Halide solid electrolytes (SEs) have garnered significant attention due to their decent ionic conductivity and exceptional high-voltage stability. However, their structure–property relationship remains enigmatic. In this work, we synthesize a range of Li_3−xLu_1−xZr_xCl₆ (LLZC, 0 < x < 1) solid solutions to investigate the influence of lithium ion and vacant site contents on ionic conductivity. Contrary to our previous understanding, it is found that achieving a balance in lithium ion and vacant site content, rather than aliovalent substitution-induced structural changes that introduce stacking faults, plays a crucial role in optimizing ion transport in a hexagonal close packing (hcp) anion framework. This balance culminates in the highest ionic conductivity (1.5 mS cm⁻¹) and lowest activation energy (0.285 eV) of LLZC. In addition, aliovalent substitution strengthens the oxidative stability of halide SSEs but reduces their reduction stability. Using LLZC as SEs and LiMn₂O₄ as cathodes, cobalt-free all-solid-state batteries undergo 1000 cycles of stable cycling at 0.3 C with negligible capacity decay.