A thermodynamic investigation of the LiBH₄–NaBH₄ system

Abstract

The LiBH 4 -NaBH 4 pseudo-binary system has been investigated by X-ray diffraction, temperatureprogrammed photographic analysis, and differential scanning calorimetry, in order to establish the phase diagram. The polymorphic orthorhombic-to-hexagonal phase transition of LiBH 4 was observed at 94 °C in samples containing NaBH 4 , i.e. 15 °C lower than for pure LiBH 4 , which indicates the dissolution of sodium into LiBH 4 . The formation of solid solutions was confirmed by powder X-ray diffraction measurements performed as a function of temperature. A new eutectic composition between Li 0.65 Na 0.35 BH 4 and Li 0.70 Na 0.30 BH 4 , with a melting temperature of 216 °C, is observed. Ab-initio calculations have been performed to establish the relative stabilities of the pure compounds in orthorhombic, hexagonal and cubic structure. The obtained experimental and calculated data were compared with available literature values and they were used for a thermodynamic assessment of the LiBH 4 -NaBH 4 system by the Calphad method. The enthalpy of mixing for solid and liquid solutions has been estimated on the basis of experimental data.The LiBH4-NaBH4 pseudo-binary system has been investigated by X-ray diffraction, temperature programmed photographic analysis and differential scanning calorimetry, in order to establish the phase diagram. Ab-initio calculations have been performed to establish the relative stabilities of the pure compounds in various structures. The obtained experimental and calculated data were compared with available literature values and they were used for a thermodynamic assessment of the LiBH4-NaBH4 system by the Calphad method.

Complex Hydrides for Energy Storage, Conversion, and Utilization

“…There are five ways commonly applied in the compositional modifications to M(BH 4 ) m and M(NH 2 BH 3 ) m : 1) Cation replacement, i.e., changing the thermodynamic properties of the complex hydride by forming monometal, bimetal and even trimetal hydrides; 2) Anion partial substitution by halide ion or [NH] 2− /[NH 2 ] − , etc . Recently, a series of new isostructural mixed‐cations and mixed‐anions borohydride chlorides were synthesized, e.g., LiSm(BH 4 ) 3 Cl, LiYb(BH 4 ) 3 Cl, Ca((BH 4 ) 1− x Cl x ) 2 ; 3) Coordinating metal cation with neutral molecules such as NH 3 , N 2 H 4 and NH 3 BH 3 ; 4) Compositing with reactive hydrides (mainly for M(BH 4 ) m ); 5) Forming eutectic or comelting mixture by combining two or more metal borohydrides to achieve a low melting point to assist mass transfer during hydrogen desorption . Some representative and newly developed M‐B‐H systems involved in the compositional modifications can be found in Table 1 .…”

Reactive Hydride Composite of Mg2NiH4 with Borohydrides Eutectic Mixtures

“…Figure 4 presents the results obtained for the MgNiLiNa system. Upon heating in the 1st cycle (Figure 4b, 1st cycle), we observe the polymorphic transition of LiBH 4 at T peak = 98 • C, stabilized for the presence of NaBH 4 [36] and the melting of the LiNa eutectic at T peak = 228 • C. Both phase transformations are reversible and slightly undercooled on cooling. In the 2nd cycle (Figure 4b, 2nd cycle), after the previous phase transformations, also the polymorphic transition of Mg 2 NiH 4 is observed at T peak = 246 • C, followed by the main hydrogen release from the RHC mixture at T peak = 327 • C under 2.7 bar H 2 .…”

“…In fact, a broad peak of hydrogenation Figure 4 presents the results obtained for the MgNiLiNa system. Upon heating in the 1st cycle (Figure 4b, 1st cycle), we observe the polymorphic transition of LiBH4 at Tpeak = 98 °C, stabilized for the presence of NaBH4 [36] and the melting of the LiNa eutectic at Tpeak = 228 °C. Both phase transformations are reversible and slightly undercooled on cooling.…”

Complex hydrides for energy storage

“…showing the formation of solid solutions, resulting from cationic [28,[170][171][172] or anionic [173][174][175][176][177][178][179][180][181] substitutions, as well as hydrogen-fluorine exchange [182][183][184][185]. For some binary and ternary mixtures of borohydrides, corresponding thermodynamic properties and phase diagrams have been recently assessed, allowing the determination of the enthalpy of mixing in solid and liquid phases [172,185].…”