Architecting a Better Lithium-Ion Battery

Since the early 2000s, lithium-ion (Li-ion) batteries have become the predominant rechargeable power source for many mobile devices, electric vehicles, renewable-energy storage grids, and more. But the Li-ion batteries in commercial use carry both safety risks and limitations. Julia R. Greer, the Ruben F. and Donna Mettler Professor of Materials Science, Mechanics and Medical Engineering at Caltech, whose lab has been working to improve Li-ion batteries, now has established a new path toward making them less dangerous and more environmentally friendly, while also boosting performance.

"We've developed a versatile way to make three-dimensional architected battery electrodes from safer materials," says Yingjin Wang (PhD '26), a graduate student in Greer's lab and first author of a recent paper published in ACS Energy Letters that describes the novel electrode. "Using lithium iron phosphate, commonly referred to as LFP, combined with a carbon matrix, we eliminated the use of dangerous cobalt, while improving the mechanical resilience of the battery."

Li-ion batteries contain five major parts: anodes, or negative electrodes; cathodes, or positive electrodes; electrolytes, a chemical medium that transports ions between electrodes; a permeable separator between the two electrodes that allows ions to pass through without the electrodes touching; and current collectors, which allow for harvesting the electricity produced via electrochemical ionic exchange.

Greer and Wang's study focused on developing a cathode, with a unique feature: it is 3D architected and produced via additive manufacturing. Most current Li-ion electrodes are planar, or relatively thin and flat; adding a third dimension and shaping the electrode provides efficient ion and electron transport pathways.

"If you make a battery that is 3D architected instead of planar, every lithium ion is going to have an active surface available to it as it's transporting through the electrolyte," says Greer. These surfaces are where chemical energy held in the ions converts into electrical energy. "We think this is advantageous because you can decouple the solid-state versus liquid-state diffusion distance. The electrolyte is liquid, so as it's channeling through this architecture, which is like a labyrinth it has a solid surface available to it anywhere."

By having less tortuosity, or shorter distances for the ions to follow between the cathode and the separator, the architecture allows for a higher power density, Greer says, which determines how fast stored energy can be released.

Another major issue with current Li-ion batteries—especially those in laptops and smart phones—is that many of their cathodes contain cobalt, a metal found in isolated regions of the world where it is often mined using unethical practices. LFP has a much better safety profile than cobalt; if you overcharge it, it is far less likely to catch on fire or short circuit as cobalt cathodes can.

"LFP by itself is not a new material, but using this additive manufacturing, or 3D-printing approach to create an architected electrode that doesn't contain cobalt, is a new thing," Greer says, noting that cobalt is also expensive and hard to recycle, so moving away from its use has many benefits.

Wang and Greer say the next step is to design a complementary 3D-architected LFP anode to realize a battery with fully 3D-architected electrodes that is both energy dense and power dense. But that will be no small feat.

"Architected LFP electrodes are still a relatively new area of research," says Wang. "The fabrication part is really challenging because there are so many parameters to get right. Ideally, if we can also add a polymer or polymer-based electrolyte to this system, which would make it a real solid-state battery, that would be awesome."

Solid-state batteries are the end goal for several reasons, including an increased safety profile and the potential to be extremely lightweight. This would mean they could be utilized to electrify medium- and heavy-duty transportation vehicles such as spacecraft.

"I'm a big fan of solid-state batteries, and I think that eventually we are all going to transition to the solid-state world," Greer says. "Our architected electrode is another stepping stone toward enabling solid-state batteries someday."

Yuchun Sun (PhD '24), a former graduate student in the Greer Lab, was also a co-author on the ACS Energy Letters paper, "Structure–Transport Relationships in Microarchitected LiFePO4–Carbon Li Ion Battery Electrodes." The research was supported by funding from the Defense Advanced Research Projects Agency (DARPA) and the President's and Director's Research and Development Fund at the Jet Propulsion Laboratory, which Caltech manages for NASA, and Caltech.