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

Abstract

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.