An energy landscape shaped by renewable sources requires large amounts of stationary energy storage to balance supply and demand. Within Europe, storage options on various time scales and with a total capacity on the TWh scale will be necessary. None of the technologies available today fulfils all the requirements for an ideal stationary energy storage system. New technologies therefore need to be developed – with a focus on abundance and price of the raw materials as well as a circular economy.
The Horizon 2020 project SOLSTICE answers the quest for stationary energy storage with two Na-Zn molten salt battery concepts, which operate at elevated temperature. The first concept benefits from the existing and successful ZEBRA® (sodium-nickel chloride) technology. Replacing their Ni-electrode by cheap and abundant Zn will only minimally affect other system parts thereby ensuring fast commercialisation. The second approach, an all-liquid cell, will apply the same chemistry, but does not require a ceramic electrolyte thus reducing cost further.
The Na-Zn technology is exceptionally performant as it promises similar efficiency and depth of discharge as Li-ion cells, but under extreme current densities. Featuring molten electrodes, Na-Zn cells actually work better when being cycled, as operation keeps them warm; several cycles per day and a lifetime exceeding 10,000 cycles can be expected. Na-Zn storage is perfectly sustainable: the raw materials, table salt and Zn, are abundant in the EU, cheap and not harmful. The environmental impact of Zn-mining and battery production is expected to be minimal. Finally, recycling is greatly simplified due to the large, molten electrodes. The most valuable element, Zn, can simply be recovered as pure metal and reused after dismantling the cells.
The overall objective of the SOLSTICE project was to develop both battery concepts to finally obtain sustainable molten salt batteries for use as large-scale energy storage. The project has been divided into four large working areas. Firstly, the Na-Zn chemistry was investigated to obtain batteries with high performance and long life-time. Secondly, the battery system itself – including materials, housing and manufacturing – has been further developed. Thirdly, the Na-Zn battery and its potential as large-scale storage has been assessed, e.g. by a cost and social impact assessment. Finally, its sustainability has been improved by working on life cycle assessment and on the details of the recycling strategies.
