Sodium Beta / Sodium Sulfur / Aqueous Sodium Batteries.

Imagine a battery that doesn't even start working until it is around 600F, hotter than the broiler in your oven.  Containing molten Sulfur and Sodium, this would not at first glance seem an ideal choice for battery technology, especially if you remember that Sodium metal will generally ignite spontaneously just from the water vapor in air.  However, Sodium Sulfur, or "NaS" batteries are the most successful of the new technology batteries, and are now being used in utilities in the USA and more in Japan, Europe and the Middle East.  Sold presently only by NGK Insulators of Japan, these batteries have a cycle life of 2500 100% DOD or 4500 80% DOD cycles.

A NaS battery cell is somewhat bullet shaped, as shown below, with molten Sodium metal forming the inner anode 'electrode', and molten Sulfur forming the outer cathode 'electrode' (I put 'electrode' in quotes for those that find, as I do, the idea of a liquid electrode rather strange).

The 'electrolyte' is the solid part, oddly enough.  Actually a ionic membrane of B-Alumina, or 'Beta', which allows the transfer of Sodium ions required for the energy storage reaction.  As this solid electrolyte may be used with other cathode materials, these are sometimes referred to as 'Sodium Beta' batteries.

Not shown in the drawing below is a carbon mesh electrode that fills the molten Sulfur area, and actually transfers the electrical current, as Sulfur itself is an insulator.

An issue with NaS batteries is the careful control of temperature required.  Too cool, and the reaction is inefficient or stops altogether.  Too hot, and many of the surrounding components may quickly thermally degrade.  Because of this, much effort has been made in understanding the State of Charge of the battery, as a single overcharged cell may quickly increase in temperature.  NaS batteries do exhibit a very high efficiency, especially when compared to most flow battery designs.

The units appear not to be very scalable, presently over 20,000 cells per MW are required.  The bAlumina separator is fragile, prone to heat shock and very expensive to manufacture. Making the separator thicker would make it stronger and more reliable, but would reduce the conductivity, and so the total current density.  These trade offs limit the scalability of NaS technology.

Despite these issues, NGK Insulators sold roughly 880MWh of storage in FY2009, mostly to non-USA customers, although NaS batteries have been sold on the MW scale to both American Electric Power (AEP) and Xcel Energy.  Given that the present cost of NaS batteries is very high, and that NaS batteries have a long but limited cycle life, unless costs come down significantly it seems unlikely that NaS batteries will continue to dominate this new sector.  However, for present NGK has done an excellent job of producing a genuine utility scale product, and in selling this product widely.

There is potential new competition in NaS batteries, as China State Grid Corporation, one of the two major utilities in China, announced they have developed their own Sodium-Sulfur grid-scale battery.

Another high-temperature liquid Sodium battery is the Zebra battery manufactured by MES-DEA of Switzerland.  Using Nickel Chloride instead of Sulfur, the Zebra battery could be safer, run at a somewhat lower temperature, and, given all the valuable Nickel metal in it, would provide some revenue on recycling.  Presently, Zebra is only being sold into motive power applications.
 

As for lower costs, a recent article on Ceramatec, describes how they are working on a 'warm' NaS battery, where the Sodium would stay solid at an operating temperature of a mere 200F.  This advance could make NaS batteries extremely competitive in the utility storage world.  Ceramatec is a division of CoorsTek, yes, the beer-related company.  Despite the beer beginnings, CoorsTek has produced some of the best ceramic, semiconductor and material science development in the USA.   

Even more interesting is technology for a safe, low-cost sodium-ion battery system coming out of Carnegie Mellon University, and recently funded by the DOE at $10 million.  This technology is run at room temperature with an aqueous sodium (basically sea water) electrolyte, and, if high cycle life can be obtained, could represent a winning solution.