Phase transitions in block copolymer electrolytes induced by ionic current

Abstract

We study the effect of steady ionic current on the morphology of polystyrene-block-poly(ethylene oxide) (SEO) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). At equilibrium, SEO/LiTFSI mixtures exhibit a variety of phases that depend on the salt concentration: disordered (DIS), lamellae (LAM), gyroids (GYR), hexagonally-packed cylinders (HEX), and body-centered spheres (BCC). The application of ionic current through any electrolyte results in salt concentration gradients, and this may induce phase transitions. We find that certain phase transitions – DIS to LAM, LAM to DIS, LAM to GYR, and HEX to GYR – can be induced by ionic current. These transitions were observed when the local salt concentration in the electrochemical cell crossed the equilibrium phase boundary. The timescales required to initiate these phase transitions are compared to diffusion timescales. The transition from GYR to HEX was not observed in spite of the fact that the equilibrium phase boundary was crossed. The BCC to HEX transition was also not observed, but in this case, the cell potential diverged as the salt concentration approached the equilibrium phase boundary.