The synergistic effect of poly(ethylene glycol)-borate ester on the electrochemical performance of all solid state Si doped-poly(ethylene glycol) hybrid polymer electrolyte for lithium ion battery

Abstract

Si doped-poly(ethylene glycol) (Si-PEG) hybrid polymer electrolyte with SiOC bonds in the main chain obtains high ionic conductivity even at room temperature because of the more flexible molecule chains. In this work, poly(ethylene glycol)-borate ester (B-PEG) is further covalent bonded into the crosslinked Si-PEG network to form the hybrid solid polymer electrolyte (HSPE). Comparing to previous works in which B-PEG is just added as a plasticizer, the Lewis acidity of boron (B) in the network of polymer chains is less shielded by the connected PEG chains, which is more effective to improve lithium ion transference number (tLi+) and reduce the interfacial polarization of LiFePO4/HSPE/Li cells by attracting the anions of lithium salt. The tLi+ increases monotonously along with the increase of B-PEG contents in HSPE, while the ion conductivity has a peak value. When B-PEG content is 23.1 wt%, the highest ionic conductivity of the hybrid polymer electrolyte reaches up to 1.6 × 10−4S/cm and tLi+ is 0.68 at 25 °C. And the initial capacity of LiFePO4/HSPE/Li cells are 120 mAh/g and 90 mAh/g at 0.5C and 1C at 25 °C. After 100 cycles under 1C at 25 °C, the capacity retention of the LiFePO4/HSPE/Li cell is 84%, which is higher than that of 65% for LiFePO4/PSPE/Li cell. (PSPE indicates the Si-PEG polymer electrolytes with B-PEG just as plasticizer).