Studies on Li3AlF6 thin film deposition utilizing conversion reactions of thin films

The ternary lithium aluminum fluoride Li3AlF6 is formed from two optically interesting fluorides, LiF and AlF3. It has been reported to have a large electronic bandgap with a reasonable lithium-ion conductivity at room temperature, making it a potential electrolyte material for solid state lithium-ion batteries. Because of complications during attempts at direct atomic layer deposition of Li3AlF6, we have studied the deposition of the material using two conversion processes. In Process 1, a conversion reaction takes place when Al(thd) 3 (thd=2,2,6,6-tetramethyl3,5-heptanedionate) and TiF4 are sequentially pulsed onto LiF films. The Li3AlF6 films contained LiF as an impurity phase, as determined with grazing incidence X-ray diffraction (GIXRD), and a large amount of titanium impurity, as determined with time-of-flight elastic recoil detection analysis (ToF-ERDA). In Process 2, AlF3 films are exposed to Lithd vapor, resulting in a conversion reaction that produced Li3AlF6 with some LiF. These films have also been studied with GIXRD and ToF-ERDA, and contained much smaller amounts of titanium and other impurities. The Li:Al metal ratios vary depending on the extent of Lithd exposure. Field emission scanning electron microscopy (FESEM) revealed that the Li3AlF6 films are quite porous. (C) 2017 Elsevier B.V. All rights reserved.