Ammonia, a Switch for Controlling High Ionic Conductivity in Lithium Borohydride Ammoniates

Lithium borohydride ammoniates can be readily generated by LiBH4 absorbing ammonia at room temperature. Li(NH3)(n)BH4 (0 < n <= 2) performed high ionic conductivity near room temperature, e.g., 2.21 x 10(-3) S cm(-1) for mono-ammoniate at 40 degrees C. A drastic increase in ionic conductivity occurs around 38 degrees C due to the structural change resulting from ammonia desorption. Moreover, the jump of the ionic conductivity is reversible because ammonia absorption/desorption acts as a switch for the structural change. The experimental results show that the stable electrochemical window for the sample is similar to 4 V. The crystal structure of Li(NH3)nBH(4) (0 < n <= 1) can be easily changed while the equilibrium dynamically changes with temperature. First-principles calculation results also indicate that Li(NH3)BH4 exerts an expected compatibility in contact with lithium iron phosphate cathode material. Finally, by introducing a gas instead of substituting an element, a dynamic change of crystal structure is achieved, accompanied with the improvement of lithium ionic conductivity.