Structure and ionic association interactions in solid polymer and liquid electrolytes

Electrolytes are a key component of electrochemical energy storage devices and the properties of the electrolyte often dictate the performance limitations of a device. Yet the molecular interactions which occur in the solvent-salt mixtures comprising electrolytes remain poorly understood. Characterization is a difficult task because of the amorphous nature of the ionically conducting phases of these materials. An examination of the related phase behavior and structural properties provides insight into the molecular interactions.; Work in the present thesis explores the phase behavior of poly(ethylene oxide) (PEO)-lithium salt mixtures which have been under scrutiny for two decades for use as solid polymer electrolytes. Differences in the phase behavior with different salts are attributed to the widely varying ionic association of the ions when the salts are solvated by ether solvents. The elucidation of the crystal structures of the PEO-salt phases has been far from trivial, often requiring complicated X-ray powder diffraction solution techniques. The structures for many of the phases have only been determined in the last few years. In the present thesis, it is demonstrated that single crystals of complexes formed between lithium salts and relatively long polyether chains may be prepared and the structures of the crystals determined. This may offer a new approach to the problems encountered in polymer electrolyte structural characterization.; Work in the present thesis also explores the phase behavior of low molecular weight polyether (e.g., glyme)-lithium salt mixtures. This work adds depth to the ionic association argument presented to explain PEO-salt phase behavior. A wealth of information regarding the influence of the anions upon solvates formed, the effect of varying glyme chain lengths on solvate structures, solvate stability and crystallization kinetics becomes available. But perhaps even more significantly, these studies permit one to view solvate structures which may form in an amorphous PEO-salt phase but which cannot crystallize as PEO-salt phases due to restrictions on the long-range packing or conformations of the longer PEO chains.