By rotating a battery cell and scanning it with different x-ray energies as it charges, researchers have been able to produce a three-dimensional map of the chemical reactions taking place.
The technique developed by researchers at the US Department of Energy's Brookhaven National Laboratory is claimed to be an improvement on two-dimensional x-ray imaging, which can’t separate one layer from the next.
"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," explains Jun Wang, a physicist at the National Synchrotron Light Source II who led the research.
The work, which was supported by the US Department of Energy, is reported in the 12 August issue of Nature Communications.
Using a lithium-ion battery, Wang and her team tracked the phase evolution of the lithium iron phosphate within the electrode as the battery charged. By combining tomography with a technique known as x-ray absorption near edge structure spectroscopy that is sensitive to chemical and local electronic changes, they were able to generate a ‘five dimensional’ image - a full three-dimensional image over time and at different x-ray energies.
To make this chemical map in 3D, they scanned the battery cell at a range of energies that included the ‘x-ray absorption edge’ of the element of interest inside the electrode, rotating the sample a full 180 degrees at each x-ray energy, and repeating the procedure at different stages as the battery was charging. Each three-dimensional pixel, or ‘voxel’ produces a spectrum like a fingerprint that identifies the chemical and its oxidation state in the position represented by that voxel. Fitting together the fingerprints for all voxels generates a chemical map in 3D.
This 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 occurs in only certain directions. But as the battery becomes more highly charged, the evolution proceeds in all directions over the entire material.
A two-dimensional model wouldn’t show this effect, says Wang. "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."
Modelling will help the team explore the way the spread of the phase change occurs and how the strain on the materials affects this process.