The chemical phase within the battery evolves as the charging time increases; these cut-away views reveal a change from anisotropic to isotropic phase boundary motion. Images: Jun Wang.
The chemical phase within the battery evolves as the charging time increases; these cut-away views reveal a change from anisotropic to isotropic phase boundary motion. Images: Jun Wang.

Researchers at the US Department of Energy (DOE)'s Brookhaven National Laboratory have created a new imaging technique that can probe the internal makeup of a battery with different x-ray energies while the battery cell is rotated. The technique produces three-dimensional (3D) chemical maps that allow the researchers to track chemical reactions in the battery as it charges and discharges. The work is published in a paper in Nature Communications.

Getting an accurate image of the activity inside a battery as it charges and discharges is a difficult task. Often even x-ray images don't provide researchers with enough information about the internal chemical changes in a battery material, because two-dimensional images can't separate one layer from the next. Imagine taking an x-ray image of a multi-story office building from above. You'd see desks and chairs on top of one another, several floors of office spaces blending into one picture. This would make it difficult to know the exact layout of any one floor, let alone to track the movements of one person throughout the day.

"It's very challenging to carry out in-depth study of in situ energy materials, which requires accurately tracking chemical phase evolution in 3D and correlating it to electrochemical performance," said Jun Wang, a physicist at the National Synchrotron Light Source II, who led the research.

Using a working lithium-ion battery, Wang and her team tracked the phase evolution of lithium iron phosphate within the cathode as the battery charged. They combined tomography (a kind of x-ray imaging technique that displays the 3D structure of an object) with x-ray absorption near edge structure (XANES) spectroscopy (which is sensitive to chemical and local electronic changes). The result was a ‘five dimensional’ image of the battery during operation: a full 3D image over time and at different x-ray energies.

To produce this chemical map in 3D, the researchers scanned the battery cell at a range of energies that included the ‘x-ray absorption edge’ of each element of interest inside the cathode. This involved rotating the sample a full 180° at each x-ray energy, and repeating this procedure at different stages as the battery was charging. With this method, each 3D pixel, known as a voxel, contains a spectrum that acts like a chemical-specific ‘fingerprint’ for identifying the chemical and its oxidation state in the region represented by that voxel. Fitting together the fingerprints for all the voxels generates a chemical map in 3D.

This map revealed that, during charging, the lithium iron phosphate transforms into iron phosphate, but not at the same rate throughout the battery. When the battery is in the early stage of charging, this chemical evolution only occurs in certain directions. But as the battery becomes more highly charged, the evolution proceeds in all directions over the entire material.

"Were these images to have been taken with a standard two-dimensional method, we wouldn't have been able to see these changes," Wang said.

"Our unprecedented ability to directly observe how the phase transformation happens in 3D reveals accurately if there is a new or intermediate phase during the phase transformation process. This method gives us precise insight into what is happening inside the battery electrode and clarifies previous ambiguities about the mechanism of phase transformation," Wang said.

He added that modeling will help the team to explore how the spread of the phase change occurs and how the strain on the materials affects this process.

This work was completed at the now-closed National Synchrotron Light Source (NSLS), which housed a transmission x-ray microscope (TXM) developed by Wang using DOE funds made available through the American Recovery and Reinvestment Act of 2009. This TXM instrument will be relocated to Brookhaven's new light source, NSLS-II, which produces x-rays 10,000 times brighter than its predecessor. Both NSLS and NSLS-II are DOE Office of Science User Facilities.

"At NSLS-II, this work can be done incredibly efficiently," Wang said. "The stability of the beam lends itself to good tomography, and the flux is so high that we can take images more quickly and catch even faster reactions."

This story is adapted from material from Brookhaven National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.