19 April 2017
The longstanding mystery of why brittle electrode materials don’t crack under the strain of expansion and contraction cycles has been resolved by a team of researchers at MIT, the University of Southern Denmark, Rice University, and Argonne National Laboratory.
The secret is apparently in the electrodes’ molecular structure. While the electrode materials are normally crystalline, with all their atoms arranged in a regular, repetitive array, when they undergo the charging or discharging process, they transform into a disordered, glass-like phase that can accommodate the strain of the dimensional changes.
The scientists looked at potential battery cathodes, called phospho-olivines, for sodium-ion batteries, and specifically at sodium-iron-phosphate.
They found it is possible to fine-tune the volume changes over a wide range — changing not only how much the material expands and contracts, but also the dynamics of how it does so. For some compositions, the expansion is slow and gradual, but for others it can increase suddenly.
“We know that brittle compounds like this would normally fracture with less than a 1% volume change,” Professor Yet-Ming Chiang explained. “So how does this material accommodate such large volume changes? What we found is that the crystal gives up and forms a disordered glass instead of maintaining its precisely ordered lattice.”
“This is a mechanism that we think might apply more broadly to other compounds of this kind,” he said. “The finding may represent a new way to create glassy materials that may be useful for batteries.”
Once the change to a glassy composition takes place, its volume changes become gradual rather than sudden, and, as a result, may be longer-lived.
The findings could provide a design tool for those trying to develop longer-lived, higher-capacity batteries. It could also lead to possible applications in which the volume changes could be used, for example, as robotic actuators or as pumps to deliver drugs from implantable devices.
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