Lithium Insertion into Li2MoO4: Reversible Formation of (Li3Mo)O4

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Lithium insertion into Li2MoO4: reversible formation of (Li3Mo)O4 with a disordered rock-salt structure D. Mikhailova, A. Voss, S. Oswald, A. A. Tsirlin, M. Schmidt, A. Senyshyn, J. Eckert, and H. Ehrenberg Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b01633 • Publication Date (Web): 01 Jun 2015 Downloaded from http://pubs.acs.org on June 8, 2015

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Chemistry of Materials

Lithium insertion into Li2MoO4: reversible formation of (Li3Mo)O4 with a disordered rock-salt structure D. Mikhailova1,2,3*, A. Voss2, S. Oswald2, A. A. Tsirlin4, M. Schmidt3, A. Senyshyn5, J. Eckert2,6, H. Ehrenberg1 1

Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 2 IFW Dresden, Institute for Complex Materials, Helmholtzstr. 20, D-01069 Dresden, Germany 3 Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, D-01187 Dresden, Germany 4 National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia 5 Forschungsneutronenquelle Heinz Maier-Leibnitz FRM-II, Technische Universität München, Lichtenbergstr. 1, D-85747 Garching, Germany 6 TU Dresden, Institute of Materials Science, Helmholtzstr. 7, D-01062 Dresden, Germany

Abstract During Li-insertion in some complex transition metal molybdates with a NASICON structure, which serve as cathodes in Li-ion rechargeable cells, a formation of a cubic rock-salt-type phase was often detected between 1 and 2 V vs. Li+/Li. Detailed information about elemental composition and stability of this compound was missing and suggestions were made toward a solid solution composed of lithium oxide and two-valence transition metal oxide MO with M a 3d element. In the present work, we showed that Li2MoO4 with a phenacite-type structure without any additional transition metal can reversibly accommodate Li-ions at room temperature with the formation of the NaCl-type compound. Reversible Li-incorporation into the Li2MoO4 structure is accompanied by a reduction of Mo ions and changes in their oxygen coordination. Li-ions are shifted from a tetrahedral to an octahedral site, resulting in the formation of a cubic (Li3Mo)O4 framework with a random distribution of Li and Mo on one site. This mixed occupancy is remarkable because of significant charge and size differences between Li+ and Mo5+. The novel compound shows Li-deficiency at least up to xLi = 0.2, which can be deduced from charge flow in the galvanostatic cycling of the electrochemical cells with a (Li3Mo)O4 cathode between 1.5 and 2.75 V vs. Li+/Li. An increase in the cell potential above 3 V leads to the oxidation of (Li3Mo)O4 back to Li2MoO4 with phenacite-type structure. The reaction of (Li3Mo)O4 to Li2MoO4 also occurs upon a short exposure to air.

Introduction Complex polyanionic molybdates of the general formula Li3M(MoO4)3 or Li2M2(MoO4)3 with a 3d transition metal M can be considered as intercalation materials for storage of Li-ions in stationary battery devices due to the presence of large channels in the structure, which are 1 Environment ACS Paragon Plus


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sufficient for significant Li-ion mobility, and due to the ability of a 3d metal to change its oxidation state while keeping the oxygen coordination nearly intact.1 The NASICON-like structure of these molybdates is built on a three-dimensional framework of (M,Li)O6octahedra sharing corners with MoO4-tetrahedra. The electrochemical behaviour of such quaternary molybdenum oxides with 3d transition metals as cathode materials was studied in numerous works.1-6 The oxidation and reduction processes during lithium extraction and insertion are associated with a change in the oxidation state of the 3d transition metal, either alone or with molybdenum simultaneously.6 In oxide compounds, molybdenum typically features oxidation states between 3+ and 6+ and adopts different oxygen surroundings such as tetrahedra, tetragonal pyramids and octahedra. Detailed structural studies of deep Li-insertion into ternary molybdenum oxides with 3d transition metals often registered an unknown phase with a cubic rock-salt type structure with a lattice parameter of about 4.12 Å.3, 6-9 For example, such a compound was found during Liinsertion into MnMoO4 (Ref. 7) and ZnMoO4 (Ref. 8). A formation of a defect LixZn1-xO rock-salt structure was proposed by Leyzerovich et al.8 A cubic NaCl-type compound was also detected by in situ synchrotron measurements during Li-intercalation into isostructural Li3V(MoO4)3 (Ref. 6), or Li3+xFe(MoO4)3 with x > 1 (Ref. 4) materials. A compound with a similar cubic lattice parameter was also observed after chemical lithiation of Fe2(MoO4)3, where a formation of iron oxide with a small amount of Li was proposed.9 Up to now, the compositions of these cubic compounds, conditions of their formation and their electrochemical activity remained unclear. The phases could consist of i) a 3d metal oxide MO with a small amount of lithium, or ii) a mixed oxide containing lithium, molybdenum and a 3d element, or iii) a lithium molybdenum oxide. The latter case would be most interesting because of the large difference in the ionic radii of Li and Mo cations (for example, 0.76 Å of Li+ and 0.59 Å of Mo6+ for octahedral oxygen coordination10). Several Li,Mo-containing oxides with structures related to the NaCl-type structure are known. However, they all show a cation ordering. For example, Li4Mo5O17 with Mo6+ forms an ordered triclinic rock-salt-type structure after accommodation of 8 Li atoms per formula unit. 11

The cation ordering in the structure results from very different ionic radii of the cations.11

Other lithium molybdates, LiMoO2 (Ref. 12) and Li2MoO3 (Ref. 13) with Mo3+ and Mo4+, respectively, adopt the α-NaFeO2-type structure with a cubic close-packed arrangement of oxygen anions and alternating layers of LiO6- and MoO6-octahedra (LiMoO2) or LiO6- and (Li,Mo)O6-octahedra (Li2MoO3), which can also be seen as a NaCl-derivative. Only Liextraction without drastic structural changes is possible for these compounds since the pristine 2 Environment ACS Paragon Plus

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crystal structures exhibit no voids suitable for Li-insertion. Recently, carbon-coated Li2MoO4 nanotubes were studied as a potential anode material for Li-ion batteries.14 The authors reported that the structure of Li2MoO4 irreversibly transforms into Li0.98MoO2 and metallic Mo upon deep Li+-insertion during cell discharge down to 0.38 V vs. Li+/Li, and only metallic Mo and Li2O were seen after cell discharge down to 0.1 V vs. Li+/Li.14 A hypothesis about the formation of rock-salt-type compounds was discussed a short time ago by Pralong:15 they can be obtained after Li-insertion into three-dimensional (3D), twodimensional (2D) and one-dimensional (1D) hosts, provided that the anionic framework is a cubic close-packed structure, and transition-metal ions adopt a suitable oxidation state. However, the formation of the rock-salt structure after the decomposition of ternary molybdates may be hampered by the large difference in the ionic radii of Li+ and Mo5+ and by the proclivity of molybdates to the formation of a very stable three-dimensional MoO4framework. In contrast, we show in the present work that the rock-salt-type structure with random cation distribution is formed upon lithium insertion at 1 V vs. Li+/Li into commercially available Li2MoO4. The parent compound crystallizes in a phenacite-type rhombohedral framework (R3 space group, a = 14.330 Å, c = 9.548 Å, Z = 18), formed by corner-sharing LiO4- and MoO4-tetrahedra,16 see Fig. 1. Two types of channels running along the c-axis are formed. Narrow and wide channels feature 4-member rings with an average diameter of 3.49 Å and 6member rings with an average diameter of 5.47 Å, respectively. These channels provide sufficient space for the insertion of guest species, either cations or molecules. Both electrochemical and chemical lithium insertion into Li2MoO4 were studied. The crystal structure of the newly obtained cubic (Li3-xMo)O4 compound was investigated by synchrotron and neutron powder diffraction. This new compound was also tested in electrochemical test cells with Li as a counter electrode.

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5.47 Å

3.49Å

Figure 1. Crystal structure of the rhombohedral modification of Li2MoO4. Blue and yellow spheres represent Mo and Li ions, respectively. Narrow and wide channels along the c-axis feature 4-member rings and 6-member rings, respectively.

Experimental Preparation of cubic (Li3-xMo)O4 by a chemical route Chemical insertion of Li-ions into the powdered Li2MoO4 (Alfa Aesar, 99.99%) host compound was performed at room temperature in an Ar-filled glove-box according to the reaction (1), using double or triple excess of n-butyllithium (n-LiC4H9) in hexane: Li2MoO4 + (1-x) LiC4H9 (Li3-xMo)O4 + (1-x)/2 C8H18

(1)

The reaction time was varied between one and four weeks. Attempts to use lithium iodide in acetonitrile instead of n-LiC4H9 did not lead to lithium insertion in significant amounts. After reaction with n-LiC4H9 the (Li3-xMo)O4 samples were washed with pentane to remove traces of unreacted agents and dried under vacuum at room temperature. The amounts of Li and Mo in the samples were quantitatively determined by the ICP-OES analysis (IRIS Intrepid II XUV, Thermo Fisher) using a 3:1 mixture of HCl (37%, p.a. Fa. Merck) and HNO3 (65%, p.a. Fa. Merck) for dissolving the samples. Thermal stability of (Li3Mo)O4 was


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investigated in an STA 449 (Netzsch, Selb, Germany) under Ar-atmosphere during heating in alumina crucibles with 10 K/min rate from room temperature up to 1073 K. Electrochemical synthesis and characterization Electrochemical studies on Li2MoO4 and (Li3-xMo)O4 as positive electrode (cathode) materials in Li-ion cells were performed with a multichannel potentiostatic-galvanostatic system VMP3 (Bio-Logic, France) in standard Swagelok-type cells with metallic lithium as the negative electrode (anode). For the positive electrode, a mixture of Li2MoO4 or (Li3xMo)O4,

carbon black and polyvinylidene fluoride (PVdF) as polymer binder in an 80:10:10

weight ratio was pressed on Al-meshes with 8 mm diameter and dried in vacuum at 373 K for Li2MoO4 and at room temperature for (Li3-xMo)O4. A 1 M solution of LiPF6 in a mixture of ethylene carbonate (EC) and dimethylcarbonate (DMC) (1:1 v/v) was used as electrolyte. The cells were assembled in an Ar-filled glove-box with H2O and O2 contents less than 1 ppm. Electrochemical synthesis of cubic (Li3-xMo)O4 was performed in two-electrode Swageloktype cells at 1V vs. Li+/Li during 100 h. Phase analysis and structure characterization Phase analysis and determination of (Li3-xMo)O4 cell parameters at room temperature after electrochemical or chemical lithium insertion were carried out using X-ray powder diffraction (XPD) with a STOE STADI P diffractometer (Cu-Kα1-radiation, λ = 1.54059 Å) in transmission mode. All diffraction experiments were performed without any contact of the sample with air. Room-temperature neutron powder diffraction (NPD) studies on (Li3-xMo)O4 were performed on the high-resolution powder diffractometer SPODI at the research reactor FRM-II (Garching, Germany) with monochromatic neutrons of 1.5481(1) Å wavelength17 in DebyeScherrer geometry. The vanadium container for measurements was filled in an Ar-atmosphere glove box with the sample prepared through the reaction of Li2MoO4 and n-LiC4H9 during four weeks. High-temperature structural investigations of the (Li3-xMo)O4 powdered sample in an Ar atmosphere were performed between 295 K and 623 K by synchrotron diffraction at HASYLAB/DESY (Hamburg, Germany) at the beam-line B2 (Ref. 18) in Debye-Scherrer mode using the on-site readable image-plate detector OBI19 and a STOE furnace equipped with a EUROTHERM temperature controller and a capillary spinner. The wavelength of 0.49962(1) Å was selected by a double-crystal monochromator and determined from the positions of 8 reflections from a LaB6 reference material. X-ray photoelectron spectroscopy (XPS) 5 Environment ACS Paragon Plus


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X-ray photoelectron spectroscopy was applied to chemically prepared Li3-xMoO4 and the products after electrochemical Li-extraction from this compound. A PHI 5600 CI system with an Al Kα 350 W monochromatized X-ray source and a hemispherical analyzer at a pass energy of 29 eV were used. Two cells with the same cathode mixture consisting of rock-salt Li3-xMoO4, carbon black and PVdF with a mass of about 20 mg were charged with a C/10 rate to 2.6 V and 3.5 V and immediately disassembled. In order to remove LiPF6 from the surface the samples were washed with DMC. During XPS measurements, when necessary, surface charging was minimized by means of a low-energy electron flood gun. The system base pressure was about 10-9 mbar. The samples were transferred from the glove box in Ar atmosphere with a special transfer chamber.20

Results and discussion 1. Rhombohedral Li2MoO4: electrochemical lithium insertion and extraction First, electrochemical properties of rhombohedral Li2MoO4 were tested. Two reduction processes and one oxidation process were registered during cyclic voltammetry (CV) measurements of the electrochemical cell with the Li2MoO4 cathode and metallic Li as anode, cycled with a voltage sweep rate of 0.1 mV/s between 1.0 and 3.8 V (Fig. 2a). Different electrochemical behaviour was observed during the first cycle and the following discharge cycles: i) a strong reduction peak at ca. 1.15 V observed in the first cycle shifted to 1.0 V in following cycles, and ii) an additional reduction peak appeared in the second and following cycles at 2.0 V. The corresponding oxidation peak appears at 2.2 V. A formation of a new compound (compounds) at about 1.15 V during the first discharge of Li2MoO4 could be envisaged. This compound shows oxidation/reduction behaviour at about 2.2/2.0 V in subsequent cycles. Galvanostatic cycling with potentiostatic limitation (GCPL) between 0.8 and 4 V at constant current corresponding to the intercalation or extraction of 1Li per formula unit during 10 h (C/10 rate) showed that about one Li atom can be inserted in Li2MoO4 at about 1.15 V in the first discharge cycle (Fig. 2b). This voltage is lower than 2.4 V and 1.6 V known from the literature for the redox couples Mo6+/Mo5+ and Mo5+/Mo4+, respectively.11 A step-like feature in voltage between 1.4 and 1.1 V in the beginning of the first cycle, which corresponds to the change of the x value from 2.0 to 2.22, could be due to the solid electrolyte interphase (SEI) formation on the carbon black from the cathode mixture,21,22 or a reductive instability of the electrolyte components. For example, a reduction potential of 1.32 V vs. Li+/Li for dimethyl carbonate and 1.36 V vs. Li+/Li for ethylene carbonate was reported in 6 Environment ACS Paragon Plus

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work.23 In a more recent work, the reduction potential of ethylene carbonate was estimated to a lower value of 1.07 V vs. Li+/Li.24 In the second and following cycles, a reversible extraction/insertion of about 0.25 Li atoms takes place during the cycling between 4 and 1 V. XRD measurements of the cathode material after the discharge down to 1 V, which corresponds to three lithium atoms in the material, revealed two phases, a cubic phase with the NaCl structure type and lattice parameter a = 4.1563(2) Å, and a rhombohedral Li2+xMoO4 with lattice parameters different from these of pristine Li2MoO4, see Table 1.

4

rhombohedral Li2MoO4

0

I [mA/g]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-4

-8

1

2

3

4

+

E vs. Li /Li [V] Figure 2a. Cyclic voltammetry curves at a voltage sweep rate of 0.1 mV/s of the Li2MoO4/Li cell.



4

rhombohedral Li2MoO4 3

+

E vs. Li /Li [V]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

1

0 2.0

2.5

3.0

3.5

2+x in "Li2+xMoO4" Figure 2b. Galvanostatic cycling of the Li2MoO4/Li cell between 0.8 and 4.0 V with the C/10 rate, which corresponds to a current density of about 15 mA/g. The first charge cycle largely differs from the subsequent cycles. 2. Cubic (Li3-xMo)O4: chemical synthesis and characterization Only the reaction between Li2MoO4 and n-LiC4H9 (1:1 in molar ratio) in hexane led to the formation of a new cubic phase (Li3-xMo)O4, because the low electrochemical potential of nLiC4H9 of about 1 V vs. Li+/Li24 is close to the voltage of the reduction process observed in the GCPL curve of Li2MoO4. After the synthesis during one week the rhombohedral Li2+xMoO4 compound (about 40% w/w) was still observed in the diffraction pattern together with cubic (Li3-xMo)O4 (Fig. 3, left). However, lattice parameters of the rhombohedral phase differed from these of pristine Li2MoO4, thus reflecting a change in the composition, see Table 1. Table 1. Preparation conditions, lattice parameters and cell volumes of rhombohedral and cubic (Li3-xMo)O4, normalized to the Li2+xMoO4 formula unit. Space group, lattice parameters a, c, Å

V/Z, Å3

Pristine rhombohedral Li2MoO4

R-3, a = 14.3449(1), c = 9.59704(7)

95.014(1)

Rhombohedral Li2MoO4 discharge down to 1V

R-3, a = 14.3490(3), c = 9.5958(2)

95.056(3)

Preparation conditions

after


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Cubic (Li3-xMo)O4 after cycling between 1.3 and 3.5 V, stopped at 2.88 V Rhombohedral 100 h at 1.2 V

Li2MoO4

Fm-3m, a = 4.1563(2)

71.800(5)

R-3, a = 14.3378(6), c = 9.5897(5)

94.850(6)

Fm-3m, a = 4.1672(2)

72.364(3)

Fm-3m, a = 4.1534(4)

71.648(5)

R-3, a = 14.3298(1), c = 9.5850(1)

94.696(1)

Fm-3m, a = 4.1450(1)

71.217(3)

Fm-3m, a = 4.1494(2)

71.442(5)

after

Reaction between Li2MoO4 and n-LiC4H9, one week Reaction between Li2MoO4 and n-LiC4H9, four weeks

Detailed analysis of the lattice parameters and unit cell volumes for the rhombohedral and cubic phases revealed a huge difference in the size of the unit cell per formula unit. Additionally, lattice parameters of each phase fluctuate depending on the preparation procedure and reflect the existence of solid solutions. The lattice parameters a and c of the rhombohedral phase decrease with increasing Li-content, as can be seen from the comparison of pristine Li2MoO4 and rhombohedral Li2+xMoO4 compounds after the reaction with nLiC4H9 during one week (Table 1). The variable lattice parameter of the cubic phase reflects different Li/Mo ratios. Therefore, the change in the composition of this NaCl-type compound must be also detectable electrochemically. Chemical analysis of the product of the one-week synthesis with n-LiC4H9 yields the cation composition Li3.1(3)Mo1.0O4, which is normalized to the molybdenum content. After four weeks, only cubic (Li3-xMo)O4 was registered by XRD (Fig. 3, right). However, a somewhat higher background at low angles in the XRD pattern reflects the presence of an amorphous component in the sample. From the chemical analysis of this sample, we obtained the ratio Li/Mo =3.8(2)/1.0, which is too high for the rocksalt-type structure where only 3 Li atoms per one Mo atom can be incorporated, provided that the oxygen content does not change. One plausible reason for the presence of excess lithium would be the incorporation of n-LiC4H9 molecules into large channels of the Li2MoO4 structure. The ensuing agglomerate structure should have low thermal stability and may decompose already at 473-523 K. Indeed, the DTA-TG analysis of Li3.8(2)Mo1.0O4 showed a two-step mass loss in the TG-curve, with about 0.4% at 523 K, and 0.98% at 973-1073 K, but this mass loss of 1.4% is tiny (data not shown) and cannot explain observed Li excess in the sample. An irreversible endothermic peak was 9 Environment ACS Paragon Plus


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seen at about 723 K. Mass-spectrometric analysis revealed solely H2 and CH3 fragments as decomposition products with the maximum of decomposition in the temperature range 670773 K. No oxygen was detected during heating up to 1073 K. The XRD data of the sample after DTA revealed well-crystallized metallic molybdenum and unknown partially crystallized products of decomposition. From these results, we cannot distinguish between two processes, i) metallic molybdenum is formed during the phase decomposition at elevated temperatures, or ii) it was already present in the sample in amorphous form and became crystalline upon heating. In the last case, Li-excess in the sample could be explained as a presence of amorphous lithium oxides. All Li2+xMoO4 samples became black after lithium insertion in accordance with an oxidation state of molybdenum lower than +6. The structural model of the cubic (Li3-xMo)O4 was refined based on the rock-salt structure (Fm-3m) with oxygen atoms on the 4a site, and Li and Mo atoms randomly distributed over the 4b site. Note that the exact cation stoichiometry of the cubic phase could not be reliably extracted from powder XRD data because of a very low form factor of lithium in comparison to the one of molybdenum. Neutron powder diffraction performed on another Li2+xMoO4 sample after synthesis during four weeks showed about 90% (w/w) cubic (Li3-xMo)O4 and 10% (w/w) rhombohedral Li2+xMoO4 compounds, see Fig. 4 and Table 2. After constrain setting the total occupancies of the 4a and 4b site to 100%, the (Li3.00(2)Mo1.00(2))O4 composition was refined from the NPD data. This composition is not consistent with the chemical analysis of the same sample yielding Li/Mo = 3.8/1.0. If we fix the “(Li3.2Mo0.8)O4” stoichiometry for the cubic compound in the structural model, the resulting refinement is much worse, see inset in Fig. 4. A presence of an amorphous Li-rich oxide, which cannot be quantified by diffraction methods, is a plausible reason for this discrepancy. It is known that the usage of the strong reducing agent n-LiC4H9 often leads to additional irreversible reactions.25 Moreover, Mo-compounds are known for their ability to undergo charge disproportionation of Mo ions under reducing conditions. For example, Mo4+containing ethylenediaminetetraacetate complexes produce Mo5+ and Mo3+ upon reduction.26 The amorphization upon Li insertion is a common feature of molybdenum oxides.27,28


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4000

(Li3.01(1)Mo0.99(1))O4

2000

Intensity, counts

(Li2.95(1)Mo1.05(1))O4

Intensity, counts

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o

a = 4.1450(1) A

0

20

40

60

o

a = 4.1494(2) A

0

20

80

40

2θ [°]

60

2θ [°]

80

Figure 3. XRD patterns of Li2MoO4 after the reaction with n-LiC4H9 during different reaction times, observed and calculated curves together with their difference curve. Left: one week reaction time, 40% (w/w) Li2MoO4 (top reflection marks) and 60% (w/w) cubic (Li3-xMo)O4 with S.G. Fm-3m were included in the structural model. Right: four weeks reaction time, only cubic (Li3-xMo)O4 was observed. The compositions of the cubic (Li3-xMo)O4 phases were refined with the constraints of a 100% occupation of the oxygen lattice and also no vacancies on the cation sublattice. Table 2. Room-temperature structural parameters (from NPD) of the Li2+xMoO4 sample prepared by the chemical method during 4 weeks. The Li content x is about 1.8 from ICPOES and x ≈ 1 from the neutron refinement. Biso for the 4b site was fixed to 0.3 Å2 because of a strong correlation between Biso and the occupancy number. Atom site x y Z Biso, Å2 Occupancy d(M-O), Å O

4a

0

0

0

0.92(2)

1

Li,Mo 4b

½

½

½

0.3

0.25(2)/0.75(2)

2.0695(1)

Fm-3m, a = 4.1389(2) Å Bragg R-factor: 1.56%, Rf-factor: 0.81%, χ2 = 6.4 Synchrotron powder diffraction experiments showed the stability of the cubic (Li3Mo)O4 compound up to at least 620 K, see Fig. 5. Its thermal expansion behavior is non-linear. The temperature dependence of the lattice parameter a (Fig. 5 left) can be described between 300 and 620 K by the following equation: a(T) = 4.19687-2.63945*10-4T+4.02251*10-2T2. The thermal expansion coefficient α calculated from the formula α = (1/aT)*(daT/dT), is close to zero around room temperature, which may suggest negative expansion at temperatures below 300 K. Between 350 and 500 K, α(T) can be described as a straight line and deviates considerably at higher temperatures, similar to the thermal expansion behavior of NaCl studied by X-ray powder diffraction.29 For NaCl, the onset of “excess” in the thermal expansion above 800 K matches very well with a kink in the temperature dependence of the electrical conductivity, which was assumed to be due to intensive formation of Schottky 11 Environment ACS Paragon Plus


defects.29 The same reason could be suggested for a strong increase of α(T) for isostructural (Li3Mo)O4 above 500 K. 2

χ = 6.4

2

χ = 24.5

90000

Intensity [counts]

Intensity [counts]

90000

0

40

2θ [°]

80

0

20

40

60

80

120

100

2θ [°] Figure 4. Neutron powder diffraction pattern of cubic (Li3-xMo)O4 (main phase) and rhombohedral Li2MoO4 (about 10 % w/w), observed and calculated curves together with their difference curve. The refined cation composition of the cubic phase revealed the cation ratio very close to Li/Mo = 3:1. Inset: calculated curve with the Li/Mo ratio fixed to 4:1.

4.20

100

ααx106 [K-1]

o

a [A]

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4.18

cubic (Li3.99(1)Mo0.99(1))O4

50

4.16 0

cubic (Li3.01(1)Mo0.99(1))O4 300

400

500

600

300

T [K]

400

500

600

T [K]

Figure 5. Temperature dependence of the lattice parameter a (left) and the thermal expansion coefficient α (right) for cubic (Li3Mo)O4 from the synchrotron powder diffraction experiment. Dashed lines correspond to the best least squares fitting representing a polynomial of the second order (left curve) and third order (right curve). 3. Cubic (Li3Mo)O4: electrochemical behaviour


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Cyclic voltammetry measurements were performed on the sample prepared chemically in four weeks and containing only cubic (Li3Mo)O4 as a crystalline phase. The measurements between 1.3 and 3.5 V revealed an oxidation process at 2.35 V and a reduction process at 2.1 V in the first cycle, see Fig. 6a. The following cycles showed the oxidation and reduction at 2.3 and 2.05 V, respectively. An additional oxidation process at ~3.0 V becomes visible after several cycles. It may correspond to the oxidation of an amorphous impurity phase, charge disproportionation from Mo5+ into Mo4+ and Mo6+ or decomposition of cubic (Li3Mo)O4. In the letter case, the formation of pristine phenacite-type Li2MoO4 cannot be excluded. Transformation back to cubic (Li3-xMo)O4 should occur below 1.2 V.

40 cubic (Li3Mo)O4

I [mA/g]

20 0

-20 -40

(a)

2.0

1.5

2.5

3.0

3.5

E vs. Li+/Li [V] Mo)O cubic Li(Li MoO 2+x3 4 4 3 +

E vs. Li /Li

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(b)

2

3.2

3.4

3.6

2+x in Li2+xMoO4 13 Environment ACS Paragon Plus

3.8


Mo)O cubic Li(Li 3MoO 4 2+x 4 2.5

E vs. Li+/Li

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(c)

2.0

1.5 3.4

3.6

3.8

2+x in Li2+xMoO4 Figure 6. Electrochemical studies of (Li3Mo)O4/Li test cells. (a) Cyclic voltammetry curves at a voltage sweep rate of 0.1 mV/s between 1.3 and 3.5 V; (b) galvanostatic cycling between 1.3 and 3.5 V; and (c) galvanostatic cycling between 1.5 and 2.75 V with the C/10 rate. Cubic (Li3Mo)O4 was obtained via reaction between Li2MoO4 and n-LiC4H9 in an Ar-filled glovebox during four weeks; the starting Li-content was set according to the ICP-OES results. According to the current value from GCPL, about 0.23 Li atoms can be reversibly extracted/inserted from/into cubic (Li3Mo)O4 between 3.5 and 1.3 V. An additional plateau at about 2.9 V during charge appears upon cycling and becomes more pronounced in further cycles in line with the CV measurements. The XRD pattern of the cubic (Li3Mo)O4 cathode cycled 12 times between 1.3 and 3.5 V and stopped at 2.88 V can be interpreted as a mixture of cubic (Li3Mo)O4 and rhombohedral Li2MoO4, see Fig. 7. The GCPL measurements in the voltage range of 1.5 - 2.75 V, which is below the oxidation process at 2.9 V, shows a more stable behaviour during cycling (see Fig. 6c). A topotactical Li extraction/insertion mechanism can be proposed for this voltage range since cubic (Li3Mo)O4 demonstrates change in the lattice parameters depending on the treatment conditions, as it can be seen from Table 1, what corresponds to the change in the Li-composition.


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4000

cubic (Li3-xMo)O4 o

Intensity, counts

a = 4.1672(1) A

0

20

40

60

80

2 θ [°] Figure 7. Diffraction pattern of the cubic Li3-xMoO4 cathode cycled 12 times between 1.3 and 3.5 V with C/10, and stopped at 2.88 V. Rhombohedral Li2+xMoO4 appeared in the material after cycling (reflections of this phase are denoted by tick marks in the bottom line). 4. Quasi in situ XPS studies of (Li3-xMo)O4 in different oxidation states The changes in the electronic structure of Li3-xMoO4 due to Li-extraction were studied by “quasi in situ” X-ray photoelectron spectroscopy (XPS),20,30 see Fig. 8. Although the XPS method represents an electronic structure of the surface region only and is influenced by contaminations or surface effects, a presence of Mo in mixed valence states in (Li3Mo)O4 according to the Mo3d peak positions can be concluded in agreement with the reference data.31

pristine (Li3-xMo)O4

charged to 2.6 V charged to 3.5 V

1

0 243

pristine (Li3-xMo)O4

O1s

Mo3d

charged to 2.6 V charged to 3.5 V

normalized Intensity

1

normalized Intensity

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0 240

237

234

231

Binding Energy [eV]

228

225

536

534

532

530

Binding energy [eV]


528

526


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Figure 8. Mo3d (left) and O1s (right) spectra of the pristine rock-salt (Li3Mo)O4, electrochemically delithiated (Li3-xMo)O4 at the upper voltage limit voltage of 2.6 V, and (Li3+ xMo)O4 at 3.5 V vs. Li /Li. A similar complex Mo3d spectrum with a shoulder at about 231 eV was observed in Li4V(MoO4)3 (Ref. 6), indicating the presence of Mo in an oxidation state lower than +6. A shift of the Mo3d peaks toward higher energies for materials discharged to 2.6 V and 3.5 V as well as the complete disappearance of the lower-energy Mo3d peak at 231 eV for Li3-xMoO4 charged to 3.5 V unambiguously correspond to an increase of the average Mo oxidation state after Li extraction. The Mo3d spectrum of the material charged to 3.5 V is identical to the Mo3d spectrum of Li2MoO4 reported in work.14 Full oxidation of Mo to Mo6+ in the 3.5Vmaterial is in accordance with the appearance of phenacite-type Li2MoO4 with Mo6+ in the XRD pattern of the material. Any charge disproportionation to a lower oxidation state of Mo in the material at 3.5 V can be excluded. Although O1s peaks can represent a superposition of oxygen contributions from (Li3Mo)O4 and traces of DMC, which was used for sample washing after electrochemical experiments, they became significantly broader and are shifted to lower energies from pristine (Li3Mo)O4 to (Li3-xMo)O4 charged to 2.6 V and 3.5 V (Fig. 8 right). The higher O1s binding energy in pristine Li3-xMoO4 could be due to a partial charge transfer from oxygen to molybdenum, which leads to the formation of peroxide-like species, known from the literature for complex oxides32 with an O1s binding energy of 532-533 eV. Note that the same shift in the O1s peak position to higher binding energy values was observed in Li4V(MoO4)3 with a lower average oxidation state of Mo than +6 (Ref. 6).

Discussion Reversible Li-intercalation into the phenacite-type structure of Li2MoO4 demonstrates how new metastable materials that cannot be obtained by a conventional solid-state synthesis are formed through the chemical or electrochemical intercalation of Li+-ions. The Li2O-MoO3Mo phase diagram constructed experimentally at 830 K33 puts forward a more stable threephase mixture of Li4MoO5, Li6Mo2O7 and Li2MoO3 compounds with Mo6+, Mo4+ and Mo4+, respectively, for the composition point “Li3MoO4”. The cubic symmetry, regular geometry of Mo5+O6-octahedra and Mo,Li-cation disorder in (Li3Mo)O4 are rather surprising in the light of the significant size and charge difference of Li+ (0.76 Å) and Mo5+ (0.61 Å) for the octahedral oxygen coordination.10 The case of (Li3Mo)O4 shows that the difference in the ionic radii and valence states of cations does not necessarily lead to cation ordering, as usually supposed. 16 Environment ACS Paragon Plus

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Another and even more striking example that counters the conventional criterion of cation ordering, is Li4MoO5 with its disordered rock-salt structure. This Mo6+-containing phase is obtained through a reaction between Li2O2 and MoO3 (Ref. 34). Here, the size and charge differences between Li+ (0.76 Å) and Mo6+ (0.59 Å) are even more pronounced than in (Li3Mo)O4. Existence of Li-containing rock-salt-type structures can also be expected for other 4d metal oxides. For example, synthesis of disordered rock-salt Li0.75Nb0.25O with Nb5+ (0.61 Å) was reported by Modeshia.35 The reconstructive transformation of rhombohedral phenacite-type Li2MoO4 into cubic rocksalt (Li3Mo)O4 is accompanied by change in the oxygen coordination from tetrahedral to octahedral for both, Li and Mo cations, and by significant reduction of the formula unit volume from 95 Å3 to 72 Å3 because of elimination of tunnels in the pristine structure. These channels may also promote pressure- and temperature-induced structural changes. Indeed, a cubic spinel Li2MoO4 structure with a fcc close oxygen packing and formula unit volume of 75 Å3 can be obtained at 5 kbar in the temperature interval of 500-800 K.36 However, in the spinel structure, only Li ions change their oxygen surrounding from tetrahedral to octahedral; Mo cations keep their tetrahedral oxygen coordination unchanged. Thus, lithiation of phenacite-type Li2MoO4 results in a deeper structural transformation than “conventional” methods like temperature and/or pressure. Liu et al.14 did not see the formation of the cubic (Li3-xMo)O4 phase during in situ XRD measurements of Li2MoO4 nanotubes for the first cell discharge. It could be due to the higher current density of 30 mA/g vs. 15 mA/g used in our work. Since the phase transformation is reconstructive, including cation diffusion and change in oxygen coordination, it is strongly time-dependent. Stability regions of the rhombohedral Li2+xMoO4 and cubic (Li3-xMo)O4 phases remain somewhat unclear. The transformation of Li2MoO4 into (Li3-xMo)O4 represents a biphasic process that starts immediately in the Li-ion cells after applying current and leads to an abrupt decrease in the cell voltage from 3 V to 1.2 V. However, pristine rhombohedral Li2MoO4 with its complete cation ordering and the tetrahedral coordination of both Li and Mo is capable of accommodating small amounts of Li prior to the transition into the disordered cubic phase, where Li and Mo are octahedrally coordinated. The Rietveld refinement based on the neutron powder data revealed the composition of Li3.00(2)Mo1.00(2)O4 for the crystalline phase having the (A3B)O4 stoichiometry. Insertion of more than one Li into the structure must lead to the formation of a mixture of Li0.98MoO2 and metallic Mo at 0.38 V, and an amorphous mixture of probably Li2O and metallic Mo at 0.1 V vs. Li+/Li.14 Note that subsequent cell charge did 17 Environment ACS Paragon Plus


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not lead to the pristine phenacite-type Li2MoO4, but to a crystalline orthorhombic LixMoOy and an unknown phase.14 However, as we showed in the present work, phase transitions in the Li-Mo-O system at room temperature are extremely time-dependent. Much slower deep Liinsertion into Li2MoO4 could lead to other crystalline reaction products, or to amorphization, which is common for Mo-containing materials like MoO3 or Fe2(MoO4)3 when more than two Li atoms enter the structure and Mo is reduced. The partial amorphization of Li2MoO4 after prolonged chemical lithiation (four weeks) may be inferred from the discrepancy in the Li content determined by ICP-OES and from the structure refinement. While the former method provides the total Li-content in the sample (the ratio Li/Mo = 3.8:1), the latter method yields the Li-content in the crystalline phase (Li/Mo = 3.0:1). Galvanostatic cycling of cubic (Li3xMo)O4

obtained through the reaction with n-LiC4H9 in Li-cell at voltages below 2.8 V vs.

Li+/Li reflects a change of x = 0.2-0.25 in the Li-composition. Note that the cubic rock-salt-type oxide (Li3Mo)O4 can be potentially formed upon discharge of Li-ion electrochemical cells with other complex molybdenum oxides as electrode materials. Conclusion A new disordered cubic molybdenum oxide (Li3Mo)O4 (Fm-3m) with a NaCl-type structure can be obtained electrochemically at 1 V using a Li-anode and Li2MoO4 cathode, or through the reaction between Li2MoO4 with n-LiC4H9 in hexane. We suggest that this compound can be formed at voltages of about 1V in electrochemical Li-cells with complex molybdenum oxides as cathode materials. It participates in electrochemical processes in the potential range between 2.0 and 2.5 V. (Li3Mo)O4 is stable in an inert atmosphere up to at least 630 K and oxidizes at room temperature toward crystalline Li2MoO4 in electrochemical cells at potentials above 3 V vs. Li+/Li, or in air.

References (1) Lithium Batteries: Science and Technology, Edited by G.-A. Nazri, G. Pistoia, Springer Science + Business Media, 2009, 708 p. (2) Prabaharan, S. R. S.; Michael, M. S.; Begam, K. M. Synthesis of a polyanion cathode material, Li2Co2(MoO4)3, and its electrochemical properties for lithium batteries. Electrochem. Solid-State Lett. 2004, 7, A416-A420. (3) Alvarez-Vega, M.; Amador, U.; Arroyo de Dompablo, M. E. Electrochemical study of Li3Fe(MoO4)3 as positive electrode in lithium cells. J. Electrochem. Soc. 2005, 152, A1306A1311. 18 Environment ACS Paragon Plus

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(4) Arroyo de Dompablo, M. E.; Alvarez-Vega, M.; Baehtz, C.; Amador, U. Structural evolution of Li3+xFe(MoO4)3 upon lithium insertion in the compositional range 0

Lithium Insertion into Li2MoO4: Reversible Formation of (Li3Mo)O4

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