Electrochemical exfoliation of graphene as an anode material for ultra-long cycle lithium ion batteries

Journal Pre-proof Electrochemical exfoliation of graphene as an anode material for ultra-long cycle lithium ion batteries Xin Zhao, Hongyan Li, Fangjie Han, Mengjiao Dai, Yingjuan Sun, Zhongqian Song, Dongxue Han, Li Niu PII:

S0022-3697(19)31396-4

DOI:

https://doi.org/10.1016/j.jpcs.2019.109301

Reference:

PCS 109301

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 15 June 2019 Revised Date:

31 October 2019

Accepted Date: 5 December 2019

Please cite this article as: X. Zhao, H. Li, F. Han, M. Dai, Y. Sun, Z. Song, D. Han, L. Niu, Electrochemical exfoliation of graphene as an anode material for ultra-long cycle lithium ion batteries, Journal of Physics and Chemistry of Solids (2020), doi: https://doi.org/10.1016/j.jpcs.2019.109301. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Electrochemical Exfoliation of Graphene as an Anode Material for Ultra-long Cycle Lithium Ion Batteries

Xin Zhao

a, c

, Hongyan Li b*, Fangjie Han

a, c

, Mengjiao Dai

a, c

, Yingjuan Sun b, Zhongqian

Song a, c, Dongxue Han a, c, d*, Li Niu a, c, d

a State Key Laboratory of Electroanalytical Chemistry, c/o Engineering Laboratory for Modern Analytical Techniques, CAS Center for Excellence in Nanoscience, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China b Department of Materials Science and Engineering, College of Chemistry and Materials Science, Jinan University, Guangzhou 510632, P. R. China c University of Chinese Academy of Sciences, Beijing 100039, P. R. China d Center for Advanced Analytical Science, c/o School of Chemistry and Chemical Engineering, c/o MOE Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Guangzhou University, Guangzhou 510006, P. R. China

Abstract One of the key challenges in developing lithium ion batteries is the super-long cycles. Two-dimensional (2D) materials (especially graphene) with a layered crystal structure disclose many advantages and attract more interests in this field. Here we report a novel method through which the electrochemically exfoliated graphene (EG) is employed to construct an anode for ultra-long cycle Li-ion batteries. The high quality EG was directly sprayed onto the copper foil as a binder-free material, resulting in high conductivity, high volume capacity and bridging properties. The battery has exhibited a high capacity of 356 mA h g-1 at 1 A g-1, with ~100% capacity retention over 6500 cycles. This work demonstrates that

*

Corresponding author E-mail: [email protected], [email protected]

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the excellent graphene preparation process combined with binder-less spraying is effective strategy to significantly improve the cyclic stability of Li-ion batteries. Keywords: exfoliated graphene, anode, ultra-long cycle, lithium ion batteries, energy storage 1. Introduction Although traditional fossil energies with limited reserves are still the main energy sources, they cause a series of environmental issues, such as, greenhouse effect, acid rain, etc.[1-3] Howbeit electric energy is considered as one of the most momentous green energies and its storage and conversion is the essential focus currently. The long-period, eco-friendly, affordable lithium ion battery (LIB) is urgently needed to meet the supply of modern society at present.[4-12] Even if the LIB has been used widely in mobile devices and zero-emission vehicles, for instance, hybrid electric cars, plug-in hybrid electric cars, and electric cars, it is still a hot research topic with the aim for further improving their properties and characteristics.[13-22] Graphite is generally used as anode material in LIB. However, it’s low charge and discharge capability under high current condition and is excessive sensitivity to electrolysis should be improved. At the same time, during the battery cycle, the distance between the graphite layers varies greatly, and the graphite layer is also prone to gradual peeling.[23] Thereby, to a certain extent, the longevity of the LIB is shortened. Since then, some researchers have turned to metals and their oxides. Although certain metals and their oxides exhibit high capacity as battery electrodes, they possess some fatal problems, including the huge volumetric deformation, structural change during charge and discharge process, low electronic conductivity and large irreversibility etc. For example, Zhang et al. pointed out in their article that Sn and partial transition metal oxides showed huge volume change and large voltage hysteresis during Li ions insertion and extraction.[24] Dai et al. put forward in their study that the reason why Mn3O4 has been rarely applied as an anode material so far should 2

partly due to its extremely low electrical conductivity, which greatly limited its capacity.[25] These problems principally give rise to the poor cyclic stability.[26] In contrast, graphene seems more suitable for the LIB anode material. As a kind of 2D nanomaterial, graphene is considered as carbon layers with very low thickness and strongly bonded networks.[27] In the cellular layered structure, the graphene carbon atoms are interconnected to form hexagonal rings, where each carbon atom contributes an unbonded electron in the p-z orbital, and the p-z orbital of the neighboring atom is perpendicular to the plane to form the π bond, thus endows graphene excellent electrical conductivity. More importantly, graphene holds a huge surface areas of more than 2600 m2 g-1, good stability and so forth.[28] Therefore, the preparation method of the graphene is the focus of research because it determines the properties of the prepared graphene. Commonly used graphene preparation methods involve epitaxial growth,[29] chemical vapor deposition,[30] micromechanical exfoliation,[27] chemical exfoliation[31] and so on, but it is difficult to obtain a high quality product as a result of low yield, non-controllable and rich-defects in the process of preparation graphene.[32] Electrochemical exfoliation of graphene is a method of utilizing electric field and ion intercalation to effectively solve the above problems to obtain high quality graphene as the battery electrode.[33, 34] On the basis of the above research, herein we report a strategy for fabricating excellent battery electrode material by a facile electrochemical exfoliation of graphene sprayed to copper foil (Details are shown in Fig. 1). The exfoliated graphene anode material exhibits outstanding lifetimes and high coulombic efficiency, which might greatly promote foundation research for further application in the future. 2. Results and discussion 2.1 Structure and morphology The electrochemically exfoliation of graphene (EG) is one of the most effective method for preparing high quality graphene (Fig. 1).[35-38] Fig. 1a shows the schematic diagram and 3

corresponding mechanism of electrochemical exfoliation of graphene. The graphite foil was initially exfoliated by a home-made system including dispersing (NH4)2SO4 and TEMPO in deionized water (DIW), electrochemical exfoliation by constant +10 V, collecting with cellulose filters, and washing with DIW and alcohol to produce the EG powder. The powder was hereafter dispersed in isopropanol (IPA) for 6 h by ultrasonic treatment, resulting in a stable EG dispersion as shown in Fig. 1b. The mechanism of exfoliated graphene is because the ions in the solution enter the graphite layers under applied electric field and interact with them to increase the interlayer spacing and realize the separation of graphene. Fig. 1b discloses the excellent dispersion of the EG in IPA, which opens up the possibility for the EG to be directly spray onto copper foil.

Fig. 1.. (a) The schematic diagram of the electrochemical exfoliation of graphene and the mechanism of electrochemical exfoliation. (b) The digital photo of the dispersion of the exfoliated graphene in isopropanol (IPA). Fig. 2 illustrates the structural characterization of EG. The XRD pattern (Fig. 2a) reveals that compared with the diffraction peaks of the graphite (crystal system: Hexagonal; space group: P63/mmc(194); JCPDS. 41-1487), the EG demonstrates an angle of 26.42° for the (002) plane, an angle of 44.50° for the (101) plane and an angle of 54.59° for the (004) plane, which declares that the surface of EG contains small amounts of functional groups. To further 4

confirm the nanostructures and phase of the EG, transmission electron microscopy (TEM) is tested and the image is presented in Fig. 2b. The graphene nanosheets are found to be mainly composed of two layers of sheet stacks with ripple-like wrinkles. By further observing the EG structure by HRTEM in Fig. S1, it can be seen that the lattice fringe spacing is 0.39 nm, corresponding to (002) plane. The morphology and crystallographic properties of the sample were further understood by analyzing the diffraction spot strength – the selected area electron diffraction (SAED, Fig. 2b inset). Here a typical hexagonal honeycomb structure of diffraction spot is observed, which exhibits a bilayer graphene film determined by a stronger diffraction intensity from the (1-210) plane rather than (0-110) plane. It strongly proves that most of the graphene sheets are comprised of the two-layer graphene.[39-41] It is observed that the obvious diffraction pattern of the EG coincides with the each peak of EG in the XRD pattern. In order to explore the surface morphology of the material, the atomic force microscope (AFM, Fig. 2c) is used to characterize the sample. Graphene is in a sheet-like stack with a thickness of 2.08 nm, which is about two layers of graphene. Functional groups on the surface of graphene are detected by Fourier transform infrared spectroscopy (FTIR, Fig. 2d). At 1250-1600 cm-1, it is found to contain C=O, C-C and C-O, which strongly illustrates the presence of oxygen-containing functional groups on the surface of graphene. So as to further confirm the number of layers of graphene, Raman spectroscopy was used to test the sample in Fig. 2e. The material detected three bands, D band, G band and G' band. In particular, G’ band can be fitted into four Lorentzien peak and it is indeed a double-layer graphene. We determined the specific surface area and pore size of the material by nitrogen adsorption-desorption isotherm (Fig. 2f) and pore size distribution (Fig. 2f inset). It can be seen that the EG has a large specific surface area (1240 m2 g-1) and a large pore size (10.31 nm). The N2 adsorption-desorption isotherms of the sample can be classified as type IV in classification suggesting the presence of mesopores (2-50 nm). The shapes of the hysteresis loops are of type H3, indicating the presence of slit-like pores. These crack-like holes should 5

be generated by the stack of graphene. Fig. 2g-i show scanning electron microscopy (SEM) images of the EG loading onto Cu foil. As the SEM images enlarged from Fig. 2g to i, the thin sheets structure of the EG is clearly observed, which should provide a large surface area, more active sites and defects, simultaneously, constitute strongly bonded carbon networks for the lithium ion insertion and extraction. It is consistent with that of the observations from TEM and BET results.

Fig. 2. Characterization of the exfoliated graphene: (a) XRD pattern; (b) TEM image (inset: SAED image); (c) AFM image; (d) FTIR; (e) Raman; (f) Nitrogen adsorption desorption isotherm curve (inset: pore size distribution); (g-i) SEM images of the electrochemically exfoliated graphene sprayed on copper foil. 2.2 Electrochemical test The electrochemical characteristics of the EG for LIB are shown in Fig. 3. In the cyclic Voltammetry (CV) performance (Fig. 3a) for Li+ interaction, during the 1st cycle, an 6

irreversible cathodic peak at 1.75 V was observed, which should be ascribed to the reduction of oxygen-containing functional groups on the EG surface.[42, 43] The cathodic peak is due to an irreversible consumption of Li+ by the functional groups on the EG surface, [42, 44, 45] which is consistent with the result of the XRD. In the range of 0 to 0.25 V, it is found that Li+ interacts with the EG for charge injection during this process, which is depicted as the follow: [46] LiC72 + Li ⇋ 2LiC36

(1)

4LiC27 + 5Li ⇋ 9LiC12

(2)

LiC12 + Li ⇋ 2LiC6

(3)

In the second and third cycle, the cathodic peak at 1.75 V disappeared, further indicating the oxygen-containing functional groups on the EG were completely reduced. The three redox peaks between 0 and 0.25 V have no apparent changes. It demonstrates a reversible Li+ and intercalation/deintercalation in the EG layer. The electrochemical impedance spectroscopy (EIS) was further employed to evaluate the impedance of the EG electrode. In Fig. 3b, the high-frequency region appears as a semicircle, indicating the charge transfer process, which means that Li+ migrate through the interface between the electrode surface and the electrolyte. The low-frequency region is an approximate inclined, suggesting fast Li+ diffusion.

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Fig. 3. Electrochemical behaviors of the EG: (a) Cyclic Voltammograms of the first to the third scanning cycles; within the voltage range from 0.01 to 3.0 V (vs. Li+/Li) at 1 mV s−1, starting at 2.3 V (open circuit potential). (b) Nyquist plot (10-2-105 Hz, 2.3 V). Fig. 4a shows the LIB performances by using EG anode. The first capacity is up to 1150 mA h g-1. After 7 cycles, the capacity is stable and maintains a good capacity retention throughout the cycle at various current densities, including 0.1 to 5 A g-1. Average capacities of 413, 363, 319, 289, 255 and 134 mA h g-1 can be delivered at 0.1, 0.2, 0.5, 1, 2, and 5 A g-1, respectively. When the current density tries back to 0.1 A g-1, the capacity is found to return initial value, showing great invertibility. The cycle life of batteries is critical for the practical application. In Fig. 4b, it is observed that the cycle life over 1700 cycles at 0.1 A g-1 still has high average capacity (ca. 392 mA h g−1). During the cycles of the lithium ion battery, the thickness of the carbon anode changed, causing the lithium ion insertion or deintercalation balance to break, which in turn caused the battery cycle curve to fluctuate. [47]And in the Fig. 4c, the battery has achieved the goal of the discharge capacity retaining at 343 mA h g−1 even after 6500 cycles at 1 A g−1 as well as a high coulombic efficiency (almost 100%) and the scatter point in the middle region might originated from t the formation of unstable SEI film and the embedding and deintercalation of Li+. This phenomenon has reached a dynamic balance after a period of time, and the scatter points disappear.[48, 49] This electrochemical performance is compared with previous works related to carbon-based anodes in LIBs in the literature (Table 1). [45, 50-64] For meaningful system comparison, the capacities are given in the form of specific capacities, and the current densities are standardized into specific current densities. Through performance comparison, the EG electrodes has relatively high stability and high capacity retention at high current densities, which is close to metal-carbon composites, or even better.

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Fig. 4. The LIB performances by using EG anode: (a) Rate capability at 0.1, 0.2, 0.5, 1, 2, 5 A g−1, respectively； (b) Cycle performance and efficiency at a current density of 0.1 A g-1 between 0.01 and 3 V vs. Li+/Li; (c) Cycle performance and efficiency at a current density of 1 A g-1 between 0.01 and 3 V vs. Li+/Li. 3. Conclusions In summary, a high-quality EG anode material of LIB has been prepared by a simple electrochemically exfoliated graphene strategy. The EG shows excellent charge/discharge performances. High lithium storage capacities and extremely high stability have been verified. The battery capacity is up to 356 mA h g−1 at 1 A g−1, with a remarkable capacity retention (~100%) and more than 6500 cycles. Through comparison with some relevant studies, the EG electrode even demonstrate a preferable performance contrast with those of carbon or carbon/metal composite. Overall, a novel electrode preparation strategy combined with unique properties of graphene material is developed, through which the effective

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improvement of the battery performance is achieved and expected to further application in the future. Acknowledgements This work was supported by NSFC, China (21622509 and 21527806), the Department of Science and Techniques of Jilin Province (2160201008GX, 20170203004SF and 20170101183JC) , Science and Technology Bureau of Changchun (15SS05), the Start-up Funding of Jinan University (88016105 and 55800001) and the Fundamental Research Funds for the Central Universities (12819023).

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Table 1. Comparison of the anode electrochemical stability performance in LIBs. Capacity (mA h g-1) after cycles at Anode materials

current density (mA g-1)

Retention of initial capacity

(ref.)

Graphene (This work) Graphitic carbon/ TiO2 (ref. 50) Graphene paper/ TiO2 (ref. 51) Graphene aerogel/ SnO2 (ref. 52) MWCNT/ TiS2 (ref. 53) Graphene nanoflakes (ref. 54) Graphene ball (ref. 55) Graphene / Li4Ti5O12 foam (ref. 56) Graphene/Fe3O4 (ref. 57) Graphene/SnO2 (ref. 45) Graphene/MoS2 (ref. 58) Graphene /APS-Si (ref. 59) Graphene/Fe2O3 /CNT (ref. 60) Graphene/T-Co3O4 (ref. 61) Graphite (ref. 62) CNFWs (ref. 63) Ball milled graphite (ref. 64)

Capacity

Cycles

Current density

(%)

343

6508

1000

~ 105

~137

1000

1000

~ 92

147

100

~17

~98

~250

2000

4000

80

~340

50

100

~100

~650

150

700

~ 43

~337

500

3500

88

~155

500

5250

~96

660

50

500

~100

~506

500

340

~74

~870

200

1000

~ 80

656

200

500

~66

716

120

50

~70

250

100

200

~27

~270

100

270

~69

~600

80

100

~86

~250

20

35

~83

19

Highlights 1. A novel method is electrochemically exfoliated graphene (EG). 2. The high quality EG was directly sprayed onto the copper foil as anode. 3. The battery has exhibited a high capacity and capacity retention over 6500 cycles.

Interest statement

The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.