Lithium Battery Research
Li Ion Battery Aging, Degradation, and Failure
Stephen J. Harris  sjharris(at)lbl.gov

Lithium Dendrite Growth and Electrode Fracture upon Delithiation

 

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Dendrites and Fracture

 

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A disk-shaped graphite composite electrode (shown on the left of the video) is placed concentrically inside a ring of Li metal (not shown), and the potential of the graphite electrode is set to 1 mV with respect to the Li metal. Li+ ions dissolve from the Li metal and insert into the graphite from the outside in, changing its color first to blue, then red, then gold. The initial frame shows that the electrode is partially lithiated, with the outermost region colored gold (LiC6) and within that a region colored red (LiC12). Also visibe are some lithium metal dendrites that have grown on the outside edge of the graphite electrode during a previous charging cycle. As the video begins, the voltage on the graphite electrode has just been jumped to 1.0 volts, which pushes the Li out of the graphite (radially outward) toward the Li metal ring.

As the outer edge becomes delithiated, the gold color changes to red and then to blue/black. (Note that the interior red-gold boundary does not move--Lithium in this region has not yet felt the delithiation process.) As Li+ ions leave, their concentration in the electrolyte next to the electrode rises very substantially, because the rate of liquid phase transport away from the graphite electrode (toward the Li electrode) can't keep up with the rate of Li+ entering the electrolyte from the electrode. The result is a very substantial growth of the dendrites. It is easy to envision how such dendrite growth could lead to cell failure if the dendrite extends far enough to short the electrodes.

Graphite swells during lithiation and shrinks during delithiation. The shrinking process leads to a buildup of tensile stresses in the graphite electrode. Calculations of these tensile stresses during delithiation (and compressive stresses during lithiation) have been discussed (and see references therein). However, in most cases these stress calculations have referred to buildup of stresses in individual particles. In this case, we see the result of tensile stresses that build up in the electrode, causing the electrode to fracture along a north-south direction (best seen in full-screen mode). The fracture is seen to grow until a sliver of electrode appears to separate from the body of the electrode. Such a separation will cause a loss in capacity of the graphite electrode, which could lead to plating and cell failure. This may be the first direct in-situ obsevation of an electrode in the act of fracturing from delithiation.

The experimental arrangement for this experiment is described elsewhere. Individual images in the video are taken every 10 minutes. The diameter of the graphite (disk) electrode is 1.6 cm.