One-Step Synthesis Of Si@C-Based Anode Materials From Nano-SiO₂@C Precursor And Their Electrochemical Performance

Currently,graphite dominates the anode material market for lithium ion battery（LIBs）,but its theoretical capacity is only 372 m Ah g^-1（Li C₆）,which cannot meet the requirements of high energy-power density in electric vehicles and other fields.Si has been recognized as one of the most appealing anode candidates for use in next-generation LIBs due to its natural abundance,environmental benignity,relatively low lithiation potential（<0.4 V vs Li/Li⁺）,and high theoretical specific capacity of 4200m Ah g^-1(Li₂₂Si₅).Despite these amazing benefits,large volume changes（～300%）of Si anode materials during alloying-dealloying will lead to irreversible pulverization,poor electrical contact,unstable solid electrolyte interface layer,thus,fast capacity degradation at high rates.Recent studies demonstrate that the combination of nano-silicon with various carbon species can effectively avoid the drawbacks of pure Si anode material.In this thesis,a medium-temperature molten salt aluminothermic reduction（50mol%KCl+50mol%Al Cl₃）and a low-temperature molten salt ball-milling aluminothermic reduction（26mol%Na Cl+13mol%KCl+61mol%Al Cl₃）were designed to prepare Si@C-based anode material,using rice husk,phenolic resin（PR）and hydrophilic silica（SiO₂）as starting materials.X-ray diffraction,X-ray photoelectron spectroscopy,high resolution transmission electron microscopy,and other methods were used to characterize the phase composition,chemical composition and microscopic morphology and structure of Si@C-based materials.Electrochemical performance tests include cyclic voltammetry,galvanostatic charge-discharge and electrochemical impedance spectroscopy.The research contents and main results are as follows:（1）The Si@C-based anode material was synthesized by one step medium-temperature molten salt aluminothermic reduction using rice husks and KCl-Al Cl₃ as Si/C sources and molten salt.The obtained Si particles inherited the structures and morphologies of the rice husk,such as porous structure,amorphous/crystalline mixture structure.Electrochemical results demonstrated that the Si@C-based anode material showed excellent electrochemical performance,delivering a reversible capacity of 1514m Ah g^-1 at 0.05 A g^-1.After 200 cycles at 0.5 A g^-1,the anode still preserved a reversible capacity of 482 m Ah g^-1.Importantly,the synthetic strategy for the production of Si-based anode materials is not only more facile and cost-effective,but also provides a high-value development strategy for the utilization of abundant agricultural wastes.（2）Inspired by carbonized rice husk,the carbonized rice husk-like structure nano-SiO₂@C was prepared using the hydrophilic silica（SiO₂）and phenolic resin（PR）.Then,nano-SiO₂@C was designed to transform to Si@C-based anode material by a medium-temperature molten salt aluminothermic reduction using Al Cl₃-KCl as molten salts.The effects of different reduction temperature and time on compositions,performances and microstructure of Si@C-based material were studied.After reduction at 400 for 5 h,the Si@C-based material was composed of spherical particles with a size of about 20 nm.Electrochemical results demonstrated that the Si@C-based anode material showed excellent electrochemical performance,delivering a reversible capacity of 2576 m Ah g^-1 at 0.05 A g^-1,615 m Ah g^-1when increasing current density to 2.0 A g^-1.After 200cycles at 0.5 A g^-1,the anode still preserved a reversible capacity of 881 m Ah g^-1,showing a capacity retention of 75.4%.This synthesis strategy can be applied to the large-scale production of high performance silicon-based anode materials considering the cost control of synthesis path and experimental conditions.（3）The nano-SiO₂@C was transformed to Si@C-based materials by a low temperature molten salt and mechanical milling coactivated aluminothermic reduction,using hydrophilic silica（SiO₂）and phenolic resin（PR）as raw materials.The effects of ball milling time and Si@C ratio on the microstructure and electrochemical properties of Si@C-based materials were studied.The reaction product derived from 4 h of the ball-milling assisted aluminothermic reduction is composed of irregular porous agglomerates constructed by ultrafine particles with a particle size less than 50 nm.Electrochemical results demonstrated that the Si@C-based material showed excellent electrochemical performance,delivering a reversible capacity of 2363 m Ah g^-1 at 0.05A g^-1.After 200 cycles at 0.5 A g^-1,the anode still preserved a reversible capacity of933 m Ah g^-1.The use of molten salt combined with high-energy ball-milling significantly reduces the temperature of traditional aluminothermic reduction,improves the reduction efficiency of silicon.The synthesis strategy is simple,high efficiency,and low cost to scale up production.