Enhanced Ionic Conduction in Metal–Organic-Framework-Based Quasi-Solid-State Electrolytes: Mechanistic Insights

Metal–organic frameworks (MOFs) are promising materials for quasi-solid-state electrolytes as a result of their tunable crystal structure and ion-selective capabilities. However, the rational design of MOF-based high-energy lithium battery electrolytes still requires improvement. In this study, MOF-based quasi-solid-state electrolytes (MQSSEs) were synthesized using various MOFs, and the effects of different metal active sites and ligand groups on their electrochemical performance were systematically investigated. The results indicate that active metal sites have a more significant impact on ion transport in MQSSEs compared to ligand groups. Specifically, the NiCu–MOF-74 electrolyte, featuring multi-metal synergy, exhibited higher lithium ion conductivity (0.69 mS cm–1) and a higher lithium-ion transference number (tLi+ = 0.72) than single-metal MOFs. The conductivities of Cu-based MQSSEs using different ligands were found to be similar. In a symmetrical battery setup with a NiCu–MOF-74 electrolyte and metallic lithium, stable plating and stripping of the lithium metal anode were observed at a current density of 0.5 mA cm–2. Furthermore, the MQSSE paired with a LiFePO4 cathode demonstrated 99.73% capacity retention after 200 cycles at 0.5 C. This study provides valuable insights into the structure–property relationship of MOFs, highlighting their tunable structure and electrochemical performance for the development of high-performance solid-state electrolytes.