A year ago at Sandia National Laboratories, a nanowire wriggled and twisted like a snake hatching from an egg. Researchers watched in real time as a SnO₂ nanowire electrode charged with lithium inside a tiny electrochemical device. In the December 10, 2010 issue of Science (DOI: 10.1126/science; p. 1515), J.Y. Huang, L. Zhong, C.M. Wang, and colleagues across five institutions have reported on the design of the device inside a high-resolution transmission electron microscope and have analyzed the phase and morphological changes that occurred in the electrode. Nanowire electrodes recently have shown the potential for longer life and improved performance over other material structures for electrochemical energy storage.

In a cell with ionic liquid electrolyte, the researchers applied a negative potential to a SnO₂ nanowire with respect to a LiCoO₂ counter electrode. This spurred lithium ions to react with the tin oxide nanowire, producing a reaction front that propagated longitudinally through the single crystal. In the front’s wake, an amorphous-reacted phase actively started to bend and coil while lengthening and swelling. After fully charging, the nanowire was composed of amorphous Li₂O and nanoparticles of Li_xSn and Sn, as determined by electron diffraction and electron energy-loss spectroscopy, and had a total volume change of over 200%.

SnO₂ has promising electrochemical storage capacity, but in bulk, the material is brittle. The researchers reported, however, that the tested nanowires showed high plasticity during phase transformation and no signs of fracture after lithiation. During charging, they witnessed a high density of dislocations continuously nucleate at the amorphous-crystalline interface and subsequently consumed by the advancing reaction front (see figure). The researchers said that the very high stresses resulting from the difference in wire diameter at opposite sides of the interface nucleated the dislocations in the crystalline phase. An electrochemically-driven solid-state amorphization of the nanowire took place, possibly caused by the dislocations driving the crystal far away from equilibrium but at a temperature too low for it to become liquid. Plasticity on both the crystalline and amorphous sides of the wire prevented fracture during the phase transformation.

A dislocation cloud forms at the reaction front where the single crystal SnO₂ nanowire charges with lithium ions. The reaction front propagates to the left, leaving behind a plastically-deformed, amorphous phase of Li₂O with nanoparticles of Li_xSn and Sn. Reproduced with permission from Science 330 (2010) DOI: 10.1126/science; p. 1515. © 2010 AAAS.

The investigators also studied why the nanowire morphology showed a greater increase in length compared to diameter during charging. Using density functional theory calculations of three-dimensional bulk material, the researchers said that lithium insertion in the material contributed to volume expansion, but the elastic boundary conditions of the nanowire and the low Li ion flux from the ionic liquid at the nanowire surface resulted in preferential wire elongation.

Further experiments investigated the discharge of the nanowires, and the researchers said, “The methodology described [in this report] should stimulate real-time studies of microscopic processes in batteries and lead to a more complete understanding of the mechanisms governing battery performance and reliability[.]”

A video of the nanowire charging can be accessed at http://www.efrc.umd.edu/highlights/list.php?id=18.