Lithium metal-based batteries have the potential to completely change the battery industry. With the ultra-high capacity of lithium metal used by itself, the new types 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 Northwestern University's Jiaxing Huang. "But using an additional material 'dilutes' the battery's performance. Lithium is already a metal, so why not use lithium by itself?"
One solution is to bypass lithium’s destructive dendrites to use a porous scaffold, like those made from carbon materials, on which lithium preferentially deposits. When the battery is charging, lithium can deposit along the surface of the scaffold, which avoids dendrite growth.
But this introduces a new problem. As lithium deposits onto and then dissolves from the porous support as the battery cycles, the volume fluctuates significantly. The volume fluctuation induces stress that could break the porous support.
Huang and collaborators have solved the problem by taking a different approach that makes batteries even lighter and able to hold more lithium.
The solution lies in a scaffold made from crumpled graphene balls that can stack easily to form a porous scaffold because of their paper ball-like shape. They prevent dendrite growth and survive the stress from a large amount 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. He then made the particles 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 miniaturized paper balls.
In the new battery, 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 miniature paper balls are conductive and allow lithium ions to flow rapidly along the 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."
The paper on this research has been published in the journal Joule.