Mechanical Properties of Free-Standing Graphene Oxide Films Intercalated with Water and Lithium Ions

Graphene oxide (GO) holds tremendous promise for applications like filtration and ion-conductive membranes, multifunctional protective coatings, flexible electronics, and electrodes in lithium-ion batteries. The mechanical properties of GO can show large variations, and understanding these is essential for these applications. Here, we study the in-plane elastic modulus of 100 μm-thick free-standing GO films with different hydrated states and lithium salt loading. The moisture uptake was characterized with IR spectroscopy, sorption studies, and temporal evolution of the XRD peaks. The Young's modulus of the film varied from E ≈ 12 to 1 GPa as the film's hydration state was tuned from dry to a moisture-saturated state. The value of E for the Li-salt-loaded GO (Li-GO) films varied from ≈13 to 0.06 GPa when tuning the hydration state from dry to moisture-saturated, i.e., a reduction by an average factor of ≈212. Hydration of the Li-GO films reduced their tensile strength by 2 orders of magnitude. We attribute this wide tunability of mechanical properties partly to the stiffening effect arising from the strong Li⁺ ion: GO electrostatic interactions in the dry state. Furthermore, the synergistic effect of intercalated Li⁺ ions and moisture enhanced the hydration's softening effect, attributable to increased water uptake enabled by the alkali metal cations.