Fast Lithium Ion Migration in Room Temperature LiBH₄

Abstract

The defect structure and the Li ion diffusion mechanism of orthorhombic LiBH4 (o-LiBH4) are studied by first-principles calculations to elucidate the Li ion transport in o-LiBH4. Two metastable Li interstitial sites are identified, and the formation energies of the Schottky and Frenkel defect pair are calculated to be 1.2–1.4 eV, the former being slightly easier to form. The energy required to form intrinsic defects is higher than that of hexagonal LiBH4 (h-LiBH4). On the other hand, the migration energy barrier of the Li vacancy or interstitial ranges from 0.1 to 0.3 eV, which is comparable to that of h-LiBH4. Therefore, the higher defect formation energy mainly accounts for the much lower Li ion conductivity of o-LiBH4. The calculated overall activation barrier for the Li ion conduction is in fair agreement with the experimental activation energy. Molecular dynamics simulation demonstrates that both the interstitial and the interstitialcy mechanisms are operative for the Li interstitial diffusion and that the interconnected interstitial sites compose a fast diffusion path. The simulation results point out that the enhancement of the carrier density via defect or interface engineering may significantly raise the ionic conductivity of o-LiBH4.

Combined DFT and geometrical–topological analysis of Li-ion conductivity in complex hydrides

“…Calculated migration energies from the literature are also shown in Table 3 for comparison with our results and we can note that there is a satisfactory agreement between them. For example, Cho et al 50 reported similar values for LiBH 4 , in the range 0.1-0.3 eV, depending on the diffusion mechanism and paths. The value of 0.3 eV reported by Hoang et al 49 is matching our result.…”

“…Different distances between the interstitial atom and the vacancy are thus obtained and the formation energy can be obtained by extrapolation at infinite distance. Cho et al 50 reported the results for the formation energy of Frenkel defect pairs in LiBH 4 using this approach.…”

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.…”