Synthesis and Li+ conduction study of solid ionic liquids derived from Zwitterion modified metal-organic framework (MOF)

Construction of fast ion-conduction channels is of great significance for the performance breakthrough of Li⁺ solid state electrolytes (LiSSE). It not only can significantly improve target ion transfer, but also enhance the Li⁺ migration number. This study reports a new type of solid electrolyte of metal-organic framework (MOF) covalently modified by Li⁺ ionic liquids derived from zwitterion APS group, namely UiO-66-APS⋅xLiTFSI (zwitterion APS = amino-1-propane sulfonate, LiTFSI = bis (trifluoromethane) sulfonimide lithium salt). Wherein, the Li⁺ ionic liquid moiety (APS⋅xLiTFSI) is confined and arranged along the order channel (< 1 nm) within the framework of UiO-66 via covalent-bonded APS, leading to infinite paths and abundant lithium ion hopping sites for fast Li⁺ transmission. The enhancement of the Li⁺ conduction of UiO-66-APS⋅2.7LiTFSI is over 2 orders magnitude higher than that of the counterpart H₂BDC-APS⋅2.7LiTFSI ionic liquid within a wide temperature range. This material exhibits excellent ionic conductivity, with the conductivity of 3.5 × 10⁻⁴ S cm⁻¹ and a lithium-ion transference number (tLi⁺) of 0.83 at room-temperature. The conductivity follows an Arrhenius-type temperature dependence, increasing linearly with temperature to reach 1.33 × 10⁻² S cm⁻¹ at 100 °C. Structure and component effect on the new type of LiSSEs are investigated and fully characterized, and the features of non-flammability, non-leakage of high safety with high conduction are discussed.