Studies Of Formation Of Silicon Nanowires By Electroreduction Of Porous Nano-Ni&NiO/SiO₂ Pellets In Molten Salts

Silicon nanowires are one of the most promising silicon-based nagative materials for lithium-ion batteries.In this paper,nickel formate with water solubility is used as a catalyst precursor,which can be decomposed into two kinds catalyst(nickel nanoparticale and nickel oxide nanoparticale)in different atmospheres(N2/Air)and prepared Ni/SiO2 and NiO/SiO2 pellets.In addition,the amount of the catalyst in the pallets and the stability of the catalyst in the molten CaCl2 were investigated and straight silicon nanowires with higher purity were prepared.The structural characteristics and the electrochemical properties of silicon nanowires were analyzed by XRD,FESEM,HRTEM and SEAD.The purpose of this study is to provide theoretical guidance for the engineering technology of preparing silicon nanowires by molten salt electrolysis.In this paper,the properties of the catalyst precursor nickel formate was studied.The preparation process of Ni(HCOO)2/SiO2 pellet without nickel sequestration phenomenon was determined by using nickel formate dissolved in water and insoluble in ethanol during the mixing and drying process.According to the behavior of nickel formate TG-DTA,the sintering process of Ni(HCOO)2/SiO2 pellet was established.The pellet was sintered in nitrogen and air atmosphere for 1 h at 900 ?,and two kinds of pellets(Ni/SiO2 and NiO/SiO2)were obtained for subsequent electrolysis experiments.The different nickel content(0.40 wt.%,0.80 wt.%,1.20 wt.%,1.60 wt.%)of Ni/SiO2 porous pellet was investigated in the molten CaCl2 at 900 ? for electrolysis experiments.When the nickel content is 0.80 wt.%,a large number of straight silicon nanowire with a diameter of 75 to 250 nm and a length of 2 to tens of micrometers are obtained by electrolysis for 1 h.Furthermore,the optimal nickel content of Ni/SiO2 pellets were electrolyzed under the same conditions for different electrolysis time.When the electrolysis time was over 1 h,the amount of silicon nanowires in the electrolysis products were decreased rapidly,and mainly existed in the micron-sized granular.The stability of the Ni/SiO2 pellets in the molten CaCl2 was also studied.With the prolongation of immersion time,the particle size of nickel particles in the electrode continued to increase,which resulted in very few straight silicon nanowires in the electroreduction products.The effects of different nickel oxide content of NiO/SiO2 porous pellet on the electrolysis products were studied.When the nickel oxide content is 0.51 wt.%,a large number of straight silicon nanowire with a diameter of 50 to 160 nm were prepared at 1.5 V in a molten CaCl2 at 900 ? for 1h.The electrolysis time was prolonged to 10 h,the morphology and structure of the electrolysis product remained stable.The NiO/SiO2 pellet was immersed in 900 ? molten CaCl2 for 10 h.It was found thatthe stability of catalyst nanosized nickel oxide particles in molten salts was the key factor to keep the electrolytic products as straight silicon nanowires for a long time.The electrochemical performance of electrolysis products under optimal electrolytic conditions were characterized,and the first lithiation capacities of the sample is 3319.5 mAh/g,and the initial coulombic efficiency of the sample is 88.00%.