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

Abstract

Mechanochemical synthesis using CeCl3-MBH4 (M = Li, Na or K) mixtures are investigated and produced a new compound, LiCe(BH4)(3)Cl, which crystallizes in a cubic space group I (4) over bar 3m, a = 11.7204(2) angstrom. The structure contains isolated tetranuclear anionic clusters [Ce4Cl4(BH4)(12)](4-) with a distorted cubane Ce4Cl4 core, charge-balanced by Li+ cations. Each Ce atom coordinates three chloride ions and three borohydride groups via the eta(3)-BH3 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(BH4)(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(BH4)(3)Cl of 1 X 10(-4) Scm(-1) at T = 20 degrees C. In situ SR-PXD reveals that the decomposition products consist of LiCl, CeB6 and CeH2. The Sieverts measurements show that 4.7 wt % H-2 is released during heating to 500 degrees C. After rehydrogenation at 400 degrees 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 B-11 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 B-11 site, reflecting dipolar couplings of the B-11 nuclear spin with the paramagnetic electron spin of the Ce3+ ions.