Sodium-ion batteries are emerging as a promising alternative to today’s lithium-ion batteries.
While lithium-ion technology powers everything from smartphones to electric cars, lithium is relatively scarce and unevenly distributed around the world.
Sodium, by contrast, is abundant and inexpensive, making sodium-ion batteries a more sustainable and potentially cheaper option.
However, for sodium-ion batteries to compete with lithium-ion batteries, they must store similar amounts of energy.
One of the biggest challenges lies inside the battery’s anode, the component that stores sodium ions during charging.
Most advanced sodium-ion batteries use a material called hard carbon as the anode.
Hard carbon is a porous, disordered form of carbon with tiny nanoscale holes. Scientists believe these nanopores play a key role in storing sodium, but exactly how this happens has remained unclear.
To better understand the process, a research team led by Professor Yoshitaka Tateyama at the Institute of Science Tokyo used powerful computer simulations to study how sodium behaves inside hard carbon.
Their findings were published in the journal Advanced Energy Materials.
Using high-accuracy modeling techniques known as density functional theory-based molecular dynamics, the team simulated how sodium ions move and interact within tiny carbon structures. These simulations were run on advanced supercomputers, including Japan’s Fugaku system.
The results offered new insight into how sodium is stored. At first, sodium ions stick to the surface of the carbon in a flat, two-dimensional arrangement.
But the simulations showed that they quickly transition into three-dimensional, quasi-metallic clusters inside the nanopores. In simple terms, the sodium ions group together into tiny metallic-like clusters within the carbon’s small cavities.
The researchers also identified the ideal pore size for stable sodium storage. Their calculations showed that nanopores around 1.5 nanometers wide provide optimal conditions for forming and maintaining these sodium clusters. This prediction closely matches previous experimental observations, strengthening confidence in the model.
Another key finding explains why sodium-ion batteries often charge and discharge more slowly than desired. While sodium ions can move quickly in certain well-connected areas of the hard carbon, narrow and branching regions act as bottlenecks. Sodium ions can temporarily clog these tight spaces. Only when enough repulsive force builds up do the ions push through, creating a rate-limiting step that slows the overall process.
The study also revealed that certain sodium ions attached to defects in the carbon structure can actually help cluster formation by reducing strong sodium-carbon interactions and adjusting the available space inside the pores.
Together, these insights provide clearer design guidelines for improving hard carbon materials. By carefully controlling pore size and structure, scientists may be able to create sodium-ion batteries with higher energy density and better performance.
As renewable energy from solar and wind farms continues to grow, affordable and sustainable battery storage becomes increasingly important. Improved sodium-ion batteries could play a significant role in supporting a future powered by clean energy.
