New Battery Concepts
The Fluoride Ion Battery
In search of new concepts to build batteries with high energy densities, electrochemical cells based on metal fluorides may be promising.
We have demonstrated the first reversibly working battery cells based on fluoride shuttle. In secondary fluoride batteries, fluoride anion acts as charge transfer ion between a metal/ metal fluoride pair where it will react with metal or evolve from metal fluoride depending on the flow of current. The theoretical capacity can be several times higher than that of conventional Li ion batteries, depending on the combination of metal and metal fluoride (see also: A.M. Reddy and M. Fichtner, J. Mater. Chem. 21, 17059 (2011) ).
Sketch of the principle architecture of a Fluoride Ion Battery.
Current research focuses on:
Synthesis of nanocomposites
Investigation of the transformation mechanism
- Improvement of reversibility
We have developed a new solid state synthesis method for fluoride based cathode materials by pyrolysis of metal organic precursors. The new material allows a stable cycling behavior over hundreds of cycles on a high capacity level (approximately 250-280 mAh/g), at room temperature. This is probably the first example of a conversion material for cathodes with high cyclic stability.
A novel single step Reactive Intercalation Process allows synthesis of carbon-FeF2 nanocomposites where the active material is clamped between sheetes of graphite. Graphite fluoride and iron carbonyl are used as a metal source. FeF2 nanoparticles form inside a matrix which initially acts as an F- donor and, at the same time, transforms from a non-conducting graphite derivative into an electrically well conducting graphitic matrix. The synthesis proceeds in one step and leaves no solid or liquid by-products so that the product can be used as electrode material in a battery cell without further purification or post-treatment.
Scheme of the process and cyclic properties of product:
Nanoconfined Fe3O4 in graphitic carbon produced by a simple pyrolitic one-step synthesis of Fe(CO)5.
The composite has a high cyclic stability as anode and works close to the theoretical capacity of 926 mAh/g.
The battery activities in different media
On the radio (in German):
Conversion Materials for Hydrogen Storage
Alanate Systems for Hydrogen Storage
Pure Mg alanate was synthesized in our group and the crystal structure of the compound was determined. We also synthesized Ca alanate and investigated its hydrogen storage behavior. In isothermal kinetic studies we showed for the first time that mass transport is the rate limiting step in the kinetics of the hydrogen ab- and desorption of alanates. In kinetic isotope experiments, AlHx was found to be the mobile species in the materials transformation. We also found that Ti13 clusters are world-record catalysts for alanate materials. X ray absorption experiments (EXAFS, XANES) shed light on the development of the near order around the Ti atoms and showed that the dopant forms small Ti-Al clusters during cycling.
Boranate Systems for Hydrogen Storage
synthesis of new boranates
structure of boranates
tuning the thermodynamics
enhancing the kinetics
- mechanisms of hydrogen exchange
Amide-Imide Systems for Hydrogen Storage
Amide-imide systems have shown promising results for hydrogen storage. Most importantly, various systems have been shown to be reversible. We have been investigating various combinations of hydrides and contributed to the understanding of the role of dopants and ball milling. Current research focuses on:
new materials combinations
- upscale production
Nanoconfined Hydrides for Hydrogen Storage
In many hydrogen storage systems it has been shown that using nano sized powders is essential for high reaction kinetics. Infiltrating hydrides into nano structures can be used to control the particle diameter and study size effects of the materials. Infiltration may considerably enhance the kinetics of ab- and desorption. Thermodynamic effects have been found, too. Current research focuses on:
infiltration and encapsulation methods
synthesis and investigation of nanocarbon templates
thermodynamic and kinetic properties
- upscale production
Combining computational techniques like density theory and experimental work has in the past shown to greatly benefit the research. currently in our group DFT is used to help to understand the decomposition of Mg(BH4)2. Current research focuses on:
Thermodynamics of the decomposition of Mg(BH4)2
Identification of intermediate species during decomposition of Mg(BH4)2