New simulations reveal how tiny carbon pores can boost next wave of sodium batteries

Sodium may be the underdog of battery chemistry, but new research shows it could soon punch far above its weight. As global demand for energy storage from EVs to resilient power grids explodes, scientists are pushing hard to develop alternatives to lithium-ion. Sodium-ion batteries have emerged as one of the most promising candidates as it is cheaper, more abundant, and far less environmentally taxing. But a key technical hurdle has slowed their progress.

Researchers across the globe are scrambling to figure out what kind of anode material can store sodium efficiently. A new study from Brown University engineers takes a major step toward solving that puzzle. The work uncovers how sodium behaves inside porous carbon structures and lays out concrete design specifications for future sodium-ion anodes. This work helps us understand the mechanism of sodium storage in carbon materials for sodium-ion batteries, said Lincoln Mtemeri, a presidential postdoctoral fellow in engineering at Brown who led the study.

The research offers rare clarity in a field where even the basic structure of hard carbon , which is today s leading anode candidate, has been widely debated. Inside carbon s maze Lithium-ion batteries dominate today s electronics and EVs, but scaling them for grid resilience and mass electrification poses economic and environmental challenges.

Sodium-ion systems promise relief because sodium is plentiful and inexpensive, potentially reducing both cost and mining impacts. But commercializing the technology has been slow. Graphite, the go-to anode for lithium-ion cells, performs poorly with sodium. Hard carbon has taken its place, yet the material s microscopic structure is still so murky that If you ask 10 different people what the structure of hard carbon is, you ll get 10 different answers, said Yue Qi, a professor in Brown s School of Engineering and study co-author.

Researchers have long suspected that tiny pores within hard carbon are responsible for storing sodium. However, how sodium actually settles inside those pores — or what pore size works best — remained unknown.

Atoms in formation To investigate this, Mtemeri used zeolite-templated carbon (ZTC), a material with a precisely controlled nanopore network ideal for modeling pore behavior. With a custom algorithm and density functional theory simulations, he tracked how sodium atoms arrange themselves as they enter these nano-sized chambers.

The results reveal a two-step storage phenomenon. Sodium atoms first line the pore walls through ionic bonds. Once the walls are fully coated, additional atoms gather in the middle of the pore as metallic clusters. This dual behavior of being ionic at the edges, metallic at the center, turns out to be crucial.

The mix helps maintain a low anode voltage, boosting the battery s overall voltage, while the ionic sodium prevents dangerous metal plating that can cause short circuits. This helps us determine the optimal size for the pores, Mtemeri said. We show that a pore size of around one nanometer maintains the good balance of ionicity and metallicity that we want. These findings offer some of the first firm design rules for hard-carbon anodes, providing researchers with a blueprint for future synthesis.

Sodium is 1,000 times more abundant than lithium, which makes it a more sustainable option, Qi said. Now we understand exactly which pore features are important and that enables us to design anode materials accordingly.

The study appears in the journal EES Batteries.