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LiMO2 (M = Ni, Co) thin film cathode materials: A correlation between the valence state of transition metals and the electrochemical properties

Cherkashinin, Gennady and Ensling, David and Jaegermann, Wolfram (2014):
LiMO2 (M = Ni, Co) thin film cathode materials: A correlation between the valence state of transition metals and the electrochemical properties.
In: Journal of Materials Chemistry A, pp. 3571-3580, 2, (10), ISSN 2050-7488, [Online-Edition: http://dx.doi.org/10.1039/c3ta14509c],
[Article]

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.

Item Type: Article
Erschienen: 2014
Creators: Cherkashinin, Gennady and Ensling, David and Jaegermann, Wolfram
Title: LiMO2 (M = Ni, Co) thin film cathode materials: A correlation between the valence state of transition metals and the electrochemical properties
Language: English
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.

Journal or Publication Title: Journal of Materials Chemistry A
Volume: 2
Number: 10
Divisions: 11 Department of Materials and Earth Sciences > Material Science > Surface Science
DFG-Collaborative Research Centres (incl. Transregio) > Collaborative Research Centres > CRC 595: Electrical fatigue > A - Synthesis > Subproject A3: Boundary layers and thin films of ionic conductors: Electronic structure, electrochemical potentials, defect formation and degradation mechanisms
11 Department of Materials and Earth Sciences > Material Science
DFG-Collaborative Research Centres (incl. Transregio) > Collaborative Research Centres > CRC 595: Electrical fatigue > A - Synthesis
DFG-Collaborative Research Centres (incl. Transregio) > Collaborative Research Centres > CRC 595: Electrical fatigue
11 Department of Materials and Earth Sciences
Zentrale Einrichtungen
DFG-Collaborative Research Centres (incl. Transregio) > Collaborative Research Centres
DFG-Collaborative Research Centres (incl. Transregio)
Date Deposited: 14 Apr 2014 10:21
Official URL: http://dx.doi.org/10.1039/c3ta14509c
Additional Information:

SFB 595 A3

Identification Number: doi:10.1039/c3ta14509c
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