Les batteries lithium-ion sont présentement d’excellentes candidates pour le stockage électrochimique d’énergie du futur. Cela dit, les batteries lithium-métal pourraient présenter des propriétés électrochimiques encore plus avantageuses. Cependant, ces types de batteries présentent encore des inconvénients, notamment au niveau de leur sécurité. Un des responsables majeurs de ceux-ci est l’électrolyte liquide organique. Parmi les différentes voies exploitables pour améliorer la sécurité de ces technologies, les électrolytes solides polymères (SPE) sont largement étudiés. Classiquement, ces systèmes sont mis en forme en présence de solvants qui sont ensuite évaporés. Aussi, lorsque le processus d’évaporation de solvants est terminé, les échantillons sont habituellement réexposés à l’air ambient. Or, d’une part, malgré le séchage important d’un échantillon, il se peut qu’il reste du solvant de mise en forme résiduelle. D’autre part, l’eau atmosphérique peut s’infiltrer au sein de celui-ci. Cependant, ce ne sont pas des facteurs qui sont considérés dans la recherche présente dans le domaine. Bien que l’influence des solvants résiduels est parfois mentionnée, elle n’est jamais quantifiée de façon convenable, et cela reste un facteur mal compris et souvent omis. Dans cette étude, des échantillons de différents types de SPE ont été préparés selon des conditions standards, leur teneur en solvants résiduels a été contrôlée et analysée par différentes méthodes développées au cours de cette recherche. Pour la quantification de l’eau, un analyseur d’humidité spécifique a été utilisé, et il a été montré que l’eau résiduelle permet d’augmenter les conductivités ioniques des échantillons. Pour la quantification des solvants résiduels organiques, une méthode analytique employant la chromatographie gazeuse couplée à la spectrométrie de masse a été développée. Il a été observé que comme avec l’eau, les solvants résiduels augmentent la conductivité ionique des échantillons étudiés. Cette étude doit montrer aux chercheurs dans le domaine que le contrôle des solvants résiduels est un facteur primordial dans le développement des SPEs, et que c’est un paramètre qui doit être systématiquement évalué. / Lithium-ion batteries are today’s candidates for future long-term electrochemical storage of renewable energies. That said, lithium-metal batteries could offer even more appealing electrochemical properties. However, both types of batteries still suffer from certain technical difficulties such as safety. One of the culprits for their reduced safety is the use of an organic liquid electrolyte. Indeed, the latter is flammable and poses a risk, as numerous battery fire accidents have shown throughout the past years. Luckily, scientific research has been able to propose safer alternatives to liquid electrolytes applicable to lithium batteries by replacing the former by solid state electrolytes. Amongst these systems, solid polymer electrolytes (SPE) can be considered as a promising possibility to eliminating the safety issues. Conventionally, SPEs are prepared in a solvent that is evaporated at the end of the manufacturing. Additionally, atmospheric humidity can infiltrate these materials and alter their properties. However, residual solvent content is seldom mentioned, and even when it is, the specific experimental parameters are lacking which makes it a misunderstood and regularly omitted factor in battery performance evaluation. In this study, residual solvents are quantified in different SPE systems that are prepared according to standard and non-standard procedures. To do so, certain samples have had their solvent content artificially modified in order to control and analyse it. Firstly, water content is assessed using a specific moisture analyser. Secondly, an analytical method employing gas chromatography coupled to mass spectrometry has been developed to determine the residual SPE processing solvent. It has been concluded that, similarly to water, residual solvents also contribute to enhancing ionic conductivities of SPEs. Hopefully, this study will shed light on the importance of controlling residual solvent content in SPEs, and the necessity of systematically assessing that parameter.

No description available.

Lithium alloys with metallic or semi-metallic elements are attractive candidate materials for the next-generation rechargeable Li-ion battery anodes, thanks to their large specific and volumetric capacities. The key challenge, however, has been the large volume changes, and the associated stress buildup and failure during cycling. The chemo-mechanics of alloy-based electrode materials entail interactions among diffusion, chemical reactions, plastic flow, and material property evolutions. In this study, a continuum theory of two-way coupling between diffusion and deformation is formulated and numerically implemented. Analyses based on this framework reveal three major conclusions. First, the stress-to-diffusion coupling in Li/Si is much stronger than what has been known in other electrode materials. Practically, since the beneficial effect of stress-enhanced diffusion is more pronounced at intermediate or higher concentrations, lower charging rates should be used during the initial stages of charging. Second, when plastic deformation and lithiation-induced softening take place, the effect of stress-enhanced diffusion is neutralized. Because the mechanical driving forces tend to retard diffusion when constraints are strong, even in terms of operational charging rate alone, Li/Si nano-particles are superior to Li/Si thin films or bulk materials. Third, the diffusion of the host atoms can lead to significant stress relaxation even when the stress levels are below the yield threshold of the material, a beneficial effect that can be leveraged to reduce stresses because the host diffusivity in Li/Si can be non-negligible at higher Li concentrations. A theory of coupled chemo-mechanical fracture driving forces is formulated in order to capture the effect of deformation-diffusion coupling and lithiation-induced softening on fracture. It is shown that under tensile loading, Li accumulates in front of crack tips, leading to an anti-shielding effect on the energy release rate. For a pre-cracked Li/Si thin-film electrode, it is found that the driving force for fracture is significantly lower when the electrode is operated at higher Li concentrations -- a result of more effective stress relaxation via global yielding. The results indicate that operation at higher concentrations is an effective means to minimize failure of thin-film Li/Si alloy electrodes.

<p>Currently, fossil fuels and nuclear power are our primary energy sources. However, both have critical disadvantages due to the limited supply and the hazard issues. Renewable energy research becomes one of most important research topics in the 21st century. Nanostructured materials show unique electrochemical properties in various energy conversion or storage devices. This dissertation starts with fundamental optical studies of nanomaterials (carbon nanotubes), followed by synthesizing novel nanomaterials for energy conversion (solar cells) and storage (lithium ion batteries) devices. </p><p> (1) There is an on-going debate concerning the ability of double walled carbon nanotubes (DWNTs) to exhibit photoluminescence (PL). We aim to clearly resolve this debate through the study of carefully separated DWNTs using density gradient ultra-centrifugation (DGU). Here, we clearly show that light is emitted from the inner wall of DWNTs. Interestingly, it was found that a very narrow range of diameters of the inner walls of DWNTs is required for photoluminescence (PL) to be observable. All other diameters led to complete PL quenching in DWNTs. (2) Inexpensive dye sensitized solar cells (DSSCs) on flexible plastic substrates have a bright future, but they require low temperature annealing (< 200°C). The method to fabricate low temperature DSSCs should resolve poor electron transfer between titanium dioxide (TiO2) nanoparticles (NPs) due to their incomplete contiguity and insulating layer of organic residues from binders in the photoactive film. Here, we have developed uniform CNTs/TiO2 composites for low temperature DSSCs by using modified sol gel method. DSSCs were fabricated to study incorporating functionalized few walled carbon nanotubes (f-FWNTs) effect on TiO2 NPs. Incorporating f-FWNTs can be beneficial for the low temperature annealing process of DSSCs to overcome extremely poor electron transport through TiO2 photoactive film. Incorporating f-FWNTs with TiO2 active layer improves electrons transport in some degree, but this advantage is limited. (3) Conductive fillers, such as amorphous carbon, carbon nanotube and graphene, have been mixed with nanostructured metal oxide materials to improve the performance of electrode materials in energy storage devices. However, ineffective junctions between conductive fillers are limiting the overall conductivity of the electrode. Therefore, we developed a convenient, inexpensive and scalable method for synthesizing hybrid carbon and titanium dioxide (C/TiO2) co-gels and co-aerogels to improve their electrochemical capacity in lithium ions batteries (LIBs). The monolith of the hybrid C/TiO2 co-aerogel can be directly used as active electrodes without the addition of binders. As a result, the capacitance of LIB anodes using the hybrid co-aerogel is significantly improved over current LIBs based on carbon/titanium oxide composite. Other metal oxides could also form co-gels with carbon to improve their potentials in numerous electrochemical, photocatalytic, and photoelectronic devices.</p> / Dissertation

Three-dimensional macroporous structures (‘opals’ and ‘inverse opals’) can be produced by colloidal crystal templating, one of the most intensively studied areas in materials science today. There are several potential advantages of lithium-ion battery electrodes based on inverse opal structures. High electrode surface, easier electrolyte access to the bulk of electrode and reduced lithium diffusion lengths allow higher discharge rates. Highly open structures provide for better mechanical stability to volume swings during cycling. Silicon is one of the most promising anode materials for lithium-ion batteries. Its theoretical capacity exceeds capacities of all other materials besides metallic lithium. Silicon is abundant, cheap, and its use would allow for incorporation of microbattery production into the semiconductor manufacturing. Performance of silicon is restricted mainly by large volume changes during cycling. The objective of this work was to investigate how the inverse opal structures influence the performance of silicon electrodes. Several types of silicon-based inverse opal films were synthesised, and their electrochemical performance was studied. Amorphous silicon inverse opals were fabricated via chemical vapour deposition and characterised by various techniques. Galvanostatic cycling of these materials confirmed the feasibility of the approach taken, since the electrodes demonstrated high capacities and decent capacity retentions. The rate performance of amorphous silicon inverse opals was unsatisfactory due to low conductivity of silicon. The conductivity of silicon inverse opals was improved by crystallisation. Nanocrystalline silicon inverse opals demonstrated much better rate capabilities, but the capacities faded to zero after several cycles. Silicon-carbon composite inverse opal materials were synthesised by depositing a thin layer of carbon via pyrolysis of a sucrose-based precursor onto the silicon inverse opals in an attempt to further increase conductivity and achieve mechanical stabilisation of the structures. The amount of carbon deposited proved to be insufficient to stabilise the structures, and silicon-carbon composites demonstrated unsatisfactory electrochemical behaviour. Carbon inverse opals were coated with amorphous silicon producing another type of macroporous composites. These electrodes demonstrated significant improvement both in capacity retentions and in rate capabilities. The inner carbon matrix not only increased the material conductivity, but also resulted in lower silicon pulverisation during cycling.

Lithium-rich layered transition metal oxide cathode, represented as the chemical formula of xLi₂MnO₃ &middot; (1 - x)LiMO₂(M = Mn, Ni, Co) , retains immense interest as one of the most promising candidates for energy storage system ranging from mobile devices to electric vehicle applications (EV/HEV/PHEV). This battery type benefits from superior theoretical capacity (&gt;250 mAhg^-1), high chemical potential (&gt;4.6 V vs Li⁰), good thermal stability, high discharge capacity and lower cost compared with conventional cathodes (e.g. LiCoO₂, Li(Ni_1/3Mn_1/3Co_1/3)O₂ cathodes). However, there remain major barriers which still need to be improved in order to achieve a successful commercialization for large-scale devices or electric vehicle applications. The irreversible capacity loss of 40-100 mAhg^-1 during the initial electrochemical cycle and the battery fading phenomena (capacity fading/voltage decay) on further cycles are the major problems which have emerged. The Li⁺ ion extraction accompanied by oxygen release from the active material in the form of oxide known as lithia (Li₂O) along with the transition metal migration has been suggested as the dominant processes underlying the capacity fading mechanism. Those processes, in turn, cause a phase transition from a layered structure into a spinel within the electrode material. The interplay of the local atomic environments between Li₂MnO₃ (monoclinic, C2/m) and LiMO₂ (trigonal/hexagonal, R3m) holds the key to developing better cathodes with enhanced stability. In the present thesis, an in operando XAS study using a specially-designed cell of the graphene- coated Li(Li_0.2Mn_0.54Ni_0.13Co_0.13)O₂ cathode is employed to examine the chemical, electronic, and structural states of the transition metals (Mn, Co, and Ni) during electrochemical cycle(s). Precise oxidation states for the transition metals is evaluated by the combined analyses from the XANES and SQUID measurements. The K-edge XANES spectral shift is quantified to investigate the contribution to the charge compensation mechanism by the oxidation change. Absorption features in K-edge XANES are identified. These features describe the electronic state of the individual atoms in the cathode composite, as well as the local distortion from the octahedral structure of MO₆. The Fourier transform of EXAFS offers a satisfactory description of the local structure changes with the connection to the cation arrangement. The description is generally involved with the peak amplitude, position, shape changes (trend), and coordination numbers in the real space. Hence, similarities or discrepancies in the local atomic environments could be compared at different state of charge. Major structural parameters are deduced from the EXAFS fitting process. These parameters can be used to distinguish different atomic environments upon voltage bias levels or investigate the appearance of the Jahn-Teller effect. A new approach to understand the atomic environment upon charge-discharge is demonstrated, namely, a Continuous Cauchy Wavelet Transform (CCWT) which enables the visualization of the EXAFS spectra in three dimensions by decomposing the k-space and R-space (uncorrected for phase shift) signals. The wavelet transform analysis provides possible evidence of the precursor that leads to the spinel phase transition in this battery system.

This thesis aims to provide insight into the fundamental electrochemical processes taking place when cycling SnO2 in lithium-ion batteries (LIBs). Special attention was paid to the partial reversibility of the tin oxide conversion reaction and how to enhance its reversibility. Another main effort was to pinpoint which limitations play a role in tin based electrodes besides the well-known volume change effect in order to develop new strategies for their improvement. In this aspect, Li+ mass transport within the electrode particles and the large first cycle charge transfer resistance were studied. Li+ diffusion was proven to be an important issue regarding the electrochemical cycling of SnO2. It was also shown that it is the Li+ transport inside the SnO2 particles which represents the largest limitation. In addition, the overlap between the potential regions of the tin oxide conversion and the alloying reaction was investigated with photoelectron spectroscopy (PES) to better understand if and how the reactions influence each other`s reversibility. The fundamental insights described above were subsequently used to develop strategies for the improvement of the performance and the cycle life for SnO2 electrodes in LIBs. For instance, elevated temperature cycling at 60 oC was employed to alleviate the Li+ diffusion limitation effects and, thus, significantly improved capacities could be obtained. Furthermore, an ionic liquid electrolyte was tested as an alternative electrolyte to cycle at higher temperatures than 60 oC which is the thermal stability limit for the conventional LP40 electrolyte. In addition, cycled SnO2 nanoparticles were characterized with transmission electron microscopy (TEM) to determine the effects of long term high temperature cycling. Also, the effect of vinylene carbonate (VC) as an electrolyte additive on the cycling behavior of SnO2 nanoparticles was studied in an effort to improve the capacity retention. In this context, a recently introduced intermittent current interruption (ICI) technique was employed to measure and compare the development of internal cell resistances with and without VC additive.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Gurpreet Singh / Molecular precursor derived ceramics (also known as polymer-derived ceramics or PDCs) are high temperature glasses that have been studied for applications involving operation at elevated temperatures. Prepared from controlled thermal degradation of liquid-phase organosilicon precursors, these ceramics offer remarkable engineering properties such as resistance to crystallization up to 1400 °C, semiconductor behavior at high temperatures and intense photoluminescence. These properties are a direct result of their covalent bonded amorphous network and free (-sp2) carbon along with mixed Si/B/C/N/O bonds, which otherwise can not be obtained through conventional ceramic processing techniques. This thesis demonstrates synthesis of a unique core/shell type nanowire structure involving either siliconboroncarbonitride (SiBCN) or siliconoxycarbide (SiOC) as the shell with carbon nanotube (CNT) acting as the core. This was made possible by liquid phase functionalization of CNT surfaces with respective polymeric precursor (e.g., home-made boron-modified polyureamethylvinylsilazane for SiBCN/CNT and commercially obtained polysiloxane for SiOC/CNT), followed by controlled pyrolysis in inert conditions. This unique architecture has several benefits such as high temperature oxidation resistance (provided by the ceramic shell), improved electrical conductivity and mechanical toughness (attributed to the CNT core) that allowed us to explore its use in energy conversion and storage devices. The first application involved use of SiBCN/CNT composite as a high temperature radiation absorbant material for laser thermal calorimeter. SiBCN/CNT spray coatings on copper substrate were exposed to high energy laser beams (continuous wave at 10.6 μm, 2.5 kW CO2 laser, 10 seconds) and resulting change in its microstructure was studied ex-situ. With the aid of multiple techniques we ascertained the thermal damage resistance to be 15 kW/cm2 with optical absorbance exceeding 97 %. This represents one order of magnitude improvement over bare CNTs (1.4 kW/cm2) coatings and two orders of magnitude over the conventional carbon paint (0.1 kW/cm2) currently in use. The second application involved use of SiBCN/CNT and SiOC/CNT composite coatings as energy storage (anode) material in a Li-ion rechargeable battery. Anode coatings (~1mg/cm2) prepared using SiBCN/CNT synthesized at 1100 °C exhibited high reversible (useable) capacity of 412 mAh/g even after 30 cycles. Further improvement in reversible capacity was obtained for SiOC/CNT coatings with 686 mAh/g at 40 cycles and approximately 99.6 % cyclic efficiency. Further, post cycling imaging of dissembled cells indicated good mechanical stability of these anodes and formation of a stable passivating layer necessary for long term cycling of the cell. This improved performance was collectively attributed to the amorphous ceramic shell that offered Li storage sites and the CNT core that provided the required mechanical strength against volume changes associated with repeated Li-cycling. This novel approach for synthesis of PDC nanocomposites and its application based testing offers a starting point to carry out further research with a variety of PDC chemistries at both fundamental and applied levels.

Mgr. Martina Nádherná PhD. Thesis Applications of ionic liquids in electrolytes for amperometric gas sensors and Li-ion batteries SUMMARY The dissertation presents the results of preparation and characterisation of new aprotic electrolytes based on ionic liquids for the solid-state electrochemical gas sensors and for the electrochemical energy storage devices - secondary lithium-ion batteries. In the part dealing with the solid-state amperometric sensor for NO2 research was aimed at development of new solid electrolyte. This electrolyte is developed as a system of ionic liquid embedded in the structure of a polymer, when the ionic liquid joints the properties of a solvent and a dissolved salt. The electrolyte therefore does not contain any volatile component and is long-term chemically and electrochemically stable. Several series of electrolytes were prepared with different polymers or macromonomers and imidazolium-based ionic liquids. The composition, especially the polymer-IL ratio was optimized. The electrolytes were successfully tested in a solid-state NO2 sensor with a gold minigrid serving as the indicating electrode. The research included the determination of basic electrochemical parameters and study of the sensor behaviour under different conditions. The influence of atmosphere humidity,...

<div>Potassium-ion batteries (PIBs) recently emerged as a next-generation energy storage technology, utilizing abundant and inexpensive potassium as the charge carrier cation. PIBs operate by an analogous mechanism to lithium-ion batteries (LIBs), with reversible potassium intercalation in anode and cathode through an inorganic salt - organic solvent electrolyte medium. Despite its larger size, potassium exhibits several electrochemical advantages over sodium, including a higher affinity for intercalation into graphitic (carbonaceous) anodes, forming a stage-one KC₈ structure in graphite for a specific capacity of 279 mAh g^-1. This thesis aims to provide a thorough foundation for PIB carbon anodes, through a comprehensive experimental approach combining electrode synthesis, advanced material characterization and electrochemical-analytical techniques.</div><div><br></div><div>Safety concerns have consistently plagued LIBs despite almost three decades of widespread commercialization. Thermal runaway of LIBs can initiate as early as 80°C from exothermic breakdown of the solid electrolyte interphase (SEI) layer that covers the carbon anode surface. The subsequent reaction of lithiated carbon with electrolyte solvent leads to cathode decomposition and oxygen release for cell gassing and combustion. This thesis investigates the thermal runaway behavior of graphite anode for PIBs via differential scanning calorimetry analysis, determining the effect of electrode material, state-of-charge, and cycling history on heat generation. Notably, the PIB system emits significantly less heat overall than for LIBs, albeit an earlier and more intense onset reaction at 100°C raises safety concerns. Strategies to mitigate this exothermic reaction are presented, including electrode binder manipulation to improve graphite particle coverage and enhance SEI layer stability.</div><div><br></div><div>To further evaluate the practicality of PIBs, the electrochemical behavior of graphite anode was investigated from 0 - 40°C operating temperature, in comparison to standard LIBs. The poor rate capability of potassium is attributed to sluggish solid-state diffusion and augmented cell impedance, where 3-electrode studies revealed dramatic polarization of the potassium metal counter electrode at low temperatures. Accelerated cell aging at elevated temperatures is attributed to SEI layer growth induced by the 61% volumetric expansion of graphite during potassiation, as well as the extreme reactivity of potassium metal. A full-cell system with a Prussian blue nanoparticle cathode and graphite anode showed enhanced rate performance at low temperatures by removing potassium metal counter electrode. These results provide valuable mechanistic insight for potassium intercalation in graphite and offer a practical evaluation of temperature dependent electrochemical performance for PIBs.</div><div><br></div><div>Supplementary research includes the exploration of carbon nanofibers electrospun from polyacrylonitrile precursor with subsequent pyrolysis as PIB anode. The design of an amorphous, low density carbon with a nanoscale one dimensional morphology enables mitigation of the 61% volumetric expansion of graphite during potassiation. Remarkable stability (2000 charge-discharge cycles) is thus achieved by preventing electrode pulverization, SEI layer growth, and impedance rise during cycling. Electrochemical analysis revealed a pseudo-capacitance mechanism, enabling rapid charging through surface storage of potassium that could be enhanced by surface functionalization via plasma oxidation treatment. Moreover, two dimensional MXene transition carbonitride sheets were explored as PIB anode with X-ray diffraction and X-ray photoelectron spectroscopy used to study structural changes during potassium insertion.</div><div><br></div><div>Finally, the effect of particle morphology was investigated for LIB carbon anodes, wherein commercial graphite is compared with synthesized spherical and spiky carbons. Intercalation dynamics, side reaction rates (e.g. SEI growth), self-heating, and thermal runaway behavior were studied through a combination of electrochemical analysis and modeling by a finite volume method. Spherical particles outperform irregular commercial graphite by eliminating unstructured inhomogeneities that lead to non-uniform current distributions. Interestingly, spiky particles offer a nontrivial response, where the ordered irregularities enhance intercalation dynamics to prevent degradation at extreme operating conditions. These findings emphasize the importance of tailoring particle morphology and structure in promoting desired LIB behavior and suppressing unwanted problems.</div>