Flow of lithium ions into and out of alloy battery anodes has long been a limiting factor in how much energy batteries could hold using conventional materials. Too much ion flow causes anode materials to swell and then shrink during charge-discharge cycles, causing mechanical degradation that shortens battery life. To address that issue, researchers have previously developed hollow yolk-shell nanoparticles that accommodate the volume change caused by ion flow, but fabricating them has been complex and costly.

Now, researchers at Georgia Tech, with colleagues at ETH Zrich and Oak Ridge National Laboratory, have discovered that sufficiently small antimony nanocrystals spontaneously form uniform voids on the removal of lithium, which are then reversibly filled and vacated during cycling, allowing more ion flow without damaging the anodes. A paper on their work is published in Nature Nanotechnology.

An electron microscope image shows the antimony nanoparticles used to study the spontaneous formation of nanoscale hollow electrodes for use in batteries. (Credit: Matthew Boebinger)

Intentionally engineering hollow nanomaterials 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. The problem has been that directly synthesizing these hollow nanostructures at the large scales needed for commercial applications is challenging and expensive. Our discovery could offer an easier, streamlined process that could lead to improved performance in a way that is similar to the intentionally engineered hollow structures.

Matthew McDowell, corresponding author

The researchers made their discovery using a high-resolution electron microscope that allowed them to directly visualize battery reactions as they occur at the nanoscale.

The team also used modeling to create a theoretical framework for understanding why the nanoparticles spontaneously hollowinstead of shrinkingduring removal of lithium from the battery.

The ability to form and reversibly fill hollow particles during battery cycling occurs only in oxide-coated antimony nanocrystals that are less than approximately 30 nanometers in diameter. The research team found that the behavior arises from a resilient native oxide layer that allows for initial expansion during lithiationflow of ions into the anodebut mechanically prevents shrinkage as antimony forms voids during the removal of ions, a process known as delithiation.

The finding was a bit of a surprise because earlier work on related materials had been performed on larger particles, which expand and shrink instead of forming hollow structures.

Antimony is relatively expensive and not currently used in commercial battery electrodes. But McDowell believes the spontaneous hollowing may also occur in less costly related materials such as tin. Next steps would include testing other materials and mapping a pathway to commercial scale-up.

It would be interesting to test other materials to see if they transform according to a similar hollowing mechanism. 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.

Matthew McDowell

Though they may be costly, the self-hollowing antimony nanocrystals have another interesting property: they could also be used in sodium-ion and potassium-ion batteries, emerging systems for which much more research must be done.

This work was performed at the Georgia Tech Materials Characterization Facility and the Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542174). Support also came from the Department of Energy Office of Science Graduate Student Research Program for research performed at Oak Ridge National Laboratory.

A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Support was also provided by a Sloan Research Fellowship in Chemistry from the Alfred P. Sloan Foundation and by the Swiss National Science foundation via an Ambizione Fellowship (no. 161249). The content is solely the responsibility of the authors and does not necessarily represent the official views of the sponsoring organizations.

Resources

Matthew G. Boebinger, et al. (2020) Spontaneous and reversible hollowing of alloy anode nanocrystals for stable battery cycling Nature Nanotechnology doi: 10.1038/s41565-020-0690-9

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Spontaneous formation of hollow structures in alloy anode crystals can improve battery stability - Green Car Congress