Sodium-sulfur (NaS) batteries—also referred to as molten-salt batteries—use liquid sodium for the anode and liquid sulfur for the cathode, with a beta-alumina ceramic electrolyte between them. Unlike the battery in your phone or power drill, these cells run hot: 300–350°C, so both electrode materials stay molten and the electrochemical reaction can take place. This guide explains how they work, where they’re used, how they compare with lithium-ion, and what’s coming next with room-temperature designs.
A NaS cell has three essential components:
• Anode: Molten sodium (Na)
• Cathode: Molten sulfur (S)
• Electrolyte: A beta-alumina ceramic that only allows sodium ions (Na⁺) to pass through
Both electrodes need to remain liquid, which is why the entire cell runs at 300–350°C. The beta-alumina separator is the crucial piece—it keeps the reactive molten materials apart while enabling efficient ion transfer.
During Discharge (Delivering Power)
1. Molten sodium at the anode releases electrons into the external circuit, turning into Na⁺ ions.
2. Those ions travel through the beta-alumina electrolyte toward the sulfur cathode.
3. At the cathode, the Na⁺ ions combine with sulfur and the returning electrons to form sodium polysulfide (Na₂Sₓ).
4. The cell delivers a voltage around 2 V.
Core reaction: xS + 2Na⁺ + 2e⁻ ⇌ Na₂Sₓ
During Charging (Storing Power)
The process runs in reverse—sodium polysulfide breaks down, regenerating molten sodium and sulfur at their respective electrodes.
Key Technical Specs of NaS Batteries
• Operating temperature: 300–350°C
• Energy density (cell level): ~150–240 Wh/kg
• Theoretical maximum energy density: ~760 Wh/kg
• Charge/discharge efficiency: 75–86%
• Cycle life: 2,500–4,500 cycles
• Electrolyte: Solid beta-alumina ceramic
• High energy density for grid storage: Cell-level energy density rivals that of lithium-ion (150–240 Wh/kg), and the theoretical ceiling sits near 760 Wh/kg. This makes NaS a strong candidate for long-duration, large-scale installations.
• Abundant, low-cost raw materials: Sodium comes from seawater or salt; sulfur is a byproduct of refining. No lithium, cobalt, or nickel is needed.
• Long cycle life: Proven utility deployments show thousands of cycles with minimal degradation.
• Nearly 100% recyclable: Sodium and sulfur are easy to recover—material recovery approaches 100%.
• Minimal self-discharge: Holds its charge well when idle, which is valuable for long holding periods.
• High operating temperature: Keeping cells at 300–350°C demands insulation and heaters. Units that have been idle must be reheated before use. This confines NaS to fixed, large-scale sites.
• Safety at high temperature: Molten sodium plus sulfur plus a cracked separator creates a fire risk. The technology requires sophisticated thermal management and containment.
• Limited supply base: Historically, only NGK Insulators (Japan) manufactured them commercially. NGK ended molten NaS production in 2025, though next-generation room-temperature R&D is active worldwide.
• Not suitable for mobile use: Too heavy, too hot, and too hazardous for electric vehicles or portable electronics.
Sodium-sulfur and lithium-ion are designed for very different roles. NaS operates at 300–350°C with molten electrodes and a solid ceramic electrolyte, while lithium-ion works at ambient temperature. Both deliver high energy density, but NaS is built for utility-scale grid storage and lithium-ion for mobility and consumer devices. Raw material costs are very low for NaS and moderate to high for lithium-ion. Safety-wise, NaS needs robust containment, while lithium-ion carries a risk of thermal runaway. NaS cannot be made portable, whereas lithium-ion excels in mobile applications. On the environmental side, NaS is highly recyclable; lithium-ion faces supply-chain and sustainability concerns.
NaS has a solid track record in stationary grid storage:
• Peak shaving: Charge during off-peak hours, discharge when demand peaks
• Load leveling: Smooth out gaps between supply and demand
• Renewables integration: The largest installed system—34 MW / 245 MWh in Japan—stabilizes the output of a wind farm
• Frequency regulation: Fast-response support to maintain grid stability
High operating temperature is the biggest bottleneck, so researchers are actively developing room-temperature versions.
• Anode-free designs: A 2026 study in Nature described an anode-free NaS cell that works at room temperature, delivering around 2,021 Wh/kg at the electrode level with an estimated raw material cost of roughly $5.03/kWh.
• New electrolytes: Solid-state and gel polymer electrolytes now enable stable room-temperature cycling in the lab—some demonstrations show more than 1,400 cycles with high sulfur utilization.
• Advanced cathodes: Pushing sulfur to higher oxidation states unlocks extra capacity and voltage.
Commercial viability for the grid, and possibly even for niche portable uses, could arrive within the next decade, although scaling and air stability remain challenges.
NaS batteries are made from two of the most abundant elements on the planet, can store enormous amounts of energy, and have decades of utility-scale operational proof behind them. The need for high temperatures keeps them out of your car or backpack—but room-temperature designs on the horizon could change that. For long-duration grid storage, sodium-sulfur isn’t a technology of the past. It’s a chemistry that’s still evolving, with the potential to deliver affordable, safe, large-scale storage at a cost the energy transition urgently needs.
1. What are sodium-sulfur batteries used for?
Mainly utility-scale grid storage—peak shaving, load leveling, firming up renewables, and frequency regulation. They are not used in EVs or consumer electronics.
2. How is NaS different from lithium-ion?
NaS runs at 300–350°C with molten electrodes and a ceramic electrolyte; lithium-ion runs at room temperature. NaS is for fixed grid sites, lithium-ion for portable and mobile applications. Both have high energy density, but NaS uses much cheaper raw materials.
3. Are NaS batteries safe?
High-temperature NaS batteries carry a fire risk if the ceramic separator fails. Commercial systems use robust containment. Room-temperature designs aim to eliminate this risk entirely.
4. Why did NGK stop making NaS batteries?
NGK discontinued molten NaS production in 2025, citing durability and safety challenges. Since then, research into next-generation NaS has accelerated.
5. What’s the energy density of a NaS battery?
Conventional cells: ~150–240 Wh/kg. Theoretical maximum: ~760 Wh/kg. Experimental anode-free lab cells have exceeded 2,000 Wh/kg at the electrode level.
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