Enhanced Cyclability of Li–O2 Batteries Based on TiO2 Supported

Article pubs.acs.org/cm

Enhanced Cyclability of Li−O2 Batteries Based on TiO2 Supported Cathodes with No Carbon or Binder Guangyu Zhao, Runwei Mo, Baoyu Wang, Li Zhang, and Kening Sun* Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, China S Supporting Information *

ABSTRACT: The decomposition of carbon materials and organic binders in Li−air batteries has been reported repeatedly in recent literature. The decomposition of carbon can harm the batteries’ cyclability further by catalyzing electrolyte degrading. Therefore, there is a critical need to exploit a new catalyst support substituting carbon and develop a binder free cathode preparation strategy for Li−air batteries. Herein, TiO2 nanotube arrays growing on Ti foam are used as the catalyst support to construct carbon and binder free oxygen diffusion electrodes. After being coated with Pt nanoparticles by a cool sputtering approach, the TiO2 nanotube arrays are used as cathodes of Li−O2 batteries. Benefiting from the stability of TiO2 in the discharge/charge processes, the Li−O2 batteries realize enhanced cyclability at high current densities (for instance, more than 140 cycles at 1 or 5 A g−1), within wide discharge/charge voltage windows (for instance, 1.5−4.5 V). X-ray photoelectron spectra and a scanning electron microscope image of the cathodes after cycling at 5 A g−1 150 times indicate that the TiO2 nanotubes can remain stable in the long term cycle test. 1H nuclear magnetic resonance analysis reveals that the tetraethylene glycol dimethyl ether electrolyte has no degradation, showing enhanced stability compared with that in the carbon containing batteries.

1. INTRODUCTION Rechargeable Li−air batteries have attracted a great deal of attention due to the potential to provide 3−5 times the gravimetric energy density of conventional Li−ion batteries,1−3 since first identified by Abraham and Jiang.4 However, Li−air batteries are still in the infancy of their development, despite the advantage of superior energy density.5−10 Many different aspects of this complex system need to be studied in detail, in which materials and structures of the cathodes are the undoubted key points by affecting Li−air batteries’ performances seriously.11 Common cathodes used in the previous literature are carbon supported oxygen diffusion electrodes, and the general preparation process is adding binders into porous carbon, pasting them onto the current collectors.12 Nevertheless, both carbon materials and organic binders have been suggested degrading in Li−air batteries.13−17 Bruce and coworkers13 studied the stability of carbon and the effect of carbon on electrolyte decomposition. They found that both the hydrophobic and hydrophilic carbons are unstable in the charging process in the presence of Li2O2, decomposing to form Li2CO3. On the other hand, some recent reports15−17 revealed that the binders were also unstable in Li−air batteries. Zhang et al.16 reported that Li 2 O 2 could react with polyvinylidene fluoride (PVDF), leading it cross-linking to form a dark gel. Similar results were presented in Nazar and coworkers’ report,17 which showed that superoxide readily reacted with PVDF to give unwanted byproducts. Accordingly, either carbon materials or organic binders can cause the instability of Li−air batteries’ cathodes, harming batteries’ rechargeability. As © 2014 American Chemical Society

far as we know, the best cyclabilities of carbon cathode-based Li−air batteries are from Kang and co-workers’ hierarchical carbon nanotube paper18 (70 cycles at 2 A g−1 with 1000 mAh g−1 capacity limitation) and Scrosati and co-workers’ super P carbon layer19 (100 cycles at 5 A g−1 with 1000 mAh g−1 capacity limitation). However, they are still far from peoples’ expectations for practical application, compared with conventional Li ion batteries’ hundreds of reversible cycles. The charge voltages of these two battery systems all exceed 4.0 V, much higher than 3.5 V, above which, the carbon materials are suggested to decompose in the Li−air batteries.13 Therefore, there is a critical need to find a new catalyst support to substitute carbon and develop a binder free cathode preparation strategy for Li−air batteries. TiO2 is an attractive catalyst support, because it is stable, environmentally friendly, and biocompatible.20−24 When used as the catalyst support in a proton exchange membrane and direct methanol fuel cells, TiO2 has been studied fully, since it is more stable than carbon by preventing corrosion chemically and electrochemically in the fuel cell operation environment.25−27 In addition, the quantum chemistry study in Wei and co-workers’ report28 indicated that TiO2 could improve the highest occupied molecular orbital spatial size of noble metals and weaken the adsorption of atomic oxygen on catalysts in comparison with catalysts supported on carbon. This enhanceReceived: December 4, 2013 Revised: March 31, 2014 Published: April 2, 2014 2551

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3. RESULTS AND DISCUSSION 3.1. Physical Characterization of Pt Modified TiO2 Nanotube Arrays (Pt/TNT). Uniform nanotube arrays growing on Ti foam have been obtained by the anodic oxidation, and a layer of Pt nanoparticles can spread on the nanotube array homogeneously by the subsequent cool sputtering approach, as seen from the SEM images in Figure 1. The nanotubes can be ascribed to anatase TiO2 (JCPDS:

ment is significant for improving the catalytic ability of catalysts in electro-catalysis processes. In various TiO2 supporting materials, nanotubes have drawn considerable attention, because of their large specific surface area and abundant mass transport channels.26 When TiO2 nanotubes are used as catalyst support of Li−air battery cathodes, the large surface area can supply abundant sites to accommodate Li2O2 precipitate. Moreover, the straight channels in the nanotubes are beneficial for oxygen transport, which can enhance the specific capacity, rate performance, and cyclability of Li−air batteries.29 Herein, we prepare TiO2 nanotube arrays on Ti foam and use the treated foam as a supporting frame for Li−O2 battery cathodes. Then the Pt nanoparticle catalyst is deposited on TiO2 support directly by a cool sputtering approach, preventing the use of any organic binder. The TiO2 supported cathodes exhibit outstanding stability within much wider discharge/charge voltage windows, compared with carbon supported cathodes. As far as we know, this is the first time to apply TiO2 nanotube arrays in Li−O2 battery cathodes.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The TiO2 nanotube arrays were grown directly on a circular titanium foam plate (thickness 0.5 mm, diameter 15 mm, 99.2%) using anodic oxidation. The Ti foam used in our work is manufactured by rolling the Ti powder to porous plates, which are usually used as the catalyst support and filter for treating water. Therefore, when as a cathode of Li−O2 batteries, there are adequate channels for oxygen transport in the Ti foams, as seen in the Figure S1 of the Supporting Information. The Ti plates were initially sonicated in acetone, rinsed with pure water, and then etched in 18% HCl at 85 °C for 10 min to remove any oxides from the surface. The etched Ti plate was then submerged in a two-electrode cell containing 160 mL glycerin + 20 mL H2O + 20 mL dimethyl sulfoxide (DMSO) + 0.5 wt % NH4F and was electrochemically treated via anodization at 20 V for 3 h. The as-anodized plates were annealed in an oven at 450 °C for 3 h. Pt nanoparticles were deposited on anodized Ti foam by a Leica EM SC050 cool sputtering device at a current of 20 mA for 100 s. The weights of the deposits were measured on a microbalance (Mettler Toledo) with an accuracy of 0.01 mg. The Pt loading amount on a plate was 0.1 mg. 2.2. Instruments for Characterization. Scanning electron microscope (SEM) images were obtained on a Hitachi Su-8100. The X-ray diffraction (XRD) patterns were obtained on a PANalytical X’pert PRO X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). High resolution transmission electron microscope (HRTEM) images and energy dispersive X-ray spectrum (EDS) patterns were obtained on a FEI Tecnai G2. X-ray photoelectron spectra (XPS) were obtained with a K-Alpha electron spectrometer (Thermofish Scientific Company) using Al Kα (1486.6 eV) radiation. The base pressure was about 1 × 10−8 mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. 1H nuclear magnetic resonance (NMR) analysis is carried on a Bruker ADVANCE III 400 MHz NMR analyzer. Fourier transform infrared spectra (FTIR) were obtained on a SP100 analyzer (PerkinElmer). Gas chromatography is obtained from a SP-2100 analyzer (Beijing Beifen Analytical Instrument (group) Co., Ltd.). 2.3. Li−O2 Battery Tests. The Swagelok type Li−O2 batteries were assembled inside a MBraun glovebox. The cells were constructed by placing a 15 mm diameter Li disk on the bottom, covering it with a piece of glass fiber separator (20 mm diameter, Whatman), adding excessive electrolyte (1.0 M LiTFSI in tetraethylene glycol dimethyl ether (TEGDME)), placing an air electrode disk on the separator, and sealing the Swagelok cell. All the electrochemical measurements to the batteries were carried out in pure O2 at 1 atm (99.99%). A BTS-2000 Neware Battery Testing System was employed for charge/discharge tests.

Figure 1. SEM images of Pt modified TiO2 nanotube arrays prepared by anodic oxidation of Ti foam and subsequent cool sputtering: top views (a, b, c), side view (d).

21−1272), according to the XRD pattern in Figure S2. Figure 1a and b show that free-standing TiO2 nanotubes with a 50 nm diameter and 10 nm tube-wall thickness spread on the Ti foam surface widely. On the other hand, Pt nanoparticels at a size of several nanometers (Figure 1c) are coated on the TiO2 nanotubes homogeneously, as seen in the side view of Figure 1d. Figure S3 shows the bottom view SEM of the Pt/TNT that peeled from Ti substrates, which exhibit obvious regular arrangement of the nanotubes. TEM has been carried out to reveal the structure and crystallinity of Pt/TNT, as seen in Figure 2. Figure 2a is a vertical view of Pt/TNT, showing a structure of a vertical unobstructed nanotube array. The EDS pattern of the nanotubes indicates that Pt, Ti, and O are the primary elements in the Pt/TNT, as shown in Figure 2b. The TEM and HRTEM of a single nanotube in Figure 2c and d demonstrate that a layer of Pt nanoparticles at a size of several nanometers is coated on the TiO2 nanotubes homogeneously, consistent with the SEM results. In Figure S4, the Pt 4f of XPS is characterized by a doublet containing a binding energy of 70.8 eV (Pt 4f7/2) and 74.1 eV (Pt 4f5/2). The two peaks have a binding energy difference of 3.3 eV and a peak area ratio of 4:3, corresponding to the characteristics of Pt metal.26 3.2. Li−O2 Battery Performances of Pt/TNT Cathodes. The electrochemical characteristics of Ti foam covered with Pt/ TNT are measured as the Li−O2 battery cathodes. The battery exhibits outstanding cyclability that realizes 140 cycle reversible discharge/charge using a current density of 1 A g(Pt)−1 with a limited capacity of 1000 mAh g(Pt)−1, as shown in Figure 3a. Figure 4 illustrates the discharge/charge curves in the stages of 10th−20th, 40th−50th, 70th−80th, 90th−100th, and 120th− 130th cycles. The battery shows excellent stability in the cycling, despite the voltage gap increasing with the cycle number. The discharge/charge overpotential varying with cycle 2552

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Figure 2. TEM and EDS of Pt/TNT and single nanotube: (a) TEM image of the vertical view for Pt/TNT; (b) EDS of Pt/TNT; (c) TEM image of a single nanotube; (d) HRTEM of the Pt nanoparticles coated Ti2O nanotube.

Figure 4. Discharge/charge curves of Li−O2 battery with Pt/TNT as the cathode in the stages of 10th−20th (a), 40th−50th (b), 70th−80th (c), 90th−100th (d), and 120th−130th (e) cycles using a current density of 1 A g(Pt)−1 with a limited capacity of 1000 mAh g(Pt)−1.

reactions. O 1s XPS shown in Figure S6(b) for the same sample confirmed the formation of lithium oxides. On the basis of the previous result on the discharged sample using the same electrolyte,30 these peaks (55.5 eV for Li and 531.9 eV for O) can be assigned to Li2O2. The charge product (Li2O2) can be electro-catalyzed to decompose in the charge process. The charge voltage of the 10th cycle is around 3.4 V, which is much lower than that on carbon materials.18 The low overpotential is attributed to the strong ability of the Pt electro-catalyzing oxygen evolution reaction (OER).31 The charge voltage increases with the cycle number and remains at 3.7 V in the following 70 cycles. The most notable improvement is the enhanced cyclability of the battery with the Pt/TNT air electrode operating reversibly, even at an exceptionally high current density. While conventional carbon based Li−O2 batteries have been operated at relatively low current rates (around 50−200 mA g−1),2,31,32 the current density applied here is much higher (1 A g−1). We further demonstrate that even at higher current densities of 5 A g−1, the Pt/TNT cathode successfully cycles 150 times reversibly, as shown in Figure 5a. Although the high discharge/charge current density results in large overpotentials, as shown in Figure 5b, the discharge/charge processes still remains stable. The stability endows the battery with enhanced cyclability, realizing 150 long-term cycles. This cyclability is better than most of the Li−O2 batteries based on carbon materials, which can perform no more than 100 cycles in most case. Figure 6 exhibits the discharge/charge curves in the stages of 10th−20th, 40th−50th, 70th−80th, 90th−100th, and 120th−130th cycles, which demonstrate nearly identical profiles, showing favorable rechargeability. The enhanced cyclability can be attributed to the facilitated oxygen transport channels in the anodic oxidized Ti foam, which has the macropores between Ti sands (Figure S1b) as main channels

Figure 3. The electrochemical characteristics of the Li−O2 battery with Pt/TNT as a cathode: cyclability at a current density of 1 A g(Pt)−1 with a limited capacity of 1000 mAh g(Pt)−1 (a), discharge/ charge curves of the 10th, 40th, 70th, 100th, and 130th cycles at 1 A g(Pt)−1 (b).

number can be detected from the discharge/charge curves in Figure 3b. The discharge profiles of the 10th, 40th, 70th, and 100th cycles are almost identical, demonstrating the stability of the cathode surface. The XRD pattern of the powder scraped form the discharged cathode in Figure S5 indicates that Li2O2 is generated in the discharge processes. The existence of Li2O2 also can be verified by the XPS analysis. Figure S6(a) shows the Li 1s XPS pattern for the cathodes after discharge, from which it can be concluded that lithium oxides appear to be formed as the dominant discharge product of the electrochemical 2553

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and 6. The stability of the cathodes is discussed in the following text. 3.3. Characterization of the Cathodes and Electrolyte after Cycling. The XRD pattern of the cathodes after cycle tests in Figure S7 shows no obvious difference with that of the as-prepared one. The diffraction peaks ascribed to anatase TiO2 can be detected clearly except the Ti peaks, revealing that the crystal structure of the nanotubes is stable in the cycling. The SEM image of the cathodes after cycling in Figure S8 also exhibits no obvious variety compared with that of before cycling (Figure 1a), except some undecomposed discharge product cluster on the nanotube array surface. The XRD and SEM results demonstrate that the array structure can remain stable in the Li−O2 battery, even with a very long-term cycle test. The chemical state of the cathode surface before and after cycling is compared by the XPS measurement, as seen in Figure 7a and b. Both of the Ti 2p peaks that come before/after

Figure 5. The electrochemical characteristics of the Li−O2 battery with Pt/TNT as a cathode: cyclability at a current density of 5 A g(Pt)−1 with a limited capacity of 1000 mAh g(Pt)−1 (a), discharge/ charge curves of the 10th, 40th, 70th, 100th, and 130th cycles at the current density of 5 A g(Pt)−1 (b).

Figure 7. XPS Ti 2p patterns of the cathode before (a) and after (b) 150 cycles with the current density of 5 A g(Pt)−1.

cycling can be assigned to the TiO2 (binding energy = 458.7 eV) primarily, indicating the stability of the cathode surface state. Finally, even after 150 cycles at capacity regimes of 1000 mAh g−1 (Figure 3c), the response of the Li−O2 battery using the Pt/TNT cathode remains very stable with no evidence of TEGDME electrolyte decomposition, as confirmed by 1HNMR analysis in Figure 8. The major structure of the TEGDME molecule has no decomposition, verified by the nearly identical integral area ratio (4:2:2:3) of the four 1H peaks that come before/after cycling. Additionally, FTIR results of the cathodes that come after discharge and charge are compared with the pristine ones, as seen in Figure S9. Although the Li2O2 is not detected from the FTIR of discharged samples, for the wide absorbance of TiO2 from 400−900 cm−1 hiding Li2O2’s peaks, there are not obvious absorbances around 1500 cm−1 that can

Figure 6. Discharge/charge curves of Li−O2 battery with Pt/TNT as the cathode in the stages of 10th−20th (a), 40th−50th (b), 70th−80th (c), 90th−100th (d), and 120th−130th (e) cycles using a current density of 5 A g(Pt)−1 with a limited capacity of 1000 mAh g(Pt)−1.

and the mesopores in the TiO2 nanotubes as branch channels. The hierarchical pore structure can supply the cathodes with abundant active sites for Li2O2 depositing/decomposing and supply the ORR (oxygen reduction reaction)/OER processes developed channels for oxygen transport.27 Another reason is the unexpected stability of cathodes, which leads the battery operating normally even at a charge voltage higher than 4 V with the current densities of 5 A g−1, as shown in Figures 5b 2554

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anodic oxidized Ti foam as a promising alternative to carbon as the catalyst support for Li−O2 batteries.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of pristine Ti foam. XRD pattern of the anodic oxidized Ti foam. TEM images and XPS pattern of Pt 4f for the as-prepared Pt/TNT. XRD of the discharged cathodes. XPS patterns of Li 1s and O 1s for the cathodes after discharge. XRD pattern and SEM image of the cathode after cycling. FTIR of the pristine, discharged, and charged cathodes. Gas chromatography of the inlet and outlet gas for the batteries. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax/Tel.: +86 451 86412153. E-mail: [email protected].

Figure 8. 1HNMR spectra of the TEGDME electrolyte with 1 M LiTFSI before (a) and after (b) 150 cycles with the current density of 5 A g(Pt)−1.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (NSFC; no. 20903031 and no. 21203044).

be ascribed to Li2CO333,34 in all three spectra. The FTIR results mean that there is no Li2CO3 generating on the cathodes in the discharge/charge processes. Furthermore, gas chromatography of the exhaust gas in the charge process is supplied in Figure S10. Compared with the data of the O2 from the inlet, there is not an obvious variety in the exhaust gas, demonstrating that the product gas is O2 primarily. The NMR and meteorological chromatography results demonstrate that the TEGDME is stable on the Pt/TNT catalyst surface in the discharge/charge processes even within a very wide voltage window (1.5−4.5 V, as seen in Figure 3c). Bruce and co-workers in their recent report13 have suggested that carbon materials are unstable on charging above 3.5 V, oxidatively decomposing to form Li2CO3. Carbon also promotes electrolyte decomposition to Li2CO3 and Li carboxylates during discharge and charge in a Li−O2 battery at high voltage (for instance, above 3.5 V), both in DMSO and TEGDME. Compared with carbon materials, the TiO2 nanotube arrays in the present study, showing excellent stability within a wide voltage window, are a promising catalyst support for Li−O2 battery cathodes.

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4. CONCLUSION Pt nanoparticle-coated TiO2 nanotube arrays are used as the oxygen diffusion electrodes of Li−O2 batteries. These carbon and binder free cathodes enable superior cyclability. The batteries realize long-term rechargeability at high discharge/ charge current densities (for instance, more than 140 cycles at 1 or 5 A g−1), exhibiting favorable stability. The outstanding stability of TiO2 compared with carbon materials in the Li−O2 battery operation environment is responsible for the long-term stability of the batteries, verified by comparing the XRD and XPS patterns of the cathodes before and after cycling. Furthermore, the 1HNMR measurement indicates that the TEGDME electrolyte used in present study has no decomposition after cycling, even in a wide discharge/charge voltage window. The stability of the electrolyte on the TiO2 surface is much more enhanced than that on the carbon surface, which has been reported to catalyze electrolyte (DMSO or TEGDME) decomposing in previous reports. The accessible hierarchical porous structures and outstanding stability lead to 2555

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