High temperature, though I'm confused myself about how a 40% efficient conversion done twice can match a battery. Probably will need to read the actual paper, I thought batteries were on the order of 95%. Maybe it is more energy dense or something.

It might be about profitability, long-term energy storage and much less maintenance (since no moving parts). Ref: https://www.sciencedirect.com/science/article/abs/pii/S03062...

"Evaluated herein is one E-TES concept, called Firebrick Resistance-Heated Energy Storage (FIRES), that stores electricity as sensible high-temperature heat (1000–1700 °C) in ceramic firebrick, and discharges it as a hot airstream to either (1) heat industrial plants in place of fossil fuels, or (2) regenerate electricity in a power plant. … We report that systems of 100–1000 s MWh may be cycled daily, and discharged at a constant heat rate typically for 70–90% of the storage capacity. Traditional insulation can reasonably limit heat leakage to less than 3% per day. Preliminary cost estimates indicate a system cost near $10/kWh, substantially less expensive than batteries."