Making a sodium sulphur battery
Researchers at Vanderbilt University in Nashville, Tennessee, have developed a room-temperature sodium sulphur battery based on a microporous template made from table sugar that is competitive with lithium-ion batteries. The device, which has over 98% Coulombic efficiency with less than 0.02% decay/battery charging cycle, could be used in electric vehicles but perhaps also for extremely cheap energy storage in a renewable power grid.
The sodium beta battery was first developed in the 1960s by the Ford Motor Company and their design involved a sodium anode, a sulphur cathode and a beta-alumina solid separator. However, the battery worked at high temperatures of over 300 °C (to allow for sodium wetting on the beta alumina so that sodium ions could diffuse to the cathode). This meant that it could only be used in niche applications.
Researchers have not forgotten about sodium sulphur batteries though and have been trying to make one that works at room temperature. A team led by Cary Pint has now developed one such device using cheap, readily available ingredients. The new device is even competitive with Li-ion batteries in terms of performance – and it is cheaper than them too.
“This cost aspect is important,” explains Pint, “since even the cheapest Li-ion batteries on the market today are two to three times more expensive than the costs of residential energy (about 12 cents per kWh in the US). This means that they cannot practically be used for energy storage on electric power grids until their cost can be brought down.”
On the technical side, our work also overcomes the limitations of previous sodium sulphur battery designs, which involve a cathode/electrolyte/anode configuration that is unstable either because of the cathode or the electrolyte, he adds. “We solved this problem by confining sulphur into the micropores of the sugar-derived carbon that we made using a novel isothermal infiltration process developed in our group. We also exploited the stable sodium–electrolyte interface, made from glyme electrodes.”
The result is a battery that is very stable and which can be recharged/discharged over more than 1500 cycles at fast rates – equivalent to those that would be needed for grid-level storage.
The researchers made their battery by dehydrating ordinary table sugar using acid and heating it to make microporous carbon. They then used a vapour-phase capillary force method developed in their lab to infiltrate sulphur into these tiny pores. “We then combined this material with a glyme electrode and a sodium anode, and this is the basis of our battery design.”
Storing energy on a renewable power grid
The battery works by shuttling sodium ions from the sodium metal anode to react with the sulphur in the cathode and form sodium sulphide. “In our design, the anode and cathode form stable interfaces, so this reaction can occur many times without loss of sulphur or the risk of dendrites forming that can damage the electrodes,” explains Pint.
While our battery could be used in electric vehicles, our target is extremely cheap energy storage for use on a renewable power grid,” he told nanotechweb.org. Efficient energy storage will play a critical role in the development of intermittent renewables, but current Li-ion battery technology is still far more expensive than conventional energy sources (such as fossil fuels). We will not see a future renewable-energy grid without solving this challenge first, and we believe that our work provides the ingredients to tackle this challenge.”
Sodium sulphur battery start-up?
Spurred on by its results, the team says that it is now planning to focus on manufacturing its batteries – something that will require overcoming several challenges, such as improving microporous sulphur loading and improving areal loading of the cathode materials.
“Our goal is to produce cells that perform better than state-of-the-art Li-ion batteries and which cost less than $0.02 per kWh over their cycle lifetime. When we have reached these figures, we anticipate starting up a company to bring this innovation to the marketplace,” says Pint.
The new sugar-derived room-temperature sodium sulphur battery is detailed in Nano Letters DOI: 10.1021/acs.nanolett.6b05172.