Power/Performance Bits: Feb. 10

Balancing battery capacity and stability
Researchers at Rice University are working to develop batteries that are better geared toward electric cars and more robust off-grid energy storage by digging into why lithium gets trapped in batteries, thus limiting the number of times it can be charged and discharged at full power.

The team found that by not maxing out a battery’s storage capacity, it could provide steady and stable cycling for applications that need it.

Conventional lithium-ion batteries utilize graphite-based anodes that have a capacity of less than 400 milliamp hours per gram (mAh/g), but silicon anodes have potentially 10 times that capacity, said Rice chemical and biomolecular engineer Sibani Lisa Biswal. However, silicon expands as it alloys with lithium, stressing the anode. By making the silicon porous and limiting its capacity to 1,000 mAh/g, the team’s test batteries provided stable cycling with still-excellent capacity.

“Maximum capacity puts a lot of stress on the material, so this is a strategy to get capacity without the same degree of stress,” Biswal said. “1,000 milliamp hours per gram is still a big jump.”

The team tested the concept of pairing the porous, high-capacity silicon anodes (in place of graphite) with high-voltage nickel manganese cobalt oxide (NMC) cathodes. The full cell lithium-ion batteries demonstrated stable cyclability at 1,000 mAh/g over hundreds of cycles.

Rice University engineers built lithium-ion batteries with silicon anodes and an alumina layer to protect cathodes from degrading. (Illustration courtesy of the Biswal Lab / Rice University)

Some cathodes had a 3-nanometer layer of alumina (applied via atomic layer deposition), and some did not. Those with the alumina coating protected the cathode from breaking down in the presence of hydrofluoric acid, which forms if even minute amounts of water invade the liquid electrolyte. Testing showed the alumina also accelerated the battery’s charging speed, reducing the number of times it can be charged and discharged.

There appears to be extensive trapping as a result of the fast lithium transport through alumina, said Anulekha Haridas, a postdoctoral fellow at Rice. The researchers already knew of possible ways silicon anodes trap lithium, making it unavailable to power devices, but she said this is the first report of the alumina itself absorbing lithium until saturated. At that point, she said, the layer becomes a catalyst for fast transport to and from the cathode.

Embedding data in 3D printed objects
Researchers at Nara Institute of Science and Technology (NAIST) developed a new method to embed barcode-like information in a 3D printed object and retrieve it using a consumer document scanner, without changing the object’s shape.

The team used Fused Deposition Modeling (FDM), a common 3D printing method consisting of deposing layers of molten plastic on top of each other. The desired shape is obtained by precisely controlling the position and flow of a printing nozzle such that the deposed plastic layers have a controlled path and thickness.

Normally, the plastic flow is controlled to produce a constant layer thickness. However, the team modified the plastic flow during the print to locally change the layer thickness to embed some additional information.

In order to prevent the degradation of the external surface of the object, pairs of vertically adjacent layers are selected and the ratio of their respective thicknesses is modified while keeping constant the sum of the two layer thicknesses. Since a standard layer thickness is about 0.2mm, information can be embedded in a relatively small area ranging from several millimeters to a few centimeters.

To retrieve the embedded information, it is necessary to measure the thickness of the layers. With this method, a common document scanner can perform the measurement, without the need for any special equipment. The FDM printing process naturally produces some layering artifacts that are visible in the images obtained by a document scanner. These artifacts allowed the team to measure the thickness of the layers and extract the information.

The researchers say it is possible to embed various types of information such as an URL that can be linked to Web services, a unique ID that can be used for product tracing, and the printer ID and printing date for batch quality management.