A team at Columbia University, with colleagues from Institute Recherche d’Hydro-Québec (IREQ), has developed a new pre-lithiation method to increase the energy density of lithium (Li-ion) batteries by utilizing a trilayer structure that is stable even in ambient air. This makes the battery both longer lasting and cheaper to manufacture. The work, which may improve the energy density of lithium batteries by 10-30%, is published in the ACS journal Nano Letters.
Li-ion batteries are produced in a discharged state; however, a considerable amount of active Li+ ions are lost during the initial charge due to the formation of the solid electrolyte interphase (SEI) on the anode surface. This results in a low initial coulombic efficiency and lowers the energy density of full cells. This step is even more critical in nanostructured anodes with high specific capacity, such as Si and Sn, due to their high surface area and large volume change.
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. 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.
Prelithiation—i.e., loading the electrode with lithium-rich material—offers a viable approach to address such loss. However, most of these prelithiation reagents 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.
The Columbia team, led by Yuan Yang, assistant professor of materials science and engineering at Columbia Engineering, has developed a new trilayer structure of active material/polymer/lithium anode, which is stable in ambient air (10-30% relative humidity) for a period that is sufficient to manufacture anode materials.
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 polymer layer is gradually dissolved in the battery electrolyte, and the active materials contact with lithium to form lithiated anode. This trilayer-structure not only renders electrodes stable in ambient air, but also leads to uniform lithiation. Moreover, the degree of prelithiation can vary from compensating SEI to a fully lithiated anode.
Yang’s 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.
They also demonstrated a Li4Ti5O12/lithiated graphite cell with stable cycling performance.
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.—Yuan Yang
Yang’s results point to a possible solution to enhance the capacity of Li-ion batteries. His 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.
The study received startup funding from Columbia Engineering, and additional support from the Lenfest Center for Sustainable Energy.
Zeyuan Cao, Pengyu Xu, Haowei Zhai, Sicen Du, Jyotirmoy Mandal, Martin Dontigny, Karim Zaghib, and Yuan Yang (2016) “An Ambient-air Stable Lithiated Anode for Rechargeable Li-ion Batteries with High Energy Density” Nano Letters doi: 10.1021/acs.nanolett.6b03655