a) Thermal evaporation method is the most widely adopted method for synthesis of various nanostructured SiO_x by thermal evaporation in practice. The starting material can be pure Si powder [55] a mixture of Si and SiO₂ [22] or Si grains [56]. In 2001, in-situ SiO_x was produced by heating pure Si powder to 1373 K under Ar flow. Due to various vapor concentrations, substrate surface condition and temperature gradients, SiO_x powder with various morphologies were achieved [55]. Presently, one of the widely adopted methods on the industrial scale for preparation of SiO_x has been the thermal evaporation method. SiO_x powder is produced by sublimating and condensing a mixture of Si and SiO₂ at elevated temperatures. The silicon/oxygen ratio is controlled by manipulating the process conditions and ratio of raw materials [22].

In another study, a-SiO_x films were synthesized by reactive evaporation of Si grain with oxygen gas in vacuum. The chemical reaction is

By controlling the process parameters such as oxygen flow rate and evaporative silicon, ultrafine powder of SiO_x or film with different ratios of silicon oxygen content in SiO_x (x = 0.17, 0.51, 1.02, and 1.34) were obtained. The effect of total oxygen content on the electrochemical reactions of a-SiO_x with Li was investigated. The reaction products were determined by x ray photoelectron spectroscopy (XPS). The initial charge (lithiation) and discharge (delithiation) capacities were found to be strongly related to the value of x in SiO_x. The a-SiO_x (for x = 1.02, 1.34) displayed excellent cyclability at a range of 0.0005 - 1.05 V vs Li/Li⁺. Absence of peaks at 0.45 V in differential capacity vs voltage (dQ/dV) profiles demonstrates the suppression of crystallization of lithiated a-Si even under deep cycle conditions [56].

The synthesis methods yield in-situ SiO_x of high quality. When combined with ball milling, porous structures could be achieved for SiO_x anodes. Reactive ion evaporation method needs much higher temperature.

b) PVD (physical vapor deposition) includes vacuum evaporation, magnetron sputtering, electron beam evaporation, and ion plating. In one study, researchers synthesized SiO_x@C by PVD method at two different temperatures 200˚C and 400˚C followed by carbonization at 950˚C. Pyrolysis fuel oil was the carbon precursor and PVD was performed on pristine SiO_x. SiO_x@C-200 exhibited excellent cycling stability with a capacity retention of 90.2% after 80 cycles at 1.0 C confirming suppressed volume expansion of SiO_x [57]. In another study, metallurgical grade raw a-SiO powder was utilized to obtain Si/SiO_x nanocomposite powder by Plasma spray physical vapor deposition (PS-PVD). The procedure consisted of complete evaporation of raw SiO powder and subsequent condensation of high temperature SiO_x vapor, followed by disproportionation reaction of nucleated SiO_x nanoparticles. Half-cell batteries constructed with PS-PVD SiO_x powder with C/Si = 0.25 exhibited improved initial efficiency and maintenance of high capacity as high as 1000 mAhg⁻¹ after 100 cycles at the same time [58]. Undoped silicon target was RF sputtered to obtain Li-active, nanoporous SiO_x layer for coating of a-Si thin film [59]. Infrared nanosecond laser ablation method has also been in practice to produce ultrafine nanoparticles of SiO_x. When synthesized ultrafine nanoparticles of SiO_x were utilized as anode material for LIBs, it showed excellent cycling stability and a capacity comparable to commercialized graphite [60]. Another research group employed Infrared nanosecond laser ablation technique to synthesize silicon oxide films of various stoichiometry at different partial pressures of oxygen. The SiO_x films with varying oxygen content were analyzed by three different characterization techniques. It was observed that the composition of the film could be controlled by varying the inert gas pressure at the constant pressure of the active component in the ambient mixture [61].

PVD is conducted at a lower temperature compared to thermal evaporation. The films generally exhibit highly adaptable stoichiometry though compared to thermal oxides; deposited oxides are often sub-stoichiometric (oxygen deficient). Magnetron sputtering has the advantage of high precision over film thickness and density. It has fast deposition rate and low substrate temperature but high equipment complexity and high cost.

PSPVD and Infrared nanosecond laser ablation are higher temperature, complex processes with limited precision though both have high deposition rates.

c) CVD (chemical vapor deposition) is another deposition technique in which a thin film is formed by a chemical reaction on the surface of a substrate. It is primarily used in modification of SiO_x anode material while SiO_x is fabricated via disproportionation (d-SiO_x) or thermal reduction methods. Recently, the chemical vapor deposition (CVD) method was utilized to enhance the conductivity of SiO_x by preparing SiO_x@C composites. C₂H₂ was used as carbon source in a rotating CVD furnace to produce hard carbon coated SiO_x (SiO_x@C). At the rate of 0.5 C (1 C = 2.1 Ag⁻¹), the discharge specific capacity stayed at 1116.8 mAhg⁻¹ after 140 cycles, which is 539.8 mAhg⁻¹ higher than pristine SiO_x. The carbon coating suppressed the volume expansion leading to good electrical conductivity. It also avoided the agglomeration of SiO_x. The SiO_x @C anode exhibited excellent performance in both cycling and rate performance tests [36].

Recent studies confirm the significance of carbon precursors and deposition temperatures. Methane based carbon coating of SiO_x via CVD at 1000˚C exhibited excellent electrochemical performance. A high capacity of 778 mAhg⁻¹ at 0.75 Ag⁻¹ and a remarkable capacity retention of 92.8% after 100 cycles was observed. At industrial scale production of SiO_x, this optimized CVD method is of much use [37].

PECVD has also yielded reliable results for d-SiO_x/CN (carbon nanosheet) anodes material. The d-SiO_x@CNs composite exhibited high reversible capacity (first charge/discharge capacity of 1456.7 and 1794.4 mAhg⁻¹, respectively) and excellent cycling stability (capacity retention of 87.2% after 200 cycles at a current density of 0.4 Ag⁻¹). The electrochemical performance of the anode (d-SiO_x@CNs15/G||NCM811) in full cell with commercial NCM811 cathode was noteworthy with capacity retention of 76.3% after 200 cycles at 1C and a high energy density of 424.2 Whkg⁻¹ [62].

Thermal evaporation methods are vapor phase synthesis methods used to deposit nanostructured silicon oxide material for SiO_x anodes. The powder material obtained has excellent coating uniformity for line of sight but fails on complex 3D particles. Oxygen content is very homogenous and tunable in the dense film structure but requires ultra-high vacuum control. Besides the harsh experimental conditions such as elevated temperature and pressure, these methods are extremely expensive due to the need for large equipment, low productivity, and high energy consumption [23][54]-[62]. Standard thermal evaporation, PVD and CVD usually yield relatively dense, continuous films rather than inherently porous SiO_x networks.

Mechanical milling is a facile, low-cost technique for nanosizing advanced materials and has been in use for the last fifty years [63]. High energy ball milling (HEMM) is usually employed to synthesize SiO_x with or without other synthesis processes like CVD, metal reduction, thermal evaporation and so on [53]. The starting material can be Si powder [64][65], commercial SiO_x powder [66] or a mixture of Si and SiO₂powders [67]. One group of researchers have synthesized, high purity, with good cycling stability, amorphous SiO_x negative electrode material by high energy ball milling silicon powder in air or Ar [64]. The oxygen content in SiO_x was controlled by adjusting the exposure time of silicon powder to air or Ar. Samples were high energy balled milled for a total time of 20 hr. SiO_x with x between 0 ≤ x ≤ 0.6 were obtained. XRD, XPS, SEM and TEM analysis of the structure of SiO_x displayed nanosized Si embedded in a-SiO_x matrix. SiO_x with x < 0.6 displayed lower ICE and higher reversible capacity compared to commercial SiO_x sample [64]. Another group investigated SiO_x with various oxygen content synthesized from Si powder by reactive gas milling [65]. A mixture of Si and SiO₂ powder milling was also carried out to produce SiO_x for comparison. XRD, XPS and TEM revealed nano a-Si dispersed in a-SiO_x matrics. All samples had the same morphology. After annealing in Ar atmosphere at temperature between 300˚C - 800˚C, the SiO_x anode material exhibited improved cycling performance. The 1^st irreversible capacity of the synthesized samples reduced after high temperature annealing due to mitigating the defects, while the high reversible capacity (1500 - 2000 mAhg⁻¹ or 1600 - 1800 Ahg⁻¹), was well maintained. Study of the electrochemistry and thermal behavior of SiO_x prepared by reactive gas milling revealed SiO_0.37 to be thermally stable up to 800˚C with improved cycling performance after annealing [65]. Recently, SiOx with controlled oxygen content was obtained by ball milling crystalline silicon powder in an oxidizing medium using two different ball milling techniques. The total synthesis time extended to 30 hours. The SiO_x powder thus obtained had a large surface area [68]. A “dry powder micro granulation” procedure reported by other groups [69] has been suggested by the authors to reduce the surface area and improve the electrochemical performance of the ball milled SiO_x electrode material. Reaction-ball milling surface coating strategy also involves HEMM. The technique involves high energy milling of partially prelithiated and pristine Si microparticle samples in CO₂ atmosphere for different durations and pressures. A multicomponent amorphous layer coating of SiO_x, C, SiC and Li₂SiO₃ was observed. The four-fold coating layer successfully suppressed the pulverization of partially prelithiated Si microparticle anodes during cycling due to a stable SEI film formation on the electrode surface. The reversible capacity remained at 1439 mAhg⁻¹ at 100 mAg⁻¹ after 100 cycles. It is four times higher than that of pristine Si microparticle anode [70].

Ball milling strategy, a top-down synthesis method, is, at present, the main route for mass production of nanosized SiO_x. High-energy ball milling physically reduces the particle size to micron and nanosized silicon. It can also be used to combine various materials, but the energy supplied by grinding can also stimulate reduction processes [63]. Large surface area is created by ball milling, leading to higher consumption of Li⁺ ions during initial cycle to form SEI layer. This lowers the ICE [68]. On the other hand, HEMM may cause secondary agglomeration and impurities can be incorporated from the ball milling chamber. The method requires long synthesis time extending to many hours [24][54][63]-[70]. High heat is generated which would oxidize the silicon. High energy ball milling conducted in an inert atmosphere can be more productive [71][72].

The hydrothermal method, Metal reduction method, and the sol-gel method are the three most widely used wet-chemical methods to prepare Si/SiO_x nanoparticles. The morphology and electrochemical properties differ depending on the reaction conditions and the starting material. All these methods are often preceded by ball milling. Rice husks are the most common raw materials used [24][71][72].

a) Hydrothermal method is one of the most used methods to prepare Si/SiO_x nanoparticles which serve as the precursor to obtain SiO_x/C after carbon coating. By controlling both the physical and chemical conditions in the hydrothermal reactor, high purity nanoparticles of Si/SiO_x can be obtained. Quite a few research groups have extensively used the hydrothermal method to prepare Si/SiO_x nano particles and applied carbon/graphene coating to obtain the desired SiO_x/C anode material. The hydrothermal method is often preceded by ball milling. Nanocomposites thus produced show improved electrochemical properties [71]-[75]. One group fabricated SiO_x/C nanocomposites with controllable oxidation of silicon under mild hydrothermal conditions and synchronous carbon coating. The researchers report a high reversible capacity of 1133 mAhg⁻¹ at 0.5 Ag⁻¹ with 89.1% capacity retention after 200 cycles for the fabricated SiO_x/C nanocomposites. With 15 wt.% SiO_x/C composite, graphite-SiO_x/C hybrid electrode, a high reversible specific capacity of 496 mAhg⁻¹ and stable electrochemical cycling with a capacity retention of 90.1% for 100 cycles was observed [75].

The microwave hydrothermal method has also been reported for delicate construction of Si/SiO_x which avoids the long reaction time and elevated temperature conditions needed for the hydrothermal process [76].

The wet chemical reduction in hydrothermal methods provides a facile and homogenous liquid environment and applicable for production of nanostructured, or/and porous structures. Carbon coating is excellent for solution-based core/shell wraps. However, the need for high-cost solvents, high toxicity and air sensitive chemicals limits its applicability for commercial scale production and application in LIB [71]-[76].

b) Metal reduction methods

Metal reduction utilizes suitable metals with low melting point and suitable reduction potential to reduce silicon to SiO_x. Mg, Zn and Al are the most used metals to synthesize SiO_x [54][77]-[79]. Compared to traditional carbothermal reduction [80], the reduction temperature is lower, and the reaction conditions are mild.

Magnesiothermic reduction provides an efficient and economic method to synthesize SiO_x nanoparticles with low-cost silicates, silica, and minerals of silicon as raw materials [81]-[86]. In industry, SiO_x is manufactured using vapor deposition techniques under harsh conditions at extremely elevated temperatures.

The elevated temperature, pressure and variation in synthesis parameters affect the electrochemical performance of SiO_x. Magnesio-mechanochemical reduction of natural SiO₂ resulted in highly porous SiO_x with excellent cyclic stability up to 2000 cycles under a high current density of 4 Ag⁻¹ Blending with graphite further improved the performance. [81] In another investigation, solid SiO₂ and porous SiO₂ were utilized separately to produce porous SiO(p-SiO) with carbon shell by magnesiothermic reduction at low temperature(650˚C). The p-SiO@C displayed good cycling stability (777.1 mAhg⁻¹, at 500 mAg⁻¹ after 300 cycles) and excellent rate capability (977 mAhg⁻¹, at 1000 mAg⁻¹) [82]. Talc has also been utilized to obtain a mixture of Si and SiO_x by magnesio-thermal reduction and acid etching processes. A porous network structure of Si/SiO_x was obtained. Finally, carbonization of Si/SiO_x produced C@Si/SiO_x with particularly good cycling stability. Si/SiO_x nanoparticles thus obtained delivered initial discharge capacity of 842 mAhg⁻¹ at current density of 1C (1500 mAg⁻¹), and 470 mAhg⁻¹ after 300 cycles could be maintained. The superior performance was attributed to the porous network structure capable of mitigating the volume stress [83].

Another research group reported production of Si/SiO_x powder by magnesium thermal reduction of silica in an argon atmosphere for two hours at elevated temperature. The temperature varied from 675˚C to 825˚C followed by acid leaching for 24 hours and drying. Si/SiO_x with various oxygen content were obtained. The samples thus produced were analyzed using XPS and compared with empirical results. The optimum Si/SiO_x anode not only showed high specific capacity with acceptable ICE but also improved cycling performance [84]. Other groups have also studied the Si/SiO_x network and reported good cycling stability and rate capability utilizing magnesiothermic reduction of Si/SiO₂ mixture or SiO₂ to obtain Si/SiO_x, SiO and their nano composites [53][80]-[86].

Utilization of a combination of microwave and magnesiothermic reduction for the fabrication of ultrafast Si@SiO_x core/shell structures is the most recent approach. By adjusting the microwave assisted hydrothermal process at 50˚C, 150˚C and 250˚C thickness of the SiO_x shell layer on the Si core surface was controlled and Si@SiO_x amorphous shell thickness was optimized. After magnesiothermic reduction, oxygen vacancies developed due to the etching process of MgO. It contributed to the lithium-ion diffusion kinetics leading to high electrochemical performance [87].

Zinc has a lower boiling point (907˚C) and melting point (419.5˚C). Zinco thermic reduction is therefore in practice to obtain SiO_x and their nanocomposites [23][53][88]. One research group prepared Zn and SiO composite by HEMM and subjected to heat treatment at 900˚C. Evaporated Zn induced mesopores in the composite material. The boiling temperature of zinc leads to partial reduction of SiO. Carbon incorporation in the porous SiO_x further improved the electrochemical performance of the SiO_x/C composite electrode. The porous structure was conducive to improving cycling stability [88].

Aluminum is another active metal in use to prepare SiNP and its composites. Aluminum acts as a reducing agent for amorphous silica. The reaction time and temperature (600˚C - 900˚C) are carefully controlled that SiO₂is only partially reduced to yield SiO_x and Si core embedded in a carbon framework. The basic reaction is

Porous SiO_x@C Composites have been synthesized from rice husk by aluminothermic reduction at 700˚C in 2016 [89]. In another investigation aluminothermic reduction was combined with hydrolytic polymerization and carbon coating to obtain SiO_x/Si/C nanocomposites. The SiO_x/Si/C anode material displayed a high specific capacity and excellent cyclability (1355 mAhg⁻¹ at 0.2 Ag⁻¹ after 350 cycles). The nanocomposite had excellent structural stability. It could preserve 78.11% of its original structure after 350 cycles [90].

The carbothermic reduction process has often been combined with sol-gel process and the reduction process is carried out at high temperatures. Spherical SiO₂ powder was synthesized using TEOS as the starting material to obtain SiO_x powder in one study [91]. Homogenously dispersed micro/nano structure of SiO_x/C has been reported by another group after carbothermic reduction of silica-carbon binary xerogel. The synthesized SiO_x/C anodes have high electrochemical performance [92].

Chemical reduction processes-magnesiothermal and carbothermic reduction are elevated temperature processes, 600˚C - 900˚C and 800˚C - 1500˚C, compared to hydrothermal, alumino assisted zincothermal and aluminothermic processes. However, it often involves carbonization which is another high temperature process. All these reduction processes have slow reaction kinetics, and the energy consumption is high. Occurrence of side reaction, large amount of heat generation during the reaction process and long synthesis time are other negative aspects. The resulting elevated temperature leads to agglomeration, disappearance of pores and formation of impurities [53][54][71][72].

c) Sol-Gel method

The Sol-Gel method is comparatively mild method for preparing silicon oxides. The reaction conditions are mild, and synthesis can be executed at low temperature in solvents. Silicon organic compounds are disseminated in solvents as precursors. It is then hydrolyzed and polymerized to generate gels with certain spatial structures [53]. SiO_x can be produced directly in the solution by hydrolytic condensation of orthosilicate easters such as tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate (TMOS) [71][72]. Improved electrochemical performance of the LIB has been reported with the electrode prepared by sol-gel method [93]-[97]. A 30% improvement in the capacity of LIB was demonstrated by incorporating a C/SiO_x nanocomposite electrode having a SiO_x nanoparticle loading of 15 wt%. The synthesized C/SiO_x electrode had a reversible capacity of 383 mAhg⁻¹, and the charge discharge cycle exhibited 84% capacity retention after 100 cycles [93]. Unique structures can also be prepared using the sol-gel method [93]-[97]. In one study, a solvent-free, mass producible strategy was developed to prepare carbon encapsulated SiO_x/C@C nanocomposites utilizing sol-gel, ball milling, and CVD processes. The nanocomposites thus formed exhibited superior performance as LIB anodes [97].

Sol Gel process yields porous SiO_x allowing precise control on pore size and volume. Exceptional in-situ molecular carbon blending and flawless carbon wrapping are possible. The main constraints are the expensive raw materials used in the sol-gel process, some of the organic compounds in use are harmful to health. Long synthesis time is another constraint. The reaction conditions require strict regulation to attain the required composition and structure of SiO_x. When combined with the ball milling process, additional complexities associated with ball milling arise [93]-[97].

Other methods-Chemical etching [53][98]-[100], disproportionation [69][101]-[103], and electrochemical methods [104] have also been employed by several research groups to achieve SiO_x and their composites.

Acid or Alkaline leaching of rice husk by NaOH or KOH for extraction of silica is reported by several authors [24][53][98]-[100]. Heat treatment of rice husk followed by alkaline/acid leaching has been reported as another effective method to prepare C/SiO_x anodes. In one study, C/SiO_x was produced by heating rice husk in a tubular furnace at 600˚C for one hour under N₂ gas flow at 1 L∙min⁻¹ and then immersing the pre-carbonized rice husk in NaOH solution. By adjusting the immersion temperature and time, different SiO_x content were achieved for C/SiO_x anodes of LIB. Half coin cell studies indicated initial CE decreased with decreasing SiO_x content. More experiments with different SiO_x content proved that the SiO_x is helpful for increasing the specific capacity of C/SiO_x active material. Prelithiating the C/SiO_x anode displayed a stable anodic operation. Full cells consisting of prelithiated C/SiO_x anode and a Li (Ni_0.5Co_0.2Mn_0.3) O₂ cathode exhibited high ICE (initial coulombic efficiency (~85%)) and high discharge specific capacity displaying the maximum performance of the cathode (~150 mAhg⁻¹). The specific capacity retention increased with higher SiO_x content at increased current density. This is another elevated temperature method and needs long processing time [99]. Recently NF3 flourination has been reported to design yolk shell structured CNT/SiO_xF_y/C composites for Si-based anodes [100].

Disproportion reactions have an edge over other processes due to facile, and large-scale synthesis of SiO_x/C composites. The low electrical conductivity, low (ICE) and volume expansion issues have been successfully addressed with pre-lithiation, carbon coating and nanostructure designing. Porous, ultrapure SiO_x NP with good control on morphology has been produced by this method. A high reversible capacity and excellent electrochemical performance of the SiO_x/C anode have been reported by researchers [69][101]-[103].

However, disproportionation reactions are very high temperature, complex processes. It is often combined with HEMM and CVD for large scale production of SiO_x/C [53].

A facile synthesis of SiO_x NP (nanoparticles) for mass production has been proposed by electrochemical anodization of Si electrodes with an adjustable oxygen content. The process involves the application of a strong electric field near the surface of Si electrode to directly convert the Si electrode to SiO_x NP. The study is focused on analysis of SiO_x NP oxygen content and size. Application of the SiO_x NP in LIB is proposed, not investigated [104].

To avoid toxic organic solvents and reduce the long processing time, laser induced SiO_x (LI-SiO_x) layer derived from commercial adhesive tape has been studied by one group. The homogenous, porous SiO_x layer was directly deposited over the current collector electrode. They report an improved performance with the SiO_x coating in lithium metal batteries with a zero-excess (“anode free”) configuration and verified 100% improved performance [105].

HEMM, metallothermic reduction and metal assisted chemical etching (MACE) are cost-effective and scale to mass manufacturing. However, these methods have less control on particle size distribution, internal uniformity and oxygen distribution in SiO_x.

Thermal Disproportionation, CVD, PECVD and sol-gel reactions have excellent control on particle morphology and uniform nano silicon dispersion within the oxide matrix. Carbon coating is very uniform. Thermal Disproportionation has high yield also. Production of high-quality material at large scale makes them suitable for industry.

Presently, thermal disproportionation, combined with mechanical milling and CVD carbon coating is the main synthesis route for SiO_x in industry. By vaporization and condensation of a mixture of raw Si and SiO₂ powder in vacuum reactor at 1400˚C, crude condensate is produced. Mechanical milling is the next step for pulverizing the crude condensate followed by CVD for carbon coating [45][53].

Each synthesis method has its own characteristics as presented in Table 1. Choices are made based on the specific experimental environment and requirements. For mass production, high yield of high-quality material is the first and foremost requirement with affordable cost and safety concerns.

Table 1. Synthesis methodologies.

4.1.1. Role of Carbon Coating

The formation of Li₂O and lithium silicates counteract volume expansion of SiO_x. However, it lowers the ICE of SiO_x, leading to poor electrochemical performance. Incorporation of carbon is an effective strategy to mitigate the large volume expansion of SiO_x and improve the electrochemical properties. Carbon is an ideal coating material that has high conductivity with controlled morphology and easy synthesis [114]. The combination of carbon is the most common modification method to improve the electrochemical properties of SiO_x based anodes. However, carbon is unstable at high temperature and forms silicon carbide. The difference in morphology between SiO_x and carbon also results in poor contact [53]. Extensive research has been carried out to resolve these issues. Currently, carbon coated SiO_x/C composites [29][31][33][95][97][106][122]-[125], embedded SiO_x/C composites [126] hybrid SiO_x/C composite [127]-[129] and micro/nanosized SiO_x/C composite [89][90][99][102][106][125][126][130][131] anodes are in practice.

Coated SiO_x/C refers to composites where the SiO_x particles are coated with a protective carbon layer. Carbon reduces direct contact between electrolyte and active substances. It buffers the volume expansion of lithium intercalation, thereby inducing a stable SEI film. Depending on the various designs, porous, core shell [17][87][95][130], yolk shell [79][132], carbon nanotubes (CNT) [100][101][118][132]-[134] etc. an improved cycling performance has been reported [17][53][54][72][86][87][115][122]. It is noteworthy that co-doping with heteroatoms of greater electronegativity and electron affinity, such as C, N, F, SP, B further improves the stability and efficiency of the SiO_x anode [31][72][100][101][113][114][121]. Doped carbon coated SiO_x [135]-[140] and carbon coated doped SiO_x [141][142] material are the two approaches leading to improved electrochemical performance of the anode material.

Doped carbon layers enhance the electrical conductivity of SiO_x buffering the volume variation of SiO_x thus preventing the structural degradation of the composite material. Nitrogen doping has been most extensively applied. In one study, the SiO_x@NC displayed high reversible capacity (774 mAhg⁻¹), ideal rate capability, and long-term durability (112% capacity retention over 500 cycles) [135]. Recent studies report enhanced lithium storage performance of nitrogen enriched lignin carbon/SiO_x composites. The anode material was synthesized via a novel co-precipitation and selectively etching approach. The prepared NEC@LC-SiO_x exhibited excellent performance. A high reversible specific capacity of 802 mAhg⁻¹ and outstanding rate capability was observed [136]. Recent study has been conducted on Si/SiO_x structures with fluorine doped carbon. The Si/SiO_x@F-C electrode delivered promising electrochemical performance [137].

Co doping with multiple heteroatoms further enhanced the electrochemical performance of the composite anode. Co-doping into the C matrix provides additional active sites and defects leading to improved reactivity, rate capacity conductivity, and Li⁺ storage capacity of the material [138]-[140]. Synthesized from rice husk through a simple calcination approach, the as-fabricated SN@C/SiO_x demonstrated high reversible capacity (1150 mAhg⁻¹ at 0.1 Ag⁻¹), improved initial coulombic efficiency (CE) of 70.4%, outstanding rate capacity, and long-life cycling stability (632 mAhg⁻¹ under 1.0 Ag⁻¹ over 1200 cycles) [138]. In another study micron sized SiO_x encapsulated in B, N co doped carbon nanotubes exhibited ultra-high capacity [139].

However, carbon coating is beneficial if the layer is thin. The addition of too much carbon reduces the specific capacity of the material. Therefore, introducing as little carbon as possible that matches the material particle size and gives a uniform morphology is especially important. Other negative aspects are the requirement of complicated, multistep high temperature processes [53].

Embedded and hybrid SiO_x/C composites.

Embedded SiO_x/C composites and hybrid SiO_x/C composites embed the amorphous SiO_x nanoparticles in a carbon matrix. The highly dispersed SiO_x offers plentiful active sites for Li⁺ storage. The carbon matrix provides a transport network for Li⁺ and electrons and buffers for volume expansion. SiO_x/C composites exhibit better electrochemical performance and fast Li⁺ diffusion kinetics. Recent investigations carried out on embedded SiO_x/C composites demonstrate excellent electrochemical performance, low charge resistance with fast diffusion kinetics [114][131]-[134][139]. Researchers emphasize that encapsulation of micron sized SiO_x particles into 3D B, N co doped carbon nanotube via metal-cation assisted carbonization guaranteed slight volume variation during cycling, fast Li ion and electron transfer and electrolyte diffusion [139]. The authors report further enhancement of the lithium storage capacity when heteroatomic active sites were introduced with Sn, B, N carbon nanotubes. When the silicon sub oxide co-doped boron, carbon nanotube (SSBCN) composite material was utilized as anodes in LIB half cells, it displayed ultrahigh reversible capacities of 2072.8 mAhg⁻¹ at 0.1 Ag⁻¹, a very high-rate performance of 501.8 mAhg⁻¹ at 10 Ag⁻¹ and excellent long-term cycle stability of 400 and 1000 cycles at 1 and 5 Ag⁻¹, respectively. Moreover, SSBCN/LiFePO₄ (LFP) full batteries exhibited remarkable cycle durability with a 92.72% capacity retention after 600 cycles at 1 C [139].

Nanosized SiO_x/C composites

Nanosized SiO_x/C composites are beneficial offering more space to accommodate the volume stress due to their unique size. However, they would create more interparticle space and surfaces leading to low tap density and low volumetric capacity, not suitable for high density LIB [53][54]. To address these issues micro sized SiO_x based materials with nanostructure characteristics have been investigated. The micro-nano sized SiO_x/C composite material could be synthesized by integrating nano SiO_x with micro-sized carbon matrix [140]-[144]. Studies revealed that it effectively alleviated the inherently low electron/Li⁺ conductivity and improved the structural integrity of SiO_x. The assembled SiO_x/NC-2/LiFePO₄ full cell exhibited a capacity retention of 89% after 350 cycles at 2 C [140]. In another investigation, micro-nano structured SiO_x/C composites were synthesized by carbothermal reduction of silica carbon binary xerogel. The micron sized SiO_x/C spheres were found to be composed of many near spherical nanoparticles. The structures exhibited a high reversible capacity of 830 mAhg⁻¹ for 100 cycles and excellent rate capability [92]. In another investigation sub nanoscopic SiO_x/C composites prepared by heating precursor liquid in a sealed vessel at high temperature exhibited excellent performance. The SiO_x/C consisted of homogenously dispersed SiO_x and free carbon in SiO_x/C spheres. The structure exhibited 96.1% capacity retention after 400 cycles, outstanding rate performance (146.2 mAhg⁻¹ at 10 Ag⁻¹) and high specific capacity of 1667.3 mAhg⁻¹. A high areal capacity of 2.6 - 2.0 mAh∙cm⁻² during cycling promises practical applications [143].

Homogenous dispersion of spherical SiO_x and free C components on the atomic scale is the key requirement for preparation of SiO_x@C microspheres with nano structures or sub nanoscopic structures. Such dispersion accommodates the volume change and ensures high electrical conductivity. It could be an effective strategy for implementation of micrometer size electrode materials in practical battery systems. The commercial scale production aspect is being explored as well [53][115][139][140][143]-[146].

4.1.2. Metal Doped; Carbon Coated Structures

As discussed in the previous section, doped carbon materials enhance the electronic conductivity of SiO_x/C composites. Co-doping with heteroatoms of N, P, B, and S has already been discussed in the previous section. Doping SiO_x or carbon with metals is also in practice. Metals like Mg, Sn, Fe, Ni, Mn, Ti have been reported to enhance the electrochemical performance and structural integrity of SiO_x/C anodes materials and reviewed [147]-[153]. SiO_x/iron/nitrogen co-doped carbon (Fe-N-C) microspheres were synthesized using electrospray-carbonization strategy. Studies indicated that Fe doping contributed to advanced electrochemical reaction and storage reversibility of the anodes [151]. Recent studies establish the superiority of Mg doped carbon coated SiO_x anodes over the carbon coated SiO_x without Mg doping. Researchers utilized magnesio-thermic reduction to prepare Mg doped carbon coated SiO_x anode material. The capacity of SiO_x was improved to 1477 mAhg⁻¹ with a minimal compromise in the ICE (83.77%). The enhanced electrochemical performance was attributed to the synergistical modification of both Mg doping and prelithiation [148]. High ICE, high specific capacity and cycling stability of premagnesiated anodes, C@M-SiO_x has been reported by another group with its commercialization aspects [150].

An extensive study of the role of carbon material on the electrochemical performance of SiO_x/C composites indicates a dual role of carbon. It enhances the conductivity of the SiO_x/C composite and helps retain the integrity of the SiO_x/C anode during lithiation/delithiation process. The lithium storage capacity differs based on the carbon morphology, architecture, and properties. Researchers have established that carbon nanostructure differs based on the carbon source and type [114][115][145][154].

Carbon cladding provides an attractive option to alleviate volume expansion and low electrical conductivity of SiO_x. Research shows that the effectiveness of carbon encapsulation is limited. 0D, 1D, 2D nanoscale SiO_x/C structures mitigate the issues of low electrical conductivity and high-volume expansion [30][39][53][54][72]. However, these structures have an adverse effect on mass transfer processes within the electrode after prolonged cycling due to accumulation of the material. 3D SiO_x/C porous structures are designed to circumvent this issue [145][146]. Porous structures provide plenty of space to buffer the volumetric stress during the cycling of SiO_x/C anodes [72][88][98][100][128][146][155]-[165]. A nanoporous material has pore size ranging from 1nm to 100nm or smaller according to IUPAC. Following IUPAC, porous materials can be divided into microporous (pore size < 2 nm), mesoporous (pore size between 2 - 50 nm) and macroporous (pore size > 50 nm) material depending on the size of the pores. Porous SiO_x/C structure can be porous SiO_x, Si/SiO_x/C or combination of SiO_x and a porous framework, mostly porous carbon [72][154]-[167].

3D-ordered honeycombs like macroporous SiO_x/C structures have been extensively investigated by researchers. [156][157] Unlike the 0D - 2D structures, these 3D honeycomb like structures offer plenty of active sites for lithiation/delithiation process for a stable SEI after prolonged cycling. Carbon coating alleviates volume expansion and enhances electrical conductivity. It alleviates electrode pulverization. The synthesis process of 3D honeycomb structure is also simple, based on wet chemistry and CVD. Quite a few studies are carried out on full cell structures [154][156][157].

The most recent study proposes encapsulation of SiO_x in a joint structure of carbon shell and carbon nanotube with excellent electrochemical performance. LPCVD and spray drying were employed to construct the composite anode material. On half cells, the anode SiO_x@C@CNTs exhibited excellent cycling stability (624.7 mAhg⁻¹ after 1000 cycles at 2 Ag⁻¹) and rate performance (790.3 mAhg⁻¹ reversible lithium storage capacity at 4 Ag⁻¹). Full cell study with SiO_x@C@CNTs anode and NCM 811 cathode material provided a substantial energy density of 401.8 Whkg⁻¹ with excellent cycling stability of 134.8 mAhg⁻¹ after 100 cycles at a rate of 1C. The capacity retention rate was 80.7% [161].

The porous SiO_x/C structures-macroporous, mesoporous, and nanoporous SiO_x/C structures, provide large surface area with an abundant number of pores for Li⁺ transportation leading to improved rate performance and cycling stability. Other than pore size, various other pore properties, such as pore structure, pore alignment, specific surface area, symbiosis, and defects influence the performance of the porous SiO_x/C anodes [156]-[170].

Material cost and tap density are the two main concerns for commercialization. Though nanosized porous structures have the advantage of mitigating the volume expansion, excessive specific surface area and side reactions result in low ICE of the anode. The low tap density is another disadvantage limiting practical applications. Micro sized porous material with nano characteristics is therefore best suited for production of composite anodes [171].

Recent research proposes a novel SiO_x/G/C composite with SiO_x nanoparticles, graphite, and carbon nanotube. The structure was achieved by a simple ball milling and annealing process [169]. The dual carbon framework connection with SiO_x via C-O-Si bonds offered enhanced reaction kinetics with reduced volume fluctuations. Excellent cycling stability and performance rate were demonstrated by the composite anode. A capacity retention of ~700 mAhg⁻¹ over 500 cycles at 1.0 Ag⁻¹ was observed. In a full-cell configuration (SiO_x/G/C//LiNi₁/₃Co₁/₃Mn₁/₃O₂), this system exhibited a reversible capacity of 113 mAhg⁻¹ over 100 cycles at 1.0 mA∙cm⁻² emphasizing its superior performance.

Another group recently reported fabrication of porous Si/SiO_x assembled microspheres interweaving CNT network (CNT/SFDP-Si/SiO_x@C microspheres) utilizing bottom-up inverse water-in-oil microemulsion method and magnesiothermic reduction. Silica fume (SF), an industrial byproduct or solid waste of the manufacture of silicon metal and ferrosilicon alloys was added as the precursor to form nano-Si. The particle size of SF ranged from 50 nm to 250 nm. N doped outer carbon shell improved the electronic conductivity and induced the formation of stable SEI to enhance the overall structural integrity [170]. The approach was facile compared to yolk-shell or core-shell structure fabrication methods. The structure had an excellent electrochemical performance. A long cycling lifespan with a reversible capacity of 675.8 mAhg⁻¹ after 1000 cycles at 2 Ag⁻¹ was achieved. A very small decay rate per cycle of 0.018% was observed with a high-rate performance. The prelithiated LiFePO₄||CNT/SFDP-Si/SiO_x@C full cell performance was also analyzed. It exhibited the reversible capacity of 129.4 mAh g at 0.2 C.

Synthesis of porous structures:

Porous SiO_x-based structures can be either porous SiO_x or SiO_x with a porous carbon shell. The synthesis of porous SiO_x can be divided into two groups. The top-down type of porous SiO_x prepared by etching technology and the bottom-up type of porous SiO_x formed by weak interaction self-assembly according to the different construction strategies. Therefore, there are different preparation processes such as etching, template-assisted method, chemical exfoliation and treatments etc. [53][54]. Most of the synthesis methods to prepare porous SiO_x are based on magnesiothermic reduction methods. Recently, low temperature zinc-thermal reduction has been utilized to fabricate porous SiO_x@C (pSiO_x@C) composites. When the pSiO_x@C composite material is used as an anode in LIB, it displayed stable cyclic stability even after 1300 charge/discharge cycles at 0.3 Ag⁻¹, and superior rate capability even at a high current density of 3.0 Ag⁻¹. Full cells exhibited superior Li-ion storage attaining a high reversible capacity of 147 mAhg⁻¹ at 0.2 Ag⁻¹ [171].

These synthesis methods have been discussed earlier in Section 2.3.

Porous 3D SiO_x/C structures successfully mitigate large volume expansion and have long cycle life. However, there are challenges at industrial scale production as wet processes like magnesiothermic reduction, hydrothermal reduction, Aluminothermic or zinc thermic reduction have achieved small scale production.

Another porous structure is SiO_x encapsulated in a porous carbon shell [139][155][156][160][162][172]. Biomass carbon is the potential source for large scale synthesis [77][128][138][140][173]. Researchers have successfully utilized bamboo charcoal to produce porous carbon shell embedded SiO_x composite anode for LIB. The synthesis followed a top-down route [70][152][159][160]. Catkins derived, N, P co doped, SiO_x/C nanosheets (N/P-SiO_x/C-Nss) architecture exhibit ultra long cycle life with about 340 mAhg⁻¹ and almost no capacity decay after 10,000 cycles at 10 Ag⁻¹. The studies utilized chemical exfoliation and treatments to fabricate the anodes [172]. In the most recent work, mesoporous SiO_x@C spheres (m-SiO_x@CS) were encapsulated in a three-dimensional ordered macroporous carbon (3DOMC) framework. Sol-gel and soft templating methods were utilized to obtain the nanocomposite (3DOMC/m-SiO_x@CS). The anode thus prepared displayed excellent structural stability, electrochemical performance, and cycling stability. A comparison with some other SiO_x/C anodes has been presented by the authors in Tab. S4 and Tab. S5 [173] presented here as in Table 2 and Table 3.

Table 2. Comparison of the rate performances of the 3DOMC/m-SiO_x@CS electrode with previously reported SiO_x-based electrodes.

Table 3. Electrochemical performance comparison of the 3DOMC/m-SiO_x@CS electrode with previously reported SiO_x-based electrodes.

Reproduced from Ref. [173] with permission from American Chemical Society.

Ongoing research on porous structures based SiO_x anodes establishes their superiority over other anodes. However, the etching methods involve expensive toxic chemicals. On the other hand, the sacrificial template approach remains a challenge for industrial scale production due to the complex processes involved and high production cost [53][54][113].

Recently, solvent free approaches to developing SiO_x/C anodes have also been explored [97][174][175]. Though the approach offers reduced cost and less environmental impact, inhomogeneous mixing of SiO_x nanoparticles within the carbon matrix, limited control on the particle size of SiO_x nanoparticles, and controlling the morphology of SiO_x/C composites remains the key challenges [53][97][169][176].

Carbon coating buffers the volume change of SiO_x, improves the conductivity of the material as well as its electrochemical performance. However, carbon layer diffusion to Li⁺ is limited, hindering the formation of stable SEI [107][113]. The surface modification with metal oxides reduces the direct contact of the SiO_x core and the electrolyte, thereby reducing the consumption of Li⁺ ion to form stable SEI [23][72][107][153][167]. Therefore a promising surface modification strategy is to combine hybrid coating with transition metal oxides [96][119][153][174][177]-[184] or other metal oxides [185]-[190]. Nanostructured transition metal compounds with high specific surface area and appropriate pore size distribution display high ion diffusion coefficients suitable for Li⁺ diffusion in SiO_x leading to improved electrochemical performance [174]. One group synthesized a 3D hierarchical SiO_x@C@CoO mosaic structure nanosheets via high energy ball milling and hydrothermal reduction process. The electrode material displayed good cyclic performance with a reversible discharge capacity of 1079 mAhg⁻¹ over 250 cycles at a current density of 1 Ag⁻¹. It also exhibited an excellent rate capability (with an average specific reversible discharge capacity of 777.8 mAhg⁻¹ at 2000 mAg⁻¹ and 480 mAhg⁻¹ at 5000 mAg⁻¹). The improved performance is attributed to the specific structure and the synergistic effect between the two materials [177]. In another study, sponge-like structures of SiO_x@C@CoO, were synthesized via spray drying and electrostatic self-assembly strategy. The novel design containing ultrathin nanosheets of CoO on SiO_x@C improved the conductivity of SiO_x, shortened the diffusion length and provided large surface area for enhanced Li⁺ diffusion. Moreover, the sponge like structure successfully accommodated the large volume change of SiO_x. An improved and stable electrochemical performance was observed during the charging/discharging process with the multifunctional composite anode. When employed in half cells, a reversible specific capacity of up to 1287 mAhg⁻¹ at a current density of 0.1 Ag⁻¹ and retaining 714 mAhg⁻¹ after 750 cycles at 1 Ag⁻¹ with a capacity retention of 98.9% was observed. The full pouch-type cells, employing LiNi_0.8Co_0.1Mn_0.1O₂ (NCM 811) as the cathode, exhibited an excellent reversible capacity of 206 mAhg⁻¹ and a long-term cycling stability with a capacity retention of 85.9% after 200 cycles [178]. Similarly, TiO₂ coating has been an efficient and feasible strategy to effectively improve the electrochemical performance of SiO_x based anodes. TiO₂ has strong mechanical stability, favorable electronic conductivity and ionic conductivity properties contributing to efficient charge transport within the material leading to small volume change in SiO_x electrode after lithium insertion. Synthesis routes ranged from sol-gel deposition/carbonization methods, ball-milling and surface coating during electrospinning to CVD [96][119][174][179]-[181][184]. Hierarchical mesoporous structures with hybrid MnO-SiO_x@C microspheres, obtained via spray pyrolysis and subsequent annealing, proved to effectively enhance the cyclic stability and rate performance of SiO_x anode. Researchers concluded that the Mn₂SiO₄ phase was attributed to the improvement of structural stability [182]. It is crucial to highlight that multilayer coatings are more effective in mitigating the volume change of SiO_x. A double-coated composite structure exhibited improved cycling performance. Synthesized by elevated temperature heat treatment, the double layer composite SiO_x/C/Cu₂O exhibited excellent long-cycle ultra-high stability [183]. Similarly, dual-shell SiO_x/C@Sn@NC anode exhibited excellent lithium storage ability. A combination of the hydrolysis method and polymer coating technology was utilized to synthesize the hybrid material. SEM and TEM analysis confirmed the uniformly layered, coated structure with clear lattice stripes of Sn and N-doped amorphous carbon on the outermost layer [185]. Ternary composites with Tin (IV)oxide, SiO_x@SnO₂@C [187] and (SiO_x/G/SnO₂)@C [188] displayed high lithium-ion diffusivity and lower volume expansion. In a comparative study of dry coated SiO_x/C with metal oxides, Al₂O₃-coated SiO_x/C anodes also displayed improved electrochemical properties [186]. Researchers have obtained SiO_x/MgO/Mg₂SiO₄/C composite anode material after magnesium thermal reduction and carbonization. The material effectively maintained structural integrity during cycling leading to improved electrochemical performance of the SiO_x anode [189]. Double layered SiO_x/Mg₂SiO₄/SiO_x composites also displayed enhanced ICE and good control on volume expansion [190].

Single layer carbon coating of the SiO_x material is always beneficial and universally accepted practice. Adding multiple layers gives rise to additional complexities at manufacturing level affecting the cost-effective processes.

To counteract the volume expansion problem of SiO_x anode, there are two widely studied approaches 1) Nanosizing SiO_x particles [23][24][45][53][54][72][113][120]; 2) Doping with heteroatoms, metals, and carbon [31][140][147][148][150][173]. The first approach uses the internal void space to Si and SiO_x structures to expand and contract without disrupting the connection between the particles. The second approach helps achieve better conductivity than SiO_x as the atoms replace some of the silicon atoms in the structure or occupy interstitial positions in the material. It modifies the physical and chemical properties of the material and establishes a stronger skeleton structure [176][191]. Non-metallic doping as well as metal atom doping improve the performance of SiO_x material. Nonmetallic dopants significantly alter the surface chemistry of SiO_x making it more stable during charge/discharge cycles. A finer control over the chemical and electronic properties of SiO_x can be achieved with non-metallic dopants without the fear of unwanted metal oxide formation [153]. On the other hand, certain metal dopants, like Ni, Ti, Fe, or Mg can enhance the Li⁺ diffusion kinetics and mechanical stability of the 3D doped SiO_x framework [147][148][152]. Studies carried on Al doped 3D Si/SiO_x porous structures confirm good structural stability, buffered volume change, and increased cycling life of the anodes [191]. In another investigation, porous SiO_x@Li₂SiO₃/C composites were synthesized by one pot carbonization method. The hierarchical porous structure displayed improved electrochemical performance [192]. Co-modification with carbon and lithium borate hybrid coating simultaneously improved the electronic and ionic conductivity of SiO_x. The alloy type anode structure SiO_x@C@Li₃BO₃ displayed superior cycling performance (81% capacity retention after 500 cycles at 1 Ag⁻¹). The enhanced electrochemical performance is attributed to the synergistic effect of carbon and Li₃BO₃ [193]. The most recent investigation reports high ICE of 85.4% and relatively stable cycling performance of 759.2 mAhg⁻¹ after 100 cycles at 0.5 C with the capacity retention of 59.8% for MgSiO₃ doped SiO_x carbon coated, MgSiO₃-SiO_x@C nano architectures [194].

Structural modification due to dopant insertion in the various 3D structures such as porous and pomegranate structures may prove crucial as it increases Li⁺ diffusivity resulting in more homogenous lithiation across the material. It also mitigates volume expansion, improving the ICE of the SiO_x anode. However, due to the different element ratios in SiO_x, exploration of the microscopic mechanism of Si, O, Li, and doping elements in the battery reaction process is very much required for industrial applications. Simulation of the process is a good option as it saves the cost of trial and error [195].

Recent development of “Nanoskeleton” Si-SiO_x/C anode supported by CNTs has exhibited enormous potential for improved cycling stability in full cells [196]. Another recent approach has been uniform nano-Si dispersion to achieve Si-rich SiO_x materials. Electrochemical analysis revealed silicon sub oxides achieved through high energy mechanical milling demonstrated a higher reversible capacity of 2558 mAhg⁻¹ and an ICE of 89%. Carbon incorporation further enhanced the cycling stability [197].