Structural Evolution of Li - American Chemical Society

Article pubs.acs.org/JPCC

Structural Evolution of Li2Fe1‑yMnySiO4 (y = 0, 0.2, 0.5, 1) Cathode Materials for Li-Ion Batteries upon Electrochemical Cycling Ruiyong Chen,† Ralf Heinzmann,† Stefan Mangold,‡ V.S. Kiran Chakravadhanula,†,§ Horst Hahn,† and Sylvio Indris*,†,§ †

Institute of Nanotechnology, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany § Electrochemical Energy Storage, Helmholtz Institute Ulm (HIU), 89069 Ulm, Germany ‡

S Supporting Information *

ABSTRACT: Nanocrystalline Li2Fe1‑yMnySiO4 (y = 0, 0.2, 0.5, 1) powders with in situ carbon coating have been prepared by a sol−gel route and their structural evolution during the first electrochemical charge−discharge cycles has been investigated in detail by X-ray diffraction in combination with 7Li MAS NMR, 57Fe Mössbauer spectroscopy and in situ X-ray absorption spectroscopy. These results provide detailed information about the amorphization, phase transitions and the partially reversible change in the local environment of Li nuclei upon cycling. The redox processes of the metal cations responsible for the cycling performance of the first few cycles and for the cyclability are also derived from the combined structural characterizations.

1. INTRODUCTION Polyoxyanion-type lithium metal silicates (Li2MSiO4, M = Fe, Mn) have attracted broad interest recently as cathode materials for electrochemical energy storage in new generations of lithium-ion batteries, owning to their promising properties such as the abundance of iron, manganese and silicon, nontoxicity and high theoretical capacity (330 mA h g−1).1,2 However, limited success has been achieved so far in the attempt to obtain the reversible two-lithium-ion capacity due to the structural instability of the materials during cycling.3−5 Rapid capacity fading in the first several cycles is usually observed for the silicates due to structural rearrangement upon cycling.6,7 The knowledge about the structural changes of the silicate family upon cycling is essential to get insight into the structurerelated cyclic performance and also to guide the design of new materials for large-scale applications in the future. Three common polymorphs have been reported for Li2MSiO4: monoclinic P21/n,8 orthorhombic Pmn21,2,9 and orthorhombic Pmnb.10,11 They differ in the linking manner of the LiO4, MO4 and SiO4 tetrahedra, as shown in Figure 1. In addition, the Li site in Pmn21 and Li(2) site in P21/n polymorph are surrounded by four M2+ ions, whereas the Li site in Pmnb and Li(1) site in P21/n polymorph are surrounded by three M2+ ions (insets in Figure 1). Nuclear magnetic resonance (NMR) spectroscopy has been proved as a powerful tool to distinguish different polymorphs and to identify specific lithium sites for both Li2FeSiO4 and Li2MnSiO4,5,11−13 and also for Li2Fe0.8Mn0.2SiO4 and Li2Fe0.5Mn0.5SiO4.14 However, © 2012 American Chemical Society

previous efforts were mainly made to disclose the structure of the as-prepared materials. Detailed information about the structural rearrangement, phase transitions and the change in the local environment of the lithium nuclei during the charge/ discharge process remains unclear. The present work is motivated and performed to trace the structural changes of the Li2Fe1‑yMnySiO4 (y = 0, 0.2, 0.5, 1) silicates upon electrochemical charge/discharge (i.e., Li removal/reinsertion) cycles by means of X-ray diffraction (XRD), Mössbauer spectroscopy, 7Li magic-angle spinning (MAS) NMR and Xray absorption spectroscopy (XAS).

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Nanocrystalline Li2Fe1‑yMnySiO4 (y = 0, 0.2, 0.5, 1) powders with in situ carbon coating were prepared by a modified sol−gel route using polyvinylpyrrolidone (PVP) as chelating agent and carbon source.15 137.4 mg PVP (average molecular weight of 1 300 000) was dissolved in a mixture of 30 mL of 2-propanol and propionic acid (1:1 volume ratio) under stirring. Afterward, stoichiometric amounts of LiCH3COO·2H2O, Fe(CH3COO)2, Mn(CH3COO)2·2H2O, and Si(OCH2CH3)4 were added under vigorous stirring. The solution was then refluxed at 423 K for 1 h. After cooling down, the solution was mixed with another Received: November 5, 2012 Revised: December 10, 2012 Published: December 18, 2012 884

dx.doi.org/10.1021/jp310935j | J. Phys. Chem. C 2013, 117, 884−893

The Journal of Physical Chemistry C

Article

using an Elementar vario micro cube device (Elementar, Germany) via a catalytic combustion method. Scanning electron microscopy (SEM, LEO 1530) was used to analyze the morphological features. High resolution transmission electron microscope (HR-TEM, Titan 80−300, FEI) images were taken by dispersing the powders onto carbon coated Au grids in an argon-filled glovebox. 7 Li MAS NMR was performed on a Bruker Avance 200 MHz spectrometer (B0 = 4.7 T) using a 1.3 mm zirconia rotor at 298 K and a sample rotation frequency of 60 kHz with a rotorsynchronized Hahn-echo pulse sequence of the form π/ 2−τ−π−τ−acquire. Typical values for the recycle delay and the π/2 pulse length were 4 s and 2 μs, respectively. The 7Li chemical shifts were referenced to 1 M aqueous solution of LiCl at 0 ppm. The spectra were fit to Gaussian/Lorentzian line shapes using Dmfit program.16 57 Fe Mössbauer spectra were measured in transmission mode at room temperature. 57Co was used as a γ-ray source. The velocity scale and isomer shifts were calibrated with metallic αFe at room temperature. The spectra were fitted by using WinNormos software (Wissel GmbH). For in situ XAS measurements of Li2Fe1‑yMnySiO4 (y = 0, 0.5), a custom built cell consisting of two aluminum plates with rectangular apertures (14 × 2 mm) in the center and two sheets of Kapton windows (25 μm in thickness) glued on both sides was used. A slurry mixture of active material, carbon black and poly(vinylidene fluoride) (in a weight ratio of 6:3:1) in Nmethylpyrrolidone (NMP) was cast onto an Al foil and dried overnight. Li foil as counter electrode was mounted onto a circular Cu foil current collector. 1M LiPF6 in EC:DMC (1:1) was used as electrolyte. A porous polypropylene/polyethylene film (Celgard) was soaked in the electrolyte and used as separator. The in situ cell was sealed with a rubber O-ring and assembled in a glovebox. In situ XAS studies were carried out at the XAS beamline (2.5 GeV, injection current 80−150 mA) of ANKA Synchrotron Light Source, Karlsruhe, Germany in the transmission and fluorescence modes using a Si(111) double crystal monochromator. Three consecutive ionization chambers filled with an appropriate partial pressure of N2 and He were used to measure the intensity of X-rays. The fluorescence radiation was measured with a 5-element germanium detector and an x-map detector electronic was used for the data acquisition in mapping mode. The sample cell was located between the first and second ionization chamber. Energy calibration was obtained by using Fe and Mn reference foils placed between the second and third ionization chamber. XAS scans covering Fe and Mn K-edges were taken every 5 min during charging/discharging.

Figure 1. Crystal structures of Li2Fe1‑yMnySiO4 polymorphs: (a) space group P21/n (no. 14), (b) space group Pmnb (no. 62), (c) space group Pmn21 (no. 31). LiO4 tetrahedra, pink and brown; Fe(Mn)O4 tetrahedra, green; SiO4 tetrahedra, blue. Insets show the linkage manner of LiO 4 tetrahedra with the surrounding Fe(Mn)O 4 tetrahedra.

solution of 137.4 mg PVP in 2 mL of 2-propanol and kept stirring for 1 h more. The mixture was transferred into a Petri dish and allowed for gelation at room temperature for 2 days. The xerogels were collected and dried under vacuum at 363 K for 7 h. The products were ground and sintered subsequently at 973 K (with a heating rate of 2.5 K min−1) for 6 h under flowing argon and cooled down to room temperature. 2.2. Electrochemical Tests. The galvanostatic charge/ discharge measurements were performed using a standard twoelectrode Swagelok cell in the voltage range 1.5−4.8 V at room temperature. The working electrodes were prepared by grinding the as-obtained powders and carbon black at a weight ratio of 2:1. The electrolyte was 1 M LiPF6 in a 1:1 volumetric mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Pure Li foil was used as counter electrode. Two sheets of Whatman glass microfiber were used as separator. The cells were assembled in an argon-filled glovebox. A Bio-Logic VMP3 multichannel potentiostat was used to collect the electrochemical data. The specific capacities were calculated based on the mass of active material by excluding the native carbon content. For ex situ structural analysis of the cycled materials, the Swagelok cells were disassembled and the electrode was rinsed with DMC several times to remove the LiPF6 electrolyte and then dried at room temperature. 2.3. Structural Characterization. XRD patterns were collected on a Philips X’Pert diffractometer using Cu Kα1,2 radiation (45 kV, 40 mA) in the 2θ range 10−90°. The phase structure and lattice parameters were refined by performing whole pattern fitting with Le Bail method using the TOPAS software (Bruker AXS). Raman spectra were recorded using a He−Ne laser emitting light at 632.8 nm with a maximum power of 10 mW. The amount of native carbon was measured

3. RESULTS AND DISCUSSION 3.1. Structure of Pristine Materials. XRD patterns of the as-prepared Li2Fe1‑yMnySiO4 (y = 0, 0.2, 0.5, 1) powders are shown in Figure 2a. All patterns show broad peaks due to the nanocrystalline character of the studied materials (Figure 3). Little amount of Li2SiO3 impurity phase was observed for Mncontaining materials. Indexing of the crystal structure from the powder diffraction data was obtained by performing Le Bail refinement (Figure S1 in Supporting Information).17 The determined space groups, refined lattice parameters and lattice volumes are summarized in Table 1. The indexed space groups for Li2FeSiO4 (P21/n) and Li2MnSiO4 (Pmn21, Pmnb, P21/n) are consistent with the 7Li NMR results, as discussed in section 3.4. The refined lattice parameters for Li2FeSiO4 and 885



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lattice parameters confirm the solid-solution behavior of Li2Fe1‑yMnySiO4 over the investigated composition range. SEM and TEM images show that the synthesis route yields well dispersed particles with an average size of 20−30 nm (Figure 3a,d). The total amount of the native carbon for the asprepared materials was about 6−8 wt % measured by elemental carbon analysis. The crystallization of active phase is concomitant with the pyrolysis of organic chelating agents during thermal treatment, which could result in the formation of an in situ carbon matrix surrounding the active particles. The carbon distribution with a high spatial resolution was obtained by the energy filtered TEM (EFTEM) elemental mapping technique. The carbon coating has a thickness of about 2 nm, as observed from the EFTEM carbon maps (Figure S2 in the Supporting Information). The structural properties of the carbon deposit on the active particles were determined further by Raman spectra (Figure S3 in the Supporting Information). The bands at 1604 and 1358 cm−1 are the characteristic graphite bands and defect bands, respectively. The bands at 1533 and 1282 cm−1 are characteristic of highly disordered carbon films.18 These results suggest that the in situ formation of a carbon matrix can be achieved by the direct use of PVP as carbon source in the starting sol solutions. The vibration modes from the Li2Fe1‑yMnySiO4 lattice cannot be observed since the Li2Fe1‑yMnySiO4 core is shielded by the surrounding carbon layers.19 3.2. Electrochemical Measurements. The electrochemical performance of Li2Fe1‑yMnySiO4 was evaluated in the voltage range 1.5−4.8 V (vs Li+/Li) at room temperature (Figure 4, Figures S4 and S5 in the Supporting Information). Li2FeSiO4 delivers an initial discharge capacity of about 180 mA h g−1, and after five cycles it exhibits a stable charge/discharge capacity of about 160 mA h g−1 (corresponding to extraction/ insertion of one Li+ per formula unit) over the subsequent 100 cycles (Figure 4a). Two charge plateaus located at about 3.2 and 4.3 V were observed for Li2FeSiO4 in the first charge process (Figure 4b). The charge plateau at 4.3 V disappears in the subsequent charge processes (data not shown), revealing that the subsequent cycling is dominated by the Fe2+/Fe3+ redox process. This observation is consistent with a previous report.6 The lithium deinsertion voltages for the reactions Li2M2+SiO4 → LiM3+SiO4 + Li+ + e− and LiM3+SiO4 → M4+SiO4 + Li+ + e− are about 3.15 and 4.8 V for M = Fe, and about 4.1 and 4.5 V for M = Mn.20 Recently, Eames et al.21 surveyed systematically the voltage/structure relationships of various Li2FeSiO4 polymorphs for the extraction/insertion of one Li+ ion and made a comprehensive comparison with the previous experimental and theoretical data. Therefore, the oxidation of Mn2+ to Mn4+ occurs more easily than that of Fe2+ to Fe4+ in the applied voltage range of 1.5−4.8 V. The Li2MnSiO4 exhibits the highest first charge capacity (about 190 mA h g−1, corresponding to extraction of 1.15 Li+ ions per formula unit from the pristine host) among the studied samples, with a charge plateau at about 4.2 V (Figure 4b). However, Li2MnSiO4 exhibits poor cyclability (Figure S4d in the Supporting Information), due to the Jahn−Teller distortion effect of Mn3+ cations and the collapse of its crystal structure during lithium deinsertion.13 As observed by electron diffraction and predicted by density functional theory calculations,13 the structural change of Li2MnSiO4 upon first delithiation with the extraction of one Li+ proceeds via a phase separation process with the formation of a collapsed amorphous-like MnSiO4 structure, where Si has a 5-fold

Figure 2. XRD patterns of (a) as-prepared Li2Fe1‑yMnySiO4 materials and (b) as-prepared and cycled Li2Fe0.8Mn0.2SiO4. The asterisks mark the impurity phase Li2SiO3.

Figure 3. (a−c) SEM and (d−f) TEM images for the (a, d) asprepared, (b, e) fully charged, and (c, f) fully discharged Li2Fe0.8Mn0.2SiO4. Insets show the selected area electron diffraction patterns.

Li2MnSiO4 agree well with previously reported data10,11 (Table S1 in the Supporting Information). Two mixed polymorphs were identified for Li2Fe0.8Mn0.2SiO4 (P21/n, Pmnb) and Li2Fe0.5Mn0.5SiO4 (P21/n, Pmn21), respectively, and the refined 886



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Table 1. Determined Space Group and Refined Crystallographic Parameters for As-Prepared and Cycled Li2Fe1‑yMnySiO4a y 0

ap fc fd

0.2

ap fc

fd

a

0.5

ap

1.0

ap

space group

a/Å

b/Å

c/Å

β

V/z/Å3

Rwp

Rp

P21/n P21/n Pmn21 P21/n Pmn21 P21/nb Pmnb P21/n Pmnb Pmn21 P21/n Pmnb Pmn21 P21/n Pmn21b P21/n Pmnb Pmn21b

8.244(8) 8.208(2) 6.937(9) 8.157(9) 6.760(4) 8.264(7) 6.591(6) 8.128(3) 6.625(6) 6.110(3) 8.266(1) 6.580(2) 6.019(0) 6.986(8) 6.284(0) 6.305(7) 5.929(5) 6.299(9)

5.004(4) 5.068(6) 4.898(4) 4.956(3) 5.310(1) 5.020(7) 9.462(7) 5.014(9) 10.155(1) 5.266(0) 5.016(7) 9.426(8) 5.015(6) 8.030(9) 5.391(9) 11.098(7) 10.622(0) 5.381(5)

8.244(8) 8.135(7) 5.741(2) 8.157(9) 5.020(6) 8.264(4) 4.988(1) 8.128(3) 5.024(5) 4.920(8) 8.266(1) 4.979(6) 5.012(6) 5.990(4) 5.015(1) 5.114(9) 4.961(1) 4.974(2)

99.25(4) 94.12(1)

83.93(9) 84.40(2) 97.55(7) 80.97(6) 90.11(5) 84.67(6) 77.78(3) 82.09(9) 84.51(6) 79.16(7) 84.60(9) 77.22(2) 75.66(4) 83.68(5) 84.96(2) 89.41(2) 78.11(7) 84.32(2)

0.62 1.05

0.47 0.82

0.89

0.68

0.60

0.44

0.65

0.50

0.94

0.72

0.74

0.51

1.14

0.78

100.89(4) 99.00(6) 97.63(5)

99.13(5)

95.19(8) 92.44(0)

Samples at different states of charge/discharge (ap, fc, fd) correspond to Figure 4b. bMajor phase.

For the fully discharged Li2FeSiO4 and Li2Fe0.8Mn0.2SiO4, the crystal lattice parameters and lattice volume for each polymorph (P21/n, Pmnb) derived from the XRD patterns are close to those of the pristine samples (Table 1), indicating a reversible structural change and the partial remain of the longrange order after the first charge/discharge cycle. The partially reversible structural change for the Li2Fe1‑yMnySiO4 materials is also observed from the NMR isotropic shift (Figure 5) and in situ XAS measurements (Figure 7,8), as discussed below. The P21/n lattice experiences small change in the lattice volume (∼3%), in comparison with the Pmnb and Pmn21 lattice upon cycling (Table 1). This indicates that the P21/n polymorph may be the ideal structure for Li2Fe1‑yMnySiO4, and thus the capacity fading during the first few cycles could be related to the undesirable phase transition from P21/n to Pmn21. For the fully discharged Li2FeSiO4 with Pmn21 polymorph, the refined lattice parameters match the reported values for Pmn21 polymorphic LiFeSiO4 (a = 6.722 Å, b = 5.204 Å, c = 5.060 Å, V/z = 88.53).22 Besides, a lattice volume contraction of about 8% was observed from its fully charged state to fully discharged state, indicating that the reaction LiFe3+SiO4 → Fe4+SiO4 + Li+ + e− is involved in the first charge/discharge cycle since large unit cell volume change (up to 30%) occurs in case that more than one lithium is exchanged per formula unit.13 Furthermore, the existence of Fe4+ at the first charged Li2FeSiO4 and Fe3+ at the first discharged Li2FeSiO4 are confirmed from Mössbauer measurements (see Figure 6a, Table S3 in the Supporting Information). The change of oxidation states for Fe and Mn cations was also analyzed by in situ XAS based on the absorption edge and pre-edge peak energies, as will be discussed in section 3.6. This large volume change in the Fe3+/Fe4+ redox reaction is a barrier to achieve stable two Li+ extraction/insertion performance.3 These findings can explain the observed capacity fading for Li2FeSiO4 during the first few cycles in Figure 4a. 3.4. 7Li MAS NMR. The local environments of Li nuclei at the atomic scale and its distribution in the structure of the asprepared and cycled Li2Fe1‑yMnySiO4 are examined by 7Li MAS NMR. The isotropic shift, δiso, is sensitive to the local Li environment, such as the surrounding transition metal species

coordination environment. A compromise between the good capacity retention and high discharge voltage was observed for Li2Fe0.8Mn0.2SiO4 and Li2Fe0.5Mn0.5SiO4 with mixed occupation of metal cations in the crystal structure (Figure 4b, Figure S4b,c in the Supporting Information). To gain further insight into the lithium insertion-extraction mechanism for Li2Fe1‑yMnySiO4 upon electrochemical cycling, samples with different states of charge/discharge during the first cycle were collected (labeled as “ap”, “cp”, “fc”, “dp” and “fd” in Figure 4b) for ex situ structural analysis, as discussed below. 3.3. Long-Range Structural Evolution. In order to obtain the average long-range structural information for the materials after the charge/discharge cycle, the XRD pattern refinement was performed to investigate the phase transition and the change in lattice parameters. Phase transition and the decrease in the X-ray diffraction peak intensities were observed for the Li2Fe1‑yMnySiO4 materials upon cycling (Figure 2b, Table 1). Li2FeSiO4 converts partially from the initial P21/n to the Pmn21 structure during charge (Table 1). Armstrong et al.2 reported that after 10 cycles the initial Li2FeSiO4 P21/n polymorph transforms largely into Pmn21, which is the electrochemically stable phase during cycling and is responsible for the subsequent cycling performance. Furthermore, combined Xray and neutron diffraction data indicated a significant change in the linkage manner of the (Li/Fe/Si)O4 tetrahedra after cycling and consequently a different Li+ migration path.2 In the resulting Pmn21 polymorph, all (Li/Fe/Si)O4 tetrahedra point into the same direction along the c-axis and are linked by corner-sharing oxygen ions (Figure 1c). 2 The Pmn2 1 polymorph is also formed for Li2Fe0.8Mn0.2SiO4 after cycling. Some particles with a size of about 3 nm were observed from the TEM image of Figure 3e for the fully charged Li2Fe0.8Mn0.2SiO4 sample, which can be assigned to the newly formed Pmn2 1 phase (Figure S1b in the Supporting Information). This fact contributes to the broadening of the X-ray diffraction peaks, as observed in Figure 2b and Figure S1 in the Supporting Information. Fo r t he cycled Li2Fe0.5Mn0.5SiO4 and Li2MnSiO4, the diffraction intensities are too weak for a refinement to be performed. 887



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Figure 4. (a) Cyclability data of Li2FeSiO4 at a current density of 8.25 mA g−1 (i.e., C/40 according to the theoretical capacity of 330 mA h g−1). (b) First charge−discharge curves of Li2Fe1‑yMnySiO4. The samples at different states of charge/discharge (ap, cp, fc, dp, fd) were collected for ex situ structural analysis.

(Fe2+, Fe3+, Mn2+, Mn3+), their amount and the linking geometry of the tetrahedra. In addition to the strong peak at ∼0 ppm from diamagnetic species (Li2SiO3 impurity as determined by XRD or residue of the LiPF6 electrolyte), well-resolved peaks with strong negative shifts can be observed for the as-prepared samples (Figure 5), indicating the presence of Li + -O-Fe 2+ and Li + -O-Mn 2+ interactions. These negative isotropic shifts provide the fingerprint information on the lithium sites within their crystallographic units.5,11,14 Note that previous work to identify the Li+ local environment was mainly based on 6Li MAS NMR spectra,5,11,14 which allows higher spectral resolution with fewer spinning sidebands because the 6Li isotope has a lower magnetogyric ratio, smaller electric quadrupolar moment, and weaker homonuclear dipolar coupling compared to 7Li. In this work, the assignment of the 7Li isotropic positions to their respective polymorphs was done by correlating the XRD data and referring to the reported 6Li NMR spectra.5,11,14 A single isotropic shift at −55 ppm was observed for the asprepared Li2FeSiO4 (Figure 5a), which can be assigned to the

Li(2) site of the corner-shared Li(2)O4 tetrahedra (surrounded by four FeO4 tetrahedra, Li4Fe2+) in the P21/n polymorph (inset in Figure 1a).5 Note that two crystallographically nonequivalent lithium sites (Li(1) and Li(2) sites) exist in the P21/n polymorph, as shown in Figure 1a. The isotropic shift corresponding to the Li(1) site (located at −7 ppm5) on the one edge-shared Li(1)O4 tetrahedron (surrounded by three FeO4 tetrahedra, Li3Fe2+) was not resolved due to the severe overlap with the peak at 0 ppm. Three isotropic shifts at −126, −105, and −62 ppm were observed for the as-prepared Li2MnSiO4 (Figure 5d), indicating a mixture of three polymorphs Pmn21, Pmnb, P21/n, respectively.11 The isotropic shift corresponding to the second lithium site from the polymorph P21/n (reported at −94 ppm11) was not observed due to the overlap of these peaks. These observed NMR resonances are consistent with the crystallographic data analyzed from the XRD patterns. A mixture of two polymorphs (P2 1 /n, Pmnb) can be assigned for the as-prepared Li2Fe0.8Mn0.2SiO4 and a mixture of two polymorphs (P21/n, Pmn21) can be assigned for the as-prepared Li2Fe0.5Mn0.5SiO4 888



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Figure 5. 7Li MAS NMR spectra of as-prepared and cycled (a) Li2FeSiO4, (b) Li2Fe0.8Mn0.2SiO4, (c) Li2Fe0.5Mn0.5SiO4, and (d) Li2MnSiO4, measured at a spinning frequency of 60 kHz. The sharp resonance at ∼0 ppm arises from the diamagnetic species. The samples at different states of charge/discharge (ap, cp, fc, dp, fd) correspond to Figure 4b.

(Figure 5b,c), taking into account their XRD data and also NMR data for the two end members of Li2FeSiO4 and Li2MnSiO4. This tentative assignment of the 7Li MAS NMR isotropic positions for Li2Fe0.8Mn0.2SiO4 and Li2Fe0.5Mn0.5SiO4 is also quite consistent with the reported 6Li MAS NMR spectra (−46, −105 ppm for Li2Fe0.8Mn0.2SiO4 and −52, −104 ppm for Li2Fe0.5Mn0.5SiO4).14 In addition, from y = 0 to 1, the phase fraction of P21/n decreases, whereas the amount of Pmn21 increases, as observed from the intensity of NMR resonance signals. This is consistent with the XRD data. For the as-prepared materials, the Li resonances at positive shifts with broad line width indicate that a certain amount of Li+ has Fe3+ or Mn3+ next nearest neighbors. The existence of Fe3+ in the asprepared samples is also confirmed from the Mössbauer spectra (Figure 6, Table S3 in the Supporting Information). The extraction/insertion of the first lithium out of/into the host structure corresponding to the reaction Li2M2+SiO4 → LiM3+SiO4 + Li+ + e− is accompanied by the gradual change of the local environment of Li nuclei from initial Li+-O-M2+ to Li+O-M3+ bonds (for instance: Li4M2+→Li3M2+M3+→Li2M2+2M3+→ LiM2+3M3+→Li4M3+ for the corner-shared tetrahedra) and then backward upon battery discharging. Figure 5 depicts elaborately the evolution of the local environments of Li nuclei during charge/discharge. The negative isotropic lines disappear completely for the iron-containing samples at the chargeplateau state corresponding to the oxidation of M2+ to M3+. Accordingly, for Li2Fe1‑yMnySiO4 broad peaks with positive isotropic shifts appear at 340 and 148 ppm for y = 0, at 176 ppm for y = 0.2, and at 149 ppm for y = 0.5, corresponding to the mixed environment of M2+ and M3+ surrounding the LiO4 tetrahedra. At the fully charged state, broad isotropic shifts at 340 ppm (i.e., a shift of approximate 85 ppm per Li+−O-Fe3+

interaction assuming roughly all the Li sites are surrounded by four Fe3+ ions), 231 ppm, and 214 ppm were observed for y = 0, 0.2, and 0.5, respectively, corresponding to the complete oxidization of M2+ to M3+ (see Mössbauer spectra in Figure 6). The substantial line broadenings in the NMR spectra are caused by the multiple Fe2+/Fe3+/Mn2+/Mn3+ local environments surrounding Li nuclei, the partial amorphization in the crystal structure and the decrease in the averaged M-O bond lengths from Li2M2+SiO4 to LiM3+SiO4.23 Since several polymorphs coexist at the fully charged state as deduced from XRD data, it is difficult to link the respective NMR isotropic shift to its originating local environment even under the performed ultrafast MAS conditions (60 kHz) due to the line broadening and overlap of resonances in these phases. Difficulties in NMR studies of materials containing paramagnetic Fe3+ species have been addressed.24 During discharge, the NMR signals shift back to the right, indicating a reversible structural change (as marked with dashed lines in Figure 6). Unlike the initial structure, at fully discharged state a broad isotropic shift at −7 and −17 ppm was observed for Li2Fe0.8Mn0.2SiO4 and Li2Fe0.5Mn0.5SiO4, respectively, suggesting a partially reversible process. The phase transition, the decrease in the crystal domain size upon cycling, and the residual M3+ could explain the difference in the observed NMR spectra between the fully discharged state and the pristine state. Similar structural changes were observed for the Li2MnSiO4, except for the fact that the initial structure (the negative shift signals) partially remains unchanged upon cycling and the signal with the highest isotropic shift (at 207 ppm) was observed at the charge-plateau state (Figure 5d). A less intense resonance at about 588 ppm was observed exclusively for the Li2 FeSiO 4 samples and this signal 889



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Figure 7. (a) Voltage profile during in situ XAS experiment for the first charge/discharge cycle of Li2FeSiO4 against Li metal in the voltage range 1.5−4.7 V at a current density of 20.6 mA g−1 (i.e., C/16) together with component 1 of the PCA. The corresponding Fe K-edge absorption spectra during (b) charge and (c) discharge.

6, the spectra have to be described in terms of two doublets with Mössbauer parameters characteristic of Fe2+ and Fe3+ for the as-prepared materials. The fitted parameters (isomer shift, IS; quadrupole splitting, QS) are given in Table S3 in the Supporting Information. The values of Mössbauer parameters indicate that the iron components are in tetrahedral coordination. About 57 atom % of the total iron was in the Fe3+ state in the pristine Li2FeSiO4. This could explain the weak NMR signal with positive isotropic shift observed at 119 ppm in Figure 5a. It also explains the low first charge capacity compared to the first discharge capacity observed in Figure 4a. In contrast, Fe2+ was the majority component for the asprepared Li2Fe0.8Mn0.2SiO4 and Li2Fe0.5Mn0.5SiO4. Note that the doublets decrease in intensity and become less well resolvable in some cases after cycling. This may originate from the loss of crystallinity of the nanostructured samples after cycling resulting in a strong reduction of the Debye−Waller factor.25 This effect is very pronounced for the Fe-rich

Figure 6. Mössbauer spectra for the as-prepared and cycled (a) Li2FeSiO4, (b) Li2Fe0.8Mn0.2SiO4, and (c) Li2Fe0.5Mn0.5SiO4.

disappeared at its fully charged state (Figure 5a). This signal may be assigned to the Li site of Li+-O-Fe3+ in the Pmn21 symmetry and the disappearance of this signal at the fully charged state can be explained by the fact that this Li site is completely emptied (Pmn21 symmetry may exist as Fe4+SiO4 at fully charged state as discussed in Section 3.3) due to its extraction from the host structure. This Li site is reoccupied during reinsertion of lithium into the host structure during discharge, as can be seen by the reappearance of the peak at 568 ppm. 3.5. Mössbauer Measurements. The local environment of iron in FeO4 tetrahedra and the oxidation state of iron in the as-prepared and cycled Li2Fe1‑yMnySiO4 materials were characterized by Mössbauer spectroscopy. As shown in Figure 890



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Figure 8. (a) Voltage profile during in situ XAS experiment for the first charge/discharge cycle of Li2Fe0.5Mn0.5SiO4 against Li metal in the voltage range 1.6−4.8 V at a current density of 16.5 mA g−1 (i.e., C/20) together with component 1 of the PCA of both edges. The corresponding Fe K-edge absorption spectra during (b) charge and (c) discharge and Mn K-edge absorption spectra during (d) charge and (e) discharge.

In contrast, for Li2Fe0.8Mn0.2SiO4 and Li2Fe0.5Mn0.5SiO4, Fe2+ components were largely oxidized to Fe3+ at fully charged state (Figure 6b,c). When the cell voltage reached 1.5 V, only part of the Fe3+ was reduced to Fe2+. Because of the difference in the redox activity between Fe and Mn in the Li2Fe1‑yMnySiO4, the reduction of Mn3+ to Mn2+ is faster than that of Fe3+ to Fe2+, as will be discussed in Section 3.6. This can explain that part of Fe remains in the trivalent state at the fully discharged state (1.5 V) for Li2Fe0.8Mn0.2SiO4 and Li2Fe0.5Mn0.5SiO4. Fe3+ can be further reduced below 1.5 V.26 These results also indicate that redox reactions of the Mn species are responsible for the cycling capacity in the first few cycles and this role has been taken over gradually by Fe species in the subsequent cycles. The capacity fading in the first few cycles is due to the instability of Mn3+ species.13 3.6. In situ XAS. The series of Fe and Mn K-edge absorption spectra during first charge/discharge cycles for Li2FeSiO4 and Li2Fe0.5Mn0.5SiO4 are shown in Figure 7 and 8, respectively. A background subtraction and normalization of the measured spectra has to be done with extremely high reproducibility. Changes in the average valence states and local site symmetries of both cations can be deduced from the

compounds (y = 0, 0.2) whereas for y = 0.5 already the pristine material shows weak absorption. In that case, the mixed transition metal composition leads to an increased structural disorder. At fully charged state, the Fe2+ and Fe3+ components were oxidized to Fe3+ and Fe4+ (major component, IS = 0.10 mms−1) for Li2FeSiO4 (Figure 6a, Table S3 in the Supporting Information), similar to the reported observation when the charge voltage limit was 4.6 V.6 At fully discharged state, the Mössbauer spectrum shows that the LiFe3+SiO4 and Fe4+SiO4 were reduced to Fe3+ (majority) and Fe2+. We should stress that the quantitative analysis of the doublet areas do not match strictly the measured charge/discharge capacities, due to the low signal-to-noise ratio of the Mössbauer spectra for the cycled materials and possibly different Debye−Waller factors for these room-temperature spectra. Thus, the first few charge/discharge cycles in Figure 4a were partially related to the Fe3+/Fe4+ redox couple and the capacity fading in the first few cycles is related to the large change in lattice volume, as discussed in Section 3.3. The Pmn21 polymorphic Li2FeSiO4 is responsible for the subsequent stable cycling performance in Figure 4a based on the Fe2+/Fe3+ redox reaction.2 891



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MAS NMR spectra confirmed that these materials exhibit partially reversible structural changes upon cycling. A competitive redox reaction between the Mn and Fe species were deduced from the Mö s sbauer measurements for Li2Fe0.8Mn0.2SiO4 and Li2Fe0.5Mn0.5SiO4. The redox processes of Mn species were involved in the first few cycles. However, due to the instability of Mn3+ species, rapid capacity fading was observed. The charge/discharge capacity in the subsequent cycles is dominated by the redox reaction of the Fe species. In situ XAS clearly shows the high conversion rate from Fe2+ and Mn2+ to oxidation state 3+ in the first charge, while the presence of a small fraction (

Structural Evolution of Li - American Chemical Society

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