Nano-engineering unlocks sodium-ion batteries

Working towards a cheaper alternative to lithium ion batteries for storing solar and wind energy, researchers are looking at sodium ions.

"Sodium is the next best choice, but the sodium-sulphur batteries currently in use run at temperatures above 300°C making them less energy efficient and safe than batteries that run at ambient temperatures," said the US Pacific Northwest National Laboratory (PNNL).

Working with visiting researchers from Wuhan University in China, the PNNL team has attempted to build the sodium analogue of manganese oxide lithium-ion cells.

"The free movement of lithium ions allows a lithium ion battery to hold electricity or release current. But simply replacing the lithium ions with sodium ions is problematic because sodium ions are 70% bigger than lithium ions and don't fit in the crevices as well," said PNNL.

What was needed was manganese oxide with bigger pores, so the researchers turned to nano-structuring.

"The team mixed two different kinds of manganese oxide atomic building blocks, one whose atoms arrange themselves in pyramids, and another whose atoms form an octahedron," said the lab. "They expected the final material to have large S-shaped tunnels and smaller five-sided tunnels through which the ions could flow."

PNNL describes the result as a form of nanowire, whose final efficacy is strongly affected by heat treatment.

Heat treating the mixture to 750°C, said the lab, gave the best structure: "The 750°C wires looked uniform and very crystalline. Lower, and the crystals appeared flaky, too high and the crystals turned into larger flat plates. Manganese oxide heated to 600°C had pockmarks in the nanowires that could impede the sodium ions."

The material has a peak capacity of 128mAh/g of electrode.

"It held up well to cycles of charging and discharging. After 100 cycles, it lost only 7% of its capacity," said the PNNL. "Even after 1,000 cycles, the capacity only dropped about 23%."

600°C and 900°C material lost around 37 and 25% of capacity respectively after 100 cycles.

Charge rate experiments suggest ion flow is still restricted, suggesting even smaller nanowires are necessary.