Effect of Confinement on the Structure–Conductivity Relationship in PEO/LiTFSI Electrolytes in 3D Microporous Scaffolds

Because 3D batteries comprise solid polymer electrolytes (SPEs) confined to porous scaffolds with high surface areas, the interplay between polymer confinement and interfacial interactions on SPE total ionic conductivity must be understood. This paper investigates contributions to the structure-conductivity relationship in poly(ethylene oxide) (PEO)-lithium bis(trifluorosulfonylimide) (LiTFSI) complexes confined to microporous nickel scaffolds. For bulk and confined conditions, PEO crystallinity decreases as the salt concentration (Li⁺:EO (<i>r</i>) = 0.0125, 0.0167, 0.025, 0.05) increases. For pure PEO and all <i>r</i> values except 0.05, PEO crystallinity under confinement is lower than in the bulk, whereas the glass transition temperature remains statistically invariant. At 298 K (semicrystalline), total ionic conductivity under confinement is higher than in the bulk at <i>r</i> = 0.0167 but remains invariant at <i>r</i> = 0.05; however, at 350 K (amorphous), total ionic conductivity in confinement is lower than in the bulk for both salt concentrations. Time-of-flight secondary ion mass spectrometry indicates selective migration of ions toward the polymer-scaffold interface. In summary, for the 3D structure studied, polymer crystallinity, interfacial segregation, and tortuosity play important roles in determining total ionic conductivity and, ultimately, the emergence of 3D SPEs as energy storage materials.