Polymer Electrolyte Based on Cyano-Functionalized Polysiloxane with Enhanced Salt Dissolution and High Ionic Conductivity

Polymer electrolytes (PEs) offer a promising avenue toward safer, more mechanically robust and high power density lithium-ion batteries. In PEs, conduction is achieved through the dissolution and subsequent transport of the lithium cation and organic anion, yet only lithium transport provides useful current between the two electrodes and must be maximized. As such, PEs are rationally designed to include solvation groups that only moderately interact with the Li+ cations to enable high ionic conductivity (σ) and a high Li+ transference number (t+). Herein, we report a polysiloxane-based PE grafted with cyano-containing side chains that exhibits a total ionic conductivity of 6.9 × 10–4 S/cm and a Li+ transference number of 0.48 at 90 °C, demonstrating significant performance improvements compared to the typical poly(ethylene oxide) (PEO) benchmark. Wide-angle X-ray scattering data indicate that there is no ion aggregation in these systems up to a salt loading of r = [Li+]/[CN] = 0.3. The high ion dissolution ability of the present PE is attributed to its high dielectric permittivity and the modest Li+-side-chain interaction due to the introduction of the polar cyano group, as probed by electrochemical impedance spectroscopy and infrared/Raman spectroscopy, respectively. Moreover, the side-chain length is critical to ion transport and cation selectivity. With a short alkyl chain length, the polymer matrix effectively solvates salt ions and offers good cation selectivity, while the ion mobility is limited by the chain rigidity; with a longer chain length, the polymer segmental motion increases, while the salt dissolution ability drops and the polymer is less cation-selective. These results demonstrate the vast potential of nonpolar flexible polymers grafted with polar side chains as host materials for PEs.