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Posted on 01 October 2019

Tailor-made Lithium Polymer Batteries

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Flexible in Shape and Size

The growing use of mobile electronic devices is generating an ever-increasing demand for high performance primary (disposable) and secondary (rechargeable) batteries. Applications with special needs regarding performance and design of the electrical energy storage unit call for solutions, which cannot be provided by standard battery products.

By Dr. Peter Gulde, Integrated Power Systems, Fraunhofer Institut für Siliziumtechnologie

 

Standard battery products are available in large quantities for high volume products in information and communication technologies (e.g. mobile telephones). High energy density (long operating time), but particularly the price plays an important role. If, however, specific requirements regarding electrical performance, design or application requirements are placed on the electrochemistry energy storage unit then the battery, like all other system components, must be specially designed and manufactured to match these very specific profiles. The Fraunhofer Institut für Siliziumtechnologie (Fraunhofer ISIT) offers a construction kit for the design of application optimized lithium polymer secondary batteries.

Battery Construction Kit

Experts at the Integrated Power Systems division of the Fraunhofer Institut für Siliziumtechnologie (Fraunhofer ISIT) in Itzehoe, Germany have developed a construction kit for tailor-made lithium polymer batteries, which makes ‘rapid prototyping’ possible. This construction kit addresses industrial customers with requirements that cannot be matched by standard battery products, but instead call for a battery with specific characteristics. For example:

- a customer specific electrical performance profile (e.g. voltage requirements, energy density, operating environment)
- specific reliability requirements (e.g. longterm stability, cycle stability, self-discharge)
- safety requirements (thermomechanical stability, short-circuit safety, leakage safety)
- a housing technology and/or design to match the usage profile,
- a high degree of environmental compatibility
- a safe and cost-effective production

The Fraunhofer Institut offers complete services ranging from design consultancy to prototype production of lithium polymer (Li- Polymer) batteries (Table 1). The first step is the definition, and as far as possible the quantification of the most important characteristics of the electrical energy storage unit in close cooperation with the customer. Following this, the specialists prepare a feasibility study with the goal of finding an optimal solution to meet the stipulated priorities and technical realization of the respective application.

Capabilities at Fraunhofer ISIT

The Fraunhofer Institut’s flexible production system is unique. It encompasses, for example, the design of new electrode foils and the prototype production of the batteries in various shapes and sizes. Format changes can be performed very simply, because only a few tools need to be replaced. Fraunhofer ISIT can manufacture, test and - if required - characterize up to 100 samples. For an industrial scale production, the design is transferred to Leclanché Lithium GmbH, a recently established company from a Fraunhofer-ISIT spin-off.

Lithium Battery Technology

Compared with other battery types, lithium battery systems offer a high energy density (Figure 1). Lead acid batteries, which are mostly used in automobiles, are particularly inexpensive, but their energy density is in the lowest range. Environmental concerns cause a gradual disappearing of nickel cadmium batteries from the market. However, because of their high performance capability and good low temperature characteristics they are currently still the first choice for some applications. The outstanding energy density of rechargeable Lithium batteries is a result of higher voltage in combination with a very good charge density (energy = voltage x charge). A number of compounds containing lithium allow for an utilization of a wider range of the electromotive series and therefore, depending on the choice of the electrode materials, a particular discharge voltage (e.g. system graphite/LiCoO2 with 3.7 V per cell).

Comparison of different battery technologies in terms of volumetric (Wh-l) and gravimetric (Wh-kg) energy density

A rechargeable lithium battery consists of a positive cathode (ion source), generally a lithium metal oxide, and an anode, usually graphite (Figure 2). A separator in between separates anode and cathode and only allows ions to travel through it, but not electrons. The thickness of this ion conducting separator, ranging from 10 µm to 50 µm, defines the distance between anode and cathode. In the charge mode of the battery, positive lithium (Li+) ions travel from the cathode through the separator to the anode. Simultaneously, negative electrons migrate through the external circuit from the cathode to the anode. In the discharge mode of the battery, the external electric current is used to power electronic devices.

Functional principle of lithium ion, ‘rocking chair’ battery

The Lithium Ion Battery

Lithium ion batteries use a wound ‘jelly roll’ construction. The anode, cathode and separator are wound up on a reel core. A liquid electrolyte, consisting of organic solvents in which lithium salt is dissolved, is used in lithium ion batteries to achieve high ionic conductivity. This solvent is, however, very sensitive to moisture and the battery must therefore, be hermetically sealed. A rigid metal casing is usually used to prevent leakage and ensure a good electrical contact of the layers. In addition to the shape and size restrictions related to this technology there is also, particularly for smaller batteries, a weight disadvantage.

Lithium Polymer Batteries – Flexible Form Factor

Lithium polymer batteries consisting of foil layers contain active material, which is embedded in a polymer matrix. These foils are arranged like a ‘sandwich’. The so-called bicell design consists of a central anode coated on both sides. This electrode is covered by separators, which ensure the separation from the outermost cathode (Figure 4). This configuration substantially increases the energy density of the battery. Experts at the Fraunhofer Institut use a specially developed, robust separator, which is filled with ion conducting ceramic and therefore, increases the effect of the liquid electrolyte. The foils are laminated. Following this, the battery cell is packed in a multi-layer aluminum plastic foil, whereby the metal terminals for electrical contact to the collector are brought outside of the foil packaging (Figure 3). This is then filled with a liquid electrolyte, which is completely absorbed just like in a sponge. The electrolyte cannot leak out, even if the light weight, flexible housing is mechanically damaged. The collectors are metal wire mesh so that the electrolyte can distribute itself in the battery cell very quickly and evenly, also in large area batteries.

Set-up of lithium polymer battery with foil material

Due to the layered construction, lithium polymer batteries with completely flexible designs and in very large formats are possible. Additionally, the systems designed by ISIT are very robust. They are bendable up to a battery thickness of approximately 1.5 mm. This is an important feature for many new applications, for example in smart clothes.

Electrode films and pre-processed collectors for the production of ‘sandwich type’ lithium polymer batteries

Batteries for Medical Applications

The Fraunhofer Institut ISIT currently offers two lithium polymer battery systems, which differ in the anode materials used. 3.7 V (nominal discharge voltage) optimized batteries with a high energy density of greater than 350 Wh/l and a high performance density use a graphite anode and cobalt oxide cathode. They are suited for applications requiring high energy content with a small size, e.g. hearing aids (Figure 5). When robustness and durability are the main criteria, the 2.3 V system with a lithium titanium anode is the preferred choice. The cycle stability of this system is better than 85 % / 4,000 cycles and the self-discharge is well below 5% per month. These rechargeable energy storage units have been designed for medical applications, such as implants, where a minimum lifespan of 10 years is required.

Rechargeable lithium polymer battery for hearing aid

 

 

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