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

Abstract 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.