Structure–Conductivity Relationship for Peptoid-Based PEO–Mimetic Polymer Electrolytes

Polymer electrolytes offer great potential for application in lithium batteries. In order to systematically optimize the performance of these materials, atomic level synthetic control over the polymer chemical structure is desired. In this study, we designed a series of chemically defined, monodisperse peptoid polymers to explore the impact of side-chain structure on the thermal and electrical properties. A series of comblike peptoid homopolymers with ethylene oxide (EO)_n side chains of varying length were synthesized by a rapid solid phase synthetic method. The electrical properties of these materials with dissolved lithium salt were characterized by ac impedance. The temperature dependence of the ionic conductivity of the polypeptoid electrolytes is consistent with the Vogel–Tamman–Fulcher equation. The optimum ionic conductivity of 2.6 × 10^–4 S/cm achieved for oligo-N-2-(2-(2-methoxyethoxy)­ethoxy)­ethylglycine–Li salt complex at 100 °C, is approximately 10-fold lower than the analogous PEO–salt complex. It is, however, nearly 2 orders of magnitude higher than previously reported comblike PEO–mimetic polypeptides. The ionic conductivities of these side chain analogs vary by 3 orders of magnitude, but this variation is entirely governed by the proximity of the system to the glass transition temperature. This investigation shows that polypeptoids provide a unique platform for examining the structure–property relationships of solid polymer electrolytes.