A company that two years ago was one of the most promising U.S. innovators in the clean-fuel auto industry was rescued from collapse Wednesday. Its buyer: A Chinese auto-parts company.

Wanxiang Group Corp., one of China's biggest parts makers, offered a $450 million lifeline to A123 Systems Inc., a maker of advanced batteries for electric vehicles that received U.S.-government backing. The deal would put the firm's lithium-ion technology and its U.S.-funded manufacturing plant into the hands of a company that has slowly acquired...