Preparation And Modification Of Silicon Carbon Anode Materials For Lithium Ion Batteries

The development of a new generation of lithium-ion battery anode materials with excellent performance is the main research direction to improve the energy density of lithium-ion batteries.Silicon is considered to be the best next-generation lithium battery anode material due to its ultra-high theoretical capacity,non-toxic,harmless and abundant resources.select.However,the silicon-based anode material undergoes huge volume deformation(>300%)during the cycle,resulting in powdering of the active material,destruction of the electrode structure,and loss of effective electrical contact,resulting in a rapid decay of reversible capacity and even serious safety issues that hinder the silicon-based Application and promotion of anode materials.And silicon is a semiconductor material,and its low electronic conductivity leads to poor rate performance.Therefore,the silicon nanostructure design and the method of composite with carbon materials are often used to alleviate the influence of volume deformation on the electrode structure and effectively improve the electrical conductivity of the composite material.This thesis optimizes and improves the electrochemical performance of silicon carbon anode materials through the design of silicon nanostructures,the structure design of the outer carbon layer,and the modification of the outer carbon layer.The main research work is to analyze and discuss the relationship between the structure of the material and its electrochemical performance.details as follows:(1)Preparation of coral-like three-dimensional porous silicon-carbon composite material and research on lithium battery performance:In order to solve the problems of large deformation and low conductivity of silicon-based anode materials during cycling.Silica nanotubes with unique coral-like nanostructures were prepared by a micellar method using ethyl orthosilicate(TEOS)as the silica source,and the effect of different solvent alcohol-water content on the nanostructures was investigated to determine the optimal process for the preparation of silica nanotubes by a micellar method.The reaction kinetics of Mg₂Si solid-phase reduction of Si O₂ was effectively improved by high-temperature magnesium thermal reduction,and CL-Si with high reduction purity(95.95%)and coral-like nanostructure was successfully prepared.The mechanism of the combined effect of silicon nanostructure design and C/r GO double coating modification on the electrochemical performance of the Si-C composites was investigated.It is shown that the produced CL-Si@C/r GO composites still have a reversible capacity of 910 m Ah g^-1 after 100 cycles at a current density of 1 A g-1 and capacity retention of 739.1 m Ah g^-1 even at high current densities(2 A g^-1),demonstrating excellent cycling stability and outstanding multiplicative performance.The process of this study is simple,avoids the hydrofluoric acid pickling and impurity removal steps,optimizes the process parameters of magnesium thermal reduction,and provides a theoretical basis and technical reference for the preparation of special structure silicon-based anode materials by magnesium thermal reduction.(2)Structural and electrochemical properties study of nitrogen-doped graded porous silicon-carbon composites prepared by biomass fermentation:To solve the problems of high cost and complicated process for the preparation of high-performance silicon-carbon anode materials.Nitrogen-doped silicon carbon-composite(Si/N-PC)with graded porous structure was designed and prepared by anaerobic fermentation with CTAB as the carbon source and cationization of silicon particles on the surface to deposit uniformly in wheat flour under the action of electrostatic force,and controlled fermentation temperature and time.Its porous carbon structure and nitrogen doping in the form of pyridine nitrogen,etc.,synergistically enhance the cycling stability and lithium-embedded activity of the composite.In the half-cell test,696.1 m Ah g^-1 reversible capacity was retained after300 cycles at a current density of 1 A g^-1,which was much better than that of the Si/N-PC composite(164.6 m Ah g^-1)obtained without anaerobic fermentation treatment.It even retains 946.1 m Ah g^-1 at2 A g^-1,demonstrating excellent cycling stability and multiplicative performance.The variable-sweep CV test found that the charge storage is controlled by the capacitance,which also demonstrates the excellent de/embedded lithium performance.The process of this research is simple,using renewable biomass materials as carbon sources,and preparing silicon-carbon composite materials with excellent electrochemical performance through simple biomass fermentation treatment,which provides solutions for the preparation of low-cost and high-performance silicon-carbon anode materials.(3)Research on the preparation and electrochemical performance of nitrogen-sulfur co-doped silicon-carbon composites:to solve the problem of limited lithium-ion diffusion when covered with thick carbon layers,and low electrochemical lithium insertion activity of some Si.The effects of nitrogen and sulfur co-doping modification on the electrochemical properties of silicon-carbon composites were investigated using thiourea as the nitrogen and sulfur sources.The effect of thiourea addition on the structure and electrochemical properties of the composites was also investigated.The results showed that the surface of the coated carbon layer gradually evolved from smooth to folded porous structure with the increase of thiourea addition,the charge transfer resistance of the nitrogen-sulfur co-doped composites decreased and the disorder of the carbon layer increased,and the electrochemical properties of the samples were optimal at 1.5 g of thiourea addition.In the half-cell test,the capacity of 897.2 m Ah g^-1 is still maintained at a current density of 2 A g^-1,and 886.7 m Ah g^-1 is still maintained at a current density of 1 A g^-1 for 200 cycles,which indicates proper nitrogen Sulfur double doping can effectively improve the electrochemical performance of silicon-carbon composites.The process of this research is simple,and it provides a solution to the problems of the silicon particles in the thick-coated carbon layer that lack lithium-intercalation activity under the condition of high electronic conductivity,resulting in a significant decrease in capacity.