Microporous polypropylene 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. 

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. 

Property Microporous Polypropylene Alone Microporous Polypropylene/ Polyacetylene Structure... 

Currently, all commercially available, spirally wound lithium-ion cells use microporous polyolefin separators. In particular, separators are made from polyethylene, polypropylene, or some combination of the two. Polyolefins provide excellent mechanical properties and chemical stability at a reasonable cost. A number of manufacturers produce microporous polyolefin separators (Table 1.)... 

The materials used in nonwoven fabrics include a single polyolefin, or a combination of polyolefins, such as polyethylene (PE), polypropylene (PP), polyamide (PA), poly(tetrafluoroethylene) (PTFE), polyvinylidine fluoride (PVdF), and poly(vinyl chloride) (PVC). Nonwoven fabrics have not, however, been able to compete with microporous films in lithium-ion cells. This is most probably because of the inadequate pore structure and difficulty in making thin (<25 /rm) nonwoven fabrics with acceptable physical properties. 

Polypropylene and polyethylene microporous films obtained by this method are available from Cel-gard48.5o,54,55 Ube. The dry process is technologically convenient because no solvents are required. However, only a uniaxial stretching method has been successful to date, and as a result, the pores are slitlike in shape and the mechanical properties of films are anisotropic. The tensile strength in the lateral direction is relatively low. 

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

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). 

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. 

Microporous polyolefin materials in current use are made of polyethylene, polypropylene or laminates of polyethylene and polypropylene. Also available are surfactant coated materials, designed to offer improved wetting by the electrolyte. These materials are fabricated by either a dry, extrusion type process or a wet, solvent based process. The properties of commercial materials, including pore dimensions, porosity, and permeability, have been reported. Commercial materials offer pore size of 0.03 im to 0.1 im, and 30 to 50% porosity, as illustrated by the SEM micrograph of a commercial material in Fig. 35.28. 

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