Inaccessible Polarization-Induced Phase Transitions in a Block Copolymer Electrolyte: An Unconventional Mechanism for the Limiting Current

Abstract

Block copolymer electrolytes that microphase separate into mechanically rigid and ion-conducting domains are promising materials for lithium metal batteries. We present experimental data on a block copolymer electrolyte composed of polystyrene-block-poly(ethylene oxide) (SEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) which, under equilibrium conditions, exhibits a hexagonally packed cylindrical (HEX) morphology at low salt concentrations and a body-centered cubic spherical (BCC) morphology at high salt concentrations. The limiting current, which causes the cell potential to diverge exponentially if exceeded, is usually reached when the salt concentration at the negative electrode approaches zero due to concentration polarization. We use in situ X-ray scattering to study the morphology of our SEO/LiTFSI electrolyte, which exhibits a BCC morphology at equilibrium, during polarization. We expected to obtain a HEX morphology near the negative electrode as the limiting current is approached and the salt concentration at the negative electrode approaches zero. Instead, we find that the cell potential diverges when the salt concentration at this electrode approaches the concentration at the BCC/HEX boundary at high current densities. The maximum current that can be carried by our electrolyte is thus not limited by the lack of salt at the negative electrode. Instead, it appears to be related to a phase transition that occurs readily in equilibrated samples but is inaccessible in a polarized cell.