Influence of lithium ions on the dielectric relaxation properties of a Nafion ionomer membrane

The relaxation properties of Nafion 212 membranes in both their protonated (H-Nafion) and lithiated (Li-Nafion) forms were investigated over a wide temperature range from −80 to 100 °C using dielectric spectroscopy. The replacement of protons with lithium ions was shown to displace the principal relaxation maximum by almost three decades toward lower frequencies to raise its activation energy from 4 to 61 kJ/mol. This signals stronger Li⁺–SO₃⁻ electrostatic coupling and the onset of a cooperative side-chain process that endures up to 100 °C, where it merges with the segmental relaxation of the polymer matrix. Although both membranes share virtually identical activation barriers for ion transport in the glassy region, Li-Nafion remains one to two orders of magnitude more conductive due to the suppression of the protonic Grotthuss mechanism as a result of deep drying, which nevertheless leaves the population of mobile Li⁺ carriers intact. The present quantitative benchmarks, which illuminate the decisive role of counter-ion identity in governing dipolar dynamics and ionic mobility, can inform the optimisation of perfluorosulfonic membranes as lithium-selective solid electrolytes and separators for advanced electrochemical devices.