Battery separators properties

The great quantity of very fine fibers in a meltblown web creates several unique properties such as large surface areas and small (<1 fiva) pore sizes. These have been used in creating new stmctures for hospital gowns, sterile wrap, incontinence devices, oil spill absorbers, battery separators, and special requirement filters. It is expected that much innovation will continue in the design of composite stmctures containing meltblown webs. 

Cold crank performance, battery life expectancy, and freedom from maintenance are generally co-affected by the separators, whereas ampere-hour capacity remains largely unaffected at a given separator thickness. The properties of the different leaf and pocket separators are compared in Table 10. These typical separator properties (lines 1-4) are reflected in the electrical results of battery tests (lines 5-8). The data presented here are based on the 12 V starter battery standard DIN 43 539-02 tests based on other standards lead to similar results. 

It can be stated generally that requirements for traction battery separators in respect to mechanical properties and chemical stability are considerably higher than for starter battery separators. This is due to the fact that a forklift battery is typically... 

A detailed description of the production process and the properties of polyethylene separators can be found in Sec. 9.2.2.1, so only the modifications, which are important for traction battery separators are covered here. 

Which separator properties are important for use in traction batteries For this aspect primarily the highly predominant application, namely forklift traction batteries,... 

Polyethylene separators offer the best balanced property spectrum excellent mechanical and chemical stability as well as good values for acid availability and electrical resistance have established their breakthrough to be the leading traction battery separator. Rubber separators, phenolic resin-resorcinol separators, and mi-croporous PVC separators are more difficult to handle than polyethylene separators their lack of flexibility does not allow folding into sleeves or use in a meandering assembly in addition they are more expensive. 

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... 

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. 

Although the material of a battery separator is inert and does not influence electrical energy storage or output, its physical properties greatly influence the performance and safety of the battery. This is especially true for lithium-ion cells, and thus the battery manufacturers have started paying more attention to separators while designing the cells. The cells are designed in such a way that separators do not limit the performance, but if the separator properties are... 

Table 7. Safety and Performance Tests for Lithium-Ion Batteries and the Corresponding Important Separator Property and Its Effect on the Cell Performance and/or Safety...

Up until more recent history, most of the separators and membranes historically used had not been specifically developed for battery applications. Thus, future research should be aimed at developing separators that are specifically tailored for battery applications. The general objectives of separator research should be as follows (a) to find new and cost-effective separators, (b) to understand the separator properties in batteries, and (c) to optimize separator properties related to specific cell performance, life. 

Separators in lithium ion batteries must separate positive electrodes and negative electrodes to prevent short circuits, and must allow passage of electrolytes or ions. Porous films and nonwoven fabrics of resins are known separators. The lithium ion battery separators are also required to exhibit stable properties at high temperatures such as in charging, and therefore high heat resistance is desired (21). 

Louis Hegedus Coming from the specialty chemicals applications, I would like to further emphasize what Sheldon Isakoff said. Take polyethylene, which our company uses extensively for a variety of applications. What do we do with it We fill it, modify it, functionalize it, formulate it, compound it, extrude it, laminate it, and end up with such vastly different products as battery separators, membranes, construction materials, insulation materials, battery electrodes, and microfilters. It is amazing what you can do with any given polymer. Much research emphasis today is on the properties of bulk polymers themselves. There is a whole science associated with com-... 

The RAI Research Corporation also offers a range of battery separators under the name of Permion (JL). These membranes are made by radiation grafting of a suitably active group onto an inert base film. The active groups include weak acids such as acrylic and substituted acrylic acid and stronger acidic groups such as sulfonated styrene. The base film can be Teflon R, polyethylene or polypropylene. They are thus not strictly perfluorinated membranes as is Nafion, but in chemical inertness and in many physical properties such as electrical conductivity and ion flux are useful as separators in batteries. 

Special Property Membranes. In the literature, there are numerous methods reported for the preparation of ion-exchange membranes with special properties,87-89 for instance, for use as battery separators, ion-selective electrodes, or in the chlor-alkali process. Especially membranes recently developed for the chlor-alkali industry are of commercial significance. These membranes are based on polytetrafluoroethylene and carry sulfone groups in the bulk of the membrane phase and carboxyl-groups on the surface as the charged moiety. They combine good chemical stability with high selectivity and low electric resistance. 

The physical properties of some commercially available ion-exchange membranes are listed in Table 8.1. The information on membrane properties was that supplied by manufacturers unless otherwise indicated. The diversity of membrane thickness is noteworthy. The membranes made by Ionics, Inc. are thick and rigid, because they must span rather wide spaces between supports in the solution compartments without deflection. The thick Ionics membranes have relatively high resistance, but these values are still low compared to the resistance of the brackish water they are designed to treat. RAI developed very thin membranes for battery separators and used the same radiation grafting techniques to make thin ED membranes with low resistance. 

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. 

---