Conductivity of 2-adamantanone with lithium bis(trifluoromethanesulfonyl)imide: Impact of residual solvent and temperature

Abstract

This study investigates the ionic conductivity of a mixture comprising 2-adamantanone and lithium bis(trifluoromethanesulfonyl)imide, with focus on the impact of temperature and residual tetrahydrofuran. Previous investigations have shown that the plastic crystal 2-adamantanone, when paired with lithium bis(trifluoromethanesulfonyl)imide, exhibits an ionic conductivity of 1.2 × 10⁻⁴ S cm⁻¹ and a considerable oxidation potential of 5.1 V. Nonetheless, the influence of any residual processing solvent on the ionic conductivity is not yet fully understood. The Design of Experiments methodology was utilized to analyze a broad spectrum of potential compositions of 2-adamantanone, lithium bis(trifluoromethanesulfonyl)imide, and tetrahydrofuran. We measured the ionic conductivity of the samples using electrochemical impedance spectroscopy and conducted structural studies via differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction and solid-state NMR. Our findings indicate that the leftover amount of THF enhances ionic conductivity more strongly than the molarity. Moreover, compared to crystallization from the solvent, ionic conductivity increases by over an order of magnitude following recrystallization from the melt. We suggest that the residual solvent is integrated into the crystal structure of the 2-adamantanone, thereby increasing the free volume and facilitating lithium-ion transport. At elevated temperatures, the optimized formulation transforms from a solid to a wax-like consistency, functioning as a solid electrolyte with a high ionic conductivity of 2.6 × 10⁻⁴ S cm⁻¹ at room temperature, making it a promising candidate for electrolyte applications.