That's a very good efficiency, I believe. I'm not certain of the limitations of this technology, but it anything that doesn't use lithium-ion is very good.
For me, this is where renewables are useful, as passive energy generators for storing energy and not for immediate energy use.
Rondo [USA] heat battery
https://rondo.com/how-it-works
This is the most thermodynamically stupid things I’ve seen in a long time. Electricity and a turning shaft are the highest value energy modes. Turning one of those into low level heat, a low value energy, without doing something more useful on the way is damn near criminal.
I’m no engineer, but it seems like pairing these with nuclear electric power would be very low cost and efficient.
Moving rocks around takes a lot of diesel fuel and a lot of carbon.
More nuclear power, if you want carbon free electricity, and natural gas generators to fill in the peak load.
Solar and wind and hot rocks are children’s toys.