Long-term stability and mixed-salt effect of MXene/vermiculite membranes for ion separation

MXene materials have demonstrated great potential for separation applications; however, their intrinsic instability severely limits their practical use. In this work, vermiculite nanosheets are uniformly incorporated into MXene membranes to enhance their long-term stability. This improvement is likely due to the negatively charged environment created by the vermiculite nanosheets, which weakens the polarization of C-Ti bonds in MXene and suppresses its oxidation. Stability tests reveal that MXene remains stable within the MXene/vermiculite membranes after heating at 200 °C in air for 2 h. In contrast, a pure MXene membrane is oxidized under the same conditions. In ion diffusion experiments, the membranes maintain stable transport of monovalent ions such as Cs + for more than ∼1,000 h. The MX-60% membrane exhibits the best separation performance, achieving ideal selectivity ratios of ∼27.8 for Li + /Cs + and ∼39.5 for Li + /Mg 2+ . However, in mixed-ion systems, the separation behavior changes markedly—the Cs + /Li + separation factor reverses to ∼5.2, indicating that Cs + diffuses faster than Li + . This unexpected mixed-salt effect is attributed to ion-ion interactions that disrupt the ordered interlayer water structure, thereby influencing ion transport within the membrane channels. Overall, this work provides an effective strategy to enhance the stability of MXene-based membranes and reveals a notable mixed-ion effect that impacts the ion separation performance of MXene/vermiculite membranes.