A separator works as a physical barrier between the anode and the cathode to prevent electrical shorts while allowing for the transportation of ions during charging & discharging cycles of Li-ion batteries. It is usually a thin, porous polymeric film that, despite its simple appearance, requires a complex combination of properties to perform its function effectively. This complexity comes from a variety of structural aspects, intrinsic material properties, & its behavior under various external stimuli. The key characteristics of separators are grouped into three distinct categories by Jang J. et. al. [1]:
1. Properties based on the 'Structure of Separator':
-Melt Integrity: Ability of separator to retain structure under high temperatures, preventing internal short circuits.
-Shutdown Temperature: The specific temperature at which the separator blocks ion transport to prevent overheating or thermal runaway.
- Dimensional Stability: Separator's capacity to maintain its shape & size under varied conditions, ensuring uniform performance.
- Tortuosity: Describes the complexity of the ionic path in the separator - high tortuosity can increase resistance & decrease ionic conductivity.
- Thermal Dimensional Stability: Separator's ability to maintain its dimensions under temperature variations, critical for consistent battery performance.
- Tensile Strength: Separator's resistance to breaking under tension, vital during battery assembly & usage.
- Puncture Strength: Measure of the force required to puncture the separator, crucial for preventing short-circuits.
2. Properties based on the 'Intrinsic Properties of Materials':
- Thickness: Influences ionic conductivity and battery's energy density - trade-off between safety (thicker) and performance (thinner).
- Pore Structure: Arrangement and size of pores impact ion transport efficiency and safety - uniform structure promotes consistent ionic conductivity.
- Porosity: Measure of void spaces, affecting ionic conductivity, separator resistance, and safety - trade-off between ionic conductivity (higher with high porosity) and mechanical strength (lower with high porosity).
3. Properties Against External Stimulus:
- Ionic Conductivity: Measure of ions' mobility through the separator - high ionic conductivity ensures efficient charge/discharge.
- Electrochemical Stability: Stability of the separator under the battery's electrochemical conditions - high stability ensures long-term battery functionality.
- Wettability: Affinity of the separator for the electrolyte, influencing ionic transport - high wettability can enhance ionic conductivity.
- Chemical Stability: Separator's resistance to degradation when exposed to the battery's chemicals - essential for lifespan and safety of the battery.
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