Dehydrogenation of Alkali Metal Aluminum Hydrides MAlH₄ (M = Li, Na, K, and Cs): Insight from First-Principles Calculations

Abstract

Complex aluminum hydrides with high hydrogen capacity are among the most promising solid-state hydrogen storage materials. The present study determines the thermal stability, hydrogen dissociation energy, and electronic structures of alkali metal aluminum hydrides, MAlH₄ (M = Li, Na, K, and Cs), using first-principles density functional theory calculations in an attempt to gain insight into the dehydrogenation mechanism of these hydrides. The results show that the hydrogen dissociation energy (E_d-H₂) of MAlH₄ (M = Li, Na, K, and Cs) correlates with the Pauling electronegativity of cation M (χ_P); that is, the E_d-H₂ (average value) decreases, i.e., 1.211 eV (LiAlH₄) < 1.281 eV (NaAlH₄) < 1.291 eV (KAlH₄) < 1.361 eV (CsAlH₄), with the increasing χ_P value, i.e., 0.98 (Li) > 0.93 (Na) > 0.82 (K) > 0.79 (Cs). The main reason for this finding is that alkali alanate MAlH₄ at higher cation electronegativity is thermally less stable and held by weaker Al-H covalent and H-H ionic interactions. Our work contributes to the design of alkali metal aluminum hydrides with a favorable dehydrogenation, which is useful for on-board hydrogen storage.

1. Introduction

The rapidly diminishing supply of fossil fuel and increasing environmental awareness have precipitated a growing demand for clean, safe, and renewable energy. Hydrogen is believed to be an ideal clean energy carrier due to its abundance, high energy density (142 MJ/kg), and clean combustion [1]. However, hydrogen is a highly flammable, explosive, and diffusible gas at room temperature and pressure. Thus, it is very important to store hydrogen safely and effectively, yet this task remains a major challenge in hydrogen utilization [2]. Currently, hydrogen storage approaches involve the storage of: (1) compressed hydrogen in high-pressure containers, (2) liquid hydrogen in cryogenic tanks, and (3) hydrogen in solid-state materials via physisorption/chemisorption. Among these approaches, solid-state hydrogen storage provides high hydrogen capacity, moderate operating pressures and temperatures, and favorable safety, and is a promising storage solution [3,4,5].

Complex metal hydrides, such as alanates, borohydrides, and amides, are considered good solid-state hydrogen storage candidates for on-board applications due to the high gravimetric and volumetric hydrogen densities required in this regard [3,4]. These complex hydrides, unfortunately, are often thermodynamically very stable and dehydrogenate at extremely high temperatures, thereby restricting their practical applications. To overcome these drawbacks, many investigations have been devoted to tuning the thermodynamic properties of complex metal hydrides [6,7,8,9,10].

Nakamori et al. [11] systematically investigated the thermodynamical stabilities of a series of metal borohydrides, M(BH₄)_n (M = Li, Na, K, Cu, Mg, Zn, Sc, Zr, and Hf; n = 1–4), via first-principles calculations combined with an experimental study. They reported that the stability of M(BH₄)_n is related to the ionic interaction between M and the [BH₄] complex, the charge transfer from the cation Mⁿ⁺ to the anion [BH₄]⁻ (Mⁿ⁺→[BH₄]⁻), and the Pauling electronegativity of the cation M. In particular, a gradual increase in cation Pauling electronegativity χ_P is accompanied by a linear decrease in the dehydrogenation temperature (T_d). Their works on other borohydrides, M(BH₄)_n (M = Ca, Sc, Ti, V, Cr, Mn, Zn, and Al; n = 2–4), also revealed a linear relation between χ_P and T_d [12]. These works by Nakamori and co-workers contribute to the design of metal borohydrides with appropriate stability for favorable dehydrogenation through the combination of M(BH₄)_n with more electronegative metals or metal compounds, which is useful for hydrogen storage applications [6,13,14,15,16,17,18].

Recently, in the review by Weidenthaler, the decomposition temperature (T_d) of alkali metal aluminum hydrides (alkali alanates) of the MAlH₄ variety (M = Li, Na, K, and Cs) is reported to decrease linearly with the Pauling electronegativity, χ_P, of alkali cations [19]. As an example from this review, the decrease in the starting decomposition temperature (the first step of dehydrogenation reaction), specifically, from 443 K (LiAlH₄) [20] < 503 K (NaAlH₄) [21] < 573 K (KAlH₄) [20] < 600 K (CsAlH₄) [19], is accompanied by an increase in cation Pauling electronegativity, specifically, from 0.98 (Li) > 0.93 (Na) > 0.82 (K) > 0.79 (Cs). From these results, the electronegativity of alkali metal cations was identified as dominant with respect to the thermal stability and the decomposition temperature of MAlH₄ (M = Li, Na, K, and Cs) [19]. However, the dehydrogenation mechanism of MAlH₄ associated with cation electronegativity was not provided in the review. Furthermore, to date, there have only been a few reports on this topic. Therefore, in this study, we apply first-principles density functional theory calculations on MAlH₄ (M = Li, Na, K, and Cs) since the calculations can reliably be used to study the micro-mechanisms of hydrogen storage materials [11,22,23]. The formation enthalpy, cohesive energy, Hirshfeld charge, hydrogen dissociation energy, density of states, charge density distribution, and Mulliken population of MAlH₄ (M = Li, Na, K, and Cs) are investigated in detail. We believe that our work can provide new insights into the dehydrogenation of MAlH₄ (M = Li, Na, K, and Cs) and is, therefore, helpful for exploring solid-state alkali aluminum hydrides with favorable H-desorption properties for hydrogen storage applications.

2. Computational Details

The present calculations on the alkali alanates of MAlH₄ (M = Li, Na, K, and Cs) were executed using the density functional theory (DFT) method with on-the-fly generation (OTFG) ultrasoft pseudopotential, as implemented in the Cambridge Serial Total Energy Package (CASTEP) code in Materials Studio 2017 [24]. The exchange-correlation function was used with the generalized gradient approximation (GGA)of Perdew–Burke–Ernzerhof (PBE) [25]. The dependence of total energy on the plane–wave cutoff energy and Monkhorst–Pack k-point mesh were tested carefully. Subsequently, a plane–wave cutoff energy of 900 eV and a k-point grid of 3 × 2 × 2 were adopted, thereby ensuring a convergence accuracy with a total energy difference below 3 meV/atom. Atomic valence electrons, namely, 1s²2s¹ (Li), 2s²2p⁶3s¹ (Na), 3s²3p⁶4s¹ (K), 5s²5p⁶6s¹ (Cs), 3s²3p¹ (Al), and 1s¹ (H), were used for corn electrons. Geometry optimization using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) method [26] allowed the lattice and all atoms to relax, with the convergence tolerance of 1.0 × 10⁻⁵ eV/atom, 0.03 eV/Å, 0.05 GPa, and 0.001 Å for energy, maximum force, maximum stress, and maximum displacement, respectively. The single-point energies and electronic structures of all the considered systems were calculated following geometric optimization.

The alkali metal aluminum hydrides of MAlH₄ (M = Li, Na, K, and Cs) considered in our studies have space groups of P21/C (monoclinic structure, LiAlH₄) [27], I41/A (tetragonal structure, NaAlH₄) [28], and Pnma (orthorhombic structure, KAlH₄ and CsAlH₄) [29,30], as shown in

Table 1. The space group (SP), relaxed lattice parameters (R) and cell volume (V), formation enthalpy (ΔH), cohesive energy (E_coh), charge transfer from cation M to anion [AlH₄] (C), and alkali cation electronegativity (χ_P) of MAlH₄ (M = Li, Na, K, and Cs).

Compounds	SP	R (Ǻ)			V (Ǻ3)	ΔH (kJ/mol)	Ecoh (kJ/mol)	C (e)	χP
		a	b	c
LiAlH4	P21/C	4.908	8.031	7.953	289.762	−117.156	−1684.32	0.21	0.98
NaAlH4	I41/A	4.986	4.986	11.178	277.905	−120.468	−1684.704	0.28	0.93
KAlH4	Pnma	8.896	5.810	7.399	382.448	−157.2	−1719.552	0.35	0.82
CsAlH4	Pnma	10.018	6.163	8.077	498.704	−171.9	−1740.768	0.42	0.79

. The geometric optimization of these aluminum hydrides gave the relaxed lattice parameters and volume (Table 1), whose level of agreement with the experimental data [27,28,29,30] is fairly high. Using relaxed MAlH₄, a 2 × 1 × 1 supercell consisting of 8 MAlH₄ units (M₈Al₈H₃₂) was established for calculations, as illustrated in Figure 1. In this figure, two [AlH₄] units in MAlH₄ bulk are considered for hydrogen desorption, with their H atoms labeled as H_A, H_B, H_C, H_D, H_E, H_F, and H_G, and Al-H and H-H distances within 1.625–1.643 Ǻ (Al-H) and 2.639–2.754 Ǻ (H-H) in each [AlH₄] unit. From these Al-H and H-H distances, the H bonding (Al-H and H-H bonding) in the [AlH₄] group can be predicted [31]. The previous experimental and theoretical studies on aluminum hydrides also provide support for the formation of Al-H and H-H bonds in the [AlH₄] unit [31,32,33].

The formation enthalpy (ΔH) and cohesive energy (E_coh) of MAlH₄ aluminum hydrides (LiAlH₄, NaAlH₄, KAlH₄, and CsAlH₄) are calculated using the following formulae (Equations (1) and (2)), wherein the zero-point energy (ZPE) correction has been considered [34]:

$$\mathsf{\Delta }\mathrm{H}=\mathrm{E}\left({\mathrm{MAlH}}_4\right)-\mathrm{E}\left(\mathrm{M}\right)-\mathrm{E}\left(\mathrm{Al}\right)-2\mathrm{E}\left({\mathrm{H}}_2\right)$$

(1)

$${\mathrm{E}}_{\mathrm{coh}}=\mathrm{E}\left({\mathrm{MAlH}}_4\right)-\mathsf{\epsilon }\left(\mathrm{M}\right)-\mathsf{\epsilon }\left(\mathrm{Al}\right)-4\mathsf{\epsilon }\left(\mathrm{H}\right)$$

(2)

where E(MAlH₄) denotes the total energy of MAlH₄ (M = Li, Na, K, and Cs); E(M/Al) represents the energy of M/Al atom in each crystal structure of bcc-Li/Na/K/Cs and fcc-Al; ε(M/Al/H) denotes the isolated M/Al/H atom’s energy. E(H₂), the energy of hydrogen molecular, is estimated to be −31.407 eV by placing two H atoms 0.741 Ǻ apart [35] in a 10 × 10 × 10 Ǻ (1000 Ǻ³) cubic box. The result is in good agreement with those of −31.292 eV [36] and −31.592 eV [37] from the literature.

The hydrogen dissociation energy is defined as the energy cost of removing hydrogen molecules (hydrogen pair) from the mother bulk, since the initial decomposition of MAlH₄ (M = Li, Na, K, and Cs) takes place via the reaction of 3MAlH₄→M₃AlH₆ + 2Al + 3H₂ with the release of hydrogen gas [19,38,39]. In this case, the hydrogen pairs used for H-dissociation include (H_A and H_B), (H_A and H_C), and (H_A and H_D) from one [AlH₄] unit, and (H_A and H_E), (H_A and H_F), (H_A and H_G), and (H_A and H_H) from two [AlH₄] units (Figure 1). In addition, the two hydrogen atoms, H_A and H_X (H_B, H_C, H_D, H_E, H_F, H_G or H_H), in these hydrogen pairs can be removed in two possible ways: (1) H_A and H_X are taken away one by one (asynchronous hydrogen desorption, expressed as H_A→H_X). That is, the atom H_A is first removed from the M₈Al₈H₃₂ bulk forming M₈Al₈H₃₁ with a H_A vacancy, and then atom H_X is removed from relaxed M₈Al₈H₃₁ bulk forming M₈Al₈H₃₀ with H_A and H_X vacancies. In this case, the hydrogen dissociation energy is calculated by Equation (3) (the first step for H_A removal, E_d-H_A), Equation (4) (the second step for H_X removal, E_d-H_X), and Equation (5) (the total energy for H_A→H_X removal, E_d-H₂); (2) H_A and H_X are taken away simultaneously (synchronous hydrogen desorption, denoted as H_A–H_X), with the hydrogen dissociation energy (E_d-H₂) determined by the following Equation (6) [40]

$${\mathrm{E}}_{\mathrm{d}}-{\mathrm{H}}_{\mathrm{A}}=[\mathrm{E}\left({\mathrm{M}}_8{\mathrm{Al}}_8{\mathrm{H}}_{31}\right)+\frac{1}{2}\mathrm{E}\left({\mathrm{H}}_2\right)]-\mathrm{E}\left({\mathrm{M}}_8{\mathrm{Al}}_8{\mathrm{H}}_{32}\right)$$

(3)

$${\mathrm{E}}_{\mathrm{d}}-{\mathrm{H}}_{\mathrm{X}}=[\mathrm{E}\left({\mathrm{M}}_8{\mathrm{Al}}_8{\mathrm{H}}_{30}\right)+\frac{1}{2}\mathrm{E}\left({\mathrm{H}}_2\right)]-\mathrm{E}\left({\mathrm{M}}_8{\mathrm{Al}}_8{\mathrm{H}}_{31}\right)$$

(4)

$${\mathrm{E}}_{\mathrm{d}}-{\mathrm{H}}_2=\left({\mathrm{E}}_{\mathrm{d}}-{\mathrm{H}}_{\mathrm{A}}\right)+\left({\mathrm{E}}_{\mathrm{d}}-{\mathrm{H}}_{\mathrm{X}}\right)$$

(5)

$${\mathrm{E}}_{\mathrm{d}}-{\mathrm{H}}_2=\left[\mathrm{E}\left({\mathrm{M}}_8{\mathrm{Al}}_8{\mathrm{H}}_{30}\right)+\mathrm{E}\left({\mathrm{H}}_2\right)\right]-\mathrm{E}\left({\mathrm{M}}_8{\mathrm{Al}}_8{\mathrm{H}}_{32}\right)$$

(6)

in which E(H₂) is the same as the previous definition; E(M₈Al₈H₃₂), E(M₈Al₈H₃₁), and E(M₈Al₈H₃₀) are the total energy of corresponding systems.

3. Results and Discussion

3.1. Thermal Stability

The formation enthalpy, ΔH, refers to the formation heat in a hydriding reaction and is helpful for evaluating the thermal stability of metal hydrides [41,42]. A negative formation enthalpy (ΔH < 0) suggests an exothermic reaction. Furthermore, a compound is more thermally stable if it has a more negative formation enthalpy [41,42,43]. Table 1 lists the formation enthalpy of MAlH₄ (M = Li, Na, K, and Cs) with ZPE correction. It has been found that the formation enthalpies, ΔH, of MAlH₄ (M = Li, Na, K, and Cs) are always negative, and the values −117.156 kJ/mol (LiAlH₄), −120.468 kJ/mol (NaAlH₄), −157.2 kJ/mol (KAlH₄) and −171.9 kJ/mol (CsAlH₄) are in reasonable agreement with the available literature findings, that is, −113.42 kJ/mol (LiAlH₄) [44], −155.5 kJ/mol (NaAlH₄) [45], and −183.7 kJ/mol (KAlH₄) [45]. In particular, these ΔH values become more negative when the Pauling electronegativity of cation M (χ_P, Table 1) decreases, with a linear relation ΔH = 296.033χ_P − 402.19 obtained via least square fitting in Figure 2. A similar linear correlation, ΔH = 248.7χ_P − 390.8, is also presented in borohydrides [11]. It is clear from the results above that MAlH₄ (M = Li, Na, K, and Cs) aluminum hydrides may have reduced negative formation enthalpy as their alkali cation M has greater electronegativity, such as ΔH = −117.156 kJ/mol and χ_P = 0.98 for LiAlH₄ vs. ΔH = −171.9 kJ/mol and χ_P = 0.79 for CsAlH₄. The cohesive energy E_och (Table 1), interestingly, has the same characteristics as the formation enthalpy ΔH, for which a linear relationship between E_och and χ_P, E_coh = 296.489χ_P − 1968.247, is also achieved. These ΔH and E_och results suggest that the thermal stability corresponds to a descending order, wherein LiAlH₄ < NaAlH₄ < KAlH₄ < CsAlH₄ [41,42,43,46], followed by the decomposition temperature [11,19].

The charge transfer of a metal cation is related to its electronegativity, and it can help assess the thermal stability of metal hydrides. As mentioned in [11,47,48], the Mⁿ⁺→[BH₄]⁻ charge transfer is an important characteristic of the stability of M(BH₄)_n borohydrides. The suppression of the charge transfer by the substitution of a metal cation with a more electronegative element is expected to tailor the stability, thereby effectively lowering the dehydrogenation temperature of borohydrides. In this regard, it is interesting to find that the M⁺→[AlH₄]^- charge (Hirshfeld charge) transfer increases from 0.21 e (LiAlH₄) to 0.28 e (NaAlH₄), 0.35 e (KAlH₄), and 0.42 e (CsAlH₄), with the cation electronegativity (χ_P) decreasing from 0.98 (Li) to 0.93 (Na), 0.82 (K), and 0.79 (Cs), as shown in Table 1. The results indicate that the increase in cation electronegativity helps to suppress the charge transfer of the alkali cation; as a consequence, the thermal stability of MAlH₄ (M = Li, Na, K, and Cs) is reduced [11,47,48]. This, combined with the analysis of the formation enthalpies ΔH and cohesive energies E_coh (as described above), leads us to the conclusion that alkali alanates of MAlH₄ (M = Li, Na, K, and Cs) containing higher χ_P are expected to use less energy for hydrogen desorption in the following hydrogen dissociation energy calculation.

3.2. Hydrogen Dissociation Energy

Dehydrogenation ability can be characterized theoretically by the hydrogen dissociation energy at which one or more hydrogen atoms are removed from a mother bulk.

Table 2. The hydrogen dissociation energy of MAlH₄ (M = Li, Na, K, and Cs), including atomic hydrogen dissociation energy (E_d-H) for H_A and H_X (H_B, H_C, H_D, H_E, H_F, H_G, H_H) desorption and molecular hydrogen dissociation energy (E_d-H₂) for H_A→H_X (outside the bracket) and H_A–H_X desorption (inside the bracket). The minimum and average values of all E_d-H₂ are also listed.

Hydrogen Dissociation Energy (eV)		LiAlH4	NaAlH4	KAlH4	CsAlH4
Atomic hydrogen desorption Ed-H	HA	1.815	1.719	1.702	1.720
	HB	−0.080	−0.129	0.080	0.142
	HC	−0.526	−0.077	−0.038	−0.006
	HD	−0.019	−0.077	−0.037	0.000
	HE	−0.862	−0.736	−0.721	−0.658
	HF	−0.971	−0.747	−0.722	−0.663
	HG	−0.788	−0.637	−0.723	−0.664
	HH	−0.786	−0.634	−0.723	−0.665
Molecular hydrogen desorption Ed-H2	HA→HB (HA–HB)	1.735 (1.733)	1.590 (1.592)	1.782 (1.773)	1.862 (1.866)
	HA→HC (HA–HC)	1.289 (1.287)	1.642 (1.545)	1.664 (1.661)	1.714 (1.726)
	HA→HD (HA–HD)	1.796 (1.704)	1.642 (1.660)	1.665 (1.664)	1.720 (1.708)
	HA→HE (HA–HE)	0.953 (0.832)	0.983 (0.997)	0.981 (0.984)	1.062 (1.057)
	HA→HF (HA–HF)	0.844 (0.844)	0.972 (0.975)	0.980 (0.982)	1.057 (1.055)
	HA→HG (HA–HG)	1.027 (0.856)	1.082 (1.085)	0.979 (0.986)	1.056 (1.053)
	HA→HH (HA–HH)	1.029 (1.029)	1.085 (1.081)	0.979 (0.993)	1.055 (1.065)
	Minimum value	0.832	0.972	0.979	1.053
	Average value	1.211	1.281	1.291	1.361

lists the atomic hydrogen dissociation energies E_d-H for H_A (Equation (3)) and H_X (H_B, H_C, H_D, H_E, H_F, H_G, and H_H) removal (Equation (4)), as well as the molecular hydrogen dissociation energies E_d-H₂ for H_A→H_X (Equation (5)) and H_A–H_X removal (Equation (6)), and their minimum and average values for MAlH₄ (M = Li, Na, K, and Cs). As can be seen in Table 2, for atomic hydrogen H_A or H_X desorption (an asynchronous H_A→H_X process with H_A releasing first followed by the release of H_X), the energy cost for H_X desorption is much lower than that for H_A desorption, such as in LiAlH₄ −0.080 eV (E_d-H_B) << 1.815 eV (E_d-H_A). This indicates that it is easier for alanates of MAlH₄ (M = Li, Na, K, and Cs) with an H_A vacancy to release another hydrogen H_X and even induce spontaneous H_X dissociation as the hydrogen dissociation energies E_d-H_X are negative (Table 2) [49,50,51]. Shi et al. [52] had proposed that the hydrogen diffusion of sodium aluminum hydrides is mediated by hydrogen vacancies. Here, the favorable H_X desorption benefits from the H_A vacancy in the MAlH₄ bulk. For molecular hydrogen H_A and H_X desorption, on the one hand, asynchronous H_A→H_X desorption delivers a hydrogen dissociation energy very close to that for the corresponding synchronous H_A–H_X desorption, as noted in the example of LiAlH₄ with E_d-H₂ = 1.735 eV (H_A→H_B) and 1.733 eV (H_A–H_B). On the other hand, both asynchronous H_A→H_X and synchronous H_A–H_X desorption achieve lower hydrogen dissociation energy as H_A and H_X from two [AlH₄] units compared to H_A and H_X from one [AlH₄] unit, e.g., in LiAlH₄ E_d-H₂ = 0.953 eV (H_A→H_E) vs. 1.735 eV (H_A→H_B). In general, the formation of a stable [AlH₃] unit/cluster followed by hydrogen release plays an important role in lowering the hydrogen removal energy in sodium alanate [53,54]. In the present work on alkali alanates of MAlH₄, [AlH₃] units are found to form upon H_A→H_X and H_A–H_X desorption. Moreover, for a given system, the formed [AlH₃] units are more stable with H_A and H_X from two [AlH₄] units because, in this case, H_A and H_X desorption has lower hydrogen dissociation energy (E_d-H₂) (as described above); thus, the corresponding H_A and H_X desorbed system has lower total energy (E) according to Equations (3)–(6). This can be observed in the examples shown in Figure 3, where H_A and H_X desorption from one/two [AlH₄] units has relative low hydrogen dissociation energy among all corresponding types of H_A→H_X and H_A–H_X desorption, including H_A–H_C and H_A–H_E desorption for LiAlH₄, H_A–H_C and H_A→H_F desorption for NaAlH₄, H_A–H_C and H_A→H_G desorption for KAlH₄, and H_A–H_D and H_A–H_G desorption for CsAlH₄ (Table 2). Obviously, our findings support the notion that the formation of stable [AlH₃] units upon H-desorption is responsible for the reduction in the hydrogen dissociation energy in the alkali alanates of MAlH₄.

In Table 2, it is worth noting that the hydrogen dissociation energies E_d-H₂ of the considered hydrides of MAlH₄ (M = Li, Na, K, and Cs), with respect to the minimum and average values, decrease in the order of LiAlH₄ < NaAlH₄ < KAlH₄ < CsAlH₄, i.e., 0.832 eV (LiAlH₄) < 0.972 eV (NaAlH₄) < 0.979 eV (KAlH₄) < 1.053 eV (CsAlH₄), for the minimum value, and 1.211 eV (LiAlH₄) < 1.281 eV (NaAlH₄) < 1.291 eV (KAlH₄) < 1.361 eV (CsAlH₄) for the average value. These characteristics are also achieved in M₄Al₄H₁₆ (primitive cell) in addition to M₈Al₈H₃₂ (2 × 1 × 1 supercell, present work). In particular, the descending order of hydrogen dissociation energies (E_d-H₂) agrees well with that of the calculated formation enthalpies (ΔH) and cohesive energies (E_coh) (Table 1), as well as the experimental onset dehydriding temperature, 443 K (LiAlH₄) [20] < 503 K (NaAlH₄) [21] < 573 K (KAlH₄) [20] < 600 K (CsAlH₄) [19] (as described above). However, this order is opposite to that of the cation electronegativity, namely, 0.98 (Li) > 0.93 (Na) > 0.82 (K) > 0.79 (Cs). The results further verify the fact that the alkali metal aluminum hydrides of MAlH₄ with more electronegative alkali cations are thermally less stable and, therefore, energetically favorable for hydrogen desorption.

3.3. Electronic Structures

As described above, Al-H and H-H bonds can be detected in the [AlH₄] groups of MAlH₄ (M = Li, Na, K, and Cs). Thus, upon dehydrogenation, the release of H_A or/and H_X from the considered [AlH₄] units (Figure 1) may be accompanied by the separation/rupture of Al-H and H-H bonds. In addition, weakened Al-H bonds are believed to be beneficial to the dehydrogenation of aluminum hydrides [55,56,57]. In consideration of these facts, determined the bonding features between Al-H and H-H according to the density of states, charge density distributions, and Mulliken populations should be help clarify the dehydriding mechanism of the alkali alanates of MAlH₄ (M = Li, Na, K, and Cs).

Figure 4 presents the total (TDOS) and partial density of states (PDOS) of MAlH₄ (M = Li, Na, K, and Cs), with a Fermi level (E_F) at 0 eV and H atoms (H_A and H_B) from the considered [AlH₄] unit in Figure 1. As can be seen, the DOS pictures of the studied hydrides of MAlH₄, especially M = Li, Na and K, are very similar to each other. There are orbital hybridizations between the Al and H atoms whether below Fermi level or above, thereby demonstrating a bonding interaction between Al-H. Similarly, the bonding precipitated by the interaction between H and H atoms is achieved through the hybridizations between the s states of the H atoms. These bonds of Al-H and H-H described in the DOS pictures (Figure 4) can also be detected by the charge density distribution of MAlH₄ (M = Li, Na, K, and Cs) (Figure 5). As shown in the Figures, the overlapping electronic clouds with appropriate distances between Al and H atoms (1.625–1.641 Ǻ), and H and H atoms (2.650–2.721 Ǻ) in Figure 5, may contribute to the formation of Al-H and H-H bonds, respectively [31,40]. In the DOS pictures, it is worth noting that the peaks of the TDOS contribution from Al and H electronic states (marked as I and II in Figure 4) tend to increase at lower cation electronegativity due to the enhanced PDOS of Al s and p states and H s states (such as KAlH₄ and CsAlH₄), so one or both Al-H and H-H bonding interactions may become stronger with a decreasing cation electronegativity [50,51,58].

To further quantitatively elucidate the bonding characteristics between Al-H and H-H, a Mulliken population analysis was performed on the MAlH₄ (M = Li, Na, K, and Cs) compounds. The results, including the average bond order (BO), the average bond length (BL), and the scaled bond order (BO^s) between Al-H and H-H, are listed in

Table 3. The Mulliken population for MAlH₄ (M = Li, Na, K, and Cs), including the average bond order (BO), average bond length (BL), and scaled bond order (BO^s) between Al-H and H-H.

Compounds	Al-H			H-H
	BO	BL (Å)	BOS (Å−1)	BO	BL (Å)	BOS (Å−1)
LiAlH4	0.778	1.634	0.476	−0.036	2.751	−0.013
NaAlH4	0.855	1.642	0.521	−0.039	2.755	−0.014
KAlH4	0.906	1.634	0.555	−0.055	2.668	−0.021
CsAlH4	0.893	1.637	0.545	−0.058	2.673	−0.022

. Here, BO indicates the overlapping electron population between atoms, and is useful when considering bonding characteristics with an ionic (BO < 0, BO = 0) or covalent nature (BO > 0) [40,41,58,59,60,61]. BO^s, defined as BO^s = BO/BL, is helpful for assessing the relative bonding strength between atoms. A bond with a higher BO^s value is expected to be stronger [40,41,58]. It can be seen from Table 3 that for all the studied compounds, Al-H bonds with a positive bond order (BO_Al-H > 0) show a covalent character, while H-H bonds with a negative bond order (BO_H-H < 0) exhibit an ionic nature. The H-H ionic interactions were found to be weakened at a higher electronegativity of the cation M. As shown, the scaled bond order between H-H (BO^s_H-H) decreases linearly (−0.013 Å⁻¹ (LiAlH₄) < −0.014 Å⁻¹ (NaAlH₄) < −0.021 Å⁻¹ (KAlH₄) < −0.022 Å⁻¹ (CsAlH₄)) with the increase in cation electronegativity (0.98 (Li) > 0.93 (Na) > 0.82 (K) > 0.79 (Cs)). The trend of the Al-H covalent interactions among the studied aluminum hydrides (except CsAlH₄) is similar to that of the H-H ionic interactions. Araújo et al. [55] reported that hydrogen atoms held by weak covalent and ionic bonds may lead to lower dissociation temperatures for complex alkali metal aluminum hydrides. In addition, many previous studies have shown that reducing the Al-H covalent bonding strength in metal aluminum hydrides facilitates their decomposition for H-desorption [55,56,57,62]. Our calculated results with respect to MAlH₄ (M = Li, Na, K, and Cs) mainly support these findings. That is, aluminum hydrides of MAlH₄ with larger cation electronegativity and weaker H-H ionic and Al-H covalent interactions exhibit lower hydrogen dissociation energies (Table 2). As an example, the LiAlH₄ hydride containing the weakest H-H ionic (BO^s_H-H = −0.013 Å⁻¹, Table 3) and the Al-H covalent interactions (BO^s_Al-H = 0.476 Å⁻¹, Table 3) show the lowest hydrogen dissociation energies for hydrogen desorption (E_d-H₂ = 1.211 eV, Table 2) relative to the other three hydrides, namely, NaAlH₄, KAlH₄, and CsAlH₄. The results described above lead us to the conclusion that weakening the ionic and covalent H bonds (such as H-H ionic and Al-H covalent bonds) by increasing cation electronegativity helps to modify the dehydrogenation performance of the alkali metal aluminum hydrides of MAlH₄ (M = Li, Na, K, and Cs). The incorporation of more electronegative elements (compared to M) into the MAlH₄ bulk could be used for this purpose.

4. Conclusions

First-principles calculations were performed on the alkali alanates of MAlH₄ (M = Li, Na, K, and Cs) to investigate their thermal stability, hydrogen dissociation energy, and electronic structures. The results show that cation electronegativity (χ_P) is a good indicator with which to assess the thermal stability and dehydrogenation ability of MAlH₄ (M = Li, Na, K, and Cs). For MAlH₄ with a higher χ_P, on the one hand, it is thermally less stable because the formation enthalpy ΔH and cohesive energy E_och become less negative and the M⁺→[AlH₄]⁻ charge transfer is suppressed. On the other hand, it is energetically favorable for hydrogen desorption (H_A→H_X and H_A–H_X, especially H_A and H_X from two [AlH₄] units), which is associated with its poor stability and weaker H-H ionic and Al-H covalent interactions. Our work provides new insights into the dehydrogenation of MAlH₄ (M = Li, Na, K, and Cs) and is useful for designing advanced aluminum hydrides with favorable H-desorption properties.