This is predicated on the assumption that Lithium is a scarce resource with new new sources available. In fact the opposite is true: Lithium is pervasively available almost everywhere[1], the problem is that it's expensive to extract and requires a bunch of processing facilities be built.

So you'd absolutely expect Li supply to grow along with demand and push prices down due to economies of scale. And that's exactly what we're seeing.

[1] Basically, go find a salt deposit -- that's a dried up ocean, which is the best concentrator we can find for Lithium compounds.