(121b) The Role of Interfaces on Ion Conduction in Well-Aligned Thin Film Block Copolymer Electrolytes

The independently tunable electrochemical and mechanical properties of block copolymer electrolytes (BCEs) make them attractive materials for next-generation energy storage devices. However, understanding how the hierarchical assembly within these systems affects ion solvation and transport is paramount to designing improved materials to meet the stringent requirements for use in high-performance devices. In particular, differentiating between micron-scale hinderances to ionic mobility, such as grain boundaries, from molecular-level effects arising from the interface between the conducting and non-conducting phases is extraordinarily difficult in standard electrochemical measurements. We have developed a new experimental approach that leverages thin film self-assembly of highly ordered lamellar-forming block copolymer electrolytes on top of interdigitated electrodes. By doing so, we can probe ionic conductivity within a single grain of a BCE, eliminating any confounding effects present in multi-grain samples.

We demonstrate this method with PS-PEO-LiTFSI, a model dry polymer electrolyte system. With this new approach, we have demonstrated striking differences in BCE conductivity as a function of lithium salt concentration when compared to the PEO homopolymer system. In particular, the intermixing between the two blocks near the block copolymer interface plays a crucial role in slowing ionic mobility. However, at higher salt concentrations, where PEO conductivity drops precipitously, PS-PEO conductivity remains relatively constant. This suggests that the decrease in segmental mobility in PEO at high ionic concentrations is absent in highly concentrated PS-PEO-LiTFSI films. This unexpected finding was made possible only by the study of well-aligned thin films and sheds new light on the role of polymer interfaces on ion transport.

We couple these thin film conductivity measurements with FT-IR and Raman spectroscopic measurements to further elucidate the solvation of lithium salt in these materials. We observe clear differences in the dissociation of ions within block copolymers, relative to the comparable homopolymer system, that correspond well to the observed differences in conductivity at high salt concentration. This experimental platform, and the insights gained from it, will help accelerate the development of new block copolymer electrolytes.