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Scientists discover nanomaterials with "spontaneous hollowing" properties that can improve the performance of high-energy lithium batteries

According to foreign media reports, an international team of scientists has discovered a material that can make lithium batteries have more energy without sacrificing battery life. The team found that antimony crystals spontaneously and reversibly hollow during charge and discharge cycles, a highly anticipated property that can promote greater energy density without compromising safety.

Lithium batteries generate electricity by transferring ions back and forth between two electrodes (negative cathode and positive anode). But in their current state, they have reached their limits. Efforts to increase the flow of lithium ions are hindered by the aging of the anode material, which expands and contracts during charging and discharging, causing greater pressure and thus reducing the life of the battery.

Scientists saw a solution in yolk-shel particles, because the hollow voids can accommodate the volume changes when the battery is charged and discharged, while providing a stable outer surface, thereby improving the cycle capacity. Replacing metal alloy anode materials with these particles has long been seen as a promising path, but it has proven problematic to manufacture them in a cost-effective way.

Intentionally engineering hollow nanomaterials has been possible for some time, and it's a promising approach to improving the lifespan and stability of high-energy-density batteries, said study author Matthew McDowell from the Georgia Institute of Technology. The problem has been that it's challenging and expensive to directly synthesize these hollow nanostructures at the large scales required for commercial applications. Our discovery could provide a simpler, streamlined process that improves performance in a similar way to intentionally engineered hollow structures.

The discovery by McDowell and his colleagues from Georgia Tech, ETH Zurich and Oak Ridge National Laboratory started with tiny particles one thousandth the diameter of a human hair. The team found that these oxide-coated antimony nanocrystals spontaneously hollowed out during battery cycling, rather than expanding and contracting as expected.

Using high-resolution electron microscopy to observe the nanoparticles in small test cells confirmed the hollowing behavior and found that it only occurred in particles smaller than about 30 nanometers in diameter. It works by having an elastic oxide layer that allows the material to expand when ions flow into the anode, but then voids when ions are removed, rather than resulting in the typical shrinking behavior.

"When we first observed the unique hollowing behavior, it was very exciting and we immediately understood that this could have important consequences for battery performance," McDowell said.

While these hollow nanoparticles are an exciting discovery, there are some challenges ahead for the team. Antimony itself is expensive, so it is not currently used to produce battery electrodes. However, the scientists suspect that other cheaper materials such as tin may also exhibit the same hollowing behavior. They now hope to explore these possibilities and carry out the study in larger batteries with a view to working towards commercial applications.

It will be very 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 that can be used in batteries. The small test cells we made showed promising charge and discharge performance, so we hope to evaluate these materials in larger batteries.

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