Rio Tinto recently struck a deal to acquire Arcadium Lithium for $6.7 billion. The acquisition represents a significant wager by the miner on EV growth in the future—but it comes as EV demand has slackened off and lithium demand has followed. It could be a $6.7-billion mistake. Then again, it could be a long-term play for dominance in a key sector of the energy system of the future. Rio Tinto first announced the acquisition in October, offering a 90% premium to the UK-based lithium producer’s share price at the time of the offer.…