reaches 4200 mAh g^-1,and is expected to be the next generation anode material to replace graphite.However,silicon anode materials have the disadvantages of low conductivity and high volume-change rate,which affects the commercialization process.Two effective measures in the modification method of silicon anode material are carbon material recombination and reserved buffer space.In this paper,two processes for preparing silicon-carbon anode materials using magnesium thermal reduction at relatively low temperatures were studied,and the electrochemical properties of different structures of silicon-carbon anode materials were compared.First,the specific parameters of precursor preparation,reduction method and pickling treatment process of magnesium thermal were explored.The SiO₂ was prepared by the modified St?ber method,and the effects of different reaction times on the size and uniformity of the synthesized SiO₂ were investigated.The SiO₂ microspheres with a particle size between 400 and 450 nm were used as the silicon source.Vacuum magnesium thermal reduction and molten magnesium thermal reduction were compared.Different configurations of stainless-steel reactors were designed and used.The yield of the molten salt magnesium thermal reduction method was found to be higher and can be used in subsequent experiments.The effect of the pickling process after magnesium thermal reaction on the purity of the product was investigated.It was found that the purity of the reduced product after three high-concentration hydrochloric acid pickling and HF treatment was higher,which was in good agreement with the XRD diffraction spectrum of pure silicon.Then,the process of preparing silicon carbon negative electrode material by two-step magnesiothermic reduction method was studied.Ni-SiO₂ was prepared by liquid phase method using NiCl₂·6H₂O as a nickel source,and SiO₂ and NiCl₂·6H₂O were respectively reduced to silicon and metallic nickel by magnesium thermal reduction reaction.The use of nickel to catalyze and adsorb carbon atoms,catalyze the reduction of triethylene glycol,and prepare a graphene-like mosaic structure of silicon carbon material.Studies have shown that in the silicon-carbon material of the mosaic structure,the structure of carbon is gauze-like,close to the graphene structure,embedded in the gap of the porous silicon particles.Electrochemical characterization of the transition product silicon-nickel alloy and the mosaic silicon carbon material after HCl treatment showed that the conductivity of the silicon-nickel alloy was higher than that of the pure silicon anode,and the cycle performance was excellent.The initial specific capacity of the mosaic structure Si-C material is higher,but the cycle performance is slightly worse.Finally,polydopamine?PODA?and resorcinol formaldehyde?RF?phenolic resin were used as carbon sources to study the preparation of Si/SiC/C hollow yolk-shell structure silicon carbon anode material by one-step magnesium thermal reduction method.Taking SiO₂/PODA as precursor,the suitable reaction temperature for one-step magnesium thermal reaction was 800°C.The reaction product structure was better when the mass ratio of SiO₂ and magnesium powder was 0.8,and the conductivity and charge-discharge performance were better.Pure silicon anode.By adjusting the concentration of resorcinol and formaldehyde solution,a variable thickness SiO₂/RF core-shell structure was obtained,which was used as a precursor.One-step magnesium thermal reduction gave a Si/SiC/C material with a hollow egg-shell structure.It is found that the Si/SiC/C material with a hollow shell structure with a carbon shell thickness of about 10 nm has better performance.After 220 weeks of current density cycling of 0.1 A g^-1,the reversible specific capacity is still higher than 480 mAh g^-1,higher than the graphite negative electrode,the pure silicon negative electrode and the mosaic structure of the silicon carbon material prepared by the two-step method.In this paper,the SiC intermediate layer is introduced into the silicon carbon material by one-step magnesium thermal reaction,which plays a role in stabilizing the material structure.Compared with the two-step magnesia method,it saves energy,and has certain enlightening significance for other directions.