Lithium amide–borohydrides Li[BH₄]_1–x[NH₂]_x possess liquid-like Li superionic conductivity at nearly ambient temperature. The fast Li⁺ diffusion facilitated by the localized motions of the anions is proposed to occur through a network of vacant tetrahedral sites, acting as conduction channels. To study the reorientational dynamics of the anions, we have performed quasielastic neutron scattering experiments on samples with different compositions (x = 2/3, 0.722, 0.737, 3/4) over a broad temperature and time range. To unambiguously disentangle the contributions of the two species, [BH₄]^− and [NH₂]^−, we took advantage of deuterium labeling and could clearly demonstrate that the quasielastic broadening is mainly determined by the [BH₄]^− reorientations. With the help of a newly developed model, supported by ab initio molecular dynamics calculations, we have identified three relaxation components, which account for generally anisotropic C₃-rotations of the [BH₄]^− tetrahedra including jumps by a small angle from the equilibrium position.

High ionic conductivity of up to 6.4 ×10^−3S cm^−1 near room temperature  (40 °C) in lithium amide-borohydrides is reported, comparable to values  of liquid organic electrolytes commonly employed in lithium-ion batteries.  Density functional theory is applied coupled with X-ray diffraction, calorimetry, and nuclear magnetic resonance experiments to shed light on the  conduction mechanism. A Li₄Ti₅O₁₂ half-cell battery incorporating the lithium amide-borohydride electrolyte exhibits good rate performance up to 3.5 mA cm^−2 (5 C) and stable cycling over 400 cycles at 1 C at 40 °C, indicating high bulk and interfacial stability. The results demonstrate the potential of lithium amide-borohydrides as solid-state electrolytes for high-power lithium-ion batteries.

Na₂(B₁₂H₁₂)_0.5(B₁₀H₁₀)_0.5, a new solid-state sodium electrolyte is shown to offer high Na⁺ conductivity of 0.9 mS cm^-1 at 20 °C, excellent thermal stability up to 300 °C, and a large electrochemical stability window of 3 V including stability towards sodium metal anodes, all essential prerequisites for a stable room-temperature 3 V all-solid-state sodium-ion battery.

The high energy of hydrogen vibrations in solids is the origin of their strong impact on thermodynamic properties. The peculiar structure of complex hydrides amplifies this impact. We shed light on the vibrational properties of three allotropes of Ca(BH₄)₂ using density-functional theory calculations, infrared spectroscopy, and inelastic neutron scattering. We show that the vibrational properties of Ca(BH₄)₂ depend on the specific phase and are hitherto the origin of their differences in stability.