Unexpected Selective Absorption of Lithium in Thermally Reduced Graphene Oxide Membranes

Lithium plays an increasingly important role in scientific and industrial processes, and it is extremely important to extract lithium from a high Mg^2+/Li^+ mass ratio brine or to recover lithium from the leachate of spent lithium-ion batteries. Conventional wisdom shows that Li^+ with low valence states has a much weaker adsorption (and absorption energy) with graphene than multivalent ions such as Mg^2+. Here, we show the selective adsorption of Li^+ in thermally reduced graphene oxide (rGO) membranes over other metal ions such as Mg^2+, Co^2+, Mn^2+, Ni^2+, or Fe^2+. Interestingly, the adsorption strength of Li^+ reaches up to 5 times the adsorption strength of Mg^2+, and the mass ratio of a mixed Mg^2+/Li^+ solution at a very high value of 500\!:\!1 can be effectively reduced to 0.7\!:\!1 within only six experimental treatment cycles, demonstrating the excellent applicability of the rGO membranes in the Mg^2+/Li^+ separation. A theoretical analysis indicates that this unexpected selectivity is attributed to the competition between cation–\pi interaction and steric exclusion when hydrated cations enter the confined space of the rGO membranes.