High-energy storage technologies have garnered extensive interests in recent decades due to the skyrocketing demand for efficient and affordable energy storage systems in stationary electricity storage, portable electronic devices and transports (such as cars, aircrafts and ships) to relieve climate changes and secure energy sustainability. However, the state-of-art lithium-ion (Li-ion) batteries are unable to meet the exponentially increasing demands in these markets, since they suffer from low energy densities and have limited potential to be further developed. In recent years, other Li metal-based battery systems with higher theoretical energy density are introduced and fully developed, particularly the non-aqueous lithium-sulfur (Li-S) batteries and lithium-air (Li-air) batt...[ Read more ]

High-energy storage technologies have garnered extensive interests in recent decades due to the skyrocketing demand for efficient and affordable energy storage systems in stationary electricity storage, portable electronic devices and transports (such as cars, aircrafts and ships) to relieve climate changes and secure energy sustainability. However, the state-of-art lithium-ion (Li-ion) batteries are unable to meet the exponentially increasing demands in these markets, since they suffer from low energy densities and have limited potential to be further developed. In recent years, other Li metal-based battery systems with higher theoretical energy density are introduced and fully developed, particularly the non-aqueous lithium-sulfur (Li-S) batteries and lithium-air (Li-air) batteries. The Li-S batteries are able to reach a theoretical energy density as high as 2567 Wh kg^-1, while Li-air batteries have demonstrated an extraordinary theoretical energy density of 11680 Wh kg^-1, both of which are ten times higher than that of the Li-ion batteries. Nevertheless, the widespread applications of these two types of battery systems have been hindered by several critical issues including the shuttle effect of Li-S batteries and poor redox kinetics in air cathode of Li-air batteries. The major goal of this thesis is to address these challenges and propel the place of commercialization of Li-based batteries through a bottom-up positive electrode design.

We design a series of bottom-up polar material nanostructures in different dimensions to suppress the diffusion of polysulfides for Li-S batteries, including 0D-boron carbide (B₄C) nanoparticles, 2D-layered-molybdenum trioxide (MoO₃) nanoflakes and 3D-zinc oxide (ZnO) yolk-shell spheres. These polar materials hold high binding energies towards long-chain polysulfides, and thus exhibit strongly chemical anchoring ability to polysulfides. These properties allow Li-S batteries to exhibit an excellent cycling stability after 3000 cycles (0D-B₄C nanoparticles decorated activated cotton fibers), a high areal sulfur loading of 8.0 mg cm^-2 (2D-layered-MoO₃ nanoflakes decorated carbon paper), and excellent rate capability and high retention capacity rate (3D-yolk-shell ZnO spheres as sulfur host). Meanwhile, in-situ Raman spectra is carried out to examine the sulfur conversion reactions during the discharge/charge process as well as the chemical bonds formed between polar materials and polysulfides.

For Li-air batteries, to obtain more uniform oxygen (O₂) /electrolyte distribution and larger electrochemically available surface area within the air electrode, we construct more abundant O₂ and electrolyte transportation paths by designing a series of bottom-up positive electrodes from 0 to 3D, in the same manner as what we have done in the Li-S batteries, including the 0D-mesoporous ultrafine tantalum pentoxide (Ta₂O₅) nanoparticles, 1D-paramecium-like ferric oxide (Fe₂O₃) nanotubes, 0D&1D-ruthenium dioxide (RuO₂)-decorated carbonized tubular polypyrrole, and 1D&1D-vanadium pentoxide-nickel dioxide (V₂O₅-NiO) composite nanowire. Based on these novel electrodes, we enlarge the reversible capacity and cycling lifespan of pure carbon cathodes with low-overpotentials and high rate capability for Li-air batteries in pure oxygen atmosphere and fortify the stable operation of Li-air batteries with carbon-free cathodes in ambient air.

Keywords: Bottom-up positive electrode design; Energy storage systems; Li-S batteries; Shuttle effect; Polar materials; in-situ Raman; Li-air batteries; Ambient air operation.