Physicochemical Properties of Lithium-Ion Battery Separators

Due to different manufacturing processes, the consistency of lithium-ion battery separators can vary significantly. This consistency includes inherent characteristics such as closure temperature and apparent consistency, such as the consistency of pore size and thickness observed under an electron microscope.

Physicochemical Properties of Lithium-ion Battery Separators

1. Wettability and Wetting Rate. Poor wettability of the separator increases the resistance of both the separator and the lithium-ion battery, affecting the cycle performance and charge/discharge efficiency of the lithium-ion battery. The wettability of the separator refers to the rate at which the electrolyte enters the micropores of the separator, which is related to the surface energy, pore size, porosity, and tortuosity of the separator.
2. Membrane Absorption Rate. Since lithium-ion battery separator materials function as electrolytes, they must meet the following conditions: sufficient to ensure unobstructed ion channels, and a high liquid absorption rate. Inevitably, many side effects occur in the battery system, consuming a large amount of electrolyte. Therefore, sufficient reserves are necessary; otherwise, it will lead to a lack of electrolyte, increasing interfacial resistance and accelerating electrolyte consumption, creating a vicious cycle. Therefore, the absorption rate of the fluid separator is an important parameter.
3. Chemical Stability. The separator should remain stable in the electrolyte for a long period and should not react with the electrolyte and electrode materials under strong redox conditions. The chemical stability of the separator is evaluated by measuring the corrosion resistance and expansion/contraction rate of the electrolyte.
4. Thermal Stability. Lithium-ion batteries release heat during charging and discharging, especially during short circuits or overcharging. Therefore, when the temperature rises, the separator should maintain its original integrity and a certain mechanical strength to continue its function of isolating the positive and negative electrodes and preventing short circuits.
5. Separator Resistance. Membrane resistivity is actually the resistivity of the electrolyte in the micropores. It is related to many factors, such as porosity, pore tortuosity, electrolyte conductivity, membrane thickness, and the wettability of the electrolyte to the membrane material. It is a commonly used AC impedance method for testing separator resistance. Because the film is very thin, defects are often present, adding to the measurement error. Therefore, we often use multi-layer samples and take the average value.
6. Self-Isolation Performance. When the temperature exceeds a certain level, an exothermic reaction occurs inside the lithium-ion battery, leading to self-heating. Additionally, charger malfunctions, safety current failures, and other issues can cause overcharging or external short circuits, all of which generate significant heat. Because of its thermoplastic polyolefin material, when the temperature approaches the polymer's melting point, the porous polymer's ion-conducting membrane becomes an insulating layer without leaks, closing off the circuit and preventing ions from forming a complete circuit, thus protecting the battery. Therefore, polyolefin separators can provide additional protection for the battery.

The primary function of the separator in a lithium-ion battery is to separate the positive and negative electrodes, preventing them from contacting and short-circuiting. Additionally, it allows electrolyte ions to pass through. It is understood that the quality of the separator directly affects the capacity, cycle capacity, and safety performance of the lithium-ion battery; therefore, superior separator performance is crucial for improving the overall performance of the battery.