Various Technologies to Mitigate Volume Expansion of Silicon Anode Materials in Lithium-Ion Batteries

Abstract

Silicon anodes for lithium-ion batteries (LIBs) offer exceptional theoretical capacity (~4200 mAh/g) but face critical challenges due to significant volume expansion (>300%) during lithiation, leading to mechanical degradation and rapid capacity fading. This review highlights recent advancements in mitigating these issues, including structural designs such as core–shell architectures, porous composites, and multidimensional encapsulation techniques that buffer mechanical stress and stabilize the solid electrolyte interphase (SEI). Binder innovations and hybrid material systems further enhance electrode integrity and cycling stability. While substantial progress has been made, challenges remain in scaling these solutions for commercial applications. This paper provides insights into current strategies and future directions for enabling silicon-based anodes in next-generation LIBs.

1. Potential and Challenges of Silicon Anodes

While lithium-ion batteries (LIBs) play a central role in modern society’s energy transition, currently commercialized graphite anodes have reached their theoretical capacity limit (372 mAh/g), creating a major obstacle for further energy density enhancement [1]. To overcome this limitation, researchers are actively investigating high-capacity anode active materials such as silicon (Si), tin (Sn), and metal oxides [2,3]. Silicon, in particular, offers a theoretical capacity more than 10 times higher than graphite, but faces challenges related to volume expansion during charge–discharge cycles and the resulting electrode structural instability. Therefore, multifaceted approaches including nanostructuring, hybridization, and surface modification are being pursued to maximize the potential of silicon anode materials [4,5], which is gaining attention as a key research direction for building sustainable future energy systems.

Beyond these large-scale applications, silicon anode technology is also paving the way for the development of compact and lightweight three-dimensional (3D) lithium-ion microbatteries for on-chip power supplies. In this emerging field, aligned silicon vertical pillar arrays, fabricated directly on silicon substrates using mature semiconductor processes, enable the monolithic integration of battery anodes with electronic circuits and MEMS. Interestingly, even in this micro-scale domain, hybrid material system approaches, such as applying a carbon coating to silicon nanowires to suppress volume expansion, are being actively investigated as a key strategy. For a comprehensive overview of this specific area, readers are referred to recent reviews on micro lithium batteries, advances in 3D silicon-based microbatteries, and the effects of carbon-coating on structured Si nanowires [6,7,8].

The aforementioned endeavors to replace conventional graphite anodes have placed Si in the spotlight as the most promising next-generation anode material [9]. Si boasts a theoretical mass-specific capacity of approximately 4200 mAh/g (based on Li_4.4Si), which is more than 10 times higher than that of graphite (372 mAh/g), a relatively low operating potential (approximately 0.4 V vs. Li/Li⁺), along with abundant natural reserves and environmental friendliness [10,11]. Figure 1 schematically illustrates the morphological evolution of silicon microparticles and nanoparticles during repeated lithiation and delithiation. Silicon undergoes extreme volumetric expansion of up to ~400% during the formation of Li_xSi alloys, which generates significant internal stress. This stress leads to particle cracking, pulverization, and continuous rupture of the solid electrolyte interphase (SEI), accelerating capacity fading and impedance growth. The figure also highlights the difference between micron-sized silicon, which tends to fail catastrophically, and nanosized silicon, which better accommodates volume changes due to shorter diffusion paths and higher surface-to-volume ratios. These insights emphasize why nanostructuring and composite architectures are crucial to enhance mechanical stability and cycling durability.

Reinforcing its sustainability, recent studies have demonstrated viable methods for recovering high-performance, porous micro-sized silicon from waste AlSi alloys, highlighting a promising direction for cost-effective and eco-friendly production [12]. The successful commercialization of silicon anodes could dramatically increase the energy density of LIBs, enabling significantly extended driving range for electric vehicles and prolonged usage time for portable devices. However, silicon anodes face serious technical challenges, primarily the extreme volume change during alloying and dealloying reactions with lithium ions [13,14]. Silicon can expand by up to 400% when forming Li_xSi alloys, causing substantial mechanical stress that leads to cracking and pulverization of active material particles [15,16], weakened adhesion between components resulting in delamination from the current collector [17], and formation of thick, unstable solid electrolyte interphase (SEI) layers at newly exposed surfaces that impede ion movement and cause capacity loss [18]. These complex problems severely degrade cycle life and electrical conductivity, creating major obstacles for commercialization [19]. Therefore, innovative solutions to control volume expansion and mitigate secondary issues are urgently needed to utilize silicon anodes’ inherent advantages.

Addressing the low initial Coulombic efficiency (ICE) is also a critical task, and recent interfacial engineering on porous SiO_x@C composites has demonstrated the possibility of achieving high ICE (93.4%) through prelithiation, offering a promising solution to this long-standing problem [20].

Figure 1. Schematic illustrations of morphology variation in Si active materials caused by charge/discharge processes. The Si active materials were (a) Si microparticles and (b) Si nanoparticle aggregates. Reproduced from ref. [15], licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Accessed on 30 May 2025.

Figure 1. Schematic illustrations of morphology variation in Si active materials caused by charge/discharge processes. The Si active materials were (a) Si microparticles and (b) Si nanoparticle aggregates. Reproduced from ref. [15], licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Accessed on 30 May 2025.

2. Concept and Principles of Hybrid Material Systems

2.1. Definition of Hybrid Material Systems

Hybrid material systems combine two or more different materials at the nanometer or micrometer level to achieve enhanced properties that single components cannot provide alone [21,22]. These systems maximize the advantages of each constituent material while compensating for individual disadvantages through synergistic effects [23].

In silicon anodes, hybrid material systems offer critical solutions to address volume expansion issues [24]. Figure 2 details a multi-step mechanochemical process starting from micrometric Si and CuO. Through high-energy and wet milling, these precursors are transformed into a composite of nanocrystalline and amorphous phases. A final carbonization step with glucose yields the carbon-coated Si/Cu₃Si composite. This illustrates a practical approach to creating a hybrid material designed to buffer volume changes and improve conductivity simultaneously. While silicon offers high theoretical capacity, its extreme volume expansion causes cracking, delamination, unstable SEI formation, and capacity/conductivity loss [16]. Hybrid material systems provide multifaceted solutions to mitigate these problems while improving electrical conductivity and interfacial stability. Key examples include Si/C composites that provide mechanical buffering and enhanced conductivity, Si/graphene systems that offer excellent electrical pathways and structural support, and Si/polymer binder combinations that maintain electrode integrity during cycling. These approaches represent key technological strategies for silicon anode commercialization [25].

2.2. Working Mechanisms of Hybrid Material Systems in Silicon Anodes

Hybrid material systems address silicon anode challenges through four core mechanisms that work synergistically to overcome the fundamental limitations of pure silicon [26,27]. The most critical function involves volume expansion mitigation, where core–shell structures with silicon nanoparticles encapsulated by carbon shells physically constrain expansion [28], while yolk–shell structures with intentional void spaces allow silicon expansion without electrode deformation [29]. Figure 3 illustrates how yolk–shell design preserves electrode integrity, showing that at low current density (10mC), the crystalline Si core experiences significant stress leading to cracking, but at higher density (50mC), the Si core becomes amorphous while the overall structure remains intact due to adequate void space for volume accommodation.

Equally important is the enhancement of electrical conductivity throughout the electrode structure. Carbon-based materials such as graphene, CNTs, and amorphous carbon serve as conductive matrices providing electron pathways between silicon particles and current collectors [30,31]. Conductive polymers and certain metal oxides also contribute to conductivity improvement [32], creating a robust electrical network that maintains connectivity even during the dramatic volume changes experienced by silicon particles.

The stabilization of the solid electrolyte interphase represents another crucial mechanism for improving silicon anode performance. Protective coating layers including carbon, Al₂O₃, and TiO₂ prevent direct silicon–electrolyte contact, stabilizing SEI formation and reducing electrolyte consumption [29,33,34]. This approach improves ICE and cycle stability [35,36], while advanced approaches include fluorinated MXene coatings that create LiF-rich, stable SEI layers [37]. These protective layers act as selective barriers that allow lithium ion transport while preventing unwanted side reactions.

Finally, mechanical stability enhancement plays a vital role in maintaining electrode integrity over extended cycling. Carbon matrices and metal oxide shells provide superior mechanical strength and flexibility compared to silicon, preventing particle pulverization and maintaining electrode integrity through strong interfacial interactions [19,30,38]. This mechanical reinforcement works in concert with the other mechanisms to create a robust electrode structure that can withstand the stresses associated with repeated lithiation and delithiation cycles.

3. Types and Characteristics of Hybrid Material Systems

Hybrid material systems can be classified in various ways according to their constituent materials and structural characteristics. The hybrid systems that have been most actively researched for improving the performance of silicon anodes can be broadly categorized by their constituent materials into silicon–carbon, silicon–metal oxide, silicon–2D material, and silicon–conductive polymer hybrid systems, among others. This section examines in detail the structure, characteristics, performance, and latest research trends of each of these types of hybrid systems. Furthermore, synergistic strategies combining different modification approaches, such as germanium (Ge) doping to enhance conductivity and liquid metal alloy surface modification to create a flexible buffer layer, are being explored to maximize the performance of micron-sized silicon anodes [39].

3.1. Classification According to Constituent Materials

3.1.1. Silicon–Carbon Hybrid Systems

Carbon-based materials are the most widely researched hybrid partners for improving the performance of silicon anodes due to their excellent electrical conductivity, high mechanical stability, chemical stability, and the various possibilities they offer for structural control [40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69]. Hybrid systems that are a combination of silicon and carbon offer multifaceted benefits: the carbon matrix effectively buffers the volume expansion of silicon, provides excellent electrical conduction pathways, and facilitates the formation of stable SEI layers. Representative silicon–carbon hybrid systems include composites with graphene [52], CNTs [53], and carbon nanofibers (CNFs) [54]. Figure 4 provides a comprehensive overview of the major structural designs for Si/C anodes. It categorizes the approaches into Embedded Structures, where Si is dispersed in a carbon matrix; Carbon-coated Structures, featuring a core–shell design; and Hollow Structures, which incorporate void spaces. The accompanying TEM images for each category offer a visual confirmation of these nanoscale architectures, showcasing the diverse strategies employed to tackle the volume expansion issue.

Graphene, a two-dimensional carbon material with outstanding electrical conductivity, a large specific surface area, excellent mechanical strength, and flexibility, has attracted attention as an ideal material for hybridization with silicon [55]. Various structures have been studied, including silicon nanoparticles wrapped with graphene sheets, sandwich structures in which silicon particles are intercalated between graphene sheets, or silicon dispersed within three-dimensional graphene networks [56]. Graphene effectively accommodates the volume expansion of silicon, prevents particle agglomeration, and provides rapid electron transfer pathways, which, in combination, result in high capacity and excellent cycle and rate characteristics [57]. For example, hybrid electrodes fabricated by depositing silicon onto three-dimensional porous graphene foam using chemical vapor deposition (CVD) demonstrated high reversible capacity and long-term cycle stability [58].

Carbon nanotubes (CNTs), as one-dimensional carbon materials with high aspect ratios, excellent electrical conductivity, and mechanical strength, have also been actively applied to silicon hybrids. CNTs form entangled networks around silicon particles to provide effective electron conduction pathways and disperse the mechanical stress that accumulates during the volume changes experienced by Si. Additionally, CNT networks contribute to enhancing the structural stability of electrodes by strengthening the adhesion between active materials, conductive additives, and binders within the electrode [59]. CNT-Si hybrid materials are produced using methods such as directly growing silicon nanoparticles on CNT surfaces or mixing silicon and CNTs to manufacture composites, and hybrid electrodes utilizing CNTs have been reported to exhibit improved rate capabilities and cycle stability [60].

Carbon nanofibers (CNFs), manufactured using methods such as electrospinning, have also been usefully employed in silicon hybrid systems. CNFs have continuous one-dimensional structures and large specific surface areas and can be hybridized by either incorporating silicon nanoparticles within their porous nanofibers or by coating their surfaces with silicon. CNFs effectively buffer the volume expansion of Si through their flexible structure, and the fiber network provides excellent pathways for electron and ion transfer [61]. In particular, structures comprising silicon embedded inside porous CNFs accommodate the volume expansion within the internal space to minimize external deformation and induce stable SEI formation, thus improving the electrochemical performance [62].

Researchers working on these various silicon–carbon hybrid systems have reported significant performance improvements in the initial capacity, Coulombic efficiency (CE), cycle stability, and rate characteristics compared to pure silicon or conventional graphite anodes [63]. A prime example of such improvements is detailed in Figure 5. The schematic in Figure 5a illustrates the synthesis of crumpled graphene-encapsulated silicon nanoparticles (cpDOPA-crGO-Si) via a spray-drying process. As quantitatively demonstrated in Figure 5b, this graphene-encapsulated structure maintained a high capacity of over 1000 mAh/g even after 100 cycles at a current density of 1 A/g [63]. This result validates that the graphene encapsulation strategy effectively buffers volume changes and ensures stable, long-term performance. Meanwhile, other approaches, such as hierarchically combined porous silicon structures with CNTs, have also exhibited excellent capacity retention rates even at high rates of 5 A/g [64]. Each hybrid system has unique advantages and disadvantages depending on the type of carbon material, structure, and bonding method with silicon, and requires the system design to be optimized according to the performance objectives required for specific applications.

Looking at recent research trends, ternary carbon-based hybrid systems, which maximize the synergistic effects beyond the performance of single carbon materials by using two or more types of carbon materials together, are being actively studied. For example, the purpose of structures that simultaneously use graphene and CNTs to wrap silicon particles combine the two-dimensional coverage provided by graphene with the one-dimensional connectivity of the CNTs to more effectively buffer the volume expansion and construct the electron transfer network [65]. Additionally, multifunctional carbon composites, which offer enhanced electrical conductivity, surface affinity, and lithium storage capability by accepting heteroatoms such as nitrogen (N), sulfur (S), and phosphorus (P) as dopants into their carbon matrices, are also attracting attention [66,67]. These latest developments are contributing to further enhancing the performance of silicon–carbon hybrid systems and increasing their potential for commercialization.

Table 1. Electrochemical performance summary of silicon-based hybrid anode systems reported in the references of Section 3.1.1. Only numerical data explicitly reported in the cited studies are included.

Si Anodes	Current Density (A/g)	Cycle Number	Remaining Capacity (mAh/g)	Source (1st Author, Year, [Ref. No.])
Silicon-Carbon
Si/C	0.1	100	941.1	Zhang, 2023, [42]
Si/C	0.5	100	605.43	Li, 2023, [43]
Si/C	2.0	400	1283	Ma, 2023, [44]
Si@C@void@C	0.5	50	1366	Xie, 2017, [45]
Meso-Si/C	1.0	1000	990	Xu, 2016, [46]
Si/graphite/C	0.5	300	~400	Wang, 2016, [47]
M-pSi@C	1.0	250	1702	Su, 2023, [48]
Porous Si/C nanotubes	0.2	200	1300	Chen, 2017, [49]
Si/C/graphene	0.2	100	760	Wu, 2015, [50]
Si/rGO	0.1	100	1433	Liu, 2015, [51]
Si/CNTs	5.0	1000	612.3	Guo, 2024, [53]
Si/CNTs	10	100	800	Gohier, 2012, [59]
Si@(POH-AOCNTs)	1.0	500	1195.8	Liu, 2024, [60]
Si@CNFs	1.0	100	658.1	Zhang, 2025, [61]
cpDOPA-crGO-Si	1.0	200	1038	She, 2021, [63]
HPC/Si@C	2.0	500	358	Cao, 2024, [64]
SiO2@NPC Y.S.	0.1	300	705	Min, 2022, [66]

summarizes the electrochemical performance of various silicon-carbon hybrid systems, presenting key performance metrics including current density, cycle number, and remaining capacity. This data quantitatively demonstrates the effectiveness of carbon-based hybrid systems in enhancing silicon anode performance.

Despite the numerous advantages, silicon–carbon hybrid systems face several challenges that hinder their widespread commercialization. The high cost and complex manufacturing processes of high-quality carbon materials like graphene and CNTs are significant economic barriers. Achieving a uniform and scalable carbon coating on silicon nanoparticles is technically demanding, and batch-to-batch consistency remains a major hurdle for industrial production [68,69].

Furthermore, there is an inherent trade-off between the carbon content and the overall energy density of the electrode. While a sufficient amount of carbon is necessary to ensure good electrical conductivity and buffer the volume expansion, an excessive carbon content reduces the proportion of active silicon, thereby lowering the specific capacity. The long-term stability of the Si-C interface during repeated cycling is another concern, as mechanical stress can lead to delamination and loss of electrical contact [38]. Finally, the potential for carbon oxidation at high operating voltages and the environmental impact associated with the production and disposal of carbon nanomaterials need to be carefully considered.

3.1.2. Silicon–Metal Oxide Hybrid Systems

In addition to carbon-based materials, various metal oxides (MO_x) are being actively researched as hybrid partners to improve the performance of silicon anodes [70]. MO_x exhibit semiconducting, insulating, or ionic conductive properties depending on their chemical composition and some possess inherent lithium storage capabilities [71]. The hybridization of Si with MO_x is beneficial in various ways: metal oxide layers can act as mechanical protective membranes to buffer the volume expansion of Si, provide surface modification effects that stabilize SEI formation, and in some cases, offer additional capacity. Notably, TiO₂ [34], Fe₂O₃ [70], and SnO₂ [72] have been incorporated into hybrid systems with silicon.

Titanium dioxide (TiO₂) is known for its chemical stability, excellent mechanical strength, and relatively good lithium ion conductivity [73]. Hybrid structures that have been studied include coating silicon nanoparticles with thin layers of TiO₂ [74] or inserting silicon into TiO₂ nanostructures. TiO₂ coating layers prevent direct contact between the silicon surface and electrolyte, which inhibits side reactions and induces stable SEI formation [75]. They also serve as mechanical buffer layers by contributing to mitigating the structural destruction caused by the volume expansion of Si [76]. For example, hybrid electrodes coated with uniform TiO₂ layers on silicon nanowire surfaces using atomic layer deposition (ALD) techniques demonstrated improved cycle stability and CE compared to uncoated silicon nanowires [77].

Iron oxide (Fe₂O₃) is a conversion reaction-based anode active material with a high theoretical capacity (approximately 1007 mAh/g), which can contribute to increasing the overall electrode capacity when composited with silicon [78]. Additionally, Fe₂O₃ has excellent mechanical strength, which, as would be anticipated, suppress the volume expansion of Si. Various approaches to preparing Fe₂O₃–Si composites have been studied, including mixing silicon nanoparticles with Fe₂O₃ nanoparticles [70], applying a coating of Fe₂O₃ to silicon surfaces [79], or dispersing silicon within Fe₂O₃ matrices [80]. However, Fe₂O₃ itself undergoes substantial volume changes during charging and discharging and has low electrical conductivity, which has resulted in parallel research to secure electrical conductivity and structural stability by additionally introducing carbon coatings and other measures [81]. Figure 6 provides a clear example of this ternary hybrid strategy. The synthesis schematic in Figure 6a shows Si nanoparticles being coated first with a metal–organic framework (MIL-88-Fe) template, which is then annealed to form a dual shell of Fe₂O₃ and carbon. The cycling performance graph in Figure 6b powerfully illustrates the synergy of this structure. While both pristine Si and Si@C anodes show rapid capacity decay, the Si@Fe₂O₃/C composite anode delivers a stable capacity of nearly 1000 mAh/g over 400 cycles. This demonstrates that the dual-shell approach, combining a mechanically robust metal oxide with a conductive carbon layer, is highly effective for achieving long-term stability.

Tin dioxide (SnO₂) has also been used as an anode active material with a high theoretical capacity (approximately 782 mAh/g, based on alloying reactions), which can increase the energy density of electrodes through hybridization with silicon. The electrical conductivity of SnO₂ is also superior to that of silicon, thus improved electron transfer effects can be expected when composited [82]. The formation of SnO₂ shells over silicon cores or fabrication of silicon composites with SnO₂ nanostructures has also been studied [83,84,85]. However, since SnO₂ also undergoes considerable volume changes during charging and discharging, it is important to secure structural stability through nanostructuring or carbon compositing [86].

Si-MO_x hybrid systems exhibit various performance improvement effects depending on the type and structure of the metal oxide. While TiO₂ coatings primarily focus on enhancing the interfacial stability, Fe₂O₃ or SnO₂ composites could be expected to simultaneously offer capacity enhancement effects. However, the low electrical conductivity and problematic volume change in the MO_x themselves are challenges that remain to be solved. Recent research has therefore focused on maximizing the advantages of each material by coating multiple layers of two or more types of MO_x to extend the benefits beyond those of a single metal oxide [71], or developing smart composites that can maintain the structural stability despite repetitive volume changes by imparting self-healing functions to MO_x [87]. Additionally, research on ternary hybrid systems utilizing both MO_x and carbon materials is being actively conducted, with efforts being made to comprehensively improve the electrical conductivity, mechanical stability, and interfacial stability [88].

While metal oxide coatings offer significant benefits, they also present several challenges. Many metal oxides have low intrinsic electrical conductivity, which can increase the overall impedance of the electrode and limit the rate capability. Although they can act as an artificial SEI, some metal oxides may still react with the electrolyte over time, leading to capacity fading [38]. The coating process itself, often involving techniques like atomic layer deposition (ALD) [89] or sol–gel methods, can be expensive and difficult to scale up.

Moreover, the mechanical properties of the metal oxide layer are crucial. If the coating is too rigid, it can fracture during the volume expansion of silicon, exposing the active material to the electrolyte. Conversely, if it is too soft, it may not provide sufficient mechanical constraint. Achieving the optimal thickness, uniformity, and mechanical properties of the metal oxide coating is therefore a key challenge in this hybrid system.

Table 2. Electrochemical performance summary of silicon-based hybrid anode systems reported in the references of Section 3.1.2. Only numerical data explicitly reported in the cited studies are included.

Si Anodes.	Current Density (A/g)	Cycle Number	Remaining Capacity (mAh/g)	Source (1st Author, Year, [Ref. No.])
Silicon-Metal Oxide
Porous Si/Fe2O3	0.2	100	697.2	Chen, 2020, [70]
Si@SnO2	0.2	200	1926	Zhu, 2022, [72]
Si@a-TiO2	0.42	200	1720	Yang, 2017, [74]
Si/TiO2	0.1	100	1010.7	Li, 2016, [75]
TiO2/SiNWs	0.1	100	~1700	Lotfabad, 2013, [77]
SiO/Fe2O3	0.16	50	1335	Zhou, 2013, [79]
Si/Fe2O3@rGO	0.2	200	1744.5	Yan, 2021, [80]
Si@Fe2O3/C	1.0	300	680.7	Wang, 2019, [81]
Si-SnO2-graphene	0.1	20	350	Li, 2017, [85]
TiGL@Si	1.0	350	1200	Huertas, 2022, [89]

presents the electrochemical performance data of hybrid systems combining silicon with various metal oxides (Fe₂O₃, SnO₂, TiO₂, etc.). The numerical data confirms the contribution of metal oxide coatings to mitigating silicon volume expansion and improving interfacial stability.

3.1.3. Silicon–2D Material Hybrid Systems

Following the successful application of graphene, various two-dimensional (2D) nanomaterials beyond graphene have been receiving significant attention due to their unique physicochemical properties, and their potential as hybrid partners for improving the performance of silicon anodes is being actively explored [90]. Two-dimensional materials exhibit atomic-level thinness, a large specific surface area, excellent mechanical flexibility and strength, and diverse electrical properties (metallic, semiconducting, insulating) depending on their type. These favorable characteristics can effectively buffer the volume expansion of Si, optimize electron and ion transfer pathways, and enhance the interfacial stability. Representative examples that are being applied to hybrid systems with silicon include MXenes [90,91,92,93,94] and transition metal dichalcogenides (TMDs) [95].

MXenes are 2D materials from the transition metal carbide or nitride family with the general chemical formula M_n+1X_nT_x (M: early transition metal, X: carbon or nitrogen, T_x: surface functional groups, n = 1–4) [96]. MXenes simultaneously possess metal-level high electrical conductivity, hydrophilic surface characteristics, and excellent mechanical properties, and therefore have great potential as hybrid partners for Si anodes [96]. MXene sheets, which are either wrapped around Si nanoparticles or are inserted between these particles, play a role in mitigating the volume expansion and providing rapid electron transfer pathways [97]. Additionally, the abundant functional groups on the surfaces of MXenes can contribute to enhancing the interfacial stability by inducing strong interactions with both silicon and the electrolyte [98]. For example, hybrid structures that self-assembled as a result of the electrostatic attraction between Ti₃C₂T_x MXene sheets and Si nanoparticles demonstrated high capacity and excellent cycle stability [98]. Figure 7 details this electrostatic self-assembly approach. The schematic in Figure 7a illustrates how positively charged Si nanoparticles (nmSi-NH₂) and negatively charged MXene sheets are mixed to form a self-assembled composite (BA electrode), which is then annealed. The performance graph in Figure 7b compares this BA electrode to a blank sample and an annealed-only (AA) electrode. The BA electrode exhibits significantly higher capacity retention over 100 cycles, confirming that the strong electrostatic interaction creates a uniform and stable coating of MXene nanosheets around the silicon, which effectively buffers volume expansion and provides a stable conductive network.

Hexagonal boron nitride (h-BN) has a 2D structure similar to that of graphene but is an electrical insulator with a wide bandgap [99]. BN is characterized by high thermal and chemical stability and excellent mechanical strength. Research in which BN was applied to silicon anodes primarily focused on exploiting the insulating properties and stability of BN to protect the silicon surface and control SEI formation [100]. Coating silicon nanoparticles with thin BN layers can help inhibit side reactions and form stable interfaces by preventing direct contact with the electrolyte. However, BN may, because of its inherently insulating nature, hinder electron transfer, thus additional compositing with other conductive materials such as carbon coatings is often necessary to ensure electrical conductivity [101].

TMDs are layered 2D materials with the chemical formula MX₂ (M: transition metal, X: chalcogen element), with MoS₂, WS₂, and MoSe₂ being representative examples [102]. TMDs exhibit semiconducting or metallic characteristics depending on their type and also possess lithium ion storage capabilities. Upon hybridization with Si, the TMD layers can act as buffer layers to mitigate the volume expansion experienced by Si, provide additional lithium storage sites, and improve the charge transfer characteristics at interfaces [103]. For example, the anode material consisting of silicon nanoparticles coated with MoS₂ nanosheets improved the cycle stability and rate characteristics [101]. However, the electrical conductivity of TMDs themselves is lower than that of graphene or MXenes, and some TMDs may exhibit structural instability during reactions with lithium, which would necessitate further research.

Table 3. Electrochemical performance summary of silicon-based hybrid anode systems reported in the references of Section 3.1.3. Only numerical data explicitly reported in the cited studies are included.

Si Anodes	Current Density (A/g)	Cycle Number	Remaining Capacity (mAh/g)	Source (1st Author, Year, [Ref. No.])
Silicon-2D Material
Si@Ti3C2 Mxene	0.2	150	188	Kong, 2018, [91]
Si@Ti3C2 Mxene	1.0	200	1342.8	Yang, 2020, [92]
nSi/MX-N	1.5	70	1106	Zhang, 2019, [93]
Si/Mxene	0.2	100	2118	Tian, 2019, [94]
MoS2@Si	0.1	300	1223	Marriam, 2023, [95]
Si/d-Ti3C2	0.5	200	1130	Zhu, 2019, [97]
nmSi-NH2/MXene	1.0	100	929.5	Kong, 2024, [98]
Si/MoS2-G	0.2	90	923	Kawade, 2019, [103]

provides performance data for hybrid systems combining silicon with 2D nanomaterials such as MXenes and MoS2. The data illustrates how the excellent mechanical flexibility and electrical conductivity of 2D materials contribute to improved cycling stability and rate performance of silicon anodes.

The use of 2D materials other than graphene also comes with its own set of challenges. Many transition metal dichalcogenides (TMDs) like MoS₂ and WS₂ are semiconductors with relatively low electrical conductivity, which can negatively impact the rate performance of the electrode. The synthesis of high-quality, large-area 2D materials can be complex and costly [104].

Furthermore, the interaction between the 2D material and the silicon surface needs to be carefully engineered to ensure good adhesion and efficient charge transfer. The long-term stability of these 2D materials in the electrochemical environment of a lithium-ion battery is also a concern, as they can undergo side reactions with the electrolyte [105]. The high cost and scalability issues associated with the production of many of these novel 2D materials are also significant barriers to their practical application.

3.1.4. Silicon–Conductive Polymer Hybrid Systems

Conductive polymers (CPs) are emerging as attractive hybrid partners for improving the performance of silicon anodes due to their unique electrical conductivity, flexibility, processing ease, and lightweight characteristics [106]. CPs can effectively encapsulate silicon particles or form three-dimensional network structures, which serve as flexible matrices that mitigate the volume expansion of Si [107]. Additionally, they provide electron and ion transfer pathways via their polymer chains, inhibit side reactions between silicon surfaces and electrolytes, and contribute to enhancing the mechanical stability of electrodes [108]. Representative conductive polymers that have been widely applied to hybrid systems with Si include polyaniline (PANI) [109], polypyrrole (PPy) [110], and poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) [111].

Polyaniline (PANI) is a representative conductive polymer that is easy to synthesize, environmentally stable, and its electrical conductivity can be adjusted by modifying its doping state [112]. The following hybrid structures thereof have been researched: directly polymerizing and coating PANI on the surfaces of Si nanoparticles or dispersing Si particles within PANI matrices [113]. PANI coating layers, with their flexible characteristics, effectively accommodate the volume changes in Si, inhibit crack formation, and help maintain the structural integrity of electrodes [114]. Furthermore, PANI can control the reactions with electrolytes to induce stable SEI formation [115]. For example, hybrid electrodes comprising uniformly thin PANI layers coated onto silicon nanoparticles demonstrated excellent cycle stability and improved rate characteristics [116].

Polypyrrole (PPy) is also a conductive polymer that is relatively simple to synthesize, has excellent biocompatibility, and possesses good electrical conductivity and environmental stability [117]. PPy can be easily coated onto Si surfaces through chemical or electrochemical polymerization and serves as a protective layer that encapsulates the Si particles [118]. PPy coatings demonstrate effects that mitigate the volume expansion of Si, improve the electrical connectivity between particles, and inhibit electrolyte decomposition, thereby enhancing the lifespan of the electrode [118]. In particular, the application of a PPy coating to Si electrodes with three-dimensional porous structures reportedly simultaneously improves the structural stability and electrochemical performance [119]. Figure 8 showcases a specific example of this strategy. The schematic in Figure 8a illustrates the preparation of porous hollow Si (PHSi) spheres, which are subsequently coated with a polypyrrole (PPy) layer via in situ polymerization. The cycling data in Figure 8b highlights the dramatic effect of the PPy coating. The coated PPy@PHSi nanocomposite maintains excellent capacity and stability for over 250 cycles, whereas the uncoated version would typically degrade much faster. This result confirms that the flexible and conductive PPy layer successfully accommodates the volume changes of silicon and prevents the pulverization of the porous structure.

PEDOT:PSS, a material known to exhibit the highest electrical conductivity among conductive polymers developed to date, has the advantage of excellent processability as it is provided in the form of an aqueous dispersion [120]. PEDOT:PSS can be used to form thin and uniform coating layers on Si nanoparticle surfaces or to build conductive networks to connect Si particles to each other. The outstanding electrical conductivity of PEDOT:PSS is effective for improving the high-rate charge–discharge characteristics by greatly enhancing the electron transfer within electrodes. Additionally, the flexible structure of PEDOT contributes to buffering the volume changes in Si and enhancing the mechanical stability of electrodes [121]. Numerous studies have reported an improvement in the performance of the Si electrode by utilizing PEDOT:PSS as both a binder and conductive additive [122].

Silicon–conductive polymer hybrid systems are able to improve the performance in various ways depending on the type and structure of the conductive polymer and the method that was used to synthesize the polymer. PANI and PPy are relatively easy to synthesize and have excellent stability, while PEDOT:PSS provides outstanding electrical conductivity and processability. These hybrid systems generally exhibit improved cycle life and rate characteristics, with the flexibility of conductive polymers playing a particularly important role in mitigating the volume expansion problems associated with Si [123]. More recently, the focus has been on developing smart responsive polymer systems that actively control the volume expansion by changing their structure or properties in response to external stimuli (pH, temperature, voltage, etc.) beyond simple coating structures or on introducing polymer composites with self-healing functions that repair themselves upon being mechanically damaged to maximize the electrode durability [124]. Additionally, attempts to simultaneously optimize the electrical conductivity and mechanical flexibility by forming ternary hybrid systems by combining CPs with carbon materials are also being actively pursued.

Conductive polymers, despite their advantages, also have limitations. Their electrical conductivity is generally lower than that of carbon-based materials, which can limit the high-rate performance of the silicon anode. The long-term stability of conductive polymers during repeated charge–discharge cycles can be an issue, as they can degrade or lose their conductivity over time [125].

The synthesis and processing of conductive polymers can also be challenging. Achieving a uniform and conformal coating on silicon nanoparticles can be difficult, and the adhesion between the polymer and the silicon surface can be weak. Furthermore, the swelling of the polymer in the electrolyte can lead to mechanical instability and dimensional changes in the electrode [125]. The cost and scalability of the polymerization process are also important considerations for commercial applications.

Table 4. Electrochemical performance summary of silicon-based hybrid anode systems reported in the references of Section 3.1.4. Only numerical data explicitly reported in the cited studies are included.

Si Anodes	Current Density (A/g)	Cycle Number	Remaining Capacity (mAh/g)	Source (1st Author, Year, [Ref. No.])
Silicon-Conductive Polymer
SiNP/PANi	1.0	1000	1200	Wu, 2013, [108]
Si-oxalic acid-PANI	1.0	1000	610	Zhou, 2020, [109]
Si@Ppy	1.0	500	1047	Zhang, 2024, [110]
Si/PANI	1.0	100	545.3	Tu, 2013, [113]
Si/PANi/C	0.1	200	1470	Mu, 2019, [114]
Si/PANi	0.1	25	1840	Cai, 2010, [116]
Si@PEDOT	0.5	100	1439.8	Li, 2021, [121]
PEDOT:PSS/SiNP	1.0	100	1950	Higgins, 2016, [122]

summarizes the electrochemical performance of hybrid systems combining silicon with conductive polymers including PANI, PPy, and PEDOT:PSS. The quantitative data confirms that the flexible nature of conductive polymers effectively buffers silicon volume changes, thereby enhancing the mechanical stability of electrodes.

3.2. Classification According to Structural Characteristics

Structural design critically determines how effectively hybrid systems address silicon’s volume expansion challenges. Core–shell structures provide direct volume containment but face scalability challenges due to complex synthesis requirements. Sandwich structures [126] offer simpler fabrication but may compromise uniform stress distribution. Three-dimensional networks [127] excel in mechanical stability and conductivity but require precise control of porosity and interconnectivity for optimal performance. This section examines the design principles, advantages and disadvantages, performance enhancement mechanisms, and latest research trends of hybrid systems according to each of these three structural types.

3.2.1. Core–Shell Hybrid Structures

The core–shell structure—a structural form in which one material (the core) is completely encapsulated by another material (the shell)—is one of the most widely researched structures for solving the volume expansion problem of silicon anodes [34]. Generally, silicon is used as the core material, and is encapsulated in a shell consisting of a material that can buffer the volume expansion, impart electrical conductivity, or enhance the interfacial stability (carbon, MO_x, CPs, etc.) to protect the silicon and improve the performance [128]. Core–shell structures can be either single- [129] or multi-shell structures [130] depending on the number of shells.

The single-shell core–shell structure is the most basic form in which the silicon core is encapsulated within a single layer of material. The shell material disperses the mechanical stress that occurs during the volume expansion of Si and prevents direct contact between the silicon surface and the electrolyte to inhibit the excessive formation of SEI layers [131]. For example, Si@C structures formed by encapsulating silicon nanoparticle cores within carbon shells exhibit improved cycle stability and rate characteristics owing to the excellent electrical conductivity and mechanical buffering effects of the carbon shells [129]. However, single-shell structures have limitations in that the shell may rupture or the shell may separate from the core during repeated extreme volume expansion.

To compensate for these disadvantages, multi-shell core–shell structures have been proposed. Multi-shell structures are formed by sequentially forming shell layers composed of two or more different materials around a silicon core [132]. Each shell layer can be designed to perform specific functions. For example, a representative structure (Si@void@Shell) contains an inner shell to create void space directly around the silicon core, and an outer shell that is mechanically robust or has excellent electrical conductivity on the outside [132]. The internal void space provides sufficient room for the volume expansion of Si and minimizes the stress impaired to the outer shell, while the outer shell maintains the stability of the entire structure and provides electrical conduction pathways [45]. These multi-shell structures have superior volume expansion mitigation effects compared to single-shell structures and are advantageous for securing long-term cycle stability. For example, various multi-shell structures such as Si@void@C [133], Si@SiO_x@C [134], and Si@TiO₂@C [135] have been reported and have demonstrated excellent electrochemical performance. Figure 9 illustrates an advanced core–shell design, the Si@void@C structure, and its superior performance. The process is shown in Figure 9a, where a dissolvable resin layer is used to create a precise void space between the Si core and the outer carbon shell. The TEM images in Figure 9b visually confirm the successful formation of this yolk–shell architecture. The cycling performance in Figure 9c is particularly impressive, showing that the Si@void@C anodes maintain stable cycling for over 1000 cycles at a very high rate of 2.0 A/g, far outperforming bare Si. This demonstrates conclusively that the internal void is the key to accommodating volume expansion without stressing and fracturing the protective outer shell.

The use of a core–shell structure is a highly effective strategy for improving the electrochemical performance by effectively controlling the volume expansion of Si and enhancing the interfacial stability. The performance can be optimized by precisely controlling the type, thickness, and number of shell materials, as well as the interfacial characteristics between cores and shells. Recent research has focused on developing functional multi-shell structures that impart specific functions to each shell layer (e.g., selective lithium ion permeation, self-healing functions) beyond simple multi-shells [136], or designing hierarchical core–shell systems where nanosized core–shell units are interconnected to form macroscopic hierarchical structures [137]. These advanced core–shell structural designs are expected to make important contributions to overcoming the performance limitations of silicon anodes to increase their commercialization potential. For instance, a multi-level confinement strategy, where porous silicon is wrapped in graphene oxide and carbon nanotubes and then coated with carbon (pSi@GO-CNTs@C), has been shown to effectively buffer volume expansion and significantly enhance long-term cycling stability [138].

3.2.2. Sandwich Hybrid Structures

Sandwich structures refer to forms in which different material layers are alternately stacked such as in a sandwich, and are attracting attention as effective structures particularly when combining 2D nanomaterials with silicon [139]. In these structures, the silicon layer primarily provides high capacity, while the other material layers (e.g., graphene, MXenes, or other 2D materials or CPs) serve as buffering layers to accommodate the volume expansion of Si, electron conduction pathways, or mechanical support structures [140]. Sandwich structures can be classified into single- [141] and multilayer sandwich structures [50] according to the number of layers.

Single-layer sandwich structures generally take the form of inserting a silicon layer between two layers consisting of another material (e.g., graphene/silicon/graphene). The outer layers constrain the volume expansion experienced by Si from both sides, maintain the structural stability of the entire electrode, and provide excellent electrical conduction pathways [141]. Particularly when using a 2D material such as graphene for the outer layers, their large specific surface area and flexibility effectively accommodate the volume changes in Si while preventing particle agglomeration and enabling rapid electron transfer. For example, structures comprising a layer of silicon nanoparticles inserted between two graphene sheets have been reported to exhibit high reversible capacity and excellent cycle stability [142]. However, an excessively thick silicon layer may impede the diffusion of lithium ions to the interior of the silicon layer, or the structure may be destroyed as the stress applied to the outer layers increases during volume expansion, which is a disadvantage.

Attempts to solve these problems have inspired research on multilayer sandwich structures. These structures consist of layers of silicon and other materials (buffering/conducting layers) alternately stacked in multiple layers. By controlling the thickness of each layer at the nanometer level, the migration distance of lithium ions and electrons can be shortened, and the stress applied to the entire structure can be minimized by dispersing the volume expansion of Si across each layer [143]. For example, multilayer structures created by alternately depositing thin silicon films and graphene layers have shown very high rate capabilities and long-term cycle stability [144]. Additionally, attempts are being made to facilitate ion transfer and more effectively mitigate the volume expansion by introducing void spaces between layers or inserting functional materials (e.g., ion-conductive polymers) [145]. Figure 10 provides a representative example of the sandwich structure approach. The fabrication process is detailed in Figure 10a, where layers of Si nanoparticles and reduced graphene oxide (RGO) are alternately deposited on a Ni foam substrate. The schematic in Figure 10b depicts how the flexible RGO layers encapsulate the Si nanoparticles, constraining their volume expansion during lithiation. The electrochemical data in Figure 10c confirms the effectiveness of this design; the multilayered Si/RGO anode shows significantly higher capacity retention and stability at a 1C rate compared to a simple Si anode, proving the dual function of RGO as both a mechanical buffer and a conductive pathway.

Sandwich structures are promising strategies for enhancing the performance of silicon anodes, particularly by maximizing the advantages of 2D materials. Layered structures effectively constrain volume expansion, provide excellent electron and ion transfer pathways, and contribute to increasing the mechanical stability of electrodes. Customized performance implementation is possible by precisely designing the structure by specifying the type, thickness, number of layers, and interfacial characteristics of the materials between layers. Recent research has focused on introducing functional interlayer materials that impart specific functions (e.g., self-healing, selective ion permeation) between layers beyond simple alternating stacking structures, or implementing more complex and multifunctional sandwich structures by utilizing nanostructures of other dimensions such as nanowires and nanotubes alongside 2D materials [146]. These advanced sandwich structure designs are expected to play an important role in simultaneously enhancing the energy density and stability of silicon anodes.

3.2.3. Three-Dimensional Network Hybrid Structures

Three-dimensional network (3D network) structures refer to structures where silicon active materials and other functional materials (e.g., CNTs, graphene, metal nanowires, etc.) are interconnected to form macroscopic 3D frameworks [147]. Unlike individual particle-level structures such as core–shell or sandwich structures, these 3D network structures offer several advantages by providing interconnected porous structures across the entire electrode. Their large specific surface area broadens the contact interface with the electrolyte, enhances their lithium ion accessibility, and the interconnected networks provide efficient electron transfer pathways throughout the electrode [148]. Additionally, existing pores within the network provide space for the volume expansion of Si and contribute to increasing the structural stability of electrodes by effectively mitigating the mechanical stress that accompanies charge–discharge cycling [148]. These 3D network structures can be classified into porous 3D networks [149] and hierarchical 3D networks [150] according to the pore size and distribution, as well as the hierarchy of the network framework. Figure 11 demonstrates an example of a hierarchical 3D network structure. The schematic in Figure 11a shows the self-assembly of Si nanoparticles and CNTs into a 3D hydrogel, which is then freeze-dried and annealed to create a CNTs-crosslinked hierarchical porous Si (CHSP) structure. The rate performance graph in Figure 11b compares this CHSP anode with a standard nano-Si anode. The CHSP structure exhibits vastly superior capacity at all current densities from 0.1 to 5.0 A/g, highlighting that the interconnected porous 3D network provides excellent pathways for both ion and electron transport while also accommodating volume expansion, which is critical for high-rate performance.

Simple porous 3D network structures consist of silicon active materials dispersed within a framework in which pores with a relatively uniform size are three-dimensionally connected. These structures can be fabricated by various methods such as electrospinning [151], freeze-drying [152], and template removal methods [153]. Examples are methods that involve impregnating a 3D non-woven matrix (formed by electrospinning CNFs) with silicon nanoparticles [154], or depositing Si onto porous carbon structures such as graphene aerogels [152]. Porous networks provide expanded surface areas and interconnected conductive frameworks, which enable high capacity and excellent rate characteristics [155]. The pore structure effectively accommodates the volume expansion of Si to enhance the cycle stability. However, the disadvantages of simple porous structures are that stress may concentrate in specific areas or ion/electron transfer pathways may be inefficient if the pore size or distribution is non-uniform [156].

Efforts to overcome these limitations have led to the proposal of hierarchical 3D network structures. Hierarchical structures refer to complex network structures, in which pores of various sizes ranging from micrometer-sized macropores to nanometer-sized micropores/mesopores, are hierarchically connected [157]. Macropores facilitate electrolyte penetration and provide pathways for rapid ion movement, while micropores maximize the contact area with active materials and effectively buffer the volume expansion [158]. Furthermore, hierarchical network frameworks can further enhance the mechanical stability. These hierarchical structures can be fabricated by mimicking biological structures (e.g., trees, sponges) or by forming precisely controlled structures using self-assembly phenomena. For example, hybrid electrodes with silicon nanoparticles uniformly dispersed within hierarchical porous carbon structures have been reported to simultaneously achieve outstanding rate characteristics and long-term cycle stability [63].

Hybrid structures consisting of a 3D network offer an effective approach for solving the volume expansion problems associated with Si at the electrode level and maximizing the electrochemical performance. Interconnected porous structures facilitate smooth ion and electron transfer, provide excellent buffering capacity for volume changes, and enhance the mechanical strength of electrodes [159]. In particular, hierarchical 3D network structures present the possibility of further optimizing the performance by including pores of various sizes and precisely controlled network frameworks. Recent research has focused on the design of biomimetic 3D structures that mimic efficient structures in nature [160], or on the development of self-assembly 3D network technologies that spontaneously form the desired structures via molecular-level interactions [148]. Additionally, research using 3D printing technology to directly fabricate customized 3D electrode structures is also actively underway [161]. These innovative approaches to design 3D network structures are expected to establish themselves as key strategies for developing high-performance, highly stable silicon anodes.

4. Synthesis Methods for Hybrid Material Systems

The performance of hybrid material systems for silicon anodes depends greatly not only on the types of constituent materials and structures but also on the synthesis methods used to process them. The synthesis method is a key element that determines the uniformity of hybrid structures, interfacial characteristics between components, and the physicochemical properties of the final material. Therefore, selecting appropriate synthesis methods and optimizing the process parameters are crucial for obtaining hybrid systems with the desired structures and performance. The synthesis of hybrid material systems can be broadly divided into solution-based synthesis methods [30], vapor deposition methods [30], mechanical synthesis methods [162], and hybrid processes combining these methods [163].

4.1. Solution-Based Synthesis Methods

Solution-based synthesis methods involve the dissolution or dispersion of the precursor materials in solvents, followed by the synthesis of the desired hybrid materials through chemical reactions (hydrolysis, condensation, precipitation, etc.) [160]. These methods offer advantages such as processing at relatively low temperatures, suitability for mass production, and the ability to produce materials in various forms (powders, thin films, fibers, etc.). Additionally, the composition, size, shape, and crystallinity of the final products can be easily controlled by adjusting the chemical reaction conditions of the solution [164]. Solution-based synthesis methods that are primarily used for fabricating hybrid systems for silicon anodes include the sol–gel method [165], hydrothermal synthesis [166], and the microemulsion method [167].

The sol–gel method involves hydrolyzing and condensing precursors such as metal alkoxides or salts in solvents (mainly water or alcohol) to form a sol state, which is subsequently converted to a gel state through additional reactions or drying processes, and then heated to obtain the final oxides or composites [168]. This method offers advantages such as that it enables uniform mixing at the molecular level for easy composition control, is favorable for forming porous structures, and enables processing at relatively low temperatures [169]. In the field of silicon anodes, it is utilized to manufacture silicon–oxide or silicon–carbon hybrid structures via sol–gel processes with silicon precursors and other metal oxides (e.g., TiO₂, SiO₂) or carbon precursors [170]; for example, the dispersion of silicon nanoparticles within porous TiO₂ matrices manufactured by the sol–gel method, or the formation of uniform SiO₂ or carbon coating layers on silicon particle surfaces [170]. Disadvantages of the sol–gel method include long processing times, the possibility of crack formation due to shrinkage during drying, and cost and environmental issues associated with organic solvents or precursor materials. Figure 12 details the synthesis of a Si@HC (Hard Carbon) yolk–shell structure using a sol–gel based method. As shown in Figure 12a, the process begins with coating Si nanoparticles with a SiO₂ and resin shell, followed by carbonization and HF etching to create the final void-containing structure. The TEM images in Figure 12b clearly visualize the formation of the void space inside the carbon shell. The electrochemical data in Figure 12c shows that the resulting anodes exhibit stable cycling and good rate capability, demonstrating that the sol–gel method is a versatile tool for fabricating complex, well-defined nanostructures for battery applications.

Hydrothermal synthesis entails the use of water or other solvents as reaction media under high-temperature and high-pressure conditions to synthesize materials [171]. Because these reaction conditions increase the solubility of reactants and the reaction rates, they are advantageous for synthesizing highly crystalline materials or materials with unique microstructures that are difficult to obtain through conventional solution reactions. Hydrothermal synthesis can be accomplished with relatively simple equipment, can produce nanoparticles of uniform size and shape, and is widely used for synthesizing various types of oxides, sulfides, metals, and other materials [171]. In the field of silicon anodes, it is applied to directly grow metal oxide (e.g., SnO₂, Fe₃O₄) nanostructures on the surfaces of silicon nanoparticles [172] or to form hybrid composites by hydrothermally treating silicon with other materials (e.g., graphene oxide) [173]. For example, graphene-wrapped silicon (Si@G) structures that were synthesized by hydrothermally reacting silicon nanoparticles with graphene oxide demonstrated enhanced electrical conductivity together with the ability to mitigate volume expansion effects [174]. Disadvantages of hydrothermal synthesis are the requirement for high-pressure reactors, which requires safety precautions to be observed, and the difficulty to precisely control reaction conditions.

The microemulsion method involves forming thermodynamically stable micro- or nanometer-sized droplets by adding surfactants to two immiscible liquid phases such as water and oil, and using these droplets as nanoreactors to synthesize materials [175]. The droplet size in microemulsions can be controlled by adjusting the type and ratio of surfactants and solvents, with the advantage of enabling the size and shape of the resulting particles to be precisely controlled. This method is effective for synthesizing nanoparticles of uniform size or fabricating complex nanostructures such as core–shell structures [175]. In silicon anode research, it is utilized to reduce the silicon precursors within microemulsion droplets to produce silicon nanoparticles of uniform size [176], or to create core–shell structures by using silicon nanoparticles as cores and forming shells of other materials (e.g., carbon, metal oxides) on their surfaces using microemulsions [177]. To its disadvantage, the microemulsion method requires relatively complex system configurations, may incur cost and environmental issues due to the use of large amounts of surfactants and organic solvents, and may be difficult to scale for mass production.

In solution-based synthesis methods, various synthesis conditions such as the type and concentration of precursors, type of solvent, reaction temperature and time, pH, and additives greatly influence the structure and physical properties of the final hybrid material. Therefore, systematically optimizing these process variables is essential to achieve the desired performance. Recent research has focused on eliminating the disadvantages of existing solution-based synthesis methods; for example, the development of synthesis methods that use environmentally friendly solvents such as water or bio-based solvents to minimize damage to the environment [178], the development of low-temperature process technologies to lower the energy consumption [179], and the utilization of microwaves or ultrasound as auxiliary means to shorten reaction times and increase the uniformity [180]. Additionally, attempts to synthesize more complex and sophisticated hybrid structures by combining multiple solution-based processes or integrating with other synthesis methods are ongoing [181].

4.2. Vapor Deposition Methods

Vapor deposition provides a solvent-free and vacuum-based way to form high-purity, conformal, and precisely thickness-controlled coatings on complex silicon architectures, which helps buffer volume change and stabilize the SEI, and the following sections will detail how conductive carbon shells grown by CVD and angstrom-level inorganic layers deposited by ALD translate these advantages into durable, high-rate silicon anodes.

CVD entails the deposition of the desired materials in thin film form through chemical reactions on a substrate surface or in the gas phase by injecting gaseous precursors into a reaction chamber [182,183]. The composition, structure, and thickness of the thin film can be controlled by adjusting the reaction temperature, pressure, precursor flow rate, and ratio. CVD offers advantages such as uniform thin film deposition over relatively large areas, applicability to substrates with complex shapes, and the ability to deposit various types of materials (metals, semiconductors, insulators, etc.) [184]. In the field of silicon anodes, CVD is primarily utilized to deposit carbon coatings (e.g., a layer of pyrolytic carbon) on silicon nanostructures or to directly grow carbon-based 2D materials such as graphene [185]. Examples are the synthesis of silicon thin films or nanowires by thermally decomposing silane (SiH₄) gas [186], or the deposition of carbon layers on silicon surfaces by decomposing hydrocarbon gases such as methane (CH₄) or acetylene (C₂H₂) at high temperatures [187]. CVD processes generally require high temperatures, and the precursor gases used are often toxic or explosive. This necessitates the use of safety equipment, which is a disadvantage.

Atomic layer deposition (ALD) is a type of CVD for the deposition of thin films consisting of atomic layers on a substrate surface via self-limiting chemical reactions by sequentially injecting the precursors and reactants in pulse form [188]. Because further reactions cannot occur once the surface reactions are saturated at each pulse step, the film thickness can be very precisely controlled at the atomic layer level by adjusting the number of pulses. ALD has the unique advantage of being able to deposit extremely thin, uniform, and pinhole-free high-quality thin films with high conformality even on complex 3D nanostructure surfaces [189]. In silicon anode research, ALD is actively applied to form very thin protective layers (e.g., Al₂O₃, TiO₂, or ZnO) on the surfaces of silicon nanoparticles to enhance the SEI stability [189], or to deposit metal coatings or conductive oxide thin films for improved conductivity [190]. For example, the uniform deposition of Al₂O₃ layers with a thickness of a few nanometers on the surfaces of silicon nanoparticles using ALD can effectively inhibit the unwanted volume expansion and greatly improve the cycle life [191]. The disadvantages of ALD are its very slow deposition rates, requirement for expensive equipment, and potentially limited selection of suitable precursors and reactants.

Sputtering is a representative PVD method that generates the ionized plasma of an inert gas, such as argon (Ar), in a vacuum chamber and accelerates these ions to collide with the surface of a target material at high energy. This causes the target atoms to be ejected and deposited onto the substrate surface [192]. Sputtering can be used to deposit almost all types of metals, alloys, and ceramic materials in the form of a thin film, can be used at relatively low temperatures, and produces thin films with excellent adhesion [192]. In the field of silicon anodes, sputtering is used to directly deposit silicon thin films or to deposit metal coatings (e.g., Cu, Ni) or alloy thin films that serve as conductivity enhancement or protective layers on silicon nanostructures. For example, electrodes have been fabricated by depositing silicon thin films on porous current collectors using sputtering [193], or the electrical conductivity and structural stability have been improved by coating thin copper (Cu) layers on the surfaces of silicon particles [194]. Among the disadvantages of sputtering processes are their relatively low deposition rates, lower conformality for substrates with complex shapes compared to CVD or ALD, and the possibility of damaging the substrate with the plasma. Figure 13 illustrates the use of sputtering to improve Si anode performance. The schematic in Figure 13a shows the deposition of a Cu layer onto Si particles via magnetron sputtering. The electrochemical performance is shown in Figure 13b,c. The results clearly indicate that both the cycling stability and rate capability of the Si anode are significantly improved after being coated with Cu (Si@Cu) and further with carbon (Si@Cu@C). This confirms that a sputtered metallic layer can effectively enhance the electrical conductivity and provide some mechanical support, thereby mitigating some of the key issues of Si anodes.

4.3. Mechanical Synthesis Methods

Mechanical synthesis methods involve synthesizing the desired hybrid materials by applying mechanical energy to raw materials in the solid state by way of grinding or mixing to induce alloying or chemical reactions [162]. Because these methods are dry processes that do not use solvents, they offer advantages such as the possibility of mass production with relatively simple equipment and applicability to combinations of various types of raw materials [162]. Particularly, they can be usefully employed to process materials that cannot tolerate high temperatures, as they can be processed at room temperature. Ball milling, also known as high-energy ball milling (HEBM), is a representative mechanical synthesis method.

In the field of silicon anodes, ball milling is widely used to manufacture hybrid composites of silicon with carbon-based materials (graphite, CNTs, graphene, etc.) or other metals/metal oxides. For example, when micrometer-sized silicon powder is ball milled with graphite powder, the silicon particles are finely ground while they are simultaneously uniformly dispersed within the graphite particles or attached to the surfaces of the graphite particles to yield silicon–graphite composites [195]. These composites can mitigate the low conductivity and volume expansion problems of Si by utilizing the excellent electrical conductivity and structural stability of graphite [195]. Research on manufacturing hybrid structures comprising silicon particles surrounded by CNT networks or graphene sheets by ball milling silicon with CNTs or graphene is also being actively conducted [196]. The mechanical forces generated during the ball milling process can induce strong bonding between silicon and the carbon materials, which contributes to lowering the interfacial resistance and enhancing the structural stability [197]. Figure 14 demonstrates the effectiveness of the ball-milling process for creating Si/CNT composites. The cycling and rate performance graphs in Figure 14a,b show that the Si30h/CNT composite, prepared by ball-milling, has vastly superior stability and rate capability compared to the Si30h material alone. The Nyquist plot in Figure 14c reveals a much lower charge-transfer resistance for the composite, which is direct evidence of the improved electrical network formed by the CNTs. The SEM/TEM images in Figure 14d visually confirm that the CNTs are well-distributed and entangled with the Si particles, providing the structural basis for the performance enhancement.

Ball milling is a relatively simple and cost-effective process suitable for mass production, but it also has several disadvantages. First, the high energy generated during the ball milling process may degrade or amorphize the crystallinity of the raw materials, which can affect the electrochemical performance [198]. Second, impurities originating from milling containers or balls may be incorporated into the final product, which can be problematic especially in battery materials that require high purity [199]. The selection of the container and ball materials and process management are therefore important. Third, particles manufactured by ball milling may have non-uniform sizes or shapes, and agglomeration can easily occur, and may potentially require additional dispersion processes [200].

Research being conducted to overcome these disadvantages to improve the efficiency and controllability of ball milling processes includes, for example, methods to prevent excessive cold welding or agglomeration of particles and increase the grinding efficiency by adding small amounts of liquid (the process control agent, PCA) during milling [201]. Additionally, attempts to induce specific chemical reactions or control the crystallinity by regulating the milling temperature or atmosphere have been reported [202]. Other researchers have combined ball milling with other processes (e.g., heat treatment, chemical etching, etc.) to produce more complex and functionalized hybrid structures; for example, porous silicon structures were produced by removing the metal components through selective etching after manufacturing silicon–metal composites by ball milling [203], or by restoring the crystallinity or forming specific phases through heat treatment after ball milling [24]. Mechanical synthesis methods, especially ball milling, are important synthesis technologies with the potential to mass-produce silicon-based hybrid anodes, and their range of applications is expected to expand further through process optimization and integration with other technologies.

4.4. Hybrid Process Methods

Hybrid process methods involve combining two or more of the previously described solution-based synthesis methods, vapor deposition methods, and mechanical synthesis methods [204]. These methods were designed to manufacture hybrid material systems with complex structures or characteristics that are difficult to implement with a single process alone. By taking the advantages of each process and compensating for their disadvantages, hybrid materials for silicon anodes with more precisely controlled structures and excellent performance can be developed. Hybrid processes can be designed in various ways depending on the types and sequences of the processes that are combined, which significantly influence the structure and characteristics of the final material.

One of the most common hybrid processes combines solution-based synthesis methods with vapor deposition methods. For example, support structures with porous structures (e.g., carbon aerogels, metal oxide nanostructures) can first be fabricated using solution processes such as sol–gel or hydrothermal synthesis, after which silicon thin films or nanostructures can be deposited on these support structures using vapor deposition methods such as CVD or sputtering [205]. This approach can simultaneously exploit the advantages of 3D porous structures obtained through solution processes (large surface area, volume expansion mitigation space) and the advantages of high-quality silicon thin films deposited using vapor deposition methods (uniformity, high initial capacity) [205]. Conversely, hybrid processes to first synthesize silicon nanostructures (e.g., nanowires, nanotubes) using vapor deposition methods, followed by the formation of protective layers or conductivity enhancement layers on their surfaces using ALD or solution-based coating methods (e.g., sol–gel coating, polymer coating) are also widely used [191]. For example, coating extremely thin and uniform Al₂O₃ protective layers on the surfaces of silicon nanowires grown by CVD using ALD can greatly enhance the electrochemical performance and stability [206].

Hybrid approaches in which mechanical synthesis methods are combined with other processes are also effective. For example, silicon can be mechanically mixed with carbon materials using ball milling to produce composite powders, which are then calcinated to form structures in which the silicon particles are dispersed within a carbon matrix or to enhance the crystallinity [163]. In this case, ball milling is responsible for uniform mixing and particle size reduction, whereas the heat treatment plays a role in controlling carbonization, crystallization, or phase formation. Additionally, spherical secondary particles or fiber-form electrode active materials were manufactured by combining silicon-based composite powders by ball milling with solution-based processes such as spray drying or electrospinning [207]. These hierarchical structures can enhance the ease of handling powders and improve the electrode manufacturing processability, while simultaneously enhancing the electrochemical performance by controlling the micro-/nanostructure [208].

Recently, more complex hybrid processes that combine three or more processes in sequence or in parallel have been developed. For example, highly precisely controlled multi-shell or multicore structures can be implemented through multistage hybrid processes such as creating porous carbon supports using solution processes, depositing silicon on them using vapor deposition methods, and then performing surface modification using solution coating or ALD [209]. These complex structures, which are designed to maximize the functions of each component and rely on synergistic effects, enable performance improvements at levels that were difficult to realize with existing single or dual structures.

Hybrid process methods provide powerful tools to overcome the performance limitations of silicon anodes by integrating the advantages of various synthesis technologies. However, disadvantages such as process variables becoming complicated when combining multiple processes, the importance of compatibility and optimization between each stage, and the potential increase in overall process costs must also be considered. Therefore, an important aspect of designing effective hybrid processes is to optimize the process combinations and sequences based on a deep understanding of each unit process, along with clear designs for the final target structures and performance. Future research is expected to proceed in directions that enhance precise control technologies for each process stage, maximize synergistic effects between processes, and increase the efficiency and economic feasibility of overall processes. Notably, attempts to efficiently explore vast process variable spaces and predict optimal hybrid process conditions using artificial intelligence (AI) or machine learning technologies are also attracting attention [210].

5. Advantages, Disadvantages, and Challenges of Hybrid Material Systems

The various hybrid material systems mentioned above have been developed to mitigate the chronic problem of volume expansion in silicon anodes to enhance their electrochemical performance. These hybrid approaches have greatly contributed to maintaining the advantageous high theoretical capacity of Si while complementing its disadvantages and imparting new functionalities through synergistic effects with heterogeneous materials such as carbon, metal oxides, 2D materials, and CPs. In this section, the main advantages, disadvantages, and limitations of hybrid material systems are examined, the challenges that need to be overcome for commercialization are discussed, and possible solutions are proposed.

5.1. Advantages of Hybrid Material Systems

The greatest advantage of hybrid material systems is that they can be designed to effectively control the volume expansion of Si. Carbon coating layers [211] or matrices [212], metal oxide shells [213,214], and graphene wrapping [215] provide physical buffer spaces around silicon particles or impart mechanical constraint forces to disperse the stress generated during charging and discharging to inhibit the destruction of the electrode structure. Figure 15 provides a clear and powerful demonstration of this principle. The graph compares the cycling performance of an uncoated Si anode with anodes coated with 5 nm and 10 nm of TiO₂. While the uncoated anode’s capacity fades rapidly, especially at higher C-rates, both TiO₂-coated anodes exhibit remarkable stability. This directly confirms that even an ultrathin metal oxide shell can act as an effective mechanical buffer, successfully mitigating volume expansion and drastically improving the cycle life of the electrode. The ability to control the volume expansion is of key importance for greatly improving the low cycle stability, the most serious problem presented by pure silicon anodes. The ability to control the volume expansion is of key importance for greatly improving the low cycle stability, the most serious problem presented by pure silicon anodes. Hybrid systems are also able to effectively compensate for the low electrical conductivity of Si. Compositing with conductive materials such as carbon or CPs secures electron transfer pathways to lower the internal resistance of electrodes, which leads to high rate capabilities and low voltage polarization [40]. Consequently, rapid charging and discharging become possible, which improves the energy efficiency. The ion transfer characteristics can also be enhanced through structural design. Various hybrid structures such as core–shell, sandwich, and 3D networks facilitate ion transfer by shortening the lithium ion diffusion distances or optimizing the contact areas between the active material and electrolyte [67]. This is particularly important in applications that require high power. Furthermore, certain hybrid partner materials provide additional functions. For example, some metal oxides or 2D materials can contribute to improving the battery life and safety by inducing stable SEI layer formation on silicon surfaces or by serving as protective layers that inhibit electrolyte decomposition [216,217]. In this way, hybrid material systems offer the potential to increase the electrochemical performance to vastly beyond that of pure silicon by complementing the disadvantages and maximizing the advantages of Si by combining various materials structural designs.

5.2. Disadvantages and Limitations of Hybrid Material Systems

Despite their many advantages, hybrid material systems have several inherent disadvantages and limitations. The most representative is the trade-off relationship between capacity and stability. Although inactive materials constituting hybrid systems such as carbon and metal oxides contribute to mitigating the volume expansion and increasing the conductivity of Si, they themselves have low or almost no lithium storage capacity [218]. Therefore, an increase in the amount of hybrid partner materials necessarily lowers the reversible capacity of the entire electrode [219]. This dilemma arises when the proportion of inactive materials is increased to enhance the electrode stability, but dilutes the advantage of the inherently high capacity of Si. A major disadvantage is that the synthesis of hybrid structures can be complex and costly. Particularly in cases requiring elaborate nanostructure control such as core–shell structures or multilayer structures, or hybrid process methods that require multiple sequential processes, low synthesis yields or high process costs can become obstacles to mass production and commercialization [220]. Even materials that deliver excellent performance at the laboratory scale may be difficult to apply in practice without economically viable mass production technologies. Interface issues within hybrid systems are also important limitations. Interfaces between silicon and hybrid partner materials play important roles in electron and ion transfer, stress dispersion, and other functions, but the interfacial resistance may be high or the bonding strength may be weak due to physical and chemical mismatches between the two materials. The occurrence of delamination or cracks at the interfaces during repeated charging and discharging may obstruct conduction pathways to cause the performance to rapidly deteriorate. The formation and maintenance of stable and efficient interfaces therefore remains a technical challenge. Lastly, the still low ICE is also a limitation [221]. Due to the large surface area of Si and the complexity of hybrid structures thereof, the amount of lithium consumed during SEI formation in the first charge is often high, with the result that the ICE is lower compared to that of graphite anodes [197]. Figure 16 provides a direct comparison that highlights this critical issue. The graph shows that while the Sn-Si hybrid composite anode delivers a higher specific capacity than the graphite anode, its first-cycle Coulombic efficiency is significantly lower (below 70%). This starkly contrasts with graphite’s efficiency of over 90%, visually confirming that the large irreversible capacity loss from SEI formation on high-surface-area hybrid materials is a major hurdle to overcome for practical applications. This, in turn, reduces the energy density of full cells and lowers the utilization of the cathode materials, a problem that still requires attention.

5.3. Challenges to Overcome and Possible Solutions

Continuous research and development are necessary to overcome the disadvantages and limitations mentioned above to ensure the successful commercialization of silicon anodes based on hybrid material systems. The main challenges and possible solutions are as follows. First, the materials and structural designs need to be optimally combined to simultaneously achieve high capacity and high stability. Rather than simply increasing the inactive material content, strategies are needed to develop new hybrid partner materials that provide effective buffering and conductivity enhancement effects even in small amounts, or to control the structure of Si (e.g., porous, nanowire) to minimize the volume expansion [222]. Currently, research is actively underway to simultaneously secure the structural stability and ion/electron transfer characteristics while maintaining high silicon content through 3D hierarchical structures or spherical secondary particle designs [64,223]. Second, low-cost, high-efficiency mass synthesis processes need to be developed urgently. Instead of complex laboratory-scale processes, the development of simple and scalable synthesis methods that would be industrially applicable is essential. For example, processes that include mechanical synthesis methods (ball milling) need to be optimized, solution-based processes should be simplified [224], the yield of vapor deposition methods needs to be enhanced, and the development of practical hybrid processes that combine the advantages of each process is also important [225,226]. Taking a long-term perspective, inexpensive raw materials need to be identified and recycling technologies should be developed. Third, interfacial engineering research to lower the interfacial resistance and strengthen the bonding is important. Attempts are being made to induce chemical bonding between silicon and hybrid partner materials or to improve the interfacial stability and ion/electron transfer characteristics by introducing ultrathin artificial interfacial layers using ALD and other methods [227,228]. The utilization of advanced analytical techniques to elucidate detailed reaction mechanisms at interfaces should also be conducted in parallel. Fourth, strategies to improve the ICE are needed. Technologies such as pre-passivation treatment of silicon surfaces, artificial SEI layer formation, or pre-lithiation techniques that add limited amounts of lithium in advance are being researched [229]. These technologies can contribute to enhancing the ICE by reducing irreversible lithium consumption in the first cycle and improving the full-cell performance. Figure 17 illustrates a material-level approach to this problem. By comparing the initial charge–discharge profiles of a Si/C anode and a Si/Graphite/C (Si/G/C) anode, the graph shows that the Si/G/C composite has a smaller irreversible capacity (the difference between the first charge and discharge capacities). This suggests that incorporating stable, low-surface-area graphite into the composite can mitigate excessive side reactions, thus serving as a practical strategy to improve the ICE.

Fifth, securing long-term reliability in actual operating environments and improving the electrode processability are necessary. The performance of the electrode in full cells should be optimized based on the half-cell evaluation results and comprehensive verification including high/low temperature characteristics, lifetime prediction, and safety assessment should be conducted [230]. Additionally, the development of technologies to control the powder properties (density, particle size, flowability, etc.) such that the hybrid materials that are developed are well compatible with existing electrode manufacturing processes (slurry preparation, coating, calendering, etc.) is also important [231]. Figure 18 showcases an advanced material design that considers these practical aspects. The schematic details a method to create a HPC/Si@C composite by embedding Si within a hierarchical porous carbon (HPC) host. This approach not only tackles the volume expansion issue at the particle level but also results in larger, more uniform secondary particles, which can improve powder flowability and slurry characteristics, making it more compatible with existing electrode manufacturing lines. Solving these challenges is expected to require the active utilization of computational science and data-based research methodologies, along with convergence research across various fields such as materials science, electrochemistry, and process engineering.

Figure 19 illustrates a particularly important strategy for commercialization: developing low-cost, scalable manufacturing processes. The schematic shows the fabrication of porous Si (P-Si) by simply etching aluminum from an inexpensive, metallurgical-grade Si-Al alloy, followed by a carbon coating. This method bypasses the need for costly Si nanoparticles as a starting material. By demonstrating a pathway to produce high-performance porous silicon anodes from abundant, low-cost raw materials, this approach directly addresses the economic barriers that often hinder the mass production of advanced battery materials.

5.4. Recent Industrial Progress and Commercial Cases

Despite the challenges, significant progress has been made in the commercialization of silicon anodes, with several companies leading the charge. This section delves into the specific strategies and achievements of key players in the industry.

(i) Tesla: As a trailblazer in the EV market, Tesla has been at the forefront of battery innovation. While initial reports suggested that their 4680 cells would feature a high-silicon anode, recent teardowns have revealed that the current generation of 4680 cells still relies on a graphite-based anode. This highlights the immense difficulty in achieving stable, long-lasting performance with high silicon content in a mass-produced cell. However, Tesla continues to invest heavily in R&D to overcome these hurdles. Their patents and research papers indicate a focus on developing novel electrode structures, such as silicon nanowires or porous silicon, combined with advanced binders and electrolyte additives to mitigate the volume expansion issue and improve cycle life. The company’s long-term goal is to gradually introduce more silicon into their anodes to further increase the energy density and driving range of their vehicles.

(ii) Sila Nanotechnologies: Sila is a prominent US-based company that has developed a silicon-dominant anode material called “Titan Silicon”. This material is a nano-composite silicon (NCS) powder that is designed to replace graphite in existing battery manufacturing processes. Sila’s approach involves creating a porous, engineered structure at the nanoscale that can accommodate the volume expansion of silicon without fracturing. The company claims that its material can increase the energy density of lithium-ion batteries by 20–40% compared to graphite-based anodes. Sila has already achieved commercial success in the consumer electronics market, with its material being used in the WHOOP 4.0 fitness tracker. The company is now focused on scaling up its production for the automotive market and has established a large-scale manufacturing plant in Moses Lake, Washington, which is expected to start production in 2025. Sila has also signed a supply agreement with Panasonic to provide its Titan Silicon material for EV batteries.

(iii) Panasonic: As a major battery manufacturer and a key partner of Tesla, Panasonic is actively pursuing the integration of silicon anodes into its products. The company has a two-pronged strategy. First, it is working on gradually increasing the silicon content in its existing graphite-based anodes. This incremental approach allows them to improve energy density while maintaining the reliability and safety of their batteries. Second, Panasonic is collaborating with silicon anode startups like Sila and Nexeon to develop next-generation high-energy-density batteries. In 2023, Panasonic signed a purchase agreement with Nexeon, a UK-based company that produces a range of silicon anode materials with different morphologies and performance characteristics. This partnership will enable Panasonic to produce higher-energy-density EV batteries in the US, with commercial production expected to begin in 2025.

(iv) CATL: The world’s largest battery manufacturer, CATL, is also making significant investments in silicon anode technology. The company’s Qilin battery, which boasts an impressive energy density and fast-charging capability, is rumored to incorporate a silicon-based anode. CATL’s R&D efforts are focused on developing low-cost, high-performance silicon–carbon composite materials. They are also exploring innovative electrolyte formulations and cell designs to improve the overall performance and safety of their silicon-anode batteries. CATL’s vast manufacturing scale and expertise give them a significant advantage in bringing silicon anode technology to the mass market at a competitive cost.

6. Conclusions and Future Perspectives

6.1. Conclusions

This review has systematically examined the landscape of hybrid material systems designed to mitigate the severe volume expansion of silicon anodes, a critical bottleneck hindering their widespread adoption in next-generation lithium-ion batteries. While the theoretical capacity of silicon is an order of magnitude greater than graphite, its practical application is plagued by mechanical degradation, unstable solid electrolyte interphase (SEI) formation, and rapid capacity fade. Our analysis of the literature confirms that hybrid strategies are not merely incremental improvements but a vital and effective approach to creating structurally resilient and electrochemically stable silicon anodes.

Among the various strategies explored, those integrating carbonaceous materials have demonstrated the most significant and consistent success. Specifically, hierarchical structures that provide multi-level protection have proven most effective. For instance, yolk–shell Si@void@C structures and three-dimensional porous graphene frameworks encapsulating silicon nanoparticles have shown exceptional promise. These architectures physically buffer the ~300% volume change by incorporating internal voids, while the external carbon shell provides electrical conductivity and a stable surface for SEI formation. Similarly, sandwich-like structures, such as Si/graphene composites, effectively constrain silicon particles and maintain electrical contact throughout the electrode during cycling. These approaches have led to anodes that can maintain high capacity over hundreds of cycles, a significant improvement over bare silicon.

Despite this progress, critical challenges persist. The initial Coulombic efficiency (ICE) remains a primary obstacle, with much of the initial lithium inventory being consumed in the formation of the SEI layer on the high-surface-area nanostructured silicon. While strategies like pre-lithiation of SiO_x@C composites have achieved an impressive ICE of 93.4%, these methods add complexity and cost to the manufacturing process. Furthermore, ensuring the long-term stability of the SEI over thousands of cycles, especially in the presence of continuous volume changes, has not been fully resolved. Finally, the scalability of the most promising synthesis methods remains a major concern. Complex, multi-step processes like chemical vapor deposition (CVD) or atomic layer deposition (ALD), while effective at the lab scale, face significant hurdles in terms of cost, throughput, and industrial feasibility. The development of simpler, more scalable methods, such as the mechanochemical synthesis of Si/Cu3Si composites or the etching of metallurgical-grade Si-Al alloys, represents a crucial and promising direction, but these methods often require further optimization to match the performance of more intricate nanostructures.

6.2. Future Perspectives

Building on the current understanding, future research must pivot from demonstrating proof-of-concept to addressing the practical barriers of commercialization. The path forward requires a multi-faceted approach focused on material design, process engineering, and fundamental understanding.

First, the optimization of hybrid structures must continue, with a focus on balancing electrochemical performance with material and process complexity. Future designs should aim to minimize the content of inactive materials (like carbon shells or metal oxides) without compromising structural integrity. For example, creating covalently bonded Si–carbon interfaces or utilizing multifunctional binders that also participate in charge transport could reduce the reliance on bulky conductive additives and coatings. Computational modeling and machine learning will be instrumental in accelerating the discovery of novel material combinations and predicting the long-term performance of complex architectures, reducing the need for extensive trial-and-error experimentation.

Second, a concerted effort is needed to develop scalable and cost-effective manufacturing processes. While lab-scale synthesis has produced impressive results, the focus must shift to methods compatible with existing battery production lines. Wet-ball milling of industrial-grade silicon with carbon precursors, followed by thermal treatment, is a promising route. Further research into single-step synthesis techniques, such as the direct carbonization of silicon–polymer composites, could dramatically lower production costs. Additionally, developing robust quality control metrics to ensure batch-to-batch consistency of these hybrid materials at an industrial scale is a critical, yet often overlooked, research area.

Third, resolving the interfacial challenges of low ICE and SEI instability is non-negotiable for practical application. Research into advanced electrolyte formulations, including additives that promote the formation of a thin, flexible, and LiF-rich SEI, is paramount. Combining these electrolyte solutions with surface-engineered silicon, such as through ultra-thin ALD coatings of Al₂O₃ or TiO₂, could create a synergistic effect that dramatically enhances cycle life and efficiency. Furthermore, developing practical and scalable pre-lithiation techniques that can be seamlessly integrated into the electrode manufacturing workflow is essential to compensate for the initial lithium loss.

Finally, the research community must move towards evaluating these materials under commercially relevant conditions. This includes testing in full-cell configurations (not just half-cells) with high mass loadings, lean electrolyte, and under a wide range of temperatures and C-rates. Only by validating performance under these stringent conditions can the true potential of hybrid silicon anodes be realized, paving the way for their integration into the next generation of high-energy-density batteries that will power our future.