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Evidence for a Solid-Electrolyte Inductive Effect in Superionic Conductors

preprint
submitted on 09.10.2020 and posted on 12.10.2020 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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