Article
pubs.acs.org/cm
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.
■
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
© XXXX 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
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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.
■
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,
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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.
■
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
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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,
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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.
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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
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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 highG
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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
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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
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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.
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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.
■
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.
■
REFERENCES
(1) Whittingham, M. S. Lithium Batteries and Cathode Materials.
Chem. Rev. 2004, 104, 4271−301.
(2) Padhi, A. K.; Nanjundaswamy, K.; Goodenough, J. B. Phosphoolivines as Positive-Electrode Materials for Rechargeable Lithium
Batteries. J. Electrochem. Soc. 1997, 144, 1188−1194.
(3) Whittingham, M. S. Ultimate Limits to Intercalation Reactions
for Lithium Batteries. Chem. Rev. 2014, 114, 11414−11443.
(4) Kerr, T. A.; Gaubicher, J.; Nazar, L. F. Highly Reversible Li
Insertion at 4 V in ε-VOPO4/α-LiVOPO4 Cathodes. Electrochem.
Solid-State Lett. 2000, 3, 460−462.
(5) Song, Y.; Zavalij, P. Y.; Whittingham, M. S. ε-VOPO4:
Electrochemical Synthesis and Enhanced Cathode Behavior. J.
Electrochem. Soc. 2005, 152, A721−A728.
(6) Chen, Z.; Chen, Q.; Chen, L.; Zhang, R.; Zhou, H.; Chernova, N.
A.; Whittingham, M. S. Electrochemical Behavior of Nanostructured εVOPO4 over Two Redox Plateaus. J. Electrochem. Soc. 2013, 160,
A1777−A1780.
(7) Zhou, H.; Upreti, S.; Chernova, N. A.; Hautier, G.; Ceder, G.;
Whittingham, M. S. Iron and Manganese Pyrophosphates as Cathodes
for Lithium-Ion Batteries. Chem. Mater. 2011, 23, 293−300.
(8) Bianchini, M.; Ateba-Mba, J. M.; Dagault, P.; Bogdan, E.; Carlier,
D.; Suard, E.; Masquelier, C.; Croguennec, L. Synthesis and
Crystallographic Study of Homeotypic LiVPO4F and LiVPO4O.
Chem. Mater. 2012, 24, 1223−1234.
(9) Huang, Y.; Lin, Y.-C.; Jenkins, D. M.; Chernova, N. A.; Chung,
Y.; Radhakrishnan, B.; Chu, I.-H.; Fang, J.; Wang, Q.; Omenya, F.;
Ong, S. P.; Whittingham, M. S. Thermal Stability and Reactivity of
Cathode Materials for Li-Ion Batteries. ACS Appl. Mater. Interfaces
2016, 8, 7013−7021.
(10) Harrison, K. L.; Bridges, C.; Segre, C. U.; Varnado, C. D.;
Applestone, D.; Bielawski, C. W.; Paranthaman, M. P.; Manthiram, A.
Chemical and Electrochemical Lithiation of LiVOPO4 Cathodes for
Lithium-Ion Batteries. Chem. Mater. 2014, 26, 3849−3861.
(11) Bianchini, M.; Ateba-Mba, J. M.; Dagault, P.; Bogdan, E.;
Carlier, D.; Suard, E.; Masquelier, C.; Croguennec, L. Multiple Phases
in the ε-VPO4O−LiVPO4O−Li2VPO4O System: a Combined Solid
State Electrochemistry and Diffraction Structural Study. J. Mater.
Chem. A 2014, 2, 10182−10192.
(12) Harrison, K. L.; Manthiram, A. Microwave-Assisted Solvothermal Synthesis and Characterization of Various Polymorphs of
LiVOPO4. Chem. Mater. 2013, 25, 1751−1760.
(13) Gaubicher, J.; Le Mercier, T.; Chabre, Y.; Angenault, J.;
Quarton, M. Li/β-VOPO4: A New 4 V System for Lithium Batteries. J.
Electrochem. Soc. 1999, 146, 4375−4379.
■
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.
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Article
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(35) Ong, S. P.; Richards, W. D.; Jain, A.; Hautier, G.; Kocher, M.;
Cholia, S.; Gunter, D.; Chevrier, V. L.; Persson, K. A.; Ceder, G.
Python Materials Genomics (pymatgen): A robust, open-source
python library for materials analysis. Comput. Mater. Sci. 2013, 68,
314−319.
(36) Atuchin, V. V.; Gavrilova, T. A.; Grigorieva, T. I.; Kuratieva, N.
V.; Okotrub, K. A.; Pervukhina, N. V.; Surovtsev, N. V. Sublimation
Growth and Vibrational Microspectrometry of α-MoO3 Single
Crystals. J. Cryst. Growth 2011, 318, 987−990.
(37) Scanlon, D. O.; Watson, G. W.; Payne, D. J.; Atkinson, G. R.;
Egdell, R. G.; Law, D. S. L. J. Theoretical and Experimental Study of
the Electronic Structures of MoO3 and MoO2. J. Phys. Chem. C 2010,
114, 4636−4645.
(38) Quackenbush, N. F.; Wangoh, L.; Scanlon, D. O.; Zhang, R.;
Chung, Y.; Chen, Z.; Wen, B.; Lin, Y.; Woicik, J. C.; Chernova, N. A.;
Ong, S. P.; Whittingham, M. S.; Piper, L. F. J. Interfacial Effects in εLixVOPO4 and Evolution of the Electronic Structure. Chem. Mater.
2015, 27, 8211−8219.
(39) Bonino, F.; Peraldo Bicelli, L.; Rivolta, B.; Lazzari, M.;
Festorazzi, F. Amorphous Cathode Materials in Lithium-Organic
Electrolyte Cells: Tungsten and Molybdenum Trioxides. Solid State
Ionics 1985, 17, 21−28.
(14) Dupre, N.; Gaubicher, J.; Le Mercier, T.; Wallez, G.; Angenault,
J.; Quarton, M. Positive Electrode Materials for Lithium Batteries
Based on VOPO4. Solid State Ionics 2001, 140, 209−221.
(15) Azmi, B. M.; Ishihara, T.; Nishiguchi, H.; Takita, Y. Vanadyl
Phosphates of VOPO4 as a Cathode of Li-ion Rechargeable Batteries.
J. Power Sources 2003, 119-121, 273−277.
(16) He, G.; Bridges, C. A.; Manthiram, A. Crystal Chemistry of
Electrochemically and Chemically Lithiated Layered α(I)-LiVOPO4.
Chem. Mater. 2015, 27, 6699−6707.
(17) Allen, C. J.; Jia, Q.; Chinnasamy, C. N.; Mukerjee, S.; Abraham,
K. M. Synthesis, Structure and Electrochemistry of Lithium Vanadium
Phosphate Cathode Materials. J. Electrochem. Soc. 2011, 158, A1250−
A1259.
(18) Lin, Y.-C.; Wen, B.; Wiaderek, K. M.; Sallis, S.; Liu, H.; Lapidus,
S. H.; Borkiewicz, O. J.; Quackenbush, N. F.; Chernova, N. A.; Karki,
K.; Omenya, F.; Chupas, P. J.; Piper, L. F. J.; Whittingham, M. S.;
Chapman, K. W.; Ong, S. P. Thermodynamics, Kinetics and Structural
Evolution of ε-LiVOPO4 over Multiple Lithium Intercalation. Chem.
Mater. 2016, 28, 1794−1805.
(19) Hautier, G.; Jain, A.; Ong, S. P.; Kang, B.; Moore, C.; Doe, R.;
Ceder, G. Phosphates as Lithium-Ion Battery Cathodes: An Evaluation
Based on High-Throughput ab Initio Calculations. Chem. Mater. 2011,
23, 3495−3508.
(20) Wen, B.; Chernova, N. A.; Zhang, R. B.; Wang, Q.; Omenya, F.;
Fang, J.; Whittingham, M. S. Layered Molybdenum (Oxy)Pyrophosphate as Cathode for Lithium-Ion Batteries. Chem. Mater.
2013, 25, 3513−3521.
(21) Chernova, N. A.; Roppolo, M.; Dillon, A. C.; Whittingham, M.
S. Layered vanadium and molybdenum oxides: batteries and
electrochromics. J. Mater. Chem. 2009, 19, 2526−2552.
(22) Omenya, F.; Chernova, N. A.; Upreti, S.; Zavalij, P. Y.; Nam, K.
W.; Yang, X. Q.; Whittingham, M. S. Can Vanadium Be Substituted
into LiFePO4? Chem. Mater. 2011, 23, 4733−4740.
(23) Omenya, F.; Chernova, N. A.; Zhang, R. B.; Fang, J.; Huang, Y.
Q.; Cohen, F.; Dobrzynski, N.; Senanayake, S.; Xu, W. Q.;
Whittingham, M. S. Why Substitution Enhances the Reactivity of
LiFePO4. Chem. Mater. 2013, 25, 85−89.
(24) Omenya, F.; Chernova, N. A.; Wang, Q.; Zhang, R.;
Whittingham, M. S. The Structural and Electrochemical Impact of Li
and Fe Site Substitution in LiFePO4. Chem. Mater. 2013, 25, 2691−
2699.
(25) Toby, B. H. EXPGUI, a Graphical User Interface for GSAS. J.
Appl. Crystallogr. 2001, 34, 210−213.
(26) Larson, A. C.; VonDreele, R. B. General structure analysis system
(GSAS); Los Alamos National Laboratory Report LAUR; LANL: Los
Alamos, NM, 2000; pp 86−748.
(27) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS:
data analysis for X-ray absorption spectroscopy using IFEFFIT. J.
Synchrotron Radiat. 2005, 12, 537−541.
(28) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for ab
Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys.
Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186.
(29) Blochl, P. Projector Augmented-Wave Method. Phys. Rev. B:
Condens. Matter Mater. Phys. 1994, 50, 17953−17979.
(30) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals
Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118,
8207−8215.
(31) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Erratum: “Hybrid
Functionals Based on a Screened Coulomb Potential” [J. Chem.
Phys.118, 8207 (2003)]. J. Chem. Phys. 2006, 124, 219906.
(32) Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.;
Angyan, J. G. Screened Hybrid Density Functionals Applied to Solids.
J. Chem. Phys. 2006, 124, 154709.
(33) Chevrier, V. L.; Ong, S. P.; Armiento, R.; Chan, M. K. Y.; Ceder,
G. Hybrid Density Functional Calculations of Redox Potentials and
Formation Energies of Transition Metal Compounds. Phys. Rev. B:
Condens. Matter Mater. Phys. 2010, 82, 075122.
(34) Pack, J. D.; Monkhorst, H. J. ″Special Points for Brillouin-Zone
Integrations″a Reply. Phys. Rev. B 1997, 16, 1748−1749.
L
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