Researchers converted layered MXenes into chemically bonded solids using metal intercalation, boosting thermal stability and performance in zinc-iodine batteries.
(Nanowerk Spotlight) Two-dimensional materials, despite their exceptional properties, face a structural challenge that has limited their broader application in energy devices. Most are stacked layer by layer through weak van der Waals forces, making them vulnerable to thermal degradation, mechanical deformation, and poor charge transport across layers.
Attempts to strengthen these materials by chemically linking their layers have often introduced new problems—such as collapsing their desirable nanostructures or disrupting electrical pathways.
For MXenes, a family of transition metal carbides and nitrides known for their high conductivity and surface tunability, these limitations are particularly important. MXenes offer compelling advantages in electrochemical systems, but their weakly bonded layered structure can become a liability under high temperatures or prolonged cycling.
Converting MXenes into more stable three-dimensional forms has been a major goal. Researchers have explored various surface modifications and intercalation strategies to reinforce interlayer coupling, but these efforts typically stop short of forming robust chemical bonds between layers. In some cases, metal atoms have been used to bridge adjacent layers, but the resulting architectures tend to be poorly defined or chemically unstable.
Until recently, no strategy had achieved atomic-level precision in designing a chemically bonded MXene-derived solid with both structural integrity and preserved electrochemical functionality. This gap has limited MXenes’ performance in demanding environments such as high-rate batteries, catalysis, and high-temperature applications.
A new study (Advanced Materials, “Topological Transformation of MXenes to Non-Van Der Waals Artificial Solids with Well-Defined Structure”) from researchers at Beihang University addresses this challenge by introducing a controlled method to transform MXenes into non-van der Waals solids with chemically bonded layers. The team engineered a topological transformation process in which chlorine-terminated MXenes were first modified with sulfur atoms and then intercalated with transition metals.
The result is a new class of artificial solids that preserve the layered structure of MXenes while replacing weak van der Waals interactions with strong chemical bonds mediated by sulfur and metal atoms. These chemically stitched solids exhibit enhanced thermal stability, higher electrical conductivity, and catalytic properties that outperform conventional MXenes.
Synthesis of non-van der Waals artificial solids. a) Schematic illustration of the topological transformation of Ti3C2Cl2 to produce non-van der Waals artificial solid Cu-Ti3C2S. b) HAADF-STEM image of Ti3C2Cl2, showing a layered structure with an obvious interlayer. c) HAADF-STEM image of Cu-Ti3C2S, clearly presenting the Cu atomic alignment between the Ti3C2S layers. Insets in b) and c) are the atomic models of Ti3C2Cl2 and Cu-Ti3C2S, where Ti, Cl, S, and Cu atoms are marked by blue, cyan, yellow, and orange balls, respectively. (Image: Reprinted with permission by Wiley-VCH Verlag)
The process begins with Ti₃C₂Cl₂, a well-characterized chlorine-terminated MXene. The researchers exposed this material to sulfur-containing agents under controlled heat in a salt matrix, which replaced the chlorine atoms with sulfur. This substitution increased the spacing between layers, creating room for further modification.
The sulfur-terminated MXene was then reacted with copper powder, prompting the copper atoms to spontaneously insert themselves between the layers and bind to the sulfur atoms. The result was a new material, Cu–Ti₃C₂S, in which copper atoms chemically bridge the layers of sulfur-terminated MXene, forming a robust, well-ordered structure.
Atomic-scale imaging confirmed that the copper atoms occupy precise positions between MXene sheets, creating a layered architecture that differs fundamentally from both traditional MXenes and bulk metal carbides. Spectroscopic analysis showed that the copper atoms exist as isolated single atoms, not clusters or particles, and are chemically bonded to the sulfur-terminated surfaces.
Density functional theory calculations supported the thermodynamic stability of this configuration, indicating that the intercalation is energetically favorable and likely to occur spontaneously under the reaction conditions.
The transformation yields a material with superior thermal and electrical properties. The chemically bonded Cu–Ti₃C₂S remained stable in air up to 550°C, a significant increase over the 300°C limit typical of MXenes. Its electrical conductivity reached nearly 200 S/cm, more than double that of the starting chlorine-terminated MXene. This enhancement is attributed to the formation of direct pathways for charge transfer between layers, facilitated by the inserted metal atoms and the strong sulfur-metal bonds.
These improvements in structural and electronic properties made Cu–Ti₃C₂S an effective host for iodine in zinc-iodine batteries, which store energy by cycling iodine between different oxidation states. The redox reaction of iodine is sensitive to catalyst performance and material stability, and traditional MXenes have shown promise in these systems due to their conductivity and surface area.
However, their weakly bonded layers can degrade under cycling, and their surface interactions with iodine species are often insufficient to prevent unwanted diffusion and side reactions.
Cu–Ti₃C₂S addressed both issues. When used as a host for iodine in battery cathodes, it improved the kinetics of the iodine redox reaction. Electrochemical testing showed lower voltage polarization and sharper redox peaks compared to both Ti₃C₂S and Ti₃C₂Cl₂. The material also exhibited a lower activation energy for iodine conversion steps, indicating faster electron transfer and better catalytic performance.
These features allowed the battery to operate efficiently even under fast charging conditions. At a high rate of 32C—where 1C is defined as the current that would fully charge or discharge a battery in one hour—the battery retained a capacity of 104 mAh/g, significantly higher than that of conventional MXene-based cathodes.
Cu–Ti₃C₂S also demonstrated strong adsorption of iodine-containing species. The team measured how well the material retained iodide ions, iodine molecules, and polyiodide complexes, all of which are involved in the battery’s charge-discharge cycle. Ultraviolet-visible spectroscopy showed that Cu–Ti₃C₂S effectively removed these species from solution, reducing their diffusion and suppressing the shuttle effect that often causes capacity loss in iodine batteries. Adsorption energy calculations confirmed the strength of these interactions, with values significantly higher than those seen for non-intercalated MXenes.
To understand how the material’s structure contributed to these effects, the researchers calculated the energy barriers for key redox steps. They found that the Gibbs free energy change for the conversion of I₂ to I₃⁻—the rate-limiting step in iodine reduction—was substantially lower in Cu–Ti₃C₂S than in the control materials. This reduction in energy barrier is consistent with the observed increase in reaction speed and battery efficiency.
The long-term performance of the battery was also evaluated. After 2000 charge-discharge cycles at 8C, the Cu–Ti₃C₂S-based battery retained 176 mAh/g with minimal degradation, corresponding to a per-cycle capacity loss of less than 0.003%. Structural analysis after cycling showed that the material maintained its layered architecture and suppressed parasitic reactions at the zinc anode, further supporting its durability.
Importantly, the strategy is not limited to copper. The team extended the method to other transition metals, including iron, cobalt, nickel, and tin, each producing similar non-van der Waals solids with layered structures and stable bonding. This generality suggests that the topological transformation approach could be adapted for a variety of functional applications, depending on the choice of metal and termination chemistry.
By engineering strong, directional chemical bonds between MXene layers, this study provides a new route for converting two-dimensional materials into stable three-dimensional solids without losing their electrochemical advantages. The work demonstrates that the structure and function of MXenes can be precisely controlled at the atomic scale, enabling new classes of materials with applications in batteries, catalysis, and potentially in photonic or magnetic systems.
It also reinforces the idea that subtle changes in surface chemistry—such as exchanging one termination group for another—can lead to major shifts in material behavior when guided by a clear structural strategy.
This topological transformation method redefines how layered materials can be built and used, offering a chemically controlled path to solids that combine the flexibility of 2D systems with the stability of 3D architectures. As research into MXenes and their derivatives continues, the ability to selectively bond layers through atomically designed interfaces could unlock new functions across a wide range of technologies.
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