According to the research published online in Nano Letters, Yang has developed a trilayer structure that is stable even in ambient air and that is what improves the energy density of lithium batteries by 10-30%.
"When lithium batteries are charged the first time, they lose anywhere from 5-20% energy in that first cycle. 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.
It must be noted that while charging the lithium battery for the very first time 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 generally takes place before shipping the batteries from a factory. The process lowers the energy stored in the battery and 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. This is because materials like silicon 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.
Yang has developed a new trilayer electrode structure to fabricate lithiated battery anodes in ambient air. He protected the lithium with a layer of the polymer PMMA to prevent lithium from reacting with air and moisture in these electrodes. And then, the researcher covered the PMMA with active materials like artificial graphite or silicon nanoparticles. In the next step, the PMMA layer was dissolved in the battery electrolyte, thus exposing the lithium to the electrode materials.
Asserting that this research could pave way for mass production of lithiated battery electrodes, Yang said, "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 technique helped in reducing the loss capacity in state-of-the-art graphite electrodes from 8% to 0.3%, and from 13% to -15% in silicon electrodes. 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. This outcome indicates at a possible solution to enhance the capacity of Li-ion batteries.
Yang along with his research team 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.