Re. the Wh/kg figure - If the sodium chemistry has properties comparable to lithium IRON (LiFePO4) in other respects, such as safety and robustness, then the 90Wh/kg figure is quite good in that comparison. The LiFePo4 chemistry is quite interesting and very stable and a drop-in-and-forget-about-it replacement for lead-acid battires, which is harder to do with the more sensitive and aggressive high-Wh Li-ion stuff. Afaik Tesla use LiFePo4.
Tesla is not currently using LiFePO4 cells. They've invested a lot into LiCoO2 because it has the highest specific energy, and therefore the lightest battery for the same range.
Swing and a miss! They don't use LiCoO2 because of the increased potential for fires. What Tesla uses is a modified version of NCA, or Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2). It still has the potential for fire, but it's not as volatile as LiCoO2.
Agreed. One of the reasons Li-ion (Lithium Cobalt Oxide) batteries are so expensive is the extensive quality controls that have to be performed on it. If you put 100 cells in a car and even 1% of those cells are faulty, your car catches fire. The worst that happens with LiFePO4 is the internal goo bubbles out of the battery and smells bad.
AFAICT, Tesla uses Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2) aka "NCA". It's safer than Cobalt, but not as safe as Iron.
