Diffusion-Controlled Lithium Trapping in Graphite Composite Electrodes for Lithium-Ion Batteries

Although graphite-based composite electrodes currently are widely used as negative electrodes in lithium-ion batteries due to their good cycle performances, improvements of their long-time cycling stability are still desirable. Herein, a series of lithium-metal half-cell experiments is performed to demonstrate that the diffusion-controlled lithium-trapping effect constitutes an additional, and so far, largely unrecognized, aging mechanism for graphite-based electrodes. This trapping effect, which stems from incomplete delithiation due to diffusion-controlled redistribution of intercalated lithium in graphite, is shown to account for around 30% of the total accumulated capacity loss during long-time cycling. The trapping effect is caused by the concentration gradients present at the end of the lithiation steps as these gradients result in lithium (i.e., coupled Li+ and e(-)) diffusion in the electrodes. As a result, a small fraction of the lithium becomes inaccessible on the timescale of the subsequent delithiation step. The results, however, also show that the inclusion of constant-voltage delithiation steps can increase the delithiation efficiency and decrease the influence of the lithium-trapping effect. This work consequently demonstrates that diffusion-controlled lithium-trapping effects need to be considered when trying to increase the lifetimes of graphite-based electrodes.

Automated summary

Although graphite-based composite electrodes are commonly used in lithium-ion batteries for their good cycle performance, their long-time cycling stability still needs improvement. The diffusion-controlled lithium-trapping effect, which accounts for around 30% of the total capacity loss during long-time cycling, is shown to be an additional and largely unrecognized aging mechanism in graphite-based electrodes. It is caused by incomplete delithiation due to diffusion-controlled redistribution of intercalated lithium in graphite. However, constant-voltage delithiation steps can increase the delithiation efficiency and decrease the influence of the lithium-trapping effect.