The Utilization of Carbon Nanotubes in Advanced Silicon – Carbon Anode Materials for Lithium-Ion Batteries

Abstract

The unique molecular structure of carbon nanotubes (CNTs) endows them with distinctive properties, notably exceptional conductivity. In lithium-ion batteries, CNTs are employed not only as conductive agents but are also incorporated into silicon-carbon anode materials. Beyond serving as conductive media, CNTs also function as buffer agents to mitigate the volumetric expansion of silicon during the charging and discharging cycles. This chapter will demonstrate the role of CNTs as conductive agents not just for the silicon-carbon electrode but also within hybrid particles, integrating CNTs into silicon-carbon anode materials. It will summarize the methods for assembling CNTs and silicon (Si/SiOx) through either external mixing or in situ growth and the resulting battery performance when utilizing silicon-carbon anodes enhanced with CNTs. This chapter aims to offer a reference for the industrial application of CNTs in lithium-ion battery silicon-carbon anodes by analyzing the latest research and applications of CNTs in such anodes.

1. Introduction

Carbon nanotubes (CNTs) can be imagined as one or more layers of graphite (a hexagonal lattice of carbon) rolled into cylinders. This unique structure endows CNTs with special electronic, thermal, and structural characteristics. In the past 30 years, a large amount of research has been dedicated to the preparation, properties, and applications of CNTs [1, 2, 3]. Our previous chapters have already detailed their structural characteristics. In this chapter, we primarily discuss the application of CNTs in the new energy sector, with a particular focus on their use in silicon-based anodes for lithium-ion batteries (LIBs).

In the initial stages of CNT research, many scholars identified CNTs as potential anode materials for LIBs. This is because CNTs consist of single or multiple coaxial layers of carbon sheets, creating a similar structure to graphite layers [4], which facilitates shallow insertion depths, short diffusion paths, and numerous insertion sites for lithium ions. Additionally, the excellent electrical conductivity of CNTs enables superior electron and ion transport capabilities, making them suitable as anode materials for LIBs. However, subsequent research has revealed that during the processes of intercalation/deintercalation, the peeling off of graphite layers of CNTs (typically referring to multi-walled CNTs) is a distinctive phenomenon. This exfoliation increases the surface area of the anode, thereby facilitating the formation of a solid-electrolyte interphase (SEI) layer [5]. This leads to an increase in the irreversible capacity of the anode materials. Moreover, anodes made from CNTs suffer from the absence of a stable voltage plateau and encounter issues such as potential hysteresis [6]. Consequently, current research suggests that while CNTs can act as conductive additives in LIBs, they are not suitable as the sole electrode material for LIBs.

With the increasing popularity of electric vehicles [7], the demand for LIBs with high-energy densities is growing, driven by concerns over driving range [8, 9, 10, 11]. Silicon-based anodes offer significantly larger mass/volume capacities compared to traditional materials. The theoretical specific capacity of crystalline silicon is about 4200 mAhg⁻¹, nearly 11 times that of graphite, which stands at only 372 mAhg⁻¹. Even for silicon dioxide materials, the theoretical specific capacity reaches 2600 mAhg⁻¹, approximately seven times that of graphite. Thus, silicon-based materials emerge as the most promising candidates for the next generation of lithium-ion battery anodes. However, high-capacity silicon-based anodes face two main challenges in real-world applications. For one thing, during the lithiation/delithiation process, silicon anodes exhibit significant volume changes, leading to pulverization, reduced cycling efficiency, and permanent capacity loss. The expansion and contraction of silicon anode materials caused by the insertion and extraction of lithium ions within the silicon crystal during charging and discharging result in a volume change of approximately 320%, while graphite’s volume change is only about 12%. These volume changes can lead to issues such as (i) particle pulverization, resulting in degraded cycling performance; (ii) poor contact between the active material and the conductive agent/binder; (iii) the expansion and contraction of silicon anodes, causing the binder holding silicon particles together to crack, allowing electrolyte to infiltrate between nanoparticles and form an SEI layer on the surface of silicon particles. Since the SEI is non-conductive, it impairs the anode’s ability to collect charges and consumes electrolyte and lithium sources, leading to worsened cycling performance. Second, silicon-based anodes have relatively poor electrical conductivity compared to traditional graphite anodes. To address the issue of poor conductivity, CNTs, with their small diameter, high electrical conductivity, high surface area, and graphite-like structure, offer an ideal scaffold for hosting nanosilicon materials. They can also confine the structure to accommodate volume fluctuations during the cycling process, thereby improving the thermal expansion issues and electrical conductivity of silicon-based anodes [12].

The application of CNTs in silicon anodes can be summarized into three main levels, as illustrated in Figure 1. At the first level, CNTs serve solely as conductive agents within a composite electrode structure that includes silicon nanoparticles, binders, and sometimes graphite. This approach is the simplest but yields the least effective results. The reason behind its limited effectiveness is the inherent tendency of silicon agglomerate. Within these silicon agglomerates, the silicon particles do not make contact with the CNT conductive agents, resulting in a loss of activity. This leads to underperformance in terms of anode capacity. Additionally, the volume expansion of silicon agglomerates is considerable, and the internal stress directions within these agglomerates can vary, making the electrode prone to cracking at the macro level. The second level involves a combination of CNTs, graphite, and silicon into secondary hybrid particles that serve as anode material. Due to the manufacturing process, silicon dispersion within these secondary particles is slightly improved. Moreover, graphite acts as a buffer to mitigate the volume expansion of silicon. Some of the silicon-carbon anode materials currently on the market fall into this category of composite materials. The third level aims to maximize uniformity in the distribution of CNTs and silicon, combining them as evenly as possible via some designed assembly. This strategy seeks to fully utilize the surface area of CNTs, maximize contact between silicon and CNTs, and optimally accommodate the volume changes of silicon during charging and discharging processes. This is the primary focus of our current research. By achieving such uniform integration, it is possible to harness the benefits of CNTs to their fullest extent, improving the overall performance and durability of silicon-based anodes.

Figure 1.

Three kinds of application strategies of the CNTs to Si-C anodes.

2. The application of CNTs in silicon-based anodes

2.1 CNTs as conductive agents

CNTs, known for their high aspect ratio, serve as conductive additives capable of forming extensive conductive networks within batteries. This mitigates issues such as cracking caused by the expansion and contraction of electrode materials to some extent. As conductive agents in battery materials, CNTs effectively reduce impedance. From a physical perspective, traditional carbon black forms point contacts in the conductive system due to its dot-like nature, CNTs form line contacts due to their linear structure, and graphene forms surface contacts. Compared to carbon black, CNTs have a larger specific surface area, allowing for a more thorough mix of active materials and superior conductivity. To achieve the same level of conductivity, the amount of CNTs required is only 1/6 to 1/3 of that of carbon black.

Despite the promising features of CNTs, they face limitations such as severe aggregation and entanglement caused by their low bending stiffness and van der Waals forces. These intrinsic characteristics of CNTs restrict their widespread application. It has demonstrated the formation of a 2D segregated CNT network using CNT dispersion as a conductive agent. The resulting CNT network, formed via van der Waals bonding, showed significant improvements in electrical conductivity (as shown in Figure 2). The formation of a segregated CNT network was validated for specific micron-sized bare active materials, such as silicon microparticles. The issue of silicon agglomeration, alongside the resulting low capacity utilization and problems related to the microscopic volume changes of active materials and macroscopic electrode cracking, has yet to be sufficiently resolved, thereby limiting the allowable silicon content [13].

Figure 2.

Hybrid conductive networks of CNTs for Si-based anode [13].

Compared to multi-walled carbon nanotubes (MWCNTs), single-walled carbon nanotubes (SWCNTs) consist of a single layer of carbon atoms [14]. The strong carbon-carbon bonds grant them a higher current carrying capacity, with an electrical current density exceeding that of metals such as copper by more than a thousand times. Due to their high conductivity and excellent mechanical reinforcement properties, SWCNTs possess a great elastic modulus and lack inherent twisting characteristics [15]. This allows them to maintain structural stability and prevent collapse even when the volume of the electrode changes, thanks to their mechanical reinforcement effect. Compared to MWCNTs, SWCNTs exhibit stronger van der Waals forces with silicon. As illustrated in Figure 3, SWCNTs can maintain good contact with SiO_x@C under tensile stresses up to 6.2 GPa, whereas MWCNTs lose electrical contact under alternating compressive stress up to 8.9 GPa and tensile stress up to 2.5 GPa during long-term cycling. Under such significant stresses, the flexibility of SWCNTs and their stronger van der Waals forces ensure that SiOx@C maintains effective contact with SWCNTs [16].

Figure 3.

Illustration of how multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) respond to the volume changes in silicon during charging and discharging processes [16].

It is worth mentioning that incorporating SWCNTs into silicon anodes significantly enhances their conductivity and cyclic stability, mitigating the issue of capacity fading. This effectively supports the industrial application of silicon as a high-capacity anode active material. Moreover, the quantity of SWCNTs required is substantially less than that of MWCNTs, while significantly lowering the bulk resistivity. Thanks to SWCNTs, silicon-oxygen anodes can explore using up to 90% silicon materials, aiding in the industrialization of batteries with energy densities reaching or even surpassing 360 kWhkg⁻¹. Therefore, employing SWCNTs as a conductive agent represents a trend. However, the current production cost of SWCNTs is very high. In a market-driven context, MWCNTs remain the most cost-effective option available.

Further, to enhance the contact area and points between CNTs and silicon, numerous academic experiments have adopted spray drying techniques. This involves using nanosilicon, CNTs, and other conductive agents like graphene oxide as precursors, which are then processed through spraying and drying to form composite microspheres. To further minimize the exposure of silicon to the electrolyte, the surfaces of these microspheres undergo a carbon-coating process, as illustrated in Figure 4. The following example clearly describes this process. This method of creating core–shell structured Si-based composite anodes offers a significant advancement in the development of next-generation LIBs with outstanding electrochemical performance. The composite anode of C-Si/CNT/GO (grapheme oxides) demonstrates exceptional electrochemical performance, exhibiting high specific charge and discharge capacities (2921 and 2395 mAhg⁻¹ at 100 mAg⁻¹, respectively), prolonged cycle life (1542 mAh g⁻¹ at 200 mAg⁻¹ after 100 cycles, with a capacity retention of 64%), and high charge/discharge rates (1506 mAhg⁻¹ at 6 Ag⁻¹). The elements of the C-Si/CNT/GO composite anode synergistically enhance its electrochemical performance for LIB applications [18]. The silicon nanoparticles contribute to the high electrochemical capacity; the CNTs and GO facilitate the creation of electrical networks and spherical frameworks, thanks to the dispersibility of the CNTs, thereby improving the electrical conductivity of the 3D architecture and providing adequate buffer space to accommodate the volume expansion of the silicon nanoparticles. By the way, the carbon-coating layer enhances the electrical conductivity and mechanical stability of the composite, prevents electrolyte permeation, and suppresses side reactions and the formation of an SEI [17]. In practical applications, when using composite microspheres obtained through spray drying to make batteries, we noticed that the silicon-carbon microspheres produced in this manner are prone to fragmentation. After breaking, the increased contact area between the active material and the electrolyte leads to a significant decrease in battery capacity. Typically, a certain proportion of silicon-carbon spheres is mixed with commercial graphite anode materials to achieve a commercial silicon-carbon/graphite composite product. Therefore, it is recommended to add an organic compound precursor to the microsphere precursors, since after the microspheres are fragmented, the silicon particles within the fragments remain encapsulated within the carbon-coating layer.

Figure 4.

Schematic diagram of the preparation of silicon-carbon composite particles using the method of spray drying combined with carbon coating [17].

In the industrial sector, some factories producing silicon-carbon anodes have utilized existing graphite manufacturing processes. They directly mix a small amount of nanosilicon and CNTs with small-sized graphite, then go through processes such as granulation to prepare silicon-carbon graphite composite materials (as shown on the left in Figure 5) with a capacity of 500 Ahg⁻¹. However, although this method of producing silicon-carbon materials is straightforward, the cyclic performance of the silicon-carbon anodes remains unstable. As shown on the right in Figure 5, while structurally, the CNTs effectively contact the silicon and graphite materials, enhancing conductivity and buffering effects, the exposed silicon still undergoes side reactions with the electrolyte. This continuously thickens the SEI layer, decreases battery capacity, and deteriorates the battery’s cyclic performance. Industrial technology development is already focused on successfully attempting to coat the entire composite particle, aiming to address these issues.

Figure 5.

SEM images of Si/CNTs/graphite composite particles at different magnifications.

2.2 Incorporating CNTs and silicon into secondary hybrid particles

To maximize the capacity and buffer the volume changes of Si, one of the most effective techniques is to maximize the contact between Si and CNTs. Academically, there has been extensive research on assembling nanosilicon with CNTs. Generally speaking, there are two methods. One is mixed assembly, where existing CNTs and silicon are used as starting materials and deliberately assembled together, as shown in Figure 6I. The other method involves growing silicon on CNTs or growing CNTs on Si, as shown in Figure 6II. The assembled composite structures of CNTs and silicon can maximize the contact between CNTs and Si, reducing the agglomeration of both silicon and the nanotubes themselves. This effectively increases reactive sites and buffers the volume change of Si.

Figure 6.

Two kinds of solutions of Si and CNTs incorporating.

2.2.1 Mix via assembly

In terms of hybrid assembly, significant academic research has focused on assembling silicon and CNTs through methods such as physical adsorption or chemical bonding. For example, a synthetic route was developed for the creation of a high-energy stable anode material known as Si/CNT@C. This composite material consists of carbon-coated silicon nanoparticles confined in a three-dimensional conductive carbon nanotube matrix. The fabrication process involved in situ polymerization of resorcinol formaldehyde resin with the presence of silicon nanoparticles and CNTs, followed by carbonization at 700°C as shown in Figure 7. The resulting carbon shell, derived from the resin, encapsulated the silicon nanoparticles and CNTs, preventing silicon nanoparticle agglomeration by attaching them to the interconnected carbon nanotube matrix. This unique three-dimensional architecture provided pathways for lithium-ion diffusion and electron transportation, enabling rapid lithiation and delithiation of the silicon nanoparticles, while also buffering volume changes and stabilizing the solid-electrolyte interphase film. The Si/CNT@C hybrid demonstrated excellent lithium storage performance, with initial discharge capacity reaching 1925 mAhg⁻¹ at 0.1 Ag⁻¹ and maintaining 1106 mAhg⁻¹ after 200 cycles. Even at an increased current density of 1 Ag⁻¹, a reversible capacity of 827 mAhg⁻¹ was retained, owing to its high silicon content of 62.8 wt%. This study presents a promising approach for utilizing silicon-based anode materials in high-energy stable LIBs [19]. Similarly, in related research, hybridizing nano-sized silicon with carbon-based nanostructures such as CNTs, carbon frameworks, and graphene has been shown to enhance cycling performance and rate capability. The 3D hierarchical RH-Nano Si@C/CNT (RH-Nano-Si means that the Si is made from rice husk) structure demonstrates high structural integrity, rapid electron and ion channels, and considerable electrochemical stability. When used as an anode in LIBs, RH-Nano Si@C/CNT exhibited outstanding electrochemical performance, including a high capacity of 1601 mAh g⁻¹ at 0.1C, cycle stability with a capacity of 989.5 mAh g⁻¹, a capacity retention rate of 86.6% after 1000 cycles at 0.5C, and rate performance with a capacity of 345 mAh g⁻¹ at a high rate of 3C. This work showcases the potential of silicon-based anode materials for enhancing the performance of LIBs [20].

Figure 7.

Schematic diagram of the assembly of CNTs/Si composite material through the combination of functionalized CNTs and Si [19].

2.2.2 In situ growth

When discussing research on in situ growth, let us explore a few examples. Chemical vapor deposition (CVD) is a common technique for growing nanoscale silicon on CNTs. A study reported the synthesis of 1D heterostructures consisting of vertically aligned multiwall carbon nanotubes (VACNTs) containing nanoscale amorphous/nanocrystalline silicon droplets. These heterostructures were created using a simple two-step liquid injection CVD process. The silicon clusters were anchored to the VACNTs by an interfacial amorphous carbon layer, allowing for reversible electrochemical alloying and dealloying with lithium. These heterostructures exhibited stable capacities of approximately 2050 mAhg⁻¹ with good rate capability and a first-cycle irreversible loss of around 20%, showing promise as high-capacity Li-ion anodes (Figure 8a) [21]. In comparison to CVD, electrochemical deposition is a simpler method for growing silicon on CNTs. A study demonstrated the creation of a paper-like ultrathin and flexible Si/carbon nanotube composite anode for LIBs through electrodeposition of a thin silicon layer on CNTs at ambient temperature. This method is cost-effective and scalable compared to other deposition techniques, such as sputtering or CVD. The ultrathin and flexible Si/CNT composite is highly attractive as an anode material for flexible LIBs (Figure 8b) [22]. On the contrary, the technology for growing CNTs on silicon surfaces has also been reported. However, the growth of CNTs requires the addition of catalysts. For example, an Si-multi-walled carbon nanotubes (MWNT) nanocomposite synthesized via a CVD process showed high reversible capacity and stable cycling performance due to the conductive network maintained by the direct growth and pinning of MWNTs on Si particles (Figure 8c) [23]. Furthermore, the introduction of MOF-derived carbon materials was used to enhance conductivity and buffer the volume change of silicon [25]. This composite structure exhibited excellent rate capacity and enhanced cycling stability, indicating significant application potential (Figure 8d) [24]. It is not recommended to grow CNTs directly on silicon or SiO_X surfaces, as the addition and removal of catalysts can damage the Si/CNT structure. However, if catalysts are not needed, the strong connection between in situ-grown CNTs and Si/SiO_x materials shows great potential for Si/CNT materials.

Figure 8.

Silicon grown on the surface of CNTs via the CVD method [21] (a), silicon grown on the surface of CNTs via the electrodeposition method [22] (b), CNTs grown on the surface of Si [23] (c), and CNTs grown on the surface of Si with further modifications [24] (d).

At present, we have also achieved impressive work in the in situ assembly of Si and CNTs. The obtained Si/CNT materials have been successfully synthesized for the first time via a novel procedure that starts with the creation of CNT structures through spray drying, followed by the deposition of nano-Si onto/into the CNT structures using Chemical Vapor Deposition (CVD) in a fluidized bed reactor (FBR) [12]. The process is illustrated in Figure 9, to begin, a mixture of CNT powders and a small amount of soluble phenolic resin was dispersed in a solvent of ethanol and water (mixed in a 50/50 ratio) with constant stirring. Subsequently, CNT microspheres (CNTS) were created via spray drying. It is suggested that commercial carbon nanotube conductive pastes be utilized directly as precursors for CNTS, as this approach is deemed more cost-effective and conducive to large-scale production. The carbonized resin, formed after undergoing high-temperature treatment, serves as an adhesive to reinforce the structure of carbon tube microspheres by linking the carbon tubes together. The resulting CNTS were introduced into a fluidized bed reactor (FBR) [26], where a mixture of SiH₄ and C₂H₂ was introduced into the FBR along with the CNTS as the deposition substrate. The FBR allows solid substrates to flow within the furnace, aiding in the penetration of pyrolytic gases, thereby addressing the low yield issue typically associated with traditional nano-Si preparation through CVD. Hence, the FBR process demonstrates the potential to emerge as the dominant CVD reactor for nano-Si production, owing to its simple equipment, efficient reaction process, and cost-effectiveness, thus facilitating mass production [27]. Additionally, in this synthesis, the particle size of the powder can be controlled by adjusting the growth time of the powder in the reactor [28]. The obtained microstructures effectively mitigate the volume expansion of nano-sized silicon during charge/discharge cycles and furnish the nano-Si with a three-dimensional conductive network.

Figure 9.

Schematic diagram of the process used for the preparation of CNT microsphere materials via CVD in an FBR [12].

Figure 10 illustrates the initial charge–discharge profiles of the prepared samples in coin-type half cells. At a cut-off voltage of 3.0 and 1.5 V, the specific capacity reaches 555 and 530 mAhg⁻¹, respectively, with corresponding first Coulombic efficiencies of 94 and 90%. Remarkably, even with the cut-off voltage set at 0.8 V, a capacity of approximately 490 mAhg⁻¹ can still be maintained, showcasing the significant potential of the Si/CNTS/graphite composite for practical applications. The cycling performance was initially evaluated in half cells. As depicted in Figure 10b, the specific capacity consistently exceeds 500 mAhg⁻¹ over the first 100 cycles, with Coulombic efficiencies surpassing 99.7%. However, the presence of excess lithium metal as the anode and fluoroethylene carbonate (FEC) additives in the electrolyte may mask the inherent defects of the Si-based material. To further assess the feasibility of this material, pouch-type full cells were assembled using Si/CNTS/graphite as the anode and LiNi_0.8Co_0.15Al_0.05O₂ (NCA) as the cathode. Concurrently, pouch cells comprising a commercial graphite anode and LiCoO₂ (LCO) cathode were prepared for comparison (both electrodes are commercial components used in Samsung Lithium-ion batteries). As demonstrated in Figure 10c, the pouch cell’s capacity (Si/CNTS/graphite vs. NCA) is nearly 1.3 times greater than that of the pouch cell with graphite vs. LCO, despite their similar total mass. Moreover, it is evident that the performance of Si/CNTS/graphite significantly surpasses that of the hybrid Si/graphite with CNTs as the conductive agent. By analyzing the structural information of pouch cells illustrated in Figure 10d, e, it becomes apparent that integrating this Si/CNTS composite as an additive to graphite when paired with NCA can substantially enhance the practical mass-energy density of lithium-ion batteries.

Figure 10.

Graphical representations illustrating the charging and discharging curves at 0.1C for the half cell (a), detailed analysis of the cycling performance and Coulombic efficiency at 0.1C for the half cell (b), comprehensive evaluation of cycling performance at 0.5C for pouch cells employing Si/CNTS/graphite compared to NCA and graphite versus LCO (c), in-depth information regarding the compared pouch cells (d), and a visually striking photo showcasing the pouch cell (e) [12].

Therefore, the Si/CNT materials show great promise as practical anode additives for high-energy-density batteries. Additionally, this novel synthesis strategy for Si/CNTs, especially the application of CVD in FBR for nano-Si deposition, is poised to significantly reduce costs and facilitate the industrial-scale production of Si-based anode materials. This process holds great promise for industrialization.

3. Conclusions

CNTs have garnered significant attention in enhancing the performance of silicon-based (Si-based) anodes for LIBs, owing to their unique properties, such as high electrical conductivity, exceptional mechanical strength, and excellent flexibility. The integration of CNTs in Si-based anodes serves dual pivotal roles: as conductive agents and as components of material particles. This summary outlines these roles and their implications for improving battery performance.

One of the principal challenges with Si-based anodes is their poor electrical conductivity, which hampers electron transport during the charge and discharge cycles. CNTs, known for their superior electrical conductivity, can effectively bridge this gap. When incorporated into Si-based anodes, CNTs create a percolating conductive network that facilitates rapid electron transfer throughout the electrode. This enhanced electron mobility significantly improves the rate capability of the battery, allowing it to support high-power applications. Furthermore, the incorporation of CNTs helps in maintaining electrical connectivity within the electrode. Silicon undergoes significant volume changes during lithiation and delithiation processes, which can lead to particle pulverization and loss of electrical contact. CNTs, with their flexibility and tensile strength, can accommodate these volume changes and preserve the integrity of the conductive network, thereby ensuring sustained electrical connectivity even under extensive cycling. Apart from acting as conductive bridges, CNTs also play a crucial role as components of material particles in Si-based anodes. They can be used to construct composite materials where CNTs are either coated with silicon or mixed with silicon particles. Such composites leverage the high surface area and the mechanical properties of CNTs to buffer against the detrimental volume expansion of silicon during lithiation. This buffering capability mitigates mechanical stresses and prevents the disintegration of the anode material, enhancing the cycle life of the battery.

In summary, as conductive agents, CNTs enhance the electrical conductivity and resilience of Si-based anodes. As components of material particles, they improve structural stability and energy density. Together, these roles underscore the importance of CNTs in the development of high-performance Si-based anodes for the next generation of LIBs. The synergistic effect of silicon for high capacity and CNTs for structural integrity and conductivity culminates in Si-based anodes that exhibit improved energy density, cycle stability, and rate performance. Finally, let us look forward to the further development of in situ assembly techniques, namely the technologies allowing the growth of CNTs on silicon surfaces or silicon on CNT surfaces. Such advancements are expected to significantly enhance the buffering of silicon’s volume changes and supplement its electrical conductivity through the synergistic use of CNTs.

Acknowledgments

This work is supported by the high-end foreign experts project (G2022030071L) and The Guangdong Key Laboratory for Hydrogen Energy Technologies (2018B030322005).