Battery Separator Films

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... 

Figure 14.18 Battery separator film and process diagram...

A ZSK 92 Me twin-screw extruder can provide throughputs of approximately 600 kg/h for vehicle battery separator films. 

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... 

Lithium polymer electrolytes formed by dissolving a lithium salt LiX (where X is preferably a large soft anion) in poly(ethylene oxide) PEO can find useful application as separators in lithium rechargeable polymer batteries.Thin films must be used due to the relatively high ionic resistivity of these polymers. For example, the lithium-ion conductivity of PEO—Li salt complexes at 100 °C is still only about Viooth the conductivity of a typical aqueous solution. 

For battery separators, regenerated cellulose is placed on the surface of nonwoven so that the nonwoven is available to promote the wicking of the electrolyte. The nonwovens should not allow the penetration of viscose into itself. Suitable nonwovens are made from polypropylene, poly(vinyl alcohol), and hardwood hemps. Regenerated cellulose films are commonly used in alkaline manganese cells, both primary and secondary, in NiCd industrial batteries, as well as in silver—zinc batteries. 

Although many radiation-grafted materials have been discovered, only a limited number of them have been commercially utilized. One of the first successful applications was the use of grafted films in battery separators. Other possibilities are in ion exchange resins and membranes for separation processes. The textile industry represents opportunities in improving... 

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

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). 

The reaction of lithium with the electrolyte to form a surface film significantly modifies its behaviour. On the one hand, the film confers chemical stability and useful shelf life on the system. On the other, it is responsible for greatly depressed exchange currents and the consequent phenomenon of voltage delay, as discussed in Chapter 3 in connection with magnesium aqueous batteries. It is convenient to discuss separately film formation with insoluble and with liquid and soluble cathode systems. 

Solution-cast film is produced on a larger scale for medical applications, battery separators, or other specialty uses with machinery of the type shown in Figure 3.2 [2], Viscous film is made by this technique. The solution is cast onto the surface of a rotating drum or a continuous polished stainless steel belt. These machines are generally enclosed to control water vapor pickup by the film as it dries and to minimize solvent vapor losses to the atmosphere. 

Figure 9 A simple T cell for tests in battery configuration. The two electrodes and the separator film are held in a parallel plate configuration [18]. (Reprinted with copyright from The Electrochemical Society Inc.)...

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. 

---