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 (Althd)₃ (thd = 2,2,6,6-tetramethyl-3,5-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.