This study highlights the importance of the anion in the electrochemical uptake of lithium by metal phosphides. It is shown through a variety of <em>in-situ</em> and <em>ex-situ</em> analytical techniques that the redox active centers in three different systems (MnP<i>₄</i>, FeP<i>₂</i>, and CoP<i>₃</i>) are not necessarily cationic but can rest almost entirely upon the anionic network, thanks to the high degree of covalency of the metal-phosphorus bond and strong P-character of the uppermost filled electronic bands in the phosphides. The electrochemical mechanism responsible for reversible Li uptake depends on the transition metal, whether a lithiated ternary phase exists in the phase diagram with the same M:P stoichiometry as the binary phase, and on the structure of the starting phase. When both binary and lithiated ternary phases of the transition metal exist, as in the case of MnP<i>₄</i> and Li<i>₇</i>MnP<i>₄</i>, a semi-topotactic phase transformation between binary and ternary phases occurs upon lithium uptake and removal. When only the binary phase exists two different behaviours are observed. In the FeP<i>₂</i> system, lithium uptake leads to the formation of an amorphous material in which short-range order persists; removal of lithium reforms some the long-range order bonds. In the case of CoP<i>₃</i>, lithium uptake results in phase decomposition to metallic cobalt plus lithium triphosphide, which becomes the active material for the subsequent cycles.

In this thesis, electrode binders as vital components in the fabrication of composite electrodes for lithium-ion (LIB) and lithium-sulfur batteries (LiSB) have been investigated. Poly(vinylidene difluoride) (PVdF) was studied as binder for sulfur-carbon positive electrodes by a combination of galvanostatic cycling and nitrogen absorption. Poor binder swelling in the electrolyte and pore blocking in the porous carbon were identified as origins of low discharge capacity, rendering PVdF-based binders an unsuitable choice for LiSBs. More promising candidates are blends of poly(ethylene oxide) (PEO) and poly(N-vinylpyrrolidone) (PVP). It was found that these polymers interact with soluble lithium polysulfide intermediates generated during the cell reaction. They can increase the discharge capacity, while simultaneously improving the capacity retention and reducing the self-discharge of the LiSB. In conclusion, these binders improve the local electrolyte environment at the electrode interface. Graphite electrodes for LIBs are rendered considerably more stable in ‘aggressive’ electrolytes (a propylene carbonate rich formulation and an ether-based electrolyte) with the poorly swellable binders poly(sodium acrylate) (PAA-Na) and carboxymethyl cellulose sodium salt (CMC-Na). The higher interfacial impedance seen for the conventional PVdF binder suggests a protective polymer layer on the particles. By reducing the binder content, it was found that PAA-Na has a stronger affinity towards electrode components with high surface areas, which is attributed to a flexible polymer backbone and a higher density of functional groups. Lastly, a graphite electrode was combined with a sulfur electrode to yield a balanced graphite-sulfur cell. Due to a more stable electrode-electrolyte interface the self-discharge of this cell could be reduced and the cycle life was extended significantly. This example demonstrates the possible benefits of replacing the lithium metal negative electrode with an alternative electrode material.

A novel method to detect and quantify the growth of the solid electrolyte interphase (SEI) on battery electrode materials using in-situ spectroscopic ellipsometry (SE) is presented. The effects of additives in 1 M LiPF6/EC:DEC (1:2) electrolyte on the SEI were studied. Thin film electrodes of a-Si, Ni, and TiN were prepared by magnetron sputtering for use with a custom-designed tubular in-situ electrochemical cell. Li/a-Si and Li/Ni in-situ cells in 0.1 M LiPF6/EC:DEC (1:2) were studied by galvanostatic chronopotentiometry. Large changes in the ellipsometric parameters, ? and ?, were observed for both materials. These changes were closely related to the state of charge of the in-situ cell. The formation of an a-LixSi alloy, the formation of an SEI layer, or both contributed to these large changes for a Li/a-Si in-situ cell. For a Li/Ni in-situ cell, a thin transparent surface layer was observed. The surface layer, presumably made from SEI species and species from the displacement reaction between NiO and Li, increased to roughly 17 nm during the first discharge. During the first charge, the surface layer thickness decreased to roughly 5.5 nm and could not be removed, even at high potentials. The effect of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) additives on SEI formation were studied using a Li/TiN in-situ cell in 1 M LiPF6/EC:DEC (1:2) by potentiostatic chronoamperometry. SEI thicknesses for cells containing no additives, VC, and FEC were roughly 18 nm, 25 nm and 30 nm, respectively, after a 10 h hold at 0.1 V. SE is a useful technique for measuring thin film growth in-situ on electrode materials for Li-ion batteries.

Electrolyte additives are generally used in commercial Li-ion cells to improve capacity retention and calendar life. Although it is apparent that electrolyte additives play an important role, the details of how they work are poorly understood. In order to be able to distinguish the effect of an additive on the positive or negative electrodes, an experimental method has been developed based on electrochemical impedance spectroscopy of symmetric cells constructed from electrodes of disassembled full cells similar to the method described by previous workers. This technique proved to be useful and showed that the effects of additives on both electrodes depend strongly on their concentration. It also showed that in some cases, when two additives are introduced in the same cell, both additives contribute to the formation of the surface layer of both electrodes. In other cases, each additive controls the formation of the surface layer of only one electrode.

<p>The current work focuses on the synthesis and characterization of nano-devices with potential application in alkaline electrolysis and secondary polymer lithium ion batteries.</p>

Because of its high theoretical specific capacity (4200mAh/g) and natural abundance (2nd most abundant element on earth), silicon is considered a promising anode candidate for high energy density lithium-ion batteries. However, the dramatic volume changes (up to 400%) that occur during lithiation/delithiation and the relative low electrical conductivity of silicon prevent the implementation of this material. In this work, a nano-silicon/polyaniline/reduced graphene oxide composite was synthesized via a two-step process: in-situ polymerization of polyaniline (PANi) in the presence of nano-silicon followed by combination of the prepared n-Si/PANi binary composite with reduced graphene oxide (RGO), to form a n-Si/PANi/RGO composite. Electron microscopy reveals the unique nano-architecture of the n-Si/PANi/RGO composite: silicon nanoparticles are well dispersed within a PANi matrix, which in turn is anchored to the surface of RGO sheets. The n-Si/PANi/RGO ternary composite delivered an initial capacity of 3259 mAh/g and 83.5% Coulombic efficiency. The new composite displayed better rate performance and capacity recovery than either nano-Si or n-Si/PANi. Structural and morphological studies combined with AC impedance analysis suggest that the n-Si/PANi/RGO composite has higher electrical conductivity than the other two component materials, yielding better performance at high current densities or C rates. The good rate performance, high initial specific capacity and stable Coulombic efficiency of n-Si/PANi/RGO make it a promising anode material for high energy density lithium-ion batteries.

Because of its high theoretical specific capacity (4200mAh/g) and natural abundance (2nd most abundant element on earth), silicon is considered a promising anode candidate for high energy density lithium-ion batteries. However, the dramatic volume changes (up to 400%) that occur during lithiation/delithiation and the relative low electrical conductivity of silicon prevent the implementation of this material. In this work, a nano-silicon/polyaniline/reduced graphene oxide composite was synthesized via a two-step process: in-situ polymerization of polyaniline (PANi) in the presence of nano-silicon followed by combination of the prepared n-Si/PANi binary composite with reduced graphene oxide (RGO), to form a n-Si/PANi/RGO composite. Electron microscopy reveals the unique nano-architecture of the n-Si/PANi/RGO composite: silicon nanoparticles are well dispersed within a PANi matrix, which in turn is anchored to the surface of RGO sheets. The n-Si/PANi/RGO ternary composite delivered an initial capacity of 3259 mAh/g and 83.5% Coulombic efficiency. The new composite displayed better rate performance and capacity recovery than either nano-Si or n-Si/PANi. Structural and morphological studies combined with AC impedance analysis suggest that the n-Si/PANi/RGO composite has higher electrical conductivity than the other two component materials, yielding better performance at high current densities or C rates. The good rate performance, high initial specific capacity and stable Coulombic efficiency of n-Si/PANi/RGO make it a promising anode material for high energy density lithium-ion batteries.

Lithium-ion (Li-ion) batteries are the most popular energy storage technology in consumer electronics and electric vehicles and are increasingly applied in stationary storage systems. Yet, concerns about safety and reliability remain major obstacles, which must be addressed in order to improve the acceptance of this technology. The gradual degradation of Li-ion cells over time lies at the heart of this problem. Time, usage and environmental conditions lead to performance deterioration and cell failures, which, in rare cases, can be catastrophic due to res or explosions. The physical and chemical mechanisms responsible for degradation are numerous, complex and interdependent. Our understanding of degradation and failure of Li-ion cells is still very limited and more limited yet are reliable and practical methods for the detection and prediction of these phenomena. This thesis presents a comprehensive approach for the diagnosis and prognosis of degradation in Li-ion cells. The key to this approach is the extraction of information on electrode-speci c degradation through open circuit voltage (OCV) measurements. This is achieved in three stages. Firstly, a parametric OCV model is created, which computes the OCV of each electrode. Secondly, a diagnostic algorithm is devised, through which the OCV model is tted to OCV measurements recorded on Li-ion cells at various stages throughout their cycle life. The algorithm identi es the nature and quanti es the extent of degradation experienced by the cells. Lastly, the outputs of the algorithm are used to identify the likely failure modes of the cells and predict their end-of-life. The presented methods improve safe operation and predictions of remaining useful cycle life for commercial Li-ion cells. Greater certainty about the reliability, safety, required maintenance and depreciation of Li-ion battery systems can signi cantly enhance the competitiveness of battery electric storage in both automotive and stationary applications. The ndings presented in this work are therefore not only of technological but also of commercial interest.

The research work presented in this thesis comprises a detailed investigation of conductivity mechanism in crystalline polymer electrolytes and development of a new class of ceramic-polymer composite electrolytes for Li-ion batteries. Firstly, a robust methodology for the synthesis of monodispersed poly(ethylene oxides) has been established and a series of dimethyl-protected homologues with 13, 15, 17, 28, 29, 30 ethylene oxide repeat units was prepared. The approach is based on reiterative cycles of chain extension and deprotection, followed by end-capping of the oligomeric chain ends with methyl groups. The poly(ethylene oxide) homologues show a superior level of monodispersity to previous work and were subsequently used to prepare crystalline PEO6:LiPF6 polymer electrolytes. A correlation between the number of ether oxygens in the polymer chain and the ionic conductivity of crystalline polymer electrolytes has been established. The structure and dynamics of the monodispersed complexes were studied using solid-state NMR spectroscopy for the first time. The results are in agreement with the proposed mechanism of ionic conductivity in crystalline polymer electrolytes. A new class of composite solid electrolytes for all-solid-state batteries with a lithium metal anode is reported. The composite material consists of a 3D interpenetrating network of a ceramic electrolyte, Li₁.₄Al₀.₄Ge₁.₆(PO₄)₃, and an inert polymer (polypropylene), providing continuous pathways for the ionic transport and excellent mechanical properties. 3D connectivity of this novel composite was confirmed using X-ray microtomography and AC impedance spectroscopy.

>Magister Scientiae - MSc / Architecturally enhanced electrode materials for lithium ion batteries (LIB) with permeable morphologies have received broad research interests over the past years for their promising properties. However, literature based on modified porous nanoparticles of lithium manganese phosphate (LiMnPO₄) is meagre. The goal of this project is to explore lithium manganese phosphate (LiMnPO₄) nanoparticles and enhance its energy and power density through surface treatment with transition metal nanoparticles. Nanostructured materials offer advantages of a large surface to volume ratio, efficient electron conducting pathways and facile strain relaxation. The material can store lithium ions but have large structure change and volume expansion during charge/discharge processes, which can cause mechanical failure. LiMnPO₄ is a promising, low cost and high energy density (700 Wh/kg) cathode material with high theoretical capacity and high operating voltage of 4.1 V vs. Ag/AgCl which falls within the electrochemical stability window of conventional electrolyte solutions. LiMnPO₄ has safety features due to the presence of a strong P–O covalent bond. The LiMnPO₄ nanoparticles were synthesized via a sol-gel method followed by coating with gold nanoparticles to enhance conductivity. A magnesium oxide (MgO) nanowire was then coated onto the LiMnPO₄/Au, in order to form a support for gold nanoparticles which will then form a thin film on top of LiMnPO₄ nanoparticles crystals. The formed products will be LiMnPO₄/Mg-Au composite. MgO has good electrical and thermal conductivity with improved corrosion resistance. Thus the electronic and optical properties of MgO nanowires were sufficient for the increase in the lithium ion diffusion. The pristine LiMnPO₄ and LiMnPO₄/Mg-Au composite were examined using a combination of spectroscopic and microscopic techniques along with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Microscopic results revealed that the LiMnPO₄/Mg-Au composite contains well crystallized particles and regular morphological structures with narrow size distributions. The composite cathode exhibits better reversibility and kinetics than the pristine LiMnPO₄ due to the presence of the conductive additives in the LiMnPO₄/Mg-Au composite. This is demonstrated in the values of the diffusion coefficient (D) and the values of charge and discharge capacities determined through cyclic voltammetry. For the composite cathode, D= 2.0 x 10⁻⁹ cm²/s while for pristine LiMnPO₄ D = 4.81 x 10⁻¹⁰ cm2/s. The charge capacity and the discharge capacity for LiMnPO₄/Mg-Au composite were 259.9 mAh/g and 157.6 mAh/g, respectively, at 10 mV/s. The corresponding values for pristine LiMnPO₄ were 115 mAh/g and 44.75 mAh/g, respectively. A similar trend was observed in the results obtained from EIS measurements. These results indicate that LiMnPO₄/Mg-Au composite has better conductivity and will facilitate faster electron transfer and therefore better electrochemical performance than pristine LiMnPO₄. The composite cathode material (LiMnPO₄/Mg-Au) with improved electronic conductivity holds great promise for enhancing electrochemical performances, discharge capacity, cycle performance and the suppression of the reductive decomposition of the electrolyte solution on the LiMnPO₄ surface. This study proposes an easy to scale-up and cost-effective technique for producing novel high-performance nanostructured LiMnPO₄ nanopowder cathode material.