We report the lithium ionic conductivities of closo‑type complex hydrides synthesized from various molar ratios of lithium borohydride (LiBH4) and decaborane (B10H14) as starting materials. The prepared closo‑type complex hydrides comprised [B12H12]2−, [B11H11]2−, and [B10H10]2− complex anions. In addition, increasing the LiBH4 content in the starting materials increased the amounts of [B11H11]2− and [B10H10]2−, leading to an improved ion conductivity of the prepared sample. The present study offers useful insights into strategies for controlling the complex anion composition in emerging solid electrolytes of closo-type complex hydrides at the molecular level, and improving their ionic conductivities.