SAN JOSE, Calif. --- The element of the day is sodium.

The somewhat volatile metal, number eleven on the periodic table of the elements, is increasingly finding its way into the business plans and prototypes of energy storage companies. Sodium batteries -- which contain a metal electrode, a sodium-based electrode and often a molten sodium electrolyte -- already exist: the Third Reich powered the V-2 rocket with molten salt batteries. But in the future, you will see them become more prevalent in industrial and utility applications, argue advocates. You’ll also see a wider variety of sodium chemistries.

Sodium’s selling point seems to be in durability, low cost, efficiency and safety, depending on whom you talk to and the chemical formula.

“We believe we can sell at under $300 a kilowatt-hour profitably once we are in mass manufacturing,” said Jay Whitacre, CTO of Aquion Energy, a Carnegie Mellon University spin-out that wants to produce a sodium-ion battery with a water-based electrolyte, during a presentation at the Energy Storage Association conference taking place this week in San Jose. (More on Aquion below.)

Lithium-ion battery packs sell for closer to $650 a kilowatt hour now, and some believe that a $250 per kilowatt-hour price for lithium-ion batteries, a goal of the DOE, will be tough to meet. Flow batteries, which still tend to be about as easy to find as leprechauns and unicorns, cost even more.

The downside: molten salt electrolytes typically need to get heated to 245 degrees Celsius or hotter to work.

Fiamm Sonik, the company that acquired the intellectual property for making sodium metal halide batteries from a European company called MES-DEA in April, says its sodium metal batteries can last for more than 4,500 cycles, according to chief strategic officer Peter Thomas. The batteries operate in a wide temperature range, can sit on shelves for years without degrading and are fully recyclable.

“There are no rare earth elements in sodium nickel batteries,” he said. “They are three times lighter and smaller than lead acid batteries.”

Fiamm has factory capacity to produce 100 megawatt-hours of batteries a year and hopes to expand that to 2 gigawatt hours over the next few years by adding plants in the U.S. and maybe China. Sodium batteries based on MES-DEA/Fiamm’s technology power buses in Europe and the United Arab Emirates and can be found in power plants in Italy that are owned by Enel.

General Electric will open a 1-gigawatt-hour sodium metal battery plant in the U.S. next year. The batteries will get deployed for grid storage and powering locomotives next year. Both GE’s and Fiamm’s batteries derive from the research behind the Zebra in the mid-'80s.

Japan’s NGK has made sodium sulfur batteries for grid storage for a number of years. Approximately 250 megawatts of NGK’s sodium sulfur batteries have been installed in Japan. (Thirteen megawatts' worth of the batteries have been installed in the U.S.; Xcel Energy uses a bank of NGK sodium sulfur batteries at a wind farm.) Roughly 230 megawatts of the batteries were in the area of Japan affected by the recent earthquake, but all came back on line quickly

The efficiency is around 85 percent, said NGK’s Tetsuya Hatta. (The efficiency number relates to DC efficiency and discounts any AC to DC conversions required to charge or discharge the battery.)

Aquion, meanwhile, has a battery that consists of an anode made of activated carbon, a cathode made from sodium and magnesium oxide, and a water-based electrolyte. By late 2011 or early 2012, Aquion wants to have a low-volume production line in place to produce samples for large customers. In 2013 and 2014, it wants to build a factory with a capacity of 500 megawatt hours worth of batteries a year. The plan is to build it in the U.S.

In 2015, the company hopes to replicate that factory in other parts of the world.

The batteries will get sold to utilities and will provide two to six hours of storage. The density of the company’s batteries is 25 to 30 watt hours per liter, Whitacre said. That puts it between flow batteries and lead acid batteries.

Aquion’s battery can endure 5,000+ charging cycles and exhibits an 85 percent (DC) efficiency. Plus, like all of the technologies on the list, the main ingredient, sodium, is plentiful and easy to find. Kleiner, Perkins is an investor.

Finally, Pacific Northwest National Labs said it is working on a planar sodium metal halide battery. The planar design, an improvement over cylindrical cell batteries, comes from a solid oxide fuel cell created by PNNL earlier. (Talk about recycling.) It hopes to have a 5-kilowatt prototype in one to two years. The lab is also experimenting with a sodium-ion battery.

But don’t count lithium out yet. PNNL also has a lithium battery prototype with a lithium phosphate cathode and a titanium oxide anode, which could cost $250 a kilowatt hour if and when it comes out.