The Li ⁺ conductivity of Li₇La₃Zr₂O₁₂ (LLZO) solid electrolytes was optimized by controlling the addition of the dopant (Al) into the crystal structure. A solution-based synthesis method was used to minimize Al segregation and ensure a homogeneous substitution into the garnet framework. Neutron and x-ray diffraction were used to monitor the structural transition from tetragonal to cubic LLZO. It was found that the critical dopant concentration (x_c) required to stabilize the cubic phase was 0.18 mol pfu. The maximum ionic conductivity of 5.54 × 10⁻⁴ S/cm was obtained at the critical composition. The enhanced conductivity was achieved by accurately controlling the vacancy density at the active Li octahedral sites. Neutron structure refinements were used to validate the changes in Li vacancy distribution at different dopant concentrations. Impedance measurements reveal a strong dependence of Li⁺ transport properties on Al concentration, specifically around the critical dopant concentration x_c = 0.18 mol pfu. These findings demonstrate the importance of fine-tuning composition and provide guidance for engineering improvements in ionic conductivity.