## 1 Introduction  
Currently, silicon and silicon-based composite materials are widely used in microelectronics, solar energy devices, as well as in the manufacture of portable energy storage devices [1, 2, 3]. In particular, a new lithium-ion battery (LIB) with a silicon-based anode is being actively developed. The theoretical capacity of such an anode (4200 mAh\(\cdot\)g\({}^{-1}\)) is an order of magnitude higher than the widely used graphite anode [3, 4]. However, during cycling, silicon is subject to huge volumetric expansion (up to 400% [5]), which leads to mechanical destruction of the electrode and significant loss of the original capacity. To avoid this, it is recommended to use anodes based on nanosized silicon particles. Nanosized silicon cannot be obtained by the existing methods of the carbothermal reduction of quartz to silicon with subsequent purification of the silicon by hydrogenation-dehydrogenation [6]. The use of nanosized silicon particles, mainly fibers and tubes within composite materials, can significantly reduce the problem of volumetric expansion. Suitable polymer binders also reduce mechanical degradation of the electrode, but they do not affect the silicon expansion [7, 8]. Another method that can improve silicon's performance is the coating (mainly carbon based) of silicon particles and creation of a \"core-shell\" structure [9, 10]. Doped silicon anodes have also shown improved electrochemical performance and cyclic resistance in comparison with pure silicon [11, 12, 13]. Methods for preparing silicon nanofibers can be divided into two groups. The first group comprises methods based on removing excess material-laser ablation [14], ion etching [15], chemical etching [16; 17]. These methods allow one to obtain high-purity components with the required sizes. However, they require expensive equipment, catalysts (gold is most often used) and are poorly suited for industrial scale production. The second group of methods includes silicon deposition on the substrates. Among these, the most commonly used method is the vapor-liquid-crystal deposition [18; 19]. Silane is usually used as a source of silicon. The disadvantages of these methods also include the use of gold as a catalyst and the difficulty in scaling and hardware design of the process. To obtain silicon suitable for microelectronics, several stages of purification are needed, one of which is represented by a group of methods based on zone crystallization from molten silicon [20]. One of the cheapest and promising methods for producing nanosized silicon is electrolytic reduction from molten salts. To date, many studies have been carried out aimed at the electrolytic production of silicon from various molten electrolytes with K\({}_{2}\)SiF\({}_{6}\), Na\({}_{2}\)SiF\({}_{6}\), SiO\({}_{2}\), and SiCl\({}_{4}\) additives in a temperature range mainly from 550 \({}^{\circ}\)C to 750 \({}^{\circ}\)C [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39]. All these studies show the fundamental possibility of using electroreduction for the production of silicon deposits of various morphologies, including continuous coatings up to 1 mm thick, submicron films (0.5-1 \(\upmu\)m), micro-sized dendrites, nano- and micro-sized disordered fibers. One of the most promising electrolytes for silicon production is the KCl-KF melt with a KF molar fraction of up to 66% [27; 28; 29; 30; 31], which is a good solvent for K\({}_{2}\)SiF\({}_{6}\) and SiO\({}_{2}\) and is water soluble. However, this system also has a number of disadvantages, including relatively high aggressiveness of KF to reactor materials, the need to remove impurities such as H\({}_{2}\)O and HF from KF when preparing a molten KCl-KF mixture, and a good solubility of oxides. Despite the positive results of the work performed, all of the above factors can impede the deposition of high-purity silicon, especially without oxygen inclusions.