What are carbon nanotubes for lithium batteries?

As lithium battery technology continues to advance, many materials that are unfamiliar to the average person actually play a crucial role. Carbon nanotubes are one of them.

What are carbon nanotubes?

Carbon nanotubes (CNTs) are tubular nanomaterials composed of carbon atoms. Single-walled carbon nanotubes can be imagined as a seamless cylinder rolled from a "graphene sheet" of carbon atoms, while multi-walled carbon nanotubes are composed of multiple layers of coaxially nested graphene. Their diameter is typically between a few nanometers and tens of nanometers, but their length can reach the micrometer scale or even longer.

This unique structure and arrangement of carbon atoms gives them several outstanding properties:

1. Excellent electrical conductivity and low resistance to electron transport

2. High mechanical strength and good flexibility

3. High aspect ratio, easily forming network structures

Carbon nanotubes used in lithium batteries are mainly divided into multi-walled carbon nanotubes and single-walled carbon nanotubes. Multi-walled carbon nanotubes have relatively lower manufacturing costs and are more widely used; single-walled carbon nanotubes have better performance but are more expensive.

What do carbon nanotubes do in lithium-ion batteries?

The conductivity of the positive electrode in a lithium-ion battery is often insufficient, requiring the addition of a small amount of conductive agent to improve electron transport. While the negative electrode material itself has acceptable conductivity, conductive agents are often added to further optimize performance in high-rate applications or silicon-based negative electrodes. The primary role of carbon nanotubes is as a conductive agent.

Traditional conductive agents are generally particulate materials such as carbon black and conductive graphite. These are like scattered pebbles, requiring a relatively large quantity to form conductive pathways. Carbon nanotubes, however, are different. They are long, thin, and flexible; a small amount can build a long-distance conductive network between electrode material particles.

This network structure brings several direct benefits:

Reducing battery internal resistance and improving charge/discharge efficiency

Improving rate performance, allowing for smoother high-current charging and discharging

Reducing the amount of conductive agent used, freeing up space for active materials, thus helping to increase energy density

Enhancing electrode flexibility and maintaining structural stability during repeated charge/discharge cycles

How does it differ from traditional conductive agents?

Carbon black particles primarily rely on point contact for conductivity, resulting in high contact resistance and requiring a high addition ratio. One-dimensional long fibers of carbon nanotubes can interlock to form continuous, long conductive pathways, constructing a conductive network. To achieve the same conductivity, the amount of carbon nanotubes used is typically only a fraction of that of carbon black.

While graphene also exhibits excellent conductivity, its sheet-like structure makes it prone to surface-to-surface stacking in electrodes. Carbon nanotubes, on the other hand, possess a tubular morphology and a greater aspect ratio. However, it's important to note that carbon nanotubes also face dispersion challenges, requiring specialized dispersion processes in industrial applications to fully realize their performance advantages. With good dispersion, carbon nanotubes are more easily used to construct three-dimensional conductive networks, resulting in better performance when used in conjunction with particulate materials.

In actual production, many battery manufacturers combine carbon nanotubes with carbon black, graphene, and other materials to balance performance, cost, and processing difficulty.

In summary, carbon nanotubes are an upgraded option for conductive agents in lithium-ion batteries, but they are not a panacea. Improving battery performance requires coordinated progress across multiple components, including the positive electrode, negative electrode, electrolyte, and separator. The advantages of carbon nanotubes are mainly reflected in the construction of the conductive network and the reduction of internal resistance. Their contribution to energy density is primarily achieved by reducing the proportion of inactive materials and freeing up space for active materials.

From an industry trend perspective, with the maturation of preparation technologies and the decrease in costs, the penetration rate of carbon nanotubes in lithium-ion batteries is gradually increasing. However, in practical applications, issues such as dispersion stability, batch consistency, and compatibility with different electrode materials still need to be addressed.