| The development of green and renewable energy is the focus of the current research.Photovoltaic solar energy is a relatively ideal new energy with the characteristics of rich reserves,clean and environmental protection.Silicon,as the main material of photovoltaic cells,can convert solar energy into electrical energy,which plays an important role in the photovoltaic industry.The thesis uses NaF-LiF(molar ratio 40:60)as the electrolyte(solvent),SiO2 and Na2SiO3 as the active material(solute),silver flakes as the cathode and graphite as the anode.The process and behavior of electrodeposited silicon were studied by molten salt electrolysis.The electrochemical behavior of Si(Ⅳ)on the cathode was investigated by electrochemical methods such as cyclic voltammetry,chronoamperometry and square wave voltammetry using a three-electrode system.The electrodeposited product was characterized by XRD,SEM,Raman,and other characterization methods.Cyclic voltammetry tests were carried out on molten salt systems 39.8NaF-59.8LiF-0.2SiO20.2Na2SiO3(mol%)and 39.8NaF-59.8LiF-0.4SiO2(mol%)at 750℃.The results show that the reductive peak potentials of the two molten salt systems are similar.The results show that the reduction potentials of silicon in the two molten salt systems are similar.The precipitation process of silicon proceeds in two steps:Si(Ⅳ)→ Si(Ⅱ)at a potential of-0.9 V(vs.Pt),and a potential of-1.3 V(vs Pt)when Si(Ⅱ)→Si.In the 39.8NaF-59.8LiF-0.2SiO2-0.2Na2SiO3(mol%)molten salt system,the electrochemical reduction of Si(Ⅳ)is an irreversible process and the reaction process is controlled by diffusion.The diffusion coefficient of D1 is 1.18×10-8 cm2/s(Si(Ⅳ)→Si(Ⅱ)),D2 is 1.22×10-6 cm2/s(Si(Ⅱ)→Si).The number of transferred electrons during the Si(Ⅳ)reduction process was calculated by square wave voltammetry of the 39.8NaF-59.8LiF-0.2SiO2-0.2Na2SiO3(mol%)molten salt system.The potentiostatic electrolysis experiments were carried out on the 39.4NaF59LiF-0.8SiO2-0.8Na2SiO3(mol%)and 39.4NaF-59LiF-1.6SiO2(mol%)molten salt systems using a three-electrode system with a silver plate as the cathode.In the XRD test,silicon can be obtained when the electrolytic potential of the two molten salt systems was lower than-1.0 V(vs.Pt),which is consistent with the cyclic voltammetry test results.Raman spectroscopy shows that crystalline silicon can be obtained by electrodeposition of two molten salt systems.The Raman shift has two broad peaks at 300 and 955 cm-1,corresponding to the scattering of the transverse optical two-phonon peak and the transverse acoustic two-phonon peak,respectively,indicating that nanosized crystalline silicon exists in the product obtained by electrodeposition.Compared the microstructures of electrodeposited silicon between 39.4NaF59LiF-0.8SiO2-0.8Na2SiO3(mol%)and 39.4NaF-59LiF-1.6SiO2(mol%)molten salt systems,it can be found that when only the active material SiO2 is present in the molten salt system,the electrodeposited silicon showed a lamellar structure.When Na2SiO3 and SiO2 are simultaneously added to the molten salt system,the electrodeposited product exhibited both lamellar and nano-sized spherical structures.It indicates that the spherical nano-silicon is derived from sodium metasilicate.Meanwhile,the microstructures of electrodeposited silicon is also affected by the electrolytic potential.Si@C,a composite of silicon and carbon,is used as the anode of lithium-ion battery.The initial charge specific capacity is 1260 mAh/g,but the battery capacity is severely atteuated.After 100 cycles,the specific capacity of the battery is maintained at 200 mAh/g. |
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