Exfoliation Mechanism of Graphite Cathode in Ionic Liquids - ACS

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Exfoliation Mechanism of Graphite Cathode in Ionic Liquids Haiping Lei, Jiguo Tu, Zhijing Yu, and Shuqiang Jiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03306 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

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ACS Applied Materials & Interfaces

Exfoliation Mechanism of Graphite Cathode in Ionic Liquids Haiping Lei, Jiguo Tu*, Zhijing Yu, Shuqiang Jiao* State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, PR China *Email. [email protected] (S Jiao), [email protected] (J Tu) Tel. +86-10-62333617, Fax. +86-10-62333617

ABSTRACT: Graphene has been successfully electrochemical exfoliated by electrolysis of cathode graphite in the aluminum-ion battery with ionic liquid electrolyte comprising AlCl3 and 1-ethyl-3-methylimidazalium chloride ([EMIm]Cl). The AlCl4-, Al2Cl7-, etc., intercalation into graphite flake in ionic liquid of the aluminum-ion battery by different electrolysis processes to exfoliate graphite have been researched in detail. As a result of the enhanced structural flexibility, the intercalant gallery height increases in the less than five-layer graphene film, providing more free space for AlCl4-, Al2Cl7-, etc. transport. Therefore, a quantity of 3-5 layers rather than 1-2 layers graphene can be obtained. The results clearly demonstrate that graphene has been produced in the graphite cathode in AlCl3/EMImCl ionic liquids, significantly meaningful for accelerating the theoretical research and industrialized application of graphene. Meanwhile, it has a vitally important role for promoting the recycling Al-ion batteries.

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KEYWORDS: Al-ion battery, graphene, ionic liquid, electrochemical exfoliation, intercalation. 1. INTRODUCTION Graphene material with a two-dimensional single atomic layer of sp2-bonded carbon sheet shows the great application potential for the special nanometer structure and performance in electronics, catalysis, sensors, energy storage,1-8 etc. Since graphene was discovered by Geim et al.,9 it has been widely used in a variety of fields, such as energy storage. Various researches on application of graphene have been done by Gao et al. who made a breakthrough by using graphene cathode in aluminum-ion battery.10-12 Graphene preparation technology was the key to the research and application of graphene, quite lots of different techniques to produce graphene sheets were reported during the last decade which including mechanical cleavage,9 graphitization,13 chemical vapor deposition (CVD),14-17 and liquid-phase exfoliation of graphite,18-25 etc. Although there were so many methods to produce graphene sheets, it was still the hot topic for researchers to find the efficient and facile methods to produce high output and high quality graphene sheets. Compared with these reported preparation methods, liquid-phase exfoliation of graphite is low-cost and facile. Especially ionic liquids were reported to be effective media for the exfoliation of graphene sheets from graphite.26-29 Exfoliation of graphite into graphene in solvent were repeatedly reported by Coleman et al.30 Besides, Fray et al. reported that large-scale and high-quality graphene can be prepared by high temperature insertion of hydrogen into the interlayer space of graphite.31 Recently, Dai’s group made a great achievement in aluminum-ion battery (AIB) which exhibits stable capacity, long cycled life, and low flammability by using a three-dimensional graphitic-foam cathode and an ionic liquid electrolyte.32-34 They also made a contribution to the


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application of graphene.35-36 Last year, we found graphene sheets in the cathode graphite of aluminum-ion batteries with ionic liquid as the electrolyte and by direct conversion of CO2 into graphene via molten salts electrolysis.37-38 Electrolyzing graphite cathode to obtain graphene in aluminum-ion battery is simple to operate and subsequent graphene product can be very well separated from the cathode. The discovery of small amount of graphene in graphite cathode provides conditions for recycling Al-ion batteries with graphite cathode. Therefore, graphene can be prepared under different electrolytic conditions on the basis of previous work. In this work, we deeply discussed the exfoliation mechanism of graphite cathode by intercalation ions, such as AlCl4-, Al2Cl7-, etc. under different electrolysis processes, and we have successfully obtained few-layer graphene by electrolysis of graphite cathode in the aluminum-ion battery with the constant voltage of above 2.0 V. 2. EXPERIMENTAL SECTION 2.1 Electrochemical exfoliation Electrochemical exfoliation graphene was performed in the aluminum-ion batteries which high purity Al foil as the anode (4 cm × 4 cm × 0.05 mm), graphite paper (made by natural layered structure flake graphite powder from Beijing Jing long Special Carbon Co. Ltd.) as the cathode (4 cm × 4 cm × 0.1 mm, 0.33 g), glass fiber (GF/A) (5 cm × 5 cm × 0.26 mm) as the separator and 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) and AlCl3 mixture as the ionic liquid electrolyte (~5 mL). The positive current collector used molybdenum foil (6 cm × 0.8 cm × 0.03 mm) which was put on the surface of the graphite. The soft-package aluminum-ion batteries were assembled in a high purity argon atmosphere glove-box for subsequent exfoliation of graphene sheets.


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The electrochemical experiments were carried out using the Neware BTS-53 tester. The assembled cells were performed by constant voltage electrolysis and galvanostatic charged/discharged by the tester, performed in a voltage rang of 0.3-2.3 V vs. Al3+/Al at the same current density of 10 mA g-1. The detailed electrolysis processes were as follows: Firstly, the cells were cycled ten times by charging to 2.3 V and fully discharging to 0.3 V at a constantcurrent density of 10 mA g-1; then, the cells were cycled ten times by charging to 2.3 V, electrolyzing for 20 hours at a constant voltage of 1.8 V, 2.0 V, 2.2 V, 2.4 V, and fully discharging to 0.3 V. Finally, the cells were removed in discharge state and cleaned for 20 minutes by ultrasound (KQ2200E, 40 KHz) in ethanol at room temperature, then dried at 120 oC in drum wind drying oven (DHG-9140A) after separating by centrifuging the supernatant liquid (DT5-6B) for the late testing to research the changes and differentia of the products. The graphite products electrochemically exfoliated were floating on the surface of solution after ultrasound in the ethanol. 2.2 Characterization The structure and morphology of the prepared cathode powder were characterized by X-ray diffraction (XRD, Rigaku, D/max-RB), field emission scanning electron microscopy (FESEM, JEOL, JSM-6701F), high resolution transmission electron microscopy (TEM, JEOL, JSM-2010), X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD), Raman spectroscopy (Horiba-labram HR evolution) with an excitation at 532 nm and atomic force microscopy (AFM, Bruker, Bioscope Catalyst). 3. RESULTS AND DISCUSSION


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The anticipated mechanism representation of electrochemical exfoliation of graphite in ionic liquid of Al-ion batteries was presented in the Scheme 1. The entire electrochemical exfoliation of graphite was performed in the soft-package aluminum-ion battery system. Scheme 1a is the micro schematic display of constant voltage electrolysis in the aluminum-ion battery. As the battery is treated by constant voltage electrolysis, the anions (AlCl4-, Al2Cl7-, etc.) are gathered to the graphite cathode and intercalated into the graphite layer. A portion of the intercalation anions are participated in chemical reaction, the other part is adsorbed on the surface of the graphite layer just for capacitor behavior.39 Scheme 1b shows that various ions intercalate into the cathode graphite. With the constant voltage electrolysis, the anions are continuously absorbed and intercalated into the cathode graphite (Scheme 1d). As the continuous increase of the anions in the graphite layers, the graphite layer is exfoliated into multilayer graphene nanoplates (shown in the Scheme 1c). Figure S1 shows the 1st, 5th, 10th charge/discharge curves of aluminum-ion batteries at current density of 10 mA g-1 before electrolysis. The discharge capacity of four batteries are about 50 mA h g-1 and each battery’s charge/discharge curves are almost overlapped, suggesting that the capacity were kept well before 10 cycles. The electrolysis under different voltage was performed in the batteries with the voltage of 1.8 V, 2.0 V, 2.2 V, 2.4 V, respectively. The charge and discharge curves after different voltage electrolysis were shown in Figure 1. Figure 1a, b, c, d shows the charge/discharge curves at a current density of 10 mA g-1 after the constant voltage electrolysis of 1.8 V, 2.0 V, 2.2 V, 2.4 V for 20 h, respectively. In each graph from Figure 1, the straight lines y1, y2, y3, y4 respectively were the fitting lines of the charge and discharge curves (curves line y1, y2, y3, y4 in the Figure 1a) and the tables in the pictures present the values of the intercept and slope. The lines y1 and y4 represent the capacitor behavior


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and lines y2 and y3 represent battery and capacitor behaviors.39 As is well known, the capacity of capacitor can be due to the adsorption of ions onto the surface of electrode materials, so the response to changes in potential without diffusion limitations is rapid, the electrochemical process occurring in battery is diffusion-controlled and the redox reactions are slow.40,41 From Figure 1a-c, there are just little changes on the slope of the charge and discharge curves. Obviously, it can be found from Figure 1d that the intercept and slope changes of lines y1, y2, y4 are the biggest. The results demonstrate that there appears the biggest damage to graphite structure at the 2.4 V constant voltage electrolysis, leading to the largest interlayer distance of graphite. The corresponding internal surface area of graphite largens, so that capacitor behavior increases with the more anions adsorbed on the surface of the graphite layer. With higher constant voltage electrolysis for 20 h, the AlCl4-, Al2Cl7-, etc. continuously intercalate into the carbon paper cathode, until van der Waals force between the adjacent graphite layers is conquered, allowing the exfoliation of graphite and the followed formation of graphene. XRD measurements of graphite under different voltage electrolysis were illustrated in Figure S2. The pristine graphite peak is at 2θ=25.98° and the four electrolysis graphite peaks are all at about 2θ=26.90°, showing that the peak slightly shifts to the right. The slightly shifts don’t show the significant changes with different voltage electrolysis indicating that the electrolyzed graphite still exhibits typical graphite structure. The cathode graphite doesn’t participate in forming common chemical bonds with the intercalated anions, only providing large interlayer space for the intercalation and deintercalation of anions. According to previous work, the graphite peak (2θ=25.98°) would split into two sharp peaks after charging, but didn’t split into two sharp peaks after discharging to 0.5 V.32-33 The 2 theta =26.90 degree didn’t split into two sharp peaks because the tested cathode samples were in the fully discharged stage.


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The microstructure of graphite charged in the different voltage electrolysis was presented in Figure S3. It shows that the size of graphite after 1.8 V electrolysis (see in the Figure S3a) is more than 10 µm, and the morphology of the graphite is comparatively complete. The size of graphite after 2.0 V electrolysis is above 10 µm, but it is thinner than that of 1.8 V electrolysis because that the graphite is curled up (shown in the Figure S3b). The size and thickness of the graphite with 2.2 V and 2.4 V electrolysis are less than the graphite with 2.0 V electrolysis. The SEM images show that the higher electrolysis voltage can get more products of smaller and thinner graphite. To obtain further insights into the structural features of the generated nanosheets after different voltage electrolysis, transmission electron microscopy techniques were employed to investigate the cathode material under different voltage electrolysis (1.8 V, 2.0 V, 2.2 V, 2.4 V) shown in Figure 2a-i. TEM images of graphite under different voltage electrolysis shown in Figure 2a, c, e, g revealed that the observed large quantity of flakes were all thin and nearly transparent. HRTEM image of the graphite cathode electrolyzed by 1.8 V was shown in Figure 2b and the many layers graphite edge can be observed in the enlarged HRTEM images. The inset describes a fast Fourier transform (FFT) of the image of Figure 2b which is equivalent to an electron diffraction pattern. The results disclose that the less than five layers graphene have not been found in the cathode graphite under 1.8 V electrolysis, and the exfoliation of graphite is not enough under 1.8 V electrolysis. HRTEM images of the graphite cathode electrolyzed by 2.0 V were shown in Figure 2d and the 4-5 layers graphene edge can be observed in the enlarged HRTEM images. The enlarged HRTEM image shows the about 4 layers graphene observed under 2.2 V electrolysis (see in Figure 2f). HRTEM images of the graphite cathode electrolyzed by 2.4 V were shown in Figure 2h-i and the 2-5 layers graphene edge can be observed in the


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enlarged HRTEM images at electrolysis voltage rang of 2.0-2.4 V. Meanwhile, the inside enlargement of the sheet with different voltage electrolysis was shown in Figure S4. The wrinkled and bumpy structure of graphene can be found in the all sheets with electrolysis of 1.82.4 V. The layer number of graphene does not follow the trend in one direction with the increasing of voltage probably because that the elastic stiffness of few-layered graphene is low which offers more free volume for the AlCl4- diffusivity in the below five-layer graphene films.42 So it is hard to continue the exfoliation of the below five-layer graphene. X-ray photoelectron spectroscopy (XPS) was employed to evaluate the surface composition of each type of samples, as shown in Figure S5. The XPS spectra of Al 2p peaks (74.9 eV) and Cl 2p peaks (197.9 eV, 199.3 eV) after different voltage electrolysis processes are attributed to the residual trapped/adsorbed chloroaluminate anion species in the graphite electrode. And then Raman spectroscopy was used to measure the crystalline quality of the obtained graphene samples after different voltage electrolysis (represented in Figure 3). Raman spectroscopy has a unique advantage in characterizing the layer and the defects of graphene by the typical Raman scattering features, such as D-band (~1350 cm-1), G-band (~1580 cm-1), and 2D-band (~2700 cm1 19,43

).

There is no obvious D-band found in the pristine graphite materials, but a weak D-band

(~1351 cm-1) can be found in the graphite electrolyzed by 1.8 V, 2.0 V, 2.2 V, 2.4 V electrolysis processes. This demonstrates that the defects do not appear on the pristine graphite materials and the observed D-band may be dominated by edge effects. The pristine G-band (1579 cm-1) shifts to a little higher Raman shift (1582 cm-1) after electrolyzing the battery by different electrolysis processes as shown in the Figure 3b. Compared to the pristine graphite, the little Raman shift difference for G-band can be found for the four electrolyzed graphite cathodes, probably attributed to the decrease degree of graphitization.44 The 2D-bands (Figure 3c) of the four


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samples after different voltage electrolysis strongly demonstrate that exfoliation flakes of the graphite is less than 5 layers graphene.43 The typical AFM images of 20 µm × 20 µm scan of the graphite sheets results shown in Figure S6 disclose that the thickness of graphite sheets with different electrolytic voltages (1.8 V, 2.0 V, 2.2 V, 2.4 V) is different. Figure S6a is a typical AFM image of the obtained graphite after 1.8 V electrolysis and the thickness of the graphite sheets is mostly 10-40 nm (see the inset picture in Figure S6a). The shape of graphite is irregular and the distributions of the height have great difference, demonstrating that there is no graphene generating. The typical AFM image of the obtained graphite after 2.0 V electrolysis (see Figure S6b) reveals that the graphene appears. The AFM image of graphite after 2.2 V electrolysis was shown in Figure S6c. The quantity of graphene is more than 2.0 V, but the size is smaller from the 20 µm × 20 µm scan. Figure S6d is a typical AFM image of the obtained graphite after 2.4 V electrolysis. It is obvious that the quantity of the obtained graphene after 2.4 V electrolysis is more than that of the others (presented in Figure S6a-d). The quantity of the obtained graphene products is more as the electrolytic voltage getting higher. Figure 4 is the specific thickness of the graphene found in the Figure S6. The observed AFM images (see in Figure 4a) of the larger view from the circle partially area in the 20 µm × 20 µm scan of the graphite sheets from the 2.0 V electrolysis process (corresponding to Figure S6b). The appearance of the obtained graphene products is plate-like and the thickness is 2-3 nm (Figure 4d1). It is clear from Figure 4b and d2 that the thickness of the products after the 2.2 V electrolysis is below 5 nm and it appears different thickness in the one graphene flake. Figure 4c is a typical AFM image of the obtained graphene under 2.4 V electrolysis. Figure 4d3, 4 shows the thickness of graphene corresponding to Figure 4c, about 3 nm. It was reported that the


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apparent height of graphene monolayers was measured by AFM to be about 1 nm.45 This demonstrates that the layers of the observed graphene under 2.0 V, 2.2 V, 2.4 V electrolysis are about or less than 5 layers. Therefore, the few-layer graphene (FLG) can be existed in the cathode graphite after 2.0 V, 2.2 V or 2.4 V electrolysis. The CV curves by previous work of the Al-ion battery showed that the current density at 1.8 V was much less than that under 2.0 V, 2.2 V, 2.4 V, and the current densities under 2.0 V, 2.2 V, 2.4 V were almost the same.32, 46 This indicates that the intercalation anions are little reacted with graphite in the 1.8 V, so graphene can’t be obtained. Nevertheless, it can be obtained in the electrolysis voltage of 2.0 V, 2.2 V, 2.4 V, respectively. Moreover, Han et al. used first-principles calculations to suggest that the structural flexibility of graphitic foam begins to increase when the number of graphene layers reduces to five.42 As a result of the enhanced structural flexibility, the intercalant gallery height increases in the less than five-layer graphene film, providing more free space for AlCl4-, Al2Cl7-, etc. transport and thus lowering the activation energy barrier. Therefore in this work, it can be obtained that there are large quantity of the less than 5-layer graphene. The exfoliation of graphene layers displayed in Figure 4 is in good correlation with the elasticity trend. Furthermore, the results of the AFM images show that the large thickness of such flakes were consistent with the results of Raman spectroscopy from Figure 3. By comparison to other preparation graphene methods, the exfoliation graphene in the aluminum battery is very simple and the graphene layer is almost less than five layers at the electrolysis voltage of above 2.0 V. 4. CONCLUSIONS


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In summary, we have successfully obtained few-layer graphene by electrolysis of graphite cathode in the aluminum-ion battery with the constant voltage of above 2.0 V, and the production of graphene gets more with the higher electrolysis voltage. The obtained less than 5layer graphene by direct electrochemical exfoliation of graphite in AlCl3/[EMIm]Cl ionic liquids is resulting from the enhanced structural flexibility in the less than five-layer graphene film. In addition, the high-production and uniform thickness of the prepared graphene require further investigation of that potential. By comparison to other preparation graphene methods, the exfoliation of graphene in AlCl3/[EMIm]Cl ionic liquids is apparently facile and efficient and the graphene layer is almost less than five layers. It is particularly remarkable that preparation method can be used for recycling the waste aluminum-ion batteries. ASSOCIATED CONTENT Supporting Information. The 1st, 5th, 10th charge-discharge curves of the battery before electrolysis. The XRD patterns of pristine graphite and different voltage electrolysis graphite. SEM images of graphite cathode electrolysis by different voltage. XPS data of pristine graphite and different voltage electrolysis graphite. The AFM images of a 20 µm × 20 µm scan of the graphite sheets from the different electrolysis voltage in Al-ion battery. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], * Email: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Fundamental Research Funds for the Central Universities (FRFTP-15-002C1). REFERENCES (1) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-Based Composite Materials. Nature 2006, 442, 282-286. (2) El Rouby, W. M. A. Crumpled Graphene: Preparation and Applications. RSC Adv. 2015, 5, 66767-66796. (3) Chen, C. H.; Yang, S. W.; Chuang, M. C.; Woon, W. Y.; Su, C. Y. Towards the Continuous Production of High Crystallinity Graphene via Electrochemical Exfoliation with Molecular in situ Encapsulation. Nanoscale 2015, 7, 15362-15373. (4) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Two-Dimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197-200. (5) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of Single-Layer Graphene. Nano Lett. 2008, 8, 902-907.


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(22) Matsumoto, M.; Saito, Y.; Park, C.; Fukushima, T.; Aida, T. Ultrahigh-Throughput Exfoliation of Graphite into Pristine 'Single-Layer' Graphene using Microwaves and Molecularly Engineered Ionic Liquids. Nat. Chem. 2015, 7, 730-736. (23) Srivastava, P. K.; Yadav, P.; Ghosh, S. Dielectric Environment as a Factor to Enhance the Production Yield of Solvent Exfoliated Graphene. RSC Adv. 2015, 5, 64395-64403. (24) Ciesielski, A.; Samorì, P. Supramolecular Approaches to Graphene: From Self-Assembly to Molecule-Assisted Liquid-Phase Exfoliation. Adv. Mater. 2016, 28, 6030-6051. (25) Lee, O. S.; Carignano, M. A. Exfoliation of Electrolyte-Intercalated Graphene: Molecular Dynamics Simulation Study. J. Phy. Chem. C 2015, 119, 19415-19422. (26) Fu, Y.; Zhang, J.; Liu, H.; Hiscox, W.; Gu, Y. Ionic Liquid-Assisted Exfoliation of Graphite Oxide for Simultaneous Reduction and Functionalization to Graphenes with Improved Properties. J. Mater. Chem. A, 2012, 1, 2663-2674. (27) Yang, Y.; Lu, F.; Zhou, Z.; Song, W.; Chen, Q.; Ji, X. Electrochemically Cathodic Exfoliation of Graphene Sheets in Room Temperature Ionic Liquids n-Butyl, Methylpyrrolidinium bis (trifluoromethylsulfonyl) Imide and Their Electrochemical Properties. Electrochim. Acta 2013, 113, 9-16. (28) Du, W.; Jiang, X.; Zhu, L. From Graphite to Graphene: Direct Liquid-Phase Exfoliation of Graphite to Produce Single-and Few-Layered Pristine Graphene. J. Mater. Chem. A 2013, 1, 10592-10606. (29) Huang, X.; Li, S.; Qi, Z.; Zhang, W.; Ye, W.; Fang, Y. Low Defect Concentration FewLayer Graphene using a Two-Step Electrochemical Exfoliation. Nanotechnology 2015, 26, 105602.


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(30) Coleman, J. N. Liquid-Phase Exfoliation of Nanotubes and Graphene. Adv. Funct. Mater. 2010, 19, 3680-3695. (31) Kamali, A. R.; Fray, D. J. Large-Scale Preparation of Graphene by High Temperature Insertion of Hydrogen into Graphite. Nanoscale 2015, 7, 11310-11320. (32) Lin, M. C.; Gong, M.; Lu, B.; Wu, Y.; Wang, D. Y.; Guan, M.; Angell, M.; Chen, C.; Yang, J.; Hwang, B. J.; Dai, H. An Ultrafast Rechargeable Aluminium-Ion Battery. Nature 2015, 520, 324-328. (33) Wang, D. Y.; Wei, C. Y.; Lin, M. C.; Pan, C. J.; Chou, H. L.; Chen, H. A.; Gong, M.; Wu, Y.; Yuan, C.; Angell, M.; Hsieh, Y. J.; Chen, Y. H.; Wen, C. Y.; Chen, C. W.; Hwang, B. J.; Chen, C. C.; Dai, H. Advanced Rechargeable Aluminium Ion Battery with a High-Quality Natural Graphite Cathode. Nat. Commun. 2017, 8, 14283. (34) Wu, Y.; Gong, M.; Lin, M. C.; Yuan, C.; Angell, M.; Huang, L.; Wang, D. Y.; Zhang, X.; Yang, J.; Hwang, B. J.; Dai, H. 3D Graphitic Foams Derived from Chloroaluminate Anion Intercalation for Ultrafast Aluminum-Ion Battery. Adv. Mater. 2016, 28, 9218-9222. (35) Jiao, L.; Zhang, L.; Wang, X.; Diankov, G.; Dai, H. Narrow Graphene Nanoribbons from Carbon Nanotubes. Nature 2009, 458, 877-880. (36) Wang, H.; Robinson, J. T.; Diankov, G.; Dai, H. Nanocrystal Growth on Graphene with Various Degrees of Oxidation. J. Am. Chem. Soc. 2010, 132, 3270-3271. (37) Hu, L.; Song, Y.; Jiao, S.; Liu, Y.; Ge, J.; Jiao, H.; Zhu, J.; Wang, J.; Zhu, H.; Fray, D. J. Direct Conversion of Greenhouse Gas CO2 into Graphene via Molten Salts Electrolysis. ChemSusChem 2016, 9, 588-594.


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(38) Jiao, S.; Lei, H.; Tu, J.; Zhu, J.; Wang, J.; Mao, X. An Industrialized Prototype of the Rechargeable Al/AlCl3-[EMIm]Cl/Graphite Battery and Recycling of the Graphitic Cathode into Graphene. Carbon 2016, 109, 276-281. (39) Long, J. W.; Sassin, M. B.; Fischer, A. E.; Rolison, D. R.; Mansour, A. N.; Johnson, V. S.; Stallworth, P. E.; Greenbaum, S. G. Multifunctional MnO2−Carbon Nanoarchitectures Exhibit Battery and Capacitor Characteristics in Alkaline Electrolytes. J. Phys. Chem. C 2009, 113, 17595-17598. (40) Augustyn, V.; Come, J.; Lowe, M. A.; Kim, J. W.; Taberna, P. L.; Tolbert, S. H.; Abruña, H. D.;

Simon, P.; Dunn, B. High-Rate Electrochemical Energy Storage through Li+

Intercalation Pseudocapacitance. Nat. Mater. 2013, 12, 518-22. (41) Simon, P.; Gogotsi, Y.; Dunn, B. Where Do Batteries End and Supercapacitors Begin? Science 2014, 343, 1210-1211. (42) Jung, S. C.; Kang, Y. J.; Yoo, D. J.; Choi, J. W.; Han, Y. K. Flexible Few-Layered Graphene for the Ultrafast Rechargeable Aluminum-Ion Battery. J. Phys. Chem. C 2016, 120, 1338413389. (43) Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Raman Spectroscopy in Graphene. Phys. Rep. 2009, 473, 51-87. (44) Hu, C.; Sedghi, S.; Silvestre-Albero, A.; Andersson, G. G.; Sharma, A.; Pendleton, P.; Rodríguez-Reinosob, F.; Kanek, K.; Biggs, M. J. Raman Spectroscopy Study of the Transformation of the Carbonaceous Skeleton of a Polymer-based Nanoporous Carbon Along the Thermal Annealing Pathway. Carbon 2015, 85, 147-158. (45) Lotya, M.; Hernandez, Y.; King, P. J.; Smith, R. J.; Nicolosi, V.; Karlsson, L. S.; Blighe, F. M.; De, S.; Wang, Z.; McGovern, I. T.; Duesberg, G. S.; Coleman, J. N. Liquid Phase


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18

(a)

A Al3+

AlCl4-

（＋）

Al2Cl7-

19

(a)

Charge y2

2.0 y3

1.5

Discharge

y1 y = a + b*x

1.0

1.8 V

0.5

10 mA g-1

y1 y2 y3 y4

Intercept 1st 0.65225 1.81899 5th 1.9951 10th 20.9942

y4

Slope 1.08474 0.00729 -0.0099 -0.47073

Voltage (V vs. Al3+/Al)

2.5

0.0

2.5

Charge

y1

y4


(c)

y2

2.0 y3

1.5 1.0

y3

Discharge 1st Slope Intercept 0.69385 5th 1.09319 1.81957 0.00885 10th-0.01117 2.04751

y4

20.31718

y = a + b*x y1

2.2 V

0.5

10 mA g-1

0.0

0

y2

-0.41967

5 10 15 20 25 30 35 40 45 50 -1

Specific capacity (mA h g )

Charge

y1

y4

y2

1.5

y3

Discharge

1.0

y = a + b*x

2.0 V

0.5

10 mA g-1

0

Special capacity (mA h g )

2.5

Page 20 of 25

2.0

0.0

0 5 10 15 20 25 30 35 40 45 50 55

(b)

-1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



2.5

(d)

Intercept

Slope

y1

0.65447

1.25291

y2

1.82382

0.00736

y3

2.00044

-0.00995

y4

23.83149

-0.54657

7 14 21 28 35 42 49 -1 Specific capacity (mA h g )

y1

y2

Charge

2.0 1.5

y3

Discharge

1.0

2.4 V

y1

1.30699

y2

1.95002

10 mA g-1

y3

2.0278

1st Slope 1.36879 5th 0.02009 10th -0.02364

y4

6.47534

-0.12241

y = a + b*x Intercept

0.5 0.0

y4

0 5 10 15 20 25 30 35 40 45 50 55 -1 Specific capacity (mA h g )

Figure 1. The fitting slopes of the aluminum-ion batteries charging and discharging curves after different voltage electrolysis. (a) after 1.8 V electrolysis. (b) after 2.0 V electrolysis. (c) after 2.2 V electrolysis. (d) after 2.4 V electrolysis.


20

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Figure 2. TEM images of graphite cathode tested by different voltage electrolysis. (a) TEM image under 1.8 V electrolysis. (b) HRTEM image under 1.8 V electrolysis. Inset: Fast Fourier transform (equivalent to an electron diffraction pattern) of the image. (c) TEM image under 2.0 V electrolysis. (d) HRTEM image under 2.0 V electrolysis. Inset: Fast Fourier transform of the image. (e) TEM image under 2.0 V electrolysis. (f) HRTEM image under 2.2 V electrolysis.


21


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Page 22 of 25

Inset: Fast Fourier transform of the image. (g) TEM image under 2.4 V electrolysis. (h), (i) HRTEM image under 2.4 V electrolysis. Inset: Fast Fourier transform of the image.


22

Page 23 of 25

G

(a)

(b)

1582

D

Pristine 1.8 V 2.0 V 2.2 V 2.4 V

Intensity

2D 2.4 V electrolysis 1582

Intensity

2.2 V electrolysis 1582 1300

1400

1500

1600

1700

Raman shift (cm-1)

2.0 V electrolysis

(c)

1582

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.8 V electrolysis 1579

Pristine 1.8 V 2.0 V 2.2 V 2.4 V

Pristine graphite 1000

1250

1500

1750

2000

2250

Raman shift (cm-1)

2500

2750

3000 2400

2500

2600

2700

2800

2900

Raman shift (cm-1)

Figure 3. (a) Raman spectra of graphite under different voltage electrolysis process (pristine, 1.8 V, 2.0 V, 2.2 V, 2.4 V). (b) G-band of the different graphite samples. (c) 2D-band of the different graphite samples.


23


(a)

2.0 V

(d)5 4 3 2 1 0 -1 nm 0.0 5

1

1 0.5

1.0

1.5

2.0

2.5

3.0

4.0 um

3.5

4

(b)

3

2.2 V

2

Height/nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

2

1 nm 0.0 2

2 0.4

0.8

1.2

um

1.6

1 0 -1 -2

(c)

2.4 V

3

4

-3 nm 0.0

3 0.5

1.0

1.5

2.0 nm

1 0 -1 -2 nm 0.0

4 0.3

0.6

0.9

Distance

1.2

nm

Figure 4. The AFM images of the larger view from the circle part in Figure S6. (a) The obtained graphene under 2.0 V electrolysis. (b) The obtained graphene under 2.2 V electrolysis. (c) The obtained graphene under 2.4 V electrolysis. (d) The thickness of the obtained graphene. Note: Part 1 is corresponding to the thickness through the line in Figure (a). Part 2 is corresponding to the thickness through the line in Figure (b). Parts 3 and 4 are corresponding to the thickness through the lines in Figure (c).


24

Page 25 of 25

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Constant voltage charging A Al3+

AlCl4Continue Intercalation

Exfoliation

Graphene

Al2Cl7-

TOC Few-layer graphene can be obtained by electrolysis of graphite cathode in the aluminum-ion battery.


25

Exfoliation Mechanism of Graphite Cathode in Ionic Liquids - ACS

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