A bamboo-inspired design helps ions move faster through thick energy storage materials, improving performance and enabling more efficient, scalable batteries and supercapacitors.
(Nanowerk Spotlight) Getting charged particles to move through solids quickly and efficiently remains one of the most basic and stubborn problems in energy storage. It matters most in the design of electrodes, the materials that store and release energy in batteries and capacitors. Thicker electrodes can hold more energy, but they also make it harder for ions to travel through. As the material thickens, ions slow down or get trapped, and much of the stored capacity becomes inaccessible. This is not a limitation of chemistry but of structure. The internal pathways for ion transport are too long, too crowded, or poorly connected.
To fix this, researchers have tried building smaller and more ordered spaces for ions to move through. Narrow channels just a few nanometers wide can speed up transport by focusing the flow and reducing interference. But most designs rely on symmetrical, uniform geometries that treat every part of the material the same. That creates a mismatch between what is needed near the surface, where ions must enter quickly, and what works deeper inside, where more confinement improves storage. The result is a structural compromise that limits performance.
A research team in China has proposed a different approach. Drawing inspiration from the internal structure of bamboo, they designed an electrode with a gradient of channel sizes that taper as ions move inward. The channels are not isolated but connected by horizontal pores that help ions migrate between layers. This layout creates a coordinated system for ion transport that changes gradually with depth. The material itself is MXene, a class of conductive layered compounds well suited to this kind of structural engineering.
The work, published in Advanced Materials (“Nature‐Inspired MXene Electrode with the Highly Interconnected Gradient Nanoconfined Architecture”), introduces a way to build thick electrodes without sacrificing speed or efficiency. It avoids the usual tradeoff between capacity and ion mobility by structuring space deliberately rather than uniformly. Instead of forcing ions to push through a dense block of material, the design gives them a route shaped to their needs. This shifts the problem of transport from a constraint to a design parameter, opening new possibilities for scalable energy storage.
A comparison of ion transport in two electrode structures. Model 1 shows the bamboo-inspired design with tapered vertical channels and horizontal connections between layers, which guide ions efficiently through thick material. Model 2 shows a conventional structure with uniform layers and no cross-layer links. Simulations demonstrate that Model 1 allows smoother and faster ion movement both vertically and horizontally, while Model 2 creates bottlenecks that slow transport and reduce performance. (Image: reprinted with permission by Wiely-VCH Verlag) (click on image to enlarge)
The electrode is built from Ti₃C₂Tx MXene, a conductive material with a layered structure and tunable surface chemistry. The researchers modified it in two key ways. They introduced in-plane pores to link the layers horizontally and inserted cellulose nanofibers between layers to vary the spacing. These changes created a gradient architecture that mirrors how bamboo transports water. In this configuration, ions first move into wider openings at the surface, then travel downward through gradually narrowing channels. Horizontal pores connect the vertical paths, giving ions the option to move laterally as well.
To understand how structure influences ion movement, the team simulated four different models. The configuration with both gradient channel sizes and in-plane pores allowed the fastest and most selective ion transport. It showed a high net current and a lower resistance to ion flow compared to symmetrical or static designs. The gradient structure helped reduce tortuosity, which refers to how twisted and indirect the ion paths are. Shorter and more direct routes allow for faster and more efficient movement. The combination of spatial confinement and selective passage helped the system favor positive ion transport while blocking unwanted species.
The researchers then built experimental versions of the model. They prepared films of pristine MXene, holey MXene with added pores, MXene combined with cellulose nanofibers, and the full gradient nanoconfined structure. Electron microscopy confirmed that pores were successfully created and that the interlayer spacing could be precisely tuned. The cellulose nanofibers increased the gap between layers from about 1.2 to 2.9 nanometers, depending on concentration.
They tested the materials in supercapacitor devices to evaluate charge storage and ion movement. The gradient design showed the highest area-specific capacitance, reaching 221.7 millifarads per square centimeter in thinner films and 20.7 farads per square centimeter in thicker ones. These values reflect the amount of charge the material can store in a given area. The electrode also showed improved cycling stability, retaining over 85 percent of its performance after ten thousand charge and discharge cycles. Ion diffusion coefficients confirmed faster transport in the gradient structure, supported by both simulation and physical testing.
One key innovation in the work is that the structure is not fixed once it is made. The researchers developed a post-fabrication treatment that allows the interlayer spacing to be adjusted by manipulating surface charge. In this process, known as in situ deprotonation and reprotonation, the material is first exposed to a dilute acid that triggers ionization of surface groups. This increases repulsion between the layers and pushes them apart. A stronger acid is then used to restore the surface chemistry, but the expanded spacing remains. The result is a quasi-permanent widening of the channels that allows for easier ion entry and movement.
Applying this treatment to different MXene-based films led to significant increases in capacitance. In the gradient structure, the capacitance increased by roughly 400 percent after treatment. The adjustment works by modifying the electrostatic forces between layers without damaging the overall architecture. This makes the design not only effective but also tunable, which is important for adapting to different device requirements.
To address the practical issue of scalability, the team developed a method to assemble thick films using water as a temporary bonding agent. Layers with varying pore and spacing configurations were stacked together and joined by water-assisted welding. Water enters the contact region between layers and softens the structure. As it evaporates, the layers restack and lock into place. This method allowed the team to build a 400 micrometer thick electrode that preserved the gradient architecture throughout.
Tests on these thick films showed that performance did not degrade with size. The thickest electrode retained high capacitance and energy density, reaching up to 1080 microampere-hours per square centimeter in a three-electrode test setup. The horizontal and vertical pathways remained active across the full thickness of the material. This confirms that the bamboo-inspired structure is not only effective in principle but also works at larger scales where conventional designs typically fail.
The study reframes the design of thick electrodes by treating structure as a variable that can be shaped with intent. It moves beyond uniform layering and toward spatially differentiated architectures that match transport needs to location. By combining pore connectivity, graded confinement, and tunable spacing, the work shows how internal geometry can be used to solve a core limitation in electrochemical systems.
While further testing is needed to confirm how these electrodes perform in real-world environments and under thermal or mechanical stress, the structural strategy offers a clear path toward more efficient and scalable energy storage.
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