Chloride-based electrolytes have shown promising results in electrodepositing high-purity silicon. Some frequently studied chloride-based melts for Si deposition are CaCl₂, KCl, NaCl, LiCl-KCl, and NaCl-KCl. Majority of the studies conducted to produce Si using SiO₂ as a raw material used CaCl₂-based electrolytes. CaCl₂ melts have advantages such as high solubility in water (Garrett, 2004), low corrosivity (Janz and Tomkins, 1979) and high oxygen ions solubility (Wenz et al., 1969). Moreover, CaCl₂ is non-toxic, inexpensive and widely available. Si electrodeposition is possible at a low temperature (500°C) in eutectic LiCl-KCl-CaCl₂ melt (Yasuda et al., 2005a). The low-temperature electrodeposition process reduces energy consumption but lowers the process rate due to slower oxygen diffusivity in a solid phase.

Nohira et al. (2003) reported that the solid SiO₂ can be electrochemically reduced to crystalline Si in CaCl₂ molten salts. Electrons are transferred to the SiO₂ electrode through a pinpoint electrical contact by conducting metal (Mo). When the potential applied is more negative than the reduction potential of SiO₂, the electrochemical reaction occurs, and the following reaction is expected to take place: S i O 2 s + 4 e − → S i s + 2 O 2 − (2)

As the deoxygenation of SiO₂ results in Si formation, the gaps formed between the Si atoms are filled with constituents of the molten salt. The new conducting medium is Si (transferring electrons to SiO₂), which has good electrical conductivity (50 Ω⁻¹cm⁻¹at 850°C) at high temperatures. Three-phase interface between SiO₂, molten salt and Si and continuous formation of Si takes place. Si formed is amorphous and transforms into crystalline Si at high temperatures through the bond-breaking process (Yasuda et al., 2005b).

Silicon electrodeposition can be achieved by reducing SiO₂ nanoparticles (NP) in CaCl₂ molten salts (Cho et al., 2012). Silicon layers were fabricated on Mo substrate in molten salts by directly adding and dissolving SiO₂ NP at 850°C. The deposited Si layer had high purity and was crystalline, but the deposited Si was not a continuous film. Toba et al. reported the direct electrolytic reduction of SiO₂ in molten CaCl₂ at 850°C to produce SOG-Si (Toba et al., 2013). The concept they proposed is similar to the Hall-Heroult process for aluminium production. Granular/powder SiO₂ is electrochemically reduced to Si, deposited at the cathode, and slurry containing Si and molten CaCl₂ is continuously tapped (Figure 1A). Crystalline Si was produced with applied potentials between 0.5 and 0.7 V (vs. Ca²⁺/Ca). During the electrolysis with potentials 0.5 and 0.6 V, the rate-determining step was the diffusion of oxygen ions through CaCl₂ melt in the produced Si layer on the unreduced SiO₂ core. Experimental results suggest that it is possible to deposit a continuous SOG-Si layer in CaCl₂ melt. During CaCl₂-SiO₂ electrolysis with a graphite foil cathode and graphite anode, the cell voltage determines the morphology of the Si deposited (Xie et al., 2018). For instance, at cell voltage below 2.0 V, Si nanowires with diameters between 50 and 100 nm are prepared. Under 2.2 V, Si nanoparticles are deposited on substrates and between 2.4 and 2.6 V, a dense Si film is deposited. In addition to cell voltage, the substrate also plays a vital role in depositing Si films. A dense Si film (a thickness of 3.5 µm) is deposited on a graphite substrate with silicon carbide acting as an interface between the Si film and substrate. Meanwhile, a porous (a thickness of 7 µm) Si film is deposited on the Ag substrate, resulting in the crushing of Si films due to porosity (Zhao et al., 2016).

The Si electrodeposition can be enhanced by adding CaO as a dopant to the molten CaCl₂ (Yang et al., 2017). The cyclic voltammetry curves confirm that the cathode peak associated with Si reduction intensifies with the addition of CaO 1 mol% (Figure 1B). Further, a dense crystalline layer of Si with a thickness of 35 µm was successfully deposited on a graphite substrate during potentiostatic electrolysis at 0.7 V. Optimal electrolyte composition for Si electrodeposition was reported as CaCl₂−(4.8 mol%) CaO−(3.9 mol%) SiO₂. The addition of CaO can accelerate the Si electrodeposition as CaO helps in the continuous ionisation of silicon dioxide to form silicate ions (SiO₃ ²⁻, SiO₄ ⁴⁻, etc.) (Zou et al., 2019; Zou et al., 2020; Li et al., 2022). Zou et al. deposited Si films with a purity of 99.99989 in CaCl-CaO-SiO₂ melt (Zou et al., 2019). The presence of CaO enables the following reactions (3–5): x S i O 2 + y C a O C a 2 + , O 2 −   → C a y S i x O 2 x + y y C a 2 + , S i x O 2 x + y 2 y − (3) S i x O 2 x + y 2 y − + 4 x e − → x S i + 2 x + y O 2 − (4) x S i O 2 + y O 2 − → S i x O 2 x + y 2 y − (5)

The melting point of pure CaCl₂ is 772°C, and electrolysis at such high temperatures would need more energy, resulting in corrosion of the electrolysis cell and thermal shock of electrodes. Including other chloride salts in the CaCl₂ melt could substantially reduce the liquidus temperature of the mixture. For instance, Sakanaka et al. (2017) reported electrolytic deposition of Si on Mo substrate in a eutectic BaCl₂-CaCl₂-NaCl-SiO₂ mixture at 650°C. For 1 h of potentiostatic electrolysis at −1.60 V, a dense Si film was deposited with a thickness of 1 µm. However, with an increase in electrolysis time to 3 h, a porous and less crystalline Si layer was obtained. The powdery and dendritic deposits with prolonged electrolysis time as some of the deposited Si interacts with Si(IV) and forms divalent silicon ions, which can be ascribed to the following reactions (6, 7): S i I V + 4 e − → S i (6) S i + S i I V → 2 S i I I (7)

At low temperatures, SiO₂ has low solubility in chloride melts which would affect the morphology of Si films. CaSiO₃ could be used as an alternative Si precursor in low-temperature chloride melts (Figure 1C). The CaSiO₃ easily dissolves in this chloride-based melt and generates Ca²⁺ and SiO₃ ²⁻ ions. Dong et al. reported electrolytic production of Si nanowires in CaCl₂-MgCl₂-NaCl-CaO using CaSiO₃ as raw material at 650°C (Dong et al., 2017). The Si nanowires production had a high yield and were crystalline but only had a purity of 99.58%, which is metallurgical grade.

Si nanotubes can be electrodeposited by co-reduction of SiO₂ and AgCl on Ni substrate in NaCl-CaCl₂ (Weng et al., 2020). Initially deposited Ag on the Ni will facilitate the formation of liquid Ag-Si intermetallic, promoting Si nanowires’ growth. Ag and Si are separated during the cooling process, where Ag is deposited on Ni. Si nanowires can also be electrodeposited in KCl-containing K₂SiF₆ at temperatures below 800°C (Trofimov et al., 2022). The morphology of the Si nanowires highly depends on the overpotential applied during the potentiostatic electrolysis. Adding CsCl to molten KCl-containing K₂SiF₆ can reduce the liquidus temperature of the melt and the diameter of Si nanowires. The decrease in cathodic current density would increase the diameter of Si nanowires (Gevel et al., 2022a). In KCl-K₂SiF₆ melts, the Si(IV) reduction process comprises a reversible 4-electron process, Si nanofibers with a diameter of 200 to 300 nm and a purity of 99.99% can be deposited (Gevel et al., 2022b). The main advantage of using the KCl-K₂SiF₆ system is that the anionic composition does not change. That results in the stability in energy characteristics of Si-containing electroactive ions and control of the morphology of Si deposits (Gevel et al., 2021).

The Si electrodeposition in various molten fluorides, like LiF-KF (Cohen and Huggins, 1976; Rao et al., 1981) and LiF-NaF-KF (Elwell and Rao, 1982) containing K₂SiF₆ precursor, has been studied since the 1970s. One major advantage of fluoride melts is the high solubility of SiO₂ or other Si precursors, enabling the high thickness of deposited Si films. For instance, Si film deposited by galvanostatic electrolysis for 3 h at 40 mAcm⁻² and 800°C in eutectic LiF-KF melt with K₂SiF₆ (5 mol%) has a thickness of 150 µm (Haarberg et al., 2013). The Si deposition was a two-step process, i.e., Si(IV)→Si(II)→Si. A loss of current efficiency (ranging from 85% to 95%) is related to the existence of Si(II) (reaction 6). Similar results were reported by Boen and Bouteillon, where the Si depositions involve Si(IV) and Si(II) species in LiF-NaF-KF melts containing Na₂AlF₆ (Boen and Bouteillon, 1983).

Bieber et al. reported that of all the eutectic fluoride melts containing SiF₄(g), Si(IV) has the best stability in the NaF-KF mixture, and Si(IV) volatility increases with an increase in the acidic nature (most basic to most acidic: NaF–KF < LiF–KF < NaF–MgF₂ < NaF–CaF₂ < LiF–NaF < LiF < LiF–CaF₂) of the melt (Bieber et al., 2011). Eutectic NaF-KF melts containing Na₂SiF₆ have been used in Si electrodeposition on various substrates (Ag, Ni, and C) and operating conditions (Bieber et al., 2012). Out of films formed on Ag, Ni, and C, Si films deposited on C were the most coherent and had the highest purity, and a SiC interface was observed between the Si film and C. At high current densities, the limiting diffusion rate of Si-ions strongly influences the Si film growth, leading to dendritic and rough deposits.

The solubility of SiO₂ is enhanced by adding O²⁻ ions in the fluoride melts, similar to that of chloride melts. Suzuki et al. (2019) investigated the influence of Li₂O addition to KF, LiF-KF, and LiF-NaF-KF molten salts on SiO₂ electroreduction . Compared to Si films deposited in melts without LiO₂, Si films deposited in melts containing Li₂O were around 6 to 23 times thicker and were obtained with better current efficiency, depending on the applied potential during the electrodeposition. The phenomenon is due to the capability of O²⁻ (sourced through Li₂O) to break the Si-O-Si bond of SiO₂ and complex silicate ions (e.g., Si₂O₅ ²⁻) to promote the formation of easily reduced silicate ions (e.g., SiO₃F³⁻). The presence of multiple Si ions combined with fluoride, oxyfluorides, and oxides would negatively affect the current efficiency of Si deposition. For instance, Sakanaka and Goto (2015) electrodeposited Si films in eutectic NaF-LiF-KF melt with a current efficiency of 10%. Si(IV) ions in multiple forms would have different reduction potentials, and when broader potentials are applied for electroreduction, alkali metals tend to co-deposit along with Si.

In BaF₂-CaF₂-SiO₂ melts, the electrodeposition of Si on Mo substrate was a one-step process, i.e. Si(IV)→Si (Hu et al., 2013). SEM examination of the deposits after 8 h of galvanostatic polarisation at −0.118 Acm⁻² revealed that an intermetallic phase MoSi₂ (thickness <10 µm) was formed, which acted as an interface between the Si layer (thickness >50 µm) and the Mo substrate. Even though a thick Si film is deposited in this melt, the process is performed at very high temperatures (1,300°C), which requires a lot of energy and harms the electrolysis setup.

For Si electrodeposition, molten chloride-fluoride salts are better electrolytes than individual chloride and fluoride melts. It is because the Si films deposited in chloride melts have a thickness of less than 10 µm and the solubility of Si species is low. On the other hand, fluoride melts are not water soluble, and the salts adhering to the Si films are difficult to remove. Moreover, molten fluorides are corrosive and could damage the reactor walls (Xu and Haarberg, 2013). KF-KCl mixtures are considered promising electrolytes for Si electrodeposition as the mixture has sufficient thermal stability, is less aggressive than molten fluorides, is water-soluble (until KF mol% is 66%) and has good Si species solubility at low temperatures (at 650°C) (Zaikov et al., 2013; Maeda et al., 2014; Maeda et al., 2015).

Si(IV) reduction is a one-step process in KF-KCl-K₂SiF₆ melt, where the Si(IV) reduction is governed by a quasi-reversible-reversible electron transfer reactions mechanism (Maeda et al., 2015). Zhuk et al. (2017) deposited a crystalline silicon film (purity 99.9%) on Ag, C, and Ni substrates in KF-KCl melts containing K₂SiF₆. The authors confirm that SiC was not formed on graphite during the Si deposition, whereas the presence of SiC (thickness of 10 µm) intermetallics was reported by Bieber et al. (2012). Adding KI to KF-KCl melt would reduce the melt’s aggressiveness and liquidus temperature but decrease the electrical conductivity (Laptev et al., 2021). However, the reduced electrical conductivity would contribute to forming more compact and adhesive Si films (Laptev et al., 2020). It has been stated that during the electrodeposition on energetically non-homogeneous surfaces, an increase in the specific electrical resistance of the melt can cause the smoothing effect at the electrodeposition from the melt (Laptev et al., 2020).

The semiconductor characteristics of Si films deposited can be modified based on the Si precursor metallic dopant added to the melt. For instance, Si films electrodeposited in KF–KCl–K₂SiF₆ molten salts exhibit n-type semiconductor characteristics, while Si films deposited in KF–KCl–SiCl₄ melts exhibit p-type semiconductor characteristics (Yasuda et al., 2021). The difference in the semiconductor characteristics can be attributed to the different impurities present in the Si films. Both Si films had a purity of 99.99% (4 N). A dense crystalline Si film (thickness up to 60 µm) has been deposited in KF-KCl-K₂SiF₆ melts containing Sn-dopant (0.020–0.035 wt%) (Peng et al., 2018). Co-deposited Sn along with Si could induce the lateral growth of Si thin films. A four-step mechanism is involved in the Si film formation: nucleation, island formation, island aggregation and film formation (Figure 1D). The tin-assisted Si film deposits show n-type semiconductor characteristics.

Li et al. (2018) investigated the electrochemical behaviour of Si(IV) on W and Mo (Li et al., 2019) electrodes in CaCl₂-CaF₂-CaO-SiO₂ melts and found that on both the electrodes Si(IV) reduction involves one-step reduction mechanism and follows an instantaneous nucleation mode. MoSi₂ intermetallic phase is formed when the Si films are deposited on the Mo electrode (Li et al., 2019), similar to what Hu et al. obtained during the electrodeposition in fluoride salts (Hu et al., 2013).

The morphology of Si films also depends on the type of alkali melt used during the electrolysis. Norikawa et al. (2022) studied the electrochemical behaviour of Si(IV) in MF-MCl-M₂SiF₆ melts (where M = Li, Na, K, Cs) at 800°C. For Li-systems, it was noted that no peaks related to Si were observed during the cyclic voltammetry, which is attributed to the formation of Li-Si intermetallics. In Na-systems, whisker-like Si films were deposited, while in K and Cs systems, smooth and adherent films were deposited on Ag substrates. Further examination using short-term galvanostatic polarisation revealed that the cathodic potential rapidly changed in Na-system. At the same time, the cathodic potential remained constant in K-systems, which means side reactions are taking place in Na-system.