[NeoVeles]

There are four stages of identifying and use of a resource.

Theoretical quantity - That would be the 20ppm as you said. Based on knowledge and estimates.

Identified reserves - How much of this stuff do we actually know about and is in a suffciently enough pooled location. This would be less than the 20ppm - how much less is a different question.

Technically available - How much of the known resource could we actually extract? It is ok if we know about it but if it is 10KM below a lake, could we get to it?

The most important stage after those three - Economic availability. Can we actually afford it and have people pay for it?

The argument from absurdity I use on this one is that there is effectively near infinite clean energy in the form of hydrogen in the sun. No one owns it - now go get it! The technical and economic scale ruins the argument a fair bit.

I'm not even arguing against lithium here, it seems to be more output restricted than resource limited. The two elements in batteries I worry about is Cobalt and Nickle - they could become the weakness. That said it does look like some folks are working on some neat alternatives in that space.