Block Copolymer Electrolytes for Battery Applications

Energy density is the greatest limiting factor in batteries for electric vehicle applications. Lithium metal batteries address this limitation because lithium has the highest theoretical energy density in terms of both mass (Wh/kg) and volume (Wh/L). Dry solid polymer electrolytes (SPEs), consisting of a lithium salt solvated in a polymer host, are the leading candidate materials for these batteries because they can interface stably with lithium metal. The primary failure mechanism in SPE batteries is the growth of lithium metal dendrites at the anode during recharging. Theoretical work has suggested that improving the mechanical strength of the polymer may prevent dendrite growth – thus forming the basis for our present work. We are investigating the block copolymer poly(styrene)-block-poly(ethylene oxide) (PS-PEO). The polymer microphase separates – forming microdomains of PS and of PEO – and is designed so that the PS phase suppresses lithium dendrite growth, while the PEO phase solvates lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt to form conducting pathways. Complete electrochemical characterization of PS-PEO/LiTFSI mixtures requires measurement of conductivity, salt diffusion coefficient, and lithium ion transference number. The present study covers PS-PEO copolymers that exhibit lamellar and cylindrical morphologies in the absence of salt. The addition of salt affects morphology but the relationships between morphology and electrochemical characteristics have not yet been clarified. Some aspects of these relationships will be presented.