Sinergy Flow, a deep tech start-up, is working on innovating the stationary storage sector by developing a sustainable, low-cost, high-efficiency flow cell battery capable of storing electricity for more than 20 hours. The solution ( patent pending) is a flow cell battery using innovative chemistry based on abundant and inexpensive materials, in full compliance with Circular Economy principles. In fact, the possibility of exploiting sulphur-rich by-products from industrial processes guarantees an extremely low installation cost, up to 30 times lower than solutions on the market today. The technology is characterised by high round-trip efficiency (> 70%), long lifetime and very low installation costs (30-150 $/kWh) compared to the main technologies available on the market today (200 $/kWh of Li-ions, 500-600 $/kWh of Vanadium and 700-900 $/kWh of Zinc-Bromium). Sinergy Flow is mainly targeting the market of long-term stationary storage, for off-grid applications and for integration with medium- and large-scale (kWh-MWh) renewable generation plants (wind, solar) in order to provide green electricity 24/7 to all end-users, thus supporting the current energy transition. The start-up is now in discussions with possible partners to conduct field tests to validate the technology.