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SOLSTICE answers the quest for stationary energy storage with two Na-Zn molten salt batteries, which operate at elevated temperature. The first concept benefits from the existing and successful ZEBRA® 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. Both battery concepts shall be brought to TRL5, and validated by four demonstrators, operating in a realistic environment at the end of the 4-year project. The demonstrators will be equipped with a self-learning battery management system and will be accompanied by upscaling, system integration and public acceptance studies. Na-Zn technology is exceptionally performant as it promises similar efficiency and depth of discharge as Li-ion cells, but 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 legitimately 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. Based on the existing knowledge on ZEBRA® battery production, the storage price of Na-Zn batteries is expected to approach 1 cent/kWh/cycle by 2030 - including balance-of-plant and recycling cost. Summing up, the Na-Zn technology is the cheapest molten-salt battery, is fully sustainable, fulfils all criteria of the call - and is even realistic to be commercialised by 2030.