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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01830 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018
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Chemistry of Materials
Observation of a Phonon Softening Effect on Li Ion Conduction in Mixed-Anion Chalcogenide Glasses
Maxwell A.T. Marplea, Bruce G. Aitkenb, Sangtae Kima, Sabyasachi Sena a
Department of Materials Science and Engineering, University of California at Davis, Davis CA 95616, USA b
Science & Technology Division, Corning Inc., Corning, NY 14831, USA
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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 non-bridging 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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Chemistry of Materials
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, compositional flexibility, and 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 low energy barriers for ionic transport. The ionic conductivity can be further modified via introduction of isovalent components, which can lead to non-linear 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 non-linear variation in the ionic conductivity.11-13 Recently nonlinear compositional effects on ionic conduction have been explored in mixedanion (O, S, or Se) systems of glass and crystalline ionic conductors.14-22 It is observed that doping of a small amount of O (< 7 anion%) into a sulfide glass enhances its stability against moisture attack, and at the same time, can have a beneficial effect on ionic conductivity.23, 24 The latter effect was attributed to an expansion of the “doorway radius” for ion migration within the framework of the Anderson-Stuart model, in which a mixed anion doorway between Li hopping sites is found to be somewhat larger than that in the oxygen-free system, which leads to a lowering of the migration activation energy. However, this beneficial effect quickly diminishes 3 ACS Paragon Plus Environment
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with further increase in the O concentration, due to the higher electrostatic binding energy of O with Li. Substitution with highly polarizable isovalent anions can also enhance the ionic conductivity and therefore, substitution of S with Se in chalcogenide electrolytes is a possible option. However, systematic studies of the mixed-anion effect in sulfo-selenide systems are rare in the literature and are all limited to rather small amounts of mixing (
