Enhanced Li Ion Conductivity in LiBH₄–Al₂O₃ Mixture via Interface Engineering

Abstract

A new solid-state Li ion conductor composed of LiBH 4 and Al 2 O 3 was synthesized by a simple ball-milling process. The element distribution map obtained by transmission electron microscopy demonstrates that the LiBH 4 and Al 2 O 3 are well mixed and form a large interface after ballmilling. The ionic conductivity of the mixture reaches as high as 2 × 10 −4 S cm −1 at room temperature when the volume fraction of Al 2 O 3 is approximately 44%. The ionic conductivity of the interface between LiBH 4 and Al 2 O 3 was extracted by using a continuum percolation model, which turns out to be about 10 −3 S cm −1 at room temperature, being 10 5 times higher than that of pure LiBH 4 . This remarkable rise in conductivity is accompanied by the lowered activation energy for the Li ion conduction in the mixture, indicating that the interface layer facilitates Li ion conduction. Near-edge X-ray absorption fine structure analysis reveals the presence of B− O bondings in the mixture, which was not detected by X-ray diffraction. This disruption of the chemical bondings at the interface may allow an increase in carrier concentration and/or mobility thereby resulting in the pronounced enhancement in conductivity. This result provides a guideline for designing fast Li ion conductor through interface engineering.

Phase Stability and Fast Ion Conductivity in the Hexagonal LiBH4–LiBr–LiCl Solid Solution

“…30,36 Another approach is to increase the ionic conductivity of LiBH 4 by mixing it with oxides or by means of nanoconfinement. [37][38][39][40] As described above, halogenation represents a successful strategy to stabilize the LiBH 4 conducting phase at RT. However, the ionic conductivity alone is not sufficient for designing effective SEs, but it must be flanked by an elevate (electro)chemical compatibility between SE and electrodes.…”

The influence of silica surface groups on the Li-ion conductivity of LiBH4/SiO2 nanocomposites

“…27,28 A similar increase in conductivity was reported for LiBH 4 /SiO 2 and LiBH 4 /Al 2 O 3 composites prepared by mechanical milling. [30][31][32] Nanoconfinement of LiBH 4 was originally motivated by the idea that it could stabilize the high temperature (conductive) phase. 33 However, the high room temperature ionic conductivity was also observed in nanocomposites in which the structural phase transition took place well above room temperature.…”

“…39 It is currently believed that the increased ionic conductivity is related to interface effects, such as the presence of a space charge layer and/or (partial) reaction at the LiBH 4 /metal oxide interface causing a different LiBH 4 structure or stoichiometry. [30][31][32][39][40][41][42][43] The space-charge effect is the accumulation or depletion of mobile charge carriers near an interface between two materials with different Fermi levels, due to a local electric field. 42,[44][45][46] This effect is held responsible for the high ionic conductivity of binary mixtures of inorganic ion conductors such as AgX or LiX (X = F, Cl, Br, I) and non-conducting materials such as metal oxides and ceramics.…”

“…[51][52][53] As far as we are aware, the impact of space charge effects on the ionic conductivity of complex hydrides/metal oxide composites has not yet been proven. [30][31][32] Another effect that could play a role is reaction between LiBH 4 and the metal oxide surface. Although it has been shown that reaction to form stable silicates is kinetically limited by the presence of hydrogen during the synthesis of LiBH 4 /SiO 2 nanocomposites, reaction is expected to be particularly favourable with reactive surface groups, such as hydroxyl groups.…”

Electrolytes for Energy Dense Batteries

“…[26][27] The study suggested that the high mobility of Li + was not related to stabilization of the hexagonal phased but presumably caused by high density of defects and low diffusion barriers at the interface between the two solids, which resulted from disorder, strain and spacecharge regions. [27][28] Follow up studies demonstrated similar effect using Al2O3 29 and C60 30 additives, note that the electronic conductivity in the C60 30 composites could be higher than that required from a solid electrolyte. Formation of composites with sulfides was first reported for LiBH4:P2S5, where the preparation process resulted in a material that went beyond a composite formation evident from a new crystalline 90LiBH4:10P2S5 phase that exhibited a high conductivity in the order of 10 -3 S cm -1 around 300 K. 31 Other composites that yielded varying degrees of conductivity improvements, albeit less than those achieved with the sulfides, included composites with other borohydrides as Ca(BH4)2 34 , NaBH4, 35 with hydrides such as MgH2, 36 or borohydridehydride ternary mixtures, 37 and mixtures with halides such as NaCl.…”