Enhancing Li-ion conduction and mechanical properties via addition of fluorine-containing metal–organic frameworks in all-solid-state cross-linked hyperbranched polymer electrolytes

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) are commonly used in lithium metal batteries (LMBs) for their good Li-salt solvating ability and easy processability. However, the relatively low Li-ion conduction ability hinders their further development. In this work, a novel hyperbranched-polyether-type composite solid polymer electrolyte (CSPE) is prepared via a quick cross-linking reaction between aldehyde-terminated polyethylene glycol (PEG) and hyperbranched poly(ethylene imine) (HPEI) in the presence of lithium salt and fluorine-containing Zr-based metal–organic framework (MOF) UiO-66-(F)₄. The hydrogen bonds between the fluorine atoms and amino groups in the electrolyte help to the better dispersion of UiO-66-(F)₄ in the polymer matrix, which is beneficial to solving the problem of aggregation of nanofillers. Besides, the CSPEs with the functional MOF fillers show improvements in both electrochemical and mechanical properties. Notably, the Li-ion transference number (t ${}_{{\mathrm{L}\mathrm{i}}^{+}}$) is considerably enhanced from 0.23 to 0.54. All-solid-state LMBs based on the CSPE also present good cycling performances. A high specific discharge capacity of 141.4 mAh·g⁻¹ is remained after 200 cycles at 0.2 C. This study not only provides an effective synthesis method of the cross-linked hyperbranched polymer electrolyte, but also puts forward a new strategy for uniform dispersion of inorganic fillers in CSPEs.