Combined Effects of Anion Substitution and Nanoconfinement on the Ionic Conductivity of Li-Based Complex Hydrides

Abstract

Solid-state electrolytes are crucial for the realization of safe and high capacity all-solid-state batteries. Lithium-containing complex hydrides represent a promising class of solid-state electrolytes, but they exhibit low ionic conductivities at room temperature. Ion substitution and nanoconfinement are the main strategies to overcome this challenge. Here, we report on the synthesis of nanoconfined anion-substituted complex hydrides in which the two strategies are effectively combined to achieve a profound increase in the ionic conductivities at ambient temperature. We show that the nanoconfinement of anion substituted LiBH 4 (LiBH 4 −LiI and LiBH 4 −LiNH 2 ) leads to an enhancement of the room temperature conductivity by a factor of 4 to 10 compared to nanoconfined LiBH 4 and nonconfined LiBH 4 −LiI and LiBH 4 -LiNH 2 , concomitant with a lowered activation energy of 0.44 eV for Li-ion transport. Our work demonstrates that a combination of partial ion substitution and nanoconfinement is an effective strategy to boost the ionic conductivity of complex hydrides. The strategy could be applicable to other classes of solid-state electrolytes.

Li-Ion Diffusion in Nanoconfined LiBH4-LiI/Al2O3: From 2D Bulk Transport to 3D Long-Range Interfacial Dynamics

“…The pore size was determined by N 2 physisorption and turned out to be 8.7 nm. 11 The extra 30% was added to enable interconnectivity between the insulating alumina particles. The calculation was done after the specific pore volume has been determined via N 2 physisorption.…”

“…During a dwell time of 30 min, LiBH 4 and LiI built a solid solution and infiltrate the pores of the alumina support. 53 For a more detailed description, we refer to our recent study, 11 which also includes the characterization of the reference samples LiBH 4 -LiI, i.e., a sample without alumina.…”

“…Quite recently, some of us reported on enhanced ion dynamics in nanoconfined LiBH 4 -LiI. 11 Nanoconfinement was achieved by melt infiltration of LiBH 4 -LiI in the nanopores of alumina, Al 2 O 3 . The combination of both (i) substitutions of iodide anions for the BH 4 – units and (ii) spatial confinement resulted in a composite material with promising ion conductivities at room temperature.…”

“…In our recent study on this system, 11 we shed light on the correlation between morphology and Li-ion conductivity by means of X-ray diffraction, diffuse reflectance infrared Fourier transform spectra, N 2 physisorption, and scanning electron microscopy. Impedance spectroscopy was used to characterize long-range ion transport in the nanoconfined composite.…”

Anion and Cation Dynamics in Polyhydroborate Salts: NMR Studies

“…Recent studies of combined effects of anion substitution and nanoconfinement [ 97 ] have shown that, for mixed-anion systems Li(BH 4 ) x I 1- x and Li(BH 4 ) x (NH 2 ) 1- x confined in nanoporous SiO 2 or Al 2 O 3 , the Li + conductivity at room temperature is higher than both the conductivities of the corresponding bulk materials and the conductivity of nanoconfined LiBH 4 . These results demonstrate that combining partial anion substitution and nanoconfinement may be a promising approach to reach high ionic conductivity at room temperature in complex hydrides.…”