Improved Mechanical Strength without Sacrificing Li-Ion Transport in Polymer Electrolytes

Next-generation batteries demand solid polymer electrolytes (SPEs) with rapid ion transport and robust mechanical properties. However, many SPEs with liquid-like Li+ transport mechanisms suffer a fundamental trade-off between conductivity and strength. Dynamic polymer networks can improve bulk mechanics with minimal impact to segmental relaxation or ionic conductivity. This study demonstrates a system where a single polymer-bound ligand simultaneously dissociates Li+ and forms long-lived Ni2+ networks. The polymer comprises an ethylene oxide backbone and imidazole (Im) ligands, blended with Li+ and Ni2+ salts. Ni2+–Im dynamic cross-links result in the formation of a rubbery plateau resulting in, consequently, storage modulus improvement by a factor of 133× with the introduction of Ni2+ at rNi = 0.08, from 0.014 to 1.907 MPa. Even with Ni2+ loading, the high Li+ conductivity of 3.7 × 10–6 S/cm is retained at 90 °C. This work demonstrates that decoupling of ion transport and bulk mechanics can be readily achieved by the addition of multivalent metal cations to polymers with chelating ligands.

Methods
A summarized methodology is as follows: Polymerized were prepared via anionic ring-opening polymerization and subsequently functionalized with imidazole ligands. Characterization of polymer synthesis includes nuclear magnetic resonance spectroscopy and gel-permeation chromatography. Polymers were solvent-casted with Li and Ni salts. Salt-loaded polymer electrolytes were then characterized with oscillatory shear rheology, differential scanning calorimetry, electrochemical impedance spectroscopy, chronoamperometry, and pulsed field gradient nuclear magnetic resonance spectroscopy.

See supporting information from the published main text for more information: https://doi.org/10.1021/acsmacrolett.4c00158