An international team of scientists has found a material that could enable lithium ion batteries to hold far more energy without sacrificing battery lifetime. The team discovered antimony crystals spontaneously and reversibly hollow out during the charge-discharge cycle, a much-desired characteristic that can facilitate greater energy density without compromising on safety.
Lithium-ion batteries produce electricity by shuttling ions back and forth between two electrodes, the negatively-charged cathode and the positively-charged anode. But in their current state they are stretched to their limits. Efforts to increase this flow of lithium ions have been held back by the deterioration of the anode material, which tends to swell and shrink during charging and discharging, resulting of greater stress that reduces the lifetime of the battery.
Scientists see a solution in “yolk-shell” particles that improve cycling ability thanks to hollow voids that can accommodate volume changes as the battery charges and discharges, while offering a stable outer surface. Swapping out the metal alloy anode material for these particles has long been seen as a pathway forward, but manufacturing them in a cost-effective way has proven problematic.
“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,” says study author Matthew McDowell from the Georgia Institute of Technology. “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.”
The discovery made by McDowell and his colleagues from Georgia Tech, ETH Zürich and Oak Ridge National Laboratory starts with tiny particles a thousand times smaller than the width of a human hair. These oxide-coated antimony nanocrystals, the team found, would spontaneously hollow out during battery cycling, rather than expand and shrink as expected.
This hollowing behavior was confirmed using high-res electron microscopes to observe the nanoparticles in small test batteries, and was only found to occur in particles less than around 30 nanometers in diameter. It works via the resilient oxide layer that allows the material to expand as the ions flow into the anode, but creates voids as the ions are removed, rather than leading to the typical shrinking behavior.
“When we first observed the distinctive hollowing behavior, it was very exciting and we immediately knew this could have important implications for battery performance,” McDowell says.
While these hollowed-out nanoparticles are an exciting discovery, there are a few challenges ahead for the team. Antimony is itself expensive and for that reason isn’t currently used in the production of battery electrode. However, the scientists suspect other, cheaper materials, such as tin, could exhibit the same hollowing behavior. They now hope to explore these possibilities and conduct studies on bigger batteries with a view to working toward commercial applications.
“It would be interesting to test other materials to see if they transform according to a similar hollowing mechanism,” says McDowell. “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.”
The research was published in the journal Nature Nanotechnology.
Source: Georgia Tech
