Rio Tinto has doubled down on its lithium bet with a $US2.5 billion ($3.9 billion) investment in its first large-scale brine project, just months after agreeing to buy Arcadium Lithium for $US6.7 billion.

The iron ore miner’s near $US10 billion outlay on lithium – despite a near 20 per cent collapse in prices for the commodity over the year – is an ambitious bet that underscores Rio’s confidence in the long-term prospects for the battery commodity, which is crucial for global decarbonisation efforts.