It is also worth noting that you can buy sodium ion batteries if you want to play around with them. The only trick is charging method is not the same as lithium ion, so they need a specialized charger. There are some sources on aliexpress. One listing sells a ~700Wh battery for around $76. So not quite $50/KWh but not awful either.

The discharge curve is also different than lithium ion, so it's easy to see if you actually have a lithium ion battery or a sodium ion.

That $52/kWh is for more than batteries, it's for a fully packaged bulk storage system. So it's quite possible that we're already at $40/kWh for batteries.