UCLA Researchers Developed a Tiny Li-ion Battery the Size of 100 Grains of Salt

20 May 2018

Electronic gadgets are getting tinier by the day, but the power needed to drive them is stagnant or increasing.

Figure 1. 3D batteries developed by researchers at UCLA. Source: UCLAFigure 1. 3D batteries developed by researchers at UCLA. Source: UCLA

With the advent of the internet of things (IoT), there is a big push in creating smaller gadgets, including wearable medical devices and wireless sensors. It estimated that internet-connected devices will reach 60 billion by 2025. To reach this goal, the availability of small batteries with high energy density is needed.

Last week a team of researchers at the University of California Los Angeles (UCLA) announced, using non-traditional methods, the development of a powerful 3D lithium-ion battery as small as 100 grains of salt.

Figure 2. Cross-section of a 3D battery. The orange thin l ayer shown is the electrolyte. Source: UCLAFigure 2. Cross-section of a 3D battery. The orange thin l ayer shown is the electrolyte. Source: UCLA

"For small sensors, you need to re-design the battery to be like a skyscraper in New York instead of a ranch house in California," says senior author Bruce Dunn, a professor of materials science and engineering at UCLA. "That's what a 3D battery does, and we can use semiconductor processing and a conformal electrolyte to make one that is compatible with the demands of small internet-connected devices."

To make a typical 2D battery, one packs an electrolyte between the anode and cathode, but this process limits the shape the battery can take. On the other hand, a 3D battery can be manufactured in many ways. The 3D cathode and the 3D anode can be joined together like a puzzle of Lego pieces, giving way to innumerable layouts.

The UCLA researchers chose a novel approach: what they call a “concentric-tube” design. The tubes are evenly spaced anode vias (posts) and covered by a thin layer of electrolyte. The region between the posts is filled with cathode material, as is shown in Figure 2.

The novel approach is the use of semiconductor fabrication techniques to build the battery. The array of posts are made by dry etching crystalline silicon wafers, a process that is a typical chip fabrication method. To cover the posts with electrolyte, they use a standard photolithography to cover the vias with a micron thick layer of electrolyte using SU-8 photoresist, and to complete the battery they poured a standard lithium-cathode material. The novelty of this design is the use of silicon as both the substrate (scaffold) of the chip and as the negative electrode material for the battery, a process that maximizes the energy density, according to the results of tests made by the researchers. Figure 3 shows the steps used in the fabrication of the battery.

Figure 3. Fabrication steps. Source: UCLAFigure 3. Fabrication steps. Source: UCLA

Summary of the fabrication steps:

(A) Silicon wafer is coated with oxide and array pattern is etched.

(B) 3D post array is etched into silicon.

(C) Scanning electron microscopic image of silicon array.

(D) SU-8 photoresist is selectively cross-linked around the silicon posts by photolithography.

(E) Un-crosslinked SU-8 is removed in a developer bath and base layer is cross-linked.

(F) Scanning electron microscopic image of SU-8-coated array.

(G) Vacuum infiltration of cathode slurry.

(H) Charging schematic of the complete 3D battery.

(I) Scanning electron microscopic image of the full 3D battery.

The final product resulted in a 3D micro battery with an energy density of 5.2 milliwatt-hours per cm2, and a footprint of only 0.09 cm2. The battery can withstand 100 cycles of charging and discharging.

For Dunn and colleagues, this is not the final version of their battery. They expect to increase its energy density and to find a solution to packaging the battery.

"Another challenge with batteries is always the packaging," Dunn says. "You need to seal them up, keep them small, and make sure they function just as well in the real world as in the glovebox."

An abstract of the paper was published in the journal Joule.

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