MEMS/MatSci Seminar: “Dynamics as a Design Principle for Charge Transport in Functional Solid-State Materials”

Dynamic disorder plays a substantial role in dictating the functional properties of solid-state materials. This talk will highlight several studies that are distinct in their related applications yet are united by the common themes of fundamental structure-dynamics-property relationships and leveraging dynamic disorder as a design principle for the next-generation functional solid-state materials. Part I: Anharmonicity and Octahedral Tilting in Hybrid Perovskite Semiconductors In contrast to the rigid covalent structures of conventional "diamond-like" semiconductors (e.g., Si, CdTe, GaAs), hybrid perovskite halides exhibit a complex dynamic landscape owing to the soft and deformable metal-halide octahedral sublattice coupled with the presence of dynamic organic cations such as methylammonium (MA+) or formamidinium (FA+). Part II: Lowering the Activation Barriers for Lithium-Ion Conductivity through Orientational Disorder in the Cyanide Argyrodite Li6PS5CN All-solid-state batteries hold the potential to transform electrochemical energy storage technologies. Replacing the flammable liquid electrolyte with a solid-state ion conductor can improve battery safety and may further increase battery energy density when paired with lithium metal anodes. The halide argyrodites Li6PS5X (X = halide, pseudohalide) are a promising family of candidate solid electrolytes, as they can achieve ionic conductivities that are nearly competitive with liquid electrolytes. These materials are an excellent framework to understand structure-property relationships that promote high ionic conductivity in the solid state. In this work, we have discovered the new solid electrolyte Li6PS5CN in which the halide site is occupied by the orientationally-disordered cyanide ion. The new cyanide argyrodite exhibits lower activation barriers for Li-ion conductivity compared to the current champion argyrodite Li6PS5B and comparable room temperature lithium-ion conductivities. Structurally, the similar sizes of cyanide and bromide ions produce nearly identical lithium conduction pathways. We attribute the lower activation barriers to orientational disorder of the quadrupolar cyanide ions within the structure that softens the energetic landscape surrounding the lithium ions.