Electrochemical properties of iron oxide by cyclic voltammetry and electrochemical impedance spectroscopy

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Preparation of iron oxide thin films

Pure (99.99 wt%) iron plates (1 mm thick) were purchased from Goodfellow and then cut into square (8  8 mm2) substrate samples. The surfaces were prepared by mechanical polishing with diamond spray down to 1/4 μm and then rinsed in ultrasonic baths of acetone, ethanol and Millipore© water (resistivity > 18 MΩ cm) for 2 min and dried in a flow of compressed air.
Iron oxide thin films were prepared by thermal oxidation in air. The samples were placed in a quartz tube that could be inserted in a cylindrical oven set in temperature at 300°C ± 5 oC. The samples were placed in the pre-thermalized oven for 5 min and then the quartz tube was quenched with 0oC water for cooling. The average thickness of the pristine oxide thin films was 85 – 95 nm as estimated from XPS depth profile analysis. Raman spectroscopy (Horiba Xplora system, Ar+ laser, 􀈜 = 532 nm) was employed for phase identification of the thermal oxide.

Electrochemical measurements

All electrochemical measurements (CV and EIS) were performed in an Ar-filled glovebox (Jacomex) with H2O and O2 contents lower than 1 ppm. A home-made anode for lithium-ion batteries three electrode glass cell (see Appendix 1) similar to typical commercial cells was used with the iron oxide thin film (prepared as described above) as working electrode and two Li foils (Sigma-Aldrich) used as reference and counter electrodes (~2 cm2). The surface of the counter or reference electrode was around 2 cm2. All potentials hereafter are given versus Li/Li+. The cell was operated at room temperature using an Autolab (AUT30) electrochemical workstation. The geometrical working electrode area was delimited to 0.28 cm2 by a O-ring. The electrolyte was 1 M LiClO4 in propylene carbonate (1 M LiClO4-PC, Sigma-Aldrich). Cyclic voltammograms were recorded in the potential range 0.01 – 3.0 V at a scanning rate of 0.2 mV s-1, starting form OCP (~3.0 V) into the cathodic direction. For EIS measurements, the frequency range was 10 mHz to 1 MHz and the potential perturbation was 5 mV. The cell was kept at selected potential value for more than 30 min before performing EIS and the equilibrium of the cells was deemed to be reached when the change of voltage was less than 0.01 V in 10 min.
For surface and depth profile analysis the thin films were electrochemically treated at various stages of the first cycle. After scanning the potential to the selected value, the cell was disassembled and the samples were rinsed with acetonitrile (99.8%, Sigma-Aldrich) and dried with Ar flow. The samples were then transferred directly from the glovebox to the ultra-high vacuum XPS analysis chamber.25 Thereafter, they were transferred in an air-tight vessel under argon atmosphere from the XPS to the ToF-SIMS system.

X-ray photoelectron spectroscopy (XPS)

XPS analysis was carried out on a VG ESCALAB 250 spectrometer with a UHV preparation chamber directly connected to the glovebox. Base pressure during analysis was 10-9 mbar. An Al Kα monochromatized radiation (hν = 1486.6 eV) was employed as X-ray source. Take-off angle of the photoelectrons was 90o. Survey spectra were recorded with a pass energy of 100 eV at a step size of 1 eV and high resolution spectra of the Fe2p, O1s, C1s core level and valence band (VB) regions were recorded with a pass energy of 20 eV at a step size of 0.1 eV. The data processing (curve fitting) was performed using the Avantage software provided by Thermo Electron Corporation. An iterative Shirley-type background and Lorentzian/Gaussian peak shape at a fixed ratio of 30/70 were used. Binding energies were calibrated by setting the C1s hydrocarbon (-CH2-CH2-) component peak at 285.0 eV. XPS depth profiling was performed by combining analysis with Ar+ sputtering performed in the preparation chamber. A 2 keV Ar+ sputter beam giving 0.2 μA mm-2 of sample current yielded a calibrated etching rate of about 2.5 nm min-1. The depth profiles were stopped after etching about 150 nm.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS)

Depth profiling was also performed using a ToF-SIMS 5 spectrometer (Ion Tof – Munster, Germany). The operating pressure of the spectrometer was about 10-9 mbar. A pulsed 25 keV Bi+ primary ion source was employed for analysis, delivering 1.2 pA current over a 100 × 100 μm2 area. Depth profiling was carried out using a 1 keV Cs+ sputter beam giving a 70 nA target current over a 300 × 300 μm2 area. Ion-Spec software was used for acquiring and processing the data. Negative ion depth profiles were recorded for better sensitivity to fragments originating from oxide matrices.

Modification of iron oxide thin film upon the first lithiation/delithiation shown by XPS

The XP C1s, O1s, Fe2p, Fe3p-Li1s core levels and VB region spectra for the pristine iron oxide thin film and samples treated at the selected potentials are shown in Figure 4. Binding energies (EB), full widths at half-maximum (fwhm) and relative intensity of the component peaks obtained by peak fitting are given in Table 1.

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XPS depth profile analysis of sample lithiated at 0.01 V

Figure 5 presents the XP C1s, O1s, Fe2p and Fe3p-Li1s core levels and VB region spectra of the sample lithiated to 0.01 V for increasing Ar+ sputtering time. The EB and fwhm values and relative intensities of the component peaks are compiled in Table 2.
As discussed above, C1s and O1s spectra before sputtering are indicative of the composition (Li2CO3 and ROCO2Li) of the SEI layer formed at the surface of the electrode. The photoelectrons emitted by the converted electrode are fully attenuated by the SEI surface layer and, as a consequence, the Fe3p-Li1s core level and VB (3-30 eV) spectra are characteristic of the SEI layer (no Fe3p component).
After 2 min Ar+ sputtering (about 5 nm etched), the C1sA component markedly decreases in intensity, indicating removal of the outer part of the surface carbonaceous layer mostly consisting of hydrocarbons. The decrease of the relative intensity of the O1sC component indicates that ROCO2Li species are also preferentially removed. Complete attenuation of the Fe2p intensity is still observed indicating that profiling is still in the bulk SEI layer region. Consistently, the Fe3p-Li1s core level and VB (3-30 eV) spectra are still those of the SEI layer with slight changes in the relative intensities of the VB caused by removal of the outermost hydrocarbons.

Table of contents :

Abstract…
Chapter 1. State of the art and objectives
1 Lithium-ion batteries (LIBs)
1.1 Principle of LIBs
1.2 Structure of LIBs
1.3 Reversible energy storage mechanisms in LIBs
2 Surface and interface science in LIBs – SEI layer
2.1 Formation of SEI layer – mechanism and features
2.2 Variation of SEI layer – influential factors
2.3 Characterization of SEI layer – research methods and techniques
3 Iron oxide for LiBs – state of the art
3.1 Structure and electrochemical performance of hematite (α-Fe2O3).
3.2 Nanostructured iron oxide
3.3 Thin-film iron oxide
3.4 Scientific issues of iron oxide in LIBs
4 Objectives of this work
5 Contents of the thesis
References
Chapter 2. Combined surface and electrochemical study of the lithiation/delithiation mechanism of iron oxide thin film anode for lithium-ion batteries
1 Introduction
2 Experimental section
2.1 Preparation of iron oxide thin films
2.2 Electrochemical measurements
2.3 X-ray photoelectron spectroscopy (XPS)
2.4 Time-of-flight secondary ion mass spectrometry (ToF-SIMS)
3 Results and discussion
3.1 Raman phase identification
3.2 Electrochemical properties of iron oxide by cyclic voltammetry and electrochemical impedance spectroscopy
3.3 Modification of iron oxide thin film upon the first lithiation/delithiation shown by XPS
3.4 XPS depth profile analysis of sample lithiated at 0.01 V
3.5 ToF-SIMS depth profiles analysis of pristine, lithiated and delithiated samples
4 Conclusions
References
Chapter 3. Aging-induced chemical and morphological modifications of thin film iron oxide electrodes for lithium-ion batteries
1 Introduction
2 Experimental methods
3 Results and discussion
3.1 Galvanostatic cycling
3.2 First 15 cyclic voltammograms of the iron oxide thin-film electrode.
3.3 Surface chemistry upon cycling studied by XPS
3.4 Surface and bulk modifications analyzed by ToF-SIMS
3.5 Morphology modifications studied by SEM and AFM
4 Conclusions
References
Supporting Information for Chapter 3
References
Chapter 4. Kinetics evaluation of thin film α-Fe2O3 negative electrode for lithium-ion batteries
1 Introduction
2 Experimental methods
3 Results and discussion
3.1 Structure and composition
3.2 Diffusion evaluation from cyclic voltammetry
3.3 Galvanostatic discharge-charge
3.4 Diffusion evaluation from EIS
3.5 Diffusion evaluation from ToF-SIMS
3.6 Influence of surface modifications of the iron oxide on kinetics
4 Conclusions
References
Chapter 5. Binary (Fe, Cr)-oxide thermally grown on stainless steel current collector as anode material for lithium-ion batteries
1 Introduction
2 Experimental methods
2.1 Preparation of (Fe, Cr)-binary oxide thin films
2.2 Electrochemical measurements
2.3 Spectroscopic analysis
2.4 Microscopic characterization
3 Results and discussion
3.1 Composition and phases
3.2 Conversion mechanism of binary oxide showed by cyclic voltammetry
3.3 Cycling performance by galvanostatic discharge/charge
3.4 XPS analysis upon cycling
3.5 ToF-SIMS depth profiling
3.6 SEM characterization
3.7 AFM characterization
4 Conclusions
References
Chapter 6. Conclusions and perspectives
1. Conclusions
2. Perspectives
Appendix 1
1. Sample preparation
1.1 Mechanical polishing
1.2 Thermal oxidation
2. Electrochemical measurements
2.1 Cyclic voltammetry (CV)
2.2 Electrochemical impedance spectroscopy (EIS)
2.3 Galvanostatic charge-discharge
3. X-ray photoelectron spectroscopy (XPS)
3.1 Principles
3.2 Instrument
4. Time-of-flight secondary ion mass spectrometry (ToF-SIMS)
4.1 Principles
4.2 Instrument
5. Scanning electron microscopy (SEM)
5.1 Principles
5.2 Instrument
6. Atomic force microscopy (AFM)
6.1 Principles
6.2 Instrument
7. Raman spectroscopy
7.1 Principles
7.2 Instrument
References
Appendix 2
1. ALD iron oxide nanomaterial for LIBs
2. Fe-Air batteries
References

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