High ionic conductivity and ion conduction mechanism in ZIF-8 based quasi-solid-state electrolytes: a positron annihilation and broadband dielectric spectroscopy study

The low ionic conductivity and electrode-electrolyte interface instability issues with solid polymer electrolytes jeopardize their electrochemical performances in lithium-ion batteries (LIBs). The use of quasi-solid-state electrolytes (QSSEs) with concentrated Li salt embedded inside the pore networks of metal organic frameworks (MOFs) can successfully address the aforementioned issues. Owing to the sieve effect of zeolitic imidazolate framework-8 (ZIF-8) towards selective cation permeability over anions through the interconnected pore network, a unique QSSE with LiTFSI salt concentrated in the ZIF-8 skeleton used as a filler in poly(ethylene oxide) has been synthesized. LiTFSI gets embedded inside the interconnected pore network of ZIF-8 that furnishes unhindered pathways for Li+ ion migration leading to a very high ionic conductivity of ∼6 × 10-4 S cm-1. The higher ionic conductivity is directly related to the Li+ ion conduction through the pore network of ZIF-8 which has been experimentally evident from complementary methods viz. Positron annihilation and broadband dielectric spectroscopy. The design route towards these types of QSSEs encompassing porous MOFs paves the way for realizing Li superionic conductors suitable for practical application in commercial LIBs with high safety and stability.