Silicon-based Negative Electrodes Defects
Silicon-based negative electrodes have the advantages of high specific capacity and excellent fast charging performance, but they also have certain defects:
1- Poor conductivity
The conductivity of silicon-based negative electrodes is worse than that of traditional graphite negative electrodes. The reason is that silicon is a semiconductor material. Its conductivity is poor, affecting the transmission speed of lithium ions and electrons. Silicon has poor contact with conductive agents and negative electrode binders. This also leads to poor overall conductivity of the electrode.
2- High volume expansion rate
Due to the material properties of silicon-based negative electrodes, they will produce huge expansion and contraction during charging and discharging. Its maximum volume expansion rate can be as high as 300%, much higher than the 10-12% of graphite negative electrodes. Huge volume expansion will bring a series of problems. Therefore, the application of silicon-based negative electrodes has strong requirements for the safety and stability of battery cell structure and material matching.
3- Low cycle life
The cycle life of silicon-based negative electrodes is 300-500 times, much lower than the more than 1,500 times of graphite negative electrodes. The reason is that the volume expansion rate of silicon-based negative electrodes is high. The huge expansion during the charge and discharge process will cause the active particles to break and powder, the surface SEI film structure will be unstable and continue to grow, and the serious electrode structure will collapse. This causes the electrochemical performance of the silicon negative electrode to decay rapidly, thus affecting the cycle life of the battery.
Carbon Nanotubes: The Most Suitable Conductive Agent for Silicon-Based Negative Electrodes
Silicon-based negative electrodes have the defects mentioned above. So, high-performance conductive agents must be added to improve their conductivity and compensate for the defects. They can better play the performance advantages of high specific capacity and high rate. Carbon nanotubes are the most suitable conductive agent for silicon-based negative electrodes. Carbon nanotubes (especially single-walled carbon nanotubes) are the most suitable conductive agent for silicon-based negative electrodes. They have the following advantages:
1- Excellent conductivity
Its one-dimensional structure makes it easier to build an effective conductive network. This can make up for the poor conductivity of silicon-based negative electrodes.
2- High elasticity and strong mechanical properties
Single-walled carbon nanotubes have better elasticity (3-10 times that of multi-walled carbon nanotubes. This can improve the stability of the structure of silicon-based negative electrode materials, and the structure is not easily destroyed under the action of external forces.
3- Large specific surface area and excellent hollow structure
Carbon nanotubes are considered to be the best material for solving the expansion of silicon-based negative electrodes. This can alleviate the stress of volume change of silicon-based negative electrodes during charging and discharging, reduce material collapse, and improve cycle life.
4- Improve rate characteristics, high and low-temperature performance
Application of Carbon Nanotube Conductive Agent in Solid-State Batteries
Developing new batteries, especially solid-state batteries, will drive the demand for silicon-based negative electrodes. This will drive the demand for carbon nanotube conductive agents. Solid-state batteries are globally recognized as the next generation of lithium batteries. They have great development prospects. They are in the initial stage of industrialization and have attracted much attention from the market. Solid-state batteries’ solid electrolyte has high ionic and low electronic conductivity. The requirements for carbon nanotube products are higher in the current research and development system of solid-state batteries. The amount of carbon nanotube conductive agents added to solid-state batteries is higher than that of liquid lithium batteries. The application of carbon nanotube conductive agents in solid-state batteries can improve their conductivity and structural stability. They can also extend the cycle life of batteries and increase energy density. These all provide essential support for solid-state battery technology development.
The performance advantage of single-walled carbon nanotubes is more significant.
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 larger the aspect ratio of carbon nanotubes, the thinner the diameter, the longer the length, and the higher the carbon purity, the better the conductivity of carbon nanotubes.
| Parameter | Single-Walled Carbon Nanotubes (SWCNT) | Multi-Walled Carbon Nanotubes (MWCNT) |
|---|---|---|
| Typical Diameter | 1–2 nm | 7–100 nm |
| Typical Length | Up to 1 nm | Up to 1 mm |
| Aspect Ratio | Up to 10,000 | 50–4,000 |
| Elastic Modulus | 1,000–3,000 GPa | 300–1,000 GPa |
| Tensile Strength | 50–100 GPa | 10–50 GPa |
| Thermal Conductivity at 300K | 3,000–6,000 W/(m*K) | 2,000–3,000 W/(m*K) |
Single-walled carbon nanotubes have a more significant effect on improving battery performance and are more suitable for silicon-based negative electrodes. Currently, the carbon nanotube products on the market are basically multi-walled carbon nanotubes. The market share accounts for about 80%. However, single-walled carbon nanotubes have a single-wall structure, a smaller diameter, and a larger aspect ratio, so their physical and chemical properties are better. They have better conductivity, and their conductivity efficiency is 10 times that of multi-walled carbon nanotubes. They also have excellent performance in tensile strength, toughness, and elasticity. The amount of addition is also less.
Single-walled carbon nanotubes can be used in both positive and negative electrode materials of lithium batteries. They significantly improve the energy density, rate performance, cycle life, safety, etc. of the battery. And they are more suitable for silicon-based negative electrode materials.
| Advantage | Description |
|---|---|
| Excellent safety performance | Under high-temperature multi-week cycles at 45°C, 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 battery has a lower risk of fire. |
| Improved electrode adhesion | The single-walled carbon nanotube network connects the negative electrode material particles together, 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. |
| Simple structure,
Stable chemical properties |
During the formation of multi-walled carbon nanotubes, the layers between the layers are easy to become trap center planes to capture various defects. Single-walled carbon nanotubes have a simple structure, good uniformity, few defects, and stable chemical properties. |
| Small addition amount,
Excellent conductivity |
Single-walled carbon nanotubes have a large aspect ratio and can form a three-dimensional conductive network at extremely low addition amounts. Single-walled carbon nanotubes have a layer of carbon atoms, which can exhibit metal or semiconductor properties according to the spiral characteristics of the space. Its current density can be more than 1,000 times higher than that of metals such as copper. |
| Good elasticity,
High mechanical strength |
Single-walled carbon nanotubes have greater flexibility and can bend, twist or kink better. Its elastic modulus and tensile strength are significantly better than multi-walled carbon nanotubes. |
| 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. |
