Spontaneously-hollowing antimony particles could increase the capacity of lithium ion cells, according to the Georgia Institute of Technology.

GaTech-antimony-Liion-detail-600

“Intentionally engineering hollow nano-materials has been done for a while now, and it is a promising approach for improving the lifetime and stability of batteries with high energy density,” GaTech research engineer Matthew McDowell. “The problem has been that directly synthesising these hollow nano-structures at the large scales needed for commercial applications is challenging and expensive. Our discovery could offer a process that could lead to improved performance in a way that is similar to the intentionally engineered hollow structures.”

The issue is anode structures crumbling when too much lithium is inserted and removed during battery use.

“Hollow and yolk-shell nano-structures have been used to increase the cycling stability by providing an inner void space to accommodate volume changes and a mechanically stable outer surface,” according to ‘Spontaneous and reversible hollowing of alloy anode nanocrystals for stable battery cycling’, which describes the project in Nature Nanotechnology.

GaTech-antimony-LiionSuch structures can be synthesised, but the GaTech antimony crystals, if sufficiently small,  spontaneously form uniform voids on the removal of lithium, which are then reversibly filled and vacated during cycling, according to the paper.

A resilient native oxide layer is responsible for the formation – allowing an initial expansion as lithium is inserted, but mechanically preventing shrinkage when the lithium is removed – when the voids first form. The process works in 15nm particles, but not in particles larger than ~30nm.

“When we first observed the distinctive hollowing behaviour, it was very exciting and we immediately knew this could have important implications for battery performance,” said McDowell.

Antimony is not cheap, so the team is now looking for similar behaviour in less costly materials.

“It would be interesting to test other materials to see if they transform according to a similar hollowing mechanism,” McDowell said. “This could expand the range of materials available for use in batteries. The small test batteries we fabricated showed promising charge-discharge performance, so we would like to evaluate the materials in larger batteries.”

According to GaTech, the self-hollowing antimony crystals might even be able to accommodate sodium or potassium ions – sodium-ion batteries could offer large capacity from cheap materials, if only a way could be found to stop the large ions from wrecking electrodes within a few charge cycles – many companies are working on this issue.

Georgia Tech worked with ETH Zürich and Oak Ridge National Laboratory. Between them, a theoretical framework has been created to explain why the particles spontaneously hollow during lithium removal rather than shrinking.

‘Spontaneous and reversible hollowing of alloy anode nanocrystals for stable battery cycling can be found here‘ – abstract only available without paying.