Two-Dimensional Substitution: Toward a Better Understanding of the Structure–Transport Correlations in the Li-Superionic Thio-LISICONs

A deeper
understanding of the relationships among composition–structure–transport
properties in inorganic solid ionic conductors is of paramount importance
to develop highly conductive phases for future employment in solid-state
Li-ion battery applications. To shed light on the mechanisms that
regulate these relationships, in this work, we perform a “two-dimensional”
substitution series in the thio-LISICON family Li4Ge1–xSnxS4–ySey.
The structural modifications brought up by the elemental substitutions
were investigated via Rietveld refinements against high-resolution
neutron diffraction data that allowed a precise characterization of
the anionic framework and lithium substructure. The analyses show
that the anionic and cationic substitutions influence the polyhedral
and unit cell volumes in different fashions and that the size of the
polyanionic groups alone is not enough to describe lattice expansion
in these materials. Moreover, we show that the lithium disorder that
is crucial to achieve fast ionic mobility may be correlated to the
lithium polyhedral volumes. The correlation of these structural modifications
with the transport properties, investigated via electrochemical impedance
spectroscopy and 7Li nuclear magnetic resonance spin-lattice
relaxation measurements, shows a nonmonotonic behavior of the ionic
conductivity and activation energy against the lithium polyhedral
volumes, hinting to an optimal size of the conduction pathways for
the ionic diffusion. Ultimately, the results obtained in this work
will help to establish new guidelines for the optimization of solid
electrolytes and gain a more profound understanding of the influence
of the substituents on the structure and transport properties of Li-ion
conductors.