Operando X-ray Absorption Spectroscopy and Emission KÎ²1,3 Study

Article Cite This: J. Phys. Chem. C 2018, 122, 29586−29597

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Operando X‑ray Absorption Spectroscopy and Emission Kβ1,3 Study of the Manganese Redox Activity in High-Capacity Li4Mn2O5 Cathode Maria Diaz-Lopez,*,† Yves Joly,† Melanie Freire,‡ Claire Colin,† Olivier Proux,† Valerie Pralong,‡ and Pierre Bordet† †

Université Grenoble Alpes, Institut Néel, CNRS, Institut Néel, F-38000 Grenoble, France Laboratoire de Cristallographie et Sciences des Matériaux CRISMAT, ENSICAEN, Normandie Université, CNRS, Université de Caen Normandie, 6 Bd Maréchal Juin, F-14050 Caen, France

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‡

S Supporting Information *

ABSTRACT: The electrochemical performance of nanostructured Li4Mn2O5 (or rather the 0.93Li3.6−xMn2.4O5.4− 0.07Li2O composite) displaying an outstanding charge capacity of 350 mA h/g was recently reported. Interestingly, the removal of lithium from Li4Mn2O5 is found to take place beyond the oxidation limit of +4 for Mn in an octahedral environment. To characterize the nature of this extra capacity, we have approached the study of the redox chemistry and local structure of manganese in Li4Mn2O5 via a combination of X-ray absorption and emission spectroscopies at the manganese K-edge. To support our results, we have thoroughly characterized the composition of the materials at several potential values by inductively coupled plasma and online electrochemical mass spectrometry. Additionally, operando X-ray absorption near-edge structure studies, in excellent agreement with ex situ data, were carried out for the charge and discharge of the battery. Our results unequivocally rule out the participation of the Mn4+/Mn5+ redox couple and indicate the participation of oxygen in the electrochemistry. After the first charge, the battery cycles reversibly between the charged and discharged states, where the lithium exchange is mainly compensated by anionic redox.

1. INTRODUCTION In years to come, lithium-ion batteries (LIBs) will play a key role in the transition from fossil fuels into renewable power sources. Thus far, significant technological advancements in this field have allowed for the implementation of LIBs in portable power sources and electric vehicles with zero emissions. However, a further increase in the energy density of LIBs is necessary to adapt this technology to current demands. LIBs allow for the storage and release of electricity by its conversion into chemical energy via electrochemical reactions that alternate between the charge and discharge states. The energy density of LIBs is determined by the capacity of the electrodes to store Li+ cations and the discharge potential of the cell. As the current anode materials offer Li+ cation storage capacities superior to that of cathode materials (e.g., 450 mA h/g for carbon negative electrodes or 600 mA h/ g for Li3−xCoxN1 vs 250−300 mA h/g for the best performing Li-rich layered cathodes2−4), the latter limits the performance of LIBs. Therefore, many researchers are exploring ways to improve the capacity of cathode materials. A new class of positive electrode materials has emerged in recent years, utilizing the solid-state redox reactions of oxide anions along with that of transition-metal (TMs) cations.5−7 © 2018 American Chemical Society

Although the solid-state oxidation by nonmetal anions has been known for decades for sulfide anions which can oxidize to persulfide,8 the charge compensation of oxide anions is a more recently discovered phenomenon, which is not yet fully understood. The reason behind this lies in the difference in size between the two anions: the oxidation of sulfide anions is an easier process because of the larger polarizability of the outer 3p electrons. Recently, numerous studies have been dedicated to understanding the structural and chemical origin of the solid-state redox reactions of oxide anions to obtain the knowledge required for the rational design of the next generation of cathode materials. Li1.2+[Co0.133+Mn0.544+]O2 has been extensively investigated as an archetype LIB cathode material, exhibiting an extra capacity associated to anionic redox.9−12 The exchange of 0.4 electrons on the oxygen anions per formula unit of Li1.2+[Co0.133+Mn0.544+]O2 during the first charge was first demonstrated in ref 10. Later on, Luo et al.11 demonstrated that this charge compensation occurred via the formation of Received: September 26, 2018 Revised: December 7, 2018 Published: December 7, 2018 29586

DOI: 10.1021/acs.jpcc.8b09397 J. Phys. Chem. C 2018, 122, 29586−29597

Article

The Journal of Physical Chemistry C

could vary upon different fitting procedures (e.g., probed temperature range and magnetic impurity subtraction). The oxidation of manganese beyond the tetravalent state was questioned in ref 25, where we studied the local structures of Li4 and the delithiated compounds by X-ray pair distribution function analysis (X-PDF). The X-PDF data of Li0 did not show the short Mn5+−O distances expected for Mn5+ in a tetrahedral environment (the only type of coordination reported to date for Mn5+ cations). Moreover, such modification of the manganese cation coordination from octahedral to tetrahedral would entail major structural rearrangements that were not observed by X-PDF. In this work, we have approached the study of the redox activity and structural evolution of manganese by means of a combined X-ray emission spectroscopy (XES) and XAS at the Mn K-edge, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). XES and ex situ XAS, or rather high-energy resolution fluorescence-detected XAS (HERFD-XAS), were acquired using a crystal analyzer spectrometer (CAS) with a ∼1 eV energy resolution.26 We have studied several compositions ex situ, including the pristine Li4−Li2O (P), delithiated Li0 after the first charge (C1), and, for the first time, we report the study of other compositions beyond the first charge, including the relithiated compounds obtained after the first and third discharges (D1 and D3, respectively). The EXAFS region of ex situ data was fitted using rock salt models with modified occupancies to extract valuable information on the average local structures via the quantification of interatomic distances and structural disorder. Additionally, operando XANES data were collected for a full charge− discharge cycle and a second charge to avoid spurious observations because of the potential high instability of the lithium-depleted phases from ex situ experiments. This study gave us a clear insight into the change in the OS of manganese as well as the local geometry of the absorber atom, which allowed for a better understanding of the cathode performance.

localized electron holes on O atoms coordinated by Mn4+ and Li+ ions. The “labile” electrons in oxygen could have originated from the formation of Li−O−Li configurations, that is, one oxygen atom linearly bonded to two Li, where the oxygen orbital along Li−O−Li contributes to large projected density of states (DOS) close to the Fermi level.13 Oxygen anionic redox participation is also a well-studied phenomenon for the layered Li2MnO3 and “Li2MnO3-like” positive electrode materials. Monoclinic Li2MnO3 has been extensively investigated because of its outstanding charge capacity of 260 mA h/g.14 As the oxidation of tetravalent manganese to higher oxidation states (OSs) is challenged, a part of the O2− anions irreversibly oxidizes to O2 gas that is released and permanently removed from the cell15 and also appears to reversibly transform into peroxide and superoxide (O22−/O2−) species, as experimentally observed by soft X-ray absorption spectroscopy (XAS).16 The reversible redox participation of oxygen is widely accepted for Li2RuO317 and Li2IrO3,18 isostructural with Li2MnO3. The higher degree of covalency of the larger 4d and 5d TMs allowed for the stabilization of the metal in an octahedral environment, suppressing unfavorable phase transitions observed for Li2MnO319 and allowing for a higher reversibility of the charge capacity.20 The reversible redox participation of oxygen in Li2MO3, with M = Ru and Ir, is stabilized through the formation of “peroxo-like” species consisting of O−O dimers with unusual interatomic distances: ∼2.3 and ∼2.5 Å for Ru21 and Ir,22 respectively. Recently, a cation-ordered rock salt with a further lithium excess, Li3NbO4, has been reported showing a high reversible capacity of 300 mA h/g at 50 °C originated from anionic redox.23 The aim of this work is to investigate the intervening redox species in the recently discovered 0.93Li3.6−xMn2.4O5.4− 0.07Li2O composite cathode material with a record discharge capacity of 355 mA h/g.24,25 Prior to the identification of the Li2O component, the outstanding capacity of the composite was attributed to the sole participation of Li3.6−xMn2.4O5.4 (Li4) with a disordered MnO-type rock salt structure, when, in fact, the measured capacity of the same compound prepared without any trace of Li2O was markedly lower, ∼200 mA h/g. After the first charge to 4.4 V, ∼3 Li per formula unit can be extracted from the composite that is electrochemically oxidized to Li0Mn2.4O5.4 (Li0), also displaying a rock salt structure, with a contraction of the lattice parameters with respect to the pristine Li4. Interestingly, ex situ X-ray diffraction data showed no traces of Li2O after the first charge, pointing to the participation of Li2O in the electrochemical performance of the cathode.25 However, the mechanism by which Li3.6−xMn2.4O5.4 and Li2O are able to interact during the first charge with the resulting enhancement of the capacity is still unknown. After the first charge, the fully delithiated Li0 shows a reversible capacity of ∼190 mA h/g for up to 45 cycles investigated.25 From simple electroneutrality considerations, the complete removal of lithium from Li4 cannot be reached by the sole participation of the Mn3+/Mn4+ redox couple, and the electrochemical activity of at least one additional pair must be considered: either Mn4+/Mn5+ and/or O2−/O2 and/or O2−/O22−. In a previous work,24 the Mn valence of the delithiated compound was evaluated by magnetic susceptibility measurements. However, magnetic susceptibility measurements are somewhat inaccurate, as they are greatly sensitive to the presence of magnetic species at even trace concentrations, and the calculated effective magnetic moments

2. EXPERIMENTAL SECTION 2.1. Synthesis and Sample Preparation. The nanostructured 0.93Li3.6−xMn2.4O5.4−0.07Li2O powder from ref 25 (hereafter denoted as P) was cycled in a Swagelok cell under the same conditions specified in ref 25. Three new samples studied ex situ were carefully prepared by stopping the cell at the desired potential in Figure 1: at the end of the first charge to 4.4 V (sample C1) and at the end of the first and third discharges to 1.2 V (samples D1 and D3, respectively). Samples P and C1 were previously characterized using synchrotron powder diffraction.25 Sample D1 was studied by powder X-ray diffraction (PXRD) using a Bruker D8 ADVANCE diffractometer equipped with a monochromatized MoKα1 X-ray source, (λ = 0.70930 Å). The data were collected from 2θ = 10 to 150° with a 0.01 step size. These data allowed us to evaluate the purity of the samples and to accurately determine the lattice parameters used in the simulations of the XANES and XES spectra. The Li/Mn ratios were determined by inductively coupled plasma optical emission spectroscopy (ICP−OES) elemental analysis, following the procedure in ref 25, and are reported in Table 1, along with those from electrochemistry. The Mn/O ratio for sample P, containing 7 wt % of Li2O, was determined via the combined Rietveld refinement of neutron and PXRD data, where the overall Li/Mn ratio in the composite sample 29587


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XAS allows to obtain XANES spectra with almost a core-hole lifetime-free broadening, that is, providing optimal energy resolution.26,27 2.3. Operando XAS. Operando XANES data at the Mn Kedge were acquired at the FAME French CRG beamline (BM30b) at the ESRF. The beam energy was selected using a cryo-cooled double-crystal monochromator, whose energy value was calibrated with a standard Mn foil. For the in situ experiment, the electrochemical cell from ref 28 equipped with two beryllium windows was used. Although a good signal could be recorded in transmission for the first few hours (∼15 h) of the experiment, this signal was significantly weakened because of the local radiation damage impinged to the electrolyte by the highly energetic beam. Therefore, only the fluorescence signal recorded by a 30-element Ge solid-state point detector has been introduced in this work. The Mn K-edge XAS spectra were collected in operando conditions without any current interruption. Each spectrum collected in operando corresponds to the data accumulated over 15 min, with a “beam-off” interval time of also 15 min between the recorded spectra. As an additional measure to protect the battery constituents from an overexposure to the high-energetic beam that could have ultimately lead to radiation damage and battery failure, the position of the beam impact on the sample was changed by displacing the cell between each spectrum, which also provided with a confirmation of homogeneity within the sample. 2.4. XAS Data Treatment. All the spectra introduced here correspond to at least six scans that were averaged, background-corrected, and normalized with the ATHENA software.29 2.4.1. XANES Simulation. The XES and HERFD-XAS spectra were simulated for all the compositions studied using the FDMNES software. The simulations were done selfconsistently using the finite difference method (FDM) for atoms of fixed radius, considering a large cluster of 6 Å around the absorber atoms, as the simulated spectra using a cluster size with a radius greater than 6 Å were identical to the former. Within the chosen radius, the photoelectron interacts with ∼90 atoms. The structural models used for simulating the XANES spectra of the standard samples were performed using the published data. For the cathode materials, we built four initial models by the expansion of an average MnO rock salt model into a 2 × 2 × 2 supercell accommodating substitutional disorder to match the compositions Li19Mn13O29 (P), Li5Mn13O 29 (C1), Li16 Mn13 O29 (D1), and Li14 Mn13 O29 (D3). The cell parameters of the models used in the simulations published in this work were accurately determined by the Rietveld refinement of X-ray data30 (see Figure SI1 for the refinement of the newly reported discharged compositions). As the output of the FDMNES simulations corresponds

Figure 1. Galvanostatic curve for the Li4−Li2O composite half-cell over three charge/discharge cycles. The circles indicate the potential values at which the cells were stopped to extract the compositions studied.

was in excellent agreement with ICP.25 All samples studied ex situ were diluted in BN (5 wt %), thoroughly mixed and uniaxially pressed. Thereby, the obtained pellets were mounted on a sample holder with Kapton windows to allow the entry and exit of X-rays. The handling and preparation of the airsensitive cathode materials were carried out in an inert atmosphere inside an Ar-filled glovebox. For the operando XAS experiment, the P sample was diluted with 80% carbon black to optimize the signal collected in standard fluorescence mode. The XANES spectra of the sample diluted with 80% of C measured in this way perfectly overlapped with the signal collected in transmission mode. 2.2. Ex Situ XES and HERFD-XAS. The aforementioned electrosynthesized samples and four commercial standards with Mn in various OSs, Mn2+O, Mn23+O3, Mn4+O2, and Ba3Mn25+O8, were investigated ex situ by XES and HERFDXAS at the FAME-UHD French CRG beamline (BM16) of the European Synchrotron Radiation Facility (ESRF) using angledispersive analysis of the fluorescence X-ray signal emitted from the sample. The spectrometer was equipped with five spherically bent Si(440) crystal analyzers focusing the diffracted beams onto a silicon drift detector for the recording of the Mn Kβ lines. The energy and experimental setup were calibrated by recording the Mn foil spectrum. The energy resolution of the spectrometer, including contributions from both the monochromator and the CAS, was estimated by the recorded width of a pseudo-elastic peak. For each sample, the spectrometer was positioned at the maximum of the Mn Kβ emission line prior to the recording of the XANES spectra. HERFD-XAS spectra were then collected at the selected emission, which allowed discrimination of the detected photons with an energy resolution of 0.8 eV, smaller than the core-hole lifetime of the fluorescence line. Thus, HERFDTable 1. Measured Li Content and Charge/Discharge Capacities x in Li3.6−xMn2.4O5.4

expected Mn OSb

capacity (mA h/g)

sample ID (abbreviation)

from electrochemistry

from ICP−OESa

pristine (P) charged (C1) discharged 1st cycle (D1) discharged 3rd cycle (D3)

0 2.9 0.55 0.9

0(1) 3(0.2) 0.5(3)

charge 375 375 332

discharge

290 331

cumulative

85 1

3 4.21 3.23 3.38

The ICP−OES results are the average of at least five samples, where each sample was analyzed in three replicates. bThe expected OSs were 2− calculated from the electrochemically derived formula as Li3.6−x+MnOS 2.4 O5.4 . a

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3.1.2. HERFD−XANES. In XAS, an incoming X-ray excites a core electron to a higher nonoccupied orbital. The XAS spectrum reflects the symmetry-projected partial DOS of those excited states allowed by selection rules. For the K-edge (i.e., the generation of a 1s electron hole) of first row TMs, the first formally allowed electric dipole transition is the 1s−4p transition. With the increase in the OS, the positive charge of the cation increases, and more energy is required to create an electron hole. As a result, we observe a shift of the 1s−4p transitions to higher energies with the increase of the OS of the standard samples with well-known manganese valences in Figure 2. However, the edge shape is also strongly dependent

to the relative energy values, the Mn2O3 standard was used as a reference to assign the absolute energy of the simulated spectra. 2.4.2. EXAFS Fitting. The theoretical EXAFS paths were calculated by the program FEFF6.0 embedded in ARTEMIS29 using the models described above. Additionally, the FEFF software also calculated the scattering amplitude, phase shift, and mean free path of the photoelectron. A single parameter E0 was refined for all paths to align the wavenumber grids of the data with those calculated by FEFF. The number of refined parameters was limited to a maximum of 12 variables by the Nyquist criterion (2ΔkΔR/ π) for a fitting range of the Fourier transform, Δk [2:11.5 Å−1], and the range in R over which the fit was evaluated, ΔR [1:4 Å]. k weights of 1, 2, and 3 were simultaneously fitted to distribute the sensitivity of the evaluation of χ2 over the entire k range and to make better use of the data available. S02 was refined for the known standards (MnO and MnO2), and the refined value of 0.56 was constrained in the unknown samples.

3. RESULTS AND DISCUSSION 3.1. Ex Situ. 3.1.1. Characterization of Li3.6−xMn2.4O5.4 Samples Studied ex Situ. The structure and composition of P and C1 were thoroughly characterized in ref 25. The samples D1 and D3 corresponding to the discharged compositions introduced here for the first time were characterized by ICP− OES, which gave an Li/Mn ratio in good agreement with the one determined from electrochemistry (see Table 1), and by PXRD (see Figure SI1). The PXRD pattern of D1 indicates that this sample also displays a disordered rock salt MnO-type structure without the presence of Li2O or other impurity phases. Thus, the further cycles after the first charge present a higher reversibility and occur between the compositions Li∼3.0Mn2.4O5.4 and Li∼0.7Mn2.4O5.4 (as determined for D1 and C1, respectively) without major modification of the disordered rock salt structure or the participation of Li2O. The ICP data for all the compositions studied are given in Table 1 and are in good agreement with the estimated content of lithium that is removed/reinserted in the battery from the electrochemical curve. To address the effects of the small variations in lithium content from electrosynthesis, a second replicate of the delithiated material was synthesized after the first charge at 4.4 V (sample C1′), with the expected formula of Li0.9Mn2.4O5.4, and subjected to the same analysis. Interested readers can refer Figure SI2 for a comparison of the HERFD− XANES, XES, and EXAFS data between C1 and C1′, which demonstrates that the small deviations in the compositions do not entail significant differences in the structure and OS of manganese. The release of oxygen at high potentials was evaluated by online electrochemical mass spectrometry (OEMS). The occurrence of unfavorable reactions during battery cycling, that is, decomposition of the electrolyte or the conductive carbon additive, was analyzed by the recording of the masses of several other gases including CO2, CO, POF3, or C2H4 as well as continuously recording the mass spectra of m/z values between 0 and 100. From the results of OEMS introduced in Figure SI3, we have ruled out the irreversible transformation of O2− ions into O2 gas during the full charge−discharge cycle of the battery. Thus, oxygen stoichiometry was kept identical to that of the pristine material for the models of the cycled compositions used in our simulations.

Figure 2. Experimental (left) and simulated (right) XANES spectra of standards (top) and cathode materials (bottom). The inset shows the pre-edge comparison, where the increment in the intensity of the eg states with the emptying of lithium in the cathode material has been indicated.

on the local geometry around the absorbing atoms. Thus, we have performed simulations of the XANES spectra of the reference materials to validate the method used for the simulation of the unknown samples and, more importantly, to demonstrate that the observed 1s−4p energy shift is not motivated by structural effects but mainly by the charge effect. Our simulations in Figure 2 (right) reliably reproduce the shape and relative energy values of the experimentally observed transitions. For the cathode materials with unknown manganese valences, a shift to higher energies is observed with the expected increase in the OS of manganese from P to C1, whereas only a slight shift to lower energies is observed from C1 to the discharge compositions. As for D1 and D3, no shift in energy could be appreciated between the two almost identical spectra. In Figure 3, the 1s−4p transition energy extracted from the maxima of the edge slopes in Figure 2 is plotted as a function of the expected OS of manganese (see Table 1). This plot shows the nonlinear trend of the 1s−4p transitions in the cathode material in clear contrast with the linear evolution of the edge energy for the MnO, Mn2O3, and MnO2 standards, with the total energy shift of ∼0.2 eV from P and C1 being smaller than that of ∼0.4 eV between Mn3+ and Mn4+. The smaller than expected 1s−4p energy shift proves the partial oxidation of Mn3+ to Mn4+ and rules out the participation of the Mn4+/Mn5+ redox pair hypothesized in ref 24. 29589


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Figure 4. P XANES simulations with increasing cluster radius (R).

Figure 3. Kedge 1s−4p transition energies extracted from the maximum of the first derivative in Figure 2 for the standards (black squares) and cathode materials (empty circles) plotted against the expected OS of manganese. The black line calculated by linear regression of the standard values with R2 = 0.996 is given as a guide to the eye. The black square corresponding to Mn2O3 lies underneath the symbol for P.

ligand environment and are thus sensitive to structural effects.31 Therefore, to more reliably quantify the OS of manganese in the various compositions studied, we present below the Mn−O bond lengths determined by the fitting of EXAFS and the Kβ XES spectra measured during the same experiment, less dependent of structural effects. 3.1.3. EXAFS. The XAS signal, which extends above the absorption edge (EXAFS), was used to determine the average atomic environment around manganese. The comparison between the χ(k) and Fourier transform (FT) χ(R) signals of all the compositions studied is given in Figure 5b,c. In the latter, the Mn−O and Mn−Mn peaks

For TMs with an open 3d shell, pre-edge features can be observed from the quadrupole, and thus dipole-forbidden 1s− 3d transitions. In 3d elements, the intensity of pre-edge peaks is small for centrosymmetric environments and becomes strong in noncentrosymmetric environments because of the hybridization of 3d and 4p orbitals. See, for example, the large preedge of Ba3Mn2O8 with a tetrahedral geometry (Mn5+ has only ever been reported in a tetrahedral geometry) against the small pre-edge of the other standards with manganese in an octahedral geometry. For all the compositions investigated, we observe a small pre-edge characteristic of octahedral environments in good agreement with ref 30. Notice that the conclusions from our previous study of the two end compositions of samples P and C130 still hold for the intermediate compositions D1 and D3, where the contraction of the cell in D1 determined by Rietveld is in good agreement with the expansion of XANES on the energy scale according to Natoli’s rule (see Figure 2). It could be once again noted that the presence of local disorder is evident from the flattening of the XANES spectra at higher energies with respect to the one of the highly ordered MnO having the same average structure. Our simulations in Figure 2 were able to satisfactorily reproduce the experimentally observed shape of the spectra of the cathode materials as well as the shift in energy with increased manganese valence. In the XANES simulations, the shoulder in the white line present at 6550 eV in P is completely absent in C1 and reappears for D1 and D3 with a decreased intensity with respect to P, as experimentally observed. The presence of this shoulder in the white line is attributed to the interactions between the photoelectron and the other atoms at 4−6 Å from the absorber (see Figure 4). In this new experiment, the improved sensitivity of HERFDXAS gave better resolved pre-edges, in which we clearly distinguish the quadrupole transitions toward the t2g and eg states (see Figure 2), whereas in previous XAS experiments, there was a partial overlap of the eg state with the white line.30 The increased intensity of the transition probing the eg states in C1 in the inset of Figure 2 could correspond to the emptying of electrons in the corresponding level because of the oxidation of Mn3+ (d4) to Mn4+ (d3) and provides further evidence of the oxidation of Mn in the charged material. Because of the large overlap between the 4p orbital of the TM and the oxygen ligands, the 1s−4p transitions observed in XANES are not only dependent on the OS but also on the

Figure 5. (a) Theoretical backscattering amplitude vs the photoelectron wave vector k calculated by FEFF using the average rock salt model. Experimental χ(k) (b) and FT (c) EXAFS spectra.

corresponding to the paths of the first and second scatterers have been highlighted. The oscillations of the signal in χ(k) are increased during the first charge from P to C1, as reflected by an increase in the amplitude of the FT signal. A loss of intensity is observed when moving from C1 to D1, and a small gain in intensity is observed between the two discharged compositions D1 and D3, which otherwise appear very similar to each other. The reduced intensity for the Li-rich compositions could be correlated with the increased atomic 29590


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Table 2. Description of the Paths Included in the Modela

disorder introduced by the incorporation of Li in the material. This observation is supported by the increased refined meansquare displacement (σ2) values determined during the fitting of the EXAFS data. In addition, an overall damping of higher order shells at distances >3 Å is observed for Li-rich compositions compared to the charged phase (see Figures 5 and SI2). The increased intensity and the different profile shape of χ(R) at the higher R region more sensitive to manganese contributions (see Figure 5a) could be an indication of a densification process during Li extraction in the first charge from P to C1. The migration of Mn cations to the vacancies generated by the Li removal could explain the partial irreversibility of Li insertion/extraction (the material is able to incorporate less lithium in the successive cycling after the first charge) and the nonlinear evolution of the lattice parameters with the Li content in Figure 6. The densified

pathb

Mn−O1 Mn−Mn1 Mn−O2 Mn−Mn2 Mn−O−Mn

MnO Reff (Å) 2.240 3.168 3.880 4.480 4.480

P

Ndegen

Amp (%)

Reff (Å)

6 12 8 6 12

100 300 31 61 155

2.083 2.946 3.608 4.166 4.166

Ndegen 6.0 4.92 6.77 3.08 6.15

± ± ± ±

1.55 0.83 1.75 3.51

Amp (%) 100 113 25 31 80

Initial path lengths from the theoretical models “Reff”, degeneracy of the paths “Ndegen”, and amplitude ratio relative to the first paths calculated by FEFF “Amp”. bSingle scattering paths are denoted as Mn-X, where X is O1, Mn1, O2, and Mn2 for the first (1) and second (2) O and Mn shells. The multiple scattering path Mn−O−Mn is from the absorber atom to an O1 atom and then to the opposite Mn atom aligned with the absorber and O1 atoms. a

Figure 7. FT EXAFS spectra of MnO (black) and P (green). Figure 6. Nonlinear evolution of the lattice parameters (Mn−Mn2 distance) as a function of the composition in the cathode materials.

attributed to the strong structural disorder of the samples synthesized by high-energetic milling. The passive electron reduction factor (S02) was calculated from the refinement of the crystalline MnO and MnO2 standards with well-known structures and path degeneracies (Ndegen). In both the refinements of MnO and MnO2 (see Figure SI6), the fitted value of S02 converged to 0.56 and was fixed to this value in the EXAFS refinements of the cathode material samples. The high degree of correlation among S02, Ndegen, and the mean-square atomic displacements (σ2) makes the refinement of Ndegen in highly disordered systems particularly challenging. Thus, Ndegen were held constant to the values reported in Table 2. The parameters determined in the fit corresponded to the interatomic distances, and σ2 parameters for all the paths and k-weights of 1, 2, and 3 were simultaneously fitted. The “breathing” of the ordered manganese framework was added in the refinements by the inclusion of the volume expansion coefficient “α”. The singlescattering paths involving manganese for the second and fourth nearest neighbors (Mn−Mn1 and Mn−Mn2 in Table 2) were then refined as ΔR = αReff. The best-fit models and refined parameters are given in Figure 8 and Table 3 respectively. Good fits to the data were produced by the average rock salt structure with substitutional disorder for all the samples with the exception of P. Bond Valence Sum constraints were included in the refinement of P, with a well-characterized valence of Mn (III), to avoid unphysical Mn−O distances. Although sensible Mn−O distances were obtained by the inclusion of such constraints, the fit (Rw ≈ 15%) was still considerably worse than for the

material could then accommodate the reversible insertion/ extraction of ∼2.5 Li in further electrochemical cycles following the first charge. It should be noted that cation rearrangement and densification processes have been reported for other positive electrode materials (e.g., Li2MnO332 and Li1.12(Ni0.425Nb0.425Co0.15)0.88O2).33 The average rock salt structure with modified occupancies was also used for the refinement of the EXAFS data, where the lattice parameters of the initial models were adjusted to those values previously determined by Rietveld refinement. Given its very small scattering power, lithium was not included in the refinement (see Figure SI4), and only those paths involving manganese and oxygen were considered (see Table 2). Four single scattering paths and a forward scattering path (multiple linear scattering paths are known to contribute significantly to the EXAFS spectra of cubic close-packed structures) were included in the refinement (see Table 2 and Figure SI5). In Li3.6−xMn2.4O5.4, the first shell of oxygen remains fully occupied, whereas the content of manganese and oxygen in the outer shells is decreased with respect to MnO because of the substitution of Mn by Li and the incorporation of oxygen vacancies, respectively. The effect of the substitutional disorder in P can be observed in Table 2 as a significant decrease in the amplitude ratio relative to the first path with respect to MnO and by the comparison of the FT χ(R) of P with that of MnO in Figure 7. The overall damping of the higher order shells at distances >3 Å in the FT χ(R) of the cathode materials is in high contrast with the intense signal observed for the crystalline MnO standard in the same region and can be 29591


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Figure 8. EXAFS data (circles) and best fits (red line). Left: magnitude of FT χ(R)·k3 with the inset showing the real part; right: χ(k)·k2. The sample labels and the R-factors are given in the graphs. Dashed vertical lines indicate the fit range “ΔR” and data range “Δk” of [1−4] and [2−11.5] Å, respectively.

Table 3. Best-Fit Interatomic Bond Distances and Mean-Square Atomic Displacementsa P

C1

D1

D3

path

R

σ (Å )

R

σ (Å )

R

σ (Å )

R

σ2 (Å2)

Mn−O1 Mn−Mn1 Mn−O2 Mn−Mn2 Mn−O−Mn

1.96(2) 2.946 3.63(9) 4.166 4.23016

0.0075 0.012(5) 0.0075 0.012(5) 0.012(5)

1.89(1) 2.878(4) 3.50(4) 4.071(4) 4.15(3)

0.0040(8) 0.004(1) 0.004(1) 0.005(4) 0.013(5)

1.89(1) 2.892(9) 3.46(8) 4.090(9) 4.18(3)

0.006(1) 0.011(3) 0.02(1) 0.008 0.017(4)

1.89(2) 2.905(8) 3.47(7) 4.108(8) 4.19(6)

0.005(1) 0.010(2) 0.015(11) 0.01 0.012(8)

2

2

2

2

2

2

Values without uncertainties were held constant during the fit.

a

other samples (Rw ≈ 2−3%). It comes as no surprise that the average local structure of pristine Li4 could not be successfully described via the nonlinear least-square fitting of an average model given the large degree of structural disorder and distribution of interatomic distances at the local scale previously reported in 30. In Figure 7, the cell parameter (Mn−Mn2 distances in Table 3) shows a contraction of the lattice from 4.17 to 4.07 Å between P and C1. After the first charge, the cell parameter remains roughly constant with only a slight increase to 4.09− 4.11 Å for samples D1 and D3. In the rock salt structure, the ratio between the interatomic distances, in Table 3, and the lattice parameters are 1/2 for Mn−O1/Mn−Mn2, 2 /2 for Mn−Mn1/Mn−Mn2, and 3 /2 for Mn−O2/Mn−Mn2. For all samples, the Mn−Mn1/Mn−Mn2 ratio is found equal to 2 /2 to the third decimal place, which shows that the Mn cation lattice remains face-centred cubic. This is a clear confirmation of the “breathing” mechanism deduced from the neutron and X-PDF analysis reported in 25. For sample P, the Mn−O1/Mn−Mn2 distance ratio is only 94% of the expected value of 1/2, indicating a shortening of the average Mn−O bond length with respect to the undistorted MnO structure, as also observed by PDF. This value becomes slightly smaller for the three electrochemically synthesized samples (92.8% for C1, 92.5% for D1, and 92.2% for D3). The Mn−O2/Mn−Mn2 ratios vary from 100.6 to 99.2, 97.8, and 97.7% of the expected 3 /2 for P, C1, D1, and D3, respectively. The increased distortion of the local structure upon cycling, in good

agreement with the operando XANES data in Figure 12c, could have originated from the densification of the cathode material after the first charge. In Table 3, the refined σ2 values, generally associated with atomic disorder, increased with the higher content of lithium in the sample. This observation is in good agreement with the broadening of Li- and O-contributing peaks in the PDF of P.25 The refined distances of the first Mn−O shell for P and C1 are in good agreement with the Mn(III) and (IV) valences determined by HERFD−XANES, respectively. However, for D1 and D3, this distance remains at the same value of 1.89 Å found for C1, corresponding to the shorter Mn−O bond length expected for Mn4+ cations (1.88(1) Å bond length was refined for MnO2). To dissipate any doubts on the OS of manganese in these compositions, we introduce the Kβ XES spectra next. 3.1.4. Kβ XES Spectra. Contrary to XANES, XES is a second-order process and can be explained as the occurrence of two first-order processes as follows. For the Kβ emission of the first row TM, first, a 1s electron is excited into the continuum, leading to the formation of an intermediate 1s1 3dN state. In the second process, the system in the excited state decays radiatively to a final 3p5 3dN state. The 3p−3d exchange interaction between the unpaired 3p and 3d electrons leads to the splitting of the Kβ peak into a high-multiplicity Kβ1,3 and a low-multiplicity Kβ′ transition, motivated by the two possible orientations of the 3p hole with respect to the spin-up d electrons. 29592


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Therefore, XES is less sensitive to structural effects than XANES. In this study, we use the more intense Kβ1,3 transition as a gauge of OS and the Kβ′ transition as an indication of the spin state, as it appears as a well-defined peak in high-spin compounds and as a lower energy tail in low-spin compounds.34 See also how the Kβ′/(Kβres + Kβ1,3) integrated intensity ratio decreases as with the formal number of unpaired d electrons in Table 4, as reported in ref 35. The ratio is larger for the high-spin MnO and lower for the high-spin Mn2O3 and MnO2. An accurate quantification of this effect in Table 4 was hampered by the overlapping of Kβ′ with the Kα transitions. Therefore, the intensity of this peak has only been addressed qualitatively to discern between the high- and low-spin states for the unknown cathode materials. The OSs of the cathode materials in Table 5 were obtained by interpolation of the Kβ1,3 transition position with a linear fit

The magnitude of the 3p−3d exchange interaction depends on the number of unpaired 3d electrons, the spin state, and covalency/charge-transfer contributions. Figure 9a and Table 4

Table 5. Expected vs Experimentally Estimated OS experimentally estimated OS

Figure 9. Normalized XES spectra of the Mn standards (a) and cathode materials (b) in different OSs.

sample

expected OS

XANESa

XESb

P (Li3.6) C1 (Li0.7) D1 (Li3.05) D3 (Li2.7)

3 4.2 3.2 3.4

2.9(1) 3.7(1) 2.9(1) 2.9(1)

3.1(3) 3.7(6) 3.6(9) 3.6(9)

a The XANES data were linear fitted between the Mn(II) and (V) standards (E0 = 6536.62 + 3.89·OS). bThe XES data were linear fitted between the Mn(II) and (IV) standards (Kβ = 6.49368−4.51983 10−4·OS).

illustrate this effect, showing the XES of the standard samples. As the OS of manganese increases from MnO (Mn2+) to Ba3Mn2O8 (Mn5+), fewer unpaired 3d valence electrons are available to interact with the 3p hole, and the magnitude of the 3p−3d spin-exchange interaction becomes smaller. As a result, the Kβ′−Kβ1,3 splitting becomes smaller (i.e., the Kβ1,3 transition shifts to a lower energy and the Kβ′ transition to a higher energy). The values reported in Table 4 were obtained by the curve-fitting of the XES spectra using three symmetric pseudo-Voigt functions to describe Kβ′, Kβres, and Kβ1,3 (see Figure SI7), where Kβres accounted for the residual intensity that can be observed in the spectra because of the spin-flip excitation in the 3d band. Note that the Kβ spectrum is a more accurate indication of the OS than the previously reported XANES 1s−4p energy transition. The 3p orbitals are smaller than the 4p orbitals and have less overlap with the ligand orbitals, as they lay deeper within the electronic shells.

between the same transition for the MnO (Mn2+) and MnO2 (Mn4+) standards. From the analysis of the Kβ1,3 transition (see Figure 9b and Table 4), an increase of the manganese OS from +3 to ∼+3.7 between P and C1 is observed. Beyond C1, there is no noticeable change in the OS of manganese (within the estimated error) for D1, and for D3, contrary to our previous observations from XANES when using the 1s−4p transition as a probe. In fact, for the C1, C1′, D1, and D3 samples, the transitions are observed exactly at the same position within experimental precision (see also Figure 10). In lights of these results, we can conclude that only the Mn3+/Mn4+ redox pair is active during the first charge, providing a direct proof for the absence of Mn5+ in C1. As the change of charge in manganese cannot amount to the total charge of Δq ≈ 1.2 v.u. exchanged during the first charge from P to C1, the participation of anionic redox is inferred. In the

Table 4. Curve-Fitted Peak Positions and Intensitiesa MnO Mn2O3 MnO2 Ba3Mn2O8 P (Li3.6) C1 (Li0.9) C1′ (Li0.7) D1 (Li3.05) D3 (Li2.7)

expected OS

Kβ′ (keV)

Kβres (keV)

Kβ1,3 (keV)

Kβ′/(Kβres + Kβ1,3) ratio (%)

2 3 4 5 3 4.1 4.2 3.2 3.4

6.4773(1) 6.4772(3) 6.479(1)

6.4908(3) 6.4898(3) 6.4897(8) 6.4908(1) 6.4899(2) 6.4894(5) 6.4894(4) 6.4896(5) 6.4895(8)

6.49279(6) 6.4922(1) 6.4919(1) 6.484(4) 6.49229(7) 6.4920(1) 6.4920(1) 6.4921(1) 6.4920(2)

37.4(4) 22.5(5)a 28(1) 0 41(1) 43(2) 49(3) 16.8(5)a 12.8(5)a

6.4774(3) 6.4772(7) 6.4772(5) 6.4776(7) 6.477(1)

a

The smaller than expected intensity for these particular compositions could be explained in terms of a partial overlap of Kβ′ with the Kα transition at a lower energy value, resulting in the apparent loss of intensity of the Kβ′ transition. 29593


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Figure 10. Kβ1,3 energies of standards (black squares) and cathode materials (circles). Note that the symbol for Mn2O3 lies underneath that of P.

Figure 11. Mn−O bond lengths fitted by EXAFS for Li3.6−xMn2.4O5.4 (circles), MnO and MnO2 standards (black squares), and interatomic distances from ref 37 (gray squares) vs the Mn valence estimated from XES.

successive cycles, that is, from C1 to D1 and later on to D3, with a higher reversibility and a total charge exchange of Δq ≈ 0.9 v.u., the minimal change in the OS of manganese probed by emission spectroscopy also points to the participation of anionic redox. The intensity of the Kβ′ peak of all the samples studied, comparable to that of MnO (see Table 4), suggests a high-spin configuration of the Mn cations. We have used the structural model and simulation parameters introduced in the aforementioned XANES simulations and calculated the Kβ energy values shown in Figure SI8. Our calculations successfully described the observed experimental trend to decrease the emission energy with the increase in the OS of manganese in the standards. Moreover, the simulations exactly matched the Kβ 1,3 experimental value of Mn4+ of interest in this study. For the cathode materials, a good agreement between the simulated Kβ and the experimental value was obtained for P with Mn in OS +3. As for the cycled compositions, the OS predicted by FDM, which successfully described the XANES spectra in Figure 2, does not go beyond +3 as expected and remains constant at ∼+2.5. Taking charge-transfer effects into consideration could justify the discrepancies between the simulated Kβ energies and the experimentally determined values for the charged material. However, the charge-transfer effects cannot be completely accounted for in the ab initio approach used in this work. Our DFT method does not properly calculate the excited state with the exciton properties which demand theory beyond DFT.36 As for empirical methods, the multiplet ligand theory is found to be able to describe the charge-transfer effect. Unfortunately, the large amount of parameters in the present case makes the use of multiplet ligand field theory unfeasible. Note that the shorter Mn−O interatomic distances obtained by EXAFS for D1 and D3 are in good agreement with the higher Mn valence determined by XES of ∼3.6+ (see Figure 11). From the superposition of the Kβ1,3 lines between C1, D1, and D3 and the shorter Mn−O bond distances determined, we can conclude that the shift of the K-edge in the XANES spectra between the oxidized and discharged materials cannot be attributed to the change in the OS, but to changes within the local structure that promote or prevent the appearance of a shoulder in the edge for the Li-rich compositions. According to our simulations, the shoulder at 6550 eV comes from the interaction of the photoelectron with the atoms located within 4−6 Å from the absorber (Figure 4). The superposition of this shoulder with the 1s−4p transition in D1 and D3 results in the broadening of the white line, even with the increased energy

resolution achieved by CAS, hindering the reliable determination of the OS of manganese by HERFD−XANES alone. Instead, the complementary techniques EXAFS and Kβ1,3 spectroscopy were employed to accurately determine the OS of the discharged compositions. Therefore, we can reliably conclude from our combined XANES, EXAFS, and Kβ spectroscopies that the OS of manganese is not fully reduced to Mn(III) during discharge. 3.2. Operando XANES. An electrochemical cell from ref 28 was employed to study the Li4−Li2O composite (P) in operando during a full charge−discharge cycle and a second charge over the course of >100 h. The measured voltage profile (see Figure 12) neatly replicates the profile obtained with conventional coin or Swagelok-type cells in previous studies.25 The monotonous change of the potential between the selected cutoff voltages is consistent with the XANES results. Figure 12b,c reveals a typical solid solution behavior, where the concentration of lithium continuously changes across a single phase. This observation is in good agreement with the absence of a plateau in the electrochemical curve characteristic of the separation of the electrode into two phases with different concentrations. Note that the comparison of the operando and ex situ data at the same potential values was in excellent agreement, validating our conclusion from the ex situ study above (see Figure SI9 for a comparison of these data). A broadening of the XANES features as a function of the lapsed time is shown in Figure 12c. Notice the difference between the broader shoulder of the reduced compositions at the fully discharged state of 2.2 V during the second charge and that of the discharge cycle at the same potential, where the same shoulder feature is more clearly resolved. This observation indicates an increase of the structural disorder in the cathode material upon cycling. The structural amorphization accompanying battery cycling is not unusual and has been reported for several other cathode materials,38−40 oftentimes being associated to a fading of the capacity after extended cycling. However, in the present study, the same capacity value was retained between the discharge I and charge II cycles, and no loss of performance could be detected.

4. CONCLUSIONS During the first oxidation of the 0.93Li3.6−xMn2.4O5.4− 0.07Li2O composite, an increase of the OS of manganese from +3 to ∼+4 was determined ex situ by the shift of the Kβ1,3 line between P and C1. This result rules out the participation 29594


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Figure 12. (a) Voltage profile. (b) Operando XANES spectra. A red dotted line has been overlapped in the graph to highlight the shift in the white line to higher energies upon charging. (c) Selected spectra with ΔV = 1 for charge II, discharge I, and charge I from top to bottom.

of the Mn4+/Mn5+ redox couple as formerly hypothesized. The fact that the manganese redox alone cannot amount to the total charge exchange during the electrochemical oxidation points to a possible anionic redox mechanism in the cathode. Although our study does not provide with a direct experimental evidence of oxygen redox, this seems the most viable explanation for the fact that the measured change in the OS of manganese is markedly smaller than the charge exchange during the electrochemical oxidation. O K-edge and Kβ″ spectroscopy experiments are underway to provide a direct observation of the mechanism for anionic redox couple activity. The determination of the OS of manganese in this work was achieved by the combination of complementary absorption and emission spectroscopy data collected both ex situ and in operando conditions. The use of several complementary probes allowed us to confirm the higher OS of the discharged compositions, which would be otherwise mistakenly assigned a lower value according to the analysis of XANES data alone. The irreversible contraction of the cell parameters during the first charge determined by EXAFS, in good agreement with previous PDF studies, was associated to the densification of the cathode material, motivated by the migration of Mn cations to the vacancies formed upon the removal of Li. This densification process would in turn explain the partial irreversibility of lithium exchange in the material, that is, ∼3 Li extracted during the first charge versus only ∼2.5 Li in the later cycles. Time-resolved XAS allowed for a full understanding of the nonequilibrium processes occurring upon battery cycling and provided us with a validation of the ex situ data. The study of the Mn K-edge in operando suggests that the centrosymmetric octahedral coordination around manganese cations remains constant during the oxidation/reduction processes.

■

■

EXAFS fitting of MnO and MnO2 standards; curvefitting of XES; simulated Kβ1,3 energies; and comparison of XANES data, ex situ and operando (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Maria Diaz-Lopez: 0000-0001-5670-1859 Yves Joly: 0000-0002-1872-6112 Claire Colin: 0000-0003-1332-7929 Valerie Pralong: 0000-0003-4644-8006 Pierre Bordet: 0000-0002-1488-2257 Author Contributions

M.D., C.V.C., and P.B. were the experimentalists in the experiments at FAME and FAME-UHD; M.D. and Y.J. performed the XANES and XES simulations; M.D. and O.P. performed EXAFS fittings; M.F. and V.P. contributed to the synthesis of the material; and M.D, Y.J., and P.B. contributed to writing the paper. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the ANR grant ANR15-CE050006-01 DAME. The FAME-UHD project is financially supported by the French “grand emprunt” EquipEx (EcoX, ANR-10-EQPX-27-01), the CEA-CNRS CRG consortium, and the INSU CNRS institute. The authors acknowledge the help of Paul Chometon with the OEMS setup.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b09397. XRD data of the discharged material; HERFD−XANES and FT EXAFS of charged material; in situ OEMS; simulated Li contribution to the FT-EXAFS data; schematic representation of the paths fitted in EXAFS; 29595


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Operando X-ray Absorption Spectroscopy and Emission KÎ²1,3 Study

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