J. Tyler Mefford, Stanford University
Operando Electrochemical Scanning Transmission X-Ray Microscopy of Energy Conversion and Storage Electrode Materials
Written by Hortense Le Ferrand
Following the local oxidation states of oxides during electrochemistry brings more understanding about the reaction paths and subsequently on how to make and use more efficient batteries.
Using soft x-rays from a synchrotron source, with energy of 100-200 eV, Tyler Mefford and his colleagues have mapped the oxidation states across the 2-µm-wide surface of hexagonal crystals of cobalt hydroxide while electrochemistry is occurring. These crystals are 75 nm in thickness, thus opaque in transmission electron microscope (TEM) but thin enough to be studied by soft x-rays. These x-rays are particularly interesting as they provide fingerprint spectra of oxides states, otherwise not available using hard x-rays of higher energies.
Adapting to the x-ray set-up in situ holder designed to perform electrochemistry in TEM, the crystal is raster scanned with a pixel size of 25 nm. Each pixel contains the absorption energy spectra from which the oxidation states are extracted. Combining spatial with temporal resolution, the researchers have discovered that the two oxidation states of the cobalt hydroxide during water splitting are +2 to +2.65, then +2.65 to +3, instead of the transitions +2 to +3 then +3 to +4 as it was previously predicted. The very high spatial resolution revealed also that the edges of the crystals are the most active.
Using this technique to map battery materials during function, the researchers also study the effects of charging and discharging rates on the distribution of atoms, phase separation, or the propagation of defects. One major output of their research is that lithium batteries are more robust when charged at fast rates.