β-FeOOH decorated highly porous carbon aerogels composite as a cathode material for rechargeable Li

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b-FeOOH decorated highly porous carbon aerogels composite as a cathode material for rechargeable Li–O2 batteries
Wei Chen, Yanqing Lai, Zhian Zhang,* Yongqing Gan, Shaofeng Jiang and Jie Li
A composite of b-FeOOH–carbon aerogels (FCAs) was designed and prepared via a simple hydrothermal method. The samples were characterized by X-ray di?raction, ?eld emission scanning electron microscopy and N2 adsorption/desorption measurements. It is found that the composite possesses a large pore volume and, especially, has a high proportion of mesopores. The material was studied as an O2 electrode for non-aqueous lithium–O2 batteries and the electrochemical performance of the electrode was evaluated by using galvanostatic discharge–charge processes and cyclic voltammetry (CV). The FCAs electrode exhibits excellent performance and delivers a discharge capacity of up to $10 230 mA h gcarbon+catalyst?1 at 0.1 mA cm?2 and a high C-rate performance with a discharge capacity of $6110 mA h gcarbon+catalyst?1 at 0.2 mA cm?2 and $4270 mA h gcarbon+catalyst?1 at 0.5 mA cm?2, it also exhibits a stable discharge voltage plateau at 2.75 V and a charge voltage plateau of $3.75 V, and shows a good cycle performance. The excellent electrochemical performance of the FCAs is attributed to the synergistic e?ect of the good catalytic activity of b-FeOOH and the high porosity of the carbon aerogels. These results demonstrate that the carbon aerogels are suitable for the skeleton material to load the catalyst and the FCAs can be an outstanding material for the cathode of Li–O2 batteries.

Received 14th December 2014 Accepted 18th February 2015 DOI: 10.1039/c4ta06879c www.rsc.org/MaterialsA

1

Introduction

High energy secondary battery technologies are of great importance due to the rapidly increasing development of portable electronics and electric vehicles.1,2 As one of the most promising secondary battery technologies, rechargeable Li–O2 batteries have attracted extensive attention owing to their high energy density of 5200 W h kg?1 (3212 W h kg?1 as O2 included), which is almost equal to gasoline and is 5–10 times higher than state-of-the-art Li-ion batteries.3,4 However, the slow kinetics of both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) on the cathode result in several challenges that need to be addressed before the practical application of Li–O2 batteries, including large over-potentials, short cycle life and low rate capability.5,6 Since the cathodic catalysts play a key role in promoting both OER and ORR in Li– O2 batteries, extensive research e?orts have been devoted to develop e?cient electrocatalysts, such as precious metals,7–9 transition metal oxides10–16 and metal complexes,17 to promote the electrochemical performance of Li–O2 batteries. As the most promising catalysts, transition metal oxides and oxyhydroxides have received more and more interest due to their excellent catalytic activity and low cost.15,18–20

School of Metallurgy and Environment, Central South University, Changsha 410083, China. E-mail: zza75@163.com

In recent years, b-FeOOH, which has a characteristic 2 ? 2 tunnel structure in its lattice, has been found to possess a good intrinsic catalytic activity. The inner tunnels can accommodate both Li+ and O2? and enable the reversible formation and decomposition of Li2O2. The research reported by Bruce et al. also demonstrated that the special 2 ? 2 tunnels present in aMnO2 are one of the main reasons for its excellent catalytic activity.14 b-FeOOH has been applied as a catalyst for the oxygen reduction reaction in borohydride fuel cells and for water splitting, it exhibits excellent performance which con?rms the good intrinsic catalytic activity of b-FeOOH.21,22 Jung et al. synthesized b-FeOOH by ultrasonic-irradiated chemical synthesis. A?er being mechanically mixed with Ketjen black, the b-FeOOH was used as a catalyst for lithium–O2 batteries, and the cells showed superior performance.23 However, there are still a lot of challenges in improving the catalytic ability of bFeOOH and the performance of the Li–O2 batteries. As the skeletal material for the cathode of Li–O2 batteries, carbon materials also have a signi?cant impact on the cell performance. The pores of the carbon materials not only supply room for the insoluble discharge product (Li2O2) deposition, but also act as the means of transport of oxygen and Li+ during the charge–discharge process, which is related to the formation or decomposition of Li2O2.24–26 Therefore, the design of the pore structure of the carbon materials is very important, and a large pore volume is essential for an excellent carbon material for use in Li–O2 batteries. It is worth noting that the mesopores in

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carbon materials have a larger contribution to these two aspects. Previous studies have proved that small pores would be easily choked by Li2O2 precipitation, and then the transport of reactants (O2 and Li+) would be hindered, which will prevent the pore volume from being utilized further.25,27,28 The speci?c surface area of the carbon materials is another critical factor determining the cell performance. It is found that the insoluble and insulating discharge products will form a solid ?lm on the surface of the carbon materials in the discharge process, which will lead to electrical passivation of the cathode and then cause serious polarization.29,30 So it has previously been argued that the electrical passivation of the cathode by the discharge products terminates further reaction on the cathode, rather than pore clogging.29,31,32 Thus, carbon materials with larger speci?c surface area are more conducive to achieving higher discharge capacity. Furthermore, carbon materials are usually the carrier of the catalyst in Li–O2 batteries, it has been shown by many studies that the pore structure, the speci?c surface area and the conductivity of carbon materials all have a certain in?uence on the catalyst performance.9,33–35 Here, in our work, a composite of b-FeOOH–carbon aerogels (FCAs) was designed and studied as an O2 electrode for nonaqueous lithium–O2 batteries. Carbon aerogels (CAs) are a carbon material with versatile properties, such as high porosity, large surface area, outstanding electrical conductivity, and especially their high proportion of mesopores.36–38 These properties make CAs bene?cial to the deposition of the solid discharge products (Li2O2) of lithium–O2 batteries, the transportation of abundant O2 and Li+ in the charge–discharge process, and the exhibition of the catalytic ability of the catalyst. Therefore, these characteristics make CAs an ideal cathode carbon material for lithium–O2 batteries, but they have rarely been reported in lithium–O2 batteries as far as we know.39,40 A?er a simple hydrothermal process, b-FeOOH was grown on the surface of CAs via the in situ synthesis route, which is conducive to a more uniform distribution of b-FeOOH. This composite combines the excellent catalytic performance of bFeOOH with the excellent porous structure of CAs and can deliver a really high discharge capacity of up to $10 230 mA h gcarbon+catalyst?1. These results show that highly porous CAs decorated by b-FeOOH composite would be a promising material for the cathode of lithium–O2 batteries.

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A?er magnetic stirring for 20 min, 0.15 g of CAs was dispersed in the solution. Following 60 min of sonication and 60 min of magnetic stirring, the resulting solution was transferred into a 60 ml Te?on-lined stainless steel autoclave. The autoclave was kept at 145  C in an oven for 9 h. When the autoclave was cooled down to room temperature, the product was removed from the reaction solution, washed with deionized water and ethanol 3 times by centrifugation, and dried in an oven at 60  C for 12 h. As a control, a b-FeOOH–Super P composite (FSP) was prepared by the same procedure. 2.3 Materials characterizations

X-ray di?raction patterns (XRD) were collected by a Rigaku3014 using graphite-monochromated Cu Ka radiation. Field-emission scanning electron microscopy (SEM) images taken on a Nova NanoSEM 230 and transmission electron microscopy (TEM) images obtained by using a Tecnai G2 20ST were employed to observe the morphology of the samples and/or the electrodes before and a?er discharge/charge. Atomic absorption spectroscopy (AAS) was performed on a TAS-986. Raman spectra (Raman) were recorded on a Jobin-Yvon LabRAM HR800 Raman spectrometer. N2 adsorption/desorption measurements were performed at 77 K by using a Quantachrome instrument (Quabrasorb SI-3MP). 2.4 Electrochemical tests

2 Experimental
2.1 Materials Ferric chloride hexahydrate (FeCl3$6H2O) and glycerol were obtained from Sinopharm (Shanghai) Chemical Reagent Co., Ltd., China. CAs were purchased from Deruifengkai (TianJin). Other chemical reagents were all analytical grade and were used without any further puri?cation. 2.2 Synthesis of b-FeOOH–carbon aerogels composite

The FCAs is fabricated via a simple solvothermal route. 1.365 g of FeCl3$6H2O (5 mmol) was added into a mixed solvent, which was composed of 6 ml of glycerol and 44 ml of deionized water.

Electrochemical measurements were carried out in Li–O2 coin cells with 3 ? F1.5 mm holes in the center of the positive pans in an evenly distributed pattern to allow O2 passage. A Li foil was used as the anode for the Li–O2 batteries, which was separated from the O2 electrode by a glass micro?ber separator. The O2 electrodes (10 mm in diameter) were prepared by casting a mixture containing the as-prepared b-FeOOH–carbon aerogels composite (80 wt%), Super P (10 wt%) and PVDF (10 wt%) onto a Ni foam current collector, followed by drying at 50  C for 12 h in a vacuum oven, and the obtained electrode is denoted here as the FCAs electrode. The same method was used to fabricate the b-FeOOH–Super P electrode, and this electrode was named the FSP electrode. The mass loading of carbon and catalyst on the O2 electrode is $1 mg cm?2. The mass percentage of b-FeOOH in the composite measured by AAS analysis was $30%. The electrolyte consisted of 1 M LiTFSI in tetraethylene glycol dimethyl ether (TEGDME). The Li–O2 cells were constructed in an argon atmosphere glove box (Universal 2440/750) with oxygen and water contents of less than 1 ppm. The galvanostatic discharge/charge measurement of the Li–O2 battery was performed at a current density of 0.1–0.5 mA cm?2 in the potential range of 2.0–4.4 V under a LAND CT2001A system. It is noted that the speci?c capacity was calculated based on the total mass of carbon and catalyst. Cyclic voltammetry (CV) was carried out with Solartron 1470E electrochemical measurement system. CV measurements were conducted at a scan rate of 0.2 mV s?1 in the voltage range of 2.0–4.4 V. All measurements were undertaken in 1 atm dry oxygen to avoid any complications related to H2O and CO2.

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3 Results and discussion
The structure of b-FeOOH with a typical 2 ? 2 tunnel is illustrated in Fig. 1a. In the walls of the tunnel, the edge-shared double chain of Fe3+O6 octahedra shares its corners with the neighboring double chain. The XRD patterns of the pure bFeOOH, CAs, as-prepared FSP and as-prepared FCAs are shown in Fig. 1b. It can be observed that CAs show two broad di?raction peaks at around 23 and 43 , which can be indexed to the (002) and (100)/(101) planes of graphitized carbon, respectively. This result suggests that CAs are a short-range-ordered partial graphitized carbon material.36,41 In the XRD patterns of FSP and FCAs, apart from the peaks at $23 and $43 which result from the SP and CAs of the composites, respectively, all the marked peaks can be ascribed to the well-crystallized tetragonal bFeOOH phase (space group I4/m, JCPDS card no. 34-1266) without any impurities.42 This result demonstrates the successful synthesis of the composite. The morphology of pure b-FeOOH, pure SP, pure CAs, FSP, and FCAs was characterized through SEM, and the obtained SEM images are presented in Fig. 2a–e. From the SEM image of b-FeOOH shown in Fig. 2a, it can be observed that the nanospindles acquire an average length of 100–180 nm and an average width of 20–40 nm. An image of the SP is shown in Fig. 2b. It can be seen that the particle size of Super P is smaller than CAs (Fig. 2c), but the surface of the SP particles is smooth, with no open pores. This kind of morphology is unfavorable for Li2O2 deposition and reactant transport. Fig. 2c indicates that the CAs particles exhibit a loose and porous structure. There is an abundance of disordered open pores on the surface of the irregular CAs particles. During the charge–discharge process, the pores could supply room for the deposition of discharge products and act as the O2 and Li+ transport highways in the electrode. The image of the as-prepared FSP shown in Fig. 2d shows that the spindle-like b-FeOOH particles formed in the solvothermal reaction are distributed among the Super P particles. From the image of the FCAs (Fig. 2e), it can also be observed that the b-FeOOH nanospindles integrate well with the CAs particles and are anchored on the surface of the CAs particles uniformly. And compared with the pure CAs, there are no obvious changes in the morphology of the CAs particles in the as-prepared FCAs. This result indicates that the FCAs composite has been synthesized. The detailed morphology and

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Fig. 2 SEM images of the pure b-FeOOH (a), SP (b), CAs (c), FSP (d), FCAs (e), TEM images of the FCAs (f), and HRTEM of the FCAs (inset).

Fig. 1 (a) Schematic illustration of the akaganeite structure of bFeOOH (projection along [001]). (b) XRD patterns of CAs, b-FeOOH and the as-synthesized FCAs and FSP.

microstructure of the FCAs composite can be further con?rmed by transmission electron microscopy (TEM) as shown in Fig. 2f. It can be observed that the CAs particles possess a porous structure and are decorated by b-FeOOH particles, which is consistent with the SEM image of the FCAs. A high resolution TEM (HRTEM) image of the b-FeOOH nanospindles in the asprepared FCAs is shown in the inset of Fig. 2f. The regular interplanar spacing of $0.229 nm corresponds to the (301) plane of the b-FeOOH (JCPDS card no. 34-1266) crystal lattice. These results further con?rm that the FCAs composite has been successfully synthesized. And the SEM and TEM images of the FCAs reveal that the FCAs possesses huge porosity and a large speci?c surface area. The pore structure of the FCAs and FSP was further investigated by N2 adsorption–desorption measurements. The obtained N2 adsorption–desorption isotherms and pore-size distribution (PSD) curves are shown in Fig. 3. As shown in Fig. 3a, the two samples exhibit typical type IV N2 sorption isotherms following the IUPAC classi?cation with distinct H3 hysteresis loops.43–45 The total speci?c surface area of the FCAs calculated by multi-point Brunauer–Emmett–Teller (BET) theory is about 961.5 m2 g?1. This value is much higher than the total speci?c surface area of many other carbon materials in previous reports, which was also calculated by multi-point BET theory.30,46 Meanwhile, the FCAs possesses a large pore volume of 1.21 cm3 g?1. The surface area and the pore volume of the

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Fig. 3

(a) The nitrogen adsorption/desorption isotherms of the FCAs and FSP; (b) the pore size distribution of the FCAs and FSP.

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FCAs are much higher than those of FSP (52.3 m2 g?1 and 0.22 cm3 g?1, respectively). This conclusion is fully supported by the PSD curves shown in Fig. 3b, which are obtained from the N2 isotherms based on nonlocal density functional theory (NLDFT) calculation. From the PSD curves (Fig. 3b), it can also be suggested that the FCAs possesses an abundance of mesopores (2– 50 nm). The mesopore volume and surface area of mesopores of the FCAs are 1.05 cm3 g?1 and 566 m2 g?1, respectively. It can be noted that the FCAs also possesses a larger amount of mesopores than the FSP. As shown in Fig. 3b, the mesopore sizes of the FCAs range from 2 to 30 nm with the dominant pore size around $15 nm, which is similar to the pore size distribution of pure CAs. In comparison with pure CAs, the amount of gas absorbed by the FCAs decreased. This means that when bFeOOH was combined, the porosity of the FCAs dropped.47 The surface area and the pore volume of CAs are 1579.5 m2 g?1 and 1.81 cm3 g?1, respectively, and the mesopore volume and surface area of mesopores of the CAs are 1.49 cm3 g?1 and 832.1 m2 g?1, respectively. The decrease in pore volume and surface area is ascribed to part of the pores being taken over or blocked by b-FeOOH. The large pore volume, especially the high proportion of mesopores, helps to hold more solid discharge products (Li2O2) and to ensure the transport of O2 and Li+ during the charge–discharge process.21,22 And the large surface area of the FCAs can provide more surface for the deposition of insulating discharge products and defer the electrical passivation of the cathode.25,27–29,48 For the same reason, the large surface area can also defer the covering of the b-FeOOH by the insulating discharge products and avoid the degeneration of the catalytic activity of b-FeOOH, and so allows the excellent intrinsic electrocatalytic ability of the b-FeOOH to be more e?ectively utilized.29,49–51 Therefore, the FCAs may be an excellent cathode material for Li–O2 batteries. In order to investigate the ORR and OER performance of the FCAs in Li–O2 batteries, cathodes with FCAs were prepared and galvanostatic charge–discharge measurements were performed in an oxygen atmosphere at the voltage range of 2.0–4.4 V (vs. Li+/Li) at di?erent current densities. For comparison, the performance of FSP was also tested under the same conditions. The discharge and charge curves of the two di?erent electrodes are presented in Fig. 4. The FCAs electrode delivers a discharge capacity of $10 230 mA h gcarbon+catalyst?1 at a current density of 0.1 mA cmcarbon+catalyst?2, which is much higher than the FSP electrode ($6060 mA h gcarbon+catalyst?1). The FCAs electrode also shows a very stable discharge voltage plateau at $2.75 V,

Fig. 4 Discharge/charge voltage curves of (a) the FCAs electrode and (b) the FSP electrode in the voltage range between 2.0 and 4.4 V at di?erent current densities.

which is not only higher than the FSP electrode ($2.66 V) by about 0.09 V, but also higher than that of previously reported catalysts.10,34,52,53 Meanwhile, the FCAs electrode exhibits a charge voltage for the OER process at $3.75 V vs. Li+/Li. The charge curve of the FCAs electrode exhibits two voltage plateaus, which is similar to previous reports.54–57 The charge voltage of the FCAs electrode is considerably lower than the FSP electrode ($4.03 V) by about 0.28 V and is comparable to that of noble metal catalysts and other reported metal oxide catalysts.58,59 Moreover, based on the discharge and charge potential plateaus, the discharge and charge curves shown in Fig. 4 reveal that the roundtrip e?ciency of the FCAs electrode is $73.3%, while the FSP electrode exhibits only $65.8%. The charge capacities of the FCAs electrode and the FSP electrode are $10 210 mA h gcarbon+catalyst?1 and $6050 mA h gcarbon+catalyst?1, respectively. The result reveals the excellent catalytic activity of the b-FeOOH for ORR and OER processes.35,52,60–62 It should also be noted that the FCAs electrode exhibits much larger charge– discharge capacities and lower polarization than the FSP electrode. It is rationally deduced that the large charge–discharge capacities and low polarization of the FCAs electrode is attributed to the synergistic e?ect of the good catalytic activity of bFeOOH and the high porosity of the CAs. The larger pore volume of CAs, especially the high proportion of mesopores, not only supplies abundant room for the deposition of solid discharge products (Li2O2) which causes the large discharge capacity, but can also o?er more abundant transportation paths of O2 and electrolyte which may facilitate the transport of O2 and Li+

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during the charge–discharge process.21,22 In addition, the large surface area of the FCAs could defer the electrical passivation of the cathode by providing more surface for the deposition of passivated ?lm which is composed of insulating discharge products (Li2O2).25,27–29,48 Furthermore, the large surface area can also prevent the b-FeOOH from being covered prematurely, which could decrease its catalytic activity according to previous reports.29,49–51 So the porous structure of CAs allows the excellent intrinsic electrocatalytic ability of b-FeOOH to be fully displayed. As the current density increases, the FCAs electrode presents a better rate capability than the FSP electrode. It is not surprising to observe that the potential di?erence between the charging and discharging potentials becomes higher, which originates from the higher ohmic loss caused by the higher current density.24,63 It can also be observed that the discharge capacity ($6110 mA h gcarbon+catalyst?1 at 0.2 mA cmcarbon+catalyst?2 and $4270 mA h gcarbon+catalyst?1 at 0.5 mA cmcarbon+catalyst?2, respectively) and charge capacity ($6090 mA h gcarbon+catalyst?1 at 0.2 mA cmcarbon+catalyst?2 and $4230 mA h gcarbon+catalyst?1 at 0.5 mA cmcarbon+catalyst?2, respectively) decreased with the rise in current density owing to the oxygen di?usion limitation in the nonaqueous electrolyte.24,64 For further investigation, the cyclic voltammograms (CVs) of Li–O2 batteries with these two di?erent cathodes were measured under an O2 atmosphere, the results are shown in Fig. 5. The two di?erent cathodes both exhibit ORR and OER peaks. Compared with the FSP electrode, a higher ORR onset potential can be observed for the FCAs electrode. The FCAs electrode also has a higher ORR peak current response. Meanwhile, the current in the OER process for the FCAs electrode is larger than for the FSP electrode. These results indicate that the FCAs possesses a better catalytic activity than the FSP. And the enhanced ORR and OER kinetics for the FCAs electrode may be attributed to the synergistic e?ect of the good catalytic activity of b-FeOOH and the high porosity of the CAs. This result con?rms that the FCAs electrode can be an excellent and e?ective cathode for Li–O2 batteries.65

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Limiting the charge and discharge capacities is widely used to improve the cycle stability of Li–O2 batteries.13,54,66 In this study, the cycling performance of the FCAs electrode was also investigated by con?ning the discharge/charge capacities to 800 mA h gcarbon+catalyst?1, at a current density of 0.1 mA cm?2 in the voltage window between 2.0 and 4.4 V. For comparison, a cell with the FSP electrode was also examined under the same capacity-limited mode. The results are shown in Fig. 6. As shown in Fig. 6a, the FCAs electrode exhibits good cycle performance over 60 cycles with stable reversible capacities. From Fig. 6b, it is found that the discharge/charge potential plateaus of the FCAs electrode for the ?rst cycle are $2.80 V and $3.60 V, respectively, further con?rming the reasonable roundtrip e?ciency and the good reversibility of the FCAs electrode

Fig. 5 CV curves of Li–O2 batteries with the FCAs electrode and the FSP electrode.

Fig. 6 (a) Cyclic performance of the air electrode tested under a speci?c capacity limit of 800 mA h gcarbon+catalyst?1 in the voltage range between 2.0 and 4.4 V at a current density of 0.1 mA cm?2; and charge/discharge behavior of (b) the FCAs electrode and (c) the FSP electrode when curtailing the capacity to 800 mA h gcarbon+catalyst?1 in the voltage range between 2.0 and 4.4 V at a current density of 0.1 mA cm?2.

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for the ORR and OER processes. The discharge–charge potential di?erence of the FCAs electrode increased slightly during cycling. Although a little dramatic increase in the discharge– charge potential di?erence of the FCAs electrode is observed a?er 50 cycles, the discharge voltage of 2.42 V and the charge voltage of 4.15 V at the 60th cycle still remain acceptable. Meanwhile, as shown in Fig. 6a and c, the FSP electrode can only sustain 42 cycles, and it also exhibits a larger discharge– charge potential di?erence than the FCAs electrode (with a discharge voltage of 2.69 V and a charge voltage of 3.91 V for the ?rst cycle, respectively). These results clearly demonstrate the excellent cycle stability of the Li–O2 cell with the FCAs electrode. To further investigate the discharge and charge process of the Li–O2 batteries with FCAs cathodes, the FCAs electrode was studied by SEM before discharge, a?er discharge and a?er recharge. As shown in Fig. 7b, before discharge, the surface of the pristine FCAs cathode is uneven, on which the porous CAs particles with abundant pores densely distributed throughout their surfaces are loosely dispersed. And it can be observed that the b-FeOOH nanoparticles are distributed on the CAs particles. Fig. 7c shows that, a?er discharge, the insoluble discharge products precipitate on the surface of the electrode. A?er recharging, the solid discharge product disappeared and the porous structure and uneven morphology are regained on the FCAs electrode (Fig. 7d). But many studies indicate that even the relative stable ether-based electrolyte we used here is still not completely stable in Li–O2 battery systems due to the high reactivity of the lithium superoxide formed in the discharge process.67 And the decomposition products of the electrolyte are also solid. More importantly, the decomposition of electrolyte may contribute to the capacity of Li–O2 batteries and it is hard to decompose in the charge process.68–70 In order to verify whether the discharge and charge capacity of batteries with the FCAs electrode are derived from the generation and

decomposition of Li2O2, which is the discharge product of reversible Li–O2 batteries, ex situ XRD measurements were carried out. As shown in Fig. 7a, the XRD patterns of the pristine, discharged, and recharged electrodes were obtained. According to Fig. 7a, two peaks corresponding to the Li2O2 phase were observed at 33.1 and 34.8 in the XRD pattern of the discharged electrode, a?er the battery was discharged to 2.0 V at a current density of 0.1 mA cm?2.3,13 When the battery was fully recharged to 4.3 V, it was found that the two peaks fully disappeared, indicating that the Li2O2 phase formed during the discharging has been reversibly decomposed in the subsequent charging process. The results demonstrate that the observed close-packed solid product in Fig. 7c is believed to be Li2O2 and further con?rms that the discharge and charge capacity of the Li–O2 batteries with FCAs electrodes are mainly derived from the generation and decomposition of Li2O2, instead of the decomposition of the electrolyte. These results again suggest the superior electrochemical performance of the FCAs electrode.

4 Conclusions
In summary, we designed and prepared a composite of bFeOOH–carbon aerogels (FCAs) by a simple hydrothermal method. The composite, which possesses a large pore volume and, especially, has a very high proportion of mesopores, was studied as an O2 electrode for non-aqueous lithium–O2 batteries. The FCAs electrode exhibits outstanding electrochemical performance and delivers a discharge capacity of up to $10 230 mA h gcarbon+catalyst?1 at 0.1 mA cm?2 and a high C-rate performance, with a discharge capacity of $6110 mA h gcarbon+catalyst?1 at 0.2 mA cm?2 and $4270 mA h gcarbon+catalyst?1 at 0.5 mA cm?2, and also shows a good cycle performance. The excellent electrochemical performance of the FCAs is attributed to the synergistic e?ect of the good catalytic activity of b-FeOOH and the high porosity of the CAs. The porous structure is bene?cial to the deposition of the solid discharge product (Li2O2), and can o?er more abundant transportation paths for O2 and the electrolyte. The large surface area of the FCAs originating from the high porosity of the CAs could prevent the cathode from being passivated prematurely. Consequently, these results suggest that the FCAs can be an outstanding material for the cathode of Li–O2 batteries. Loading catalyst on the CAs would be a promising strategy to fabricate cathode materials for Li–O2 batteries.

Notes and references
1 F. Cheng and J. Chen, Nat. Chem., 2012, 4, 962. 2 P. Hartmann, C. L. Bender, M. Vracar, A. K. D¨ urr, A. Garsuch, J. Janek and P. Adelhelm, Nat. Mater., 2012, 12, 228. 3 X. Han, Y. Hu, J. Yang, F. Cheng and J. Chen, Chem. Commun., 2014, 50, 1497. 4 P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J. M. Tarascon, Nat. Mater., 2012, 11, 19. 5 F. J. Li, T. Zhang and H. S. Zhou, Energy Environ. Sci., 2013, 6, 1125.

Fig. 7 (a) XRD patterns of a pristine, discharged and recharged air electrode with the FCAs; SEM images of the FCAs electrode: (b) pristine, (c) after 1st discharge and (d) after 1st recharge.

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6454 | J. Mater. Chem. A, 2015, 3, 6447–6454

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