BU-210a: Why does Sodium-sulfur need to be heated

Sodium batteries, also known as molten salt or thermal battery, come in primary and secondary versions. The battery uses molten salts as an electrolyte and gains conductivity by heating the stack to a temperature of 400–700°C (752–1,292°F). Newer designs run at a lower 245–350°C (473–662°F) temperature.

Conceived by the Germans during World War II and used in their V-2 rockets, the electrolyte of the molten salt batteries is inactive when cold and has a long storage of more than 50 years. Once activated with a heat source, the battery can provide a high power burst for a fraction of a second or deliver energy over several hours. High power is made possible by the good ionic conductivity of the molten salt. Primary sodium batteries are almost exclusively used for the military as a “one-shot” engagement in guided missiles, but the interest lies in the rechargeable version.

The rechargeable sodium-sulfur (NaS) gained worldwide attention during the 1970s and 1980s, but short service life and high cost dampened the enthusiasm. The sodium-nickel-chloride battery, also known as ZEBRA, came to the rescue and today this battery is being used successfully in many applications. (ZEBRA stands for Zeolite Battery Research Africa Project.)

ZEBRA has a nominal cell voltage of 2.58 volts and a specific energy of 90–120Wh/kg, a level comparable with Li-manganese and Li-phosphate. The service life is about eight years and 3,000 cycles. It can be fast-charged, is non-toxic and the raw materials are abundant and low-cost. ZEBRA batteries come in large sizes of 10kWh or higher and typical applications are forklifts, railways, ships, submarines and electric cars.

A growing market for sodium-based batteries is load leveling, also known as grid storage. The Think City EV had a choice of ZEBRA and Li-ion. ZEBRA has advantages when operating at extreme temperatures and when the battery is in continuous use, such as in taxis and delivery vans.

The ZEBRA battery must be heated to 270–350°C (518–662°F), a temperature that is lower than the original sodium-sulfur battery. Even though special insulation minimizes heat loss, heating consumes 14 percent of the battery’s energy per day. Since the energy to keep the battery hot is taken from the battery, the resulting parasitic load, or self-discharge, is 18 percent.

An active ZEBRA battery should be either plugged in and on charge or in use. It takes 3–4 days to cool down, and reheating takes about two days depending on the SoC at time of shutdown.

Common failures include electrical shorts due to corrosion of the insulators, which then become conductive, as well as growth of dendrites, which increases self-discharge. ZEBRA batteries are safer than Sodium-sulfur and a short circuit does not cause complete failure of the battery.

Last Updated: 22-Oct-2021
Batteries In A Portable World
Batteries In A Portable World

The material on Battery University is based on the indispensable new 4th edition of "Batteries in a Portable World - A Handbook on Rechargeable Batteries for Non-Engineers" which is available for order through Amazon.com.

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On March 15, 2013, ghifari wrote:
i think if you use hydrogen gas as the source, then the people who want to bring it to the plane must ask to empty the fuel cartridge or change it to a methanol mode if the liquid not exceed such a limit this may can be optional to them
On February 18, 2013, bharat wrote:
are fuel cells comparable to diesel thermal plant to produce electricity in grid level please clarify which one will be cheaper and reliable
On February 10, 2013, Jack wrote:
I think these are dangerous to carry on planes. Once you allow them, then there is possibility that several of them can be on a flight. This may be a major security and safety concern because of potential high heat each can generate.
On May 6, 2011, scott wrote:
It all seems so simple Solar/Wind> Electrolysis > Hydrogen If we just build enough solar panels, solar thermal, and wind turbines; we can have clean abundant cheap hydrogen.
On March 9, 2011, sanjay agarwal wrote:
dear sir , Mercedes is using this technology in it's car
On March 8, 2011, Cadex Electronics Inc. wrote:
This article has been updated as of March 8, 2011.
On February 11, 2011, masroor wrote:
1) For PEMFC in particular, where pure H2 and O2 is needed, source is still a challenge . 2) water storage is not at all a problem, lets say that it is being used in autos then, there can be a small storage tank for water which can be later used in drinking also, as it was used in gemini space station. 3)running cost u can chk on website, actually methanol fuel cell is being produced commercially and people are buying it and yes its really portable, u can buy it and discover the advantages, this answes your last question about it being commercial.
On January 28, 2011, sanjay wrote:
dear sir I can not understand how it can be use portable , my questions are following. 1. the source for oxygen and hydrogen 2. storage of water 3. running cost(how much electric can produce by how much gas) 4. can it be use commercially