Solid Polymer Electrolytes Based on Statistical Polyurethanes

ABSTRACT Poly(ethylene oxide) (PEO) is one of the most widely used materials for solid polymer electrolytes (SPEs). However, the high crystallinity and low ionic transference numbers of conventional PEOs are hindering their utilization in lithium metal batteries, except at temperatures above the melting point of PEO at ca. 60°C. Different strategies have thus been examined over the years to decrease the crystallinity of PEO. Here, we use α,ω‐dihydroxy PEO of low molar mass and a diol containing an aromatic pyridine group as comonomers in a step‐growth polymerization with a diisocyanate to obtain statistical polyurethanes. SPEs are obtained by mixing the polyurethanes with different amounts of LiTFSI. We thoroughly characterize the different investigated combinations by differential scanning calorimetry to determine crystallinity, rheology to assess mechanical properties, and temperature‐dependent ionic conductivity, linear sweep voltammetry, and lithium‐ions transference number measurements to discriminate the best candidates for SPEs. We demonstrate that the statistical distribution of the comonomers in the PUs effectively decreases the crystallization degree of PEO segments, while the presence of urethanes and aromatic amine functional groups allows better transport of lithium ions.