Lithium hydroxide halide antiperovskite Li-ion conductors are ideal model systems for the systematic investigation of the effect of grain, grain boundary and interfacial resistance on the total Li-ion conductivity in solid-state batteries. Their low melting point (<300°C) empowers the use of melting and solidification to prepare pellets with high relative density without additional sintering steps and with control over grain size. The tunability of the halogen anion site enables control over grain conductivity and interfacial chemistry, with minimal structural perturbation. In this study, we conduct a comprehensive investigation of Li-ion conduction in Li2OHCl(1-x)Brx antiperovskites. We identify Li2OHCl0.9Br0.1 as the composition with the highest Li-ion conductivity of 2.52 E-3 mS/cm at room temperature. We highlight how the thermal expansion coefficient can serve as an indicator for the presence of structural defects hard to probe directly with X-ray techniques and essential in improving bulk Li-ion conduction. The detrimental effect of grain boundaries on ionic conductivity is demonstrated by atomistic calculations and validated experimentally by electrochemical impedance spectroscopy on pellets with controlled grain size. In-situ X-ray photoelectron spectroscopy experiments of Li2OHCl0.9Br0.1 demonstrate its chemical stability in contact with metallic lithium at room temperature. These insights provide design principles to improve Li-ion conductivity of lithium hydroxide halide antiperovskites.