Study of solid polyurethane electrolytes synthesized from HDI and PEO of different molecular weight

The development of solid polymer electrolyte (SPE) is important for the fabrication of safe and high performance lithium ion battery of the next generation. A series of polyurethanes (PUs) based SPEs synthesized from hexamethylene diisocyanate (HDI) and poly(ethylene glycol)s (PEGs) of varying molecular weight (MW = 300 ~ 1500 corresponding to PU03 ~ PU15) via one-step approach are investigated for the conductivity of lithium ion and other electrochemical properties. These synthetic PUs exhibit semi-crystalline structures when the MW of PEG is higher than 300, and turn into amorphous after the addition of LiClO4. The ionic conductivity decreases with the increasing concentration of LiClO4. The highest lithium ion conductivity was found to be 3.42 × 10−5 S/cm for PU15 with 10 wt% LiClO4 at 50 °C. However, its lithium ion transference number is low by 0.15. All of the PU-based SPEs containing 10 wt% LiClO4 are electrochemically stable up to 4.2 ~ 4.5 V versus Li+/Li, and show very good cycling stability in charge/discharge experiments. In the assembled battery with LiFePO4 as the cathode, PU15 displays higher performances than PU03, indicating good correlation to the molecular weight of PEG.