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Reduced graphene oxide allows you to grow flat lithium layer

Time:2020/01/06 丨 source:未知 丨 visit count:

Worried about the lithium dendrite? Reduced graphene oxide allows you to grow flat lithium layers with ease.

[ Research Background ]

The high theoretical specific capacity (3860mah g - 1) and low electrochemical potential (- 3.04v vsNHE) of Lithium (Li) metal anode have great potential in rechargeable lithium metal batteries (LMBs) , especially lithium sulfur (Li-s) batteries. However, the uncontrolled growth of lithium dendrites hinders the application of Lithium anode in high performance and safe batteries. For this reason, Professor Xie Keyu of the Northwestern Polytechnical University works with Zhenhai Xia of the University of North Texas and Bingqing Wei of the University of Delaware, the in-plane lithium layer was grown on self-assembled reduced graphene oxide (rGO) using the electrocrystallization properties of lithium metal. The experimental and simulation results show that when Li atoms are deposited on Rgo, each layer of Li atoms grows along the (110) crystal plane of Li because of the good planar lattice matching between Li and RGO. The results were published in Advanced Materials, a leading international journal, under the title "Reduced-Graphene-Oxide-Guided Directional Growth of Planar Lithium Layers".

[Illustration ]

1. Deposition of lithium on rGO substrate
Because of the difference in solution between graphene oxide (GO) and copper reduction potential, GO will self-reduce on copper foil, so a self-assembled rGO substrate can be prepared simply by immersing commercial copper foil in GO aqueous solution. The self-assembled rGO substrate, stripped from the copper foil, is flexible enough to fold without damage and has a highly crystalline graphene structure. The electronic conductivity of rGO substrate is about 61s m- 1. The good conductivity ensures the electron transportation and is favorable for the uniform deposition of lithium. A smooth planar lithium surface can be obtained by using a self-assembled RGO (Fig. 1b-c) with a smooth, laminated structure and 23 m thickness as the substrate for Li electrodeposition. As shown in Fig. 1d-g, the diameter of Li increases significantly (20 - 133 m) with the increase of Li deposition capacity from 1 to 5 Mah cm - 2, but the thickness of Li increases only slightly (9 - 25.3 m) , which shows the morphology of Li. Xrd And polarimetric analysis (Fig. 1h-j) show that Li preferentially grows along (110) crystal plane when rGO substrate is used for electrodeposition, which is consistent with the calculated results of texture coefficient formula.

Fig. 1 structure and morphology characterization of lithium deposition on rGO substrate

2. In Situ optical observation of Li electrodeposition on rGO substrate and bare copper foil
The morphology and uniformity of lithium in large area plane were studied by optical microscope. Based on the relatively uniform brightness of the optical image, the surface of rGO is covered by a large area of Planar Li (Fig. 2a) . In contrast, Li deposition on bare copper foil is extremely uneven (Fig. 2b) , and even some lithium dendrites as long as several hundred microns exist. In order to study the dynamic deposition process of lithium on copper foil and self-assembled rGO substrates, in-situ optical observation was carried out without diaphragm. For the bare copper foil electrode, a large number of rod-like Li dendrites appeared at the deposition time of 8min, which resulted in heterogeneous and porous Li deposits. After 40 Min, the thickness of the electrode increased by about 66.7 um, as shown in Fig. 2d. Even after 40min, the deposition of Li on self-assembled rGO substrates is relatively smooth and dense, and the deposition thickness is much smaller than that of Cu foils, as shown in Fig. 2e. The mechanism of Li-oriented growth was investigated from the atomic configuration of the (110) crystal plane of Li and graphene. The lengths of the two Li atoms along the (110) crystal plane (4.96A) coincide with the lengths of the two carbon hexagons along the Zigzag direction of Graphene (4.92A) , in-plane lattice matching may result in epitaxial alignment of Planar Li and rGO.

Fig. 2 SITU optical observation of Li electrodeposition on rGO substrate and bare copper foil

3. Molecular Dynamics
In order to further understand the experimental results of Li Crystal Orientation on rGO, the simulation of Li Crystal on rGO was carried out by using Ab-MD simulation in VASP. The simulation results show that the lattice constant of Li Crystal is about 3.72Å, which is close to the experimental value. Fig. 3a shows the plane of the Li surface layer in contact with rGO, where the Li atoms on rGO are distributed in the same way as the (110) crystal plane in the BCC structure. The lengths of the two Li atoms along the Li (110) crystal plane (d1, 4.96Å) coincide well with the lengths of the two carbon hexagons along the Zigzag direction of the graphene (4.92Å)(lattice mismatch 0.8%) , in addition, the length of the other two lithium atoms (2d2,7.0Å) along the Li (110) crystal plane also matches that of graphene (7.1Å)(lattice mismatch ratio 1.4%) . The calculated results are in good agreement with the experimental results of Li Crystal Orientation on rGO.

Fig. 3 distribution of Li atoms on rGO after geometric optimization

4. rGO@Electrochemical properties of Lithium anodeTo assess the cycling stability of the rGO@Li battery, an asymmetric rGO | Li battery was assembled. As shown in Fig. 4a, the coulomb efficiency (CE) of the rGO based anode was kept at 99% and 98% respectively after 300 cycles with an increase in current density from 1.0 to 2.0 mA cm-2, much higher than that of the copper based anode. In addition, as can be seen from Fig. 4b, during Li plating / stripping of 1.0 and 2.0 mA cm-2, all the overpotential based on the rGO substrate anode is lower than that of the bare copper foil, which fully demonstrates the stability of Rgo@Li. As shown in figure 4c, the voltage of the symmetric rGO@li | | Cu battery began to rise after 190h, while the symmetric rGO@Li | rGO battery showed a fairly stable voltage-time curve even after 500h, with no evidence of short circuit. This further confirms the long-term Cyclic stability of rGO-induced Directional growth of Planar Li.

Fig. 4 rGO@Li negative electrode and copper foil-Li negative electrode
To evaluate the viability of rGO@Li , the performance of rGO@Li in a full battery was designed and tested, as shown in figure 5a. The positive electrode was 3D rGO foam@S with a high sulfur load (≈4.5 mg cm -2) and a sulfur ratio (≈70%) , and the negative electrode was rGO@Li . As shown in figure 5c, the full battery showed good active material utilization and significant capacity stability, with a capacity retention rate of 81% after 400 cycles, much higher than previously reported. In addition, the Li-S battery shows the high energy density of 754 Wh kg -1 and the power density of 377 W kg -1 , respectively.

Fig. 5 Electrochemical performance of Li-S battery with 3D rGO Foam@S as positive electrode and rGO@Li as negative electrode
[ Conclusion ]

In this work, the epitaxial growth of Li layer on self-assembled rGO thin film substrate was first reported, and the morphology evolution of Li electrodeposition was characterized by in-situ optical observation, in the end, the basic mechanism is explained by Molecular dynamics simulation. Experimental and first-principles simulations show that the lengths of the two Li atoms along the Li (110) crystal plane (4.96Å) coincide with the lengths of the two carbon hexagons along the zigzag direction of graphene (4.92Å) , the in-plane lattice matching results in the epitaxial alignment of Li and rGO, and the in-plane lithium layer can be grown directionally. Due to its inherent dendrite-free property, rGO-guided Planar Li anode exhibits excellent electrochemical performance in all asymmetric and symmetrical batteries at ultra-high current density of 20 mAcm-2. In addition, the highly flexible Li-S battery with only 300% excess Li on rGO substrate shows the high energy density of 754kg Wh -1 and the power density of 377W kg-1, respectively. This work provides new strategies and basic insights to address the greatest challenges to the commercialization of LMBs.
Nan Li, Kun Zhang, Keyu Xie, Wenfei Wei, Yong Gao, Maohui Bai, Yuliang Gao, Qian Hou, Chao Shen, Zhenhai Xia, Bingqing Wei. Reduced-Graphene-Oxide-Guided Directional Growth of Planar Lithium Layers. Advanced Materials 2019, 1907079, DOI:10.1002/adma.201907079

The source from Energist


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