Atomic-Scale Origin of the Grain Boundary Capacitance in Polycrystalline Solid Electrolytes: Transient Nanoscale Ionic Waves

Solid electrolytes, also known as fast ion or superionic conductors, exhibit exceptionally high ionic conductivities and are critical to the success of all-solid-state fuel cells and batteries. The electrical response of these polycrystalline materials is commonly modeled using an RC-circuit. A thorough physical understanding of the circuit parameters based on the material structure, grain boundaries (GBs), point defects, and dopant distribution has not been fully achieved yet. To address this gap, we probe the capacitor-like behavior using voltage-controlled molecular dynamics simulations of ion transport in single GB defect. These simulations reveal that upon applying an electric field, dynamic redistribution of charge manifests at the atomistic scale as spatiotemporal ionic waves which traverse the grain interiors and the GB defects. Using linear response theory, we show that this spatially nonlocal, collective, emergent phenomenon determines the capacitor characteristics. These findings advance our fundamental understanding of ionic conduction processes in solid electrolytes.