Nov 5 (Reuters) - Lithium producer Livent Corp said on Thursday it had extended its supply agreement with Tesla Inc through the end of 2021 and would form a joint venture to buy Quebec's Nemaska Lithium Inc.

The company also posted a third-quarter net loss of $11.8 million, or 8 cents per share, compared with net income of $18 million, or 12 cents per share, in the year-ago period. (Reporting by Ernest Scheyder; Editing by Leslie Adler)