​​Thermal Nanoimprinting: A Novel Solution for Thickness-Constrained Electrochemical Energy Storage Electrodes​

Professor Ge Haixiong, our Chief Scientist, has published a research paper titled "Vertically Aligned and Ordered Arrays of 2D MCo2S4@Metal with Ultrafast Ion/Electron Transport for Thickness-Independent Pseudocapacitive Energy Storage"in the prestigious journal ACS Nano. This work was completed in collaboration with Professor Tang Shaochun's research group at the School of Modern Engineering and Applied Sciences.

Research Summary

The study focuses on electrode structure optimization, utilizing thermal nanoimprinting technology combined with microcurrent electrodeposition to develop a novel array-structured electrode with high capacity, high rate capability, and long cycle life. This breakthrough provides a new solution to overcome the long-standing challenge of thickness limitations in electrochemical energy storage.

Background & Significance

With the rapid development of flexible electronics, flexible supercapacitors have shown tremendous potential in applications such as flexible displays, portable energy storage, and wearable devices, attracting significant attention from both academia and industry. The key to enhancing supercapacitor energy density lies in developing high-performance flexible electrodes and asymmetric all-solid-state devices.

Pseudocapacitive electrode materials store energy through reversible faradaic redox reactions at the electrode/electrolyte interface, delivering significantly higher capacitance than traditional electric double-layer carbon materials. However, electrolyte diffusion in these materials is typically limited to 20 nm, leaving the remaining material volume inactive ("dead volume"). By improving ion diffusion rates, effective volume utilization, and electron transport efficiency, pseudocapacitive supercapacitors could achieve energy densities comparable to secondary batteries.

Yet, challenges remain:

✔ Low intrinsic conductivity of pseudocapacitive materials

✔ High "dead volume" ratio

✔ Long electron/ion transport distances, resulting in actual capacity far below theoretical values

This research addresses these limitations through innovative electrode design and scalable fabrication techniques.

Figure1. Ion/Electron Transport Pathways in Novel Array Electrodes for Electrochemical Energy Storage

The research team has developed a groundbreaking array electrode structure to address critical challenges in electrochemical energy storage. While two-dimensional nanomaterials show promise for high-capacity energy storage by preventing stacking when grown on substrates, existing nanoarray structures often suffer from dense and disordered configurations. A major limitation has been the persistent thickness dependence of energy storage performance, which becomes particularly severe when electrode thickness reaches or exceeds 10 micrometers. Slow ion movement within thick electrodes leads to rapid capacity decay under high charge-discharge rates.

To overcome these challenges, the team proposed an innovative design featuring dual vertical ion channels and rapid charge storage/transport to enhance electrochemical reaction kinetics. As shown in Figure 1, the novel electrode consists of vertically ordered pillar arrays with pseudocapacitive NiCo2S4 nanosheets grown on high-aspect-ratio pillar current collectors. The electrode architecture incorporates two types of vertical ion channels: large inter-pillar spacings that serve as unobstructed ion highways during electrochemical energy storage, and porous nanochannels formed by vertically aligned, ultrathin NiCo2S4 nanosheets that significantly accelerate ion diffusion throughout the electrode. Furthermore, the nanoimprint fabrication process creates pillar current collectors that are monolithic with the bottom nickel layer, eliminating charge transfer interfaces. The NiCo2S4 nanostructures not only regulate surface cation redox reactions but also rapidly transfer electrons from active sites to the conductive current collector, thereby promoting ion diffusion, increasing effective contact area, and shortening faradaic charge transfer distances.

To achieve this breakthrough, the team employed cost-effective, scalable thermal nanoimprinting technology to fabricate high-aspect-ratio, vertically ordered nickel arrays (Figures 2a-b), followed by microcurrent electrodeposition to grow ultrathin nanosheets perpendicular to the curved pillar surfaces (Figures 2c-d). The resulting structure features active two-dimensional transition metal sulfide MCo2S4 (M = Ni, Co, Fe, Mn, etc.) with a mere 5 nm thickness standing on pillar current collectors (Figures 2e-f), where intersecting nanosheets form vertical pore channels that facilitate ultrafast directional ion diffusion and transport within the electrode. Remarkably, the developed electrodes achieve thicknesses up to 20 micrometers while demonstrating linear capacity increase with MCo2S4 coating thickness - overcoming the common limitation where capacity begins to decrease beyond certain thickness values - thereby effectively solving the long-standing problem of thickness-dependent capacity limitations in energy storage electrodes.

Figure 2a shows the SEM image of the nano thermal imprint template interface；Figure 2b presents the pillar nickel micro current collector； Figures 2c-d display the MCo2S4nickel array prepared by microcurrent deposition；Figures 2e-g show local magnification SEM images of surface nanosheets；Figure 2g illustrates the EDS mapping element distribution of the array electrode；Figures 2h-j provide TEM SAED and HRTEM analysis of MCo2S4 nanosheets

The team developed a novel MCo2S4nickel array electrode achieving a capacity of 486.9 mAh g⁻¹ at 1 A g⁻¹ current density outperforming previously reported NiCo2S4 electrode materials. Even when the current density increased to 100 A g⁻¹ the electrode maintained a capacity of 150 mAh g⁻¹. Remarkably after 5000 charge-discharge cycles at a high current density of 10 A g⁻¹ only 3.8 percent capacitance loss was observed as shown in Figure 3a. The morphology of the nanosheet coating and the nanopore microstructure showed no significant changes see inset SEM image demonstrating the electrode’s exceptional cycling stability. This is attributed to the electrode structure effectively securing the distribution of two-dimensional nanosheets and pore architecture while the independent micro current collectors and interwoven two-dimensional NiCo2S4 flakes significantly mitigate volume changes caused by redox reactions during charging and discharging.

Thanks to its unique array structure this electrode exhibits outstanding flexibility distinct from other electrodes. Under substantial bending at angles of 45° 90° 135° and 180° the electrochemical capacity not only remained stable but even increased slightly as depicted in Figure 3b. In an asymmetric all-solid-state device using the NiCo2S4nickel electrode as the positive electrode and graphene foam as the negative electrode the energy density reached 66.5 Wh kg⁻¹ at a power density of 400 W kg⁻¹ surpassing the energy density reported in literature for other NiCo2S4-based asymmetric devices.

Figure 3a shows the capacitance retention rate of the MCo2S4nickel array single electrode after 5000 charge-discharge cycles at a current density of 10 A g⁻¹ with the inset SEM image displaying the post-cycling morphology. Figure 3b presents the CV curves of the electrode under different bending angles.

This research provides a new solution to address the challenge of thickness limitations in electrochemical energy storage electrodes. Thermal nanoimprinting technology offers high throughput low cost and simple processes making it a key technique for large-scale device fabrication. Notably the nickel arrays in this thermal nanoimprinting process can be reused like molds. The study confirms the effectiveness of this new preparation method which has been extended to other pseudocapacitive materials such as MFe variants providing an efficient pathway for the design and scalable production of high-performance flexible pseudocapacitive electrodes.

We welcome interested industry partners to contact us for further collaboration on related applications.

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