In this work, Li₃AlH₆ powder was synthesized in-situ by ball milling LiH and LiAlH₄. Approximately 15 g mixed powder of LiH and LiAlH₄ was milled each time with a molar ratio of 2:1, a ball to powder ratio of 25:1, and a milling speed of 300 rpm. To suppress temperature rise in the ball milling, every 30 min was set as an operation cycle, which contained a stop time of 6 min, periodically. After ball milling for 20 h, XRD patterns were obtained and examined to characterize the mixed powder. Figure 1 shows the XRD results of pure LiH powder, LiAlH₄ powder, and the LiH and LiAlH₄ powder after ball milling for 20 h. No impurity phases were observed in any of the samples, which indicates that the samples were highly pure and did not oxidize. As shown in Figure 1C, a few Li₃AlH₆ diffraction peaks are present, while the rest are of LiAlH₄ and LiH. This shows that the synthesis reaction was not completed. Due to long milling time, LiH peaks were much lower than before. The LiH content was lower, and an amorphous structure was formed.

Further ball milling was carried out since the synthesis reaction to Li₃AlH₆ was not completed. The milling samples were taken for XRD analysis after every 10 h. Figure 2 shows the XRD results of the milled powders after 20, 30, 40, and 50 h. Strong double peaks at 22° and 23° are detected, which are the characteristic peaks of Li₃AlH₆. After ball milling for 30 h, most of the diffraction peaks are Li₃AlH₆; however, some LiAlH₄ and LiH peaks can still be found at 27°, 43°, and 46°. As the ball milling time increased, the LiH and LiAlH₄ content decreased, which shows that the synthesis reaction proceeds as the milling time increases. Nevertheless, the diffraction intensity of Li₃AlH₆ decreased, which shows that the longer milling time, the more likely the production of an amorphous phase becomes.

FTIR analysis was used to investigate the synthesized powder. In pure LiAlH₄ (Figure 3A), the bending modes around 1,780 and 1,610 cm⁻¹ correspond to the A-H stretching band, as was previously shown by Chen et al. (2001). After 50 h ball milling, this A-H stretching band disappeared in the 2LilH-LiAlH₄ mixtures (Figure 3B). A new A-H stretching band was generated around 1,380 cm⁻¹, which belongs to Li₃AlH₆. This indicates the completion of the synthesis reaction.

The 2LiBH₄ + M (M = Al, LiAlH₄, Li₃AlH₆) composite systems were prepared by ball milling, respectively. The ball-milling time was 1 h, with a mole ratio of 2:1. The as-milled mixtures are presented in Figure 4. In the 2LiBH₄-Al composite, the narrow and sharp diffraction peaks are of Al, while the other diffraction peaks are of LiBH₄. The intensity of the LiAlH₄ peaks in the 2LiBH₄-LiAlH₄ composite are also strong, but both LiBH₄ and Li₃AlH₆ have attenuated diffraction peaks in 2LiBH₄-Li₃AlH₆. The absence of other peaks indicated that there were no side reactions that could generate impurities or by-products during ball milling.

The dehydrogenation performances of the samples were investigated. The four samples were heated to 550°C at a heating rate of 5°C/min in an argon atmosphere. The DSC/TG results are shown in Figure 5. In the pure LiBH₄ sample (Figure 5A), the endothermic peak at 120°C denotes that LiBH₄ transformed from an orthorhombic to a hexagonal crystal system. The endothermic peak at 286°C is attributed to the melting of LiBH₄. According to the TG curve, LiBH₄ began to release hydrogen and lose weight at 400°C. There is a dehydrogenation peak at around 469°C, and the rate of dehydrogenation slowed down after that. Both the transformation and the melting peak of LiBH₄ for each composite sample are shown in Figures 5B–D. All samples have lower transformation and melting temperatures than pure LiBH₄ (transformation temperature: T_d < T_c < T_b < T_a; melting temperature: T_c < T_d < T_b < T_a). In the as-milled 2LiBH₄-Al sample (Figure 5B), the wide endothermic range around 445°C represents the decomposition of LiBH₄. The last peak at 529°C is the decomposition of LiH. As shown in Figure 5C, peaks at 190 and 198°C indicated the dehydrogenation of LiAlH₄, which were verified in our previous study (Li et al., 2012). LiAlH₄ decomposed into Li₃AlH₆ at 190°C, and Li₃AlH₆ started to generate LiH, Al, and release H₂ at 198°C. LiBH₄ and LiH started to decompose at 435 and 472°C, respectively. Figure 5D shows that the decomposition peak of Li₃AlH₆ is at 198°C, which is consistent with the 2LiBH₄-LiAlH₄ sample. The decomposition peak of LiBH₄ (416°C) is lower than that of the 2LiBH₄-LiAlH₄ (435°C) or 2LiBH₄-Al samples (445°C). In addition, the decomposition peak of LiH, which is around 452°C, is also lower than that of the other composite systems. These results showed that the LiBH₄ became more unstable due to the addition of Li₃AlH₆ and that hydrogen was released at a lower temperature. The main reason for this is that the Al present in Li₃AlH₆ was active enough to stimulate the dehydrogenation reaction.

The temperature-programmed desorption (TPD) method was implemented to study the dehydrogenation performance of pure LiBH₄, 2LiBH₄-Al, 2LiBH₄-LiAlH₄, and 2LiBH₄-Li₃AlH₆; the results are shown in Figure 6. The samples were heated to 400°C and held for 5 h. The pure LiBH₄ began to release H₂ at around 400°C. Its desorption rate slowed down when 5 wt.% of H₂ had been released. The 2LiBH₄-Li₃AlH₆ sample released 9.1 wt.% H₂ within 150 min, which is the fastest reaction kinetics in this work. Its decomposition process includes two main steps, with the release of 3 and 6.1 wt.% H₂ as the first and second steps, respectively. In the 2LiBH₄-LiAlH₄ sample, LiAlH₄ began to decompose at ~130°C and released 3.9 wt.% H₂. Subsequently, its desorption rate was retarded. This composite released a total of 8 wt.% H₂. The 2LiBH₄-Al composite had a slower desorption rate. Only 5.7 wt.% H₂ was released after 6 h, which is much lower than the 8.6 wt.% predicted by theoretical capacity. Previous research (Friedrichs et al., 2009) has reported that Al is an effective catalyst to activate LiBH₄, and this phenomenon can also be observed in this study. However, the Al element generated from 2LiBH₄-Li₃AlH₆ is much more effective than for the other samples. Since aluminum can be easily oxidized, an oxide film can easily form on the surface of the Al powder, which can slow down the dehydrogenation kinetics. In contrast, the Al in 2LiBH₄-Li₃AlH₆, decomposed from Li₃AlH₆ exhibits high purity and is non-oxidized, resulting in the superior reaction kinetics of LiBH₄.

The XRD patterns of the dehydrogenated samples are shown in Figure 7. In Figure 7A, prominent diffraction peaks of LiH and Al are present. However, the LiBH₄ phase was not observed, as its diffraction intensity is relatively weaker. In Figure 7B, residual peaks of LiBH₄ are observed, which demonstrates incomplete dehydrogenation. In Figure 7C, besides LiH and Al, the dehydrogenation products contain AlB₂, which was reported as the reversible phase (Friedrichs et al., 2009). AlB₂ can accelerate the formation of LiBH₄ in the reverse reaction. The AlB₂ content was higher in 2LiBH₄-Li₃AlH₆ than in the other samples, which signifies higher reversibility of 2LiBH₄-Li₃AlH₆. There is an unexpected peak near 50° both in the 2LiBH₄-LiAlH₄ and 2LiBH₄-Li₃AlH₆ samples, which was not identified but was also detected in previous reports (Yang et al., 2007).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.