Non-stoichiometry in LiMn2O4 thin films by laser ablation

Solid State Ionics 138 (2001) 213–219www.elsevier.com/ locate / ssi

Non-stoichiometry in LiMn O thin films by laser ablation2 4

a,b a , b c d a* `M. Morcrette , P. Barboux , J. Perriere , T. Brousse , A. Traverse , J.P. Boilota ` ´Physique de la Matiere Condensee, Ecole Polytechnique, 91128 Palaiseau, France

b ´Groupe de Physique des Solides, Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris, Francec ´ ´Laboratoire de Genie des Materiaux, Isitem, Rue Christian Pauc, BP 90604, 44306 Nantes, France

dLURE Synchrotron Facility, Bat 209D, University Paris-Sud, 91408 Orsay, France

Received 18 May 2000; received in revised form 28 August 2000; accepted 9 October 2000

Abstract

We have demonstrated that thin films of LiMn O deposited by laser ablation are stoichiometric when deposited in an2 4

oxygen pressure of 0.2 mbar on a substrate heated at 5008C. For higher oxygen pressures, the pure spinel structure is stillobtained but non-stoichiometric. We show that the use of the unit cell volume determined by X-ray diffraction can be anaccurate tool to determine the oxidation state of manganese in thin films. These results are confirmed by nuclear reactionanalysis and X-ray absorption spectroscopy. Furthermore, we have tested the electrochemical properties of such films as acathode in lithium batteries. The mobility of lithium within the structure determined by cyclic voltammetry depends on thequality of the films and is highest for the stoichiometric films. 2001 Elsevier Science B.V. All rights reserved.

Keywords: LiMn O ; X-ray absorption; Thin films; Laser ablation; Non-stoichiometry2 4

1. Introduction deposited using radio frequency sputtering [4], elec-tron-beam evaporation and more recently by laser

In a bulk form, LiMn O has been proposed as a ablation [3,5].2 4

cathode in lithium battery. It can reversibly dein- The LiMn O compounds present a direct spinel2 4

tercalate one lithium per formula unit at high po- structure with a Fd3m space group and a lattice˚tential. This is associated with a change of the parameter of 8.247 A. Oxygen ions are placed on a

manganese oxidation state from 3.5 to 4. Thin films face centered cubic array. The lithium ions occupyof LiMn O can also find interesting applications as the tetrahedral 8a sites of the oxide network whereas2 4

cathodes in microbatteries [1,2] as well as in electro- the Mn are placed in the octahedral 16c sites.chemical ionic sensors such as selective electrodes However, because of the particular conditions offor the accurate determination of lithium concen- fabrication (high vacuum, high kinetic energy of thetration in solution [3]. Such films have been already incident species) the LiMn O thin film electrodes2 4

grown by various methods may present a large non-stoichiometry and structural defects. These defects

*Corresponding author. Tel.: 133-1-6933-4663; fax: 133-1-(cationic disorder, oxygen non-stoichiometry, etc.) as6933-3004.well as those introduced upon aging in the electro-E-mail address: [email protected] (P. Bar-

boux). chemical charge and discharge cycles strongly alter

0167-2738/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0167-2738( 00 )00796-7

214 M. Morcrette et al. / Solid State Ionics 138 (2001) 213 –219

the electrochemical properties of this material. For tion. The final target was composed of the pureexample, in the case of films deposited by sputtering, LiMn O phase as indicated by X-ray diffraction.2 4

unusual electrochemical properties have been attribu- Laser ablation was performed with a pulsed22ted to cationic disorder. Thus, a defect rocksalt Nd:YAG laser quadrupled at 266 nm (2 J cm with

structure has been proposed with Mn randomly a pulse duration of 7 ns and a repetition rate of 526distributed among all the octahedral sites (16c and Hz). The operating oxygen pressure was from 10

16d) of the oxygen network. These defects can be to 1 mbar. No post-deposition annealing or increas-also considered as an intergrowth of two types of ing of the oxygen pressure during cooling werebuilding blocks, one with the Li Mn O classical performed after deposition in order to maintain the8a 16d 4

spinel lattice and the second with the Li Mn O composition of the as-grown films. Films were8a 16c 4

framework [4,6]. deposited simultaneously onto Si(001) and poly-In the case of the laser ablation method, we have crystalline Pt substrates heated at 5008C. The silicon

observed that the composition of the films strongly substrates were only used to allow a quantitativedepends on the deposition parameters [3,7]. Films determination of the film thickness and the com-deposited at 5008C present a large lithium and position by Rutherford backscattering (RBS) andoxygen loss for oxygen pressures below 0.2 mbar nuclear reaction analysis (NRA) whereas the Ptand, on the contrary, a manganese deficiency above substrates were used for the electrochemical experi-this threshold pressure. The spinel LiMn O phase is ments.2 4

obtained as a pure phase only for oxygen pressures The average thickness of the films determined byabove 0.2 mbar, despite of the Mn loss. In this work RBS is of the order of hundred nanometers. Becausewe present a refinement of previous analysis of our of the collisions with the gas in the chamber, higherfilms in terms of composition and oxidation state of oxygen pressures lead to thinner films for identicalmanganese [3]. We also discuss the effect of the deposition times and laser focusing (Table 1).non-stoichiometry on the diffusion coefficient of By RBS, we also verified that at 5008C, there is nolithium in the films. interdiffusion between the films and the substrates

and we determined the manganese content in thefilms. By the complementary use of nuclear reaction

2. Experimental analysis (NRA), the lithium and oxygen contentcould also be determined for the whole range of

Films have been deposited from a stoichiometric oxygen pressures. They were obtained with the7 4LiMn O target. The target was prepared by mixing Li(p,a) He reaction at the energy of 1.5 MeV for2 4

16 17Li CO and MnO powders in stoichiometric lithium and the O(d,p) O reaction at 850 keV for2 3 2

amounts. The powders were reacted at 9008C for oxygen with precise references of these elements.24 h, then pressed into a pellet and sintered at The phase composition and the lattice parameters12508C for 2 h in order to prevent lithium evapora- have been determined by X-ray diffraction on a

Table 1Unit cell parameter, oxidation state deduced from the composition Li Mn O obtained by RBS and NRA, as well as oxidation state from thex y 4

analysis of the X-ray absorption data

Deposition Film Lattice Composition from Oxidation state Oxidation state˚conditions thickness (nm) parameter (A) NRA and RBS by RBS by XAS

a0.1 mbar 405 8.240 Li 3.4 –0.84

0.2 mbar 330 8.240 Li Mn O 3.560.1 3.50.95 2 4

0.3 mbar 250 8.230 Li Mn O 3.560.1 3.551.02 1.95 4

0.4 mbar 95 8.215 Li Mn O 3.860.1 3.651.09 1.82 4

Target – 8.247 Li Mn O 3.560.03 3.5 (assumed)1 2 4

a Multiphase system.

M. Morcrette et al. / Solid State Ionics 138 (2001) 213 –219 215

Philips X-Pert diffractometer. The cell parametershave been obtained by a linear fit of the peakpositions (Table 1). The films are polycrystallinewhen deposited onto Si or Pt. A precise structuredetermination with Rietvelt refinement is difficultbecause of the low signal /noise ratio and a smalleffect of preferred orientation along the (111) direc-tion.

The X-ray absorption studies were performed atthe LURE synchrotron facility. Experiments werecarried out at the Mn K-edge in the electron de-tection mode using a Si(311) double crystal mono-chromator. The X-ray absorption near edge structure(XANES) spectra were recorded, at room tempera- Fig. 1. Oxygen and lithium composition as a function of theture, in the energy range 6520–6640 eV per step of oxygen pressure after deposition at 5008C as determined by0.3 eV. Energies were calibrated using a Mn foil, nuclear reaction analysis.

LiMn O and l-MnO powders. l-MnO has been2 4 2 2

obtained by treating LiMn O with 1 M of HCl for 22 4

days following the method described by Hunter [8].The oxidation state of manganese for these twodifferent powders have been determined by theiodometric redox titration method and is 3.5 and 4.0,respectively, for LiMn O and l-MnO . The back-2 4 2

ground was removed by fitting the pre-edge absorp-tion with a linear function. Spectra were normalizedin intensity at the inflexion point of the first oscilla-tion after the maximum absorption peak.

Films were electrochemically characterized bycycling in the 3.5–4.5 V region versus a metalliclithium electrode used both as reference and counterelectrode. The lithium electrode and the film wereseparated by a glass fiber filter wetted with LiPF 16

M in a 50:50 mixture of ethylcarbonate and di-methylcarbonate as the electrolyte. The sweep rates

21were changed between 0.09 and 2.8 mV s for thedetermination of the diffusion coefficients of lithiumin the films.

3. Results and discussion

3.1. Composition of the films

The oxygen pressure has an important effect onthe composition of the films (Fig. 1). At low oxygenpressures, the films are both oxygen- and lithium- Fig. 2. (a) X-ray absorption spectroscopy at the Mn–K edge ofdeficient. X-ray diffraction shows that the films are films grown in various conditions and reference powders. (b)mixtures of LiMn O spinel and MnO or Mn O Enlarged part at the absorption edge.2 4 3 4


when the growth is performed at pressures lower different valence states or a Jahn–Teller effect31than 0.2 mbar. Upon increasing the oxygen pressure associated to the presence of Mn ions. This

26from 10 mbar up to 0.2 mbar, the composition shoulder is probably associated to the splitting of 4pincreases from Li Mn O (average Mn oxidation levels caused by different association of MnO0.6 2 3 6

state near 2.5) to LiMn O (stoichiometric com- octahedra in the solid state. Indeed, it is absent in the2 4

pound). For these last films, a lattice parameter of XAS spectra of perovskite compounds with corner˚ sharing octahedra such as LaMnO [9] and present in8.24 A has been measured in good agreement with 3

˚ systems with both edge and corner sharing octahedrathe stoichiometric LiMn O powder (8.247 A).2 4

such as Mn O and MnO . The distortion of the MnHowever, for still higher oxygen pressures, the Li 2 3 2

coordination octahedra lifts the degeneracy of theand O content increase rapidly to yield a composition4p levels and induces the splitting of 1s → 4pLi Mn O for an oxygen pressure of 0.4 mbar. x,y,z1.09 1.82 4

transitions that may account for this shoulderThis corresponds to a manganese oxidation state of[10,11]. Thus, in the LiMn O compound, X-ray3.8. However, the spinel structure is still observed as 2 4

31absorption is unable to differentiate between Mna single phase. The oxygen and lithium excess could41and Mn that all appear equivalent. This indicatesbe located in a phase not detected by X-ray such as

that electronic jumps between neighboring Mn sitesan amorphous carbonate. But, this should be visibleare probably faster than ionic relaxations althoughin the RBS spectra that did not detect any carbon inthe LiMn O system does not exhibit a high con-the film. Moreover, the cell parameter of the spinel 2 4

˚ ductivity. At the phonon frequency, all Mn octahedraphase decreases from 8.24 to 8.21 A as the oxygenare identical as they appear in the static and averagedpressure increases from 0.2 to 0.4 mbar (Table 1).cubic spinel structure. Therefore, the edge position,This confirms a non-stoichiometry effect in thedefined here as the energy at half maximumspinel phase.E of the edge, can be a precise determination of1 / 2max

the average oxidation state of Mn, similar to a3.2. Oxidation state of Mn and non-stoichiometrychemical shift [9]. In Fig. 2b, we enlarged the part ofthe spectrum corresponding to the absorption edgeThe XANES spectra of the films obtained at 0.2emphasizing a shift of 1.75 eV between LiMn Oand 0.4 mbar are shown in Fig. 2a. They are 2 4

and l-MnO . Assuming the same linearity than incompared to the reference powders LiMn O and 22 4

the case of perovskite structures (La Ca )MnOl-MnO . Each spectrum consists of weak pre-edge 12x x 32

[12] we can obtain by interpolation the actual Mnfeatures between 6540 and 6550 eV ascribed tooxidation state for the films grown under differentexcitations from the (1s) core level to differentconditions. The results are reported in Table 1.configurations of the hybridized 3d levels of the Mn

The spinel structure of LiMn O may accept aion. In purely octahedral coordination, these transi- 2 4

wide range of non stoichiometry towards the highertions are forbidden but they can be allowed by aoxidation states of Mn (V(Mn) . 3.5) between thedistortion of the site and also by covalent mixinglimiting compositions LiMn O –Li Mn O –between Mn3d and 02p states. The main absorption 2 4 4 5 12

Li Mn O . In the bulk form (studied by solid stateline around 656062 eV arises from Laporte-allowed 2 4 9

synthesis of powders) the first line of the phasetransitions from Mn(1s) core level to the continuumdiagram between LiMn O and Li Mn O is ob-states starting with the 4 p levels. In the post-edge 2 4 4 5 12

tained by the substitution of Li for Mn in thestructure, absorption oscillations result from multipleoctahedral sites and corresponds to the nominalscattering of the photo-excited electron on the neigh-composition Li Mn O (0 , x , 0.33) [13]. Aboring atoms. 11x 22x 4

second line of the phase diagram can be obtained byThe absorption edge presents a shoulder atlow temperature synthesis which yields spinel struc-655561 eV in all spectra (emphasized in Fig. 2b).tures with vacancies in both cationic sitesThis shoulder is present in the LiMn O composition2 4

31 41 Li Mn O up to x 5 0.11 corresponding to pure(mixture of Mn and Mn ) as well as in l-MnO 12x 222x 4241 41which only contains Mn . Therefore, this cannot be Mn in the Li Mn O composition [14].2 4 9

a separation between two manganese ions with Because all these phases are located in the part of


comparison between the films and the powdersthe phase diagram which contains more Mn(IV) than probably holds because there is no interaction be-Mn(III), they do not exhibit a Jahn–Teller distortion. tween the film and the substrate. Indeed, a strongTherefore, they all present a cubic structure and their interaction could affect the texturing and cause acell parameter linearly depends on the Mn oxidation strain within the film, large enough to affect the cellstate as shown in Fig. 3. This is easily understood by parameters.the fact that the MnO octahedra maintain the6

cohesion of the structure, the interstitial sites of 3.3. Chemical diffusion of lithiumwhich are filled with lithium ions. Therefore, there isa straightforward relationship between the cell pa- Cyclic voltammetry has been performed in the

21rameter and the oxidation state of manganese. But range of sweep rates from 0.09 to 2.8 mV s .the cell parameter does not bring any information on Lithium extraction occurs in a two-step process asthe nature of the defects causing the change of shown by the two oxidation peaks at 4.05 and 4.15 Voxidation state. We have also added in Fig. 3 the which indicate an electrochemical behavior similar toresults obtained on our films, taking the oxidation the conventional powder material [15]. These stepsstate from the X-ray absorption analysis and the cell correspond to the two successive reactions betweenparameters from the X-ray diffraction diagram. two solid phases:There is a good agreement between the oxidation

1 2LiMn O → Li Mn O 1 0.5Li 1 0.5estate deduced from X-ray diffraction and from X-ray 2 4 0.5 2 4

absorption spectroscopy. Note, however that theand

1 2Li Mn O → 2l-MnO 1 0.5Li 1 0.5e0.5 2 4 2

Before cycling, the open cell potential measuredbetween the film and the lithium reference electrodeincreases with the oxygen pressure during the depo-sition (Table 2). This is in good agreement with thehigher oxidation state of the films deposited atpressures above 0.2 mbar. The 2.0 V potentialobtained for the film deposited at 0.1 mbar is too lowfor a LiMn O starting composition. However, its2 4

behavior between 3.4 and 4.4 V is characteristic ofthe stoichiometric spinel. Therefore, the films proba-bly contain impurity phases responsible for the lowstarting potential but playing a minor role duringcycling in these conditions.Fig. 3. Correlation between the unit cell parameter and the

manganese oxidation state in powders as compared to our films. As compared to conventional powders, the quality

Table 21 2 21Open-cell potential (V), the Li /Li electrode and electrochemical diffusion coefficients (in cm s ) of lithium in films obtained at various

oxygen pressures

Oxygen Open-cell potential Oxidation Reduction1pressure (V vs. Li /Li )

(mbar) LiMn O →Li Mn O Li Mn O →l-MnO l-MnO →Li Mn O Li Mn O →LiMn O2 4 0.5 2 4 0.5 2 4 2 2 0.5 2 4 0.5 2 4 2 4

211 211 212 2120.1 2.0 3310 1310 3310 8310210 210 211 2110.2 2.9 1310 2310 5310 6310211 211 212 2120.3 3.3 3310 5310 3310 9310211 211 212 2110.4 2.9 1310 1310 5310 1310


of the voltammograms is poor as shown in Fig. 4. or electronic. One should also remind that, to theExcept for the film deposited at 0.2 mbar, the contrary of conventional powders, no carbon hasoxidation and reduction peaks are broad and poorly been added to increase the electronic conductivity inresolved. This may be attributed to a disorder the materials.induced by the non-stoichiometry as already dis- Increasing the potential sweep rate increases thecussed by Xia et al. [13]. The disorder also causes a height of the oxidation and reduction peak since their

1low mobility for electrical charges either ionic (Li ) integration over voltage divided by the scan rateyields the capacity of the material in the film whichis a constant (Fig. 5a). But, upon increasing the scanrate, the current peaks broaden. The height of eachpeak shows a square root dependence on the sweeprate as shown in Fig. 5b. This is expected for

Fig. 5. (a) Cyclic voltammetry of a LiMn O film deposited at2 4

5008C under 0.2 mbar oxygen pressure for various rates. (b) Plotof I as a function of the scan rate for a LiMn O film depositedmax 2 4

21Fig. 4. Cyclic voltammetry at a rate of 0.2 mV s of a LiMn O at 5008C under a 0.2 mbar oxygen pressure. The lines correspond2 4

film deposited at 5008C under a pressure of 0.1 mbar, 0.2 mbar to the linear fits to the data. The a, b, c and d marks correspond tooxygen and 0.4 mbar. the peaks labeled in (a).


diffusion-limited intercalation processes. If we con- low oxygen pressure, films are multiphased whereassider that the limiting step for intercalation is the above 0.2 mbar the spinel phase is maintained butdiffusion of lithium into the semi-infinite layer of the with a lower lattice parameter. Such films containfilm, the current peak height is given by the Randles more oxygen and lithium than in the ceramic targetequation: and manganese in a higher oxidation state. The

electrochemical properties of the films grown under1 / 2nF ` 1 / 2 1 / 2]S DI 5 2 0.4463nF C SD y (1) 0.2 mbar are comparable to LiMn O whereas forp 0 2 4RTother oxygen pressures, they are strongly affected

where n and F are the Avogadro and the Faraday either by impurity phases and by the non-stoichiome-constants, respectively, I is the current peak (A) forp try.the different oxidation and reduction reactions,

2whereas S the electrode surface (cm ), n is the21 `scanning rate (in V s ), and C the number of0 Acknowledgements

lithium sites expressed in moles per unit volume23(mol cm ). The results are reported in Table 2. We wish to thank T. Dantas for providing stan-

Higher diffusion coefficients are observed in the case dards for the NRA analysis and F. Villain for herof oxidation reactions than for reduction. The orders technical help at the Lure synchrotron facility.of magnitude of the diffusion coefficients observedin the different intercalation steps are similar to thosedetermined for powders [15,16] as well as for thin Referencesfilms [17].

The mobility of lithium is highest for the stoichio- [1] F.K. Shokoohi, J.M. Tarascon, J.M. Wilkens, Appl. Phys.metric film obtained at 0.2 mbar. The film deposited Lett. 10 (59) (1991) 1260.

[2] Y.-S. Park, S.H. Lee, B.I. Lee, S.K. Joo, Electrochem. Solidat 0.1 mbar contains impurity phases associated toState Lett. 2 (1999) 58.the presence of Mn O . Since this parasitic phase3 4 `[3] M. Morcrette, P. Barboux, J. Perriere, T. Brousse, Solid Statemay have been formed by evaporation of Li from theIonics 112 (1998) 249.

film during the deposition, it probably builds a [4] M.M. Thackeray, J. Electrochem. Soc. 144 (1997) L100.diffusion barrier at the surface of the film. On the [5] A. Rougier, K.A. Stiebel, S.J. Wen, E.J. Cairns, J. Electro-

chem. Soc. 145 (1998) 2975.other side, films obtained at pressures higher than 0.2[6] M.M. Thackeray, M.F. Mansueto, J.B. Bates, J. Powermbar present a lower mobility of Li. In this case, the

Sources 1 (68) (1997) 153.spinel structure is obtained as a pure phase but, as[7] M. Morcrette, Mater. Res. Soc. Symp. Proc. 548 (1999) 213.

discussed above, may contain manganese vacancies [8] J.C. Hunter, J. Solid State Chem. 39 (1981) 142.that artificially increase the oxidation state of Mn as [9] M. Croft, D. Sills, M. Greenblatt, C. Lee, S.W. Cheong, K.V.

Ramanuchawary, D. Tran, Phys. Rev. B 55 (1997) 8726.compared to the actual lithium content. The origin of[10] H. Yamaguchi, A. Yamada, H. Uwe, Phys. Rev. B 58 (1998)this lower mobility may be attributed to the presence

8.of a local disorder such as some Mn in the tetra-`[11] B. Ammundsen, D.J. Jones, J. Roziere, G.R. Burns, Chem.

hedral or a minor part of the Li occupying the 16c Mater. 8 (1996) 2799.sites of the spinel structure that block the diffusion [12] I. Maurin, P. Barboux, Y. Lassailly, J.-P. Boilot, F. Villain, J.

Magn. Magn. Mater. 211 (2000) 139.out of the tetrahedral sites.[13] Y. Xia, M. Yoshio, J. Electrochem Soc. 12 (144) (1997)

4186.[14] C. Masquelier, M. Tabuchi, K. Ado, R. Kanno, Y.

4. Conclusion Kobayashi, Y. Maki, O. Nakamura, J.B. Goodenough, J.Solid State Chem. 123 (1996) 255.

[15] D. Guyomard, J.M. Tarascon, J. Electrochem. Soc. 139A pure and stoichiometric LiMn O spinel phase2 4(1992) 937.can be grown as a thin film by laser ablation. This

[16] G. Pistoia, G. Wang, C. Wang, Solid State Ionics 58 (1992)requires to carefully adjust the temperature and285.

pressure during the growth. By X-ray absorption [17] K.A. Striebel, C.Z. Deng, S.J. Wen, E.J. Cairns, J. Electro-combined with RBS and NRA studies, we have also chem. Soc. 143 (1996) 1821.checked the oxidation state of Mn in the films. At

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Non-stoichiometry in LiMn2O4 thin films by laser ablation