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Materials Chemistry and Physics 149-150 (2015) 209e215

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Hydrogen reduction of vanadium in vanadium-doped LiMnPO4

D.G. Kellerman a, b, *, Yu.G. Chukalkin c, N.I. Medvedeva a, M.V. Kuznetsov a,N.A. Mukhina d, A.S. Semenova a, V.S. Gorshkov d

a Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 620990 Ekaterinburg, Russiab Ural Federal University, 620002 Ekaterinburg, Russiac Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, 620990 Ekaterinburg, Russiad Eliont LLC, 620137 Ekaterinburg, Russia

h i g h l i g h t s

* Corresponding author. Institute of Solid State CheEkaterinburg 620990, Russia.

E-mail address: kellerman@ihim.uran.ru (D.G. Kell

http://dx.doi.org/10.1016/j.matchemphys.2014.10.0080254-0584/© 2014 Elsevier B.V. All rights reserved.

g r a p h i c a l a b s t r a c t

� The samples of LiMnPO4 doped withvanadium were prepared.

� The samples were annealed underhydrogen atmosphere.

� The V4þ presence was proved.� The ab initio calculations revealed themost favorable position for oxygenvacancy.

� The presence of V4þ changes mag-netic properties and Raman spectraof LiMnPO4.

a r t i c l e i n f o

Article history:Received 2 June 2014Received in revised form24 September 2014Accepted 7 October 2014Available online 12 October 2014

Keywords:Inorganic compoundsRaman spectroscopy and scatteringMagnetic propertiesab initio calculationsNeutron scattering and diffractionPhotoelectron spectroscopy

a b s t r a c t

The samples of vanadium-doped LiMnPO4 have been synthesized by the solid-state reaction technique.To reduce the oxidation state of vanadium from V5þ to V4þ, the samples have been additionally annealedunder hydrogen atmosphere. The tetravalent state of vanadium was proved by analysis of V 2p X-rayphotoelectron spectra. Raman spectroscopy, first-principle studies and magnetic measurements wereused to determine the effect of V4þ on the LieMn phosphate properties.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Since the demonstration of reversible electrochemical lithiuminsertioneextraction for LiFePO4 [1], the lithium metal phospho-olivines, LiMPO4 (M ¼ Mn, Co, Ni, Fe), have been attracting more

mistry, Pervomaiskaya st., 91,

erman).

and more attention as materials for positive electrodes of high-performance rechargeable Li-ion batteries [2e6]. All of thempromise good specific capacity, structure stability, cyclability, andsafety. Moreover, LiFePO4 now supersedes even such a recognizedcathode material as LiCoO2 in many application areas. Though therest of the phospho-olivines, LiMPO4 (M ¼ Mn, Co, Ni), offer ahigher operating voltage versus Li (4.1e5.2 V) compared with thatof LiFePO4 (3.5 V), they have not yet found wide practical applica-tions and are significantly less studied (except the magnetic prop-erties [7e10]). As for LiMnPO4, its low intrinsic electronic

D.G. Kellerman et al. / Materials Chemistry and Physics 149-150 (2015) 209e215210

conductivity resulting in poor electrochemical performance [11] isthe main obstacle to the practical use. According to the recenttheoretical study [12] the poor electrochemical performance ofLiMnPO4 can be explained by the existence of vacancy-polaroncomplexes with high energy of lithium vacancy formation. It isshown that lithium vacancy and a corresponding hole-polaronform the complex owing to lattice distortion and Coulomb inter-action between them.

Some efforts were devoted to improvement of the transportproperties of LiMnPO4 by different ways, such as deposition ofcarbon coatings [13e15], or by production of small-size particles[16e18]. A traditional way to increase the conductivity is isovalentor heterovalent substitution of metallic positions. The effect of thedoping element on the properties of the matrix depends on thecrystallographic positions of the dopant. LiMnPO4 like all com-pounds of this series has orthorhombic structure with space groupPnma [19]. It is built of corner-sharing MnO6 octahedra and edge-sharing LiO6 octahedra that are linked by PO4 tetrahedra (Fig. 1).There have been numerous attempts to improve the electro-chemical performance of LiMnPO4 by partial replacement of octa-hedrally surrounded manganese ions by other ions: Mg, V, Fe, Co,Gd [20]; Cu [21]; Zn [22]; Ti, Mg, Zr [23]; Co [24]; Fe [25,26]; Ni[27,28], etc. The authors of Ref. [29] managed to replace a part ofoctahedral lithium by sodium. Some dopants, for example vana-dium, can occupy the tetrahedral positions in the LiMnPO4 struc-ture too. The arrangement of vanadium was investigated using anumber of diffraction and spectral methods [30e32], which, likethe ab initio calculations, clearly indicated that the vanadium atomsin the vanadium-substituted LiMnPO4 occupy the phosphorus sites.According to Ref. [31], this increases the grain conductivity of the V-substituted LiMnPO4 by 1e2 orders of magnitude. It is possible toassume that the change in the vanadium state from pentavalent totetravalent one could promote additional growth of conductivitydue to the increase in the charge carriers concentration. The pur-pose of this work is to verify whether tetravalent vanadium can bepresent in the V-substituted LiMnPO4.

2. Experimental details

The samples of pure and vanadium-doped LiMnPO4 were pre-pared by the solid-state reaction technique using the powder re-agents Li2CO3, Mn2P2O7, MnO and NH4VO3 as starting materials.The appropriate amounts of the carefully dried reagents weremixed andmilled until the average particle size was less than 1 mm.The samples were compacted and thermally treated two timesunder pure argon flow for 6 h at 820 �C. The X-ray diffraction datawere collected at room temperature using a transmission STADI-P

Fig. 1. Fragment of the LiMnPO4 structure (a); antiferromagnetically ordered zigzag layers fothe c-axe (c).

(STOE) diffractometer equipped with a scintillation detector. Poly-crystalline silicon (a¼ 5.43075(5) Å) was used as internal standard.The lattice parameters were refined with the full-profile Rietveldanalysis using the FULLPROF software package. The phase purity ofthe samples was proved by comparing their XRD patterns withthose in the PDF2 database (ICDD, USA, release 2009).

Neutron diffraction experiments were carried out on D2(wavelength l ¼ 1.805 Å and D3 l ¼ 2.432 Å) neutron diffractom-eters (IWW-2M reactor, Zarechny).

The magnetization curves were collected by means of avibrating sample magnetometer VSM-5T (Cryogenic Ltd.) in thetemperature interval from 2 to 300 K with magnetic field rangingbetween 0 and 50,000 Oe.

The Raman spectra were collected with a Renishaw-1000spectrometer. The excitation wavelength of an Arþ ion laser was514.5 nm. The laser power at the focus was equal to 25 mW.

Thermal analysis measurements were carried out with asimultaneous TG-DTA apparatus SETARAM SETSYS Evolution 1750.

X-ray photoelectron spectroscopy (XPS) was applied to studythe surface composition and chemical states of ions in V-dopedLiMnPO4. The XPS measurements were performed using an ESCA-LAB II spectrometer (VG, UK) with nonmonochromatedMg Ka x-raysource (hv ¼ 1253.6 eV) operating at 250 W. The working pressurewas less than 7 � 10�8 Pa. The survey spectra were collected at100 eV pass energy, 0.5 eV$step�1, and 1 s$step�1. The detailedspectra were recorded for O1s and V2p at 20 eV pass energy,0.1 eV$step�1, 1 s$step�1. XPSpeak4 software was used for spectraanalysis.

The electronic structure calculations were carried out in thiswork using the Vienna ab initio simulation package (VASP) [33,34],and the projector augmented wave pseudopotentials with thegeneralized-gradient approximation (GGA) for exchan-geecorrelation functional according to PerdeweBurkeeErnzerhof(PBE) [35] were employed. Previously [31,36,37], the GGA þ Uapproach was found to provide a more proper description of thelocalized character of 3d electrons and the band gap in olivine thanthe LDA or GGA methods. In the present study, we used theGGAþ U formalism, inwhich two parameters: the on-site Coulombinteraction U and the exchange parameter J were included to cor-rect the self-interaction energy.We choose U¼ 2 eV and J¼ 1 eV forMn in accordance with the previous calculations of manganeseoxides [38e40]. To study the V3d-3d orbital correlations, whichmay have a considerable effect on the electronic properties, asshown for vanadium oxides [41e43], we tested a set of U correc-tions (0, 2 and 4 eV). The energy cutoff of 450 eV was used toexpand the valence electron orbitals in plane waves. The k-pointssampling was performed within the Monkhorst-Pack scheme [44]

rmed by Mn2þ (b); ferromagnetic Mn2þeO2�eP(V4þ)eO12�eMn2þ spin exchange along

Fig. 3. Unit cell parameters for vanadium-substituted LiMnPO4: as-prepared - tri-angles and reduced in hydrogen e circles.

D.G. Kellerman et al. / Materials Chemistry and Physics 149-150 (2015) 209e215 211

for 6� 6� 6 k-point meshes in the Brillouin zone (BZ) and with theuse of the tetrahedron method with Bl€ohl corrections. We used a28-atomic supercell (Li4Mn4P4O16) containing 4 formula units ofLiMnPO4 to study the oxygen vacancies (Li4Mn4P3VO15) as well asvanadium substitution in different atomic sites (Li4Mn4P3VO16,Li4Mn3VP4O16 and Li3VMn4P4O16) at 6.25% and 25% concentrations,respectively. The experimental lattice parameters were taken for allthe calculations, whereas the atomic positions were relaxed byminimizing the HellmaneFeynman forces on all atoms, which wereless than 0.1 eV/Å for equilibrium structures.

3. Results and discussion

Powder X-ray diffraction (XRD) analysis showed that the syn-thesized samples of LiMn(PO4)1-x(VO4)x with 0 � x � 0.15 weresingle phases with orthorhombic structure of space group Pnma(No. 62), the same as that of LiMnPO4 [19]. The diffraction pattern ofpure LiMnPO4 (V-0) as well as of the sample with the highest va-nadium content (V-0.15) are presented in Fig. 2. The lattice pa-rameters (Fig. 3) of the solid solutions, in which pentavalentvanadium replaces phosphorus in tetrahedral positions, increasewith the dopant concentration due to the larger ion radius of V5þ ascompared with P5þ. Similar changes in the unit cell dimensions aredescribed in Ref. [31].

To obtain tetravalent vanadium in V-doped LiMnPO4, the sam-ples have been additionally annealed under hydrogen atmosphereat a temperature of 820 �C for 6 h. After this treatment the structureof the LiMn(PO4)1-x(VO4)x solid solutions with 0 � x � 0.10remained unchanged and no additional lines in the XRD patterns ofthe hydrogen treated samples in comparison with LiMnPO4 weredetected. Fig. 2 demonstrates the diffraction pattern recorded forthe composition with x ¼ 0.1 after reducing annealing (hereinafterreferred to as V-0.1H).

The results of the Rietveld refinement (Fig. 4) reveal that allvanadium ions in the hydrogen treated samples are located in the4c positions and they replace only phosphorus ions in the sameway as was found previously for the parent solid solutions [30]. Theappropriate data for V-0.15 and V-0.1H are shown in Table 1.

As can be seen from Fig. 3, the lattice parameters for the bothseries vary similarly. The lack of difference suggests that the

Fig. 2. XRD patterns of pure LiMnPO4 (V-0), V-doped LiMnPO4 (V-0.15), and V-dopedLiMnPO4 treated in the hydrogen atmosphere (V-0.1H).

hydrogen treatment does not produce tetravalent vanadium.However, this conclusion can be refuted by comparing a number ofproperties that are drastically different for the two series of thesamples. First of all, in order to determine the valence states ofvanadium, we examine the V 2p X-ray photoelectron spectra of thetwo samples with the same vanadium concentration: V-0.1 and V-0.1H. The 2p X-ray photoelectron spectra of vanadium are located inthe energy range 516e526 eV close to the intensive O 1s peak ofoxygen. The O 1s satellite governed by nonmonochromatic Mg Ka

Fig. 4. X-ray diffraction pattern (circles) of V-0.1H and its Rietveld refinement fitting(solid line) e a; Difference between observed and calculated diffraction patterns e b;Data of crystalline LiMnPO4 with orthorhombic structure from ICDD card 77-178 e c.

Table 1Refined structural parameters (atomic coordinates, isotropic DebyeeWaller factors,and occupancy factors) determined in combined Rietveld refinement of the neutronand X-ray data for V-0.15 and V-0.1H samples.

Atom Wyckoff site Atom positions B Occupation

x y z

V-0.15; c2 ¼ 5.58, Rb ¼ 4.80%, Rwp ¼ 5.71%.Li 4a 0 0 0 5.53(44) 1.000(0)Mn 4c1 0.2806(6) 0.2500(0) 0.9722(16) 0.82(14) 1.000(10)P 4c2 0.0924(5) 0.2500(0) 0.4066(11) 1.41(12) 0.845(7)V 4c2 0.0924(5) 0.2500(0) 0.4066(11) 1.41(12) 0.155(7)O1 4c3 0.0959(4) 0.2500(0) 0.7371(9) 1.74(9) 1.003(8)O2 4c4 0.4533(4) 0.2500(0) 0.2156(10) 1.75(10) 0.999(7)O3 8d 0.1624(3) 0.0425(4) 0.2768(7) 1.55(6) 1.993(16)V-0.1H; c2 ¼ 5.61, Rb ¼ 4.85%, Rwp ¼ 5.73%.Li 4a 0 0 0 5.55(44) 1.000(0)Mn 4c1 0.2809(8) 0.2500(0) 0.9816(16) 0.83(15) 1.000(10)P 4c2 0.0920(6) 0.2500(0) 0.4039(14) 1.39(12) 0.889(8)V 4c2 0.0920(6) 0.2500(0) 0.4039(16) 1.39(12) 0.111(8)O1 4c3 0.0980(6) 0.2500(0) 0.7344(10) 1.78(9) 1.001(8)O2 4c4 0.4517(6) 0.2500(0) 0.2138(10) 1.78(10) 0.999(7)O3 8d 0.1613(4) 0.0440(6) 0.2777(7) 1.49(6) 1.973(18)

Fig. 5. X-ray photoelectron spectra (XPS) V2p3/2 of the V-doped LiMnPO4.

Fig. 6. Raman scattering for pure and vanadium-substituted LiMnPO4.

D.G. Kellerman et al. / Materials Chemistry and Physics 149-150 (2015) 209e215212

X-ray source is superimposed on the low-intensity vanadiumspectrum and complicates its analysis. However, we managed tosingle out the V p3/2 signals that were not damaged by the O 1ssatellite for the both samples (Fig. 5). The obtained data indicatethat V-0.1 contains pentavalent vanadium only when vanadium inV-0.1H is largely tetravalent. It should be noted that the values ofthe peaks corresponding to different vanadium states in V-0.1H donot reflect the true ratio of V4þ and V5þ in V-0.1H, since the XPSgives the information about the surface of the sample to a depth ofz3 nm where the highest valence of the metal always dominatesdue to easy oxidation in air. Nevertheless, the presence of a largeamount of V4þ in the hydrogen treated samples is proven. Heatingof the hydrogen treated samples in air should lead to oxidation oftetravalent vanadium. Indeed, according to the results of ther-mogravimetric analysis, the increase in mass after heating of V-0.05H and V-0.1H up to 700 �C in air was 0.18% and 0.39%, respec-tively. This means that the average oxidation state of vanadium inthese samples is þ4.2. This confirms the results of the XPS analysis.It should be noted that reduced oxidation state, V4þ, being in thevanadate group, was detected in apatites, Ca10(PO4)6-x(VO4)x(OH)2,treated with hydrogen at high temperatures [45].

Some useful information about the tetrahedral (PO4)3� or(VO4)3� groups in the phospho-olivines can be obtained byanalyzing the Raman scattering. Fig. 6 shows the spectra of undo-ped LiMnPO4 (V-0), as well as those of the samples containing va-nadiume V-0.1 and V-0.1H. Many authors discussing the vibrationalspectra (Raman and IR) of phosphates consider the vibrations oftetrahedral groups (PO4)3� and the external lattice vibrationsseparately [46e48]. This approach is based on the fact that thechemical bond between oxygen and phosphorus is much strongerthan that between other ions. As a result, the oscillation frequencyof the phosphate groups is practically identical in different com-pounds and does not differ noticeably from that of the free tetra-hedral AB4 molecule [49].

The V-0 spectrum displayed in Fig. 6 shows all typical features ofthe olivine spectrum described in the literature [47]. The spectrumcan be clearly divided into two areas: the frequencies above400 cm�1 correspond to internal modes, while those below400 cm�1 reflect external (lattice) oscillations. The singlet A1g atn1 ¼948 cm�1 is the strongest line among the internal vibrations; itcorresponds to the totally symmetric stretching vibration of thetetrahedral anion (PO4)3�. The band at n2 ¼ 4375 cm�1 refers to thebending vibration and the bands of 587 and 623 cm�1 are from the

n4 mode. Both high-frequency lines at 1066 cm�1 and 1005 cm�1

are due to the antisymmetric stretching vibrations n3. Translationalvibrations of MnO6 and LiO6 octahedra are observed in the lowwavenumber region (below 300 cm�1).

As vanadium occupies phosphorus positions in the structure ofV-doped LiMnPO4, one should expect some additional lines relatedto the (VO4)3� tetrahedral groups in the Raman spectrum for V-0.1.Indeed, there are two sets of vibrations in the frequency ranges750e950 cm�1 and 350e400 cm�1. They reflect mainly a stretchingand a bending modes of the (VO4)3� tetrahedron respectively [50].In the Raman spectrum of the V-0.1H sample, the peaks reflectingthe vibrations of the (VO4)3� tetrahedral groups become practicallyundetectable, and only a very broad signal centered atz 800 cm�1

is seen. It can be assumed that the disappearance of a well-resolvedspectrum of tetrahedrally surrounded vanadium is caused by somelocal structure distortions, which in turn are due to the appearanceof oxygen vacancies and to a change in the oxidation state of va-nadium. The observed broad signal of low intensity apparentlyreflects the presence of a certain amount of V5þ in the V-0.1Hsample, and it does not contradict the XPS results (Fig. 5). Toconfirm this conclusion, further experimental and theoretical

D.G. Kellerman et al. / Materials Chemistry and Physics 149-150 (2015) 209e215 213

studies are required. It should be noted that the position of themain lines relating to the LiMnPO4 matrix for the both samples (V-0.1, V-01H) remain practically unchanged. This proves that invanadium-substituted LiMnPO4 tetravalent vanadium, as well aspentavalent vanadium replace phosphorus rather than the transi-tion element, as it occurs for example in LiFePO4 [51].

3.1. First-principle calculations

Our calculations demonstrated that the band gap Eg in LiMnPO4is formed by splitting of the spin-up and spin-down Mn 3d states,and Eg is sensitive to the on-site Coulomb interaction on Mn atoms[32]. For the optimized structures, the GGA calculations predict theband gap of 2.4 eV, whereas the GGA þ U approach with U ¼ 2 eVand J¼ 1 eV increases Eg up to 3.0 eV (Fig.1a). The spin-up t2g and egstates are almost filled, whereas the spin-down t2g and eg states arecompletely empty (Fig. 7b). The manganese atoms have a high-spinstate with a magnetic moment of 4.5 mB, which does not depend onthe presence of Hubbard U.

We have calculated the total energies of LiMnPO3.75 for oxygenvacancy in the O1, O2 and O3 sites and found that the O3 site is themost favorable position with an energy gain of 0.29 eV and 0.10 eV,respectively, as compared with vacancies in the O1 and O2 sites.This is in good agreement both with the results of our structurerefinement clearly indicating the reduction of the occupancy factorfor this site (Table 1) and with the data reported in Ref. [52].

In the previous works [30e32], the V atom was shown to sub-stitute for P rather than for Mn, although the atomic radii of V(1.35 Å) and Mn (1.26 Å) are similar, while phosphorus has a muchsmaller radius (1.10 Å). To determine which site (Mn or P) isfavorable for V substitution, we compared the formation energiesof Li(Mn,V)PO4 and LiMn(P,V)O4 relative to those of LiMnPO4. Forthis purpose, we calculated DEform(V/ X)¼ Etot[LiMnPO4]� Etot[Li(Mn,P,V)O4)] þ E(V) � E(X), where E(V) and E(X) are the total en-ergies of V and X (X ¼ Mn, P) elemental phases in their groundstates. We found that in stoichiometric LiMnPO4, vanadium prefersto replace phosphorus, but not manganese or lithium atoms withenergy preference of 0.34 eV. Similar calculations were performedfor V-doped LiMnPO3.75 with oxygen vacancy in the O3 site. Herewe are restricted to the configuration when the oxygen vacancy isclose to the V atom. In this case, the V / P substitution remainsmore preferable as compared to V/Mn substitutions with 0.45 eVfor LiMnPO3.75. As is seen, oxygen deficiency does not affect thepreferable site for vanadium in olivine.

In LiMn(PO4)0.75(VO4)0.25, the PeO distances do not change, theLieO and MneO distances are increased and the MnO6 octahedraare distorted stronger than in LiMnPO4. The optimized VeO dis-tances in the VO4 tetrahedron are longer (1.74 Å) than the PeOdistances (1.55 Å) in the PO4 group and are close to the typical VeOdistances in vanadium oxide compounds. The oxygen vacancy in

Fig. 7. Total densities of states (a) and partial densities of Mn3d

the most favorable O3 site in vanadium-doped olivine, LiMn(-PO4)0.75(VO3)0.25, does not affect the PeO distances, whereas itresults in the shortening of the LieO and MneO distancescompared with those in the stoichiometric LiMn(PO4)0.75(VO4)0.25.

Our GGA þ U calculations demonstrate that the band gap is notvery sensitive to the on-site Coulomb interaction on V atoms. Wefound that the band gap increases from 0.93 eV (U ¼ 0) to 1.08 eV(U ¼ 2 eV) and to 1.25 eV (U ¼ 4 eV) in LiMn(PO4)0.75(VO4)0.25.Fig. 8b shows the density of states for typical U ¼ 4 eV, which wasestablished to be the most appropriate value for prediction of theband structure of vanadium oxides [41e43]. As is seen, thecontribution of occupied V3d states is very small and the top of thevalence band in the V-doped compound is determined by theMn3dstates as in undoped LiMnPO4. The bottom of the conduction bandis composed of the V3d states, which are shifted to lower energiesas compared with the empty Mn3d states and the value of Eg isdetermined by the position of the unoccupied V3d states, see Fig. 8.As a result, the V doping strongly decreases Eg which becomesdependent on the U parameter for V atom. A strong decrease in theband gap with vanadium doping is supported by the observed in-crease in conductivity by 1e2 orders of magnitudes in vanadiumdoped samples [30]. The small contributions of the V3d states to theMn3d states in the valence band and of theMn3d states in the rangeof empty V3d states point to weak MneV interaction.

For V-doped olivine with oxygen vacancy, LiMn(-PO4)0.75(VO3)0.25, the GGAþU calculations show greater occupancyof the V3d states and stronger admixing of theMn3d states. We findalso that the distance between the Mn and V atoms decreases from2.85 Å to 2.74 Å. The DOS at the bottom of the valence band isdominated by both the V3d andMn3d states, whereas the top of theconduction band is composedmostly of the V3d states. As is seen inFig. 9, the value of the band gap does not depend on the appearanceof O3 vacancy in the V-doped olivine.

Magnetic interactions depend only slightly on the Hubbard U.Within both GGA and GGA þ U schemes, V is nonmagnetic instoichiometric olivine, but has a magnetic moment of 0.22 mB inoxygen-deficit LiMn(PO4)0.75(VO3)0.25. The magnetic moment onthe Mn atom is about 4.5 mB and it depends little either on UV andUMn or on V-doping or oxygen deficit.

We performed a charge-partition analysis using Bader's “atomsin molecules” theory [53] to assign charges to atoms. The Baderanalysis indicates that the charges of the Li and P atoms in LiMn(P1-yVy)O4-x are þ1 and þ5 (that corresponds strictly to their formalvalences) for all the considered ranges of x and y (x ¼ 0, 0.25; y ¼ 0,0.25), and both vanadium substitution and oxygen vacancy in O3site do not affect the charge states of Li and P ions. The charges ontheMn and O atoms in LiMnPO4 and LiMnPO3.75 areþ1.6 e and�1.9e, respectively, whereas the V/P substitution increases the elec-tron depletion from the O2p states, and the charges of O1 and O3atoms in VO4 decrease down to �1.4 e. The vanadium valence

states (b) in LiMnPO4 calculated within GGA þ U approach.

Fig. 8. Total densities of states (TDOS) (a) and the partial densities of Mn3d and V3d states in LiMn(PO4)0.75(VO4)0.25 (b).

Fig. 9. Total densities of states (TDOS) (a) and the partial densities of Mn3d and V3d states in LiMn(PO4)0.75(VO3)0.25 (b).

Fig. 10. DC magnetization versus temperature curves (FC and ZFC) for pure LiMnPO4

(V-0), as-prepared LiMnPO4 with 10% of vanadium (V-0.1) and for hydrogen treatedLiMnPO4 with 10% of vanadium (V-0.1H), under an applied field of 400 Oe: insert:magnetic susceptibility versus temperature for pure LiMnPO4 (V-0) under an appliedfield of 50000 Oe.

D.G. Kellerman et al. / Materials Chemistry and Physics 149-150 (2015) 209e215214

states are partly occupied due to the covalent V3d-O2p bonding,and the charge of V atom (þ2.78 e) differs from its formal valenceof þ5. In LiMn(PO4)0.75(VO3)0.25, where the O3 site near the V atomis vacant, the V atom becomes less positively charged (þ1.95 e) thatcan be related to the formal change from V5þ to V4þ. We also findthat the Mn atom near the O3 vacancy in the V-doped olivine getsadditional electrons that testify to the mixed valence of Mn. Thesignificant charge redistribution is also confirmed by the densitiesof states for LiMn(PO4)0.75(VO4)0.25 and LiMn(PO4)0.75(VO3)0.25(Figs. 8 and 9). The integration of projected DOSs for the V3d or-bitals within the range from�2 eV to 0 (EF) provides 0.26 e and 1.3 efor LiMn(PO4)0.75(VO4)0.25 and LiMn(PO4)0.75(VO3)0.25, respectively,which corresponds to a transition from V5þ to V4þ. Thus, electronsreleased by oxygen vacancy are not localized completely at thevacancy and partly transfer to the V andMn atoms. This descriptionagrees with the scheme of improved conductivity [31], where theformation of mixed valence Mn states was suggested to open apathway for electron hopping conductivity. Besides, the transfer ofelectrons and oxygen vacancies creation in the hydrogen treated V-doped LiMnPO4 can change the disorder in the Mn sublattice. Thelatter affect the mobility of the Li in LiMnPO4, and therefore theperformance of this phosphate as a cathode material. It should beemphasized that the authors of [54] from 7Li and 31P NMR obtainedexperimental evidence that the movement of Li within LiMnPO4 isreally significantly coupled to lattice distortions.

Magnetic properties are a good illustration of the presence oftetravalent vanadium in the hydrogen treated V-doped LiMnPO4.Like other members of the olivine family with Pnma symmetrygroup, LiMnPO4 undergoes long-range AFM transitions (TN z 34 K[55]). The local magnetic moments (Mn2þ; S ¼ 5/2) in a collinearmagnetic structure are oriented along the a-axe [56], Fig. 1. Anti-ferromagnetically ordered zigzag layers formed by manganese ionsare shown in Fig. 1b. According to powder neutron data, the mag-netic structure of V-doped LiMnPO4 is the same as for pureLiMnPO4 [30]. The similarity of the magnetic structures is well

understood since pentavalent vanadium in the phosphoric sub-lattice hardly perturbs the indirect Mn2þeO2�Mn2þ exchangeresponsible for magnetic ordering.

The susceptibility (c) of LiMnPO4 as well as of V-substitutedsamples can be well described with the CurieeWeiss law onlyabove 70 K.When temperature is lowered, the susceptibility clearlyshows a peak characteristic of antiferromagnetic ordering (Fig. 10,insert for V-0). A distinctive feature of the magnetic ordering in

D.G. Kellerman et al. / Materials Chemistry and Physics 149-150 (2015) 209e215 215

LiMnPO4 described in Ref. [30] is a divergence between magneti-zations measured under zero field cooled (ZFC) conditions andthosemeasured under field cooled (FC) conditions. This fact, as wellas the abrupt increase in FC magnetization below ~40 K shown inFig. 10, are indicative of the presence of a ferromagnetic compo-nent, which is often typical of systems with weak ferromagnetism.As is evident from Fig. 10, the temperature dependences of themagnetization obtained for both vanadium-substituted LiMnPO4demonstrate the above peculiarities of the magnetic behavior too.However, while for V-0 and V-0.1 compositions both the level ofmagnetization and the FC-ZFC discrepancy are almost identical, allthe above-mentioned effects for the hydrogen treated V-01H sam-ple are essentially strengthened. Detailed discussion of the effect ofvanadium on the magnetic properties of LiMnPO4 will be a subjectof our special article. In the present work, we only relate the dif-ferences between magnetizations of V-0.1 and V-0.1H samples withthe existence of tetravalent vanadium in them. In our opinion, it isV4þ that presumably leads to the observed enhancement of theferromagnetic components for V-0.1H because its partially filledorbitals get involved into ferromagnetic Mn2þeO2�eP(V4þ)eO12�eMn2þ spin exchange along the c-axe (Fig. 1).

4. Conclusions

The hydrogen treatment of V-doped LiMnPO4 keeps the initialstructure unchanged but reduces the oxidation state of vanadiumlocated on phosphorus sites. The disappearance of a well-resolvedRaman spectrum of tetrahedrally surrounded vanadium in thehydrogen treated samples is caused by local structure distortions,which in turn are due to the appearance of oxygen vacancies and toa change in the oxidation state of vanadium. The ab initio calcula-tions revealed that the O3 site is the most favorable position for thevacancy with an energy gain of 0.29 eV and 0.10 eV, respectively, ascompared with vacancies in the O1 and O2 sites. From the analysisof magnetic data it follows that observed enhancement of theferromagnetic components for hydrogen treated V-doped LiMnPO4is due to participation of partially filled orbitals in ferromagneticMn2þeO2�eP(V4þ)eO1

2�eMn2þ spin exchange along the c-axe.

Acknowledgments

The work was supported by the Russian Foundation for BasicResearch (Grant No. 13-03-00135-a) and by the Programs of the UBRAS (Nos. 12-M-23-2032, 12-P-3-1003). The Raman spectra wererecorded at the Multiple Access Center Composition of Materials atthe Institute of High-Temperature Electrochemistry of the UB RAS.

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