Article Cite This: Chem. Mater. 2018, 30, 5896−5903
pubs.acs.org/cm
Observation of a Phonon Softening Effect on Li Ion Conduction in Mixed-Anion Chalcogenide Glasses Maxwell A. T. Marple,† Bruce G. Aitken,‡ Sangtae Kim,† and Sabyasachi Sen*,† †
Department of Materials Science and Engineering, University of California at Davis, Davis, California 95616, United States Science & Technology Division, Corning Inc., Corning, New York 14831, United States
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ABSTRACT: The structural evolution and Li-ion transport in 40%Li2S−60%Ge(S,Se)2 glasses are investigated as a function of S/(S + Se) ratio, using Raman, 7Li nuclear magnetic resonance (NMR), and electrochemical impedance spectroscopic techniques. The structure of these glasses is primarily composed of corner-shared GeS4/2, GeSe4/2, and mixed-anion GeSx/2Se(4−x)/2 tetrahedra with nonbridging Se and S, where Li plays the role of a modifier cation. 7Li NMR line shape analysis indicates that the electrical conductivity is entirely due to the hopping transport of Li ions. While these glasses are found to have comparable ionic conductivity that varies little with composition, the activation energy and the pre-exponential factor display a nonlinear variation with the S/ (S + Se) ratio. The observed compositional trends of the pre-exponential factor and the activation energy are rather similar and are shown to be related to the progressive phonon softening as Se replaces S in a mixed-anion network. When taken together, these results suggest that the phonon softening of the structural network of solid electrolytes, induced via compositional modification, can be used to tune their electrical properties.
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INTRODUCTION Glassy ionic conductors are an important class of materials for their potential use in solid-state electrochemical devices. These materials are characterized by several favorable properties including high ionic conductivity, absence of grain boundaries, wide electrochemical stability windows, and compositional flexibility, and they can potentially stabilize “superionic” crystalline phases in a glass ceramic.1−8 Glasses with large concentrations of mobile network modifiers such as Li or Na cations often display high room-temperature ionic conductivities, owing to the formation of percolation pathways with lowenergy barriers for ionic transport. The ionic conductivity can be further modified via introduction of isovalent components, which can lead to nonlinear compositional effects, despite maintaining the same structural characteristics. This is most notable in the mixed-alkali effect where introduction of a second-alkali species to a single-alkali glass at a fixed modifier concentration typically leads to a dramatic decrease in the ionic conductivity.9,10 Another such compositional effect that has been explored relatively recently is the mixed network-former effect, wherein mixing of two different network-forming components led to significant nonlinear variation in the ionic conductivity.11−13 Recently, nonlinear compositional effects on ionic conduction have been explored in mixed-anion (O, S, or Se) systems of glassy and crystalline ionic conductors.14−22 It is observed that doping of a small amount of O (
