Quasi-Solid-State Electrolytes Engineered by Metal-Organic Frameworks: A Synergic Effect from Pore Characteristics and Ligand Functionalities

Abstract

Lithium metal batteries (LMBs) hold great promise for next-generation energy storage solutions but are hindered by the lagged development of high-performance solid-state electrolytes (SSEs) that are instrumental for cyclic stability and operational safety. In virtue of the tunable porosity and chemistry, Li⁺-conducting metal–organic frameworks (MOFs) have emerged as a compelling candidate. To interrogate the structure-performance correlation of MOF-based SSEs, this study fabricates a set of freestanding composite membranes comprising the UiO-series MOFs of varying ligand length and functional motifs. Comprehensive electrochemical assessments unveiled that both the functionalized linkers and larger pore structure facilitate Li⁺ transport within the MOF channels by promoting lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) decomplexation while reducing the steric hindrance. Consequently, the best SSE comprising the bipyridine ligand demonstrates a high ionic conductivity of 1.18 × 10⁻³ S cm⁻¹, a high Li⁺ transference number of 0.81, and a high potential window up to 4.97 V. Symmetric cells achieve a prolonged operation for 2660 h at 1 mA cm⁻² with a low cycling overpotential of 24.8 mV. LMB full-cells further showcase a stable operation at 1 C for 940 cycles with 90.7% of capacity retention, outperforming the majority of MOF-based SSEs reported in literature. This work, by capitalizing on the tailorable topological and chemical structure, offers a useful guideline for the design and fabrication of MOF‐based quasi-solid-state electrolytes for crafting high-performance LMBs.