Demand for electricity fluctuates all the time, and power output has to be adjusted accordingly. With more renewable energy sources, the need for load balancing measures is increased even more. Grid energy storage would be one way to handle this, but with current battery technology, costs would be high. Liquid metal batteries, where electrodes made of cheap and abundant materials are kept molten at high temperature, might be a solution.

There are many ways to generate electricity – biomass, fossil fuels, hydroelectric power, nuclear, solar and wind, for instance. At all times, the electricity generated must match the current demand. Peaks or spikes in power demand are handled by peaking power plants; though responsive, they are often less efficient and generating higher emissions. Renewable electricity production is often intermittent, adding to the variability.

An efficient grid storage buffer would reduce the demand for peaking power plants and at the same time make it easier to incorporate large amounts of renewable energy. Large-scale batteries with high storage capacity and fast response times would be a convenient solution. (There are also other possible ways – read more in the article “Grid Energy Storage“.)

Battery development from a different angle

Solving grid energy storage with batteries would require improvements in technology beyond what is available today, though. The batteries would have to be long-life, able to handle very high voltages, and at the same time very cost-effective. But an MIT research concept – liquid metal batteries – might be a step in the right direction.

Ever since Alessandro Volta invented the first battery in 1800, the basic principle has been to put two metal electrodes in a conducting solution, an electrolyte, allowing ions to move from one electrode to the other. In Volta’s pile, the electrodes were pairs of silver and zinc discs, separated by salt water-soaked cardboard.

Since then, batteries have been developed by searching for new and exotic materials with interesting electrochemical properties – and then hoping for a more efficient manufacturing process to create a downward pressure on price. Professor Donald Sadoway’s idea, leading to the concept of liquid batteries, was to instead focus on price from the onset.

If you want to make something dirt cheap – make it out of dirt!

Sadoway realized that a cost constraint would make many rare and expensive materials unfeasible – no matter how good their electrochemical properties were. His approach was to look for cheap and abundant materials instead, and to try and find a simple, straight-forward manufacturing process. The production of aluminum came to mind.

An aluminum smelter, where aluminum is extracted from its oxide, consists of a long row of reduction pots electrically connected in series – a setup resembling Volta’s pile. In the electrolytic process, electricity drives the chemical reaction – and the metal and electrolyte are molten, maintained at high temperature.

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The liquid metal battery is maintained at high temperature. The molten electrodes and electrolyte separate into layers due to their difference in density.

Sadoway set out to create a high temperature battery, with liquid metal electrodes and a molten salt electrolyte, all kept in a container. With materials of different density, they would naturally separate into layers with the electrolyte in between the electrodes.

Lower temperature – higher capacity

In the first liquid batteries, magnesium and antimony were used for the electrodes. In later versions, the materials were replaced with a lead-antimony alloy together with lithium, lowering the melting point from 700 to 450 degrees. Larger and larger working batteries have been developed, and the aim is to connect refridgerator-sized modules in shipping containers for deployment, with a capacity of 2 MWh per container. This corresponds to the daily energy need of about 200 households.

The start-up Ambri has brought the battery design closer to the market, but there are still technical hurdles to overcome. For instance, designing the refractory lining and sealing to withstand the high temperatures has been more difficult than anticipated.

Whether the Ambri design will be successful or not remains to be seen. No matter what, there are many ways to make batteries, and the focus on low-cost, earth-abundant materials is an interesting approach. The responsiveness and flexibility of liquid metal batteries has brought large-scale grid energy storage closer to reality. Such applications differ from electric vehicles and consumer electronics in the capabilities needed, and liquid metals, distinct from other battery technologies, seem to offer unique opportunities.

The article was published in November 2015