Common Conductive Agents for Lithium Batteries and Their Mechanisms

Overview: Experimental data indicate that early lithium-ion batteries could not meet the required electron transmission rate with their intrinsic conductivity alone. The main role of adding conductive agents to lithium batteries is to enhance their conductivity.

During the charge-discharge cycles of a lithium battery, current passing through the anode plate can lead to a pure reaction, resulting in a loss of the electrode’s original equilibrium state and causing the electrode potential to deviate from the equilibrium potential. This phenomenon is known as polarization, which can be classified into ohmic polarization, electrochemical polarization, and concentration polarization. Polarization voltage is an important parameter reflecting the internal electrochemical reaction of lithium-ion batteries. Prolonged unreasonable polarization voltage can accelerate the precipitation of lithium metal on the cathode, potentially leading to a short circuit by penetrating the separator.

Mechanism: Conductive materials situated between the active materials collect microcurrents and reduce the electrode’s contact resistance, thereby increasing the electron mobility in the battery and reducing polarization. Additionally, conductive agents can improve the working performance of lithium-ion plates and enhance the permeability of lithium-ion electrolytes.

Common conductive agents for lithium batteries can be divided into traditional conductive agents (such as carbon black, conductive graphite, and carbon fibers) and new conductive agents (such as carbon nanotubes, graphene, and their mixed conductive pastes). The types of conductive agents available in the market include SPUERLi, S-O, and KS-.

1. Carbon Black

Carbon black particles, appearing as chains or grape-like structures in scanning electron microscope (SEM) images, have a very high specific surface area (700m²/g). The close contact between large carbon black particles forms a conductive network in the electrode. High specific surface area and tight dispersion are technical challenges. To improve the dispersibility and ensure that the amount of carbon black remains within a certain range (typically no more than 1.5%), it is crucial to optimize the mixing process of active materials and conductive agents.

Conductivity Mechanism: The conductivity of carbon black is closely related to its internal microstructure, surface characteristics, particle size, and structure. Variations in the internal microstructure of carbon black particles significantly influence their conductivity. Graphitized carbon black has higher conductivity. Impurities such as volatiles or tar on the surface of carbon black can form an insulating layer that increases resistance. By heating carbon black in a vacuum or inert gas to remove oxygen-containing volatiles and oil impurities, resistance can be significantly reduced. Smaller particle sizes lead to increased dispersibility, reducing resistance as the number of particles per unit volume increases. Hence, fine particles of carbon black exhibit excellent conductivity. The structure of carbon black, such as chain-like or fibrous formations, also plays a crucial role in its conductivity. Conductivity is typically measured by the resistance of compressed carbon black powder.

2. Conductive Graphite

Conductive graphite also exhibits good conductivity, with particle sizes similar to those of active materials. Particles form a point-contact conductive network structure, enhancing both conductivity and the capacity of cathode and anode materials.

3. Carbon Fibers (VGCF)

Conductive carbon fibers have a linear structure that easily forms a conductive network within the electrode, reducing electrode polarization and resistance, thus improving battery performance. The contact form between charged materials and conductive agents in the battery is less than point contact, which helps to improve electrode conductivity, reduce the amount of conductive agents needed, and increase battery capacity.

4. Carbon Nanotubes

Carbon nanotubes, which can be single-walled or multi-walled, have a one-dimensional fiber-like structure and are hollow cylindrical internally. Using carbon nanotubes as conductive agents helps to form a comprehensive conductive network through point and line contact with the electric material. This significantly impacts battery capacity (by improving sheet compression density), performance, cycle life, and interface impedance reduction.

Currently, lithium battery products that use carbon nanotubes as conductive agents show excellent results. Carbon nanotubes can be grown under two conditions: herringbone type and array type. Issues in lithium battery applications can be addressed through high-speed shearing, adding dispersants, dispersing slurry, and ultrafine grinding during the static dispersion process.