The Role and Importance of Conductive Agents in Lithium Batteries
One of the critical functions of adding a conductive agent to a lithium battery lies in enhancing its efficiency during charge and discharge cycles. During these cycles, positive and negative currents flow through the electrodes, leading to chemical reactions that can cause the electrodes to deviate from their equilibrium potential, resulting in what’s known as polarization. This polarization can be categorized into ohmic polarization, electrochemical polarization, and concentration polarization. The polarization voltage is a key indicator of the internal electrochemical reactions occurring within a lithium-ion battery. Prolonged exposure to an unreasonable polarization voltage can accelerate the deposition of lithium metal on the negative electrode, potentially causing the diaphragm to puncture and leading to a short circuit. Initial experimental data from lithium batteries reveal that the conductivity of active materials alone is insufficient to meet the demands of electron migration rates. To address this, conductive agents are incorporated to expedite electron movement.
The primary role of conductive agents is to enhance electronic conductivity by facilitating the collection of microcurrents between active materials and between active materials and the current collector. This reduces the contact resistance of the electrode, increases electron migration rates, and minimizes battery polarization. Additionally, conductive agents improve the processability of electrode sheets, promoting electrolyte infiltration into the electrode, thereby extending the battery's lifespan.
Common Types of Lithium Battery Conductive Agents
Traditional conductive agents, such as carbon black, conductive graphite, and carbon fibers, have been widely used. Newer options include carbon nanotubes, graphene, and their mixed conductive pastes. Some commercially available conductive agents include SPUER Li, S-O, KS-6, KS-15, SFG-6, SFG-15, 350G, acetylene black (AB), Ketjen black (KB), and vapor-grown carbon fiber (VGCF).
Carbon black, observed under a scanning electron microscope, appears either in chains or clusters, with individual particles having a significant specific surface area (around 700 m²/g). While this high surface area facilitates the formation of a conductive network among particles, it also presents challenges in dispersion and oil absorption. These issues can be mitigated by optimizing the mixing process and controlling the carbon black content within a range of 1.5% or less.
Conductive graphite exhibits excellent conductivity and aligns well with the particle size of active materials. Its point contact with particles forms a conductive network, enhancing conductivity and increasing the capacity of the negative electrode.
Carbon fibers (VGCF), characterized by their linear structure, form a robust conductive network in electrodes, reducing internal resistance and improving overall battery performance. Their point-line contact with active materials improves conductivity while reducing the required amount of conductive agent.
Carbon nanotubes (CNT), including single-walled and multi-walled varieties, offer a one-dimensional structure that facilitates the formation of a highly efficient conductive network. Their point-line contact with active materials enhances battery capacity, rate performance, and cycle life while reducing interface impedance. Leading companies like BYD and AVIC have successfully implemented CNTs in their batteries. Dispersal challenges are addressed using methods such as high-speed shearing and ultra-fine grinding.
Graphene, with its unique two-dimensional structure, enables point-to-surface contact with active materials, maximizing conductivity and minimizing the required amount of conductive agent. However, its high cost, difficulty in dispersion, and impact on lithium ion transmission have hindered widespread industrial adoption.
Binary and ternary conductive pastes, combining CNTs, graphene, and carbon black, represent the latest advancements in conductive agent technology. These mixtures are essential for industrial applications and leverage synergistic effects between components.
The Future of Conductive Agents
The evolution of conductive agents—from granular carbon black to carbon fibers and CNTs, to the current graphene-based solutions—reflects ongoing improvements in microstructure and conductivity. While carbon black remains dominant due to its maturity, CNTs have demonstrated success in power batteries. Graphene, despite its promise, faces challenges related to cost and processing. The future likely lies in multi-component conductive pastes, which combine the strengths of various agents, offering enhanced performance and reliability.
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