Dominant Li+ ion conduction in Li+/Na+ mixed crystal, 15NaI∙LiBH4

In the present work, the conduction ions in 15NaI∙LiBH4 are investigated by secondary ion mass spectroscopy (SIMS) and galvanostatic measurements. An all-solid-state half-cell is constructed using 15NaI∙LiBH4 and a Sn film as the solid electrolyte and working electrode, respectively. SIMS depth profiles in a charged Sn film indicate that 7Li+ is the dominant charging ion, while the intensity of 23Na+ ions is one order of magnitude less than that of 7Li+. These results clearly indicate that the dominant conduction ion in 15NaI∙LiBH4 is Li+ despite its low concentration (6 mol%). The possibility of Na+ conduction is further investigated by charging a Na/Sn cell using a three-layered electrolyte with 15NaI∙LiBH4 sandwiched by a known Na+ conductor, 75Na2S∙25P2S5. The charging of Sn is interrupted for the three-layered electrolyte, while Sn can be smoothly charged using 75Na2S∙25P2S5 as the single electrolyte. These results confirm that the conduction of Na+ in 15NaI∙LiBH4 is not as significant as that of the guest Li+ ions and strongly suggest that the Li+ ions are the dominant conduction species. The results show the potential of Na compounds as the base materials in novel solid electrolytes for all-solid-state lithium batteries.