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The lowdown on batteries: Sodium sulfur

Posted: 04 Aug 2014 ?? ?Print Version ?Bookmark and Share

Keywords:vacuum tube? sodium sulfur? NS? insulator? battery?

Several months ago, Max Maxfield roped me into his ongoing robot project. This led to my writing this series of articles on the various battery technologies available to us.

Now, I don't know about you, but I am old enough to remember the old B-plus battery used in vacuum tube-based devices. Max has an antique vacuum tube radio we are in the process of testing and refurbishing. This doesn't use batteries, but seeing it did take me down memory lane ...

The sodium sulfur (Na2S4) battery
Let's start with a little history. Here are a few bullet points in the development of sodium sulfur (NaS) battery technology:

???The technology was begun in 1966 by Joe Kummer and Neil Weber at the Ford Motor Company.
???One test version flew on the space shuttle.
???One of these batteries was used to power the Ford Ecostar demonstration vehicle in 1991. (This vehicle never made it into production.)
???The world's largest sodium-sulfur battery in service is in Texas. It is used for supplementing the power grid.
???A fire was caused by a failed NGK-manufactured battery installed outside of Mitsubishi Materials Corporation's Tsukuba Plant.
???A lower-temperature version was announced in 2011 by Kyoto University and Sumitomo Electric Industries.

This battery technology is known for its very high energy density, excellent cycle life, low-cost materials, and high efficiency. However, this comes with the disadvantages of needing high temperatures to operate (molten sodium), the fact that the insulator can be brittle, and safety concerns with regard to the need to protect the sodium from moisture. Another common problem with the technology is that many chemistries form undesirable Na2S2 solids with higher depth-of-discharge operation. The main application area has been for grid storage. However, this technology also has potential for buildings and transportation.

Specific energy: approximately 150-760 Wh/kg (high-temperature version)
Energy density: approximately 151 Wh/L
Specific power: 200 W/kg
Discharge efficiency: approximately 72% to 90%
Energy/consumer-price: 0.4 Wh/dollar
Service or shelf life: Two to five years for most types (some to 15 years), hot
Cycle durability: >4,200 at 80% DOD
Nominal cell voltage: 2V
Cut-off voltage: 1.78 to 1.9V
Temperature: internal 350C (original version), 100C to 150C (new versions)

2Na + 4S Na2S4 or
2Na + 3S Na2S3 or
2Na + (SSCH2CH2)n Na2SSCH2CH2 for 90C to 100C melting temperatures, poly(ethylenedisulfide)

As the cells charge and discharge, they typically exchange sulfur forms between S5, S4, S3, and S2.

The following figures illustrate some interesting applications and characteristics associated with sodium sulfur batteries.

Figure 1: A grid application and the structure of a typical NaS battery. (Source: NGK)

Figure 2: Pulsed power verses discharge time of a large, high-temperature NaS battery module.

Figure 3: Cell voltage verses state of charge with respect to Na2S3. (Source: J. L. Sudworth, 1981)

Figure 4: Cell voltage verses state of charge with respect to Na2S3. (Source: J. L. Sudworth, 1981)

In my next article, we'll look at some more tips and tricks and consider another battery technology. In the meantime, as always, I welcome any questions or comments.

About the author
Ivan Cowie is the Chief Engineer at MaxVision.

To download the PDF version of this article, click here.

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