A novel intrinsic flame-retardant and flexible polyurethane solid electrolyte for lithium batteries

An effective approach is proposed to enhanced the safety of lithium batteries by an intrinsic flame-retardant polyurethane (FR-PU) solid electrolyte, which is constructed by ethylene glycol methyl phosphonate ethylene glycol propionate (EMPEP), polyethylene glycol 800 (PEG800) and hexamethylene diisocyanate (HDI) through a simple polymerization reaction. The FR-PU not only displays benign flame retardancy with the limit oxygen index value of 27.6, but also exhibits excellent flexibility, sufficient mechanical strength and heat resistance which are beneficial to their dimensional stability. Infrared spectroscopy results reveal the complicated coordination relationship between FR-PU and lithium ion. The ionic conductivity of solid electrolyte with FR-PU containing 20 wt% lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI) can up to 1.51 × 10−4 Scm−1 at 70 °C. The FR-PU-20% Li solid electrolyte possesses a wide electrochemical stability window of 0–4.81 V (vs. Li+/Li). Moreover, the cells which fabricated with lithium iron phosphate (LiFePO4), FR-PU-20% Li, and Li anodes exhibits both superior discharge specific capacity and cycle performance (discharge specific capacity of 155.1 mAhg−1 and coulombic efficiency of 98.3% after 120 cycles at the rate of 0.2 C) at 70 °C. Therefore, FR-PU could be a promising candidate as a high performance polymer electrolyte to improve the safety of lithium batteries.