EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Solid-state electrolytes (SSEs) for battery systems is a fast developing field. Here, we focus on a new class of solid-state electrolytes based on metal organic frameworks (MOFs). In this study MIL-121 (Al centres linked by pyromellitic acid) was synthesized by a hydrothermal route [1], post synthetically modified with lithium acetate and sodium acetate and subsequently soaked with $\mathrm{LiClO}_4$ or $\mathrm{NaClO}_4$ in propylene carbonate in order to increase the ion content even further.
X-ray powder diffraction (XRD) revealed that the structure of pristine MIL-121 could be largely maintained after lithiation or sodiation together with a small loss in crystallinity. At 303 K a conductivity of $4.6 \cdot 10^{−6}$ $\mathrm{S/cm}^{−1}$ for Li ions and $1.2 \cdot 10^{−4}$ $\mathrm{S/cm}^{−1}$ for Na ions was measured; in line with other MOF based SSEs.[2] Interestingly, activation energies were different at higher and lower temperatures. The kink in the Arrhenius curve could not be assigned to structural changes or phase transitions. Hence, the observed non-Arrhenius behaviour was attributed to a change from correlated to uncorrelated motion as suggested in the model of Ngai.[3] $^7$Li NMR line shapes and spin-lattice relaxation (SLR) NMR suggested two different conduction processes pointing out the vital role of the liquid electrolyte. $^1$H SLR NMR measurements revealed that indeed the alkali metal ion is the moving species in the material. A correlation between the alkali metal ion and hydrogen might, however, play a major role in the diffusion process at lower temperatures. The successful modifications (lithiation, sodiation) of MIL-121 lead to encouraging conductivities and proved the potential suitability in batteries of this young class of solid-state ion conductors.