Pyrolytic carbon derived from coffee shells as anode materials for lithium batteries

ARTICLE IN PRESS

Journal of Physics and Chemistry of Solids 68 (2007) 182–188 www.elsevier.com/locate/jpcs

Pyrolytic carbon derived from coffee shells as anode materials for lithium batteries Yun Ju Hwang, Soo Kyung Jeong, Kee Suk Nahm, Jae Sun Shin, A. Manuel Stephan1 School of Chemical Engineering and Technology, Chonbuk National University, Chonju 561-756, S Korea Received 30 August 2006; received in revised form 7 October 2006; accepted 17 October 2006

Abstract Disordered carbonaceous materials have been obtained by pyrolysis of coffee shells at 800 and 900 1C with pore-forming substances such as KOH and ZnCl2. X-ray diffraction studies revealed a carbon structure with a large number of disorganized single layer carbon sheets. The structure and morphology of the materials have been greatly varied upon the addition of porogens. The prepared carbon materials have been subjected to cycling studies. The KOH-treated products offered higher capacity with improved stability than those with untreated and ZnCl2-treated one. r 2006 Elsevier Ltd. All rights reserved. Keywords: C. Electron microscopy; C. X-ray diffraction; D. Electrochemical properties; D. Microstructure

1. Introduction Lithium metal is found to be an attractive anode material for lithium secondary battery that provides a larger specific capacity of 3800 mAh g 1 which is about 10 times higher than that of carbon-based anode (372 mAh g 1) with a composition of LiC6 [1,2]. However, the cycle life of lithium metal secondary cells is very short due to the low cycling efficiency of lithium metal anode as it reacts with both aprotic and protic solvents at its surface [3–6]. Many reasons have been offered for this poor cycling, which include the electrochemical reactions between the anode and the electrolyte and loss of electronic contact between the electrode and dentritic lithium [7–9]. Nevertheless, graphitic carbons have a theoretical specific capacity of a tenth of lithium metal, lithium-ion batteries with carbon as anode materials have been commercialized one decade ago. Furthermore, their application is being gradually extended to the transport sector where they are envisaged as the sole power source and in hybrid modes Corresponding author. Fax: +82 63 270 2306.

E-mail address: [email protected] (K.S. Nahm). On leave from Central Electrochemical Research Institute, Karaikudi 630 006, India. 1

0022-3697/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.10.007

with fuel cells and super capacitors. Therefore, relatively cheap electro-active materials with high-rate capability materials are required for high-power lithium ion batteries. The development of disordered carbon has drawn the attention of many researchers due to it appealing properties such as, (i) higher lithium intake than the theoretical limit of 372 mAh g 1 for perfectly graphitic materials; (ii) good cycling properties and (iii) dependency of their structural properties with their organic precursors, pyrolysis temperature and soaking time. Kim et al. [10], studied the microstructural and electrochemical properties of sulfuric acid-treated poly (acrylonitirle) PAN-based carbon anode. In a similar study, Fey et al. [11] examined the compositional, structural and electrochemical characteristics of carbonaceous products obtained from acrylonitrile-butadiene-styrene (ABS) terpolymer precursors. Nevertheless, the structural and other properties carbonaceous materials change with the nature of the precursors, the carbonaceous materials obtained with polymer precursors are expensive. Reports are also available on the carbonaceous materials obtained from biomass precursors. Fey et al. [12–14] examined the electrochemical properties of carbon materials derived from sugar [12] and rice husk [13]. Studies have also been made on other natural precursors like sugar [14–17] walnut and almond shells [18],

ARTICLE IN PRESS Y.J. Hwang et al. / Journal of Physics and Chemistry of Solids 68 (2007) 182–188

peanut shells [19], lignin, cotton and wool [20], starch and oak [18]. In the present study, we report lithium- insertion properties of pyrolytic carbons obtained from coffee shells treated with KOH or ZnCl2 as a porogen.

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2. Experimental procedure

3. Results and discussions 3.1. Thermal analysis The TG curves recorded with the porogen-treated and untreated coffee shell samples between room temperature and 800 1C are depicted in Fig. 1. The weight loss observed around 100 1C in Fig. 1a is due to the loss of superficial moisture from the shells. Around 200–300 1C a major weight loss was observed associated with the destructive distillation of the shells to yield low-volatile organics. Further the weight loss continues until 450 1C which is attributed to the loss of volatile vapors and aromatic condensation processes which are part of the pyrolytic reactions. The decomposition of the precursor is completed around 450 1C and that any change that occurs beyond this temperature is attributed to carbon layer organization [11].

80

Weight (%)

Good quality dry coffee shells were purchased from the local sources. The shells were ground into a fine powder fibers and then treated with concentrated solutions of ZnCl2 or KOH for 5 days at room temperature at a shell: porogen ratio of 1:5 by weight, and dried. Pyrolysis was carried out by heating under flowing nitrogen at 800 and 900 1C at a heating ramp of 10 1C min 1 for a hold period of 1 h. Pyrolytic carbons were also obtained without treatment with the porogen, in which case, the fibers simply heat-treated carbons under the above conditions. Elemental analysis was done by a Perkin-Elmer CHN 2000 elemental analyzer. Powder X-ray diffraction patterns were recorded between 101 and 801 on a X-ray diffractometer, model (Rigaku D/Max 2500) fitted with a nickel-filtered Cu-Ka radiation source. The morphologies of the pyrolytic carbons were examined by a (FESEM S-4700, Hitachi) scanning electron microscope. BET surface area measurements were carried out on a (MicromeriticsASAP2010) surface area analyzer. Carbon electrodes for electrochemical lithium insertion studies were prepared by bladecoating a slurry of 90 wt% of the pyrolytic carbon, 8 wt% of poly(vinylidene fluoride) and 2 wt% carbon black of dispersed in N-methyl-2-pyrrolydone on a copper foil, followed by drying at 110 1C in an air oven, rollerpressing the dried sheets, and punching out circular sheets. The carbon electrodes were coupled with lithium (Cyprus Foote Minerals) with an electrolyte of 1 M LiPF6 in a 1:1 (v/v) mixture of EC-DMC in 2032 coin cells in an argonfilled glove box (OMNI- LAB system). Galvanostatic charge–discharge profiles were made between 2.500 and 0.002 V on a computer-controlled battery testing unit (BTS 2004, Japan).

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60

b

c

40

a 20

0 100

200

300

400

500

600

700

800

Temperature (°C) Fig. 1. TG- traces of precursors: (a) coffee shells without porogen; (b) treated with KOH; (c) treated with ZnCl2.

On the other hand, the thermal behavior of the porogentreated coffee shells differs from the untreated one. Major weight loss was observed even before 200 1C which may be associated with the decomposition of porogenic material (Fig. 1b and c). The subsequent carbonization starts at 500 and 650 1C, respectively, for the samples with KOH and ZnCl2 as porogen. 3.2. SEM analysis Fig. 2 depicts the SEM micrographs of coffee shells obtained with and without porogen treatment. The porogen-free carbon shows a flake-like structure (Fig. 2a). On the other hand, the carbon upon treatment of the shells with KOH seems to have small particle size (Fig. 2b). More interestingly, the ZnCl2-treated fiber carbons have a loose, disjointed structure without any particular shape (Fig. 2c). However, the particle size of these carbon materials has not been reduced like KOH-treated coffee shells. A similar trend was observed for the samples pyrolyzed at 900 1C (Fig. 2d–f). 3.3. Surface area studies Table 1 displays the values of average pore diameter and BET surface area of the pyrolytic carbons. It can be seen from the Table 1 that the surface area and the pore diameter have been greatly varied due to the influence of the porogens. For example, the pore diameter increased

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Fig. 2. SEM micrographs of pyrolytic carbons derived from: (a) untreated coffee shells; (b) shells treated with KOH; (c) shells treated with ZnCl2 at 800 1C (d) untreated coffee shells; (e) shells treated with KOH; (f) shells treated with ZnCl2 at 900 1C.

Table 1 The BET surface area, pore diameter and H/C ratio of the carbon materials pyrolyzed at 800 and 900 1C Pore diameter (A˚)

H/C ratio

156.2

27

39

123.6

24

32

Coffee shells (800 1C) (KOH treated)(900 1C)

2589.1

72

38

2457.2

68

28

Coffee shells (800 1C) (ZnCl2 treated)(900 1C)

172.3

35

29

165.9

32

25

BET surface area (m2g 1) Coffee shells (800 1C) (untreated) (900 1C)

from 24 A˚ for the untreated fiber carbon to 68 and 32 A˚ for the ZnCl2 and KOH treated products, respectively. Subsequently, the BET surface area was also increased from 107.9 to 165.9 and 2457.2 m2g 1 for the ZnCl2 and KOH treated, respectively. The pore size has been increased nearly three- fold, while the surface area increased 1.5 (ZnCl2-treated) and 19.9 (KOH-treated) times because of the action of the porogens. The microstructural evolutions can significantly influence the electrochemical characteristics of the carbons.

It is possible that the heat generated during the decomposition of the porogen or any chemical reaction of the products of decomposition of the porogen with the shells might have altered the chemical composition of the carbonaceous products. Porosity in carbonaceous materials can be generated by use of several chemical activation agents such as Na2CO3, K2CO3, NaOH, KOH, H3PO4, AlCl3, MgCl2, LiCl and ZnCl2 are employed. In this work we have used KOH and ZnCl2 for activation. Despite the large volume of literature on chemical activation, the mechanisms of the processes are yet to be fully established [21–29]. It is, however, generally believed that ZnCl2 acts as a dehydrating agent [23,30,31] and, upon carbonization, leads to charring and aromatization of the carbon skeleton and creation of pore structure [23,30]. ZnCl2 at high concentrations is known to give Bronsted acidity to the activation solution, and to dissolve cellulosic constituents of biomass [28]. In the case of KOH, the oxygen in the alkali removes the cross-linking and stabilizes the carbon atoms in crystallites [31]. Metallic potassium obtained at the pyrolysis temperature may intercalate and force apart the lamellae of the crystallites [31]. Removal of metallic potassium by, simply washing with water creates microporosity in the new structure [31,32]. The activation process involves the oxidation of carbon and gasification. According to Hsu and Teng [33], activation with KOH results in lower carbon yields than that with ZnCl2. However, the porosity of the carbon products would be higher with KOH than with ZnCl2. They suggested that ZnCl2 being acidic, was not

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Intensity (a.u)

suitable for preparing high-porosity carbons, while bases like KOH should yield carbons with very high porosity. However in the present study both porogens have been employed to compare their role in the surface area and electrochemical studies. The H/C ratios are smaller than those of the products derived from untreated coffee shells. The variation of the elemental analysis of the carbon materials is rather difficult to explain. According to Fey et al. [17] that heat generated during decomposition of the porogen or any chemical reaction of the porogens with the shells might have altered the composition of the final product.

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3.4. XRD analysis

3.5. Charge–discharge studies Fig. 4 shows the first galvanostatic charge–discharge (0.2 C rate) curves obtained for the carbons derived from the untreated and porogen-treated coffee shells at 0.2 C— rate at 30 1C. The first lithium insertion capacity of 524 and 603 mAh g 1 has been obtained for the untreated samples pyrolyzed at 800 and 900 1C respectively (Fig. 4a and d). However, only 250 and 300 mAh g 1 equivalent of lithium was retrievable. This corresponds to an irreversible capacity of about 50%. However, in the subsequent cycles the coulombic efficiencies were 90% or more and reaching

10

20

30

40

50

c

R = 1.49

b

R = 2.47

a

60

70

80

2θ degree

Intensity (a.u)

Fig. 3(a–f) displays the X-ray diffractogram for the pyrolytic carbons obtained with and without porogens at 800 and 900 1C. The typical characteristic peaks that appear at (0 0 2) and (1 0 0) increase with the increase of temperature. The reflections at (1 0 0) around 431 indicate the presence of honeycomb structures formed by sp2 hybridized carbons. The broad reflections of (0 0 2) between 201 and 301 indicate the small domains of coherent and parallel stacking of the graphene sheets. As indicated by Liu et al. [34], the quantity of single layers in carbon materials pyrolyzed at low temperatures can be estimated from the empirical R-factor. Generally R-factor value indicates the concentration of non- parallel single layers of carbon and in the present study, the value of R has been found to be o2 for the untreated and ZnCl2-treated one. This R-factor value increases with the increase of temperature which indicates that at higher temperatures the single layers become mobile resulting in their parallel orientation with respect to one another. Subsequently this single carbon layers align themselves in to small domains of ordered structures, the carbon tends to be increasingly graphitic in nature [17]. As discussed by Fey et al. [17] the porogens do not alter the crystallographic parameters of the products however, an increase in the d002 values has been observed for carbons derived from the porogen-treated coffee shells which is attributed to the turbostratic disorder arises due to the evolution of gaseous decomposition products which in turn increases the average values for the d002 spacings.

R = 2.76

10

20

30

40

50

R = 2.85

f

R = 1.94

e

R = 2.59

d

60

70

80

2θ degree Fig. 3. X-ray diffractograms of the carbon materials prepared at different pyrolysis temperature. Coffee shells (a) untreated (b) KOH-treated shells (c) ZnCl2-treated at 800 1C (d) untreated (e) KOH-treated shells (f) ZnCl2treated 900 1C.

100% at fourth cycle. Interestingly, on the other hand the first lithium insertion capacity of 1150 and 1200 mAh g 1 has been observed for the KOH-treated coffee shells pyrolyzed at 800 and 900 1C (Fig. 4b and e), respectively. An irreversible capacity of about 56% and 62% has been observed during the first cycle. A similar trend was observed for the ZnCl2-treated shells (Fig. 4c and f). However, the specific capacity has not been improved much as observed for KOH-treated carbonaceous materials. These results are in accordance with the BET- surface area results. Fig. 5 illustrates the variation of capacity as a function of cycle number for the carbonaceous materials pyrolyzed at 800 and 900 1C. It is quite obvious from the figure that the capacity fade of carbon materials depends on the pyrolysis temperature and the nature of the porogen. The large loss

ARTICLE IN PRESS Y.J. Hwang et al. / Journal of Physics and Chemistry of Solids 68 (2007) 182–188

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e

c 3

3

2

2

2

1

Voltage (V)

3

Voltage (V)

Voltage (V)

a

1

0

0

0 0

200

400

600

800

0

1000 1200

200

Capacity (mAh/g)

400

600

800

1000 1200

0

2

2

2

200

400

600

800

1000 1200

Voltage (V)

3

Voltage (V)

3

0

600

800

1000 1200

f

d

1

400

Capacity (mAh/g)

3

0

200

Capacity (mAh/g)

b

Voltage (V)

1

1

0 0

200

Capacity (mAh/g)

400

600

800

Capacity (mAh/g)

1000 1200

1

0 0

200

400

600

800

1000 1200

Capacity (mAh/g)

Fig. 4. Charge-discharge characteristics of carbon materials prepared at different pyrolysis temperature. Coffee shells (untreated) (a) 800 1C (b) 900 1C; KOH treated (c) 800 1C (d) 900 1C; ZnCl2-treated (e) 800 1C (f) 900 1C.

1400 (a) (b) (c) (d) (e) (f)

Discharge capacity (mAh/ g)

1200 1000 800 600 400 200 0

0

5

10

15

Cycle number Fig. 5. Capacity as a function of cycle number for the carbonaceous materials. Coffee shells (a) untreated (b) KOH-treated (c) ZnCl2-treated at 800 1C. Coffee shells (d) untreated (e) KOH-treated (f) ZnCl2-treated at 900 1C.

in capacity means that a significant part of the lithium is unavailable for reversible reactions, rendering a significant part of the active material deadweight. Moreover, since the

primary source of lithium is the lithiated cathode, low anode efficiency means inability to completely re-lithiate the cathode. The coulombic efficiencies in the subsequent cycles gradually increase and stabilize to nearly 100%. In a couple of cases, the deintercalation capacities are slightly larger than the amount of lithium inserted in the previous charging. It is believed that some of the lithium inserted goes into deep sites in the carbon matrix, which renders them irretrievable. However, the larger deinsertion capacity suggests that there is a constant realignment of the single layer carbon layers, during which some of the inserted lithium get ‘‘exposed’’ and become available for the electrochemical cycling. The high first-cycle insertion and irreversible capacities are attributed to: (i) high H/C ratios (Table 1) [25,26]; (ii) presence of a large number of nanopores [27–29]; (iii) large surface areas with reactive functional groups that provide ample sites for passivation. Hydrogen-containing carbons exhibit large capacities proportional to the hydrogen content; they also exhibit large hysteresis in their chargedischarge profiles [25,26,35]. Hydrogen in the carbons is believed to ‘‘saturate’’ the dangling bonds on the edge carbon atoms, generating polyaromatic hydrocarbons [21]. Dahn et al. [36,37] attributed the large hysteresis in hydrogen-containing carbons to (H–C)–Li bridging, by

ARTICLE IN PRESS Y.J. Hwang et al. / Journal of Physics and Chemistry of Solids 68 (2007) 182–188

which a two-coordinated edge carbon would transform from the sp2 to sp3 hybridized form. As we have seen earlier, these disordered carbons have a predominance of unorganized single carbon layers, which can provide sites for accommodating lithium during charge. According to Dahn’s ‘‘falling cards’’ model [38], at low pyrolytic temperatures at which these carbons are formed, the thermal energy insufficient to rotate graphene sheets into parallel alignment and into stacks. Thus, low-temperature carbons have a large number of non-parallel, unorganized single layers of hexagonal carbons, a fact reflected in the low values of the R parameter. According to Mabuchi et al. [39], the ‘‘excess’’ capacity is due to lithium occupying sites inside nanopores that are present between layers in disordered carbons from which it is often difficult to retrieve the inserted lithium. Such pores facilitate insertion of large amounts of lithium, part of which is lost due to extensive passivation [40,41]. The products of passivation gradually clog the openings of the pores and cavities, rendering the lithium trapped in the pores unavailable for further cycling. The large first-cycle insertion capacity realized with the carbon produced from the ZnCl2-treated fiber is commensurate with the large surface area of the carbon. However, the large surface area can also expose a large surface for passivation. Additionally, the large pores that are generated in this carbon do not serve as lithiumaccommodating cavities. Instead, they allow access to electrolyte, which can in turn passivate the pore surface, raising the irreversible capacity. Unlike the SEI, clogging of the pores must necessarily involve formation of pores into thick layers, which impede electrolyte access to the active carbon surface. Thus, the masking of the carbon surface would lead to a lower retrieval of stored lithium in the carbons. It is thus clear that large pores are pejorative to carbon-based anodes. Such pores facilitate simultaneous interaction of lithium with surface functional groups and plating on the carbon surface followed by passivation upon reaction with the electrolyte. On the other hand, small can be active lithium-accommodating sites [41–43].

4. Conclusions Disordered carbons obtained by the pyrolysis of coffee shells have shown higher capacities for lithium intake. The surface area of the carbon materials has been increased to 1.3 and 19.9 orders upon the addition ZnCl2 and KOH as porogen respectively. The high capacities in these disordered carbons are attributed to the binding of lithium on extra surfaces of the single layers of carbon and in the nanocavities formed by the porogens. Nevertheless these carbons exhibit high capacities, their high irreversible capacity and loss in capacity upon cycling prevent their practical application. We believe that optimization of conditions of materials preparation can overcome these advantages.

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Acknowledgment This work was supported by Ministry of Commerce Industry and Energy (MOCIE), South Korea.

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