New Electrolytes with High Energy Density

Currently, the maximum energy densities of redox flow batteries are 70 Wh/L, which is substantially lower than those of Li ion batteries (200 Wh/kg, 400-500 Wh/L). There are three main targets to improve for the liquid systems: i) high molar concentration of the electroactive compound, ii) high number of electrons per formula unit of the electroactive component and iii) large electrochemical potential(s) of the involved steps. The demand for high durability and facile maintainability relates to high chemical stability and high resistance toward water or oxygen.

This led to the consideration of ionic liquids (ILs), which are readily available and highly chemical stable. In total, we anticipate that an increase by one order of magnitude is possible. Item i) we will be addressed by inclusion of the electroactive compound into the ionic liquid itself, either as the anionic or a cationic component. Considering typical concentrations of neat ionic liquids of 4-5 mol/L, this value is doubled compared to the concentrations of current H2SO4 electrolytes.

First targets will be anionic vanadium based polyoxometallates (POMs), which are known for their high electro-/chemical stability. Due to the multi-vanadium centers in these clusters more than one electron can be transferred per formula unit. Typically at least two and up to five electrons will be delivered. This represents a dramatic increase compared to V(II)/V(III) or V(IV)/V(V) one-electron systems, which are currently employed in redox flow batteries. The non-aqueous ionic liquid electrolytes provide typically large electrochemical windows. Combined with the large electrochemical potentials of the POMs, this will provide another boost for these systems.

To the best of our knowledge only one recently reported (Fe(II/III)) system by a research group of the Sandia National Laboratory is based on this approach. The proposed project is therefore at the frontier of research and will strongly profit from our experience in quantumchemical modeling and classical molecular dynamics, which will be applied to the simulation of the ionic liquids. Our own cyclic voltammetric studies of the systems will be supported by electrochemical BMBF partner and experts at the Karl-Winnacker Institut