First-principles study of the paths of the decomposition reaction of LiBH₄

Abstract

A clear description of the paths of thermal decomposition of complex borohydrides represents a crucial step forward to their utilisation as a reservoir of hydrogen and hence as materials for solid state hydrogen storage. We present in this work a theoretical study of the possible paths of decomposition of LiBH₄ by means of density functional theory approach. Our first-principles calculations showed the possibility to form linear chains of tetraborate of lithium in the residue of decomposition, among other thermodynamically competitive reactions. Their analytical formula LiBH _x agreed with the quantitative analysis already reported by Schlesinger and co-workers in the 1940s. The structure showed the formula unit Li₄B₄H₁₀, and the analytical formula LiBH_2.5, of which the Gibbs free energy of formation was −111.76 kJ mol⁻¹. The lattice stability was confirmed by the phonon calculations, which revealed all positive normal modes. Comparatively, the formation of lithium dodecaborate(12) is presented and discussed. The calculated standard Gibbs free energy of the decomposition reactions considered in the present work were in the range (158,286) kJ mol⁻¹ of LiBH₄ decomposed.

Keywords:

• hydrogen storage
• DFT calculations
• lithium borohydride
• lithium borides
• lithium tetraborate

Acknowledgements

The authors would like to thank Dr Adem Tekin for valuable discussions and gratefully acknowledge the financial support from the European Commission DG Research RTN Marie Curie Actions-Hydrogen (contract MRTN-CT-2006-032474, until February 2009) and the 7.Serie Interner F+E-Projekte, EMPA funding. The computer facilities IPAZIA at EMPA in Dübendorf, Switzerland are highly appreciated (http://ipazia.empa.ch).

Notes

Notes

1. Early in 1941 H.I. Schlesinger and co-workers, among them H.C. Brown, worked on the synthesis of metal borohydrides, to obtain, at that time, new volatile compounds even of heavy metals in particular, uranium. Most of the work was carried out under contracts with the National Defense Research Committee. Recently, interest has been revived for their potential use as hydrogen reservoir materials, as shown by the increasing amount of work published in the last decades worldwide.

2. The solid state synthesis of some of the lithium borides has been reported by R. Nesper and co-workers.

3. The heat of formation of LiBH₄ was already determined by Davis in the 1949 via the Hesse cycle in solution. Recently, it was reported by Chase in 1998, as cited on the NIST reference page. In all those works, nothing is reported regarding the relationship between structure and heat of formation. Our calculations, reported in the present work, refer to the lowest orthorhombic energy structure found by our modelling approach. Our calculated entropy value is +86.583 J mol⁻¹ comparable, anyway with the reported value in the NIST web book, which is equal to +75.88 J mol⁻¹.

4. Regarding the formation of boranes, even at the solid state, a systematic research has been done since the first study by Alfred Stock in 1912, and since then there has been a steady stream of other boron hydride formations also at the solid state 46.

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