Influence of Porosity of Polybenzimidazole Separator Membranes on Lithium-Ion Diffusion and Local Mobility

Abstract

The influence of porosity on the diffusion of lithium-ions across macroporous and mesoporous PBI membranes was examined. The kinetics of diffusion of lithium-ions across porous PBI membranes were compared with those of the commercially available and widely used Celgard-2325 separator. A faster rate of lithium-ion diffusion across the macroporous and mesoporous PBI was observed when compared with Celgard-2325. This was attributed to the nature of the pores. Lithium ions solvated by the carbonate solvent fill the pores of the membrane and are partially immobilized by weak ionic interactions with the imidazole groups present on the pore walls of PBI. To characterize the dynamic lithium response across macroscopic to mesoscopic pore scales, local ion mobility within the electrolyte-swollen PBI networks was evaluated using single-frequency 7Li NMR spin–lattice (T1) and spin–spin (T2) relaxation time measurements at 9.4 T. In neat liquid electrolytes and the inert Celgard matrix, the 7Li relaxation signatures are governed by the fast, isotropic rotational mobility of the ethylene carbonate (EC) and dimethyl carbonate (DMC) solvated lithium-ions within the bulk phase (τc ∼ 500 ps). A solvent-driven relaxation response is similarly observed within the nonporous and macroporous electrolyte-swollen PBI membranes, where matrix confinement introduces a mild, localized mobility deceleration. The observed enhancements in lithium NMR relaxation are rationalized using the relaxation enhancement (ΔR = R2 – R1) and T1/T2 anisotropy parameters to illustrate a clear pore-size-dependent microdynamic response. The relaxation efficiency peaks within the mesoporous PBI membranes, indicating a dominant surface-driven relaxation mechanism due to prominent lithium coordination at imidazole nitrogen sites, which homogenizes local flux and facilitates efficient, long-range translational mobility. Ultimately, a semiquantitative analysis of the T1/T2 parameters using a 2D surface translational diffusion model successfully isolates the Surface Affinity Index of the mesoporous networks, providing definitive microdynamic evidence of a continuous, chemically directed coordination pathway for lithium-ion migration along the mesoporous polymer boundaries.