Noble metal-free cathode and electrolyte materials for low-cost, efficient Li-CO₂ batteries
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IIn this dissertation, noble metal-free cathode and electrolyte materials are developed to improve the energy efficiency, capacity, and cycle life of lithium - carbon dioxide (Li-CO₂) batteries. These performance enhancements are achieved by reducing the overpotentials of the carbon dioxide reduction reaction (CDRR) during discharge and the carbon dioxide evolution reaction (CDER) during charge. In Chapter 1, an overview of the Li-CO₂ battery system is provided, including a description of Li-CO₂ electrochemistry, Li-CO₂ battery requirements and challenges, and a summary of prior noble metal-free catalysts reported to date. In Chapter 2, general experimental details are outlined, including the cell design, general materials, and characterization instruments used. In Chapter 3, a nanocomposite of anatase TiO₂ nanoparticles grown onto carbon nanotubes and mixed with carbon nanofibers (TiO₂-NP@CNT/CNF) is employed as a gas diffusion cathode (GDC) in Li-CO₂ batteries to improve CDRR kinetics and cycling stability. In Chapter 4, phenyl disulfide (PDS) is introduced as an Li-CO₂ battery electrolyte additive to allow for homogeneous CO₂ utilization. A reaction mechanism involving the formation of the intermediate S-phenyl carbonothioate (SPC⁻) is proposed and supported with experimental evidence. In Chapter 5, a composite of MoS₂ nanosheets grown onto multi-walled carbon nanotubes and mixed with single-walled carbon nanotubes (MoS₂-NS@MWNT/SWNT) is employed as an efficient Li-CO₂ GDC to reduce CDRR and CDER overpotentials. A mechanism is proposed in which the oxalate intermediate C₂O₄²⁻-Mo⁶⁺S₂⁴⁻ is formed, lowering the energy barrier to Li₂CO₃ decomposition on charge. Finally, in Chapter 6, freestanding vanadium nitride nanowires (VN-NW) are employed as a carbon-free Li-CO₂ GDC. The discharge product morphology is greatly improved compared to that of a control MWNT GDC, leading to reduced charge voltage and improved cycle life. Finally, a summary of all the work carried out in this dissertation and a brief perspective on future Li-CO₂ research is given in Chapter 7.