A study provides new guidance for designing sodium-ion batteries, which are emerging as a less expensive and more environmentally friendly complement to lithium-based batteries.
PROVIDENCE, R.I. [Brown University] - As the world's need for energy storage increases, sodium-ion batteries are emerging as a less expensive and more environmentally friendly complement to lithium-based batteries. Research by Brown University engineers sheds new light on how sodium behaves inside these batteries, providing new design specifications for anode materials that maximize stability and energy density for sodium-ion batteries.
"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. "That provides some guidelines for synthesizing the desired anode materials for these batteries that maximize overall performance."
The research is published in EES Batteries.
Lithium-ion batteries are currently used in the lion's share of rechargeable electronics and electric vehicles. They work well, but increasing demand for energy storage, particularly in adding resilience to power grids, requires additional options. Sodium-ion offers an alternative with some major potential upsides. Sodium is cheap and abundant, which could reduce production costs and the need for destructive mining.
Commercialization of sodium-ion batteries is in its infancy, however, and researchers are still tweaking the basic design. One outstanding question is what material structure works best as a sodium-ion anode - the side of the battery that stores sodium atoms during charging. Lithium-ion anodes are generally made of graphite, but research has shown that graphite performs poorly for sodium storage. So scientists have turned to "hard carbon" - a material that can be made by heating any number of carbon-bearing materials, from wood to sugar.
"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. "The ambiguous structures are a major problem for designing the anode materials because of the lack of knowledge of the structure-property relationship."
Qi is deputy director of Brown's Initiative for Sustainable Energy, which focuses on the development of renewable energy, sustainable fuels and materials, and energy efficiency technologies.
Previous research suggests that sodium storage probably occurs in tiny pores that form in hard carbon structures. But exactly how that storage takes place - or how the size of the pores might enhance it - wasn't known. For this new study, Mtemeri investigated a carbon material known as zeolite-templated carbon (ZTC), which can be made with a well-defined network of nanopores. Using ZTC as a model for the hard carbon pore framework and a custom algorithm to simulate pore filling, Mtemeri used a computational technique called density functional theory to investigate the behavior of sodium within the nanopores.
The research showed that as sodium atoms gravitate into the pores, they first line the walls of each pore with ionic bonds. After the walls are covered, additional sodium atoms fill the middle of the pore in metallic clusters. The dual modes of sodium storage - ionic along the walls and metallic toward the centers of the pores - are critical, the researchers say. The mixed ionic and metallic sodium helps to keep the anode voltage low, which increases the overall voltage of the battery (a battery's overall voltage is equal to the cathode voltage minus the anode voltage, so lower anode voltage is better). Meanwhile, the ionic sodium prevents sodium metal plating, a condition that can create short circuits between anode pores.
"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."
The findings, the researchers say, offer some of the first concrete design specifications for making hard carbon anodes - or any carbon materials with this kind of porous structure - in the lab. That could help pave the way for future commercial use of sodium-ion batteries.
"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."
