Mobility and dynamics in the complex hydrides LiAlH4 and LiBH4

Area of Science:

• Materials Science
• Solid-State Chemistry
• Spectroscopy

Background:

• Complex hydrides like lithium borohydride (LiBH4) and lithium aluminum hydride (LiAlH4) are crucial for hydrogen storage and ion conduction.
• Understanding the dynamics and bonding of these materials is essential for optimizing their performance.
• Vibrational spectroscopy offers a powerful tool to probe atomic and molecular motions within these compounds.

Purpose of the Study:

• To investigate the dynamics and bonding of LiBH4 and LiAlH4 using a combination of vibrational spectroscopy techniques.
• To elucidate the diffusion mechanisms and coefficients of hydrogen-containing species within these complex hydrides.
• To explore the potential for tailoring material properties through anion exchange.

Main Methods:

• Infrared (IR) spectroscopy, Raman spectroscopy, and inelastic neutron scattering (INS) were employed.
• Deuterated samples were used to study isotope effects and tracer diffusion.
• Nuclear magnetic resonance (NMR) and temperature-dependent Raman spectroscopy were used to study Li-ion conductivity and phase transitions.

Main Results:

• Distinct hydrogen mobility was observed between LiBH4 and LiAlH4, despite their iso-electronic structures.
• Diffusion coefficients were quantified: D ≈ 7 x 10⁻¹⁴ m²/s at 473 K for LiAlH4 and D ≈ 5 x 10⁻¹⁶ m²/s at 348 K for LiBH4.
• Anion exchange in LiBH4 (e.g., with I⁻ or NH₂⁻) can tune phase transition temperatures and influence Li-ion conductivity.

Conclusions:

• Vibrational spectroscopy provides comprehensive insights into the lattice and anion dynamics of LiBH4 and LiAlH4.
• Anion exchange offers a viable strategy to modify the physical properties of LiBH4, particularly its ionic conductivity.
• While anion exchange impacts Li-ion conductivity, its influence on hydrogen sorption properties appears limited.