Elektrochemie der Batterien
Innovative Electrochemical Energy Storage
Novel electrochemical energy storage technologies like rechargeable lithium-ion batteries (LIBs) represent a step further with respect to more conventional chemistries (Ni-MH, Ni-Cd, and lead-acid batteries). Up to this point, LIBs are the storage technology of choice for consumer electronics (cell phones, laptop computers, etc.) as well as for large-scale applications, such as short-range electric and hybrid vehicles and stationary energy storage.
However, new concepts are needed to reduce the cost of the battery production as well as to develop new cell chemistries capable of offering high capacity and/or power in connection with improved safety and the use of environmentally friendly materials.
Fields of Research
For the improvement of current lithium-ion batteries technology, our group focuses on the investigation, optimization and development of existing and new active and inactive materials. We design, synthesize and investigate new nanostructured active materials and active material composites, allowing increased specific capacities and enhanced power densities. Although energy is a must, safety, recycling and environmental friendliness are required for sustainable large-scale markets. The use of safer conducting electrolyte salts, non-toxic materials and green processing are under development in our group.
As the lithium-air batteries battery offers a very high theoretical specific energy, the concept requires, for practical applications, solutions to numerous challenges. To cite a few: Li electrode operation, carbon cathode and electrolyte oxidation, as well as slow mass transport of O2 to the cathode, for which a flow cell is being developed to allow the operation with a fully flooded (and fully loaded) cathode to reach practical target.
Sulfur as a cheap, abundant and high capacity conversion material is a very promising cathode. The challenge in this battery system is the solubility of lithium polysulfides, formed during the charge, which leads to rapid degradation of performance. Starting from Li2S, we synthesize carbon coated particles to encapsulate the sulfur and therefore enhance the cycle life of lithium-sulfur batteries.
Sodium-ion batteries represent a relatively unexploited research field which has gained scientific and commercial interest within the last years. Indeed, it offers an appealing combination of lower cost and good electrochemical performance, providing an alternative to lithium-ion batteries in low cost applications or in fields where energy density is not a crucial issue.
In this exciting field we are working on the characterization and understanding of sodium-ion batteries, the development and improvement of battery components like positive and negative electrode materials and the use of various electrolytic solutions.
Magnesium might be an attractive alternative for lithium-ion battery technology because Mg is abundant, cheap and lightweight. However, the present electrolytes for this cell chemistry are neither safe nor environmentally friendly. Ionic liquids (see below) might solve these issues. In our group, we investigate and characterize mixtures of magnesium salts and ionic liquids as Mg electrolytes and develop methods for their characterisation.
Electrostatic charge storage offers high power performance. Electrochemical double-layer capacitors (EDLC) and lithium-ion capacitors (LIC) can provide large current in a very short time, resulting in high power density. Our efforts in this field are focused on the synthesis of graphene-based active materials and fluorine-free ionic liquid electrolytes as well as on the development of environmentally friendly methods for the production of composite electrodes, containing natural polysaccharide-based binders.
Deep understanding of insertion chemistry is indispensable for the improvement and development of positive and negative electrode materials for secondary batteries and lithium-ion capacitors.
We focus on the synthesis (co-precipitation, solid state, xerogel, etc.) and the structural and electrochemical investigation of various transition metal compounds as well as carbonaceous and titanium based materials.
“Next generation” high energy batteries such as lithium-air or lithium-sulfur are generally deviating from the classical lithium-ion intercalation and insertion chemistry and store lithium by a so-called “conversion reaction”.
On the anode side, conversion anodes such as transition metal oxides are attracting interest for li-ion batteries, due to their high specific capacities exceeding that of graphite. The initial electrochemical reduction of the oxide forms transition metal nanograins that enable the reversible formation of Li2O.
A new class of lithium-ion anodes combines the conversion and alloying mechanisms. These materials offer significantly increased specific capacities, enhanced rate performances, stabilized cycling, as well as the tailoring of the operating potential range. Examples are, for instance, carbon-coated ZnFe2O4 nanospheres or transition metal-doped zinc oxide nanoparticles, which show specific capacities of about 1000 mAh g-1 and exceptional rate capability.
Most of the electrodes manufactured and investigated for application in batteries are composite electrodes. These electrodes are mainly composed of “active” (cathode or anode) materials, which are primarily responsible for electrochemical energy storage, but also contain smaller quantities of “inactive” compounds like conducting agents, binders or other additives. The synergy of these different compounds significantly influences the mechanical and electrochemical properties of battery electrodes. Within recent years we particularly focused on the research and development of environmentally benign, cost-efficient and biocompatible materials allowing advantageously the aqueous processing of lithium-ion electrodes.
Ionic Liquid Electrolytes
We design, synthesise and investigate room temperature molten salts (“ionic liquids”). They combine high electrochemical stability and ionic conductivity with low vapour pressure and low flammability, all being properties that are desirable for safer battery electrolytes.
While allowing lithium metal anode use, polymer electrolytes feature desirable properties such as thermal and mechanical stability, safety, as well as good processability. Integration of ionic liquids into polymeric matrices improves the ionic conductivity, especially at low temperatures. Our main efforts are focused on the improvement of PEO and new polymer electrolyte systems for leakage-free batteries.
These devices are characterized by the active materials being dissolved or suspended in a liquid phase. This allows external storage of the ‘liquid electrodes’ limiting the weight of inactive components and, in addition, decoupling power and energy. Moreover, flow batteries offer the possibility of adjusting the chemistry of the system during the operating life.