Energy storage system developer Form Energy announced a $450 million Series E financing round led by TPG’s global impact investing platform, TPG Rise. 

The financing includes amounts that are subject to regulatory approval and allocations to existing investors.

Marc Mezvinsky, partner at TPG, is also joining Form Energy’s board of directors.

Also joining the Series E round were GIC and Canada Pension Plan Investment Board, along with existing investors ArcelorMittal, Breakthrough Energy Ventures, Capricorn Investment Group, Coatue, Energy Impact Partners, MIT’s The Engine, NGP ETP, Temasek, Prelude Ventures, and VamosVentures.

Form Energy is working to produce what it said is a low-cost iron-air battery technology. Its goal is for the technology to store electricity for 100 hours at system costs competitive with legacy power plants.

The company said the battery’s basic principle of operation is reversible rusting. While discharging, the battery takes in oxygen from the air and converts iron metal to rust.  While charging, an electrical current is applied that converts the rust back to iron and the battery releases oxygen.

Each ibattery module is about the size of a side-by-side washer/dryer and contains a stack of 50 one meter-tall cells. The cells include iron and air electrodes, which enable the electrochemical reactions to store and discharge electricity. Each cell is filled with water-based, non-flammable electrolyte, like the electrolyte used in AA batteries.

Form said it has narrowed the site selection process to three states for its manufacturing facility, and expects to make an announcement before the end of the year.