The Structural and Electrochemical Impact of Li and Fe Site

The Structural and Electrochemical Impact of Li and Fe SiteSubstitution in LiFePO4

Fredrick Omenya,† Natasha A. Chernova,‡ Qi Wang,‡,§ Ruibo Zhang,‡ and M. Stanley Whittingham*,†,‡,∥

†Department of Chemistry, Binghamton University, Binghamton, New York 13902-6000, United States‡Institute for Materials Research, Binghamton University, Binghamton, New York 13902-6000, United States§Brookhaven National Laboratory, Upton, New York 11973, United States∥Northeastern Center for Chemical Energy Storage, Department of Chemistry, Stony Brook University, Stony Brook, New York11794-3400, United States

ABSTRACT: The crystal structure and delithiation mechanism of Li-site substitutedLiFePO4 have been revealed by investigation of supervalent V3+ substitution. The combinedX-ray and neutron powder diffraction data analysis surprisingly shows that the substitutingaliovalent vanadium ions occupy the Fe site while some of the Fe resides at the Li site,probably as sarcopside, which leads to an increase in the unit cell volume. Such substitutionreduces the miscibility gap at room temperature and also significantly lowers the solidsolution formation temperature in the two-phase region. The effect of the phase diagrammodification results in improved kinetics, leading to better rate performance. Suchsubstitution, however, significantly lowers the LiFePO4 capacity at moderate currentdensities.

KEYWORDS: olivine, site substitution, Li electrochemistry, sarcopside

■ INTRODUCTION

Since the discovery of LiFePO4 as a cathode material,1 manystudies have been devoted to the optimization of itselectrochemical performance through carbon coating,2 sub-stitution,3 and nanosizing of the particles. At present, LiFePO4is capable of cycling at remarkably high rates despite itsinherently low ionic and electronic conductivities. Recent focuson the material has shifted toward understanding the reactionmechanism of the LiFePO4; is it a two-phase or single-phasemechanism, and what controls the Li-ion intercalation−deintercalation processes?The possibility and effect of aliovalent substitution has

attracted much interest following the earlier report by Chung etal.3 that an improvement in electronic conductivity by about 8orders of magnitude was observed when aliovalent cationsbelow 1 at % were doped at the Li (M1) site. This dramaticincrease in conductivity was later attributed to the formation ofa conductive coating of Fe2P

4 and/or carbon5 on the surface ofthe LiFePO4 during synthesis at temperatures above around650 °C, rather than to an increase of the conductivity of theLiFePO4 structure itself. Following the earlier debate on thepossibility and the effect of aliovalent substitution at the M1site, Chiang’s6,7 and Nazar’s8 groups have recently revisited thisarea. Using combined X-ray and neutron diffraction data,Wagemaker et al.8 showed the incorporation of a lowconcentration of aliovalent ions (less than 3%) at the M1site, which was balanced by vacancies at the same site.Meethong et al.,6 on the other hand, have recently claimed thepossibility of higher level aliovalent cation substitution at the

M1 site. In their work, in which the synthesis was done at 700°C, the formation of a second NASICON phase in the Zrsubstituted samples was observed. The X-ray diffraction lines ofthe Zr-NASICON phase were detected even at a very low Zrconcentration, 1.5 mol % of Zr. These intensities of theNASICON peaks were observed to increase with the increasein the Zr content. In addition, the above work did not show anycrystallographic evidence of the decrease in the miscibility gapapart from relying on the PITT data7 to determine thesolubility limit. In such instances, it might not be possible toisolate the role of the second phase and that of the substitutedone in the electrochemical behavior. Many researchers havealso investigated substitution at the Fe (M2) site.9−17 We havepreviously reported the possibility and role of aliovalentsubstitution at the Fe site on the crystallographic andelectrochemical properties of LiFePO4. From our results, upto 10 mol % of aliovalent ions could be substituted andvacancies were found to be located at the M2 site. Theelectrochemical performance is improved upon aliovalentsubstitution,13 demonstrating the role of proper stoichiometriccontrol and reduced synthesis temperature in enhancing thealiovalent solubility. Recently, Harrison et al.17 confirmed thetemperature dependence of the aliovalent substitution with upto 20% substitution of V at the Fe site using microwavesynthesis. Most of the research on vanadium substitution has

Received: April 20, 2013Revised: June 9, 2013Published: June 12, 2013

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© 2013 American Chemical Society 2691 dx.doi.org/10.1021/cm401293r | Chem. Mater. 2013, 25, 2691−2699

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focused on the Fe site, with only one paper on the Li site.18 Inthe latter report, single-phase olivine phosphate could only beobtained for a vanadium concentration below 2%.In this work, we show unambiguous proof of high levels of

aliovalent substitution at the Li site through crystallographicand electrochemical evidence. This sheds a new light on thefundamental understanding of substitution in olivine materials:the possibility of substitution, the site occupancy of thesubstituent, the impact of such substitution on the phasediagram, and its effect on the electrode kinetics.

■ EXPERIMENTAL SECTIONThe LiFePO4 samples were synthesized by solid-state reaction ofLi2CO3, FeC2O4·2H2O, NH4H2PO4, and NH4VO3. These reactantswere mixed together in amounts to give the transition-metal-richcomposition Li1−3y[VyFe]PO4, where V + Fe > 1 > Li, assuming Livacancies for charge compensation, and for y = 0, 0.025, 0.05, 0.10, and0.20. After adding 5 wt % carbon black, the mixture was planetary ball-milled in acetone for 12 h. The acetone was evaporated and dried. Theresulting precursors were preheated at 350 °C for 8 h before beingsintered at 550 °C for 10 h in an 8.5% H2/He atmosphere. This lowtemperature was chosen so that only the effects of substitution werestudied, as at higher temperatures, second phases such as Li3V2(PO4)3and the conductive Fe2P are formed, which impact the electro-chemistry.13

The phase composition and the crystal structure of the synthesizedsamples were determined by powder X-ray diffraction (XRD) using aScintag XDS2000 θ-θ diffractometer equipped with a Ge(Li) solid-state detector and a Cu Kα sealed tube (λ = 1.54178 Å). The datawere collected in the range of 2θ = 10−80° with a step size of 0.02°while spinning the sample to minimize preferred orientation. High-resolution synchrotron powder XRD data were collected usingbeamline 11-BM at the Advanced Photon Source (APS), ArgonneNational Laboratory, with an average wavelength of 0.413612 Å. Insitu high-temperature X-ray diffraction analysis was performed at theNational Synchrotron Light Source, beamline X7B, wavelength 0.3196Å, with a heating rate of 5 °C min−1 under a helium gas flow. X-rayabsorption data was collected at beamline X23A2, NationalSynchrotron Light Source, as described in our earlier work.13 Neutronpowder diffraction (NPD) measurements were performed using thehigh-resolution neutron powder diffractometer (BT-1) at the NISTCenter for Neutron Research, where measurements were taken with aneutron wavelength of 1.54030 Å using a Cu(311) monochromator.Time-of-flight (TOF) neutron diffraction data were collected on thePOWGEN instrument in the Spallation Neutron Source (SNS) at OakRidge National Laboratory (ORNL). XRD and neutron powderRietveld refinements were performed to determine the latticeparameters and site occupancy for the synthesized samples using theGSAS/EXPGUI package.19,20

The cathode materials for the electrochemical tests were preparedby mixing 80% active material, 10% carbon black (total amount ofcarbon in the electrode), and 10% polyvinylidene fluoride (PVDF)with 1-methyl-2-pyrrolidinone solvent. The slurry formed was thencast onto an Al foil current collector before drying. The driedelectrodes, with an area of 1.2 cm2, containing 3−5 mg of activematerial were placed in 2325-type coin cells in a He-filled gloveboxwith pure lithium foil (Aldrich, thickness = 23 μm) as the counter andreference electrodes, and Celgard 2400 as the separator. LiPF6 (1 M)in a 1:1 volume ratio of ethylene carbonate (EC) and dimethylcarbonate (DMC) (LP30 from EM Industries) was used as theelectrolyte. The cells were tested using a VMP2 multichannelpotentiostat (Biologic). The galvanostatic charge and dischargeexperiments were performed at current densities of 0.05, 0.1, 0.25,1, 2.5, and 5 mA/cm2 corresponding to 0.1, 0.2, 0.5, 1, 5 and 10 Crates (1 C corresponds to 160 mAh/g) over a 2.0−4.3 V voltage range.The cyclic voltammetry (CV) tests were done at different scan rates of0.02, 0.05, 0.1, 0.5, 1, 5, and 10 mV/s in a voltage range of 2.2−4.3 Vvs Li+ /Li. The potentiostatic intermittent test (PITT) technique was

applied after one complete charge and subsequent discharge of theelectrodes to 2 V. A 5 mV step was then applied from 2 to 4.3 V; thecurrent at each step was measured until the current dropped to lessthan 5 μA/cm2. The partially delithiated samples for ex situ XRD wereinitially electrochemically delithiated at a current density of 0.1 mA/cm2. These samples were then disassembled in a He-filled gloveboxand washed several times with DMC to remove all the electrolytebefore being packed in a Kapton capillary tube for room-temperaturediffraction or in a quartz capillary tube for in situ high-temperaturediffraction.

■ RESULTS AND DISCUSSIONThe Li site substitution was achieved by controlling thestoichiometric composition of the precursors so as tohypothetically allow V3+ substitution and vacancies at the Li+

site as in the formula Li1−3y[VyFe]PO4. The as-synthesizedsubstituted compounds show single olivine phases similar tothe unsubstituted LiFePO4, with no observable impurity for upto 10 mol % substitution. The XRD patterns are shown inFigure 1. The as-synthesized samples with various vanadium

concentrations in the structure show very small differences inthe diffraction patterns; only a close assessment of the differentdiffraction peaks for the different vanadium concentrationsreveals noticeable changes, as observed in Figure 1, inset. Thedifferent diffraction peaks shift in different directions withdifferent magnitudes. For example, the 200 and the 201diffraction peaks shift to the higher angles with an increase invanadium content in olivine structures, but with varyingdegrees, and the 101 peak, on the other hand, shows asignificant shift to the lower angles. The normalized intensitiesof the as-synthesized compounds also show slight variations; forexample, the 200 and the 101 diffraction peaks show anobservable decrease in normalized intensities with an increasein vanadium concentration, whereas the intensities of certaindiffraction peaks, such as 210, are not affected much with thevanadium substitution level. These are strong indications ofstructural modification upon vanadium substitution. It is worthpointing out that, in Meethong’s work,6 formation of theNASICON phase in Zr substituted LiFePO4 was observed atvery low Zr concentration; at 1.5 mol % of Zr, they couldobserve the formation of the NASICON phase with the

Figure 1. High-resolution X-ray diffraction data for Li1−3y[VyFe]PO4, y= 0, 0.05, and 0.1 (λ = 0.413612 Å). The insets are magnifications ofthe 200, 101, and 210 reflection peaks.

Chemistry of Materials Article

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intensity of this phase increasing with the increase of Zr added.This is in good agreement with our previous study,13 where weshowed that, at temperatures ≥ 650 °C, some of the vanadiumsubstituted in the olivine structure is ejected, forming theNASICON phase. When the precursor concentration ofvanadium was increased to 0.2 mol, we observed the formationof pyrophosphates as the second phase, contrary to the Fe sitesubstitution where high vanadium contents lead to theformation of the NASICON phase.The Rietveld refinement results further support vanadium

solubility into the olivine structure; the lattice parametersshown in Table 1 follow Vegard’s law, as shown in Figure 2.

The a and b lattice parameters show a small, but consistent,decrease with the increase in the vanadium content in theolivine structure, whereas the c lattice parameter shows asignificant increase as compared to the change in the a and blattice constants. The consequence of the combined effect ofthe anisotropic lattice parameter changes leads to an overalllinear increase in the unit cell volume as a result of the largerchange in the c lattice constant, which overcompensates for thesmall changes in the a and b parameters. In contrast,substitution on the Fe site, on the other hand, shows asignificant decrease in both the a and the b lattice parameterswith an almost negligible increase in the c parameter, leading to

an overall decrease in the unit cell volume. There is, therefore, asignificant difference between the Li site and Fe sitesubstitution; substitution on the Fe site leads to a unit cellvolume decrease, whereas the attempted Li-site substitution byvanadium leads to a unit cell volume increase. Theseanisotropic lattice parameter changes are consistent with thechanges observed in the diffraction peak position alreadyillustrated in Figure 1 for the different vanadium concen-trations.After establishing the possibility of vanadium substitution in

the olivine lattice, the next issue to address is the siteoccupancy. To determine the site occupancy of the substitutingion and the vacancies in the substituted olivine structure, wecarried out both a high-resolution XRD and an NPD datarefinement. The XRD data refinement reveals a nearly fulloccupancy of the M2 site. However, due to the almost identicalX-ray scattering powers of iron and vanadium, no informationon the detailed site occupancy between the two could beobtained; the full occupancy of the M2 site could suggest eitherfull occupancy by the Fe or the occupancy by both Fe andvanadium. The results further revealed that, close to thetargeted composition of the transition metal, Fe or vanadium orboth reside at the M1 site. The lithium and vacancies weremostly on the M1 site, while the concentration could not beprecisely determined from the XRD refinement due to the lowscattering of X-rays by lighter atoms. The XRD data thus givesthe formula as Li1−3y[MyM]PO4, where M = Fe + V. The siteoccupancies of the various atoms were further evaluated byneutron powder diffraction. In contrast to XRD, neutrondiffraction has sensitivity to light atoms, such as Li, and alsoprovides good contrast between Fe and V since Fe has apositive scattering, whereas the vanadium has a small negativescattering factor. A combination of XRD and NPD techniquesallows an accurate determination of the lithium, vanadium, andvacancies site occupancies. In the NPD data refinement, shownin Figure 3, the unit cell parameters of the substitutedcompound were constrained to the values obtained from thehigh-resolution X-ray diffraction data. The site occupancyrefinement from the neutron diffraction data revealed about

Table 1. Lattice Parameters of “Li1−3y[VyFe]PO4”, y = 0, 0.05, and 0.1

y in Li1−3y[VyFe]PO4 a (Å) b (Å) c (Å) V (Å3)

0 10.3210(1) 6.0041(1) 4.6933(1) 290.8470.05 10.3197(1) 5.9984(1) 4.7019(0) 291.0580.1 10.3151(1) 5.9873(0) 4.7146(1) 291.203

Figure 2. Lattice constants and unit cell volume comparison ofLi1−3y[VyFe]PO4 and LiFe1−3y/2VyPO4 as a function of vanadiumconcentration.

Figure 3. Neutron diffraction data refinement for the 0.1 molvanadium-substituted LiFePO4 (Rwp = 0.037, Rp = 0.0314, and χ2 =1.18).



94% occupancy of the M2 site. Since the XRD data hadrevealed a full occupancy of the M2 site, the neutronrefinement result, therefore, suggests that the vanadium, withalmost zero scattering factor, occupies the Fe site. Thus, thesemetal-rich olivines are best represented by the formulaLi1−3yFey[Fe1−yVy]PO4, where most, if not all, of the highervalent vanadium is on the Fe site. This formulation is used inthe remainder of the paper. In most of the previouspublications,3,6,8 the substituting ions have been proposed tooccupy the M1 site; the use of vanadium as a substituting ionallows for accurate refinement of its site occupancy due to thegood contrast between V and either Fe or Li in the neutrondiffraction.The thermal evolution of the molar magnetic susceptibility of

the pure LiFePO4 and 0.1 mol vanadium-substituted LiFePO4were studied in the temperature range of 2−350 K with a 1000Oe magnetic field. Both the substituted and the unsubstitutedsamples exhibit paramagnetic behavior in the high-temperatureregion and antiferromagnetic ordering at temperatures below50 K. The Neel temperature TN is observed to decrease withsubstitution, from 49.2 K for LiFePO4 to 47.6 K for the 0.1 molvanadium-substituted material. In addition, the substitutedcompounds show a more pronounced increase in the molarsusceptibility values at lower temperatures below the Neeltemperature as compared to the unsubstituted phase (Figure4). The Curie−Weiss law was used to fit the paramagnetic

region, 150−350 K, to determine the Curie constant and theCurie−Weiss temperature (results summarized in Table 2).Both the Curie constant and the absolute value of the Curie−Weiss temperature decrease with substitution. These changes inthe magnetic properties support the incorporation of vanadiumions in the olivine structure, even more specifically, onto the Fesite, which weakens the magnetic interactions between the

transition-metal ions and thereby lowering the magnetictransition and Curie−Weiss temperatures. It should be notedthat the magnetic behavior of these vanadium-substitutedcompounds differs markedly from that of an Fe-excess singlecrystal, where a sharp upturn of the magnetic susceptibility wasobserved right at the Neel temperature.21 It supports the idea ofthe iron being distributed between the Fe and Li sites, resultingin a more disordered structure and the absence of aferrimagnetic transition.X-ray absorption near-edge structure was used to determine

the oxidation state and the local environment of vanadium inthe olivine structure. By comparison with the references ofdifferent oxidation states, the V K-edge XANES spectra for theseries of vanadium-substituted LiFePO4 (vanadium concen-tration = 0.05, 0.075, and 0.1 mol) exhibits an absorption edgeat a similar energy to that of Li3V2(PO4)3, suggesting that the Vis approximately trivalent in these substituted LiFePO4materials (Figure 5). The Figure 5 inset shows a weak pre-

edge absorption, characterized by a triplet peak in all threesamples. Such a feature arises from 1s−3d transitions, which is aformally electric dipole-forbidden transition and gains itsintensity through electric quadrupole coupling and p−d orbitalhybridization in the distorted MO6 octahedra. Crystal fieldsplitting in a quadrupole transition has been attributed to theformation of multiplet pre-edge peaks, as well as a possiblecontribution from the dipole transition. The relatively lowintensities in the V-LFP series corroborate that vanadiumresides at the octahedral site (M1 or M2) in the substitutedcompounds. This pre-edge feature is observed to decrease inintensity upon the increase of vanadium concentration in theolivine structure, suggesting that the distorted octahedron

Figure 4. Temperature dependence of the magnetic susceptibility ofLiFePO4 and 0.1 mol vanadium-substituted LiFePO4. The inset showsinverse molar susceptibility corrected for the temperature independentcontribution and their fit to the Curie−Weiss law.

Table 2. Magnetic Parameters of Pristine and 0.1 mol Vanadium-Substituted LiFePO4a

y in Li1−3y[VyFe]PO4 TN, K χ0, 10−4 emu/mol θ, K C, emu K/mol μeff

exp, μB μefftheor, μB

0 49.2 1.42 −85.6 3.24 5.09 5.090.1 47.6 1.45 −75.4 3.21 4.83 4.92

aTN is determined as an inflection point of the M(T) dependence. μeffexp is determined using μ = [8C/(1 + y)]1/2. In calculations of μeff

theor = [(yμV2

+ μFe2)/(1 + y)]1/2, the magnetic moment of Fe2+ is assumed to be μFe = 5.09 μB, as in LiFePO4, since it is difficult to account for its orbital

contribution. The magnetic moment of V3+ is assumed as spin-only μV = 2.82 μB.

Figure 5. Normalized vanadium K-edge XANES for vanadium-substituted LiFePO4 samples together with standards, VCl2, Li3V2-(PO4)3, VO4, and V2O5, corresponding to vanadium in +2, +3, +4, and+5 valence states. The inset shows the magnified pre-edge region forthe 0.05, 0.075, and 0.1 mol of vanadium-substituted LiFePO4.



becomes more symmetric in geometry with the increase insubstitution level (Figure 5, inset)We further studied the valence state of the vanadium in

electrodes during the charge process. It is observed that thevanadium edge feature remains unchanged in shape andposition at different states of charge, which suggests that thevanadium does not participate in the electrochemical process.The XANES of the 0.1 V substituted sample charged to 4.3 V isplotted in Figure 5 along with pristine samples to illustrate thiscomparison.The effect of lithium nonstoichiometry upon delithiation was

further examined by XRD on partially delithiated samples todetermine the impact of substitution on the miscibility gap. Thediffraction profiles from synchrotron-based XRD are shown inFigure 6 for both pure and vanadium-substituted LFP

electrodes at the 50% state of charge (Li1−3y−xFey[Fe1−yVy]PO4,for y = 0, 0.025, 0.05, 0.1 and x = 0.5, 0.45, 0.35, 0.3,respectively). At the given states of charge, the samples in therelaxed state at room temperature unquestionably show thecoexistence of the two phases, triphylite and heterosite, withoutthe existence of any other crystalline phase. Taking the 200peaks, for example (inset in Figure 6), as the concentration ofthe vanadium is increased in the olivine structure, the mismatchbetween the heterosite and the triphylite 200 diffraction peaksdecreases. For example, at 0.1 mol of vanadium substitution,the two diffraction peaks appear as one broad single peak. Thisclearly demonstrates that the miscibility gap is a function of thealiovalent substitution level. The miscibility gap shrinkssystematically with the increase in vanadium solubility in theolivine structure, reaching a concentration where the two peakscannot be readily resolved and they appear as one broad singlepeak. The mismatch in these samples is much smaller ascompared to the substitution targeting the Fe site.

The lattice mismatch results were further investigated by aRietveld refinement of the partially delithiated phases. Theserefinement results, shown in Table 3, indicate a systematicdecrease in the a and b lattice parameters and of the unit cellvolume with the increase in the vanadium concentration in theolivine structure. The calculated volume changes between theLi-poor and the Li-rich phases coexisting in the partiallydelithiated samples show a systematic decrease from 6.0%, to5.3%, to 4.5%, corresponding to LiFePO4, Li0.925Fe0.025[V0.025-Fe0.975]PO4, and Li0.85Fe0.05[V0.05Fe0.95]PO4, respectively.The crystallographic changes of the single phase Li0.7V0.1-

FePO4 were further studied for Li solubility upon electro-chemical delithiation to different states of charge. The partiallydelithiated samples lead to a much smaller lattice mismatchbetween the Li-rich and the Li-poor phases such that the nextneighboring peaks (for example, the 200 peaks) belonging tothe heterosite and triphylite phases could not be resolved bythe XRD instrument, as shown in Figure 7. Considering the

200 reflection peaks of the two phases of the partiallydelithiated sample, the two peaks appear as a single broaddiffraction peak observed at around 2θ = 3.5° at all states ofcharge, as shown in Figure 7, inset. The unresolved peaks canbe mistaken to be a single olivine phase. However, comparisonof these peaks with those peaks of the same partially delithiatedsample that are not affected by the contribution from the twophases, heterosite and triphylite, or with the similar diffractionpeaks of the pristine material, shows that the broadness is aresult of overlapping unresolved peaks. Because of theseunresolved peaks of Li-rich and Li-poor phases, it may not befeasible to refine these phases of the partially delithiatedLi0.7−xV0.1FePO4. This observation is strong evidence thatcontrolling the stoichiometric composition of the starting

Figure 6. X-ray diffraction patterns of partially delithiated Li1−3y−x-Fey[Fe1−yVy]PO4, y = 0, 0.025, 0.05, and 0.1 and x = 0.5, 0.45, 0.35,and 0.3, respectively (λ = 0.3196 Å).

Table 3. Lattice Parameters of Partially Delithiated Li1−3y−xFey[Fe1−yVy]PO4

sample a (Å) b (Å) c (Å) V (Å3) Rwp

LiFePO4, 50% charged T 10.309(1) 5.999(1) 4.697 (1) 290.505 0.023H 9.835(1) 5.805(1) 4.783(1) 273.048

Li0.925Fe0.025[V0.025Fe0.975]PO4, 45% charged T 10.291(1) 5.987(1) 4.702(1) 289.704 0.038H 9.862(1) 5.818(1) 4.777(1) 274.067

Li0.85Fe0.05[V0.05Fe0.95]PO4, 35% charged T 10.273(1) 5.978(1) 4.711(1) 289.287 0.028H 9.906(2) 5.840(1) 4.775(1) 276.278

Figure 7. Ex situ synchrotron XRD patterns of Li(0.7−x)Fe0.1[Fe0.9V0.1]-PO4 at different states of charge: x = 0.0, 0.25, 0.3, and 0.46. The insetshows the magnified section of the 200 reflections of the triphylite andheterosite phases (λ = 0.3196 Å).



precursors leads to the substitution of the transition metal atthe M1 site, which affects the electrochemical performance ofthe LiFePO4.In addition to the XRD crystallographic information, PITT

was used to study the delithiation mechanism to furtherunderstand the effect of substitution on the reaction pathwayand the reaction kinetics. This technique has been usedpreviously to determine the lithium nonstoichiometriccompositions and the phase transformation of LiFePO4 ondoped or nano-LiFePO4.

6,7,22 The PITT curves for thesubstituted samples differ markedly from that of theunsubstituted one. First, a current maximum is observed inLiFePO4 but is absent in the substituted samples; a currentrelaxation maximum is indicative of a first-order phasetransformation. Second, the current relaxation time at the“plateau region” for pure LiFePO4 is the longest, whereas thoseof the substituted materials are much shorter and decrease withthe increase of the vanadium content. Such current relaxationtimes can be qualitatively used to understand the reactionkinetics in this class of materials. On the basis of the current−time response, the two-phase behavior is observed to decreasewith the increase in the vanadium concentration (Figure 8).The diffusion-like behavior and continuous variation in the

voltage curve observed in the 10% vanadium material areindicative of a single-phase solid solution. Our XRD results,however, show that these partially delithiated samplesdemonstrate the coexistence of two phases, but with a narrowlattice mismatch between them. The current response in PITTis, therefore, very closely related to the mismatch of theheterosite and triphylite phases.To understand the effect of vanadium substitution on the

LiFePO4 phase diagram, we performed an in situ high-temperature synchrotron XRD. The thermal behavior ofpartially electrochemically delithiated samples of y = 0.0,0.025, and 0.05 at x = 0.5, 0.4, 0.35, and 0.3, respectively, wereinvestigated. From our XRD data in Figure 9a, no matter thecontent of vanadium in the structure, at room temperature,these partially delithiated samples existed as a mixture of twophases, triphylite and the heterosite. The mismatch between thetwo phases at room temperature, however, decreases with theincrease in vanadium solubility into the structure, as alreadydiscussed. The diffraction peaks between 3.5−4°, 6−6.5°, and7−7.7°, are characteristic of the two-phase nature. Li0.5FePO4was used as a baseline case. At temperatures below 150 °C, noobservable transformation is found in the diffraction pattern, asshown in the inset in Figure 9a; the observable transformationstarts at a considerably high temperature above 200 °C. Asignificant solid solution phase is observed at around 250 °C,where a single unresolved peak is observed, as evident in the200 reflection. The complete transition to the single solidsolution phase is not complete till temperatures above 300 °C.Introducing the vanadium into the olivine structure tends to

change the solid solution formation temperature. When as littleas 0.025 mol of vanadium is introduced into the olivinestructure, the transformation temperature is slightly lowered,and at 150 °C, a small change can be observed in the 200 peak,as shown by the magnified 200 peaks (Figure 9b, inset).Compared to LiFePO4, a considerable solid solution formationis observed at 200 °C, and on further heating the sample, acomplete single-phase solid solution is formed at a temperaturemuch below 300 °C. When the concentration of the vanadiumin the LiFePO4 structure is further increased to 0.05 mol(Figure 9c), the solid solution formation temperature islowered and is observed at temperatures below 100 °C and aconsiderable solid solution is observed at 150 °C. Attemperatures below 250 °C, a complete solid solution isformed, which is persistent up to temperatures around 500 °C.This shows that, upon increasing the vanadium concentrationin the olivine structure, the solid solution formation temper-ature is systematically lowered, allowing the system to accessthe single-phase solid solution at fairly low temperatures ascompared to the unsubstituted LiFePO4.The electrochemical properties based on galvanostatic charge

discharge for the different vanadium substitution levels areshown in Figure 10. For all the curves between 2.0 and 4.3 V,we only observe a single plateau at around 3.45 V, which ischaracteristic of the LiFePO4. There is no additional redoxplateau from the vanadium substitution. At low currentdensities, there is no significant difference between the chargeand the discharge polarization in the vanadium-substitutedsamples and unsubstituted LiFePO4. Also, at the lowest currentdensities, we observe a decrease in the electrochemical capacitywith the increase of the vanadium content in the olivinestructure; this is consistent with the lower lithium contentcaused by the occupancy of a fraction of the Li sites bytransition metals, and the lithium vacancies needed for the

Figure 8. PITT graphs of the 0.0, 0 5, and 10 mol % vanadium-substituted LiFePO4; the blue curve shows a voltage step of 5 mV andthe corresponding current relaxation time in red.



transition-metal charge compensation. On increasing thedischarge current densities from low rates, 0.1 C, to highrates, 10 C, an increase in polarization is observed as expected.At a 10 C current density, the unsubstituted LiFePO4 has thehighest polarization and the polarization decreases with theincrease of the vanadium content. There is thus a much less

capacity drop in the highly substituted vanadium electrodes, asthe current is increased. The kinetics of reaction are muchimproved by vanadium substitution, but the overall capacity isreduced, as clearly shown in Figure 10a.The reaction kinetics dependent on the vanadium concen-

tration in the olivine structure were further evaluated usingcyclic voltammetry (CV). Figure 11 shows the CV curves forthe vanadium-substituted LiFePO4 over a range of scan rates.The CV profiles were normalized with the active mass of eachelectrode for comparison of the effect of substitution on theelectrochemical properties. The CV curves show only onedistinct anodic and cathodic peak at all scan rates within the2.2−4.3 V range. At slower scan rates, 0.02 or 0.05 mV/s, thepeak current is observed to decrease with the increase in thevanadium substitution level. The peak positions show only avery small shift, if any, at such slow scan rates, as shown inFigure 11. This suggests good electrochemical performance ofLiFePO4 at such a slow scan rate. As the scan rate increases,first, the current peak intensities, which are smaller for the 5and 10 mol % of vanadium-substituted samples, significantlygrow relative to the low vanadium substitution with theincrease in the scan rate. For example, at 0.1 C, the y = 0 and0.05 have similar current densities and, at an even much higherscan rate, 1 or 5 mV/s, the peak current of the y = 0.05 is higherthan y = 0. The 0.1 mol % vanadium-substituted sample has thesmallest current peak; however, at a higher scan rate, it isobserved to significantly increase relative to the unsubstitutedand 0.05 mol % vanadium-substituted one. This shows that thekinetics of the higher vanadium-substituted LiFePO4 is superiorto the low-level substituted and unsubstituted LiFePO4.Second, the peak separation between the anodic and thecathodic peaks is generally similar at slow scan rates, but as thescan rate increases, the 10 mol % vanadium-substituted samplehas the least peak separation, followed by the 5%, whereas the

Figure 9. In situ XRD patterns of (a) LiFePO4, (a) Li0.925−xFe0.025-[Fe0.975V0.025]PO4, x = 0.4, and (c) Li0.85−xV0.05FePO4Li0.85−xFe0.05-[Fe0.95V0.05]PO4, x = 0.35, collected under a He atmosphere; therectangular boxes show peaks affected by solid solution formation. Theinsets show the 200 peaks at 25, 100, and 150 °C (λ = 0.3196 Å).

Figure 10. Galvanostatic charge−discharge curves of Li1−3yFey-[Fe1−yVy]PO4 discharged at different rates: (a) 0.1 C and (b) 10 C.The inset shows the OCV of the different samples.



unsubstituted LiFePO4 has the highest peak separation. Thesepeak potential shifts are attributed to polarization as a result ofthe kinetic limitations. Our results are consistent with the lowerpolarization observed in galvanostatic charge discharge curvesat higher current densities for increasing the vanadiumconcentration in the structure. At a 5 mV/s scan rate, theunsubstituted LiFePO4 shows the least electrochemical activityas compared to other vanadium-substituted counterparts at thesame scan rate due to the shift of its redox peaks and veryreduced peak intensities.The rate performance of the vanadium-substituted LiFePO4

was also evaluated. The cells were charged at the same currentdensities to 4.3 V and then discharged at different currentdensities from 0.2 to 10 C; the results are shown in Figure 12.In the comparative analysis of the systematic variation ofvanadium concentration level in the structure, we observe that,at the lower rates of 0.1 and 0.2 C, the unsubstituted LiFePO4

shows the highest electrochemical capacity. The substitutedsamples exhibit inferior capacities at such low rates, as expectedbecause of their lower lithium content; this was also noted byMeethong et al.7 for Mg and Zr substitution. At higher currentdensities, the unsubstituted LiFePO4 loses much more of itscapacity to such an extent that, at 1 C, the capacities of the2.5% and 5% vanadium-substituted samples outperform theLiFePO4. The capacity drop as the rate increases is much lowerin the vanadium-substituted samples, similarly for Mg and Zrsubstitution.7 The higher the vanadium content in the olivinestructure, the better the rate performance. If the capacities ofthe various systems are normalized with the achievable capacityat 0.1 C for each system, then, at higher current densities, 10 C,the LiFePO4, 2.5, 5, and 10 mol % vanadium-substitutedcompounds have a capacity relative to the 0.1 C value ofaround, 40, 55, 60, and 70%, respectively. This shows that thecapacity drop decreases with the increase in the vanadiumcontent. The faster kinetics upon vanadium substitution isconsistent with the CV curve results. The improved rates onsubstitution can be attributed to the reduced lattice mismatchbetween the Li-rich and the Li-poor phases, which decreaseswith the increase of vanadium content in the structure and alsothe ease of accessibility of the solid solution at lowertemperatures as compared to LiFePO4, as already discussed.It is clear from the high rates achieved in these metal-rich

materials that the iron atoms at the lithium sites are notblocking the lithium ions from fast diffusion. This implies thatmost of the diffusion pathways, tunnels, must remain free of Featoms, and that these iron atoms are probably clustered in thestructure, forming nanodomains of an iron-rich phase, such assarcopside, Fe3(PO4)2. The high rates in these materials may beassociated with these nanodomains introducing disorder intothe structure, thereby reducing the overpotential required forsingle-phase formation, or the nucleation energy to form thesecond phase. Studies are underway to determine the exactnature of this phase.23 It is likely that, whenever the substituention does not form a sarcopside phase, then that ion goes to theFe site, displacing Fe to the sarcopside phase.

Figure 11. Normalized CV curves for different concentrations of vanadium in Li1−3yFey[Fe1−yVy]PO4 at 0.02, 0.1, 1, and 5 mV/s scan rates.

Figure 12. Rate perfomance as a function of vanadium concentrationin the olivine structure, Li1−3yFey[Fe1−yVy]PO4, y = 0, 0.025, 0.05, and0.1 mol of vanadium.



■ CONCLUSION

We have systematically and unambigously demonstratedcrystallographic evidence for higher aliovalent substitution inLiFePO4 and its effect on the electrochemical properties.Unlike previous reports on aliovalent substitution at the Li site,our combined XRD and NPD analysis shows the distribution ofaliovalent V to the Fe site with some Fe occupying the Li site.These Fe atoms on the Li site are clustered in microdomains ofa sarcopside-like phase, leaving the Li tunnels free for fast Li-iondiffusion. The overal effect leads to an increased Li solubilityrange accompanied by a decrease in the unit cell volume changebetween the heterosite and the triphylite phases, which mayfacilitate the Li-ion mobility. Such substitution lowers the solidsolution temperature as a function of the concentration ofsubstituted transition-metal ions in the structure. The reactionkinetics are also improved with substitution; however, thiscomes at the expense of lowering the electrochemical capacity.

■ AUTHOR INFORMATION

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We thank Shirley Meng for the initial suggestion of ironclustering in LiFePO4. This research is supported as part of theNortheastern Center for Chemical Energy Storage, an EnergyFrontier Research Center funded by the U.S. Department ofEnergy, Office of Science, Basic Energy Sciences, under AwardNumber DE-SC0001294. Use of the Advanced Photon Sourceat Argonne National Laboratory and the National SynchrotronLight Source at Brookhaven National Laboratory is supportedby the U.S. Department of Energy, Office of Science, BasicEnergy Sciences, under Contract Nos. DE-AC02-06CH11357and DE-AC02-98CH10886, respectively. We also acknowledgesupport from Oak Ridge National Laboratory’s SpallationNeutron Source sponsored by the Scientific User FacilitiesDivision, Office of Basic Energy Sciences, U.S. Department ofEnergy, and the National Institute of Standards andTechnology, U.S. Department of Commerce, in providing theneutron research facilities.

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