Lithium-ion diffusivity in complex hydrides: Pulsed-field-gradient NMR studies of LiLa(BH4)3Cl, Li3(NH2)2I and Li-1-CB9H10

The bimetallic borohydride-chloride LiLa(BH4)(3)Cl, the lithium amide-iodide Li-3(NH2)(2)I, and the lithium monocarba-closo-decaborate Li-1-CB9H10 represent complex hydrides showing superionic conductivity at room temperature or slightly above it. To study the Li-ion diffusivity that is closely related to the ionic conductivity, we have measured the diffusion coefficients of Li+ cations in these compounds using the pulsed-field-gradient (PFG) spin-echo technique over the temperature range of 298-403 K. The experiments have revealed fast Li+ diffusivities in all these complex hydrides: at 400 K, the measured diffusion coefficients exceed 10(-7) cm(2)/s for LiLa (BH4)(3)Cl, 5 x 10(-8) cm(2)/s for Li-3(NH2)(2)I, and 10(-6) cm(2)/s for Li-1-CB9H10. For LiLa(BH4)(3)Cl and Li-3(NH2)(2)I, the diffusion coefficients are found to follow the Arrhenius behavior over the entire temperature ranges studied with the activation energies of 268(6) meV and 224(6) meV, respectively. For Li-1-CB9H10, the Arrhenius behavior with the activation energy of 265(6) meV is observed in the disordered high-temperature polymorph (360-403 K), whereas below 360 K the measured diffusivity drops significantly due to the transition to the ordered phase. Comparison of the measured Li+ diffusion coefficients with the ionic conductivity results and the data on the cation and anion jump rates provides new insights into the unusual dynamical properties of these superionic materials.

Automated summary

The study examined the diffusion of Li+ cations in complex hydrides, revealing fast Li+ diffusivities and superionic conductivity at room temperature or slightly above it. The experiments found that Li+ diffusion coefficients in the compounds follow Arrhenius behavior, providing new insights into the unusual dynamical properties of these superionic materials.