Lithiation Process in LiFe0.4

Article pubs.acs.org/JPCC

Mechanism of the Delithiation/Lithiation Process in LiFe0.4Mn0.6PO4: in Situ and ex Situ Investigations on Long-Range and Local Structures Ilham Bezza,†,‡ Maximilian Kaus,‡ Ralf Heinzmann,§ Murat Yavuz,‡,∥,◊ Michael Knapp,‡,∥ Stefan Mangold,@ Stephen Doyle,@ Clare P. Grey,⊥ Helmut Ehrenberg,‡,∥ Sylvio Indris,‡,∥ and Ismael Saadoune*,† †

LCME, FST Marrakech, University Cadi Ayyad, Av. Abdelkrim Khattabi, BP 549, 40000 Marrakech, Morocco Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Karlsruhe, Germany § Bruker Biospin GmbH, Silberstreifen 4, 76287 Rheinstetten, Germany ∥ Helmholtz Institute Ulm for Electrochemical Energy Storage(HIU), Albert-Einstein-Allee 11, 89069 Ulm, Germany ⊥ Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB21EW, United Kingdom @ ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany ◊ Materials Science, Technical University of Darmstadt, Alarich-Weiss-Strasse 2, 64287 Darmstadt, Germany ‡

S Supporting Information *

ABSTRACT: LiFe0.4Mn0.6PO4 olivine was prepared by a sol−gel route, using citric acid as a chelating agent and NH4H2PO4 as a phosphorus source. Sucrose was used as the source for the carbon-coating of the particles. The correlation between the physicochemical and the electrochemical properties of this positive electrode material was investigated. The electrochemical tests showed an initial discharge capacity of 121 mAh/g at a C/20 rate with a good reversibility of the lithiation/delithiation reactions. In situ XRD on LixFe0.4Mn0.6PO4 reveals the occurrence of new phases upon cycling, which disappeared again at the end of discharge. The single phase observed after one complete cycle is identical to the pristine one. In situ XAS spectroscopy in combination with 57Fe Mössbauer and 7Li NMR spectroscopy were used to investigate the changes in the local structure and the oxidation states of the transition metals and thus to complete the overall characterization of the lithiation/delithiation mechanism. All results reveal a high reversibility of the reactions in this electrode material.

mal, and hydrothermal techniques.4−7 They crystallize generally in the orthorhombic olivine-type structure with the space group Pnma, sometimes also described in the equivalent space groups Pbnm and Pmnb. Li, M (Fe, Mn...) and P occupy octahedral 4a, octahedral 4c, and tetrahedral 4c sites, respectively. The major advantage of these compounds is the presence of the strong

1. INTRODUCTION Iron phosphates are very attractive as electrode materials for Liion batteries. Extensive research has been reported on iron phosphate cathodes for a lithium-ion battery with an open framework for fast diffusion of mobile ions. Their main features justifying this interest are the safety, the low cost of the raw materials, and their nontoxicity.1−3Phospho-olivine compounds with the general formula LiMPO4 (M: transition metal Mn, Ni, Fe, and Co...) were prepared with different methods, including solid state route, sol−gel synthesis, coprecipitation, solvother© XXXX American Chemical Society

Received: December 31, 2014 Revised: March 31, 2015

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DOI: 10.1021/jp513032r J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Microscopy (SEM) images were collected using an FE-SEM MERLIN microscope (Zeiss GmbH). Raman measurements were carried out by a Lab RAMARAMIS spectrometer equipped with a 785 nm diode laser. Electrochemical properties of the LiFe0.4 Mn0.6PO4/C composite were tested in a coin cell CR2032 at room temperature (25 °C). For the preparation of working electrodes, a mixture of LiFe0.4Mn0.6PO4/C powders, super P conductive carbon, and polyvinylidene fluoride (PVDF) at a weight ratio of 80:10:10 was ground in a mortar with an adequate amount of N-methylpyrrolidone (NMP) to form a slurry. The slurry was cast on Al foil (20 μm thickness) and dried under vacuum at 120 °C. The electrolyte was 1 M LiPF6 in 1:1 ethylene carbonate (EC): dimethyl carbonate (DMC). Metallic lithium was used as the counter electrode while polypropylene/polyethylene foil (Celgard 2325) was used as a separator. The assembly of the coin cells was done inside an argon-filled glovebox. The structural evolution occurring in the studied olivine during the lithium extraction/insertion was observed within a dedicated test cell (Figure 1) with two Kapton windows (14 ×

P−O bonding which confers a good stability of the structure even at high temperature and also leads to a high operating potential through the M−O−P inductive effect by stabilizing the antibonding of M2+/M3+.8 Nevertheless, one drawback of these phosphates is their poor electronic conductivity ( 0.75. Furthermore, a carbon coating from different sources10,14 was investigated in order to improve the electronic conductivity. PPG (polypropylene glycol) based coating, showed a remarkable improvement of the electrochemical properties of LiMn1−xFexPO4.15 Another factor that could assist in enhancing electronic and ionic transport was minimizing the particle size of such compounds and thus shortening the lithium diffusion pathways. For this reason, many synthesis methods were involved to reduce this structural parameter.6,16,17 Recently much research has focused on Mn-rich LiFe1−xMnxPO4 electrode materials and several techniques as X-ray photoelectron spectroscopy (XPS), and 31P NMR nuclear magnetic resonance (NMR) spectroscopy are implemented to study the surface and the bulk structure.18,19 Herein, we report the synthesis of LiFe0.4Mn0.6PO4 as a cathode material for Li-ion batteries, and we use several in situ techniques to understand the mechanism of the electrochemical reaction of lithium extraction and insertion in LiFe0.4Mn0.6PO4 with the aim to establish a correlation between the structural changes occurring in the studied cathode material and the electrochemical features by using in situ X-ray diffraction (XRD), in situ X-ray absorption spectroscopy (XAS), 57Fe Mössbauer, and 7Li NMR spectroscopy.

Figure 1. Experimental setup used at PDIFF beamline for in situ experiments on two electrochemical cells in parallel (i.e., alternating measurements on both cells during charging/discharging).

3 mm, 25 μm thickness) in transmission mode.20,21 The measurements were performed at the PDIFF beamline at the ANKA synchrotron in Karlsruhe. The flat cells consist of two aluminum plates that are sealed with an O-ring and contain two electrode foils and 1 M LiPF6 in EC/DMC as an electrolyte. The Celgard film was soaked with the electrolyte and used as a separator. This setup allows performing in situ XRD measurements on two complete battery cells simultaneously (i.e., both cells will be discharged or charged within typically 20 h and XRD measurements will be performed alternatingly on both cells every 5 min). To check the reproducibility, the measurements were performed on two identical cells and no significant differences were detected. The reaction mechanism involved upon lithium extraction/ insertion was also characterized by in situ XAS spectroscopy. The in situ XAS measurements were performed at the XAS beamline at the ANKA synchrotron in transmission and fluorescence mode using a Si(111) double crystal monochromator. The custom-built cell described above (Figure 1) was used. Two complete electrochemical test cells are discharged or charged within typically 20 h and XAS measurements are performed every 5 min. Again, no differences were found for two identical cells.

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. LiFe0.4Mn0.6PO4 was prepared via a sol−gel route. The starting materials are lithium acetate, iron acetate, manganese acetate, and NH4H2PO4 salt as the phosphorus source. Citric acid was used as a chelating agent. All these chemicals were purchased from Sigma-Aldrich with a purity of more than 99%. The precursors were mixed according to the stoichiometric compositions and dissolved in a minimum quantity of distilled water. The obtained solution was stirred and heated at 80 °C until the formation of a green gel. The obtained gel was dried at 120 °C for 12 h in air before grinding and heating at 800 °C for 12 h under gas-mixture 90% Ar and 10% H2 to prevent the oxidation of Fe2+ to Fe3+. The LiFe0.4Mn0.6PO4/C composite was obtained by mixing the asprepared material with sucrose and calcinating for 2 h at 400 °C. The quantity of sucrose was chosen in such a way as to obtain a composite with 10% (w/w) of carbon. 2.2. Materials Characterization. The crystal structure of the as-prepared samples was determined by XRD analysis using a STOE STADI P diffractometer with curved Ge 111 monochromator and Mo Kα1 radiation and a Mythen strip detector (50 μm strip pitch DECTRIS). Scanning Electron B


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The Journal of Physical Chemistry C Table 1. Structural Information of Pristine LiFe0.4Mn0.6PO4/C from Rietveld Refinement on X-ray Diffraction Pattern atom

x/a

site

Li1 4a Mn1 4c Fe1 4c P1 4c O1 4c O2 4c O3 8d lattice parameters: a = 10.4016(4) Å, b Rp: 10.6% Rwp: 10.1% Bragg R-factor: 4.71%

y/b

z/c

0 0 0.282 (1) 1/4 0.282 (1) 1/4 0.094 (1) 1/4 0.095 (3) 1/4 0.454 (3) 1/4 0.162 (2) 0.051 (2) = 6.0636(3) Å, c = 4.7220(2) Å

57

3. RESULTS/DISCUSSION 3.1. Structure and Morphology. The Rietveld refinement based on the XRD pattern of the pristine LiFe0.4Mn0.6PO4 sample was carried out by assuming a Pnma space group with a statistical distribution of Mn2+ and Fe2+ ions on the 4c Wyckoff site. Figure S1 of the Supporting Information shows the comparison between the observed and calculated diffraction patterns while Table 1 gives the structural parameters deduced from the Rietveld refinement. The pattern confirms the presence of a single phase belonging to an orthorhombic olivine-type structure. The obtained unit cell parameters are a = 10.4016(4) Å, b = 6.0636(3) Å, and c = 4.7220(2) Å. These results are in good agreement with the literature.22 Table 2 reports some selected bond distances corresponding to the MO6 (M: Fe, Mn), LiO6, and PO4 polyhedra. In comparison with the interatomic distances of LiFePO4 reported by Delmas23 and also by Goodenough,24 we observed an increase of the interatomic distances of LiFe0.4Mn0.6PO4, which is readily explained by the incorporation of Mn on the 4c site.

2.157 2.187 1.534

(3) (2) (3) (4) (4) (3)

P1−O1 P1−O2 P1−O3 × 2

1.519 (4) 1.559 (4) 1.528 (3)

Shannon values39 Li−O Fe(Mn)-O P−O

2.160 2.045 1.570

1 0.6 0.4 1 1 1 1

Figure 2. Raman spectrum of LiFe0.4Mn0.6PO4/C sample.

coating, especially the D and G bands of carbons reflecting sp3 and sp2 hybridization, respectively.25−27 The bands at 1592 and 1334 cm−1 are attributed to the E2g vibration mode of crystalline graphite (band G) and the A1g mode of amorphous carbon (D band), respectively, and the band at 943 cm−1 belongs to the symmetric stretching of the PO43− anion. The intensity ratio ID/IG is related to the disorder of the coated carbon layer. A decreasing ID/IG ratio correlates with increasing the amount of amorphous carbon in the sample.32 All peaks in the Raman spectra correspond to the LiFePO4/C composite and they confirm that no significant impurity phase is present.28−30 3.3. Electrochemical Tests. The electrochemical performance of the cathode material was evaluated using cyclic voltammetry (CV) and galvanostatic charge/discharge tests. Figure 3 shows the first ten discharge curves of LiFe0.4Mn0.6PO4/C at C/20 in the voltage range of 2.5−4.6 V (i.e., complete charge−discharge within 20 h, at room temperature). A specific capacity of 121 mAh/g is obtained,

Table 2. Different Interatomic Distances in LiFe0.4Mn0.6PO4 Determined from Rietveld Refinement of the X-ray Diffraction Pattern 2.204 2.107 2.161 2.247 2.107 2.259

(1) (1) (1) (3) (3) (2)

occupancy

3.110 1.080 1.080 1.173 1.098 1.045 0.971

Figure S2 of the Supporting Information shows SEM pictures of LiFe0.4Mn0.6PO4/C. These pictures indicate the agglomeration of particles and an average grain size of about 900 nm. Elemental mappings demonstrate a uniform distribution of Mn, Fe, P, O, and C throughout the LiFe0.4Mn0.6PO4. This confirms the statistical distribution of the transition metal ions within the structure as assumed by the Rietveld refinement. 3.2. Raman Spectroscopy. Raman spectroscopy on LiFe0.4Mn0.6PO4 (Figure 2) was used to analyze the carbon

Fe Mössbauer spectroscopic measurements were performed in transmission mode at room temperature using a constant acceleration spectrometer with a 57Co(Rh) source. Isomer shifts are given relative to that of α-Fe at room temperature. 7 Li magic-angle spinning (MAS) NMR was performed on a Bruker Avance 200 MHz spectrometer (B0 = 4.7 T) using 1.3 mm zirconia rotors in a dry nitrogen atmosphere at a spinning speed of 60 kHz and a rotor-synchronized Hahn-echo pulse sequence. An aqueous 1 M LiCl solution was used as the reference for the chemical shift of 7Li (0 ppm). Typical values for the recycle delay and the π/2 pulse length were 5 s and 2 μs, respectively.

Li1−O1 × 2 Li1−O2 × 2 Li1−O3 × 2 Fe(Mn)−O1 Fe(Mn)−O2 Fe(Mn)−O3× 2 mean bond distances Li1−O Fe(Mn)−O P1−O

0 0.974 0.974 0.412 0.734 0.207 0.280

B (Å)2

C


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Figure 3. Voltage profile of LiFe0.4Mn0.6PO4 /C acquired during discharging at C/20 rate.

which is less than the theoretical capacity (170 mAh/g). The reason for this is the low electronic conductivity of the composite, which might be improved by optimization of the carbon coating. The curves show two distinct plateaus at 4.05 and 3.5 V, which are attributed to redox reactions of Mn2+/ Mn3+ and Fe2+/Fe3+, respectively, already assigned by Padhi et al.31 The characteristic curve of the CV is reported in Figure 4. The Fe2+/Fe3+ redox couple shows an oxidation potential at

Figure 5. In situ XRD analysis of LiFe0.4Mn0.6PO4/C measured at C/ 20 charge/discharge rate.

potential in the used cell for the synchrotron study. Upon lithium extraction, the peaks of the pristine material successively disappear and are replaced by new peaks at slightly larger diffraction angles. Thus, a new, partly delithiated phase appears (LixFe0.4Mn0.6PO4), which is isostructural to the pristine; the reflections from both phases are close to each other due to similar lattice parameters.32 During further charging, the intensity of the peaks of the new phase decreases again and a third phase appears revealing that the second is an intermediate one. At the last step of charging, three phases coexist, the pristine “phase 1” (Pnma space group), LixFe0.4Mn0.6PO4 “phase 2” (Pnma space group), and a third phase which belongs to MPO4 (M = Fe3+0.4Mn3+0.6) “phase 3” (Pnma space group). For this last phase, the lattice parameters are different, but it is also an olivine-like structure. The coexistence of three phases hints at a nonequilibrium state and thus at slow kinetics of the phase transformations. To confirm this kinetics effect, we performed an ex situ XRD analysis on cathode material recovered from a Swagelock cell cycled until the composition corresponding to the middle of the discharge (i.e., to the point E of Figure 9). The obtained pattern (Figure S3 of the Supporting Information) was refined with two orthorhombic phases having the same space group Pnma. The mixture contains 45% of phase 1 (a = 10.3873(5) Å; b = 6.0552(4) Å; c = 4.7251(3) Å, and RB = 4.2%; RF = 2.8%) and 55% of phase 2 [a = 10.3108(5) Å; b = 6.0289(4) Å; c = 4.7500(3) Å and RB = 4.0%; RF = 3.0%]. During discharging (i.e., lithium reinsertion), the reflections shift back to lower 2θ angles. Finally, after a complete electrochemical cycle, a single phase that belongs to the fully lithiated compound is observed. Figures 6 and 7 show the Rietveld refinement of one selected pattern where the coexistence of the three phases is observed together with the changes in the phase content and the unit cell volume. The data are deduced from the Rietveld refinement using Fullprof.33 The appearance and disappearance of new phases during charge and discharge is followed by changes in the phase fractions. The variation of the unit cell volume is due to the Jahn−Teller effect, caused by Mn3+ ions, which leads to a

Figure 4. Cyclic voltamogramm (CV) of LiFe0.4Mn0.6PO4/C for the first cycle (scan rate 0.01 mV/s in the voltage range of 2.5−4.6 V).

3.60 V and a reduction potential at 3.45 V versus Li/Li+. For the Mn2+/Mn3+ redox couple, an oxidation potential at 4.12 V and a reduction potential at 4.00 V is observed. 3.4. In Situ XRD. The in situ synchrotron diffraction analysis was used to detect structural changes of LiFe0.4Mn0.6PO4 during the cell charge/discharge. The diffraction patterns displayed in Figure 5 reveal the occurrence of new phases during cell charge and discharge. A specific capacity of 86 mAh/g at C/20 and between 2.5 and 4.6 V, corresponding to an extraction of 0.52 Li per formula unit, could be obtained in these in situ cells. Note that due to the large cell polarization, the potential step detected around 3.7 V when using the coin cell (Figure 3), is observed rather at higher D


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for Mn and Fe are shown in Figure 8. Both elements reveal reversible shifts of the absorption edges to higher binding

Figure 6. Rietveld refinement of LixFe0.4Mn0.6PO4/C at the end step of charge with coexistence of three phases.

Figure 7. Phase fractions of the pristine LiFe0.4Mn0.6PO4 (phase 1), LixFe0.4Mn0.6PO4 (phase 2), and Fe0.4Mn0.6PO4 (phase 3) (top); changes in the unit cell volume upon charge and discharge (botom).

Figure 8. (a) Fe and Mn K-edge XAS spectra of LiFe0.4Mn0.6PO4/C recorded during first charge, first discharge, second charge, and second discharge. (b) The voltage profile and the Fe and Mn K edge positions of the sample during the two first cycles.

distortion of the O-octahedral sites as reported by Yamada et al.34 3.5. In Situ XAS Analysis. Synchrotron radiation absorption spectroscopy was used to determine the oxidation state of both Mn and Fe and to reveal their coordination environment.35 The K-edge X-ray absorption spectra measured

energies during charging (i.e., oxidation of Fe2+ to Fe3+ and Mn2+ to Mn3+) and to lower binding energies during discharging. For the first charge, the Fe/Mn edge shifts show that Fe is active during the whole charge while oxidation of Mn sets in later. During the first and the second discharge, the Mn edge is E


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The Journal of Physical Chemistry C moving, while the Fe edge is almost constant and then the Fe edge is moving while the Mn edge is almost constant. This reveals the subsequent reduction of Mn3+ + e− → Mn2+ and Fe3+ + e− → Fe2+ during discharge. During the second charge, Mn and Fe are then oxidized in inverse order which also becomes apparent in the two steps in the voltage profile: at 3.75 and 4.25 V for Fe and Mn, respectively. While the movement of the Mn edge occurs almost exclusively at high potentials above 4.0 V, the shift of the Fe edge takes place mainly at potentials below 4.0 V, but weaker shifts are also observed above 4.0 V. This is in good agreement with the Mössbauer results (see below), which also show some overlapping of the two redox processes of the two elements. The changes in the shape of the Fe absorption edge during the first charge (Figure 8a) and also the corresponding changes in the position of this Fe edge (Figure 8b) suggest that the first charge includes some irreversible changes (the original shape of the Fe edge and also its original position is not recovered), while the following charge/discharge steps reveal a high reversibility with respect to the Fe oxidation state and also the local Fe environment. The fact that the Fe Mössbauer measurements show very high reversibility (see below) suggests that these irreversible changes have to be assigned to a modified local structure and not to changes in the oxidation state. 57 Fe Mössbauer and 7Li NMR spectra of LiFe0.4Mn0.6PO4 were collected at different charge and discharge states which are shown in Figure 9.

Figure 10. 57Fe Mössbauer spectra of LiFe0.4Mn0.6PO4 at different lithiation states. The labels A−F correspond to the different charge states as depicted in Figure 9. Fe2+ and Fe3+ subspectra are given as blue and red doublets, respectively.

disappears again and the broad doublet is reformed. After the complete discharge (spectrum F), the spectrum looks very similar to that of the starting material. This is confirmed by the Mössbauer parameters IS = 1.24 mm/s and QS = 2.95 mm/s, which are very close to those of the starting material and thus reveal the high reversibility of the Fe2+/Fe3+ oxidation/ reduction during electrochemical cycling. Observation of the intermediate states C and E, which correspond to charging to 4.25 V and discharging to 3.75 V, respectively, reveals that both Fe2+ and Fe3+ are present. Therefore, the clear separation of Fe and Mn oxidation/ reduction indicated by the clear steps in the voltage profile during charging/discharging is not supported by these spectra. They rather exhibit an overlapping of these processes with a small amount of Fe2+ still present after charging to 4.25 V (spectrum C) (i.e., above the upper voltage plateau normally assigned to the redox activity of Mn) and also large amounts of Fe2+ already present after discharging to 3.75 V (spectrum E) (i.e., in between the two voltage plateaus). According to the results of the X-ray absorption spectroscopy described above and the Mössbauer results, it can be concluded that oxidation and reduction of the transition metals occurs with a high degree of reversibility and both elements are active during charging/discharging in different voltage regimes. 3.7. 7Li NMR. 7Li NMR spectroscopy is used to investigate the local environment of Li in LiFe0.4Mn0.6PO4 (Figure 11). The initial state (spectrum A) is characterized by a broad peak

Figure 9. Electrochemical cycling curve and points at which 57Fe Mö ssbauer and 7Li NMR spectra of LixFe0.4Mn0.6PO4/C were recorded.

3.6. 57Fe Mö ssbauer. The 57Fe Mössbauer spectra acquired at different lithiation states are shown in Figure 10. The asprepared state (state A) can be well defined with a broad doublet with an isomer shift IS of 1.24 mm/s and a quadrupole splitting QS of 3.01 mm/s (Table 3). These parameters confirm that exclusively Fe2+ is present in state A.36 During charging against Li metal (spectra A−D) (i.e., delithiation of the cathode), the broad doublet successively disappears and a new, narrow doublet is formed. After complete charge, this narrow doublet is the only component in the spectrum, and it is characterized by IS = 0.44 mm/s and QS = 1.07 mm/s. These values are characteristic of Fe3+, and thus, this spectrum shows that the Fe2+ ions have been oxidized completely to Fe3+. During discharging the battery (spectrum D−F) (i.e., reinsertion of Li into the cathode material), the narrow doublet F


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Table 3. Fitting Parameters of the Contributions Used to Fit the Mössbauer Spectra of LiFe0.4Mn0.6PO4 for Different Charge States during the First Galvanostatic Cyclea IS A B C D E F a

Fe2+ Fe2+ Fe3+ Fe2+ Fe3+ Fe3+ Fe2+ Fe3+ Fe2+

1.239 1.240 0.463 1.219 0.466 0.443 1.232 0.457 1.240

± ± ± ± ± ± ± ± ±

QS 0.001 0.004 0.005 0.005 0.001 0.004 0.001 0.003 0.002

3.007 2.996 0.992 2.994 1.105 1.070 2.950 0.943 2.949

± ± ± ± ± ± ± ± ±

0.001 0.007 0.010 0.009 0.002 0.008 0.002 0.006 0.004

line width

area fraction

± ± ± ± ± ± ± ± ±

100.0% 41.3% 58.7% 57.4% 42.6% 100.0% 61.3% 38.7% 100.0%

0.268 0.278 0.446 0.196 0.398 0.440 0.249 0.363 0.307

0.000 0.010 0.014 0.013 0.003 0.011 0.003 0.009 0.006

Isomer shift (IS), quadrupole splitting (QS), and line width, are given in mm/s. The labels A−F correspond to the different charge states (Figure 9).

be consistent with the in situ XAS results which show only partial oxidation/reduction of Mn during cycling in contrast to Fe, which is completely oxidized/reduced (see Mössbauer results). The larger shift can also be ascribed, at least in part, to the increased positive shifts associated with residual local environments nearby Fe3+ ions.21 The reaction mechanism of LiFe 0.4 Mn 0.6 PO 4 during delithiation/lithiation determined from the various techniques can be described in the following way: LiFe 2 +0.4Mn 2 +0.6PO4 ⇌ LixFe3 +0.4(Mn 2 +, Mn 3 +)0.6 PO4 + (1 − x)Li+ + (1 − x)e− LixFe3 +0.4(Mn 2 +, Mn 3 +)0.6 PO4 Figure 11. 7Li NMR spectra of LiFe0.4Mn0.6PO4 at different lithiation states. The labels A−F correspond to the different charge states as mentioned in Figure 9.

⇌ Fe3 +0.4Mn 3 +0.6PO4 + x Li+ + x e−

The in situ XRD results show that the intermediate phase LixFe0.4Mn0.6PO4 does not disappear completely after the full charge, and also the phase fraction of the product Fe0.4Mn0.6PO4 remains small. Thus, the second step of this reaction is far from being complete, which is consistent with the specific capacity obtained during electrochemical cycling which corresponds to extraction/insertion of only about 0.7 Li per formula unit. Since the oxidation/reduction of Fe2+/Fe3+ is complete, as evidenced from Mössbauer spectroscopy, the incomplete conversion of the intermediate phase corresponds to only a partial oxidation/reduction of Mn2+/Mn3+. The in situ XAS results show that this Mn2+/Mn3+ oxidation/reduction has a reasonable reversibility for each cycle, but the extent of this conversion decreases rapidly during prolonged cycling.

covering the range from +100 ppm to −50 ppm. For the compounds LiMnPO4 and LiFePO4, shifts of about +70 ppm and small negative shifts, respectively, have been reported.37,38 Therefore, in the present case, the broad peak covering the range spanned by LiMnPO4 and LiFePO4 is consistent with the presence of multiple environments around the Li ions due to the statistical arrangement of Fe/Mn on the 4c site, while the resonances giving rise to the large positive shifts largely represent Mn-rich environments. Those giving rise to negative shifts being dominated by the Fe-rich environments. During charging (spectra A−C), in a first step, the intensity of this broad peak is decreased and a new strong peak close to 0 ppm occurs. After full charge (spectrum D), the intensity of this new peak decreases again and some residual intensity remains around 68 ppm. Therefore, the sequence of these spectra also reveals a two-step reaction in agreement with the voltage profile and the in situ XAS results. The evolution of the spectra during discharging (spectra D−F) reveals a reversible mechanism. The strong peak close to 0 reappears after discharging to 3.75 V (spectrum E) and disappears again after the complete cycle (spectrum F). It therefore represents a local environment, present in an intermediate phase which is consistent with the in situ XRD results. After one complete cycle, spectrum (F) looks very similar to that of the pristine material. The fact that the residual intensity after the complete charge (spectrum D) occurs in the region around +70 ppm might be explained in different ways. One possibility is that in particular the Li ions in Mn-rich environments are difficult to be removed. This would

4. CONCLUSION The investigations described above present a comprehensive study of the reaction mechanism occurring during electrochemical Li extraction/insertion from/into LiFe0.4Mn0.6PO4. Overall, a reaction mechanism consisting of two subsequent two-phase steps (including an intermediate, partially delithiated phase) has been observed. This behavior is similar to that observed for LiCoPO4.20,40 Furthermore, changes in the lattice constants and in the unit cell volume have been observed. In situ XAS revealed reversible oxidation and reduction of Fe and Mn in different voltage regimes. While the oxidation/reduction of Fe2+/Fe3+ is complete, as also evidenced from Mössbauer spectroscopy, Mn is only partially oxidized/reduced. The reversibility of these reactions is confirmed by observations on the local Li environments by NMR spectroscopy. G


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ASSOCIATED CONTENT

S Supporting Information *

Rietveld refinement of the X-ray diffraction pattern of LiFe0.4Mn0.6PO4/C, SEM image of LiFe0.4Mn0.6PO4/C, the elemental mapping images EDX of C, O, P, Mn, Fe, and ex situ XRD of LixFe0.4Mn0.6PO4/C composition recovered at point E. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was financially supported by IRESEN (Institut de Recherche en Energie Solaire et Energies Nouvelles, MOROCCO) (Grant P1.2.2). C.P.G. thanks the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012583 for support.

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Lithiation Process in LiFe0.4

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