Relationship between morphology and conductivity of block-copolymer based battery separators

Nanoporous battery separators were made by blending a polystyrene-block-polyethylene-block-polystyrene copolymer (SES) and polystyrene (PS) homopolymers, casting films of the blend, and selectively dissolving the homopolymer. The efficacy of the separators thus obtained was determined by measurement of the ionic conductivity of separators soaked in 1 M lithium hexafluorophosphate in ethylene carbonate/diethyl carbonate (1:1, v/v, Novolyte Technologies, Inc.), a standard lithium battery electrolyte. We focus on the effect of chain length of the sacrificial homopolymer on separator morphology and ion transport. In highly porous separators with a nominal pore volume fraction of 0.43, conductivity peaked at α = 0.22, where values as high as 0.39 mS cm−1 were achieved (α is the molecular weight of the PS homopolymer normalized by that of the PS block in the SES copolymer). Nitrogen adsorption experiments and scanning electron microscopy were used to determine the underpinnings of this observation. At α = 0.12, extremely small pores with low surface area are formed. Increasing α to 0.22 results in a film with well-connected nanoscale pores. A further increase in α to 2.02 results in films with micron-sized pores that are not effective for ion transport.