Lithium ion batteries have become the go-to energy storage medium for everything from EVs to renewable energy reservoirs at the substation level. While flow batteries and other devices are being developed and are on the horizon, none have the cost effectiveness of current Li-ion storage solutions.
The drawback to current lithium ion batteries is that the lithium is usually atomically distributed in another material such as graphite or silicon in the anode. This can dilute battery performance. Scientists have spent years trying to address this challenge. As lithium gets charged and discharged in a battery, it starts to grow dendrites and filaments, which can cause a number of problems, leading to rapid degradation of battery performance. At worst, it can cause the battery to short or even catch fire, which recently resulted in restrictions on battery transport by aviation.
Developing true lithium metal-based batteries offers the potential to turn the battery industry upside down by providing ultra-high capacity compared to what we have now. This would mean a huge boost in power and range for EVs and other kinds of vehicles, among other applications.
One solution to bypass lithium’s destructive dendrites is to use a porous scaffold such as those 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. As lithium deposits onto and then dissolves from the porous support as the battery cycles, its volume fluctuates significantly. This volume fluctuation induces stress that could break the porous support.
A team at Northwestern University’s McCormick School of Engineering led by Jiaxing Huang has solved this problem by taking a different approach — one that even makes batteries lighter, and able to hold more lithium.
The solution lies in a scaffold made from crumpled graphene balls, which can stack with ease to form a porous scaffold, due to their paper ball-like shape. They not only prevent dendrite growth but can also survive the stress from the fluctuating volume of lithium. The research was featured on the cover of the January issue of the journal Joule.
“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 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 Huang’s team’s 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 itself, and then readily reassemble when the lithium is depleted. Because 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.”
Compared to batteries that use graphite as the host material in the anode, Huang’s solution reduces weight and stabilizes a higher load of lithium during cycling. Whereas typical batteries encapsulate lithium that is just tens of microns thick, Huang’s battery holds lithium stacked 150 microns high.
Huang and his collaborators have filed a provisional patent.
The research was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Tianjin, China, the State Key Laboratory of Chemical Engineering, and the Office of Naval Research.