http://phys.org/news/2015-03-na-ion-batteries-closer-li-ion.html

This should lead to a significant cost-effectiveness improvement as lithium is much more expensive than sodium.

But I still say that Mg-S batteries (if not Mg-air batteries ) will one day make most other types of batteries obsolete including both Na-ion and Li-ion batteries. This is only in part because the theoretical maximum energy density of Mg-S batteries is higher than that for any metal-ion battery that can be economical ( + both Magnesium and sulfur are relatively cheep and extremely abundant )