Battery separators porosity

The dependence of separator electrical resistance on porosity for the selected SLI battery separator (0.25 mm backweb thickness) and the practical approximation T P = 1 can be seen in Fig. 6. 

The production process and the principal properties of this system have been described in detail in the section on traction battery separators (see Sec. 9.2.3.1). The outstanding properties, such as excellent porosity (70 percent) and resulting very low acid displacement and electrical resistance, come into full effect when applied in open stationary batteries. Due to the good inherent stiffness the backweb may even be reduced to 0.4 mm, reducing acid displacement and electrical... 

The process for making lithium-ion battery separators can be broadly divided into dry - and weH processes. Both processes usually employ one or more orientation steps to impart porosity and/or increase tensile strength. The dry process involves melting a polyolefin resin, extruding it into a film, thermally... 

Porosity. It is implicit in the permeability requirement typically lithium-ion battery separators have a porosity of 40%. Control of porosity is very important for battery separators. Specification of percent porosity is commonly an integral part of separator acceptance criteria. 

The testing of battery separators and control of their pore characteristics are important requirements for proper functioning of batteries. Mercury porosim-etry has been historically used to characterize the separators in terms of percentage porosity, mean pore size and pore size distribution. In this method, the size and volume of pores in a material are measured by determining the quantity of mercury, which can be forced into the pores at increasing pressure. Mercury does not wet most materials, and a force must be applied to overcome the surface tension forces opposing entry into the pores. 

The ideal battery separator would be infinitesimally thin, offer no resistance to ionic transport in electrolytes, provide infinite resistance to electronic conductivity for isolation of electrodes, be highly tortuous to prevent dendritic growths, and be inert to chemical reactions. Unfortunately, in the real world the ideal case does not exist. Real world separators are electronically insulating membranes whose ionic resistivity is brought to the desired range by manipulating the membranes thickness and porosity. 

Producing polyethylene microporous film with a porosity of 20 to 80% for battery separators. ... 

Preferably, the lithium ion battery separators range in average fiber diameter from 1-3 p, and in basis weight from 10-20 g m-2 The average fiber diameter and basis weight are substantially the same before and after the pressing. The lithium ion battery separator desirably has a porosity of 40 to 50%, and a thickness of 20-45 p. A lithium ion battery separator with this porosity value provides a low internal resistance and does not pass electrode substances to prevent short circuits. The thickness in the above range is suitable for the separator to be applied to small sized lithium ion batteries (21). 

Battery separator films, as shown in Fig. 14.18, are used in vehicle batteries to produce a defined resistance for electrical insulation between the individual lead plates. At the same time, the defined porosity of these films guarantees the necessary electron exchange. The films normally consist of ultra-high molecular weight polyethylenes (PE-UHMW) and... 

Porous UHMW polymer is made by sintering to produce articles of varied porosity. It has found growing use for controlled-porosity battery separators. Patents have been issued on the production of ultrahigh strength, very lightweight fibers from UHMW polymer by gel spinning. 

Costa, C.M., Rodrigues, L., Sencadas, V, Silva, M., Rocha, J.G., Lanceros-Mndez, S., 2012. Effect of degree of porosity on the properties of polyfvinylidene fluoride-trifluorethylene) for Li-ion battery separators. 

Teijin Solufill Limited produces Solufill a high porosity PE engineering film formerly made by a joint venture with DSM whose interests were bought out by Teijin. Solufill is filled with a special ceramic powder. Solufill was formerly used in the manufacture of bulk multilayer ceramic capacitors but in 2005 Teijin decided to withdraw from this application due to weakening electronic component prices and a worsening market environment. However, Solupor, a sister product is being supplied for use as a battery separator. 

NKK has approximately 15 types of ceUulose-based separators for supercapacitors and batteries. Cellulose-based separators have higher thermal stability than their polyolefin counterparts. An example version of an NKK cellulose-based film (used as a battery separator or as a separator for EDLCs (electric double layer capacitors)) has features such as 35—45 pm thickness, 14.5 basis weight (gsm), 66% porosity (mercury poros-imetry), and ionic resistance of 0.58 (ohms-cm ) (Fig. 11.15). " ... 

Polyolefin materials used for the battery separator are based on a homopolymer or a blend of polyethylene (PE) and polypropylene (PP) in a number of combinations between high density polyethylene (HOPE) and ultrahigh molecular weight polyethylene (UHMWPE). The methods for manufacturing the microporous polyolefin membranes can be divided into the dry process and wet process. Both processes contain an extmsion step to produce a thin film, and employ one or more orientation steps to impart porosity and increase tensile strength. The membranes made by diy process show a distinct slit-pore microstmcmre, while those by wet process feamre interconnected spherical or elliptical pores. 

Battery separators are characterized by numerous properties, including material nature, membrane stractural and functional properties. Material nature includes chemical stability, crystalline structure, hydrophilicity, thermal shrinkage, melting point, M and Mv,/M of polyolefin materials. Structural properties include thickness, porosity, pore size, pore shape, pore tortuosity, and pore distribution. Functional properties include mechanical strength, electrical resistivity, air permeability, thermal shutdown, electrolyte wettability and retention. Many of the above properties are affected with each other and may be in a trade-off relationship. For example, the mechanical strength is affected in opposite manner by the thickness, porosity and permeability, as required by the battery performance. 

Figure 11.6 Electrical resistance as a function of porosity. Reprinted from W. Bohnstedt, Automotive lead/acid battery separators a global overview./ Power Sources, 1996, 59, 45-50, with kind permission from Elsevier Science S.A., Lausanne [3].

The porosity, at 70%, is excellenf and it also achieves strikingly good values for acid displacement and electrical resistance for an industrial battery separator (0.12 cm ). 

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