Ionic Conductivity of and Structure and Property Relationships in Li₂S + SiS₂ + LiPO₃ Glassy Solid Electrolytes

Li2S-SiS2-LiPO3 mixed oxy-sulfide (MOS) glassy solid electrolytes (GSEs) have been studied to examine the effects of the systematic mixing of anions, O and S, and glass formers, Si and P, on the short-range order (SROs) structures and the Li+ ionic conductivity, glass transition temperature (Tg), crystallization temperature (Tc), working range (ΔT ≡ Tg – Tc), density, and molar volume. In this work, LiPO3 was added to these GSEs to form the series 0.58Li2S + 0.42[(1 – x)SiS2 + xLiPO3], where 0 ≤ x ≤ 0.5. The SRO structures of these GSEs were investigated using a combination of Raman, Fourier transformation-infrared (FT-IR), and 29Si and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Tg, Tc, and ΔT maximized in value at x = 0.2 while the density increased, and the molar volume decreased monotonically. However, the molar free volume exhibited a maximum at x = 0.1 before decreasing. This behavior is also observed in the room-temperature (25 °C) Li+ ionic conductivity, which reaches a maximum value of 1.4 mS/cm at x = 0.1 before decreasing to 0.2 mS/cm at x = 0.5. By deconvoluting the MAS NMR spectra, it was found that the overall trend in the SRO structures of these GSEs is for the pure sulfide SRO units to decrease in concentration. In contrast, the concentration of MOS SRO units increases with x. Interestingly, while oxygen was added to these GSEs in the form of LiPO3, at low x, 0 ≤ x ≤ 0.2, nearly all the oxygen is amassed by the Si away from the P to create high concentrations of Si–O bonds, leaving the displaced S to bond to P. With the further LiPO3 addition and reduced silicon concentration, this P–O bearing species becomes the dominant SRO species. The maxima in the Tg, Tc, and ΔT are associated with maxima in the concentrations of stronger Si–O bonds compared to weaker P–O bonds. The maximum conductivity at x = 0.1 is associated with a maximum in the calculated free volume in these GSEs that is suggested to create larger interstitial doorways connecting the Li+ tetrahedral coordination sites.