Synthesis, characterization, and performance of electrodes for lithium-ion and lithium/O2 batteries

Abstract

Lithium ion (Li-ion) batteries have been extensively used in our daily life. However, current Li-ion batteries cannot satisfy the requirements of safety, long-term cyclability, and rapid recharging. In addition, their theoretical limits fall short of the high energy density target for electric vehicles. Therefore, the development of other battery systems becomes necessary. Lithium titanate (Li4Ti5O12) is an appealing anode material for Li-ion batteries, which exhibits outstanding safety performance and cyclability. Its main drawback is poor electronic conductivity. In this thesis, an interconnected hierarchical porous carbon (HCMS), which possesses an ordered hollow macrocore-thin mesoporous shell structure, was introduced as a conductive matrix for dispersing Li4Ti5O12 nanoparticles. Electrochemical impedance spectroscopy on symmetric cells was further employed to investigate the correlation between electrode structure, electrode process, and performance. Results show that as-synthesized Li4Ti5O12-HCMS composite delivers higher capacity, lower polarization, and enhanced high-rate capability due to synergistic contribution of efficient electron transport and fast ion migration. The HCMS carbon supplies a network with a high electrical conductivity. The hierarchical porosity provides not only sites for dispersion of Li4Ti5O12 nanoparticles but also short paths for fast ion transport. The emerging technology of non-aqueous lithium/oxygen batteries exhibits a high energy density rivaling that of gasoline. However, they suffer from a large kinetic barrier in oxygen reduction/evolution reactions, and the battery components, including the carbon materials and binders, tend to be corroded or decomposed during battery operation. In this thesis, the stability of four metallic meshes (stainless steel (type 304), aluminum, nickel and titanium) were evaluated as cathode current collectors in two commonly used electrolytes. Their anodic electrochemical behaviors depend on the electrolyte and the gaseous atmosphere. The stainless steel and titanium meshes are applicable in both electrolytes under oxygen atmosphere because of the protective surface layers formed protecting from anodic corrosion. By using the stainless steel mesh as a current collector, carbon-free and binder-free manganese oxides nanostructures were successfully synthesized via an electrodeposition-calcination procedure, and their electrochemical behaviors and catalytic effects were investigated. The MnO2 and Mn2O3 nanostructures demonstrate a low discharge over-potential (0.08 V), due to the abundant Mn3+ and Mn4+ ions on the surface, and a multiple-stage recharge process with medium average charge voltages. Benefiting from its excellent oxygen reduction activity, and the firm adhesion to the substrate, resulting from the pre-calcination of stainless steel, the MnO2 electrode endures an excellent cycling performance (250 cycles at a cut-off capacity of 500 mAh/g). (398 words)