As the new energy vehicle and energy storage markets mature, the performance requirements for lithium-ion batteries are gradually increasing. This drives lithium-ion battery technology to upgrade to high energy density and safety. Conductive agents are the components of lithium batteries, and upgrading lithium batteries also requires higher performance of conductive agents.
Carbon nanotubes are one-dimensional quantum materials formed by curling single or multi layer graphene layers around the central axis at a certain spiral angle. As a new type of conductive agent, it has the excellent function of strong conductivity.
It can improve the conductivity of positive electrode active materials more than traditional conductive agents (such as carbon black, conductive graphite, etc.). It can improve battery energy density and battery cycle life.
The aspect ratio and carbon purity of carbon nanotubes are two core indicators that affect conductivity. They directly determine the product performance of carbon nanotubes. The thinner the diameter and the longer the length, the better the conductivity.
The difference in the number of graphene layers divides them into single-walled carbon nanotubes and multi-walled carbon nanotubes.
Advantages of single-walled carbon nanotubes
1. Excellent safety performance:
Under 45°C high-temperature multi-cycle, the internal resistance growth of the soft-pack battery with single-walled carbon nanotubes is significantly lower than that of the battery with other conductive agents. This indicates that the risk of battery fire is lower.
2. Improve the adhesion of the pole piece:
The single-walled carbon nanotube network connects the positive electrode material particles together, thereby increasing the connection strength between the particles. This feature is particularly important for silicon-based negative electrodes that are easy to pulverize and fall off.
3. Simple structure and stable chemical properties:
During the formation process of multi-walled carbon nanotubes, the layers between layers can easily become trap centers and capture various defects. Single-walled carbon nanotubes have a simple structure, good uniformity, few defects, and stable chemical properties.
4. Small addition amount and excellent conductivity:
Single-walled carbon nanotubes have a high length-diameter ratio. It can form a three-dimensional conductive network at a very low addition amount. At the same time, single-walled carbon nanotubes have a layer of carbon atoms and can exhibit metal or semiconductor properties according to the spiral characteristics of the space. In addition, its strong carbon-carbon bond enables it to have a higher current carrying capacity, and the current density can be more than 1,000 times higher than that of metals such as copper.
5. Good elasticity and high mechanical properties:
Single-walled carbon nanotubes have stronger flexibility. It can bend, twist, or twist better. The elastic modulus and tensile strength are significantly better than multi-walled carbon nanotubes.
6. Good thermal conductivity:
The thermal conductivity per unit mass of single-walled carbon nanotubes is higher than that of multi-walled carbon nanotubes. At the same time, both can withstand high temperatures above 750°C.
The large-scale mass production of single-walled carbon nanotubes currently has the following difficulties:
1. It is difficult to control product performance precisely:
The performance of single-walled carbon nanotubes is closely related to their diameter and chirality. Most samples are composed of multiple single-walled carbon nanotubes with different diameters and chirality. They are entangled with each other and difficult to separate. Therefore, it is difficult to achieve precise control of their performance.
2. It is challenging to prepare a uniform and stable conductive slurry:
Due to the presence of a certain strength of van der Waals force between carbon nanotubes, they are easy to agglomerate. This affects the monomer characteristics of single-walled carbon nanotubes. All the dispersion stability problems need to be solved.
3. The cost of single-walled carbon nanotube generation is too high:
On the one hand, the catalytic efficiency of the catalyst is not high, resulting in a large amount of impurities in the sample. It needs to be purified later before it can be used; on the other hand, the preparation efficiency and yield need to be further improved, and the process parameters need to be further optimized.
Single-walled carbon nanotubes have a broad market in the future
1. Excellent performance:
The excellent performance of single-walled carbon nanotubes. Single-walled carbon nanotubes have a high specific surface area, extremely fine tube diameter and ultra-long tube length, better graphitization degree, and other physical and chemical characteristics. This makes it have many excellent properties that multi-walled carbon nanotubes do not have, and the conductivity efficiency is more than ten times that of multi-walled carbon nanotubes. Single-walled carbon nanotubes can be used in both positive and negative electrode materials of lithium batteries, greatly improving the energy density, rate performance, safety, cycle life, and other performance of lithium batteries;
2. Fast charging trend:
“Fast charging” is the development trend of the new energy vehicle industry, and single-walled carbon nanotubes can improve the rate performance of lithium batteries better than multi-walled carbon nanotubes and traditional conductive agents. Many companies will now choose to add single-arm and multi-arm matching methods for batteries above 3C. On this basis, 4C batteries will add 2% to 4% of single-walled carbon nanotubes separately to improve fast charging performance.
3. Synergy between silicon-based negative electrode materials and single-walled carbon nanotubes
“Silicon-based” negative electrodes are more suitable for single-walled carbon nanotubes. Silicon-based negative electrode materials themselves are not good conductors. As negative electrode materials, they must use many conductive agents, and high-end conductive agents, such as single-walled carbon nanotubes, can improve their conductivity. At the same time, the high mechanical strength of single-walled carbon nanotubes can improve the stability of the structure of silicon-carbon negative electrode materials, and the structure is not easily destroyed under the action of external forces. In addition, the particle pulverization caused by the volume change of silicon-carbon negative electrode materials during the charge and discharge cycle will lead to rapid capacity decay. As an excellent one-dimensional conductive material, single-walled carbon nanotubes can more effectively help build a long-range conductive network, greatly improving its cycle performance and increasing the service life of the battery.
4. Demand for large-size batteries and solid-state batteries drives the growth of the single-walled carbon nanotube market
“Large cylindrical” batteries and “solid-state batteries” will gradually increase in volume in the next few years, increasing the use of silicon-based negative electrodes, thereby promoting the growth of the single-walled carbon nanotube industry;
5. Cost reduction:
At present, due to high technical barriers and high production difficulties, the production cost and price of single-walled carbon nanotubes are too high (the current price of single-walled carbon nanotube powder is 10-15 million yuan/ton), but as the single-walled carbon nanotube process technology and technology gradually mature, the unit production efficiency and process automation degree are further optimized, thus bringing about economies of scale. In the future, the price of single-walled carbon nanotube conductive agents will gradually decrease, which will be conducive to the use of carbon nanotube conductive agents.
According to the Starting Point Research Institute (SPIR), high-performance and stable single-walled carbon nanotubes are currently in short supply. At the same time, its domestic customers’ single-walled carbon nanotube orders have reached hundreds of tons. With the gradual mass production of large cylindrical and solid-state batteries, single-walled carbon nanotubes are ushering in a volume expansion stage. The Starting Point Research Institute (SPIR) predicts that in 2030, the global single-walled carbon nanotube slurry market will reach 17.8 billion yuan, with an average annual compound growth rate of 49.4% in the next six years. The future prospects of single-walled carbon nanotubes are broad.
