"$600K per MWh" sounds like the cost per peak (initial) storage capacity; as the batteries can be recharged something like six thousand times before wearing down to half capacity, that's more like adding $100 per MWh ($0.1/kWh) if you throw them out at that point or $50 per MWh (5¢/kWh) if they keep wearing down at the same rate until they're negligible.

> And are there that numbers of batteries available

Not yet — but there are enough people investing in each stage of mining, processing, and making battery factories, that this is probably not going to be an issue.

> Currently dependent on availability of cobalt, lithium,manganese and nickel with China as the main supplier of these and world’s largest producer of graphite, the primary anode material for Li-ion batteries.

Technically, I think, yes; but that's just where we are today rather than a fundamental property of world geography.