Sept 15 (Reuters) - Sibanye Stillwater Ltd is buying half of ioneer Ltd's Nevada lithium mine project for $490 million, one of the largest deals ever for U.S. supply of the electric vehicle battery metal as demand is poised to soar later this decade.

Australia-based ioneer said it will form a joint venture with the South African miner to develop the Rhyolite Ridge mine, roughly 220 miles (355 km) north of Las Vegas. (Reporting by Ernest Scheyder Editing by Chris Reese)