Over the course of several battery charge/discharge cycles, particularly when the battery is cycled at a fast rate, lithium dendrites sprout from the surface of the lithium electrode and spread like kudzu across the electrolyte until they reach the other electrode. The passage of current through these structures can result in ignition of the electrolyte and catastrophic failure.

These 3D reconstructions show how dendritic structures that can short-circuit a battery form deep within a lithium electrode, break through the surface and spread across the electrolyte. Source: Harry et al. Click to enlarge.

Efforts to prevent dendrite growth have primarily focused on blocking these protrusions, the Berkely Lab team notes.

This Article demonstrates the presence of subsurface structures within the lithium electrode that lie underneath the dendrites. Furthermore, the formation of the subsurface structures dominates early stages of dendrite growth in polymer electrolyte cells at 90 °C, suggesting that the key to preventing dendrite formation is the elimination of filamentous cavities inside the electrode.

The electrodes in today’s lithium ion batteries are porous, and about 30 vol% of the electrode is taken up by inactive phases. With a lithium metal anode however, the simplicity of the reactions at the lithium electrode and facile transport of electrons within the metal eliminates the need for designing porous electrodes containing separate phases for transporting ions and electrons. Furthermore, many of the high-energy-density battery technologies being researched today, such as lithium–sulphur and lithium–air batteries, assume the presence of a lithium metal anode. It is thus not surprising that researchers have used a variety of tools to study dendrite formation in lithium batteries. These include optical and electron microscopy, nuclear magnetic resonance, magnetic resonance imaging and so on.

These techniques have detected ‘tree-like’ or ‘moss-like’ structures that emanate from the lithium surface and protrude into the electrolyte. A key advance in this Article is the use of synchrotron hard X-ray microtomography that enables the imaging of structures residing on either side of the lithium metal electrode/electrolyte interface, illuminating the presence of subsurface structures in the lithium anode beneath dendritic protrusions.

—Harry et al.

The Berkeley Lab team discovered that during the early stages of development, the bulk of dendrite material lies below the surface of the lithium electrode, underneath the electrode/electrolyte interface. Using X-ray microtomography at Berkeley Lab’s Advanced Light Source (ALS), a team led by Nitash Balsara, a faculty scientist with Berkeley Lab’s Materials Sciences Division, observed the seeds of dendrites forming in lithium anodes and growing out into a polymer electrolyte during cycling. It was not until the advanced stages of development that the bulk of dendrite material was in the electrolyte. Balsara and his colleagues suspect that non-conductive contaminants in the lithium anode trigger dendrite nucleation.

Contrary to conventional wisdom, it seems that preventing dendrite formation in polymer electrolytes depends on inhibiting the formation of subsurface dendritic structures in the lithium electrode. In showing that dendrites are not simple protrusions emanating from the lithium electrode surface and that subsurface non-conductive contaminants might be the source of dendritic structures, our results provide a clear prescription for the path forward to enabling the widespread use of lithium anodes.

—Nitash Balsara

This is the first study to employ microtomography using monochromatic beams of high energy or “hard” X-rays, ranging from 22 to 25 keV, at ALS beamline 8.3.2. This technique allows non-destructive three-dimensional imaging of solid objects at a resolution of approximately one micron.

We observed crystalline contaminants in the lithium anode that appeared at the base of every dendrite as a bright speck. The lithium foils we used in this study contained a number of elements other than lithium with the most abundant being nitrogen. We can’t say definitively that these contaminants are responsible for dendrite nucleation but we plan to address this issue by conducting in situ X-ray microtomography.

—Katherine Harry, lead author

Balsara and his group also plan further study of the role played by the electrolyte in dendrite growth, and they have begun to investigate ways to eliminate non-conductive impurities from lithium anodes.

This research was funded by the DOE Office of Science.

Resources

• Katherine J. Harry, Daniel T. Hallinan, Dilworth Y. Parkinson, Alastair A. MacDowell and Nitash P. Balsara (2014) “Detection of subsurface structures underneath dendrites formed on cycled lithium metal electrodes.” Nature Materials 13, 69–73 doi: 10.1038/nmat3793

• Wu Xu, Jiulin Wang, Fei Ding, Xilin Chen, Eduard Nasybulin, Yaohui Zhangad and Ji-Guang Zhang (2014) “Lithium metal anodes for rechargeable batteries.” Energy Environ. Sci. doi: 10.1039/C3EE40795K

• J.-M. Tarascon and M. Armand (2001) “Issues and challenges facing rechargeable lithium batteries.” Nature 414, 359-367 doi: 10.1038/35104644