  Alkali metal anodes paired with solid ion conductors offer promising avenues
for enhancing battery energy density and safety. To facilitate rapid ion
transport crucial for fast charging and discharging, it is essential to
understand point defects within these conductors. In this study, we investigate
the heterogeneity of defect distribution in Li$_3$OCl solid ion conductor,
quantifying the defect formation energy (DFE) of lithium vacancies and
interstitials as a function of distance from the surface through
first-principles simulations. Our results reveal that the surface DFE is
consistently lower than bulk except for one surface termination, indicating
significant defect aggregation at surfaces. This difference can cause the
defect density to be up to 14 orders of magnitude higher at surfaces compared
to the bulk. Moreover, we unveil the transition in DFE when moving from the
surface to the bulk through the DFE function, which exhibits an exponentially
decaying relationship. Incorporating this exponential trend, we develop a
revised model for the average behavior of defects that offers a more accurate
description of the influence of grain size. Surface effects dominate for grain
sizes $\lesssim$ 1 $\mu$m, highlighting the importance of surface defect
engineering and the DFE function for accurately capturing ion transport in
devices. We further explore the kinetics of defect redistribution by
calculating the migration barriers for defect movement between bulk and
surfaces. We find a highly asymmetric energy landscape for the lithium
vacancies, exhibiting lower migration barriers for movement towards the surface
compared to the bulk, while interstitial defects exhibit comparable kinetics
between surface and bulk regions. These insights underscore the importance of
considering both thermodynamic and kinetic factors in the design of solid ion
conductors.
