To make electric vehicles more practical and attractive to consumers, lithium-ion batteries need some improvements. Mainly, they need to hold much more energy per weight to allow vehicles to go longer distances, and the batteries need to charge fast, ideally in under 15 minutes, all while keeping battery costs low.
Mechanical engineer Kun Fu of the University of Delaware believes that 3D printing could be a way to meet those goals. Last week at the American Chemical Society Spring 2022 meeting, his graduate student Soyeon Park presented results on new 3D-printed battery electrodes with a carefully designed structure that can store more than three times as much charge as conventionally made electrodes.
If battery makers want to increase charge storage in their devices, they could make the batteries’ graphite anodes thicker to hold more lithium ions, said Park, who presented the new work at a Division of Polymeric Materials Science and Engineering session. The anodes also would need an organized microscopic structure to let ions travel faster and farther for fast charging and high power, she says. But the current method to make these electrodes, which involves coating a graphite slurry onto a sheet, does not allow precise control of electrode geometry. The electrodes are usually a few hundred micrometers thick, and making the coating thicker just increases the electrode weight without improving performance.
A few researchers have recently explored 3D printing to make thick, structured electrodes. They have typically used inks made of graphite particles and a polymer binder. But while these teams have made electrodes with complex geometries, they have not considered how the graphite nanoflakes end up positioned in the material, so the flakes can be randomly oriented, which doesn’t optimize the flow of lithium ions, Park said.
So she, Fu, and their colleagues developed a 3D-printing technique that orients the graphite flakes in the same direction across the thickness of the electrode. “This alignment of graphite gives a path for lithium ions to easily move from top to bottom of the electrode,” she said.
The team starts by making pellets of a composite of graphite, polylactic acid (PLA), and a small amount of carbon black and carbon nanotubes as additives for mechanical strength. As the molten composite passes through the small heated nozzle of a 3D printer, the force acting on the liquid aligns graphite nanoflakes in the printing direction.
By printing layer upon layer in a zig-zag pattern, the researchers made 1 mm-thick electrodes. After heating their printed patterns at 800 °C to burn away the PLA, the researchers are left with a pure carbon electrode. Tests showed that the electrodes have a charge capacity of about 77 milliampere-hours per gram. Electrodes with a similar thickness made with a conventional slurry-coating technique had a capacity of 25.5 mAh/g.
Combining nanomaterial orientation with 3D printing is a novel approach, said Kai Narita, a materials scientist and battery engineer at battery startup 24M Technologies in Boston. But while 3D-printed battery electrodes may beat traditional ones in energy density and charging speed, they still aren’t practical enough for commercial use, he said. “Regarding cycle life, safety, and cost, traditional electrodes win over 3D printed electrodes.”
The idea of 3D-printed batteries is still relatively new, Park said. For now, the Delaware team is working on improving the electrodes’ capacity and strength by increasing the amount of graphite and including new types of additives.