The major conductive ions in 15NaI LiBH4 were identified by the following three experiments. First, the ionic conductivity of 15NaI.LiBH4 was two orders of magnitude larger than that of 15NaI.NaB4 ("Li-free" solid solvent) and their activation energies were clearly different from one another, indicating that the small Li+ ions introduced as a dopant play a major role for the ion conduction. Second, the variations of the cation ratio in 15NaI.LiBH4 were investigated before and after conducting DC current through the all-solid-state cell composed of Li | 15NaI.LiBH4 | In. The lattice parameter of the solid electrolyte was not changed by the DC current flow, suggesting that the substitution between host Na+ ions in 15NaI.LiBH4 and electrochemically doped Li+ ions rarely occurs. This gives evidence that the observed steady state current is mainly determined by the electrochemical reactions of Li -> Li+ + e(-) and Li+ + e(-) -> Li coupled with Li+ ion conduction in 15NaI LiBH4. Finally, Li was mainly detected inside the charged a-Si film by secondary ion mass spectrometry (SIMS), which also supports the dominant Li+ ion conduction in 15NaI LiBH4. Based on the above experiments, the mechanism of ion conduction and the conductive species in "Li-free" solid solvent of 15NaI.NaBH4 with Li+ doping has been revealed. (c) 2018 Elsevier Ltd All rights reserved.