LiCe(BH₄)₃Cl, a New Lithium-Ion Conductor and Hydrogen Storage Material with Isolated Tetranuclear Anionic Clusters

Abstract

Mechanochemical synthesis using CeCl 3 -MBH 4 (M = Li, Na or K) mixtures are investigated and produced a new compound, LiCe(BH 4 ) 3 Cl, which crystallizes in a cubic space group I4̅ 3m, a = 11.7204(2) Å. The structure contains isolated tetranuclear anionic clusters [Ce 4 Cl 4 (BH 4 ) 12 ] 4− with a distorted cubane Ce 4 Cl 4 core, charge-balanced by Li + cations. Each Ce atom coordinates three chloride ions and three borohydride groups via the η 3 −BH 3 faces, thus completing the coordination environment to an octahedron. Combination of synchrotron radiation powder X-ray diffraction (SR-PXD), powder neutron diffraction and density functional theory (DFT) optimization show that Li cations are disordered, occupying 2/3 of the 12d Wyckoff site. DFT calculation indicates that LiCe(BH 4 ) 3 Cl is stabilized by higher entropy rather than lower enthalpy, in accord with the disorder in Li positions. The structural model also agrees well with the very high lithium ion conductivity measured for LiCe(BH 4 ) 3 Cl of 1 × 10 −4 Scm −1 at T = 20°C. In situ SR-PXD reveals that the decomposition products consist of LiCl, CeB 6 and CeH 2 . The Sieverts measurements show that 4.7 wt % H 2 is released during heating to 500°C. After rehydrogenation at 400°C and p(H 2 ) = 100 bar for 24 h an amount of 1.8 wt % H 2 is released in the second dehydrogenation. The 11 B MAS NMR spectra of the central and satellite transitions for LiCe(B(D/H) 4 ) 3 Cl reveal highly asymmetric manifolds of spinning sidebands from a single 11 B site, reflecting dipolar couplings of the 11 B nuclear spin with the paramagnetic electron spin of the Ce 3+ ions.

Characterization of Gas‐Solid Reactions using In Situ Powder X‐ray Diffraction

“…[25] The Li + conductivity was found to be 1.03 ϫ 10 -4 S cm -1 at T = 20°C, further supporting the structural model. Figure 3 illustrates the anions within the structure of LiCe(BH 4 ) 3 [25] In conclusion, structural evaluation of LiCe(BH 4 ) 3 Cl was only possible by the combined use of the three techniques SR-PXD, PND and DFT and likely not by any combination of two methods alone. The mixed-metal and anion-substituted rare earth metal borohydrides have been studied extensively.…”

“…ball milling of samples of CeCl 3 ϪLiBH 4 in various ratios. [25,43,44] The LiCe(BH 4 ) 3 Cl was eventually indexed and found to crystallize in a cubic structure, a = 11.7204(2) Å, with space-group I43m. Structural evaluation was performed on data obtained at approx.…”

“…The mixed-metal and anion-substituted rare earth metal borohydrides have been studied extensively. [25,[43][44][45][46][47] Among them, other fast Li + ion conductors isostructural to LiCe(BH 4 ) 3 Cl have been discovered i.e. LiLa(BH 4 ) 3 Cl and LiGd(BH 4 ) 3 Cl.…”

“…[23,24] Structures containing both light and heavy elements are usually challenging to investigate as observed for a new compound, LiCe(BH 4 ) 3 Cl, discussed later in this review. [25] In such cases supplementary techniques e.g. powder neutron diffraction, spectroscopy or theoretical modelling are required to obtain full understanding of the crystal structure.…”

Hydrogen Sorption in Erbium Borohydride Composite Mixtures with LiBH4 and/or LiH

“…Another class of materials to be considered is rare earth (RE) borohydrides, with hydrogen capacities varying between 9.0 wt % for Y(BH 4 ) 3 and 5.5 wt % for Yb(BH 4 ) 3 [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Li-containing RE borohydrides are also considered as solid state electrolytes for new battery applications, due to their high Li-ion conductivities [20][21][22][23]. RE borohydrides have shown novel properties such as luminescence, and a magnetocaloric effect has also been recently published [24][25][26].…”