Developing next-gen batteries using Si-based electrodes

A team of researchers from the Georgia Institute of Technology and three other research organisations has conducted a detailed nano-mechanical study of mechanical degradation processes in silicon structures containing varying levels of lithium ions. The result offers good news for researchers aiming to create reliable next-generation rechargeable batteries using silicon-based electrodes.

Anodes based on silicon can theoretically store up to ten times more lithium ions than conventional graphite electrodes, making the material attractive for 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, the researchers 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.

Shown is a sample holder used to test samples of lithiated silicon to determine its nano-mechanical properties. The device was used to develop a detailed nano-mechanical study of mechanical degradation processes in silicon thin films. (Credit: Rob Felt, Georgia Tech)

"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 the George W. Woodruff School of Mechanical Engineering at Georgia Tech. "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."

Lithium ion batteries are used today in a range of applications from hand-held mobile devices up to laptop computers and electric vehicles. A new generation of high-capacity batteries could facilitate expanded transportation applications and large-scale storage of electricity produced by renewable sources.

The challenge is to get more lithium ions into the anodes and cathodes of the batteries. Today's lithium batteries use graphite anodes, but silicon has been identified as an alternative because it can store substantially more lithium ions per atom. However, storing those ions produces a volume change of up to 280 per cent, causing stress that can crack anodes made from pure silicon, leading to significant performance degradation. One strategy is to use a composite of silicon particles and graphite, but that does not realise the full potential of silicon for boosting battery capacity.

In an effort to understand what was happening with the materials, the research team used a series of systematic nano-mechanical tests, backed up by molecular dynamics simulations. To facilitate their study, they used silicon nanowires and electrochemical cells containing silicon films that were about 300nm thick.

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