In this work, a new type of addition reaction between La(BH4)3 and LiX, X = Cl, Br, I, is used to synthesize LiLa(BH4)3Cl and two new compounds LiLa(BH4)3X, X = Br, I. This method increases the amounts of LiLa(BH4)3X and the sample purity. The highest Li-ion conductivity is observed for LiLa(BH4)3Br, 7.74 × 10−5 S/cm at room temperature (RT) and 1.8 × 10−3 S/cm at 140 °C with an activation energy of 0.272 eV. Topological analysis suggests a new lithium ion conduction pathway with two new different types of bottleneck windows. The sizes of these windows reveal an opposite size change with increasing lattice parameter, that is, increasing size of the halide ion in the structure. Thus, we conclude that the sizes of both windows are important for the lithium ion conduction in LiLa(BH4)3X compounds. The lithium ion conductivity is measured over one to three heating cycles and with different contacts (gold or carbon) between the electrodes and the electrolyte. Moreover, 11B MAS NMR is used to verify the contents of the samples, whereas thermogravimetric analysis shows 4.8 and 3.6 wt % of hydrogen release for LiLa(BH4)3Cl and LiLa(BH4)3Br in the temperature range RT to 400 °C.