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.

Solid electrolytes based on LiBH 4 receive much attention because of their high ionic conductivity, electrochemical robustness, and low interfacial resistance against Li metal. The highly conductive hexagonal modification of LiBH 4 can be stabilized via the incorporation of LiI. If the resulting LiBH 4 -LiI is confined to the nanopores of an oxide, such as Al 2 O 3 , interface-engineered LiBH 4 -LiI/Al 2 O 3 is obtained that revealed promising properties as a solid electrolyte. The underlying principles of Li + conduction in such a nanocomposite are, however, far from being understood completely. Here, we used broadband conductivity spectroscopy and 1 H, 6 Li, 7 Li, 11 B, and 27 Al nuclear magnetic resonance (NMR) to study structural and dynamic features of nanoconfined LiBH 4 -LiI/Al 2 O 3 . In particular, diffusion-induced 1 H, 7 Li, and 11 B NMR spin–lattice relaxation measurements and 7 Li-pulsed field gradient (PFG) NMR experiments were used to extract activation energies and diffusion coefficients. 27 Al magic angle spinning NMR revealed surface interactions of LiBH 4 -LiI with pentacoordinated Al sites, and two-component 1 H NMR line shapes clearly revealed heterogeneous dynamic processes. These results show that interfacial regions have a determining influence on overall ionic transport (0.1 mS cm –1 at 293 K). Importantly, electrical relaxation in the LiBH 4 -LiI regions turned out to be fully homogenous. This view is supported by 7 Li NMR results, which can be interpreted with an overall (averaged) spin ensemble subjected to uniform dipolar magnetic and quadrupolar electric interactions. Finally, broadband conductivity spectroscopy gives strong evidence for 2D ionic transport in the LiBH 4 -LiI bulk regions which we observed over a dynamic range of 8 orders of magnitude. Macroscopic diffusion coefficients from PFG NMR agree with those estimated from measurements of ionic conductivity and nuclear spin relaxation. The resulting 3D ionic transport in nanoconfined LiBH 4 -LiI/Al 2 O 3 is characterized by an activation energy of 0.43 eV.

Solid‐state electrolytes (SSEs) can leapfrog the development of all‐solid‐state batteries (ASSBs), enabling them to power electric vehicles and to store renewable energy from intermittent sources. Here, a new hybrid Li+ and Na+ conducting SSE based on the MIL‐121 metal‐organic framework (MOF) structure is reported. Following synthesis and activation of the MOF, the free carboxylic units along the 1D pores are functionalized with Li+ or Na+ ions by ion exchange. Ion dynamics are investigated by broadband impedance spectroscopy and by 7Li and 23Na NMR spin‐lattice relaxation. A crossover at 50 °C (Li+) and at 10 °C (Na+) from correlated to almost uncorrelated motion at higher temperature is observed, which is in line with Ngai's coupling model. Alternatively, in accordance to the jump relaxation model of Funke, at low temperature only a fraction of the jump processes are successful as lattice rearrangement in the direct vicinity of Li+ (Na+) is slow. 1H NMR unambiguously shows that Li+ is the main charge carrier. Conductivities reach 0.1 mS cm−1 (298 K, Na+) while the activation energies are 0.28 eV (Li+) and 0.36 eV (Na+). The findings pave the way towards development of easily tunable and rationally adjustable high‐performance MOF‐based hybrid SSEs for ASSBs.