Abstract
Energy-dependent full field transmission soft X-ray microscopy (TXM) is able to give a full picture at the nanometer scale of the chemical state and spatial distribution of oxygen and other elements relevant for battery materials, providing pixel-by-pixel absorption spectrum. We show different methods to localize chemical inhomogeneities in Li1.2Mn0.56Ni0.16Co0.08O2 particles with and without VOx coating extracted from electrodes at different states of charge. Considering the 3d(Mn,Ni)-2p(O) hybridization, it has been possible to discriminate the chemical state of Mn and Ni in addition to the one of O. Different oxidation states correspond to specific features in the O-K spectra. To localize sample regions with specific compositions we apply two different methods. In the first, the pixel-by-pixel ratios of images collected at different key energies clearly highlight local inhomogeneities. In the second, introduced here for the first time, we directly correlate corresponding pixels of the two images on a xy scatter plot that we call phase map, where we can visualize the distributions as function of thickness as well as absorption artifacts. We can select groups of pixels, and then map regions with similar spectral features. Core-shell distributions of composition are clearly shown in these samples. The coating appears in part to frustrate some of the usual chemical evolution. In addition, we could directly observe several further aspects, such as: distribution of conducting carbon; inhomogeneous state of charge within the electrode; molecular oxygen profiles within a particle. The latter suggests a surface loss with respect to the bulk but an accumulation layer at intermediate depth that could be assigned to retained O2.
Highlights
Battery cathodes probing the oxygen chemical state of the involved reaction products by a full XANES spectrum per pixel is a good example of the transmission Soft X-ray microscopy (TXM) capabilities [1,2,3,4,5]
We presented different strategies to analyze and represent data obIn this report, we presented different strategies to analyze and represent data obtained tained from energy-dependent full-field transmission microscopy at the O-K edge on infrom energy-dependent full-field transmission microscopy at the O-K edge on intercalation tercalation oxides
Given the substantial amount of stray light present in the MISTRAL microscope microscope, it is important to apply a correction as the, it is important to apply a correction as the one explained one explained in Appendix A and establish appropriate intervals of sufficient absorbin Appendix A and establish appropriate intervals of sufficient absorbance reliability if ance reliability if quantitative analysis needed
Summary
Battery cathodes probing the oxygen chemical state of the involved reaction products by a full XANES spectrum per pixel is a good example of the TXM capabilities [1,2,3,4,5]. Oxygen is formally the anion in the host material and its oxidation is often referred to as anion redox, in contrast to the more established and understood redox chemistry of the TM cations. This supplemental activity from the anion provides additional capacity to cathode material, and larger energy density, which explains the strong interest for these phenomena, beyond the fundamental point of view. The most studied group of this class of electrode active materials is the lithium-rich and cobalt poor nickel, manganese, cobalt oxide, with general formula Li[Li0.2 M0.8 ]O2 , where M represents one or a few TMs
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