Lithium Americas is buying Argentina-focused Millennial Lithium for $400-million in stock and cash, eclipsing an offer from China's Contemporary Amperex Technology Co Ltd (CATL) as demand for the electric vehicle (EV) battery metal surges worldwide.

The deal, announced on Wednesday, comes a day after a deadline expired for CATL to respond to the Lithium Americas offer for Millennial. CATL is the world's largest EV battery manufacturer, but does not produce any lithium.

Shares of Lithium Americas rose 3.5% on Wednesday, while shares of Millennial fell about 3%.

Both companies have tussled in recent weeks over Millennial, though Lithium Americas said it believes the fact that it is based in Canada worked to its advantage.

"This transaction is a lower regulatory risk than CATL or another Chinese company. Like it or not, there's critical mineral strategies by the US, Canadian and Australian governments that could have played a part in this," Jon Evans, the Lithium Americas chief executive, told Reuters.

Evans said he does not expect CATL to challenge the deal.

"This is a great expansion opportunity for us in Argentina," he said, adding the company hopes to start construction on Millennial's Pastos Grandes lithium brine project within two years. The project is expected to produce 24,000 tonnes annually of battery-quality lithium carbonate for 40 years.

Lithium Americas has been building the Cauchari lithium project with Ganfeng Lithium Co - its largest shareholder - near the Millennial lithium deposit, which made the buyout even more appealing, Evans said.

"I don't think you're going to see opportunities like this going forward, at least not at valuations that we have here," he said.

Lithium Americas is also developing the Thacker Pass lithium mine in Nevada, though that project has faced legal setbacks. The company will publish a definitive feasibility study on Thacker Pass in 2022, Evans said.

CATL declined to comment when asked by Reuters if it would challenge the deal. Millennial had given CATL until November 16 to revise its offer to match the proposal from Lithium Americas.

"CATL will invest in major upstream mineral resources in an all-around way in order to ensure the supply chain security and the long-term stable delivery of our products, and avoid the impact of skyrocketing prices of certain upstream minerals," it said in an e-mailed statement.