Preparation Of Silicon-based Composite Materials By Molten Salt Electrochemistry And Research On Their Lithium Storage Performanc

This paper aims to address the issue of low electrical conductivity and volume expansion of silicon during the lithium storage process,which leads to poor cycle stability and capacity decay.With silicon nanosizing,alloying and compounding as the research guide,nanoscale silicon wires were obtained by molten salt electrolysis of SiO₂,the effects of various electrolysis parameters on the phase and morphology of electrolysis products were investigated,and the phase transfer and morphology evolution of SiO₂during electrodeoxygenation.The addition of metal Ag to the SiO₂ cathode powder was explored to enhance the electrical conductivity of the cathode and accelerate the electrode deoxidation rate of SiO₂ to obtain Ag-Si composites with nanoscale linear structures,and the effects of silver content and electrolysis temperature on the electrolysis of Ag/SiO₂ products,then the growth and nucleation mechanism of metallic silver-catalyzed silicon nanowires was proposed.Cu₂O mixed in SiO₂ powder and electrolytically prepared to obtain composites with Cu₃Si particles attached to silicon nanowires,the effects of copper content,electrolysis time and electrolysis voltage on the electrolytic Cu₂O/SiO₂ products were investigated.Finally,the above three silicon-based materials were assembled into batteries as anode materials for lithium-ion batteries and tested for electrochemical performance.The specific work and results are as follows:Under the CaCl₂-Na Cl molten salt system,nano-spherical SiO₂ prepared by sintering was used as the cathode,graphite rod as the anode,and molybdenum rod as the conducting collector.The optimal preparation conditions of SiO₂ particle size of 170 nm,electrolysis temperature of 850°C,electrolysis voltage of 2.6-2.8 V（two-step method）,and electrolysis time of 6 hours,bent-like silicon nanowires with diameters of 200-500nm were obtained.The electrochemical performance test was carried out with silicon nanowires as the anode material,and the results showed that the first discharge specific capacity was as high as 3172 m Ah·g^-1 at a 400 m A·g^-1 discharge current density.However,the first coulomb efficiency was only 67.71%,and the capacity retention rate was 13.58%after 100 cycles.The reversible specific capacities of the materials were1851.6,1237.8,717.8,411.6和708.8 m Ah·g^-1 at current densities of 200,400,800,1600 and 400 m A·g^-1.The Ag/SiO₂ mixed cathode powder was prepared by introducing silver nanoparticles into SiO₂ using a simple silver mirror reaction.Under the optimized preparation conditions of 0.17 g of silver per 1 g of SiO₂,electrolysis temperature of850°C,electrolysis voltage of 2.6-2.8 V,and electrolysis time of 6 h,two types of linear structures of silicon nanowires were obtained.One type was curved silicon nanowires with spherical silver-silicon alloy at the growth end,while the other type was long straight silicon nanowires with silver nanoparticles attached to the surface.A proposed electrochemical nucleation mechanism of silver-catalyzed silicon involves two nanowire structures growing under two three-phase reaction interface models（3PIs）of Mo/SiO₂/CaCl₂-Na Cl and Ag/SiO₂/CaCl₂-Na Cl.As the reaction progresses to the interior,the 3PIs are transformed into Si/SiO₂/CaCl₂-Na Cl and Ag-Si/SiO₂/CaCl₂-Na Cl,respectively.The silicon nuclei stack and grow at the two new reaction interfaces,resulting in two structures of silicon nanowires.After 100 charge-discharge cycles of Ag-Si nanowires,the discharge specific capacity was 694.2 m Ah·g^-1,with a capacity retention rate of 25.73%.These results suggest that the cycling stability of Ag-Si negative electrode material is enhanced.Cu（NO₃）₂/SiO₂ reacted with ammonia water to get Cu（OH）₂/SiO₂,then get Cu O/SiO₂ through hydrothermal reaction,and finally get Cu₂O/SiO₂ by sintering.Under the optimal experimental conditions of a Cu/Si molar ratio of 3:8,electrolysis temperature of 850°C,electrolysis voltage of 2.4-2.6 V for 3 h,cross-distributed linear silicon nanowires were obtained,and Cu₃Si alloy particles were enriched on the silicon nanowires.At 2.6-2.8 V or electrolysis for more than 6 h,Cu₃Si bulk with a thin film-like structure stacking was obtained.The first discharge specific capacity of copper-silicon nanowires was 2231.6 m Ah·g^-1 with a first coulomb efficiency of 88.04%.After100 charge-discharge cycles,the discharge specific capacity was 1007.4 m Ah·g^-1 with a capacity retention rate of 45.14%.These results indicate that copper-silicon nanowires have good cycling stability.