Improved Li<sup>+</sup> Transport in Polyacetal Electrolytes: Conductivity and Current Fraction in a Series of Polymers
Rachel L. Snyder, Youngwoo Choo, Kevin W. Gao, David M. Halat, Brooks A. Abel, Siddharth Sundararaman, David Prendergast, Jeffrey A. Reimer, Nitash P. Balsara, Geoffrey W. Coates
Abstract
Polymer electrolytes mitigate safety concerns surrounding flammable liquid electrolytes in lithium-ion batteries. Poly(ethylene oxide) (PEO) electrolytes demonstrate viable conductivity values (∼1 × 10–3 S/cm) at elevated temperatures (>70 °C) but a relatively low Li+ current fraction (≤0.2) because strong Li+ coordination inhibits cation mobility. We have developed a series of polyacetal electrolytes by systematically varying methylene oxide (MO) and ethylene oxide (EO) units in the polymer backbone. These materials maintain high oxygen-to-carbon ratios like PEO but offer improved ion transport, revealing trends of decreasing conductivity and increasing current fraction with respect to polymer composition. In particular, the increasing current fraction measured via the Bruce–Vincent method suggests that MO units improve Li+ mobility relative to anion mobility. We calculate an overall efficacy (product of conductivity and current fraction) for each polymer/salt composition and identify two polymers—P(EO-MO) and P(EO-2MO)—that outperform PEO at high and low salt concentrations, respectively.