MEM Charge Density Study of Olivine Li - American Chemical

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MEM Charge Density Study of Olivine LiMPO4 and MPO4 (M = Mn, Fe) as Cathode Materials for Lithium-Ion Batteries Yuji Mishima,*,†,§ Takuma Hojo,† Takahisa Nishio,† Hideaki Sadamura,† Noboru Oyama,‡ Chikako Moriyoshi,§ and Yoshihiro Kuroiwa§ †

R & D Division, Toda Energy Materials Company, Todakogyo Corporation, 1-1-1 Shinoki, Sanyoonoda, Yamaguchi 756-0847, Japan Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan § Department of Physical Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan ‡

S Supporting Information *

ABSTRACT: We have investigated the electron-density distributions in LiMPO4 and MPO4 (M = Mn, Fe) by analyzing high-energy synchrotron-radiation powder-diffraction data using the maximum entropy method (MEM)/ Rietveld method to deepen understanding of the electrochemical properties in designing a promising cathode material. The Jahn−Teller Mn3+ ion in a high electric field induced by the adjacent (PO4)3− ion in MnPO4 forms anisotropic chemical bonding with the neighboring oxide ions. The net charge analysis shows that the electrons assigned to both the Mn and P atoms influence the delithiation for LiMnPO4, while the electrons of the Fe atom in LiFePO4 mainly contribute to the delithiation. The effects of the anisotropy of the d-electron density of the Mn3+ ion and the role played by the charge transfer of the P atom are discussed on the instability at high temperature and the metastability at room temperature, respectively.

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dized.2,16−20 The P−O bonding in FePO 4 sufficiently suppresses oxygen release from the crystals and therefore leads to the thermal stability for the batteries.21,22 Although the higher capacities even at high rates can be acquired in LiFePO4 cathodes,4 the voltage does not satisfy the demands for the high-energy batteries. On the other hand, recent reports have shown the advantages on the electrochemical properties of quasi-twophase reaction between LiMnPO4 and MnPO4 with the olivine structures as 4.1 V cathodes.18,25−28 It is possible to achieve high reversible capacities at low rates using the cathodes of carbon-composite LiMnPO4 with small particles,26 compared with the LiFePO4 cathode,4,8 because of inherently slow kinetics.18,28 The primary cause of the corresponding kinetics is under discussion in terms of the low electronic and ionic conductivities,28 mismatch of phase boundaries,7 and structural instability.18,23 In the case of the Mn compounds, the problems we must overcome are the metastability at room temperature (i.e., amorphization18) and the instability at high temperature (e.g., oxygen release at 210 °C23) attributed to the lattice distortion induced by the Jahn−Teller active Mn3+ ion. The structural instability, resulting in ignition sources with organic

INTRODUCTION Lithium iron phosphate LiFePO4 (space group Pnma) as a promising electrode for safety and long-term-cyclable batteries has attracted researchers and engineers since Padihi and coworkers’ reports in 1997.1−5 Many papers have been devoted to study the electrochemical and powdery characteristics of LiFePO4 composited or coated with an electronic conducting carbon as cathode materials.1−22,24,58−60 LiFePO4 is transformed to isostructural FePO4 through the delithiation reaction with the Fe2+ oxidation, while the lithiation of FePO4 reversibly takes place. The mobile Li+ ions are easily migrating through the channels formed by the LiO6 and vacancy-O6 chains along the b-axis in LiFePO4 and FePO4, respectively.10,11,14 The phase boundaries on the bc-planes are moved along the a-axes during the delithiation process.12 Two-phase reaction models using shrinking core−shell,1 platelet-type,12,15 and dominocascade models6,24 have been reported for comprehensive understanding of the reaction process between LiFePO4 and FePO4. The voltage plateau at 3.5 V versus Li/Li+ associated with the electrochemical reaction represents the difference between the energy level of the Fe2+/Fe3+ redox couple of the 3d orbitals and the Fermi energy level of lithium in the band structure.2,5 An electron inductive effect of (PO4)3− in LiFePO4 enhances the voltage to the useful level with less overlapping between the Fe-3d and the O-2p orbitals in the valence band, whereas the orbitals in the delithiated FePO4 are hybri© 2013 American Chemical Society

Received: October 11, 2012 Revised: December 28, 2012 Published: January 17, 2013 2608

dx.doi.org/10.1021/jp310075w | J. Phys. Chem. C 2013, 117, 2608−2615

The Journal of Physical Chemistry C

Article

Figure 1. Rietveld profile fitting patterns of (a) LiMnPO4, (b) MnPO4, (c) LiFePO4, and (d) FePO4. The deviations between the observed intensities (+) and the calculated intensities (solid line) are drawn at the bottoms of the figures with indications of the peak positions (|) inserting the enlarged high-angle regions.

profile using the Scherrer’s equation (X-ray diffraction (XRD), RINT 2500, Rigaku Corp., and a program RIETAN-2000).38 An asymmetric pseudo-Voigt function of Thompson et al. by a procedure of Finger et al. was used as a peak profile function for the refinements,38−40 and the structural parameters reported previously were used as initial parameters.41,42 The amount of the residual carbon was 2.65 wt % measured by a carbon−sulfur analyzer (EMIA-820, Horiba, Ltd.). FePO4 with the olivine structure was synthesized by chemical delithiation of LiFePO4 produced by sintering the identical precursor at 800 °C for 5 h under flowing N2. The chemical oxidation was performed at room temperature for 24 h in acetonitrile with nitronium tetrafluoroborate (NO2BF4, Aldrich, >95%) as an oxidizing agent using a recipe given in the literature.18 The Rietveld analysis detected 0.3 wt % Fe2P as an impurity from the produced FePO4 whose crystallite size was 3.1 × 102 nm. LiMnPO4 was synthesized by the solid-state reaction method with equimolar amounts of ( 1 / 2 )Li 2 CO 3 and Mn(CH3COO)2·4H2O (Kishida Chemical Co., Inc., 99%) and (NH4)2HPO4 (Wako Pure Chemical Industries, Ltd., 98%). The precursor preparation and the heat treatment for synthesizing LiMnPO4 were carried out in the same way as LiFePO4. The crystallite size of the product was 90 nm with a single phase. Carbon-composite LiMnPO4 was prepared by annealing of mechanochemical reacted LiMnPO4 and carbon black (acetylene black, Denki Kagaku Kogyo Kabushiki Kaisha) with the weight ratio of 90.9 to 9.1 at 400 °C. As it was reported that fully delithiated MnPO4 through the chemical reaction was apt to be amorphization,18 partial electrochemical delithiation of LiMnPO4 was employed. MnPO4 was obtained

electrolytes, is undesirable in developing safe batteries. Nowadays, LiMn1−xFexPO4 (x ≥ 0.6) is proposed as a candidate of high power and energy cathodes.8,18,30,61 The lower Mn2+/Mn3+ redox potential in LiMnPO4 can make up for the deficiency in LiFePO4. In the present paper, we carry out the structural studies for LiMPO4 and MPO4 (M = Mn, Fe) by analyzing the high-energy synchrotron-radiation (SR) powder-diffraction data using the maximum entropy method (MEM)/Rietveld method8,31−37,48 to reveal the structural characteristics essential in designing a promising cathode material. The MEM charge density study enables us to visualize the chemical bonding nature between atoms accurately, and it reveals the charge transfers between atoms. We have obtained clear structural evidence on the anisotropic Mn3+−Ok (k = 1−3) bonding in MnPO4 affected by both the Mn3+ Jahn−Teller effect and the (PO4)3− inductive effect leading to a charge transfer from the Mn to the P atoms. The present results will be helpful in understanding the delithiation mechanism in the two-phase reactions.

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EXPERIMENTAL SECTION Sample Preparation. LiFePO4 was prepared by a conventional carbothermic reaction. Li2CO3 (Kanto Chemical Co., Inc., 99%) and FePO4 (heat treatment of FePO4·2H2O) were mixed in a 1:2 mol ratio and were added to 10 wt % sucrose. The raw materials were dispersed into ethanol and were ground by a high-energy ball milling for 24 h. After drying, the mixtures were sintered at 700 °C for 5 h under flowing N2. The crystallite size was 1.4 × 102 nm with a single phase estimated by the Rietveld refinement for the X-ray powder-diffraction 2609



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after dismantling the cell constituted in the LiMnPO4−carbon composite cathode with 85% charging state at 0.1 C at 50 °C. The sample was prepared in argon gas owing to no moisture resistance and to being readily soluble in water, which would be connected with the metastability. The electrochemical conditions and the results are written in the Supporting Information (SI) in detail. The product containing MnPO4 and LiMnPO4 in an 83:17 mol ratio was regarded as a MnPO4 sample. The crystallite sizes of the obtained MnPO4 and LiMnPO4 in the sample were 19 and 23 nm, respectively. The four samples of LiMPO4 and MPO4 (M = Mn, Fe) used for the MEM/Rietveld analysis were battery grades (scanning electron microscope images of the four samples in Figure S1). MEM/Rietveld Technique. The samples were sealed in quartz capillaries of 0.2 mm internal diameter after grinding them in an agate mortar. The high-energy SR powderdiffraction experiments at 298 K were performed using a large Debye−Scherrer camera and an imaging plate as a detector installed at BL02B2 in SPring-8. High-energy SR with ca. 25 keV [wavelength λ = 0.49646(4) Å] was adopted as incident X-rays. A split pseudo-Voigt function43 was used as a peak profile function. A computer program SP enabled us to remove complicated background contributions statistically.37 The charge density distributions were determined by the MEM analysis using the observed structure factors and standard deviations (a program ENIGMA).32 The unit cells were divided into 256 × 128 × 128 pixels for the MEM charge density analysis. The crystal structures and the charge density distributions were visualized using a program VESTA.44 Bond valences, whose values decrease exponentially with values of interatomic distances Rij, and the sums for the cation45,46 were calculated using the values Rij estimated in the MEM/Rietveld analysis. Bond valences correspond to empirical bond strengths which modify the Pauling’s bond strength and provide empirical bond strengths between identical cation and anion according to an electromagnetism calculation.45,47 As values of minimum-charge densities ρmin between atoms also decrease exponentially with values Rij,34 ρmin has a connection with the empirical bond strength. Each atom was assigned a fraction of the charge density; the space partitioning for the atoms was performed by considering spaces surrounded by planes perpendicular to straight lines, which link the atoms and neighboring atoms, through positions of ρmin on the lines.8,31,34 Net charge is computed by the difference in the atomic number and the number of electrons in the enclosed spaces of the continuous charge density distributions. We consider that the net charge analysis supplies more informative results.31,34

summarized in Table 1. The reliability factors (R-factors) based on the weighted profile RWP were 1.80, 3.31, 1.80, and 2.73%; Table 1. Structural Parameters for LiMPO4 and MPO4 (M = Mn, Fe) in Orthorhombic System with Olivine Structure at 298 Ke atom

x

y LiMnPO4

Li Mn P O1 O2 O3 Mn P O1 O2 O3

0 0.28176(3) 0.09271(5) 0.09620(14) 0.45319(14) 0.16199(10)

z

Ueq (10−2 Å2)

0 0.97170(8) 0.40869(12) 0.73360(26) 0.21105(26) 0.27608(18)

1.9(3) 0.69(2) 0.67(4) 0.72(10) 0.83(10) 0.80(7)

a

0 0.25 0.25 0.25 0.25 0.04800(17) MnPO4b

0.2853(2) 0.0984(2) 0.1175(5) 0.4372(6) 0.1642(5)

0.25 0.25 0.25 0.25 0.0423(9) LiFePO4c

Li Fe P O1 O2 O3

0 0.28227(3) 0.09508(5) 0.09633(13) 0.45494(14) 0.16662(11)

0 0.25 0.25 0.25 0.25 0.04576(16) FePO4d

Fe P O1 O2 O3

0.27468(4) 0.09352(7) 0.12158(14) 0.43942(15) 0.16910(11)

0.25 0.25 0.25 0.25 0.04206(18)

0.9132(4) 0.3925(5) 0.722(1) 0.150(1) 0.2478(6)

0.56(1) 0.65(7) 0.5(2) 0.5(2) 0.5(2)

0 0.97444(8) 0.41781(11) 0.74609(24) 0.20596(24) 0.28457(17)

1.3(2) 0.61(1) 0.53(3) 0.68(9) 0.55(9) 0.57(6)

0.95013(7) 0.39693(14) 0.71154(30) 0.16077(29) 0.24927(20)

0.52(2) 0.54(4) 0.69(11) 0.64(11) 0.55(7)

a

a = 10.44843(6) Å, b = 6.10419(3) Å, c = 4.74477(3) Å, and V = 302.618(5) Å3. bThe analysis was performed under the constraint conditions U(O1) = U(O2) = U(O3). The mol fraction of MnPO4 was 83.2 mol %. a = 9.6237(7) Å, b = 5.9019(3) Å, c = 4.7711(3) Å, and V = 270.99(5) Å3. ca = 10.32525(4) Å, b = 6.00594(2) Å, c = 4.69246(2) Å, and V = 290.993(3) Å3. da = 9.81508(6) Å, b = 5.79121(3) Å, c = 4.78392(3) Å, and V = 271.924(5) Å3. eThe refined anisotropic atomic displacement parameters are tabulated in Table S1 of the Supporting Information (SI). The bond angles and the averaged interatomic distances and bond angles are listed in Tables S2 and S3 of the SI, respectively. The distortion parameters for the bond length50,51 and bond-angle variance44 between the transition metals and neighboring oxide ions are also summarized in Table S3 of the SI. The observed and the calculated structure factors through the MEM/ Rietveld technique are listed in Table S4 of the SI.

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RESULTS AND DISCUSSION Rietveld Refinements of SR Powder-Diffraction Data for LiMPO4 and MPO4 (M = Mn, Fe). The diffraction intensity data were collected in the range of sin θ/λ < 1.02 Å−1 (d > 0.49 Å), and totally, 1385, 1247, 1329, and 1271 reflections were measured for LiMnPO4, MnPO4, LiFePO4, and FePO4, respectively. The Rietveld analyses in the MEM/ Rietveld method were carried out assuming the previously reported structure parameters as initial parameters of LiMPO4 (M = Mn, Fe).41,42 The Li atoms are settled at the 4a sites; the M, O1, and O2 atoms at the 4c sites; and the O3 atoms at the 8d sites in the space group of Pnma. The Li sites in the LiMPO4 crystal are regarded to be vacant after the delithiation. The fitting results for LiMPO4 and MPO4 are shown in Figure 1, and the structural parameters for LiMPO4 and MPO4 are

the Bragg integrated intensities RI were 1.83, 3.37, 1.35, and 2.23%; and the structure factors RF were 1.27, 2.72, 0.98, and 1.48% for the analyses of LiMnPO4, MnPO4, LiFePO4, and FePO4, respectively. All the R-factors were sufficiently small, although the two crystal phases of 83.2 mol % MnPO4 and 16.8 mol % LiMnPO4 coexisting were detected in the analysis of the MnPO4 sample. The experimentally obtained structural parameters for MnPO4 with excellent accuracy are so important as there are few reports about them because of the metastability.18,51 The crystal structure of LiMnPO4 and the local configuration of the MnO6 octahedron and PO4 tetrahedron are drawn in Figure 2. The unit cell of LiMnPO4 in the orthorhombic system contains the four chemical formula units (Li4Mn4P4O16) 2610



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Visualization of Three-Dimensional (3D) Charge Density Distribution of LiMPO4 and MPO4 (M = Mn, Fe) and Their Crystallographic and Electronic Properties on the Chemical Bonding. The isosurfaces of the 3D charge density distributions in LiMPO4 and MPO4 (M = Mn, Fe) for the unit cells are shown in Figure 3 in order to understand the atomic positions in space and the bonding nature between the cation and the neighboring oxide ions. The isosurfaces of Li, M, P, and Ok atoms (k = 1−3) are colored red, flesh, green, and yellow, respectively, in the abovementioned manner in the Experimental section. The Li atom in LiMPO4 is ionic and is isolated in the charge density distributions, while the large electron density distributions (i.e., the large nodes of the distributions) between the P and the O atoms and the M and the O atoms can be seen in all the four compounds because of their covalency. The delithiation causes the contraction in the unit cell along the a-axis because of the relative stable P−O3− M olivine networks50−52 on the bc-planes. The M−Ok bonding (k = 1, 2) inclining toward the a-axis in MPO4 becomes more covalent than that in LiMPO4. The structural parameters revealed in the MEM/Rietveld analyses are listed in Table 2 on the specific chemical bonds for LiMPO4 and MPO4 (M = Mn, Fe). We pick up the interatomic distance Rij between the cation i and the neighboring oxide ion j and the minimum charge density ρmin on the bonding from Table 2, and we exhibit the correlations in Figure 4. The charge densities ρmin on each bonding change according to a formula (Rij/R0)−N as indicated with the fitting curves in Figure 4 by which ρmin might be in harmony with the empirical bond strength,34,45,47 where R0 and N are optimized values for the fitting. The plotted data on the P−O bonding are gathered at the upper left of the figure, which denotes high covalency on the P−O bonding. The experimentally estimated ionic radii of bonded oxide rO on the P−O bonding in Table 2 are rather smaller than 1.38 Å of the Shannon and Prewitt (S−P) ionic radii,55,56 which also supports high covalency on the P−O bonding. The two curves for the Li−O bonding are located at the lower left side in the figure, and rO on the Li−O bonding is close to the S−P ionic radii (1.4 Å), which indicates that the

Figure 2. Crystal structure of LiMnPO4 and local configuration of MnO6 octahedron and PO4 tetrahedron with thermal displacement ellipsoids at 99% probability drawn by using parameters in Table 1 and in S1 of the SI. Symmetry codes: (i) x, −y + 1/2, z; (ii) −x + 1/2, y + 1/2, z + 1/2; (iii) −x + 1/2, −y + 1, z + 1/2; (iv) x, y + 1, z; (v) −x + 1/2, −y + 1, z − 1/2; (vi) −x + 1, −y + 1, −z + 1. The blue plane at y = 3/4 and the red plane through the O3 atoms in MnO6 and PO4 are colored for analyses of two-dimensional (2D) charge density distributions in Figure 5. The blue plane corresponds to a mirror plane of MnO6 octahedron and PO4 tetrahedron and contains Mn−P bonding. The red plane can also expresses Mn−P bonding as well as physically toughened olivine network which has impact on the electronic conduction and magnetic properties.50−52 MnO6 octahedron is formed by two axial oxide ions (O1 and O2) and four equatorial ones (O3).

composed of the LiO6 octahedron and the MnO6 octahedron and the PO4 tetrahedron on the mirror plane parallel to the acplane. The so-called olivine-type networks are constituted along the bc-planes with sharing the corners and edges of the MnO6 and PO4 polyhedron. The cation and the neighboring oxide ions are connected by the gray sticks. The blue and red planes are illustrated to discuss the charge transfer between the Jahn− Teller Mn3+ ion and the electron induction (PO4)3− ion by mapping the MEM charge density distributions on the planes (see Figure 5). The chains of the LiO6 octahedra matched with the diffusion path of the Li+ ions are not drawn to emphasize the two planes.

Figure 3. Three dimensional (3D) charge density distributions at isosurface of 0.48e Å−3 in the unit cells for (a) LiMnPO4, (b) MnPO4, (c) LiFePO4, and FePO4 corresponding to Figure 2. Electron distributions of Li, M, P, and O atoms are colored red, flesh, green, and yellow, respectively, according to an extended Mulliken scheme.8,31,34 The cross section is colored with blue. Electron clouds of the Li atoms in LiMPO4 are isolated from others, which indicates the Li atom is ionic, while bonding electrons are seen between the M and the O atoms and the P and the O atoms, which indicates the M−O bonding and the P−O bonding are covalent. The bonding electron densities on the M−O1v and M−O2iii bonds are increased after the delithiation with the contraction of the a-axis. 2611



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Table 2. Interatomic Distance Rij, Radii of Bonded Oxide Ion rO,a and Minimum-Charge Densities ρmin in LiMPO4 and MPO4 (M = Mn, Fe) and Stretching Rate for Rij between LiMPO4 and MPO4b LiMnPO4

MnPO4

bond

Rij (Å)

rO (Å)

ρmin (e Å−3)

Li− O1 Li− O2 Li− O3 M− O1 M− O2 M− O3i M− O3ii P−O1 P−O2 P−O3

2.2219(10)

1.290

2.1090(9)

LiFePO4 Rij (Å)

rO (Å)

ρmin (e Å−3)

0.28

2.1595(9)

1.262

0.28

1.224

0.34

2.0916(8)

1.230

0.33

2.1602(10)

1.254

0.31

2.1951(10)

1.291

0.28

2.2439(14)

1.100

0.48

1.855(5)

0.887

0.97

2.1987(13)

1.078

2.1209(14)

1.040

0.57

1.848(6)

0.877

0.96

2.0878(14)

2.1251(10)

1.017

0.57

1.958(5)

0.960

0.75

2.2742(10)

1.097

0.49

2.326(4)

1.122

1.5421(14) 1.5650(20) 1.5621(11)

0.847 0.835 0.851

1.71 1.57 1.56

1.582(5) 1.565(6) 1.543(5)

0.838 0.804 0.787

Rij (Å)

rO (Å)

ρmin (e Å−3)

stretching ratec

FePO4 Rij (Å)

rO (Å)

ρmin (e Å−3)

M = Mn (%)

M = Fe (%)

0.51

1.8870(14)

0.910

0.81

−17.3

−14.2

1.015

0.58

1.9052(15)

0.926

0.74

−12.9

−8.7

2.0561(10)

1.016

0.62

2.0220(10)

0.999

0.68

−7.9

−1.7

0.48

2.2469(10)

1.112

0.48

2.1382(11)

1.048

0.54

2.3

−4.8

1.48 1.70 1.74

1.5405(12) 1.5592(15) 1.5624(10)

0.858 0.850 0.858

1.73 1.62 1.53

1.530(2) 1.537(2) 1.5810(11)

0.804 0.868 0.874

1.67 1.62 1.46

2.6 0.0 −1.2

−0.7 −1.4 1.2

a

The values rO are defined as distances from the nucleus of the oxide ions to positions of minimum-charge densities, which use bonded radii of O atoms defined in literature49 as reference. bThe symmetry codes are shown in Figure 2. cThe values are taken as the difference divided by the value of lithiated compounds.

between the P and O1 atoms. The phenomenon will be elucidated in detail using the net charge analysis in the final subsection of Results and Discussion. The stretching rates for the interatomic distance Rij are calculated and added in Table 2 to see how the delithiation affects the bond lengths. According to the rates, Rij between the M and the axial Ok (k = 1, 2) atoms in the MO6 octahedron is remarkably shortened most of all after the delithiation, and the rates for M−O1 and M−O2 bonding in M = Mn are higher than those in M = Fe. These results imply that the electrons in the dz2 orbital of LiMnPO4 and in the dyz or dzx orbital of LiFePO4 contribute to the delithiation, since the electronic configurations with high spin states18 for the Mn2+ ion in the MnO6 octahedron and for the Fe2+ ion in the FeO6 octahedron are denoted by t23e2 and t24e2, respectively. The plotted data for the Fe−O bonding in FePO4 are distributed at the upper left side compared to the data for that in LiFePO4 on the green and blue lines, respectively, in Figure 3, which gives experimental evidence that the orbital hybridizations between the Fe−3d and O−2p orbitals in FePO4 are stronger as mentioned in the Introduction.16−20 The lattice deformation caused by the Mn3+ Jahn−Teller effect connected with the (PO4)3− inductive effect in MnPO4 provides a wide distribution in the variation of the Mn−O bond lengths from 1.848 to 2.326 Å. Only the Mn−O3ii bonding among the M−O bonds lengthens and weakens after the delithiation. It is interpreted that the number of the cations bonded to each O1 and O2 atom in MPO4 becomes two and that the number of the cations bonded to the O3 atoms becomes three without marked changes for the original bond angle centered upon the Ok atoms with the four bonded cation in thermally stable LiMPO4.41,42 Thus, the fact that only the Mn−O3ii bonding lengthens and weakens among the M−O bonding (M = Mn, Fe) leads to the collapse of the M−O−P olivine network and, therefore, to the instability of MnPO4 at the high temperature because the number of the bonded cations of the O3 atoms in MnPO4 is regarded as almost two because of the very weak Mn−O3ii bonding.

Figure 4. Minimum-charge density ρmin vs interatomic distance Rij, where the data points between the cations and the bonded oxide ions are fitted by solid lines using the form (Rij/R0)−N as a guide for eyes.34,45.

Li−O bonding is ionic and which implies that the majority of the space assigned to the Li atoms is very low electron density distribution. In this situation, migration of the Li+ ions may be allowed in the solid, which cannot be explained by bond valence plotted against Rij since bond valence provides empirical bond strength on the same cation−anion bonds.45,47 It has been reported that the PO4 tetrahedron in H3PO4 possesses one shorter double bonding PO and three longer single bonding P−OH.55,56 The O atoms associated with the hydrogen bonding are known to be negatively charged because of the inductive effect. In the case of LiMPO4 and MPO4 similarly to H3PO4, the shortest P−O1 bonding of the PO4 tetrahedron in LiMnPO4, LiFePO4, and FePO4 in Table 2 can be considered to hold the nature of the partial double bonding, respectively. In the three materials, the isosceles triangle formed by the O2, O3ii, and O3iii atoms of the PO4 tetrahedron (see Figure 2) attracts electrons and becomes negatively charged. However, the PO4 tetrahedron in MnPO4 shows the lowest value of ρmin on the longest P−O1 bonding and the highest value of ρmin on the shortest P−O3 bonding. Thus, we consider that the delithiation reaction of LiMnPO4 causes unexpected change of the electronic configurations of the PO4 tetrahedron because of the disappearance of the partial double bonding 2612



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Figure 5. Visualization of anisotropic M−O bonding and specific M−P bonding for LiMPO4 and MPO4 (M = Mn, Fe): Two-dimensional (2D) contour maps of charge density distributions with equidensity lines drawn from 0.4e to 3.0e Å−3 on y = 3/4 planes and the planes through the O3 atoms in MO6 octahedron and PO4 tetrahedron for (a, e) LiMnPO4, (b, f) MnPO4, (c, g) LiFePO4, and (d, h) FePO4. The M and P atoms labeled with italic fonts in e−h are located at ca. 0.1 Å and ca. 0.25 Å out of the plane, respectively. To clarify the charge density distributions induced from M ions by (PO4)3− ions, the equidensity lines in the range of 0.4−0.5e Å−3 are divided more closely into three.

Table 3. Bond-Valence Sums and Net Charges of Atoms in LiMPO4 and MPO4 (M = Mn, Fe) bond-valence sum LiMnPO4 MnPO4 LiFePO4 FePO4

net charge

Li

M

P

Li

M

P

O1

O2

O3

0.917(1)

2.051(5) 3.17(2) 2.007(5) 3.081(8)

4.53(2) 4.53(4) 4.55(7) 4.55(2)

0.8

1.5 2.4 1.6 2.1

2.5 1.8 2.5 2.5

−1.2 −1.0 −1.2 −1.0

−1.0 −1.1 −1.1 −1.3

−1.0 −1.0 −1.1 −1.1

0.954(2)

0.7

Direct Observation of Lattice Distortion by Jahn− Teller Mn3+ Ion in MnPO4 and Specific M−P Bonding in LiMPO4 and MPO4 (M = Mn, Fe) and the Effects on the Electrochemical Properties. Figure 5 shows the 2D counter maps of the charge density distributions with equidensity lines drawn from 0.4e to 3.0e Å−3 on y = 3/4 planes and the planes through the O3 atoms in the MO6 octahedron and PO4 tetrahedron. The planes of Figure 5a−d correspond to the blue plane in Figure 2, and those in Figure 5e−h correspond to the red plane. The axial M−Ok bonding (k = 1, 2) in Figure 5a−d shows the higher charge densities in MPO4 than those in LiMPO4 owing to the stronger hybridizations between the M− 3d and the Ok−2p orbitals. The 2D counter maps in Figure 5e−h display the lower charge density distributions on the M− O3ii bonding than those on the M−O3i bonding owing to the longer M−O3ii bonding caused by the electron induction (PO4)3− ion. Especially, the anisotropic Mn−O3 bonding is highlighted. The M−P bonds between the M and the nearest P atoms in all samples are evidently visualized on the 2D maps in the range from 0.4e to 0.48e Å−3 in Figure 5a−h. The specific M−P bonds are also ascribed to the inductive effects of the (PO4)3− ions leading to the lower M2+/M3+ redox potentials. The 2D maps reveal slightly higher electron density distributions between the M and P atoms in the delithiated MPO4 (M = Mn and Fe) than those of LiMPO4; ρmin between the M and P atoms in LiMnPO4 and MnPO4 is 0.42e and 0.46e Å−3, respectively, and that between LiFePO4 and FePO4 is 0.44e and 0.45e Å−3, respectively. It is found that delithiated MPO4 is significantly affected by the electron induction of (PO4)3− ion. Charge Analysis for Atoms in LiMPO4 and MPO4 (M = Mn, Fe) and the Effects on the Electrochemical Properties. Bond-valence sums and net charges of the atoms in LiMPO4 and MPO4 (M = Mn, Fe) are summarized in Table 3. The values of the bond-valence sums for the Li, M, and P

atoms fairly coincide with those of the ideal oxidation states, that is, +1 for Li and +2 for Mn and Fe in LiMnPO4 and LiFePO4, respectively, +3 for Mn and Fe in MnPO4 and FePO4, respectively, and approximately +5 for P, and agree with the precedent crystallographic studies.41,50,51 According to the classic and rough categorization based on the bond-valence sums,45 the P−O and Li−O bondings show covalent and ionic natures, respectively, and the M−O bonding shows the intermediate types. The values of the net charges in Table 3 are less than those of the bond-valence sums because the outermost electrons are shared between the atoms; the difference between net charges and the bond-valence sums for the same cation increases with the covalency. The value of the Li atom in LiFePO4 is almost the same value as the difference in the values of the Fe atoms between LiFePO4 and FePO4, which supports the regular delithiation reaction. While the values of the net charges for the Li, M, P, and Ok (k = 1−3) atoms in LiMnPO4 agree with those in LiFePO4, the values for only the Ok atoms in MnPO4 agree with those in FePO4. Namely, it is found that the Mn atom in MnPO4 is more ionized than the Fe atom in FePO4 and that the number of electrons assigned to the P atom in MnPO4 is largest among the P atoms in the four olivine materials. Hence, the results suggest that the electrons assigned to not only the Mn atom but also the P atom are involved in the delithiation of LiMnPO4, which gives evidence that the charge transfer from the Mn to the P atoms occurs. The more active P atom in MnPO4 may lead to the metastability at room temperature and may allow to be dissolved in water. There is no evidence for such a drastic change for the PO4 units in the delithiation of LiMnPO4 except for theoretical prediction that the electronic structure around the (Li)PO4 units in olivine LiCoPO4 is significantly affected by the delithiation with charge donation of 0.3e from the Co atom to the P atom.57 2613



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CONCLUSION The structural characteristics in LiMPO4 and MPO4 (M = Mn, Fe) as cathode materials are investigated by SR powder diffraction through the MEM/Rietveld technique. The delithiated MnPO4 is less stable at high temperature and is metastable at room temperature. The present study directly reveals that the anisotropic Mn−O bonding in MnPO4 is derived from the inductive effect of the adjacent (PO4)3− ion, which causes the lattice distortion and, therefore, which induces less thermal stability of MnPO4. The net charge analysis shows that the number of electrons assigned to the P atoms increases in the delithiation process of LiMnPO4, which suggests that more active P atoms are attributed to the metastability of MnPO4 such as the amorphization at room temperature. It is essential to suppress the lattice distortion and charge transfer in designing a promising cathode material.

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ASSOCIATED CONTENT

* Supporting Information S

Electrochemical conditions and the results. 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 The authors thank Dr. J. Kim and Dr. N. Tsuji for their help in the experiment at SPring-8. The SR experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal nos. 2009B0084, 2010A0084, and 2010B0084).

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MEM Charge Density Study of Olivine Li - American Chemical

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