The reason that they use Lithium over Sodium in batteries is due to the energy density that the Lithium contains. Here's and article from ezine...
Lithium Ion Batteries - What You Need to Know About Li-Ion Technology
Your question, MPaul, isn't specifically answered. However, if you are a student of chemistry, you know that both Sodium and Lithium react Violently when in the presence of water, and if I recall correctly Lithium is just more stable in that arena than Sodium. In fact I am not sure if it's possible to create NaFe2PO3 like the Lithium can. Ignore the above ramblings, it pre-coffee time....
Let's hope we all find an answer to that question soon, it's going to bug me all day.