Carbon nanofibers (CNFs) and carbon nanotubes (CNTs) are prominent types of 1D carbon materials that have garnered significant attention for their use in LIB applications. The composition of Si with either CNTs or CNFs brings several advantages to the table, notably increased electrical conductivity, as well as exceptional mechanical and thermal stability [38]. As such, CNFs and CNTs have proven more effective in constructing conductive networks and providing a superior host matrix for Si-based anode materials that face volume changes during the charging and discharging process.

Before delving into specific studies, it's crucial to acknowledge the significance of ICE in the performance evaluation of silicon-based anodes. Nanosilicon, often used in conjunction with carbon materials like CNTs, can induce more side reactions and form thicker SEI due to its high specific surface area and energy, potentially affecting the ICE. Hence, alongside capacity metrics, the ICE is a vital parameter to assess the initial energy efficiency of these composites. Wang et al. [39], in a pioneering effort, used chemical vapor deposition (CVD) to grow CNTs on both crystalline and amorphous Si. The structure that resulted from this technique offers the rapid transportation of ions and electrons, achieving a high CE and a notable reversible capacity of 2000 mAh g⁻¹. Significantly, the ICE of this composite was reported at 80.3%, highlighting its efficiency in the first cycle and indicating a balance between capacity and initial energy retention. Here, the incorporation of CNTs by Wang et al. demonstrates the essential role of 1D carbon structures in enhancing the electrical conductivity and mechanical stability of Si. The CNTs form a conductive network that efficiently manages electron transport and accommodates the volumetric expansion of Si during lithiation, crucial for the observed high CE and capacity. Similarly, Zhang et al. [40] innovated a three-dimensional (3D) Si/CNT composite, ensuring that Si nanoparticles are effectively interconnected electrically via CNTs. This composite, when utilized as an anode for LIBs, showcased a capacity of 943 mAh g⁻¹ post 100 cycles. Notably, the ICE for this composite was reported at 56%, underscoring the impact of its design on initial energy efficiency. Such data are crucial for understanding the balance between capacity retention and initial energy utilization in nanosilicon composites. Taking another innovative approach, Su et al. [41] crafted a 3D porous material constituted of SiO₂ and Si that was seamlessly decorated with CNTs in situ, termed as pSS/CNT. This unique composite offers superior stability during cycling and boasts an improved CE. Importantly, the ICE of the pSS/CNT composite is reported to be 70%. This figure is crucial as it provides a measure of the initial energy efficiency of the composite, highlighting its effectiveness in the first cycle and contributing to a comprehensive understanding of its overall electrochemical performance. Peng et al. [42] ventured to design a chain-like Si CNT architecture (C-L-SC), integrating ZIF-67 derived porous carbon encapsulating Si nanoparticle with CNTs acting as connectors between the carbon shells. The manufacturing procedure of C-L-SC involves the growth of CNTs upon SiNPs via CVD to generate a tangled cluster structure first, and the growth of ZIF-67 on the surface, followed by carbonization (Fig. 3a). Figure 3b showcases the morphology of pure Si NPs, which exhibit a mean diameter of ~ 80 nm and a relatively uniform distribution. After CVD process, the CNTs uniformly grew on the nanosized Si, intertwining among themselves as seen in Fig. 3c. Figure 3d, e illustrates ZIF-67 crystals growing along the Si@CNTs, in which a chain-like structure formed. Upon carbonization, the C-L-SC retained its original structure, with ZIF-67 crystals maintaining their dodecahedral morphology as depicted in Fig. 3f, g. Figure 3h, i provides a comparative analysis of the cycling performances and rate capability of various compositions, including pure Si, Si@CNTs, C-ZIF-67, and C-L-SC electrodes. Impressively, the C-L-SC composite exhibited significant rate capacity and cycling stability, maintaining a capacity of 732 mAh g⁻¹ at 2 A g⁻¹ and retaining 72.3% of this capacity over 100 cycles at 1 A g⁻¹. To assess the anode material's stability, SEM imaging captured the morphological evolution of the C-L-SC electrode before (Fig. 3j) and after cycling (Fig. 3k). Remarkably, even after 100 cycles, the electrode maintained a complete and unaltered structure, devoid of new surface cracks. This series of studies further emphasizes the significance of 1D carbon structures. The mechanical stability provided by CNTs and CNFs is key in alleviating the strain associated with Si’s volume changes during battery operation. This contributes to improved cycle stability and capacity retention, as evidenced in these Si/C composites.

In a similar realm of innovation, Deng et al. [43] conceived a structure by embedding Si nanoparticles within the core of double-layered CNTs, resulting in a sandwich-like configuration. This configuration significantly boosted the electrochemical attributes of Si electrodes. In a noteworthy methodological shift, CNT arrays were synthesized using liquid paraffin, replacing traditional unsaturated hydrocarbons via the CVD method. This unique design ensured that the resultant material exhibited stellar rate performance and excellent cycling stability, retaining capacities of 1310 mAh g⁻¹ at 0.1 A g⁻¹ post 100 cycles and 1050 mAh g⁻¹ at 1 A g⁻¹ post 500 cycles, alongside a 98% CE. In another commendable effort, Zhang et al. [44] synthesized Si/Cu₃Si@C composites encased within a mesh of CNTs (SCC-CNTs) by amalgamating ball milling and CVD techniques. Comprising of conductive Cu₃Si, an amorphous carbon layer, interconnected CNTs, and etched pores, these elements collaboratively enhance electronic conductivity and Li⁺ diffusion. The electrochemical processes involved also counteract the volume expansion of the Si anode. When juxtaposed against pure Si, SCC-CNTs composites manifest considerably superior electrochemical performance, recording a discharge capacity of 2171 mAh g⁻¹ at 0.4 A g⁻¹, an ICE of 85.2%, and a retained capacity of 1197 mAh g⁻¹ after 150 cycles. In these examples, the 1D carbon matrix notably boosts electron conductivity while providing a mechanical buffer against Si volume expansion during cycling. This synergy between the conductive carbon network and Si nanoparticles is instrumental in achieving enhanced rate performance and cycling stability in the Si/CNT and Si/Cu₃Si@C composites.

CNFs, due to their distinct properties, have been extensively incorporated in LIB applications. Their robustness in terms of chemical and thermal stabilities is complemented by their commendable electrical conductivity. Xu et al. [45] employed a fusion of electrospinning and electrospray techniques to produce 3D Si/C fiber paper, exhibiting a formidable electrochemical performance of 1200 mAh g⁻¹ after a span of 100 cycles. In a similar vein, Lee et al. [46] demonstrated the potential of Si/CNF composite films for anode applications. These films, developed through a filtration method followed by a thermal reduction process, showcased a significant enhancement in LIB performance, as depicted in Fig. 4a. The SEM images (Fig. 4b, c) reveal a dense, intertwined architecture where CNFs support Si particles, forming a robust three-dimensional matrix. This configuration is conducive to efficient electron and ion transport and effectively manages the volumetric alterations of Si particles throughout charge/discharge cycles, ensuring exceptional mechanical stability. TEM analyses (Fig. 4d, e) further confirm the dense packing and uniform distribution of Si particles within the CNFs. This structural design is pivotal for achieving superior specific capacity and excellent cycle and rate performances.

In summary, while 1D Si/C composites such as Si/CNTs and Si/CNFs show significant potential in enhancing charge transfer and lithium-ion diffusion for improved electrode cycling stability, they present unique challenges. Their complex fabrication processes require innovative and cost-efficient strategies, and their adaptation to high-performance applications demands careful consideration of mass loadings and economic feasibility. These challenges not only highlight the critical need for ongoing research and development within the realm of 1D structures but also pave the way for investigating the next frontier in carbon-Si composites—the realm of two-dimensional (2D) structures. The transition to 2D carbon-Si composites presents an opportunity to build on the foundations laid by 1D composites while exploring new structural benefits and solutions to persisting challenges.

2D materials offer distinct advantages as anodes. They can shield electrodes from abrasion, enhance the synergy of Li-ion deposition, assist Li ions in traversing through electrolytes and electrodes, and bolster stability at elevated temperatures. Among these materials, graphene stands out as the predominant 2D carbon substance, boasting a surface area of 2630 m² g⁻¹ and exceptional mechanical, electrical, and thermal properties [47,48]. It’s a favored choice for energy storage, extensively utilized in LIBs, Na-ion batteries, supercapacitors, and Li-S batteries [49]. The significant transformation brought about by 2D materials in Si-based anodes can be attributed to several key mechanisms. The planar structure of materials like graphene provides a large surface area for uniform Li-ion deposition and facilitates rapid electron and ion transport. This not only improves the electrical conductivity within the electrode but also effectively manages mechanical stresses during lithiation/delithiation processes. The synergy between the 2D carbon structure and Si contributes to both the electrochemical efficiency and mechanical resilience of the anode. Such structural benefits are critical in overcoming the challenges of electrochemical inefficiency commonly experienced with Si anodes. Su et al. [50] devised an efficient approach to creating a Si@G composite with a spherical morphology using a simple spray drying method, eliminating the need for post-annealing. This composite, characterized by its core-shell structure, significantly improves both electric and ionic conductivities due to the synergistic interaction between the spherical shape and the multi-layered graphene arrangement. A crucial element of this approach is the formation of a void within the Si@G structure, designed to accommodate Si's inherent volume expansion during battery operation. The resulting robust Si@G composite demonstrates an impressive initial discharge capacity of 2882.3 mAh g⁻¹ and an admirable 86.9% ICE at 0.2 A g⁻¹.

In a related study, Jamaluddin et al. [51] presented a novel Si@N-G composite, where Si nanoparticles (SiNPs) serve as the core and electrochemical exfoliation-derived nitrogen-doped graphene (N-ECG) forms the shell. This composite underwent further annealing with H₂ and NH₃ to optimize its properties as an anode material. The incorporation of N-ECG, led to significantly improved electrical conductivity and ion mobility. These enhancements were particularly prominent in the Si@N-ECGB microball structure, demonstrating a high initial discharge capacity of 2604.5 mAh g⁻¹, and a CE of 85.2%. The superior electrochemical performance can be attributed to its facilitation of an effective lithiation/delithiation process. Lin et al. [52] devised an innovative approach to fabricate Si deposited graphene nanowalls (GNWs) within Ni foam matrix. As illustrated in Fig. 5a-c, GNWs were first grown on Ni foams using a plasma-enhanced tube furnace deposition system. Then Si materials were precisely deposited onto the as-grown GNWs via magnetron sputtering, ensuring a uniform, smooth, and conformal coverage of Si thin film on both sides of the GNWs, as depicted in Fig. 5f. The unique architecture of these 3D GNW networks facilitates enhanced electron transport. Notably, the Si thin film with a thickness of several tens of nanometers, acts as a buffer against the pulverization issue arising from volume expansion. Figure 5g, h showcases remarkable adaptability across a diverse range of current rates when the GNWs@Si composite applied in LIBs. The composite retained a commendable discharge specific capacity of 1450 mAh g⁻¹ at 190 mA g⁻¹ even after seventy dynamic cycles. Such exemplary performance underscores the potential of the GNWs@Si nanocomposite, attributing its efficiency to the three-dimensional network structure of GNWs, which bolsters the conductivity and minimizes electrode polarization at elevated rates. For instance, the Si@G composite developed by Su et al. demonstrates how the 2D graphene structure, with its high conductivity and mechanical stability, synergistically interacts with Si to enhance the composite's overall performance, particularly in terms of capacity and cycle life. MXenes, another breed of 2D carbon materials, consist of nitrides and carbides with multi-atomic layer thicknesses, finding uses in various battery components [53,54,55,56]. Guo et al. [57] introduced the MXene/Si@SiO_x@C composition, which displayed robust cycling performance influenced by Si concentration. At 72.8 wt% of Si, the anode exhibited a stellar capacity of 470 mAh g⁻¹ at 10 C across 1000 cycles, credited to the MXene layers' high electronic conductivity, modifiable layer spacing, and abundant covalent bonding.

In summary, 2D materials have significantly transformed the landscape of anode enhancements, particularly with their incorporation into Si-based anodes. Their unique structural properties, combined with impressive electrical conductivity, provide a tactical advantage in overcoming the common setbacks experienced with Si anodes, notably regarding electrochemical inefficiency. By enhancing electron conductivity within electrodes, these 2D materials, including graphene and other layered materials, have demonstrated substantial advancements in battery performance metrics. These enhancements are evident in their impressive specific and volumetric capacities, along with and their ability to withstand substantial mechanical stresses encountered during lithiation/delithiation process.

However, although 2D materials represent a revolutionary stride forward, they constitute just one facet of the broader exploration into spatially complex carbon-Si composites. Progressing from 2 to 3D frameworks ushers in a new era of possibilities in energy storage technology. The subsequent section will shift our discussion to 3D carbon-Si composites, where we will investigate how these intricate structures continue to push the boundaries of electrochemical performance. As we delve into 3D configurations, we anticipate unveiling innovative strategies to navigate the inherent challenges of Si-based anodes, potentially leading to even more robust, efficient, and durable LIBs.

Porous carbon and graphite stand out as the two primary categories of 3D carbon materials extensively utilized in LIBs. The widespread use of porous carbon as a support material in LIBs is attributed to its ability to facilitate rapid ion and electron mobility, coupled with its generous void space. A groundbreaking study led by Li’s group [58] detailed the fabrication process of gigaporous carbon microspheres. This process involved emulsifying poly (styrene-co-vinylbenzyl chloride), oleic acid, and Si powder, resulting in Si-embedded porous microspheres that was subsequently encapsulated in situ with poly (urea-formaldehyde) (PUF). By incorporating an extra carbon source and adjusting the pH to accomplish the carbon encapsulation, carbonizing the resulting material was carbonized, giving rise to the final carbon-encapsulated porous Si/C microsphere. Prior to Si implantation, these microspheres exhibited a profusion of micropores and were infused with increasing quantities of micron-sized Si powder. They meticulously analyzed both micron-sized (u-Si) and nanosized (n-Si) Si particles. The improved retention of n-Si over u-Si is attributed to its reduced volume fluctuation. Furthermore, the extra carbon layer on the Si/C microsphere fortified the structural integrity of the electrode. The carbon-encapsulated n-Si/C microsphere debuted with an initial capacity of 3320 mAh g⁻¹ and remarkably retained 90% of this capacity over after 100 cycles, benefiting from the leveraging the robust porous carbon framework.

Si/graphite composites are particularly captivating because graphite augments the composite's capacity while serving as a Si matrix, dispersing volume expansion and facilitating electrical connection. Xiao et al. [59] pioneered in the realm of electrode material design with their innovative Si/graphite/carbon (Si-G/C) composite. As schemed in Fig. 6a, a Si/graphite precursor was first fabricated by thorough mixing of nano-Si particles, graphite, pitch and polyvinyl pyrrolidone (PVP), which was then underwent a spherification process in a fusion machine, followed by a heat treatment at 900 °C, resulting in the Si-G/C composite. Figure 6b exemplifies the TEM depictions of the Si nanoparticles with 30-50 nm in diameter, exhibiting tendencies of aggregation owing to their elevated surface energies. A comprehensive structural profile of the Si-G/C composite is presented in Fig. 6c. Figure 6d indicates that the full cell, integrating the Si-G/C composite anode with the NCA cathode, demonstrates an initial cell capacity of 3000 mAh and maintains an impressive 81% of this capacity over 1200 cycles. This underscores the potential of the Si-G/C composite as a robust and reliable component in advanced battery assemblies.

Elsewhere, Li et al. [60] developed a porous Si/C composite, which was then integrated into a graphite-blended anode. This composite consistently achieved a capacity of 550 mAh g⁻¹ across 200 cycles. Wu et al. [61] innovatively harnessed pinecones to craft an environmentally benign porous carbon (PPC) envisioned as an ideal matrix for Si/carbon composite anodes. Leveraging the intrinsic porous architecture of pinecones, they proficiently encased Si nanoparticles within the PPC substrate, culminating in the PPC/Si composite's birth. This composite, bolstered by the electro-conductive PPC network and minuscule Si nanoparticles, enhances both electron and ion movements. Its porous attributes not only guarantee optimal electrolyte permeation but also make room for Si’s volume expansion during cycling. Consequently, the PPC/Si composite exhibits a stellar rate performance-478.4 mAh g⁻¹ at 2 A g⁻¹ and remarkable cycling endurance-720.6 mAh g⁻¹ at 0.2 A g⁻¹ after 300 cycles.

To encapsulate, 3D carbon coatings, when applied to Si surfaces, are meticulously engineered to shield Si particles, preventing their direct exposure to electrolytes. Instead of interacting with Si directly during electrochemical processes, the electrolyte constructs a stable SEI film in tandem with the carbon layer. This robust carbon layer absorbs the volume expansion of Si, ensuring the prevention of Si disintegration. The unique yolk-shell structure of Si/3D carbon composites creates ample space for Si particle fluctuations, negating their detachment of the particle from the electrode, thereby preserving their electrochemical vitality. 3D carbon coatings have undeniably demonstrated their efficacy in shielding Si anodes and sustaining exceptional battery performance. However, it’s important to note that producing 3D carbon-coated Si tends to be costlier, and refining the process demands meticulous attention and precisions.

In summary, 3D carbon-Si composites represent a pinnacle in anode engineering, harnessing the structural benefits of three-dimensional carbon materials to address fundamental issues associated with Si anodes. These composites excel in accommodating Si’s volume expansion, enhancing electrical connectivity, and ensuring structural integrity during rigorous cycling. Innovations, particularly in the synthesis of gigaporous carbon microspheres and unique Si/graphite/carbon composites, have underscored the feasibility and performance superiority of these 3D constructs. While challenges in cost and fabrication complexity underscore the need for further optimization, the advancements within this domain lay a solid foundation for the next phase of anode material research. Table 3 provides the cycle stability of various Si/C-based for LIBs.

In 3D carbon-Si composites, the structural benefits of three-dimensional carbon materials play a crucial role in enhancing the performance of Si anodes. The porous nature of materials like gigaporous carbon microspheres and graphite provides extensive void space, facilitating rapid ion and electron mobility. This architecture is instrumental in managing the volume expansion of Si during cycling, a key challenge in Si anode utilization. Additionally, the 3D carbon framework ensures structural integrity and consistent electrical connectivity, further contributing to the robustness and efficiency of the composite. These structural advantages, combined with the unique yolk-shell configurations, offer an effective buffer against mechanical stresses and electrolyte interactions, thereby significantly enhancing the durability and electrochemical performance of Si anodes.

As we transition into new strategic explorations, the upcoming sections will focus on the burgeoning field of nanostructured-based Si anode materials. The shift toward nanotechnology signifies our continual pursuit of refining performance and longevity in LIBs, where even finer control at the molecular level opens new horizons for material capabilities and electrochemical outcomes.

Si nanoparticles, classified as zero-dimensional Si materials, have become highly promising candidates for LIBs for two primary reasons:

1. Their diminutive size is essential for alleviating stress and preventing Si damage during the lithiation/delithiation processes, enhancing cycling performance [84,85].

2. Their synthesis techniques are well-established, making them commercially accessible [86].

Ge and colleagues [87] crafted porous Si nanoparticles using through electroless etching and boron doping. Upon incorporating with graphene, the resulting material exhibited a commendable capacity of 1000 mAh g⁻¹ after 200 cycles. Wang’s team [88] introduced a mesoporous Si/C composite, synthesized using a self-assembly method induced by evaporating a triblock copolymer in a resorcinol-formaldehyde (RF) resin. This composite featured 100 nm Si nanoparticles evenly dispersed within a mesoporous carbon matrix, boasting a capacity of 1018 mAh g⁻¹ over 100 cycles. Epur et al. [89] innovated a binder-free Si/MWCNT anode for LIBs, demonstrating an initial discharge capacity of 3112 mAh g⁻¹ at 300 mA g⁻¹. Remarkably, it retained 76% of this capacity after 50 cycles, attributed to the robust CNT/Si connection, which ensured minimal volume variation and robust electrical conductivity. Liu et al. [90] introduced an innovative and effective synthesis strategy to produce composite Cu₂MoS₄/SiNS materials, achieved through the self-assembly of silicon nanospheres and a two-dimensional Cu₂MoS₄ material (Fig. 7a). The resulting Cu₂MoS₄/SiNS composite exhibited clear adherence of silicon nanospheres to small particles on the surface of the sheet material (Fig. 7b). TEM analysis further revealed the distinctive sheet structure of the Cu₂MoS₄/SiNS composite, showcasing the efficient dispersion of porous silicon within the two-dimensional layered structure (Fig. 7c). This dispersion plays a crucial role in releasing volume expansion and alleviating mechanical stress, providing additional active sites and fast channels for Li⁺ transmission, ultimately enhancing the material’s conductivity. When employed as the anode in LIBs, the Cu₂MoS₄/SiNS material displayed significantly improved electrochemical performance. Pushing the boundaries, the material was subjected to fast charging at an increased current density of 2.0 A g⁻¹. Impressively, it demonstrated a specific capacity of 1180 mAh g⁻¹ with 69.2% capacity retention even after 400 cycles (Fig. 7d). The composite material's remarkable stability and enhanced capacity underscore its potential for advancing the construction of high-performance LIBs.

Jung et al. [91] employed a one-pot hydrothermal technique to produce a SiC composite material, incorporating etching-modified Si nanoparticles and sucrose as a carbon precursor. The proposed SiC composite, which is meso-macroporous and houses a significant quantity of Si nanoparticles (40 wt%) within a 3 μm diameter carbon sphere, presented a high initial capacity of 1300 mAh g⁻¹, with an impressive 90% retention after 200 cycles and rapid charge/discharge abilities of just 12 min. Zhou and colleagues [92] developed SiNP@G, achieving a capacity of 1205 mAh g⁻¹ over 150 cycles. Liu’s team [93] utilized CVD to design a yolk-shell carbon structure on Si nanoparticles, facilitating Si expansion without rupture during charging and discharging processes.

Si nanoparticles, characterized by their porous and hollow configurations, yield superior electrochemical performance due to several factors:

1. Voids and pores accommodate volume expansion, effectively safeguarding against the volume alterations during charge and discharge processes. This unique feature ensures the structural integrity of the material, allowing it to endure repeated cycles without significant degradation.

2. The porous architecture truncates electron and lithium pathways, minimizing polarization and amplifying rate efficiency [94]. By reducing the distances lithium ions and electrons need to travel, the material can charge and discharge rapidly, making it ideal for high-performance applications.

3. Such structures decrease local current density, leading to reduced stress gradients near particle surfaces and thus enhanced electrochemical performance. By distributing the electrochemical reactions more evenly throughout the material, these nanostructures can sustain their performance over numerous cycles.

Fang et al. [95] unveiled a Si@TiO₂ core-shell structure, encapsulating Si nanoparticles within a TiO₂ void. This design demonstrated an impressive capacity of 804 mAh g⁻¹ at 0.1 C over 100 cycles, highlighting the potential of core-shell structures in enhancing Si-based electrode materials. Furthermore, through mechanical mixing, Park and associates [96] produced Si/Ti₂O₃/rGO, forming ternary nanocomposite with a capacity of 950 mAh g⁻¹ after 100 cycles at 100 mA g⁻¹. This versatile approach demonstrates the continuous efforts to innovate Si nanostructures, addressing mechanical challenges associated with lithiation and delithiation processes. Their smaller scale not only facilitates rapid Li transfer but also effectively mitigates stress, ultimately leading to improved overall performance.

In the realm of 0D Si-Based Anode Materials, the diminutive size of Si nanoparticles plays a pivotal role. Their nanoscopic scale fundamentally changes the kinetics of lithium insertion and extraction processes. This smaller scale allows for rapid Li-ion transfer and effectively mitigates mechanical stress during the lithiation/delithiation cycles. Additionally, the porous and hollow configurations of these nanoparticles provide voids that accommodate volume expansion, safeguarding the structural integrity during charge and discharge processes. This feature is crucial in preventing significant material degradation over numerous cycles. Furthermore, their synthesis techniques, such as electroless etching and self-assembly, enable the production of Si nanostructures with features like mesoporosity and carbon coating, which further enhance their electrochemical characteristics.

In conclusion, zero-dimensional Si nanoparticles emerge as a revolutionary stride in the realm of LIB anode materials. The strength of these materials lies in their nanoscopic scale, which fundamentally alters the kinetics and mechanics of lithium insertion and extraction processes. Through various innovative approaches, researchers have successfully synthesized Si nanostructures with enhanced electrochemical characteristics. Techniques such as electroless etching, self-assembly, and chemical vapor deposition have led to the development of Si nanostructures with integral features such as mesoporosity, carbon coating, and yolk-shell architectures. These structural refinements address the critical challenges associated with Si’s volume expansion and mechanical degradation during battery operation.

Particularly notable are the strategies involving the incorporation of conductive additives, protective coatings, and structurally reinforcing elements that contribute to prolonged cycle life and improved capacity retention. Examples, including Si/CDs, Si@TiO₂, and Si/Ti₂O₃/rGO composites, have underscored the versatility and adaptability of Si nanoparticles in various composite configurations. Moreover, the deliberate design of voids and pores in these particles serves multiple purposes, from accommodating physical expansion to enhancing ion transport and reducing polarization effects.

As we advance to explore one-dimensional (1D) Si-based materials, the insights gained from 0D Si nanoparticle research set a compelling precedent. The quest continues to optimize Si anodes’ resilience and efficiency, with nanoscale engineering remaining at the forefront of these innovative efforts. The ensuing section will delve into the prospects offered by 1D Si structures, reflecting on their unique geometries and the implications thereof in the continuing evolution of LIB technology.

Si nanowires (SiNWs) and Si nanotubes (SiNTs) are prominent 1D structures in LIBs due to their unique attributes: Firstly, they possess the capability to mitigate mechanical strain during volume changes in the radial direction, preventing material pulverization. Secondly, they facilitate their facilitation of efficient electron transit, ensuring rapid charge transfer and, consequently, enhanced electrochemical performance. Significant efforts have been devoted to developing 1D-structured Si anodes using various innovative strategies. In 2008, Chan’s group [17] synthesized Si nanowires with a diameter of 90 nm utilizing a CVD. These nanowires exhibited commendable CE of 73% and improved cycle life, attributed to their well-structured nature accommodating substantial volume changes. Zhao et al. [97] introduced a novel 1D tubular silicon-nitrogen-doped carbon composite (Si@NC) with a core-shell structure, utilizing silicon magnesium alloy and polydopamine as a template and precursor (Fig. 8a). The morphology of the Si@NC composite was further analyzed using SEM (Fig. 8b), revealing that the diameter of silicon particles is approximately ~ 100 nm. The Si@NC composite demonstrates a remarkable specific capacity and ultrafast redox kinetics, exhibiting outstanding cycling stability with a fine capacity of 583.6 mAh g⁻¹ at 0.5 A g⁻¹ over 200 cycles (Fig. 8c). The engineered nanotube structure and silicon confined in nitrogen-doped carbon effectively alleviate volume expansion and confer superior stability.

Chen et al. [98] pioneered a novel self-supporting electrode with high mass loading capacity and enduring cycle life. It was based on carbon-coated SiNWs grown in situ on highly conductive, flexible carbon fabric substrates via a nickel-catalyzed one-pot atmospheric CVD technique. The superior quality of these carbon-coated SiNWs resulted in an impressive reversible specific capacity of 3500 mAh g^-1 at 100 mA g^-1. Beyond NWs, NTs represent another potential 1D configuration for Si anode materials. Song et al. [99] fabricated SiNT arrays on a stainless-steel substrate, utilizing a synthesis method combining templating and CVD. This structure demonstrated an admirable electrochemical performance of 2500 mAh g⁻¹ after 50 cycles, attributed to the interspersed vacuum spaces between successive NTs. Wu and his team [100] designed a distinctive double-walled Si-SiO_x nanotube (DWSiNT) anode, featuring an active Si inner wall and a protective SiO_x outer layer. As an anode for LIBs, it maintained 94% of its original capacity after 500 cycles, with its structural design playing a pivotal role in this elevated electrochemical performance.

In 1D Si-based anode materials, the structural attributes of Si nanowires (SiNWs) and Si nanotubes (SiNTs) play a crucial role. Their elongated, one-dimensional form allows for efficient accommodation of volumetric expansion during the lithiation/delithiation processes, preventing material pulverization. Furthermore, the 1D structure facilitates rapid electron transport, significantly enhancing charge transfer and electrochemical performance. These unique properties, combined with advanced fabrication techniques like CVD and MACE, lead to controlled dimensions and improved electrochemical characteristics, thereby addressing the challenges associated with the use of Si in LIBs.

In summary, the exploration of one-dimensional (1D) Si-based anode materials has marked a significant milestone in enhancing the electrochemical performance of LIBs. Both Si nanowires (SiNWs) and Si nanotubes (SiNTs) have demonstrated exceptional promise owing to their structural attributes that enable effective accommodation of volumetric expansion and efficient electron transport, critical to high-performance LIBs.

Innovative fabrication techniques, such as CVD and metal-assisted chemical etching (MACE), have paved the way for the synthesis of highly ordered SiNWs and SiNTs with controlled dimensions and desirable electrochemical characteristics. The development of 1D structures like mesoporous Si nanowire arrays and double-walled Si-SiO_x nanotubes epitomizes the ongoing efforts to harness Si's high specific capacity while mitigating the adversities of volume expansion. Furthermore, the integration of carbon coatings and flexible substrates has emerged as a viable strategy to reinforce structural integrity and enhance electrical conductivity, factors paramount to sustaining long-term cycling stability.

Noteworthy are the reports of prolonged cycle lives and high specific capacities achieved by these 1D Si anodes, underscoring the effectiveness of these nanoengineered configurations. For instance, the pioneering work on self-supporting electrodes based on carbon-coated Si nanowires and the impressive performance of double-walled Si-SiO_x nanotube anodes exemplify the synergistic benefits of combining robust mechanical design with electrochemically active materials.

As we transition into the discussion of two-dimensional (2D) Si-based anode materials, it becomes evident that the lessons learned and successes garnered in the realm of 1D Si structures are invaluable. They not only serve as a testament to the advancements in nanomaterial synthesis and application but also set a solid foundation upon which further explorations into multidimensional Si-based materials for next-generation LIBs can be built.

Si thin film, characterized as a two-dimensional structure, has garnered widespread attention as an anode material for LIBs. Its allure stems from its ability to mitigate the mechanical degradation challenges encountered during charge and discharge cycles. Greatz et al. [101] employed a 100-nm-thick amorphous Si thin film, revealing a specific capacity of 2000 mAh g⁻¹ over 50 cycles, a remarkable achievement attributed to its nanoscale dimension. Chen’s team [102] applied a 275-nm-thick layer of amorphous Si on a Cu substrate for anode purposes, registering a reversible capacity of 2200 mAh g⁻¹ after 500 cycles. Placke et al. [103] synthesized Mg-Si thin films by merging Si and Mg via magnetron sputtering, embedding the Si within an active magnesium-silicate matrix. This structure showcased a capacity of 1000 mAh g⁻¹, retaining 96% of it over 400 cycles. Sun's team [104] utilized electrodeposition to produce a Si film on Ni foam, displaying an impressive reversible specific capacity exceeding 2800 mAh g⁻¹ at a current of 360 mA g⁻¹ through 80 cycles.

In 2D Si-Based Anode Materials, the planar structure of Si thin films plays a pivotal role. This structure offers a high surface-area-to-volume ratio, which is beneficial for reducing electron transport and lithium diffusion distances. This, in turn, minimizes polarization and enhances rate performance. Additionally, the thin-film form of Si provides effective management of mechanical stresses during the lithiation/delithiation cycles. This planar geometry allows for even distribution of stress and volume expansion, which is crucial for maintaining the structural integrity of the anode material over numerous cycles.

In summary, thin films emerge as a paramount solution to overcome Si's dual impediments: limited conductivity and pronounced volume expansion. Conventionally, the thickness of thin films spans from a fraction of a nanometer to several micrometers. The inherent advantages of this approach include higher average output voltage, reduced electrode mass, and extended cycle longevity compared to traditional LIBs. The structure of thin films, featuring with a superior surface-area-to-volume ratio, expedite lithiation and delithiation due to substantially minimized electron transport and lithium diffusion distances. Consequently, this curtails polarization, bolstering the rate performance. Moreover, this configuration adeptly manages the mechanical stresses arising from lithium-ion movement.

In retrospect, the realm of 2D Si-based anode materials has witnessed a transformative evolution, driving forward the efficacy and resilience of LIBs. Si thin films, epitomizing 2D structures, have captivated researchers due to their inherent ability to navigate the notorious mechanical deterioration prevalent during electrochemical cycling. This is predominantly facilitated by their planar geometry that significantly minimizes deleterious stress associated with lithiation and delithiation, thereby preserving structural integrity.

The breakthroughs chronicled within this domain exemplify meticulous material engineering and novel synthesis approaches. From employing amorphous Si layers of varying thicknesses to innovatively coupling Si with other elements such as magnesium, researchers have unfolded diverse strategies to enhance both the specific capacity and cycle stability of LIBs. For instance, the adoption of a multi-layered approach, leveraging active matrices like magnesium silicate, underscores the creative integration of materials aimed at stabilizing Si during battery operation.

Thin films’ characteristic high surface-area-to-volume ratio is pivotal, serving dual purposes: it shortens lithium ion and electron transport pathways, mitigating polarization and enhancing rate capability, and it proficiently accommodates volumetric expansions, a chronic setback with Si utilization in battery applications. This has culminated in demonstrable improvements in specific capacities, cycling life, and overall battery performance metrics.

Furthermore, the synthesis methods, ranging from electrodeposition to magnetron sputtering, reflect a sophisticated evolution of thin-film technology, reinforcing the versatility and adaptability of 2D Si-based anodes. This versatility extends beyond material composition to encompass advancements in structural design, including hybrid architectures that meld the benefits of Si with synergistic materials.

As we venture into the exploration of three-dimensional (3D) Si-based anode materials, the insights garnered from 2D structures are indispensable. They delineate a pathway rife with potential for overcoming the long-standing obstacles associated with Si anodes. Thus, the transition to 3D configurations is a logical progression, building on the foundational understanding established through the study of 2D Si anodes, and symbolizing a quest to further exploit Si's volumetric charge storage capacity without compromising structural and electrochemical stability.

3D macroporous Si-based structures have attracted significant attention as anode materials for LIBs due to several reasons: (1) the microporosity accommodate volume variations during the lithiation/delithiation process without compromising the structure or integrity of the material; (2) their continuous network of electrode materials enhances electrical conductivity; and (3) the macropores, spanning hundreds of nanometers, facilitate rapid liquid-phase Li-ion transport, reducing polarization and boosting electrochemical performance. Bang et al. [105] developed 3D macroporous Si materials by integrating electroless metal deposition through a galvanic displacement reaction with a metal-assisted chemical etching method, utilizing commercially available bulk Si powders. This material exhibited a reversible capacity of 2050 mAh g⁻¹. Yang et al. [106] fabricated a 3D macroporous Si with a consistently interconnected architecture via magnesiothermic reduction. They achieved adjustable morphological control of 0D hollow nanospheres by simply altering reaction conditions. As anode materials in LIBs, the void space in both architectures efficiently accommodates the significant volume changes linked to lithium insertion and extraction. Due to the robustness of the interconnected porous structure, the 3D Si@C electrode displayed superior electrochemical characteristics, including a reversible capacity of 1058 mAh g⁻¹ after 800 cycles and a capacity retention rate of 91%.

Liu and colleagues [107] utilized 3D porous silica derived from natural reed leaves as precursors, subsequently producing a 3D ultra-porous SiC hierarchical structure through magnesiothermic reduction combined with a carbon coating method. This resulting architecture adeptly buffers considerable volume alterations and supports both electron and ion transport, achieving 420 mAh g⁻¹ after 4000 cycles. Yao’s team [108] adeptly created interconnected, hollow Si nanospheres by using silica nanoparticles as templates. This process involved depositing Si layers on silica particles via CVD and then etching away the silica with HF to achieve hollow structures, depicted in Fig. 9a. The cross-sectional and side-view SEM images (Fig. 9b, c) showcase an orderly arrangement of the interconnected hollow Si spheres, revealing their substantial wall thickness and uniformity. Figure 9d exhibits a closer look at both the inner and outer surfaces of the hollow spheres, confirming their interconnectivity via shared shells, which is essential for maintaining structural resilience and enhancing electrochemical performance. The TEM image in Fig. 9e further confirms the hollow nature of the spheres and details their size and the amorphous quality of the Si material. Figure 9f exhibits the GCD profiles, illustrating impressive capacity retention and stability. Figure 9g shows the plots of the reversible Li discharge capacity and CE over numerous cycles, evidencing high efficiency and sustained capacity over 700 cycles-a significant improvement over conventional graphite anodes. Yu et al. [109] employed a silver mirror reaction to fabricate silver-coated 3D macroporous Si featuring consistent macropores around 200 nm in diameter, boasting a discharge capacity of 3585 mAh g⁻¹ and an ICE of 81%.

The unique properties of 3D macroporous Si structures are central to their effectiveness as anode materials in LIBs. These structures provide substantial void space to accommodate the significant volume changes associated with lithium insertion and extraction, thus maintaining structural integrity. Their interconnected porous network enhances electrical conductivity, crucial for rapid electron transport and efficient charge transfer. Furthermore, the macropores facilitate rapid liquid-phase Li-ion transport, reducing polarization and boosting electrochemical performance. These combined attributes enable 3D Si anodes to overcome the challenges of volumetric expansion and unstable electrochemical behavior, characteristic of traditional Si anodes.

Reflecting on the progress within the sphere of 3D Si-based anode materials, it’s evident that this arena marks a significant departure from conventional approaches, addressing some of the most persistent challenges faced by Si anodes in LIBs. The introduction of 3D macroporous structures represents a paradigm shift, strategically tackling Si’s inherent issues related to volumetric expansion and unstable electrochemical behavior during battery operation.

The research initiatives discussed highlight a multifaceted approach to material and structural engineering. Key methodologies employed in the synthesis of 3D Si anodes—such as magnesiothermic reduction, galvanic displacement reactions, and chemical vapor deposition—underscore the intricate processes involved in producing these complex architectures. These techniques have not only enabled the creation of robust and highly interconnected porous networks but also facilitated unprecedented control over the morphological properties of the materials, as seen in the deliberate fabrication of hollow nanospheres and ultra-porous hierarchical structures.

One of the standout attributes of 3D macroporous Si is its ability to accommodate substantial volume changes during lithiation/delithiation. This is made possible by the presence of macropores and the unique structural integrity of the 3D framework, which collectively contribute to maintaining electrochemical stability over extended cycles. The impressive performance metrics—including high reversible capacity, admirable rate capability, and long cycle life—are testaments to the advantages conferred by this innovative material design. Table 4 provides the cycle stabilities of various composites anodes comprised of different Si and carbon materials for LIBs.

Moreover, these studies have illuminated the critical role of void spaces and continuous electron/ion transport pathways in these structures. The strategic integration of carbon coatings and other elements further enhances conductivity and structural resilience, directly impacting the anodes’ overall performance.

Beyond the advancements in 3D Si-based anode materials in half-cell configurations, it is crucial to consider their performance in full-cell setups to provide a more comprehensive evaluation. Full-cell studies offer insights into the practical applicability of these anodes in real-world battery systems. For instance, a study detailed in ‘Nature Communications’ [110] discusses the preparation and characterization of Silicon Fuzz@Graphene (SF@G). This research includes comprehensive electrochemical characterization in both half-cells and full-cells, emphasizing the importance of full-cell performance in addition to half-cell data. Such full-cell analyses are imperative in assessing the overall efficiency and feasibility of 3D Si-based anodes in commercial LIBs.

As we transition to discussing SiO_x/C type composites and the incorporation of Si with non-carbonaceous materials, the insights garnered from 3D Si-based anodes are invaluable. They serve as a robust foundation for future explorations aimed at optimizing composite materials that can harness the full potential of Si while mitigating its drawbacks. The journey ahead in battery material innovation beckons, promising new synergies, especially in composite anode configurations, that could redefine performance benchmarks in next-generation LIBs.

This method involves altering the composition of the electrolyte to promote the formation of a more robust and uniform SEI layer. For instance, the introduction of additives like gamma-butyrolactone in traditional electrolytes can selectively dissolve lower-modulus components of the SEI, resulting in a layer predominantly composed of lithium fluoride and polycarbonates. This method has shown to significantly enhance the mechanical properties of the SEI, as demonstrated in the work of Tian et al. [155]. The process is visually illustrated in Fig. 12a, where the selective dissolution strategy for in situ regulation of the SEI's mechanical properties is depicted. This novel approach has shown promising results, with raw micron-sized Si anodes retaining 87.5% capacity after 100 cycles at 0.5 C (1500 mA g⁻¹, 25 °C), as shown in Fig. 12b. Furthermore, when applied to carbon-coated micron-sized Si anodes, the cycle life could be extended to more than 300 cycles, evident from the data in Fig. 12c.

This method entails depositing inorganic materials such as lithium phosphorus oxynitride (LiPON) onto Si particles to form a protective layer. LiPON and similar materials offer superior ionic conductivity and electrochemical stability, preventing electrolyte decomposition and mitigating volume expansion during cycling. Liang’s group [146] highlighted that a critical LiPON layer thickness of 40-50 nm is essential for optimal performance.

This strategy involves coating Si particles with organic materials like citric acid, polyacrylic acid (PAA), or a carboxymethyl cellulose-citric acid (CMC/CA) composite. These coatings can improve the mechanical flexibility of the SEI and enhance its electrochemical stability. Patolsky et al. [156] demonstrated that a poly p-xylylene (parylene) coating can form a nanometrically thin, highly resilient SEI layer, significantly extending the lifespan of the anodes.

The method developed by Wang et al. [157] for creating an ASEI on a ferrosilicon/carbon (FeSi/C) composite anode is delineated in Fig. 13a. The process commences with the adsorption of sulfur onto the FeSi/C composite via a vapor transfer method under vacuum. This sulfur-adsorbed composite is then processed into an electrode film and integrated into lithium coin cells with an electrolyte containing vinylene carbonate (VC) and fluoroethylene carbonate (FEC), where the ASEI layer in situ forms during the initial discharge phase. Further analysis using SEM, and TEM, was conducted on both untreated and sulfur-adsorbed FeSi/C composites. As depicted in Fig. 13b, the unmodified FeSi/C composite exhibits spherical aggregates, while the HRTEM image in Fig. 13c reveals a core-shell structure within the primary particles. Introduction of sulfur results in an enhanced external layer, as detailed in Fig. 13d, e.

The comparative cycling performance of the pristine and ASEI-modified FeSi/C anodes is presented in Fig. 13f. After initial activation at 100 mA g⁻¹, the anodes were cycled at 500 mA g⁻¹. The ASEI-modified anode demonstrated a remarkably stable capacity over 650 cycles, with a considerable retention of 74.2%. Additionally, the CE of the modified anode showed substantial improvement, stabilizing at 99.8% by the 200th cycle, a notable contrast to the pristine anode. Further investigation into the practicality of this technique revealed that a sulfur-adsorbed FeSi/C anode, with a high areal capacity of 4.1 mAh cm⁻², maintained its performance when cycled at a current density of 1.0 mA cm⁻², suggesting a promising approach to enhancing Si-based anodes in LIBs.

In summary, this section highlights the pivotal role of the SEI in LIBs, particularly concerning Si-based anodes where conventional SEIs prove insufficient due to their mechanical frailty and reactivity with the electrolyte. The reviewed research emphasizes the progress made in developing advanced ASEIs as a practical solution to the inherent challenges of Si anodes. ASEIs, with their tailored properties such as enhanced elasticity, toughness, and electrochemical stability, have demonstrated the potential to significantly improve the durability and safety of Si-based anodes. Innovations such as the incorporation of LiPON layers and the use of advanced materials like parylene have shown promising results in protecting anodes against electrolyte decomposition and handling the substantial volume changes during cycling. Particularly notable is the vapor transfer method used to create a sulfur-adsorbed ASEI on FeSi/C composites, which has yielded a remarkable increase in capacity retention and cycling stability. These advancements, verified through diverse imaging techniques and electrochemical performance tests, underscore the effectiveness of ASEIs in achieving more robust, reliable, and high-performance Si anodes, presenting a path forward for the commercial viability of high-capacity LIBs.

Emerging developments in binder technology have introduced multifunctional binders, such as self-healing and conductive binders, to address these limitations:

·Self-healing Binders: These binders can recover from mechanical damages, ensuring the long-term integrity of the electrode. For example, a study by Wang et al. [165] demonstrated the application of a self-healing binder in a Si-based anode, showing improved cycling stability and mechanical resilience.

·Conductive Binders: Enhancing the electrical conductivity of the anode material, conductive binders contribute to enhanced cycle stability and overall battery performance. A representative study by Liu et al. [166] highlighted the effectiveness of a conductive binder in a Si-based anode, resulting in significant improvements in electrical performance and battery life.

Additionally, alternative binders like alginate [167,168], carboxylic methyl cellulose (CMC) [169], and polyacrylic acid (PAA) [170] to address the shortcomings of traditional binders. Kovalenko et al. [171] employed alginate as a binder for a nano-Si anode, achieving a remarkable cyclic performance of 1700 mAh g⁻¹ at 140 mA g⁻¹. This achievement was attributed, in part, to the evenly distributed carboxyl groups within the alginate polymer chain, enhancing its interaction with Si and consequently improving anode stability during cycling. Wu et al. [172] explored various alginate-based binders (Al-alg, Ba-alg, Zn-alg, Ca-alg, and Mn-alg) with Si as the anode material. Among these, the Al-alg binder exhibited exceptional performance, reaching a capacity of 2100 mAh g⁻¹ at 4200 mA g⁻¹ after 300 cycles, while the Ba-alg binder retained a capacity of 840 mAh g⁻¹ after 200 cycles. Another notable advancement came from Wang et al. [173], who improved the cyclic stability of a Si-based anode by introducing a self-healing polymer (SHP) binder. This SHP binder demonstrated superior electrochemical performance compared to PVdF, CMC, and alginate binders, owing to its mechanical and electrical self-healing properties. This nuanced advantage of SHP binders, especially in comparison to PVdF, CMC, and alginate, marks a significant leap forward, promising to address the enduring challenges associated with Si-based anodes.

Han et al. [174] introduced a groundbreaking binder solution for porous Si anodes by combing poly(acrylic acid) (PAA) and poly-(ethylene-co-vinyl acetate) (EVA) latex (PAA/EVA). This binder effectively addressed electrode pulverization and electronic contact loss caused by volume fluctuations during charge and discharge cycles. PAA’s abundant carboxyl groups enhanced the cohesion between porous Si particles, while EVA’s inherent elasticity improved binder ductility. The high-ductility PAA/EVA binder accommodated Si volume changes, ensuring electrode integrity during cycling. The unique EVA latex structure, as seen in Fig. 15b, allowed electrolytes to infiltrate and populate voids (Fig. 15a), enhancing binder performance. Mechanical evaluations revealed PAA/EVA’s superior tensile properties, with a rupture threshold of 58%, attributed to EVA rubber cavitation and stress concentration effects (Fig. 15c).

Electrochemical assessments demonstrated PAA/EVA’s excellence, achieving specific capacities of 3578 and 3185 mAh g⁻¹ at 50 mA g⁻¹ current density during discharge and charge respectively (Fig. 15d). This outperformance resulted from the superior ductility and electrolyte retention of the PAA/EVA binder. PAA/EVA’s stable cycling behaviors, with 80% retention after 140 cycles (Fig. 15e), highlighted the synergistic effect of PAA and EVA. The adaptable EVA colloid shape prevented binder fracturing, supported pulverized Si, maintained electrode integrity, and slowed capacity fading by minimizing direct Si-electrolyte interactions. In contrast, the PAA electrode rapidly deteriorated, retaining only 26% capacity 100 cycles.

The evolution from traditional PVdF binders to cutting-edge multifunctional and composite binders reflects the ongoing advancements in battery materials science. While alginate and its derivatives marked a departure from the inadequacies of PVdF, the emergence of multifunctional binders like self-healing and conductive binders ushers in a new era for Si-based anodes in LIBs. These developments are crucial in accommodating the physical changes of Si, indicating potential pathways for future research in binder technology and, consequently, the performance benchmarks of Si-based anodes.

In our thorough review, we have navigated the intricate domains of Si-based anodes for LIBs, delving into the depths of Carbon-Si composites, nanostructured Si materials, SiO_x/C composites, their integration with non-carbonaceous materials, and other crucial factors influencing anode performance. We have rigorously examined the evolving roles of ASEIs, the transformative impact of prelithiation techniques, and the revolutionary advancements in binder technology. These discussions have not only highlighted the scientific ingenuity behind these developments but also their practical implications in enhancing the efficiency and durability of Si-based anodes. To complement our extensive narrative, we present Table 6 through 9 at the culmination of our analysis. These tables synthesize our findings into a coherent overview of the scalability and commercial viability of advanced materials and technologies for LIBs, serving as a crystallized reflection of our exploration. Table 6 details Carbon-Si composites, highlighting potential enhancements in LIBs' energy density and stability. Table 7 delves into nanostructured Si anodes, examining the influence of size and shape on performance. Table 8 explores SiO_x/C composites and non-carbonaceous integrations, showcasing strategies to overcome traditional Si anodes' limitations. Table 9 examines advancements in Si-based anodes, focusing on the role of ASEI, binders, and the crucial aspect of prelithiation of anode materials, emphasizing strategies for offsetting initial capacity loss and enhancing silicon anodes' lifecycle in commercial applications. Together, these tables underscore the balance between material innovation and commercial applicability in LIBs.