New method may boost smartphone battery life

Scientists have developed a new method to increase the energy density of lithium batteries, an advance that may lead to longer-lasting and cheaper batteries for smartphones, tablets and electrical vehicles.

The work may improve the energy density of lithium (Li-ion) batteries by 10-30 per cent, researchers said. Researchers at Columbia University in the US built a trilayer structure that is stable even in ambient air, which makes the battery both longer lasting and cheaper to manufacture.

"When lithium batteries are charged the first time, they lose anywhere from 5-20 per cent energy in that first cycle,"said Yuan Yang, assistant professor at Columbia.

"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," said Yang.

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 about 10 per cent for state-of-the-art negative electrodes, but can reach as high as 20-30 per cent 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 of these nanostructured electrodes.

The traditional approach to compensating for this loss has been to put certain lithium-rich materials in the electrode. However, most of these materials are not stable in ambient air. Manufacturing batteries in dry air, which has no moisture at all, is a much more expensive process than manufacturing in ambient air. Yang has developed a new trilayer electrode structure to fabricate lithiated battery anodes in ambient air.

In these electrodes, he protected the lithium with a layer of the polymer PMMA to prevent lithium from reacting with air and moisture, and then coated the PMMA with such active materials as artificial graphite or silicon nanoparticles. The PMMA layer was then dissolved in the battery electrolyte, 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, so the trilayer-structured electrode can be operated in ambient air.

This could be an attractive advance towards mass production of lithiated battery electrodes," Yang said.

The method lowered the loss capacity in state-of-the-art graphite electrodes from eight per cent to 0.3 per cent, and in silicon electrodes, from 13 per cent to minus 15 per cent.