Solarpunk

Really gives me extra hope that this isn't another vaporware battery technology.

I'll take it with a grain of salt:

• From the summary I didn't see if the energy density is a cell level value or a system level value. We have to compare system level energy densities.
• Also energy density isn't the only thing lithium ion batteries are good at. Firstly, if the cell at its core is a reversible fuel cell with nafion membrane, the dynamics of the cell will be comparable to PEM fuel cells. Lithium-ion batteries can deliver full power instantly without an issue. With PEM fuel cells you have to be more careful: it's possible to flood the membrane with water or dry it up if you change power too quickly, which is why their response time (time from set point change to actual delivery of full power) is usually limited to about one minute. Secondly, how good or bad is the calendric and cyclic aging and recyclability of the new battery? Lithium-ion has come a long way concerning lifetime - recyclability not yet, sadly.
• With Sodium-ion batteries there's another, more mature and similarly environmentally-friendly technology already on the market. And while I wouldn't have thought that it made sense, chinese manufacturer CATL already has built Sodium-ion batteries for cars, and (sadly) that's the far more important market for batteries compared to stationary ones.