The problem with rechargeable batteries is that repeated cycles of charging and discharging lead to a drop in performance over time, no thanks to constantly expanding and shrinking during a cycle. Tsinghua University and MIT researchers, however, have discovered how to address the problem by taking advantage of a "yolk and shell" arrangement.
In a study published in Nature Communications, researchers detailed a novel way involving creating a nanoparticle electrode with a "yolk" inside and a solid shell. The yolk can repeatedly change size without affecting the shell, drastically improving cycle life and boosting power and capacity for a battery.
Lithium-ion batteries are the most widely used of rechargeable batteries and most of them use graphite anodes. Researchers have studied other options for many years, seeking a material that will offer better energy storage given a certain weight. They considered lithium metal because it can store around 10 times as much energy for every gram but, unfortunately, the material was volatile, capable of short-circuiting or catching fire. Tin and silicon were also good candidates because they have high capacities but their capacity levels drop immensely when batteries are charged and discharged frequently.
The researchers settled on aluminum but faced the problem of mechanical stress due to expansions and contractions disconnecting electrical contacts. Additionally, liquid electrolytes decompose when they come into contact with aluminum, creating a solid-electrolyte interphase (SEI) layer. Still, repeated expansions and contractions caused shedding in SEI particles, rendering aluminum as also a bad option, until the researchers came up with the idea of confining aluminum in a yolk-shell configuration.
This yolk-shell configuration features a void between the yolk and the shell, giving the "yolk" enough room to expand and contract without affecting the stability and dimensions of the "shell." For the study, the researchers made a shell out of titanium oxide, separating aluminum from the liquid electrolyte situated between two electrodes in a battery. Since shell won't expand or contract, the SEI layer stays stable, ensuring electrical contacts remain connected.
The study received funding support from the National Natural Science Foundation of China and the National Science Foundation. Authors include Sa Li, Ju Li, Junjie Niu, Chang An Wang, Yu Cheng Zhao, Chao Wang and Kang Pyo So.
