Ionic Conductivity of K-ion Glassy Solid Electrolytes of K₂S-P₂S₅-KOTf System

Ternary glassy electrolytes containing K₂S as a glass modifier and P₂S₅ as a network former are synthesized by introducing a new type of complex and asymmetric salt, potassium triflate (KOTf), to obtain unprecedented K⁺ ion conductivity at ambient temperature. The glasses are synthesized using a conventional quenching technique at a low temperature. In general, alkali ionic glassy electrolytes of ternary systems, specifically for Li⁺ and Na⁺ ion conductivity, have been studied with the addition of halide salts or oxysalts such as M₂SO₄, M₂SiO₄, M₃PO₄ (M = Li or Na), etc. We introduce a distinct and complex salt, potassium triflate (KOTf) with asymmetric anion, to the conventional glass modifier and former to synthesize K⁺-ion-conducting glassy electrolytes. Two series of glassy electrolytes with a ternary system of (0.9-x)K₂S-xP₂S₅-0.1KOTf (x = 0.15, 0.30, 0.45, 0.60, and 0.75) and z(K₂S-2P₂S₅)-yKOTf (y = 0.05, 0.10, 0.15, 0.20, and 0.25) on a straight line of z(K₂S-2P₂S₅) are studied for their K⁺ ionic conductivities by using electrochemical impedance spectroscopy (EIS). The composition 0.3K₂S-0.6P₂S₅-0.1KOTf is found to have the highest conductivity among the studied glassy electrolytes at ambient temperature with the value of 1.06 × 10^-7 S cm^-1, which is the highest of all pure K⁺-ion-conducting glasses reported to date. Since the glass transition temperatures of the glasses are near 100 °C, as demonstrated by DSC, temperature-dependent conductivities are studied within the range of 25 to 100 °C to determine the activation energies. A Raman spectroscopic study shows the variation in the structural units PS43-, P2S74-, and P2S64- of the network former for different glassy electrolytes. It seems that there is a role of P2S74- and P2S64- in K⁺-ion conductivity in the glassy electrolytes because the spectroscopic results are compatible with the composition-dependent, room-temperature conductivity trend.