Understanding the complex structural and chemical factors that influence ionic conduction mechanisms is paramount for developing advanced inorganic superionic conductors in all-solid-state batteries, particularly halide solid electrolytes with excellent electrochemical oxidative stability and mechanical sinterability. Herein, contrasting ionic conduction behaviors in I^- and Br^- substituted Li₂ZrCl₆ are revealed by combining experimental structural analyses and theoretical calculations. The inter-slab distance along the c-axis, which varies with the anion substitution and M2-M3 site disorder, is a key factor for opening the ab-plane conduction and facilitating the overall Li⁺ conduction. Increased M3 site occupancy generally leads to contracted inter-slab distance. The substantial increase in Li⁺ conductivity upon I substitution (from 0.40 to 0.91 mS cm^-1) originates from a sufficiently expanded lattice volume owing to its large ionic radii (I^- = 2.20 Å), particularly inter-slab distance that facilitates the ab intra-plane Li⁺ conduction, which also benefits from decreased M2-M3 disorder. In contrast, Br (Br^- = 1.96 Å) substitution results in insufficiently expanded Li⁺ channels, which, exacerbated by increased M2-M3 disorder, leads to degradation in Li⁺ conductivity. Implementing I^- substituted Li₂ZrCl₆ resulted in superior electrochemical performance in LiCoO₂||Li-In cells compared to those with an unsubstituted catholyte.