Mapping Salt Distribution and Ion Mobility in Mixed-Polymer Electrolytes for Solid-State Batteries Using Nuclear Magnetic Resonance Spectroscopy

Polymer-based electrolytes, which tend to have higher mechanical and chemical stability than liquid electrolytes, are promising candidates for next-generation lithium batteries. Despite these advantages, there has been minimal large-scale adoption of this technology, attributable to electrolyte performance being severely limited by the interplay between polymer crystallinity and ionic conductivity. In this work, the full composition space of a ternary thermoplastic vulcanizate (TPV) electrolyte composed of poly(caprolactone) (PCL), hydrogenated nitrile butadiene rubber (HNBR) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was systematically mapped. Ionic conductivity between 25 and 80 °C was correlated with thermal properties obtained from differential scanning calorimetry and molecular-level insights from solid state NMR spectroscopy, including quantification of salt distribution between the PCL and HNBR phases and lithium ion mobility. Ion transport was found to be governed by the balance between PCL crystallinity, amorphous phase fraction and LiTFSI concentration. PCL-rich compositions containing 20–30 wt % LiTFSI exhibited the highest ionic conductivities at 60–80 °C, reaching values above 10 –4 S cm –1 . At 25 °C, the most conductive samples had more balanced PCL/HNBR ratios and higher salt content. Solid-state NMR showed that conductivity maxima coincided with enhanced lithium mobility in the amorphous PCL phase and a temperature driven shift in salt distribution between the two polymers. Conductivity, crystallinity and NMR-derived mobility metrics were integrated across the entire composition range, to establish how polymer phase behavior, salt distribution and lithium dynamics collectively control ion transport in ternary polymer electrolytes. These results provide a quantitative framework for identifying optimal formulation windows and guide the rational design of multiphase polymer electrolytes for lithium battery applications.