Nonrigid Band Behavior of the Electronic Structure of LiCoO2 Thin

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Nonrigid Band Behavior of the Electronic Structure of LiCoO2 Thin Film during Electrochemical Li Deintercalation D. Ensling,† G. Cherkashinin,*,† S. Schmid,† S. Bhuvaneswari,† A. Thissen,† and W. Jaegermann†,‡ †

Institute of Materials Science, TU Darmstadt, Jovanka-Bontschits str. 2, D-64287 Darmstadt, Germany Center of Smart Interfaces, TU Darmstadt, Jovanka-Bontschits str. 2, D-64287 Darmstadt, Germany

‡

S Supporting Information *

ABSTRACT: In this study, a comprehensive experimental in situ analysis of the evolution of the occupied and unoccupied density of states as a function of the charging state of the Lix≤1CoO2 films has been done by using synchrotron X-ray photoelectron spectroscopy (SXPS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and O K- and Co L3,2-edges XANES. Our experimental data demonstrate the change of the Fermi level position and the Co3d−O2p hybridization under the Li+ removal and provide the evidence for the involvement of the oxygen states in the charge compensation. Thus, the rigid band model fails to describe the observed changes of the electronic structure. The Co site is involved in a Co3+ → Co4+ oxidation at the period of the Li deintercalation (x ∼ 0.5), while the electronic configuration at the oxygen site is stable up to 4.2 V. Further lowering of the Fermi level promoted by Li+ extraction leads to a deviation of the electronic density of states due to structural distortions, and the top of the O2p bands overlaps the Co3d state which is accompanied by a hole transfer to the O2p states. The intrinsic voltage limit of LiCoO2 has been determined, and the energy band diagram of Lix≤1CoO2 vs the evolution of the Fermi level has been built. It was concluded that LixCoO2 cannot be stabilized at the deep Li deintercalation even with chemically compatible solid electrolytes.

1. INTRODUCTION The transition metal (TM) oxides exhibit a wide variety of physical and chemical properties that have found their application in the field of spintronics,1 catalysis,2 and renewable energy storage devices (rechargeable Li- ion batteries (LIB),3 capacitors,4,5 hydrogen-evolving solar cells,6 oxygen production from water splitting,7 etc.) involving internal redox reactions of the oxide. Unique properties of the TM oxides are caused by an ability of the bound transition metals to alter their electronic configuration under external stimuli like temperature, pressure, voltage, chemical reactions, etc. In a simplified chemical view, only the changed oxidation state of the transition metal is considered in the physical consideration. The occupation of electron states leads to a shift in Fermi level, which may be described in terms of the rigid band model (no changes in the distribution and energy positions of the involved electrons density of states)8 or structural modifications may alter the DOS.9 The complexity of phenomena determined by the coupling between charge, orbital, and spin of valence electrons stimulates a further interest in a deeper understanding of the electronic structure changes of these oxides. The electronic structure of the transition metals plays a key role in LIB, since the cathode material (a transition metal oxide) determines, in fact, the specific capacity and the energy density of a battery cell,10 as well as the intrinsic voltage limit © XXXX American Chemical Society

which defines a stability of the whole battery cell under reversible electrochemical Li extraction/insertion from/to the host material.11 A LIB operates stably if the removal (insertion) of a Li+ ion from (to) the host material is compensated by a hole (an electron) transfer to the 3d occupied electron state resulting in the TMn ↔ TMn+1 one-electron redox process, where n is a formal oxidation state of the TM ion. However, a more detailed understanding needs a complete consideration of the involved electronic modification. The hole transfer caused by a Li+ removal lowers the chemical potential μ (or Fermi level EF) of the TM oxide and can lead to the situation possibly also involving changes of the respective energy bands where the occupied 3d state crosses the top of the O2p band that leads to the decomposition of the cathode material. The intrinsic voltage limit is inherent for a relevant cathode material, since it depends on the electronic configuration given by the coupling of the metal 3d states and p states of the anion. Generally, the degradation of LIB is a quite complex phenomenon12 and is related to many factors such as an instability of the crystallographic structure of the cathode13,14 and anode materials15,16 induced by the lattice stress during the LiReceived: April 23, 2014 Revised: June 13, 2014

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dx.doi.org/10.1021/cm501480b | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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the ultrahigh vacuum (UHV) deposition chamber for the growth of LiCoO2 was below 5 × 10−8 mbar. The polycrystalline LCO thin films were deposited on Ti foil (99.95%, Alfa Aesar) heated at 550 °C in an Ar/O2 sputter gas mixture [Ar (5.0), O2(4.8), Air Liquide] in a po/ar= 8 × 10−3 to 2 × 10−2 mbar pressure range by using the radio frequency (RF) sputtering method.45 A 2 in. target of the commercial stoichiometric Li1.0Co1.0O2 (99.9%, FHR Anlagenbau GmbH) was used for this aim. The sputtering rate of the Li1.0Co1.0O2 material was around 2 nm min−1. The thickness of the LCO thin film was around d ∼ 200 nm as estimated from scanning electron microscopy (SEM) images. More details of the LCO thin film parameter deposition are reported elsewhere.36,45 In-Situ Photoelectron Spectroscopy and Electrochemical Experiments in the Surface Science Group. After the LCO films growth, a sample was transferred under high vacuum conditions, p ∼ 1 × 10−9 mbar, through the distribution chamber to the XPS analysis chamber with pxps ∼ 5 × 10−10 mbar. The electronic structure of the LCO thin films was studied by using a PHI5700 (Physical Electronics) spectrometer of the DAISY-MAT. The core-level electrons and valence electrons were excited by using a monochromatic AlKα (hν = 1486.6 eV) source, and the photoelectron spectra were collected with the electron escape angle of 45°. The energy resolution of the system is ∼0.4 eV. Ultraviolet (UV) photoelectron experiments have been done by using He I (hν = 21.2 eV) and He II (hν = 40.8 eV) with the electron escape angle of 90°. The secondary electrons at the cutoff were measured by using hν = 21.2 eV. The binding energies are referred to the Fermi level of a Ag foil. The background under the XPS spectra was subtracted using a Tougaard-type function; the photoelectron peak positions and areas were obtained by a weighted leastsquares fitting of model curves (70% Gaussian, 30% Lorentzian) to the experimental data. The XPS quantitative analysis was carried out according to the equation

deintercalation/intercalation under/overcharge or overdischarge conditions, electrochemical reduction/oxidation of the electrolyte,17,18 and a degradation of the cathode/anode surfaces19−21 caused by chemical reactions at the electrode− electrolyte interface, which leads to the formation of a solid electrolyte interface (SEI) layer.22−25 Most of these factors are also strongly influenced by electronic structure changes. Since the advent of the rechargeable LIB cell in 1991,26 the electronic structure and electrochemical properties of the layered cathode material LiCoO2 (LCO) have been extensively studied. However, the majority of these investigations deal with powder materials synthesized by different chemical methods (see, for example refs 27−33). The intensive development of the LCO thin film cathode material started relatively recently34−38 due to its potential application for all solid state batteries.39−41 Thin film materials grown under high vacuum conditions have an advantage for investigating in situ electronic properties by surface science techniques, since (1) the pristine film is contamination free, and therefore, its electronic and crystallographic structures are not affected by impurities which are often present in the synthesized powder materials; (2) the surface electronic structure is not perturbed by the gas adsorption species from the ambient environment; (3) protection of a battery cell against the contact to the atmosphere during the electrochemical/photoemission experiments allows one to avoid unwanted chemical reactions of the battery cell components with the ambient environment, which makes the interpretation of the electronic and electrochemical properties unambiguous. In this paper, we report on a study of stability of the electronic configuration of Lix≤1CoO2 thin film layered oxide cathode material under electrochemical Li deintercalation. The local electronic structure at the Co and oxygen sites is in situ studied by using Co L2,3-, O K- XANES, soft X-ray photoelectron spectroscopy (SXPS), conventional X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) using a specially developed experimental setup (SoliAS) for the investigation of electrochemical interfaces of BESSY II. The results obtained can perfectly be compared to electronic structure calculations, e.g., using DFT approaches.9,42,43 Starting from a Li1.0Co1.0O2.0 film with Co3+ (3d6, t2g3↑t2g3↓eg0) with a low spin (LS) state, we build up the energy band diagram of stoichiometric LCO and study the evolution of the occupied and unoccupied states of the LCO film versus Li+ deintercalation. We show that the Li+ removal is accompanied by the charge compensation at the Co site, leading to the Co4+ (3d5, t2g3↑t2g2↓eg0) with a LS state, while the oxygen electronic configuration is stable up to a battery voltage of 4.2 V. Above 4.2 V, the Co3+/Co4+ redox reaction overlaps with the top of the O2p bands that leads to a hole transfer to the O2p states, indicating the intrinsic voltage limit for the LCO film cathode material. To the best of our knowledge, this study is the first comprehensive experimental in situ analysis of the evolution of the electronic configuration of the LCO-electrolyte interface as a function of the charged state/Li content of the LCO film cathode material.

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n ASFA CA = A = × 100% I ∑ ni ∑ ASFi i

The analysis depth of photoelectron spectroscopy was estimated according to the universal curve of the inelastic mean free path (IMFP) of electrons, λ, vs their kinetic energy, Ekin.46 After XPS analysis, a LCO thin film was transported in the Ar filled self-made transferrable chamber to a glovebox (MBraun, Germany) for the battery cell’s assembling under an argon atmosphere. The electrochemical experiments of the LCO thin film cathode materials were performed using Swagelok-type two electrode cells and a VMP3 multichannel potentiostat (Bio-Logic Science Instruments, France). A 1 M solution of the LiPF6 salt in a 1:2 (w/w) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC; Ube Ltd.) was used as an electrolyte. A metallic lithium foil served as the anode. The cathode materials were cycled between 2.7−4.2 and 2.7−4.4 V at room temperature by using the voltammetry mode. A fast transfer of a LCO film from the XPS chamber to the glovebox for 1−2 min under air conditions does not influence on the electrochemical properties of the cathode material, as was found by our observations. In-Situ Photoelectron Emission Spectroscopy and Electrochemical Experiments at BESSY. The SXPS and O K- and L-edges XANES in situ experiments have been carried out at the undulator beamline U49/2 PGM-2 with a plane grating monochromator. The beamline is equipped with the integrated UHV system for solid liquid interface analysis (SoLiAS).44 The spectrometer is equipped with the SPECS PHOIBOS 150 MCD-9 electron analyzer. The accessible energy range is hν = 100−1900 eV with an overall energy resolution of better than 400 meV and about 150 meV below photon energies of 800 eV.45 The UHV thin film deposition chamber was transported to BESSY-II from Darmstadt and connected directly to SoLiAS. Thus, contamination free LCO thin films were deposited in the chamber and transferred under UHV conditions to the analysis chamber (p = 2 × 10−10 mbar). The intensities of the measured SXP spectra were normalized to the incident X-ray flux. The binding energies of the spectra have been referenced to the Fermi level of sputter cleaned Au-

2. EXPERIMENTAL SECTION The LiCoO2 (LCO) thin film deposition and photoelectron spectroscopy experiments were carried out in either the Darmstadt Integrated System for Material Research (DAISY-MAT) of the Surface Science group (TU Darmstadt) or the SoliAS (ref 44) at the synchrotron source in BESSY-II (Berlin). The base pressure, pbase, in B



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and/or Ag- polycrystalline foils. Since the analysis depth of the photoelectron emission is 3λ cos θ (here, θ is the electrons escape angle with respect to the surface normal), the variation of photon energies allows us both to reach the maximal surface sensitivity and to collect the photoelectrons from the same depth. The secondary electrons cutoff was measured with hν = 150 eV. The SXPS quantitative analysis was carried out by using the photoionization cross-section reported in ref 47. After measuring the electronic structure of the pristine LCO, in situ electrochemical Li-deintercalation/intercalation experiments of a LCO thin film cathode material were performed in a specifically developed glass cell under an Ar atmosphere attached to the UHV system via a buffer chamber (see Figure 1). In this cell, a capillary tube containing the electrolyte and

parameters reported in refs 36, 45, 49, and 50. Special attention was paid concerning unwanted carbon species, which are a common impurity, since they are easily adsorbed on the surface even under high vacuum conditions. No carbon adsorbate species on the pristine LCO thin film were detected by SXPS or XPS (see Supporting Information, Figure 1Sa,b). Figure 2 shows the secondary electrons cutoff (the left side). Since electrons cannot overcome the surface potential barrier

Figure 2. In-situ experimental electronic structure of stoichiometric LCO thin film (EF = 0 eV). From the left to the right: the secondary electron cutoff (hν = 150 eV, Ekin = hν − |Ebin| − eϕ); the VB structure (hν = 120 eV) below EF; the unoccupied states above EF are measured by O K XANES. DOS and density of unoccupied states are plotted on a unified energy scale. The spectral feature A* of the O K-edge was referred to the O1s photoelectron line with Ebin= 529.3 eV for the lattice oxygen of LCO.

Figure 1. (a) A schematic sketch of the glass-cell for the electrochemical experiments at BESSY-II (ref 43) and (b) a photo of the glass-cell which is attached to SoLiAS.

with the kinetic energy lower than the work function of a solid, linear extrapolation of the cutoff to the kinetic energy scale gives us the work function, which has been found to be eϕLCO = 5.0 ± 0.10 eV for stoichiometric LCO. A unique opportunity for electron spectra recorded at synchrotron facilities is the ability to obtain information about the electronic properties in the whole energy range (see Figure 2). The valence band (VB) structure of stoichiometric LCO film is dominated by the spectral feature A (Figure 2, middle). According to first principle calculations43,51,52 and experimental photoelectron spectroscopy results,45,53,54 the sharp emission A and the spectral feature B are assigned to occupied Co3d (t2g) states of Co3+ (3d6, t2g3↑t2g3↓eg0) with low spin (LS) state electronic configuration. The broad spectral features marked as C, D, and E are due to O2p states, namely, the C emission is assigned to t1u orbitals with a slight admixture of the Co4p states; bands D and E are due to the O2p−Co3d (eg) hybridization and the O2p−Co4s, Co4p (a1g, t1u) hybridized states, respectively.52,53 The F satellite is interpreted as a poorly screened Co3d hole (3d5 final state). A sharp structure, A*, of the O K-edge corresponds to the electron transition from the occupied O 1s core level to empty O2p−Co3d (eg*) hybrid states (see Figure 2, the right side). The oxygen 2p states are highly hybridized with the Co3d (eg*) states, thereby leading mostly to metal character of these empty bands. Overall occupation of t2g states leads to a low spin electronic configuration (t*2g6, 1A1g) for octahedral (Oh) symmetry.51,54,55 The broad structure in the O K edge spectrum above 5 eV is attributed to transitions into O2p states which are hybridized with Co4s and Co4p states. The band gap Eg, that is defined as the minimum energy required to remove an electron (En−1 − En) plus the minimum energy required to add an electron (En+1 − En),51 can be estimated from the A and A* difference (Figure 2). The obtained 3d(eg*) − 3d(t2g) energy distance is very close to 2.1 eV, identified as the valence band to conductance band d−d

the counterelectrode as well as a reference electrode can be attached to the thin film cathode. Metallic lithium was used for reference and as a counterelectrode. A 1 M solution of LiClO4 salt in propylene carbonate (PC; Aldrich, 99%) was used as an electrolyte. With a connected potentiostat, the degree of Li intercalation can be electrochemically adjusted. After washing the sample with acetonitrile in order to remove remainders of the electrolyte followed by drying the sample in an Ar flow, the sample has been retransferred into the UHV system for the SXPS and XANES analysis. The choice of PCLiClO4 electrolyte has been dictated by (a) an easy way to rinse the surface of a film after a certain step of the electrochemical experiment by using acetonitrile solvent, (b) a lower boiling point of acetonitrile BP = 82 °C, 48 and (c) not disturbing the CxHyF SEI layer to be formed on the cathode.21 All these factors allow us to reduce significantly the time to reach good ultrahigh vacuum conditions, after the electrochemical treatment step.

3. RESULTS AND DISCUSSION 3.1. Electronic Properties of Stoichiometric LiCoO2 Film. The high temperature phase of the layered LiCoO2 (HTLCO) with the R3m ̅ space group symmetry is formed after annealing of the deposited LCO thin films in the 400−650 °C range.36,45 A Co and Li ion in such a structure are surrounded by six oxygen atoms forming CoO6 and LiO6 octahedral coordination. The edge sharing CoO6 octahedra form alternative CoO2 layers separated by layers of octahedral coordinated lithium ions which lead to a two-dimensional triangular lattice. Li ions are ionically bound in the van der Waals gap formed by the O2− ions which allows the reversible Li extraction/insertion from/to the host material. Previous studies of the LCO thin films have revealed that the optimal temperature of the growth of the stoichiometric HT-LCO is TLCO = 550 °C.45 Temperatures higher than 550 °C can lead to the reduction of the trivalent Co ions,49 while TLCO < 400 °C leads to the low temperature (LT) phase of LCO. Therefore, the stoichiometric LCO film was grown under the same C



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Figure 3. Pristine LCO film: the valence states of LCO excited by Al Kα (a), ultraviolet He II (b), and He I (c). The secondary electron cutoff (hν = 21.2 eV) gives eϕLCO = 5.0 ± 0.10 eV. (d) DOS (black curve) and PDOS for Co (blue curve) and O (red curve) in LixCoO2 for different values of x calculated using the LDA + U functional (reproduced with permission from ref 43. Copyright 2009, Royal Society of Chemistry). The energy of the highest occupied state EF was chosen as 0 of the energy scale.

Figure 4. Energy diagram derived from work function, eϕLi, of a Li-foil59 (a); photoemission and O K XANES of a pristine LCO film (b) and of LCO film under the charging potential of 4.2 V (c). EF(4.2 V) is the Fermi level position at the charging state of 4.2 V.

(t2g−eg) transition of the semiconducting LCO obtained from optical absorption measurements.56 The reported band gap values of 1.7, 2.5, and 2.7 eV were obtained from the optical spectroscopy,57 photocurrent experiments,58 and a combination of bremsstrahlung isochromat spectroscopy and XPS measurements,51 respectively. It is worth it to note that a scatter in the experimentally determined band gap values might be assigned to a non-stoichiometric composition of previously studied LCO or to their purity, which is a common problem in the case of synthesized powder materials and which is a crucial parameter for the correct determination of the electronic properties by using surface sensitive techniques. The electrons collected deeper from the surface by changing the excitation energy give similar spectral features of the valence band structure [Figure 2 (middle) and Figure 3a]. Therefore, the electronic configuration of LCO is constant over the film thickness. The contribution of occupied O2p states to the valence band spectra becomes dominant with the decrease of photon energy (Figure 3b,c), since the photoionization crosssection of O2p is higher than Co3d for hν < 100 eV.47 The valence band maximum ELCO VBM is determined by the linear

extrapolation of the onset of the valence band emission to the baseline. The onset energy of valence electrons was determined by using four different photon energies (Figures 2, 3) giving EF LCO − ELCO = 5.4 ± VBM = 0.4 ± 0.2 eV. The ionization potential IP 0.20 eV is determined as the energy distance between ELCO VBM and the vacuum level, Evac (the latter corresponds to the secondary electrons cutoff). Figure 4 shows the energy band diagram of stoichiometric LCO estimated from the presented photoelectron spectroscopy and O K XANES experiments in relation to the EF level of a Li foil. The energy values plotted in the band diagram of Figure 4b are averaged ones over the data which are experimentally obtained on a number of stoichiometric LCO films by using UPS, XPS, and XANES. The relevant errors in the band diagram, Figure 4b, take into account the scatter in these values, as well as the spectrometer resolution. The energetic condition of the anode (Li foil) in contact with a vacuum and in relation to the vacuum level is given from tabulated values of the work function as eϕ = 2.5 eV.59 The values of the pristine LiCoO2 are deduced from our measurements with eϕLCO = 5.0 eV, which translates to a D



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Figure 5. Evolution of (a) the occupied Co3d and O2p states and (b) unoccupied Co3d−O2p hybrid states as a function of the charging state of a LCO film. (c) A schematic show of the density of Co3d states with respect to the Fermi level and the variation of the Ed−d gap vs Li content (x ∼ 0.5 for 4.2 V is taken from ref 65). Above the intrinsic voltage limit, the O− (2p) states are formed due to a hole transfer to the O2− (2p) states (c).

difference in electron chemical potential of Δμ = 2.5 eV, a value which is close to the rest potential difference measured as 2.8 V. It must be noted at this stage that in an electrochemical cell an unknown value of an electrochemical dipole potential drop must be considered in addition for both interfaces Li/ electrolyte and LiCoO2/electrolyte. Also shown in this figure are the energetic position of the conduction band related to the Co3+(3d, eg*)−O2−(2p6) hybrid states as well as Co3+(4s0/ 4p0)−O2−(2p6) hybrid states. The experimentally determined densities of states are in reasonable agreement with published DFT calculations.43 3.2. Stability of the LCO Films under the Electrochemical Li-Deintercalation/Intercalation and Related Changes of Electronic Structure. Li+ extraction from the host material is accompanied by the removal of an electron from (add a hole to) the occupied states to satisfy charge neutrality of the material. In LCO, a valence electron is removed, at first, from the occupied t2g state which is close to EF (see Figure 3), which results in the chemical potential shift to low energies shown recently on a single crystal LCO by photoemission experiments.60 Changes in the valence band spectra measured as a function of the charging state of our thin films are shown in Figure 5a. The VB shift to higher binding energy after forming the contact to the electrolyte (rest potential 3.5 V) is more probably associated with the formation of solid-electrolyte interface (SEI) layer at the cathode surface. The origin of SEI may result from the fact that an electrolyte, where the electron energy level of the highest occupied molecular orbital (HOMO) has a lower ionization potential than the chemical potential of the cathode material, donates an electron until the double layer potential drop compensates this potential difference. The relative energy position of the chemical potential of LCO is expected to be slightly above the highest occupied molecular orbital (HOMO) of a majority of carbon-based electrolytes.61 Therefore, electron transfer from the HOMO to the cathode is not expected. It may also be possible that a difference in the chemical potential of the Li ions in the cathode vs the anode may lead to the formation of double layer as recently shown in adsorption experiments.62 As our photoemission results evidence the SEI formation on the surface of a LCO film after contact with the PC-LiClO4 electrolyte (see Supporting Information, Figure 2S), we attribute this potential shift to the formation of a double

layer induced by Li-ion transfer. Similarly, the formation of a surface layer consisting of organic and inorganic species on the LCO/(EC-DMC-LiPF6) electrolyte interface is established by earlier publications.21,63,64 The valence band photoemission changes considerably when a charging potential is applied, as shown in Figure 5a. With further decreasing x, the position of the Co3d (t2g) is shifted to EF, and the peak becomes broader and is decreased in its intensity with respect to the O2p valence band. The most pronounced changes in the valence electron emissions are observed beyond an applied potential of 4.2 V. A decrease in the intensity of the A spectral feature with respect to the occupied O2p band can be explained by a decrease of the electron population of the Co3d states resulting from the creation of holes in the t2g band in agreement with published data,63 while the broadening of VB structure is caused by an increase of the Co3d−O2p hybridization due to a shortening of the Co−O distance in a CoO6 octahedron promoted by the Co3+ → Co4+ oxidation upon Li deintercalation. On the basis of the SXPS quantitative analysis, the lithium content of x ∼ 0.2 was estimated for the charging potential of 4.2 V, instead of x ∼ 0.5 as predicted by Faraday’s law and experimentally obtained for LCO.65 Underestimated Li concentrations of LCO are more probably assigned with uncertainty of the electron’s IMFP, which depends upon the nature of the material, as well as with an inaccuracy of the numerical calculations of the photoionization cross-sections. It is interesting to compare the valence band spectra to the evolution of O K XANES as a function of the charging voltage state which is shown in Figure 5b. Up to a 4 V charging potential, no visible changes in the unoccupied Co3d−O2p hybridized states are observed, which indicates that the charge compensation caused by a Li+ removal only involves the Co3d (t2g) states and does not affect the oxygen site. Therefore, in this potential range, the chemical description of a Co3+/4+ redox reaction is consistent with a physical argumentation of a downward shift of the Fermi level. However, an increase of the charging voltage by 0.2 V leads to a drastic alteration of the spectral feature A*, which is accompanied by the appearance of the shoulder at the lower energies of the O K edge. The lowering of EF beyond a certain critical value leads to the situation where the Co3d (t2g) states (Co3+/4+ redox couple) cross the top of O2p bands which is accompanied by a hole transfer to the O2p states, as shown in Figure 5b,c. The removal of a significant amount of electrons E



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Figure 6. Evolution of Co2p photoelectron emission (a) and of Co L3 XANES (b) as a function of the charging state of the LCO film. A schematic view of the Co3+ (3d6, t2g3↑t2g3↓eg0) and Co4+ (3d5, t2g3↑t2g2↓eg0) spin configurations for stoichiometric LCO and LixCoO2, respectively, is shown in the inset.

from the O2p band will result in an oxidation of O2− and oxygen loss from the lattice, which is a known reason for degradation of the cathode material.11,66−68 This effect clearly indicates that the charge transfer process in LiCoO2 cannot be treated in terms of a rigid band model. With Li+ deintercalation, the changes in the structural arrangement, e.g., the increase of the c-lattice parameter of the unit cell for x ≥ 0.4 (ref 69) caused by Coulombic repulsion between the CoO2 layers and accompanied by a reduction of the CoO6 slab distance leads to a stronger hybridization and related shift of the Co3d (t2g) toward O2− (2p6) bands, as is schematically shown in Figure 4. The related changes are evidenced by the strongly modified valence band spectra for the charging voltage beyond 4.2 V. It is evident that the Co3d related feature A approaches the O2p bands [Figures 4b,c and 5a], and at the same time the hybridization is increased, which is evident from the equalization of photoemission intensity (for the applied excitation energy of hν = 175 eV, the photoemission cross-section of the 3d states is considerably higher than that of the O2p states, see also ref 45). These experimental results are in close correspondence to DFT calculations of LiCoO2 deintercalation and related changes in electronic structure.9,42,43 The approximation in energy of t2g and O2p bands determines the critical charging voltage potential or “intrinsic voltage limit” for the LCO cathode material.11,61,70 The evolution of Co2p photoelectron emission versus the charging state is shown in Figure 6a. The well-defined Co2p photoelectron emissions of pristine LCO at Ebin ∼ 780 eV and ∼795 eV are due to the Co2p3/2 and Co2p 1/2 spin−orbital splitting, respectively. A low intensity charge transfer satellite, S1, at ΔEm,s ∼ 10 eV from the main Co2p photoelectron line is typical for Co3+ (3d6, t2g3↑t2g3↓eg0) with the LS state electronic configuration. The main Co2p photoelectron emission arises from the 2p53d7L final state, whereas the satellite S is related to the sum of the 2p53d6 and 2p53d8L2 final state configurations (here L is a hole in the ligand p orbital).49,50,71 The electrochemical Li+ deintercalation leads to an asymmetric broadening (tailing) of the main Co2p emission, which starts to be visible at the rest potential of 3.5 V. The tailing of the main peak is attributed to the formation of the Co4+ (3d5, t2g3↑t2g2↓

eg0) electronic configuration with a binding energy higher than the Co2p photoemission line of the Co3+ oxidation state.72 This picture is consistent with the fact that the related tailing to high energies increases with decreasing Li content x. This tailing of the main Co2p emission is accompanied by a loss of the originally given satellite structure. It is interesting to note that the vanishing of the S1 satellite peak (Ebin ∼790 eV) and an increase of a satellite peak S2, which ends in rather structureless satellite region S with enhanced intensity, is typical for mixed Co2+/3+ oxidation states, since Co2+ (3d7,t2g3↑t2g2↓eg2) with HS state will form a very intensive satellite structure, S2 with ΔEm,s′ ∼ 6 eV.49,73 The formation of divalent Co at the surface of the LCO film is related to the surface decomposition reaction involving electron transfer from the electrolyte to the cathode material or due to the chemical reaction at the cathode− electrolyte interface to form the SEI layer. Co L3 XANES spectra of the LCO film as a function of the charging state are shown in Figure 6b. The Co L3 XANES of LCO is dominated by the transition of the Co2p electron to the unoccupied 3d state. The shape of the Co L3 edge of the pristine LCO accounts well to a Co3+ t2g6 (1A1) with a LS (S = 0) ground state configuration, as shown by Montoro et al.,74 and is in agreement with numerous investigations of LCO.54,55,75,76 The low energy shoulder ACoL* is an inherent spectral feature of Co3+ with a LS state (see, for example, refs 54, 55, and 74]. The peak BCoL* and the shoulder CCoL* are assigned to the 2p5t2g6eg1 and 2p5t2g5eg2 final states, respectively, as shown by first principle calculations of Kumagai et al.76 The overall shape of the Co L3 XANES is not markedly influenced by Li+ deintercalation aside from some spectral broadening. Therefore, most Co3d electrons remain in the LS state. However, the relative spectral weight of the shoulder ACoL* is increased after charging LCO above 3.5 V, thereby arguing that Li+ deintercalation induces 3d holes at the Co site. First principle calculations of Co L XANES of Co4+ (t2g5) with LS state (the case of a totally Li+- deintercalated CoO2) and of Co3+ with a HS state show the spectral feature which lies in a similar energy range to that for Co3+ LS ions.74,77 The main difference of the low energy spectral features of Co3+ (LS) and Co4+ (LS)/Co3+ (HS) configurations is the low intensity of F



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Figure 7. CV characteristics of the battery cells with Li1.0Co1.0O2 film deposited on a heated Ti-substrate at TLCO = 550 °C followed by the electrochemical cycling in the 3.0−4.2 V range (a) and in the 3.0−4.4 V range (b). LiFP6-EC-DMC electrolyte is used and a Li foil as the anode material. The scan rate was 0.05 mV/s (a) and 0.1 mV/s (b). The marked area denotes the instability region of LCO.

ACoL* for Co3+ (LS) (see Supporting Information: Figure 3S reproduces the theoretical Co L2,3 XANES from ref 77). The cyclic voltamograms (CV) of the LCO films cycled electrochemically to 4.2 and 4.4 V, respectively, are plotted in Figure 7a and b. The stoichiometric LCO films exhibit the wellknown oxidation and reduction peaks at around 3.9 V. The narrowness of the CV curves is a sign of the well-ordered crystallized structure of the films. No distinctive differences in the intensity, the width, and the polarization of the oxidation and reduction peaks are observed for the LCO film cycled only to 4.2 V, suggesting a good electrochemical reversibility (Figure 7a). In this potential regime, chemical decomposition of LiCoO2 does not occur, and only a hole transfer to Co3d (t2g) states was observed in our photoemission experiments. The slight capacity drop after the first electrochemical cycling of the LCO thin film battery cell is assigned to the formation of the solid-electrolyte interface (SEI) layer. In contrast, the CV curve of a LCO film charged to >4.2 V demonstrates a weak rise of the current [denoted as the marked area]. At the same time, the following reduction peak is less intensive compared to that for the oxidation peak. Both these observations are related and are known as instability of LixCoO2 for x < 0.5.69 In this potential regime, we have observed the marked changes of the electronic structure of the LCO film, as deduced from our photoelectron spectroscopic and O K and Co L3 XANES studies. Thus, the limited capacity of LiCoO2 in practical cells (only half of the lithium can be removed) is related to instability of LCO associated with the intrinsic voltage limit. This instability is induced by the change in electronic structure with Li+ deintercalation. For values of x < 0.5 in LixCoO2, the Co3d (t2g) and O2p states start to shift energetically, approaching each other and forming a stronger hybridized state. As a consequence, hole transfer also affects the O2−(2p) states. Thus, the observed degradation of the LCO film exposed to the overcharged potential, Figure 7b, is mostly related to the O2− → O− oxidation after exceeding the intrinsic voltage limit of the cathode material. Subsequently, these highly reactive (oxidative) surface species of the LixCoO2 film may also be involved in the formation of a thicker SEI film, which is evident from many XPS studies.21,63,64,78 Our electron spectroscopy results are in agreement with the works of the Manthiram’s group (see, for example, ref 66) where it was shown that the oxidation state of cobalt for the chemically delithiated LixCoO2 is increased in the range of 0.5 ∼ x < 1.0, whereas oxygen loss occurs at deeper lithium extraction.

4. CONCLUSIONS The evolution of the electronic structure during Li+-electrochemical deintercalation of the LixCoO2 thin film cathode materials has been investigated using a dedicated sample transfer setup from the electrolyte to UHV. The combination of SXPS, XPS, and O K and Co L XANES provides access to the occupied core level and valence band density of states, as well as the unoccupied conduction bands. The gradual low spin Co3+ (3d6, t2g3↑t2g3↓eg0) to low spin Co4+ (3d5, t2g3↑t2g2↓eg0) change of electronic configuration starts at the beginning of Li+ deintercalation and continues up to an x value of about 0.5 related to electrode potential of 4.2 V. The Co3+ → Co4+ oxidation is accompanied by lowering chemical potential (Fermi level) of LixCoO2 leading to higher ionization potentials and by an increase of Co3d−O2p hybridization. The oxygen 2p6 states are involved in charge compensation at deeper Li+ deintercalation (x < 0.5, the potential >4.2 V), when the Co3+/4+ redox couple is pinned at the top of the O2p bands, leading to a hole transfer to the O2−(2p6) states. The observed changes of the electronic configuration of the oxygen site establish an intrinsic voltage limit for LiCoO2 above which the decomposition of the cathode material takes place. The charge compensation reaction can be written as 1.0 > x > 0.5

Li+Co3 +(O2 −)2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Li+x (Co3x +Co14−+x )(O2 −)2 x < 0.5

4+ ⎯⎯⎯⎯⎯→ Li+x (Co30.5+Co0.5 )(O2 −O−)2

As this charge compensation mechanism is mostly independent of any subsequent reactions with the electrolyte and is determined by intrinsic voltage limit, LiCoO2 cannot be stabilized even with solid electrolytes, ergo in all solid state batteries.

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ASSOCIATED CONTENT

S Supporting Information *

(Figure 1 S) High resolution SXPS and XPS of the carbon binding energy range of a pristine LCO film. (Figure 2 S) The O 1s and C 1s photoelectron spectra of the evolution of the SEI layer versus the charging/discharging state. The spectra are measured at the enhanced surface sensitivity. (Figure 3 S) Theoretical Co L2,3 XANES of Co2+, high-spin Co3+, low-spin Co3+, high spin Co4+, and low spin Co4+ reproduced with from ref 77. This material is available free of charge via the Internet at http://pubs.acs.org. G



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by Deutsche Forschungsgemeinschaf t (German Research Foundation) in the frame of the Research Collaborative Centre SFB 595 “Electrical Fatigue in Functional Materials.” The authors thank Dr. Ralf Hunger, Dr. Patrick Hoffman, Prof. Christian Pettenkofer from HMI Berlin, Prof. Dieter Schmeisser from BTU Cottbus, and BESSY staff for supporting the synchrotron beamtimes.

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Nonrigid Band Behavior of the Electronic Structure of LiCoO2 Thin

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