Commercial lithium ion batteries use liquid electrolytes because of their high ionic conductivity (>10-3 S/cm) over a broad range of temperatures, high dielectric constant, and good electrochemical stability with the electrodes (mainly the cathode cathode). The disadvantages of their use in lithium ion batteries are that they react violently with lithium metal, have special packing needs, and have low lithium ion transference numbers (tLi+ = 0.2-0.3). These limitations prevent them from being used in high energy and power applications such as in hybrid electric vehicles (HEVs), plug in electric vehicles (EVs) and energy storage on the grid. Solid polymer electrolytes (SPEs) will be good choice for replacing liquid electrolytes in lithium/lithium ion batteries because of their increased safety and ease of processability. However, SPEs suffer from RT low ionic conductivity and transference numbers. There have been many approaches to increase the ionic conductivity in solid polymer electrolytes. These have focused on decreasing the crystallinity in the most studied polymer electrolyte, polyethylene oxide (PEO), on finding methods to promote directed ion transport, and on the development of single ion conductors, where the anions are immobile and only the Li+ ions migrate (i.e. tLi+ = 1). But these attempts have not yet achieved the goal of replacing liquid electrolytes with solid polymer electrolytes in lithium ion batteries. In order to increase ionic conductivity and lithium ion transference numbers in solid polymer electrolytes, I have focused on the development of multi-ionic lithium salts. These salts have very large anions, and thus are expected to have low tanion- and high tLi+ transference numbers. In order to make the anions dissociative, structures similar to those formed for mono-ionic salts, e.g. LiBF4 and lithium imides have been synthesized. Some of the multi-ionic salts have Janus-like structures and therefore can self-assemble in polar media. Further, it is possible that these salts may not form non-conductive ion pairs and less conductive ion triplets. First, we have prepared nanocomposite electrolytes from mixtures of two polyoctahedral silsesquioxanes (POSS) nanomaterials, each with a SiO1.5 core and eight side groups. POSS-PEG8 has eight polyethylene glycol side chains that have low glass transition (Tg) and melt (Tm) temperatures and POSS-phenyl7(BF3Li)3 is a Janus-like POSS with hydrophobic phenyl groups and -Si-O-BF3Li ionic groups clustered on one side of the SiO1.5 cube. The electron-withdrawing POSS cage and BF3 groups enable easy dissociation of the Li+. In the presence of polar POSS-PEG8, the hydrophobic phenyl rings of POSS-phenyl7(BF3Li)3 aggregate and crystallize, forming a biphasic morphology, in which the phenyl rings form the structural phase and the POSS-PEG8 forms the conductive phase. The -Si-O-BF3- Li+ groups of POSS-phenyl7(BF3Li)3 are oriented towards the polar POSS-PEG8 phase and dissociate so that the Li+ cations are solvated by the POSS-PEG8. The nonvolatile nanocomposite electrolytes are viscous liquids that do not flow under their own weight. POSS-PEG8/POSS-phenyl7(BF3Li)3 at O/Li = 16/1 has a conductivity, σ = 2.5 x 10-4 S/cm at 30°C, 17 x greater than POSS-PEG8/LiBF4, and a low activation energy (Ea ~ 3-4 kJ/mol); σ = 1.6 x 10-3 S/cm at 90°C and 1.5 x 10-5 S/cm at 10°C. The lithium ion transference number was tLi+ = 0.50 ± 0.01, due to reduced mobility of the large, bulky anion and the system exhibited low interfacial resistance that stabilized after 3 days (both at 80°C). Secondly, solid polymer electrolytes have been prepared from the same salt, POSS-phenyl7(BF3Li)3 and polyethylene oxide (PEO). These exhibit high ambient temperature conductivity, 4 x 10-4 S/cm, and transference number, tLi+ = 0.6. A two-phase morphology is proposed in which the hydrophobic phenyl groups cluster and crystallize, and the three -BF3- form an anionic pocket, with the Li+ ions solvated by the PEO phase. The high ionic conductivity results from interfacial migration of Li+ ions loosely bonded to three -BF3- anions and the ether oxygens of PEO. Physical crosslinks formed between PEO/Li+ chains and the POSS clusters account for the solid structure of the amorphous PEO matrix. The solid polymer electrolyte has an electrochemical stability window of 4.6 V and excellent interfacial stability with lithium metal. In order to further enhance the ionic conductivity of solid polymer electrolytes, we have made two improvements. First, we have used so called half cube structures, T4-POSS, that contain 4 phenyl groups on one side of a Si-O- ring, and 4 ionic groups on the other side, and so are true Janus structures. They contain a 4/4 ratio of phenyl/ionic groups, unlike the previous structures that contain 7 phenyl groups/3 ionic groups. At the same O/Li ratio, the ionic conductivity of [PhOSi(OLi)]4 with POSS-PEG8 is higher than POSS-phenyl7Li3 because of more Li+ dissociation in the former case. Second, we have increased the dissociation of the lithium salts by replacing the Si-O-BF3Li groups with Si-(C3H4NLiSO2CF3)4. Both T4-POSS-(C3H4NLiSO2CF3)4 and POSS-(C3H4NLiSO2CF3)8 have been synthesized and characterized, with some preliminary conductivity data obtained. / Chemistry

This thesis concerns the use of advanced solid-state NMR methods to investigate local structural features and ion dynamics in a series of paramagnetic cathode materials for lithium and sodium ion batteries. A variety of polyanionic phosphate and fluorophosphate derivatives were explored to identify characteristics that ultimately improve battery performance. Solid-state NMR is an excellent method to probe such materials, as it offers the unique ability to track the charge-carrying alkali ion (Li or Na) over the course of the electrochemical process, adding insight not obtainable by bulk characterization techniques. Selective inversion exchange experiments were used to elucidate ion diffusion pathways in low-mobility Li ion conductors Li2MnP2O7 and Li2SnO3. Contrasting experimental results highlight significant differences observed when the method is applied to paramagnetic versus diamagnetic systems, with the former being much more complicated to study with traditional exchange spectroscopy methods. Selective inversion was similarly applied to a new lithium iron vanadate framework, LiFeV2O7, where the changing ion dynamics as a function of electrochemical state of charge were quantified, allowing for the development of a model to explain the corresponding phase changes in the material. This represents the first example of an ex situ Li-Li exchange study for a cathode material, particularly where the conductivity changes are linked directly to a change of ion exchange rates. Additionally, 23Na NMR spectroscopy was additionally used to investigate Na2FePO4F as a potential Na ion battery cathode, where ex situ NMR measurements successfully determined the local Na ion distribution in the electrode as a function of electrochemical cycling. In combination with density functional theory (DFT) calculations, the NMR results lead to the construction of a biphasic desodiation model for Na2FePO4F cathodes. Finally, possible defect formation in sodium iron fluorophosphate was investigated with a variety of methods including 23Na NMR, DFT calculations, powder X-ray diffraction and Mössbauer spectroscopy. / Thesis / Doctor of Philosophy (PhD) / Lithium ion batteries are considered to be at the forefront of current energy storage development, offering high energy density in a small and lightweight package. This thesis delineates the investigation of materials for both lithium and sodium ion batteries via nuclear magnetic resonance methods. Slow Li ion dynamics were investigated and quantified in three lithium-conducting materials: Li2MnP2O7, Li2SnO3, and LiFeV2O7 via the use of selective inversion NMR experiments. In the case of the latter, the ion dynamics were probed ex situ during the course of battery cycling, where a maximum in Li mobility is observed approximately half way through the charge-discharge cycle. Additionally, a potential Na ion cathode material, Na2FePO4F, was found by ex situ methods to reveal a biphasic mechanism for the desodiation of the electrode during charging. This mechanism and the NMR data used to discover it were further supported by ab initio calculations.

Energy storage is a necessity for the electrification of the modern world and the progression towards renewable energy. Designing new and innovative energy storage alternatives is one of the many challenges taken on by the Nuckolls group at Columbia University. More precisely, organic materials for energy storage with facile synthesis methods, non-toxic materials, and compatibility with aqueous electrolytes are a focus of this research. For this purpose, Perylenediimide (PDI) is the chosen primary molecular building block, that has enabled design of redox active materials due to its versatility as a structural unit, as well as its remarkable electrochemical performance. In this thesis 3 classes of materials based on PDI - small molecules, polymer networks, and COF materials - are compared; providing insights into how their design impacts electrochemical performance. Chapter 1 provides an overview of existing organic materials for energy storage. In particular, explaining the limitations, challenges, current landscape, and future of organic materials for battery and pseudocapacitive applications. This research area confronts current traditional energy storage strategies, such as lithium-ion batteries, with new organic alternatives that offer opportunities that could be more eco-friendly alternatives to lithium-ion batteries in specific applications. Chapter 2 describes the synthesis and characterization of PHATN, the highest performing aqueous n-type pseudocapacitor based on the PDI molecular backbone integrated into a 3-dimensional polymer network, and the relationship between electrochemical performance and structural contortion generated because of the molecular design. This is accomplished by benchmarking against a non-contorted linear polymer and comparing their electrochemical properties. This work provides the foundation for chapters 3 and 4. Chapter 3 expands on the use of molecular contortion as a design principle for molecular electronics, associating molecular contortion to electrochemical performance by generating helical inspired PDI polymers. This design reveals that the helical motif allows for enhanced electronic communication between the redox moieties and leads to higher device performance. Chapter 4 utilizes linear PDI polymers as a non-contorted control in comparison to the helical inspired polymers described in chapter 3. This linear motif reveals the competing design principle of increased surface area for electrolyte access to redox sites which is shown to increase device performance. Chapter 5 discusses a COF inspired redox active 2-dimensional polymers (RA-2DP) based on PDI materials and how the structural motif and conductive linkers can improve electrochemical performance. This chapter validates the design criteria outlined in chapter 4 and explains how these RA-2DPs and similar structures can enhance energy storage in organic materials. Collectively, this work provides a structured story of PDI materials, their potential as energy storage materials, and the design principles that have led to increased performance. The work completed in this thesis points towards structured and porous redox active organic materials as next generation energy storage alternatives. With the consideration of renewable energy and challenges with existing energy storage options, it is our hope that organic materials will contribute to this ever evolving and growing research area to create a more sustainable and environmentally friendly future.

京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24925号 / エネ博第467号 / 新制||エネ||87(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 萩原 理加, 教授 佐川 尚, 教授 野平 俊之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

<p dir="ltr">Energy storage has become a promising solution to stabilize renewable energy outputs and to solve the peak/off-peak issues of the power grid. Redox flow battery (RFB) possesses separated energy and power, high capacity, long cycle life and safety, and therefore is regarded as a potential candidate of energy storage. In this thesis, we have researched the degradation pathway of TEMPO derivative redoxmers, obtained long-time stable cycling of a non-aqueous RFB with synthetic redoxmers and permselective ceramic membranes, and extended the redox flow approach to the field of water desalination.</p><p dir="ltr">The properties of redoxmers are the main elements that affect RFB performance. Organic redoxmers come to sight due to their facile property improvement based on structural diversity and molecular tailorability. But the majority of reported redoxmers are anolytes; catholytes are less developed. Also, the mechanism of limited long-time cycling stability is still not well understood. In our experiment, we have progressively unraveled a series of degradation mechanisms of TEMPO-based redoxmers, including oxidation, crossover, ring-opening and possibly deoxygenation. The initial candidate, 4-hydro-TEMPO (TEMPOL), presents combined decomposition pathways. The charged oxoammonium species oxidizes the alcohol group (-OH) in its structure to a ketone (C=O) bond and also undergoes a protonation-induced ring-opening side reaction forming an alkene structure, evidenced by the characteristic 13C NMR chemical shifts of C=O and C=C groups. Due to its non-ionic structure, crossover through the anion exchange membrane used in flow cells is another issue that causes capacity loss. A hydroxyl-free TEMPO derivative bearing an anionic sulfonate group (‒SO3‒) also suffers from deprotonation-induced ring opening. By eliminating nucleophilic moiety, we have designed the third TEMPO derivative that has a cationic tetraalkylammonium end group. This molecule exhibits greatly improved cycling stability in flow cells, yet still with slow capacity fading that may hypothetically be a result of parasitic deoxygenation reaction. With the carefully designed analyses, the obtained mechanistic understanding of molecular decomposition has paved the way for rationale structural design toward stable TEMPO catholyte candidates.</p><p dir="ltr">Nonaqueous RFBs hold promise for higher cell voltage and energy density given their wider electrochemically stable voltage windows, but their performance is often plagued by the crossover of redox compounds. In this study, we used permselective lithium superionic conducting (LiSICON) ceramic membranes to enable reliable long-term cycling of organic redox molecules in nonaqueous flow cells. With different solvents on each side, enhanced cell voltages were obtained for a flow battery using viologen-based negolyte and TEMPO-based posolyte molecules. The thermoplastic assembly of the LiSICON membrane realized leakless cell sealing, thus overcoming the mechanical brittleness challenge. As a result, stable cycling was achieved in the flow cells, which showed good capacity retention over an extended test time (e.g. two months).</p><p dir="ltr">Desalination of saline water is becoming an increasingly critical strategy to overcome the global challenge of drinkable water shortage, but current desalination methods are often plagued with major drawbacks of high energy consumption, high capital cost, or low desalination capacity. To address these drawbacks, we have developed a unique continuous-mode redox flow desalination approach capitalizing on the characteristics of redox flow batteries. The operation is based on shuttled redox cycles of very dilute Fe2+/Fe3+ chelate redoxmers with ultralow cell overpotentials. The air instability of Fe2+ chelate is naturally compensated for by its in situ electrochemical generation, making the desalination system capable of operations with electrolytes at any specified state of charge. Under unoptimized conditions, fast desalination rates up to 404.4 mmol·m−2·h−1 and specific energy consumptions as low as 7.9 Wh·molNaCl−1 have been successfully achieved. Interestingly, this desalination method has offered an opportunity of sustainable, distributed drinkable water supplies through direct integration with renewable energy sources such as solar power. Therefore, our redox flow desalination design has demonstrated competitive desalination performance, promising to provide an energy-saving, high-capacity, robust, cost-effective desalination solution.</p>

京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24853号 / エネ博第462号 / 新制||エネ||87(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 野平, 俊之, 教授 萩原, 理加, 教授 佐川, 尚 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

The present thesis demonstrates and discusses polymeric ion and mixed ion-electron conductors for rechargeable batteries based on lithium viz. lithium-ion and lithium-sulphur batteries. The proposed polymer ion conductors in the thesis are discussed primarily as potential alternatives to conventional liquid and solid-crystalline electrolytes in lithium-ion batteries. These discussions are part of Chapters 2-4. On the other hand, the polymer based mixed ion-electron conductor is demonstrated as a novel electrode for lithium-Sulphur battery in Chapter 5. Possibility of application of polymer ion conductors is discussed in the context of Li-S battery in Chapter 6. A distinct correlation between the physical properties and electrochemical performance of the proposed conductors is highlighted in detail in this thesis. Systematic investigation of the ion transport mechanism in the polymeric ion conductors has been carried out using various spectroscopic techniques at different time and length scales. Such detailed investigations demonstrate the key structural and physical parameters for design of alternative polymer conductors for rechargeable batteries. Though the thesis discusses the various polymeric conductors in the context of lithium-based batteries, it is strongly felt that the design strategies are equally likely to be beneficial for different battery chemistries as well as for other electrochemical generation and storage devices. A brief discussion of the contents and highlights of the individual chapters are described below: The thesis comprises of six Chapters. Chapter 1 briefly reviews the important developments and materials of lithium-based batteries, with specific focus on Li-ion and Li-S batteries. It starts with discussions on different types of liquid, solid crystalline and solid-like electrolytes. Their materials characteristics, advantages and disadvantages are discussed in the context of secondary batteries such as lithium-ion and lithium-sulphur batteries. As prospective alternative electrolytes polymer based soft matter electrolytes are discussed in detail. Special emphasis is given to the recent developments in polymer electrolytes and their ion conduction mechanism, which are central themes to this thesis. The importance of investigation of charge transport, typically ion, on electrochemical processes is also briefly discussed in Chapter 1. A brief discussion about the characteristics, materials and non-trivialities of the electrochemical storage process in Li-S battery is also reviewed. Chapter 2A demonstrates a binary polymer physical network based gel (PN-x) electrolyte, comprising of an ionic liquid confined inside a binary polymer system for electrochemical devices such as secondary batteries. The synthesis, physical property and electrochemical performances are studied as a function of content of one of the polymers in this Chapter. A physical network of two polymers with different functional groups leads to multiple interesting consequences. The polymer physical network characteristics determine all physical properties including electrochemical property of the ionic liquid integrated PN based GPE. The conductivities of the proposed gel are nearly an order in magnitude higher than the unconfined ionic liquid electrolyte and displays good dimensional stability and electrochemical performance in a separator-free battery configuration. The ac-impedance spectroscopy, steady shear viscosity measurement, dynamic rheology are employed to study physical properties of the proposed gel polymer electrolyte. Chapter 2B discusses the detailed investigations of the ion transport mechanism of the gel polymer electrolyte, as discussed in Chapter 2A. Ion conduction mechanism is investigated in the light of ion diffusion and solvent dynamics of the entrapped ionic liquid inside the polymer. The studies reveal a heavy influence of network characteristics on the ion conduction mechanism. The influence of solvent dynamics on the ion transport is drastically altered by polymer physical network. Consequently, a drastic change in the ion mobility and nature of predominant charge carrier is observed in the polymer physical network based gel electrolyte. A clear transformation from dual ion conductivity to a predominantly anion conductivity is observed on going from single polymer to a dual polymer network. The spectroscopic tools such as pulsed field gradient nuclear magnetic resonance (PFG–NMR), Brillouin light scattering spectroscopy, ac-impedance spectroscopy, FT-Raman and FTIR spectroscopy were used to elucidate the ion transport mechanism in the Chapter. Chapter 3 demonstrates a simple design strategy of gel polymer electrolyte comprising of a lithium salt (lithium bis(trifluoromethanesulfonyl) imide, LiTFSI) solvated by two plastic crystalline solvents, one a solid (succinonitrile, abbreviated as SN) and another a (room temperature) ionic liquid (1-butyl-1-methyl-pyrrolidinium bis(trifluoromethane sulfonyl) imide, (abbreviated as IL) confined inside a linear network of poly(methyl methacrylate) (PMMA). The concentration of the IL component determines the physical properties of the unconfined electrolyte and when confined inside the polymer network in gel polymer electrolyte. Intrinsic dynamics of one plastic crystal influences the conduction mechanism of gel polymer electrolytes. The enhanced disordering in the plastic phase of succinonitrile by IL doping alters both the local ion environment and viscosity. The proposed plastic crystal electrolytes show predominantly anion conduction (tTFSI ≈ 0.5) however, lithium transference number (tLi ≈ 0.2) is nearly an order higher than the ionic liquid electrolyte (IL-LiTFSI) (tLi ≈ 0.02-0.06), discussed in Chapter 2. The gel polymer electrolyte displayed high mechanical compliability, stable Li-electrode | electrolyte interface, low rate of Al corrosion and stable cyclability. The promising electrochemical performance further justifies simple strategy of employing mixed physical state plasticizers to tune the physical properties of polymer electrolytes requisite for application in rechargeable batteries. Chapter 4A proposes a novel liquid dendrimer–based single ion conducting liquid electrolyte as potential alternative to conventional molecular liquid solvent–salt solutions and conventional solid polymer electrolytes for rechargeable batteries, sensors and actuators. The physical properties are investigated as a function of peripheral functionalities in the first generation poly(propyl ether imine) (G1-PETIM)–lithium salt complexes. The change in peripheral group simultaneously affects the effective physical properties viz. viscosity, ionic conductivity, ion diffusion coefficients, transference numbers and also the electrochemical response. The specific change from ester (–COOR) to cyano (–CN) terminated peripheral group resulted in a remarkable switch over from a high cation (tLi+ = 0.9 for –COOR) to a high anion (tPF6- = 0.8 for –CN) transference number. Chapter 4B presents an analysis of the frequency dependent ionic conductivity of single ion dendrimer conductors by using time temperature scaling principles (TTSPs) and dielectric modeling of the electrode polarization. The TTSP provides information on the salt dissociation and number density of mobile charges and hence provides direct insights into the ion conduction mechanism. Summerfield and Baranovskii–Cordes scaling laws, which are well known TTSPs, have been applied to analyze the ion conductivity. The electrode polarization, which quantifies the number density of mobile charges and ionic mobility, is studied using Macdonald-Coelho model of electrode polarization. The combination of these two theoretical investigations of the experimental data emanating from one technique i.e. ac– impedance spectroscopy, predicts independently the contributions of the effect of mobile ion charges and ionic mobility to ion conduction mechanism. In Chapter 5 focus shifts from polymer ion conductors to polymer mixed ion-electron conductor. The polymer mixed ion-electron conductor is demonstrated as a novel electrode material for Li-S battery. A simple strategy to overcome the challenges towards practical realization of a stable high performance Li–S battery is discussed. A soft mixed conducting polymeric network is utilized to configure sulphur nanoparticle. The soft matter network provides efficient and distinct pathways for lithium and electron conduction simultaneously. A lithiated polyethylene glycol (PEG) based surfactant tethered on ultra-small sulphur nanoparticles and wrapped up with polyaniline (PAni) (abbreviated as S-MIEC) is demonstrated here as an exceptional cathode for Li–S batteries. The S-MIEC is characterized by several methods: powder-X-ray diffraction (PXRD), thermo gravimetric analysis (TGA), fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), ac-impedance spectroscopy and dc current-voltage measurements are performed to evaluate conductivity of S-MIEC cathode. Electrochemical studies such as cyclic voltammetry, galvanostatic charge-discharge cycling, galvanostatic intermittent titration (GITT) are performed to demonstrate feasibility of S-MIEC in the Li–S battery performance. Chapter 6 provides a brief summary of the work carried out as part of this thesis and also demonstrates the future perspective of the present work. Potential of the polymer physical network based gel polymer electrolytes, which are discussed in Chapter 2A-B for lithium-ion batteries, are demonstrated in Li-S battery. The proposed polymer physical network confines higher order lithium polysulfides (typically Li2S8) dissolved in tetraethylene glycol dimethyl ether (TEGDME) based electrolyte (TEGDME-1M LiTFSI). The three dimensional polymer network is proposed to be formed by physical blending of the poly(acrylonitrile) (PAN) with the copolymer of AN and poly(ethylene glycol) methyl ether methacrylate (PEGMA), [ P(AN–co–PEGMA)]. We extend here the similar synthetic approaches as described in Chapter 2A. The approach proposed and demonstrated in this concluding Chapter is expected to mitigate some of the major issues of Li-S chemistry. The proposed Li2S8 confined gel electrolyte exhibits moderately high values of ionic conductivity, 2 × 10-3 Ω-1cm-1 and shows a stable capacity of 350 mAhg-1 over 30 days in a separator free Li-S battery.

Subsequent to the studies on precursor lead-acid systems by Daniel, Grove and Sindesten, practical lead-acid batteries began with the research and inventions of Raymond Gaston Planté in France as early as in 1859, and, even to-day, lead-acid battery remains the most successful battery system ever developed, and no other battery is yet able to compete with lead-acid batteries on cost grounds, albeit batteries based on other chemistries are rapidly catching up. In the past, although lead-acid battery designs have been optimized in several different directions, there are still certain new challenges facing the lead-acid battery designers as additional failure modes become evident in various use modes. There are three types of lead-acid batteries in common use: (a) batteries with flooded or excess electrolyte, (b) low-maintenance lead-acid batteries with a large excess of electrolyte, and (c) batteries with immobilized electrolyte and a pressure-sensitive valve usually referred to as absorptive glass-microfibre (AGM) valve-regulated lead-acid (VRLA) batteries. The flooded-electrolyte lead-acid battery requires checking of specific gravity of electrolyte, periodic addition of water to maintain electrolyte level above the plates and recharge soon after discharge to prevent hard sulfation that causes loss of capacity. The emission of acid fumes corrodes metallic parts in the vicinity of the battery, and the seepage of acid on the top cover of the batteries leads to leakage current resulting in increased self-discharge and ground-shunt hazards. To overcome these problems, AGMVRLA batteries based on oxygen-recombination cycle have emerged. These batteries offer the freedom of battery placement, cyclability without the addition of water or checking the specific gravity, increased safety, and superior performance in some instances. Both flooded-electrolyte and AGM-VRLA batteries can suffer from acid stratification. But, AGM-VRLA batteries are especially susceptible to failures owing to the heat generated by oxygen recombination within the cells as well as due to cell-to-cell variations in electrolyte volumes. Indeed, partial heating of AGM-VRLA batteries could cause dry-out with grid corrosion and even lead to thermal runaway. Consequently, mitigating temperature variations in AGM-VRLA batteries becomes seminal to their commercial success. A dissipation of local heat within the AGM-VRLA batteries can be achieved by adequately filling the void volume in the battery with a thermally conducting gel, such as a gel formed from colloidal silica and sulfuric-acid electrolyte. Although, conventional lead-acid batteries are considered rather a matured technology, significant research and development efforts are currently under way to enhance their performance. Indeed, many improvements have been made in the lead-acid battery since its invention, and although the essential electrochemistry remains unchanged, the modern lead-acid batteries have little semblance to those produced 50 years ago. Over the years, seminal advances have been made in the lead-alloys used, in the materials and design of separators, in battery packaging and in their construction methods, which have led to lead-acid batteries with improved performance, lighter weight and lower cost. This thesis is an attempt to develop lightweight hybrid-VRLA batteries.

Energy storage is crucial for intermittent renewable energy sources, electric vehicles, and portable devices. The continuously increasing energy consumption in these industries necessitates the enhancement of commercial lithium-ion batteries (LIB), especially regarding their safety and energy density. Historically, aqueous electrolytes were the norm in the battery industry. Prior to the development of lithium batteries, most commercially significant batteries used water as the solvent. In the past decade, "highly concentrated" electrolytes resurrected the notion of an aqueous lithium-ion battery (ALIB). Significant efforts have been made since then to comprehend the interfacial stability of these high-concentration electrolytes, and make them suitable for use in batteries especially high voltage ones. Another candidate for future batteries is All-Solid-State Batteries (ASSB) as they have the potential to double, or even triple, the energy density figures we currently achieve in LIBs mainly due to their ability to utilize lithium metal anode which has the highest specific capacity among anodes (3860 mAh g⁻¹), lowest reduction potential (-3.04 V vs SHE), and low density (0.53 g cm⁻³). This thesis first proposes a phenomenological model to describe the microstructure of aqueous electrolyte and the relation between their phase diagrams with ionic conductivity; highlighting a common correlation between the eutectic composition and peak ionic conductivity in conductivity isotherms. we then propose an empirical model correlating ionic conductivity with both molar concentration and temperature. The aim of this portion of the thesis is to provide an in depth understanding of aqueous electrolytes' physical properties in a way that can help researchers optimize the energy density and the cost of ALIBs. Moving further, the thesis presents two novel composite solid electrolytes (CSE) that were developed and fully characterized. Both of which were composed of the following four components; polyethylene oxide (PEO), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, lithium lanthanum titanate (LLTO) perovskite inorganic ceramic and the polymer plasticizer succinonitrile (SN). The careful formulation of these CSEs was based on the trade-off between film forming ability and ionic conductivity. The optimized polymer rich CSE proved to have better characteristics when compared to its ceramic rich alternative. ASSBs employing both CSEs were successfully charged and discharged when coupled with lithium metal anode and in-lab prepared composite cathode. The developed thin and flexible CSEs could be utilized in small applications (Wh-KWh) such as in consumer electronics and flexible biomedical devices (e.g., pacemakers) or larger applications (kWh-MWh) such as in EVs and large format storage for the electrical grid.

The global EV stock is expected to increase from 7.2 million in 2019 to nearly 140 million vehicles by 2030. So, the demand for the battery also increases due to the increase in the number of EVs. In any EV, battery degradation is an unavoidable phenomenon and EV batteries are assumed to arrive at their end-of-life in EV application when the state of health reaches 80 %, repurposing the eligible EV batteries after end of first life is expected to extend their lifetime by another 5-15 years in the second life applications.  This thesis aims to conduct a techno-economic study on the usage of second life EV batteries as an alternative storage option in off-grid PV systems compared to lead-acid batteries and new Li-ion batteries. A single-family house with an annual demand of 2245 kWh/year located in Athens was chosen as the primary location, the off-grid PV system is pre-sized for Athens and based on the pre-sizing results and what is state of art in the market. The system components were chosen for system design (4 kW bi-directional inverter, 2.9 kW PV array, 7.2 kW genset and three battery bank options i.e., 16.5 kWh of lead-acid, 8 kWh new Li-ion and 12.6 kWh of second life EV battery). PV off-grid system with different storage options is simulated using HOMER for both locations and the results are compared.   The simulation results show that the designed off-grid PV system can reach a solar fraction of 90 % in Athens and 73 % in Gotland when 16.5 kWh of lead-acid batteries are used with an allowed depth of discharge of 50 %. When a new Li-ion battery of 8 kWh with an allowed depth of discharge of 80 % is used then the achievable solar fraction is 84 % in Athens and 71 % in Gotland, When the second life EV battery of 12.6 kWh with an allowed depth of discharge of 60 % is used then the achievable solar fraction is 90 % in Athens and 74 % in Gotland. Sensitivity analysis is performed on the depth of discharge and results showed that the solar fraction can be increased by allowing the battery to discharge more, but it also decreases the battery lifetime.   The simulation results also show that the net present cost was lower in Athens for all the reference cases compared to Gotland. Net present cost and levelized cost of electricity for the off-grid system are 25.3 k€, 0.9 €/kWh in Athens and 29.2 k€, 1.0 €/kWh in Gotland when a lead-acid battery is used. When a new Li-ion battery is used then 26.2 k€, 0.9 €/kWh in Athens and 29.3 k€, 1.0 €/kWh in Gotland, when the second life EV battery is used then 26.7 k€, 0.9 €/kWh in Athens and 30.7 k€, 1.1 €/kWh in Gotland.   Overall, the net present cost and levelized cost of electricity are lower in Athens in all cases compared to Gotland. For the reference house in Athens, lead acid battery system has shown slightly lower net present cost than new Li-ion battery and second life EV battery. For the reference house in Gotland, both lead acid battery and new Li-ion battery system have shown similar net present cost and they are slightly lower than second life EV battery.   Also, the second life EV battery levelized cost of electricity is fairly comparable to the new Li-ion and lead acid battery system. In future, the massive inflow of used batteries from EV are expected to be available on the market for the second life application at a lower price than today. Thus, in future, second life EV batteries can become economically viable.