Room-temperature sodium-sulfur (RT Na-S) batteries represent a significant shift from conventional high-temperature sodium-sulfur (HT Na-S) systems, offering potential advantages in safety, cost, and operational flexibility. Traditional HT Na-S batteries operate at 300–350°C, utilizing molten electrodes and a solid ceramic electrolyte (beta-alumina). In contrast, RT Na-S batteries function near ambient temperatures, employing liquid or solid electrolytes and avoiding the need for complex thermal management. This transition introduces new challenges and opportunities in materials science, interfacial engineering, and electrochemical performance.

The fundamental difference between HT and RT Na-S systems lies in their electrochemical mechanisms and material requirements. HT Na-S batteries rely on the high ionic conductivity of molten sodium and sulfur species, with beta-alumina serving as a sodium-ion conductor. The high operating temperature ensures low viscosity and rapid reaction kinetics but demands robust thermal insulation and raises safety concerns. RT Na-S batteries, however, face sluggish reaction kinetics due to the insulating nature of sulfur and the poor solubility of sodium polysulfides in most electrolytes at room temperature. Overcoming these limitations requires innovative electrode and electrolyte designs.

Liquid electrolytes in RT Na-S batteries typically consist of sodium salts (e.g., NaClO4 or NaTFSI) dissolved in organic solvents (carbonates, ethers, or glymes) or ionic liquids. These electrolytes must balance ionic conductivity, polysulfide solubility, and electrochemical stability. Ether-based electrolytes, such as tetraglyme, have shown promise due to their ability to solvate polysulfides and facilitate redox reactions. However, polysulfide dissolution leads to the shuttle effect, where long-chain polysulfides migrate to the sodium anode, causing active material loss and rapid capacity fade. Solid electrolytes, including sodium superionic conductors (NASICON) and sulfide-based glasses, offer potential solutions by physically blocking polysulfide diffusion while enabling sodium-ion transport. Yet, their brittleness and high interfacial resistance remain unresolved issues.

Cathode materials for RT Na-S batteries focus on sulfur-carbon composites to enhance electronic conductivity and confine polysulfides. Porous carbon matrices, such as microporous carbon, carbon nanotubes, or graphene, provide high surface areas for sulfur loading and mitigate volume changes during cycling. Heteroatom doping (nitrogen, oxygen) further improves polysulfide adsorption through polar interactions. For example, sulfur embedded in nitrogen-doped carbon has demonstrated capacities exceeding 1,000 mAh/g in early cycles, though practical cycling stability remains below 500 cycles with significant capacity decay. The formation of solid-electrolyte interphases (SEI) on both electrodes also critically impacts performance. A stable SEI on the sodium anode prevents parasitic reactions with polysulfides, while cathode SEI layers influence charge transfer and polysulfide trapping.

Anode stabilization is another major challenge. Sodium metal anodes suffer from dendrite growth and volume expansion, leading to short circuits and electrolyte depletion. Strategies include artificial SEI layers (e.g., Al2O3 coatings), three-dimensional current collectors, and alloy-based anodes (e.g., Na-Sn or Na-Sb). Alternatively, sodium-ion anodes like hard carbon avoid dendrite formation but sacrifice energy density. Recent work on hybrid anodes, combining sodium metal with protective interlayers, has shown improved cycling efficiency over 1,000 hours in symmetric cells.

Energy density comparisons reveal the tradeoffs between RT and HT Na-S systems. HT Na-S batteries achieve theoretical energy densities of 760 Wh/kg, with practical values near 150–240 Wh/kg at the cell level. RT Na-S batteries theoretically reach 1,275 Wh/kg based on sodium and sulfur masses, but actual cells report 200–400 Wh/kg due to excess electrolytes and inactive components. Cycling stability also lags, with most RT Na-S systems limited to 200–500 cycles at moderate rates (0.1–0.5C), compared to HT Na-S batteries enduring 4,500 cycles in grid storage applications. The table below summarizes key metrics:

System Operating Temp Energy Density (Wh/kg) Cycle Life Coulombic Efficiency
HT Na-S 300–350°C 150–240 4,500+ >99%
RT Na-S 20–60°C 200–400 200–500 95–98%

Remaining scientific hurdles for RT Na-S batteries include polysulfide shuttle mitigation, sodium anode stabilization, and electrolyte optimization. Advanced characterization techniques, such as in-situ X-ray diffraction and cryo-electron microscopy, are essential to understand degradation mechanisms at interfaces. Scaling viable materials to commercial formats while maintaining performance is another critical step. Despite these challenges, RT Na-S batteries offer a compelling pathway for grid storage and electric vehicles if material innovations can bridge the gap between laboratory prototypes and practical systems. The progress in sulfur hosts, solid electrolytes, and anode protection continues to push the boundaries of this emerging technology.