New Li Ion Conductors and Solid State Hydrogen Storage Materials: LiM(BH₄)₃Cl, M = La, Gd

Multiple reaction mixtures with different composition ratios of MCl 3 –LiBH 4 (M = La, Gd) were studied by mechano-chemical synthesis, yielding two new bimetallic borohydride chlorides, LiM(BH 4 ) 3 Cl (M = La, Gd). The Gd-containing phase was obtained only after annealing the ball-milled mixture. Additionally, a solvent extracted sample of Gd(BH 4 ) 3 was studied to gain insight into the transformation from Gd(BH 4 ) 3 to LiGd(BH 4 ) 3 Cl. The novel compounds were investigated using in situ synchrotron radiation powder X-ray diffraction, thermal analysis combined with mass spectroscopy, Sieverts measurements, Fourier transform infrared spectroscopy, and electrochemical impedance spectroscopy. The two new compounds, LiLa(BH 4 ) 3 Cl and LiGd(BH 4 ) 3 Cl, have high lithium ion conductivities of 2.3 × 10 –4 and 3.5 × 10 –4 S·cm –1 ( T = 20 °C) and high hydrogen densities of ρ m = 5.36 and 4.95 wt % H 2, and both compounds crystallize in the cubic crystal system (space group I -43 m ) with unit cell parameter a = 11.7955(1) and a = 11.5627(1) Å, respectively. The structures contain isolated tetranuclear anionic clusters [M 4 Cl 4 (BH 4 ) 12 ] 4– with distorted cubane M 4 Cl 4 cores M = La or Gd. Each lanthanide atom coordinates three chloride ions and three borohydride groups, thus completing the coordination environment to an octahedron. The Li + ions are disordered on 2/3 of the 12 d Wyckoff site, which agrees well with the very high lithium ion conductivities. The conductivity is purely ionic, as electronic conductivities were measured to only 1.4 × 10 –8 and 9 × 10 –8 S·cm –1 at T = 20 °C for LiLa(BH 4 ) 3 Cl and LiGd(BH 4 ) 3 Cl, respectively. In situ synchrotron radiation powder X-ray diffraction (SR-PXD) reveals that the decomposition products at 300 °C consist of LaB 6 /LaH 2 or GdB 4 /GdH 2 and LiCl. The size of the rare-earth metal atom is shown to be crucial for the formation and stability of the borohydride phases in MCl 3 –LiBH 4 systems.