In this study, we found optimizing the ZIF-8 content is an effective strategy to induce the soggy-sand effect while mitigating surface roughness of the membrane. (The optimal composition: 30 vol% ZIF-8)
Three Li⁺ transport pathways coexist: conduction through the polymer matrix, migration along the continuous pathways between interconnected ZIF-8 particles, and penetration through the ZIF-8 cages. The presence of these multiple pathways enhances the overall ionic conductivity, reduces the activation energy, and increases the lithium transference number. When Li⁺ penetrates the ZIF-8 cages, structural expansion occurs through elongation of the Zn–N bonds and rotation of the 2-methylimidazole ligands.
As a result, the battery performance is significantly enhanced, with the rate capability increasing from 86 to 125 mAh g⁻¹ at 2 C, achieving a capacity retention of 65.8% after 1500 cycles at room temperature.
However, further increasing the ZIF-8 volume fraction leads to the formation of numerous cracks in the polymer matrix and increases the surface roughness of the membrane. These structural defects increase the interfacial resistance, which decreases the exchange current density and increases the overpotential of the LiCoO2∥Li cell.
These findings provide valuable insights into the design of composite membranes and highlight the importance of controlling filler content and interfacial interactions to achieve enhanced electrochemical performance.