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
5.7.The surface of the conductive carbon has a hydroxyl group, a carboxyl group and a small
amount of carbonyl group, which is optimized by adjusting the surface functional groups.
5.8.The direction of development of conductive agents
Conductive coatings
Applying conductive paint on top of copper and aluminum foils:
5.8.1.1On the one hand, improve the adhesion between the active substance and the
substrate to prevent the detachment between the active substance and the substrate in
the middle and late stages of long-term circulation;
5.8.2.On the other hand, by reducing the impedance between the active material and
the substrate, improved the comprehensive performance of the battery.
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7.
Q & A
In this Q&A session, Dr. Ke also provided answers to everyone's questions one by one.
Lithium-Sulfur: "Hello, Dr. Ke! I have a question. Does the pore structure in conductive agents help or hinder the retention of the electrolyte?"
Dr. Ke:
"Hello, this needs to be considered in two parts. First, pores are essential because without them, the electrolyte cannot be absorbed, and the ion transport channels would be lost. However, having too many pores or high porosity isn't always better, because it can lead to poor electron conductivity between the particles. Additionally, the electrodes might become too thick, reducing the energy density."
Changda New Energy - Ternary Cathode: "What are the specific manifestations of physical polarization and electrochemical polarization?"
Dr. Ke:
"Physical polarization can be thought of as the conductivity of electrons in a conductor, while electrochemical polarization refers to the electrochemical reaction of lithium de-intercalation and intercalation."
Gentle but Not Noisy: "Can conductive agents help form a stable SEI?"
Dr. Ke:
"Although some conductive agents can form a thin layer on the surface of particles, if the electrolyte comes into contact with the active material, an SEI will still form at the interface. So, I believe it's difficult for conductive agents to form a truly stable SEI."
Lithium-Sulfur: "Thanks! Another question: Is it ion conductivity or electronic conductivity that primarily affects rate performance and capacity? And as the viscosity of the electrolyte increases due to electrochemical reactions, will the efficiency of electrolyte transport in the conductive agent be affected?"
Dr. Ke:
"Both electronic and ionic conductivity affect rate performance and capacity, so improving rate capability depends on the specific problem. If the viscosity of the electrolyte increases, the conductivity decreases, making lithium-ion transport more difficult, which in turn reduces rate performance."
Thousand Questions: "Why can't SFG be applied to the cathode? And since it's sheet-shaped, why does the shape of the cathode and anode need to match?"
Dr. Ke:
"SFG can be applied to the cathode, but it’s more expensive. KS6 can meet the needs of the cathode, so SFG is less commonly used."
Lithium-Sulfur: "If a conductive agent with a mesh structure is used, can the binder usage be reduced?"
Dr. Ke:
"Yes, the addition of CNTs can significantly reduce the amount of binder needed."
Wasted Talent: "Does the fact that conductive agents like KS-6 have specific capacity affect the calculation of active material capacity?"
Dr. Ke:
"This is a good question. KS6 does have capacity, which theoretically could affect the capacity of the electrode. However, in commercial applications, the amount of conductive agent added is generally quite low, so it's often neglected. In academic settings, where the additive amounts might exceed 5%, or even 10%, this needs to be considered."
Worship: "Are there any other good conductive agents, or is CNT alone better, or CNT/SP?"
Dr. Ke:
"This depends on your application and requirements. There are many products available. For example, in the cobalt oxide systems used in mobile phone batteries, pure CNT is used because adding SP would reduce the active material. These products focus more on energy density, aiming to use as much active material as possible. However, in other products, where cost-performance ratio or low temperature performance is considered, composites are used. The composite reduces cost, and SP has better liquid absorption, which is beneficial for low temperatures."
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