High Li⁺ and Na⁺ Conductivity in New Hybrid Solid Electrolytes based on the Porous MIL-121 Metal Organic Framework

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.