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Free standing flexible graphene oxide + α-MnO2 composite cathodes for Li–Air batteries
Solid State Ionics 286 (2016) 34–39
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Free standing flexible graphene oxide + α-MnO2 composite cathodes for Li–Air batteries Seyma Ozcan ⁎, Mahmud Tokur, Tugrul Cetinkaya, Aslihan Guler, Mehmet Uysal, Mehmet Oguz Guler, Hatem Akbulut Engineering Faculty, Department of Metallurgical & Materials Engineering, Sakarya University Esentepe Campus, 54187 Sakarya/TURKEY
a r t i c l e
i n f o
Article history: Received 5 October 2015 Received in revised form 2 December 2015 Accepted 9 December 2015 Available online xxxx Keywords: Graphene oxide α-MnO2 Air breathing cathode Li–air batteries
a b s t r a c t The most important factor regarding to influence the electrochemical performance of lithium–air battery is to choose an effective catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, α-MnO2 (manganese oxide) nanowires were used as an active electrocatalyst synthesized by microwave hydrothermal method. Graphene oxide (GO) and α-MnO2 nanowires were combined to obtain free standing paper cathodes and α-MnO2 + GO free standing paper cathodes were produced with dispersing different amounts of α-MnO2 nanowires (10 wt.%, 30 wt.%, 50 wt.%, and 70 wt.%) by using vacuum filtration method. The morphologies and structures of cathodes are evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. The electrochemical tests were carried out at the voltage window between 1.0 and 4.5 V in ECC-Air test cell. α-MnO2 +GO composite paper containing 70wt.% α-MnO2 catalyst showed the highest specific discharge capacity value of 2900 mAh g−1. Moreover, the experimental findings showed that dispersing of α-MnO2 catalyst in GO structure results in remarkable improvement of the electrochemical performance of Li–air battery cathodes. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Lithium–air secondary batteries consisting of lithium metal as the anode active material and oxygen as the cathode active material have been considered as a novel promising energy source after first proposal by Abraham et al. in 1996 [1,2]. Rechargeable commercial lithium–ion batteries generally exhibiting capacity of 130 mAh g− 1 are widely used in mobile phones, laptop computers, and similar electronic devices. However, their energy density is still not sufficient to use in electric vehicles [3,4]. Therefore, several metal–air batteries have been investigated such as iron–air, aluminum– air, and zinc–air batteries due to the promising energy densities. However, their energy densities cannot meet the requirements of many high-energy applications [5]. Recently, lithium–air batteries have attracted much attention because of their extremely high specific energy density [6]. It is well known that 1 kg of lithium air battery releases energy of 11,680 Wh kg−1, not much lower than the energy density of gasoline is 13,000 Wh kg−1 [7]. This high energy density of Li–air battery has the potential to be the power source for the advanced electric vehicles [8]. In practical applications, lithium–air batteries provide 3–5 times higher gravimetric energy density when compared with conventional Li–ion batteries [9].
⁎ Corresponding author. E-mail address: [email protected] (S. Ozcan).
http://dx.doi.org/10.1016/j.ssi.2015.12.016 0167-2738/© 2015 Elsevier B.V. All rights reserved.
Although Li–air batteries are promising candidates as high-energydensity batteries, they still suffer one serious problem, in that the solid reaction products such as Li2O or Li2O2 are insoluble in organic electrolytes [3]. The insoluble nature of Li2O2 in organic electrolytes makes them more prone to clog the porous structure of the air electrode. Thus the practical capacity of the Li–O2 cells is much lower than theoretical discharge capacity and is always considered as cathode limited [10]. It is critical to choose a carbon matrix with a microstructure providing large surface area and pore volume to facilitate a Li/O2 reaction and to hold a maximum amount of discharge products [11]. The air breathing cathode must have optimum porosity and carbon content. When the carbon content is increased, the porosity of the cathode decreases, which leads to decreasing oxygen diffusion length and the specific capacity [5,12]. Owing to these needs, graphene oxide (GO) is useful cathode material for lithium–air batteries. The GO structure possesses a layered structure containing oxidized graphene sheets having epoxide and hydroxyl groups at their basal planes; carbonyl and carboxyl groups at the edges [13,14]. The as-formed GO shows a high mechanical stiffness and strength compared to other paper-like membrane materials [15]. Besides, the major challenge is development of an efficient, low cost, and porous catalyst to provide reversible Li2O2 reaction during charging and discharging [11]. MnO2 is an attractive functional metal oxide for catalysis applications because of its structural flexibility, low cost, high average voltage, and it is environmentally friendly, which enables to MnO2 structure to be used as catalysts for lithium air batteries [16,17]. In our previous
S. Ozcan et al. / Solid State Ionics 286 (2016) 34–39 Table 1 Sample codes of produced air breathing cathodes. Composition of starting material (wt.%)
Sample code
10 α-MnO2 + 90 GO 30 α-MnO2 + 70 GO 50 α-MnO2 + 50 GO 70 α-MnO2 + 30 GO
10MG 30MG 50MG 70MG
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the authors' knowledge, this study is the first to use α-MnO2 +GO composite free-standing paper electrode as a lithium-air-breathing cathode for Li–air batteries. This study proves that α-MnO2 catalyst is very effective to improve oxygen reduction (ORR) and oxygen evaluation reactions (OER) and increasing capacity retention of the GO cathodes. 2. Experimental details 2.1. Preparation of graphene oxide
Fig. 1. XRD patterns of GO and α-MnO2.
work, it was carried out an electrochemical test of the GO paper electrode for Li–O2 batteries [15] and reported that GO paper shows very effective cyclability and specific capacity. But, this high capacity value was obtained under low current density due to the low conductive nature of GO structure. In this study, it was targeted to improve electrochemical reaction of the GO paper by production of smooth and flexible αMnO2 + GO nanocomposite free-standing air breathing cathode using vacuum filtration technique. Although α-MnO2 has been used as catalyst with different carbon materials in Li–air batteries, to the best of
In the present work, graphene oxide was prepared by modified Hummers' method by using pre-treated graphite flakes as starting material. For the pre-treatment process, 1.0 g of pristine graphite flakes (Alfa Aesar, + 100 mesh in size) was exposed to 50 mL of HNO3/ H2SO4 (nitric acid 65%, Merck, and sulfuric acid 95–97%, Sigma–Aldrich) (volume ratio of 1:3) solution under vigorous stirring for 2 h and then washed with distilled water until pH becomes neutral and dried in air at 70 °C for 24 h. After drying the product, it was subsequently then heat treated in air atmosphere at 850 °C for 120 s. For the graphite oxide production, 1.0 g of pre-treated graphite flakes and 0.5 g NaNO3 (Sigma–Aldrich, ≥ 99.0%) were added into 23 mL of H2SO4 acid and stirred for 2 h by magnetic stirrer until a homogenous mixture was obtained. The mixture was then cooled down to 0 °C by using an ice bath and 3.0 g KMnO4 was added slowly into the mixture while keeping the reaction temperature below 20 °C. After addition of KMnO4, the ice bath was removed and the mixture was heated up to 35 °C in 30 min. 46 mL H2O was slowly added into the mixture and the temperature was increased to 98 °C due to exothermic reaction and it was kept at 98 °C for 15 min to maintain the reaction temperature. Next, 140 mL of hot water and then 10 mL H2O2 aqueous solution were added into the mixture and a homogeneous suspension was obtained with dark yellow color after stirring the mixture for 2 h. Further, the product washed with distilled water and centrifuged until pH become neutral. The product was dried at 60 °C in vacuum oven and graphite oxide was obtained. To produce graphene oxide, graphite oxide was exfoliated by ultrasonication (Hielscher UP400S). For this, 30 mg of as-synthesized graphite oxide was dispersed in 100 mL of distilled water and sonicated for 2 h to increase the interlayer distances between graphite oxide
Fig. 2. Surface SEM images of 10ΜG, 30MG, 50MG, and 70MG cathode electrodes.
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S. Ozcan et al. / Solid State Ionics 286 (2016) 34–39
Fig. 3. Cross-sectional SEM images of 10ΜG, 30MG, 50MG, and 70MG cathode electrodes.
layers to obtain graphene oxide. Then, the solution was filtered on PVDF membrane (Millipore, Durapore Membrane, hydrophobic) by vacuum filtration technique.
2.2. Preparation of α-MnO2 In our previous work, synthesizing of α-MnO2 nanowires was reported in details [17]. In brief, 2 mmol of KMnO4 (Merck) and 3 mmol manganese sulfate MnSO4.H2O (Alfa Aesar) were dissolved in 80 mL distilled water. Then the resulted solution was transferred into a Teflon (PTFE)-lined autoclave sealed and placed in the microwave oven (Milestone ROTOSYNTH). The hydrothermal reaction was carried out for 30 min at 140 °C. Then the autoclave was cooled down to room temperature and the as-prepared black precipitate was filtered and washed several times with distilled water. α-MnO2 nanowires were obtained after drying at 60 °C in vacuum oven.
Fig. 4. Raman spectra of 10ΜG, 30MG, 50MG, and 70MG cathode electrodes.
2.3. Preparation of α-MnO2 +GO free-standing electrodes To obtain α-MnO2 +GO free standing air breathing cathode paper, 40 mg of as-specified amounts α-MnO2 and GO were dispersed into 100 mL of distilled water and sonicated for 1 h to form a welldispersed suspension and then the suspension was vacuum filtered through the PVDF membrane filters (220 nm pore in size) and free standing α-MnO2 +GO cathode was precipitated on membrane. After then, the obtained solid was washed up for several times and the αMnO2 + GO film was peeled-off from the PVDF membrane and a flexible, free-standing α-MnO2 + GO paper was obtained. In order to compare the effect of α-MnO2 content on the electrochemical performance of the cathodes, four different α-MnO2 + GO composite freestanding paper cathodes were prepared with dispersing different amounts of α-MnO2 (10 wt.%, 30 wt.%, 50 wt.%, and 70 wt. %). The free-standing cathodes prepared with different amount of constituents of GO and α-MnO2 air breathing electrode with their codes are shown in Table 1.
Fig. 5. First full discharge profile of 10ΜG, 30MG, 50MG, and 70MG cathode electrodes.
S. Ozcan et al. / Solid State Ionics 286 (2016) 34–39
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Fig. 6. Galvanostatic 10 h charge – 10 h discharge profiles of (a) 10MG, (b) 30 MG, (c) 50 MG, and (d) 70 MG cathode electrodes.
2.4. Physical and electrochemical characterization of the α-MnO2 +GO papers
with electrolyte, cyclic voltammetry test was carried out at a scan rate of 0.1 mV s−1.
The phase structures of the samples were determined by powder Xray diffraction (XRD) with a Rigaku D/MAX 2000 X-ray generator and diffractometer with CuKα radiation. The diffraction patterns were collected in step scan mode and recorded in 1° (2θ) steps at 1 min per step between 5° b 2θ b 80°. Scanning electron microscopy (SEM) (Jeol 6060 LV) was used to investigate the microstructure of samples. To further investigation of phase composition of the products, Raman spectroscopy analysis was performed by Kaiser Raman RXN1 system. An ECC-Air test cell (purchased from EL-Cell Company in Germany) was used for electrochemical characterization of the α-MnO2 + GO composite cathodes. 1 M LiBF4 (Sigma–Aldrich, 98%) dissolved in Nmethyl-2-pyrrolidone solution (NMP) (Sigma–Aldrich, 99 +% / Sigma–Aldrich, ≥99%) (1:1 by volume) was used as the electrolyte. Produced GO + α-MnO2 composite cathodes were used as the cathode. Anode and cathode were separated using glass fiber separator. Electrochemical characterization of the α-MnO2 +GO nanocomposite cathode was performed between 1 and 4.5 V at a constant current density of 0.1 mA cm−2 using a computer-controlled battery tester (MTI BST8MA). In order to investigate electrochemical reaction of the cathode
3. Results and discussion
Table 2 Open circuit voltages and over potential values of composite cathodes. Samples
Open circuit voltage
MG10 MG30 MG50 MG70
2.49 V 2.75 V 2.66 V 2.82 V
Over potential values 1st cycle
2nd cycle
5th cycle
2.75 V 2.7 V 2.3 V 2.4 V
3.3 V 3.5 V 3.3 V 2.5 V
3.4 V 3.5 V 3.05 V 2.05 V
The crystal structures of GO and α-MnO2 are identified using X-ray diffraction (Fig. 1). GO shows a strong peak centered at 2θ = 10.4°, corresponding to the (002) plane and its inter-planar spacing is 0.848 nm [13,15,18]. The typical reflection peak of α-MnO2 is observed at 2θ values of 12.7°, 18.0°, 28.6°, 36.7°, 38.6°, 41.9°, 49,7o, 56.4°, 60.2°, 65.4°, 69.6°, and 72.9° [19] corresponding to (110), (200), (310), (400), (211), (420), (301), (600), (521), (002), (541), and (312) planes of α-MnO2 crystals. The morphologies of the as-prepared α-MnO2 +GO cathodes were investigated by SEM. Figs. 2 and 3 show the surface and cross-sectional SEM images of 10MG, 30MG, 50MG, 70MG cathodes. As can be clearly seen from Fig. 3, α-MnO2 products possess wire-shaped structure and α-MnO2 nanowire catalysts (average diameter ranging between 40 and 60 nm) were homogenously dispersed and anchored on the surface and interlayer space of GO sheets. Formation of a network structure with increasing the amounts of α-MnO2 nanowires in the composite cathodes is also acquired, which allows the rapid electrolyte transportation into the cathode [20]. The expected network structure that formed by dispersing α-MnO2 nanowires between the GO nanosheets is not only beneficial for flowing electrolyte easily between the GO channels but also significant to allow oxygen permeation inside the cell. In this case, it is normally expected to get high charge–discharge and also increasing the catalyzing effects. Fig. 4 shows the Raman spectra of as-prepared cathodes. The D, G, and Gı bands of GO were observed at 1306, 1590, and 2600 cm−1, respectively. As can be concluded from Fig. 4, D band corresponds to defects and vacancies in the structure while the G band represents the oxygenation of graphite [15,21]. However peaks at 645 and 572 cm−1
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S. Ozcan et al. / Solid State Ionics 286 (2016) 34–39
Fig. 7. Cyclic voltammetry curves of 10MG and 70ΜG composite paper cathodes.
correspond to the pure α-MnO2 structure [17,22]. Moreover, increasing α-MnO2 nanowires in composite structure leads to increasing intensity of 645 and 572 cm− 1 peaks due to increasing content of α-MnO2 in composite structure. Fig. 5 shows the first full discharge profiles of as-prepared cathodes, performed between 1 and 4.5 V at a constant current density of 0.1 mA cm−2. It is obviously seen that dispersing and increasing content of αMnO2 catalyst in composite paper leads to increase oxygen reduction reaction (ORR) plateau of cathode from 1.5 to 2.0 V. This increment shows that increasing content of α-MnO2 in composite provides better ORR, which is beneficial to obtain higher discharge capacity [23]. Moreover, when α-MnO2 content is increased in composite structure, a new ORR plateau about 1.2 V is observed. It is speculated that high content of α-MnO2 structure in composite not only increases ORR but also provides large ORR potential window. Among composite cathodes, while 10MG exhibits 895 mAh g− 1, 30MG exhibits 1345 mAh g−1 and 50MG exhibits 172 mAh g−1, 70MG composite cathode demonstrates 2900 mAh g−1 specific capacity in full discharge, which obviously proves that the α-MnO2 has an excellent catalyze and improve the Li– O2 reaction. To investigate cycle life of the cathodes, further electrochemical cycling test was carried out at the state of 10 h charge–discharge cycles and the first, second, and fifth cycles are shown in Fig. 6. It has been reported that time controlled charging and discharging mode limits are a widely useful technique to investigate cycling stability of Li–Air batteries [24]. It is obviously seen that increasing content of α-MnO2 in composite cathode leads to increase discharge and charge capacities of the cathodes. Moreover, increasing α-MnO2 content in composite cathode caused improving capacity retention between discharge and charge, which shows effective reversible reaction during ORR and OER [25].
Fig. 8. Cycle performances of 10ΜG, 30MG, 50MG, and 70MG cathode electrodes.
When the ORR and OER plateau of the cathodes were investigated in Fig. 6, it is obviously seen that dispersing and increasing α-MnO2 catalyst in the composite cathode provide increasing ORR plateau and decreasing OER plateau during discharging and charging, which shows α-MnO2 catalyst is very effective to produce reversible reaction in the composite [6]. Eventually, 70MG shows the best electrochemical cycling test behavior and demonstrates about 50 mAh g−1 discharge capacity after 5 cycles. Another feature can be observed from Table 2 that, in general, increasing α-MnO2 content results in decreasing the overpotential. Taking into account the first cycle, 10MG nanocomposite electrode exhibits an overpotential of 2.75 V and this overpotential decreases almost continuously with increasing α-MnO2 content and shows an overpotential of 2.4 V for the 70MG nanocomposite electrode. This decrease is more prominent with increasing cycle number. For example; for the 10MG nanocomposite electrode, the overpotential is 3.4 V after fifth cycle and decreases to 2.05 V for the 70MG nanocomposite electrode. Since the cathode ORR in Li–air batteries is much slower than that of the anode Li oxidation reaction. This means the cathode reaction overpotential is much higher than that of the anode reaction. Ma et al. [26] reported that the overpotential increase during the discharge process with the growth of lithium peroxide was owing to insulativity of Li2O2 which could lead to a decreased discharge voltage. This decreased overpotential by increasing the α-MnO2 content is attributed to the improved surfaces that catalyze oxygen. Similarly, Song et al. [27] reported that Li–air batteries using α-MnO2 nanowire catalysts possess low overpotentials. This was because large amount of Mn3 + exposed on the nanowire surface and a unique mechanism for deposition of discharge products cause decreasing overpotential. They also found that lithium–air cells including nanoparticles, nanowires, and nanotubes as the cathode catalysts all show significantly reduced overpotentials compared to the catalyst-free cathode, which make their ORR potentials close to the thermodynamic equilibrium potential of Li2O2 formation. In order to investigate the effect of α-MnO2 content on the electrochemical reaction of the cathode against oxygen, cyclic voltammetry test was performed in the voltage range of 1.0–4.5 V at a scan rate of 0.1 mV/s. The cyclic voltammetry tests of 10MG and 70MG composite paper cathodes are shown in Fig. 7. In 10MG cathode, the peak observed between 2.5 and 1.5 V at first cathodic sweep shifting toward to 2.2 to 1.0 V at the second cathodic sweep corresponding to the formation of Li2O2 and the peak observed between 3.5 and 4.5 V related to the decomposition of Li2O2 compound. In 70MG cathode, the peak observed between the 2.2 and 2.0 V at the first cathodic sweep shifting toward to 2.2 and 1.5 V at the second cathodic sweep attributed to the formation of Li2O2 and the peak observed between 3.5 and 4.2 V related to the decomposition of Li2O2. The obtained cyclic voltammetry curves clearly overlapped with charge–discharge profile of cathodes (see in Fig. 6). As can be seen from the cyclic voltammetry curves, when the content of α-MnO2 is increased from 10 to 70 wt.%, the polarization
S. Ozcan et al. / Solid State Ionics 286 (2016) 34–39
between cathodic and anodic peaks decreases. Moreover, the current peak intensity is higher in 70MG cathode than that of 10MG cathode. These results show that the dispersing of α-MnO2 in composite structure enhances the reversibility of Li2O2 formation/decomposition reactions [28]. The electrochemical cycling performances of α-MnO2 + GO cathodes over the first 15 cycles at the state of 10 h charged–discharged are shown in Fig. 8. After a significant capacity drop in the first five cycles, the cathode electrodes exhibited stable cycling capacity during the subsequent charge/discharge cycles. The average capacity losses less than 10 mAh g−1 per cycle from 5th to 15th cycles for each electrodes. After the 15th cycle, 70MG cathode still evidently has the highest capacity, attributed to the one-dimensional shape of MnO2 nanowires providing more active sites to reduce the oxygen and contact between the cathode and electrolyte [29,30]. 4. Conclusion α-MnO2 +GO composite papers containing 10, 30, 50, and 70 wt.% α-MnO2 were produced by using vacuum filtration technique used as a cathode material for Li–air batteries. 70MG nanocomposite cathode showed the best electrochemical result. In this composite, an initial specific discharge capacity of 2900 mAh g−1 was obtained at full discharge state. This cathode showed very high capacity retention and 405 mAh g− 1 discharge capacity was obtained after 15 cycles at the 10 h charge–discharge state. Improving α-MnO2 content caused to decrease overpotential between the charge and discharge. The decrease in the overpotentials was found more prominent in the case of increased cycles. This study prove that reinforcing of α-MnO2 in composite cathode provides very effective reversible reaction during charging and discharging and increase specific capacity of the composite cathodes The α-MnO2 +GO paper could be a promising cathode electrode material for lithium–air batteries due to its high reversible capacity. Acknowledgments This research has received funding from the Seventh Framework Programme FP7/2007-2013 (Project STABLE – Stable high-capacity Lithium-Air Batteries with Long cycle life for Electric cars) under grant agreement 314508.
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Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Free standing flexible graphene oxide + α-MnO2 composite cathodes for Li–Air batteries Seyma Ozcan ⁎, Mahmud Tokur, Tugrul Cetinkaya, Aslihan Guler, Mehmet Uysal, Mehmet Oguz Guler, Hatem Akbulut Engineering Faculty, Department of Metallurgical & Materials Engineering, Sakarya University Esentepe Campus, 54187 Sakarya/TURKEY
a r t i c l e
i n f o
Article history: Received 5 October 2015 Received in revised form 2 December 2015 Accepted 9 December 2015 Available online xxxx Keywords: Graphene oxide α-MnO2 Air breathing cathode Li–air batteries
a b s t r a c t The most important factor regarding to influence the electrochemical performance of lithium–air battery is to choose an effective catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, α-MnO2 (manganese oxide) nanowires were used as an active electrocatalyst synthesized by microwave hydrothermal method. Graphene oxide (GO) and α-MnO2 nanowires were combined to obtain free standing paper cathodes and α-MnO2 + GO free standing paper cathodes were produced with dispersing different amounts of α-MnO2 nanowires (10 wt.%, 30 wt.%, 50 wt.%, and 70 wt.%) by using vacuum filtration method. The morphologies and structures of cathodes are evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. The electrochemical tests were carried out at the voltage window between 1.0 and 4.5 V in ECC-Air test cell. α-MnO2 +GO composite paper containing 70wt.% α-MnO2 catalyst showed the highest specific discharge capacity value of 2900 mAh g−1. Moreover, the experimental findings showed that dispersing of α-MnO2 catalyst in GO structure results in remarkable improvement of the electrochemical performance of Li–air battery cathodes. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Lithium–air secondary batteries consisting of lithium metal as the anode active material and oxygen as the cathode active material have been considered as a novel promising energy source after first proposal by Abraham et al. in 1996 [1,2]. Rechargeable commercial lithium–ion batteries generally exhibiting capacity of 130 mAh g− 1 are widely used in mobile phones, laptop computers, and similar electronic devices. However, their energy density is still not sufficient to use in electric vehicles [3,4]. Therefore, several metal–air batteries have been investigated such as iron–air, aluminum– air, and zinc–air batteries due to the promising energy densities. However, their energy densities cannot meet the requirements of many high-energy applications [5]. Recently, lithium–air batteries have attracted much attention because of their extremely high specific energy density [6]. It is well known that 1 kg of lithium air battery releases energy of 11,680 Wh kg−1, not much lower than the energy density of gasoline is 13,000 Wh kg−1 [7]. This high energy density of Li–air battery has the potential to be the power source for the advanced electric vehicles [8]. In practical applications, lithium–air batteries provide 3–5 times higher gravimetric energy density when compared with conventional Li–ion batteries [9].
⁎ Corresponding author. E-mail address: [email protected] (S. Ozcan).
http://dx.doi.org/10.1016/j.ssi.2015.12.016 0167-2738/© 2015 Elsevier B.V. All rights reserved.
Although Li–air batteries are promising candidates as high-energydensity batteries, they still suffer one serious problem, in that the solid reaction products such as Li2O or Li2O2 are insoluble in organic electrolytes [3]. The insoluble nature of Li2O2 in organic electrolytes makes them more prone to clog the porous structure of the air electrode. Thus the practical capacity of the Li–O2 cells is much lower than theoretical discharge capacity and is always considered as cathode limited [10]. It is critical to choose a carbon matrix with a microstructure providing large surface area and pore volume to facilitate a Li/O2 reaction and to hold a maximum amount of discharge products [11]. The air breathing cathode must have optimum porosity and carbon content. When the carbon content is increased, the porosity of the cathode decreases, which leads to decreasing oxygen diffusion length and the specific capacity [5,12]. Owing to these needs, graphene oxide (GO) is useful cathode material for lithium–air batteries. The GO structure possesses a layered structure containing oxidized graphene sheets having epoxide and hydroxyl groups at their basal planes; carbonyl and carboxyl groups at the edges [13,14]. The as-formed GO shows a high mechanical stiffness and strength compared to other paper-like membrane materials [15]. Besides, the major challenge is development of an efficient, low cost, and porous catalyst to provide reversible Li2O2 reaction during charging and discharging [11]. MnO2 is an attractive functional metal oxide for catalysis applications because of its structural flexibility, low cost, high average voltage, and it is environmentally friendly, which enables to MnO2 structure to be used as catalysts for lithium air batteries [16,17]. In our previous
S. Ozcan et al. / Solid State Ionics 286 (2016) 34–39 Table 1 Sample codes of produced air breathing cathodes. Composition of starting material (wt.%)
Sample code
10 α-MnO2 + 90 GO 30 α-MnO2 + 70 GO 50 α-MnO2 + 50 GO 70 α-MnO2 + 30 GO
10MG 30MG 50MG 70MG
35
the authors' knowledge, this study is the first to use α-MnO2 +GO composite free-standing paper electrode as a lithium-air-breathing cathode for Li–air batteries. This study proves that α-MnO2 catalyst is very effective to improve oxygen reduction (ORR) and oxygen evaluation reactions (OER) and increasing capacity retention of the GO cathodes. 2. Experimental details 2.1. Preparation of graphene oxide
Fig. 1. XRD patterns of GO and α-MnO2.
work, it was carried out an electrochemical test of the GO paper electrode for Li–O2 batteries [15] and reported that GO paper shows very effective cyclability and specific capacity. But, this high capacity value was obtained under low current density due to the low conductive nature of GO structure. In this study, it was targeted to improve electrochemical reaction of the GO paper by production of smooth and flexible αMnO2 + GO nanocomposite free-standing air breathing cathode using vacuum filtration technique. Although α-MnO2 has been used as catalyst with different carbon materials in Li–air batteries, to the best of
In the present work, graphene oxide was prepared by modified Hummers' method by using pre-treated graphite flakes as starting material. For the pre-treatment process, 1.0 g of pristine graphite flakes (Alfa Aesar, + 100 mesh in size) was exposed to 50 mL of HNO3/ H2SO4 (nitric acid 65%, Merck, and sulfuric acid 95–97%, Sigma–Aldrich) (volume ratio of 1:3) solution under vigorous stirring for 2 h and then washed with distilled water until pH becomes neutral and dried in air at 70 °C for 24 h. After drying the product, it was subsequently then heat treated in air atmosphere at 850 °C for 120 s. For the graphite oxide production, 1.0 g of pre-treated graphite flakes and 0.5 g NaNO3 (Sigma–Aldrich, ≥ 99.0%) were added into 23 mL of H2SO4 acid and stirred for 2 h by magnetic stirrer until a homogenous mixture was obtained. The mixture was then cooled down to 0 °C by using an ice bath and 3.0 g KMnO4 was added slowly into the mixture while keeping the reaction temperature below 20 °C. After addition of KMnO4, the ice bath was removed and the mixture was heated up to 35 °C in 30 min. 46 mL H2O was slowly added into the mixture and the temperature was increased to 98 °C due to exothermic reaction and it was kept at 98 °C for 15 min to maintain the reaction temperature. Next, 140 mL of hot water and then 10 mL H2O2 aqueous solution were added into the mixture and a homogeneous suspension was obtained with dark yellow color after stirring the mixture for 2 h. Further, the product washed with distilled water and centrifuged until pH become neutral. The product was dried at 60 °C in vacuum oven and graphite oxide was obtained. To produce graphene oxide, graphite oxide was exfoliated by ultrasonication (Hielscher UP400S). For this, 30 mg of as-synthesized graphite oxide was dispersed in 100 mL of distilled water and sonicated for 2 h to increase the interlayer distances between graphite oxide
Fig. 2. Surface SEM images of 10ΜG, 30MG, 50MG, and 70MG cathode electrodes.
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Fig. 3. Cross-sectional SEM images of 10ΜG, 30MG, 50MG, and 70MG cathode electrodes.
layers to obtain graphene oxide. Then, the solution was filtered on PVDF membrane (Millipore, Durapore Membrane, hydrophobic) by vacuum filtration technique.
2.2. Preparation of α-MnO2 In our previous work, synthesizing of α-MnO2 nanowires was reported in details [17]. In brief, 2 mmol of KMnO4 (Merck) and 3 mmol manganese sulfate MnSO4.H2O (Alfa Aesar) were dissolved in 80 mL distilled water. Then the resulted solution was transferred into a Teflon (PTFE)-lined autoclave sealed and placed in the microwave oven (Milestone ROTOSYNTH). The hydrothermal reaction was carried out for 30 min at 140 °C. Then the autoclave was cooled down to room temperature and the as-prepared black precipitate was filtered and washed several times with distilled water. α-MnO2 nanowires were obtained after drying at 60 °C in vacuum oven.
Fig. 4. Raman spectra of 10ΜG, 30MG, 50MG, and 70MG cathode electrodes.
2.3. Preparation of α-MnO2 +GO free-standing electrodes To obtain α-MnO2 +GO free standing air breathing cathode paper, 40 mg of as-specified amounts α-MnO2 and GO were dispersed into 100 mL of distilled water and sonicated for 1 h to form a welldispersed suspension and then the suspension was vacuum filtered through the PVDF membrane filters (220 nm pore in size) and free standing α-MnO2 +GO cathode was precipitated on membrane. After then, the obtained solid was washed up for several times and the αMnO2 + GO film was peeled-off from the PVDF membrane and a flexible, free-standing α-MnO2 + GO paper was obtained. In order to compare the effect of α-MnO2 content on the electrochemical performance of the cathodes, four different α-MnO2 + GO composite freestanding paper cathodes were prepared with dispersing different amounts of α-MnO2 (10 wt.%, 30 wt.%, 50 wt.%, and 70 wt. %). The free-standing cathodes prepared with different amount of constituents of GO and α-MnO2 air breathing electrode with their codes are shown in Table 1.
Fig. 5. First full discharge profile of 10ΜG, 30MG, 50MG, and 70MG cathode electrodes.
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Fig. 6. Galvanostatic 10 h charge – 10 h discharge profiles of (a) 10MG, (b) 30 MG, (c) 50 MG, and (d) 70 MG cathode electrodes.
2.4. Physical and electrochemical characterization of the α-MnO2 +GO papers
with electrolyte, cyclic voltammetry test was carried out at a scan rate of 0.1 mV s−1.
The phase structures of the samples were determined by powder Xray diffraction (XRD) with a Rigaku D/MAX 2000 X-ray generator and diffractometer with CuKα radiation. The diffraction patterns were collected in step scan mode and recorded in 1° (2θ) steps at 1 min per step between 5° b 2θ b 80°. Scanning electron microscopy (SEM) (Jeol 6060 LV) was used to investigate the microstructure of samples. To further investigation of phase composition of the products, Raman spectroscopy analysis was performed by Kaiser Raman RXN1 system. An ECC-Air test cell (purchased from EL-Cell Company in Germany) was used for electrochemical characterization of the α-MnO2 + GO composite cathodes. 1 M LiBF4 (Sigma–Aldrich, 98%) dissolved in Nmethyl-2-pyrrolidone solution (NMP) (Sigma–Aldrich, 99 +% / Sigma–Aldrich, ≥99%) (1:1 by volume) was used as the electrolyte. Produced GO + α-MnO2 composite cathodes were used as the cathode. Anode and cathode were separated using glass fiber separator. Electrochemical characterization of the α-MnO2 +GO nanocomposite cathode was performed between 1 and 4.5 V at a constant current density of 0.1 mA cm−2 using a computer-controlled battery tester (MTI BST8MA). In order to investigate electrochemical reaction of the cathode
3. Results and discussion
Table 2 Open circuit voltages and over potential values of composite cathodes. Samples
Open circuit voltage
MG10 MG30 MG50 MG70
2.49 V 2.75 V 2.66 V 2.82 V
Over potential values 1st cycle
2nd cycle
5th cycle
2.75 V 2.7 V 2.3 V 2.4 V
3.3 V 3.5 V 3.3 V 2.5 V
3.4 V 3.5 V 3.05 V 2.05 V
The crystal structures of GO and α-MnO2 are identified using X-ray diffraction (Fig. 1). GO shows a strong peak centered at 2θ = 10.4°, corresponding to the (002) plane and its inter-planar spacing is 0.848 nm [13,15,18]. The typical reflection peak of α-MnO2 is observed at 2θ values of 12.7°, 18.0°, 28.6°, 36.7°, 38.6°, 41.9°, 49,7o, 56.4°, 60.2°, 65.4°, 69.6°, and 72.9° [19] corresponding to (110), (200), (310), (400), (211), (420), (301), (600), (521), (002), (541), and (312) planes of α-MnO2 crystals. The morphologies of the as-prepared α-MnO2 +GO cathodes were investigated by SEM. Figs. 2 and 3 show the surface and cross-sectional SEM images of 10MG, 30MG, 50MG, 70MG cathodes. As can be clearly seen from Fig. 3, α-MnO2 products possess wire-shaped structure and α-MnO2 nanowire catalysts (average diameter ranging between 40 and 60 nm) were homogenously dispersed and anchored on the surface and interlayer space of GO sheets. Formation of a network structure with increasing the amounts of α-MnO2 nanowires in the composite cathodes is also acquired, which allows the rapid electrolyte transportation into the cathode [20]. The expected network structure that formed by dispersing α-MnO2 nanowires between the GO nanosheets is not only beneficial for flowing electrolyte easily between the GO channels but also significant to allow oxygen permeation inside the cell. In this case, it is normally expected to get high charge–discharge and also increasing the catalyzing effects. Fig. 4 shows the Raman spectra of as-prepared cathodes. The D, G, and Gı bands of GO were observed at 1306, 1590, and 2600 cm−1, respectively. As can be concluded from Fig. 4, D band corresponds to defects and vacancies in the structure while the G band represents the oxygenation of graphite [15,21]. However peaks at 645 and 572 cm−1
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Fig. 7. Cyclic voltammetry curves of 10MG and 70ΜG composite paper cathodes.
correspond to the pure α-MnO2 structure [17,22]. Moreover, increasing α-MnO2 nanowires in composite structure leads to increasing intensity of 645 and 572 cm− 1 peaks due to increasing content of α-MnO2 in composite structure. Fig. 5 shows the first full discharge profiles of as-prepared cathodes, performed between 1 and 4.5 V at a constant current density of 0.1 mA cm−2. It is obviously seen that dispersing and increasing content of αMnO2 catalyst in composite paper leads to increase oxygen reduction reaction (ORR) plateau of cathode from 1.5 to 2.0 V. This increment shows that increasing content of α-MnO2 in composite provides better ORR, which is beneficial to obtain higher discharge capacity [23]. Moreover, when α-MnO2 content is increased in composite structure, a new ORR plateau about 1.2 V is observed. It is speculated that high content of α-MnO2 structure in composite not only increases ORR but also provides large ORR potential window. Among composite cathodes, while 10MG exhibits 895 mAh g− 1, 30MG exhibits 1345 mAh g−1 and 50MG exhibits 172 mAh g−1, 70MG composite cathode demonstrates 2900 mAh g−1 specific capacity in full discharge, which obviously proves that the α-MnO2 has an excellent catalyze and improve the Li– O2 reaction. To investigate cycle life of the cathodes, further electrochemical cycling test was carried out at the state of 10 h charge–discharge cycles and the first, second, and fifth cycles are shown in Fig. 6. It has been reported that time controlled charging and discharging mode limits are a widely useful technique to investigate cycling stability of Li–Air batteries [24]. It is obviously seen that increasing content of α-MnO2 in composite cathode leads to increase discharge and charge capacities of the cathodes. Moreover, increasing α-MnO2 content in composite cathode caused improving capacity retention between discharge and charge, which shows effective reversible reaction during ORR and OER [25].
Fig. 8. Cycle performances of 10ΜG, 30MG, 50MG, and 70MG cathode electrodes.
When the ORR and OER plateau of the cathodes were investigated in Fig. 6, it is obviously seen that dispersing and increasing α-MnO2 catalyst in the composite cathode provide increasing ORR plateau and decreasing OER plateau during discharging and charging, which shows α-MnO2 catalyst is very effective to produce reversible reaction in the composite [6]. Eventually, 70MG shows the best electrochemical cycling test behavior and demonstrates about 50 mAh g−1 discharge capacity after 5 cycles. Another feature can be observed from Table 2 that, in general, increasing α-MnO2 content results in decreasing the overpotential. Taking into account the first cycle, 10MG nanocomposite electrode exhibits an overpotential of 2.75 V and this overpotential decreases almost continuously with increasing α-MnO2 content and shows an overpotential of 2.4 V for the 70MG nanocomposite electrode. This decrease is more prominent with increasing cycle number. For example; for the 10MG nanocomposite electrode, the overpotential is 3.4 V after fifth cycle and decreases to 2.05 V for the 70MG nanocomposite electrode. Since the cathode ORR in Li–air batteries is much slower than that of the anode Li oxidation reaction. This means the cathode reaction overpotential is much higher than that of the anode reaction. Ma et al. [26] reported that the overpotential increase during the discharge process with the growth of lithium peroxide was owing to insulativity of Li2O2 which could lead to a decreased discharge voltage. This decreased overpotential by increasing the α-MnO2 content is attributed to the improved surfaces that catalyze oxygen. Similarly, Song et al. [27] reported that Li–air batteries using α-MnO2 nanowire catalysts possess low overpotentials. This was because large amount of Mn3 + exposed on the nanowire surface and a unique mechanism for deposition of discharge products cause decreasing overpotential. They also found that lithium–air cells including nanoparticles, nanowires, and nanotubes as the cathode catalysts all show significantly reduced overpotentials compared to the catalyst-free cathode, which make their ORR potentials close to the thermodynamic equilibrium potential of Li2O2 formation. In order to investigate the effect of α-MnO2 content on the electrochemical reaction of the cathode against oxygen, cyclic voltammetry test was performed in the voltage range of 1.0–4.5 V at a scan rate of 0.1 mV/s. The cyclic voltammetry tests of 10MG and 70MG composite paper cathodes are shown in Fig. 7. In 10MG cathode, the peak observed between 2.5 and 1.5 V at first cathodic sweep shifting toward to 2.2 to 1.0 V at the second cathodic sweep corresponding to the formation of Li2O2 and the peak observed between 3.5 and 4.5 V related to the decomposition of Li2O2 compound. In 70MG cathode, the peak observed between the 2.2 and 2.0 V at the first cathodic sweep shifting toward to 2.2 and 1.5 V at the second cathodic sweep attributed to the formation of Li2O2 and the peak observed between 3.5 and 4.2 V related to the decomposition of Li2O2. The obtained cyclic voltammetry curves clearly overlapped with charge–discharge profile of cathodes (see in Fig. 6). As can be seen from the cyclic voltammetry curves, when the content of α-MnO2 is increased from 10 to 70 wt.%, the polarization
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between cathodic and anodic peaks decreases. Moreover, the current peak intensity is higher in 70MG cathode than that of 10MG cathode. These results show that the dispersing of α-MnO2 in composite structure enhances the reversibility of Li2O2 formation/decomposition reactions [28]. The electrochemical cycling performances of α-MnO2 + GO cathodes over the first 15 cycles at the state of 10 h charged–discharged are shown in Fig. 8. After a significant capacity drop in the first five cycles, the cathode electrodes exhibited stable cycling capacity during the subsequent charge/discharge cycles. The average capacity losses less than 10 mAh g−1 per cycle from 5th to 15th cycles for each electrodes. After the 15th cycle, 70MG cathode still evidently has the highest capacity, attributed to the one-dimensional shape of MnO2 nanowires providing more active sites to reduce the oxygen and contact between the cathode and electrolyte [29,30]. 4. Conclusion α-MnO2 +GO composite papers containing 10, 30, 50, and 70 wt.% α-MnO2 were produced by using vacuum filtration technique used as a cathode material for Li–air batteries. 70MG nanocomposite cathode showed the best electrochemical result. In this composite, an initial specific discharge capacity of 2900 mAh g−1 was obtained at full discharge state. This cathode showed very high capacity retention and 405 mAh g− 1 discharge capacity was obtained after 15 cycles at the 10 h charge–discharge state. Improving α-MnO2 content caused to decrease overpotential between the charge and discharge. The decrease in the overpotentials was found more prominent in the case of increased cycles. This study prove that reinforcing of α-MnO2 in composite cathode provides very effective reversible reaction during charging and discharging and increase specific capacity of the composite cathodes The α-MnO2 +GO paper could be a promising cathode electrode material for lithium–air batteries due to its high reversible capacity. Acknowledgments This research has received funding from the Seventh Framework Programme FP7/2007-2013 (Project STABLE – Stable high-capacity Lithium-Air Batteries with Long cycle life for Electric cars) under grant agreement 314508.
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