Date of Award
Doctor of Philosophy
Alexei P. Sokolov
Joshua Sangoro, Jaan Mannik, Steven Johnston
The rate of advancement for mobilized electronic technologies is outpacing the development of small efficient batteries. Lithium-ion batteries are currently the most widely used energy storage device for consumer electronics. Traditional lithium-ion batteries use a liquid electrolyte to separate the cathodes and anodes; however, conventional liquid electrolytes have inherent problems, such as consisting of flammable carbonate components, hazardous material, and have a significant cost/weight in the battery. In addition, the liquid electrolyte cannot prevent the growth of lithium dendrites during the charge/discharge cycle of the lithium-ion battery. These dendrites can connect the anode to the cathode of the battery cell through the liquid electrolyte separator, which will lead to high self-discharge currents through a low resistance path, igniting the flammable liquid electrolyte and causing fires or explosions. These problems have motivated the research community to resolve these related safety issues by using cheaper novel materials, such as polymerized electrolytes, to replace the traditional liquid electrolyte. Polymerized electrolytes can solve and alleviate some of the safety risks posed by liquid electrolytes. The main challenges regarding the use of polymerized electrolytes are an insufficient understanding of the ion transport mechanism and the inability to reach the desired industrial standard for conductivity of higher than 10-3 S/cm at ambient temperature.
This dissertation presents the findings of experimental studies of several different polymerized electrolytes using broadband dielectric spectroscopy, Brillouin light scattering, differential scanning calorimetry, and rheology. Varying the mobile ion size with different chemical structures of polymerized electrolytes allowed extensive analysis. The study analyzed the charge and mass transport of several polymerized electrolytes and one monomeric precursor, which led to a proposed approach to estimate ionic diffusivities from the characteristic times of the conductivity relaxation and ion concentration without any adjustable parameters.
Using the new and modified approach to estimate ionic diffusivities revealed that the charge transport is about ten times slower compared to that of ion diffusion, suggesting that a strong ion-ion correlation reduces ionic conductivity in polymerized electrolytes. Study of the activation energy of the ion diffusion shows a non-monotonous dependence on the mobile ion size, which indicates competition between coulombic and elastic forces controlling ion transport. This finding proposes that a simple qualitative model describing the activation energy for the ion diffusion would result in an increase in the dielectric constant of polymerized electrolytes can lead to a significant enhancement of conductivity of small ions (e.g., Li and Na).
Stacy, Eric Wayne, "Understanding the Fundamentals of Ionic Conductivity in Polymer Electrolytes. " PhD diss., University of Tennessee, 2020.