Toward High Energy and High Efficiency Secondary Lithium Batteries

Lithium ion batteries and lithium sulfur batteries demonstrate great potentials to be the substitutes for fossil fuels in electric cars. The first part of this dissertation focused on the development of Li-Mn-rich composite cathode materials and high voltage spinel cathode materials for lithium ion batteries. Li-Mn-rich composite materials were synthesized using various methods to optimize their performances, including a polymer-assisted method, a co-precipitation method in a continuous stirred tank reactor (CSTR) and an aerosol method. The highest first discharge capacity of these composite materials was over 300 mAh/g. High voltage spinel cathode material LiNi0.24Mn 1.76O4 was prepared through a simple solid state method and the as-prepared material demonstrated excellent rate capabilities and cycling stabilities. High voltage spinel cathode material LiNi0.5Mn 1.5O4 in a nanowire structure was prepared through an electrospinning method followed by a heat-treating procedure. To obtain secondary lithium batteries with even higher capacities, the second part of the dissertation went beyond the lithium-ion concept to the lithium-sulfur chemistry. Novel lithium sulfur systems were designed and they delivered a capacity five times as much as lithium ion batteries can offer. A novel polysulfide-based electrolyte that prevents the performance degradation inherent to Li-S batteries was designed. By creating a dynamic equilibrium between the dissolution and precipitation of lithium polysulfides at the sulfur/electrolyte interface, the Li-S cells were capable of delivering a superior capacity (1450 mAh/g, sulfur), which along with the high coulombic efficiency and excellent cycle life make our cells among the best performing Li-S cells.