Non-aqueous lithium-oxygen batteries have been attracting increasing attention over the past years, primarily due to its high capacity for two reasons. First, lithium is the lightest metal and has the highest specific capacity of 3.86 × 10³ mAh g^-1. Second, the cathode active material, oxygen, can be retrieved from ambient air without occupying the battery volume. To make this technology commercially viable, however, a number of technical barriers, such as low practical capacity, low energy efficiency, short cycling life, and poor stability in ambient air, need to be addressed. The primary objective of this thesis is to address these issues for performance improvement. Based on state-of-the-art non-aqueous electrolytes, we first investigate the operating conditions on the batte...[ Read more ]

Non-aqueous lithium-oxygen batteries have been attracting increasing attention over the past years, primarily due to its high capacity for two reasons. First, lithium is the lightest metal and has the highest specific capacity of 3.86 × 10³ mAh g^-1. Second, the cathode active material, oxygen, can be retrieved from ambient air without occupying the battery volume. To make this technology commercially viable, however, a number of technical barriers, such as low practical capacity, low energy efficiency, short cycling life, and poor stability in ambient air, need to be addressed. The primary objective of this thesis is to address these issues for performance improvement. Based on state-of-the-art non-aqueous electrolytes, we first investigate the operating conditions on the battery performance, and then focus on designing cathode electrodes for non-aqueous lithium-oxygen batteries with increased practical capacity and energy efficiency, improved cycling life, and high stability in ambient air.

To understand the effects of operating conditions on the battery performance, the relationships among product morphology, current density, operating temperature, and oxygen pressure are investigated. It is found that the shape of the solid product Li₂O₂ is governed by the discharge current density, while its size depends on the operating temperature. The operating temperature affects the capacity, charge voltage, and cyclability. In addition, a new type of product morphology that closely resembles furrowed toroid particles is formed at high operating oxygen pressures. Further investigations show that this morphology is attributed to an increase in the proportion of the intermediate product LiO₂, which improves the effective electrical conductivity and leads to a lower charge voltage.

To increase the practical capacity, two carbon-based cathodes are designed, including a composite cathode made of carbon powder and nanotubes and a gradient cathode with a stepwise pore distribution. The respective volumetric and gravimetric capacity of the composite cathode are 67.2% and 36.3% higher than those of the cathode made of pure carbon powder. The gradient cathode enables the capacity to be 19.2% higher than that of a uniform porous cathode. In addition, a RuO₂ nanoparticle-decorated buckypaper (weaved with carbon nanotubes) cathode is fabricated, which is free of binders and current collectors compared with conventionally slurry-formed cathodes. The battery with this cathode demonstrates a high discharge capacity of 4.72 mAh cm^-2 (1150 mAh g_cathode ^-1) and exhibits the energy efficiency as high as 71.2%, 65.4%, and 58.0% at the current densities of 0.2, 0.4, and 0.8 mA cm^-2, respectively.

To improve the cycling life of carbon-based cathodes, NiO and RuO₂ nanoparticles are decorated onto carbon surfaces. Unlike catalyst-decorated carbon cathodes with activities for oxygen reduction and evolution reactions reported in the literature, the NiO-RuO₂ nanoparticle-decorated carbon electrode also promotes the decomposition of the irreversible side products (e.g. Li₂CO₃) at the active surfaces. As a result, the present cathode is able to operate for 50 cycles without capacity decay, which is more than twice the cycle number of that of the pristine carbon cathode.

To address the poor stability in ambient air, a cathode composed of RuO₂ nanoparticle-decorated NiO nanosheets is designed. This cathode not only catalyzes oxygen reduction and evolution reactions, but also promotes the decomposition of LiOH and Li₂CO₃ formed from H₂O and CO₂ in ambient air, enabling the battery to be operated for 200 cycles (800 h) with stable coulombic efficiency (100%) and high energy efficiency (~75%). Moreover, the effects of moist air on the cycling performance are studied based on this cathode. In the dry air, the discharge and charge terminal voltages are around 2.51 and 4.12 V, respectively, but change to 2.79 and 3.87 V when the relative humidity reaches 84%. The energy efficiencies corresponding to the dry air and the relative humidity of 84% are 66.2% and 73.8%, respectively.

Keywords: Non-aqueous lithium-oxygen battery; operating conditions; performance; practical capacity; cycling life; ambient air.