Australasian Science: Australia's authority on science since 1938

Sodium Batteries to the Rescue

By Stephen Luntz

There is a new entrant in one of the great races of our times – the quest to find cheap ways to store electricity. Murdoch University scientists believe they have the answer in the form of sodium ion batteries.

“The central obstacle facing sustainable energy is unreliability. Wind turbines don’t turn on a still day. Solar doesn’t work at night and can be hampered in the day by cloud, dust or snow coverage,” says Dr Manickam Minakshi of the School of Chemical and Mathematical Sciences.

“To provide power at non-generation times, excess energy needs to be stored in batteries but storage technologies now being considered, such as molten salt or molten sulphur, work at high temperatures, making them expensive and impractical.”

Lithium ion batteries are popular due to their power density for weight and size, but lithium is an expensive metal and laptop batteries are inflammable. “Lithium is not listed as a rare metal,” says Minakshi, “but it will be rare if the whole world wanted to use it for energy storage”.

Sodium, which sits immediately below lithium on the periodic table, has similar chemical properties, a much lower price and exists by the ocean-full. However, sodium atoms are 2.5 times lithium’s size and the ions cannot pass out of the cathode and into the anode of existing batteries to form a current.

After experimenting with a variety of possible replacements, Minakshi found success with manganese dioxide cathodes and olivine sodium phosphate anodes.

“While the technology is too bulky for portable devices, it has excellent potential for large-scale use, including storing energy from wind turbines and solar farms for later feeding into local electricity grids, as well as use in industry,” Minakshi says. All the main ingredients are globally abundant and relatively cheap, although there are concerns about the future availability of phosphate.

Minakshi describes his water-based sodium ion batteries as environmentally benign, in contrast to the potential for leaks from vanadium (AS, June 2011, p.37) or zinc/bromine redox flow batteries. He is also confident they can fully discharge without damaging the battery, in contrast to lithium versions.

More importantly, while his batteries have so far only been charged and recharged 100–150 times, he anticipates they will meet the industry expectation of 2000 cycles.

Most crucially, Minakshi anticipates that the eventual cost of storage will be around $100–150/kWh, close to the estimated $100/kWh sometimes quoted as the point where intermittent energy sources will be able to compete with fossil fuels.

Murdoch University is seeking venture capital to bring its sodium ion batteries to market while continuing to explore additives that may increase energy density.