Morphology-controlled graphene nanosheets as anode material for lithium-ion batteries

Electrochimica Acta 132 (2014) 172–179

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Morphology-controlled graphene nanosheets as anode material for lithium-ion batteries Wook Ahn a,b , Hoon Sub Song c , Sang-Hoon Park a , Kwang-Bum Kim a,1 , Kyoung-Hee Shin b , Sung Nam Lim d , Sun-Hwa Yeon b,∗ a

Department of Materials Science & Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-Gu, Seoul, 120-749, Korea Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-Gu, Daejeon, 305-343, Korea c Department of Chemical Engineering, University of Waterloo, 200 University Ave W. Waterloo, ON, N2L3G1, Canada d Department of Chemical & Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea b

a r t i c l e

i n f o

Article history: Received 27 December 2013 Received in revised form 14 March 2014 Accepted 14 March 2014 Available online 2 April 2014 Keywords: Lithium ion batteries Anode Reduced graphite oxide Graphene nanosheets Morphology-controlled

a b s t r a c t Morphology-controlled graphene nanosheets can be easily synthesized as anode material for application in high-capacity lithium-ion batteries. A modified version of an improved method for higher degree of oxidation of graphite oxide (GO) has been developed and characterized. X-ray diffraction analysis shows that GO prepared using this method has a higher degree of oxidation than that of using the improved method. The interlayer d-spacing increases from 0.87 nm (using the improved method) to 0.92 nm (using the modified-improved method). Also, it is confirmed by XPS analysis that the O/C ratio in GO increases from 2.51 (improved method) to 8.27 (modified-improved method). It is hypothesized that GO, which has a higher degree of oxidation, is more reducible to graphene. The more reduced graphene has a larger amount of free -bonds and fewer layers, and it can be easily altered to morphology-controlled graphene. Graphene nanosheets prepared using the modified-improved method exhibits discharge capacities of 1079 mAh g−1 (at a constant current of 40 mA g−1 ) and 1002 mAh g−1 after 50 cycles. The capacity retention of the synthesized graphene nanosheets is 1070 mAh g−1 at a current of 40 mA g−1 after the rate capability test, and their rate capability is 463 mAh g−1 at a current of 400 mA g−1 . The morphologycontrolled graphene nanosheets prepared by the modified-improved method shows better discharge performance compared to graphene prepared by the improved method. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Lithium rechargeable batteries require increasingly high power and energy density owing to the development of high-performance portable electric devices, electric vehicles, and energy-storage systems. To meet these challenges, an innovative material is needed. Graphite is widely used as an anode material for lithium-ion batteries because of its high columbic efficiency and acceptable specific capacity by forming intercalation compounds (LiC6 ) [1–3]. However, graphite has theoretically a Li storage capacity of ∼370 mAh g−1 by the limited Li-ion storage sites within the sp2 carbon structure [4–7]. In order to enhance its energy and power density, another anode material has been desired. One of the candidates is Si- and Sn-based materials. Silicon and its alloys have been studied

∗ Corresponding author. Tel.: +82 2 2123 2839; fax: +82 2 312 5375. E-mail addresses: [email protected] (K.-B. Kim), [email protected] (S.-H. Yeon). 1 Tel.: +82 42 860 3618; fax: +82 42 860 3133 http://dx.doi.org/10.1016/j.electacta.2014.03.078 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

as alternative high-capacity anode materials to graphite but these materials have severe problems such as volume expansion during charging-discharging [8–13]. Another potential candidate is graphene. Graphene is considered a potential alternative material of graphite as an anode in lithium-ion batteries [14–17]. It has been receiving intense attention recently because it has a single twodimensional (2D) atomic layer (sp2 carbon configuration and – complexation) in a honeycomb lattice. Graphene also has many advantages, such as high electronic conductivity, large surface area, and high mechanical strength [18–26]. In addition, graphene with high reversible capacities ranging from 500–1200 mAh g−1 for Li storage capacities has been reported [27–30]. The graphene nanosheets obtained from various fabrication routes have already been utilized as the anode materials for lithium ion batteries [14–17,27,28]. The representative methods for graphene synthesis are microcleavage, chemical vapor deposition, and wet chemistry [31–33]. Microcleavage and chemical vapor deposition can produce high-quality graphene, but the yield is very low. The wet chemistry methods are inexpensive and advantageous for electrochemical

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Table 1 Preparation of graphite oxide by improved-method and modified improved method. Wook Ahn el al. Sample name

GOI GOK RGOI RGOK

Precursor

Commercial graphite Commercial graphite GOI GOK

Oxidant

H2 SO4

H3 PO4

H2 O2

360 mL 350 mL None None

40 mL 50 mL None None

3 mL 20 mL None None

device applications which require mass production because the final product can be in the form of a powder [34]. This synthesis method has critical drawbacks, however, such as low quality of the graphene produced [35]. The Hummers method is generally considered a typical wet chemistry method for producing graphite oxide, using a mixture of sulfuric acid (H2 SO4 ), sodium nitrate (NaNO3 ), and potassium permanganate (KMnO4 ). This method is still widely used, often with a modified version of it [33–36]. However, the graphene prepared by Hummers method or modified Hummers method have exhibited multi-layered graphene of low quality by lots of stacks. Generally, these low quality graphene nanosheets can result from low oxidation degree of the graphite oxide in conventional Hummers method. In our previous report, the “modified-improved method” for graphite oxide synthesis showed the enlarged interlayer spacing (d-spacing) using phosphoric acid as a second acid with first sulfuric acid [37,38]. The addition of phosphoric acid to this reaction produced RGO (reduced graphite oxide) with more intact graphitic basal planes indicating graphite oxide with fewer defects in the basal plane, as compared to graphite oxide prepared by the Hummers method. Also, the protocol for running the reaction does not involve a large exotherm and produces no toxic gas. Moreover, the improved method yields a higher fraction of well-oxidized hydrophilic carbon material [39]. In this study, we focus on the strategic synthesis of morphologycontrolled graphene nanosheets by using simple post-process treatment (ultra-sonication), and the morphology of graphene nanosheets from graphite oxide having high degree of oxidation prepared optimized reaction process could be easily controlled. Also, we demonstrate significantly improved electrochemical capacity of the prepared graphene as an anode electrode for Li ion battery application.

Post-treatment (ultra-sonication)

Microwave susceptor

Microwave condition for RGO

None None 30 min 30 min

None None Acetylene black Acetylene black

None None 90 sec 90 sec

mixture was transferred to the ice bath, and the flask was slowly filled with 400 mL of de-ionized (DI) water to accommodate the highly exothermic nature of the reaction. Next, 20 mL of H2 O2 was added and the color of the mixture turned to bright yellow and generated many bubbles. After the reaction began, the mixture was stirred for 48 h to complete the reaction. The resulting paste was centrifuged at 8000 rpm for 5 min. The remaining solid paste was washed five times with a mixture containing 100 mL of H2 O, 100 mL of hydrochloric acid (37%, ACS reagent, Sigma-Aldrich), and 100 mL of ethanol solution. It was then washed with DI water until the pH reached 5. Centrifugation at 8000 rpm was conducted for each washing cycle. After the final washing step, the product was first frozen and then dried overnight in a vacuum freeze dryer at -55 ◦ C. Details of the fabrication process for GOI are the same as that for GOK except for the amount of oxidant (H2 O2 : 3 mL, H2 SO4 : 360 mL, H3 PO4 : 40 mL) and the reaction time (24 h after adding of H2 O2 ).

2. Experimental 2.1. Preparation of graphite oxide We refer to the prepared graphite materials as GOI, GOK, RGOI, and RGOK, as shown in Table 1. In order to prepare graphene nanosheets, graphite oxide is first produced by a technique developed in our previous research called the “modified-improved method” [37,38]. The resulting graphite oxide is called “GOK.” This procedure is modified to optimize the reaction condition from the “improved method” (Marcano et al. [39]), which produces graphite oxide we refer to as “GOI.” The fabrication process for GOK is as follows: 3 g of graphite (<45 ␮m, ≥99.99%, Sigma-Aldrich) powder was mixed in a round-bottomed flask with 350 mL of sulfuric acid (95.0–98.0%, ACS reagent, Sigma-Aldrich) and 50 mL of phosphoric acid (≥85 wt% in H2 O, ACS reagent, Sigma-Aldrich), as a solvent, in an ice bath. When the temperature dropped below 10 ◦ C, 18 g of potassium permanganate (99.3%, Samchun Chemical) was gradually added to the mixture. Then, the mixture was transferred to a heating mantle to provide an oxidation process with isothermal conditions at 45 ◦ C and maintained for 12 h before it was cooled to room temperature. When the oxidation procedure was over, the

Fig. 1. XRD patterns of (a) graphite oxide (GOI and GOK) and (b) reduced graphite oxide (RGOI and RGOK) using microwave-induced thermal reaction.

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Table 2 XRD results of GOI, GOK, RGOI and RGOK including interlayer d-spacing (nm). Wook Ahn et al. sample

GOI GOK RGOI RGOK

(002) plane 2␪ (◦ )

Lc (nm)

d-spacing (nm)

Number of layers

10.14 9.76 24.1 23.6

35 39 2.3 1.5

0.87 0.92 0.37 0.38

40.3 42.3 6.2 3.9

2.2. Preparation of reduced graphene nanosheets The graphene nanosheets, RGOK and RGOI, were prepared from graphite oxide, GOK and GOI, respectively, using a microwaveinduced thermal reduction process: 1.0 g of graphite oxide was mixed with 0.1 g of acetylene black (microwave susceptor) using an agate mortar. The mixture was transferred to a microwave reactor and the reduction procedure was carried out at a power of 750 W for 2 min. After vigorous thermal reduction, the color of the product changed from yellow to black. The volume of the graphene nanosheets also expanded enormously. When the reduction was complete, the graphene powder was dispersed in DI water. Then, in order to alter the product to morphology-controlled graphene nanosheets, ultra-sonication was carried out at energy of 100 kJ. After the energy-induced process, the graphene powder was dried overnight in a vacuum chamber at room temperature. The acetylene black, which was used as microwave susceptor for the graphene nanosheets, was not removed after the reduction process because of a limitation in removal process. The acetylene black included in the graphene acts as only a conducting agent when the garphene nanosheets are applied to anode electrode as active material in Li ion battery. 2.3. Characterization: physical and electrochemical properties To characterize the distance of interlayers (d-spacing) from the (002) plane, X-ray diffraction (XRD) experiments were carried out with an automated HPC-2500 XRD diffractometer (Gogaku)

using Cu K˛ radiation ( = 0.15405 nm) in the 2 value range of 5–60◦ in steps of 0.02◦ . The morphologies of the synthesized composites were analyzed using field-emission scanning electron microscopy (SEM; S4700, Hitachi). The specific surface area determination of prepared materials was performed using the Brunauer–Emmett–Teller (BET) method with an adsorptionmeter (BELSORP-max, BEL Japan Inc). Field-emission transmission electron microscopy (TEM; JEM-1010, JEOL Ltd.) was carried out to confirm the microscopic images of RGOK (morphology-controlled graphene) and RGOI. To ascertain each bonding group and verify the oxidation state of the C–C bond and the C–O or C = O bonds, X-ray photoelectron spectroscopy (XPS) was conducted (MultiLab 2000, Thermo). The anodes were prepared by dissolving 10 wt.% of Super P® conductive carbon black (TIMCAL Graphite and Carbon), 10 wt.% of the copolymer of vinylidene fluoride and hexafluoropropylene (PVDF-co-HFP; Kynar® 2801, Arkema), as a binder, and 80 wt.% of the prepared graphene nanosheet in N-methyl-2-pyrrolidone (NMP) until a slurry was obtained. As mentioned before, the prepared graphene nanosheet already includes the conducting material, 8.3 wt% acetylene black as microwave susceptor. Therefore, the anodes are really composed of 73.4 wt% of active material (graphene sheets), 16.6 wt% of conducting agent, and 10 wt% of binder, respectively. The slurry was coated on a piece of Cu foil (20 ␮m) using a doctor blade and dried at 70 ◦ C under vacuum conditions. Finally, the electrode was pressed with a twin-roller. To investigate the electrochemical properties, CR2032 coin-type cells were tested. Lithium foil was used as the counter electrode and

Fig. 2. SEM images of (a), (b) graphite; (c), (d) graphite oxide; (e), (f) graphite oxide prepared using the improved method (GOI) and the modified-improved method (GOK), respectively.

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Fig. 3. TEM images of (a) RGOI (200 nm) and (b) RGOK (200 nm) before ultra-sonication. After ultra-sonication, (c) RGOI (200 nm) and (d) RGOK (200 nm).

1.0 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC; v/v 1:1) was used as the electrolyte. The entire procedure for the coin cell assembly was performed in an Ar-filled glove box, and the cells were both charged and discharged at 40 mA g−1 . The C-rate test was also conducted using a Maccor 4000 battery test system and the rate range was 40–400 mA g−1 .

3. Results and discussion 3.1. Structure features and physical properties The XRD patterns of graphite oxide using the modifiedimproved method (GOK) and the improved method (GOI) are

Fig. 4. TEM after sonication (a), (b); Enlarged images of RGOK (10 nm and 2 nm). Enlarged images of flattened center and honeycomb lattice of morphology-controlled RGOK (1 nm).

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Fig. 5. XPS C1s spectra of (a) GOK prepared using the modified-improved method; (b) GOI prepared using the improved method; (c) RGOK and RGOI reduced using microwave-induced thermal reaction.

shown in Fig. 1(a). The interlayer d-spacing between the graphitic layers should be the ideal indicator of each type of graphite oxide’s degree of oxidation. It is known that the oxygen-containing functional groups (the hydroxyl, epoxy, and carbonyl groups) on and between the graphite layers enlarge the interlayer spacing and turn the graphite oxide to an sp3 configuration. Therefore, a larger d-spacing should indicate a higher degree of oxidation of the graphite oxide. The d-spacing was calculated from Braggs’ law (n = 2d sin ), and the number of layers was calculated from average crystallite (La ) of the Scherrer–Debye Equation (La = ˇ0.89 ). cos  Generally, the distance between the graphite layers (graphenes) is 0.335 nm, in which the number of layers on the prepared graphene sheets (RGOI and RGOK) can be estimated based on the grapheme distance (0.335 nm) and average crystallite (La). The calculated interlayer spacing of GOK was 0.92 nm, which is higher than the value, 0.87 nm, for GOI (Table 2). It means that GOK produced by the modified-improved method had a higher oxidation state than GOI produced using the improved method. GOK and GOI were reduced using a microwave-induced thermal reduction process to form RGOK and RGOK, respectively, as described in section 2.2. The XRD results of reduced graphite oxide are shown in Fig. 1(b). Both of them show broad amorphous peak and RGOK had lower intensities in its XRD data compared to that of RGOI, and RGOK had fewer layers than RGOI. The number of layers was 3.9 in RGOK and 6.2 in RGOI (Table 2), probably because the functional groups on the surface and in the interlayers of RGOK could be removed more easily than RGOI since the number of oxygen-containing functional groups in the interlayer enlarged the interlayers of graphite oxide. From this result, we can deduce that RGOK was more easily exfoliated and reduced than RGOI.

Fig. 2a (b) shows the SEM images of graphite. The particle size was about 10–20 ␮m and the particles were in the form of flakes. Fig. 2(c), (d) and (e), (f) show GOI and GOK, respectively. Fig. 2 reveals that the thickness of the GOK layers prepared by the modified-improved method was lower than that of the GOI layers prepared by the improved method. The thickness of a layer of GOI was about 17 nm and the thickness of a layer of GOK was 12 and 14 nm, indicating that the more oxidized graphite oxide consisted of fewer layers and had more exfoliated layers. The BET characterization of RGOK and RGOI was conducted to measure the surface area. The BET specific surface area (BET SSA) of RGOK (728 m2 g−1 ) is higher than that of RGOI (443 m2 g−1 ). The RGOK possesses much larger BET SSA, indicating that the average particle size of RGOK is smaller than RGOI. The graphite oxide (GOK) existing in a high oxidation state could be easily exfoliated than GOI of a low oxidation state. Figs. 3 and 4 show the TEM images of reduced graphite oxide (RGO) prepared from GOI and GOK. The reduced graphene oxide formed from the oxidation and reduction process had a wrinkled shape. When used in a battery system, the wrinkles could affect the electrical conductivity because graphene has a single 2D atomic layer (with sp2 carbon configuration and – complexation) in a honeycomb lattice, and electricity can be conducted through the – bond. Therefore, the wrinkling could have been adversely affecting the electrical conductivity. In order to enhance the electrical conductivity, the wrinkles should be flattened. Ultra-sonication treatment on the prepared graphenes greatly influenced the morphology of RGOI and RGOK, as shown in Fig. 3. After reduction of the graphite oxide, we induced ultra-sonication at energy of 100 kJ to the RGOK, and the morphology of the graphene nanosheets

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Fig. 6. Charge-discharge profiles of (a) RGOI and (b) RGOK. (c) dQ/dV curve of RGOK. (d) Comparison of cycle performance of RGOI and RGOK.

Table 3 O/C ratios of GOI, GOK, RGOI and RGOK from XPS C1s result. Wook Ahn et al. O/C ratio

C-O/C-C

sample GOI

GOK

RGOI

RGOK

2.51

8.27

0.149

0.107

was changed. Fig. 3 shows RGOI and RGOK before and after ultrasonication. While there were no changes in the RGOI morphology of the wrinkles, the wrinkles in the center of the RGOK nanosheets were flattened along with rolling the ends of the nanosheets such as multi-walled carbon nanotubes (MWCNT). It is thus proposed that the edge side of a graphene nanosheet could roll up while the energy is imparted to the graphene nanosheet. The rolled edge side of the graphene nanosheet could spontaneously pull the wrinkled center of the nanosheet until it is flattened. However, RGOI had a thicker layer than RGOK and this thick layer has some difficulty with forming the rolled edge of the graphene nanosheet. This means that the wrinkles of RGOK could be easily transformed rather than those of RGOI. The enlarged images of the flattened sheet of RGOK is shown in Fig. 4(a), (b) and (c), which confirms that a honeycomb lattice was obtained, and morphology-controlled graphene has the potential to be applied in the manufacturing of composites through simple techniques such as wrapping graphene with metal oxide. The XPS results are presented in Fig. 5 and Table 3. From the binding energies of the XPS result, we were able to confirm the presence of each bonding group [40–45]. The degree of oxidation of graphite oxide was determined from the area ratio between the sum of the peak areas for C–C, C = C and the sum of the peak areas for C–O, C = O. The dominant oxygen-containing functional groups in graphite oxide are sp3 carbon, namely C–O, epoxides, carbonyls (C = O), and hydroxyl groups (C–OH). In the XPS spectrum of the

sample of graphite oxide (Fig. 5), the peaks of the carbon–oxygen groups had higher intensity than that of the carbon–carbon group. However, depending on the graphite oxide (GO), the ratio of oxygen to carbon (O/C) varied. The results in Table 3 show that the O/C ratio of GOK was more than three times higher than that of GOI, which means that the degree of oxidation of GOK was higher, implying that there were more functional groups between the interlayers on the GOK surface. As shown in Fig. 5(c) and (d), the quantitative fractions (ratios of C–O bonds to C–C bonds) of RGOK and RGOI were 0.149 and 0.107, respectively, confirming that RGOK reduced from GOK had a lower degree of oxidation than RGOI reduced from GOI. 3.2. Electrochemical properties In order to compare the electrochemical performances of prepared materials, we carried out charge–discharge and rate capability tests of coin-type cells using RGOI and RGOK as electrode materials. It is generally known that the first discharge capacity of a cell is related to the formation of the solid–electrolyte interface (SEI) [14]. Thus, the second cycle is considered the initial capacity. Further details can be obtained from an analysis of the dQ/dV curves. Fig. 6(a) and (b) show the charge–discharge profiles of RGOI and RGOK. Graphene (RGOK) prepared by the modifiedimproved method had better discharge performance than RGOI. The initial discharge capacity of RGOI was 906 mAh g−1 and that of RGOK was 1079 mAh g−1 at a constant current density of 40 mA g−1 , which corresponds to 0.1 C-rate based on the theoretical capacity of graphite (380 mAh g−1 ). The capacity of RGOI decreased dramatically within 5 cycles and its capacity retention declined significantly (74% after 50 cycles) as the cycling test continued. However, the prepared graphene (RGOK) exhibited good

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graphene by post-process treatment (ultra-sonication). In the XRD result, the graphite oxide synthesized by the modified-improved method (GOK) had a higher d-spacing than that prepared by the improved method (GOI). Additionally, its reduced graphite oxide, RGOK, had XRD data with lower density, and it consisted of fewer layers than RGOI. The XPS data showed the degree of oxidation as well as the C/O ratios of GOK, GOI and RGOK, RGOI. The TEM and SEM analyses confirmed the microscopic images and the thickness of graphene and morphology-controlled images after modifiedimproved method and ultra-sonication process. The anode material prepared using the modified-improved method (RGOK) exhibited excellent battery performances. The initial discharge capacity of RGOK was 1079 mAh g−1 at a constant current density of 40 mA g−1 and exhibited good cycleability of 1002 mAh g −1 after 50 cycles with good capacity retention of 93%. It was thus concluded that the morphology-controlled RGOK, prepared from the GOK synthesized by the modified-improved method, is a promising anode material for rechargeable lithium-ion batteries. In addition, morphology-controlled graphene has the potential to be applied in the manufacturing of composites via simple procedures such as wrapping graphene with metal oxides. Acknowledgement This research was financially supported by the Ministry Of Trade, Industry & Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT) through the Inter-ER Cooperation Projects. References

Fig. 7. Rate capability of (a) RGOI and (b) RGOK.

cycleability of 1002 mAh g −1 after 50 cycles and its capacity retention was 93%. In the dQ/dV curve (Fig. 6(c)), RGOK displayed two wide and sharp peaks at the first cycle. The reduction steps above and around 1.0 V were generated from the reaction of the residual oxygen (contained in graphene’s functional groups) with lithium. At a plateau below and around 0.5 V, the high irreversible capacity of the graphene electrode in the first cycle could be explained by the formation of an SEI film. The SEI film formation consumed many lithium ions in the first cycle, leading to a high capacity loss [14]. In addition, the irreversible peak disappeared after the first cycle. Fig. 7 shows the rate capability of RGOI and RGOK. RGOK and RGOI exhibited discharge capacities of 437 mAh g−1 and 351 mAh g−1 , respectively, at a current density of 400 mA g−1 . They showed capacity retentions of 1036 mAh g−1 in RGOK and 786 mAh g−1 in RGOI, respectively, at a current density of 40 mA g−1 after the rate capability test. It means the capacity of RGOK is recovered after the rate capability test when compared with the first cycle. In addition, RGOK had better cycle performance than RGOI. These results all indicate that graphene RGOK had fewer layers and fewer functional groups than RGOI on the surface or in the interlayer, and it displayed good electrochemical properties. 4. Conclusion Graphene nanosheets were manufactured as anode material for lithium-ion batteries using a simple modified-improved method and a microwave reduction process. Morphology-controlled and wrinkle-free graphene was also easily manufactured from prepared

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