Article pubs.acs.org/JPCC
Elucidating the Nature of Pseudo Jahn−Teller Distortions in LixMnPO4: Combining Density Functional Theory with Soft and Hard X‑ray Spectroscopy L. F. J. Piper,*,†,‡ N. F. Quackenbush,† S. Sallis,‡ D. O. Scanlon,§ G. W. Watson,∥ K.-W. Nam,⊥ X.-Q. Yang,⊥ K. E. Smith,# F. Omenya,∇ N. A. Chernova,∇ and M. S. Whittingham∇ †
Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, New York 13902, United States Materials Science & Engineering, Binghamton University, Binghamton, New York 13902, United States § University College London, Kathleen Lonsdale Materials Chemistry, Department of Chemistry, 20 Gordon Street, London WC1H 0AJ, U.K. ∥ School of Chemistry and CRANN, Trinity College Dublin, Dublin 2, Ireland ⊥ Brookhaven National Laboratory, Upton, New York 11973, United States # Department of Physics, Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215, United States ∇ Chemistry and Materials, State University of New York at Binghamton, Binghamton, New York 13902, United States ‡
ABSTRACT: A combination of soft and hard synchrotron-based spectroscopy with firstprinciples density functional theory within the GGA + U framework is used to investigate the distortion of the Mn local environment of LixMnPO4 as a function of electrochemical delithiation (x = 1.0, 0.75, 0.5, 0.25) and its effect on the electron and hole polaron formation. Analysis of the soft X-ray absorption spectroscopy (XAS) of the Mn L3,2-edges confirmed the evolution from the Mn2+ to the Mn3+ charge state as a two-phase reaction upon delithiation; the corresponding Mn K-edge extended X-ray fine structure measurements clearly revealed a splitting of the Mn−O nearest-neighbor distances with increasing Mn3+ character. In addition, the O K-edge absorption and emission spectra confirmed the corresponding orbital lifting of degeneracy accompanying the distortion of the MnO6 octahedra in the Mn3+ state. Our GGA + U calculations show that the distortion is not a strict Jahn−Teller distortion but is instead a preferential elongation of two of the equatorial Mn−O bonds (edge-sharing with the PO4), which results in a Mn−O−P induction driven hybridization of the unoccupied states (i.e., a pseudo Jahn−Teller distortion). Excellent agreement between the calculated electronic structure and our soft X-ray measurements of the electrochemically delithiated LixMnPO4 nanoparticles verifies the link between the preferential structural distortion and the resultant hybridization of the unoccupied 3d dxz and dx2−y2 orbitals. Our analysis of the corresponding calculated electron and hole polaron supports claims that the elongation of the equatorial bonds (edge-sharing with the PO4) in the Mn3+ charge state (i.e., the pseudo Jahn−Teller distortion) is responsible for increasing the activation energy for polaron migration and the formation energy of the electron (hole) lithium ion (vacancy) complex of the Mn olivine compared to the Fe olivine.
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INTRODUCTION The drive for alternative cathode materials to supersede the costly and environmentally unfriendly layered LiCoO2 systems currently used within rechargeable lithium-ion batteries has generated extensive research activity in recent years into olivine nanocomposite cathodes.1−3 The olivine LiFePO4, first identified by Goodenough et al.,4 has attracted the most interest despite having a two-phase character combined with low electrical conductivity (as low as 10−8 S cm−1 in their bulk form). Nevertheless, the use of a carbon coating and reduced particle size have improved performance toward the theoretical Li capacity of 170 mAh g−1 with a lithium intercalation potential of 3.4 V.5 The success of LiFePO4 initially motivated interest in other olivines. Higher energy densities could be obtained with © XXXX American Chemical Society
LiMnPO4 and LiCoPO4, which have similar theoretical capacities, but with higher voltage plateaus due to the larger redox potentials of Mn3+/4+ (∼4.1 eV) and Co3+/4+ (∼4.7 eV). However, the extremely sluggish kinetics of LiMnPO4 means that, within any reasonable range of charge/discharge rates, the effective energy density of LiMnPO4 is far inferior to that of its Fe counterpart.6 Yonemura et al. concluded that the ratelimiting step was the extremely low ionic and electronic conductivity of the material (as low as 10−10 S cm−1 in bulk form). The origin of this is thought to be due to the Jahn− Teller active Mn3+ ions that introduce both large local lattice Received: December 12, 2012 Revised: April 10, 2013
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dx.doi.org/10.1021/jp3122374 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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polaron. In this sense, the polaron formation in Fe and Mn olivines is the same.9,10 However, the Mn3+ ion is considered 3 Jahn−Teller active due to its high spin t2g (↑) e g1(↑) configuration, whereupon removing the electron will lead to an additional distortion in order to lift the degeneracy of the eg manifold. For a perfect octahedral site symmetry (Oh), the d orbital energies split into a triplet t 2g and doublet e g corresponding to the degree of orbital lobe overlap in the direction of the ligands. The removal of an electron in going to the Mn3+ charge state results in the removal of the degeneracy formed in the eg state, whereby changes in bond lengths effectively reduce the coordination number from 6 to 4 and further split both the t2g and, more importantly, the eg state. This is typically an elongation of the axial Mn−O bond lengths with the contraction of the equatorial Mn−O bond lengths, resulting in the preferred deoccupation of the dx2−y2 state, as shown schematically in Figure 1. As a result, one should
deformations, which further localize the electron and hole polarons compared to the Fe counterpart.6 Conductivity in LiFePO4 and LiMnPO4 is governed by a small polaron diffusion mechanism;7,8 however, the barriers to diffusion of free and bound, hole and electron polarons in LiMnPO4 have been calculated to be significantly higher than that of LiFePO4.9−11 Experimental studies, typically Mn K-edge extended X-ray fine structure studies (EXAFS) and/or X-ray diffraction, have shown that all the bond lengths of FeO6 vary in accordance with the ionic radii (i.e., shrinking from Fe2+ to Fe3+), whereas for MnO6, two equatorial bond lengths significantly increase while the remaining bonds shrink upon oxidation (e.g., ref 12). Qualitatively, the observed difference in local structure around the transition-metal ion between Fe and Mn has been supported by density functional theory (DFT) calculations,9−11 often concluding that the large local distortion required between the Jahn−Teller inactive Mn2+ and the Jahn− Teller active Mn3+ phase is responsible for further localizing the polaron. However, these experimental and theoretical studies are treated separately, and DFT calculations require the use of species-specific +U corrections in order to treat the electron correlation in the localized d states that are typically over delocalized within the local density approximation (LDA) and generalized gradient approximation (GGA).7−11 To better understand the polaron formation within LixMnPO4, these computational efforts need to be evaluated by experimental studies of the electronic structure. So far, these have been limited to comparing the optical gap of LiFePO4,7 and valence band X-ray photoemission spectroscopy (XPS) of fully lithiated olivines.11 We have chosen to employ soft X-ray spectroscopy, namely, X-ray absorption and emission spectroscopy (XAS/XES), in addition to typical electrochemical studies, local structure measurements (Mn K-edge EXAFS), and GGA + U calculations, to examine the variation in charge state (Mn L3,2 XAS) and electronic structure near the Fermi level (O K-edge XES/XAS) during the delithiation of LixMnPO4. While hard Kedge (1s → 4p) XAS is routinely employed for the study of real cathodes (often during operation) largely due its bulkelectronic and local structural sensitivity, soft X-ray spectroscopy studies have been limited due to the need for ultrahigh vacuum (UHV) and ideal crystal films. Since 2005, the application of soft X-ray spectroscopy has grown for studying battery compounds,13,14 with recent advances including that of an in situ electrochemical cell to measure both the transitionmetal L3,2 and oxygen K absorption edges.15 We demonstrate that soft X-ray spectroscopy provides a means to validate our first-principles calculations and support our hard X-ray studies. We conclude that the rate-limiting conductivity of LiMnPO4 is associated with the preferential elongation of the PO4 edgesharing Mn−O bonds. This large distortion upon going between the Mn2+ and the distorted Mn3+ ion will further localize both the electron and hole polaron and does account for the extremely low kinetics of LixMnPO4.
Figure 1. Schematic representation of a Jahn−Teller distortion (JTD) involving the elongation of the axial Mn−O bonds to lift the orbtial degeneracy of the Mn3+ configuration.
observe a splitting in the local neighbor distances accompanying the change in charge state, along with the emergence of an unoccupied state. Nie et al. have predicted that such a situation occurs.16 However, this is rather too simplistic a scenario and will be discussed further below. Soft XAS is an ideal probe of the Jahn−Teller distortion in manganites, as highlighted by refs 17−20. XAS provides information regarding the unoccupied (final) states of the photoexcited system subject to the dipole selection rule (Δl = ± 1). For a 3d transition-metal (TM) oxide typical of Li-ion cathodes, the lowest unoccupied states will be associated with the partially filled d orbitals, with higher transitions accessing the metal−oxygen sp bands. The TM K-edge (∼6550 eV) is fundamentally ill-suited to electronic studies of the d states compared to the TM L-edge and even the O K-edge. The dipole selection rule means that information regarding the d orbital in TM K-edge XA spectra is restricted to the cases where there exists finite p−d hybridization on the TM atom, and manifests as a small prepeak region before the abrupt white line associated with the dipole-allowed transitions. It is better suited to transitions with the covalent metal sp−oxygen p
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BACKGROUND Starting with LiMnPO4, as lithium is extracted, an electron is also removed (i.e., hole formation) from a Mn site nearby (i.e., oxidation of Mn occurs). The delithiation results in a volume shrinkage associated with the reduction of the ionic radius from the 2+ to the 3+ ion. The hole is then localized on the MnO6 in a such a way to distort the ion from Mn2+ to Mn3+ (i.e., hole polaron). Likewise, lithiating MnPO4 will form an electron B
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antibonding states in the conduction band and higher energy transitions (sensitive to local scattering). The latter has meant that TM K-edge XAS is an excellent probe of short-to-medium range structure with this variant referred to as extended X-ray fine structure and is ideal for probing the change in the Mn local nearest-neighbor distances associated with the Jahn− Teller distortion, but not the electronic structure.21 Further information can be extracted by employing soft X-ray absorption to access to the O K-edge (∼530 eV) and Mn L3,2-edge (∼640 eV) to complement the Mn K-edge EXAFS. The O K-edge (1s → 2p*) of metal oxides is well-known to map mainly the unoccupied electronic structure at the metal sites, due to hybridized states above the Fermi level with mostly metal weight and some O 2p character. A single-particle description can be employed where the resultant spectra largely reflects the unoccupied O 2p partial density of states (PDOS). As a result, it is a straightforward approach to determining the conduction band electronic structure of metal oxide (e.g., ref 22). It has been successfully employed to study Jahn−Teller distortion effects in Mn3+/4+ manganite perovskites and spinels,17−19 by studying the emergence and dichroism of the unoccupied state associated with the degeneracy lifting. The TM L-edge cannot be described within a single-particle description. Following the photoexcitation from the core p level to the partially occupied d states, d−d and core p−hole d interactions lead to many-electron multiplet states, due to the strong atomic overlap between the core and valence wave functions.22,23 As a result, the Mn L3,2-edge XA spectra do not reflect the unoccupied Mn 3d PDOS; instead, the spectra reflect the dipole-allowed transitions between the 2p63dn initial and 2p53dn+1 final state multiplets. The sensitivity to spin−orbit coupling, the dn occupation, and crystal field symmetry22,23 results in the Mn L3,2-edge XAS being an excellent indicator of Mn oxidation state and coordination. Until recently, soft X-rays were considered to be ill-suited for studies of real battery components, especially during their operation. This stems from the low energies of the photons and electrons involved and, therefore, the requirement for ultrahigh-vacuum environments for measurements, meaning that such studies are better suited for ideal surfaces (i.e., atomically clean and well-ordered single crystals). As a result, a majority of core-level spectroscopy techniques of battery electrodes thus far have primarily used hard XAS of the metal K-edge, which circumvents the need for UHV and allows for real electrodes and in-operandum studies. This has recently been addressed by in situ soft XAS setups.15 Meanwhile, the use of soft X-ray spectroscopy using ex situ (i.e., careful cell disassembly) has also proven useful for studying battery electrodes,13,14 including olivines.24
Figure 2. Rietveld refinement of the LiMnPO4 XRD pattern.
on the XRD pattern of the LiMnPO4 based on the orthorhombic Pnma space group. The refined lattice parameter is a = 10.4389(2) Å, b = 6.0984(1) Å, c = 4.7408(1) Å, and V = 301.8 Å3, in agreement with earlier studies.25,26 The cathode materials for the electrochemical tests were prepared by mixing 88% active material, 7% carbon black, and 5% polyvinylidene fluoride (PVDF) with 1-methyl-2-pyrrolidinone solvent. The slurry formed was then cast onto an Al foil current collector before drying. The dried electrodes, with an area of 1.2 cm2, containing 5 mg of active material were placed in 2325-type coin cells in a He-filled glovebox with pure lithium foil (Aldrich, thickness = 23 μm) as the counter and reference electrodes, and Celgard 2400 as the separator. LiPF6 (1 M) in a 1:1 volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) (LP30 from EM Industries) was used as the electrolyte. The cells were tested using a VMP2 multichannel potentiostat (Biologic). Figure 3 shows the galvanostatic
Figure 3. Galvanostatic intermittent charge curves for the LiMnPO4 charged to 75, 50, and 25% states of charge.
intermittent charge curves for LixMnPO4 for x = 0.25, 0.5, and 0.75 measured at 25 °C with a successive charge at a current density of 9.5 μA cm−2 for 1 h, followed by 1 h of resting time. The end point phase MnPO4 could not be realized in our study with electrochemical delithiation, although we were able to with chemical delithiation (not reported here). Ex situ measurements were performed by carefully disassembling the partially delithiated cells at different states of charge in an argon-filled glovebox. The electrodes were soaked in anhydrous dimethyl carbonate for 3 hours and later dried in the vacuum chamber of the glovebox. Once mounted in the glovebox, the samples were directly inserted into the UHV chamber at the X1B endstation for soft X-ray measurements. Following the soft X-ray experiments, hard Xray experiments were performed at beamline X23A2 using the same cathodes that were sealed in Kapton tape. X-ray Spectroscopy. Soft X-ray experiments were performed at the undulator beamline X1B at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. The beamline is equipped with a spherical grating
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EXPERIMENTAL SECTION Sample Preparation. LiMnPO4 was synthesized by a solidstate method by reacting stoichiometric amounts of manganese acetate tetrahydrate (C2H3O2)2Mn·4H2O, ammonium dihydrogen phosphate NH4H2PO4, and lithium acetate LiC2H3O2. The precursors were mixed by wet ball-milling in acetone for 12 h. Carbon black (5%) was later added to the mixture and milled for an additional 6 h. The acetone was then evaporated and dried. The resulting precursors were preheated at 350 °C for 8 h before sintering at 550 °C for 10 h in a nitrogen atmosphere in a temperature-controlled tube furnace. The phase purity of the stoichiometric LiMnPO4 sintered at 550 °C was confirmed by X-ray diffraction (XRD). Figure 2 shows Rietveld refinement C
dx.doi.org/10.1021/jp3122374 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 4. Crystal structures of (top) LiMnPO4 and (bottom) MnPO4. Unit cells are indicated by the black cuboids in both cases. Li, Mn, P, and O ions are turquoise, black, gray, and red, respectively. The MnO6 octahedra are indicated by green polyhedra, with the PO4 tetrahedra indicated by gray polyhedra. The structures were displayed using the VESTA software.41
profile of the carbon black (not shown) was used to background subtract for the VB scan of the Li1.00MnPO4. The Mn 2p XPS profile (not shown) was consistent with a Mn2+ charge state.
monochromator and a Nordgren-type grazing-incidence spherical grating spectrometer. The emission measurements of the O K-edge were carried out using a spectrometer resolution of 370 meV (or better). The energy axes were calibrated from Zn metal Lα,β emission lines. The XAS spectra were recorded in total electron yield (TEY) mode by measuring the sample drain current and were normalized to the current from a reference Au-coated mesh in the incident photon beam. The energy resolution was set at 180 and 260 meV for the O K and Mn L3,2 edges, respectively. The energy scale of the XAS measurements was calibrated using first- and second-order diffraction Ti L3,2-edge absorption features of rutile TiO2.27 The hard X-ray absorption data were acquired in the transmission mode at beamline X23A2, which is a bending magnet beamline with a Si(311) monochromator at the NSLS. Energy calibration was carried out by using the first inflection points in the spectra of Mn metal foil (Mn K-edge = 6539 eV) as reference. X-ray photoemission spectroscopy of Li1.00MnPO4 and carbon black references was performed using a Phi VersaProbe 5000 system at the Analytical and Diagnostics Laboratory (ADL) at Binghamton University. The samples were mounted using Ta foil to improve conductivity (and in contact with a Au foil). The samples were heated at 200 °C in a UHV preparation chamber with a base pressure of
