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.

Nanoconfinement-Induced Phase Segregation of Binary Benzene–Cyclohexane Solutions within a Chemically Inert Matrix

“…It is notable that fusion temperatures of dilute NaCl solutions confined within MCM-41 fall below the solidus temperature of the unconfined solution, suggesting that these solutions are unable to achieve the eutectic composition. LiBF 4 and LiPF 6 dimethyl carbonate solutions also experience concentration-dependent melting points when confined within MCM-41 and/or SBA-15 silicas, regardless of the presence or absence of an external reservoir. , …”

“…LiBF 4 and LiPF 6 dimethyl carbonate solutions also experience concentration-dependent melting points when confined within MCM-41 and/or SBA-15 silicas, regardless of the presence or absence of an external reservoir. 29,30 For dilute solutions, such as those encountered at high and low mole fractions of benzene in Figure 2, a substantial amount of solid phase will accumulate before the external reservoir freezes entirely. It is possible that the excessive amounts of solid material will block the pore openings, thereby preventing the confined solution from undergoing mass exchange with the unconfined solution.…”

Functional polymers for lithium metal batteries

“…In addition, in GPEs the melting point of the solvent can be significantly lowered when the solvated electrolyte is trapped in a confined environment. As a consequence, the GPE can exhibit sufficiently high conductivity at sub-zero temperatures, widening the application range of the electrolyte [144] .…”