Power/Performance Bits: Nov. 23

Increasing lithium battery density

Researchers at Columbia University developed a new method to increase the energy density of lithium batteries using a trilayer structure that is stable in ambient air.

“When lithium batteries are charged the first time, they lose anywhere from 5-20% energy in that first cycle,” said Yuan Yang, assistant professor of materials science and engineering at Columbia Engineering. “Through our design, we’ve been able to gain back this loss, and we think our method has great potential to increase the operation time of batteries for portable electronics and electrical vehicles.”

During the first charge of a lithium battery after its production, a portion of liquid electrolyte is reduced to a solid phase and coated onto the negative electrode of the battery. This process, usually done before batteries are shipped from a factory, is irreversible and lowers the energy stored in the battery. The loss is approximately 10% for state-of-the-art negative electrodes, but can reach as high as 20-30% for next-generation negative electrodes with high capacity, such as silicon, because these materials have large volume expansion and high surface area. The large initial loss reduces achievable capacity in a full cell and thus compromises the gain in energy density and cycling life.

Graphite/PMMA/Li trilayer electrode before (left) and after (right) being soaked in battery electrolyte for 24 hours. Before soaking in electrolyte, the trilayer electrode is stable in air. After soaking, lithium reacts with graphite and the color turns golden. (Source: Yuan Yang, Columbia Engineering)

To address this, the team developed a new trilayer electrode structure. In these electrodes, the lithium is protected with a layer of the polymer PMMA to prevent lithium from reacting with air and moisture, and then coated the PMMA with active materials such as artificial graphite or silicon nanoparticles. The PMMA layer was then dissolved in the battery electrolyte, thus exposing the lithium to the electrode materials. “This way we were able to avoid any contact with air between unstable lithium and a lithiated electrode,” Yang explained, “so the trilayer-structured electrode can be operated in ambient air. This could be an attractive advance towards mass production of lithiated battery electrodes.”

The method lowered the loss capacity in state-of-the-art graphite electrodes from 8% to 0.3%, and in silicon electrodes, from 13% to -15%. The -15% figure indicates that there was more lithium than needed, and the “extra” lithium can be used to further enhance cycling life of batteries, as the excess can compensate for capacity loss in subsequent cycles.

The group is now trying to reduce the thickness of the polymer coating so that it will occupy a smaller volume in the lithium battery, and to scale up his technique.

Improving perovskite solar cell fabrication

Researchers at the Australian National University (ANU), Monash University, and Arizona State University found a new way to fabricate high-efficiency semi-transparent perovskite solar cells.

The new method involves adding a small amount of indium into one of the cell layers during fabrication, which along with increasing efficiency, the team hopes will stabilize the cells so they can be used in traditional solar applications.

“The prospect of adding a few additional processing steps at the end of a silicon cell production line to make perovskite cells is very exciting and could boost solar efficiency from 25% to 30%,” said Tom White, of the ANU Research School of Engineering.

(Source: Jack Fox, ANU)

As we’ve seen in other recent record-breaking solar reports, the team used tandem solar cells. But rather than perovskite-perovskite, they used perovskite-silicon. Perovskite solar cells are good at making electricity from visible light – blue, green and red – while conventional silicon solar cells are more efficient at converting infrared light into electricity.

Over all, the team was able to reach a steady-state efficiency of 16.6% for the semi-transparent perovskite cell on its own, while the team’s perovskite-silicon tandem cell achieved a steady-state efficiency of 24.5%.