Composite Electrolytes from Molecular Crystals and Polymer Gels: Absence of Interface Effects

Abstract Composites where the total conductivity is a linear combination of the conductivity of the two components must have no interfacial resistance between them. In composites formed from molecular crystals and polymer gel electrolytes, this can be accomplished since molecular crystals (composed of Li salts and low molar mass ligands) have fluid grain boundaries that can be swollen with excess ligands, either the same or different than the one used in the crystal. The chains of the polymer can entangle with the grain boundary layer thus allowing easy Li+ ion migration between the fluid grain boundaries and the fluid phase of the polymer gel. Here, in a one pot synthesis, the components of the molecular crystal (Adpn)2LiPF6, poly(ethylene oxide) (PEO), additional LiPF6 and succinonitrile (SN) were dissolved in a common cosolvent, which after evaporation formed a uniform dispersion of (Adpn)2LiPF6 crystals in a gel matrix of PEO/LiPF6/SN. The composite was stiffened by the molecular crystals but brittle, and the conductivity was a linear combination of the conductivities of the crystal and gel phases. However, in another system using the same molecular crystal, a block copolymer of polyacrylonitrile/polystyrene (PS-co-PAN) and a trinitrile ligand, 1,3,6-hexanetricarbonitrile (HCTN), after evaporation of the cosolvent a single gel phase was formed, which had good elastomeric properties due to phase separation of the PS block. This highlights the necessity of using lithium salts that are not very soluble in the gel phase but which have a strong tendency to form molecular crystals with the ligand.