Microwave-Irradiation-Assisted Combustion toward Modified Graphite

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Microwave Irradiation Assisted Combustion towards Modified Graphite as Lithium Ion Battery Anode Kunfeng Chen, Hong Yang, Feng Liang, and Dongfeng Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16418 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Applied Materials & Interfaces

Microwave Irradiation Assisted Combustion towards Modified Graphite as Lithium Ion Battery Anode Kunfeng Chena, Hong Yangb, Feng Liangc and Dongfeng Xue*,a a

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b

School of Mechanical and Chemical Engineering, The University of Western Australia, WA 6009, Australia

c

Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China *E-mail: [email protected]

Abstract A rapid method to high yield synthesis of modified graphite by microwave irradiation of partially oxidized graphite (oxidized by H2SO4 and KMnO4) is reported. During the microwave irradiation, electrical arc flame induced combustion of Mn2O7 and vaporization and decomposition of H2SO4 to form O2 and SO2, which helped to decompose graphite within 30 s. The modified graphite boosts its ability to support the intercalation and diffusion of Li+ ions. As an anode material for lithium-ion batteries, the modified graphite displays high reversible capacity of 373 mAh/g, approaching the theoretical value of 372 mAh/g. Long cycling performance of 410 charge-discharge cycles shows the capacity is retained at 370 mAh/g, demonstrating the superior stability. The improved cycling stability is attributed to the formation of a stable SEI film with the help of in-situ formed S-based compounds on graphite sheet. This work demonstrated a simple and effective method to alter carbon structures for improving energy storage ability. Keywords: modified graphite, lithium ion battery, microwave synthesis, KMnO4 oxidation, combustion 1

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1. Introduction Energy shortage has become one of the most challenging global concerns, which penetrates all walks of life. Various energy storage devices including lithium-ion rechargeable batteries, supercapacitors and their hybrids have been developed to conserve energy and to utilize intermittent energy sources.1-3 The development of highly active electrode materials is the center subject for high-energy devices.1,2 In commercial lithium-ion batteries, graphite is the dominant anode material due to its low-cost, safety, low working potential close to that of metal Li and high coulombic efficiency.4-6 Graphite has a layered structure and in pristine state its interlayer spacing is 0.34 nm. During electrochemical intercalation reaction, the interlayer spacing of graphite expands in order to accommodate Li+ ions. In the fully charged state corresponding to the formation of LiC6, the interlayer spacing of graphite increases to 0.37 nm, equivalent to about 10 % volume expansion.7,8 This large volume expansion is responsible for the loss capacity of graphite anode after long-time charge-discharge cycling. Extensive research has been devoted to solve this problem through structure modification of graphite. For example, porous graphite was prepared to buffer the volume change during cycling9 and more directly, the development of expanded graphite with large interlayer spacing was used to reduce volume change during cycling.10-14 However, the overall process involves multiple steps of intercalation, washing and high temperature processing, and is thus a long process with high energy consumption and associated waste production.15-19 Cost reduction is crucial on the development of high energy devices,20,21 and thus, it demands facile, low-cost and fast production method for electrode materials. Compared 2


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with conventional heating methods, microwave irradiation has the advantage of short processing time and less energy consumption, which can shorten the process time from hours to seconds.22-24 In this work, we developed a one-step rapid synthesis route to produce modified graphite (MG) using microwave irradiation from partially oxidized graphite (oxidized by H2SO4 and KMnO4). The strong exothermic reaction involved in the process leads to additional benefit in producing functionalized MG. 2. Experiment section All chemicals, H2SO4, KMnO4, polyvinylidene fluoride (PVDF), N-methyl-2-pyrrolidone (NMP), were used as purchased without further treatments. Natural flake graphite (average particle size of 100 µm) was purchased from Beijing Chemical Company in China. Typically, 1 g of flake graphite was mixed with 0.5 g H2SO4 (98 %). Different amounts of KMnO4 (0.07, 0.1, 0.13 or 0.15 g) was then added into above mixtures with stirring in glass beaker to form the final mixtures and allow to react for 3 h. After that, the mixtures in the glass beaker were transferred directly into a microwave oven (Midea, made in China) and underwent microwave irradiation at 700 W for 30 s. During microwave irradiation, the volume of graphite mixture was found to increase rapidly, resulting in the formation of modified graphite (Figure S1 and S2). The morphology, structure and phase composition of as-prepared sample were characterized by a field emission scanning electron microscope (FESEM, Hitachi S4800), transmission electron microscope (TEM, TECNAI G2), and X-ray diffractometer (XRD, Bruker D8 Focus). Button battery was assembled to test the electrochemical performance of the resulted MG. Metallic Li served as anode, whilst the prepared MG electrode served as working 3



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cathode. A polypropylene membrane was used as the separator and 1 M LiPF6 in EC/DMC/DEC, solvent (1:1:1 vol %) served as electrolyte. The working electrode was prepared by blending modified graphite, carbon additive and PVDF in mass ratio of 8:1:1 and NMP as solvent to form slurry. The slurry was then coated onto a copper foil, dried at 100 °C for 24 h. The mass loading of active materials is about 2 mg/cm2. The 2032-type cell was assembled in an argon (Ar) filled glove box. The galvanostatic charge-discharge measurements were carried out by LANHE CT2001A cell tester. 3. Results and discussion Scheme 1a illustrates the synthesis process using microwave irradiation on partially oxidized graphite to produce MGs. Firstly, H2SO4 and KMnO4 were intercalated into flake graphite interlayer by oxidation of carbon layer edge and the following ion diffusion. The following chemical reactions may occur during oxidation step: 2KMnO4 + H2SO4 → Mn2O7 + K2SO4 + H2O

(1)

Mn2O7 + H2SO4 → MnO3+ + HSO4− + HMnO4

(2)

2Mn2O7 → MnO2 + O2

(3)

The dimanganese heptoxide (Mn2O7) and permanganyl cation (MnO3+) produced are strong oxidizing agents, which can oxidize graphite to form graphene oxide with various oxygen-containing groups (Scheme 1b).14 In addition, C atoms were partially oxidized and thus removed from graphite sheets via reactions between C and MnO4−, Mn2O7 or MnO3+, e.g. 14 −C=C− → −C=O + −C=O

(4)

−C=C− → −C-OH + −COOH

(5)

4


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4MnO4− + 3C + H2O → 4MnO2 + 2HCO3− + CO32−

(6)

Under microwave irradiation, in-situ formed gas molecules, e.g. SO2 and O2, and other compounds, e.g. Mn2O7, helped to further modify and break down the initial flake graphite. Figure S1 shows the photographs taken at different reaction times during the microwave irradiation process. Electrical arc induced flame can be seen. The microwave irradiation on graphite produced high temperatures, which ignite the combustion of Mn2O7, a highly volatile and explosive compound. The volume of graphite was hugely increased, indicting the expansion and breaking down of graphite (Scheme 1a and Figure S2). After microwave irradiation, as-formed oxygen-containing groups were reduced and the final product consists of C defect and S-modified structures (Scheme 1b). It has been reported that partially reduced graphene oxide can absorbed microwave leading to its rapid heating within 1-2 s.22 This rapid heating process can promote atom doping on graphite sheet. During oxidation stage, H2SO4 and Mn-based compounds were intercalated into interlayer of flake graphite. With the application of microwave irradiation, the S- and Mn-modified graphites were in-situ formed. Figure 1 shows the X-ray diffraction (XRD) patterns of the original graphite and modified graphite samples. Table S1 summarizes the 2θ and corresponding interlayer spacing d002 of the main diffraction peak for all samples. The d002 value was found to decrease slightly from 0.339 to 0.334 nm with increased amount of KMnO4. The results indicated that the as-obtained products consist a large piece of small-size graphites which were formed by microwave-assisted explosion reaction. Figure S3 shows the SEM images of MG samples. When the amount of KMnO4 is less than 0.10 g, layered graphite is clearly 5



seen (Figure S3a and S3b). With increased KMnO4 amount, thin graphite nanosheets formed (Figure S3c and S3d). TEM image and SEAD pattern show that the MG samples display single crystal characteristics (Figure 2a). The d-spacing value of (002) planes was 0.338 and 0.365 nm as shown in HRTEM (Figure 2b). At the edge of nanosheets, the expanded graphite with large d002 value is formed. Figure S4 shows IR spectra of MG samples. It confirms that O- and S-containing groups are present on graphite sheets. Elemental mapping was performed to show the atom distribution (Figure 2c-2h). The final samples include C, S, O and Mn elements, which indicated that S- and Mn-based compounds or the oxygen-containing functional groups can co-exist on the MG basal plane (Figure S5).25,26 C/S and C/Mn ratios are 149 and 8, respectively (Table S2). The existence of these atoms can provide additional properties when applied in lithium ion batteries, for example the formation of stable SEI on graphite anode. The degree of graphite oxidation is expected to affect the microstructures of the as-resulted MG. It was reported that KMnO4 to H2SO4 ratio affects the oxidation degree of graphite.14 Increased the KMnO4/H2SO4 ratio promoted the formation of Mn2O7 and MnO3+, which are responsible for the oxidization of graphite basal plane. Large amount of carbon defect sites would form on graphite during oxidation, which may act as locations for microwave assisted expansion. In this work, the effect of H2SO4 addition (being 0.5, 1.0 and 1.5 g) on the resulted MG samples is also studied. As shown in Figure 3, a large amount of graphite nanosheets were found compared with Figure S2. With adding less amount of KMnO4 (< 0.2 g), thick sheets proved that the expanding degree of graphite was lower (Figure 3a and 3b). Thinner nanosheets were present in the samples with adding more 6


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amount of KMnO4 (Figure 3c-3e), indicating the expanding degree of graphite was higher. When the amount of H2SO4 increased to 1.5 g, thin nanosheets were obtained with adding different amounts of KMnO4 (Figure 3f-3j). EDS spectra also confirm the formation of O, S and Mn-based doping (Figure S5 and S6). Electrochemical performance of the MG samples was systematically evaluated as a lithium-ion battery anode. Figure 4a shows the galvanostatic discharge–charge curves, showing both charge and discharge voltages of < 0.5 V, which are in consistent with the reported values.27,28 As shown in Figure 4b and S8, the 1st discharge capacity of MG-0.1 was 710 mAh/g, whilst the 2nd discharge capacity decreased to 373 mAh/g, a value very close to the theoretical capacity of 372 mAh/g. 1st discharge voltage is higher than that in charge process (Figure 4c), indicating the formation of solid electrolyte interface (SEI). 29,30 In addition, the capacity loss in the 2nd cycle is also due to the formation of SEI. Figure 4b shows cycling performances of tested samples at a current density of 100 mA/g. The first discharge capacities of all samples can reach 700-800 mAh/g. After 400 charge-discharge cycles, flake graphite anode shows a capacity of 193 mAh/g. The capacities of MG-0.07, MG-0.10, MG-0.13, MG-0.15 are 249, 371, 280 and 214 mAh/g, respectively, displaying high capacity and cycling stability than flake graphite. It should be noted that pure flake graphites used in this work have the average particle size of 100 µm, which is higher than that often used in literatures. After microwave irradiation, their electrochemical performance can be significantly enhanced. The cells were also evaluated at the current density of 200 mA/g for long-time life testing. The capacity of MG-0.1 sample can keep at 370 mAh/g after 410 charge-discharge 7



cycles (Figure 4d). It should be noted that capacity fluctuate during the cycle is due to the temperature fluctuate. The formation of nanographite from flake graphite and expanded graphite edge contributes to increased capacity. In-situ formed O-, S- and Mn-based groups on graphite sheets may help to the formation of stable SEI, leading to the high cycling performance. Electrochemical performance of the MG samples synthesized with the addition of more H2SO4 and KMnO4 were also studied. As shown in Figure S9, all samples displayed high cycling stability after 200 charge-discharge cycles. Samples with large amount of both H2SO4 and KMnO4 showed low capacities. However, the capacity of MG-0.4 sample with adding 0.15 g H2SO4 can reach 460 mAh/g (Figure S9), which is higher than all other samples, due to the formation of graphene structure. The improved cycling stability was attributed to the formation of stable SEI film on graphite anode with the help of S and Mn doping.31-33 The present results confirm that modifying surface of graphite in synthesis process can enhance their final electrochemical performances.34,35 4. Conclusions In summary, microwave irradiation was used to in-situ modify the partially oxidized graphite (oxidized by KMnO4 and H2SO4), leading to the production of S-modified graphite samples. As tested for lithium-ion batteries, the modified graphite materials displayed the high reversible capacity of 373 mAh/g at a current density of 100 mA/g, approaching the theoretical value of 372 mAh/g. The capacity of 370 mAh/g was obtained even after 410 charge-discharge cycles. The high capacity and excellent cycling stability can be attributed to the formation of nanographite from flake graphite, expanded graphite edge and stable SEI film. This method provided a rapid and low-cost route to synthesize high-performance 8


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graphite- based electrode materials for energy storage. Acknowledgments Financial support from the National Natural Science Foundation of China (grant nos. 91434118, 21521092, 21601176), the External Cooperation Program of BIC, Chinese Academy of Sciences (grant no. 121522KYS820150009), and CAS-VPST Silk Road Science Found 2018 (GJHZ1854) is acknowledged. Yang also wishes to acknowledge the finacial support of Research Collaboration Awards (RCA 2016) of the Unviersity of Western Australia. Liang acknoledge the finacial support of Open Project of Earth Resources Utilization National Key Laboratory (under project No. RERU2016019). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Photographs of synthesis, SEM, and electrochemical performances of modified graphite electrode materials. References: (1) Chu, S.; Cui, Y.; Liu, N. The Path towards Sustainable Energy. Nat. Mater. 2017, 16, 16-22. (2) Grey, C. P.; Tarascon, J. M. Sustainability and in situ Monitoring in Battery Development. Nat. Mater. 2017, 16, 45-56. (3) Zeng, Y.; Lin, Z.; Meng, Y.; Wang, Y.; Yu, M.; Lu, X.; Tong, Y. Flexible Ultrafast Aqueous Rechargeable Ni//Bi Battery Based on Highly Durable Single-Crystalline Bismuth Nanostructured Anode. Adv. Mater. 2016, 28, 9188–9195. 9



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a

b

O

HS HS

HO O

O-S-O

OH

Mn-O

Mn

Mn

O

Graphite sheet

Partly oxidized graphite sheet

Reduced graphite sheet

Scheme 1 (a) Schematic drawing of microwave assisted synthesis of modified graphite from graphite-H2SO4-KMnO4 system and the Li+ intercalation reaction for lithium ion battery anodes. Photograph shows the flame produced in the microwave irradiation. Inset shows photographs of graphite and modified graphite after microwave treating. (b) At the molecular scale, illustrates the evolution of local chemical structure of graphite during chemical oxidation and microwave assisted reduction.

15



a (002) MG-0.15

Intensity

MG-0.13 MG-0.10 MG-0.07 Graphite 10

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b (002) MG-0.15

MG-0.13

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MG-0.10 MG-0.07 Graphite

24

25

26

27

28

29

30

2θ (degree)

Figure 1 (a, b) XRD patterns of graphite and modified graphite samples with adding different amounts of KMnO4. (b) The enlarged (002) reflection peaks. MG = modified graphite (addition of 0.5 g H2SO4).

16


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a

b 0.338 nm

0.365 nm (002) 200 nm

10 1/nm

d

c

5 µm f

5 µm

5 nm C

S

5 µm

5 µm O

e

g

K

h

Mn

5 µm

5 µm

Figure 2. Structure characterization of MG-0.15 (addition of 0.5 g H2SO4). (a) TEM image with SEAD pattern inset, (b) HRTEM image, (c) SEM image and (d-h) elemental mapping of C, S, O, K, and Mn.

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a

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b

1 µm c

1 µm d

1 µm e

2 µm f

2 µm g

1 µm h

1 µm

5 µm i

j

1 µm

1 µm

Figure 3 SEM images of MG-0.07 (a), MG-0.1 (b), MG-0.2 (c), MG-0.3 (d), MG-0.4 (e) with addition of 1 g H2SO4. SEM images of MG-0.07 (f), MG-0.1 (g), MG-0.2 (h), MG-0.3 (i), MG-0.4 (j) with addition of 1.5 g H2SO4.

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a

b

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3.0


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Potential (V vs. Li+/Li)

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


0

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500

600

Cycle number (n)

Figure 4. Lithium ion battery anode performance of original graphite and MG samples with addition of 0.5 g H2SO4. (a) galvanostatic discharge–charge curves after 200 cycles, (b) cycling performance at a current density of 100 mA/g, (c) 1st galvanostatic discharge–charge curves, (d) cycling performance at a current density of 200 mA/g.

19



TOC Art

800

Graphite


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

400

200 Current denisty: 200 mA/g 0

Modified graphite

MG-0.07 MG-0.10 MG-0.13 MG-0.15

600

0

100

200

300

400

500

600

Cycle number (n)

20


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Microwave-Irradiation-Assisted Combustion toward Modified Graphite

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