US scientists used density functional theory to reveal how sodium ions are stored in nanoporous carbon anodes for sodium ion batteries, identifying dual ionic and metallic storage mechanisms in the pores. Their findings provide practical design guidelines to improve battery voltage, safety and commercial viability of stationary energy storage.
Researchers from Brown University in the United States have investigated the behavior of sodium storage in carbon materials used in sodium ion batteries, with the aim of improving their commercial viability for stationary renewable energy storage applications.
“Our work provides guidance for synthesizing anode materials that maximize overall battery performance,” said lead author Lincoln Mtemeri. “Our findings provide some of the first concrete design specifications for the production of hard carbon anodes (or other carbon materials with similar porous structures) in the laboratory. This could pave the way for the future commercial use of sodium ion batteries.”
The researchers explained that hard carbon is widely considered a promising anode material for sodium ion batteries due to its unique combination of structure, chemistry and transport properties. Its disordered, porous, and conductive nature enables efficient ion storage, rapid charge transport, and long-term electrochemical stability. However, the team noted that the sodium mechanism in hard carbon remains poorly understood due to the complexity of its structure. This lack of understanding has also limited the development of theoretical models that can accurately quantify the open-circuit voltage of the material.
In the study “Structural descriptors controlling the pore filling mechanism in the hard carbon electrode during post-treatment”, published in ESS batteries, the researchers examined carbon with zeolite template (ZTC). ZTC is a nanoporous carbon material synthesized using zeolites as hard templates, allowing precise control over pore size and well-defined ion diffusion pathways.
The team used density functional theory (DFT), a quantum mechanical computational method used to calculate the electronic structure of atoms, molecules and solids, to analyze the sodium behavior in the nanopores. The simulations showed that when sodium atoms enter the pores, they initially bind to the pore walls through ionic interactions. Once the pore surfaces are fully occupied, additional sodium accumulates in the pore centers, forming metal clusters.
The researchers found that two sodium storage mechanisms – ionic adsorption along the pore walls and metal aggregation in the pore centers – play a crucial role in battery performance. The coexistence of ionic and metallic sodium helps maintain a low anode potential, increasing the overall battery voltage since the cell voltage is defined as the cathode potential minus the anode potential. At the same time, ionic sodium suppresses sodium metal plating, which could otherwise cause a short circuit between adjacent pores.
“This helps us determine the optimal pore size,” says Mtemeri. “We show that a pore size of approximately one nanometer maintains the desired balance between ionicity and metallicity.”
Looking ahead, the researchers say the descriptors developed in the research work, including pore size, specific volume and carbon topology, could serve as practical design guidelines for optimizing carbon-based electrodes in sodium ion batteries.
“Sodium is a thousand times more abundant than lithium, making it a more sustainable option,” said co-author Yue Qi. “Now we understand exactly which pore characteristics are important and that allows us to design anode materials accordingly.”
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