How some battery materials expand without cracking Featured

    Brittle electrodes handle expansion by going glassy, study shows.

    MIT Battery Expansion 1 Web
    These images, made by transmission electron microscopy, show the progression of the sodium-olivine electrode material, first in the original starting material in powdered form [a]; after sodium is inserted in different concentrations [b and c]; and after an amorphous, glassy structure forms in between tiny areas of microcrystalline structure [d and e]. Image courtesy of the researchers.

    When you charge a battery, or when you use it, it’s not just electricity but also matter that moves around inside. Ions, which are atoms or molecules that have an electric charge, travel from one of the battery’s electrodes to the other, making the electrodes shrink and swell. In fact, it’s been a longstanding mystery why fairly brittle electrode materials don’t crack under the strain of these expansion and contraction cycles.

    The answer may have finally been found. A team of researchers at MIT, the University of Southern Denmark, Rice University, and Argonne National Laboratory has determined that the secret is in the electrodes’ molecular structure. While the electrode materials are normally crystalline, with all their atoms neatly arranged in a regular, repetitive array, when they undergo the charging or discharging process, they are transformed into a disordered, glass-like phase that can accommodate the strain of the dimensional changes.

    The new findings, which could affect future battery design and even lead to new kinds of actuators, are reported in the journal Nano Letters, in a paper by MIT professor of materials science and engineering Yet-Ming Chiang, graduate students Kai Xiang and Wenting Xing, and eight others.

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    David L. Chandler | MIT News Office
    April 12, 2017