(597a) Imaging the Lithium Distribution within Nanostructured Polymer Electrolytes

Polymer membranes with high ionic conductivity are important for a variety of applications, including lithium batteries. These membranes must have a high modulus to prevent the catastrophic formation of dendrites. However, since the conductivity of homopolymer/salt mixtures is inversely related to the modulus, it is not feasible to increase the modulus of a homopolymer the necessary three orders of magnitude to prevent dendrite formation. Thus, our strategy is to decouple the mechanical and conductive properties through a second, non-conductive component. This is achieved through the use of poly(styrene)-block-poly(ethylene oxide) (PS-PEO) copolymers, where PEO forms the conducting ion channels and PS provides the rigid matrix. The ionic conductivity of these nanostructured electrolytes is comparable to that of the conducting part of the copolymer, and the elastic modulus is mainly governed by the hard insulating phase. We found that the conductivity of the copolymers increases with the molecular weight of the PEO block, although the conductivity of homopolymer PEO decreases with increasing molecular weight. In order to determine the role of structure on the ionic conductivity of these materials, we perform energy-filtered transmission electron microscopy experiments to directly measure the lithium distribution within our nanostructured copolymer. This is the first instance of successful imaging of amorphous lithium. We found that the lithium salt segregated itself to the middle of the conductive channels, and that this effect is more pronounced for higher molecular weight copolymers. It appears that thinner lithium lamellae lead to higher ionic conductivity.