The liquid metal battery

03 February 2012 - Concepts

A magnesium-antimony Liquid Metal Battery? Sure, why not. Here is how Bradwell, Kim, Aislinn, Sirk and Sadoway made one (DOI) starting from a 90 mm hollow carbon rod. At the bottom of the cylinder is a tungsten pin as the positive current collector. The cylinder (diameter 40 mm) is lined with a layer of boron nitride for insulation, filled with a layer of pure antimony, then a layer of electrolyte salt (MgCl2-KCl-NaCl), then filled with a layer of magnesium metal and finally filled with a capping epoxy layer. A stainless steel pin on the top is the negative current collector.
This battery has an operating temperature of 700°C. In discharging mode magnesium is oxidized to Mg2+ ions which dissolve in the electrolyte salt. On the other side these magnesium ions are again reduced to magnesium which dissolves in the antimony phase. Both electrolyte salt and the resulting alloy at 700°C are liquids but do not mix. The driving force for the battery to run is a difference in chemical potential between solid magnesium and a magnesium / antimony alloy.
Compare this mode of operation to the way a common nickel-cadmium battery works: cadmium is oxidized to cadmium hydroxide on one end and nickel(III) oxide is reduced to nickel(II) hydroxide on the other side. Common potassium hydroxide is the electrolyte in between.
The main attraction of the liquid metal battery according to the researchers is that it is resistant to cracking, in this way it is a self-healing material. This type of battery already existed but this particular one uses less expensive metals.