Electrochemical Energy Storage Materials

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Insertion and Intercalation Materials

The vast majority of commercial lithium batteries is based on the use of insertion-type or intercalation-type electrode materials. Such materials can reversibly host the charge carrier cations without substantial changes (or the degradation) of the crystal structure. Increasing the reversibility of this process and enhancing the stability of the interface with the electrolyte, especially at very low and very high voltages, is of essential importance to further improve the performance of such materials. In addition to the development of a better understanding and the modification and optimization these compounds (including the realization of fine-tuned electrolyte compositions), we are working on a new insertion-type mechanism, for which we are introducing highly redox-active elements into such structurally stable frameworks in order to boost the achievable capacity by reversibly hosting substantially more charge carrier cations.

Conversion, Alloying and Conversion/Alloying Materials

Targeting higher specific capacities and (potentially) higher energy and power densities, we are also studying alternative charge carrier storage mechanisms, including conversion-type and alloying-type materials as well as materials that combine these two mechanisms in one single compound – so-called conversion/alloying materials. While (understoichiometric) silicon (oxide), for instance, has already reached the commercial stage – despite the extensive volume variation upon cycling, conversion materials are still at the research & development level. Of particular importance in this field is the identification of the origin of the extensive voltage hysteresis between the charge and discharge process, which is intrinsically limiting the eventual energy efficiency of the corresponding full-cells. One approach to overcome the related challenges is the transition to conversion/alloying materials with the goal to synergistically combine the two mechanisms and their rather complementary benefits, including the development of completely new materials based on an enhanced comprehension of the reaction mechanism and the impact of the different elements comprised.

Lithium-Metal Electrodes and “Anode-free” Concepts

Lithium-metal electrodes and so-called “anode-free” concepts, which are eventually also resulting in lithium-metal electrodes upon charge when the lithium from the positive electrode is plated at the negative electrode, are the “holy grail” of lithium battery research, as they promise the highest energy density possible in combination with suitable cathode active materials. Our group is studying the impact factors that are determining the homogeneity and reversibility of the lithium deposition, with the objective to realize highly efficient lithium plating and stripping by stabilizing the interface between the electrode and the liquid or solid electrolyte.

Organic Active Materials

The replacement of inorganic active materials by more sustainable organic materials has attracted a continuously increasing attention in recent years due to the great progress that has been achieved in this field. In addition to the development of new organic active materials and completely metal-free organic (solid-state) battery cells, a major focus of our work is dedicated to the development of an in-depth understanding of the electrochemical charge storage and charge transport processes occurring in such materials – at the molecular, particle, electrode and cell level – in combination with the design of advanced electrode architectures.d cost efficiency. This work is accompanied by the investigation of advanced hierarchical secondary particle architectures and the implementation of nanostructured, electronically conductive matrices to ensure long-term cycling stability and high rate capability. Eventually, the overall target is the realization of demonstrator full-cells, comprising “next generation” electrolyte systems and cathode active materials.

Electrode Composites and Inactive Materials

Most state-of-the-art (rechargeable) battery electrodes are composed of the active material and additional (inactive) components such as a conductive carbon additive and a polymer binder as well as a (metallic) current collector. In fact, essential improvement of the energy and power density of commercial battery cells has been achieved by improving the electrode composition and the electrode manufacturing process. We are specifically investigating the potential use of environmentally friendly processes and the potential use of bio-derived inactive components, especially for lithium-ion cathode materials, for which this is particularly challenging.

Polymer and Hybrid Electrolytes

Solid-state electrolytes such as polymers are considered key for the use of metallic anodes such as lithium, potentially hindering dendritic metal deposition. Additionally, they may enable an enhanced safety as well as good processability and adhesion to the metal electrode surface. Polyether-based electrolytes are, as a matter of fact, commercial already, but the operation of such lithium-metal-polymer batteries is limited to elevated temperatures around the melting point of the polymer. Moreover, the rather low stability towards oxidation commonly does not allow for the use of high-energy cathode materials operating at potentials beyond 4 V. Besides a further understanding of the charge transport mechanism in these electrolyte systems and the processes taking place at the interface with the metallic anode, our work is particularly focused on single-ion conducting polymer-based electrolytes, which enable the use at ambient temperatures and the combination with state-of-the-art lithium-ion cathode materials (e.g. NMC₈₁₁), including the realization and in-depth understanding of alternative or modified charge transport mechanisms.

As science ideally does not know any barriers, we are collaborating with several international research groups, each of them having great expertise in their field, and we will be more than grateful to further extend these collaborative activities in future.