If Cui succeeds in making the new materials work, it will be partly because he can see inside the experimental batteries while they are operating. This "in situ" technology, providing extraordinary black-and-white images, has become readily available only in the past several years and is improving all the time. High-energy, extra-bright X-rays generated at the SLAC National Accelerator Laboratory can capture a material expanding and breaking, helping researchers probe batteries closely. The X-rays travel through helium-filled pipes so that they are not absorbed too soon, and then get focused through a condenser lens on the tiny battery sample that had been prepared in an argon-filled box, away from moisture and oxygen.
Johanna Nelson Weker, an associate staff scientist at the Stanford Synchrotron Lightsource within SLAC, has been x-raying batteries for Cui and others for more than three years. She has seen some fascinating things. One battery, made from the element germanium, expanded and contracted before her eyes, as the lithium ions moved between the cathode and anode while the battery charged and discharged. "We can watch them essentially breathe," she says. "As you lithiate them, they expand, and as you de-lithiate them, they contract." A recent X-ray image showed one of Cui's new experiments with silicon particles in a self-healing polymer—the silicon expands and breaks the polymer, which mends itself. The image was black-and-white and appeared to a lay viewer the way clouds do from a plane. Soon, Weker hopes to add another dimension to the images, so that they will appear in color, reflecting their chemical state.
With that ability, researchers could "see where the lithium is going, really," she says. "And that's what everybody wants to know. We can't actually see the lithium, but we can see this interact with the metal." The goal is to "see the lithium basically changing a particle from red to green as it lithiates, and then green to red as it de-lithiates."
The trick is "getting the information you want without damaging your sample," Weker says. That means making sure the batteries are not overexposed to radiation through too frequent X-rays. Some samples stay in the X-ray booth for more than a day, with X-rays running every half-hour.
Electron microscopy, available at Stanford rather than SLAC, produces even higher-resolution images of atoms. However, in order to absorb the electrons, the batteries must be so thin that they are "not close to a normal operating battery," Weker says. Still, she says, the technique is good for very thin materials such as the silicon nanowires Cui has used as a building block.
Cui is bullish on imaging technology. "It can further improve," he says, but "even with the current capability, there's a lot of battery problems we can study."
