Solid Electrolytes in the N-Propyl-N-methyl-pyrrolidinium Tetrafluoroborate—Lithium Tetrafluoroborate System

Solid electrolytes prepared by the addition of LiBF4 to the plastic phase of [N13pyr]BF4 were prepared, and their physical and electrical properties were investigated. The electrolytes [N13pyr]BF4-LiBF4 containing 8–20 wt% LiBF4 are solid at temperatures below 80 °C and have a high ionic conductivity ~10−3–10−2 S cm−1 at 60 °C. Based on the results of DSC and conductivity studies, the phase diagram of the [N13pyr]BF4-LiBF4 binary system was plotted, and the formation of a new compound, 3[N13pyr]BF4·2LiBF4 was proposed. The existence of the new phase was supported by X-ray diffraction data. Electrochemical measurements of cells with lithium electrodes were carried out to test the applicability of these materials in lithium batteries. The electrochemical window was determined to be more than 5 V. In contrast to earlier data obtained for similar systems, the preconditioning effect was not observed. Nevertheless, the solid electrolyte [N13pyr]BF4-LiBF4 system has high ionic conductivity and may be used in solid-state lithium-ion batteries.