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

Si doped-poly(ethylene glycol) ( Si-PEG) hybrid polymer electrolyte with Si O C 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 ( t L i + ) and reduce the interfacial polarization of LiFePO 4 /HSPE/Li cells by attracting the anions of lithium salt . The t L i + 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 −4 S/cm and t L i + is 0.68 at 25 °C. And the initial capacity of LiFePO 4 /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 LiFePO 4 /HSPE/Li cell is 84%, which is higher than that of 65% for LiFePO 4 /PSPE/Li cell. (PSPE indicates the Si-PEG polymer electrolytes with B-PEG just as plasticizer).