Heterogeneity in\nPoint Defect Distribution and Mobility\nin Solid Ion Conductors

Alkali metal anodes paired with solid ion conductors\noffer promising\navenues for enhancing battery energy density and safety. To facilitate\nrapid ion transport crucial for fast charging and discharging of batteries,\nit is essential to understand the behavior of point defects in these\nconductors. In this study, we investigate the heterogeneity of defect\ndistribution in two prototypical solid ion conductors, Li<sub>3</sub>OCl and Li<sub>2</sub>PO<sub>2</sub>N (LiPON), by quantifying the\ndefect formation energy (DFE) as a function of distance from the surface\nand interface through first-principles simulations. To simulate defects\nat the electrode–electrolyte interface, we perform calculations\nof Li<sup>+</sup> vacancy in Li<sub>3</sub>OCl near its interface\nwith lithium metal. Our results reveal a significant difference between\nthe bulk and surface/interface DFE which could lead to defect aggregation/depletion\nnear the surface/interface. Interestingly, while Li<sub>3</sub>OCl\nhas a lower surface DFE than the bulk in most cases, LiPON follows\nthe opposite trend with a higher surface DFE compared to the bulk.\nDue to this difference between bulk and surface DFE, the defect density\ncan be up to 14 orders of magnitude higher at surfaces compared to\nthe bulk. Further, we reveal that the DFE transition from surface/interface\nto bulk is precisely characterized by an exponentially decaying function.\nBy incorporating this exponential trend, we develop a revised model\nfor the average behavior of defects in solid ion conductors that offers\na more accurate description of the influence of grain sizes. Surface\neffects dominate for grain sizes ≲1 μm, highlighting\nthe importance of surface defect engineering and the DFE function\nfor accurately capturing ion transport in devices. We further explore\nthe kinetics of defect redistribution by calculating the migration\nbarriers for defect movement between bulk and surfaces. We find a\nhighly asymmetric energy landscape for the lithium vacancies, exhibiting\nlower migration barriers for movement toward the surface compared\nto the bulk, while interstitial defects exhibit comparable kinetics\nbetween surface and bulk regions. These insights highlight the importance\nof considering both thermodynamic and kinetic factors in designing\nsolid ion conductors for improved ion transport at surfaces and interfaces.