Dendrite-free High Amount of Li Grown on Top of Crumpled Graphene Ball Electrodes. (A) Li (3 mA hr cm−2) deposited on 8-μm thick crumpled graphene ball (CGB) electrode. (B and C) Li deposited on a 40-μm thick CGB electrode at (B) 6 mA hr cm−2 and (C) 12 mA hr cm−2. Liu et al. Click to enlarge.

A number of strategies have been developed to address the problems associated with Li filaments. For example, one can make the battery structure more robust by employing solid electrolytes that are not easily pierced by Li dendrites and strengthen the SEI by adjusting the formulation of the liquid electrolytes. Alternatively, an ion-permeable blocking layer can be introduced to prevent the growing Li filaments from penetrating the separator. However, this does not prevent the fluctuation of the apparent volume of Li metal layer during filament growth/disappearance, which tends to weaken or even delaminate the Li/electrolyte interface or the blocking layers during cycling, thus allowing additional growth of dendrites.

In another type of strategy, an insulating porous network made of polymer gels or glass fibers can be added, through which Li filaments can only grow along the tortuous network of pores. Unfortunately, such tortuous Li filaments tend to break and become disconnected from the electrodes during cycling. Using scaffolds can help to minimize the volume fluctuation of electrodes. Such host material needs to be porous, electrically conductive, chemically and mechanically stable, and have a low interfacial energy with Li metal for preferential deposition to suppress filament growth. Various porous forms of Cu have been demonstrated as an effective host to support Li. However, the main issue in using Cu concerns its high density (8.9 g cm⁻³) in regard to Li (0.53 g cm⁻³), which drastically decreases the overall energy density of the electrode.

Porous carbon nanostructures, including graphene-based materials, are attractive lightweight Li host materials. Here we report the use of crumpled graphene balls as scaffolds to stabilize Li metal anodes.

—Liu et al.

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.

The researchers addressed this problem by using 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.

One general philosophy for making something that can maintain high stress is to make it so strong that it’s unbreakable. 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.

—Northwestern Engineering’s Jiaxing Huang, co-corresponding author

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 a 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 their surface, the scaffold creates a continuously conductive, dynamic, porous network for lithium.

Since they do not aggregate, crumpled graphene balls can readily form a closely packed, continuous solid, which has highly uniform thickness and pore-size distribution. They are lithiophilic and can support fast Li-ion diffusion.

Crumpled paper ball-like graphene particles have scalable Li loading up to 10 mAh cm⁻² within tolerable volume fluctuation. The researchers achieved high Coulombic efficiency of 97.5% over 750 cycles (1,500 hr). Plating/stripping Li up to 12 mAh cm⁻² on crumpled graphene scaffold does not experience dendrite growth.

Closely packed, the crumpled graphene balls operate like a highly uniform, continuous solid. We also found that the crumpled graphene balls do not form clusters but instead are quite evenly distributed.

—Jiayan Luo, the paper’s co-corresponding author and professor of chemical engineering at Tianjin University

Compared to batteries that use graphite as the host material in the anode, Huang’s solution is much lighter weight and can stabilize 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 through Northwestern’s Innovation and New Ventures Office (INVO).

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.

Resources

• Shan Liu, Aoxuan Wang, Qianqian Li, Jinsong Wu, Kevin Chiou, Jiaxing Huang, Jiayan Luo (2018) “Crumpled Graphene Balls Stabilized Dendrite-free Lithium Metal Anodes” Joule 2, 1 184-193 doi: 10.1016/j.joule.2017.11.004