Carbon nanotubes (CNTs) have emerged as critical components in lithium-ion battery electrodes, serving as conductive additives or active materials to enhance performance. Their unique structural and electronic properties make them ideal for improving electron transport, accommodating volume changes during cycling, and extending battery life. This article explores the role of CNTs in lithium-ion batteries, synthesis methods, functionalization strategies, and their application in both anode and cathode materials.

CNTs contribute significantly to electron transport within battery electrodes due to their high electrical conductivity and large aspect ratio. When incorporated into electrode materials, CNTs form a percolating network that facilitates rapid electron transfer, reducing internal resistance and improving rate capability. This is particularly beneficial for high-loading electrodes or materials with inherently low conductivity, such as silicon or lithium iron phosphate. The interconnected CNT network ensures efficient charge collection and distribution, enabling faster charging and discharging without significant capacity loss.

Volume expansion of active materials during lithiation and delithiation is a major challenge in lithium-ion batteries, particularly for high-capacity anodes like silicon or tin. CNTs mitigate this issue by providing a flexible, mechanically robust scaffold that accommodates strain without fracturing. In silicon-CNT composites, for example, the CNT matrix buffers the substantial volume changes of silicon particles, preventing electrode disintegration and maintaining electrical contact. This structural resilience enhances cycling stability, allowing such composites to achieve hundreds of cycles with minimal capacity fade. Additionally, the porous nature of CNT-based electrodes facilitates electrolyte penetration, ensuring uniform lithium-ion transport and reducing concentration polarization.

The cycle life of lithium-ion batteries is prolonged by CNTs through several mechanisms. Their chemical stability minimizes side reactions with the electrolyte, while their mechanical strength prevents electrode degradation. Furthermore, CNTs can inhibit the dissolution of transition metals in cathodes, a common issue in high-voltage systems. By maintaining electrode integrity and conductivity over extended cycling, CNTs contribute to long-term performance retention.

CNTs are synthesized primarily through arc discharge and chemical vapor deposition (CVD). Arc discharge produces high-quality multi-walled CNTs (MWCNTs) or single-walled CNTs (SWCNTs) by vaporizing carbon in a plasma arc between graphite electrodes. This method yields CNTs with fewer defects but requires post-processing to remove impurities like amorphous carbon or metal catalysts. CVD, on the other hand, offers better control over CNT morphology and scalability. In this process, carbon-containing gases decompose on metal catalysts at elevated temperatures, forming CNTs with tunable diameters and lengths. Catalytic CVD can be optimized to produce vertically aligned CNTs or entangled networks, depending on electrode requirements.

Functionalization of CNTs is often necessary to enhance their compatibility with electrode materials and electrolytes. Covalent functionalization introduces oxygen-containing groups (e.g., carboxyl or hydroxyl) via acid treatment, improving dispersion in polar solvents and strengthening interactions with active materials. Non-covalent functionalization employs surfactants or polymers to stabilize CNTs without altering their electronic properties. For battery applications, functionalized CNTs can improve wetting by electrolytes, promote uniform coating during electrode fabrication, and reduce interfacial resistance. However, excessive functionalization may compromise conductivity, requiring careful optimization.

In anode applications, CNTs are combined with high-capacity materials like silicon, tin, or metal oxides to enhance performance. Silicon-CNT composites are particularly promising due to silicon’s high theoretical capacity (3579 mAh/g). The CNT matrix not only conducts electrons but also alleviates silicon’s volume expansion (up to 300%), enabling stable cycling. For instance, silicon nanoparticles embedded in a CNT network have demonstrated capacities exceeding 1000 mAh/g over 500 cycles. Similarly, CNT-tin oxide hybrids exhibit improved rate capability and cycle life compared to pure tin oxide. These composites leverage CNTs’ conductivity and mechanical support to overcome the limitations of alloying or conversion-type anodes.

CNTs also play a vital role in cathodes, particularly in systems with low electronic conductivity, such as lithium iron phosphate (LFP) or sulfur. In LFP-CNT composites, CNTs form a conductive backbone that enhances charge transfer, enabling high-rate performance and full capacity utilization. Sulfur cathodes benefit from CNTs’ ability to trap polysulfides and maintain electrical connectivity despite sulfur’s insulating nature. By confining sulfur within CNT networks or coating CNTs with conductive polymers, researchers have achieved sulfur cathodes with capacities above 800 mAh/g and improved cycling stability.

The energy density and rate capability of CNT-based electrodes depend on their composition and architecture. In anodes, the combination of high-capacity active materials and CNTs can deliver energy densities surpassing conventional graphite (372 mAh/g), though trade-offs exist between capacity and cycle life. Cathodes incorporating CNTs often exhibit enhanced rate performance due to improved kinetics, making them suitable for fast-charging applications. The lightweight nature of CNTs also contributes to higher gravimetric energy densities compared to traditional carbon black additives.

Comparative studies highlight the advantages of CNTs over other conductive additives. For example, electrodes with CNTs typically require lower additive loadings (1-5 wt%) than those with carbon black (10-20 wt%) to achieve similar conductivity, freeing up more space for active material. Additionally, CNT-based electrodes often show better mechanical cohesion, reducing the need for binders and further increasing energy density. However, the cost and scalability of CNT production remain challenges for widespread commercialization.

In summary, CNTs serve as versatile conductive additives and active materials in lithium-ion battery electrodes, addressing key challenges related to electron transport, volume expansion, and cycle life. Their synthesis and functionalization can be tailored to optimize performance in both anode and cathode applications, enabling higher energy densities and improved rate capabilities. While challenges persist in cost and large-scale production, ongoing advancements in CNT technology continue to strengthen their role in next-generation lithium-ion batteries.