Molybdenum Substituted Vanadyl Phosphate Îµ-VOPO4 with Enhanced

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Molybdenum Substituted Vanadyl Phosphate ε‑VOPO4 with Enhanced Two-Electron Transfer Reversibility and Kinetics for Lithium-Ion Batteries Bohua Wen,† Qi Wang,‡ Yuhchieh Lin,§ Natasha A. Chernova,† Khim Karki,† Youngmin Chung,† Fredrick Omenya,‡ Shawn Sallis,†,‡ Louis F. J. Piper,†,¶ Shyue Ping Ong,§ and M. S. Whittingham*,† †

NECCES, Binghamton University, Binghamton, New York 13902, United States Department of NanoEngineering, University of California San Diego, 9500 Gilman Drive #0448, La Jolla, California 92093, United States ‡ Materials Science and Engineering, Binghamton University, Binghamton, New York 13902, United States ¶ Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, New York 13902, United States §

ABSTRACT: We have investigated the possibility of molybdenum substitution into ε-VOPO4 structure and its effects on the electrochemical performance of this material as a cathode in Li-ion battery. We have found that up to 5% of Mo can substitute V upon hydrothermal synthesis at 180 °C with further annealing at 550 °C. The substitution is confirmed by the increase of the unit cell volume with Mo content. A combination of X-ray absorption and photoelectron spectroscopy, magnetic studies, and density functional theory calculations indicates an Mo6+ oxidation state which is charge compensated by reduction of the same amount of V to 4+. Mosubstituted samples show much smaller particle size as compared to unsubstituted ε-VOPO4 and significantly improved electrochemical behavior. ε-V0.95Mo0.05OPO4 shows the initial reversible capacity ∼250 mAh/g (∼1.6 Li) and ∼80% retention for up to 20 cycles at C/25. Sloping voltage profile, faster kinetics, and lower voltage hysteresis of Mo substituted VOPO4 are demonstrated by the galvanostatic intermittent titration technique. This enhanced electrochemical performance is attributed to the smaller particles and possible existence of partial LixMoyV1−yOPO4 solid solution supported by X-ray diffraction, which leads to less abrupt and completely reversible structure changes upon Li cycling evidenced by X-ray absorption spectroscopy.

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INTRODUCTION Currently, layered oxides and olivine LiFePO4 are the most common cathode materials in commercially available lithiumion batteries (LIBs).1,2 Oxides have safety issues due to oxygen release in the charged state, whereas the safer iron-based phosphates are low in volumetric energy density. One strategy to improve the energy density of polyanionic phosphates is to involve more than one-electron transfer per redox center.3 Two-electron redox couples of V5+/V4+ and V4+/V3+ in phosphates have been demonstrated, and the possibility of Fe2+/Fe3+and Fe3+/Fe4+ in pyrophosphates have been explored, with major challenges originating from structural reversibility and the electrolyte decomposition.4−8 The ε-VOPO4− LiVOPO4 system has been regarded as one of the most promising and safe candidates to provide two-electron reaction with high theoretical capacity of ∼318 mAh/g and specific energy over 900 Wh/g coming from two redox potentials of V4+/V5+ and V3+/V4+ at about 4.0 and 2.5 V, respectively.9−11 VOPO4 and LiVOPO4 have been investigated as cathodes for LIBs for over a decade, and the performances of several phases with different structures have been reported. 4−18 The delithiated VOPO4 has seven polymorphic modifications with © 2016 American Chemical Society

few reports on their electrochemical properties. An early report by Kerr et al. pointed out that chemically lithiated ε-VOPO4 can extract ∼0.8 lithium at a high-voltage region for up to 100 cycles, while no good cycling data of ε-VOPO4 was reported.4 In addition, our group found that even though ε-VOPO4 has displayed high initial discharge capacity covering both a highand low-voltage plateau, it suffers from large capacity loss after the first cycle.5,6 The phase formed upon chemical and electrochemical lithiation of ε-VOPO4 has been known as αLiVOPO4; however, we have recently rebranded it as εLiVOPO4 to keep the names consistent through the lithiation process ε-VOPO4 ↔ ε-LiVOPO4 ↔ ε-Li2VOPO4.3 The structure of the second lithiated phase ε-Li2VOPO4 from both chemical and electrochemical lithiation has been reported only recently, confirming the possibility of intercalating the second lithium.5,11,12,17,18 Mo is another well-known multivalent element, which is predicted to display very close redox potentials to vanaReceived: March 2, 2016 Revised: April 6, 2016 Published: April 10, 2016 3159

DOI: 10.1021/acs.chemmater.6b00891 Chem. Mater. 2016, 28, 3159−3170

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Chemistry of Materials

Figure 1. (a). High resolution X-ray diffraction patterns of MoyV1−yOPO4 (y = 0, 0.0125, 0.025, 0.0375, and 0.05) in the wavelength of λ = 0.78013 Å. (b) Magnified selected peaks of low substituted MoyV1−yOPO4 (y = 0, 0.0125, 0.025) and dependences of lattice parameters on Mo content. as reported by Song et al.5 VCl3 (Sigma-Aldrich, 97%), MoCl5 (Aldrich, 95%), and P2O5 (Sigma-Aldrich, ≥98%) were dissolved in 30 mL of ethanol (190 proof). The solution was placed in 4748 Type 125 mL PTFE-lined reactor (Parr Instrument Co.) and kept at 180 °C for 3 days. Then, the hydrothermal products were filtered with ethanol and heated at 550 °C in oxygen for 3 h. In the synthesis of Mosubstituted compounds, the aimed molar amounts of Mo precursor replaced corresponding molar amounts of V precursor. The substituted samples termed as MoyV1−yOPO4 turned out to be greenish in color, while VOPO4 is yellow. Electrodes of VOPO4 and MoyV1−yOPO4 were prepared by mixing the compounds with carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 80:10:10 using 1-methyl-2-pyrrolidinone (NMP) as solvent. Then, slurries were cast onto an Al foil 144 current collector and dried in air at 80 °C. The dried electrodes of area 1.2 cm2 containing 5−6 mg of active material were placed in 2325-type coin cells in a He-filled glovebox with pure lithium foil (Aldrich, thickness 0.38 mm) as the counter and reference electrodes. Typical electrolyte 1 M LiPF6 (lithium hexafluorophosphate) dissolved in a mixture solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:1 and Celgard 2400 separator (Hoechst Celanese) were used. The electrochemical properties of the compounds were evaluated using a VMP multichannel potentiostat (Bio-Logic) at current densities of C/25 and C/50 (1 C corresponds

dium.19,20 Mo and V have similar crystallographic properties in oxides and phosphates, such as ionic radii and coordinations, and therefore may be expected to substitute each other.21 We have previously investigated vanadium substitution in LiFePO4, and found that the solid state solubility of vanadium in this structure depends on the synthesis temperature, and up to 10% of vanadium can be substituted if the synthesis temperature is limited to 550 °C. The substituted materials show improved rate capability due to the increased range of solid solution during lithium removal and insertion.22−24 To our knowledge, there is no report on metal substitution in the ε-VOPO4 phase. Here, we systematically study the possibility of Mo substitution in VOPO4 and its effects on structure, morphology, and electrochemical behavior. We are interested in knowing whether substitution can alter the reaction mechanism over either high- or low-voltage Li insertion/removal reactions, possibly leading to faster kinetics and/or smaller voltage difference between the two processes.

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EXPERIMENTAL SECTION

VOPO4 and MoyV1−yOPO4 (y = 0, 0.0125, 0.025, 0.0375, 0.05) were synthesized through the hydrothermal method followed by annealing, 3160


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Chemistry of Materials Table 1. Lattice Parameters of ε-VOPO4/MoyV1−yOPO4 y

a (Å)

b (Å)

c (Å)

V (Å3)

β (°)

Rwp

0 0.0125 0.025

7.2692(8) 7.2697(8) 7.2694(3)

6.8799(2) 6.8825(8) 6.8895(5)

7.2639(7) 7.2630(1) 7.2646(6)

328.26(9) 328.27(6) 328.49(8)

115.36(3) 115.39(8) 115.46(2)

0.07 0.05 0.06

to 159 mA/g and ∼0.04 mA/cm2). GITT measurements were conducted in the voltage window of 2.0−4.5 V by applying current at C/50 for 1.5 h and followed by 24 h of relaxation. An inductively coupled plasma (ICP) test was conducted on a Varian Vista-MPX Axial ICP-OES instrument. The structure of the samples was characterized by powder X-ray synchrotron diffraction at beamline X14A of National Synchrotron Light Source (NSLS) in Brookhaven National Lab with wavelength of 0.7801 Å. The Rietveld refinement of the X-ray diffraction patterns was done using the GSAS/ EXPGUI package.25,26 The Superconducting Quantum Interference Device (SQUID) magnetometer (Quantum Design MPMS XL-5) was used to measure the dc magnetic susceptibility (χ) of the samples from 350 to 2 K in a 1000 Oe magnetic field. High resolution X-ray photoemission spectra (XPS) were obtained with a PHI 5000 Versa Probe using a monochromated Al Kα source and a constant 23.5 eV pass energy, which corresponds to an energy resolution of ∼0.5 eV. Transmission electron microscopy (TEM) characterization of the particle samples was performed at Brookhaven National Laboratory. All TEM imaging, selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDX) were obtained with a JEM-2100F (JEOL) operated at 200 kV. X-ray absorption spectroscopy (XAS) experiments were performed at beamline X18A at NSLS, Brookhaven National Laboratory. A double-crystal Si(111) monochromator was used to scan X-ray energy from −200 eV to +1000 and +1200 eV relative to Mo K edge (20 000 eV) and V K edge (5465 eV), respectively. The electrode samples loaded with 10−15 mg of active material were discharged to various charge states. The cells were disassembled in a helium glovebox, and 12 mm diameter electrode samples on an aluminum current collector were washed, dried, and press-sealed between Kapton tape. The samples were stored in the glovebox prior to being subjected to XAS detection. Fine powders of reference compounds (e.g., MoO2, MoO3, and V2O5) were brushed uniformly onto a Kapton tape which was then folded several times to achieve a suitable total thickness for the measurement. Transmission XAS measurements were carried out with the pure V metal foil measured in reference mode simultaneously for X-ray energy calibration and data alignment; the Mo test was done in a fluorescence mode. Respective metal foil was measured in reference mode simultaneously for X-ray energy calibration and data alignment. The in situ experiment was done in coin cell with Kapton windows. The cell was cycled at C/30, discharging from OCV (∼3.85 V) to 2.0 V and then charging to 4.3 V. XAFS data were analyzed by using IFEFFIT software package.27 The first-principles calculations were performed using the Vienna ab initio simulation package (VASP)28 with the projector augmentedwave approach.29 The Heyd-Scuseria-Ernzerhof (HSE06)30−32 screened hybrid functional33 with a plane-wave energy cutoff of 520 eV was used. The first Brillouin-zone was sampled using a 1 × 2 × 1 Monkhorst-Pack grid.34 All the parameters were chosen to ensure total energy convergence. All analyses were carried out using the Python Materials Genomics package.35 The calculations were performed using a 1 × 2 × 2 supercell of ε-VOPO4 (16 formula units). For Mo doping, one of the V atoms is removed, and a Mo atom is introduced, which yields an effective Mo doping concentration of 6.25% to mimic the experimental doping concentration of 5% while keeping calculations at a reasonable cost. We investigated two kinds of potential sites for Mo. The first is simply a direct substitution of V for Mo in the VOPO4 structure. The second type of site includes interstitial positions that are within 3 Å from the introduced vacancy. Special care was taken to ensure proper charge ordering in the structures by introducing appropriate initial distortions of the VO6 octahedral. Two scenarios were considered: (i) all V and Mo are in the 5+ oxidation state and (ii)

the Mo is in the 6+ oxidation state and one of the vanadium atoms is in the 4+ oxidation state, with the rest of the vanadium atoms remaining in the 5+ oxidation state. The resulting lowest energy configuration determined by density functional theory (DFT) calculations was selected as the starting structure for subsequent analyses.

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RESULTS AND DISCUSSION Figure 1a shows the high resolution X-ray diffraction patterns of ε-MoyV1−yOPO4 with different substituted amounts of Mo (y = 0, 0.0125, 0.025, 0.0375, and 0.05). ε-VOPO4 is monoclinic with space group Cc (PDF 04-014-1224), and its diffraction pattern shows sharp peaks implying high crystallinity; however, the compounds with 0.0375 and 0.05 Mo substituted show much broader XRD peaks. After normalizing the peak intensity with respect to the (111) peak, selected strong peaks of εMoyV1−yOPO4 (y = 0, 0.0125, and 0.025) are compared in Figure 1b. Generally, the diffraction peaks are wider with more Mo substituted, and (111̅), (020), and (021) peaks slightly shift to low angle. Since the refinement of too broad peaks is not reliable, Table 1 and Figure 1b only summarize the refinement results of y ≤ 0.025 samples. Upon Mo substitution, b lattice parameter slightly increases as does the unit cell volume V and β, while a and c do not change much. This increase of unit cell volume is consistent with the substitution of Mo into the structure. The contents of Mo in this series of samples have been determined by the ICP test, the resulting ratios of V to P and Mo to P are listed in Table 2. ε-VOPO4 shows a slight Table 2. Element Analysis of ε-VOPO4/MoyV1−yOPO4 y

P

0 0.0125 0.025 0.0375 0.05

1 1 1 1 1

Mo/P

V/P

0.014 0.027 0.042 0.057

1.043 0.995 0.989 0.986 0.971

excess of V. Upon Mo substitution, the ratio of Mo to P increases, while that of V to P decreases, which supports the Mo substitution of V. The ICP test has been completed at least three times with reproducible results. TEM has been conducted to investigate the Mo substitution effects on the morphology of as-synthesized products. The unsubstituted ε-VOPO4 shows a large particle size in the 200− 300 nm range as illustrated in Figure 2a. The corresponding SAED pattern (inset) taken from the boxed area of the particle shows strong diffraction spots indexed along the zone axis (314), thus proving high crystallinity of the material. With the increase in the Mo amount, particles are reduced to smaller sizes. The particles in the sample with 1.25% of Mo substituted are about 100 nm (Figure 2b), and the particles in the 5% Mo substituted samples are even smaller (Figure 2c). With the Mo substitution, the primary particles tend to agglomerate into larger secondary particles, as is apparent from the TEM images. The SAED pattern (inset) of the 5% Mo-substituted sample shows broadened diffraction spots implying that most of the 3161


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Figure 2. TEM images of MoyV1−yOPO4 particles for (a) y = 0, (b) y = 0.0125, and (c) y = 0.05 with corresponding SAED patterns taken from the yellow boxed areas (insets). (d) TEM images and EDX elemental mappings of MoyV1−yOPO4 (y = 0.0125 and 0.05).

Figure 3. V K-edge XANES spectra: (a) Normalized absorption coefficient of ε-VOPO4 and Mo0.05V0.95OPO4 with that of reference V2O5; (b) Fourier transformed k3 weighted EXAFS of ε-VOPO4 and Mo0.05V0.95OPO4.

smaller crystallites share a common orientation. This tendency of smaller particles getting agglomerated explains the broadening of diffraction peaks discussed before. The EDX elemental mappings of two of the substituted samples are shown in Figure 2d. The mappings demonstrate that both Mo and V are distributed uniformly throughout the particles. To study the oxidation state of Mo in the substituted compounds and the effect of the substitution on V oxidation

state and local coordination, we combined X-ray absorption and X-ray photoelectron spectroscopies, magnetic studies, and DFT calculation techniques. First, we have compared the oxidation states and local geometry of V in the substituted Mo0.05V0.95OPO4 and ε-VOPO4 from XANES spectra (Figure 3a). The absorption edge position, which is sensitive to the oxidation state, does not change with Mo substitution and is essentially the same as in the reference compound V2O5, 3162


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The fitting results show that the VO6 octahedra in our εVOPO4 are similar to those in the reported structure (PDF 04014-1224), albeit a slight elongation in V−Oeq bonds (1.907(7) vs. ∼1.876 Å). The expansion of the vanadyl bond and concurrent contraction of the trans bond are observed in Mo0.05V0.95OPO4, compared to their counterparts in ε-VOPO4. The finding suggests that VO6 octahedra become more symmetric upon Mo substitution. The same approach was applied to study the substituted Mo atom in the structure. The comparison of the Mo K-edge position in Mo0.05V0.95OPO4 to that in MoO3 and MoO2 standards shows that it matches closely to the MoO3 edge, suggesting the Mo6+ oxidation state in the substituted sample (Figure 4a). The pre-edge peak in Mo compounds is mainly attributed to the electronic excitation of Mo 1s → 4d. Such features are formally forbidden while gaining the intensity by p−d hybridization in the final states as a result of the noncentrosymmetric environment around the central metal. The pre-edge characteristics exhibited by Mo0.05V0.95OPO4 resemble that of MoO3 in both position and shape, pointing to an asymmetrical octahedron geometry centered in Mo. All this evidence attests that Mo prefers octahedral sites over the tetrahedral ones, which eliminates the possibility for Mo to substitute for phosphorus residing at tetrahedral sites of vanadyl phosphates. The real part of Fourier transformed k3 weighted Mo K-edge EXAFS spectra of Mo0.05V0.95OPO4 in comparison with those of MoO2 and MoO3 is presented in Figure 4b. Visually, there is much more resemblance between Mo0.05V0.95OPO4 and MoO3 spectra, again confirming the Mo6+ oxidation state. Table 3 lists the fitting results of Mo EXAFS of Mo0.05V0.95OPO4. The model with two types of Mo−O bonds was found to produce the best fit. Introduction of the third, longest, Mo−O3 bond by analogy with VO6 coordination increases the R-factor of the fitting but still produces a plausible fit. Both types of Mo−O bonds are longer than the same type of V−O bonds, and the difference between Mo−O1 and Mo−O2 is smaller than the two extreme bonds of MoO3, which are 1.67 and 2.08 Å, respectively.36 Hence, the pre-edge peak of Mo K-edge in substituted VOPO4 is depressed compared with MoO3. To further confirm the Mo oxidation state, we have also acquired core-level XPS spectra of Mo-substituted VOPO4, which show a Mo 3d5/2−3d3/2 doublet typical of Mo6+, confirming the Mo oxidation state found from XANES (Figure 5a).37 Since the presence of Mo6+ requires some form of charge compensation and V5+ oxidation state was observed in XANES,

suggesting approximately a V5+ oxidation state. Both compounds show noticeable pre-edge peak arising from the 1s → 3d transition, which gains the intensity from p−d hybridization due to the noncentosymmetric V coordination. ε-VOPO4 shows comparable pre-edge intensity to that of reference V2O5, in which the short VO vanadyl bond in the structure leads to a significant VO6 octahedral distortion. In contrast, the pre-edge peak of Mo0.05V0.95OPO4 is less prominent than in εVOPO4, suggesting more symmetric VO6 octahedra in the substituted compounds. The above-observed geometrical modification to V by Mo incorporation is corroborated by V K-edge X-ray absorption fine structure (EXAFS). To investigate the local V environment in more detail, the k3 weighted Fourier transformed V EXAFS of ε-VOPO4 and Mo0.05V0.95OPO4 were examined (Figure 3b). In the case of ε-VOPO4, VO6 octahedra have one short V−O bond ∼1.6 Å and one long bond ∼2.6 Å at the opposite position. The other four V−O bonds, close in length ∼1.9 Å, are in the equatorial plane (Table 3). In Figure 3b, the first two Table 3. Coordination Numbers and Bond Lengths from EXAFS Fitting samples

bonds

CN

R (Å)

σ2 (Å2)

ε-VOPO4 theory

VO V−Oeq V−Otrans V−P1 VO V−Oeq V−Otrans VO V−Oeq V−Otrans Mo−O1 Mo−O2

1 4 1 4 1 4 1 1 4 1 1 4

1.572 avg 1.876 2.556 avg 3.227 1.576 (5) 1.907 (7) 2.50 (8) 1.601 (6) 1.91 (1) 2.43 (7) 1.683 (7) 1.95 (1)

0.0000 (2) 0.0035 (3) 0.012 (9) 0.0004 (7) 0.0049 (7) 0.011 (10) 0.000 (2) 0.007 (4)

pristine VOPO4

MoVOPO4

peaks below 2 Å can be assigned to the first V−O coordination shell, followed by an additional 3−4 peaks extending to 3.5 Å which include V−P and V−O single scattering paths along with various multiple scattering paths contributing the amplitude to these outer shell peaks. Focusing on the first coordination shell, a model considering one short V−O bond, four medium V− Oeq bonds, and one long V−Otrans bond was utilized to analyze the EXAFS spectra and compare the local V environments in εVOPO4 and Mo0.05V0.95OPO4.

Figure 4. (a) Mo K-edge XANES spectra and (b) Fourier transformed k3 weighted EXAFS of Mo0.05V0.95OPO4 in comparison with MoO3 and MoO2. 3163


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Figure 5. (a) Mo 3d and (b) V 2p XPS core spectra of Mo0.05V0.95OPO4. (c) Temperature dependences of the magnetic susceptibility of pristine and Mo-substituted Mo0.05V0.95OPO4 and their fit to the Curie−Weiss law.

we have used the V 2p XPS spectrum and temperature dependence of the magnetic susceptibility (Figure 5b,c) to determine whether a small amount of V4+ ions (d1, S = 1/2) is present in the substituted compounds, which are undetected by other techniques. We have used 5% substituted sample, where the amount of magnetic V4+ ions is expected to be higher. The V 2p XPS spectra reveal a low-energy shoulder attributed to V4+ ions,38 which is significantly more pronounced in the Mo substituted VOPO4 case. The amount of V4+ was further quantified from the temperature dependence of the magnetic susceptibility, which indeed shows higher magnetic susceptibility in Mo-substituted VOPO4 than in the unsubstituted one; the latter is not expected to contain magnetic ions (V5+ is d0, S = 0). A small increase of magnetic susceptibility observed in εVOPO4 at low temperatures corresponds to about 1% of V4+ ions as found from the fit to the Curie−Weiss law, possibly due to surface reduction.38 Upon Mo substitution, the content of S = 1/2 ions increases to 4% as found from the Curie−Weiss law fit. Since both V5+ and Mo6+ have no unpaired electrons, we attribute this increase to the formation of V4+. This result suggests that Mo6+ substitution is compensated by the presence of an equal amount of V4+ ions. From DFT calculations, we find that the lowest energy structure of the Mo-substituted ε-VOPO4 is one where the Mo directly substitutes for V. To determine the oxidation states of Mo and V, we plotted the spherically integrated spin-polarized charge density of all transition metals as a function of cutoff radius in Figure 6. We observe that one of the V ions has an integrated spin of ∼1/2 μB beyond a radius of 1 Å, indicating that this V is likely in the 4+ oxidation state. The rest of V and the Mo ion have very small integrated spins, which suggests that oxidation states of V and Mo are likely 5+ and 6+, respectively, which is consistent with our conclusions from XAS, XPS, and magnetic data.

Figure 6. Integrated spin charge density as a function of radius cutoff (Å) around Mo and V in MoV15P16O80.

Figure 7 shows the local MoO6 and VO6 environments for the lowest energy structure. The average Mo6+−O bond length is 1.930 Å, with the shortest Mo−O bond at 1.680 Å and the longest bond at 2.226 Å. This is consistent with the Mo−O bond lengths observed in MoO3 (also Mo6+)36 and with Mo− O bond lengths obtained from EXAFS data analysis. We also observe that the shortest bond for the V4+O6 (1.625 Å) octahedron is significantly longer than that of the V5+O6 octahedral (average: 1.567 Å). In the case of V, the EXAFS data is averaged over V5+−O and V4+−O bonds. Therefore, the observed V−O bond lengthening in the substituted compound is in agreement with the DFT predictions and is consistent with the presence of V4+ ions. Next, we have investigated the effect of Mo substitution on the electrochemical performance. Figure 8a compares the initial discharge−charge profiles of ε-VOPO4, Mo0.025V0.975OPO4, and Mo0.05V0.95OPO4. Despite the fact that ε-VOPO4 was cycled at C/50, the first full discharge capacity is only ∼200 mAh/g and the second lithium insertion involving V4+/V3+ transition only delivers ∼65 mAh/g. This total amount of capacity in the first 3164


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Figure 7. DFT calculated bond lengths (Å) of (a) Mo6+−O in MoO6, (b) V4+−O in VO6, and (c) V5+−O in VO6 of MoV15P16O80.

Figure 8. (a) 1st Galvanostatic discharge−charge curves of ε-VOPO4 at C/50 and MoyV1−yOPO4 at C/25 (y = 0.025 and 0.05); (b) cycling of Mo0.05V0.95OPO4 at C/25 and C/50 for 20 cycles.

Figure 9. GITT of ε-VOPO4 and Mo0.05V0.95OPO4, current of C/50 was applied for 1.5 h followed by 24 h relaxation: (a) capacity−voltage profiles with DFT-calculated values, (b) the magnified dotted region, and (c) time−voltage graph of discharge at high-voltage plateau.

discharge corresponds to ∼1.26 Li (the theoretical capacity of 1 lithium for VOPO4 is ∼159 mAh/g). Further cycling results in fast capacity loss. However, with the substitution of Mo, MoyV1−yOPO4 (y = 0.025 and 0.05) reaches a reversible

capacity ∼250 mAh/g at C/25, equivalent to 1.6 Li. The discharge process delivers ∼100 mAh/g at high-voltage and 150 mAh/g at the low-voltage region, whereas upon charge, the high-voltage capacity is ∼170 mAh/g. An even longer high3165


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Figure 10. High resolution X-ray diffraction patterns (wavelength λ = 0.78013 Å) of ex situ ε-Mo0.05V0.95OPO4 at different states of charge.

Figure 11. V K-edge in situ XAS spectra of Mo0.05V0.95OPO4: (a) high-voltage discharge, (b) low-voltage discharge, (c) 1st discharge−charge profile of the in situ cell, and (d) Mo K-edge XANES spectra of ex situ electrodes (pristine, partially discharged at 2.55 V and fully discharged at 2.0 V) together with MoO3 and MoO2 references.

considerably in several cycles. Thus, 5% Mo substituted sample will be further discussed as the model compound to study the Mo substitution effects on the electrochemical performance. The intercalation kinetics of ε-VOPO4 and Mo0.05V0.95OPO4 are compared through GITT in Figure 9. Both materials show very high discharge/charge hysteresis in the region between V3+/V4+ and V4+/V5+ transitions. The open circuit voltage (OCV) curve of ε-VOPO4 displays a flat plateau at ∼3.9 V corresponding to transition of V5+/V4+ and two steps in the low-voltage region, which are also observed in the case of triclinic LiVOPO4, indicating the existence of intermediate phase Li1.5VOPO4.11,18 However, the OCV profile of the substituted Mo0.05V0.95OPO4 is sloping at both high- and lowvoltages. The voltage of V 4+ /V 5+ redox couple in Mo0.05V0.95OPO4 is lower than in unsubstituted VOPO4,

voltage charge plateau is observed at higher Mo substitution, comparing the two samples of MoyV1−yOPO4 (y = 0.025 and 0.05). This difference of the charge/discharge capacities in these two continuous processes causes a wide gap in the charge/discharge curve. Furthermore, substituted samples display sloping plateaus at both the high- and low-voltage region, hinting that a solid solution reaction may be involved. Figure 8b displays the cycling performance of Mo0.05V0.95OPO4 at C/50 and C/25. The capacity loss after the first cycle is greatly reduced indicating enhanced reversibility. At C/50, the capacity is above 200 mAh/g for 20 cycles, while at C/25 the capacity is lower and initially decays faster, still with 80% of the capacity being maintained after 20 cycles. Even though only a small difference between 2.5% and 5% Mo is observed in the first charge, the capacity of 2.5% Mo substituted VOPO4 fades 3166


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Figure 12. (a) Fourier transformed k3 weighted EXAFS of V and Mo at different charge states of Mo0.05V0.95OPO4: pristine, 2.0 V discharged, and 4.5 V fully charged; (b) plots of bond length with errors of Mo0.05V0.95OPO4 EXAFS fitting results at different charge states; (c) phase fractions obtained from PCA of the full XAS data set; the bottom abscissa is the percentage of 2 lithium insertion.

which is supported by the DFT calculations. In the V4+/V3+ region, the DFT calculations predict lower voltage in the substituted compound partly due to the contribution from Mo reduction to Mo3+, while the experiment shows that this process onsets at about the same voltage in both compounds. The average voltage is higher in Mo0.05V0.95OPO4 due to the absence of voltage drops associated with phase transitions. This raises a question whether the lithiation/delithiation of the substituted compound goes through a different reaction mechanism and whether Mo is electrochemically active. Both points will be addressed later in the paper. The magnified dotted region at high-voltage in Figure 9b shows the cell polarization, i.e., the potential gap between charge and discharge. Mo0.05V0.95OPO4 exhibits decreased cell polarization, 187 mV versus 274 mV of ε-VOPO4. Moreover, from the time-voltage graph in Figure 9c, where both samples have been relaxed for 24 h before current application, the substituted sample is faster to reach equilibrium and shows lower overpotential, 21 mV, in contrast to 90 mV of ε-VOPO4. Thus, substitution of Mo in VOPO4 improves the kinetics of V5+/V4+ and V4+/V3+ transitions. In order to understand the reasons for improved kinetics and reversibility of the Mo-substituted VOPO4, we have investigated the structural evolution of the compounds upon electrochemical lithiation/delithiation by X-ray diffraction and absorption techniques. Figure 10 shows the X-ray diffraction patterns of the ex situ electrodes of ε-Mo0.05V0.95OPO4 at different discharge/charge states. The diffraction peaks of assynthesized ε-Mo0.05V0.95OPO4 are very broad due to the very small particle size. Moreover, the main peaks of VOPO4, LiVOPO4, and Li2VOPO4 in the range of 13−14° (2θ) are very close, and these broad peaks can not be well resolved. Thus, we can only roughly identify the phase transformations. Upon the first Li insertion, the XRD peaks in the 13−14° region shift to

higher angles and that in the 9−10° region shift to lower angles, which suggests that the LixMo0.05V0.95PO4 phase may exist in this system (point B). Points C and D can be identified as εLiVOPO4 and point E matches ε-Li2VOPO4. It should be noted that only 1.6 Li is inserted at this point, so the end phase might as well be similar to Li1.5VOPO4, since it is not possible to tell the difference between these two phases from our broadened XRD patterns. The charge process displays the reverse change. Generally, the phase transformation follows the same route as unsubstituted VOPO4, i.e., from ε-VOPO4 to εLiVOPO4 and then ε-Li2VOPO4, but a tendency to solid solution formation is evident from the XRD peak shifts over the first Li insertion, consistent with the sloping voltage profile found in GITT. In situ X-ray absorption (XAS) at V and Mo K-edges was further applied to understand the effects of substitution on atomic local structure and the structural reversibility upon lithium cycling in Mo0.05V0.95OPO4. The first discharge−charge profile in Figure 11 shows the state of charge points extracted from the whole in situ XAS measurements to represent the vanadium reduction process at both high- and low-voltage regions. At the high-voltage lithiation region, the pre-edge peak forms a low-energy shoulder (0.1 Li) and splits into two peaks (0.28 Li), and then, the intensity of the low-energy peak increases, which reflects the reduction of V5+ to V4+. The edge position continuously shifts to lower energy as V5+ is being reduced to 4+. There are no clear isosbestic points observed upon lithiation, consistent with the solid solution reaction proposed from GITT and XRD data. With the insertion of the second lithium at the low-voltage region, the pre-edge peak intensity decreases without position shift, indicative of more symmetric VO6 octahedra. The main absorption edge is consistently shifting to lower energy, suggesting the continuous reduction of V oxidation states. No isosbestic points are 3167


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Figure 13. V K-edge spectra of ε-VOPO4 and Mo0.05V0.95OPO4: (a) XANES of pristine, 2.0 V discharged, and 4.5 V charged after full discharging; Inset: the magnified pre-edge peaks; (b) k3 weighted Fourier transformed EXAFS of pristine and fully charged phases after the 1st cycle.

Principal component analysis (PCA) has been conducted for the whole collection of in situ Mo0.05V0.95OPO4 spectra upon discharge to determine the number of independently varying components. It was performed in energy space over the full spectra, and the components A, B, and C could not be described as a linear combination of each other. The missing data in the plot are due to the beamline down at 18B in BNL, which used to happen around every 12 h. In Figure 12c, the xaxis is defined as the lithiation percentage of ideally 2 lithium, and since ∼1.6 Li inserted at 2.0 V, the maximum of the scale is 80%. PCA reveals three independent components (A, B, and C) which can be attributed to contributions from vanadium ions in 5+, 4+, and 3+ oxidation states, respectively, judging from how the component fractions vary upon lithiation (Figure 12c). The fraction of component A linearly decreases while that of component B linearly increases between 0 and 40% lithiation, where only component B is observed. Upon further lithiation, the fraction of component B linearly decreases, while that of C linearly increases, C being the final lithiation product. It should be noted that XAS is a bulk average technique, and it is hardly possible to tell from the PCA analysis alone whether these spectral components form separate phases or whether different vanadium oxidation states coexist in one phase. In order to compare reduction extent of V at 2.0 V as well as the structural reversibility in ε-VOPO4 and Mo0.05V0.95OPO4, V K-edge XAS spectra at different charge states are plotted in Figure 13. The V K-edge XANES parts of pristine, fully lithiated at 2.0 V, and fully charged at 4.5 V after the initial cycle of both samples are compared in Figure 13a. VO6 octahedra of the fully discharged Mo substituted phase is almost symmetric as seen from its reduced pre-edge intensity as opposed to that of ε-VOPO4, which maintains a significant pre-edge, indicative of a more distorted VO6 at 2.0 V (inset). A higher V oxidation state in discharged ε-VOPO4 is also apparent based on its relative edge shift to higher energies at this stage. This edge shift is in agreement with the 50 mAh/g smaller discharge capacity compared to Mo0.05V0.95OPO4. Furthermore, the preedge peaks as well as main edge peak positions of both pristine and fully charged samples are very close in the XANES part, but the EXAFS part in Figure 13b quantitatively displays the more reversible atomic displacement of Mo substituted ε-VOPO4 after the first cycle. Magnitudes of the Fourier transformed k3 weighted data reveal the coordination shell surrounding V center below 2 Å, which corresponds to V−O bonds and can quite reversibly change back after the initial cycle in Mo0.05V0.95OPO4 as shown in Figure 12b and is consistent

observed at the low-voltage lithiation process, further supporting a single-phase lithiation process. Figure 11d displays the ex situ Mo K-edge XANES spectra of pristine Mo0.05V0.95OPO4 and at different states of discharge. As discussed before, the oxidation state of Mo in the pristine compound is close to Mo6+, and the half electrochemically lithiated phase at 2.55 V shows nearly an overlapped spectrum, suggesting that the high-voltage reaction does not involve molybdenum reduction. This is consistent with Mo reduction potentials observed in other Mo-based cathodes upon Li insertion. For example, the average potential of Mo6+/Mo5+ reduction is 2.5 V in MoO3.39 In (MoO2)2P2O7, Mo6+/Mo5+ reduction occurs in two steps at 3.2 and 2.6 V with further reduction to Mo4+ at 2.1 V.20 In Mo0.05V0.95OPO4, the spectrum of the 2.0 V lithiated phase reveals the shift toward lower energy of both the pre-edge feature and the main absorption peak, proving that Mo is active in the low-voltage lithium insertion region. This reduced Mo can be estimated as Mo5+ by comparison with references MoO2 and MoO3. The local structure changes are best observed from EXAFS, which is affected by the coordinating atoms surrounding the photoabsorber. The EXAFS spectra of pristine, discharged to 2.5 V, fully discharged to 2 V, and charged back to 4.5 V Mo0.05V0.95OPO4 phases were selected and fitted to further investigate the lithiation process (Figure 12). The fitting results of Mo−O and V−O bonds with errors are plotted in Figure 12b. In both cases, a model with two types of M−O bonds is found to produce the best fit over the whole lithiation process. The longest V−O bond included in the EXAFS fitting model earlier in the paper contributes a small peak at the high-R end of a double peak between 1 and 2 Å (Figure 12a), and it becomes progressively more difficult to fit it reliably as shorter V−O bonds elongate upon lithiation (Figure 12b), so it was not used here. With intercalation of lithium, both V−O and Mo−O bonds are elongated continuously, and Mo−O bonds change more with the second lithium insertion. These fitting results are different from the report by Allen, which indicates both short and equatorial V−O bonds of LiVOPO4 increase drastically in the low voltage discharge, while at the highvoltage they vary slightly.17 The continuous instead of abrupt change of the structural local geometry in the Mo-substituted sample may be one of the reasons behind better reversibility in the Mo substituted sample. Figure 12a also compares Mo and V EXAFS of pristine phases with fully charged sample. Good agreement of these spectra is qualitative proof that the bond lengths of V−O and Mo−O bonds are reversible throughout the whole Li insertion/removal process. 3168



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with the fitting results. In contrast, the low-R peak of ε-VOPO4 loses the shape after one cycle. To summarize our findings from the electrochemical and reaction mechanism studies, we observe improved initial capacity, good capacity retention, and faster kinetics upon Mo-substitution of ε-VOPO4. The improved initial capacity comes from the low-voltage process, while the high-voltage discharge capacity is smaller in the substituted compound. Both high- and low-voltage processes show a sloping electrochemical profile in clear contrast with the voltage plateau at high-voltage reaction of pristine ε-VOPO4 and with low-voltage reaction of ε-LiVOPO4 showing two additional voltage plateaus. X-ray diffraction shows peak shift upon Li insertion, and in situ X-ray absorption data does not show isosbestic points typical of the two-phase reaction, supporting the hypothesis that Mo substitution may stabilize LixMoyV1−yOPO4 solid solution. The exact range of Li solubility in substituted VOPO4 will be a subject of further research. We have recently reported38 that the second Li intercalation into ε-VOPO4 starts before the intercalation of the first Li is complete, resulting in a pronounced Li gradient in the particles. The Li2VOPO4 phase was reported to form only on the particle surface, which limits the electrochemical capacity. ε-Mo0.05V0.95OPO4 reported in this paper has much smaller particle size than the pristine ε-VOPO4. However, this does not seem to extend the first Li intercalation to its theoretical limit. The PCA analysis shows that the third component, presumably corresponding to V3+ species, forms already at 40% lithiation; therefore, we assume that the second Li reaction also starts from the surface before the completion of the first reaction, even in such small particles. However, the small particle size does promote the second Li intercalation, where we observe the capacity increase.

Article

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DESC0012583. Partial support to Q.W. was provided by the New York State Energy Research and Development Authority (NYSERDA), as matching funding to NECCES. Part of this work was performed at NSLS beamlines X14A and X18A. Use of the NSLS at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0298CH10886. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

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REFERENCES

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CONCLUSIONS Molybdenum substituted ε-VOPO4 was prepared from hydrothermal synthesis with subsequent annealing at 550 °C in oxygen. TEM images show very fine primary particles forming 300−500 nm agglomerates. The substitution of Mo into the structure is proved by slight lattice changes evident from the Xray diffraction pattern refinement. Mo is found in the 6+ oxidation state, compensated by an equal amount of V4+ by combination of X-ray absorption and photoelectron spectroscopies, magnetic studies, and DFT calculations. EXAFS data analysis reveals more symmetric VO6 octahedra in the substituted compound, which undergo gradual V−O bond lengthening upon lithiation. Both V and Mo were found to be electrochemically active, with their local structure completely reversible upon Li cycling. The sloping voltage profile is observed upon the first Li insertion in contrast with the twophase reaction of unsubstituted VOPO4. No additional intermediate phases were detected upon the second Li insertion in contrast with Li intercalation into ε-LiVOPO4, where two intermediate phases are formed. It suggests that Mo substitution can stabilize LixMoyV1−yOPO4 solid solution to some extent. This can be related to faster kinetics and better cyclability of Mo0.05V0.95PO4, which in the voltage range of 2.0− 4.5 V can deliver reversible capacity of about 250 mAh/g in the initial cycle at C/25, which remains about 200 mAh/g at C/25 in 20 cycles. Moreover, GITT shows lower discharge/charge hysteresis as well as shorter relaxation time indicating the faster kinetics in the substituted compound. 3169


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Molybdenum Substituted Vanadyl Phosphate Îµ-VOPO4 with Enhanced

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