Lithium-ion batteries are the best thing on the market when it comes to powering things like laptops, cell phones and tablets. As a material, lithium is fantastic capacity and an unmatched ability to move lithium ions and electrodes in and out of an electrode. It’s the perfect material for anodes, were it not for its tendency to short-circuit and overheat. Instead, graphite is used for the anodes, significantly limiting the battery’s energy density.

The problem lies with “dendrites,” microscopic hair-like protrusions of lithium that sprout and spread from lithium anodes as the battery moves through charge and discharge cycles. Until now, researchers using electron microscopes have been stumped, as they only observed dendrite formation at the surface at the anodes. However, 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 inside lithium anodes and growing out into a polymer electrolyte during cycling.

“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,” Balsara says. “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.”

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,” says Katherine Harry, a member of Balsara’s research group. “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.”

Moving forward, the team is investigating ways of removing the non-conductive contaminates from the lithium anodes. Hopefully, their research will pave the way for more efficient, longer-lasting batteries in the future.