Core-Shell MOF-in-MOF Nanopore Bifunctional Host of Electrolyte for High-Performance Solid-State Lithium Batteries

Solid-state lithium-ion batteries with high safety are the encouraging next-generation rechargeable electrochemical energy storage devices. Yet, low Li⁺ conductivity of solid electrolyte and instability of solid-solid interface are the key issues hampering the practicability of the solid electrolyte. In this research, core-shell MOF-in-MOF nanopores UIO-66@67 are proposed as a unique bifunctional host of ionic liquid (IL) to fabricate core-shell ionic liquid-solid electrolyte (CSIL). In the current design of CSIL, the shell structure (UIO-67) has a large pore size and a high specific surface area, boosting the absorption amount of ionic liquid electrolyte, thus increasing the ionic conductivity. Nevertheless, the core structure (UIO-66) has a small pore size compared to the ionic liquid, which can confine the large ions, decreasing their mobility, and selectively boost the transport of Li⁺ . The CSIL solid electrolyte exhibits considerable enhancement in the lithium transference number (t_Li ⁺ ) and ionic conductivity compared to the homogenous porous host (pure UIO-66 and UIO-67). Additionally, the Li|CSIL|Li symmetric batteries maintain a stable polarization of less than 28 mV for more than 1000 h at 1000 µA cm^-2 . Overall, the results demonstrate the concept of core-shell MOF-in-MOF nanopores as a promising bifunctional host of electrolytes for solid-state or quasi-solid-state rechargeable batteries.