1.Introduction to conductive agents
1.1.The normal charging and discharging process of lithium battery is the process in which lithium ions and electrons participate in the reaction, which requires that the electrode of the lithium-ion battery must be a mixed conductor of ions and electrons, and the electrode reaction can only occur at the junction of electrolyte, conductive agent and active material. However, the conductivity of the positive and negative active materials of lithium-ion batteries is not satisfactory.
1.2.Cathode: mostly transition metal oxide or transition metal phosphate, which is a semiconductor or insulator with poor conductivity.
1.3.Anode: Graphite is the main material with good conductivity,but the graphite base surface is mainly conductive, and at the same time, the expansion and contraction of the graphite material during the charging and discharging process will lead to the deterioration of the contact between the particles, the increase of the gap, and the increase of battery polarization.
To sum up, both positive and negative electrodes need to be added and used with conductive agents
2.The role of conductive agents and the conductive mechanism
2.1.The role of conductive agents
2.1.1.Improve electrical conductivity, reduce polarization, improve the capacity of the material (increase the depth of reaction), and improve the discharge platform
2.1.2.Improvement of battery rate characteristics (charge + discharge)
2.1.3.Improves battery cycle life
2.1.4.Reduce the generation of lithium dendrites and improve the safety performance of batteries
2.1.5.Some conductive agents form a connection network inside the electrode to improve the adhesion of the electrode
2.1.6.Some conductive agents improve the rolling performance of the electrode and increase the compaction density
2.2.Conductive mechanism of conductive agents
2.2.1.Electronic conductivity: Construct short- and long-range conductive networks through self or physical connections to improve the physical polarization of the battery
2.2.2.Ionic conductivity: By absorbing and holding the electrolyte, it provides a reaction interface for the lithium ion charging and discharging process and reduces electrochemical polarization
3.Common types of conductive agents
3.1.Carbon black conductive agents
3.1.1.SUPER P Li(abbreviated as SP)
SP is currently the most widely used conductive agent in China, is a conductive carbon black similar to the furnace black method ("MMM" method preparation), is a diameter of about 40nm of primary particles agglomeration into a secondary structure of 150~200nm, through soft agglomeration and artificial compression and other subsequent processing molding, the whole is a staphylococcus chain.
3.1.2.Acetylene carbon black (abbreviated as AB)
AB is prepared by calcium carbide method, that is, acetylene gas is made from calcium carbide first, and then purified to isolate the air and crack it at a high temperature of about 1400 °C.
3.1.3.Superconducting carbon black
Generally, the carbon black of BET more than 800m2/g is called superconducting carbon black, and the surface of the primary particles of general superconducting carbon black is very rough, such as ENSACO 350G primary particles present a hollow open structure, due to the ultra-high BET, in order to facilitate dispersion, it is often granulated, and it is recommended to use a dry mixing process when dispersing, and add a dispersant.
TEM for SP SEM for SP 350G SEM (Hollow Structure)
3.1.4.SP conductive agent has good processing performance, low price, and is most widely used in commercialization
Carbon black | Particle size/nm | Oil absorption value/(ml DBP/100g) | BET/(m2/g) |
Super P Li | 45 | 290 | 62 |
Acetylene carbon black | 36 | 250 | 80 |
350G | 40 | 320 | 770 |
3.1.5.The KS series is more recommended for positive electrodes
3.1.5.1.Provides good electrical conductivity
3.1.5.2.The morphology is more matched and conducive to positive electrode compaction
3.1.5.3.Low gram capacity and first effect, and it is not suitable for the negative electrode
3.1.6.The SFG series is more recommended for negative electrodes
3.1.6.1.Provides good electrical conductivity
3.1.6.2.The flake morphology is more compatible with spherical and spherical graphite and conducive to negative electrode compaction
3.1.6.3.High degree of graphitization, high capacity and first effect
3.2.Fiber-based and nanotube-based conductive agents
3.2.1.It has ultra-high electrical conductivity and thermal conductivity, which can effectively help the battery improve the rate performance and improve the temperature consistency during the high-rate discharge of the battery
3.2.2.Having a large aspect ratio, on the one hand, it is conducive to forming a conductive network, improving adhesion. On the other hand, it can use the fibrous characteristics to improve the softness of the electrode sheet
3.2.3.Using the characteristics of fibers to reduce the expansion and contraction of the electrode during charging and discharging processes, and improve cycling performance
3.2.4.VGCF structure: fibrous
3.2.5.CNT structure: hollow, single-armed, multi-armed
3.3.Improvement Mechanism
Conductive agents such as CNT and VGCF form an effective conductive network on the surface of the material, and the expansion and contraction states can be maintained
4.Recommendations for the use of conductive agents
4.1.The conductive agent is selected based on the morphology of the positive and negative electrodes, and a conductive network is formed using three types of conductive agents: point, line, and plane, to improve conductivity.
4.2.Select conductive agents based on the application needs and cost of the battery, such as rate performance, low temperature performance and high temperature performance
4.3.CNT, VGCF, ECP-600JD and other nano-level conductive agents are easy to agglomerate and difficult to disperse, so they can be improved by step-by-step addition, special dispersants, and high-speed dispersion. At present, it is recommended to use a conductive adhesive solution that has been dispersed
5.The direction of development of conductive agents
5.1.Low cost: SP and domestic alternative conductive carbon
5.2.High energy density: CNT (multi-wall and single-arm CNT)
5.3.Long cycle: composite conductive carbon (KS6+SP, CNT+SP, etc.)
5.4.Magnification type: composite conductive carbon, Balancing electronic and ionic
conductivity
5.5.Silicon anode: CNT, VGCF, etc. are easy to form a conductive network, on the one hand,
to maintain good conductivity, on the other hand, to maintain the conductivity of the
electrode after expansion and contraction (too much amount of addition will affect the first
effect, required balance adjustment)
5.6.Easy-to-process conductive agent: short dispersion time and good coating effect
