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.

Schematic of the process to prepare ambient air-stable lithiated anode: a) Li is deposited on Cu film, b) polymer (e.g. PMMA) is coated on Li surface to make it air-stable, c) anode materials are coated onto Li with a solvent that does not dissolve PMMA, d) the anode and the cathode are assembled with organic electrolyte injected, which dissolves the polymer coating, and e) anode materials are lithiated as they contact with Li due to pressure inside cell. Credit: ACS, Cao et al. Click to enlarge.

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.

Resources

• 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