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Non-aqueous rechargeable lithium-air (O₂) batteries are receiving intense interest because of their high theoretical specific energy, which are several times greater than that of lithium-ion batteries. To achieve it, the highly reversible formation/decomposition of Li₂O₂ is required to occur in the cathode during cycling. Due to the reactivity of reduced O₂ species, the aprotic electrolyte and carbon electrode substrate would be attacked and then decomposed. The organic carbonate decomposed on discharge, forming C₃H₆(OCO₂Li)₂, Li₂CO₃, HCO₂Li, CH₃CO₂Li, CO₂ and H₂O. Part of these by-products decomposed on the subsequent charge process and the rest remained and blocked the electrode surface. Finally, the cell cycling stopped because of the depletion of electrolyte and the passivation of the electrode surface. Possible mechanisms are proposed for reactions on discharge and charge. Some other types of aprotic solvents were investigated in the same way. Ethers, amides, sulfones, dimethyl sulfoxide (DMSO), etc reveal better stability than organic carbonates. Reversible formation/decomposition was observed together with minor side-reactions. Besides electrolytes, carbon substrate of electrode also slightly decomposed. Several other substrate materials were studied. If the carbon electrodes were replaced with the nanoporous gold electrodes, less side-reaction was observed in the cells, and the cell sustained 100 cycles without severe polarisation and capacity fading. The charge performance of a Li-O₂ cell remains a challenge. Great voltage polarisation even at modest rate was observed because of the difficulty of charge transfer between solid electrode surface and solid Li₂O₂. Redox mediators were used in a Li-O₂ cell, which transported the charge between electrode surface and solid Li₂O₂, acting as an electron-hole transfer agent. The oxidation of solid Li₂O₂ was facilitated and the cell with mediator demonstrated 100 charge/discharge cycles.

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The a.c. conductivity of the solid electrolyte beta-alumina with blocking Au electrodes has been examined by complex plane analysis using the complex impedance (Z*), admittance (A*), modulus (M*) and permittivity (epsilon*) formalisms. The electrode response which gives rise to the characteristic "spikes" in the Z* plots is attributed to a highly resistive layer on the surface of beta-alumina. This is found on single crystal and polycrystalline materials, and is sensitive both to the effects of polishing and the uptake of water vapour. Water vapour does not affect the bulk properties. For the first time, both the conductivity and the modulus spectra of single crystal beta"-alumina have been reported. It is confirmed that beta"-alumina has a much higher activation energy (~27kJ/mole) than beta-alumina (16.0kj/mole) and thus has a lower conductivity at lower temperatures (100°C). On this basis, an attempt has been made to explain the behaviour of the (commercial) polycrystalline sinters which are mixtures of beta and beta" phases. While the grain boundary activation energy remains constant at ~ 26kJ/mole, the bulk activation energy varies with beta/beta" content. The mixtures of approx 50-70% beta" content have the lowest bulk activation energy. These effects may be related in some way to the method of charge compensation, and in particular to the interstitial oxygen content (Oi2-). A "mixed-alkali" effect has been discovered in Na/Ag beta-aluminas which is remarkably similar to the effects found in glass. Both conductance minima and modulus (M") peak "narrowing" are observed. The results add some support to the weak electrolyte theory of beta-alumina, and also offer an interesting new exception to Jonscher's "universal" dielectric behaviour.

The importance of energy storage worldwide is increasing with the use of renewable energy sources and electric vehicles. With the intermittent nature of wind and solar power, large-scale grid storage is an extremely important progression needed to reduce the use of fossil fuels. For this to become a reality, rechargeable batteries beyond existing Li-ion technologies need consideration. The development of such batteries requires improvement of understanding their component materials. Modern computer modelling techniques enable valuable insights into the fundamental defect, ion transport and voltage properties of battery materials at the atomic level. Atomistic simulation and ab initio density functional theory (DFT) techniques have been used to study a number of potential cathode materials for Na-ion and Mg batteries. Firstly, the olivine and maricite forms of NaFePO4 are considered in terms of their defect formation energies and Na ion diffusion. The atomistic study indicates that anti-site disorder is the most favourable type of intrinsic defect. The activation energies for Na-ion migration in the olivine and maricite materials are 0.4 eV and 1.6 – 1.8 eV respectively. Moreover, molecular dynamics (MD) studies reveal that there is only substantial Na-ion diffusion in the olivine structure, with diffusion coefficients (DNa) at 300 K of 7 x 10−13 cm2s−1 for maricite and 4 x 10−9 cm2s−1 for olivine NaFePO4. The presence of anti-site defects is shown to decrease Na+ diffusion within the olivine structure, which is of relevance to its rate behaviour. Secondly, the effect of lattice strain on ion transport and defect formation in olivine-type LiFePO4 and NaFePO4 is investigated as a means to enhance their ion conduction properties. It is predicted that lattice strain can have a remarkable effect on the rate performance of olivine cathode materials, with a major increase in ionic conductivity and decrease in blocking defectsat room temperature. Thirdly, DFT techniques have been used to examinesurface and grain boundary formation in P2-NaCoO2. The coordination lossexperienced by ions present at surfaces is found to influence the resultingsurface energy. Layered oxide cathode materials were further investigated byconsidering the effect of Mg2+ doping on P2-Na2 [Ni1 Mn2 ]O2. Na vacancy 333formation energies decreased with 10% Mg2+ doping on the Ni site and an increase in Na diffusion was predicted with MD calculations. This positive effect on Na ion conductivity is caused by displacement of the Mg ions from the transition metal layer and the resulting change in electrostatic potential. Finally, Mg ion conduction, doping and voltage behaviour of MgFeSiO4 were studied. The Mg-ion migration activation energy is relatively low for an olivine-type silicate, and MD simulations predict a diffusion coefficient (DMg) of 10−9 cm2s−1, suggesting favourable electrode kinetics. Partial substitution of Fe by Co or Mn could increase the cell voltage from 2.3 V vs Mg/Mg2+ to 2.8 - 3.0 V.

This dissertation focuses on the synthesis and electrochemical testing of new electrolyte salts for rechargeable multivalent ion batteries. In chapters 2 and 3 the synthesis of Mg and Ca hexafluoropnictogenate salts as well as the electrochemical behaviour of Mg(PF6)2 is presented. Pure samples of Mg(EF6)2 (E = P, As, and Sb) can be synthesized using Mg metal and NOPF6/NOSbF6 in CH3CN or via a ammonium salt deprotonation route using Me3NHAsF6 and Bu2Mg. The NOPF6 method was extended to the Ca variant, but isolation of a pure Ca(PF6)2 material required the presence of a crown ether. Electrochemical and microscopy measurements of THF-CH3CN solutions of Mg(PF6)2 show that the electrolyte good electrochemical stability and can facilitate the plating/stripping of Mg. Further, this electrolyte system can be cycled in a full cell using the Chevrel phase Mo6S8 cathode. The electrochemical stability of the AsF6− and SbF6− salts is lower than that of the PF6− salt and electrolyte decomposition is observed when cycling on Mg electrodes. In chapter 4 the development of a series of Mg aluminates [Mg(AlOR4)2] using a general synthetic platform based on Mg(AlH4)2 and various alcohols is presented. Preliminary electrochemical studies performed on these aluminate salts in dimethoxyethane identify the phenoxy and perfluoro-tert-butoxy derivatives as promising electrolyte systems. Electrochemical cycling of these electrolytes using gold and Mg electrodes show that systems containing chloride, brought through to the product from the starting material in the form of NaCl, exhibit lower plating/stripping overpotentials and higher Coulombic efficiencies than systems from which chloride had been removed. Further, these two electrolytes can be used in Mg full cells containing the Chevrel phase cathode. Solid-state 23Na NMR analysis as well as DFT calculations show that chloride-containing electrolytes facilitate the co-insertion of Na into the cathode material. In chapter 5 the hydroboration of pyridines and CO2 in the presence of pinacolborane is presented. An optimized system employing NH4BPh4 and HBpin is developed and a mechanism of pyridine hydroboration is proposed based on multinuclear NMR spectroscopy. The catalytic reaction was found to be catalyzed by a boronium salt, which was structurally characterized in the solid-state by single crystal X-ray diffraction. This new catalytic method is shown to be tolerant to a number of functional groups in the 3-position on pyridine as well as quinoline, and CO2, producing the hydroboration products in good yields.

Metal air battery has captured the spotlight recently as a promising class of sustainable energy storage for the future energy systems. Metal air batteries offer many attractive features such as high energy density, environmental benignity, as well as ease of fuel storage and handling. In addition, wide range of selection towards different metals exists where different energy capacity can be achieved via careful selection of different metals. The most energy dense systems of metal-air battery include lithium-air, aluminum-air and zinc-air. Despite the choice of metal electrode, oxygen reduction (ORR) occurs on the air electrode and oxidation occurs on the metal electrode. The oxidation of metal electrode is a relatively facile reaction compared to the ORR on the air electrode, making latter the limiting factor of the battery system. The sluggish ORR kinetics greatly affects the power output, efficiency, and lifetime of the metal air battery. One solution to this problem is the use of active, affordable and stable catalyst to promote the rate of ORR. Currently, platinum nanoparticles supported on conductive carbon (Pt/C) are the best catalyst for ORR. However, the prohibitively high cost and scarcity of platinum raise critical issues regarding the economic feasibility and sustainability of platinum-based catalysts. Cost reduction via the use of novel technologies can be achieved by two approaches. The first approach is to reduce platinum loading in the catalyst formulation. Alternatively platinum can be completely eliminated from the catalyst composition. The aim of this work is to identify and synthesize alternative catalysts for ORR toward metal air battery applications without the use of platinum re other precious metals (i.e., palladium, silver and gold). Non-precious metal catalysts (NPMC) have received immense international attentions owing to the enormous efforts in pursuit of novel battery and fuel cell technologies. Different types of NPMC such as transition metal alloys, transition metal or mixed metal oxides, chalcogenides have been investigated as potential contenders to precious metal catalysts. However, the performance and stability of these catalysts are still inferior in comparison. Nitrogen-doped carbon materials (NCM) are an emerging class of catalyst exhibiting great potential towards ORR catalysis. In comparison to the metal oxides, MCM show improved electrical conductivity. Furthermore, NCM exhibit higher activity compared to chalcogenides and transition metal alloys. Additional benefits of NCM include the abundance of carbon source and environmental benignity. Typical NCM catalyst is composed of pyrolyzed transition metal macrocycles supported by high surface area carbon. These materials have demonstrated excellent activity and stability. However, the degradation of these catalysts often involves the destruction of active sites containing the transition metal centre. To further improve the durability and mass transport of NCM catalyst, a novel class of ORR catalyst based on nitrogen-doped carbon nanotubes (NCNT) is investigated in a series of studies. The initial investigation focuses on the synthesis of highly active NCNT using different carbon-nitrogen precursors. This study investigated the effect of using cyclic hydrocarbon (pyridine) and aliphatic hydrocarbon (ethylenediamine) towards the formation and activity of NCNT. The innate structure of the cyclic hydrocarbon promotes the formation of NCNT to provide higher product yield; however, the aliphatic hydrocarbon promotes the formation of surface defects where the nitrogen atoms can be incorporated to form active sites for ORR. As a result, a significant increase in the ORR activity of 180 mV in half-wave potential is achieved when EDA was used as carbon-nitrogen precursor. In addition, three times higher limiting current density was observed for the NCNT synthesized from ethylenediamine. Based on the conclusion where highly active NCNT was produced from aliphatic hydrocarbon, similar carbon-nitrogen precursors with varying carbon to nitrogen ratio in the molecular structure (ethylenediamine, 1, 3-diaminopropane, 1, 4-diaminobutane) were adapted for the synthesis of NCNT. The investigation led to the conclusion that higher nitrogen to carbon ratio in the molecular structure of the precursors benefits the formation of active NCNT for ORR catalysis. The origin of such phenomena can be correlated with the higher relative nitrogen content of the resultant NCNT synthesized from aliphatic carbon precursor that provided greater nitrogen to carbon ratio. As the final nitrogen content increased in the molecular structure, the half-wave potential of the resultant NCNT towards ORR catalysis was increased by 120 mV. The significant improvement hints the critical role of nitrogen content towards ORR catalysis. To further confirm the correlation between the nitrogen content and ORR activity, another approach was used to control the final nitrogen content in the resultant NCNT. In the third investigation, a carbon-nitrogen precursor (pyridine) was mixed with a carbon precursor (ethanol) to form an admixture. The relative proportion of the two components of the admixture was varied to produce NCNT with different nitrogen content. By adopting this methodology, potential effect of different carbon-nitrogen precursors on the formation of NCNT can be eliminated since the same precursors were used for NCNT synthesis. Based on the electrochemical evaluations, the nitrogen content can be positively correlated to ORR activity. Among the NCNT samples, 41% higher limiting current density was achieved for 0.7 at. % increase in overall nitrogen content. Furthermore, the selectivity of the NCNT catalyst with higher nitrogen content favours the production of water molecule—the favourable product in metal-air battery by 43%. ORR catalyst is an outer-sphere electron transfer reaction whereby the reactants interact with the surface of catalysts. Consequently, the surface structure can be a determining factor towards the ORR activity of the NCNT in addition to the nitrogen content. In the forth investigation, the surface structure of NCNT was tailored to differentiate the ORR activity of smooth and rugged surface while controlling the overall nitrogen content to be similar. NCNT having different surface structures but similar nitrogen content (approximately 2.7 to 2.9 at. %) were successfully synthesized using different synthesis catalysts. Comparison of the two NCNT catalysts showing different surface structure resulted in a 130 mV increased in half-wave potential favouring the NCNT with more rugged surface structure. This study provided insights to the potential effects of synthesis catalyst towards directing the surface structure and the ORR activity of NCNT. Through a series of studies, the important parameters affecting the ORR performance of NCNT were elucidated and the most active NCNT catalyst synthesized was used for testing in a prototype zinc-air battery. The fifth study evaluated the performance of NCNT catalyst in different concentrations of alkaline electrolyte and at different battery voltage. An increase in the electrolyte’s alkaline strength improved the battery performance to a certain degree until the increasing viscosity impeded the performance of the battery system. The zinc-air battery employing NCNT as ORR catalyst produced a maximum battery power density of 69.5 mWcm-2 in 6M potassium hydroxide. The fifth study illustrated the great potential of NCNT towards the ORR catalysis for metal-air batteries. In combination, the series of investigations presented in this document provide a comprehensive study of a novel material and its application towards ORR catalysis in metal air batteries. Specifically, this report provides insights into the fundamentals of NCNT synthesis; the origins of ORR activity and the optimal operating conditions of NCNT in a prototype zinc-air battery. The excellent performance of NCNT warrants further studies of this material in greater details, and the information presented in this document will create a basis for future investigations towards ORR catalysis.