Exploring Novel Methods for Enhancing Ionic Conductivities of Complex Borohydrides (Li, Na, K and Mg)

Abstract

Since the discovery of the high ionic conductivity in lithium borohydride at high-temperature, turning complex borohydrides into fast ion conductors, has become an extensive research area. However, the low ionic conductivity at ambient temperature of borohydrides remains a major hurdle for their application in real systems. In addition to creating defects in the intrinsic structure of the complex borohydrides by introduction doping with elements of large anionic radius and high polarizability, nanoconfinement has shown some intriguing effects in enhancing the ionic conductivity of lithium borohydride (LiBH4). According to the initial report on nanoconfined LiBH4 in MCM-41, a ionic conductivity of ~10-3 S cm-1 can be achieved for Li+ conduction at 140 ℃. To date, the effect of such an encapsulation method on the ionic conductivity has only been reported for LiBH4. Herein, we thus investigated the potential of nanoconfinement on enhancing the conductivity of other borohydrides including NaBH4 and KBH4. Our results on the conductivity of nanoconfined borohydrides also triggered our attention on the possibility of increasing the ionic conductivity of complex borohydrides at room temperature, through a simple oxidation process. Hence, we have found that by simply exposing NaBH4 to air, the resulting partial surface oxidation enabled a significant improvement in the ionic conductivity of NaBH4 at room temperature. This behaviour was also observed for LiBH4 and Mg(BH4)2 with several-folds increase in their ionic conductivity. The current work reports on these findings and future challenges associated with the application of such observations.