Lithium Ion Battery Conductive Agents

Brief introduction

Conductive Agents

As an important part of the lithium ion battery, the conductive agents, although occupy a small amount in the battery, they greatly affect the performance of the lithium ion battery. Besides, it plays a very important role on improving battery cycle performance, capacity development, rate performance, etc.

Like lithium ion battery electrode materials, conductive agents are constantly evolving. From the earliest carbon black materials, it is characterized by punctate conductive agents. It can also be called zero-dimensional conductive agents, which improve conductivity mainly by point contact between particles. Later, conductive carbon fibers and carbon nanotubes have been gradually developed, a type of conductive agent with a one-dimensional structure. Due to its fibrous structure, it increases the contact with the electrode material particles. What’s more, it greatly improves the conductivity of the electrode and reduces the resistance of the pole piece.

Recently, the hot graphene material has gradually become a new type of conductive material for lithium-ion batteries. Because graphene has a two-dimensional layered structure. And it has the following advantages:

greatly increases the contact between electrode particles
improves conductivity
reduces theamount of conductive agent used
improvethe energy density of the lithium ion battery is improved

1.The function of conductive agent

The primary role of conductive agent is to improve electronic conductivity. In order to ensure that the electrode has good charge and discharge performance, a certain amount of conductive agent is usually added during the production of the pole piece. And that can collect the micro current between the active materials, the active material and the current collector. Then, it reduces the contact resistance of the electrode and accelerate the movement rate of electrons. In addition, the conductive agent can also improve the workability of the pole piece, promote the infiltration of the electrolyte into the pole piece. At the same time, it can effectively increase the migration rate of lithium ions in the electrode material. Also reduce the polarization, thereby improving the charging and discharging efficiency of the electrode and the service life of the lithium battery.

2. Comparative analysis of conductive agents

Conductive agents mainly include:

granular conductive agents such as acetylene black, carbon black, etc.
conductive graphite is mostly artificial graphite
fibrous conductive agents such as metal fibers, vapor-grown carbon fibers, carbon nanotubes, etc.
new types of graphene and its mixed conductive slurry, etc.

These conductive agents have their own advantages and disadvantages. The following is a comparison of the physical and chemical parameters of some common conductive agents:

The Comparison of the Physical and Chemical Parameters of Common Conductive agents

The following introduces several types of conductive agents mainly used in lithium-ion batteries: conductive carbon black Super-P Li, which has branched Ketjen black ECP, conductive graphite KS-6, SFG-6, vapor-grown carbon fiber VGCF, carbon nanotubes CNTs and graphene and their composite conductive agent.

(1)Carbon black

Carbon black is chain-like or grape-like under the scanning electron microscope. A single carbon black particle has a very large specific surface area. It has better ionic and electronic conductivity than graphite. The high specific surface area and tight packing of carbon black particles are conducive to close contact between the particles, forming a conductive network in the electrode. That is conducive to the adsorption of electrolytes and improves ionic conductivity.

In addition, the primary particles of carbon agglomerate to form a branched structure, which can form a chain conductive structure with the active material. And it helps to improve the electronic conductivity of the material. The process problem caused by the larger specific surface is the difficulty of dispersion and strong oil absorption. This requires improving the dispersibility of the mixing process of active substances and conductive agents and controlling the amount of carbon black within a certain range (usually 1.5% or less). In the battery, it can absorb and retain liquid.

At present, conductive carbon black is still dominated by conventional conductive agent SP. There is a Ketjen black among conductive carbon blacks, such as EC-300J, Carbon ECP and ECP-600JD. Compared with other conductive carbon blacks used in batteries, Ketjen black has a unique branched morphology. The advantage of this form is that the conductor has many conductive contact points and the branched chain forms more conductive paths. Therefore, a very high conductivity can be achieved with a small amount of addition. Other carbon blacks are mostly spherical or flakes. Therefore, a high amount of addition is required to achieve the required electrical properties.

(2)Conductive graphite

The graphite conductive agent is basically artificial graphite. Compared with the negative electrode material artificial graphite, the artificial graphite has a smaller particle size, generally 3~6μm. Besides, it has more developed pores and specific surface, and also has better conductivity. Its particles are closer to the particle size of the living material, and the particles are in point contact. It can form a certain scale of conductive network structure, which is beneficial to improve the compaction of the pole piece particles and increase the ion and electronic conductivity. When used in the negative electrode, the capacity of the negative electrode can be increased. Conductive graphite has better compressibility and dispersibility. Because of that, it can increase the volume energy density of the battery and improve the process characteristics of the pole piece, and is generally used with carbon black.

Graphite conductive agents include: KS-6, KS-15, SFG-6, SFG-15, etc. KS-6: Large-grain graphite powder, feather-like, with a certain lithium storage function, used in the actual production of the positive electrode. SFG-6: It is more suitable for the negative electrode as a conductive agent. The scaly artificial graphite can improve the surface performance of the negative electrode.

(3)Carbon fiber (VGCF)

Conductive carbon fiber has a linear structure, it is easy to form a good conductive network in the electrode, and exhibits good conductivity. Thus it reduces electrode polarization, reduces battery internal resistance and improves battery performance. In the battery with carbon fiber as the conductive agent, the contact form of the active substance and the conductive agent is point-line contact. Compared with the point-to-point contact form of conductive carbon black and conductive graphite, it not only helps to improve electrode conductivity, but also reduces the amount of conductive agent and increase battery capacity.

VGCF has very few impurities and can be used in positive electrode additives. If VGCF is added to the electrodes (positive, negative), VGCF has a large aspect ratio. Even after the positive and negative active materials expand and contract, the gap between the active material particles can be bridged by VGCF. The transmission of electrons and ions will not be interrupted, which can greatly improve the conductivity of the electrode. Because of the hollow microstructure of the nano-carbon fiber VGCF, the positive and negative electrodes can absorb more electrolyte. So that lithium ions can be inserted smoothly and quickly, which is conducive to high-rate charging and discharging.

VGCF is a high-strength fibrous material with a large aspect ratio, which can increase the windability of the electrode plate. The binding force between the positive and negative active material particles is stronger, and will not crack due to bending. In addition, the strength of the electrode and the characteristics of high electrical and thermal conductivity can be improved. The positive electrode active material has poor electrical conductivity. Adding carbon nanofibers can improve the active conductivity of the positive electrode. And also improve the thermal conductivity of the positive and negative electrodes, which is good for heat dissipation. The above effects can greatly improve the characteristics (cycle characteristics, output characteristics, etc.) of lithium-ion batteries. VGCF is the most suitable additive material for automotive lithium-ion batteries that require long life and high output.

(4)Carbon Nanotubes (CNT)

CNTs can be divided into single-walled CNTs and multi-walled CNTs. Carbon nanotubes with a one-dimensional structure are similar to fibers in a long columnar shape with a hollow interior. Carbon nanotubes have good electronic conductivity. The fibrous structure can form a continuous conductive network in the active material of the electrode. It is also in the form of point-line contact with the active material. It has a great effect on improving battery capacity (increasing the compaction density of pole pieces), rate performance, battery cycle life and reducing battery interface impedance. After adding carbon nanotubes, the pole piece has higher toughness. It can improve the spalling caused by the volume change of the material during the charging and discharging process, and increase the cycle life. Carbon nanotubes can greatly improve the penetration ability of the electrolyte in the electrode material.

As a conductive agent, CNT can form a large number of conductive contact sites between the active material particles of the lithium battery electrode. So that it can reduce the contact resistance between the electrode material particles, and act as a “wire” in the conductive network. And it has an electric double layer effect, which can give play to the high rate characteristics of supercapacitors. Its good thermal conductivity also helps to dissipate heat during battery charging and discharging. Then, it reduce battery polarization, improve battery high and low temperature performance, and enhance battery cycle performance. However, due to its small diameter and large aspect ratio, under the action of van der Waals force, agglomeration easily occurs, which affects its conductive effect.

Therefore, as a conductive agent for lithium ion batteries, the main problem that needs to be solved is the dispersibility of CNT, which requires good dispersion in the slurry. At present, it can be achieved by high-speed shearing, adding dispersant, making dispersion slurry, and electrostatic dispersion of ultrafine grinding beads. CNT can more effectively improve the overall performance, which makes it a research hotspot and one of the most potential application directions of conductive agents for lithium batteries.

(5)Graphene

When graphene alone is used as a negative electrode material, although its initial capacity is high, the capacity of the battery decays rapidly with charging and discharging. This may be due to a large specific surface area and many structural defects. And that leads to more side reactions between the graphene and the electrolyte, resulting in a higher irreversible capacity. Therefore, the current applications of graphene in lithium-ion batteries are mainly focused on adding graphene as a conductive agent to improve conductivity and preparing graphene composite materials. For example, graphene is combined with Si material to prepare Si-G composite material with porous structure.

Graphene as a new type of conductive agent, due to its unique sheet structure (two-dimensional structure), the contact with the active material is point-to-surface contact instead of the conventional point-to-point contact form. By which it can maximize the role of the conductive agent and reduce the amount of conductive agent. So that more active materials can be used to increase the capacity of the lithium battery. The effect as a conductive agent is closely related to the amount of it added. In the case of a small amount of addition, the effect of graphene is much better than that of conductive carbon black because it can better form a conductive network. However, graphene with thicker layers will hinder the diffusion of lithium ions and reduce the ionic conductivity of the pole pieces (6-9 layers are generally considered the most appropriate).

In the latest research progress, the conductive agent selected for some lithium batteries is a binary or ternary conductive paste that is a mixture of two or three of CNT, graphene, and conductive carbon black. It is the requirement of industrial application to compound conductive agent into conductive paste, and it is also the result of mutual cooperation and excitation between conductive agents. Whether it is carbon black, graphene or CNT, it is already very difficult to disperse them when they are used alone. If you want to mix them with the active substance uniformly, you need to disperse them before stirring the electrode slurry, and finally put it into use.

3.The effect of conductive agent content on electrical performance.

The role of conductive agent in the electrode is to provide a channel for electrons to move. A proper content of conductive agent can achieve higher discharge capacity and better cycle performance. If the content is too low, there will be fewer electronic conductive channels, which is not conducive to high current charging and discharging. While too high will reduce the relative content of active materials and reduce the battery capacity.

The conductive agent can affect the distribution of the electrolyte in the battery system. Due to the space limitation of the lithium ion battery, the amount of electrolyte injected is limited, generally in a poor state. And the electrolyte is used as the internal connection of the battery system. The distribution of extremely polar ions has a vital influence on the migration and diffusion of lithium ions in the liquid phase. When the content of conductive agent in one electrode is too high, the electrolyte is concentrated in this electrode. And that makes the lithium ion transmission process of the other electrode is slow. Then, the polarization is high, and it is prone to failure after repeated cycles, thereby affecting the overall performance of the battery .

When the content of the conductive agent reaches a turning point, too much will only reduce the electrode density and decrease the capacity. While too little will result in a low utilization rate of the active material in the electrode and a decrease in the high-rate discharge performance.

4.Application and Prospect of Conductive Agent

The development of conductive agent will focus on the following aspects: In the aqueous system or in the solvent of the NMP organic system, the conductive agent should have good dispersibility. And the highly conductive carbon nanotubes, Graphene and other new carbon materials are compounded to reduce the use ratio of conductive agents and improve performance. Also, it increases the specific surface area and electrolyte adsorption capacity, and further improves the ionic conductivity of the pole piece.

Whether it is carbon nanotubes or graphene composite materials, compared with traditional materials, it’s necessary to reduce costs to meet actual needs. Taking into account the dispersibility of the above two conductive agents, the currently marketed carbon nanotubes and graphene are provided in the form of pre-dispersed conductive paste. And its price is much more expensive than ordinary carbon black SP. Carbon black is a very mature conductive agent with a relatively stable price. As the scale effect of CNT and graphene increases in the future, there is a relatively large room for price drop, and the future application prospects are objective.

Conductive agents have different shapes and types, and their microstructure is an important factor affecting conductivity. From the granular carbon black to the one-dimensional structure of carbon fiber and CNT to the current two-dimensional sheet structure of graphene, this is a process of continuous improvement. In practical applications, carbon black has been widely used as a conductive agent, the process is also very mature with a relatively stable price. The application of CNT as a conductive agent has also been tested and mass-produced by many manufacturers and achieved good results.

Graphene has not been widely used in the conductive agent industry due to its cost and process problems. However, with the gradual maturity of graphene preparation technology and the continuous reduction of production costs, the application of graphene as a conductive agent in lithium-ion batteries has entered practical applications stage. Each kind of conductive agent has its own advantages. Multi-mixed conductive pastes that have taken in the strengths of different conductive agents will be the mainstream development direction of conductive agents in the future.