Sodium-Sulfur (NaS) Battery

The sodium-sulfur (NaS) battery is a high-temperature rechargeable chemistry built around molten sodium and molten sulfur electrodes, separated by a solid ceramic electrolyte. It was first demonstrated in 1966 by Joseph Kummer and Neill Weber at Ford Motor Company, who were originally targeting electric vehicle propulsion [1]. That automotive ambition never materialised commercially – the cell’s operating temperature and thermal management needs ruled out mobile use – but the chemistry found a lasting home in stationary grid-scale energy storage, an application it has now served for over two decades, led commercially by the Japanese manufacturer NGK Insulators.

Electrochemistry

A NaS cell places a molten sodium negative electrode and a molten sulfur positive electrode either side of a solid beta-alumina (β″-Al₂O₃) ceramic wall. Beta-alumina is unusual in being a ceramic that conducts sodium ions well while remaining an electrical insulator, so it can act simultaneously as the separator and the electrolyte. On discharge, sodium at the negative electrode gives up an electron and the resulting Na⁺ ion migrates through the ceramic to react with sulfur at the positive electrode, forming a sequence of sodium polysulfides (Na₂Sₓ) as discharge proceeds; charging reverses the process. The nominal open-circuit voltage is around 2.1 V, though it drifts down through several plateaus as the polysulfide mix shifts.

Both electrodes have to stay molten for the cell to function, and beta-alumina only conducts sodium ions well once hot, so the whole cell is run at roughly 300–350°C. In practice a working battery bank is largely kept at temperature by its own internal resistance losses during regular cycling, with supplementary heaters covering idle periods.

Key Performance Characteristics

Nominal open-circuit voltage: ~2.1 V, declining through discharge as the polysulfide composition changes
Operating temperature: ~300–350°C, needed to keep both electrodes molten and the ceramic electrolyte conductive
Theoretical specific energy: up to ~780 Wh/kg at the cell level [2]
Round-trip efficiency: roughly 75–80% AC-to-AC [2]
Cycle life: on the order of 4,500 full discharge cycles, with a 15-year calendar life quoted by NGK for its containerised systems [2]
Response time: within tens of milliseconds once the system is online, useful for grid ancillary services [2]

Advantages

Higher specific energy than lead-acid or most flow battery chemistries used at grid scale, so less floor space and structure per MWh installed
Long cycle and calendar life suited to multi-decade stationary infrastructure
No memory effect, and abundant, low-cost raw materials (sodium and sulfur) relative to lithium- or vanadium-based systems
Fast electrical response once at temperature, suitable for frequency regulation and firming variable renewable output

Limitations

Requires continuous heat input to stay above the sodium and sulfur melting points; if a bank is left idle and allowed to cool and solidify, restarting it is a slow, deliberate reheating process, and auxiliary heating draws parasitic power that erodes the round-trip efficiency figures above during low-use periods
Molten sodium is highly reactive with water and air, and molten sulfur is corrosive, which makes cell sealing and module-level containment a first-order engineering problem rather than an afterthought
A cell-level fault can propagate to neighbouring cells if not isolated quickly – see the safety history below
The thermal management overhead and cell mass rule out mobile or space-constrained use; every commercial deployment today is stationary

Applications and Commercial Deployment

Every NaS deployment today is grid-scale stationary storage: renewable time-shifting and firming, peak shaving, load levelling, transmission and distribution deferral, and frequency or voltage support for utilities. NGK has shipped NAS systems since 2002 and has installed capacity across multiple countries; the largest single site is reported at 50 MW / 300 MWh [2]. In 2019 NGK formed a European sales partnership with BASF, and a 950 kW / 5.8 MWh NAS system supplied through that partnership was commissioned at a BASF production site in Antwerp, Belgium [3].

Safety History

In September 2011, an NGK NAS installation at a Mitsubishi Materials Corporation site in Tsukuba, Japan, caught fire. NGK’s investigation traced the cause to a manufacturing defect in a single cell that produced an internal short circuit, with the resulting heat propagating to neighbouring cells in the module. NGK paused new shipments while it redesigned the battery modules to add fuses and short-circuit prevention plates, and added fire-suppression measures to previously installed systems, resuming production in 2012 [4]. The incident is a useful illustration of the point made above: a chemistry that runs hot, molten and reactive needs module-level fault isolation engineered in from the start, not bolted on afterwards.

References

Kummer, J.T. & Weber, N. “A Sodium-Sulfur Secondary Battery.” SAE Technical Paper 670179 (1967). doi.org/10.4271/670179
NGK Insulators, Ltd. “Sodium-Sulfur (NAS) Battery” – product overview and system specifications. ngk.co.jp/nas/specs
“BASF switches on 5.8MWh NGK sodium-sulfur battery storage system in Belgium.” Energy Storage News (2023). energy-storage.news
“NGK Insulators recalling sodium-sulfur batteries to make safety modifications; resuming production later this year.” Green Car Congress (2012). greencarcongress.com
Sudworth, J.L. “The sodium/sulphur battery.” Journal of Power Sources 11, 143 (1984). doi.org/10.1016/0378-7753(84)80080-4