Iron flow battery startup ESS raised an additional $30 million to take its technology from pilots to commercial scale. Since 2011, the company has been developing a low-cost, nonflammable long-duration storage technology to compete across domains where the dominant lithium-ion battery chemistries are weaker. The bigger potential for flow batteries is large-scale renewable energy-shifting, and that requires moving into megawatt-scale territory. This marks the second energy storage investment from Japanese investment firm SoftBank. It made its first foray into the sector this summer with a splashy $110 million investment in the block-stacking gravity storage startup Energy Vault.