Not a battery scientist, but both do seem to mention Sodium Nickel Chloride as a key feature so that may be exactly right. As the first article said, the chemistry is old (1968) - the part that is new is that they found a way to create a membrane that doesn't wear out quickly, a problem which handicapped the total number of charge cycles originally.
Edit: Found an
actual Zebra battery here. Says it is only good for 1,000 cycles, which agrees with the first article that original design was limited on charge cycles. Also, battery needs to be kept 270° and 350 °C. Probably more suitable to grid storage...