Source:University of Science and Technology of China, School of Chemistry and Materials ScienceOfficial Website
As an important raw material in fields such as lithium-ion battery anodes and graphene preparation, graphite generally contains two lattice structures: hexagonal phase (Hexagonal, graphene stacked in ABAB… pattern, referred to as 2H phase) and rhombohedral phase (Rhombohedral, graphene stacked in ABCABC… pattern, referred to as 3R phase). Among them, the 2H phase has lower energy and occupies a higher proportion in the powder; the 3R phase has higher energy and generally occupies a lower proportion in the powder. However, as graphite particles break down and the sheet diameter decreases, the proportion of the 3R phase gradually increases, reaching up to 50%. Recent studies have shown that the band structure of 3R phase graphite includes three-dimensional Dirac cones (3D Dirac cones), where the electronic states have a gap, and at low temperatures, electron transport is dominated by surface states; the flat band of three-layer graphene in the 3R stacking form contributes to strong correlation phenomena, leading to spontaneous symmetry breaking, such as ferromagnetic ordered states and surface superconducting states. In existing studies on graphite phase transitions, it often requires high-temperature and high-pressure graphitization treatment to transform all 3R phases in graphite powder into 2H phase, or to use methods such as laser heating and Joule heating, which have harsh conditions and high energy consumption.
Professor Zhu Yanwu’s team from the University of Science and Technology of China has found through long-term research that by adding a small amount of lithium nitride (Li3N) crystal powder to graphite powder, a complete transformation of the 3R phase to the 2H phase can be achieved at lower temperatures (as low as 350 ℃) for a wide range of sheet sizes (1 ~ 60 μm) in macroscopic graphite powder (hundred grams level). With the collaborative research help from teams at Northwestern Polytechnical University, National University of Defense Technology, Institute of Semiconductors of the Chinese Academy of Sciences, and the University of Manchester, the team discovered that the mechanism of graphite phase transition under these conditions is: the difference in work function causes the lithium nitride crystal powder to transfer some electrons to the conjugate π electron cloud of graphite upon contact, leading to an abnormal increase in interlayer spacing of graphite, thereby significantly reducing the sliding energy barrier between graphite layers, allowing the 3R phase to transition to the 2H phase under milder conditions.
The team members proposed the interlayer sliding path from 3R graphite to 2H graphite through in-situ X-ray studies combined with first-principles calculations, and confirmed the change in charge state through the Raman signal variation of thin-layer graphite before and after contact with Li3N. The results of this study indicate that by regulating the shape of the graphite π electron cloud, it is possible to achieve precise control over the stacking morphology and properties of graphite, providing new ideas for the structural regulation of other carbon-based materials and the preparation of new carbon materials.
Figure 1. (a) Atomic models of 2H and 3R phase graphite; (b) Initial graphite and XRD results after heat treatment with and without Li3N; (c) XRD fitting to distinguish between 3R and 2H phases; (d) Reaction conditions and phase diagram of remaining 3R phase in graphite.
Figure 2. (a) and (b) In-situ XRD results of graphite heating process with and without Li3N; (c) Evolution of peak intensity of graphite 3R (101) and 2H (101) with temperature with and without Li3N; (d) Evolution of interlayer spacing of graphite with temperature with and without Li3N.
Figure 3. (a) Two possible sliding paths from AB stacking to AC stacking; (b) Interlayer spacing of AB, AA, and AA’ stacked graphite as a function of charge injection amount; (c) Charge differential density of 3R stacked graphite during charge injection; (d) Variation of interlayer sliding energy barrier of graphite under different charge injection conditions.
Figure 4. (a) Optical photos of mechanically exfoliated thin-layer graphite before and after coverage with Li3N; (b) Scanning image of Raman G peak position difference (intensity in red dashed box in (d) minus intensity in red dashed box in (c)); (c) Scanning image of Raman G peak of thin-layer graphite without Li3N coverage; (d) Scanning image of Raman G peak of thin-layer graphite in the same region with Li3N coverage.
References
Phase-Changing in Graphite Assisted by Interface Charge Injection. Nano Letters. 2021
https://pubs.acs.org/doi/10.1021/acs.nolett.1c01225
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