Peering Into A Battery

A tiny lithium-ion rechargeable battery built inside a transmission electron microscope shows for the first time how a SnO₂ nanowire electrode swells and distorts as it is charged, researchers report in Science (DOI: 10.1126/science.1195628).

The open-cell battery uses an ionic liquid electrolyte with a low vapor pressure to resist evaporation under vacuum in the microscope. Calling the experiment “ingenious,” Yet-Ming Chiang, a professor of materials science and engineering at Massachusetts Institute of Technology, says in a commentary accompanying the paper that a better understanding of how SnO₂ accommodates the strain of charging “should contribute to the design of nanoscale electrodes that fully exploit the potential of ultrahigh-capacity storage materials.”

A crystalline SnO₂ nanowire made up the battery anode in the study, with bulk LiCoO₂ serving as the cathode. Researchers from Sandia and Pacific Northwest National Labs, the University of Pittsburgh, the University of Pennsylvania, and China’s Xi’an Jiaotong University used a microscope to watch what happens to the nanowire as it is first reduced. In the reduction step, four Li⁺ ions and four electrons react with a molecule of SnO₂ to produce two Li₂O molecules and metallic Sn.

The group observed that a reaction front migrates along the wire from the electrolyte, converting the wire from crystalline SnO₂ to amorphous Li₂O with metallic Sn and Li_xSn dispersed throughout as nanocrystals. The reaction front, which the researchers call the “Medusa zone,” contains a high density of crystallographic defects, or dislocations. The researchers propose that the dislocation points may be lithium transport sites. After the first charge, the Li₂O becomes a permanent part of the electrode.

As the wire’s composition and morphology change, its dimensions also alter. A wire initially 16 μm long and 188 nm in diameter gets about 60% longer and 45% wider, for a total volume increase of about 240%. The wire also bends and coils. Despite the magnitude of the changes and the accompanying strain on the wire, however, it neither cracks nor fractures.

In addition to providing a better understanding of the transformation of the nanowire’s composition, phase, and plasticity under strain, visualizing the effects could have consequences for battery assembly, says Jianyu Huang, a scientist at Sandia and the paper’s lead author. The bending, coiling, and twisting of electrode wires will have to be accommodated in battery design to prevent loss of electrical contact and shorting across different electrode wires, Huang says.