Whiskers, surface growth and dendrites in lithium batteries

As our love of gadgets grows, so do demands for longer lasting batteries. But there’s a problem. To make a longer-lasting battery, it needs to be bigger, and bigger isn’t better when it comes to cell phones or electric cars—not to mention pacemakers. Lithium ion batteries already have a less-than-stellar reputation: think exploding cell phones or fires on airplanes. Beyond these existing problems, when researchers attempt to shrink these batteries without compromising the performance, the results are even more unstable and prone to short-circuiting; engineers have not been able to move past these issues. Researchers at Washington University in St. Louis have new insights into the cause—or causes—of these issues, paving the way for smaller, safer, more energy-dense batteries. The result of their work has recently been published online in the journal Joule. Peng Bai, assistant professor in the School of Engineering & Applied Science, has identified three key current boundaries when it comes to these energy-dense lithium metal batteries. It turns out, engineers had been looking for one solution to what turns out to be three problems. A lithium ion battery is made of three layers: one layer of low-voltage material (graphite) called the anode; one of high-voltage material (lithium cobalt oxide) called the cathode; and a layer of porous plastic which separates the two. The separator is wetted by a liquid called an electrolyte. When the battery discharges, lithium ions empty out of the anode, passing through the liquid electrolyte, and move into the cathode. The process is reversed as the battery charges. “With half of the lithium-ion-hosting electrode materials empty at all times,” Bai said, “you are wasting half of your space. Engineers have known that they could build a more energy-dense battery (a smaller battery with a similar output capabilities) by discarding some of the dead weight that comes with half of the host materials always being empty. They have been minimally successful by removing the graphite anode, then reducing the lithium ions with electrons during recharge, a process which forms a thin plating of lithium metal.

Phys.org, 215 October 2018 ; http://phys.org