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Uniform second Li ion intercalation in solid state ϵ-LiVOPO4Linda W. Wangoh, Shawn Sallis, Kamila M. Wiaderek, Yuh-Chieh Lin, Bohua Wen, Nicholas F. Quackenbush,Natasha A. Chernova, Jinghua Guo, Lu Ma, Tianpin Wu, Tien-Lin Lee, Christoph Schlueter, Shyue Ping Ong,Karena W. Chapman, M. Stanley Whittingham, and Louis F. J. Piper Citation: Applied Physics Letters 109, 053904 (2016); doi: 10.1063/1.4960452 View online: http://dx.doi.org/10.1063/1.4960452 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/109/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The role of electronic and ionic conductivities in the rate performance of tunnel structured manganese oxides inLi-ion batteries APL Mater. 4, 046108 (2016); 10.1063/1.4948272 Li transport in fresh and aged LiMn2O4 cathodes via electrochemical strain microscopy J. Appl. Phys. 118, 072016 (2015); 10.1063/1.4927816 A high pressure x-ray photoelectron spectroscopy experimental method for characterization of solid-liquidinterfaces demonstrated with a Li-ion battery system Rev. Sci. Instrum. 86, 044101 (2015); 10.1063/1.4916209 Communications: Elementary oxygen electrode reactions in the aprotic Li-air battery J. Chem. Phys. 132, 071101 (2010); 10.1063/1.3298994 Pt metal- CeO 2 interaction: Direct observation of redox coupling between Pt 0 / Pt 2 + / Pt 4 + and Ce 4 + / Ce 3+ states in Ce 0.98 Pt 0.02 O 2 − δ catalyst by a combined electrochemical and x-ray photoelectron spectroscopystudy J. Chem. Phys. 130, 114706 (2009); 10.1063/1.3089666
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Uniform second Li ion intercalation in solid state �-LiVOPO4
Linda W. Wangoh,1 Shawn Sallis,2 Kamila M. Wiaderek,3 Yuh-Chieh Lin,4 Bohua Wen,5
Nicholas F. Quackenbush,1 Natasha A. Chernova,5 Jinghua Guo,6 Lu Ma,3 Tianpin Wu,3
Tien-Lin Lee,7 Christoph Schlueter,7 Shyue Ping Ong,4 Karena W. Chapman,3
M. Stanley Whittingham,5 and Louis F. J. Piper1,2,a)
1Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton,New York 13902, USA2Materials Science and Engineering, Binghamton University, Binghamton, New York 13902, USA3X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne,Illinois 60439, USA4Department of NanoEngineering, University of California San Diego, 9500 Gilman Drive 0448,La Jolla, California 92093, USA5NECCES, Binghamton University, Binghamton, New York 13902, USA6Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA7Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE,United Kingdom
(Received 28 June 2016; accepted 22 July 2016; published online 4 August 2016)
Full, reversible intercalation of two Liþ has not yet been achieved in promising VOPO4 electrodes. A
pronounced Liþ gradient has been reported in the low voltage window (i.e., second lithium reaction)
that is thought to originate from disrupted kinetics in the high voltage regime (i.e., first lithium
reaction). Here, we employ a combination of hard and soft x–ray photoelectron and absorption
spectroscopy techniques to depth profile solid state synthesized LiVOPO4 cycled within the low
voltage window only. Analysis of the vanadium environment revealed no evidence of a Liþ gradient,
which combined with almost full theoretical capacity confirms that disrupted kinetics in the high
voltage window are responsible for hindering full two lithium insertion. Furthermore, we argue that
the uniform Liþ intercalation is a prerequisite for the formation of intermediate phases Li1.50VOPO4
and Li1.75VOPO4. The evolution from LiVOPO4 to Li2VOPO4 via the intermediate phases is
confirmed by direct comparison between O K–edge absorption spectroscopy and density functional
theory. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4960452]
There is a demand for higher capacity, safer Li-ion bat-
tery (LIB) cathodes that are capable of reversibly intercalat-
ing more than one Liþ per redox center whilst remaining
compatible with existing LIB architecture. Vanadyl phos-
phates can reversibly intercalate more than one Liþ per vana-
dium with the added benefit of better thermal and chemical
stability than their layered oxide counterparts, thereby mak-
ing them attractive.1,2 In particular, �–VOPO4 has a theoreti-
cal capacity of 332 mA h/g but the full two lithium ion
capacity has not yet been achieved. Depth-profile studies of
�–VOPO4 as a function of discharge have shown that the
second reaction occurs at the surface before the first lithium
has been fully incorporated into the bulk.3 The inability to
reach full capacity was explained in terms of not realizing the
expected Li2VOPO4 endpoint phase within the bulk when
starting from VOPO4. In contrast, full incorporation of the
second Liþ can be achieved when starting from �–LiVOPO4,
with capacities reaching close to the theoretical limit of
165 mA h/g that can be retained upon extended cycling within
the low voltage window (3.5–1.5 V).4 In addition, well-
defined intermediate phases are observed under these condi-
tions,4,7 which are absent in the aforementioned studies of
VOPO4.3 Interestingly, when �–LiVOPO4 is cycled over
both voltage windows (4.5–1.5 V), rapid capacity fading is
observed upon cycling in tandem with the smearing of
plateaus associated with the intermediate phases.4 Taken
together, these studies suggest that the loss of capacity and
smearing of intermediate phases emanate from disrupted
kinetics in the high voltage window, which is evident from a
pronounced Liþ gradient observed in the low voltage win-
dow. As a result, a Liþ gradient is not expected to evolve for
�–LiVOPO4 when cycled within the low voltage window.
In this letter, we perform depth-profile analysis of solid
state �–LiVOPO4 electrodes cycled only within the low volt-
age (1.5–3.5 V) window to correlate Li–ion uniformity with
well-defined intermediate phases as well as isolate the effects
of disrupted kinetics in the high voltage window. We employ
soft and hard x–ray photoelectron spectroscopy (XPS and
HAXPES) together with complementary x–ray absorption
near edge structure (XANES) to monitor changes in the
vanadium oxidation state at the surface, subsurface, and
bulk. Oxygen K-edge X–ray absorption spectroscopy (XAS)
was also employed to confirm the formation of intermediate
phases (i.e., Li1.5VOPO4 and Li1.75VOPO4) previously pre-
dicted by density functional theory (DFT).4 We report a uni-
form vanadium oxidation state at various stages of charge
and discharge which reflects the lack of a pronounced Liþ
gradient. These data support our hypothesis that poorer
kinetics in the high voltage window are responsible for the
formation of a Liþ gradient, which smears out plateaus asso-
ciated with intermediate phases. Furthermore, our work high-
lights the need to address the kinetics in the high voltagea)Electronic mail: [email protected]
0003-6951/2016/109(5)/053904/4/$30.00 Published by AIP Publishing.109, 053904-1
APPLIED PHYSICS LETTERS 109, 053904 (2016)
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14:10:21
window in order to achieve the full theoretical capacity of
reversible two Liþ intercalation that VOPO4 promises.
�–LiVOPO4 was prepared via previously reported solid
state synthesis4 and the phase purity confirmed by X–ray dif-
fraction (Figure S1). Fabrication details of the ex–situ and
operando electrodes can be found in supplementary material.
Surface studies were performed using monochromated
Al Ka XPS and XAS (TEY mode), which have an effective
probing depth of 2–3 nm and 2–5 nm, respectively.5 While
the subsurface and bulk studies were performed using
HAXPES (with an effective probing depth of 5–20 nm) and
XANES (measured in transmission mode), respectively. In
addition, operando XANES employing the AMPIX cell6 was
performed to account for differences between samples under
equilibrium (ex–situ) and non-equilibrium (operando).
The corresponding electrochemical data for the LiVOPO4
electrodes are plotted in Fig. 1. The capacity of our electrodes
reached 162 mA h/g, i.e., almost full theoretical capacity of
165 mA h/g, after the second Liþ insertion. Three plateaus are
observed in our electrochemical curves (and associated deriva-
tive curves), consistent with the formation of intermediate
states highlighted in previous studies.4,7 The set of electrodes
used for ex–situ spectroscopy studies were disassembled at
different voltages indicated in Fig. 1. We note that the electro-
chemical data of the LiVOPO4 electrodes within the AMPIX
cell showed similar electrochemical performance (capacity
and plateaus) to the disassembled cells.
The combination of the XPS, HAXPES, and XANES
(both ex-situ and operando) was used to depth–resolve the
vanadium oxidation state at various stages of discharge and
charge (Figs. 2 and S2). The soft XPS was used to determine
the vanadium oxidation states at the surface of the pristine
LiVOPO4, Li2VOPO4 and recovered LiVOPO4 occurring at
the endpoints based on the relative energetic difference
between the O 1s (531.8 eV) and V 2p3=2.3 As the reaction
progresses, the initial vanadium oxidation state of V4þ for the
pristine LiVOPO4 electrode progressively reduces to V3þ
when discharged to 1.6 V (Li2VOPO4) and reversibly recovers
to V4þ upon charging to 3.5 V (LiVOPO4). The corresponding
vanadium oxidation state of the subsurface was determined
from the same analysis of the V 2p3=2 core-level measured by
HAXPES. In Fig. 2, the HAXPES shows the same oxidation
state as from XPS at all stages of discharge and charge. The
V L edge XAS, which was measured simultaneously with the
HAXPES, was used to further confirm the vanadium oxida-
tion state at the surface, refer to Fig. S3(a). We note that in
the case of the hydrothermally synthesized �–VOPO4, the V3þ
endpoint phase was only achieved at the surface.3 Finally, in
Fig. 2(b) both ex–situ and operando V K edge XANES meas-
urements show strong pre–edge peaks that are good indicators
of bulk V4þ and V3þ oxidation states at the endpoints.8 The
evolution of the XANES spectra is shown in supplementary
information Figs. S2(a) and S2(b). Figure 2(b) reveals that the
pristine LiVOPO4 pre–edge peak for both ex–situ and oper-ando cases is centered around 5470 eV, as expected for the
V4þ oxidation state.9–11 The pre–peak intensity reduces and
shifts to lower energy upon discharging to 1.6 V, consistent
with expected V3þ oxidation.4 For both ex–situ and operando
FIG. 1. (Left) First cycle discharge/charge curve of electrochemically cycled
�–LiVOPO4 for both the ex–situ and operando cases. (Right) The associated
derivative curves of the ex-situ case are shown alongside.
FIG. 2. Comparison of the (a) V 2p3=2 core–level (XPS and HAXPES) and
(b) V K edge ex–situ and operando endpoints. (c) Summarized results of
vanadium oxidation state from V K–edge (bulk) XANES and V 2p3=2 core–
level (surface) XPS spectra of the cycled �–LiVOPO4 electrodes. The vana-
dium is reducing from V4þ to V3þ upon discharge and back to V4þ when
charged in both instances. The dotted lines indicate the expected fractions of
V3þ and V4þ at 1.50 and 1.75 Li.
053904-2 Wangoh et al. Appl. Phys. Lett. 109, 053904 (2016)
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14:10:21
cases, the V4þ oxidation state is recovered upon charging
back to 3.5 V. The consistency across these multiple
approaches and length-scales indicates uniform Liþ intercala-
tion and extraction within this voltage window.
To better illustrate the uniformity in the surface and
bulk, fitting of the V 2p3=2 and the V K edge pre–peak was
performed at various stages of discharge and charge. (Fitted
data are provided in supplementary material.) The inelastic
background of XPS spectra was subtracted using a
Shirley–like profile before fitting the V 2p3=2 core–level.
The surface V4þ and V3þ ratio for each electrode was then
determined following a procedure described earlier,3 and is
shown in Fig. 2(c). Representative end member XANES
spectra were extracted using an algorithm based on
Bayesian non–negative matrix factorization with volume
constraints.12,13 They were then fit to the XANES data
using least squares linear combination fitting to determine
the bulk average vanadium oxidation state of the electrodes,
also plotted in Fig. 2(c). The vanadium oxidation states
of the surface and bulk are similar throughout the first
low voltage cycle (Fig. 2(c)). This would suggest that lith-
ium ion diffusion is uniform for solid state �–LiVOPO4
within the low voltage window. This explains why almost
full reversible capacity of 165 mA h/g is realized in this
electrode.
Having established the quality of the �–LiVOPO4 and
uniform Liþ distribution within our electrodes, we turn to
the formation of intermediate phases with compositions
of Li1.5VOPO4 and Li1.75VOPO4 that have been previously
predicted from DFT.4 Figure 3(a) shows the O K edge
XAS for selected voltages reflecting vanadium oxidation
states expected for the four phases (i.e., LixVOPO4 where
x ¼ 1.00, 1.50, 1.75, and 2.00). We have ruled out possible
spectral contamination (refer to Fig. S3) to ensure that
direct comparison with the O 2p partial density of states
(PDOS) was valid. (Computational details are provided in
supplementary material.) The O 2p PDOS in Fig. 3(a),
obtained from raw O 2p in Fig. 3(b), was broadened
by Gaussian (0.4 eV) and Lorentzian (0.3 eV) profiles
to account for instrumental and lifetime broadening,
respectively. The photon and binding energy axes were
aligned by the O 1s binding energy. The evolution of the
XAS spectra is reproduced by the DFT in Fig. 3(a). The
O K-edge pre–edge region (528 eV–535 eV) reflects
changes in V 3d t�2g and e�g orbital occupation with Liþ
insertion, as shown in Fig. 3(c). Feature A (�534 eV)
is predominantly associated with the Li 2s orbital in
Fig. 3(d). The features above 535 eV result predominantly
from O 2p–P 3sp hybridized states, with minor contribu-
tions from O 2p–V 4sp hybridization.14–16 No major
changes are observed in these regions confirming the
robustness of the PO3�4 groups.
In summary, we correlated a uniform vanadium oxida-
tion (and therefore Liþ) profile with the formation of inter-
mediate phases in solid state �–LiVOPO4 electrodes cycled
within the low voltage (3.5–1.5 V) window. In addition,
using O K-edge XAS and DFT, we confirmed the formation
of the well-defined intermediate phases, Li1.5VOPO4 and
Li1.75VOPO4. These data suggest that uniform Liþ intercala-
tion is a prerequisite for the formation of these intermediate
phases. Our previous report of discharged �–VOPO4 revealed
a pronounced Liþ gradient in the low voltage window, which
was thought to originate from disrupted kinetics in the high
voltage window.3 Other studies have shown that the electro-
chemistry of LiVOPO4 does not degrade when cycling is
restricted to the low voltage window.4 We argue that the uni-
form lithium ion insertion and extraction observed here is
because we avoid the high voltage window. As a result, the
depth-profile analysis of the first cycle shown here is consid-
ered to be representative of subsequent cycles. Taken
together, our data reinforce the view that disrupted kinetics
in the high voltage regime hinder the full two Liþ capacity
of �–VOPO4 by forming a Liþ gradient profile within the
electrode. Therefore, a better understanding of the kinetics in
the high voltage regime is required to realize the full two
Liþ capacity of VOPO4.
FIG. 3. (a) The O K edge XAS of the end points (pristine and 1.6 V) and intermediate phases of (2.35 V and 2.1 V) �–LixVOPO4 plotted on a photon energy
axis (top). Plotted underneath is the corresponding convoluted O 2p PDOS plotted against binding energy (bottom, 0 eV refers to the Fermi level). The two
energy axes are different by the O 1s binding energy. The calculated spin up and spin down orbital and elementally resolved PDOS of (b) O p, (c) V d, and (d)
Li s states of �–LixVOPO4 (x¼ 1.00, 1.50, 1.75, and 2.00). The V d is broken down into the eg and t2g states.
053904-3 Wangoh et al. Appl. Phys. Lett. 109, 053904 (2016)
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See supplementary material for details of the electrode
fabrication, spectroscopy measurements and curve fitting
analysis. Additional XRD, V K-edge XANES and V L-edge
XAS analysis are also provided.
This work was supported as part of NECCES, an Energy
Frontier Research Center funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences
under Award No. DE-SC0012583. This research used
resources of the Advanced Photon Source, a U.S. Department
of Energy (DOE) Office of Science User Facility operated for
the DOE Office of Science by Argonne National Laboratory
under Contract No. DE-AC02-06CH11357. We thank
Diamond Light Source for access to beamline I09 (SI12546)
that contributed to the results presented here. The Advanced
Light Source is supported by the Director, Office of Science,
Office of Basic Energy Sciences, of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231.
1M. S. Whittingham, “Ultimate limits to intercalation reactions for lithium
batteries,” Chem. Rev. 114(23), 11414–11443 (2014).2K. L. Harrison, C. A. Bridges, C. U. Segre, C. D. Varnado, Jr., D.
Applestone, C. W. Bielawski, M. P. Paranthaman, and A. Manthiram,
“Chemical and electrochemical lithiation of LiVOPO4 cathodes for
lithium-ion batteries,” Chem. Mater. 26(12), 3849–3861 (2014).3N. F. Quackenbush, L. Wangoh, D. O. Scanlon, R. Zhang, Y. Chung, Z.
Chen, B. Wen, Y. Lin, J. C. Woicik, N. A. Chernova, S. P. Ong, M. S.
Whittingham, and L. F. J. Piper, “Interfacial effects in �-LixVOPO4 and evo-
lution of the electronic structure,” Chem. Mater. 27(24), 8211–8219 (2015).4Y.-C. Lin, B. Wen, K. M. Wiaderek, S. Sallis, H. Liu, S. H. Lapidus, O. J.
Borkiewicz, N. F. Quackenbush, N. A. Chernova, K. Karki, F. Omenya, P. J.
Chupas, L. F. J. Piper, M. S. Whittingham, K. W. Chapman, and S. P. Ong,
“Thermodynamics, kinetics and structural evolution of �-LiVOPO4 over mul-
tiple lithium intercalation,” Chem. Mater. 28(6), 1794–1805 (2016).5N. F. Quackenbush, J. W. Tashman, J. A. Mundy, S. Sallis, H. Paik, R.
Misra, J. A. Moyer, J.-H. Guo, D. A. Fischer, J. C. Woicik, D. A. Muller,
D. G. Schlom, and L. F. J. Piper, “Nature of the Metal insulator transition
in ultrathin epitaxial vanadium dioxide,” Nano Lett. 13(10), 4857–4861
(2013).6O. J. Borkiewicz, B. Shyam, K. M. Wiaderek, C. Kurtz, P. J. Chupas, and
K. W. Chapman, “The AMPIX electrochemical cell: A versatile apparatus
for in–situ x-ray scattering and spectroscopic measurements,” J. Appl.
Crystallogr. 45(6), 1261–1269 (2012).7M. Bianchini, J. M. Ateba-Mba, P. Dagault, E. Bogdan, D. Carlier, E.
Suard, C. Masquelier, and L. Croguennec, “Multiple phases in the
�–VPO4O–LiVPO4O–Li2VPO4O system: A combined solid state electro-
chemistry and diffraction structural study,” J. Mater. Chem. A 2(26),
10182–10192 (2014).8J. Wong, F. W. Lytle, R. P. Messmer, and D. H. Maylotte, “K–edge
absorption spectra of selected vanadium compounds,” Phys. Rev. B
30(10), 5596 (1984).9C. J. Allen, S. Mukerjee, and K. M. Abraham, “Li2–xFe0.5 (VO)0.5 (PO4)F0.5,
a new mixed metal phosphate cathode material,” J. Electrochem. Soc.
159(10), A1659–A1663 (2012).10C. J. Allen, Q. Jia, C. N. Chinnasamy, S. Mukerjee, and K. M. Abraham,
“Synthesis, structure and electrochemistry of lithium vanadium phos-
phate cathode materials,” J. Electrochem. Soc. 158(12), A1250–A1259
(2011).11F. Omenya, N. A. Chernova, S. Upreti, P. Y. Zavalij, K.-W. Nam, X.-Q.
Yang, and M. S. Whittingham, “Can vanadium be substituted into
LiFePO4?” Chem. Mater. 23(21), 4733–4740 (2011).12M. Arngren, M. N. Schmidt, and J. Larsen, “Bayesian non–negative matrix
factorization with volume prior for unmixing of hyperspectral images,”
in IEEE International Workshop on Machine Learning for SignalProcessing, 2009. MLSP 2009 (IEEE, 2009), p. 1–6.
13M. Arngren, M. N. Schmidt, and J. Larsen, “Unmixing of hyperspectral
images using Bayesian non–negative matrix factorization with volume pri-
or,” J. Signal Process. Syst. 65(3), 479–496 (2011).14F. M. F. De Groot, M. Grioni, J. C. Fuggle, J. Ghijsen, G. A. Sawatzky,
and H. Petersen, “Oxygen 1s x–ray absorption edges of transition–metal
oxides,” Phys. Rev. B 40(8), 5715 (1989).15M. G. Willinger, D. S. Su, and R. Schl€ogl, “Electronic structure of
b–VOPO4,” Phys. Rev. B 71(15), 155118 (2005).16L. F. J. Piper, N. F. Quackenbush, S. Sallis, D. O. Scanlon, G. W. Watson,
K.-W. Nam, X.-Q. Yang, K. E. Smith, F. Omenya, N. A. Chernova, F.
Omenya, N. A. Chernova, and M. S. Whittingham, “Elucidating the nature
of pseudo Jahn–teller distortions in LixMnPO4: Combining density func-
tional theory with soft and hard x-ray spectroscopy,” J. Phys. Chem. C
117(20), 10383–10396 (2013).
053904-4 Wangoh et al. Appl. Phys. Lett. 109, 053904 (2016)
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