Magazine July 2015

Focus on the textile world: from batteries to artificial turf

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Potential applications of battery systems

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The energy transition and the trends in electromobility are demanding improvements in existing materials, or innovations with completely new approaches. As a result, in numerous research projects and in the battery industry, the focus is on new material profiles in combination with industrial plant technology for cost-effective and efficient energy storage.

This development is being accompanied by an increase in demand for technical textiles, especially nonwovens and their diverse applications – such as functioning as separators in battery systems.

Requirements placed on separators

The task of the separator is to separate the cathode and anode in batteries (the negative and the positive electrodes in accumulators) both electrically as well as physically and to absorb an electrolyte. This barrier is important for insulating the poles and preventing short-circuits in the batteries, while absorption of the electrolyte is a prerequisite for the ion flow.

The requirements placed on the separator are:

  • 1.Permeability

    If the permeability of the medium is not met, the efficiency of the battery is adversely affected.

  • 2.Porosity

    It represents the ratio of pores to solids and is important for absorption of the electrolyte.

  • 3.Thickness

    The thicker the separator, the better the handling in production of the battery. However, the space for the anode and cathode may be adversely affected if the separator is too thick.

  • 4.Moisture absorption

    This should be high, so that the separator can quickly accumulate the electrolyte.

  • 5.Electrolytic absorption and binding

    Ion flow is only assured if this is good.

  • 6.Swelling

    The separator must not swell, because this has a negative impact on battery performance.

  • 7.Shape and dimensional accuracy

    It must be ensured that the separator retains its dimensional stability, otherwise this can cause a short circuit within the battery.

  • 8.Chemical resistance

    This should be ensured throughout the entire life of the battery, so that the separator loses none of its strength and structure and no impurities are formed in the battery during the chemical process.

  • 9.Dielectric strength

    The electrode must not pass through the separator, since this would trigger a short circuit.

  • 10.Thermal stability

    This has to be ensured, because the separator has to be heated during the manufacturing process.

  • 11.Tensile strength

    This is important during the production of certain types of battery, since the separator must not contract during winding of the electrode / separator pack.

  • 12.Pore size

    This should be as small and as uniform as possible, so that the barrier properties exist but do not hinder the ion transport.

  • 13.Curvature of the edges

    The edges should be completely straight, so that no contact with the electrodes is possible.

  • 14.Shutdown (as a safety feature)

    From about 130°C, the separator should interrupt the flow of ions yet still retain its mechanical stability. This effect is to protect against a short circuit of the battery and overcharging of the cell.

Nonwovens on their way to the electrotechnical industry

Meeting these numerous requirements constitutes a major challenge for separators. Possible battery separators include microporous polymer membranes, inorganic composite membranes, and nonwovens. Price is a further important criterion in the selection of a suitable separator. In this emerging industry, constant pressure on prices, the ever more economical production of nonwovens, diversity in the choice of raw materials, and the high number of possible variations are all sharply increasing demand for nonwovens.

Separators that have to remain stable over many charge/discharge cycles are made of higher-quality materials than those used in inexpensive, disposable batteries. Today's battery industry is now inconceivable without powerful separators and thus without nonwovens.

Battery production primarily uses carded nonwovens, wetlaid nonwovens, meltblown fabrics and spunbond fabrics as separators. These are usually still needled or hydroentangled to obtain certain properties, e.g. a larger surface by means of felting. Depending on the respective manufacturing process and field of application, nonwovens are subsequently further equipped in order to generate additional special effects: Hydrophilization (increase in fluid intake capability) and ceramic equipment are just two options for increasing the efficiency of a battery.

Separators for rechargeable or secondary batteries

In lead-acid batteries, materials are used which withstand the highly acidic and oxidative conditions. Extruded or sintered separators made of polyethylene, sintered PVC or mats made from microfiber fleece are used here. For nickel-cadmium batteries in the strongly alkaline milieu of potash, separators made of polyamide and polyethylene/polypropylene combinations are predominantly used. Here, nonwovens are almost exclusively in use today.

The conditions for nickel-metal hydride batteries are the same as for nickel-cadmium batteries, except that the battery separator also has to be able to reduce self-discharge. This is achieved through functionalization of the nonwoven surface by means of chemical treatment, e.g. surface treatment with acrylic acid or sulfonation.

Just an imageApplications in electromobility: high capacity, rapid chargeability

With rechargeable lithium batteries (Li-batteries), membranes are generally employed. These are microporous films which can partly consist of several layers. They have low temperature resistance and, despite the use of ceramic layers in separators, can break very easily. Research is currently being conducted into materials based on very fine nonwoven fabric that has been ceramic-coated. This makes the separators more temperature-resistant and less prone to breakage, and promises increased safety for rechargeable storage media that have long-term stability and are operationally reliable – especially for applications in electric vehicles.

Separators for non-rechargeable or primary batteries

With non-rechargeable Li batteries or Li-primary batteries, either nonwovens or microporous films are used. In non-rechargeable alkaline manganese batteries, nonwovens are mainly used as separators in a mixture of polyvinyl alcohol microfibers (PVA) and cellulose. Similarly, laminates made from nonwoven fabrics and membranes such as cellophane are also utilized.

The ideal needle for diverse applications

Depending on the requirements placed on nonwovens as battery separators, Groz-Beckert offers numerous opportunities to achieve the required properties or optimize the separator through utilization of the right needle. Due to the high process stability of Groz-Beckert products and the resulting continuously high level of quality, effects can be generated in the manufacturing process that are just as continuous and reproducible. This is reflected directly in the separator's performance in the battery.

The choice of suitable needle type depends on the raw material used, the fiber fineness, and the desired effect. Here Groz-Beckert, with its many years of experience in the nonwovens sector and numerous exclusive needle types, can make application recommendations. The GEBECON®, EcoStar and CrossSTAR® needles are just a few of the needle types that have proven themselves in separator production.

A common needle-machine configuration in the nonwoven line for producing battery separators often consists of pre-needling followed by the needling process itself.

In pre-needling, conical needles with a barb distance of 4.80 mm and the RF barb shape are often used. Conical needles with these specifications combine a high needling effect with defined and gentle fiber transportation. Groz-Beckert felting needles with a conical working part differ from standard working-part needles because of their much higher stability.

When using very fine fibers, microfibers or ceramic fibers, the HL barb is recommended for the pre-needling phase, since it is very gentle on the fibers and minimizes breakage and elongation. GEBECON® needles are also suitable for pre-needling, especially in the needling of fine fibers and microfibers, since they reduce the risk of needle breakage to a minimum.

In the main needling process itself, standard felting needles with a C barb distance and RF barb shape are mostly used. These guarantee effective compaction while providing the opportunity to precisely adjust the pore size and the resulting permeability.

During the main needling process, with EcoStar and CrossSTAR® needles, certain effects (porosity, pore size, permeability, hydrophilization, etc.) can be influenced in the product.

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Jet strips

Groz-Beckert jetstrips are ideal for battery separators that are hydroentangled in order to produce a larger surface area. With sustainable care and expertise for functional parts, Groz-Beckert jetstrips are available in all standard dimensions and hole geometries.