July 12 (Bloomberg) — Ioxus Inc., a closely held maker of energy-storage devices, received $15 million in an equity offering with six unnamed investors, according to a filing.

The company is seeking to raise $20 million, according to a document filed with the U.S. Securities and Exchange Commission today. Chief Financial Officer Henry Barber didn’t immediately return a phone message seeking comment.

Ioxus, based in Oneonta, New York, is backed by Braemar Energy Ventures, Northwater Capital Management Inc., Aster Capital and Energy Technology Ventures, a joint venture between General Electric Co., ConocoPhillips and NRG Energy Inc. Aster is sponsored by Alstom SA, Schneider Electric SA and Solvay SA, according to its website.

The company makes ultracapacitors that rapidly absorb and release high-voltage power, according to its website. The devices can store power generated by solar and wind farms or be paired with batteries in electric vehicles to capture energy from braking.

To contact the reporter on this story: Andrew Herndon in San Francisco at aherndon2@bloomberg.net

To contact the editor responsible for this story: Reed Landberg at landberg@bloomberg.net