Polyethylene separator

However, it has to be conceded that after battery life cycle tests at such temperatures polyethylene separators also reach their limits (although this fact does not yet reflect in failure-mode studies [49]) even in locations with extreme ambient temperatures. The tendency toward using ever-thinner backwebs cannot be continued, however, without seeking protective measures. Suitable provisions have to be made, especially with respect to the separator s oxidative stabihty at elevated temperature. The leading producers of polyethylene separators have recently presented solutions [41, 47] which, even at 150 (xm backweb, provide for oxidative stability and puncture strength in excess of that for the standard product at 250 (xm backweb [41]. 

Without any doubt, the microporous polyethylene pocket wiU meet aU requirements of modern starter batteries for the foreseeable future. Whether and to what extent other constructions, such as valve-regulated lead-add (VRLA) batteries, other battery systems, or even supercapacitors, will find acceptance, depends - besides the technical aspects - on the emphasis which is placed on the ecological or economical factors. 

Separators for Industrial Batteries 11.2.3.1 Separators for Traction Batteries 

It can be stated generally that requirements for traction battery separators in respect to mechanical properties and chemical stability are considerably higher than those for starter battery separators. This is due to the fact that a forklift battery is typically operated for about 40 000-50 000 h in charge-discharge service, whereas a starter battery for only about 2000 h. The requirements for electrical resistance are lower because of the typically lower current densities for traction batteries. These differences are of course reflected in the design of modem traction battery separator material. 

Industrial battery separators are often supplied in cut-piece form, that is, they have to have a certain stiffness and robustness in order to withstand the assembly 

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As a safety specialty, polyethylene separators are used to shut down run-away (shorting) Li cells by simply melting and creating a high-resistance barrier. The speed of resistance gains increases, approaching the melting point. Bilayers of polyethylene (PE) and polypropylene (PP) are even faster (Hoechst) (see also Chapter III, Sec. 10). 

In the second half of the 1960s, at the same time but independently, three basically different plastic separators were developed. One was the polyethylene separator [16] already referred to in starter batteries, used only rarely in stationary batteries, but successful in traction batteries. The others were the microporous phenolic resin separator (DARAK) [18] and a microporous PVC separator [19], both of which became accepted as the standard separation for stationary batteries. They distinguish themselves by high porosity (about 70 percent) and thus very low electrical resistance and very low acid displacement, both important criteria for stationary batteries. 

Polyethylene separators Phenol- formald.- resorcinol separators PVC separators Rubber separators Micro fiber glass mat separators Total... 

The term polyethylene separator is somewhat misleading, since this separator consists mainly of agglomerates of precipitated silica, held within a network of extremely long-chained, ultrahigh-molecular weight polyethylene molecules. The raw materials, precipitated silica (Si02 — about 60 percent), ultrahigh-... 

Figure 7. Polyethylene separator production process (I) Mixing and Extrusion...

The alternative is hexane, which because of the explosion hazard requires a more expensive type of extractor construction. After the extraction the product is dull gray. The continuos sheet is slit to the final width according to customer requirements, searched by fully automatic detectors for any pinholes, wound into rolls of about 1 m diameter (corresponding to a length of 900-1000 m), and packed for shipping. Such a continuous production process is excellently suited for supervision by modern quality assurance systems, such as statistical process control (SPC). Figures 7-9 give a schematic picture of the production process for microporous polyethylene separators. 

Filled polyethylene separators are the only pocket material that has been able to meet all requirements of a starter battery reliably [39-48]. It is flexible and weldable into three-sided closed pockets, making the previously usual mud room at the bottom... 

Figure 14. Polyethylene separator cross-ribs in the margin area...

Figure 16. Polyethylene separator intermediate vertical ribs...

Surprisingly the water consumption of a starter battery, provided it contains anti-monial alloys, is affected by the separator. Some cellulosic separators as well as specially developed polyethylene separators (e.g., DARAMIC V [76]) are able to decrease the water consumption significantly. The electrochemical processes involved are rather complex and a detailed description is beyond the scope of this chapter. Briefly, the basic principle behind the reduction of water loss by separators is their continuous release of specific organic molecules, e.g., aromatic aldehydes, which... 

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. 

Voltage Control Additive [83]) allow significant improvement of the top-of-charge performance of batteries, helping polyethylene separators to gain acceptance in the great majority of applications. 

Flexible polyethylene separators have facilitated a novel cell construction the separator material, supplied in roll form, is wound so that it meanders around electrodes of alternating polarity (Fig. 23), requiring ribs in the cross-machine direction such profiles are available commercially [60],... 

Figure 21 Polyethylene separator diagonal industrial battery profile.

Characteristic data for polyethylene separators in comparison with competing systems are discussed later in this section (Table 11). 

Polyethylene separators Rubber separators Phenol formaldehyde-resorcinol separators Microporous PVC separators... 

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 for polyethylene separators (Sec. 9.2.2.1) as well as the characteristic properties (see Sec. 9.2.2.1 and 9.2.3.1) have already been described in detail above. Deviating therefrom, the desire for low acid displacement has to be added for separators in open stationary batteries. This can be met either by decreasing the backweb thickness or by increasing the porosity the latter, however, is at the expense of separator stability. 

Table 12 shows the physicochemical data of separators used in open stationary batteries. Since the emphasis is on low acid displacement, low electrical resistance, and high chemical stability, the phenolic resin-resorcinol separator is understandably the preferred system, even though polyethylene separators, especially at low backweb, are frequently used. For large electrode spacing and consequently high separation thickness, microporous as well as sintered... 

Polyethylene separators Phenol-Formal dehyde-resorcinol Separators Microporous PVC separators Sintered PVC separators Rubber separators... 

Among the separator varieties described, the phenol-formaldehyde-resorcinol separator (DARAK 2000) [60] as well as the microporous PVC separator [86] have proven effective for this construction. For applications without deep discharges, concessions may be made with the respect to porosity and pore sizes of the separator therefore polyethylene separators or a spe-... 

One version of the microporous, filled polyethylene separator ( PowerSep ) [113], which is so successful in the lead-acid battery, is also being tested in nickel-cadmium batteries. This separator is manu-... 

We experienced no safety problems during the external short tests because of the Polyswitch inside the cell. We confirmed that even if the Polyswitch fails to operate, the short-circuit current stops flowing before thermal runaway occurs because the micropores are closed by the polyethylene separator, which melts at 125 °C ("separator shutdown"). 

They are fabricated from a variety of inorganic, organic, and naturally occurring materials and generally contain pores that are greater than 50—100 A in diameter. Materials such as nonwoven fibers (e.g. nylon, cotton, polyesters, glass), polymer films (e.g. polyethylene (PE), polypropylene (PP), poly(tetrafluo-roethylene) (PTFE), poly (vinyl chloride) (PVC)), and naturally occurring substances (e.g. rubber, asbestos, wood) have been used for microporous separators in batteries that operate at ambient and low temperatures (<100 °C). The microporous polyolefins (PP, PE, or laminates of PP and PE) are widely used in lithium based nonaqueous batteries (section 6.1), and filled polyethylene separators in lead-acid batteries (section 7.3), respectively. 

ENTER Membranes LLC has developed Teklon— a highly porous, ultrahigh molecular weight polyethylene separator for lithium-ion batteries. At the writing of this publication, the separator is available in small quantities. Pekala et al. characterized Celgard, Setela, and Teklon separators in terms of their physical, mechanical, and electrical properties. ... 

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