Water molecules replace mechanical force to exfoliate defect-free MXene nanosheets at kilogram scale, enabling record conductivity and lithium extraction from seawater.
(Nanowerk Spotlight) Certain materials gain extraordinary properties when thinned down to a single atomic layer. Graphene, a one-atom-thick sheet of carbon isolated in 2004, was the first famous example. MXenes, a family of two-dimensional transition metal carbides first produced in 2011, may be the most versatile.
A single MXene sheet, roughly one nanometer thick, conducts electricity like a metal yet attracts water naturally, a rare pairing that has made it attractive for applications ranging from energy storage and flexible electronics to MXene membranes for selective ion transport.
The problem is getting those sheets out intact. In their bulk form, MXene layers cling together through electrostatic forces and hydrogen bonds and pulling them apart has always required at least some mechanical energy. High-yield methods, including recent efforts toward scalable MXene synthesis methods, rely on aggressive force: ultrasonic cavitation, high-speed shaking, or impact milling. That energy separates the layers but also damages them, punching holes into the atomic lattice, fracturing sheets into small fragments, and oxidizing their edges. Gentler approaches preserve quality but produce very little material, leaving researchers with a persistent trade-off between pristine nanosheets and usable quantities.
Mechanism and scalability of the water-mediated exfoliation strategy. a) Schematic illustration of the water-mediated exfoliation strategy. b) Time-dependent SEM evolution of multilayer MXene during the hydration process (0–12 hours), showing the progressive “accordion-like” expansion. c) XRD patterns corresponding to the hydration timeline. d) TEM image of a typical large-area single-layer MXene nanosheet. e) AFM topography and height profile (inset) confirming a monolayer thickness of ≈1.5 nm with a smooth surface. f) Photograph of the kilogram-scale production of high-concentration MXene dispersion (5 mg mL−1). g) Comparison of scale of production versus electrical conductivity for MXene nanosheets produced by this work (red star) and other reported methods. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The approach, termed water-mediated scission, begins with a conventional step: intercalating lithium ions between the MXene layers. In standard protocols, those lithium ions sit in a partially hydrated or naked state, forming electrostatic bridges that reinforce interlayer cohesion and still demand external force for exfoliation. The new method instead immerses the lithium-intercalated material in water and allows diffusion to do the work.
Over a 12-hour soaking period, water molecules migrate into the interlayer galleries and transform the lithium ions through three coupled processes. Some lithium diffuses out into the bulk solution, and water fills the resulting vacancies, screening the electrostatic attraction between remaining cations and the negatively charged MXene surface. The residual lithium ions capture incoming water molecules, swelling into large hydrated clusters that physically prop the layers apart.
Meanwhile, the intercalated water constructs a dynamic hydrogen-bond network that replaces rigid electrostatic coupling with a fluid hydration layer, lowering the energy barrier for separation. Time-resolved imaging confirmed this progression, showing the multilayer stack opening into an accordion-like structure over the soaking period as interlayer spacing steadily increased.
After soaking, only mild vortex shear, far gentler than ultrasonic cavitation, was needed to complete exfoliation. The resulting nanosheets had lateral dimensions averaging 10.46 µm, uniform monolayer thicknesses of approximately 1.5 nm, and an overall yield of approximately 84.7%.
Computational modeling reinforced the proposed mechanism. Molecular dynamics simulations showed lithium ions spontaneously detaching from the MXene surface and becoming encapsulated by water molecules within 100 picoseconds. The hydration enthalpy released by these clusters exceeded the interlayer van der Waals and electrostatic forces, thermodynamically driving spontaneous layer separation.
A control experiment using acetonitrile, a polar solvent comparable to water in dipole moment but incapable of hydrogen bonding, produced negligible expansion and yielded only fragmented, defect-laden particles. Yield and conductivity scaled linearly with the water fraction in mixed solvents, confirming that the cooperative lithium-water hydrogen-bond network is indispensable.
The lattice integrity of the water-mediated nanosheets stood in sharp contrast to conventionally produced material. Atomic-resolution imaging showed that sheets exfoliated by cavitation, oscillation, or shear alone were riddled with lattice vacancies and impurity clusters, with measured strains ranging from 2.95% to 6.23%. The water-mediated nanosheets showed only 0.36% strain and displayed a perfectly ordered hexagonal atomic lattice with no visible vacancies or oxidation sites.
These microscopic qualities carried through to macroscopic performance. Membranes assembled from the defect-free nanosheets achieved the highest lamellar ordering among all tested groups, with a tensile strength of 72.27 MPa, roughly three times that of cavitation-exfoliated membranes. Electrical conductivity reached approximately 11 000 S cm⁻¹, which the researchers attribute to large lateral dimensions minimizing resistive inter-flake junctions and near-zero strain eliminating electron scattering centers. The defect-free lattice also conferred superior oxidation stability and enough flexibility for the membrane to be folded into complex shapes without fracture.
The team produced kilogram quantities of single-layer nanosheets and established a continuous roll-to-roll coating line integrating coating, tunnel drying, and winding to generate membrane rolls exceeding 100 meters in length. Measurements at more than 30 randomly distributed sites across a large-area membrane confirmed tight consistency in both lithium permeation rate and selectivity, a prerequisite for industrial module fabrication.
One application demonstrated why defect-free channels matter. The ocean holds an estimated 230 billion tons of lithium, dwarfing terrestrial reserves, but seawater concentrations are vanishingly low and competing ions such as magnesium outnumber lithium roughly thousandfold. Separating one from the other demands membranes with angstrom-level precision, exactly the kind of flawless channel that defect-free MXene can provide.
Upon hydration, the membrane swelled to a stable interlayer spacing of 16.0 Å, creating nanofluidic channels of approximately 6 Å, wide enough for small lithium ions but punishingly tight for larger hydrated magnesium and calcium ions. Selectivity for lithium over magnesium reached 177, surpassing conventional MXene membranes by an order of magnitude, and performance remained stable over 150 hours. Tested with real seawater, the membrane achieved 36% lithium extraction efficiency in a single pass, with less than 3% permeation of competing ions.
By showing that exfoliation can be driven by solvation chemistry rather than mechanical force, this work removes the quality penalty that has constrained MXene’s transition from laboratory prototype to industrial material. The defect-free membranes break the permeability-selectivity trade-off that has limited membrane-based lithium extraction from brine and seawater alike, and offer a scalable route to harvesting critical minerals from the ocean.
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