In the current commercial lithium-ion battery system, the limiting factor is mainly the electrical conductivity. In particular, the insufficient conductivity of the positive electrode material directly limits the activity of the electrochemical reaction. It is necessary to add a suitable conductive agent to enhance the conductivity of the material and construct the conductive network to provide a fast channel for electron transport and ensures that the active material is fully utilized. Therefore, the conductive agent is also an indispensable material in the lithium ion battery relative to the active material.
The performance of a conductive agent depends to a large extent on the structure of the materials and the manners in which it is in contact with the active material. Commonly used lithium ion battery conductive agents have the following characteristics:
(1) Carbon black: The structure of carbon black is expressed by the degree of aggregation of carbon black particles into a chain or a grape shape. The fine particles, the densely packed network chain, the large specific surface area, and the unit mass, which are beneficial to form a chain conductive structure in electrode. As a representative of traditional conductive agents, carbon black is currently the most widely used conductive agent. The disadvantage is that the price is high and it is difficult to disperse.
(2) Graphite: Conductive graphite is characterized by a particle size close to that of the positive and negative active materials, a moderate specific surface area, and good electrical conductivity. It acts as a node of the conductive network in the battery, and in the negative electrode, it can not only improve the conductivity , but also the capacity.
(3) P-Li: Super P-Li is characterized by small particle size, similar to conductive carbon black, but moderate specific surface area, especially in the form of branches in the battery, which is very advantageous for forming a conductive network. The disadvantage is that it is difficult to disperse.
(4) Carbon nanotubes(CNTs): CNTs are conductive agents that have emerged in recent years. They generally have a diameter of about 5nm and a length of 10-20um. They can not only act as “wires” in conductive networks, but also have double electrode layer effect to give play to the high-rate characteristics of supercapacitors. Its good thermal conductivity is also conducive to heat dissipation during battery charge and discharge, reduce battery polarization, improve battery high and low temperature performance, and extend battery life.
As a conductive agent, CNTs can be used in combination with various positive electrode materials to improve the capacity, rate, and cycle performance of material/battery. The positive electrode materials that can be used include: LiCoO2, LiMn2O4, LiFePO4, polymer positive electrode, Li3V2(PO4)3, manganese oxide, and the like.
Compared with other common conductive agents, carbon nanotubes have many advantages as positive and negative conductive agents for lithium ion batteries. Carbon nanotubes have a high electrical conductivity. In addition, CNTs have large aspect ratio, and lower addition amount can achieve a percolation threshold similar to other additives (maintaining the distance of electrons in the compound or local migration). Since carbon nanotubes can form a highly efficient electron transport network, a conductivity value similar to that of a spherical particle additive can be achieved with only 0.2 wt% of SWCNTs.
(5) Graphene is a new type of two-dimensional flexible planar carbon material with excellent electrical and thermal conductivity. The structure allows the graphene sheet layer to adhere to the active material particles, and provide a large number of conductive contact sites for the positive and negative electrode active material particles, so that the electrons can be conducted in a two-dimensional space to form a large-area conductive network. Thus it’s considered as the ideal conductive agent currently.
The carbon black and the active material are in point contact, and can penetrate into the particles of the active material to fully increase the utilization ratio of the active materials. The carbon nanotubes are in point line contact, and can be interspersed between the active materials to form a network structure, which not only increases conductivity, At the same time, it can also act as a partial bonding agent, and the contact mode of graphene is point-to-face contact, which can connect the surface of the active material to form a large-area conductive network as a main body, but it is difficult to completely cover the active material. Even if the amount of graphene added is continuously increased, it is difficult to completely utilize the active material, and diffuse Li ions and deteriorate the electrode performance. Therefore, these three materials have a good complementary trend. Mixing carbon black or carbon nanotubes with graphene to construct a more complete conductive network can further improve the overall performance of the electrode.
In addition, from the perspective of graphene, the performance of graphene vary from different preparation methods, in the degree of reduction, the size of the sheet and the ratio of carbon black, the dispersibility, and the thickness of the electrode all affect the natures of conductive agents greatly. Among them, since the function of the conductive agent is to construct a conductive network for electron transport, if the conductive agent itself is not well dispersed, it is difficult to construct an effective conductive network. Compared with the traditional carbon black conductive agent, graphene has an ultra-high specific surface area, and the π-π conjugate effect makes it easier to agglomerate in practical applications. Therefore, how to make graphene form a good dispersion system and make full use of its excellent performance is a key problem that needs to be solved in the widespread application of graphene.