ION TRANSPORT IN POLYMER ELECTROLYTES

Batteries with superior performance will advance many technologies, such as the field of energy storage and electrochemical devices. Traditional lithium ion batteries based on liquid electrolytes have intrinsic problems such as leaking, dendrite growth, and those problems are associated with fire or even explosion hazard. Extensive efforts have been devoted into the development of solid polymer electrolytes (SPEs), which would not only reduce the size and weight of the batteries, but also solve safety related issues. However, none of current dry SPEs have reached the desired conductivity of 10^-3 [0.001] S/cm at ambient temperature. The ion conductivity is controlled by two parameters, the free ion concentration and ion diffusivity. Despite the generally accepted theory that ion diffusion is facilitated by the segmental relaxation of the polymer, the mechanism of ion transport in SPEs is not completely understood.

In this dissertation, the ion transport in different SPEs systems were studied with a combination of experimental techniques: dielectric spectroscopy, differential scanning calorimetry and rheology. The ion transport mechanism was investigated in poly(propylene glycol) (PPG) doped with LiClO₄ [lithium perchlorate]. A comprehensive analysis was performed by systematically varying the temperature, pressure, polymer molecular weight and salt concentration. It was found that the ion transport was controlled by the segmental relaxation of the “ion-rich” phase in the system, which obeyed the traditional theory. On the contrary, decoupling was observed in several carbonate and styrene based polymer electrolytes. Analysis indicated that the decoupling feature might be related to the packing frustration in those systems.

Polymerized ionic liquids (PolyILs) offer an opportunity of combining the high conductivity of ionic liquids and the superior mechanical strength of polymer. Unlike their small molecule analogue-aprotic ionic liquids, decoupling feature was observed in studied PolyILs. The variation of the pendant group structures altered the fragility index of the samples and thus the degree of decoupling.

Unraveling the mechanisms of the ion transport and structure-property relationship in SPEs is of obvious fundamental and industrial importance. Findings in this work suggested new routes for future polymer electrolytes design of desired properties.