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Evidence for a Solid-Electrolyte Inductive Effect in Superionic Conductors

preprint
submitted on 09.10.2020, 17:54 and posted on 12.10.2020, 12:42 by Sean Culver, Alex Squires, Nicolo Minafra, Callum Armstrong, Thorben Krauskopf, Felix Boecher, Cheng Li, Benjamin Morgan, Wolfgang Zeier

Identifying and optimizing highly-conducting lithium-ion solid electrolytes is a critical step towards the realization of commercial all–solid-state lithium-ion batteries. Strategies to enhance ionic conductivities in solid electrolytes typically focus on the effects of modifying their crystal structures or of tuning mobile-ion stoichiometries. A less-explored approach is to modulate the chemical-bonding interactions within a material to promote fast lithium-ion diffusion. Recently, the idea of a solid-electrolyte inductive effect was proposed, whereby changes in bonding within the solid-electrolyte host-framework modify the potential-energy landscape for the mobile ions, resulting in an enhanced ionic conductivity. This concept has since been invoked to explain anomalous conductivity trends in a number of solid electrolytes. Direct evidence for a solid-electrolyte inductive effect, however, is lacking—in part because of the challenge of quantifying changes in local bonding interactions within a solid-electrolyte host-framework. Here, we consider the evidence for a solid-electrolyte inductive effect in the archetypal superionic lithium-ion conductor Li10Ge1−xSnxP2S12, using Rietveld refinements against high-resolution temperature-dependent neutron-diffraction data, Raman spectroscopy, and density functional theory calculations. Substituting Ge for Sn weakens the {Ge,Sn}–S bonding interactions and increases the charge-density associated with the S2- ions. This charge redistribution modifies the Li+ substructure causing Li+ ions to bind more strongly to the host-framework S anions; which in turn modulates the Li-ion potential-energy surface, increasing local barriers for Li-ion diffusion. Each of these effects is consistent with the predictions of the solid-electrolyte inductive effect model. Density functional theory calculations further predict that this inductive effect occurs even in the absence of changes to the host-framework geometry due to Ge → Sn substitution. These results provide direct evidence in support of a measurable solid-electrolyte inductive effect and demonstrate its application as a practical strategy for tuning ionic conductivities in superionic lithium-ion conductors.

Funding

Deutsche Forschungsgemeinschaft ZE 1010/4-1

Royal Society grant nos. UF130329 & URF\R\191006

Faraday Institution (faraday.ac.uk) (EP/S003053/1), grant no. FIRG003

History

Email Address of Submitting Author

wzeier@uni-muenster.de

Institution

University of Muenster

Country

Germany

ORCID For Submitting Author

0000-0001- 7749-5089

Declaration of Conflict of Interest

none

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