I think it might literally mean "operates at room temperature" as in 25-35 degrees C. Not really sure how it works but there seems to be a distinction between high, intermediate, and room temperature for the Na-S battery's operating conditions.

Quote from: https://www.tandfonline.com/doi/full/10.1080/21663831.2022.2...

    1.1. History of Na-S batteries

    Research on Na-S batteries originated in the 1960s, with the first research focused on High-Temperature Sodium-Sulfur (HT-Na/S) batteries, which operate around 300–350 °C. A molten Na anode (melting point=98 °C), a molten sulfur cathode (melting point = 118 °C) and ceramic β'-Al2O3 as solid electrolyte are assembled into the HT-Na/S batteries [11]. HT-Na/S batteries avoid the dendrite problem and have high electrical conductivity. However, it also has the defects of high working temperature, high risk, low energy density and high operation cost. And then, the Intermediate-Temperature Sodium-Sulfur (IMT-Na/S) batteries were innovated in the 1970s and operate between 120–300 °C. The IMT-Na/S batteries also eliminated the dendrite problem, but the electronic conductivity and the utilization of sulfur also decreased. Researchers have been intensively investigating Room-Temperature Sodium-Sulfur (RT-Na/S) batteries, which operate around 25 °C-35 °C. RT-Na/S batteries can completely convert S8 to Na2S, so they have a high theoretical energy density (1274 Wh kg−1)