To enhance the safety of Li-ion batteries, many separators are designed with a critical feature known as the shutdown temperature. This feature is based on the inherent thermal properties of the separator's material, which is typically a type of polymer. Separators are typically thin, porous sheets made from polymeric materials that serve as an essential component between the battery's anode and cathode. They play two vital roles: allowing the movement of ions between the electrodes for the battery's operation while preventing any direct electrical connection between the two that could lead to short-circuiting.
Polymeric materials used in separators, such as polyethylene (PE) and polypropylene (PP), have a characteristic called the melting point. When the temperature exceeds this point, the material starts to melt. In the case of separators, this melting leads to pore closure, essentially shutting down ion movement between the electrodes, hence the term 'shutdown temperature'. This phenomenon creates an internal open circuit within the battery, inhibiting any further electrochemical reactions that may lead to thermal runaway, a process involving rapid, uncontrolled temperature increase that can ultimately result in battery fires or explosions.
The separator's shutdown temperature is typically set at a value above the battery's normal operating temperature but below the onset temperature for thermal runaway. When the shutdown temperature is reached or exceeded, the pores of the separator begin to close due to the melting of the polymeric material.
As the separator's pores close, the ionic conductivity, which allows the battery to function, decreases dramatically. This essentially cuts off the electrical pathway between the anode and cathode, halting the battery's electrochemical reaction and subsequent heat production.
The shutdown temperature feature significantly enhances the safety of Li-ion batteries, especially in applications where the batteries may be exposed to high temperatures or harsh operating conditions. However, it's important to note that this safety mechanism is a last-resort measure to prevent catastrophic failure, not a routine operational feature.
Once the shutdown temperature feature has been activated and the separator's pores have closed, the battery's functionality is effectively lost, as the electrochemical pathway cannot be restored. Therefore, while this feature is a critical aspect of battery safety, other measures such as careful battery design, thermal management systems, and safe charging practices should also be implemented to prevent overheating from occurring in the first place.