Polyethylene Glycol Boric Acid Ester Solid Electrolyte For Lithium Ion Batteries

Solid polymer electrolyte plays a significant role for safety performance improvement of lithium ion batteries due to high thermal stability. In this thesis, a kind of polyethylene glycol boric acid ester (B-PEG) with a symmetric Y-shaped structure was synthesized and used as solid electrolyte matrix for LiFePO4/Li batteries. The batteries can work in the temperature range of25-160℃. The main results are as follows:1. B-PEG was synthesized from polyethylene glycol and borane, and its chemical structure was verified by FTIR,1H NMR, and gel chromatography. The decomposition temperature of375℃was measured by TG-DSC, showing a high thermal stability of B-PEG.2. B-PEG based polymer electrolyte thin film was prepared with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as lithium salt. When LiTFSI is18.8wt%, the ionic conductivities of the film are2.95×10-5S cm-1,5.4×10-4S cm-1and1.28×10-3S cm-1, and the electrochemical oxidation potentials are5.23V,5.05V and4.88V, respectively at25℃,80℃and120℃.3. The cycling performance of LiFePO4/Li batteries assembled with B-PEG based electrolyte was investigated. Under the operating conditions of25℃/0.3C,80℃/1C and120℃/2C, the specific capacities are about78mAh g-1,125mAh g-1and123mAh g-1respectively, without decline during200charging/discharging cycles.At140℃and160℃, the discharge specific capacities for the first cycle are164mAh g-1and161mAh g-1at3C, and the capacity retention ratios are77.4%and64.3%after200cycles, respectively.