The value of x (0 < x < 2) affects the electrochemical performance of SiO_x, as shown in Table 1 [49–57]. The specific capacity and ICE of SiO_x tend to decrease with increasing x. Considering the structural model of SiO_x, a higher x value indicates an increase in the SiO₂-like regions, which can change the intrinsic properties of the material. Al-Maghrabi et al. designed combinatorial SiO_x thin films to identify the electrochemical property changes according to the oxygen ratio of SiO_x [53]. Based on the voltage profiles and dQ/dV plots of these thin-film electrodes, they proposed three models for the electrochemical mechanism. In the model most consistent with the experimental data, Si reacts with lithium to produce Li_3.75Si, and SiO₂ reacts with lithium to produce Li₄SiO₄ as a Li-inactive phase, indicating that increasing the x value results in a higher irreversible capacity. Because those authors suspected that the formed Li₄SiO₄ phase enhances the diffusion of Li into Si and buffers volume expansion, they proposed that an optimal oxygen content could improve the electrochemical performance of the SiO_x anode material. Haruta et al. used radio frequency (RF) magnetron sputtering to synthesize SiO_x with x = 0.02, 0.21, 0.48, 1.09, and 1.78 and compared their electrochemical performance and morphology changes [50]. Their results revealed that a higher x increased the irreversible reaction of Li₄SiO₄ formation, which led to a drop in ICE but better cycle performance. Through the buffering role of the Li₄SiO₄ phase, cross-sectional SEM images of the cycled SiO_x electrode showed less morphological changes and crack formation compared to the pure Si electrode. Cho et al. controlled the surface oxygen content of SiO_x using RF magneton sputtering and investigated the resulting structural changes in SiO_x thin films. X-ray photoelectron spectroscopy (XPS) revealed that the SiO_x film underwent chemical changes during cycling, with the most significant changes observed at the highest oxygen content (x = 2). These results suggest that the solid electrolyte interphase (SEI) layer can be engineered by tuning the surface oxygen content of SiO_x [58]. During ball milling, the value of x in SiO_x can also be tuned by controlling the air exposure time or varying the mixing ratio of Si and SiO_x [57,59,60]. These findings demonstrate that the electrochemical performance of SiO_x varies significantly with the x value. Because of the trade-off relationships among ICE, specific capacity, and cycle performance, an appropriate x value for SiO_x could be important for achieving the required electrochemical performance.

During disproportionation, amorphous SiO_x separates into crystalline Si and SiO₂ phases, which can affect the electrochemical performance [61–64]. Disproportionation can be induced thermally by annealing SiO_x powder with or without a catalyst (Fig. 2). In the XRD data of annealed SiO, Bragg peaks for crystalline Si phase were observed at temperatures above 800°C. In the presence of NaOH as a catalyst, both crystalline Si and SiO₂ phases were observed in the XRD data of porous SiO at temperatures above 700°C. Another study reported the emergence of a crystalline SiO₂ phase at above 1350°C without a catalyst [61]. Park et al. reported the characteristics and electrochemical behavior of SiO, whose degree of disproportionation was varied by heat treatment at different temperatures [35]. Based on the XRD, high-resolution TEM (HRTEM), and ²⁹Si MAS NMR data of disproportionated SiO and pristine SiO, those authors proposed that the crystalline Si phase emerges at 800°C, whereas the crystalline SiO₂ phase does not emerge until 1200°C. Upon increasing the heat-treatment temperature, the average crystallite size of Si increased from 5 nm (1000°C) to 30 nm (1200°C). Because of the uniformly dispersed crystalline Si particles and surrounding silicon suboxide matrix, electrodes with SiO heat-treated at 1000°C showed the highest ICE and capacity retention. However, electrodes with overly disproportionated SiO (1200°C) showed poor electrochemical properties, because the Li-inactive SiO₂ phase hinders the alloying reaction of Si with Li. Such poor electrochemical performance may be related to the larger domain size of the SiO₂ phase in the disproportionated SiO. It has been reported that high-energy mechanical milling (HEMM) can reduce the particle size of disproportionated SiO, re-activate Si, and improve the cycle performance compared to pristine SiO electrodes [65].

SiO_x also can be disproportionated via electrochemical reactions with Li during cycling. Laser-assisted atom probe tomography (La-APT) was used to visualize Si domains in SiO before and after 419 cycles [66]. The results showed that the Si domains, which were randomly scattered in the SiO matrix before cycling, agglomerated and formed larger ones after cycling, implying that electrochemical disproportionation can occur because of the unstable structure of a-SiO.

The reaction mechanism between SiO_x, especially SiO, and lithium is challenging to investigate because of (1) the uncertain amorphous structure of SiO_x and (2) the difficulty in detecting the light Li atoms using electron beam analysis. Furthermore, because the electrochemical mechanism of SiO_x is affected by its structural and compositional properties, such as the x value and degree of disproportionation, it has only been investigated in case studies (Table 2) [55,66–79]. Nagao et al. reported a negative peak for lithiated SiO in the RDF pattern derived from neutron elastic scattering (NES), which is related to lithium oxide compounds apart from the Li_xSi alloy formed by the reaction of silicon with lithium [67]. Kim et al. reported that Li₄SiO₄ and Li₂O were formed during the lithiation of SiO, based on ²⁹Si-NMR and ⁷Li-NMR analyses [70]. The SiO₂ in SiO reacts irreversibly with Li to form the Li₄SiO₄ phase, whereas Si reacts reversibly with Li to form the Li_xSi alloy phase. This two-phase reaction of SiO was also confirmed by dilatometric analysis. Kim et al. reported that SiO reacts with Li to form Li₁₅Si₄, Li-Si-O (especially Li₄SiO₄), and Li₂O phases, based on the dQ/dV plot, ⁷Li MAS NMR and ²⁹Si MAS NMR spectra, and HRTEM images [71]. From these results, they suggested that SiO undergoes a three-phase reaction following the revised random mixture model (a-Si, interphase boundary layer, and a-SiO₂), in which the interphase boundary layer reacts with lithium to form the Li₂O and Li₁₅Si₄ phases. Using a thermodynamic approach, Yasuda et al. calculated the reaction pathway for the lithiation of SiO and SiO₂ with respect to the Li–Si–O terminal phase diagram [75]. After comparing the voltage window and capacity for each reaction stage, they determined that the possibly formed phases were Li₄SiO₄, Li₂O, and Li₁₃Si₄. The Li₂₂Si₅ phase, considered to be a fully lithiated form of lithium silicon alloy, could not be formed thermodynamically; instead, Li₁₃Si₄ was the favorable phase for the lithiation of SiO. Recently, Kitada et al. reported phase changes in SiO during cycling through in-situ ⁷Li as well as ex-situ ⁷Li and ²⁹Si MAS NMR techniques [79]. They compared pure Si, a-SiO, and disproportionated SiO (d-SiO) electrodes by marking the lithiation process of Si in three major steps 1–3, as shown in Fig. 3a. While the insitu ⁷Li MAS NMR data of the pure Si electrode was asymmetric during charging and discharging, that of the a-SiO electrode was symmetric (Fig. 3b). Their finding suggested that pure Si transformed into the c-Li₁₅Si₄ phase during the lithiation process (phase 3), but a-SiO did not form the c-Li₁₅Si₄ phase and instead formed a metallic m-Li_xSi phase (x = 3.44), which did not undergo a phase transition to the crystalline lithium silicide phase. This difference in electrochemical process between pure Si and a-SiO electrodes could explain the improved cycle performance of a-SiO, in that a-SiO does not suffer severe structural changes caused by c-Li₁₅Si₄ formation. Those authors also suggested that Li₄SiO₄ was produced during the initial lithiation reaction, and that Li₂SiO₃ was produced during the subsequent cycling of a-Li_xSi and Li₄SiO₄.

The internal Si in SiO_x undergoes a reversible reaction with the lithium-silicon alloy, and the remaining SiO_x or SiO₂ regions react with lithium to form a Li-Si-O phase, which is generally accepted as Li₄SiO₄. However, the formation of lithium compounds and the value of x of Li_xSi in the fully lithiated SiO remain uncertain. For example, Li₂SiO₃ could emerge during the delithiation of Li₄SiO₄ or due to the electrolyte decomposition reaction [80]. These differences in the experimental data may be related to the different structures and compositions of SiO_x materials used in each study.

Carbon has good intrinsic electronic conductivity and forms electronically conductive networks to enable fast electron transport [81,82]. In SiO_x-carbon composites, carbon coating has been used to increase the electrical conductivity and minimize the direct contact of active materials by forming a protective layer [55,82,83]. The coating processes are highly scalable and cost-effective, with the additional advantage of stabilizing the SEI layer [84]. Carbon coatings also reduce volume expansion by relieving stress owing to their ductility, sufficient porosity, and flexibility [85]. Another strength of carbon coating is the ease of application and flexibility compared to other coating methods [32,86]. Wet-chemistry synthesis using the pyrolysis of organic materials is considered one of the most useful, scalable, and easiest approaches. Chemical vapor deposition (CVD) and physical vapor deposition (PVD), which vaporize carbon or carbon precursors to deposit carbon on a substrate, are capable of forming uniform and stable coating layers. Research on carbon-coated Si electrodes showed that coating eliminated the parasitic reaction between the electrode and ethylene carbonate (EC)/propylene carbonate (PC), and the formed surface contained an electronic contact network that enhanced the electrochemical properties [87,88]. Due to its low intrinsic electrical conductivity, SiO_x is especially suited for carbon coating to enhance its electrochemical performance [89,90]. In an early report in 2007, Kim et al. coated carbon on SiO via wet chemical pyrolysis using polyvinyl alcohol [32]. The coated electrode showed a capacity of 800 mAh g^–1 and excellent cycle performance, which can be explained by the electronic contact bestowed on each particle through carbon coating that remained despite the volume change. Zhai et al. prepared polypyrrole (PPy) to prepare carbon-coated SiO composites via in-situ polymerization and high-temperature carbonization. The SiO/M@cPPy showed an ICE of 63.1% and an initial discharge capacity of 1657.9 mAh g^–1 [91]. Using a similar approach, researchers doped the carbon layers with heteroatoms by using nitrogen-, fluorine-, or phosphorus-containing polymers via wet-chemistry pyrolysis, effectively enhancing the electrochemical properties [92,93]. You et al. synthesized Si/SiO_x@F-C composites by in-situ pyrolysis of polyvinylidene fluoride (PVDF), which served as both carbon and fluorine source [92]. High-temperature decomposition of PVDF created a fluorine-doped carbon layer on the Si/SiO_x surface. XPS analysis confirmed that the fluorine-doped coating significantly increased the LiF concentration in the SEI layer, contributing to enhanced interfacial compatibility. The Si/SiO_x@F-C electrode showed an ICE of 79% and a high capacity retention of 82.12% after 2400 cycles.

Liu et al. used benzene as a carbon precursor to fabricate uniform carbon-coated SiO composites in a fluidized bed CVD reactor [86]. The carbon coating introduced a conductive network and pre-set voids, which compressed the polarization, enhanced the cycling performance, and reduced the irreversible electrode expansion. Simultaneously, secondary particles formed by the agglomeration of carbon-coated SiO particles acted as an additional volume buffer. The C-SiO composite electrode showed excellent capacity retention of 88% after 50 cycles. Yi et al. compared SiO anodes coated with pitch- and acetylene-derived carbon via CVD, and demonstrated that the acetylene-based coating featured a higher degree of graphitization and more uniform surface coverage [94]. These features contribute to improved electrical conductivity and structural integrity, leading to superior Li⁺ transport kinetics and cycling performance. Similarly, Zhang et al. reported on a dynamic CVD method that uses C₂H₂ to deposit a dense amorphous carbon layer on SiO, which improved both the electrochemical and mechanical performance by enhancing structural strength (Young’s modulus: 35 GPa) and suppressing volume expansion (9.3%) [95]. In 18650-type cells, C₂H₂@SiO achieved a high ICE of 87.58% and maintained 85.7% of its capacity after 1000 cycles at 25°C. In another study, the same researchers constructed a low-impedance carbon shell on SiO_x via sequential gas-phase coating using C₃H₈ and C₂H₂ [96]. The resulting SiO@C₃H₈@C₂H₂ structure featured short-range vertically aligned and long-range lamellar carbon networks, which reduced the charge transfer resistance (Rct: 13.3 Ω) and contact resistance (Rc: 1.6 Ω) compared to pristine SiO_x (Rct: 47.5 Ω, Rc: 7.2 Ω). This architecture showed a high ICE of 83.7% and achieved excellent capacity retention of 86.6% after 1000 cycles during full-cell evaluation at 0.5 C.

Exploiting the aforementioned good intrinsic properties of carbon materials, various SiO_x-carbon composite structures have been actively studied, such as yolk-shell, sugar apple-shaped, and nanowire structures [90,97,98]. Among them, composites formed from organosilica materials have the strength of naturally coexisting silica and carbon coating precursors, while being eco-friendly, easily scalable, and do not require expensive reagents [99–101]. For example, rice husks (RH) contain both carbon precursors (such as lignin, cellulose, and hemicellulose) and Si-based inorganic compounds [101–103]. Cui et al. used RH to fabricate porous SiO_x@C composites via carbonization and aluminothermic reduction [102]. The microporous SiO_x@C composite electrode exhibited a capacity of 1230 mAh g^–1 (0.01–3 V vs. Li/Li⁺) and excellent cycle life with less than 0.5% capacity loss after 200 cycles. Recently, Zhu et al. fabricated atomic-level distributed carbon in a SiO_x matrix using an organoalkoxysilane precursor [104]. While existing wetchemistry or physical mixing methods result in an inhomogeneous carbon distribution, their approach produced a homogeneous SiO_x-carbon composite. In organosilica nanospheres created by organic-inorganic hybrid composition, the organic alkly group of phenylene-bridged mesoporous organosilicas (PBMOs) is homogeneously distributed, and a homogeneous carbon matrix can be achieved through carbonization (Fig. 4a). A uniform carbon material was formed by graphitization of aromatic rings, as revealed by Raman spectroscopy and HAADF-STEM with energy-dispersive X-ray spectroscopy (EDS) (Fig. 4b,c). In the synthesized carbon composite with homogeneous atomic distribution, the molecularly interconnected carbon facilitated electron transport, and the micropores increased the Li⁺ diffusion rate, resulting in a high initial discharge capacity of 765 mAh g^–1 and a superior cycle capacity of 501 mAh g^–1 after 300 cycles.

In addition to conventional SiO_x-carbon composites, Si/C hybrid structures have been developed to combine the mechanical robustness and electrical conductivity of carbon materials with the high theoretical capacity of silicon-based materials [105–108]. These hybrid designs incorporate graphite, graphene, or engineered carbon shells to enhance the interfacial stability and ensure long-term cycling performance. Wang et al. introduced a dual interfacial reinforcement strategy employing graphene and carbon coatings to simultaneously enhance the electronic/ionic transport and structural stability during extended cycling [105]. The Gr-SiO_x@C electrode retained a discharge capacity of 652 mAh g^–1 over 1000 cycles at 1 C with a capacity retention of 97%. Xiao et al. developed a spherical natural graphite(SNG)/hollow-SiO_x@C composite by integrating hollow-structured SiO_x with natural graphite and a pitch-derived carbon coating [106]. The pitch functioned as both a conductive matrix and a binder, effectively maintaining cohesion between the SiO_x and graphite particles. This composite achieved an initial capacity of 465mAh g^–1 and maintained excellent cycling stability with a capacity retention of 93% over 500 cycles at 0.5 C.

Heteroatom doping adds a specific element to the SiO_x matrix through two types of approaches. In the first one, the substance is added to SiO_x to form a simple composite with improved electrochemical performance due to the intrinsic characteristics of the additive, such as high conductivity and mechanical stability [109–118]. In the second approach, the ICE and cycle performance are enhanced through reaction of the heteroatom (M) to form lithium-inactive phases before cycling [119–122].

A major advantage of composite materials is their ability to combine characteristics from different components to match the requirements for diverse applications. Ball milling is a simple process that can create a wide variety of composites. Because heteroatom doping is intended to compensate for the disadvantages of SiO_x, the heteroatom sources tend to have high electrical conductivities and stable structures to maintain an electric network with buffering roles. Tin (Sn) can undergo Si-like Li alloy reaction and shows good ductility and high electronic conductivity [109]. A SiO_x/Sn/C hybrid composite was formed by adding Sn particles to a 2D SiO_x sheet, followed by carbon coating to improve the electronic conductivity, tap density, and inner structural stability. The hybrid composite electrode showed an improved cycle performance of 838.5 mAh g^–1 after 100 cycles, and the introduction of Sn enhanced the tap density to 0.75 g cm^–3. Gu et al. fabricated a SiO_x@SnO₂@C composite by utilizing a lithiated SnO₂ material with high conductivity and excellent structural stability, resulting in suppressed Si pulverization and improved cycle performance of the electrode [110]. Fu et al. fabricated a Sn/SiO composite by ball milling nanoscale Sn and SiO together [123]. The re-activation of Li⁺ in lithium irreversible phase Li₂O was possible owing to the high electrical conductivity of Sn particles. Therefore, the ICE of the Sn/SiO composite electrode was higher than that of the pristine SiO electrode (85.5% vs. 66.5%). Copper (Cu) also has a high electrical conductivity (5.8 × 10⁷ S cm^–1) and good ductility for use as a dopant in SiO_x anode materials [112]. A SiO/Cu/expanded graphite (EG) composite was synthesized to exploit the buffering role of EG and Cu as conductive additives. The composite electrode had a high capacity and cycle performance of 836 mAh g^–1 at the 100^th cycle, because the Cu network and EG had excellent ductility and maintained facile electrical conduction while effectively alleviating stress in the electrode. TiO₂ has also been used as a dopant to improve the electrochemical performance of SiO_x [113–117]. Using the sol-gel method, Li et al. trapped TiO₂ nanoparticles in a SiO_x matrix and then applied carbon coating to create a watermelon-like SiO_x–TiO₂–C composite (Fig. 5a) [113]. Anatase TiO₂ exhibits high Li⁺ conductivity, excellent electrical conductivity in the lithiated form, and fast Li⁺ insertion/extraction through its (001) plane. The prepared SiO_x–TiO₂–C composite electrode showed a great cycle capacity of 910 mAh g^–1 after 200 cycles (Fig. 5b). Bae et al. coated Si/SiO_x with black TiO_2-x, and the corresponding electrode showed a high reversible capacity of 1200 mAh g^–1 with excellent performance up to 100 cycles [117]. TiO₂-based materials also have the unique ability to improve thermal reliability [116]. The discharge capacity of black TiO_2-x-coated Si/SiO_x electrode was improved by 3%, and its capacity recovery rate was 15% higher than that of the pristine Si/SiO_x electrode after staying in the charged state at 60°C for 48 h.

Some dopants enhance the electrochemical performance by modulating the SEI through the formation of stable lithium compounds or reversible conversion reactions. Zhao et al. fabricated a Se-doped C/SiO_x composite via solid-phase sublimation, in which Se was deposited on both the interior and exterior pore walls [124]. Se doping promoted the formation of Li₂Se in the SEI, thereby suppressing Li₂O formation and reducing the interfacial resistance. The Se@C/SiO_x electrode achieved a high ICE of 82.4% with a discharge capacity of 844.4 mAh g^–1 at 0.1 C. Similarly, Ling et al. synthesized a Co-doped SiO_x composite by coating SiO₂ spheres with a Co²⁺-doped polymer, followed by carbonization. The embedded Co nanoparticles enhanced charge transfer kinetics and contributed to additional reversible capacity through the conversion of Li₂O [125]. The SiO_x@NC-Co electrode showed an ICE of 62.2% with a discharge capacity of 890.95 mAh g^–1 at 0.1 A g^–1.

After the chemical reaction of SiO_x with the heteroatom element M, the resultant MO_x or M–Si–O Li-inactive phases may increases ICE. These phases could further improve the electrochemical properties through their own characteristics such as good mechanical strength. Zhang et al. fabricated a C–SiO–MgSiO₃–Si anode material with high ICE by reacting MgO with SiO to form MgSiO₃ [120]. The Li-inactive MgSiO₃ buffer phase and the mesoporous secondary particles improved the ICE and cycle performance of the C–SiO–MgSiO₃–Si secondary particle electrode, with a high ICE of 78.3% and a great capacity of 1608 mAh g^–1. Similarly, Youn et al. prepared a Si/Mg₂SiO₄ composite material via the MgH₂-driven magnesiation of SiO, with Mg₂SiO₄ acting as a Li-inactive buffer phase [126]. The endothermic dehydrogenation of MgH₂ effectively suppressed coarsening of the Si domains. As a result, the Si/Mg₂SiO₄ nanocomposite electrode showed a high ICE of 89.5% and an initial discharge capacity of 1108 mAh g^–1. Using a different doping element, the same group demonstrated a porous Si/Ca–Si–O nanocomposite via the calciothermic reduction of SiO using CaH₂, in which the Li-inactive Ca–Si–O domains served as the buffer phase [127]. Subsequent HEMM resulted in a porous microstructure capable of accommodating volume changes in Si during cycling. This porous Si/Ca–Si–O composite electrode showed a high ICE of 86.4% and maintained a discharge capacity of 55% after 200 cycles. Alumina also has sufficiently high structural stability to suppress volume expansion during the Li_xSi alloying reaction, and it can compensate the low electrical conductivity of SiO_x [128,129]. Kim et al. prepared a carbon-coated Si–SiO_x–Al₂O₃ composite via a two-step process: (1) aluminothermic reduction of SiO₂ to prepare a Si-embedded SiO_x–Al₂O₃ composite, and (2) carbon coating using naphthalene as a carbon precursor. The carbon-coated Si–SiO_x–Al₂O₃ composite electrode showed a high ICE of 74.3% and excellent cycle performance of 500 mAh g^–1 after 500 cycles [122].

Prelithiation is under active investigation as a way to increase the ICE of SiO_x electrodes through heteroatom doping. The goal is to reduce active lithium loss (ALL) due to the initial irreversible formation of lithium oxide and Li–Si–O in SiO_x, which decreases the ICE [130]. Because a full cell contains a limited amount of Li⁺, this irreversible reaction causes internal capacity loss and a lower energy density [130]. Prelithiation works by inserting lithium before cycling. Unlike other heteroatom doping methods, prelithiation artificially creates phases that are also formed during the actual electrochemical reactions of cells, such as lithium silicates, lithium oxide, and even SEI layers. Besides an electrochemically irreversible phase, it can also create a reversible Li_xSi alloy phase that enables the ICE to even exceed 100% [131].

We divide prelithiation methods into five types: (1) contact, (2) impregnation, (3) short circuiting, (4) vapor deposition, and (5) thermal. In the contact method, Li metal is directly attached to the SiO_x anode. This method is scalable, easily processable, and allows the formation of an SEI layer on the electrode. Using the contact method, researchers reported improved performance of SiO_x full cells and lithium-ion capacitors [132,133]. However, its utilization remains difficult due to poor reaction control, possible Li-dendrite formation, inhomogeneous solid-solid reaction, and safety issues associated with Li metal. Recently, a homogeneous and controllable contact method was proposed to prelithiate SiO_x electrodes by using polyvinyl butyral-coated carbon nanotube as a resistance buffer layer (RBL) [134]. The RBL inhibits direct contact between the Li metal foil and the SiO_x anode. By making contact prelithiation homogeneous and controllable, RBL-regulated prelithiation improves the ICE from 79% to 89% and shows stable cycle performance up to 200 cycles in full-cell applications, compared to the electrode without prelithiation.

Impregnation or organochemical prelithiation utilizes the characteristics of Li metal melted with benzene-based substances in organic solvents. In this method, electrodes or anode materials are prelithiated by immersion in lithium-containing benzene-organic solutions [135–137]. The impregnation approach is scalable and could achieve relatively uniform reaction. Tabuchi et al. performed organochemical prelithiation by immersing electrodes in a Li-organic complex solution consisting of Li metal and naphthalene in butyl methyl ether [135]. After 72 h, the prelithiated SiO electrode showed a discharge capacity of 670 mAh g^–1 without charging. Huang et al. used a prelithiation agent in an SLMP-SBR-toluene (SST) suspension, and the ICE was improved from 66% to 70%–120% [138]. Li et al. immersed negative electrodes in a LAC (N-LAC and B-LAC) solution for a controlled period and at specific temperatures [139]. For N-LAC solution at 60 min, the cells showed an ICE of 93.5%. The cell prelithiated with N-LAC also had a longer cycle life and a higher energy density. In another study, the SiO electrode was prelithiated by immersion in a solution of lithium-2-fluorobiphenyl-2-methyltetrahydrofuran (Li-FBP-MeTHF) [140]. The SiO electrode prelithiated with a LiF oligomer layer exhibited high hydrophobicity and maintained good air stability. Its ICE also increased to 101.7%.

In the short-circuit prelithiation method, artificial external short circuits are created between the Li source and the electrode. This method is both scalable and controllable [131,141]. Kim et al. reported a roll-to-roll short-circuit prelithiation process using Li metal foil, as shown in Fig. 6a,b [131]. The penetration speed and degree of reaction were controlled by changing the resistance applied to the external circuit. XPS and time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyses confirmed that the artificially formed SEI layer was identical to that formed during the actual electrochemical process. The longer the prelithiation time, the higher the ICE (up to 107.9%), but the optimal efficiency was probably only 94.9% owing to Li dendrite formation caused by overcharging, which degraded the electrochemical properties (Fig. 6c,d).

In vapor deposition, a Li metal source is vaporized to induce a gas-phase reaction on the electrode. Takezawa et al. vaporized metallic Li using a thermal evaporator to manufacture prelithiated a-SiO_x electrodes with different x values (0.51, 0.68, and 1.02), by adjusting the required amount of Li metal according to the oxygen ratio [142]. The prelithiated SiO_1.02 electrode showed a high ICE of 118% and a capacity of 1041 mAh g^–1.

In the thermal method, heat treatment is applied to induce chemical reactions between SiO_x and Li source, thereby directly prelithiating the active material [74,143–145]. Zhao et al. prepared Li_xSi nanodomains in a Li₂O matrix through the reaction between SiO and Li metal [144]. The resultant electrode showed an excellent capacity of 1240 mAh g^–1 without charging, even after exposure to a humidity of 40% RH for 6 h. Yom et al. demonstrated a prelithiated SiO electrode with 82.1% ICE by producing Li₂SiO₃ and Li₄SiO₄ as irreversible phases through heat treatment of Li powder and SiO_x [74]. In the Li metal-free prelithiation method proposed by Chung et al., LiH is dehydrated at elevated temperatures above 470°C [146,147], and the released Li acts as a precursor for the preemptive formation of lithium silicates (the primary cause of low ICE for SiO) [146]. Prelithiation of SiO induces a phase transformation into Si and Li₂SiO₃ with an optimized topological arrangement [147]. The material treated at 650°C exhibited ~90% ICE and stable cycling over 300 cycles. This improvement in performance is due to the uniform nanoscale distribution of the active and inactive phases, which significantly reduces volume expansion.

Structural modification aims to improve the electrochemical performance by morphological changes, such as pore generation and nanostructuring, in order to control volume expansion and enlarge the surface area to reduce the Li diffusion length and improve the kinetics. In 2007, Kim et al. adjusted the particle size of SiO_x through a ball-milling process to increase the mechanical stability and significantly improve the cycle performance [32]. The ball-milled SiO–C composite electrode showed an excellent cycle life because of the homogeneous volume expansion, owing to the uniform particle size and shape with free voids created by the carbon coating. Intensive research efforts have been devoted to enhancing the electrochemical properties of SiO_x through structural modifications by creating ball-milled SiO_x–C composites [86,148–150].

A porous structure facilitates the transport of Li⁺ through a larger surface area and improves the cycle life through pre-set voids [25,151]. Park et al. prepared a c-Si-embedded Si/SiO_x complex via the hydrothermal reduction of hydrogen silsesquioxane (HSQ) prepared by the sol–gel process involving triethoxysilane (Fig. 7a) [22]. This non-toxic and highly scalable sol-gel process produced a nanosized porous structure with superior expansion control. As a result, the electrode showed 78% capacity retention after 100 cycles and 20.8% volume expansion after two cycles (Fig. 7b,c). Similarly, Lee et al. formed porous SiO particles through HF etching of Ag-deposited SiO, which was formed by the reaction between AgNO₃ and SiO. The porous SiO electrode delivered a high specific capacity of 1520 mAh g^–1 with stable capacity retention of 1490 mAh g^–1 after 50 cycles [152]. Park et al. obtained a porous SiO composite by mechanochemically oxidizing an inexpensive Si powder while simultaneously reducing ZnO powder using a high-energy ballmilling process [153]. The porous SiO composites exhibited enhanced electrochemical performance compared to pristine SiO, which was attributed to the microstructural changes. Raza et al. created double-layered SiO_x/Mg₂SiO₄/SiO_x composites through the magnesiothermic reduction of micro-sized SiO with Mg metal powder [154]. The composite electrode showed an ICE of 75.6% and a capacity retention 86% after 300 cycles. Core-shell matrices were also studied to utilize the characteristics of both the core and shell components. For example, core-shell SiO_x synthesized by gradually increasing the x value of SiO_x from the inside to the outside combines both the high CE and high specific capacity of Si and the superior cycle reliability of SiO_x [155–157].

Nanostructuring enhances the electrochemical performance of SiO_x by controlling its structure at the nanoscale [158,159]. For example, controlling the direction of SiO_x expansion can reduce crack formation during the volume expansion of Si, increase electronic conductivity by forming a conductive network, and decrease the Li diffusion length, resulting in superior Coulombic efficiency, cycle performance, and rate capability. Reported SiO_x nanostructures include nanorods [34], nanotubes [160], nanocoils [161], nanowires [162], and 3D gyroid structures [163]. Li et al. reported core–shell porous carbon-coated SiO_x nanowires (pC-SiO_x NWs) through the self-sacrificial SiO_x nanowire formation using bimodal mesoporous silica (BMS) [90]. The 1D structure effectively alleviated the strain, maintained the structure, and prevented pulverization during repeated cycles. The corresponding electrode demonstrated excellent cycle performance: the capacity did not decrease from the 2^nd to the 150^th cycle and remained at 1060 mAh g^–1 after 100 cycles.

While nanosized particle modification can create electrodes with excellent cycle performance and rate capability, excessive and irreversible SEI formation may occur because of the increased surface area. The trade-off between nanosized modification and excessive SEI formation should therefore be carefully optimized. Yu et al. proposed a hybrid structure comprising active SiO particles and carbon nanofibers (SiO/CNFs) [164]. The nanofibrous carbon skeleton prevented particle agglomeration and restricted the volume expansion of SiO. A capacity retention of 73.9% was achieved after 400 cycles, because the structure not only provided a conductive network for active SiO nanoparticles but also surmounted agglomeration sufficiently.