# Patent

## Title
XLi3N2 compounds and their hydrides as hydrogen storage materials

## Abstract

State-of-the-art electronic structure calculations provide the likelihood of the availability of ScLi3N2, TiLi3N2, VLi3N2, CrLi3N2, MnLi3N2, CoLi3N2, NiLi3N2, CuLi3N2, and ZnLi3N2 as compounds for reaction with hydrogen under suitable conditions. Reaction with hydrogen is likely to form stable hydrides of the family ScLi3N2Hn, TiLi3N2Hn, VLi3N2Hn, CrLi3N2Hn, MnLi3N2Hn, FeLi3N2Hn CoLi3N2Hn, NiLi3N2Hn, CuLi3N2Hn, and ZnLi3N2Hn, where n is an integer in the range of 1-4. These materials offer utility for hydrogen storage systems.

## Background
TECHNICAL FIELD
This invention pertains to compounds useful for solid-state storage of hydrogen. More specifically, this invention pertains to a family of new compounds, XLi3N2, which form hydrides, XLi3N2Hn, where X is a 3d transition metal.
BACKGROUND OF THE INVENTION
Considerable development effort is currently being expended on the development of hydrogen and oxygen consuming fuel cells, and there is also interest in hydrogen burning engines. Such power systems require means for storage of hydrogen fuel which hold hydrogen in a safe form at ambient conditions and which are capable of quickly receiving and releasing hydrogen. In the case of automotive vehicles, fuel storage is required to be on-board the vehicle, and storage of hydrogen gas at high pressure is generally not acceptable for such applications.
These requirements have led to the study and development of solid-state compounds for temporary storage of hydrogen, often as hydrides. For example, sodium alanate, NaAlH4, can be heated to release hydrogen gas, and a mixture of lithium amide, LiNH2, and lithium hydride, LiH, can be heated and reacted with the same effect. Despite such progress, however, no known solid-state system currently satisfies targets for on-board vehicular hydrogen storage.
SUMMARY OF THE INVENTION
FeLi3N2 is prepared by reaction of Li3N melt with elemental iron in a nitrogen atmosphere. It crystallizes in the body-centered Ibam structure (space group 72). Other ternary nitride compounds, having the same stoichiometry as FeLi3N2, are of interest as hydrogen storage materials where the ternary nitride takes up hydrogen as XLi3N2Hn.
State-of-the-art computational electronic structure methods, using FeLi3N2 as the template compound, indicate that these ternary nitrides, XLi3N2, are thermodynamically stable. Accordingly, this invention demonstrates the credible likelihood that each of ScLi3N2, TiLi3N2, VLi3N2, CrLi3N2, MnLi3N2, CoLi3N2, NiLi3N2, CuLi3N2, and ZnLi3N2 can be prepared as new materials for storage of hydrogen. The computational methods also show thermodynamic stability of the hydrides ScLi3N2Hn, TiLi3N2Hn, VLi3N2Hn, CrLi3N2Hn, MnLi3N2Hn, FeLi3N2Hn, CoLi3N2Hn, NiLi3NHn, CuLi3N2Hn, and ZnLi3N2Hn. Accordingly, this invention also provides the likelihood of a hydrogen storage compound for each of the specified ternary nitride compositional formulas.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the experimentally determined structure of FeLi3N2 that is the template for computational electronic structure methods showing the thermodynamic stability of other isostructural ternary nitride compounds, XLi3N2. In this figure, the large dark-filled circles represent the positions of nitrogen atoms, the speckled circles represent the positions of iron atoms, and the small unfilled circles represent positions of lithium atoms.
FIG. 2 illustrates the calculated structure of FeLi3N2H2 with H atoms occupying the 8j sites in the Ibam structure; it is the most thermodynamically stable hydride of FeLi3N2 identified with the computational methodology used here. In this figure, the large dark filled circles represent the positions of nitrogen atoms, the speckled circles represent the positions of iron atoms, the small unfilled circles represent positions of lithium atoms, and the small dark filled circles represent positions of hydrogen atoms.



DESCRIPTION OF PREFERRED EMBODIMENTS
State-of-the-art computational electronic structure methods implementing density functional theory (DFT) have been employed with substantial success to model hydride properties, including the crucial enthalpies of formation. That success encourages the development of strategies for harnessing the calculational tools to guide the discovery of novel hydrides. The approach in this case is to choose a compound having a known crystal structure and calculate enthalpies of formation for isostructural, hypothetical compounds constructed by elemental replacements and the addition of hydrogen to the original lattice.
In this work FeLi3N2 is selected as the template compound, and the formation of isostructural XLi3N2 phases and their XLi3N2Hn hydrides with X any of the 3d transition elements (Sc—Zn) or boron, aluminum, or gallium is considered. Searching for hydrides comprising a 3d element such as Ni or Fe to facilitate H2 dissociation and lighter elements such as Li to enhance the gravimetric hydrogen density is the strategy. This strategy is believed to be realistic because two of the most comprehensively investigated hydrides have Ni as a component: LaNi5H7 and Mg2NiH4; the former has excellent H2 sorption characteristics but is only 1.4 mass % H, while the latter requires heating to at least 250° C. for hydrogen liberation. Compounds having the XLi3N2 stoichiometry are only known for X═B, Fe, Al, and Ga, and the only known hydrides are those recently identified in the B—Li—N—H system.
Crystal Structure of FeLi3N2 
FeLi3N2 crystallizes in the body-centered orthorhombic Ibam structure (space group No. 72). The conventional unit cell, illustrated in FIG. 1, contains four FeLi3N2 formula units (f.u.). The space group allows eleven distinct crystallographic sites; the 4a, 4b, 4c, 4d, and 8e positions are fixed by symmetry, while the 8f, 8g, 8h, 8i, 8j, and 16k sites have variable coordinates and thus can be multiply occupied. In FeLi3N2, iron and nitrogen ions occupy the 4a and 8j sites, respectively, and the lithium ions fill the 4b and 8g sites. FeLi3N2 contains infinite chains of edge-sharing FeN4 tetrahedra along the c-direction. These chains are isoelectronic to the SiS2 one-dimensional macromolecule and form nearly hexagonal arrays linked by sharing common edges with LiN4 tetrahedra. Two of these tetrahedra are highlighted with speckling in FIG. 1.
Calculation Procedures
Electronic total energies E were computed for the primitive cells (containing two formula units, f.u.) with the Vienna ab initio simulation package (VASP), which implements DFT using a plane wave basis set. Projector-augmented wave potentials were employed for the elemental constituents, and a generalized gradient approximation (GGA) was used for the exchange-correlation energy functional μxc. Paramagnetic calculations were performed for all materials except those containing Fe, Co, and Ni, for which spin-polarized calculations were done to assess the possibility of magnetic states. An interpolation formula was used for the correlation component of μxc in the spin-polarized cases. For all the XLi3N2 and XLi3N2H, compounds a plane wave cutoff energy of 900 eV was imposed and (6 6 6) Monkhorst-type k-point grids having 45 points in the irreducible Brillouin zone were employed. In each case at least two simultaneous relaxations of the lattice constants and nuclear coordinates not fixed by the space group were carried out. The electronic total energies and forces were converged to 10−6 eV/cell and 10−4 eV/Å, respectively. Calculations for the H2, N2 molecules and the elemental metals Li, X were performed with the same computational machinery to the same levels of precision.
Enthalpies of formation, ΔH, were obtained from total energy differences:

ΔH(XLi3N2)=E(XLi3N2)−E(X)−3E(Li)−E(N2)  (1)

for the parent compounds, and

ΔH(XLi3N2Hn)=(2/n) [E(XLi3N2Hn)−E(X)−3E(Li)−E(N2)−(n/2)E(H2)]  (2)

for the hydrides, where n is the number of H atoms in a given configuration. Each ΔH, specified per XLi3N2 formula unit (f.u.) in equation (1) and per H2 molecule in equation (2), is the standard enthalpy of formation at zero temperature in the absence of zero point energy contributions. A negative ΔH indicates stability of the material relative to its elemental solid and molecular constituents.

Results of Calculations

XLi3N2 Parent Compounds

Table I lists ΔH(XLi3N2) calculated according to equation (1) for the 3d transition elements X═Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, as well as for X═B, Al, and Ga. The compounds are in the Ibam structure of FeLi3N2. All ΔH values are in kJ/mole formula unit. Only FeLi3N2, BLi3N2, AlLi3N2, and GaLi3N2 are known to exist.










TABLE 1








ΔH(XLi3N2)



Compound
(kJ/mole f.u.)



















ScLi3N2
−458



TiLi3N2
−446



VLi3N2
−384



CrLi3N2
−333



MnLi3N2
−295



FeLi3N2
−246



CoLi3N2
−184



NiLi3N2
−119



CuLi3N2
−28



ZnLi3N2
−117



BLi3N2
−379



AlLi3N2
−482



GaLi3N2
−285










In each case ΔH<0, suggesting that all the compounds may form. This is certainly correct for FeLi3N2, the prototype. Encouragingly, it is also true for BLi3N2, AlLi3N2, and GaLi3N2. BLi3N2 forms in at least three structures all distinct from Ibam: (i) a tetragonal P42212 low-temperature phase, (ii) a monoclinic P21/c phase often observed at high temperatures, and (iii) a body-centered tetragonal I41/amd phase only recently established. AlLi3N2 and GaLi3N2 are known to form in the body-centered cubic Ia3 structure. All the other compounds in Table I are unknown. Although the results in the table indicate compound existence, the BLi3N2, AlLi3N2, and GaLi3N2 examples emphasize the very likely possibility that the actual XLi3N2 crystal structure for X other than Fe could well differ from the Ibam FeLi3N2 template on which the calculations are based. Furthermore, ΔH in Table I sets an upper bound on the actual ΔH. If an XLi3N2 compound exists and its structure differs from that of FeLi3N2, then its ΔH will be more negative than the value in Table I. BLi3N2 also illustrates this point. The enthalpies of formation of the three known structures, also calculated via Eq. (1), are all  ̃−500 kJ/f.u., substantially lower than ΔH=−379 kJ/f.u. in Table I for BLi3N2 in the Ibam structure.
Plots of the electronic density of states (DoS) were calculated for each XLi3N2 compound. All the materials in Table I are metals with the exception of ScLi3N2 and BLi3N2, which are insulators. Analogous computations for the three known BLi3N2 phases indicate that all are also insulators with a gap of  ̃3 eV separating the valence and conduction bands.
FeLi3N2, CoLi3N2, and NiLi3N2 were found to have net magnetic moments of 0.97, 0.17, and 0.19 μB/f.u.
XLi3N2Hn Hydrides
Since the fixed-coordinate 4a and 4b sites in the Ibam structure are occupied by X and Li, respectively, the 4c, 4d, 8e, 8f, 8g, 8h, 8i, and 16k sites, and combinations of them, are available for occupation by hydrogen. All these sites other than the 4c, 4d, and 8e can be multiply occupied, so that in principle an infinite number of hydrogen configurations are possible. For each element X calculations of ΔH(XLi3N2Hn) as defined by Eq. (2) were performed to assess whether stable [ΔH(XLi3N2Hn)<0] hydride configurations exist and to find the most stable configuration, that for which ΔH(XLi3N2Hn) is a minimum. According to the van't Hoff relation

ln p/p0=ΔH/RT−ΔS/R,  (3)

where ΔS is the entropy of formation and R the gas constant, the configuration having the most negative ΔH is that which is stable at the lowest H2 pressure p.

Calculations were carried out for hydrogen occupying each of the possible individual sites and for the 4c4d, 4d8j, 8j18j2, 4d8j18j2, 8j16k, and 16k116k2 combinations. This spectrum of choices was sufficiently comprehensive to ensure that the configuration having the minimum ΔH was identified for each XLi3N2Hn. Several negative ΔH values were found for every X, suggesting the possibility of hydride formation in each case. The most stable hydrogen configuration varies through the XLi3N2Hn series. The 4d sites provide the greatest stability for X═Sc, Ti; the 8j18j2 combination for X═V, Cr; the 8j sites for X═Mn, Fe, Co, Ni, Cu; and the 16k sites in the Zn hydride. The most unstable arrangements are those for hydrogen in the 8f, 8g, 8h, and 8i sites. It appears likely that hydrogen filling of these sites destabilizes the interaction between the Li ions on the 4b and 8g sites and the N ions on the 8j sites.
No XLi3N2Hn hydrides are known for X═Sc—Zn but, as for the parent compounds, the case of boron provides encouragement that at least some may form. The calculations for BLi3N2Hn in the Ibam structure reveal BLi3N2H4 with hydrogen occupying 8j18j2 sites as the most stable; each conventional cell contains four BLi3N2H4 formula units. This result is in qualitative accord with the fact that BLi4N3H10 and other B—Li—N—H hydrides have indeed been synthesized.
Table II summarizes the minimum ΔH results from the calculations and includes the hydrogen mass percentage for each hypothetical hydride. The crystallographic sites occupied by hydrogen are indicated in parentheses in the first column. All ΔH values in kJ/mole H2. Only B—Li—N—H hydrides are known to exist. If any of these were to form in a different crystal structure or with an alternate stoichiometry the hydrogen content could certainly change. In particular, the 6.3 mass % value for BLi3N2H4 in Table II underestimates the 11.1 H mass % for the actual BLi4N3H10 hydride.











TABLE 2







XLi3N2 hydride





(H configuration in
ΔH(XLi3N2)



conventional cell)
(kJ/mole H2)
mass % H









ScLi3N2H (4d)
−439
1.1



TiLi3N2H (4d)
−331
1.0



VLi3N2H4 (8j18j2)
−198
3.9



CrLi3N2H4 (8j18j2)
−155
3.8



MnLi3N2H2 (8j)
−128
1.9



FeLi3N2H2 (8j)
−124
1.9



CoLi3N2H2 (8j)
−144
1.8



NiLi3N2H2 (8j)
−143
1.8



CuLi3N2H2 (8j)
−178
1.8



ZnLi3N2H4 (16k)
−148
3.4



BLi3N2H4 (8j18j2)
−228
6.3










To determine the effect of hydriding on the electronic structure the electronic densities of states (DoS) for the hydrides characterized by the minimum AH were calculated together with the DoS for the parents. In each case hydrogen-derived bands appear below the bottom of the valence bands of the parent, similar to the behavior of LaNi5 on hydriding. From the DoS at the Fermi energy εF it became apparent that the X═Sc, Ti, V, Cr, Fe, Ni, and Zn hydrides are metals. NiLi3N2H2 (8j) [occupied hydrogen sites in parentheses] is in fact a half-metal since the majority (↑) spin DoS is zero at εF while there is substantial minority (↓) spin DoS at the Fermi level. The X═Mn, Co, Cu, and B hydrides are all insulators (zero DoS and gaps at εF).
Most of the XLi3N2 parents are metals and remain metallic on hydrogen uptake, and BLi3N2 and its BLi3N2H4 (8j18j2) hydride are both insulators. The X═Sc, Mn, Co, Cu, and Ni materials, however, feature three types of intriguing electronic transitions on hydriding to the minimum ΔH configuration. First, ScLi3N2 is an insulator, but its ScLi3N2H (4d) hydride is a metal. Second, MnLi3N2, CoLi3N2, and CuLi3N2 are metals, while their corresponding MnLi3N2H2 (8j), CoLi3N2H2 (8j), and CuLi3N2H2 (8j) hydrides are insulators. Third, NiLi3N2 is metallic in both spin bands, but its NiLi3N2H2 (8j) hydride is predicted to be a half-metal. Were these materials to form, such transitions might serve as the bases for hydrogen sensor applications.
These state-of-the-art electronic structure calculations demonstrate the credible likelihood of the availability of ScLi3N2, TiLi3N2, VLi3N2, CrLi3N2, MnLi3N2, CoLi3N2, NiLi3N2, CuLi3N2, and ZnLi3N2 as compounds for reaction with hydrogen under suitable conditions. Reaction with hydrogen should form stable hydrides of the family ScLi3N2Hn, TiLi3N2Hn, VLi3N2Hn, CrLi3N2Hn, MnLi3N2Hn, CoLi3N2Hn, NiLi3N2Hn, CuLi3N2Hn, and ZnLi3N2Hn, where n is an integer in the range of 1-4.
The above-described synthesis of FeLi3N2, adapted for the properties of the specific other elements of the 3d transition metal group, provides a basis for the synthesis of the parent compounds.

## Claims

1. Any one or more of the hydrides of ternary nitrides of the compositional formulas selected from the group consisting of ScLi3N2Hn, TiLi3N2Hn, VLi3N2Hn, CrLi3N2Hn, MnLi3N2Hn, FeLi3N2Hn, CoLi3N2Hn, NiLi3N2Hn, CuLi3N2Hn, and ZnLi3N2Hn, where n is an integer having a value of from 1 to 4.

2. Any one or more of the hydrides of ternary nitrides as recited in claim 1 of the compositional formulas selected from the group consisting of ScLi3N2H, TiLi3N2H, VLi3N2H4, CrLi3N2H4, MnLi3N2H2, FeLi3N2H2 CoLi3N2H2, NiLi3N2H2, CuLi3N2H2, and ZnLi3N2H4.