The further development of electrical energy storage is of central importance to society since the increasing use of renewable energy sources and decentralised energy generation, for example using rooftop photovoltaic installations, demand improved storage. Lithium-ion batteries (LIB) are a promising technology for this purpose. LIBs are also being used increasingly in the field of mobile applications and in the automobile sector. Against this background, the Application Centre for Plasma and Photonics at Fraunhofer IST is working on improving the batteries. Special metal foils were developed to act as carriers for LIB electrode layers. The starting point for the corresponding project “Battery Foil with Carbon Contact Layer” (KoKon) is the minimisation of electrical resistances in electrodes. This is achieved with functional layers produced using plasma. In addition to the production of battery films, reliably determining the electrical properties of manufactured foils and battery electrodes is an important part of the project. A novel measurement system is being developed within the project in order to accomplish this: e-CON.
Resistance measurement system: e-CON
In electrical resistance measurement on objects with a rough surface, such as LIB electrodes, establishing lossless contacting is often not possible. Measuring tips or pins are typically used for this purpose. However, these are not only invasive but also cause “parasitical” voltage drops in the contact zone. Both can significantly distort the measurement result and impair the reproducibility of the measurements.
A contacting probe was developed by the Application Centre for Plasma and Photonics with the goal of realising non-destructive, stable and also low-impedance measuring contacts. Magnetizable micro-particles with a coating that is highly electrically conductive are used here. Under the influence of a carrier magnet, numerous such particles form elongated particle chains that are in contact with the magnet. Overall this results in a three-dimensional formation with antenna-like structures that exhibit high flexibility. When the probe is placed on the electrode surface, the formation deforms and adapts to the topography. Since the particles shift easily, a positive fit can be achieved without any mechanical strain on the object. With a suitable sensor for height positioning of the probe, the contact force on the measurement object can be controlled and regulated with z-axis control. Overall the largely lossless transport of electrons from the measurement object to the particle formation of the probe and therefore to the measuring device such as a micro-ohmmeter is assured. This is a key prerequisite for obtaining reliable measurements.
The project results to date show that the use of battery foil leads to an increase in the output of lithium-ion cells. In the further course of the project, the work is going to continue for foil and cell production as well as battery tests in a technical centre under industry-oriented conditions. The e-CON laboratory unit is currently being transferred to a compact, semi-automatic measurement system.
The research and development project “Battery Foil with Carbon Contact Layer” (KoKon) is conducted in cooperation between the Application Centre for Plasma and Photonics at Fraunhofer IST and the Fraunhofer Institute for Silicon Technology ISIT. Eleven companies make up the accompanying industry group. The sub-project of the Application Centre is subsidised by the AiF German Federation of Industrial Research Associations “Otto von Guerick” e. V. with € 215,750 (€ 117,600 in 2015).