Liquid metal batteries
With the growing role of solar and wind power in the German energy landscape, large scale storage becomes a key enabler for a functional power grid. In this setting, cost per unit stored energy and high number of charge and discharge cycles (low capacity fading) are the main criteria for a successful technology.
Liquid metal batteries, i.e. batteries in which both electrodes as well as the electrolyte are in the liquid state, are a very promising concept for economic storage. If abundant and cheap active materials can be used in large cells, benefiting from the economies of scale, the predicted total costs per unit stored energy are low and quite competitive.
A battery with fully liquid active interior has a number of advantages: when densities are chosen properly, the battery is self-assembling due to stable stratification. Liquid-liquid interfaces possess fast kinetics, thereby allowing for rapid charging and discharging, i.e., high rate capacity. Structureless (liquid) electrodes are insusceptible to aging providing nearly unlimited cyclability.
High current densities together with the large electrode areas of big cells imply a large total cell current and here electromagnetics together with fluid mechanics – i.e. magnetohydrodynamics – comes into play. The Lorentz force produced by the interaction of the cell current with it's own magnetic field can excite the Tayler instability (TI) as was recently demonstrated in our group by Seilmayer et al. (2012).
The vortical flow arising as a consequence of the TI may lead to a displacement of the molten salt electrolyte resulting in a direct contact between anode and cathode. A short circuit and battery failure would be the consequence. Depending on the aspect ratio of the cells, interface instabilities as known from, e.g., aluminium reduction cells might become an issue for LMBs as well.
We study these current driven instabilities experimentally and numerically and develop means to prevent them. The TI can be counteracted by the application of an additional magnetic field that superposes with the original one and modifies it's stability properties. A convenient source for the added magnetic field is the current drawn from the battery. Feeding it back through an isolated conductor at the cell axis makes the cell interior stable against any current.
Complex phenomena may arise in simulations due to the strong coupling between electrodynamics, fluid dynamics and electrochemistry and can make the computations quite time consuming. Large-scale experiments are foreseen at the DRESDYN facilty, which will provide the necessary infrastructure for safe liquid metal handling and flow measurement.
Research Group Liquid Metal Batteries
At our battery laboratory we are able to perform electrochemical testing of electrodes, molten salt electrolytes and operation of small scale cells. For scale-up and efficiency improvement, testing of different container and isolator materials is essential to enable long term operation of cells. Research activities on system integration, scale-up and on operation of liquid metal batteries are performed within the joint initiative Energy System 2050, where a network of eight Helmholtz Centres aimes to improve the understanding of energy systems and develops technological solutions for use by politics and industry.
This year, the first international workshop on liquid metal battery fluid dynamics (LMBFD 2017) is organized by our group and will be held on May 16th and 17th 2017 in Dresden. The focus is on fluid dynamics and other aspects of liquid metal batteries and related devices (e.g., aluminum reduction cells).