Mapping Heterogeneity of Pristine and Aged Li- and Na-Mnhcf Cathode by Synchrotron-Based Energy-Dependent Full Field Transmission X-ray Microscopy.

Manganese hexacyanoferrate is a promising cathode material for lithium and sodium ion batteries, however, it suffers of capacity fading during the cycling process. To access the structural and functional characteristics at the nanometer scale, fresh and cycled electrodes are extracted and investigated by transmission soft X-ray microscopy, which allows chemical characterization with spatial resolution from position-dependent x-ray spectra at the Mn L-, Fe L- and N K-edges. Furthermore, soft X-rays prove to show superior sensitivity toward Fe, compare to hard X-rays. Inhomogeneities within the samples are identified, increasing in the aged electrodes, more dramatically in the Li-ion system, which explains the poorer cycle life as Li-ion cathode material. Local spectra, revealing different oxidation states over the sample with strong correlation between the Fe L-edge, Mn L-edge, and N K-edge, imply a coupling between redox centers and an electron delocalization over the host framework.