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News Article | December 1, 2016
Site: phys.org

Batteries and super capacitors are electrochemical energy storage media, but they are as different as night and day. Both are capable of energy storage and targeted energy release – and yet there are major differences between the two. Batteries store very large amounts of energy that is released slowly but constantly. By contrast, super capacitors can only store small amounts of energy, but they release this energy much faster and more powerfully with large short-term peak currents. Stefan Freunberger at TU Graz together with a group of researchers from Université de Montpellier in Southern France had a sudden flash of insight. Why not exploit the benefits of batteries and super capacitors simultaneously and combine them in some kind of energy hybrid, they asked themselves. In the current issue of renowned scientific journal Nature Materials the group introduces its approach, describing a liquid energy storage material for the first time in a European Research Council (ERC) sponsored study. While the energy density of this material is comparable to that of a battery, its power output equals that of a super capacitor. Ions with an urge to move "Batteries release energy so slowly and take so long to charge because their energy storage materials are solid. This make it difficult for the ions to move.  But as the ions in a super capacitor move in a liquid, they are much more mobile than in a solid body," explains Stefan Freunberger from the Institute of Chemistry and Technology of Materials at TU Graz. The novel redox active ionic liquid developed by Freunberger in co-operation with the French colleagues consists of an organic salt that is liquid at a temperature of just below 30 °C – only slightly above room temperature. Similar to a solid storage medium this liquid can store many ions, but allows them to be much more mobile. The sudden flash of insight of Freunberger and colleagues culminated in a first approach to create an integrated energy supply system that offers a constant energy supply with high-power output. In some cases we are still faced with an either/or decision. Automatic doors, for example in trams or trains, are typical candidates for super capacitors. Energy is only needed for a very short time but when it is, a high-power output is of the essence. In other cases batteries are clearly the first choice. "But our principle of an energy hybrid can offer enormous advantages, for example when applied in electric vehicles. So far, electric vehicles often carry a combination of different battery types or battery systems together with super capacitors. If we had a single system that combines the benefits of both energy storage types, we could save considerable space and resources," remarks Freunberger. More information: Eléonore Mourad et al. Biredox ionic liquids with solid-like redox density in the liquid state for high-energy supercapacitors, Nature Materials (2016). DOI: 10.1038/nmat4808

Brandstatter H.,Institute of Chemistry and Technology of Materials | Wohlmuth D.,Institute of Chemistry and Technology of Materials | Bottke P.,Institute of Chemistry and Technology of Materials | Pregartner V.,Institute of Chemistry and Technology of Materials | Wilkening M.,Institute of Chemistry and Technology of Materials
Zeitschrift fur Physikalische Chemie | Year: 2015

The monoclinic polymorph of Li2TiO3 (β-form) is known to be a relatively poor Li ion conductor. Up to now, no information is available on how the ion transport properties change when going from well-ordered crystalline Li2TiO3 to a structurally disordered form with the same chemical composition. Here, we used high-energy ball milling to prepare nanocrystalline, defect-rich Li2TiO3; ion dynamics have been studied via impedance spectroscopy. It turned out that ball milling offers the possibility to enhance long-range ion transport in the oxide by approximately 3 orders of magnitude. Its effect on the oxide ceramic is two-fold: besides the introduction of a large number of defects, the originally μm-sized crystallites are decreased to crystallites with a mean diameter of less than 50 nm. This process is accompanied by a mechanically induced phase transformation towards the α-form of Li2TiO3; besides that, a significant amount of amorphous materials is produced during milling. Structural disorder in nanocrystalline as well as amorphous Li2TiO3 is anticipated to play the capital role in governing Li ion dynamics of the sample finally obtained. © 2015 Walter de Gruyter.

"Currently, single systems of photovoltaic cells which are connected together—mostly lead-based batteries and vast amounts of cable—are in use. Solar panels on the roof with a battery in the cellar. This takes up a lot of space, needs frequent maintenance and is not optimally efficient," says Ilie Hanzu from TU Graz's Institute of Chemistry and Technology of Materials. "We want to make a battery and solar cell hybrid out of two single systems which is not only able to convert electrical energy but also store it." Hanzu and his team— in cooperation with Graz Centre for Electron Microscopy (ZFE)—are entering largely unknown scientific territory. In the SolaBat project, they want to develop a new, application-relevant concept and test its capability. The key to success lies in the new combination of functional materials. Hanzu explains: "In the hybrid system, high-performance materials share their tasks in the solar cell and in the battery. We need materials that reliably fulfil their respective tasks and that are also electrochemically compatible with other materials so that they work together in one device." Instead of environmentally damaging cobalt-containing electrodes, eco-friendly titanates will be used as the active materials. Polymer-based cells—in other words, organic solar cells— could also be used. "We have to know what happens when the materials come into contact with each other. For this reason, our project partner, the Centre for Electron Microscopy, is investigating the underlying fundamental interface effects and reactions," say Hanzu. The other three work packages of the project concentrate on materials for the photovoltaic side and the battery side as well as the compatibility of materials and the assembly of both components into one device. The advantages of a "two in one" hybrid system are obvious: It would be space saving, efficient and comparatively simple to manage. In the SolaBat project, the basics are being developed and tested, but even at this early stage, a variety of potential applications of such a system are on the horizon—from mobile batteries and car batteries to larger solar panels. Hanzu explains: "Our preliminary work was very promising and I'm confident that at the end of SolaBat, we will be able to present a working concept of a photovoltaic battery hybrid. Where, exactly, such a system will find application is too early to say, but the possibilities are in any case manifold." Moreover, different applications have different needs. "With batteries in micro applications or small appliances, such as smartphones, space saving is primary and weight secondary. In the case of car batteries, in contrast, weight is the most important parameter, space not so much." Explore further: New low-cost battery could help store renewable energy

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