On the Structure–Conductivity Relationship of Confined PEO/LiTFSI Electrolytes

Emerging 3D solid–state batteries comprise polymer electrolytes filled into high–surface area porous scaffolds, underscoring the importance of polymers under confinement. Here, we report on the structure–conductivity relationship of poly(ethylene oxide)–lithium trifluoromethanesulfonylimide complexes confined to microporous scaffolds at various salt concentrations (Li⁺:ether oxygen ratio (r) = 0.0125, 0.0167, 0.025, 0.05). From thermal analysis and X–ray scattering, we observe that confinement suppresses polymer crystallinity relative to the bulk, whereas the glass transition temperature remains unaffected. Further from dielectric spectroscopy, we observe that confinement enhances ionic conductivity in dilute electrolytes (r ~ 0.0167) at 298 K, but not in concentrated electrolytes (r ~ 0.05). At 350 K (melt), all confined electrolytes are less ionically conductive than the bulk due to scaffold–ion interactions as evidenced by time–of–flight secondary ion mass spectrometry (ToF–SIMS). In summary, our findings indicate that polymer crystallinity, interfacial ion segregation, and tortuosity all play important roles in determining total ionic conductivity and, ultimately, the emergence of 3D SPEs as energy storage materials.