AN IN-SITU INVESTIGATION OF SOLID ELECTROLYTE INTERPHASE FORMATION ON ELECTRODE MATERIALS FOR LITHIUM-ION BATTERIES USING SPECTROSCOPIC ELLIPSOMETRY by Mark A. McArthur Submitted in partial fulfillment of the requirements for the degree of Master of Science at Dalhousie University Halifax, Nova Scotia August 2011 © Copyright by Mark A. McArthur, 2011 DALHOUSIE UNIVERSITY DEPARTMENT OF PHYSICS AND ATMOSPHERIC SCIENCE The undersigned hereby certify that they have read and recommend to the Faculty of Graduate Studies for acceptance a thesis entitled “AN IN-SITU INVESTIGATION OF SOLID ELECTROLYTE INTERPHASE FORMATION ON ELECTRODE MATERIALS FOR LITHIUM-ION BATTERIES USING SPECTROSCOPIC ELLIPSOMETRY” by Mark A. McArthur in partial fulfillment of the requirements for the degree of Master of Science. Dated: August 8, 2011 Supervisor: _________________________________ Readers: _________________________________ _________________________________ ii DALHOUSIE UNIVERSITY DATE: August 8, 2011 AUTHOR: Mark A. McArthur TITLE: AN IN-SITU INVESTIGATION OF SOLID ELECTROLYTE INTERPHASE FORMATION ON ELECTRODE MATERIALS FOR LITHIUM-ION BATTERIES USING SPECTROSCOPIC ELLIPSOMETRY DEPARTMENT OR SCHOOL: Department of Physics and Atmospheric Science DEGREE: MSc CONVOCATION: October YEAR: 2011 Permission is herewith granted to Dalhousie University to circulate and to have copied for non-commercial purposes, at its discretion, the above title upon the request of individuals or institutions. I understand that my thesis will be electronically available to the public. The author reserves other publication rights, and neither the thesis nor extensive extracts from it may be printed or otherwise reproduced without the author’s written permission. The author attests that permission has been obtained for the use of any copyrighted material appearing in the thesis (other than the brief excerpts requiring only proper acknowledgement in scholarly writing), and that all such use is clearly acknowledged. _______________________________ Signature of Author iii To Mom and Dad… iv Table of Contents List of Tables .................................................................................................................... vii List of Figures.................................................................................................................. viii Abstract............................................................................................................................. xii List of Abbreviations and Symbols Used ........................................................................xiii Acknowledgements.......................................................................................................... xix Chapter 1 Introduction................................................................................................. 1 1.1 Thesis Structure ............................................................................................... 3 Chapter 2 Lithium-Ion Batteries.................................................................................. 4 2.1 Introduction to Li-Ion Cells............................................................................. 4 2.2 Negative Electrode Materials......................................................................... 11 2.3 The Solid Electrolyte Interphase Model ........................................................ 13 2.3.1 Modeling the SEI ........................................................................... 15 2.3.2 Measuring Physical Properties of the SEI...................................... 19 2.4 Electrolytes and Additives for Li-Ion Batteries ............................................. 23 Chapter 3 Background and Theory of Spectroscopic Ellipsometry.......................... 26 3.1 History and Development.............................................................................. 26 3.2 Principles of Optics........................................................................................ 27 3.3 Principles of Spectroscopic Ellipsometry...................................................... 40 Chapter 4 Other Instrumentation............................................................................... 49 4.1 Magnetron Sputter Deposition....................................................................... 49 4.2 In-situ Electrochemical Cell .......................................................................... 54 Chapter 5 Experimental Details................................................................................. 58 5.1 Electrode Preparation..................................................................................... 58 5.2 Electrode Characterization............................................................................. 59 5.3 Electrochemical and Spectroscopic Ellipsometry Testing............................. 65 5.3.1 Constant Current Testing ............................................................... 66 5.3.2 Constant Potential Testing ............................................................. 67 v 5.4 Optically Modeling SE Data.......................................................................... 68 5.4.1 Refractive Indices of Nonaqueous Electrolyte............................... 71 5.4.2 Refractive Indices of the SEI ......................................................... 71 5.4.3 Modeling the Baseline and Extracting Surface Film Thickness.... 72 Chapter 6 Results and Discussion ............................................................................. 74 6.1 Preliminary Optical Measurements................................................................ 74 6.1.1 Optical Properties of Bare Electrode Materials ............................. 74 6.1.2 Optical Properties of Electrolyte and SEI...................................... 81 6.2 In-situ Electrochemical Measurements.......................................................... 84 6.2.1 a-Si Thin Film Electrodes .............................................................. 86 6.2.2 Ni Thin Film Electrodes................................................................. 93 6.2.3 TiN Thin Film Electrodes .............................................................. 99 Chapter 7 Conclusion.............................................................................................. 113 7.1 Future Work................................................................................................. 114 Bibliography ................................................................................................................... 116 vi List of Tables Table 2.1 Contrasting various commercial cell chemistries available today. Li-ion batteries provide the largest voltage, specific energy, and energy densities among the cell chemistries shown. ..................................................6 Table 2.2 List of physical properties of several common salts for Li-ion batteries..... 24 Table 5.1 Sputtering conditions for the listed target materials. ................................... 59 Table 6.1 Fitting parameters for two models from bare a-Si thin film electrodes measured using spectroscopic ellipsometry in air. Model 1 gives parameters from a single Tauc-Lorentz oscillator. Model 2 gives the parameters from a 2-term Tauc-Lorentz oscillator...................................... 76 Table 6.2 Fitting parameters for a model used to determine the optical properties and thickness of a bare TiN thin film electrode measured by spectroscopic ellipsometry in air. To model the TiN electrode, a single Drude-type and two Lorentz-type oscillators were used. ............................ 79 Table 6.3 Cauchy parameters used to determine the real part of the refractive index for various electrolytes and the SEI used in all in-situ electrochemical testing........................................................................................................... 83 Table 6.4 Descriptions of the Li/TiN cells displayed in Figure 6.20......................... 108 vii List of Figures Figure 1.1 A schematic of a commercial graphite/LiCoO 2 Li-ion cell in the discharged state just before the start of a charge. .......................................... 2 Figure 2.1 Schematic of the interior of a Li-ion coin cell used to study different cell chemistries, cycling behaviour, and more for various electrode materials.... 4 Figure 2.2 Voltage curves for Li/graphite and Li/LiCoO 2 cells cycled in the same electrolyte (1 M LiPF 6/EC:DEC(1:2))........................................................... 8 Figure 2.3 Potentiostatic chronoamperometry study on a Li/TiN cell in 1 M LiPF 6/EC:DEC(1:2) held at 0.5 V at 22ºC................................................... 10 Figure 2.4 Graphical comparison of specific capacities of negative electrode alloy materials vs graphite. (Adapted from Ref. [16])......................................... 12 Figure 2.5 Equilibrium Li-Ni phase diagram................................................................ 12 Figure 2.6 Schematic of the SEI developed on a negative electrode. Letters indicate the SEI species as described in the legend. (Adapted from [2]). .. 14 Figure 2.7 Schematic representation of the solvent diffusion model for determining the growth of the SEI on negative electrode materials. ............................... 17 Figure 3.1 Electromagnetic (light) wave propagating from a dielectric medium with n = n1 and k = 0 to a dielectric medium with n = n2 and: (a) k = 0 and (b) k > 0. ...........................................................................................................