Two-Dimensional Substitution: Towards 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. In order 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 Li₄Ge_1‒xSn_xS_4‒ySe_y. 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 the 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 non-monotonic 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 to gain a more profound understanding of the influence of the substituents on the structure and transport properties of Li‒ion conductors