A microscope image of the crumpled graphene balls. Image: Jiaxing Huang.
A microscope image of the crumpled graphene balls. Image: Jiaxing Huang.

Lithium metal-based batteries have the potential to transform the battery industry. By taking advantage of the theoretically ultra-high capacity of pure lithium metal, this new type of battery could power everything from personal devices to cars.

"In current batteries, lithium is usually atomically distributed in another material such as graphite or silicon in the anode," explains Jiaxing Huang from Northwestern University. "But using an additional material 'dilutes' the battery's performance. Lithium is already a metal, so why not use lithium by itself?"

In order to do so, however, scientists need to overcome a major obstacle. As lithium is charged and discharged in a battery, it starts to grow dendrites and filaments, "which causes a number of problems," Huang said. "At best, it leads to rapid degradation of the battery's performance. At worst, it causes the battery to short or even catch fire."

One current approach for bypassing lithium's destructive dendrites in lithium metal batteries is to use a porous scaffold made from carbon materials, on which lithium preferentially deposits. Then when the battery is charging, lithium can deposit along the surface of the scaffold, avoiding dendrite growth. This, however, introduces a new problem. Lithium depositing onto and then dissolving from the porous support as the battery cycles causes the support’s volume to fluctuate significantly. This volume fluctuation induces stress that can break the porous support.

To solve this problem, Huang and his collaborators have now developed a modified version of the scaffold approach, which can produce batteries that are even lighter in weight and able to hold more lithium. They used a scaffold made from crumpled graphene balls, which can stack with ease to form a porous scaffold, due to their paper ball-like shape. As the researchers report in a paper in Joule, these graphene balls not only prevent dendrite growth but can also survive the stress caused by the fluctuating volume of lithium.

"One general philosophy for making something that can maintain high stress is to make it so strong that it's unbreakable," said Huang, professor of materials science and engineering in Northwestern's McCormick School of Engineering. "Our strategy is based on an opposite idea. Instead of trying to make it unbreakable, our scaffold is made of loosely stacked particles that can readily restack."

Six years ago, Huang discovered crumpled graphene balls – novel ultrafine particles that resemble crumpled paper balls – which he made by atomizing a dispersion of graphene-based sheets into tiny water droplets. When the water droplets evaporated, they generated a capillary force that crumpled the sheets into miniature balls.

In the battery developed by Huang and his team, the crumpled graphene scaffold accommodates the fluctuation of lithium as it cycles between the anode and cathode. The crumpled balls can move apart when lithium deposits and then readily assemble back together when the lithium is depleted. Because the miniature graphene balls are conductive and allow lithium ions to flow rapidly along their surface, the scaffold creates a continuously conductive, dynamic, porous network for lithium.

"Closely packed, the crumpled graphene balls operate like a highly uniform, continuous solid," said Jiayan Luo, the paper's co-corresponding author and professor of chemical engineering at Tianjin University in China. "We also found that the crumpled graphene balls do not form clusters but instead are quite evenly distributed."

Formerly advised by Huang, Luo earned his PhD in materials science and engineering in 2013. Now a professor and researcher at Tianjin University, he continues to collaborate with Huang.

Compared to batteries that use graphite as the host material in the anode, Huang's solution is more lightweight and can stabilize a higher load of lithium during cycling. Whereas typical batteries encapsulate lithium that is just tens of micrometers thick, Huang's battery holds lithium stacked 150µm thick.

This story is adapted from material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.