A couple of issues I wondered about are: Do we have enough raw materials that we would need to make this many Lithium-ion batteries? Aren’t they the ones that are used in vehicles...the ones that occasionally catch fire?
I don’t know what the reserves are for lithium, but I think most are controlled by China. I doubt there is enough lithium to make backup storage for the entire world. Recycling lithium batteries is difficult, so there needs to be improvement in technology in that area too.