Lecturer Motoaki Matsuo
Institute for Materials Research, Tohoku University, Sendai, Japan

Fast Ionic Conduction in Complex Hydrides

Complex hydrides has been attracting great interest because of its various energy-related functions such as microwave absorption and lithium fast-ionic conduction as well as hydrogen storage. By microwave irradiation, LiBH₄ consisting of Li⁺ and [BH₄]⁻ complex anions, which is one of representative complex hydrides, is heated rapidly above 380 K due to a large conductive loss, at which almost all hydrogen is desorbed [1]. As a result of clarifying the mechanism of the microwave absorption, we discovered lithium fast-ionic conduction in LiBH₄ accompanied by the structural transiton (more than 1×10⁻³ S/cm over 390 K), which opened up research on lithium ionic conduction in complex hydrides [2]. Then, we have worked on the design of lithium fast ionic conductors in LiBH₄-based, LiNH₂-based, and LiAlH₄-based complex hydrides [3]. It has been recently reported that the complex hydrides can be used as electrolytes in all-solid-state lithium rechargeable batteries [4].

These findings suggest that ionic conductions of the other cation such as Na⁺, K⁺ and Mg²⁺ may also occur in complex hydrides because they are composed of various combinations of the above-described metal cations and complex anions. In fact, Na₂(BH₄)(NH₂) obtained in the NaBH₄–NaNH₂ system exhibits the sodium fast ionic conductivity of 2×10⁻⁶ S/cm at RT because of the specific antiperovskite-type structure with vacancies on the Na⁺ site [5]. Just recently, Na₂B₁₀H₁₀ and Na₂B₁₂H₁₂ were found to exhibit high ionic conductivity on the order of 0.1 S/cm above its order-disorder phase-transition at about 360 K and 530 K, respectively [6, 7].

References

1. M. Matsuo et al., Appl.Phys. Lett., 90 (2007) 232907.
2. M. Matsuo et al., Appl.Phys. Lett., 91 (2007) 224103.
3. M. Matsuo and S. Orimo, Adv. Energy Mater., 1 (2011) 161.
4. A. Unemoto et al., Adv. Func. Mater., 24 (2014) 2267.
5. M. Matsuo et al., Appl.Phys. Lett., 100 (2012) 203904.
6. T.J. Udovic et al., Chem. Comm., 50 (2014) 3750.
7. T.J. Udovic et al., Adv. Mater., in press.