Nickel-based cathode has higher energy density for longrange vehicles, for Tesla
Standard-range vehicles and stationary storage will move to iron-based battery cathodes, Musk thinks. He thinks the majority of batteries in the future will be iron-based, so there won't be any shortage. It's just a question of making the equipment to process the iron into a cell and then into a pack.
Nickel isn't rare, but there's about 10-100x as much iron as nickel.
(Cobalt-based for phones and laptops.)
Lithium makes up about 2% or so of a battery cell, but lithium is also not rare (basically its a salt, and there's a little bit everywhere).