And recall that, early lithium batteries had a fraction of the longevity that current designs have, so, it's likely that sodium batteries have plenty of room for improvement in that regard as well.
In my mind, the likely application is grid-scale storage where density doesn't matter as much but upfront cost does. Not really so much for renewables, but more so that you can store your unused base load during offpeak hours for later use.
The other way to deal with low cycle count is to keep the cells only half-charged and minimize the excursions from that. A large pack that goes from 40 to 60 percent daily will last eons compared to one cycled from 90 to 10 percent.
From what I understand, the degradation of the cell is not linear with the discharge excursion. So you may have exponentially better battery life the narrower of charge band you keep it in. If anyone has more detailed information let me know, I'd like to see better numbers.
How much cheaper are these types of batteries expected to be?
This is not true if you are talking about lithium ion batteries. It may be true for some chemistries if you are talking about lithium iron phosphate.
If they can be recycled like lithium, even better.
Please remember there are a lot of different sodium-ion chemistries.