Metal-assisted chemical etching (MACE) is a typical top-down synthesis method, which utilizes bulk silicon (such as doped silicon wafers as raw materials), and porous Si-based materials are obtained via the galvanic displacement between metal particles such as Ag and Si in HF solution. (Bai et al., 2012) The general MACE process assisted by the introduction of silver particles via adding AgNO₃ solution to the solution, was described by the following two simultaneous electrochemical reactions: 4 A g + + 4 e − → 4 A g S i + 6 F − → [ S i F 6 ] 2 − + 4 e −

As for silicon wafer raw materials, a high degree of n-/ p-doping can provide defect sites on the surface of the materials to facilitate the formation of porous structures. Ge et al. synthesized porous silicon nanowires by MACE of boron-doped silicon wafers. (Ge et al., 2012) The experiments indicated that the doping was a necessary condition for the formation of porous structures. If the silicon wafer were not doped, the obtained silicon would be solid silicon nanowires rather than porous structures. The obtained anode materials with porous silicon nanowire, which possessing with both 8 nm pore size and wall thickness, showed good long-term cycling performance as well as a reversible capacities of 1,100 mA h g⁻¹ at the rate of 4.5 C after 250 cycles. Meanwhile, they set up a mathematical model to simulate the effect of porosity and pore size on the structural stability, and concluded that high porosity and large pore size could help to stabilize its structures, which is consistent with the experimental results.

Compared with silicon wafers, metallurgical grade silicon (MG-Si) is an attractive material option as a low-grade silicon source and has been extensively studied in recent years due to its cheap price (∼1,000 $/ton) and natural abundance. Jin et al. reported a scalable process involving ball milling and modified MACE. (Jin et al., 2015) Different from traditional MACE, they adjusted the dispersion and hydrophobicity of silicon powder via adding ethanol into the etching solution to obtain porous nanostructures with different morphologies. At the same time, the etching process was accelerated by adding H₂O₂ solution, and silicon particles with micro-pores were obtained. The obtained materials with micro/nano-sized structures exhibited excellent electrochemical performance (capacity retention of 80% after 100 cycles) due to their micro-sized porous structures.

The etching-type methods for preparing porous silicon also include stain etching and electrochemical etching. Stain etching is a simple and scalable method that is widely used in the field of solar energy, (Dimova-Malinovska et al., 1997) but its practical applications in LIBs is rarely reported, because its limited control over reaction parameters results in an inability to selectively control the morphologies. Electrochemical etching is another etching-type method for preparing porous Si-based materials, and silicon wafers are also generally employed as raw materials. Different from MACE’s method of etching silicon wafers with the assistance of noble metal particles, electrochemical etching is to etch silicon wafers through HF solution at a constant current density, and the pore size, pores and other structural parameters of the materials can be varied by changing the current density. Thakur et al. reported a simple method to prepare thin films of macro-porous silicon, which can achieve a discharge capacity of 1,260 mA h g⁻¹ when combined with pyrolytic polyacrylonitrile. (Thakur et al., 2012) However, compared with MACE, this method has drawbacks in controlling the material morphologies and scaling up production, which couldn’t be widely utilized in practical applications.

Magnesiothermic reduction method possesses the potential to expand the production of porous silicon materials because of its ability to maintain the original structures of the template. Unlike conventional carbothermic reduction, the reaction temperature of this method is relatively low (500–950°C), which is much lower than the melting point of silicon (1,414°C). (Entwistle et al., 2018) The reactions that take place in the process are as follows: S i O 2 + 2 M g → 2 M g O + S i S i + 2 M g → M g 2 S i

Magnesium oxide and silicon are formed by a simple reduction reaction of silicon dioxide with magnesium, and the silica near the magnesium source will be reduced to form Mg₂Si by-product during the reaction. All by-products including excess magnesium, unreacted silicon dioxide, magnesium oxide and Mg₂Si were removed after etching by HCl and HF solution, respectively, resulting in porous silicon-based materials with specific template structures.

The reaction utilizes silica as template and starting material. It is known that silica is abundant in nature. (Zhang et al., 2016)Among various biological sources, rice husks are considered as potential silicon sources for large-scale production of porous silicon due to their low cost and recyclability. Jung et al. converted silica in rice husk into silicon with excellent electrochemical performance as anode materials, indicating that rice husk is a potential silicon source for large-scale applications. (Jung et al., 2013) Jiao et al. prepared porous Si-based materials via utilizing rice husks as silicon precursors by the same process, and uniformly inserted them into graphene by combining a condensation reaction and subsequent thermal treatment. (Jiao et al., 2016) It was found that the composites exhibited good cycling stability (830 mA h g⁻¹ at 1 A g⁻¹ after 200 cycles) and rate performance. The performance improvements stem from the buffering effect of graphene and the porous structures on volume expansion.

Although the above methods have the potential to expand production due to their low cost, however, silica materials derived from biological resources generally contain carbon, and silicon carbide may be formed in places with high local temperature during the magnesium reduction processes, leading to applications restricted. Besides, some impurities are generally present in the natural silica precursors, and additional processes are required to remove the impurities, resulting in increased cost. Artificial silica has also received extensive attention due to its high purity and controllable size. Nowadays, the most widely used artificial silica is from tetraethylorthosilicate (TEOS), because this method can obtain nanoparticles (about 30–60 nm) with excellent monodispersity. (Xiao et al., 2015) Zuo et al. synthesized nano-sized silica spheres via using TEOS as the silicon precursor, and 3D hierarchical macroscopic/mesoporous silicon powders were synthesized through the magnesiothermic reduction process. (Zuo et al., 2017) A reversible capacity of 959 mA h g⁻¹ was retained at 0.2 A g⁻¹ after 300 cycles. The macropores accommodate the drastic volume expansion of the silicon under lithiation, and the mesopores facilitate Li⁺ transport due to their larger specific surface area. Even though the production process of artificial silica is complicated, the method still possesses certain research value owing to its high purity and structural design characteristics.

The preparation of porous silicon by CVD involves etching templates, also classified as a template-etching method. Silica, ZnO nanowire arrays, porous Ni materials, etc. can be employed as porous templates to realize the preparation of porous silicon. Tesfaye et al. fabricated silicon nanotubes with thin porous sidewalls by utilizing ZnO nanowires as sacrificial templates, maintaining a reversible capacity of 800 mA h g⁻¹ at C/10 after 180 cycles. (Tesfaye et al., 2015) Harpak et al. prepared 3D, sponge-like silicon nanostructure network on nano-porous stainless steel surface by CVD. (Harpak et al., 2019) These novel anodes exhibited stable cycle performance with over 50% capacity retention after 500 cycles and high Coulombic efficiency (over 99.5%).

The uniform surface coating and growth of the materials on the substrate can be achieved by CVD, and the prepared porous Si-based composite materials possess good cycle life and rate performance, but silane as a general silicon source is expensive, toxic, and explosive, which limits its applications along with safety hazards. Therefore, it is urgent to find safer and cheaper alternative silicon sources.

Although nano-sized silicon materials possess unique advantages in relieving volume stress, the inherent shortcomings such as unstable SEI films, low tap density and excessive side reactions have severely limited their practical applications. Therefore, the porous Si-based materials with both micro-nano structures are of great practical significance. They retain advantages of nanostructures while increasing their tap density through microstructures. Recently, selective dealloying is considered as a promising method for manufacturing micro-porous silicon. (Luo et al., 2020; Zhou et al., 2020) This method employs cheap silicon-metal alloys (such as Si-Al alloys etc.) as raw materials, and porous silicon materials are obtained by removing metal elements in the alloys. Wang et al. prepared the double-layer constrained micron-sized porous Si/SiO₂/C composites by dealloying, the obtained coral-like porous silicon structures exhibited good cycle stability with the capacity of 933.4 mA h g⁻¹ after 100 cycles. (Wang et al., 2019) Although this method requires a large amount of acid for etching, the silicon-alloy precursors of this method are easy to achieve large-scale manufacture, which is promising for further applications.

In addition to the above methods, other methods such as molten salt, solution synthesis, sol-gel, preparation of porous Si-based materials using Mg₂Si as silicon precursor, etc., have also been reported in recent years. The metals magnesium or aluminum have been demonstrated to reduce silica into silicon. However, excessive reaction temperature of magnesiothermic reduction limits its applications (500–950°C). Recently, molten salt systems have been considered as a promising reaction media to reduce silica at a lower temperature. (Liu et al., 2012; Mishra et al., 2018) Lin et al. designed a low temperature molten salt system for metallothermic reduction of silica and silicates. The AlCl₃ in this system could not only provide liquid environment, but also participate in the reaction. This anode showed a reversible specific capacity of 870 mA h g⁻¹ at 3 A g⁻¹ after 1,000 cycles (Lin et al., 2015). Gao et al. prepared a silicon hollow sphere by reduction of a carbon-coated silica sphere at 300°C with metal Al powder in a molten salt. After being prelithiated, it retains 80% of the initial capacity after 1,100 cycles at 8 A g⁻¹ (Gao et al., 2018). Molten salt system enables controllable preparation of porous silicon at lower temperatures. It is noticed that AlCl₃ is very sensitive to water, the reactions need conduct in stainless steel autoclave to isolate air. The method of low temperature solution synthesis of silicon has been widely concerned because of its mild conditions. Nanostructured Si such as nanoparticles, nanosheets and porous materials have been prepared through reduction reaction of silane and silicon halide. (Wang et al., 2020b; Wang et al., 2020c) Sun et al. prepared mesoporous silicon in high yields by a simple solution chemistry method. (Sun et al., 2017) After crystallization and carbon coating, the crystalline mesoporous Si/C nanocomposites exhibited high reversible capacity with 847 mA h g⁻¹ at 2 A g⁻¹ after 320 cycles.

Besides selective dealloying, there are other methods to synthesis micro-sized porous silicon. An et al. fabricated ant-nest-like microscale porous Si with nano-porous from Mg₂Si. The porous silicon materials exhibited good cycling performance after carbon coating (a reversible capacity of 1,271 mA h g⁻¹ after 1,000 cycles). (An et al., 2018) Liu et al. prepared interpenetrating 3D porous silicon by preparing a novel silica composite gel and then performing a modified magnesium thermal reduction process. (Liu et al., 2018) This method retained the bicontinuous structures of gel, and the cycling stability and rate performance were improved (0.98 mA h cm⁻² after 200 cycles). Although the above methods can controllably obtain the micro-sized porous silicon with nano characteristics, the inevitable shortages in their synthesis process limit their applications. The use of high-cost commercial Mg₂Si during the reaction process increases production costs of porous silicon. Although sol-gel method provides an unconventional solution to the capacity fading of Si-based anodes, complex multi-step synthesis increases the production cost and production period, which is not conducive to the expansion of large-scale manufacture, and remains to be further improved.