A. V. Isakov, M. V. Laptev, A. O. Khudorozhkova, A. S. Shmygalev, A. P. Apisarov.
ABSTRACT
A promising task in the energy sector is the development of high-capacity lithium-ion batteries. The use of silicon fibers as an anode will further increase the capacity of LIB. However it becomes necessary to obtain silicon without silicon dioxide on the surface because of since the greatly reduce the capacity of the LIB anode material. In this paper, we propose a method for purifying silicon nanofibers obtained by electrolysis of molten salts from an oxide film in a reaction with ammonium fluoride using a vacuum setup developed by the authors. A technique has been developed for preparing the charge for purification and further analysis. Silicon nanofibers are obtained by electrolysis of a KF-KCl-K2SiF6-SiO2 melt. The chemical composition was studied using X-ray microanalysis combined with electron microscopy and mass spectroscopy. The structure and morphology of silicon deposits was studied by electron microscopy and X-ray diffraction. The fundamental possibility of removing the oxide film from the surface of silicon nanofibers is shown. Si-NH4F mixtures with molar ratios of 1/1, 1/0.123 and 1/0.49 were used for purification. The composition of the Si-NH4F mixture and the temperature at which the purification process is most complete are determined. The thermodynamic probabilities of the occurrence of chemical reactions in the interaction of silicon with ammonium fluoride are discussed. In the course of the study, the mechanism of interaction of ammonium fluoride with the surface oxide layer of silicon nanofibers was established. The interaction of the silicon oxide film with ammonium fluoride under experimental conditions proceeds with the formation of an ammonium hexafluorosilicate phase and easily volatile components, ammonia and water vapor. Mass ratios of silicon and oxygen in purified fibers are determined. It has been established that an increase in this ratio is an indicator of the improvement in the purification of the silicon surface from oxygen. It has been established that the treatment of the cathode silicon deposit by the proposed method using high-temperature vacuum distillation is an effective method for cleaning the silicon surface from oxide-containing compounds formed on its surface.
KEYWORDS
molten salts, potassium hexafluorosilicate, silicon nanofibers, electrolysis, high-temperature evacuation.
DOI: 10.31857/S0235010622060056