Nanoconfined LiBH₄ as a Fast Lithium Ion Conductor

Abstract

Designing new functional materials is crucial for the development of efficient energy storage and conversion devices such as all solid‐state batteries. LiBH4 is a promising solid electrolyte for Li‐ion batteries. It displays high lithium mobility, although only above 110 °C at which a transition to a high temperature hexagonal structure occurs. Herein, it is shown that confining LiBH4 in the pores of ordered mesoporous silica scaffolds leads to high Li+ conductivity (0.1 mS cm−1) at room temperature. This is a surprisingly high value, especially given that the nanocomposites comprise 42 vol% of SiO2. Solid state 7Li NMR confirmed that the high conductivity can be attributed to a very high Li+ mobility in the solid phase at room temperature. Confinement of LiBH4 in the pores leads also to a lower solid‐solid phase transition temperature than for bulk LiBH4. However, the high ionic mobility is associated with a fraction of the confined borohydride that shows no phase transition, and most likely located close to the interface with the SiO2 pore walls. These results point to a new strategy to design low‐temperature ion conducting solids for application in all solid‐state lithium ion batteries, which could enable safe use of Li‐metal anodes.

Halogenated Sodium-closo-Dodecaboranes as Solid-State Ion Conductors

“…[8][9][10][11] LiBH 4 shows fast Li-ion conductivity above a polymorphic phase transition, which can be stabilized to room temperature by anion substitution of BH4 -by I -12,13 or by nanoconfinement. 14 [15][16][17][18][19] The high conductivity in these compounds is linked to the highly disordered high temperature polymorphs, where partially occupied cation sites and fast reorientational dynamics of the anion promote excellent cation mobility. Perchlorinated lithium-closo-boranes, Li 2 B 12 Cl 12 and Li 2 B 10 Cl 10 , were proposed as liquid electrolytes in Li/SOCl 2 battery cells in the early 1980s, where the extraordinary stability of [B n Cl n ] 2--anions (n = 10, 12) towards temperature and the Li anode were a significant advantage.…”

Complex hydrides as room-temperature solid electrolytes for rechargeable batteries

“…conductivities (10 -4 S cm -1 ) at room temperature (Fig. 3) [39]. These are surprisingly high values, given that these nanocomposites comprise a large fraction (up to 50 vol%) of insulating SiO 2 and also a significant void fraction.…”

“…These are surprisingly high values, given that these nanocomposites comprise a large fraction (up to 50 vol%) of insulating SiO 2 and also a significant void fraction. The high room-temperature ionic conductivity in LiBH 4 by confinement in a mesoporous SiO 2 matrix is stable (when temperature cycling to 140°C, electrochemically cycling ±6 V) and reproducible [39].…”

“…Confinement of LiBH 4 in the oxide pores leads to limited stabilisation of the hexagonal high-temperature phase in the core of the pores. However, the high ionic mobility seems rather associated with the fraction of LiBH 4 that is close to the SiO 2 interface [39,40]. In general high defect densities and low diffusion barriers are found at the interface between two solids, due to effects such as disorder, strain, and space charge regions.…”

Fluorine Translational Anion Dynamics in Nanocrystalline Ceramics: SrF2-YF3 Solid Solutions

“…Considering nanocrystalline ionic conductors [2][3][4][5], a range of studies report on enhanced anion and cation dynamics [6,7]. Structural disorder and defects [8][9][10], lattice mismatch [11][12][13] as well as size effects [14][15][16][17][18][19], which results in extended space charge regions, are used to explain the properties of nanocrystalline compounds. Especially for nano-engineered systems, which were prepared by bottom-up procedures, such as gas condensation or epitaxial methods [20,21], space charge regions lead to non-trivial effects that may enhance ion transport.…”