Study suggests route to improving rechargeable lithium batteries

Most of today’s lithium-ion batteries, which power everything from cars to phones, use a liquid as the electrolyte between two electrodes. Using a solid electrolyte instead could offer major advantages for both safety and energy storage capacity, but attempts to do this have faced unexpected challenges. Researchers now report that the problem may be an incorrect interpretation of how such batteries fail. The new findings, which could open new avenues for developing lithium batteries with solid electrolytes, are reported in the journal Advanced Energy Materials, in a paper by Yet-Ming Chiang, the Kyocera Professor of Ceramics at MIT; W. Craig Carter, the POSCO Professor of Materials Science and Engineering at MIT; and eight others. The electrolyte in a battery is the material in between the positive and negative electrodes — a sort of filling in the battery sandwich. Whenever the battery gets charged or drained, ions (electrically charged atoms or molecules) cross through the electrolyte from one electrode to the other. But these liquid electrolytes can be flammable, and they’ve been responsible for some fires caused by such batteries. They are also prone to the formation of dendrites — thin, fingerlike projections of metal that build up from one electrode and, if they reach all the way across to the other electrode, can create a short-circuit that could damage the battery. Researchers have tried to get around these problems by using an electrolyte made out of solid materials, such as some ceramics. This could eliminate the flammability issue and offer other big benefits, but tests have shown that such materials tend to perform somewhat erratically and are more prone to short-circuits than expected. The problem, according to this study, is that researchers have been focusing on the wrong properties in their search for a solid electrolyte material. The prevailing idea was that the material’s firmness or squishiness (a property called shear modulus) determined whether dendrites could penetrate into the electrolyte. But the new analysis showed that it’s the smoothness of the surface that matters most. Microscopic nicks and scratches on the electrolyte’s surface can provide a toehold for the metallic deposits to begin to force their way in, the researchers found. This suggests, Chiang says, that simply focusing on achieving smoother surfaces could eliminate or greatly reduce the problem of dendrite formation in batteries with a solid electrolyte. In addition to avoiding the flammability problem associated with liquid electrolytes, this approach could make it possible to use a solid lithium metal electrode as well. Doing so could potentially double a lithium-ion battery’s energy capacity — that is, its ability to store energy for a given weight, which is crucial for both vehicles and portable devices.”

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