DOI: 10.5061/dryad.x3ffbg7t2
title: Data from: A formal Fe<sup>III/V</sup> redox couple in an intercalation electrode

description: Iron redox cycling between its low-valent oxidation states of FeII and
 FeIII drives crucial processes in nature. The FeII/III redox couple charge
 compensates cycling of lithium iron phosphate, a positive electrode
 (cathode) for lithium-ion batteries. High-valent iron redox couples,
 involving formal oxidation higher than FeIII, could deliver higher
 electrochemical potentials and energy densities. However, because of the
 instability of high-valent Fe electrodes, they have proven difficult to
 probe and exploit in intercalation systems. Here, we report and
 characterize a formal FeIII/V redox couple by revisiting the charge
 compensation mechanism of (de)lithiation in Li4FeSbO6 (LFSO).
 Valence-sensitive experimental and computational core-level spectroscopy
 reveal a direct transition from FeIII (3d5) to a negative charge-transfer
 FeV (3d5L2) ground state upon delithiation, without forming FeIV, or
 oxygen dimers. We identify that the cation ordering in LFSO drives a
 templated phase-transition to stabilize the unique FeV species and
 demonstrate that disrupting the cation ordering suppresses the FeIII/V
 redox couple. Exhibiting resistance to calendar aging, high operating
 potential, and low voltage hysteresis, the FeIII/V redox couple in LFSO
 provides a framework for developing sustainable, Fe-based intercalation
 cathodes for high-voltage applications.
description: # Dryad dataset Dataset DOI:
 [10.5061/dryad.x3ffbg7t2](10.5061/dryad.x3ffbg7t2) ## Description of the
 data and file structure This zip file contains all main text data for the
 publication "**A formal Fe^III/V^** **redox couple in an
 intercalation electrode**". Replication data for Main Text Figures
 1-4 associated with the mansucript, "A Formal Fe^III/V^ Redox Couple
 in an Intercalation Electrode". All files are zipped as
 'Main_Text_Figures_Data.zip'. Copied below are the figure
 captions for each figure. Figure 1: Two-phase crystallography is
 associated with two spectroscopic species. a, Slow galvanostatic cycling
 behavior of sol-gel LFSO. The shading along the voltage curve corresponds
 to the state of (de)lithiation, which is used in the following subfigures
 as a legend for the state of (de)lithiation at which samples were
 harvested for ex-situ characterization. Inset: Pristine and delithiated
 LFSO crystal structures, viewed in the ac plane, showing the *O3-O1* phase
 transition. Lithium and oxygen atoms are represented as light green and
 red spheres, respectively, and iron and antimony atoms are represented as
 light brown and dark blue octahedra, respectively. b, Ex-situ X-ray
 diffraction patterns collected for pristine and c, fully delithiated LFSO
 (with a constant voltage hold at 4.3 V). d, Ex-situ synchrotron X-ray
 diffraction patterns, with the (001) layering peak displayed to highlight
 the crystallographic phase transformation that occurs during delithiation
 and lithiation. Dashed grey lines indicate the positions of pristine and
 delithiated phases and are guides for the eye. e, ^57^Fe Mössbauer spectra
 collected on ex-situ samples across the voltage plateau, measured at 95 K.
 The data is represented by dots. Pristine and delithiated components are
 represented with orange and blue shading, respectively, and the total fit
 is shown with colored solid lines which represent the state of
 (de)lithiation. A vertical bar indicates 1% absorption for each sample. f,
 Fe *L*~III~-edge inverse partial fluorescence yield (iPFY) X-ray
 absorption spectra (XAS), collected on ex-situ samples across the voltage
 plateau. The data is represented by dots. Pristine and delithiated
 components are modelled with orange and blue shading, respectively, and
 the fit is shown with colored solid lines. Figure 2: Delithiated LFSO
 adopts a formal Fe^V^ oxidation state. The Fe *L*~III~-edge iPFY XAS
 spectrum of delithiated end member shows good agreement with that of
 La~2~LiFeO~6~ (a) and is distinct from that of SrFeO~3~ (d). We also
 observe this behavior using ^57^Fe Mössbauer spectroscopy – the comparison
 with La~2~LiFeO~6~ shown in b and SrFeO~3~ in e (reproduced with
 permission^39^). The O *K*-edge XAS spectra of the delithiated component,
 La~2~LiFeO~6~ (c) and SrFeO~3~ (f) all show a prominent peak at 528 eV.
 Note that the peak at 532 eV in the delithiated end member stems from
 SbV-O bonding, and the spectrum of Sb~2~O~5~ is overlaid (grey dashed
 trace) to highlight this. Figure 3: Electronic ground states of Fe^III^
 and Fe^V^ end members are modelled through RIXS and XAS calculations. a,
 Experimental O *K*-edge RIXS of lithiated LFSO. b, OCEAN calculation of
 the O *K*-edge of pristine LFSO. c, Integrated and experimental and
 simulated O *K*-edge XAS of pristine LFSO. d, Experimental O *K*-edge RIXS
 of delithiated LFSO. e, OCEAN calculation of the O *K*-edge RIXS of
 delithiated LFSO. f, Integrated experimental and simulated O *K*-edge XAS
 of delithiated LFSO. g, Experimental Fe *L*~III~-edge RIXS of pristine
 LFSO. h, Calculated Fe *L*~III~-edge RIXS using hybridized multiplets in a
 5-hole cluster, with a Fe^III^ (3*d^5^*) majority ground state. i,
 Integrated experimental and simulated Fe *L*~III~-edge XAS of pristine
 LFSO. j, Experimental Fe *L*~III~-edge RIXS of La~2~LiFeO~6~. k,
 Calculated Fe *L*~III~-edge RIXS of La~2~LiFeO~6~ using hybridized
 multiplets in a 7-hole cluster, with a hybridized majority Fe^V^
 (3*d^5^L^2^*) ground state. l, Integrated experimental and simulated Fe
 *L*~III~-edge XAS of La~2~LiFeO~6~. In all subfigures, the incident energy
 is denoted Ein. Figure 4: Introducing defects in LFSO suppresses Fe^III/V^
 redox and engages O-O dimerization. (a) Fe-Li disorder, quantified through
 refinement of neutron scattering data shows that as synthesized sol-gel
 LFSO has a low defect concentration compared to mechanically milled, solid
 state LFSO. Error bars represent uncertainties from Rietveld refinement of
 neutron scattering data are shown in black. (b) First cycle galvanostatic
 cycling at a current density of ≈3.55 mA/g (“≈C/50”) of sol-gel LFSO and
 the milled solid-state LFSO shows that milling decreases the charge and
 discharge capacities and increases voltage hysteresis. (c) Super partial
 fluorescence yield XAS shows that milling causes a reduction in the
 intensity of the Fe^V^-O peak at 528 eV, and a simultaneous increase in
 the intensity of the O-O dimer peak at 531 eV. Arrows are added as guides
 for the eye. (d) Quantification of capacity from Fe^III/V^ redox using
 Mössbauer spectroscopy during the first charging cycle shows that the
 milled samples have excess capacity that cannot be accounted for by
 Fe^III/V^ redox alone. The dots were obtained from phase fractions
 obtained from fitting Mössbauer spectra, and the dashed lines represent
 lines of best fit to the dots. The excess capacity region, representing
 the region where the observed capacity is greater than that from Fe^III/V^
 conversion, is shaded yellow. ### Files and variables **Figure 1.** * All
 data is presented as .txt files, and an .ipynb Jupyter notebook is
 provided that generates a Matplotlib version of the final figure utilizing
 these .txt files. * Fig1a_Echem.txt. * **Galvanostatic cycling data**
 includes the following columns: time/h, (Q-Qo)/mA.h/g, I/mA, Ewe/V, Ns.
 These correspond to the elapsed time after measurement, gravimetric
 capacity, current, voltage, and step number. * Fig1b_Pristine_APS_XRD.txt.
 * **Refined synchrotron X-ray diffraction data from the Advanced Photon
 Source, measured on pristine sample**. Columns include: d(Å), 14A_Pri.xye,
 SigmaYobs, Ycalc, Diff, Q(Å⁻¹), which correspond to the d-spacing, the
 data, the observed intensity, the calculated intensity, the difference.
 between observed and calculated intensity, and the scattering vector. *
 Fig1c_DelithiatedwCVhold_APS_XRD.txt * **Refined synchrotron X-ray
 diffraction data from the Advanced Photon Source, measured on sample
 delithiated to 4.3 V with a voltage hold**. Columns include: d(Å),
 14A_Pri.xye, SigmaYobs, Ycalc, Diff, Q(Å⁻¹), which correspond to the
 d-spacing, the data, the observed intensity, the calculated intensity, the
 difference. between observed and calculated intensity, and the scattering
 vector. * **Fig. 1d. X-ray diffraction data from the Stanford Synchrotron
 Lightsource.** Each consists of columns: Q(Å⁻¹), Sqrt. Intensity, which
 correspond to the scattering vector and the square root of observed
 intensity at a different state of charge. * Fig1d_Pri_SSRL_XRD.txt *
 Ex-situ XRD of pristine material. * Fig1d_1ov3Delith_SSRL_XRD.txt *
 Ex-situ XRD of material delithiated to 1/3 capacity. *
 Fig1d_2ov3Delith_SSRL_XRD.txt * Ex-situ XRD of material delithiated to 2/3
 capacity. * Fig1d_Delith4p3V_SSRL_XRD.txt * Ex-situ XRD of material
 delithiated to 4.3 V. * Fig1d_1ov3Lith_SSRL_XRD.txt * Ex-situ XRD of
 material delithiated to 4.3 V, then lithiated to 1/3 capacity. *
 Fig1d_2ov3Lith_SSRL_XRD.txt * Ex-situ XRD of material delithiated to 4.3
 V, then lithiated to 2/3 capacity. * Fig1d_Lith3p2V_SSRL_XRD.txt * Ex-situ
 XRD of material delithiated to 4.3 V, then lithiated to 3.2 V. * **Fig.
 1e. Mössbauer spectra collected at 95 K.** Columns may include: Velocity
 (mm/s), Data, Total Fit, Delithiated Component, Pristine Component. *
 Fig1e_Mossbauer_Pristine.txt * Ex-situ Mössbauer spectrum of pristine
 material. * Fig1e_Mossbauer_HalfDelith.txt * Ex-situ Mössbauer spectrum of
 material delithiated to 1/2 capacity. * Fig1e_Mossbauer_Delith4p3V.txt *
 Ex-situ Mössbauer spectrum of material delithiated to 4.3 V. *
 Fig1e_Mossbauer_Halflith.txt * Ex-situ Mössbauer spectrum of material
 delithiated to 4.3 V, then lithiated to 1/2 capacity. *
 Fig1e_Mossbauer_Lith3p2V.txt * Ex-situ Mössbauer spectrum of material
 delithiated to 4.3 V, then lithiated to 3.2 V. * **Fig. 1f. Fe *L*-edge
 XAS data collected in the inverse partial fluorescence mode (iPFY).**
 Columns may include: Energy (eV), Intensity, Pri_Comp, Delith_Comp,
 Total_Fit. Pri_Comp corresponds to the pristine component, Delith_Comp to
 the delithiated compoenent, and Total_Fit to the total fit. *
 Fig1f_FeLXAS_Pristine.txt * Ex-situ Fe *L*-edge spectrum of pristine
 material. * Fig1f_FeLXAS_1ov3Delith.txt * Ex-situ Fe *L*-edge spectrum of
 delithiated to 1/3 capacity. * Fig1f_FeLXAS_2ov3Delith.txt * Ex-situ Fe
 *L*-edge spectrum of delithiated to 2/3 capacity. *
 Fig1f_FeLXAS_Delith4p3V.txt * Ex-situ Fe *L*-edge spectrum of material
 delithiated to 4.3 V. * Fig1f_FeLXAS_1ov3Lith.txt * Ex-situ Fe *L*-edge
 spectrum of material delithiated to 4.3 V, then lithiated to 1/3 capacity.
 * Fig1f_FeLXAS_2ov3Lith.txt * Ex-situ Fe *L*-edge spectrum of material
 delithiated to 4.3 V, then lithiated to 2/3 capacity. *
 Fig1f_FeLXAS_Lith3p2V.txt * Ex-situ Fe *L*-edge spectrum of material
 delithiated to 4.3 V, then lithiated to 3.2 V. **Figure 2.** * **Fig. 2a
 and d. Fe *L*-edge XAS in iPFY mode.** *
 Fe_L3_XAS_LFSO_Delithiated_component_xaxis.npy * NumPy binary file which
 includes the x-axis (Energy / eV) data for the delithiated component
 spectrum. * Fe_L3_XAS_LFSO_Delithiated_component_yaxis.npy * NumPy binary
 file which includes the y-axis (Intensity / a.u.) data for the delithiated
 component spectrum. * Fe_L3_RIXS_La2LiFeO6 * Folder which contains the raw
 scan files for a RIXS scan on La~2~LiFeO~6~. *
 LLFO_FeL3_fullHR_15687-AI.txt * This is a "legend" for the X-ray
 emission scans located in the "Andor" folder. Columns include,
 Time, Frame #, BL 8 Energy, Izero, Izero 2, TFY, TEY, Sample Current, Beam
 Current, Total Flux, Counts, Background, Filename. The "BL 8
 Energy" columns. * Andor * This folder contains emission raw data
 files (ending in "-1D.txt") that can be plotted together to
 visualize the RIXS map. The iPFY data is processed from this data. *
 Fe_L3_RIXS_SrFeO3 * Folder which contains the raw scan files for a RIXS
 scan on SrFeO~3~, as for La~2~LiFeO~6~. * HR_SFO_HT_FeL3_6302-AI.txt *
 This is a "legend" for the X-ray emission scans located in the
 "Andor" folder. Columns include, Time, Frame #, BL 8 Energy,
 Izero, Izero 2, TFY, TEY, Sample Current, Beam Current, Total Flux,
 Counts, Background, Filename. The "BL 8 Energy" columns. * Andor
 * This folder contains emission raw data files (ending in
 "-1D.txt") that can be plotted together to visualize the RIXS
 map. The iPFY data is processed from this data. * **Fig. 2b and e.
 Mössbauer spectra.** * Mossbauer_SrFeO3.csv * This is a .csv file with two
 columns: Absorption (%) versus Velocity (mm/s), for SrFeO~3~, reproduced
 with permission from Gallagher, P. K., MacChesney, J. B. & Buchanan,
 D. N. E. Mössbauer Effect in the System SrFeO2.5–3.0. J. Chem. Phys. 41,
 2429–2434 (1964). * Mossbauer_LFSO_Delithiated_component.txt * This is a
 .txt file with two columns: Absorption (%) versus Velocity (mm/s), for
 Mössbauer data of the delithiated component of LFSO (determined from the
 fits of charged LFSO spectra). * Mossbauer_La2LiFeO6.txt * This is a .txt
 file with two columns: Absorption (%) versus Velocity (mm/s), for
 Mössbauer data collected on La~2~LiFeO~6~. * **Fig. 2c and f. O *K*-edge
 XAS.** * OK_XAS_Sb2O5.csv.csv * This is a .csv file with two columns:
 Energy (eV) and Intensity (a.u.) for O *K*-edge XAS collected on
 Sb~2~O~5~. * OK_RIXS_Intensities_LFSO_delithiated_component.npy * This is
 the npy file that contains a 2 dimensional array of the raw O *K*-edge
 RIXS intensities (in arbitary units) of the delithiated sample of LFSO.
 The 2 dimensions consist of Excitation energies and emission energies (see
 below). * OK_RIXS_Excitation_energies_LFSO_delithiated_component.npy *
 This is a 1 dimensional array of the excitation energies (incident
 energies) in eV used to collect O *K*-edge RIXS measurements on
 delithiated LFSO. *
 OK_RIXS_Emission_energies_LFSO_delithiated_component.npy * This is a 1
 dimensional array of the emission energies (incident energies) in eV used
 to collect O *K*-edge RIXS measurements on delithiated LFSO. *
 OK_RIXS_SrFeO3 * This folder contains emission raw data files (ending in
 "-1D.txt") that can be plotted together to visualize the O
 *K*-edge RIXS map, from which the O *K*-edge PFY XAS can be integrated,
 for SrFeO~3~. * OK_RIXS_La2LiFeO6 * This folder contains emission raw data
 files (ending in "-1D.txt") that can be plotted together to
 visualize the O *K*-edge RIXS map, from which the O *K*-edge PFY XAS can
 be integrated, for La~2~LiFeO~6~. **Figure 3.** * plot_figure_3_L_edge.py
 * Plotting script to plot Fe *L*-edge RIXS map, uses python3 and relevant
 libraries. * plot_figure_3_K_edge.py * Plotting script to plot O *K*-edge
 RIXS map, uses python3 and relevant libraries. * pristine_ocean * This
 directory contains the XAS and RIXS OCEAN calculation results for the O
 *K*-edge. For the XAS, it includes all the absspct files from OCEAN
 corresponding to the oxygen atoms in the calculation. The first column is
 the incident energy and second column the intensity. For the RIXS, the
 full map was calculated in different batches (absspct_X and rxsspct_X, for
 X = 0, 1, 2, ...). For each incident energy in any of the absspct files in
 absspct_X, there is a corresponding rxsspct file in rxsspct_X, showing the
 RIXS at that incident energy. The second to last index in the rxsspct file
 corresponds to the index of the incident energy in the absspct file. The
 first column in rxsspct is the energy loss, and the third column is the
 RIXS cross-section. * pristine_multiplet * This directory contains the XAS
 and RIXS files for the multiplet calculations, as well as other output
 from the code. The XAS files have two columns: incident energy and
 intensity. The RIXS files contains three columns, the first is the
 incident energy, the second is the energy loss, and the third is the
 intensity. This is true for both the Fe *L*-edge and O *K*-edge. *
 pristine_experiment * These are the raw experimental data files, where the
 first column are the incident energies, the first row are the emitted
 photon energies, and the cell blocks tying an incident energy to an
 emitted energy is the intensity for that particular transition. *
 charged_ocean * Same structure as pristine_ocean. * charged_multiplet *
 Same structure as pristine_multiplet. * charged_experiment * Same
 structure as pristine_experiment. **Figure 4.** * O *K*-edge XAS measured
 in the super partial fluorescence yield (sPFY) mode. The following three
 files contain the sPFY XAS data for the LFSO without ball milling
 (sPFY_0h_Ball_milling.npy), with 1 h of milling (sPFY_1h_Ball_milling.npy)
 and 20 h of milling (sPFY_20h.npy). These are obtained from integrating
 the RIXS emissions for these samples (shown in the SI) between the
 emission energies of 522-524 eV. * sPFY_20h.npy * sPFY_1h_Ball_milling.npy
 * sPFY_0h_Ball_milling.npy * Electrochemical Data_BallMilling.xlsx *
 Includes galvanostatic cycling data for the sol-gel sample
 ("SG"), solid-state synthesized ball-milled for 1 h
 ("1h"), and solid-state synthesized ball-milled for 20 h
 ("20 h") samples. Each contains a column for gravimetric
 capacity "(Q-Qo)/mA.h/g" and voltage "Ecell/V". All
 data files in this dataset can be read using Python, Numpy, and Pandas.
 Jupyter notebooks are included to read and process the data.