Lithium borohydride

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Lithium borohydride (LiBH₄) is known in organic synthesis as a reducing agent for esters.

Schematics of lithium borohydride recycling. Inputs are lithium borate and hydrogen.

It is also renowned for one of the highest energy density chemical energy carriers. By reacting with atmospheric oxygen it liberates 65.2 MJ/kg heat, and its density of 0.666 g/cm³ gives 43.4 MJ/L. In comparison gasoline is 44 MJ/kg and 29 MJ/L, while liquid hydrogen is 120 MJ/kg, but a very low 8 MJ/L because of the very low 0.0708 g/cm³. The high specific energy density of lithium borohydride has been well known, and the material has been proposed for rocket fuel, but despite the research and advocacy it has not been used widely. One of the issues with all chemical hydride based energy carriers is the recycling complexity and energy conversion efficiency. While batteries such as lithium ion carry an energy density of up to 0.72 MJ/kg and 2 MJ/L, their DC to DC efficiency is on the order of 90%. In view of the complexity of recycling mechanisms for metal hydrides,^[1] such high energy conversion efficiencies are beyond practical reach.