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Schematic illustration of Li-Li Symmetric Cell under SRS imaging CREDIT QIAN CHENGColumbia Engineering

Lithium metal batteries hold tremendous promise for next-generation energy storage because the lithium metal negative electrode has 10 times more theoretical specific capacity than the graphite electrode used in commercial Li-ion batteries. It also has the most negative electrode potential among materials for lithium batteries, making it a perfect negative electrode. However, lithium is one of the most difficult materials to manipulate, due to its internal dendrite growth mechanism. This highly complex process is still not fully understood and can cause Li-ion batteries to occasionally short circuit, catch fire, or even explode.

While researchers know that the growth of dendrites, which are needle-like lithium whiskers that form internally in battery electrodes, is affected by how ions move in the electrolyte, they do not understand how ion transport and inhomogeneous ionic concentration affect the morphology of lithium deposition. Imaging ion transport in a transparent electrolyte has proved to be highly challenging, and current techniques have been unable to capture low ionic concentrations and ultrafast electrolyte dynamics.

Columbia University researchers have used Stimulated Raman Scattering (SRS) microscopy, a technique widely used in biomedical studies, to explore the mechanism behind dendrite growth in lithium batteries and, in so doing, have become the first team of material scientists to directly observe ion transport in electrolytes. They discovered a lithium deposition process that corresponds to three stages: no depletion, a partial depletion (a previously unknown stage), and full depletion of lithium ions. They also found a feedback mechanism between lithium dendrite growth and heterogeneity of local ionic concentration that can be suppressed by artificial solid electrolyte interphase in the second and third stages. The paper is published online in Nature Communications.

For this study, Yang collaborated with Wei Min, professor of chemistry at Columbia University and the study's co-author. Ten years ago, Min developed SRS with colleagues as a tool to map chemical bonds in biological samples. Yang learned about the technique from Min's website , and realized that SRS might be a valuable tool in his battery research.

The study revealed that there are three dynamic stages in the Li deposition process:

  1. A slow and relatively uniform deposition of moss-like Li when ionic concentration is well above 0;
  2. A mixed growth of mossy Li and dendrites; at this stage, Li+ depletion partially occurs near the electrode, and lithium dendrite protrusions start to appear; and
  3. Dendrite growth after full depletion. When the surface ions are fully depleted, the lithium deposition will be dominated by "dendrite growth" and you will see the quick formation of lithium dendrites.

Stage 2 is a critical transitional point at which the heterogeneous Li+ depletion on the Li surface induces the lithium deposition to grow from "mossy lithium mode" to "dendrite lithium mode." At this stage, two regions begin to appear: a dendrite region where lithium starts to deposit dendrites at a faster and faster rate, and a non-dendrite region where the lithium deposition slows down and even stops. These results are also consistent with predictions made from simulations carried out by Pennsylvania State University collaborators, Long-Qing Chen, professor of materials science and engineering, and his PhD student Zhe Liu.

Following up on their observations, the Columbia team then developed a method to inhibit dendrite growth by homogenizing the ionic concentration on the lithium surface at both stages 2 and 3.

About the study:

The study is titled "Operando and three-dimensional visualization of anion depletion and lithium growth by stimulated raman scattering microscopy." DOI: 10.1038/s41467-018-05289-z

Labels: renewable energy,battery storage,Columbia University,ions,Raman scattering microscopy,lithium

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