Metal Deposition and Interfaces
Batteries with an electrode consisting of lithium metal exhibit a particularly high energy density and are already being sold commercially as button cells. In contrast to the lithium-ion rechargeable batteries used in portable electronics, such batteries with a pure metal electrode are not yet rechargeable. During charging, the lithium metal has to be redeposited from the electrolyte. In this process, the metal does not grow as a flat metal foil, as desired, but in the shape of a needle or dendritic, i.e., in a branched, bushy structure. Such lithium dendrites can lead to short circuiting and to overheating of the battery, posing a substantial safety risk. They can also grow like this in lithium-ion rechargeable batteries in use today if the latter are charged too rapidly or at low temperatures. HIU is therefore working to identify the mechanisms by which lithium metal is deposited on the electrodes and also how the deposition in lithium-ion rechargeable batteries can be recognized and prevented. This work is done, on the one hand, with model systems and structure calculations and, on the other, by testing commercial batteries. Also being studied are the conditions under which lithium dendrites grow and how their presence can be proven. This is ultimately supposed to make higher performance batteries possible that guarantee the highest level of safety even under nonideal conditions.
Insertion Materials and Electrode Structure
The by far most important type of rechargeable high-performance battery is the lithium-ion battery. In it, lithium-ions wander between the electrodes in one direction during charging and back during discharging. Insertion materials are used, which absorb the ions, which in turn temporarily store a host structure and release it again. The materials should be able to store as many lithium ions as possible as quickly as possible without this modifying its volume very much since larger alterations of its volume or its structure decrease a battery's lifespan. One of the goals of HIU's research is to develop new and improved insertion materials, for example ones that can store more lithium per gram of material. This would increase the storage capacity of a rechargeable battery without increasing its weight. Another goal is to develop, by means of comparing different materials and of theoretical work, an understanding of what makes a "good material". For example, we want to find out which structures the insertion material should have by figuring out, for example, which qualities limit the adsorption of lithium in different materials. To do this, we examine the structure of the materials analytically, e.g., using modern x-ray diffraction methods, and track how structures change during lithium adsorption and release. The ultimate goal is to identify, by means of knowledge-based search strategies, the materials and structures that make optimal power storage possible.
Lithium Based Conversion Materials and Alloys
In lithium-ion batteries, lithium is stored in a solid carrier, which imposes limitations on the amount of lithium stored per mass and volume. In innovative conversion materials, in contrast, chemical solid-state reactions lead to the formation of completely new phases with different properties. The hope is that this will result in a marked increase in the energy storage capacity at a lower weight. The researchers at HIU are currently concentrating on lithium-air batteries, lithium-sulfur batteries, and other innovative ideas based on, for example, metal fluorides.
In lithium-air batteries, lithium binds with oxygen from the air at the cathode. In theory, this makes a high energy density possible. The stabile operation of such batteries is guaranteed by a solvent. Scientists are therefore studying lithium-air batteries employing aqueous-nonaqueous solvents and ionic liquids. In the process, they study the complex structures and morphology of liquid (solvents), gaseous (oxygen), and solid (lithium-oxygen) combinations or phases.
In lithium-sulfur batteries, sulfur serves as the active material since it is especially inexpensive and environmentally friendly. During discharge, the sulfur is dissolved and reduced in several partial reactions. The solubility of the intermediate products poses a great challenge. The focus of the studies is on the nanostructure of the electrode material and the optimization of the solvent employed, the goal being to further enhance energy density and lifespan.
Metal fluorides offer theoretically the highest voltage and largest storage capacity at the cathode. Such systems are being studied at HIU, the focus being on iron fluorides. For their practical use, innovative scalable synthesis procedures were developed that can be used to create new composite materials with a very high capacity and good cyclical stability.
Batteries Beyond Lithium
Lithium-ion and metal hydride batteries are established systems that are currently being successfully employed for energy storage in electrically powered applications. In order to make future devices safer, less expensive, more sustainable, and more powerful, global research is looking for alternatives to the current systems. Lithium is supposed to be replaced by other elements which can also make bidirectional batteries possible. In order to attain this goal it is necessary for us to develop anew all the components of the battery and to acquire an understanding of the electrochemical processes.
Of the four new types of batteries that are currently the object of international research, which are based on using magnesium, sodium, chloride, or fluoride as the charge carriers, two (the chloride-ion and fluoride-ion batteries) were first presented by HIU. HIU developed the electrolyte that is currently the best for use in a magnesium battery; this has also made it possible to build the first magnesium-sulfur cells. With the exception of the sodium-ion battery, all of these systems have the potential of achieving markedly higher energy storage densities than the present lithium-ion batteries. HIU has played a pioneering role in these new fields of research.