First-principles study on copper-substituted lithium borohydride, (Li1−xCux)BH4

Our recent analysis using the first-principles calculations for LiBH4 [K. Miwa, N. Ohba, S. Towata, Y. Nakamori, S. Orimo, Phys. Rev. B 69 (2004) 245120], which is rather stable and desorbs hydrogen only at elevated temperatures, reveals that the charge compensation by Li+ cations is a key feature for the stability of the internal bonding [BH4]− anions. Considering this character, we have proposed the partial substitution of more electronegative elements for Li to lower the dehydriding temperature. The effect of the cation substitution is examined theoretically. We select copper as a substituent element, since the ionic radii of Cu+ and Li+ are almost the same but the electronegativity of Cu is larger than that of Li. Assuming the same crystal structure as orthorhombic LiBH4, the calculations on (Li1−xCux)BH4 are performed for x=0.25, 0.5, 0.75, and 1. It is confirmed that the heat of formation increases with increasing the Cu content x. The optimum x is predicted to be around x=0.3 for practical applications.