Ionic Liquid-Laden Zn-MOF-74-Based Solid-State Electrolyte for Sodium Batteries

Sodium batteries are receiving increasing interest as an alternative to reduce dependence on lithium-based systems. Furthermore, the development of solid-state electrolytes will lead to higher-performing and safer devices. In this work, a Zn-based metal-organic framework (Zn-MOF-74) is combined as a physical barrier against the growth of dendrites, together with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][TFSI]) ionic liquid, which provides improved mobility to sodium ions. It is demonstrated that the incorporation of the appropriate amount of ionic liquid within the pores of the MOF produces a considerable increase in ionic conductivity, achieving values as high as 5 x 10-4 S cm-1 at room temperature, in addition to an acceptable Na+ transference number. Furthermore, the developed Na[EMIm][TFSI]@Zn-MOF-74 hybrid solid electrolyte contributes to stable and dendrite-free sodium plating/stripping for more than 100 h. Finally, a more than notable extension of the electrochemical stability window of the electrolyte has been determined, being useful even above 7 V vs. Na+/Na. Overall, this work presents a suitable strategy for the next generation of solid-state sodium batteries.