Crystal Structures and Thermodynamic Investigations of LiK(BH4)2, KBH4, and NaBH4 from First-Principles Calculations

The bialkali borohydride LiK(BH4)(2) has a total H-2 content of 10.65 wt % and has been suggested as a potential candidate for hydrogen storage applications. We report first-principles calculations of the structure and reaction thermodynamics of LiK(BH4)(2). We have also examined KBH4 and its structural analog NaBH4. Both KBH4 and NaBH4 have crystal structures involving disordered H atoms on lattice sites, and we show that the configurational entropy associated with these sites plays an important role in determining the stable phases of these materials. Our calculations indicate that LiK(BH4)(2) is unstable with respect to decomposition into LiBH4 and KBH4 at almost all temperatures. We have also searched for reactant compositions that can destabilize LiK(BH4)(2), KBH4, or NaBH4. This effort identified several coreactants in mixtures with KBH4 and NaBH4 that are predicted to have reaction thermodynamics for hydrogen release within the range of other borohydride-based systems being actively considered for hydrogen storage applications.