Plug in hybrids need to haul around an engine, gas tank, transmission, exhaust system etc. They also need extra batteries over a standard hybrid.

Those items aren’t made with as much extractive materials proportionally as EV batteries. If using PHEVs means more miles travelled are electric while less lithium is mined, great.

If you’re replacing the battery then you still need to extract those materials. Thus weight is only really meaningful in terms of vehicle efficiency not materials extraction. And of course having a plug in vehicle doesn’t mean it will be driven as a plug in vehicle where EV’s are limited to pure electricity.

There’s more aluminum and steel available above ground than lithium.

There is unused capacity, but all batteries in an EV are used slightly over a commute. That can lead to a much less damaging wear-pattern on the batteries, extending their life by a lot.