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: lpiper@binghamton.edu

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.

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