The LiBH4-LiI solid solution is a good Li+ conductor and a promising crystalline electrolyte for all-solid-state lithium based batteries. The focus of the present work is on the effect of heat treatment on the Li+ conduction. Solid solutions with a LiI content of 6.25-50% were synthesized by high-energy ball milling and annealed at 140 °C. Powder X-ray diffraction and scanning electron microscopy were used for characterizing the samples and for comparing their crystallite sizes and the density of defects before and after the annealing. The Li+ conductivity was measured using impedance spectroscopy, resulting in conductivities exceeding 0.1 mS/cm at 30 °C and 10 mS/cm at 140 °C. It was found that the formation of defect-rich microstructures during ball milling increased the specific conductivities of these compounds significantly. The phase transition temperatures between the orthorhombic and hexagonal structures of LiBH 4 were measured using differential scanning calorimetry (DSC). The measured transition temperatures range from 100 to -70 °C and show a linear decrease of 70 °C for every 10% of LiI addition up to a LiI content of 25%. The relative stability of the two structures was calculated using density functional theory, and together with the DSC measurements, the calculations were used to evaluate the change in entropic difference between the structures with LiI content.