The current trend towards so-called 'green' electricity which relies on transient power sources such as wind and solar power ideally requires that an effective mechanism is harnessed to store the power once it is generated. Energy generated from these sources can be used to charge-up batteries which can then be called on when demand is high.

Conventional lithium ion rechargeable batteries are too expensive to meet this challenge and the current, next best alternative, sodium-sulphur batteries run at 300 °C, making them impractical for remote use, although they are considerably cheaper.

Researchers at the US Department of Energy's Pacific Northwest National Laboratory have been collaborating with visiting scientists from Wuhan University in China on producing a sodium-based battery which operates at ambient temperatures.

The team has used the concepts of lithium batteries in conjunction with nanotechnology to achieve their aims. Lithium is roughly a third the size of a sodium atom and is small enough to migrate through holes within the metal oxide (manganese oxide) electrode under the influence of a charge, allowing the device to store and deliver electrical energy.

The stumbling block with sodium batteries was the size limitation caused by the bigger sodium atoms. Rather than use conventional manganese oxide, the team used nanotechnology to fabricate holes within the structure. This was done by incorporating two different allotropes of the metal oxide one of which has an octahedral structure and the other a pyramidal form. The team had predicted that the material should produce "S" shaped tunnels and five-sided tunnels through which the sodium ions would be able to migrate.

The material was heated over a range of temperatures from 450 to 900 °C and then the properties were investigated. They discovered that heating the material to 750 °C created the best crystalline properties when viewed under electron microscopy. The material was then placed in contact with the electrolyte and its properties as a battery were evaluated. The new material was determined to have a peak capacity of 128 mAhr per gram of manganese oxide.

The new battery was also resistant to repeated charging and discharging cycles, losing only 7% of storage capacity after 100 cycles. At 1000 cycles, capacity fell off to 77% of the initial level. The battery was not well suited to being fast charged as the charge capacity diminished; the team hypothesised that this was due to a limitation on the speed with which sodium ions could migrate through the metal oxide tunnels.

Future research will look at producing even smaller nanowires with the aim of increasing the maximum charging and discharging rates of the battery. The research was published in the June 3^rd edition of Advanced Materials.

(Image Credit: PNNL. Caption: The uniform nanostructure of heat-treated manganese oxide provides tunnels for sodium ions to flow through, improving the performance of the electrodes.)