Modelling of mass transfer and electrode kinetics in liquid metal batteries

• Applicant: Dr.-Ing. Norbert Weber
• Subject Area: Fluid Mechanics
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 338560565

In frame of the German energy transition, 80% of all electricity shall be generated by renewable sources by the year 2050. Due to the fluctuating nature of wind and solar energy, the deployment of stationary energy storage will then become mandatory on a large scale. Such storage devices will have to fulfil a number of crucial requirements: low price (10ct/kWh), long life-time (7000 cycles), high efficiency (>80%) and for short-time storage: fast reaction times. Up to now, no technology fulfils all mentioned criteria; therefore, a development of new, or an improvement of existing storage devices will be necessary.A totally new, and very promising storage technology is the liquid metal battery (LMB) - built as a stable density stratification of two liquid metals, separated by a likewise liquid salt. Cheap raw materials, as Na or Pb and a simple construction allow for building very cheap cells. The completely liquid interior avoids micro-degradation and promises very long life-time. The optimal kinetics at the liquid interfaces allow for potentially very high current densities. These allow providing high power; however, the potentially possible current densities are not always achieved in practice.Objective of this project is an increase of current density and efficiency of LMBs. For this purpose, an electrochemical cell model shall be developed and coupled with an existing fluid dynamics model. It will be implemented in the open source CFD library OpenFOAM, also to simplify its use by other researchers. Later, an analytical and experimental validation is planned. In a first step the influence of different overvoltages (resistances as Ohmic losses, concentration polarisation, charge transfer) shall be evaluated. Subsequently, their dependence on temperature, current density, operation condition and geometry of the cell shall be explored. Finally, it is planned to analyse, how efficiency and current density can be increased by an artificial flow in the cathode. Such a mixing of the liquid interior was proposed several times for an improvement of mass transfer.The significance of the mentioned overvoltages was emphasised several times in the literature; however, their influence was never studied detailed or quantitatively, up to now. By coupling electrodynamics, fluid dynamics and electrochemistry, the model shall be the basis for a detailed simulation of LMBs. In the future it can be easily complemented, e.g. by a model for intermetallic phases, in order to study other processes in the cells.

• DFG Programme: Research Grants
• Co-Investigators: Dr. Frank Stefani; Dr.-Ing. Tom Weier