(Digital Presentation) Post Mortem Microscopical and Selective Analysis of Manganese Hexacyanoferrate Cathode Material By Transmission Soft X-Ray Microscopy

Abstract

Prussian Blue (PB) and its analogues (PBAs) are a large family of transition metal hexacyanoferrates with general formula of AxM[Fe(CN)6]γ□1-γ∙zH2O. They have open framework structure, redox-active sites and strong structural stability, which makes them perspective electrode materials, as PBAs are able to perform the rapid insertion and extraction of the ions with the lowest possible lattice strain. In addition, they are safe, non-toxic and relatively inexpensive. Among simple PBAs, manganese hexacyanoferrate (MnHCF) displays a high specific capacity and redox plateaus at a high voltage against lithium and sodium in LIBs and SIBs [1]. The synthesis of MnHCF is possible by simple co-precipitation method. In our group we synthesized and investigated its structure and electrochemical performance as a cathode material. For the characterization of MnHCF several conventional and synchrotron-based techniques were used.Generally, after cycling inside the battery some changes might occur in the cathode material, including changing the charge states of the different elements. For the investigation of this phenomenon synchrotron techniques are particularly informative, such as energy-dependent full field transmission soft X-ray microscopy (TXM), which is able to give a full picture at the nanometre scale of the chemical state and spatial distribution of the elements [2]. By using X‐rays in the “soft” energy region (<3 KeV), it is possible to access transitions from the core levels of the light elements, such as the K‐edge of nitrogen, oxygen, fluorine, as well as L- and M- edges of heavier elements [3, 4]. TXM provides pixel-by-pixel absorption spectrum, making it possible to select groups of pixels and map regions with the similar spectral features [2]. Therefore, TXM studies were conducted on our material before and after cycling, and the special protocol of the data treatment was developed and applied to the results of the analysis in order to enhance the representation of the heterogeneities, if any, and then to extract the most relevant spectra.Indeed, the inhomogeneities within the samples were identified, especially in the aged electrodes. It was observed that the distribution of the heterogeneities were different for the electrodes extracted from the Li-ion system, compared to Na. Characteristic spectra from the entire samples as well as particular regions with different oxidation states were obtained and compared to one another, which made it evident, that the treated results from the iron L-edge and manganese L-edge expressed strong correlation, however, while in the Fe K-edge the distribution of heterogeneities looked quite random, a weak bulk-border effect was observed in case of the Mn K-edge.[1] A. Mullaliu, J. Asenbauer, G. Aquilanti, S. Passerini, M. Giorgetti, Small Methods 4, (2019) 1900529.[2] A. Sorrentino, L. Simonelli, A. Kazzazi, N. Laszczynski, A. Birrozzi, A. Mullaliu, E. Pereiro, S. Passerini, M. Giorgetti, D. Tonti, Appl. Sci., no. 11, p. 2791, 2021.[3] D. Tonti, M. Olivares-Marín, A. Sorrentino, E. Pereiro, L. P. Mehdi Khodaei, Ed., IntechOpen, 22 March, 2017.[1] A. Mullaliu, J. Asenbauer, G. Aquilanti, S. Passerini, M. Giorgetti, Small Methods 4, (2019) 1900529.[2] A. Sorrentino, L. Simonelli, A. Kazzazi, N. Laszczynski, A. Birrozzi, A. Mullaliu, E. Pereiro, S. Passerini, M. Giorgetti, D. Tonti, Appl. Sci., no. 11, p. 2791, 2021.[4] D. Attwood, Cambridge: Cambridge University Press, 1999. Figure 1