Construction of High-Strength Flame-Retardant Li-SPEEK-Modified PEG Gel Polymer Electrolytes for Lithium Batteries

The low mechanical performance and high flammability of gel polymer electrolytes limit their application. Here, we prepare a high-strength and nonflammable polymer Li-SPEEK (lithium sulfonated polyetheretherketone) to modify PEG (polyethylene glycol) and construct a PEG-Li-SPEEK cross-linked network as a polymer electrolyte matrix, with an overall improvement in electrochemical performance, mechanical properties, and flame retardancy. The PEG-Li-SPEEK-ls (PEG-Li-SPEEK cross-linked network after gelation of the liquid electrolyte) electrolyte achieves a conductivity of 1.62 × 10–4 S·cm–1 at 30 °C, a wider electrochemical window of 4.5 V vs Li+/Li, and a higher lithium-ion transference number compared to the PEG-ls electrolyte. The excellent mechanical properties allow the symmetric cell-containing PEG-Li-SPEEK-ls electrolyte to exhibit better cycling performance for over 300 h at a current density of 0.2 mA·cm–2. The LFP/PEG-Li-SPEEK-ls/Li cells deliver a maximum discharge capacity of 142.1 mAh·g–1 with a capacity retention of 98.1% after 50 cycles at 0.5 C and the Coulombic efficiency remains above 99.5% throughout the cycling process. In addition, the good carbon formation properties of Li-SPEEK give the electrolyte matrix satisfactory flame-retardant properties. Thus, we validate the excellent performance of Li-SPEEK for modifying conventional polymer electrolytes and improving their mechanical, electrochemical, and flame-retardant properties.