Solid-state electrolytes for battery systems represent a fast-developing research field. Here, we focus on a new class of solid-state electrolytes based on metal organic frameworks (MOFs). An endless number of linkers and metal centers can be used to build a tremendous variety of different MOFs with tunable physical and chemical properties. MOFs are a thus a versatile platform for the development of solid state electrolytes for mono- and multivalent electrochemical systems.

In this study, MIL-121 (Al centers linked by pyromellitic acid) was synthesized by a hydrothermal route and post synthetically modified to embed lithium and sodium species in the MOF structure. After the lithiation and sodiation step, the samples were soaked with a limited quantity of LiClO₄ or NaClO₄ in propylene carbonate in order to increase the ion content even further. While some samples may contain up to 30 wt% small molecules, they remain solid materials with no visible traces of liquids; the small molecules are adsorbed in the pores of the MOF and lead to an improved ionic conductivity.

At 303 K, a high Na⁺ conductivity of 1.2 ∙10⁻⁴ S cm⁻¹ was measured. This fits nicely to the only very few previous investigations on different MOF structures as solid-state electrolytes. Low activation energies of 0.28 eV (above 323 K) and 0.36 eV (above 283 K) for Li and Na, respectively, were found in the higher temperature region. NMR spectroscopy confirmed the mobility of both sodium and lithium ions.

In conclusion, successful modifications (lithiation, sodiation) lead to encouraging ion conductivities in MIL-121 Al-based MOF. These findings pave the way towards development of easily tunable and rationally adjustable hybrid solid-state electrolytes.