Unraveling (de)lithiation dynamics inside graphene sheets at atomic scale

Abstract text (incl. figure legends and references)

Graphite has succeeded as a lithium-ion anode material in the last 30 years, since it enhances energy densities due to its low (de)lithiation potential and high capacity. LiC₆ had been known as the densest intercalation compound for the graphite anode. However, we observed a denser lithium phase inside graphene sheets in-situ during (de)lithiation by using spherical and chromatic aberration-corrected low-voltage high-resolution transmission electron microscopy. To meet the challenge of observing the (de)lithiation processes in-situ inside TEM, an electrochemical cell architecture including graphene sheets and polymer electrolyte was designed and placed on the Si₃N₄ membrane of a custom-made TEM sample carrier chip, as shown in Figure 1a. The lithium ions were inserted into the graphene sheets when applying a constant voltage during lithiation, while transferred back into the polymer electrolyte when the voltage was removed during delithiation (Figure 1b). Our HRTEM investigation confirms that the crystal structure of the lithium inside graphene sheets is fcc, which is denser than the LiC₆ intercalation compound. In addition, we observed that the shape of the lithium crystals is dramatically different during the (de)lithiation processes, as shown in Figure 2. During lithiation, lithium crystals nucleate at the positions of graphene defects, and grow with triangular shape further. During delithiation, the lithium crystals shrink with irregular shape. The reason for that is the introduction of oxygen atoms at different positions inside the lithium crystals. Some oxygen atoms are introduced during lithiation, and bond with lithium atoms at the octahedral interstitial positions. The distribution of the oxygen atoms is uniform. However, during delithiation, the oxygen atoms gather at the edge of the lithium crystals, and keep at the edge position while the lithium crystals are shrinking. The oxygen atoms lock part of lithium atoms, resulting in the formation of amorphous lithium oxide finally. Our work gives a detailed insight of the existence and diffusion of lithium inside graphene sheets, which may have relevance also in anode materials.

This work contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe) and was funded by the German Research Foundation (DFG) under Project ID 390874152 (POLiS Cluster of Excellence). We acknowledge financial support from the Baden-Württemberg Stiftung gGmbH (project CT 5) and from the Ministry of Science, Research and the Arts (MWK) of the federal state of Baden-Württemberg in the frame of the SALVE project.

Reference:

[1] Nature 564, 234-239 (2018)

[2] Nat. Nanotechnol. 12, 895–900 (2017)

Figure 1: Device layout and working principle. (a) Self-designed electrochemical cell architecture including graphene sheets and polymer electrolyte placed on the Si₃N₄ membrane of a custom-made TEM sample carrier chip. Electron beam is illustrated in blue. (b) Schematic side view of the device during the in-situ TEM investigation.

Figure 2: Overview of (de)lithiation inside graphene sheets. (a) The lithiation process. Lithium crystals nucleate and grow with triangular shape. (b) The delithiation process. The lithium crystals shrink with irregular shape.