Composition Dependence of the Na⁺ Ion Conductivity in 0.5Na₂S + 0.5[xGeS₂ + (1 – x)PS_5/2] Mixed Glass Former Glasses: A Structural Interpretation of a Negative Mixed Glass Former Effect

Abstract

A negative mixed glass former effect (MGFE) in the Na(+) ion conductivity of glass has been found in 0.5Na2S + 0.5[xGeS2 + (1 - x)PS5/2] glasses where the Na(+) ion conductivity is significantly smaller for all of the ternary glasses than either of the binary end-member glasses. The minimum conductivity of ∼0.4 × 10(-6) (Ω cm)(-1) at 25 °C occurs for the x = 0.7 glass. Prior to this observation, the alkali ion conductivity of sulfide glasses at constant alkali concentration, but variable ratio of one glass former for another (x) ternary mixed glass former (MGF) glasses, has always produced a positive MGFE in the alkali ion conductivity; that is, the ternary glasses have always had higher ion conductivities that either of the end-member binary glasses. While the Na(+) ion conductivity exhibits a single global minimum value, the conductivity activation energy exhibits a bimodal double maximum at x ≈ 0.4 and x ≈ 0.7. The modified Christensen-Martin-Anderson-Stuart (CMAS) model of the activation energies reveals the origin of the negative MGFE to be due to an increase in the dielectric stiffness (a decrease in relative dielectric permittivity) of these glasses. When coupled with an increase in the average Na(+) ion jump distance and a slight increase in the mechanical stiffness of the glass, this causes the activation energy to go through maximum values and thereby produce the negative MGFE. The double maximum in the conductivity activation energy is coincident with double maximums in CMAS calculated strain, ΔES, and Coulombic, ΔEC, activation energies. In these ternary glasses, the increase in the dielectric stiffness of the glass arises from a negative deviation of the limiting high frequency dielectric permittivity as compared to the binary end-member glasses. While the CMAS calculated total activation energies ΔEact = ΔES + ΔEC are found to reproduce the overall shape of the composition dependence of the measured ΔEact values, they are consistently smaller than the measured values for all compositions x. The new concept of an effective Madelung constant for the Na(+) ions in glass is introduced, MD(Na(+)), to account for the difference. Calculated MD(Na(+)) values necessary to bring the CMAS and experimental ΔEact values into agreement are in excellent agreement with nominal values for typical oxide crystals containing Na(+). New MD simulations of oxide glasses were performed and were used to calculate MD(Na(+)) values for Na2O + SiO2 glasses for the first time and were found to agree quite well with the values for the sulfide glasses studied here. Insights from the current study have been used to predict and design new MGF systems that may lead to a positive MGFE in the ionic conductivity.

NMR Investigations of Crystalline and Glassy Solid Electrolytes for Lithium Batteries: A Brief Review

“…From the model, it was thought that the Na ions preferred to cluster around Si atoms, and the addition of boron reduces this clustering leading to slower Na ion dynamics and lower conductivity, i.e., a negative MGFE. This effect was also seen in other mixed glass systems, such as sodium germanophosphate glasses (NGP) [26], further lending credibility to the model. the gaussian energy distribution, they calculated the threshold activation energy to be in agreement with conductivity measurements, underscoring the utility of the model.…”

The role of ceramic and glass science research in meeting societal challenges: Report from an NSF‐sponsored workshop

“…[245][246][247][248][249][250] ; however, much less is known about the structure and properties of multicomponent glasses and melts under pressure. A close collaboration between researchers in the glass science and geoscience communities is needed to adapt these in situ high-pressure/temperature characterization techniques to study multicomponent glasses [257][258][259] and melts, particularly those which are of industrial relevance and technological importance, for example, new kinds of solid electrolytes for safer and higher energy density batteries.…”

Network-Forming Units, Energy Landscapes, and Conductivity Activation Energies in Alkali Borophosphate Glasses: Analytical Approaches

“…13,15 Upon mixing of two or more glass formers, the ionic conductivity can increase (positive MGFE) 16 or decrease (negative MGFE). 17,18 Many studies in particular were conducted for alkali borophosphate glasses. 14,19−22 In a former study, our research group developed a theoretical approach to explain the MGFE in sodium borophosphate glasses with compositions of 0.4Na 2 O − 0.6[xB 2 O 3 − (1 − x)P 2 O 5 ] in the whole range of borate to phosphate mixing 0 ≤ x ≤ 1.…”

“…The MGFE received particular attention in the past decade. , Upon mixing of two or more glass formers, the ionic conductivity can increase (positive MGFE) or decrease (negative MGFE). , Many studies in particular were conducted for alkali borophosphate glasses. ,− …”