The electronic properties of LiCoO have been studied by theoretical band-structure calculations (... more The electronic properties of LiCoO have been studied by theoretical band-structure calculations (using density functional theory) and experimental methods (photoemission). Synchrotron-induced photoelectron spectroscopy, resonant photoemission spectroscopy (ResPES), and soft x-ray absorption (XAS) have been applied to investigate the electronic structure of both occupied and unoccupied states. High-quality PES spectra were obtained from stoichiometric and highly crystalline LiCoO thin films deposited ''in situ'' by rf magnetron sputtering. An experimental approach of separating oxygen- and cobalt-derived (final) states by ResPES in the valence-band region is presented. The procedure takes advantage of an antiresonant behavior of cobalt-derived states at the 3p-3d excitation threshold. Information about the unoccupied density of states has been obtained by O K XAS. The structure of the Co L absorption edge is compared to semiempirical charge-transfer multiplet calculations. The experimental results are furthermore compared with band-structure calculations considering three different exchange potentials [generalized gradient approximation (GGA), using a nonlocal Hubbard U (GGA+U) and using a hybrid functional (Becke, three-parameter, Lee-Yang-Parr [B3LYP])]. For these different approaches total density of states and partial valence-band density of states have been investigated. The best qualitative agreement with experimental results has been obtained by using a GGA+U functional with U=2.9 eV.
ABSTRACT In this study, a comprehensive experimental in situ analysis of the evolution of the occ... more 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.
ABSTRACT The electronic properties of the LiMO2 (M = Ni, Co) thin film cathode materials grown by... more ABSTRACT The electronic properties of the LiMO2 (M = Ni, Co) thin film cathode materials grown by RF sputtering/co-sputtering are in situ studied by X-ray photoelectron spectroscopy (XPS). Stoichiometric Li1.0Co1.0O2 thin films deposited on a heated substrate at T = 500–550 °C reveal the Co3+ (t2g6eg0) ground state configuration in the low spin (LS) state. Stoichiometry of the Lix(Ni,Co)O2 films and the valence and spin states of the Ni ions depend strongly on the growth conditions. The electronic configuration of stoichiometric Li1.0Ni0.5Co0.5O2 is described as the Ni3+ (t2g6eg1) LS and Co3+ (t2g6eg0) LS states. The Li-deficient Lix<1.0(Ni,Co)O2 exhibits Ni2+ (t2g6eg2) in the high spin (HS) and Co3+ (t2g6eg0) in LS states. The reduction of the trivalent Ni ions to Ni2+ (t2g6eg2) with a HS state electronic configuration is related to the evaporation of Li2O at elevated substrate temperatures coupled to a loss of O2 due to an internal oxidation reaction of O2− lattice ions induced by the strongly oxidizing Ni3+ ions. Owing to the stable Co3+ (t2g6eg0) with a LS state electronic configuration, Li1.0Co1.0O2 thin films cycled to 4.2 V exhibit a very good electrochemical reversibility. Li1.0Ni0.5Co0.5O2 films annealed at the same temperature as for Li1.0Co1.0O2 manifest a broadening of the oxidation/reduction peaks of the cyclic voltammogram (CV) curves with a strong current drop after the first step of the electrochemical Li-deintercalation. The observed irreversibility of the Li-intercalation/deintercalation process is attributed to instability of the Ni3+ (t2g6eg1) ions. Temperatures of the deposition/annealing above 750 °C lead to the phase separation of the Lix(Ni,Co)O2 films, a strong Li deficiency, the occurrence of Co2+ (t2g5eg2) with HS ions and consequently a complete degeneration of the electrochemical cyclability.
The layered oxide LiNixMnyCozO2 (x+y+z=1) is a promising cathode material for the rechargeable li... more The layered oxide LiNixMnyCozO2 (x+y+z=1) is a promising cathode material for the rechargeable lithium ion batteries. The valence state of the freshly prepared LiNixMnyCozO2 as reported by Shaju et al. [1], as well as the influence of the electrochemical cycling of LiNixMnyCozO2 based batteries on the valence state at the cathode-electrolyte interface was studied by X-ray photoelectron spectroscopy (XPS). The quantitative analysis carried out using XPS and Inductively Coupled Plasma analysis with Optical Emission Spectrometry (ICP-OES) shows that the surface of the cathodes is not stoichiometric but lithium deficient as compared to the bulk. By the joint analysis of the photoelectron peak energy position, the shape of the photoelectron spectra, as well as the satellite structure of the 3d metals, it is shown that the Li-deficiency is accompanied by a change of the valency of the cations towards oxidation resulting in the mixed valence state. The data indicate that not only Co, but M...
LiMO2 (M=Co, Ni) thin films are grown by RF sputtering/co-sputtering and used as a positive catho... more LiMO2 (M=Co, Ni) thin films are grown by RF sputtering/co-sputtering and used as a positive cathode material in Li-rechargeable batteries. Electronic properties, crystallographic structure, and performance of the batteries are investigated depending on deposition temperature, gas mixture, and other growth conditions. The films are studied by in-situ photoelectron spectroscopy (XPS, UPS, SXPS) and X-ray absorption spectroscopy (XAS), ex-situ X-ray diffraction (XRD), and electrochemical cycling of the Li(NiCo)O2 in the Swagelock-type battery cell with LiPF6/EC/DMC liquid electrolyte and a Li-foil as an anode material. The LiNi0.5Co0.5O2 films deposited at room temperature are amorphous and stoichiometic with Ni3+ and Co3+/Co4+ mixed ions. Annealing of the amorphous films at moderate temperatures results in (003)–oriented R3m stoichiometric crystal structure with Co3+. The electrochemical performance of the stoichiometric thin films is similar to the data reported for the bulk samples....
... Matter 2, 9653 (1990). D. Ensling, F. Fernandez-Madrigal, A. Thissen, and W. Jaegermann, in 2... more ... Matter 2, 9653 (1990). D. Ensling, F. Fernandez-Madrigal, A. Thissen, and W. Jaegermann, in 203rd ECS Meeting, edited by K. Zaghib, CM Julien, and J. Prakash (Proc. ECS, Paris, 2003), p. 585. ... 106, 173 (1997). T. Bredow, K. Jug, and RA Evarestov, Phys. ...
Li(x)CoO(2) and Li(x)NiO(2) (0.5 &amp... more Li(x)CoO(2) and Li(x)NiO(2) (0.5 < x < 1) are used as prototype cathode materials in lithium ion batteries. Both systems show degradation and fatigue when used as cathode material during electrochemical cycling. In order to analyze the change of the structure and the electronic structure of Li(x)CoO(2) and Li(x)NiO(2) as a function of Li content x in detail, we have performed X-ray diffraction studies, photoelectron spectroscopy (PES) investigations and band structure calculations for a series of compounds Li(x)(Co,Ni)O(2) (0 < x < or = 1). The calculated density of states (DOS) are weighted by theoretical photoionization cross sections and compared with the DOS gained from the PES experiments. Consistently, the experimental and calculated DOS show a broadening of the Co/Ni 3d states upon lithium de-intercalation. The change of the shape of the experimental PES curves with decreasing lithium concentration can be interpreted from the calculated partial DOS as an increasing energetic overlap of the Co/Ni 3d and O 2p states and a change in the orbital overlap of Co/Ni and O wave functions.
ABSTRACT X-Ray photoelectron spectroscopy (XPS) has been used to characterise the surfaces of car... more ABSTRACT X-Ray photoelectron spectroscopy (XPS) has been used to characterise the surfaces of carbon-coated Li2FeSiO4 cathodes extracted from Li-ion batteries in both a charged and discharged state. 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiPF6) based electrolytes were used with ethylene carbonate (EC) and diethyl carbonate (DEC) as organic solvents. The LiTFSI-based electrolyte exhibited high salt stability and no significant formation of LiF. However, solvent reaction products from EC were found together with lithium carbonate. A LiPF6-based electrolyte, on the other hand, showed inferior salt stability with LixPFy, LixPOyFz and LiF species formed on the surface. Solvent reaction products together with lithium carbonate were also found. There are also indications that Li2FeSiO4 is degraded by the HF formed in the electrolyte by the hydrolysis of LiPF6. A better understanding of the surface chemistry of carbon-coated Li2FeSiO4 after the first cycles in a Li-ion battery has thus been achieved, thereby facilitating the optimisation of Li-ion batteries based on this potentially cheap and electrochemically most promising cathode material giving excellent capacity retention: <3% drop over 120 cycles.
... MS Bhuvaneswari .R. Hunger .W. Jaegermann (*) Surface Science Division, Institute of Material... more ... MS Bhuvaneswari .R. Hunger .W. Jaegermann (*) Surface Science Division, Institute of Materials Science, Darmstadt University of Technology, Petersenstraße 23, 64287 Darmstadt, Germany e-mail: jaegerw@surface.tu-darmstadt.de Q.-H. Wu Department of Physics, Institute ...
... Andreas Thissen, David Ensling, F. Javier Fernández Madrigal, and Wolfram Jaegermann. Surface... more ... Andreas Thissen, David Ensling, F. Javier Fernández Madrigal, and Wolfram Jaegermann. Surface Science Institute, Department of Materials Science, Darmstadt University of Technology, Petersenstrasse 23, 64287 Darmstadt, Germany. Ricardo Alcántara, Pedro Lavela, and ...
ABSTRACT The study delivers detailed information about different lithium phases contributing to t... more ABSTRACT The study delivers detailed information about different lithium phases contributing to the complex surface chemistry of active electrodes in lithium ion batteries. Photoemission spectroscopy has been used to investigate the oxidation of metallic lithium as well as the reaction with air. From XP and UP valence band spectra characteristic emissions have been separated in order to help identifying lithium containing surface phases on intercalation electrodes. Characteristic spectral shapes of Li2CO3, Li2O2, Li2O and LiOH compounds are presented. The spectral features of a reacted lithium surface are compared to carbonate emissions on a LiCoO2 thin film cathode.
... sample preparation sample analysis Fig. 2. Schematic drawing of Darmstadt Integrated System f... more ... sample preparation sample analysis Fig. 2. Schematic drawing of Darmstadt Integrated System for Materials Research (DAISY-MAT) Page 3. ... [12] D. Ensling, FJ Fernandez-Madrigal, A. Thiûen, W. Jae-germann, in preparation. [13] SL Qiu, CL Lin, J. Chen, et al.,Phys. Rev. ...
The electronic properties of LiCoO have been studied by theoretical band-structure calculations (... more The electronic properties of LiCoO have been studied by theoretical band-structure calculations (using density functional theory) and experimental methods (photoemission). Synchrotron-induced photoelectron spectroscopy, resonant photoemission spectroscopy (ResPES), and soft x-ray absorption (XAS) have been applied to investigate the electronic structure of both occupied and unoccupied states. High-quality PES spectra were obtained from stoichiometric and highly crystalline LiCoO thin films deposited ''in situ'' by rf magnetron sputtering. An experimental approach of separating oxygen- and cobalt-derived (final) states by ResPES in the valence-band region is presented. The procedure takes advantage of an antiresonant behavior of cobalt-derived states at the 3p-3d excitation threshold. Information about the unoccupied density of states has been obtained by O K XAS. The structure of the Co L absorption edge is compared to semiempirical charge-transfer multiplet calculations. The experimental results are furthermore compared with band-structure calculations considering three different exchange potentials [generalized gradient approximation (GGA), using a nonlocal Hubbard U (GGA+U) and using a hybrid functional (Becke, three-parameter, Lee-Yang-Parr [B3LYP])]. For these different approaches total density of states and partial valence-band density of states have been investigated. The best qualitative agreement with experimental results has been obtained by using a GGA+U functional with U=2.9 eV.
ABSTRACT In this study, a comprehensive experimental in situ analysis of the evolution of the occ... more 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.
ABSTRACT The electronic properties of the LiMO2 (M = Ni, Co) thin film cathode materials grown by... more ABSTRACT The electronic properties of the LiMO2 (M = Ni, Co) thin film cathode materials grown by RF sputtering/co-sputtering are in situ studied by X-ray photoelectron spectroscopy (XPS). Stoichiometric Li1.0Co1.0O2 thin films deposited on a heated substrate at T = 500–550 °C reveal the Co3+ (t2g6eg0) ground state configuration in the low spin (LS) state. Stoichiometry of the Lix(Ni,Co)O2 films and the valence and spin states of the Ni ions depend strongly on the growth conditions. The electronic configuration of stoichiometric Li1.0Ni0.5Co0.5O2 is described as the Ni3+ (t2g6eg1) LS and Co3+ (t2g6eg0) LS states. The Li-deficient Lix<1.0(Ni,Co)O2 exhibits Ni2+ (t2g6eg2) in the high spin (HS) and Co3+ (t2g6eg0) in LS states. The reduction of the trivalent Ni ions to Ni2+ (t2g6eg2) with a HS state electronic configuration is related to the evaporation of Li2O at elevated substrate temperatures coupled to a loss of O2 due to an internal oxidation reaction of O2− lattice ions induced by the strongly oxidizing Ni3+ ions. Owing to the stable Co3+ (t2g6eg0) with a LS state electronic configuration, Li1.0Co1.0O2 thin films cycled to 4.2 V exhibit a very good electrochemical reversibility. Li1.0Ni0.5Co0.5O2 films annealed at the same temperature as for Li1.0Co1.0O2 manifest a broadening of the oxidation/reduction peaks of the cyclic voltammogram (CV) curves with a strong current drop after the first step of the electrochemical Li-deintercalation. The observed irreversibility of the Li-intercalation/deintercalation process is attributed to instability of the Ni3+ (t2g6eg1) ions. Temperatures of the deposition/annealing above 750 °C lead to the phase separation of the Lix(Ni,Co)O2 films, a strong Li deficiency, the occurrence of Co2+ (t2g5eg2) with HS ions and consequently a complete degeneration of the electrochemical cyclability.
The layered oxide LiNixMnyCozO2 (x+y+z=1) is a promising cathode material for the rechargeable li... more The layered oxide LiNixMnyCozO2 (x+y+z=1) is a promising cathode material for the rechargeable lithium ion batteries. The valence state of the freshly prepared LiNixMnyCozO2 as reported by Shaju et al. [1], as well as the influence of the electrochemical cycling of LiNixMnyCozO2 based batteries on the valence state at the cathode-electrolyte interface was studied by X-ray photoelectron spectroscopy (XPS). The quantitative analysis carried out using XPS and Inductively Coupled Plasma analysis with Optical Emission Spectrometry (ICP-OES) shows that the surface of the cathodes is not stoichiometric but lithium deficient as compared to the bulk. By the joint analysis of the photoelectron peak energy position, the shape of the photoelectron spectra, as well as the satellite structure of the 3d metals, it is shown that the Li-deficiency is accompanied by a change of the valency of the cations towards oxidation resulting in the mixed valence state. The data indicate that not only Co, but M...
LiMO2 (M=Co, Ni) thin films are grown by RF sputtering/co-sputtering and used as a positive catho... more LiMO2 (M=Co, Ni) thin films are grown by RF sputtering/co-sputtering and used as a positive cathode material in Li-rechargeable batteries. Electronic properties, crystallographic structure, and performance of the batteries are investigated depending on deposition temperature, gas mixture, and other growth conditions. The films are studied by in-situ photoelectron spectroscopy (XPS, UPS, SXPS) and X-ray absorption spectroscopy (XAS), ex-situ X-ray diffraction (XRD), and electrochemical cycling of the Li(NiCo)O2 in the Swagelock-type battery cell with LiPF6/EC/DMC liquid electrolyte and a Li-foil as an anode material. The LiNi0.5Co0.5O2 films deposited at room temperature are amorphous and stoichiometic with Ni3+ and Co3+/Co4+ mixed ions. Annealing of the amorphous films at moderate temperatures results in (003)–oriented R3m stoichiometric crystal structure with Co3+. The electrochemical performance of the stoichiometric thin films is similar to the data reported for the bulk samples....
... Matter 2, 9653 (1990). D. Ensling, F. Fernandez-Madrigal, A. Thissen, and W. Jaegermann, in 2... more ... Matter 2, 9653 (1990). D. Ensling, F. Fernandez-Madrigal, A. Thissen, and W. Jaegermann, in 203rd ECS Meeting, edited by K. Zaghib, CM Julien, and J. Prakash (Proc. ECS, Paris, 2003), p. 585. ... 106, 173 (1997). T. Bredow, K. Jug, and RA Evarestov, Phys. ...
Li(x)CoO(2) and Li(x)NiO(2) (0.5 &amp... more Li(x)CoO(2) and Li(x)NiO(2) (0.5 < x < 1) are used as prototype cathode materials in lithium ion batteries. Both systems show degradation and fatigue when used as cathode material during electrochemical cycling. In order to analyze the change of the structure and the electronic structure of Li(x)CoO(2) and Li(x)NiO(2) as a function of Li content x in detail, we have performed X-ray diffraction studies, photoelectron spectroscopy (PES) investigations and band structure calculations for a series of compounds Li(x)(Co,Ni)O(2) (0 < x < or = 1). The calculated density of states (DOS) are weighted by theoretical photoionization cross sections and compared with the DOS gained from the PES experiments. Consistently, the experimental and calculated DOS show a broadening of the Co/Ni 3d states upon lithium de-intercalation. The change of the shape of the experimental PES curves with decreasing lithium concentration can be interpreted from the calculated partial DOS as an increasing energetic overlap of the Co/Ni 3d and O 2p states and a change in the orbital overlap of Co/Ni and O wave functions.
ABSTRACT X-Ray photoelectron spectroscopy (XPS) has been used to characterise the surfaces of car... more ABSTRACT X-Ray photoelectron spectroscopy (XPS) has been used to characterise the surfaces of carbon-coated Li2FeSiO4 cathodes extracted from Li-ion batteries in both a charged and discharged state. 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiPF6) based electrolytes were used with ethylene carbonate (EC) and diethyl carbonate (DEC) as organic solvents. The LiTFSI-based electrolyte exhibited high salt stability and no significant formation of LiF. However, solvent reaction products from EC were found together with lithium carbonate. A LiPF6-based electrolyte, on the other hand, showed inferior salt stability with LixPFy, LixPOyFz and LiF species formed on the surface. Solvent reaction products together with lithium carbonate were also found. There are also indications that Li2FeSiO4 is degraded by the HF formed in the electrolyte by the hydrolysis of LiPF6. A better understanding of the surface chemistry of carbon-coated Li2FeSiO4 after the first cycles in a Li-ion battery has thus been achieved, thereby facilitating the optimisation of Li-ion batteries based on this potentially cheap and electrochemically most promising cathode material giving excellent capacity retention: <3% drop over 120 cycles.
... MS Bhuvaneswari .R. Hunger .W. Jaegermann (*) Surface Science Division, Institute of Material... more ... MS Bhuvaneswari .R. Hunger .W. Jaegermann (*) Surface Science Division, Institute of Materials Science, Darmstadt University of Technology, Petersenstraße 23, 64287 Darmstadt, Germany e-mail: jaegerw@surface.tu-darmstadt.de Q.-H. Wu Department of Physics, Institute ...
... Andreas Thissen, David Ensling, F. Javier Fernández Madrigal, and Wolfram Jaegermann. Surface... more ... Andreas Thissen, David Ensling, F. Javier Fernández Madrigal, and Wolfram Jaegermann. Surface Science Institute, Department of Materials Science, Darmstadt University of Technology, Petersenstrasse 23, 64287 Darmstadt, Germany. Ricardo Alcántara, Pedro Lavela, and ...
ABSTRACT The study delivers detailed information about different lithium phases contributing to t... more ABSTRACT The study delivers detailed information about different lithium phases contributing to the complex surface chemistry of active electrodes in lithium ion batteries. Photoemission spectroscopy has been used to investigate the oxidation of metallic lithium as well as the reaction with air. From XP and UP valence band spectra characteristic emissions have been separated in order to help identifying lithium containing surface phases on intercalation electrodes. Characteristic spectral shapes of Li2CO3, Li2O2, Li2O and LiOH compounds are presented. The spectral features of a reacted lithium surface are compared to carbonate emissions on a LiCoO2 thin film cathode.
... sample preparation sample analysis Fig. 2. Schematic drawing of Darmstadt Integrated System f... more ... sample preparation sample analysis Fig. 2. Schematic drawing of Darmstadt Integrated System for Materials Research (DAISY-MAT) Page 3. ... [12] D. Ensling, FJ Fernandez-Madrigal, A. Thiûen, W. Jae-germann, in preparation. [13] SL Qiu, CL Lin, J. Chen, et al.,Phys. Rev. ...
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