Investigating ageing behaviours in supercapacitor (cells and modules) using EEC (electrical equivalent circuit) models

This thesis contributes to the reliability and aging studies of supercapacitors for more efficient use in EV/HEV applications. This thesis demonstrates the effect of aging/failure in supercapacitor cells and module cells using accelerated tests employed to expedite the aging process. The tests, as explained below were categorized based on operational and environmental aging factors associated with supercapacitor failure in EV/HEV applications to; • Investigate supercapacitor cell performance at high temperature and constant voltage individual conditions, and also simultaneously (known as calendar test) • Investigate the effect of voltage balancing/equalization circuits on supercapacitor module cells’ performance during constant current cycling tests under certain environmental and electrical factors • Investigate supercapacitor module cells’ cycling performance in a lab-scale designed electrical DC programmable motor load system that emulates supercapacitor operational conditions in an EV/HEV application. The aging behaviors characterized by the three factors mentioned above are quantified in this thesis through the periodic monitoring of their electrical and electrochemical state of health with Electrochemical Impedance Spectroscopy, Cyclic Voltammetry, and Constant Current characterization tests. These tests help identity aging modes in supercapacitors, and it was observed that regardless of their aging factors; an increase in ESR and decrease of capacitance was determined. Although this information is required, the results from Electrochemical Impedance Spectroscopy (EIS) tests revealed more details distinctive to each aging factor. From this distinction, the aging mechanisms in relation to the aging factors, which causes the deterioration in the supercapacitor electrical performance, are identified and summarized as the following: 1. Loss of contact within supercapacitor electrode, given rise to the contact resistance due to the presence of high temperature as the main aging factor 2. Change of supercapacitor porous electrode emulating a charge transfer reaction thereby increasing its distributed resistance, caused by the effect of high voltage or cycling Mathematical models in the form of electrical equivalent circuits (EECs) distinctive of their aging factors are generated from EIS electrochemical behaviors to easily describe aging behaviors in supercapacitors. The EEC models developed using impedance modeling generated an initial model from dormant cells, which transitioned to aging models distinctive of their aging factors as soon as a 100% increase in ESR and/or an 80% decrease in capacitance is observed. The proposed EEC models were validated to show the dynamic interaction between aging of the supercapacitor cells on their electrical performance in both frequency and time domains. In summary, the EEC models encompass this thesis objective and as such considered the main contribution of this research work

Novel Multiphysics Phenomena in a New Generation of Energy Storage and Conversion Devices

The swelling demand for storing and using energy at diverse scales has stimulated the exploration of novel materials and design strategies applicable to energy storage systems. The most popular electrochemical energy storage systems are batteries, fuel cells and capacitors. Supercapacitors, also known as ultracapacitors, or electrochemical capacitors have emerged to be particularly promising. Besides exhibiting high cycle life, they combine the best attributes of capacitors (high power density) and batteries (high energy storage density). Consequently, they are expected to be in high demand for applications requiring peak power such as hybrid electric vehicles and uninterruptible power supplies (UPS). This dissertation aims to make advancements on the following two topics in supercapacitor research with the aid of modeling and experimental tools: applying various thermophysical effects to design supercapacitor devices with novel functionalities and studying degradation mechanisms upon continuous cycling of conventional supercapacitors. The prime drawback of conventional supercapacitors is their low energy density. Most research in the last decade has focused on synthesizing novel electrode materials. Although such novel electrodes lead to high energy density, they often involve complicated synthesis process and result in high cost and low power density. A new concept of inducing pseudocapacitance developed in recent years is by introducing redox additives in the electrolyte that engage in redox reactions at the electrode/electrolyte interface during charge/discharge. The first section of this dissertation reports the performance of fabricated solid-state supercapacitors composed of redox-active gel electrolyte (PVA-K3Fe(CN)6-K4Fe(CN)6). The electrochemical performance has been studied extensively using cyclic voltammetry, constant current charge/discharge and impedance spectroscopy techniques, and then the results are compared with similar devices composed of conventional gel electrolytes such as PVA-H3PO4 and PVA-KOH on the basis of capacitance, internal resistance and stable voltage window. The second section explores the utility of the thermogalvanic property of the same redox-active gel electrolyte, PVA-K3Fe(CN)6-K4Fe(CN)6 in the construction of a thermoelectric supercapacitor. The integrated device is capable of being electrically charged by applying a temperature gradient across its two electrodes. In the absence of available temperature gradient, the device can be discharged electrically through an external circuit. Therefore, such a device can be used to harvest waste heat from intermittent heat sources. An equivalent circuit elucidating the mechanisms of energy conversion and storage applicable to thermally chargeable supercapacitors is developed. A fitting analysis aids in the evaluation of model circuit parameters providing good agreement with experimental voltage and current measurements. The latter part of the dissertation investigates the factors influencing aging in conventional supercapacitors. In the first part, a new imaging technique based on the electroreflectance property of gold has been developed and applied to characterize the aging characteristics of a microsupercapacitor device. Previous aging studies were performed through traditional electrical characterization techniques such as cyclic voltammetry, constant charge/discharge, and electrochemical impedance spectroscopy. These methods, although simple, measure an average of the structures’ internal performance, providing little or no information about microscopic details inside the device. The electroreflectance imaging method, developed in this work is demonstrated as a high-resolution imaging technique to investigate charge distribution, and thus to infer aging characteristics upon continuous cycling at high scan rates. The technique can be used for non-intrusive spatial analysis of other electrochemical systems in the future. In addition, we investigate heat generation mechanisms that are responsible for accelerated aging in supercapacitors. A modeling framework has been developed for heat generation rates and resulting temperature evolution in porous electrode supercapacitors upon continuous cycling. Past thermal models either neglected spatial variations of heat generation within the cell or considered electrodes as flat plates that led to inaccuracies. Here, expressions for spatiotemporal variation of heat generation rate are rigorously derived on the basis of porous electrode theory. Detailed numerical simulations of temperature evolution are performed for a real-world device, and the results resemble past measurements both qualitatively and quantitatively. In the last chapter of the thesis, a rare thermoelectric effect called the Nernst effect has been investigated in single-layer periodic graphene with the aid of a modified Boltzmann transport equation. Detailed formulations of the transport coefficients from the BTE solution are developed in order to relate the Nernst coefficient to the amount of impurity density, temperature, band gap and applied magnetic field. Detailed knowledge of the variation of the thermoelectric and thermomagnetic properties of graphene shown in this work will prove helpful for improving the performance of magnetothermoelectric coolers and sensors

Modelling the ageing behaviour of supercapacitors using electrochemical impedance spectroscopy for dynamic applications

Diagnosis of ageing mechanisms in supercapacitors is made difficult by the enforcement of various ageing factors in the current ageing tests. The thesis presents the exact determination of the ageing mechanism by separating the impacts of high temperature, current cycling and constant voltage applications in accelerated ageing tests. The state of health (SOH) of the supercapacitors are monitored periodically with electrochemical impedance spectroscopy, cyclic voltammetry and constant current test to observe the evolution of ageing. The thesis identifies patterns of ageing from the changes at supercapacitor impedance. The thesis also presents the cause of the increase in ESR and the loss of capacitance in supercapacitors. High temperature application causes the appearance of high frequency semicircle which reflects the damage at the electrode-current collector interface. A tilt of the impedance line at low frequencies reflects modifications of electrodes and it is most sensitive to current cycling and constant voltage applications. The increase in ESR is observed to be caused by a single ageing mechanism while the capacitance loss is caused by multiple interactions of these ageing mechanisms at the same time. The thesis develops a supercapacitor model by means of electrical equivalent circuit. The model is divided into two parts based on the changes in its SOH: the baseline model represents the early stage of the supercapacitor life and the ageing model represents the phase of ageing. The models show dynamic interactions between ageing process and supercapacitor electrical performance. The supercapacitor model, in the form of fractional-order model, reduces the number of circuit components and shows excellent electrical behaviour particularly at the open circuit voltage decay and voltage recovery period. The parameterisation of model parameters shows that aged supercapacitors experience an increase of distributed resistance in the electrode pores and an increase of diffusion impedance at high temperature

Batteries and Supercapacitors Aging

Electrochemical energy storage is a key element of systems in a wide range of sectors, such as electro-mobility, portable devices, and renewable energy. The energy storage systems (ESSs) considered here are batteries, supercapacitors, and hybrid components such as lithium-ion capacitors. The durability of ESSs determines the total cost of ownership, the global impacts (lifecycle) on a large portion of these applications and, thus, their viability. Understanding ESS aging is a key to optimizing their design and usability in terms of their intended applications. Knowledge of ESS aging is also essential to improve their dependability (reliability, availability, maintainability, and safety). This Special Issue includes 12 research papers and 1 review article focusing on battery, supercapacitor, and hybrid capacitor aging

Modelling the ageing behaviour of supercapacitors using electrochemical impedance spectroscopy for dynamic applications

Diagnosis of ageing mechanisms in supercapacitors is made difficult by the enforcement of various ageing factors in the current ageing tests. The thesis presents the exact determination of the ageing mechanism by separating the impacts of high temperature, current cycling and constant voltage applications in accelerated ageing tests. The state of health (SOH) of the supercapacitors are monitored periodically with electrochemical impedance spectroscopy, cyclic voltammetry and constant current test to observe the evolution of ageing. The thesis identifies patterns of ageing from the changes at supercapacitor impedance. The thesis also presents the cause of the increase in ESR and the loss of capacitance in supercapacitors. High temperature application causes the appearance of high frequency semicircle which reflects the damage at the electrode-current collector interface. A tilt of the impedance line at low frequencies reflects modifications of electrodes and it is most sensitive to current cycling and constant voltage applications. The increase in ESR is observed to be caused by a single ageing mechanism while the capacitance loss is caused by multiple interactions of these ageing mechanisms at the same time. The thesis develops a supercapacitor model by means of electrical equivalent circuit. The model is divided into two parts based on the changes in its SOH: the baseline model represents the early stage of the supercapacitor life and the ageing model represents the phase of ageing. The models show dynamic interactions between ageing process and supercapacitor electrical performance. The supercapacitor model, in the form of fractional-order model, reduces the number of circuit components and shows excellent electrical behaviour particularly at the open circuit voltage decay and voltage recovery period. The parameterisation of model parameters shows that aged supercapacitors experience an increase of distributed resistance in the electrode pores and an increase of diffusion impedance at high temperature

Analysis of thermal and electrochemical properties of alternative electrolytes for electrical-double layer capacitors

With climate change becoming more and more pressing, a switch of energy production from fossil fuels to renewable energy sources is necessary, which leads to big challenges for our today’s society. To meet these challenges, not only the widespread expansion of renewable energies is necessary, but also of reliable and efficient energy storage, which guarantees a reliable energy supply on demand and a stable power grid. Nowadays, different energy storage devices are available for different areas of operation, the most prominent are lithium-ion batteries (LIBs) and electrical double layer capacitors (EDLCs). This thesis is dedicated to the investigation of EDLCs. The first part of this work focuses on the development of alternative electrolytes for high voltage EDLCs. Electrolytes based either on adiponitrile (ADN) or the mixture of acetonitrile (ACN) and ethyl isopropyl sulfone (EiPS) are investigated for their use in high voltage EDLCS. The second part discusses the thermal investigation of EDLCs. On the one side, the interaction of activated carbon (AC) with alternative electrolytes based on the solvents propylene carbonate (PC), butylene carbonate (BC), 3-cyano propionic acid methyl ester (CPAME) or ADN is investigated via thermogravimetric analysis (TGA). On the other side, an in-situ simultaneous thermal analysis (STA) cell is used for the investigation of thermal and electrochemical properties of EDLCs under various temperature and voltage conditions. Here the electrochemical performance of an EDLC is associated with the heat flow generated during operation

The accelerated life cycle testing and modelling of Li-ion cells used in electric vehicle applications

Li-ion batteries have become one of the chosen energy storage devices that are used in applications such as power tools, cellular phones and electric vehicles (EV). With the demand for portable high energy density devices, the rechargeable Li-ion battery has become one of the more viable energy storage systems for large scale commercial EVs because of their higher energy density to weight or volume ratio when compared to other current commercial battery energy storage systems. Various safety procedures for the use of Li-ion batteries in both consumer and EV applications have been developed by the international associations. The test procedures studied in this dissertation demonstrated the importance of determining the true capacity of a cell at various discharge rates. For this, the well known Peukert test was demonstrated. The study also showed that cells with different battery geometries and chemistries would demonstrate different thermal heating during discharge and slightly different Ragone results if different test methods were used as reported in the literature. Accelerated ageing tests were done on different cells at different Depth-of-Discharge (DoD) regions. The different DoD regions were determined according to expected stresses the electrode material in a cell would experience when discharged to specific DoD that follows the discharge voltage profile. Electrochemical Impedance Spectroscopy (EIS) was used to measure various electrochemical changes within these cells. The EIS results showed that certain observed modelled parameters would change similarly to the ageing of the cell as it aged due to the accelerated testing. EIS was also done on cells at different State-of-Charge (SoC) and temperatures. The results showed that EIS can be used as an effective technique to observe changes within a Li-ion cell as the SoC or temperature changed. For automotive vehicles that are powered by a fuel cell or battery, a supercapacitor can be coupled to a battery in order to increase and optimize the energy and power densities of the drive systems. A test procedure in the literature that evaluated the use of capacitors with Pb-acid batteries was applied to Li-ion type cells in order to quantify the increased power due to the use of a supercapacitor with a Li-ion cell. Both a cylindrical LiCoO2 cell and a VRLA Pb-acid cell showed some additional charge acceptance and delivery when connected to the supercapacitors. A LiMn2O4 pouch cell showed significant charge acceptance and delivery when connected to supercapacitors. The amount of additional charge acceptance and delivery of the different combinations could be explained by EIS, in particular, the resistance and capacitance of the cell in comparison to the combination of the cell and supercapacitor. A large capacity LiCoO2 cell showed high charge acceptance and delivery without connection with a supercapacitor. The study proved that EIS can be used to model the changes within cells under the different conditions and using different test procedures

The accelerated life cycle testing and modelling of Li-ion cells used in electric vehicle applications

Li-ion batteries have become one of the chosen energy storage devices that are used in applications such as power tools, cellular phones and electric vehicles (EV). With the demand for portable high energy density devices, the rechargeable Li-ion battery has become one of the more viable energy storage systems for large scale commercial EVs because of their higher energy density to weight or volume ratio when compared to other current commercial battery energy storage systems. Various safety procedures for the use of Li-ion batteries in both consumer and EV applications have been developed by the international associations. The test procedures studied in this dissertation demonstrated the importance of determining the true capacity of a cell at various discharge rates. For this, the well known Peukert test was demonstrated. The study also showed that cells with different battery geometries and chemistries would demonstrate different thermal heating during discharge and slightly different Ragone results if different test methods were used as reported in the literature. Accelerated ageing tests were done on different cells at different Depth-of-Discharge (DoD) regions. The different DoD regions were determined according to expected stresses the electrode material in a cell would experience when discharged to specific DoD that follows the discharge voltage profile. Electrochemical Impedance Spectroscopy (EIS) was used to measure various electrochemical changes within these cells. The EIS results showed that certain observed modelled parameters would change similarly to the ageing of the cell as it aged due to the accelerated testing. EIS was also done on cells at different State-of-Charge (SoC) and temperatures. The results showed that EIS can be used as an effective technique to observe changes within a Li-ion cell as the SoC or temperature changed. For automotive vehicles that are powered by a fuel cell or battery, a supercapacitor can be coupled to a battery in order to increase and optimize the energy and power densities of the drive systems. A test procedure in the literature that evaluated the use of capacitors with Pb-acid batteries was applied to Li-ion type cells in order to quantify the increased power due to the use of a supercapacitor with a Li-ion cell. Both a cylindrical LiCoO2 cell and a VRLA Pb-acid cell showed some additional charge acceptance and delivery when connected to the supercapacitors. A LiMn2O4 pouch cell showed significant charge acceptance and delivery when connected to supercapacitors. The amount of additional charge acceptance and delivery of the different combinations could be explained by EIS, in particular, the resistance and capacitance of the cell in comparison to the combination of the cell and supercapacitor. A large capacity LiCoO2 cell showed high charge acceptance and delivery without connection with a supercapacitor. The study proved that EIS can be used to model the changes within cells under the different conditions and using different test procedures

Flexible and High Temperature Supercapacitor Based on Laser-Induced Graphene Electrodes and Ionic Liquid Electrolyte, a De-rated Voltage Analysis.

Herein we report the fabrication and electrochemical characterization of a novel type of supercapacitor composed of laser-induced graphene (LIG) electrodes, achieved by the laser-writing of polyimide foils, and 1-Butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid as electrolyte. This combination allows the development of a flexible microsupercapacitor suitable for harsh environment application. The influence of several parameters is evaluated with the aim of maximizing the performance of the flexible pouch-bag devices, such as the laser-writing conditions, type of electrode layout and amount of nitrogen-doping. Among them, the laser writing conditions are found to strongly influence the areal capacitance allowing to achieve about 4 mF cm−2, as measured from the galvanostatic charge-discharge measurement at 10 µA cm−2, with a maximum operating potential range of 3 V at 25 °C. In order to probe the potential application of such device, i) flexible pouch architecture and ii) high temperature measurements (considering harsh environment field) are investigated. This type of flexible device exhibits energy and power density as high as 4.5 µWh cm−2 and 90.5 µW cm−2, respectively, high cycling stability as well as acceptable coulombic efficiency above 97% demonstrating good stability even at high bending condition (1.25 cm of bending radius). The electrochemical measurements increasing temperature up to 100 °C reveal a 300% of rise in capacitance and 43% of increment in energy density at de-rated voltage. The obtained energy storage performance are comparable to the best data ever reported for a microsupercapacitor for high temperature application. Moreover, a de-rated voltage analysis (DVA) is proposed as a safe procedure to characterize an energy storage device in an extended temperature range without compromising the system performances