The Effect of Annealing on the Grain Structure and Ionic Conductivity of Block Copolymer Electrolytes

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Block copolymer electrolytes that microphase-separate into rigid non-conducting domains and soft ion-conducting domains are known to exhibit stability against lithium metal anodes. In these systems, order is confined to grains with concomitant defects. When these electrolytes are annealed, the grain size typically increases, which is assumed in the literature to lead to a decrease in the ionic conductivity. In this work, we study the interplay between grain size and ionic conductivity using a block polymer electrolyte composed of a polystyrene (PS) block with a molecular weight of 19 kg/mol and a poly(ethylene oxide) (PEO) block with a molecular weight 20 kg/mol mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt at a variety of salt concentrations. The electrolytes have lamellar morphologies at all salt concentrations. At low salt concentrations, the average grain size before annealing is large and ionic conductivity decreases upon annealing. At high salt concentrations, however, the average grain size before annealing is small and ionic conductivity increases upon annealing.