Image courtesy of SLAC National Accelerator Laboratory
Depiction of superionic phase of copper (blue) diffusing through the sulfur (yellow) sublattice.
Time resolved measurements were able to decipher the movement of the copper ions and the simultaneous change in the sulfur sublattice through which it travels. Being able to discern these two key features opens the door to designing energy relevant materials such as batteries based on electrochemical storage and switching devices.
Measurements indicate how a class of phase transitions with relevance to materials for next generation electrochemical energy storage occur, and show that the switching speed at the nanoscale is determined by the process by which copper ions are transported through the nanocrystal.
Ultrafast x-ray scattering and spectroscopic measurements have been used to probe the phase transition of a type of binary nanocrystal where one atomic element becomes mobile (like a liquid) and the other elemental species remains in a crystalline lattice. The crystal class (specifically Cu2S or copper sulfide in this particular case) is called "superionic" because the copper (Cu) ionic sub-lattice "melts" allowing the Cu atoms to move more freely while the sulfur (S) sub-lattice maintains the solid crystalline container. The research confirmed the supposition that it does not take much Cu movement to disorder the Cu sub-lattice, while expanding the interatomic distances between the S atoms and making the S lattice more symmetric. The key finding is that the phase transition occurs on the time scale of a single "hopping event," defined as the time needed for Cu to move from one local sweet spot to another. This insight is useful because ion transport is a key aspect of battery function and because ionic phase transition materials can be used in electrochemical switching devices.
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory
Office of Science, Basic Energy Sciences program and utilized the Advanced Light Source, the Advanced Photon Source, and the Stanford Synchrotron Radiation Lightsource, each one a BES-supported Office of Science user facility.
T.A. Miller, J.S. Wittenberg, H. Wen, S. Conner, Y. Cui, A.M. Lindenberg, Nature Communications 4, 1369 (2013).
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