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Condens. Matter 2018, 3, x FOR PEER REVIEW 5 of 13 Condens. Matter 2018, 3, 36 5 of 13 during discharge. This rising-edge feature arises from normally forbidden dipole transitions to empty bound states involving the breakdown of the octahedral symmetry of ordered atomic shells cyanides [40–42]. More details on the fitting results and the exact correspondence of the displayed beyond the cyanides [40–42]. More details on the fitting results and the exact correspondence of the states with the collected spectra can be found in the Supplementary Materials. displayed states with the collected spectra can be found in the Supplementary Materials. Figure 4. Pre-edge data analysis for the pristine, partially charged, fully charged, partially discharged, Figure 4. Pre-edge data analysis for the pristine, partially charged, fully charged, partially fully discharged states, and their comparison. discharged, fully discharged states, and their comparison. 2.3. EXAFS Data Analysis 2.3. EXAFS Data Analysis Complementary structural information about both Fe and Co sites can be extracted from the Complementary structural information about both Fe and Co sites can be extracted from the EXAFS signals. Analysis of the pristine electrode was first done to check the reliability of the structural EXAFS signals. Analysis of the pristine electrode was first done to check the reliability of the model satsruwcteullralsmtodseltathsewreelleavsatontseptatrhaemreltevrasnftopratrhaemmetienrismfoirzathtieomn.inInimaizsaetcion.dInmaosmeceontd, tmhoemanenatl,ysis the analysis was extended to the operando dataset. For an accurate extraction of the structural was extended to the operando dataset. For an accurate extraction of the structural information, all the information, all the measurements were analyzed by multiple-edge approach [43], that is, measurements were analyzed by multiple-edge approach [43], that is, a simultaneous fitting procedure a simultaneous fitting procedure at both Fe and Co metal edges. This in turns means that the same at both Fe and Co metal edges. This in turns means that the same structural parameters are probed structural parameters are probed using two independent measurements, hence the reliability of the using two independent measurements, hence the reliability of the fitting minimization is enhanced. fitting minimization is enhanced. Figure 5 displays the details of the EXAFS analysis for the pristine, Figure 5 displays the details of the EXAFS analysis for the pristine, partially charged, fully charged, partially charged, fully charged, partially discharged, fully discharged states at Fe (top layer) and partially discharged, fully discharged states at Fe (top layer) and Co (bottom layer) K-edges. Briefly, Co (bottom layer) K-edges. Briefly, (i) only few relevant single EXAFS contributions were necessary (i) only few relevant single EXAFS contributions were necessary to simulate the overall EXAFS signal to simulate the overall EXAFS signal at each edge; (ii) not only the two-body signals, but also the at eachtherdege-e;a(niid) nthoet ofonulyr-bthodeytwMoS-btoerdmyssiagrnearlesl,ebvuanttalwsoiththientehnrseitei-esancdomthpearfaobuler-tboodthyeMfirSstesrhmelsl;are 8c 8c 8c 8c relevan(itiiw) tihthe itnwtoe-nbsoidtiyesFec-oMmpanrdabCleo-tMo thseigfinraslst sdheepleln; d(iioi)nththeetcwatoio-bnoodcycuFpea-nMcy daunridngCoch-Marge saingdnals discharge, explaining the reduced intensities observed for fully charged states; (iv) the multi-edge depend on the cation occupancy during charge and discharge, explaining the reduced intensities refinement allows one to take into consideration the oscillation of the MS terms at the Fe K-edge that observed for fully charged states; (iv) the multi-edge refinement allows one to take into consideration could overlay the Co K-edge. the oscillation of the MS terms at the Fe K-edge that could overlay the Co K-edge. The fit was performed on a representative selection of spectra by taking into account the The fit was performed on a representative selection of spectra by taking into account the XANES XANES traces and the result of the MCR-ALS analysis. The most relevant fitting results are traces and the result of the MCR-ALS analysis. The most relevant fitting results are displayed in displayed in Figure 6. Here, the difference in the trends displayed in panels a and b is noteworthy. Figure 6. Here, the difference in the trends displayed in panels a and b is noteworthy. Indeed, Indeed, even though bond distances were not significantly altered in the process, the disorder even though bond distances were not significantly altered in the process, the disorder related to Fe–N related to Fe–N bonds increased considerably near the fully charged state, that is, the state with all bondsintcerestaistieadl cionssideirnasberltyedn.eaOrnthtehefuoltlhyecrhsairdgee,dthsetaCteo,–tChaftraisg,mthenetstwataeswnietihthaerlleinletcetrsotniitciallyions deinser(atesdo.bsOernvetdhebyotherXsAidNeE,Sthoef FCigou–rCe 2fr)angomr setnrutcwtuarsallnyeaitfhfecrteedlebcytrtohneicelaelcltyro(cahsemobicsaelrvreadctiboyn.the Additionally, the number of linear Fe–NC–Co chains, labelled as CNchain, set the degeneracy of the XANES of Figure 2) nor structurally affected by the electrochemical reaction. Additionally, the number Fe–NC–Co fragments and therefore canchbaeinconsidered as an indicator of the linearity of the chain of linear Fe–NC–Co chains, labelled as CN , set the degeneracy of the Fe–NC–Co fragments and (this is due to the strong focusing effect of the linear Co–C–N and of the additional superfocusing therefore can be considered as an indicator of the linearity of the chain (this is due to the strong one carried out by the linear Co–C–N–Fe fragment). As seen from panel c, the CNchain parameter focusing effect of the linear Co–C–N and of the additional superfocusing one carried out by the linear Co–C–N–Fe fragment). As seen from panel c, the CNchain parameter oscillated around a value of 3.5 during charge, while its centroid slightly decreased in the discharge. Although the large error bars set a limit to further considerations, we might assume that the lithiation could induce a small deviation from the linearity of the orthogonal chains. Finally, the Fe coordination number to the interstitialPDF Image | XAFS and XRD Study of a Prussian Blue Analogue Cathode Iron Hexacyanocobaltate
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