Solid State Ionics 225 (2012) 737–741

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Solid State Ionics

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Electronic structure of cobalt–nickel mixed oxides

Stephan Schmidt, Dieter Schmeißer ⁎Brandenburg University of Technology, Konrad-Wachsmann-Allee 17, D-03046 Cottbus, Germany

⁎ Corresponding author. Tel.: +49 355 693145; fax:E-mail address: dsch@tu-cottbus.de (D. Schmeißer).

0167-2738/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.ssi.2012.06.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 September 2011Received in revised form 2 May 2012Accepted 6 June 2012Available online 15 July 2012

Keywords:Transition metalsOxidesResonant photoemissionX-ray absorption spectroscopy

We investigate the electronic structure of cobalt–nickel mixed oxides by means of SR-based electron spec-troscopy. We vary the Co–Ni content systematically and find that the most significant changes in the spectro-scopic data occur at a mixing ratio of 30/70. We analyze the electronic structure of this material in detail andwe are able to distinguish between pure covalent contributions, which cause a band gap of about 6 eV andempty defect states, which fill the band gap almost completely. These defects are associated with the forma-tion of 3d7L charge transfer states. These are correlated defect states and are identified by resonant photo-emission spectroscopy.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

We investigate the electronic structure of cobalt–nickel com-pounds because of its direct impact to the battery operation. Mate-rials which sufficiently meet these requirements and are widelyused commercially are LiCoO2, cobalt substituted lithium nickeloxide, and materials based on the spinel LiMn2O4 [1–5]. Replacing co-balt by large amounts of nickel is considered to reduce costs inlithium-ion-batteries while maintaining their performance. Lithiumnickel oxide, LiNiO2, is isostructural with lithium cobalt oxide buthas not been used in the pure state as a battery cathode for a varietyof reasons, even though nickel is more readily available than cobalt[6,7]. In a series of papers, the Delmas group [8-11], Zhecheva andStoyanova [12], and Molenda et al. [13] determined the structural de-tails and physical properties of the LiCoyNi1− yO2 system and showedthat there is an increased ordering as the Cobalt concentration in-creases. They found that the c/3a ratio increases monotonically from1.643 to 1.652 as y increases from 0 to 0.4 and that there is no nickelcontent on the lithium sites for y≥0.3.

In this contribution, we report on a systematic study of the elec-tronic structure of mixed Co/Ni films by varying the Co/Ni relativecomposition. We use synchrotron based electron spectroscopy to an-alyze the partial density of states (pDOS) in the valence band, theconduction band, and for states within the electronic gap, respective-ly. We find that intrinsic defects do exist which cause a filling of theband gap by oxygen derived defect states.

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2. Experimental details

Thin cobalt–nickel oxide films on metal substrates were preparedby spin coating from aqueous nitrate solutions followed by subse-quent annealing in air and UHV. Keeping the annealing temperaturesbelow 800 K we make sure to have no contributions from di-valentCoO in our films. The following cobalt-to-nickel ratios (Co/Ni) havebeen investigated: 50/50, 40/60, 30/70, and 20/80. The results arecompared with the pure NiO and Co2O3 oxide films and magnetronsputtered LiCoO2 reference samples [5].

Spectroscopic measurements on these samples were carried out atthe U49/2 beamline at BESSY II, Berlin [14]. All samples were charac-terized by means of X-ray photoelectron and absorption spectroscopy(XPS and XAS). XPS of the transition metal 2p regions was performedto obtain the relative core-level intensities and to calculate the Co/Niratio as well as to reveal the existence of charge-transfer groundstates and different oxidation states. Resonant photoemission spec-troscopy (resPES) probes the density of occupied states and was ap-plied to distinguish the contributions of the transition metal 3d andthe oxygen 2p states in the valence region. XAS at the oxygen K andtransition metal L2,3 edges respectively, projects the density of unoc-cupied states just above the Fermi energy (EF). XAS spectra weretaken in the total electron yield mode (TEY).

3. Results

In a detailed study [15], we have analyzed the core level data ofthe different mixing ratios. Indeed, there were only subtle changeswithin the O1s, the Co2p and the Ni2p core levels. In particular, wefound only minor changes to occur in the XAS data at the Co2p andNi2p edges [15]. This is why we focus in this study on the changesin the O1s XAS spectra.

Fig. 2. A detailed view of the O1s XAS data of the 30/70 mixture compared to that of thepure Co2O3 films. We focus on the first resonance; the inset shows the completespectra.

738 S. Schmidt, D. Schmeißer / Solid State Ionics 225 (2012) 737–741

In Fig. 1 we report on the changes in the O1s absorption data whenchanging the Co/Ni content from Co2O3 to NiO. For the pure Co2O3

films, we observe a strong resonance to occur at 530.5 eV while wea-ker structures appear above 534 eV. In NiO, on the other hand, theresonance appears at 532.6 eV and the features above 534 eV appearmuch stronger. Our series of different mixtures indicates that thesemain features change gradually and all these features maintain theirsignature of the pure films in the mixed phases.

In the XAS data of themixed oxidefilms,we find a strong decrease ofthe intensities of the first resonances related to pure oxides (530.5 eVand 532.6 eV, respectively) while the intensity at the features above534 eV remains comparable, although a distinct shift of these featurescan be seen. Coming again to the structure of the first resonance, it is ev-ident that there is a very remarkable additional shoulder which showsup before the main absorption band and which originates at around528 eV. Its intensity increaseswith increasingNi content and it becomesmost prominent in the 30/70 mixture ((Co30Ni70)2O3).

In Fig. 2we give a detailed comparison of theO1sXASdata of the pureCo2O3 and that of the 30/70 mixture. It demonstrates two effects in the30/70 film, the strong decrease of the intensity of the first resonance ofCo2O3 aswell as the increase of thefirst pre-edge band. The first observa-tion indicates that the number of empty states is reduced as the XAScross section directly depends on that quantity. The latter, the increaseof the pre-edge intensity at around 528 eV is most remarkable and willbe discussed in more detail below. From our analysis in Fig. 1 we deducethat these changes appear to bemost prominent in the Co/Ni 30/70 film;therefore we focus on that sample in the following exclusively.

In Fig. 3 we elucidate the influence of the Ni admixture to the O1sdata in detail and compare the O1s XAS data of the 30/70 mixture tothe corresponding data of LiCoO2 and Co2O3. In this diagram, the intensi-ty of the first resonancewas adjusted for the three samples. This compar-ison demonstrates oncemore the strong increase in the spectral width ofthe first resonance, which is found for the 30/70 mixture. It must be at-tributed to the formation of states in the electronic band gap, which arederived from O2p valence states. Obviously, the pre-edge absorption ispresent in all of the three samples; however, by the Ni admixture it be-comes stronger and is very prominent in the 30/70 sample.

Fig. 1. XAS spectra at the O1s edge of various Co/Ni compositions—compared to thecorresponding spectra of Co2O3 (bottom) and NiO (top).

For the 30/70 film, we record a complete set of energy distributioncurves (EDC) as shown in Fig. 4. We use resPES at the O1s edge to fur-ther characterize the oxygen-derived states. Here an electron out ofthe O1s core level is resonantly transferred into the empty conductionband (CB) states. It causes an excited intermediate state in which onlyoxygen derived empty states are involved due to the dipole selectionrules. The decay of these states can be used to probe the empty CBstates. The plot in Fig. 4 was acquired by variation of the photon ener-gy in the range between 520 eV and 555 eV. In this contour plot of thevalence band (VB) spectra recorded around the O1s threshold we can

Fig. 3. The O1s XAS data of LiCoO2, Co2O3, and of (Co30Ni70)2O3. The position of EF andVBM are indicated.

520 525 530 535 540 545 550 555

0

-5

-10

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-25

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-45

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photon energy / eV

bind

ing

ener

gy /

eV

1375

1.606E+04

1.481E+05

7.138E+05

Fig. 4. Resonant photoemission data of the 30/70 sample at the O1s absorption edge.The arrows indicate the 45° and 67.5° Auger lines.

Fig. 5. The combined partial density of states of the valence band and conduction band asdetermined from the UPS and XAS data, respectively. The O2p derived states (bold lines)and the Co3d derived states are given. The common energy is referred to EF. The arrowsindicate the extrapolated position of the VBM and the CBM of the covalent gap.

739S. Schmidt, D. Schmeißer / Solid State Ionics 225 (2012) 737–741

distinguish between features at constant binding (horizontal) or con-stant kinetic energy (45°). This gives us the opportunity to study theparticular decay phenomena connected with the resonant excitation.The behavior of the pre-edge region as well as the evolution of thestrongest components of the O KLL Auger lines with excitation energycan be analyzed. In this contribution, we just focus on the O-KLLAuger lines with a kinetic energy of 512 eV, which become evidentin the profile as lines following an angle of 45°. Such Auger contribu-tions are observed for photon energies above 535 eV and are indicat-ed in Fig. 4 by white arrows. However, it is evident in that plot thatthe features next to the threshold at around 530.5 eV are stronger in-clined and appear under an angle of 67.5°. These, again, are markedby white arrows in the regime between 528 eV and 534 eV. This ob-servation indicates that there must be either different mechanismsor different states involved in the resonant behavior at the O1sthreshold. We argue that in this regime we have contributions froman Auger process, which has three holes in its final state and willfocus on that process in our discussions below.

Next, we give a combined electronic structure of the respective O1s,Co2p XAS and VB data at resonance for the 30/70 sample (Fig. 5). To ob-tain this figure we have used the binding energies of the O1s (529.5 eV)and Co2p (780.4 eV) core levels and referred the XAS data accordinglyto obtain a common energy scale with respect to EF. The VB spectrumis recorded at the excitation energy of the O1s resonance at 530.5 eV.Resonant photoemission was used to distinguish the contributions ofthe transition metal 3d and the oxygen 2p states in the valence region.While the sharp feature at the O1s threshold appears next to EF, the res-onant behavior of the Co2p states shows up only at 1 eV above EF. Thelatter is assigned to the Co3+ (3d6 LS) state.

These combined data enable us to discuss the electronic structure inmore detail. The combined UPS and XAS data can be related to the Oxy-gen 2p and Cobalt 3d derived pDOS, respectively. Hence, from thesedata, we deduce directly the electronic structure of the VB and CB states.

In all of themixed oxidic systems,we expect a rather large electronicband gap. It must be in the order of 4 eV as values between 2 eV and6 eV has been reported for both, NiO [16,17] and for various Co oxides

[17,18]. In our XAS data, however, we find strong features appearingaround EF and no indication of a band gap at all. Furthermore, for the30/70 mixture these states become very prominent and broaden con-siderably to extend even below the Fermi energy. In particular, asseen from the extrapolated values for the valence band maximum(VBM), which is at −2 eV below EF, and the value of the conductionband minimum (CBM), which is at 4 eV above EF we find that the elec-tronic band gap is completely filledwith oxygen derived gap states. Ob-viously, this indicates that in our XAS data, we do not have the strongsignals from the extended (Bloch-like) covalent CB states but they rath-er do originate from localized defect states within the band gap.

It should bementioned that our XAS data and our assignment are inperfect agreementwith earlier studies [19]. These authors argue that forLiCoO2 the separation of VB and CB features is about ~2.5 eV, a valuethat is in good agreement with calculations by van Elp et al. [20].Upon Li extraction they find hole states in the O1s-XAS data which fillthe band gap completely. The authors argue that the increase of hole-states is the consequence of a phase separation upon lithium de-intercalation by the formation of metallic clusters within a semicon-ducting matrix. We find an excellent agreement with our XAS data onthe Co/Ni 30/70 samples. However, we do not agree on their conclu-sions. Instead, we argue that oxygen derived hole states are created byLi extraction as well as by the Co–Ni mixture.

4. Discussions

In this section, we focus on the electronic structure of the mixedoxides. Based on our spectroscopic data we derive the partial densityof states in the valence band and conduction band, respectively. Inparticular, the O2p derived density of states in the CB is found to bevery broad. In the mixed Co–Ni films, we find even an increase inthe width of the O1s signal at resonance which leads to a system inwhich no electronic gap is noticeable. This observation is evidentfrom the combined pDOS data as shown in Fig. 5.

It should be mentioned that there are no significant changes in theCo2p core levels when increasing the Ni amount up to 70%. This find-ing will be reported in a forthcoming report as we currently try to an-alyze the minor changes. In brief, there is no evidence for theformation of Co4+ and Ni2+ ions based on the electron transferfrom Co3+ and Ni3+ states. Although intuitively expected, this is themajor reason for our detailed study of the role of the oxygen atomsupon substitution of Ni in the Co-oxide matrix. This finding just dem-onstrates the importance of our present study dealing with the varia-tions of the O1s XAS spectra which are most prominent when varying

Fig. 6. Schematic presentation of the 3 h-Auger process. For details, see text.

740 S. Schmidt, D. Schmeißer / Solid State Ionics 225 (2012) 737–741

the Ni content in mixed Co–Ni oxidic films. We also like to state thatthis study is focused on the electronic structure of the mixed oxides.In our films we have no crystallization (annealing or sintering) proce-dures. Due to the small thickness (necessary to avoid charging ef-fects) we are working with amorphous films at all Ni/Co ratios.

To understand the significant variations in the O1s XAS signal aspresented in Figs. 2–4 we start to describe the electronic structureof the mixed Co/Ni oxides in terms of the early Goodenough [21,22]model. Here the VB and CB are formed by the tight binding molecularlevels, which are due to the O2p and Co (4 s, 4p) states, respectively.In our O1s and Co2p XAS data next to the sharp resonances, we findsome broad features, which we can attribute to transitions into theempty CB states. As indicated in Fig. 5 we can extrapolate fromthese features the VBM at 2 eV below EF and the CBM at 4 eV aboveEF. In total, the width of the covalent (Goodenough) band gap is inthe order of 6 eV (see also Fig. 1). Such a value for the electronicgap is in agreement with earlier reports [16,17]. In our data, however,we do not observe such a forbidden region, instead, we find severalcontributions originating from both, O2p and Co3d states, which ap-pear in the gap and in total will fill the gap.

To explain this behaviorwe argue that these energy levelswithin theband gapmust arise from defect states. The most common defect statesin metal oxides certainly are oxygen vacancies. For such vacancies, weexpect a homogenous distribution of energy levels rather than sharpresonant features. Actually, we do have evidence for oxygen vacanciesbut these states are not of importance for the following discussions asthey contribute to a featureless background only. Instead, we have to at-tribute the observed sharp features to arise from localized gap states.

We nowdiscuss the origin of the sharp features in the XAS data. Theseabsorption signals appear next to EF, but definitely below the covalentCBM. Localized band gap states can arise from charge-transfer (CT) reac-tions [23] inwhich an electron from theO2p6 state in theVB is transferredto an empty 3d state [24,25]. In the case of Co2O3we expect the formationof 3d7L (L=O2p5) states in addition to the three-valent Co3d6 groundstate. Such CT states are correlated by the Coulomb repulsion between ad-jacent d-d states as well as the attractive Coulomb interaction betweenthe valence hole L and the extra electronwithin the d-levels. The CT statesare usually described by quantities like the d-d repulsion U, the charge-transfer energy Δ, the valence-hole 3d Coulomb interaction, and theCoulomb attraction between the valence hole and the CT electron Uvh.Kim andGoodenough [7] have recently proposed thatwithin the covalentgap the metal ion Mx

+/Mx−1+ redox couple is pinned next to the valence

band maximum. This description resembles very well the findingsshown in Fig. 5 where the covalent gap is indicated by the arrows. Theyalso proposed that unoccupied metal states will be placed above theFermi energy and there must be a charge flow into empty O2p derivedstates. Again this is consistent with our data which we summarize inFig. 5. In particular we emphasize that the main effect is in the O2ppDOS while the metal states are almost unchanged. Because of the afore-mentioned Coulomb terms, such correlated states can no longer bedescribed in the covalent tight binding picture with band states. In calcu-lations [26] using an extended Hubbard model and calculating the elec-tron addition and removal spectra in one and two dimensions theauthors find rather high values for the on-site repulsion of U=10 eVand for the hopping integral t=0.1 eV. In particular, they also reportthat the electronic gap can be completely closed by nearest-neighborelectron interactions V between the localized O2p and metal 3d states.These nearest-neighbor contributions between the CT states cause theformation of an O2p5 sub-band in the electronic structure. The unoccu-piedpart of this sub-band ismeasured in ourO1s-derivedXAS data. In ad-dition, our study demonstrates that these defect states can be influencedby the Ni admixture. Although there might be an overlap of these differ-ent states, we can estimate that the localized states are lower in energythan the covalent states, which start at around 4 eV above EF in Fig. 5.

To summarize, the admixture of nickel does not change the oxida-tion states of the Co2p ions but rather modifies the unoccupied

contributions of the O2p states, which are attributed to defect statesbased on the ligand-to-metal charge transfer to form 3d7L states.These states cause the first sharp resonance in the O1s XAS data atabout 528 eV. The fact that the absorption starts already at 528 eVmeans that these defect states exist within the electronic (covalent)gap. They form a band with a continuous pDOS, which covers therange between VBM and CBM completely.

There is a further characteristic feature of these defect states,which is identified by the profile in the Auger processes at resonance.As shown in Fig. 4 we find that the lowest oxygen resonance shows adifferent Auger profile when compared to the states above 534 eV,which appear under 45°, and we find these states under 67.5°. We ex-plain such a behavior by the 3 h-Auger process.

The 3 h-Auger process is a final state process in which the primarycore hole at the O1s level is filled to cause three valence holes V1, V2,V3, and the resulting kinetic energy is transferred to the resonantlyexcited electron to leave as participator decay. Hence, the resultingfinal state may be denoted as (O1s–V1–V2–V3–P). The P means thatthe V3–P process always must be understood as participator decay.In Fig. 6, this process is described schematically in six steps. Thefirst step is the excitation into the empty CB from the O1s level (1)which causes the creation of the primary core hole. In the secondstep, that electron becomes trapped into an empty localized state(2). Thereby that electron is stabilized by a Coulomb energy, whichmeans that it has a longer lifetime when compared to a conductionband electron. Consequently, while it is in the localized state, its pri-mary core hole can be filled from any valence band state (3). This cre-ates the valence hole V1 and causes an Auger electron to escape (4)leaving another valence hole V2. Up to this point, the process is justtypical spectator decay. The three hole process now involves the pri-mary electron in its localized state which, before its thermal deactiva-tion (radiation-less), is to become emitted as an participator Augerelectron. This step creates the third valence hole V3, the electron ofwhich is to fill an additional, accidently appearing (bypassing) corehole (5) (not the primary one—it is already filled by another valenceelectron) and transferring its energy to the primary electron to escape(6). As a result, we have to deal with a combined spectator–participatormechanism that is enabled by the fact that the valence holes aredelocalized covalent bonded O2p states.

This mechanism becomes possible only at resonance where a highdensity of primary core holes is available. Furthermore, it requires ahigh mobility of valence band electrons and holes, which is givenonly in covalent bands. These two arguments make clear that the

741S. Schmidt, D. Schmeißer / Solid State Ionics 225 (2012) 737–741

3 h-Auger process is not an atomic-like process (as the KLL Augerdecay is) but is a multiple particle decay channel at resonance.

The main feature of the 3 h-Auger process in the context of theelectronic structure of mixed Co–Ni oxides, however, is that the pri-mary electrons need to be trapped (state E in Fig. 6) by someCoulomb interactions U. It means that the observation of the 3 h-Auger is a tool for identifying correlated defect states. We can usethe observation and evidence of the 3 h-Auger process as a methodto probe the existence of such Coulomb interactions within the pro-bed system. In the Co–Ni mixed oxides the 3 h-Auger process pro-ceeds via localized – oxygen derived – empty states within theelectronic band gap. Such states are attributed to the oxygen derivedstates which are created by the CT (3dn+1 L, O2p5=L) process. Itshould be mentioned that not the CT process with the 3d7L state itselfis involved in the 3 h-Auger process but only these empty hole(L=O2p5) states. Insofar the 3 h-Auger process is indicative of oxy-gen related correlated defect states. It should be mentioned that wealso identified such a 3 h-Auger process in organic systems [27]where the primary electron is trapped in an excitonic state.

We reconcile that the electronic structure consists of three contri-butions. First, there are the covalent interactions of O2p6 and Co3d6

valence states to form the covalent (Goodenough) gap, which is inthe order of 6 eV. Oxygen vacancies are located within that gap anddetermine the electronic transport properties like conductivity andthe position of EF. Second, there occur CT reactions in the groundstate between O2p and Co3d states to form O2p5–Co3d7 complexes.Thereby the attractive interaction between the valence hole and the3d-electron state (Uvh) comes into play. Third, we have to considerthe contributions from O2p5 states, which are correlated by the Uvh

Coulomb interaction—in contrast to the O2p6 states, which are cova-lent. The O2p5 states then form a sub-band, originating close to theVBM but also with non-bonding contributions close to the CBM.Caused by the small but finite covalent interaction between the va-lence hole and the Co3d state these levels appear broadened. In addi-tion, the value of Uvh has not a fixed value, rather it spans a range ofabout 1 to 2 eV. As a total, the O2p5 minority gives a band of states,which exists within the electronic band gap. In our XAS experimentsat the O1s threshold, we just probe both, the localized gap states andthe covalent CB states. In particular, we attribute the sharp resonancesright at the O1s threshold to these non-bonding O2p5 derived states.

Actually, pre-edge absorption bands are found in the XAS data of thepure Co2O3 and NiO phases also. In the mixed phases these states ap-pear enhanced which means that the number of such localized emptystates is enhanced or the transition probability is increased. We arguethat the role of the admixture of NiO into the Co2O3 system is twofold.It causes an increase of the width of such pre-edge states and it lowersthe correlation as for the two-valent Ni ions the ionic Coulomb contri-bution is weaker than for the three-valent Co ions. Both arguments in-dicate that there is a common band of empty oxygen derived states inthe mixed systems. From our studies we learn that these processes be-come most prominent at 30/70 samples ((Co30Ni70)2O3). This is a re-markable agreement with earlier structural data [8-10,13] in whichthe increase of the c-axis upon Ni admixture was found to show thehighest value at that 30/70 value. In a forthcoming study we will relateour findings with optical, structural data aswell aswith transport prop-erties such as conductivity measurements. This seems to be importantas for the battery application, we argue that the electron transport oc-curs via such localized O2p5 empty states and is not related to covalentband states. Furthermore, we will develop a band model which is con-sistent with our assignment of the partial DOS presented here and cor-relate the pDOS with the battery performance in Li-doped mixedoxides. Certainly, we have to correlate the existence of these O2p de-rived defect states to the structural properties before we can argue onthe transport properties and the battery operation.

5. Conclusions

In this study,we report about thinfilms ofmixed oxides of Cobalt andNickel varying from Co2O3 to NiO, which are studied by XPS and XAStechniques. We find no significant changes in the Co2p and Ni2p XPSand XAS data [15] but a very significant contribution at the onset of theO1s XAS spectra. This contribution increases with increasing Ni contentand becomes most prominent at 30/70 samples ((Co30Ni70)2O3).

For the films with the 30/70 composition we performed a detailedspectroscopic characterization and analysis; a band diagram can be de-duced (see Fig. 5) inwhichwefind that the pre-edge absorption band at528 eV in the O1s XAS data give evidence for O2p5 defect states, whichare distributed in energy throughout the electronic gap. We give evi-dence that these states are associated with 3d7L CT states which we de-rive from the observation of a 3 h final state at resonance. The 3 h-Augerprocess indicates the existence of localized empty states, which are ableto trap electrons upon resonant excitations.

Acknowledgements

This paper is dedicated to Stephan Schmidt who passed away dur-ing the finalization of the data for our studies. We like to acknowledgethe skillful technical assistance of G. Beuckert and I. Paloumpa. Weappreciate the continuous support of the BESSY staff. We thankK. Henkel for a critical reading of the manuscript. This work issupported by grants from DFG (Schm745/9-3) and BMBF (05KS7IC1).

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