Separator (battery)
A separator in electrochemical elements such as electrolytic and galvanic cells has the task of cathode and anode , i.e. This means that the negative and positive electrodes in the accumulator cells and the primary elements must be spatially and electrically separated. The separator must, however, be permeable to the ions that convert the stored chemical energy into electrical energy. The main materials used are microporous plastics and nonwovens made of fiberglass or polyethylene .
Task and structure
The separator acts as a barrier that electrically isolates the two electrodes from one another in order to avoid internal short circuits . At the same time, however, the separator must be permeable to ions so that the electrochemical reactions can take place in the cell.
A separator must be thin so that the internal resistance is as low as possible and a high packing density can be achieved. This is the only way to achieve good performance data and high capacities . Other important functions of the separator are to suck up the electrolyte and to ensure gas exchange in closed cells. While earlier u. a. Woven fabrics and paper were used, nowadays mostly very fine-pored materials such as nonwovens and membranes are in use.
A simple construction made of injection-molded plastic bars can also be used as a separator if the only aim is to keep the electrodes at a certain distance.
The tube pocket is a special form of the separator . This is made from two layers of fabric or non-woven fabric, which are first impregnated with a resin, then sewn together and shaped into a specific tube. These tubes are filled with active material and are then used as electrodes in lead-acid batteries .
Different separators have to be used for different chemical systems. Their composition depends on the electrolyte to which they are exposed over the course of their service life . Another criterion for choosing a separator is the price. Separators, which have to be stable over many charging and discharging cycles or several years, are made of higher quality materials than those used in short-lived primary cells .
Separators for rechargeable or secondary elements
- Lead accumulators
- Here materials are required that can withstand the strongly acidic and oxidative conditions. Extruded or sintered separators made of polyethylene, sintered PVC or mats made of micro glass fiber fleece (AGM) can be used here.
- Nickel-cadmium batteries
- Here, in the strongly alkaline environment of the potassium hydroxide solution , separators made of polyamide and polyethylene or polypropylene combinations are mainly used. Nowadays, nonwovens are used almost exclusively here. Hydrophobic polymers can be made hydrophilic by fluorination or wetting agents so that they eagerly take up the electrolyte .
- Nickel-metal hydride accumulators
- The requirements here are the same as for nickel-cadmium batteries, only that additional requirements are placed on the battery separator. This is because it must be able to reduce self-discharge . This is achieved by functionalizing the nonwoven surface using chemical treatment. Such can be surface treatment with acrylic acid or sulfonation .
- Lithium batteries
- Microporous membranes are used here to allow the passage of ions. These are mostly polymeric films which can also consist of several layers. Since these foils have very good electrochemical properties, but only a low temperature resistance (about 120 ° C.), heat-resistant microporous ceramic separators are also used. Their mechanical properties (susceptibility to breakage), however, severely limit the application possibilities to exclusively stationary applications.
Since around 2009, we have been working on materials that are based on a very fine non-woven fabric that has been coated with ceramic. This achieves a high level of safety through flexibility and temperature resistance, especially for use in traction batteries for electric cars and hybrid vehicles . A new development comes from the Evonik group and is used by the Saxon company Li-Tec . The Separion ceramic film developed as a separator is temperature- resistant up to around 700 ° C and is intended to reliably prevent explosive thermal runaway .
Separators for non-rechargeable or primary cells
- Lithium cells from lithium batteries
- Microporous films or nonwovens are used.
- Alkaline manganese cells
- In alkaline manganese batteries mostly nonwoven fabrics are (English nonwovens ) are used as separators. These usually consist of a mixture of polyvinyl alcohol microfibers (PVA) and cellulose . Occasionally, laminates made of nonwovens and membranes , such as cellophane, are also used. Special requirements for the separator material in alkali-manganese batteries are good wetting and high absorption capacity for the alkaline electrolyte solution . The pore diameter must be small, so-called growths of the separator by zinc - dendrites that the internal short circuit lead to avoid. A low price of the material is also important.
- Zinc-carbon elements
- With this type of battery, paper is mainly used as a separator.
Other applications
Batteries that are exposed to high temperatures require more temperature-resistant materials, e.g. B. temperature-resistant polymers or, in individual cases, asbestos .
For information on separators in fuel cells, see there.
literature
- Peter Kritzer, John Anthony Cook: Nonwovens as Separators for Alkaline Batteries . In: Journal of The Electrochemical Society . tape 154 , no. 5 , 2007, p. A481-A494 , doi : 10.1149 / 1.2711064 .
Individual evidence
- ↑ H. Wallentowitz, K. Reif: Manual automotive electronics: Fundamentals, components, systems, applications. Springer, 2006, ISBN 9783528039714 , p. 256
- ^ Carsten Herz: Daimler and Evonik are pushing battery alliance. Article in the Handelsblatt from August 11, 2010.
- ↑ MembranGuide: Separion is the new ceramic separator for lithium ion batteries , accessed June 13, 2012
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