Rahul Panat, an associate professor of mechanical engineering at Carnegie Mellon University, and a team of researchers from Carnegie Mellon in collaboration with Missouri University of Science and Technology have developed a new method of aerosol jet (AJ) 3-D printing battery electrodes that creates a 3-D microlattice structure with controlled porosity.

3-D printing this microlattice structure, the researchers show in a paper published in the journal Additive Manufacturing, vastly improves the capacity and charge-discharge rates for lithium-ion batteries.

Lattice architecture can provide channels for effective transportation of electrolyte inside the volume of material, while for the cube electrode, most of the material will not be exposed to the electrolyte. The cross-section view shows the silver mesh enabling the charge (Li+ ions) transportation to the current collector and how most of the printed material has been utilized. Credit: Rahul Panat, CMU College of Engineering. Click to enlarge.

Advances in the electrochemical energy storage technologies such as Li-ion batteries have been realized not only by introducing new high energy/power materials, but also by creating new electrode architectures that increase surface area and mitigate mechanical instability. Three dimensional (3D) porous electrode architectures with irregular or regular (e.g. lattice) geometries enhance the ingress of Li-ions into the host electrodes while reducing the total diffusion path, which can lead to an improved utilization of the electrode volume. Further, the facile transport of Li-ions leads to their uniform distribution along the electrodes, enhancing the tolerance of the battery to mechanical stress during demanding intercalation/de-intercalation cycles (similar to cellular materials such as bones).

In addition to energy storage, the 3D architected materials have been proposed as metamaterials, where mechanical, thermal and optical properties can be tuned by simple changes to their structures and shapes. Although the potential benefits of such architectures are clear, a scalable and repeatable manufacturing process that can control porosity for a wide range of battery materials remains a significant challenge.

SEM images of 3D printed electrodes for Li-ion batteries used for electrochemical cycling in the researchers’ study. Image taken from top of microlattice electrodes with height of about 250mm. Credit: Rahul Panat and Mohammad Sadeq Saleh. Click to enlarge.

… Although significant advances have been made in this area, the electrode shape control, especially in 3D, is rather limited either due to the nature of the templates or the etching processes used. Another limitation is the electrode material compatibility with chemicals used to create the porosity. Lastly, although the electrodes with special geometries such as hollow tubes have controlled shapes; they offer a very limited amount of total electrode volume which is not practical for most applications.

—Saleh et al.

Because of the nature of the 3D manufacturing process, the design of these 3-D printed electrodes is limited to just a few possible architectures. Until now, the internal geometry that produced the best porous electrodes through additive manufacturing was an interdigitated geometry—metal prongs interlocked like the fingers of two clasped hands, with the lithium shuttling between the two sides.

This paper proposes a new innovative approach for fabricating complex 3D lattice battery electrodes with hierarchical porosity. The work aims to: (1) create a new fabrication approach for battery architectures that provides adequate degrees of freedom to ensure structural integrity and electrical connectivity, (2) enable a facile electrolyte immersion through open porosity to take full advantage of the energy storage capability of the active material, and (3) relax the stress by accommodating volume change during charge/discharge cycles. Toward this, AJ 3D printing is utilized to realize highly complex 3D microlattice electrodes with a hierarchical porosity over several orders of magnitude in length scale.

—Saleh et al.

The additive manufacturing method presented in the paper represents a major advance in printing complex geometries for 3-D battery architectures, as well as an important step toward geometrically optimizing 3-D configurations for electrochemical energy storage. The researchers estimate that this technology will be ready to translate to industrial applications in about 2-3 years.

The microlattice structure (Ag) used as lithium-ion batteries’ electrodes was shown to improve battery performance in several ways such as a four-fold increase in specific capacity and a two-fold increase in areal capacity when compared to a solid block (Ag) electrode.

Furthermore, the electrodes retained their complex 3D lattice structures after forty electrochemical cycles, thereby demonstrating their mechanical robustness. The batteries can thus have high capacity for the same weight or alternately, for the same capacity, a vastly reduced weight—an important attribute for transportation applications.

The Carnegie Mellon researchers developed their own 3-D printing method to create the porous microlattice architectures while leveraging the existing capabilities of an Aerosol Jet 3-D printing system. The Aerosol Jet system—which was deployed at Carnegie Mellon University’s College of Engineering earlier this year—also allows the researchers to print planar sensors and other electronics on a micro-scale.

Until now, 3-D printed battery efforts were limited to extrusion-based printing, where a wire of material is extruded from a nozzle, creating continuous structures. Interdigitated structures were possible using this method. With the method developed in Panat’s lab, the researchers are able to 3-D print the battery electrodes by rapidly assembling individual droplets one-by-one into three-dimensional structures. The resulting structures have complex geometries impossible to fabricate using typical extrusion methods.

Because these droplets are separated from each other, we can create these new complex geometries. If this was a single stream of material, as is in the case of extrusion printing, we wouldn’t be able to make them. This is a new thing. I don’t believe anybody until now has used 3-D printing to create these kinds of complex structures.

—Rahul Panat

The team, which also includes mechanical engineering Ph.D. student Mohammad Sadeq Saleh and postdoctoral researcher Jie Li (Missouri University of Science and Technology), is also working on creating more complex three-dimensional structures, which can simultaneously be used as structural materials and as functional materials. For example, a part of a drone can act as a wing, a structural material, while simultaneously acting as a functional material such as a battery.

Resources

• Mohammad Sadeq Saleh, Jie Li, Jonghyun Park, Rahul Panat (2018) “3D printed hierarchically-porous microlattice electrode materials for exceptionally high specific capacity and areal capacity lithium ion batteries,” Additive Manufacturing, Volume 23, Pages 70-78 doi: 10.1016/j.addma.2018.07.006