Musk has addressed this a few times. If I recall correctly Cobalt production will be more of a constraint than lithium... for example, from the Q1 2014 earnings call:

the lithium, although lithium is sometimes thought of as a bigger thing than it really is for lithium ion cell.

It’s like using maybe a couple percent of the cell mass, but the biggest cost constituent is nickel

There's a great analysis of material constraints here [1].

1. http://seekingalpha.com/article/654441-ev-myths-and-realitie...