Researchers find way to make crack-free nanocellular graphene to upgrade low-cost sodium-ion energy storage systems.

Looking to solve a critical failure during the manufacture of nanocellular graphene, researchers from Tohoku University in Japan have discovered that by using this specialized form of graphene in a dealloying process effectively "heals" the cracks formed during its making.

Ever since its discovery in 2004, graphene has been revolutionizing the field of materials science and beyond. Comprised of two-dimensional sheets of carbon atoms, bonded into a thin hexagonal shape with a thickness of one atom layer, this gives graphene remarkable physical and chemical properties.

Despite this thinness, graphene is incredibly strong, lightweight, flexible, and transparent. Additionally, it also exhibits extraordinary electrical and thermal conductivity, high surface area, and impermeability to gases.

From high-speed transistors to biosensors, graphene boasts an unrivaled versatility in nearly all applications.

Advanced Materials (2024)

Schematic for the formation of nanocellular graphene during liquid metal dealloying of amorphous manganese-carbon alloy in a molten bismuth to induce selective dissolution of manganese atoms and self-organization of carbon atoms into graphene layers.

Nanocellular graphene (NCG) is a specialized form of this wonder material that achieves a large specific surface area by stacking multiple layers of graphene to allow control of its internal structure. Coveted for its potential to improve the performance of electronic and energy devices as well as sensors, its development has only been stunted due to defects that occur during the manufacturing process.

Cracks often appear when forming NCGs, and scientists have spent a great deal of time looking for new processing techniques that can ultimately fabricate homogenous, crack-free, and seamless NCGs at necessary scales.

"We discovered that carbon atoms rapidly self-assemble into crack-free NCG during liquid metal dealloying of an amorphous Mn-C (manganese-carbon) precursor in a molten bismuth," said Won-Young Park, a graduate student at Tohoku University.

Simply put, dealloying is the process of separating the molecules that make up the joined metal. Using a process that exploits the varying miscibility or liquid compatibility, such as in a molten metal bath, this process selectively corrodes certain components of an alloy while preserving others.

Park and colleagues determined that NCGs developed with this method exhibited the qualities sought after from unblemished NCGs, especially high tensile strength and high conductivity after graphitization.

The researchers tested their unblemished NCGs in an experimental sodium-ion battery.

Sodium-ion

One advantage that sodium-ion batteries hold over their more ubiquitous lithium-ion counterparts is the availability of their main ingredient – salt is everywhere.

Given the ever-increasing demand for lithium, supply is having trouble keeping up, and with that scarcity, prices continue to soar, making the dominant battery less of an ideal in what is supposed to be a clean, affordable enabler of the clean energy transition.

As a result, batteries based on sodium have been gaining traction, especially from Western companies seeking a secure supply of battery materials. However, the Achilles' heel of sodium-ion batteries is that they can effectively store about two-thirds of the energy of lithium-ion batteries of equivalent size.

Focused on long-term energy storage systems rather than mobility (for now), sodium-ion batteries with a more efficient anode may just be the breakthrough needed to place them as a salty contender to offset lithium in areas where other minerals can perform as effectively, leaving the white metal to shine in the fields it is best at.

"We used the developed NCG as an active material and current collector in a SIB (sodium-ion battery), where it demonstrated a high rate, long life and excellent deformation resistance," said Park. "Ultimately, our method of making crack-free NCG will make it possible to raise the performance and flexibility of SIBs-an alternative technology to lithium-ion batteries for certain applications, particularly in large-scale energy storage and stationary power systems where cost, safety, and sustainability considerations are paramount."