Investigation of pore structure in plastic separators and ultrathick gradient porosity electrodes for fast ionic transport in lithium-ion batteries

Benefited from their tunable porosity, bondability, high voltage stability, fast transport properties, and good electrolyte retention, plastic separators have been an attractive technology option for separators in Li-ion batteries. Despite their invention in 1994, there has been little optimization and further understanding of the plastic separator comprising of PVDF-HFP copolymer resin, a plasticizer (dibutyl phthalate), and nano filler (SiO2). This thesis explores the impact of the plastic separator’s formulation on transport and pore structure to identify the optimal pore architecture for fast ion transport. Each formulation was investigated for pore size distribution by gas adsorption analysis (BET) and He ion microscopy, ionic conductivity and tortuosity by electrochemical impedance spectroscopy (EIS), and mechanical properties by tensile testing to correlate pore size distribution to transport, mechanical properties, and formulation. The bondable, high voltage stable separator formulations created had ~280% greater conductivity than the baseline formulation and the industry standard polyolefin separator.Technologies such as electric vehicles and portable electronic devices have an ever-pressing need for improved energy density and specific energy. To improve these performance factors and reduce the cost of inactive components, ultrathick electrodes have been a high area of interest within the battery research field. However, ultrathick electrodes have not been incorporated into commercial batteries due to their sluggish kinetics, difficulty of fabrication, and poor high rate performances. To combat these challenges, this thesis explores the utilization of gradient porosity in highly loaded LiCoO2 (LCO) electrodes of 54 mg/cm2 and an extreme ~230 μm thickness. Gradient porosity electrodes were fabricated by a novel methodology to create monolithic electrodes of predesigned porosity. Homogenous porosity electrodes were also fabricated and analyzed to compare to the gradient electrodes. Transport properties, such as tortuosity and high/low rate capabilities, were investigated for the gradient and homogenous porosity electrodes. For the first time, thick gradient electrodes have demonstrated much improved high rate capacity densities over homogenous electrodes of equivalent porosity, while retaining low rate capabilities and very high capacity density.M.S.Includes bibliographical reference