Growing environmental concerns and increasing demand for energy have stimulated extensive interest in electrical energy storage. Li-air batteries are appealing in this regard as they offer much higher energy density than the current Li-ion batteries, but the nonaqueous Li-air batteries suffer from poor cycle life arising from electrolyte decomposition and clogging of the air electrode by insoluble discharge products. Interestingly, hybrid Li-air batteries in which a solid electrolyte separates the lithium-metal anode in an aprotic electrolyte from the air electrode in an aqueous catholyte could overcome these problems. Lots of efforts have been made on developing efficient bifunctional catalysts to lower the overpotential and improve the stability of hybrid Li-air batteries, but the cycle life is still limited. This dissertation focuses on the development of advanced cell configurations and high-performance catalysts for hybrid Li-air batteries. First, a buffer catholyte solution with a moderate pH, based on phosphoric acid and supporting salts, has been developed to keep the solid electrolyte stable and reduce the internal resistance and overpotential. With a high operating voltage and the utilization of all the three protons of phosphoric acid, the buffer catholyte enables a Li-air cell with high energy density. Further increase in power density has been realized by increasing the solid-electrolyte conductivity and operating temperature to 40 °C. The biggest challenge with Li-air cells is the large overpotentials associated with the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Noble-metal-free NiCo₂O₄ nanoflakes directly grown onto a nickel foam (NCONF@Ni) has been found to exhibit high OER activity that is comparable to that of the expensive, noble-metal IrO₂ catalyst. Furthermore, a novel 3-D O- and N-doped carbon nanoweb (ON-CNW) has been developed as an inexpensive, metal-free catalyst for ORR. With a hybrid Li-air cell, the ON-CNW exhibits performance close to that of commercial Pt/C. In addition, a novel hybrid Li-air cell configuration with decoupled ORR and OER electrodes has been developed. The hybrid Li-air cell with decoupled ORR and OER electrodes eliminates the degradation of ORR catalysts and carbon support in the highly oxidizing charge process and leads to high efficiency with good cycle life. / text

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Energy storage is crucial for intermittent renewable energy sources, electric vehicles, and portable devices. The continuously increasing energy consumption in these industries necessitates the enhancement of commercial lithium-ion batteries (LIB), especially regarding their safety and energy density. Historically, aqueous electrolytes were the norm in the battery industry. Prior to the development of lithium batteries, most commercially significant batteries used water as the solvent. In the past decade, "highly concentrated" electrolytes resurrected the notion of an aqueous lithium-ion battery (ALIB). Significant efforts have been made since then to comprehend the interfacial stability of these high-concentration electrolytes, and make them suitable for use in batteries especially high voltage ones. Another candidate for future batteries is All-Solid-State Batteries (ASSB) as they have the potential to double, or even triple, the energy density figures we currently achieve in LIBs mainly due to their ability to utilize lithium metal anode which has the highest specific capacity among anodes (3860 mAh g⁻¹), lowest reduction potential (-3.04 V vs SHE), and low density (0.53 g cm⁻³). This thesis first proposes a phenomenological model to describe the microstructure of aqueous electrolyte and the relation between their phase diagrams with ionic conductivity; highlighting a common correlation between the eutectic composition and peak ionic conductivity in conductivity isotherms. we then propose an empirical model correlating ionic conductivity with both molar concentration and temperature. The aim of this portion of the thesis is to provide an in depth understanding of aqueous electrolytes' physical properties in a way that can help researchers optimize the energy density and the cost of ALIBs. Moving further, the thesis presents two novel composite solid electrolytes (CSE) that were developed and fully characterized. Both of which were composed of the following four components; polyethylene oxide (PEO), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, lithium lanthanum titanate (LLTO) perovskite inorganic ceramic and the polymer plasticizer succinonitrile (SN). The careful formulation of these CSEs was based on the trade-off between film forming ability and ionic conductivity. The optimized polymer rich CSE proved to have better characteristics when compared to its ceramic rich alternative. ASSBs employing both CSEs were successfully charged and discharged when coupled with lithium metal anode and in-lab prepared composite cathode. The developed thin and flexible CSEs could be utilized in small applications (Wh-KWh) such as in consumer electronics and flexible biomedical devices (e.g., pacemakers) or larger applications (kWh-MWh) such as in EVs and large format storage for the electrical grid.