Heterogeneity in Point Defect Distribution and Mobility in Solid Ion Conductors

Solid-state ion conductors paired with alkali metal anodes offer a promising approach for improving the energy density and safety of future battery technologies. Understanding point defect behavior in these materials is critical to achieving the rapid ion transport needed for efficient battery charge and discharge. This study explores the distribution and formation energy of defects in two key solid-state ion conductors, Li₃OCl and Li₂PO₂N (LiPON), using first-principles simulations [1]. We examine how the defect formation energy (DFE) changes with distance from the surface and interfaces. Specifically, we model the Li⁺ vacancy in Li₃OCl near its interface with lithium metal. Our findings show marked differences in DFE between the bulk and surface/interface regions, suggesting a tendency for defects to either aggregate or deplete near these regions. Notably, Li₃OCl exhibits a lower surface DFE compared to its bulk, whereas LiPON displays a contrasting behavior, with a higher surface DFE. The counterintuitive behavior of defects in LiPON could offer a potential explanation for dendrite suppression in lithium metal batteries. As a result of DFE variation, defect density can differ by up to 14 orders of magnitude between the bulk and surface. We further characterize this variation by fitting it to an exponentially decaying function, offering an enhanced model that captures defect behavior across different grain sizes. This surface dominance is particularly significant for grains smaller than 1 μm, underlining the importance of surface defect manipulation in optimizing ion transport. Additionally, we investigate the kinetics of defect migration, discovering that lithium vacancies move more easily toward surfaces, while interstitials show similar mobility in both regions. These insights emphasize the need to consider both thermodynamic and kinetic factors in designing solid ion conductors with enhanced surface ion transport properties.

1. Limon, M. S. R.; Ahmad, Z. Heterogeneity in Point Defect Distribution and Mobility in Solid Ion Conductors. ACS Applied Materials & Interfaces 2024, 16 (38), 50948–50960. https://doi.org/10.1021/acsami.4c12128.