Lithium borohydride

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Lithium borohydride
Unit cell of lithium borohydride at room temperature
Unit cell of lithium borohydride at room temperature
Names
IUPAC name
Lithium tetrahydridoborate(1–)
Other names
Lithium hydroborate,
Lithium tetrahydroborate
Borate(1-), tetrahydro-, lithium, lithium boranate
Identifiers
3D model (
JSmol
)
ChemSpider
ECHA InfoCard
 100.037.277 Edit this at Wikidata
RTECS number
  • ED2725000
UNII
  • InChI=1S/BH4.Li/h1H4;/q-1;+1 checkY
    Key: UUKMSDRCXNLYOO-UHFFFAOYSA-N checkY
  • InChI=1/BH4.Li/h1H4;/q-1;+1
    Key: UUKMSDRCXNLYOO-UHFFFAOYAS
  • [Li+].[BH4-]
Properties
LiBH4
Molar mass 21.784 g/mol
Appearance White solid
Density 0.666 g/cm3[1]
Melting point 268 °C (514 °F; 541 K)
Boiling point 380 °C (716 °F; 653 K) decomposes
reacts
Solubility in ether 2.5 g/100 mL
Structure[2]
orthorhombic
Pnma
a = 7.17858(4), b = 4.43686(2), c = 6.80321(4)
216.685(3) A3
4
[4]B
Thermochemistry
82.6 J/(mol⋅K)
75.7 J/(mol⋅K)
Std enthalpy of
formation
(ΔfH⦵298)
 −198.83 kJ/mol
Hazards
> 180 °C (356 °F; 453 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Lithium borohydride (LiBH4) is a borohydride and known in organic synthesis as a reducing agent for esters. Although less common than the related sodium borohydride, the lithium salt offers some advantages, being a stronger reducing agent and highly soluble in ethers, whilst remaining safer to handle than lithium aluminium hydride.[3]

Preparation

Lithium borohydride may be prepared by the metathesis reaction, which occurs upon ball-milling the more commonly available sodium borohydride and lithium bromide:[4]

NaBH4 + LiBr → NaBr + LiBH4

Alternatively, it may be synthesized by treating boron trifluoride with lithium hydride in diethyl ether:[5]

BF3 + 4 LiH → LiBH4 + 3 LiF

Reactions

Lithium borohydride is useful as a source of

Brønsted–Lowry-acidic substances (sources of H+) to form hydrogen
gas.

Reduction reactions

As a

alkyl halides, or secondary and tertiary
amides.

Hydrogen generation

Lithium borohydride reacts with water to produce hydrogen. This reaction can be used for hydrogen generation.[8]

Although this reaction is usually spontaneous and violent, somewhat-stable aqueous solutions of lithium borohydride can be prepared at low temperature if degassed, distilled water is used and exposure to oxygen is carefully avoided.[9]

Energy storage

Volumetric vs gravimetric energy density
Schematic of lithium borohydride recycling. Inputs are lithium borate and hydrogen.

Lithium borohydride is renowned as one of the highest-

MJ/L. In comparison, gasoline gives 44 MJ/kg (or 35 MJ/L), while liquid hydrogen gives 120 MJ/kg (or 8.0 MJ/L).[nb 1] The high specific energy density of lithium borohydride has made it an attractive candidate to propose for automobile and rocket fuel, but despite the research and advocacy, it has not been used widely. As with all chemical-hydride-based energy carriers, lithium borohydride is very complex to recycle (i.e. recharge) and therefore suffers from a low energy conversion efficiency. While batteries such as lithium-ion carry an energy density of up to 0.72 MJ/kg and 2.0 MJ/L, their DC-to-DC conversion efficiency can be as high as 90%.[10] In view of the complexity of recycling mechanisms for metal hydrides,[11]
such high energy-conversion efficiencies are not practical with present technology.

Comparison of physical properties
Substance
kg
g/cm3
MJ/L
LiBH4 065.20 0.6660 43.4
Regular gasoline 044.00 0.7200 34.8
Liquid hydrogen
 120.00 0.0708 8
Lithium-ion battery 000.72 2.8000 2

See also

Notes

  1. ^ The greater ratio of energy density to specific energy for hydrogen is because of the very low mass density (0.071 g/cm3).

References

  1. ^ Sigma-Aldrich Product Detail Page.
  2. doi:10.1016/S0925-8388(02)00521-2.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  3. ^
    doi:10.1002/047084289X.rl061.pub2
    .
  4. doi:10.1002/14356007.a13_199
    .
  5. ISBN 978-0-12-126601-1
    .
  6. ISBN 978-0-08-040599-5
    .
  7. ^
    doi:10.1021/jo00371a017
    .
  8. doi:10.1016/j.ijhydene.2003.12.009
    .
  9. doi:10.1021/ja01129a048
    .
  10. ^ Valøen, Lars Ole and Shoesmith, Mark I. (2007). The effect of PHEV and HEV duty cycles on battery and battery pack performance (PDF). 2007 Plug-in Highway Electric Vehicle Conference: Proceedings. Retrieved 11 June 2010.
  11. ^ U.S. patent 4,002,726 (1977) lithium borohydride recycling from lithium borate via a methyl borate intermediate.
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