Microporous polypropylene membranes properties

In lithium-based cells, the essential function of battery separator is to prevent electronic contact, while enabling ionic transport between the positive and negative electrodes. It should be usable on highspeed winding machines and possess good shutdown properties. The most commonly used separators for primary lithium batteries are microporous polypropylene membranes. Microporous polyethylene and laminates of polypropylene and polyethylene are widely used in lithium-ion batteries. These materials are chemically and electrochemically stable in secondary lithium batteries. 

Table I. Processing Conditions and Properties of Microporous ACCUREL Polypropylene Membranes...

Samples of microporous polypropylene were prepared for treatment with the catalyst solution. Microporous polypropylene (0.2 micrometer rated) membrane properties are shown in Table I. 

The membrane properties of the microporous polypropylene/poly-acetylene structure were measured, and a comparison with the original properties is given in Table II. 

Microporous polyolefin membranes in current use are thin (< 30 pm) and are made of polyethylene (PE), polypropylene (PP), or laminates of polyethylene and polypropylene. They are made up of polyolefin materials because they provide excellent mechanical properties, chemical stability, and acceptable cost. They have been found to be compatible with the cell chemistry and can be cycled for several hundred cycles without significant degradation in chemical or physical properties. 

There are two common types of membrane material-, microporous and homogeneous. Microporous mate-riiils are manufactured from hydrophobic polymers such as polytetrafluoroethylene or polypropylene, which have a prtrosity (void volume) of about 70% and a pore size of less than I pm. Because of the nonpolar, water-repellent properties of the film, water molecules and electrolyte ions are excluded frrrm the pores gaseous molecules, on the other hand, are free to move in and out of the pores by affusion and thus across this barrier. Typically, the thickness of microporous membranes is about 0.1 mm. 

For PET track membranes treated in air plasma, a decrease in their thickness and an increase in the effective pore diameter were observed. Additionally, the pores became asymmetric. The permeability increased and depended on the pH of the filtered solution. The membrane surface was no longer smooth, because of the faster etching of the amorphous areas than of the crystalline areas (Dmitriev et al. 2002). The surface of the PET membrane becomes hydrophilic and, in properly chosen conditions, the surface properties are stable (Dmitriev et al. 1995). In polypropylene hollow fiber microporous membranes (PPHFMMs), both the O and the N functionalities were found and numerous cracks could be seen on the surface. Generally, a decrease in the flow rate was observed as a result of faster cake formation and its compaction. The main positive result of the plasma treatment was a significant improvement in the membrane regeneration characteristics (Yu et al. 2008b). 

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