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Features of ideal SEI
The formation of the SEI layer on the electrode surface will prevent further decomposition of the electrolyte in the successive cycles. Therefore, an ideal SEI layer should be a compact layer and well adhered onto the electrode material, be stable and insoluble in the electrolyte even at high temperatures, and have minimum electronic and maximum Li+ conductivity. Moreover, an ideal SEI should also have uniform morphology and composition. It should be elastic and flexible to accommodate volume variations during lithiation/delithiation process.
Influence of SEI on the electrochemical performances of LIBs
The SEI layer plays a very important role in the electrochemical performances of LIBs. The battery performance parameters affected by the SEI are irreversible charge loss (ICL), self-discharge, cyclability, rate capability, and safety. Every parameter and property of the SEI significantly affects battery performance. The composition, thickness, morphology, and compactness are a few to name.
In commercial LIBs, graphite cannot be cycled in PC based electrolyte (LiPF6/PC or LiClO4/PC) because the undesirable SEI layer is not able to prevent the co-intercalation of solvent molecules in the graphite leading to the formation of gas responsible for an increase of the pressure in graphene layers and then electrode destruction by exfoliation.33 The exfoliation phenomenon is governed by low quality SEI layer which depends on its composition and morphology. The use of EC as co-solvent with PC increases significantly the cycling ability. Indeed, the composition of the SEI influences the electronic insulating properties of the SEI layer and its chemical stability. The morphology of the SEI layer, its porosity and thickness, will govern the conduction of lithium ions through the SEI layer.
Therefore, the charge capacity and the irreversible capacity will depend strongly on the For the conversion-type and alloying-type electrode materials, which have a large volume expansion during lithiation process, the SEI layer also affects the electrochemical performances of the electrodes in LIBs. As evidenced from XPS and ToF-SIMS studies performed on the iron oxide and iron sulfide thin-film electrodes, the SEI layer formed by reductive decomposition of the electrolyte penetrated into the bulk of the oxide thin film upon repeated conversion/deconversion, which leads to deterioration of the electrode performances upon further multi-cycling.69,70 XPS analysis performed on silicon thin films-α-Si:H model electrodes shows a thicker SEI layer formed after cycling in PC-based electrolyte as compared to EC:DMC electrolyte. 71 Investigation on Si nanowires (SiNWs) shows the SEI modifications caused by the lithiation/delithiation rate and the modifications of the Si electrode upon cycling. Low lithiation/delithiation rate improves electrochemical performance due to a better penetration depth of lithium into the SiNW electrode and the formation of a homogeneous solid electrolyte interphase (SEI) layer on the SiNWs after the first cycle. However, after repeated cycling, SiNWs suffered strong mechanical stress leading to a rough or porous SiNW structure covered by a porous SEI layer.72
Variation of SEI layer – influential factors
The SEI layer, as the reaction product of the electrode material and electrolyte, is formeon the electrode surface during charging and discharging of a LIB. Its composition, structure, density and stability is mainly determined by the nature of electrodes and electrolyte, but also influenced by the temperature, cycle number and current density of charging and discharging.
Anode materials
Various properties of anode materials, including types of material, composition, structures and morphology, especially surface morphology, of the electrodes have a critical influence on the formation of SEI layers. In a commercial LIB, the SEI layer is mainly formed on the surface of carbonaceous negative active materials, thus the type of carbon significantly affects the SEI. In-depth studies of various types of carbon anode materials (i.e. pyrolytic carbon, carbon fiber, petroleum coke, artificial graphite and natural graphite) showed that the degree of graphitization and orderliness seriously affect properties of the SEI layer. Even for the same carbon material, the SEI formation can be different in composition and morphology depending on the electrode surface (i.e., basal plane, cross section and edges), and particle size.
Lithium salt
The composition and contents of the SEI layer are partially determined by the types of lithium salt in the electrolyte. Generally, the lithium salt is considered more reductive than the solvent and its reduction product becomes part of the SEI layer. The commonly used lithium salt, LiClO4, is less secure due to its strong oxidizing property. LiAsF6 is highly toxic although it has better electrochemical properties for carbon anode. LiPF6 has poor thermal stability and will decompose into LiF at 60~80 oC. Therefore, looking for new lithium salt is still in progress.
The main difference of various lithium salts is the type of anionic species, which results in different formation potential and chemical composition of the SEI layer. Reduction products of the lithium salts are present in the SEI layer when Cl and F are contained in inorganic lithium salts. Experimental results show that high contents of LiF and LiCl are found in the SEI layer due to the Cl and F elements contained in the electrolyte (lithium salt), which may be caused by the following reactions:56 LiPF (solv) + H O(l) → LiF(s) + 2HF(solv) + POF (g) 6 2 3 (1-5).
Characterization of SEI layers – research methods and techniques
The characterization of SEI layers is focused on morphology, structure, composition and electrochemical performance. Electrochemical, microscopic and spectroscopic (including electron spectroscopy, chromatography and mass spectrometry) techniques are the main research methods and approaches. In recent years, a variety of in situ study techniques have been widely used.
Electrochemical methods
Electrochemical methods (i.e., electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and electrode charge/discharge test) have been widely used for studying the SEI layer. The characteristics of the SEI layer could be speculated from some electrochemical parameters of the electrodes.
EIS technique is a powerful tool to understand the electrode processes in LIBs. EIS studies can clearly expose deposition and dissolution process of the SEI layer which corresponds to the impedance increases and decreases. At the same time, from the fit and simulation of the Nyquist plots, a suitable equivalent circuit can be obtained to further describe the structural characteristics and electrochemical behaviors of SEI layer.83-86 Cyclic voltammetry is also a powerful tool. The potential window of SEI formation could be clearly shown by using cyclic voltammetry. A strong cathodic peak is observed at around 0.65 V in the first cyclic voltammograms but disappeared in the second scan for the fresh graphite electrode. Apparently, this reduction peak is the current peak of SEI formation.
In addition, other electrochemical methods i.e., galvanostatic method, potentiostatic method, pulse voltammetry and steady state polarization method are useful to analyze SEI kinetics.
Microscopic methods
With the development of electron microscopy, the properties, structure and morphology of the SEI layer can not only be indirectly speculated from electrochemical and optical data, but also be observed directly by microscopic methods on the surface of the electrode. Microscopic techniques i.e., scanning electron microscope (SEM), 87 transmission electron microscope (TEM),88 atomic force microscope (AFM)89-91 and scanning probe microscope (SPM)92,93 have been gradually applied in the study of SEI layer. These microscopic techniques can directly display the SEI within a nanoscale of around 10 nm, which provides the most direct methodology of characterizing SEI layer.
Spectroscopic methods
Modern technology offers a variety of spectroscopic and chromatographic techniques as useful tools for the study of SEI composition. The spectroscopic and chromatographic analysis of the SEI provides not only qualitative but also quantitative information on the components. Amongst the various spectroscopic techniques used to analyze the SEI, Fourier transform infrared spectroscopy (FTIR),47,94 Raman spectroscopy, 73 X-ray photoelectron spectroscopy (XPS),64 and time-of-flight secondary ion mass spectrometry (ToF-SIMS) 57 have proved to be very useful.
Moreover, chromatography and mass spectrometry are often used for analyzing the composition of SEI. Especially, combining these two techniques, Ogumi et al. 95 analyzed the chemical constituents of the SEI layer formed on graphite negative electrode in EC-based electrolyte solutions by pyrolysis/gas chromatography/mass spectroscopy (Pyro/GC/MS), to obtain direct evidence for decomposed products of relatively high-molecular weights. The results showed that oligomers with oxyethylene units, such as ethylene glycol, di(ethylene glycol), and tri(ethylene glycol) methyl ester, were detected, and their presence suggested that the SEI layer contained polymer-like substances that have repeated oxyethylene units. Similar studies on the SEI layer were also implemented by Ota et al.
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
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
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
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
Notation
List of publications
Acknowledgements
