Mechanisms of Enhancing Solid Polymer Electrolytes Using Nanofillers and Ionic Liquid for Applications in Flexible Lithium Ion Batteries

Polymer-based electrolytes have gained much attention in recent decades due to their many advantages including high thermal and chemical stability, and the consequent enhanced safety in lithium ion batteries. Also, thin-film manufacturability and mechanical strength make the polymer-based electrolyte excellent candidate for the development of thin, flexible lithium ion battery. One main issue with polymer electrolytes is their lower ion conductivity compared to that of conventional liquid electrolytes. In this dissertation, the properties of polymer-based solid and gel electrolytes and their applications for lithium ion batteries have been investigated. The focus of this dissertation is the influence of selected additives including nanofillers, and ionic liquids on the performance of the polymer electrolytes and flexible lithium ion batteries. The effect of nanofillers on ion conductivity of polymer electrolytes is investigated using a continuum, bulk level approach. Based on the free volume theory, a model of the ion conductivity enhancement of polymer electrolyte as a function of nanofiller content is proposed. The model could fit to various experimental results of ionic conductivity enhancement and degradation. It could also be used to fit the temperature dependency of the ionic conductivity. The influence of the nanofiller is also studied at the molecular, discrete level using the molecular dynamics simulations of a polymer nanocomposite electrolyte. It is found that the embedded nanofiller can affect the salt dissociation, lithium-ion mobility, and the dynamics of polymer chains. Those effects can depend on the surface functionality and size of the nanofiller. Furthermore, the effect of ionic liquid on polymer electrolyte performance is investigated. A highly conductive ionic liquid (IL), 1-Ethyl-3-methylimidazolium dicyanamide (EMIMDCA), with ionic conductivity as high as 27 S/cm, is incorporated in poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) polymer matrix and lithium salt (i.e., lithium perchlorate) to form the polymer-IL electrolyte. The obtained electrolyte is a freestanding thin-film and exhibits solid-like appearance. Due to its high stability, the polymer-IL electrolyte film is used for a low-cost, simple lamination method to fabricate high performance flexible lithium ion batteries. The battery shows relatively stable energy delivery capability and can function in both flat and bent configurations.Mechanical Engineering, Department o