Origin of the High Capacity Manganese-Based Oxyfluoride Electrodes

Article pubs.acs.org/cm

Cite This: Chem. Mater. 2018, 30, 5362−5372

Origin of the High Capacity Manganese-Based Oxyfluoride Electrodes for Rechargeable Batteries Leiting Zhang,†,‡,§ Damien Dambournet,∥,⊥ Antonella Iadecola,⊥ Dmitry Batuk,†,# Olaf J. Borkiewicz,¶ Kamila M. Wiaderek,¶ Elodie Salager,⊥,○ Minhua Shao,‡ Guohua Chen,*,§ and Jean-Marie Tarascon*,†,⊥,∇ Chimie du Solide et de l’É nergie, Collège de France, UMR 8260, 11 place Marcelin Berthelot, Paris 75231 Cedex 05, France Department of Chemical and Biological Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China § Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China ∥ Sorbonne Universités, CNRS, Physico-chimie des électrolytes et nano-systèmes interfaciaux, PHENIX, F-75005 Paris, France ⊥ Réseau sur le Stockage É lectrochimique de l’É nergie (RS2E), FR CNRS 3459, 33 rue Saint Leu, F-80039 Amicens, France # EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium ¶ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ○ CNRS, CEMHTI UPR3079, Université d’Orléans, 1D avenue de la recherche scientifique, Orléans 45071 Cedex 2, France ∇ Sorbonne Universités, UPMC Univ Paris 06, 4 place Jussieu, 75005 Paris, France †

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S Supporting Information *

ABSTRACT: In the quest for high energy density rechargeable batteries, conversion-type cathode materials stand out with their appealing multielectron transfer properties. However, they undergo a series of complex phase transitions upon initial cycling as opposed to conventional intercalation-type materials. Within this category, iron-based mixed-anion solid solutions (FeOxF2−x) have captured the most attention of the battery community, owing to their high theoretical capacity and moderate cyclability. In the meantime, it was recently demonstrated, via a series of electrochemical cycling experiments, the in situ preparation of manganese-based mixed-anion cathode materials based on decomposition of electrolyte salt LiPF6 in the presence of MnO. To take a step forward, we herein report a routine protocol to prepare 220 mAh g−1-class composite cathodes. In addition, we provide a comprehensive understanding of the in situ fluorination and locally reversible phase transitions using complementary analytical techniques. The charged phase, with an average Mn oxidation state of ca. +2.8, consists of a highly disordered O-rich cubic-spinel-like core and an F-rich amorphous shell. Upon discharge, lithiation induces further phase transition, forming LiF, MnO, and a lithiated rocksalt-like phase. This work, which we also extended to the iron-based system, offers insights into modification of chemical and electronic properties of electrode materials by in situ fluorination.

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not possess any free Li+ diffusion channels. During cycling,

INTRODUCTION

there is a reversible formation of final products consisting of a

Following an expanding rate of electrification,1 electric vehicles with longer mileage and lower cost constitute a great global market demand. Consequently, development of high energy density rechargeable batteries made from earth-abundant and environmentally benign elements, such as Mn and Fe, receives much attention.2 Conventional cathode materials, depending on their interactions with Li + cations, mainly obey intercalation3,4 and conversion mechanisms.5−8 Intercalationtype compounds refer to materials with rigid hosting structures for Li+ diffusion, such as LiFePO4, LiCoO2, and LiMn2O4. On the other hand, conversion-type materials may © 2018 American Chemical Society

homogeneous distribution of metal nanoparticles (M) in an amorphous matrix (LinX). The redox reaction, which involves multiple electron transfer, is summarized below. MaX b + bn Li+ + bne− ↔ a M + b Li nX

(1)

Received: May 23, 2018 Revised: July 12, 2018 Published: July 17, 2018 5362

DOI: 10.1021/acs.chemmater.8b02182 Chem. Mater. 2018, 30, 5362−5372

Article

Chemistry of Materials

Electrochemical properties of the as-prepared composites were analyzed in Swagelok-type cells. A total of 12 mg of composite powders was used as the cathode, metallic Li was used as the anode, and 1 mol L −1 of LiPF 6 dissolved in ethylene carbonate (EC):dimethyl carbonate (DMC) (1:1 w/w) was used as the electrolyte (denoted as LP30 electrolyte). Cells were cycled on VMP3 potentiostats (Bio-Logic, France) between 4.8 and 1.5 V at room temperature (23 °C). In the first cycle, cells were charged initially to 4.8 V using a current density of 2.5 mA g−1 (corresponding to C/100 rate). The potential was held at 4.8 V until the current dropped to C/500. Afterward, cells were cycled at C/50 rate. Li-ion full cells were assembled by coupling the as-activated composite as the cathode and commercial lithium titanate (Li4Ti5O12) in the lithiated state (Li7Ti5O12, denoted as LLTO) as the anode. The mass of the electrodes was balanced taking into account of their respective specific capacities. Full cells were cycled using LP30 electrolyte between 0 and 3.3 V vs LLTO using a current density of 5 mAh g−1 with respect to the mass of the active material in the cathode. Crystal structures were examined by X-ray diffraction (XRD) technique using a Bruker D8 advance diffractometer in Bragg− Brentano geometry equipped with a Cu Kα radiation source (λCu‑Kα1 = 1.54056 Å, λCu‑Kα2 = 1.54439 Å) and a LynxEye detector. The applied voltage and current were 40 kV and 40 mA, respectively. In situ XRD experiment was conducted by assembling MnO-0.1LiF/C composites in a LeRiche’s cell18 using LP30 electrolyte and cycling it between 4.8 and 1.5 V under C/30 rate. Ex situ XRD experiments were carried out using a homemade XRD holder with a Kapton window. Short- and intermediate-range structural information was obtained using high-energy X-ray total scattering technique, measured at the 11-ID-B beamline at the Advanced Photon Source (Argonne National Laboratory, U.S.A.) with a wavelength λ = 0.2113 Å. MnO/C powders were charged in half cells containing LP30 electrolyte in CCCV mode at C/150 rate. Along with the pristine sample (A), samples charged for 100 h (B), 200 h (C), and 300 h (D) were obtained. Three more samples were prepared along the first discharge at C/50 rate, after discharging 17 h (E), 34 h (F), and 51 h (G). The corresponding voltage-composition profile is shown in Figure S1 in the Supporting Information (SI). The samples were sealed in Kapton capillaries before synchrotron X-ray total scattering. After correcting for background and Compton scattering, the atomic pair distribution function (PDF) G(r) was extracted following Fourier transform using PDFgetX2 software. Data refinement was performed using the PDFgui software.19 Oxidation state of manganese and local environments around Mn sites were determined by X-ray absorption spectroscopy (XAS) at Mn K-edge. The XAS experiment was performed in transmission mode at the ROCK beamline of SOLEIL synchrotron (France).20 Freestanding electrodes composed of MnO-0.1LiF/C composites with poly(vinylidene fluoride−hexafluoropropylene) (PVDF-HFP) binder were fabricated by adopting the Bellcore technique.21 The mass ratio of the active material (MnO-0.1LiF) is ca. 60% of the porous electrode. They were cycled in LP30 electrolyte subject to CCCV mode using C/50 rate and a cutoff current of C/150. Ten ex situ samples were prepared for the XAS measurements, namely, pristine (A), charged for 24 h (B), 48 h (C), end of CCCV charge at 4.8 V (D), discharged to 2.6 V (E), 1.9 V (F), end of discharge at 1.5 V (G), second charge to 3.5 V (H), 4.2 V (I), and end of second charge at 4.8 V (J), as highlighted in Figure S2. Surface properties were examined by X-ray photoelectron spectroscopy (XPS) technique in a Kratos Axis Ultra DLD multitechnique surface analysis system using focused monochromatic Al Kα radiation (1486.6 eV). MnO/C and MnO-0.5LiF/C composites were activated in LiPF6- and LiClO4-based electrolytes, respectively, using the proposed protocol. Samples after the first charge and the first discharge were analyzed. The binding energy scale was calibrated using the C 1s peak at 285.0 eV from the hydrocarbon contamination. The spectra were analyzed by XPSPEAK 4.1 software.22

Among various conversion-type cathodes, iron oxyfluorides (FeOxF2−x) have been investigated extensively.9−13 They deliver decent capacity (>300 mAh g−1 at 60 °C) with reasonably good cyclability, which is attributed to the incorporation of more covalent O2− into the insulating metal fluoride framework.9 Upon discharge, the F-rich core transforms into Fe and LiF, and a rocksalt Li−Fe−O(−F) phase forms on the O-rich surface.11 However, their average working potential is only ca. 2.2 V, hampering their practical application when coupled with a non-Li anode. In principle, this could be altered by changing the transition metal, however, none of the remaining late 3d metals (Mn, Co, Ni, Cu, Zn) has been reported to possess crystalline oxyfluoride phase.14 Indeed, our initial attempts to directly prepare manganese oxyfluorides by hydrothermal and ceramic methods failed regardless of the experimental conditions used. Such a myth was recently dispelled by a few groups beside us reporting metastable “Mn−O−F” phases with encouraging electrochemical properties.9−13 For example, Dimov and coworkers experimentally demonstrated the oxidation of Mn3O4 in the presence of LiF.15 An estimated “Mn6O8F4.5” formula unit was proposed to account for the electrochemical activity. Later, Jung and co-workers presented an elegant piece of work on the feasibility to prepare electrochemically active species based on high-energy ball-milling MnO and LiF for at least 48 h with conductive carbon.16 By enlisting various state-of-theart analytical tools, the authors proposed a surface conversion reaction mechanism, featuring the reversible incorporation of F− anions to the MnO surface. In the meantime, we reported that the electrolyte salt LiPF6 also participated in the activation of MnO by catalytic decomposition above 4.5 V following PF−6 ↔ F− + PF5

(2)

Hence, it ruled out the necessity of extended ball-milling and the presence of large amount of insulating LiF in the starting composite.17 In this paper, we present a routine protocol to prepare activated composite electrodes based on 1 h ball-milling of MnO with conductive carbon, followed by a constant-current− constant-voltage (CCCV) treatment at 4.8 V during the first charge in LiPF6-based electrolyte. The activated electrode delivers 218 and 192 mAh g−1 reversible capacities at C/50 and C/10 rates, respectively. We then focus on the mechanistic study of the formation and reversible conversion of the Mnbased mixed-anion material. An activation model is proposed based on complementary electrochemical and physicochemical characterizations, exhibiting a series of highly reversible phase transitions with cations migrating in the reconstructed anion framework. Such a complex reaction mechanism, which has been verified in iron- and sodium-based systems, offers new insights into preparing metastable oxyfluoride phases with promising electrochemical properties.

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EXPERIMENTAL SECTION

Commercially available MnO powders (99%, Sigma-Aldrich) were used as received. In a typical experiment, some 400 mg of MnO powders were placed in a stainless steel vial and ball milled in a SPEX 8000 M high energy mill for 45 min under Ar atmosphere. Some 100 mg of conductive carbon Super P (Csp) was added, and the composites were further milled for 15 min. One stainless steel milling ball was used to keep the ball-to-powder mass ratio at 21. MnO-xLiF/ C composites were prepared by adopting the same protocol, where x is the molar ratio between LiF and MnO. 5363


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especially with a heavy loading of ca. 20 mg cm−2. When cycled at C/10, the activated electrode delivers ca. 167 mAh g−1 after 50 cycles, maintaining over 90% of the initial discharge capacity. The fluctuation is attributed to the kinetically limited conversion process that is rather temperature-sensitive; hence there is the real need to perform cycling in temperature-controlled ovens. It is also worth mentioning that once the activation process is completed, no substantial electrolyte salt decomposition is required in the following cycles. We experimentally verified it by recuperating activated electrode powders and assembling them into new cells. They deliver reproducible capacities of ca. 210 mAh g−1 when cycled in fresh LiPF6-based and LiClO4-based electrolytes (Figure S3), suggesting that the in situ activation protocol can indeed stabilize electrochemically active Mn−O−F species, whose chemical nature is yet to be determined. It is worth noting that the present study deals with much larger MnO particles up to several micrometers in contrast to the previously reported work where MnO-1.2LiF/C composites prepared were highly divided by long time ball-milling (>48 h).16 The comparable reversible capacity cannot be simply rationalized by surface fluorination. Hence, we embarked on a systematic investigation on the structural and chemical properties of the active Mn−O−F phase. Structural Characterization. We first attempted in situ XRD technique to follow the dynamic structural transformation of MnO upon redox cycling at C/30. As shown in Figure S4, all Bragg peaks pertaining to MnO disappear at the end of activation without obvious formation of new peaks, resulting in a nearly featureless diffractogram. The following discharge does not regain notable crystallinity, suggesting the discharge product also remains long-range disordered. To gain better insight, we focused on ex situ XRD results that present better signal-to-noise ratio based on overnight scan. Figure 2a compares XRD patterns taken after the activation of MnO/C electrode under different cycling conditions. Overall, the diffraction intensity drastically diminishes upon activation with only diffuse intensity corresponding to short-range features. It is found that C/50 rate is not enough to fully activate the MnO/C electrode, while the C/100 rate is sufficient. Upon cycling, the bump at ca. 30° disappears with altered peak intensities of the rest, suggesting a solid-solutionlike gradual structural transition triggered by repeated electrochemical cycling. The bump at ca. 25° may be assigned to the (002) plane of Csp in the composite electrode. Representative XRD patterns of samples at different depths of discharge (DODs) are plotted in Figure 2b. A systematic peak shift toward lower diffraction angles indicates continuous lattice expansion, which is in agreement with reduction of the Mn oxidation state upon lithiation. Peaks at ca. 30° and 54° first rise and then disappear, suggesting formation of an intermediate phase with strong structural correlation with the end members. The tiny shoulders at ca. 38.7° and 45° in the 100% DOD sample may be indexed to LiF.6 Having screened the inorganic crystal structure database (ICSD), we selected cubic-spinel λ-Mn2O4, tetragonal-spinel γMn3O4, and cubic-rocksalt MnO as the model compounds. The corresponding simulated diffraction patterns plotted in Figure 2b can qualitatively reproduce most of the features of the ex situ samples. Peak shifts and intensity mismatch could be related to the defective nature of the fluorine-incorporated manganese oxides and the impact of lithiation. To avoid any further confusion, we will from now on report the discharged,

Particle morphology and spatial elemental distribution were studied using scanning transmission electron microscopy (STEM) technique. Data were obtained on an FEI Tecnai Osiris microscope equipped with a Super-X detector. Energy-dispersive X-ray spectroscopy (EDX) was acquired in STEM mode. Specimens were prepared by dipping holey carbon TEM grids into the ex situ samples inside a glovebox. Chemical environments of 19F and 7Li were analyzed by solid-state nuclear magnetic resonance (NMR) spectroscopy on a Bruker Avance III 4.7 T spectrometer with a 1.3 mm double resonance probe. Ex situ samples after the first and the twelfth cycles were tightly packed in 1.3 mm zirconia rotors in a glovebox and spun under N2 gas at the magic angle with a spinning rate of 62.5 kHz. The spectra were recorded with a rotor-synchronized Hahn-echo sequence (echo time 32 μs), a repetition time of 0.1 s and 1024 scans. 19F spectra were referenced to hexafluoroacetophenone at −106.6 ppm, and 7Li spectra were referenced to a 1 mol L−1 aqueous solution of LiCl at 0 ppm. A short repetition time was used to enhance the paramagnetic environments. All measurements were repeated with a longer repetition time (15 s for 7Li and 2 s for 19F) to ensure that no diamagnetic species were overlooked. The 19F background is negligible in this NMR probe. The NMR spectra were fitted with the DMFIT program.23

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RESULTS Electrochemistry. A reversible capacity of 175 mAh g−1 was previously reported when cycling MnO-0.1LiF/C composites in LP30 electrolyte at C/50 rate.17 By optimizing activation conditions, we succeeded in advancing the capacity to ca. 220 mAh g−1 at room temperature. Figure 1 illustrates the representative voltage−composition profile of the composite electrode with the inset showing the

Figure 1. Electrochemical cycling of MnO/C in LP30 electrolyte. The insets elucidate rate capability and cyclability of the activated composites.

Ragone plot of the activated material and the cycling performance at C/10 rate. A plateau at ca. 4.55 V reuniting several competing reactions is exclusively identified in the first charge when the C/100 rate is applied, followed by a constant potential hold until the current decays to C/500. From the first discharge, continuous voltage decay can be observed between 4.8 and 1.5 V at C/50 rate. Activated electrodes deliver sustained discharge capacities of 218, 207, 192, and 173 mAh g−1 at C/50, C/20, C/10, and C/5, respectively. Further increasing the current, the rate capability starts to worsen because of poor electrical conductivity of the loose powders, 5364


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Figure 2. Ex situ XRD results of MnO/C composite electrodes upon (a) activation and (b) discharge. CCCV@C/x-yc means the sample followed constant-current−constant-voltage cycling at C/x rate and was stopped at the yth charge.

half-charged, and fully charged samples as “MnO”, “Mn3O4”, and “Mn2O4”. The exact nature of these phases, however, is highly dependent on the extent of fluorination and/or lithiation. It is worth pointing out that the phase stabilized after the first charge (CCCV@C/100-1c) possesses features from both tetragonal- and cubic-spinel structures, and can be regarded as a spinel solid solution. Nevertheless, poor crystallinity of the ex situ samples prevents us from precisely ascertaining the structures. The synchrotron-based radial atomic PDF technique can measure the distribution of interatomic distances G(r), providing the probability of finding atomic pairs at given distance r, thus unveiling real-space structural information that is independent of the long-range ordering.24 It is especially helpful in understanding nanosized and amorphous materials, where conventional structural determination tools like X-ray and neutron diffraction become less effective. Figure 3 highlights two regions of PDF patterns of ex situ samples along the first cycle, which are characteristic of the local structure (1.5 ≤ r ≤ 4.75 Å) and intermediate/long-range orders (r ≥ 4.75 Å). Overall, intensities significantly decrease and peaks broaden upon activation, suggesting a progressive loss of long-range ordering. Such apparent amorphization of the electrode is particularly significant upon holding the voltage at 4.8 V. After the full charge, the PDF pattern shows only weak peaks at r > 4.5 Å. The local structure observed for r < 4.5 Å, however, provides structural information that extends beyond the first two coordination shells, yielding additional insight of the framework connectivity. During discharge, we observed a continuous increase of the long-range order as indicated by the increasing PDF peak intensities and a gradual change of the peak positions highlighting structural modifications. Upon charge from A to C, the first peak at 2.22 Å characteristic of the Mn2+−O distance gradually decreases with

Figure 3. PDF profiles of the ex situ samples upon initial activation (samples A to D) and the following discharge (samples E to G) processes. Description of sample natures can be found in the Experimental Section.

the concomitant appearance of two peaks located at 1.93 and 2.07 Å. Both peaks can be assigned to Mn3+−O in MnO6 and Mn2+−O in MnO4 polyhedrons, respectively, as found in “Mn3O4”. Elongated Mn3+−O distances arising from Jahn−Teller distortion in “Mn3O4” are hidden in the remaining Mn2+−O peak of MnO at 2.22 Å. The presence of octahedrally and tetrahedrally coordinated Mn sites agrees with the oxidative phase-driven transformation from MnO to “Mn3O4”. To confirm our hypothesis, the PDF patterns were fitted using a real-space refinement in Figure S5. The pristine sample was successfully refined using the cubic MnO (space group Fm3̅m, a = 4.45 Å) as a structural model. Those of partially charged samples were refined using a two-phase refinement including MnO and Mn3O4 structures. Upon charging, the 5365


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Figure 4. Ex situ Mn K-edge XANES spectra on the (a) first charge, (b) first discharge, and (c) second charge. The (d) pristine sample and samples (e) after the first charge and (f) after the first discharge are compared with references.

content of MnO gradually decreases while that of “Mn3O4” increases. It is observed that the unit cell parameter of MnO only slightly changes (sample B: a = 4.45 Å, sample C: a = 4.44 Å), suggesting that MnO readily transforms into “Mn3O4”. To model the new phase, the length of the structural coherence (spdiameter), which is the mean size of coherent X-ray scattering domains, was fitted. The refined value was 1.6 nm, indicating that the “Mn3O4” phase is highly disordered. At the final activation stage from C to D, a drastic decrease in the PDF peak intensity suggests an overall amorphization of the electrode. Based on XRD results, the electrode at the end of the first charge consists of a pseudo-solid-solution of “Mn3O4” and “Mn2O4”. Accordingly, attempts were made to fit the PDF data using a two-phase refinement. For both phases, the length of the structural coherence was around 1 nm, suggesting a continuous loss of the long-range order upon charging. The refinement allows one to capture most of the peaks but does not fully reproduce the experimental data, which might be due to the occurrence of new structural motifs that are not present in Mn3O4 nor Mn2O4. It is worth noting that the oxidation process is assisted by fluorination of the electrode. The occurrence of mixed oxy-fluorinated environment along with different oxidation states provides multiple atom connectivities that are likely to be the origin for the absence of long-range ordering. In the difference curve, one structural motif at 3.7−3.8 Å cannot be captured by the two spinel phases. It can be related to Mn−Mn distances from corner-shared MnF6 octahedrons as found in the monoclinic

structure of MnF3.25 To account for this structural motif in the PDF data, a third phase MnF3 was introduced, which yields an improvement of the fit with reliability factors Rw decreasing from 0.356 to 0.306. During discharge, three points (E, F, and G) were selected to follow the structural changes occurring upon lithiation. At point E, the PDF data were refined using Mn3O4 and Mn2O4 structures, as previously identified in the charged state. In contrast, the obtained residual curve (in green) does not point to the presence of fluorinated motifs. We observed a continuous change of the lattice parameters of the “Mn3O4” phase, which indicates the progressive lithiation process denoted as “LixMn3O4”. On the other hand, an “MnO” phasestarted to form at the expense of the disappearance of “Mn2O4” at point F. Finally, at the end of the discharge (point G), the remaining “Mn3O4” phase was converted into rocksaltlike “LiMn3O4”. The PDF refinement including “LiMn3O4” and “MnO” phases gave a good fit to the data. Moreover, the fitted coherence domains yield 3 nm for “LiMn3O4” and 2.5 nm for “MnO”, which are significantly larger than the values obtained for the fully charged sample, i.e., fitted value of 1 nm, indicating an increasing ordering upon lithiation. Chemical Characterization. Having characterized the short-range structures of the activated composites, we extended the investigation to chemical properties of the elements of interest, mainly Mn, F, and Li. Combined analytical characterizations have been employed to provide comprehensive understanding of the system. 5366


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Figure 5. Binding energies of F 1s XPS spectra from pristine and ex situ samples cycled in LiPF6- and LiClO4-based electrolytes.

We first followed the oxidation state of Mn upon cycling by XAS technique.26,27 It is a site selective and a local probe technique that provides information on the electronic as well as the local structure around the absorbing atom. In particular, the X-ray absorption near edge structure (XANES) is sensitive to the oxidation state of the absorber.28 Figure 4a−c illustrates the ex situ Mn K-edge XANES spectra of the first one-and-ahalf cycles. The position of the edge shifts to higher energy on charge and shifts back on discharge, pointing out that Mn actively participates in the reversible electrochemical redox reaction. To get more insights into the Mn oxidation state, Mn(II)O, γMn(II, III)3O4, α-Mn(III)2O3, and λ-Mn(IV)2O4 were selected as reference compounds, and their corresponding XANES spectra are compared in Figure 4d−f. A clear shift of the edge position to higher energy is in agreement with the increasing average Mn oxidation states. XANES spectra of samples A, D, and G are also plotted. Pristine sample A possesses most of the edge features from MnO reference, the marginal deviation coming from slightly decreased particle size after ball-milling, and the Mn oxidation state can be assigned to +2. The edge position of charged sample D is between that of Mn3O4 and Mn2O3, suggesting an average oxidation state between +2.67 and +3. The discharged sample G shows an oxidation state not fully reduced but close to +2. It is in good agreement with the value

calculated from electrochemical cycling, where a discharge capacity of 220 mAh g−1 corresponds to ca. 0.8 e− transfer. Since the Mn K-edge XAS spectra reveal the “bulk” oxidation state of Mn, the Mn oxidation state near surface was jointly measured by the XPS technique.29 However, the Mn 2p peaks are broad, and their shifts as a function of Mn oxidation state are relatively small.30 Nevertheless, the characteristic shake-up feature associated with Mn2+ at ca. 647 eV31 in the pristine and discharged samples readily differentiates them from the charged one (Figure S6). Estimation based on Mn 3s doublet splitting32 gives +2.81 for the charged sample. However, it has to be emphasized that the calculated value may slightly deviate from the true oxidation state owing to partial fluorine incorporation. As the presence of fluorine is of vital importance to the activation of MnO, extra attention has been paid to the investigation of the F 1s XPS spectra in Figure 5. Since MnO/ C composites were directly cycled in LiPF6 electrolyte, no trace amount of F signal is detected in the pristine sample. Upon charge, two obvious peaks can be observed at 687.6 and 684.0 eV. Curve deconvolution reveals a smaller bump at 685.4 eV buried underneath the two. The discharged compound also presents two types of fluorine at 687.2 and 684.9 eV with varied relative intensity. In the meantime, F 1s spectra of MnO-0.5LiF/C composites cycled in LiClO4 electrolyte were also measured. The pristine sample demonstrates a LiF binding 5367


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Chemistry of Materials energy of 685.4 eV. The charged one contains two ionic-type binding energies at 684.0 and 685.4 eV, which merge into a single peak at 684.9 eV in the discharged sample. By comparing the charged samples cycled in LiPF6 and LiClO4 electrolytes, we assign the peak at 687.6 eV to covalent fluorine binding energy as in an electrolyte degradation product POxFy,33 which is also supported by the P 2p signal in Figure S7. The ionic F 1s peaks should originate from Mn− F bond, since there is no Li 1s signal in the corresponding charged samples (Figure S7). The overlapping peaks indicate mixed fluorine environments, which are likely caused by locally altered oxygen/fluorine ratios and Mn oxidation states. A larger portion of the fluorinated species with binding energy of ca. 684 eV is directly associated with a higher extent of fluorination in LiPF6-based electrolyte. Nonetheless, owing to the similar ionic sizes of F− and O2−, many X-ray-based techniques (XRD, PDF, XAS, etc.) fail to distinguish lattice fluorine from oxygen. In addition, the local environment of lithium cannot be accurately measured based on X-ray interactions. Solid-state NMR spectroscopy is an element specific probe that can independently measure 19F and 17 O environments. In parallel, the 7Li signal will access the nature of the discharged sample. We first present the 19F magic angle spinning (MAS) NMR spectra of ex situ samples cycled after first charge, first discharge, and twelfth discharge against the reference LiF spectrum in Figure 6a. The spectra of ex situ samples have been normalized by total mass and number of scans. The charged sample does not show any resonance (even with a longer repetition time), while discharged samples consistently display a main resonance at −204 ppm with spinning sidebands (marked by asterisks), a broad bump at −162 ppm, and a minor contribution at −75 ppm (fit in Figure S8). It has to be remembered that signals from fluorine directly bound with paramagnetic manganese cations may be completely washed out, owing to the strong broadening issue from the interaction between the electronic spin and the nuclear spins being probed.12,34 While the −204 ppm signal in discharged samples may be assigned to the presence of LiF and the −75 ppm to O−P−F species from electrolyte degradation, the extremely broad bump located between 0 and −300 ppm suggests a distribution of 19F environments over a wide range. The 7Li MAS NMR spectra were also recorded, and the normalized ex situ results are summarized in Figure 6b. No 7Li signals can be observed in the charged sample, while both discharged samples show a strong chemical shift centered at −1 ppm with spinning sidebands. In addition, two broad bumps at ca. 50 and 220 ppm can be found in the discharged samples (fit in Figure S9). It is not surprising that the charged sample does not contain any Li, as the active electrode was prepared without any Li in the precursor. In the discharged ones, the sharp and intense resonance at −1 ppm corresponds to Li in LiF. The repetition time is short to enhance the signalto-noise ratio of the broadest and most shifted peaks, so that the intensity of the LiF peak is in reality even higher than that observed in the figure. The broad bumps are indicative of Li nuclei in close proximity to paramagnetic environments, the shift arising from the Fermi contact interaction with the unpaired electrons brought by the transition metal. This is typical for Li in the lithiated paramagnetic metal oxides or oxyfluorides.12,35 Herein, the bumps could be indicative of Li+ cations encountering a distribution of environments in terms of paramagnetic Mn2+, Mn3+ second neighbors. The determi-

Figure 6. (a) 19F and (b) 7Li MAS NMR spectra of ex situ samples after the first charge, the first discharge, and the 12th discharge. Data from reference LiF are also plotted.

nation of the exact nature of such a phase would require advanced calculations from detailed structural models.

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DISCUSSION We have briefly explored the electrochemical and physicochemical properties of the Mn−O−F system by various stateof-the-art characterization techniques. We will now focus on the nature of both charged (D) and discharged (G) samples, which should shed light on the comprehensive understanding of the reaction mechanism. The charged sample (D) consists of long-range disordered nanocomposites showing a spinel-like structure with locally fluorine-enriched “Mn−F−Mn”-type motifs as deduced by complementary XRD and PDF measurements. Moreover, we have shown by XPS the existence of at least two types of fluorine environments, namely, covalent O−P−F and ionic (O/F)−Mn−F together with an average Mn oxidation state of ca. +2.8 as confirmed by XANES technique. Worth recalling as well is our finding regarding the presence of fluorine throughout the particles, as demonstrated by EDX elemental mapping in STEM mode in Figures S10 and S11, suggesting 5368


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Figure 7. Moisture sensitivity analysis on ex situ samples. Impact of relative humidity on (a, c) structure and (b, d) fluorine environments of the charged and discharged samples, respectively.

massive anion substitution (F− vs O2−). An indirect evidence of the fluorine distribution could be deduced by placing the charged sample in a closed system with 75% relative humidity (RH) for 24 h36 and noting the rapid growth of a second phase with sharp Bragg peaks determined as rutile-type MnF2, as shown in Figure 7a. Such an observation can be rationalized by the decomposition of MnF3 according to the following reaction 2MnF3 + H2O → 2MnF2 + 2HF + (1/2)O2 as previously reported for CoF3.37 The high crystallinity is indicative of its formation via a dissolution−recrystallization process bearing in mind the marginal solubility of fluorides (1.06 g of MnF2 per 100 g of H2O at 20 °C). Based on the XRD results, the charged sample can be written as Mn1−yOFx−MnyFz, where the core (Mn1−yOFx) adopts fluorine-incorporated cation-deficient spinel-like structure and the shell (MnyFz) is mainly F-rich. Such a model is jointly confirmed by the corresponding XPS analysis in Figure 7b. The sample after the humidity test shows a significantly modified F 1s main peak at ca. 684.7 eV, 0.7 eV higher than the fresh sample, designating the transformation of the surface region into MnF2 after aging. The discharged sample (G) is composed of LiF and two Mn-based phases as deduced by combined XRD and PDF measurements with an average oxidation state for Mn being close to +2 as indicated by complementary XANES and XPS results. Moreover, NMR measurements have shown that Li+ cations are in both diamagnetic and paramagnetic environments, while F− anions possess a distribution of environments. This contrasts slightly with Jung and coworkers’ early report which solely claimed the presence of LiF and MnO in the discharge sample,16 while our 7Li NMR and PDF analyses support the presence of an additional lithiated phase. Such a Li-based phase does not come as a surprise since a limited extent of intercalation has been reported to take place prior to triggering the conversion mechanism in some metal fluorides,38 oxyfluorides,10 and oxides.39,40 For instance, initial lithiation of iron oxyfluorides (FeOxF2−x) leads to a lithiated rocksalt Li−Fe−O−F structure.10,12,41 Furthermore, House and co-workers recently

reported a cation-disordered lithium manganese oxyfluoride phase (Li1.9Mn0.95O2.05F0.95) stabilized by mechanical ballmilling, which also demonstrated the presence of 19F in the rocksalt structure.42 Like the fully charged sample, to further elucidate the structure and composition of the discharged one, a moisture aging test was also carried out. The results, recapped in Figure 7c,d, reveal a distinct conversion toward an “Mn3O4” intermediate phase after moisture exposure. The sharp peak at ca. 21.5° can be indexed to Li2CO3, whose formation is related to air oxidation and alkali metal extrusion, but we do not observe the formation of MnF2. Bearing in mind the absence of reports on the electrochemical activity of LixMnF3, one could infer that the surface fluorine has most likely been transformed into more stable products, such as LiF, upon discharge. Correspondingly, the aged F 1s XPS spectrum demonstrates less change, which is attributed to the stronger metal−fluorine ionic bond. In light of the aforementioned information regarding the fully oxidized and reduced samples, we next propose a reactive mechanism, with its schematic illustration shown in Figure 8, to account for the fluorine-assisted activation and its evolution upon cycling. Stages of the reaction pathway correspond to the seven ex situ samples (A−G) jointly studied by PDF and XAS techniques. During the activation process, pristine MnO (A) is positively polarized, adsorbing electrolyte anions (PF−6 ) to form a metastable (MnO)δ+(PF6)δ− interface. Substantial amount of POF3 species has been detected by in situ gas measurements,17 demonstrating continuous decomposition of LiPF6 and fueling of F− along the activation plateau. Like MnO that can be converted into Mn3O4 upon air oxidation,43 we believe that at a highly oxidizing potential above 4.5 V there is the buildup of a new metastable mixed anion sublattice layer (B) having a quasi-cubic close packing, owing to the presence of a thin layer of surface adsorbed fluorine. This metastable species transforms into a defective intermediate MnOFx phase (C) that is best described as MnII1−xMnIIIxOFx. When x is close 5369


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completion of the present study, we equally explored the electrochemical reactivity of FeO using LiPF6 salt as the fluorinating agent and succeeded in triggering a reversible capacity of ca. 100 mAh g−1 when cycling FeO-LiF/C composites in LP30 electrolyte at 23 °C (Figure S12). At this stage to get further insights into the proposed Mn-based model, it is interesting to compare our data with Amatucci’s work related to the electrochemical reactivity of crystalline iron oxyfluoride solid solutions (FeOxF2−x) with Li+, bearing therefore in mind that the authors were dealing with Odoped fluorides as opposed to F-doped oxides in our case. They also reported the formation of a rocksalt phase (Li−Fe− O−F) upon discharge, in addition to the presence of metallic Fe. However, we never observe extruded Mn particles from our samples. Such a distinct difference is most likely rooted in lower electromotive force of Mn2+ over Fe2+ upon reduction and the sluggish conversion kinetics for Mn.47 Unlike the FeOxF2−x phase where the most electrochemical activity comes from the conversion of F-rich core at ca. 2 V, a substantial amount of capacity in Mn−O−F composites is coming from higher potential range, which deals with more complex redox reactions. Lastly, switching to the practicality of such activated composites, Li-ion full cells were assembled. The cycling results are shown in Figure 9, and the preparation of activated

Figure 8. Schematic illustration of the in situ fluorination and subsequent cycling processes.

to 1/3, the phase corresponds to an Mn3O3F-like phase adopting tetragonal−spinel structure. MnO + x F− ↔ MnOFx + x e−

(3)

Further oxidation considerably increases the concentration of Jahn−Teller active Mn3+ cations, leading to severe lattice distortion and further rearrangement toward a more defective “Mn2O4” phase (D), since in the presence of F− there is a preference of Mn for octahedral coordination as commonly found in MnF2 and MnF3 structures. At such high oxidation potentials the absence of Mn4+, as unambiguously supported by XANES spectra, is surprising. To account for this experimental fact, a substitutional mechanism leading to MnIII2O2F2 (when x is close to 1) is considered. During this process, some manganese cations migrate to the near-surface region and are stabilized by additional F-rich clusters, forming an Mn1−yOFx−MnyFz complex, with the core region adopting cation-deficient cubic-spinel-like structure. MnOFx + z F− ↔ Mn1 − yOFx −Mn yFz + z e−

(4)

Upon discharge (E), there is a continuous lithiation of the “Mn3O4” phase toward “LixMn3O4” (F). The “Mn2O4” phase undergoes the lithiation process in parallel, which induces the stabilization of the rocksalt-type “MnO” at the end of discharge (G). Also, coming to the scene upon reduction, there is the formation of LiF (G), as deduced jointly from XRD and NMR results, which originates from surface reactions and/or LiF insolubility in Li−Mn−O(−F). This is supported by Richards and co-workers’ study which shows that, in lithium-excess transition metal oxide cathode materials, fluorine bonds preferentially to lithium over metal cations, hence resulting in a solubility limit beyond which LiF starts to form.44 Moreover, the authors concluded that high fluorination concentration can only be achieved in disordered structures. Lastly, the increase in structural ordering at the end of the discharge can be resulted from the increasing amount of LiF which could act, by analogy to its role in crystal growth, as a crystallizing flux. By now, the proposed mechanism systematically manifests the activation and reversible cycling of MnO in LiPF6-based electrolyte, providing convincing explanation to previous work on manganese-based mixed-anion composites.15−17,45 Our results are also consistent with Jung and co-workers’ recent report on the stabilization of the disordered oxyfluoride phase upon cycling of the FeO-2LiF composite.46 For the sake of

Figure 9. Voltage−composition profile of a full cell consisting of activated Mn−O−F composites as cathode, lithiated lithium titanate (LLTO) as anode, and LP30 electrolyte. The inset shows the capacity retention.

electrodes is shown in Figure S13. The first discharge delivers ca. 200 mAh g−1 capacity with the voltage−composition curve resembling that of half cells. However, nearly linear capacity decay is observed when the full cell is cycled 15 times. The poor Coulombic efficiency (86%) indicates a significant amount of side reactions that inevitably consume Li+ in the system, as supported by the diminishing capacities from the 0.5 V (vs LLTO) region corresponding to incomplete lithiation reaction. It has to be mentioned that, given the broad electrochemical activity window (ca. 2.5 V) that is typically required for conversion-type cathode materials, the practicality of such a full cell cannot immediately compete with that of conventional intercalation-type layered oxides coupled with graphite anode. The complexity of composite electrode 5370


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analysis. The authors also acknowledge Dr. Gwenaëlle Rousse, Dr. Hongjie Xu, Dr. Lu-Tao Weng, and Mr. Gaurav Assat for fruitful discussions. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. This work was supported by a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program (reference: ANR10-EQPX45). The ROCK beamline is acknowledged for providing beamtime (proposal 20170157).

reactions calls for proper surface engineering and electrolyte formulations.

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CONCLUSIONS In summary, we have reported an in situ fluorination process to prepare Mn-based mixed-anion cathode materials. The activated composite electrodes deliver reversible capacities of 218 and 192 mAh g−1 at C/50 and C/10 rate, respectively, at room temperature. A surface-fluorination-induced activation process is triggered by the decomposition of electrolyte salt LiPF6 on the surface of polarized MnO particles. Based on complementary XRD, PDF, XAS, XPS, STEM, and NMR results, the resulting highly disordered Mn−O−F species with an O-rich cubic-spinel-like core and an F-rich amorphous shell accounts for the reversible electrochemical activity in the following cycles. Preliminary cycling results on iron-based oxides seem to suggest a universal activation process. Such an approach, with proper electrode surface engineering and electrolyte formulation, points out a new way to prepare metastable mixed-anion phases that have not been stabilized using conventional synthesis methods. Moreover, we hope the present work will propel an in-depth investigation concerning the cathode−electrolyte interface, which offers an appealing opportunity for rechargeable batteries with enhanced energy density.

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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.chemmater.8b02182. Sample preparation for PDF, XAS experiments and Liion full cells, additional XRD, PDF, XPS, NMR, STEM/ EDX results, and electrochemical activity of FeO-xLiF/ C composites (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.C.). *E-mail: [email protected] (J.-M.T.). ORCID

Leiting Zhang: 0000-0003-4057-7106 Damien Dambournet: 0000-0003-3831-2643 Dmitry Batuk: 0000-0002-6384-6690 Elodie Salager: 0000-0002-5443-9698 Minhua Shao: 0000-0003-4496-0057 Guohua Chen: 0000-0001-8912-543X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.Z. acknowledges The Hong Kong University of Science and Technology and The Hong Kong Polytechnic University for sponsoring his studentship and work at the Collège de France. G.C. acknowledges the financial supports from Hong Kong Research Grants Council (Project No. 611213) and Hong Kong Polytechnic University (Project No. 1-ZE30). The authors are thankful to Dr. Mingxue Tang for measuring and analyzing preliminary NMR results, Ms. Rachelle Omnée for help with the preparation of the NMR experiments, and Dr. Dominique Foix and Mr. Nick Ho for performing XPS 5371


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Origin of the High Capacity Manganese-Based Oxyfluoride Electrodes

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