Current-induced morphological changes in block copolymer electrolytes

Block copolymers are attractive electrolyte materials for lithium metal batteries due to their ability to microphase separate into distinct ion-conducting and mechanically-reinforcing domains on the nanometer-length-scale. Although the formation of current-induced gradient crystals has been previously shown when the material was heated above the glass transition temperature, T_g, of the mechanically-reinforcing block, it has been previously assumed that these equilibrium morphologies remain intact throughout polarization under the T_g, e.g. at 90 °C. In-situ small angle X-ray scattering (SAXS) experiments were used to probe morphology and electrochemical impedance spectroscopy (EIS) was used to quantify the bulk resistance of the electrolyte throughout cycles of cell polarization and subsequent cell relaxation of lithium-lithium symmetric cells as a function of polarization voltage. The electrolyte is comprised of a polystyrene–b-poly(ethylene oxide), PS-b-PEO, copolymer mixed with a lithium salt that forms hexagonally packed PS cylinders in a matrix of salt-containing PEO in the absence of current. We hypothesize that the formation of salt-concentration gradients within the electrolyte from the cell polarization drive the changes seen in morphology and resistance.