The stability of electrode/electrolyte interfaces in Li-ion batteries is crucial to the performance, lifetime and safety of the entire battery system. In this work, interface processes have been studied in LiFePO4/graphite Li-ion battery cells.  The first part has focused on improving photoelectron spectroscopy (PES) methodology for making post-mortem battery analyses. Exposure of cycled electrodes to air was shown to influence the surface chemistry of the graphite. A combination of synchrotron and in-house PES has facilitated non-destructive interface depth profiling from the outermost surfaces into the electrode bulk. A better understanding of the chemistry taking place at the anode and cathode interfaces has been achieved. The solid electrolyte interphase (SEI) on a graphite anode was found to be thicker and more inhomogeneous than films formed on cathodes. Dynamic changes in the SEI on cycling and accumulation of lithium close to the carbon surface have been observed.    Two electrolyte additives have also been studied: a film-forming additive propargyl methanesulfonate (PMS) and a flame retardant triphenyl phosphate (TPP). A detailed study was made at ambient and elevated temperature (21 and 60 °C) of interface aging for anodes and cathodes cycled with and without the PMS additive. PMS improved cell capacity retention at both temperatures. Higher SEI stability, relatively constant thickness and lower loss of cyclable lithium are suggested as the main reasons for better cell performance. PMS was also shown to influence the chemical composition on the cathode surface. The TPP flame retardant was shown to be unsuitable for high power applications. Low TPP concentrations had only a minor impact on electrolyte flammability, while larger amounts led to a significant increase in cell polarization. TPP was also shown to influence the interface chemistry at both electrodes. Although the additives studied here may not be the final solution for improved lifetime and safety of commercial batteries, increased understanding has been achieved of the degradation mechanisms in Li-ion cells. A better understanding of interface processes is of vital importance for the future development of safer and more reliable Li-ion batteries.

Li-ion batteries are not only a technology for the future, they are indeed already the technology of choice for today’s mobile phones, laptops and cordless power tools. Their ability to provide high energy densities inexpensively and in a way which conforms to modern environmental standards is constantly opening up new markets for these batteries. To be able to maintain this trend, it is imperative that all issues which relate safety to performance be studied in the greatest detail. The surface chemistry of the electrode-electrolyte interfaces is intrinsically crucial to Li-ion battery performance and safety. Unfortunately, the reactions occurring at these interfaces are still poorly understood. The aim of this thesis is therefore to increase our understanding of the surface chemistries and stability phenomena at the electrode-electrolyte interfaces for three novel Li-ion battery electrode materials. Photoelectron spectroscopy has been used to study the surface chemistry of the anode material AlSb and the cathode materials LiFePO4 and Li2FeSiO4. The cathode materials were both carbon-coated to improve inter-particle contact. The surface chemistry of these electrodes has been investigated in relation to their electrochemical performance and X-ray diffraction obtained structural results. Surface film formation and degradation reactions are also discussed. For AlSb, it has been shown that most of the surface layer deposition occurs between 0.50 and 0.01 V vs. Li°/Li+ and that cycling performance improves when the lower cut-off potential of 0.50 V is used instead of 0.01 V. For both LiFePO4 and Li2FeSiO4, the surface layer has been found to be very thin and does not provide complete surface coverage. Li2CO3 was also found on the surface of Li2FeSiO4 on exposure to air; this was found to disappear from the surface in a PC-based electrolyte. These results combine to give the promise of good long-term cycling with increased performance and safety for all three electrode materials studied.

La société fait face à des défis tels que le réchauffement climatique et la diminution des ressources. Ils sont intimement liés à l’énergie et à son stockage, dont les batteries Li-ion sont à ce jour la technologie la plus utilisée. L’amélioration de la densité d’énergie et la sécurité, ainsi que la réduction des éléments toxiques, rares et coûteux sont recherchées. Durant cette étude, les électrodes positives basées sur des matériaux polyanioniques silicates sont considérées pour répondre à ces demandes. Deux composés sont particulièrement étudiés, Li2MnSiO4, dont la capacité spécifique est supérieure à 300mAh.g-1 et LiMnSiO4, de structure olivine, encore jamais répertorié, dont la capacité (174mAh.g-1) et le potentiel (>3.7V) théoriques sont prometteurs.Dans un premier volet, un nanomatériau Li2MnSiO4/C est synthétisé par voie sol-gel. Ses propriétés électrochimiques et structurales sont étudiées. Les différents phénomènes de dégradation observés sont discutés par la suite. Une stratégie de dopage est proposée pour limiter la perte de capacité en cyclage par stabilisation de la structure via le composé Li2-xMn1+xAlxSi1-xO4/C. Enfin l’influence du stockage à l’air de Li2MnSiO4/C est mise en évidence et un mécanisme concernant la formation de Li2CO3 est proposé.En seconde partie, une synthèse de LiMnSiO4/C en plusieurs étapes est proposée à partir de l’olivine MgMnSiO4/C, suivie d’une oxydation chimique et d’une lithiation électrochimique. Chaque étape est caractérisée pour déterminer la structure, l’état d’oxydation et le comportement électrochimique du matériau obtenu.Pour conclure cette étude, les deux matériaux optimisés ont été testés suivant les profils d’applications spatiales (satellites LEO et GEO). La meilleure cyclablité de LiMnSiO4/C est confirmée ainsi que sa légitimité en tant qu’alternative prometteuse au matériau conventionnel Li2MnSiO4/C. / The society is currently facing challenges such as global warming and rarefaction of resources. These issues have a factor in common, energy and more specifically its storage, for which lithium-ion batteries are today the state-of-the-art technology. Researchers and industries are focusing on the increase of energy density and safety and the reduction of toxic, costly and rare elements. In this study, positive electrodes based on silicate polyanionic materials are considered to fulfill these requirements. Two materials are studied, Li2MnSiO4 that exhibits appealing large capacity (>300mAh.g-1) and an unreported LiMnSiO4 with olivine structure that would have medium capacity (174 mAh.g-1) but associated with a high voltage (>3.7V).In a first part, a nanocomposite material Li2MnSiO4/C is synthesized by sol-gel route. Its electrochemical and structural properties are studied. The different degradation phenomena are discussed thereafter. Al-doped and Mn-rich Li2-xMn1+xAlxSi1-xO4/C is also proposed to lower the structural collapse during cycling. Finally the impact of its storage in air is assessed and a mechanism is proposed to explain the formation of Li2CO3.In a second part, a multistep synthesis is designed starting from olivine MgMnSiO4/C, followed by chemical oxidation and electrochemical lithiation to obtain LiMnSiO4/C. Each step is characterized to assess the structure, oxidation degree and electrochemical behavior of the final material.Finally, the testing of the two materials for space applications (LEO and GEO satellites profiles) confirms the better cyclability of LiMnSiO4/C and its validity as promising alternative to the conventional unstable Li2MnSiO4 compound

abstract: Novel materials for Li-ion batteries is one of the principle thrust areas for current research in energy storage, more so than most, considering its widespread use in portable electronic gadgets and plug-in electric and hybrid cars. One of the major limiting factors in a Li-ion battery's energy density is the low specific capacities of the active materials in the electrodes. In the search for high-performance anode materials for Li-ion batteries, many alternatives to carbonaceous materials have been studied. Both cubic and amorphous silicon can reversibly alloy with lithium and have a theoretical capacity of 3500 mAh/g, making silicon a potential high density anode material. However, a large volume expansion of 300% occurs due to changes in the structure during lithium insertion, often leading to pulverization of the silicon. To this end, a class of silicon based cage compounds called clathrates are studied for electrochemical reactivity with lithium. Silicon-clathrates consist of silicon covalently bonded in cage structures comprised of face sharing Si20, Si24 and/or Si28 clusters with guest ions occupying the interstitial positions in the polyhedra. Prior to this, silicon clathrates have been studied primarily for their superconducting and thermoelectric properties. In this work, the synthesis and electrochemical characterization of two categories of silicon clathrates - Type-I silicon clathrate with aluminum framework substitution and barium guest ions (Ba8AlxSi46-x) and Type-II silicon clathrate with sodium guest ions (Nax Si136), are explored. The Type-I clathrate, Ba8AlxSi46-x consists of an open framework of aluminium and silicon, with barium (guest) atoms occupying the interstitial positions. X-ray diffraction studies have shown that a crystalline phase of clathrate is obtained from synthesis, which is powdered to a fine particle size to be used as the anode material in a Li-ion battery. Electrochemical measurements of these type of clathrates have shown that capacities comparable to graphite can be obtained for up to 10 cycles and lower capacities can be obtained for up to 20 cycles. Unlike bulk silicon, the clathrate structure does not undergo excessive volume change upon lithium intercalation, and therefore, the crystal structure is morphologically stable over many cycles. X-ray diffraction of the clathrate after cycling showed that crystallinity is intact, indicating that the clathrate does not collapse during reversible intercalation with lithium ions. Electrochemical potential spectroscopy obtained from the cycling data showed that there is an absence of formation of lithium-silicide, which is the product of lithium alloying with diamond cubic silicon. Type II silicon clathrate, NaxSi136, consists of silicon making up the framework structure and sodium (guest) atoms occupying the interstitial spaces. These clathrates showed very high capacities during their first intercalation cycle, in the range of 3,500 mAh/g, but then deteriorated during subsequent cycles. X-ray diffraction after one cycle showed the absence of clathrate phase and the presence of lithium-silicide, indicating the disintegration of clathrate structure. This could explain the silicon-like cycling behavior of Type II clathrates. / Dissertation/Thesis / M.S. Materials Science and Engineering 2013

Nanofibrous separators use in lithium-ion batteries brings many advantages. In contrast to contemporary used commercial separators, nanofibrous ones exhibit higher temperature resistance,ionic conductivity and higher electrolyte uptake. Better ionic conductivity is ensured by porous structure and large specific surface. Fibers creates channels for the ionic species motion. Amorphous texture of nanofibers allows quick lithium ionic species motion within the polymeric matrix of separator. Furthermore, these separators exhibit higher volume of uptaken electrolyte. Further advantage of electrospinned nanofibrous separators are both high porosity and chemical stability.

Remote hybrid power systems (RHPS) serve local off-grid loads or various island grids when no grid extension is possible. They combine renewable resources, conventional generators and energy storage systems in order to balance the load at any moment, while ensuring power quality and energy security similar to large centralized grids. Modelling such a complex system is crucial for carrying out proper simulations for predicting the system’s behavior and for optimal sizing of components. The model should include an estimation of the renewable resource availability over the period of the simulation, a prediction of the load consumption and time variation, the choice of technologies, a prior dimensioning approach, an energy dispatching strategy between the components and, finally, the behavioral model of all components. This study limits its scope to the simulation of a RHPS composed of solar PV panels, a diesel generator set, and a Li-ion battery bank supplying a dynamic isolated load with a daily demand variation between 10 kW and 80 kW. Methods for building load scenarii are explained first and then, rules of thumbs for selecting the technologies and pre-sizing the components are reviewed. Commonly used dispatching strategies are described before detailing the algorithm of a Matlab behavioral model for the system’s components with an emphasis on the proper prediction of performance and aging for the Li-ion battery model. Finally, a 10-year simulation is carried out over a case study and the results are analyzed.

The thrive to achieve Sustainable Development Goal 7 is never been easy, and numbers are still showing that Sub-Saharan Africa is lagging in access to electricity index. Most of the energy poor communities residing in the rural part of the region, this by itself is a conundrum with multifaceted implications. The high capital expenditure for renewable energy technologies, the low paying ability of the society in Sub-Saharan Africa, the unavailability of anchor customer’s and so on needs new means of approaching the access problem. This study aims to enlighten policy makers on promoting energy as input to production than merely focusing on the access issue. In the report, a renewable mini-grid powering a local economic activity of a remote agrarian village in Ethiopia is discussed. Through a simulation study using PVsyst and Homer Pro tools, a yearly optimized PV diesel hybrid system with rounded up lowest LCOE of $0.17/kWh is obtained for the village in the case study. The LCOE of the mini-grid with lead acid battery and Li-ion battery is also studied at a yearly average operating temperature range of 10 to 40 ℃. The simulation-based study demonstrated that mini-grid systems with lead acid and Li-ion battery have fairly comparable LCOE between 10 to 20 ℃, however the Li-ion battery results in a lower LCOE for operating temperature beyond 25 ℃. The study has shown that mini-grids with productive energy can be cost effective option for powering areas where the grid-connection is cost and time intensive to address the energy poverty issue by 2030 or after.

This work develops three models for simulation of the high-current operation of Li-ion batteries. Simulation as a tool can provide understanding beyond what experiments can offer. Different types of electrodes such as graphite, silicon, and NMC are modeled to study cell performance and aging under aggressive operating conditions. The first part of this work focuses on the effect of electrode microscale lateral heterogeneity on the degradation of conventional Li-ion batteries, especially for fast-charge applications. The non-uniform pore distribution leads to the nonuniform current density and state of charge (SoC), which can finally result in non-uniform Li plating and aging. The interactions of electrode regions a few mm away from each other with different ionic conductivity are simulated by combining conventional models in parallel with submodels to treat additional physics. The onset and growth of lithium metal deposits on the anode are predicted. The next topic is to investigate the structure of multilayer anodes (MLA) consisting of two layers in the through-plane direction with different ionic resistances. The model is intended to simulate a commercially made cell. Simulation results demonstrate that coating a higher-density layer near the current collector and a lower-density layer near the separator provides improved accessibility to active material during cell fast charge through better ionic transport. In addition, the improved anode further augments the cathode performance in high-current discharges, leading to greater energy density and power density of the cell. The last topic is to develop a numerically efficient mechanical and electrochemical model for silicon anodes. Silicon has a much higher energy density than graphite as a material for the anode; however, it undergoes high volume expansion and contraction ($\sim$ 280\%) which affects cell thickness and electrode ionic transport. The mechanical model treats these volume-change phenomena in a continuum fashion and is integrated into a P2D model of a Si half cell. As shown by the model, the external casing material of such cells can improve or restrict electrode utilization. Different cell designs are simulated to predict the degree of lithiation.

<p>Recycling valuable cathode material from end-of-life (EOL) Li-ion batteries (LIBs) is essential to preserve raw material depletion and environmental sustainability. Direct recycling reclaims the cathode material without jeopardizing its original functional structures and maximizing return values from spent LIBs compared to other regeneration processes. This work employed two chemically delithiated lithium cobalt oxide (LCO) cathodes at different states of health (SOH), which are analogous to the spent cathodes but free of any impurities, to investigate the effectiveness of cathode regeneration. The material and electrochemical properties of both delithiated SOHs were systematically examined and compared to pristine LCO cathode. Further, those model materials were regenerated by a hydrothermal-based approach. The direct cathode regeneration of both low and high SOH cathode samples restored their reversible capacity and cycle performance comparable to pristine LCO cathode. However, the inferior performance observed in higher current density (2C) rate was not comparable to pristine LCO. In addition, the higher resistance of regenerated cathodes is attributed to lower high-rate performance, which was pointed out as the key challenge of the cathode recycling process. This study provides valuable knowledge about the effectiveness of cathode regeneration by investigating how the disordered, lithium-deficient cathode at different SOH from spent EOL batteries are rejuvenated without changing any material and electrochemical functional properties.</p>

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