Tracking salt concentration gradients and block copolymer electrolyte phase transitions using in situ SAXS during cell polarization*

Block copolymer (BCP) electrolytes with lithium salt offer promise as next generation electrolytes for Li-ion batteries. Unlike conventional liquid electrolytes, BCPs are not plagued by flammability issues. Furthermore, by incorporating a mechanically-rigid block, they can suppress deleterious dendrite formation and enable the use of higher-energy density lithium metal anodes. Here we study polystyrene-block-poly(ethylene oxide) (SEO) mixed with lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI) salt in lithium-lithium symmetric cells under constant applied current. Under equilibrium conditions, this SEO system exhibits hexagonally packed cylindrical (HEX) morphology but can display gyroid (GYR) morphology at lower salt concentrations. By using in-situ SAXS during cell polarization, we measure the formation of salt concentration gradients within the cell and simultaneously track the morphological changes as a function of position and time. While recent work found that a transition from body-centered cubic spherical (BCC) morphology to HEX in a higher molecular weight SEO was inaccessible and led to premature cell failure, here we detect the phase transition from HEX to GYR at the salt depleted electrode. Moreover, the relative prominence of HEX and GYR populations varies strongly with orientation (parallel or perpendicular to ion transport). Thus we see that certain phase transitions are accessible and elucidate an interplay between BCP grain orientation and electrochemical cell properties.