This new type of molten salt battery could make it possible to develop a whole model of seasonal energy production and storage.
Renewables are likely to continue to become more and more important as the years go by. But contrary to what one might think, it is not always easy to optimize their exploitation, in large part because of the intermittent nature of solar and wind power. To meet this constraint, a team from the Pacific Northwest National Laboratory (PNNL) demonstrated a new type of “hibernating” battery capable of retaining its charge for months.
The concept is based on a molten salt battery. They have already existed for several decades in various and varied forms. Overall, they are uncommon in consumer devices; but are regularly used in certain particular contexts such as large-scale storage on the electrical network. They have the advantage of being inexpensive and easy to produce.
A “hibernating” molten salt battery
As the name of these batteries indicates, molten salts play the role of electrolyte here; it serves as a bridge through which electrons travel between the two poles of the battery. However, to ensure the free circulation of electrons, these salts must imperatively remain in a molten state. If the electrolyte is completely frozen, the electrons can no longer circulate between the anode and the cathode.
This has a very concrete and very constraining implication in logistical terms; unlike standard Li-ion batteries, molten salt batteries must be kept at high temperatures constantly. But instead of considering this particularity as a prohibitive limit, the PNNL researchers preferred to play on it to produce a “hibernating” battery.
The battery in question consists of an aluminum anode and a nickel cathode. The whole is then immersed in a bath of molten salts doped with sulphur. To charge it, it must be heated to about 180°C. When brought back to room temperature, the molten salts harden and the flow of electrons stops instantly.
92% charge after 3 months
But what is very interesting in practice is that the charge does not disappear during this transition. On the contrary: it is even preserved with a rather impressive integrity, and the flow of electrons can resume as soon as the battery is heated again.
The prototype would thus have retained 92% of its charge over a period of approximately 3 months. A figure not really impressive at first glance compared to standard Li-ion batteries, “It’s a bit like growing vegetables in the spring, freezing them, and reheating them in the winter.”, explains Minyuan Li, one of the PNNL researchers in charge of the project.
And this is not its only advantage. It would have an energy density of 260 Wh/kg. A figure that puts them overall above most standard Li-ion batteries; most of them cover a range from 100 to 265 Wh/kg, according to the Clean Energy Institute in Washington.
Obvious potential, but to be confirmed in real conditions
It would also be a very economical way to store energy; on a large scale, this process would only cost $23 per kWh. The researchers estimate that this price could even drop to around $6 per kWh once the set is optimized. For comparison, this is almost 6 times less than the average cost of Li-Ion storage, estimated at around $137 per kWh according to Bloomberg New Energy Finance. Their prototype measures the size of a hockey puck, but the researchers believe that this technology is perfectly compatible with large-scale use.
It remains to be seen how this will work in practice. Because energy, and in particular the battery sector, is a part of the industry where sensational promises are legion… but where the results are rarely up to expectations in real conditions. Despite everything, the researchers firmly believe in their concept and have already filed a patent.
They estimate that eventually, these batteries will become a true mode of seasonal energy storage. “You can start looking at things like big batteries mounted on tractors parked on a wind farm,” says Vince Sprenkle, co-author of the study, in an interview with New Atlas. “This battery would be recharged in the spring and then transported to an electrical station where the battery could be used if needed in the heat of summer,” they conclude.
The text of the study is available here.