Synthesis of Hyperbranched Polyurethane Electrolyte for Solid-State Lithium-Ion Batteries

Hyperbranched polymers have special structures with highly branched topology, a large number of functional groups, and cavities. They have the properties of low viscosity, good solubility, and low crystallinity and can be used as solid electrolytes. Herein, a hyperbranched polyelectrolyte (HBP) was synthesized with poly(ethylene glycol) dimethacrylate and cysteine in the presence of lithium hydroxide by using “click” chemistry at first. Then, HBP was added into the polyurethane prepolymer in definite proportions. Finally, a series of hyperbranched polyurethane electrolytes were obtained by vacuum drying. The structures of the hyperbranched polyelectrolyte and hyperbranched polyurethane electrolytes were characterized by ¹H nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD) spectra. The obtained PU-DMPA₁-HBP₃₀ electrolyte has an excellent mechanical property and thermostability. Moreover, the electrolyte shows a high ionic conductivity of 0.13 mS cm^–1 at 20 °C. The discharge capacity of the LiFePO₄|PU-DMPA₁-HBP₃₀|Li cell is 113 mAh g^–1 at 0.5 °C at room temperature and can run stably more than 200 times with high Coulomb efficiency. The hyperbranched polyurethane electrolytes can be used as solid-state battery materials.