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

Abstract

Multiple reaction mixtures with different composition ratios of MCl3–LiBH4 (M = La, Gd) were studied by mechano-chemical synthesis, yielding two new bimetallic borohydride chlorides, LiM(BH4)3Cl (M = La, Gd). The Gd-containing phase was obtained only after annealing the ball-milled mixture. Additionally, a solvent extracted sample of Gd(BH4)3 was studied to gain insight into the transformation from Gd(BH4)3 to LiGd(BH4)3Cl. 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(BH4)3Cl and LiGd(BH4)3Cl, 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 ρm = 5.36 and 4.95 wt % H2, and both compounds crystallize in the cubic crystal system (space group I-43m) with unit cell parameter a = 11.7955(1) and a = 11.5627(1) Å, respectively. The structures contain isolated tetranuclear anionic clusters [M4Cl4(BH4)12]4– with distorted cubane M4Cl4 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 12d 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(BH4)3Cl and LiGd(BH4)3Cl, respectively. In situ synchrotron radiation powder X-ray diffraction (SR-PXD) reveals that the decomposition products at 300 °C consist of LaB6/LaH2 or GdB4/GdH2 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 MCl3–LiBH4 systems.