Enhanced reversible capacity of Li-S battery cathode based on graphene oxide

Journal of Energy Chemistry 22(2013)336–340

Enhanced reversible capacity of Li-S battery cathode based on graphene oxide Jin Won Kima,b ,

Joey D. Ocona,b , Dong-Won Parkb , Jaeyoung Leea,b,c∗

a. School of Environmental Science and Engineering; b. Laboratory for Energy Storage Systems; c. Ertl Center for Electrochemistry and Catalysis, RISE, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea [ Manuscript received November 2, 2012; revised December 28, 2012 ]

Abstract Lithium sulfur battery (LSB) offers several advantages such as very high energy density, low-cost, and environmental-friendliness. However, it suffers from serious degradation of its reversible capacity because of the dissolution of reaction intermediates, lithium polysulfides, into the electrolyte. To solve this limitation, there are many studies using graphene-based materials due to their excellent mechanical strength and high conductivity. Compared with graphene, graphene oxide (GO) contains various oxygen functional groups, which enhance the reaction with lithium polysulfides. Here, we investigated the positive effect of using GO mixed with carbon black on the performance of cathode in LSB. We have observed a smaller drop of capacity in GO mixed sulfur cathode. We further demonstrate that the mechanistic origin of reversibility improvement, as confirmed through CV and Raman spectra, can be explained by the stabilization of sulfur in lithium polysulfide intermediates by oxygen functional groups of GO to prevent dissolution. Our findings suggest that the use of graphene oxide-based cathode is a promising route to significantly improve the reversibility of current LSB. Key words lithiums sulfur battery; graphene oxide; capacity fading; lithium polysulfide dissolution

1. Introduction Recently, lithium-sulfur battery (LSB) and metal-air battery (i.e. Li-air battery) have been suggested as the most promising energy storage candidates [1–3] owing to their high theoretical specific energies. The capacity of Li-air battery is well known to be 5200 Wh/kg when including oxygen and 11140 Wh/kg if excluding oxygen. It is high enough to be comparable with gasoline and is ten times that of Li ion battery capacity [4]. However, Li-air battery technology had not gone beyond the initial stage of study because of several technical barriers. Among the challenges, the unclear reaction mechanism at air cathode [5], electrolyte dry-out [6], oxygen saturation, and unstable lithium anode problems [7,8] are frequently encountered. In the case of metal-air batteries using zinc, silicon, and aluminum as anodes, immense challenges in the electrical charge process [9–11] need solutions. For these reasons, LSB requires further inspection since it can be a feasible energy storage option in a wide array of applications such as electric vehicles. In LSB, the theoretical specific capacity of sulfur is 1675 mAh/gsulfur, which corresponds to three times that of Li ion battery (372 mAh/g). Fur-

thermore, sulfur is much lighter, cheaper, abundant, and nontoxic than the metal oxides present among Li ion battery cathodes. In short, these desirable qualities make a strong case for LSB’s applicability from electric vehicles, middle and large scale ESS to small-sized portable devices. Nonetheless, even with many attractive advantages, LSB suffers from serious capacity fading. The sulfur cathode has two main technical issues, first is the dissolution of reaction intermediates (lithium polysulfides) into the electrolyte and second is the low conductivity of elemental sulfur [12]. Electrochemical reaction between sulfur and lithium is shown as two potential plateaus during discharge step, at 2.3 V and 2.1 V. The former, called as upper plateau region, generally involves the formation of lithium polysulfides (Li2 Sx , 4
Corresponding author. Tel: +82-62-715-2440; Fax: +82-62-715-2434; E-mail: [email protected] This work was supported by the Core Technology Development Program for Next-Generation Energy Storage of the Research Institute for Solar and Sustainable Energies (RISE) at GIST. J. D. Ocon is grateful to the DOST UPD ERDT Faculty Development Program. ∗

Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.

Journal of Energy Chemistry Vol. 22 No. 2 2013

Thus, it is essential to study the properties of lithium polysulfides and design the cathode that can anchor it effectively. Among various approaches, use of graphene-based materials is getting spotlight for enhancing sulfur utilization. Due to the two-dimensional (2-D) sheet geometry shape, high electrical conductivity, and excellent mechanical strength, graphene is a good material to make conductive carbon and sulfur network and block the dissolution of lithium polysulfide by 2D sheet. Specifically, many research results show that sulfur wrapped in graphene derivatives exhibits enhanced cycle durability through the suppression of volume expansion and prevention of dissolution of lithium polysulfides into the electrolyte [13,14]. However, reduced graphene oxides by chemical method contain several disadvantages such as low conductivity by defects on the surfaces and poor dispersibility due to agglomeration among graphene layers. Instead of using pure graphene, we have studied the electrochemical performance in a LSB of a graphene oxide (GO)mixed sulfur cathode. According to noteworthy reports, carboxyl (HO–C = O), carbonyl (>C = O), epoxide (C–O–C) and hydroxyl (C–OH) functional groups on GO surface provide lithium polysulfide occupying sites [15,16]. These results correspond well to other studies insisting that lithium polysulfides can be easily adsorbed to relatively hydrophilic materials [17]. Although GO has high electrical resistance, the present study is focused on understanding the physicochemical behavior of GO with lithium polysulfides and how it can improve the LSB capacity reversibility. 2. Experimental GO was prepared by following the modified Hummer’s method. First, graphite powder was mixed in a solution of H2 SO4 and H2 PO4 (9 : 1, v/v) with vigorous stirring. Following the pre-oxidation step, graphite oxidation was performed by adding KMnO4 while keeping the temperature under 20 ◦ C, then slowly increasing it to 80 ◦ C and keeping the temperature again for the next 24 h. After dilution with water, aqueous H2 O2 was added into GO solution with constant stirring for 5 min. For the preparation of GO powder, GO solution was purified by two centrifugation cycles with 10 wt% HCl solution and was washed with water for several cycles. The GO powder sediment from the centrifugation process was air-dried at less than 40 ◦ C for a few days. To fabricate GO-sulfur cathode, elementary sulfur (100 mesh, 99.5%, Alfa Aesar) as active material, mixture of SuperP carbon black and GO as conductive material, polyvinylidene fluoride (PVdF) as binder, and N-methyl-2-pyrrolidone (NMP) as solvent were used. Based on initial experiments, the cathode containing GO only showed very high electrical resistance. Hence, we used a mixture of GO and carbon black, at different weight ratios of ingredients (100 : 0, 50 : 50, 0 : 100), which was dispersed in NMP solvent by an ultrasonicator for 5 h at 50 ◦ C to stabilize the GO. After mixing sulfur and PVdF in this suspension for 24 h, the prepared slurry was uniformly spread on aluminum current collector using a doctor-blade. The as-prepared cathode, which contained 50 wt% sul-

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fur, 43 wt% carbon materials (100% GO, 50% GO and carbon black or 100% carbon black for reference cell) and 7 wt% PVdF was dried at 60 ◦ C in a vacuum state for 24 h. In assembling the LSB, lithium foil (99.9%, Alfa Aesar) anode, a separator (Celgard 2320), and a cathode were stacked in the Teflon Swagelok-type cell. One molar lithium hexafluorophosphate (LiPF6 ) in tetraethylene glycol dimethyl ether (TEGDME) was used as the electrolyte and the entire assembly process was conducted under argon environment (<1 ppm). The electrochemical characteristics of the as-prepared sulfur cathode were measured. Cyclic voltammograms (CV) were taken on a potentiostat/galvanostat device (VSP, Biologic) with a voltage range from 0.5 to 3.2 V at a scan rate of 0.1 mV/s. Galvanostatic discharge and charge cycle tests were conducted using a multi-channel high performance battery cycler (Maccor 4300K), between cut-off voltages of 1.5 to 3.0 V at 0.1 C (167.5 mA/gsulfur ). During a preliminary examination, the electrode containing 100 wt% GO cathode was excluded because it has very high electrical resistance (∼MW range), making it impossible to function as an electrode. Thus, comparison of electrochemical performances was conducted only between sulfur cathode containing 0 wt% (reference cell) and 50 wt% GO. X-ray diffraction patterns were measured using an automated X-ray diffractometer (Rigaku D/MAXIIIA, Japan). For measuring Raman spectra, LabRam HR 800 UV Raman microscope (Horiba Jobin-Yvon, France) in Gwangju KBSI center with a 514 nm excitation wavelength was used. The degree of dispersion was confirmed by transmission electron microscopy (TEM, JEOL JEM-2100) at an operating voltage of 200 kV. 3. Results and discussion Figure 1(a) and Figure 1(b) show the sulfur cathode morphology with 1 : 1 ratio between GO and carbon black and Figure 1(c) and 1(d) present the XRD patterns for the three cathodes with various contents of GO. As shown in TEM images, transparent GO sheets and dark spherical sulfur and carbon black particles could be seen. The intimate interfacial contact between 2-D GO sheets and surrounding sulfur and carbon black particles can easily affect the reaction of sulfur with lithium during charging and discharging. In addition, 2-D structure gives rise to enhanced coulombic efficiency and increased number of active sites [18]. To confirm the structure and composition of the asprepared sulfur cathodes at different relative amounts of GO and carbon black, XRD measurements were performed. Elementary sulfur peaks appeared in the region between 20o to 40o among all the samples, as shown in Figure 1(c). Furthermore, clear and sharp peaks at 6.6o and amorphous GO peaks were observed in Figure 1(d) to be between 8o to 10o . In contrast to graphite and graphene peaks near 20o , amorphous GO peaks generally appear in this region [19]. Based on these results, we confirmed that the as-prepared cathodes were proper GO-sulfur mixtures.

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Figure 1. (a, b) TEM images of GO sheet, carbon black, and sulfur particles in different magnifications, (c) Comparison of XRD patterns of (1) 0%, (2) 50% and (3) 100% GO-sulfur cathode, (d) Peak changes among (1) 0%, (2) 50% and (3) 100% GO amorphous peaks between 6o and 18o

To investigate the electrochemical reaction between GOsulfur cathode and lithium at the potential range from 0.5 to 3.2 V, cyclic voltammetry was conducted and the results are shown in Figure 2. There were three distinguishable peaks in the cyclic scan. Two peaks were in the cathodic scans between 1.9 V and 2.4 V and a single peak at 2.6 V in the reverse scans, which corresponds to the reduction and oxidation of sulfur,

Figure 2. Cyclic voltammograms (dotted line: 1st cycle, dashed line: 3rd cycle, solid line: 5th cycle) of 50% GO-containing cathode as a working electrode in 1 M LiPF6 /TEGDME with a scan rate of 100 µV/s (Li metal used as counter and reference electrode). Inset figure shows enlarged CV curves of sulfur cathodic peaks (1.75−2.5 V). Arrows in figure direct the changes of potential peaks from 1st to 6th successive cycles

respectively. According to a previous report on the redox reaction between sulfur and lithium, high sulfur-numbered lithium polysulfides, Li2 S4 and Li2 S6 , are produced at around 2.4 V [13]. In addition, lithium sulfide formation generally occurs near 2.1 V. These two regions can be easily observed in the potential profiles for the discharge step as two distinguishable plateaus. As mentioned above, one of the critical issues of LSB battery is the drastic capacity fading caused by irreversible dissolution of lithium polysulfides into the ether-based electrolyte. To enhance the reversibility of LSB battery, lithium polysulfide dissolution should be minimized. Interestingly, heights of peak current at 1.9 V and 2.4 V were changed when using GO-mixed cathode, as shown in Figure 2. Peak current at 2.4 V decreased to 87.58% and peak current at 1.9 V increased to 167.43%, as the number of cycles increased, these numbers are relative to their original values. In addition, reduction peaks near 1.9 V and 2.4 V shifted to a slightly higher potential and oxidation peak near 2.6 V moved to lower potential. It is one of important evidences of increasing reversible reaction of sulfur due to the decrease in the gap between oxidation and reduction potential of sulfur. The indication of an enhanced reversibility when using GO-based cathode is in excellent agreement with a previous study [16]. In support to the CV data above, we observed enhanced reversibility in the voltage profile versus specific capacity. Carbon black (CB)-sulfur cathode exhibited a first discharge capacity of 879 mAh/g and its total capacity continuously

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decreased with cycle number. However, the voltage profile of the GO-sulfur cathode was completely different. At the first cycle, the capacity of upper potential plateau (UPP) region from OCV up to the onset of lower potential plateau (LPP) was 290.18 mAh/g. As the cycle number was increased, the capacity computed from UPP decreased. Moreover, LPP was increasingly flatter with increasing cycle number as well, while first cycle LPP continuously decreased. Obviously, these results add to the evidences that the reversibility enhancement of LSB battery was supported by GO. Table 1 shows the decreasing trend of UPP capacity to total capacity ratios when using GO-sulfur cathode. The occupation ratio of UPPs continuously decreased, from 53.7% to 40.8% following the cycles. In other words, the reversible capacity increased with cycle number due to the presence of GO. Consequently, as seen previously in the cyclic voltammogram in Figure 2, voltage profiles in Figure 3 and Table 1 clearly indicated the positive role of GO for the enhancement of LSB battery reversibility.

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black curve was assigned to the as-prepared GO-sulfur cathode and the red curve corresponded to discharged GO-sulfur cathode at a potential of 2.0 V. In both samples, there were two couple peaks; G-band was associated with the C–C vibrations of carbon with sp2 orbital structure at 1350.55 cm−1 while the D-band was connected with the disorder-induced vibration of the C–C bond at 1604.23 cm−1 , respectively. Compared with GO Raman spectra in previous studies, GO-sulfur cathode in discharged state showed a different peak at 1023.71 cm−1 position, indicating a sulfur-oxygen (S–O) band [20,21,22]. In addition, D/G band intensity ratio, which provides important information about the degree of reduction for graphene-based materials, increased from 0.84 to 1.39. The increased D/G ratio can be understood as a higher reduction state of graphenebased materials, associated with the removal of oxygen functional groups [23]. It means the oxygen functional groups on GO surface in the cathode interacted with sulfur from lithium polysulfides during the discharge step.

Table 1. Comparison of rate trends of lithium polysulfide formation relative to the total capacity for the GO-sulfur cathode Cycle numbers 1 3 5 7 9 11 13 15 17 19

Total capacity (mAh/g) 540.40 426.09 447.44 450.19 470.29 457.74 467.03 457.44 449.04 434.66

Capacity of upper plateau (mAh/g) 290.18 214.56 216.31 212.24 210.48 201.87 201.94 191.94 187.39 177.53

Occupying ratio of UPP (%) 53.7 50.35 48.34 47.14 44.76 44.10 43.24 41.96 41.73 40.84 Figure 4. Raman spectra in scanning range between 200 and 1800 cm−1 of GO-containing sulfur cathode before (1) and after (2) discharge at 2.0 V plateau at 0.1 C

Figure 3. Voltage profiles during galvanostatic discharge versus capacity of (1) 1st, (2) 11th and (3) 19th cycle at applied charge of 0.1 C

To gain a deeper insight into the enhancement of reversibility, Raman spectroscopy, a well-known powerful tool to show the chemical bonding states among elements, was employed. As shown in Figure 4, the Raman spectra clearly revealed the chemical bonding state of GO-sulfur cathode. The

Consistent with these results, Figure 5 obviously shows the difference of the performances of CB-sulfur cathode and GO-sulfur cathode. The capacity-fading trend seen in this curve was typical in LSB. The CB-sulfur cathode overall capacity continuously decreased from 868.51 mAh/g to 387.95 mAh/g after 19 cycles. In contrast, GO-sulfur cathode showed relatively low overall capacity at the initial cycles, but the overall capacity was over that of the CB-sulfur cathode after the 9th cycle keeping above 430 mAh/g up to the 19th cycle. Since the absolute value of HPP capacity associated with irreversible reaction was unstable with an increase in cycle number, the overall capacity fluctuation of GO-sulfur cathode was observed. In conclusion, oxygen functional groups of GO bind with sulfur in lithium polysulfide structures, as shown in Raman spectra for discharge step to 2.0 V. In addition, decrease in the lithium polysulfide dissolution region, HPP, can be shown in the voltage curves during discharge process. Moreover, enhancement of sulfur utilization was also observed. These arguments can be proven by the voltage curves in LPP which

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became more flat or changed at a very small rate. Thus, when oxygen functional groups exist on GO surface, its presence plays an important role as a viable enhancer of reversibility by stabilizing lithium polysulfide intermediates near the cathode surface, as illustrated in Figure 6.

for LSB electrode with enhanced capacity retention. Following from the conclusions on this work, designing sulfur cathode with hydrophilic materials in optimized ratio looks a very important step next. Thus, graphene-based materials as sulfur cathode should be further explored, keeping in mind that high electrical conductivity and high capacity reversibility are the foremost objectives. Acknowledgements The Core Technology Development Program for NextGeneration Energy Storage of the Research Institute for Solar and Sustainable Energies (RISE) at GIST supported this work. J. D. Ocon is grateful to the DOST ERDT-UPD Faculty Development Program.

References

Figure 5. Cycling performance at a constant current of 0.1 C for LSB containing 0% (1) and 50% (2) GO cathode

Figure 6. Schematic figure of lithium polysulfides bound to the oxygen functional groups on GO sheet

4. Conclusions We studied the increase in reversible capacity through the addition of GO in sulfur cathode. In contrast to CB-sulfur cathode, GO-sulfur cathode showed better cycle performance with increasing cycle number even if its initial discharge capacity was lower. In particular, continuous decrease of UPP region rate per total capacity was observed in discharge voltage curves, which comes from the decrease in lithium polysulfide dissolution. Based on our analytical results, the suppression mechanism of the dissolution of lithium polysulfides is explained by the anchoring in the oxygen functional groups on GO. All experimental observations from CV, galvanostatic discharge and charge test, and Raman spectroscopy measurement correspondingly point towards GO as a useful material

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