Eutectic melting of LiBH₄–KBH₄

Abstract

Eutectic melting in mixtures of alkali and alkali earth metal borohydrides can pave the way for new applications as fast ionic conductors, and facilitate hydrogen release by low temperature chemical reactions and convenient nanoconfinement. Here, we determine the eutectic composition for the lithium potassium borohydride system, 0.725LiBH4-0.275KBH4, with the lowest melting point, Tmelt ∼105 °C, of all known alkali and alkali earth metal borohydride mixtures. Mechanochemistry and manual mixing of LiBH4-KBH4 mixtures facilitate the formation of LiK(BH4)2. However, the melting or heat treatments used in this work do not produce LiK(BH4)2. The bimetallic borohydride dissociates into the monometallic borohydrides at ∼95 °C and partial melting occurs at ∼105 °C. Analysis of the unit cell volumes of LiBH4, KBH4 and LiK(BH4)2 in the temperature range 25 to 90 °C indicates that the formation of the bimetallic borohydride is facilitated by a more dense packing as compared to the reactants. Thus, LiK(BH4)2 is considered metastable and the formation is pressure induced. A phase diagram for the LiBH4-KBH4 system is established, which illustrates the low eutectic melting point and the stability range for the bimetallic borohydride, LiK(BH4)2.

Reactive Hydride Composite of Mg2NiH4 with Borohydrides Eutectic Mixtures

“…However, in the MgNiLiK system (Figure 3a, BM), the milling causes the formation of the LiK(BH4)2 bimetallic phase, in agreement with the literature [32]. In the MgNiLiMg system (Figure 5a, BM), LiBH4 and Mg(BH4)2 are hardly visible after milling.…”

“…As for previous studied mixtures [27][28][29][30][31], a hydrogen pressure of 100 bar is expected to fully rehydrogenate the systems. The nanoconfinement of the studied mixtures into a nanoporous scaffold to obtain nanostructured materials [13,15,19,20,[22][23][24][25]31,32] might be explored to improve the thermodynamics and kinetics of hydrogen sorption reactions and cyclability.…”

Complex and liquid hydrides for energy storage

“…200°C in a relatively wide composition range 0.6 \ x \ 0.8 [71]. 4 all displayed melting behaviour below that of the monometallic phases (up to 167°C lower) [70][71][72][73][74]. Generally, each system can behave differently with respect to their physical behaviours upon melting.…”

“…The molten phases can exhibit a range of physical changes including colour changes, bubbling and in some cases frothing, or even liquid-solid phase transitions during hydrogen release [70]. The mixture with the lowest melting point is 0.725LiBH 4 -0.275KBH 4 , T m = 105°C [74].…”

Melting Behavior and Thermolysis of NaBH4−Mg(BH4)2 and NaBH4−Ca(BH4)2 Composites

“…0.62LiBH4-0.38NaBH4 melts at Tm ~ 220 °C, as compared to pristine LiBH4 at Tm = 280 °C and NaBH4 at Tm > 500 °C, and produces a uniform clear molten phase [28,29]. 0.725LiBH4-0.275KBH4 has the lowest eutectic melting temperature at Tm = 105 °C (KBH4, Tm > 600 °C) [30]. The system xLiBH4−(1 − x)Mg(BH4), x = 0.5 to 0.6, melts at Tm ~ 180 °C (Mg(BH4)2, Tm > 280 °C) and shows improved thermodynamics and kinetics, as decomposition proceeds immediately after melting and releases 7 wt% H2 already at T = 270 °C [11,31].…”