To improve electrical conductivity of SiO _x -based electrodes, combining SiO _x with conductive carbonaceous materials is an effective strategy. Owing to their unique properties of high conductivity, good ductility, readily accessibility, and ability to form stable SEI layers, carbonaceous materials are ideal additives for electrode materials (Choi et al., 2012; McDowell et al., 2013). Guo et al. developed a graphite-like SiO _x /C composite, in which the nano-sized SiO _x particles uniformly anchored in the carbon matrix (Xu et al., 2018a). Benefiting from the unique structure, when used as anode for LIBs, the obtained graphite-like SiO _x /C derived high reversible capacity of 645 mAh·g⁻¹ with a high capacity retention of 90% for 500 cycles. Wu and his co-workers designed a two-step manufacturing process to prepare SiO_x-C composite with a hierarchical structure (Wu et al., 2015). After 100 cycles, the SiOx-C anode delivered a reversible specific capacity of 674.8 mAh·g⁻¹ and 83.5% capacity retention at 100 mA·g⁻¹. Direct growth of vertical graphene nanosheets on SiO microparticles can remarkably improve the lithium-storage performance, which showed a high capacity retention up to 93% after 100 cycles (Shi et al., 2017). More recently, an integral interface containing Li polyacrylate (Li-PAA) and carbon nanotubes (CNTs) was constructed on the carbon-coated SiO_x by Guo et al. (Li et al., 2020). The flexible Li-PAA protective layer can not only adjust the volume change of SiO_x due to its high stretchability (up to 582%), but also provide uniform Li⁺ transmission interface during charging and discharging. The embedded CNTs can provide fast electronic pathways in the Li-PAA layer, which ensures the superior electronic conductivity. Attributing to the dynamically stable interface (Figure 1), the obtained C-SiO_x/C anode showed a remarkably improved cycling ability for 500 cycles, and a high reversible specific capacity of 836 mAh·g⁻¹ can be delivered. The pea-pod structure of SiO_x/C composite via combining electrospinning and high-temperature carbonization also showed the enhanced cycling stability and rate performance, attributing to its unique structure with carbon conductive network (Zheng et al., 2020).

On account of favorable electrical conductivity and good flexibility, metals or alloys can be used to improve the lithium-storage electrochemical performance of SiO _x anode. The lithium-inactive metal of Fe, Ni or Ti was doped into SiO _x , respectively, via a co-deposition technique, which was conducive to the diffusion of Li⁺ (Miyachi et al., 2007). The obtained Ni-doped SiO _x anode delivered a high ICE of 84% and a capacity retention rate of 82% after 400 cycles when matched with an manganese oxide cathode. Similarly, a mixture of SiO, Ni, and reduced graphene oxide was prepared by a hydrothermal mixing and sintering process, and showed an improved cycle performance (Liu et al., 2018). A SiO/Cu/expanded graphite composite was fabricated by a simple electroless plating combined with ultrasonication method, and delivered a markedly improved reversible capacity and cycling stability (Zhang et al., 2017b). In addition, Xu et al. developed a carbon-coated SiO/Cu composites via Cu deposition combined with carbon coating, which also demonstrated a good cycling performance with capacity retention of 88.3% (Xu et al., 2018b). More recently, by conducting a selective alcoholysis method, Kwon et al. synthesized a vanadium-doped SiO_x composites, which exhibited an excellent reversible capacity of 1,305 mAh·g⁻¹ at 100 mA·g⁻¹, attributing that the sluggish kinetics of the electrochemical reactions between SiO _x and lithium has been accelerated by metal doping (Kwon et al., 2020). The pre-lithiation strategy of SiO_x can also greatly improve its ICE (Kim et al., 2016).

The SiO₂ phase in SiO can be partially converted into other lithium inactive metal oxides and additional silicon through chemical reduction of SiO₂ with elemental metals, which would reduce the irreversible lithiation and improve the reversible specific capacities and ICEs of the electrodes. Based on this, Jeong and his colleagues converted the SiO₂ phases in SiO into lithium-inactive alumina and new formed nano-silicon through a mechanochemical process, during which the nanostructured SiAl_0.2O material was in situ formed. The new formed silicon nanocrystallites in the matrix enhanced the ICE, while the lithium-inactive alumina improved the cycling performance (Jeong et al., 2010).

Beyond that, the electrochemically active metal can be also added into SiO_x for enhancing lithium-storage performance. For example, nanoscale Sn particles were mixed with SiO through a mechanical milling process, and the obtained hybrid delivered a significant improvement of both specific capacity by 50% and ICE from 66.5 to 85.5% (Fu et al., 2019). During the lithiation/de-lithiation process, Sn nanoparticles not only can boost reaction kinetics due to their excellent conductivity, but also play as a reagent revive intermediate Li₂O interphase by the reaction of Sn + xLi₂O → SnO _x + xLi⁺ + xe⁻, which benefits to the improvement of the lithium-storage performance. A submicron-sized hybrid SiO_x/Sn/C anode was prepared by Li et al., in which the 0D-Sn nanoparticles were reduced in situ in 2D-SiO _x inner layer and closely coated by carbon outer layer. With the introduction of Sn, the as-obtained SiO_x/Sn/C anode demonstrated a remarkable tap density of 0.74 g·cm⁻³ and an excellent long-cycle performance of 654 mAh·g⁻¹ at a current density of 1 A·g⁻¹ even after 500 cycles (Li et al., 2019).

In addition, alloys are also used as boosters for the electrochemical properties of SiO_x-based anodes. For instance, Abouimrane et al. prepared a SiO-Sn_xCo_yC_z composite by high-energy mechanical milling using 50 wt% SiO and 50wt% Sn₃₀Co₃₀C₄₀ as raw materials. The as-obtained hybrid exhibited a remarkable improved specific capacity of 900 mAh·g⁻¹ at 300 mA·g⁻¹ (Liu et al., 2012) . A nanocrystal-FeSi-embedded Si/SiO_x anode was synthesized using Fe-Si alloy as the raw material (He et al., 2017). Using the amorphous SiO_x as a buffer layer and the self-conductive nanocrystal-FeSi as a robust skeleton, the as-prepared sample showed a reversible capacity of 616.6 mAh·g⁻¹ even at a high current density of 500 mA·g⁻¹. The retractable three-dimensional porous residual Al-doped Si/SiO_x composite was also prepared from 6-μm Al-Si alloy particles (Wang et al., 2020), which displayed an attractive application prospects with specific capacity of 899.7 mAh·g⁻¹ even after 300 cycles. Recently, MXene/Si@SiO _x @C layer-by-layer superstructure with auto-adjustable function was successfully prepared by magnsiothermic reduction and pyrolytic carbon coating using two-dimensional MXene Ti₃C₂T_x (Zhang et al., 2019b). The as-obtained nanohybrids provided a reversible specific capacity of 1,674 mAh·g⁻¹ at 0.2 C with an initial coulombic efficiency of 81.3%, and superior stable lithium storage with 76.4% capacity retention even after 1,000 cycles. The superior lithium-storage performance can be attributed to the advantages of mechanical stability by the synergistic effect of SiO_x, MXene, and N-doped carbon coating, and excellent structural stability by the forming a strong Ti-N bond among the layers.

In recent years, the strategy of combining SiO _x with metal oxides has been adopted to improve the electrochemical lithium-storage performance of SiO _x -based anodes. Among them, TiO₂ is usually considered as one of the best composition of electrode materials due to the high reversibility and conductivity of the lithiated product Li _x TiO₂ (TiO₂ + nLi⁺ + ne⁻ ↔ Li _n TiO₂). Furthermore, the insertion/extraction process of lithium ions in TiO₂ layer is very rapid along (001) plane. A nano-scale and thin TiO₂ surface coating on SiO has been obtained by a facile sol–gel process (Jeong et al., 2012). According to the unique role of the TiO₂ coating, the obtained anode delivered significantly improved specific capacity, coulombic efficiency, rate capability and cycling performance, which are much higher than these of bare SiO electrode. Li et al. embedded ultrafine TiO₂ nanocrystals in SiO _x particles to form SiO_x-TiO₂ dual-phase core and then coated them with carbon shells (Li et al., 2018). The initial lithiation process of the as-obtained watermelon-like SiO_x-TiO₂@C nanoparticle is shown in Figure 2. The incorporation of TiO₂ effectively enhanced the lithium ionic and electrical conductivities, and released the structure stress during cycling. As a result, the as-obtained watermelon-like structured SiO _x -TiO₂@C nanocomposite could exhibit a high capacity of 910 mAh·g⁻¹ for 200 cycles at a current density of 100 mA·g⁻¹. Hu and his co-workers designed a nitrogen plasma-treated core-bishell structure, in which Si nanoparticles were encapsulated in SiO _x shell and N-doped TiO_2-δ shell (Hu et al., 2019). Both the SiO_x shell and N-doped TiO_2-δ shell acted as buffer components to adapt the volume change and stabilize the SEI films. In addition, increased oxygen vacancies and Ti³⁺ species can be obtained after the nitrogen plasma treatment, resulting in the improved diffusion kinetics and conductivity of Li⁺. After 300 cycles in the half-cell, the Si@SiO _x @TiO_2−δ electrode showed excellent cycling stability with a reversible specific capacity of 650 mAh·g⁻¹ at 200 mA·g⁻¹.

Meanwhile, during the charge/discharge processes, corresponding metal nanoparticles can be in situ transformed from the added transition metal oxides, which are helpful to increase the electrical conductivity of the composite materials. Zhou et al. successfully prepared a SiO/Fe₂O₃ composite via a mechanical grinding process (Zhou et al., 2013). The additional Fe₂O₃ phase can be lithiated to form metallic Fe during the discharge processes, which can improve the electrical conductivity, resulting to the enhanced reversible specific capacity and ICE from 59 to 68%. An egg-like few-layered graphene-wrapped and Fe₃O₄-pillared SiO_x anodes was established by Liao et al., which delivered a reversible specific capacity of 833.4 mAh·g⁻¹ with a high ICE of 84.9% and capacity retention ratio of 81.8% even after 500 cycles at a current density of 500 mA·g⁻¹ (Liao and Wu, 2019). The excellent performance was benefited from the comprehensive effects of integrated structure, enhanced Li⁺-diffusion kinetics, and improved pseudocapacitance behavior.

Morover, metal oxides can react with SiO _x to form metal silicate compound at high temperature, which can increase the crystallinity of SiO _x , resulting in the improvement on the ICEs of SiO _x -based anodes. For instance, SiO was coated with a olivine structure Fe₂SiO₄ layer by heating Fe₂O₃ and SiO at a molar ratio of 1:0.2, which significantly improved ICE from 70 to 90% (Yamamura et al., 2013). Zhang et al. prepared carbon-coated C-SiO-MgSiO₃-Si composites by heating the mixture of SiO, MgO and Si (Zhang et al., 2019a). The in situ formation of the MgSiO₃ phase can consume the deleterious SiO₂ phase resulting from the disproportionation reaction of SiO at high temperatures, which helps to improve the ICE to 78.3% for lithium storage. Meanwhile, as an electrochemically inert phase, MgSiO₃ can also effectively buffer the volume change during cycling, which is beneficial to enhanceing the cycling stability.