Hogrefe K, Minafra N, Hanghofer I, Banik A, Zeier WG, Wilkening HMR. Opening Diffusion Pathways through Site Disorder: The Interplay of Local Structure and Ion Dynamics in the Solid Electrolyte Li6+xP1–xGexS5I as Probed by Neutron Diffraction and NMR.
J Am Chem Soc 2022;
144:1795-1812. [PMID:
35057616 PMCID:
PMC8815078 DOI:
10.1021/jacs.1c11571]
[Citation(s) in RCA: 14] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
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Solid electrolytes
are at the heart of future energy storage systems.
Li-bearing argyrodites are frontrunners in terms of Li+ ion conductivity. Although many studies have investigated the effect
of elemental substitution on ionic conductivity, we still do not fully
understand the various origins leading to improved ion dynamics. Here,
Li6+xP1–xGexS5I served as an
application-oriented model system to study the effect of cation substitution
(P5+ vs Ge4+) on Li+ ion dynamics.
While Li6PS5I is a rather poor ionic conductor
(10–6 S cm–1, 298 K), the Ge-containing
samples show specific conductivities on the order of 10–2 S cm–1 (330 K). Replacing P5+ with
Ge4+ not only causes S2–/I– anion site disorder but also reveals via neutron diffraction that
the Li+ ions do occupy several originally empty sites between
the Li rich cages in the argyrodite framework. Here, we used 7Li and 31P NMR to show that this Li+ site disorder has a tremendous effect on both local ion dynamics
and long-range Li+ transport. For the Ge-rich samples,
NMR revealed several new Li+ exchange processes, which
are to be characterized by rather low activation barriers (0.1–0.3
eV). Consequently, in samples with high Ge-contents, the Li+ ions have access to an interconnected network of pathways allowing
for rapid exchange processes between the Li cages. By (i) relating
the changes of the crystal structure and (ii) measuring the dynamic
features as a function of length scale, we were able to rationalize
the microscopic origins of fast, long-range ion transport in this
class of electrolytes.
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