## 2 Materials and Methods  
_Melt preparation._ The investigated KCl electrolytes with additions of K\({}_{2}\)SiF\({}_{6}\) and SiO\({}_{2}\) were prepared by mixing individual salts of a chemically pure grade (Reakhim, Russia) and then melting them in a glassy carbon crucible immediately before the experiments. In a previous study [48], it was shown that the purity of KCl used is close to that of the recrystallized salt; therefore, additional purification of the prepared melt from electropositive impurities was not carried out. The K\({}_{2}\)SiF\({}_{6}\) salt was preliminarily purified from oxygen impurities by HF fluorination. For this, K\({}_{2}\)SiF\({}_{6}\) was mixed with NH\({}_{4}\)F and heated in stages to a temperature of 450 \({}^{\circ}\)C according to a previously described procedure [49]. SiO\({}_{2}\) was added to the melt in an amount of 0.34 wt.%. The KCl-K\({}_{2}\)SiF\({}_{6}\) and KCl-K\({}_{2}\)SiF\({}_{6}\)-SiO\({}_{2}\) melts were held for an hour at the operating temperature to establish equilibrium between silicon ions [50; 51]. Then, the melt samples were taken for analysis, and electrolysis was started. _Experimental setup._ Electrolysis tests were carried out in a sealed quartz retort with a high-purity argon atmosphere at a temperature of 790 \({}^{\circ}\)C (Figure 1). A glassy carbon crucible with the melt was placed at the bottom of the retort, which was hermetically closed with a fluoroplastic cap. The inner walls of the retort were additionally shielded with nickel plates to protect against fluorine-containing sublimates. Holes with fittings were made in the cap, in which a working electrode (glassy carbon), a glassy carbon counter electrode, and a silicon QRE were placed. In addition, holes were provided for an additional working electrode, thermocouple, and a tube for gas supply and loading of silicon-containing additives. During the experiments, the temperature was controlled within \(\pm\)2 \({}^{\circ}\)C by a Pt/PtRh thermocouple and a thermocouple module USB-TC01 (National Instruments, Austin, TX, USA). Figure 1: Experimental setup. 1—working electrode; 2—gas inlet; 3—quartz cell; 4 —counter electrode; 5—quasi-reference electrodes; 6—glassy carbon crucible. Electrochemical measurements and electrodepositionTo establish the effect of SiO\({}_{2}\) on the kinetics of silicon electrodeposition, voltammograms were obtained by the cyclic chronovoltammetry. Before measurement, the electrodes were kept in the melt for 30 min to establish a stable (within \(\pm\) 5 mV) potential difference between the working electrode and the quasi-reference electrode (QRE). In order to determine and compensate the ohmic voltage drop in the measuring circuit, impedance spectroscopy and current interruption (I-Interrupt) technique were used. The electrodeposition of silicon was carried out on glassy carbon plates with an area of 2 cm\({}^{2}\) at different cathode overvoltage for one hour. The electrolysis time was selected based on the results of preliminary electrolysis tests [24,25]. For electrochemical measurements and electrolysis, PGSTAT AutoLAB 302N with Nova 1.11 software (MetrOhm, Herisau, Switzerland) was used. Glassy carbon plates were used as anodes. At the end of the electrolysis, the electrode was raised above the melt for 20-30 min in order to remove excess salts included in the cathode deposit. Then, the electrode was raised to the cold zone of the retort and, after cooling in an inert atmosphere, was removed. Separation of the cathode depositThe obtained silicon deposits were washed in acidic (HCI) water solution; the pH of the solution for washing varies in the range of 2 to 4. An ultrasonic homogenizer SONOPULS UW mini 20 was used to disperse the deposits.