Miscibility of Sulphur-based compounds in LiF for Lithium ion conductivty in Lithium-ion battery.
The research on electrolytes has progressed from organic, ionic liquids and salt-in-water electrolytes, to solid-state electrolytes, with emphasis on better stability, conductivity and high operational voltage. The recent focus on solid-state electrolytes is due to its advantages over liquid electrolytes, even though liquid electrolytes show better Li-ion conductivity. Lithium solid electrolytes include LISICON, thio-LISICON, Garnet and Sulfides, most of which are heavy due to the presence of bulkier elements such as Germanium, Zirconium, Lanthanum, Tin etc. These elements as such do not contribute to conductivity but play a vital role to hold the structural framework of the crystal. In the current project, with the aim of finding better alternates to present solid-state electrolytes, an attempt has been made to prepare solid solutions of Sulfur in LiF. For the preparation of solid-state solutions, two approaches are used: Solid-state synthesis and Melt method. Solid-state synthesis involves the reaction of a polycrystalline reactant through direct reaction in solid state by proper mixing and grinding followed by heating at an increased temperature. The second, Melt method involves a chemical reaction between two compounds with either one of them having high melting point. During the mixing of two solids, the melting point decreases and form eutectic. Here, stoichiometric ratios of host and dopant are mixed to prepare solid-state solutions in different combinations. For solid-state synthesis, different combinations of LiF and Li2S are required, in which Li2S is used, the one commercially available, and LiF is synthesized in the lab. Next, a different combination of samples - 100% LiF, 97.5%LiF -2.5% Li2S, 95%LiF-5% Li2S, 90%LiF-10% Li2S - are made inside the glove box. In Melt method LiF is produced by reaction between Li2CO3 and NH4HF2 and Sulfur is incorporated by adding varying amounts of C2H5NS as impurity. In both methods, the obtained crystals after proper mixing are made into pellets and heated in tubular furnace using a platinum boat. These samples are being characterized using XRD analysis, FTIR and EIS.
Keywords: LiF, Solid electrolyte, synthesis, ionic conductivity