Operando Electrochemical Investigation on Energy Conversion Systems

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Operando Electrochemical Investigation on Energy Conversion Systems
Ko, Younghoon
Song, Hyun-Kon
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Graduate School of UNIST
Understanding the interface between electrodes and electrolyte is the essential topic of electrochemistry. Electrochemical impedance spectroscopy (EIS) is one of the most powerful analytic tools to investigate the interface where electrochemical reactions proceed because overlapped processes can be decoupled and parameters characterizing each process can be measured. The most widely used EIS is the potentiostatic EIS (PS-EIS) by which impedance spectra are obtained at a fixed potential. The data are assumed to be obtained in a stationary condition where concentrations of oxidized and reduced species are not changed significantly but only small sinusoidal waves of potential can perturb the profiles reversibly. However, it is difficult to interpret the impedance data in non-stationary situations where the concentration profiles change with time even at fixed potential or potentials are changed during EIS measurements. Two different non-stationary cases were considered in this thesis. Potentiodynamic situations in which potential is intentionally controlled (e.g., cyclic voltammetry) are the first case. To snapshot the impedance spectra at each potential, the time required for measuring a spectra of impedances should be very short. Potential would change before completing EIS measurement at a potential unless the measuring time is short. In this thesis, therefore, Fourier-transformed EIS technique (FT-EIS) was used to obtain impedance spectra during potential sweep. Galvanostatic situations are the other case for non-stationary situations. Charging and/or discharging energy storage devices such as metal-air cells and lithium ion batteries (LIBs) belongs to the non-stationary situations where potentials change continuously because the devices are galvanostatically charged and/or discharged at a fixed current. Even if the electrochemical cells for energy storage have been extensively studies by using PS-EIS, it is difficult to say that values of kinetic parameters obtained from the PS-EIS reflects operational conditions effectively. To investigate impedances in situ during chronopotentiometry, therefore, galvanostatic EIS (GS-EIS) was developed by perturbing a fixed current with a series of sinusoidal waves of current of a small amplitude. In the first part, the processes involved in electrochemical oxidation of zinc were studied in dilute alkaline solutions, 0.010 and 0.10 M KOH, employing cyclic voltammetric and real-time Fourier transform electrochemical impedance spectroscopy (FTEIS) experiments. Thermodynamic analysis of cyclic voltammetric data indicates that Zn(OH)42- is produced as a major product in both 0.10 and 0.010 M KOH although ZnO/Zn(OH)2 may also be produced as a minor product in 0.010 M. A large body of impedance data was obtained as a function of swept potential by running combined staircase cyclic voltammetry and FTEIS (SCV-FTEIS) experiments at every 10 mV and 200 ms interval, which allowed a systematic and complete analysis to be made on the interface. Analysis from the impedance data demonstrates that the charge transfer reaction occurs across the thin oxide/hydroxide film formed on the zinc surface. Also obtained were various electrode reaction kinetic parameters for oxidation of zinc by treating the impedance data thus obtained and the reaction mechanism is discussed based on the data. In the second part, we focused on in situ measurement of impedances of lithium ion batteries (LIBs) during galvanostatic charging and how to design charging rates based on the information extracted from the in situ measured impedance data. Charge transfer resistances are definitely and deeply related to kinetics of lithiation or delithiation of active materials used in LIBs. The impedance data measured by GS-EIS showed tendencies depend on C-rate. Following the course of lithiation of graphite at slow rates, charge transfer resistances decreased slightly until lithium is intercalated every two graphitic layers (stage 2L or 2). Then, the resistance increased abruptly when the alternative empty layers were filled with lithium ions (transition from stage 2 to stage 1). At the end of the transition, resistance is developed to make very resistive environments for lithiation. Based on the information about charge transfer resistance profiles along galvanostatically charging processes, a charging strategy is programmed with several different C-rates. Capacity of lithiation is significantly enhanced by the C-rate switching (CRS) strategy. As a representative example, 75 % of available capacity is charged for 50 min by a combination of 2C, 1C and 0.5C. However, only 12 % and 51 % of graphite is lithiated within the same time duration by a single charge rate at 0.1C and 0.5C, respectively. Finally, silicon-based lithium-ion batteries were investigated in operando by GS-EIS. A large body of impedance data was obtained at every set time, which allowed systematic and complete analysis of the interface. The variation of double layer capacitance (Cd) demonstrates the thickening of solid electrolyte interface (SEI) layer whereas the charge transfer resistance (RP) values show the mechanism of lithiation into the silicon structure during galvanostatic charging processes. By using this technique, nanosizing benefits of silicon nanoparticles over micro-particles were confirmed such as lower polarization resistance (Rp) and thinner solid-electrolyte interphase layer (SEI layer) over the whole lithiation range. Based on the kinetic information obtained from the non-stationary conditions, a lithiation strategy consisting of multiple galvanostatic steps was designed to lithiate silicon anodes in a faster way. Impressively, three quarters of available capacity of nano sized silicon cells were charged by a galvanostatic sequence of 4C-2C-1C-0.5C within 20 min, which cannot be achieved by a conventional single C-rate charging strategy. Also, potential profile curves of the CRS lithiation was simulated from Nernst equation for expecting lithiation time. In this study, we hereby report a novel method of analyzing electrochemical reactions by not only real-time Fourier transform electrochemical impedance (FTEIS) spectroscopy but also Galvanostatic electrochemical impedance spectroscopy (GS-EIS) and applied them to the electrochemical mechanism study of zinc electrode oxidation, lithium ion intercalation into the graphite layers, and Si alloying reaction in battery systems. A large body of impedance data obtained for battery operation help better elucidates the charge and/or discharge mechanism at the electrode/electrolyte interface.
Department of Energy Engineering
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