MELBOURNE—Prony Resources said on Wednesday Tesla Inc. had agreed to purchase around 42,000 tons of nickel in a multi-year deal—a pact that is set to make the New Caledonian miner a key supplier of the metal to the U.S. electric car maker. Prony, which bought the loss-making nickel operations in the French territory from Brazil’s Vale SA this year, added it aims to produce 44,000 tons of nickel by 2024, double its expected 2021 output. Electric vehicle makers have made securing sufficient supplies of nickel—a key ingredient in automotive batteries—a top priority and are particularly keen to seek out sustainable and low-carbon nickel supplies. The agreement with Tesla will make the carmaker by far Prony’s largest customer, Antonin Beurrier, the miner’s chief executive, said during a news conference. The 42,000-ton volume was indicative and could vary, he said, declining to specify the duration or other details of the deal. The …