Shown is a sample holder used to test samples of lithiated silicon to determine their nano-mechanical properties. The device was used to develop a detailed nano-mechanical understanding of mechanical degradation processes in silicon thin films. Photo by Rob Felt.
A detailed nano-mechanical study of degradation processes in silicon structures containing varying levels of lithium ions offers good news for researchers attempting to develop reliable next-generation rechargeable batteries using silicon-based electrodes.
Anodes, the negative electrodes, could theoretically store up to 10 times more lithium ions if they were based on silicon. This dramatic improvement over conventional graphite electrodes makes silicon attractive for use in high-performance lithium-ion batteries. However, the brittleness of the material has discouraged efforts to use pure silicon in battery anodes, which must withstand dramatic volume changes during charge and discharge cycles.
Using a combination of experimental and simulation techniques, researchers from the Georgia Institute of Technology and three other research organizations have reported surprisingly high damage tolerance in electrochemically lithiated silicon materials. The work suggests that all-silicon anodes may be commercially viable if battery charge levels are kept high enough to maintain the material in its ductile state.
“Silicon has a very high theoretical capacity, but because of the perceived mechanical issues, people have been frustrated about using it in next-generation batteries,” said Shuman Xia, an assistant professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering. “But our research shows that lithiated silicon is not as brittle as we may have thought. If we work carefully with the operational window and depth of discharge, our results suggest we can potentially design very durable silicon-based batteries.”
Ting Zhu, a professor also in the Woodruff School of Mechanical Engineering, conducted detailed molecular dynamics simulations to understand what was happening in the electrochemically lithiated silicon. As more lithium entered the silicon structures, he found, the ductile lithium-lithium and lithium-silicon bonds overcame the brittleness of the silicon-silicon bonds, giving the resulting lithium-silicon alloy more desirable fracture strength.
Supported by the National Science Foundation, the research was reported in the journal Nature Communications.
— John Toon