UC Berkeley

Block copolymers mixed with lithium salts are promising electrolyte materials for next- generation lithium batteries. One block can aid in the transport of ions, while the other block can provide improved mechanical strength to counteract lithium dendritic growth, a common failure mechanism of lithium metal batteries. Block copolymers often exhibit microphase separation of the two blocks due to thermodynamic repulsions between the two unlike chemical moieties. Understanding the correlations between structure, salt concentration, mechanical strength, and ion transport is important for improving the performance of polymer electrolytes. Significant work in characterization of structure and ion transport has been performed in all- organic diblock copolymers mixed with lithium salts, namely, polystyrene-block-poly(ethylene oxide) (PS-b-PEO; SEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). However, there is an emerging interest in replacing the mechanically rigid block with an inorganic material, such as polyhedral oligomeric silsesquioxane (POSS). POSS represents a silica caged structure with molecular weight close to 1 kg mol-1 and molecular size of 1 nm.In this dissertation, mixtures of poly(ethylene oxide)-block- acryloisobutyl polyhedral oligomeric silsesquioxane (PEO-POSS) and acryloisobutyl polyhedral oligomeric silsesquioxane)-block- poly(ethylene oxide)-block- acryloisobutyl polyhedral oligomeric silsesquioxane (POSS-PEO-POSS) with LiTFSI are studied. PEO provides good ion solvation and transport, while POSS increases the modulus. PEO-POSS/LiTFSI and POSS-PEO- POSS/LiTFSI microphase separate into distinct morphologies which are a strong function of volume fraction, chain length, salt concentration, and temperature that differ from the phase behavior of SEO/LiTFSI. We study the relationships between the morphology, salt concentration, shear moduli, crystallinity of the POSS block, and ion-transport using small/wide angle X-ray scattering, electron microscopy, ac impedance spectroscopy, chronoampotherapy and rheology. Experiments show that PEO-POSS/LiTFSI and POSS- PEO-POSS/LiTFSI tends to segregate more strongly with increasing temperature, opposite of SEO/LiTFSI. The morphology is strongly dependent upon the crystallization of the POSS block. Block structure and morphology both, in turn, effect the electrochemical properties and the limiting current density.