Introducing a Catalytic Polymerization Approach for Bottlebrush-Vinylphosphonate Solid Polymer Electrolytes with Ethylene Oxide Side Chains

In the first part of this study, we synthesized two monomers with a vinylphosphonate backbone, each functionalized with varying ethylene oxide side chains. These monomers were then catalytically polymerized using a yttrium catalyst in a rare-earth metal-mediated group transfer polymerization. The resulting polyvinylphosphonates, containing either one ethylene oxide unit (P1-VP) or two units (P2-VP), were characterized and cast into solid polymer electrolyte films using LiBF4 as the conducting salt. The onset of thermal degradation decreases with the elongation of the side chains, from 260 to 210 °C, and further decreases with the addition of LiBF4 to 225 and 160 °C. The glass transition temperature of the polymers decreased with increasing side chain length from −48 to −67 °C while showing no melting transition. X-ray diffraction confirmed the fully amorphous character of these polymers. The ionic conductivity reached 1.8 × 10–5 and 8.2 × 10–5 S cm–1 for P1-VP and P2-VP at 60 °C, respectively, with activation energies for the Li-ion hopping of 0.57 and 0.47 eV. Moreover, the polymer electrolytes showed an oxidative stability of up to 4.3 V vs Li+/Li. However, the high ratio of phosphonate units in the polymers is hypothesized to be a bottleneck, limiting the lithium transference number to 0.03 for P1-VP and 0.12 for P2-VP and the critical current density to 5 and 10 μA cm–2.