Thermal Properties and Ionic Conductivities of Confined LiBF4Dimethyl Carbonate Solutions

Abstract

Solutions of lithium tetrafluoroborate dissolved in dimethyl carbonate (DMC) are confined within MCM-41, a mesoporous silica matrix. Thermal measurements indicate that the melting points of pure DMC and the DMC solutions are significantly reduced when confined within the pores of MCM-41 compared to unconfined samples; this is an observation that is consistent with the Gibbs–Thomson equation. The melting point onsets of confined solutions are slightly lower than that of pure DMC, suggesting the dissolved salts impact the phase-transition temperature of DMC when confined within mesoporous silica. Rotational dynamics of the confined solutions are explored by doping DMC with small quantities of Tempone, an electron paramagnetic resonance (EPR)-active spin probe. Tempone rotational correlation times are an order of magnitude slower for confined liquids compared to unconfined solutions. Temperature-dependent conductivity measurements of the composite materials suggest that the liquid electrolyte solution is distributed among the MCM-41 pores and the intergrain voids between individual MCM-41 particles. Ionic conductivities of confined electrolyte solutions remain above 0.01 mS·cm–1 for temperatures greater than −50 °C. However, the ionic conductivity of the unconfined solutions (i.e., solution occupying the spaces between the MCM-41 particles) rapidly decreases over subzero temperatures. Limitations associated with directly implementing these materials as low-temperature ion conductors are discussed.