Research Article www.acsami.org
Three-Dimensional Ordered Macroporous FePO4 as High-Efficiency Catalyst for Rechargeable Li−O2 Batteries Chao Li,† Ziyang Guo,† Ying Pang, Yunhe Sun, Xiuli Su, Yonggang Wang,* and Yongyao Xia Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Fudan University, Shanghai 200433, China S Supporting Information *
ABSTRACT: The Li−O2 battery is receiving much recent attention because of its superhigh theoretical energy density. However, its performance is limited by the irreversible formation/decomposition of Li2O2 on the cathode and the undesired electrolyte decomposition. In this work, low-cost three-dimensional ordered macroporous (3DOM) FePO4 is synthesized by using polystyrene (PS) spheres template in a facile experimental condition and applied as a high-efficiency catalyst for rechargeable Li−O2 batteries, including good rate performance, high specific capacity, and perfect cycling stability. The superior performances can be attributed to the unique structure of 3DOM FePO4 cathodes, which can provide an efficient buffer space for O2/Li2O2 conversion. In addition, it is demonstrated that the Li+ intercalation/deintercalation behavior of 3DOM FePO4 in ether-based electrolyte can contribute to capacity for Li−O2 batteries over cycling. As a result, when there is no O2 in the environment, the Li−O2 cell can also be operated as a rechargeable Li-FePO4 cell with a perfect cycle capability. KEYWORDS: Li−O2 battery, three-dimensional ordered macroporous, FePO4, high cycling stability, Li-FePO4 battery
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INTRODUCTION With the urgent demand of safe and efficient energy devices, conventional lithium-ion batteries are restricted because of their limited theoretical energy density.1,2 In response, Li−O2 batteries are supposed to be the next generation of electrochemical energy storage system because of its ultrahigh theoretical energy density, which far exceed that of conventional lithium-ion batteries.3−8 In a typical nonaqueous Li−O2 battery, the porous cathode and Li metal anode are separated by a separator dipped in Li+ conducting electrolyte.9 Generally, the discharge process leads to the formation of Li2O2 deposited on the surface of porous cathode (O2 + 2Li+ + 2e− → Li2O2), while on recharge process, Li2O2 are converted back to Li and O2 (Li2O2 → O2 + 2Li+ + 2e−).10−12 One of the major challenges for rechargeable Li−O2 batteries is the selection of a suitable porous cathode, which is bifunctional to both the discharge/charge process. In practice, carbon has been extensively utilized for O2 catalytic electrode because of its high conductivity, large surface area and porous structure.9,12−16 However, the carbon reacts with Li2O2 in charge process to produce byproducts depositing on porous cathode, © 2016 American Chemical Society
which results in overpotential, specific capacity fading, and low cycling stability,5−10 thereby impeding carbon as a promising porous cathode. As a result, a large number of alternative porous cathodes to carbon were recently reported to improve the specific capacity, rate performance, energy efficiency, and cycling stability. Precious metals6,9,17,18 and transition metal oxides10,19−26 have been investigated as efficient O2 catalytic electrodes. Nevertheless, high price and complex fabrication of these catalysts limited their further application. Development of a cheap, efficient, and environmentally friendly electrocatalyst is urgent for Li−O2 batteries. It should be recognized that the materials with remarkable porosity make it a high-efficiency electrocatalyst for rechargeable Li−O2 batteries. Recent studies have revealed that the porous materials can not only provide abundant surface defects7 and a large amount of buffer space to accommodate lithium peroxide but also alleviate the volume expansion of O2 catalytic electrode Received: August 12, 2016 Accepted: October 31, 2016 Published: October 31, 2016 31638
DOI: 10.1021/acsami.6b10115 ACS Appl. Mater. Interfaces 2016, 8, 31638−31645
Research Article
ACS Applied Materials & Interfaces during formation/decomposition of Li2O2.7,13−16,23−25 Particularly, macroporous materials can function as “highways” to supply O2 over the entire pore network of cathode materials during the discharge process and facilitate a continuous oxygen flowing into O2 catalytic electrode without clogging.15,16 For example, Ru@nanoporous graphene15 was reported as a catalyst for high-performance Li−O2 batteries. The macroporous (∼250 nm) structure of graphene promotes O2 diffusion through the electrode and provides sufficient voids to accommodate discharge products. The hierarchical macroporous/mesoporous NiCo2O424 was also reported to serve as an efficient cathode catalyst for Li−O2 batteries. The increased specific capacity is due to the macroporous structure of NiCo2O4, which can provide more useful space to accommodate Li2O2. In addition to using the porous structure of cathode materials, designing a multifunctional catalyst for various energy storages sites with the same electrolyte has become a new strategy. Recently, Chen’s Group26 has reported that mesoporous MnO2 could act as working electrodes in lithium-ion batteries without O2, and decayed Li-MnO2 cells were utilized directly as rechargeable Li−air batteries when operated with O2. The dual utilization of electrode materials in various batteries motivated us to investigate whether conventional cathode materials used in lithium-ion batteries might be expanded in their applications for rechargeable Li−O2 batteries. Apart from MnO2, FePO4 was considered as a promising candidate for lithium-ion batteries because of its superior advantages, such as abundance, nontoxicity, and low cost.27−30 Especially, the lithium intercalation/extraction potential of FePO4 is also comparable to the discharge/charge platform of Li−O2 batteries, which may promote specific capacity for Li− O2 cells. Moreover, FePO4 is easy to prepare as a controlled porous structure,31 which is beneficial to O2/Li2O2 conversion. Therefore, FePO4 can be served as an electrocatalyst for Li−O2 battery, while the dual utilization of FePO4 has been never reported. Herein, to design an electrocatalyst with all these factors in mind, three-dimensional ordered macroporous (3DOM) FePO4 was synthesized by calcining polystyrene (PS) spheres template, expanding its application as a low-cost and high efficient catalyst for Li−O2 batteries. The morphology of 3DOM structure can not only provide large porous to facilitate the gas-phase oxygen but also accommodate the reversible formation/decomposition of Li2O2 on discharge/charge. Apart from favorable porous framework, Li+ intercalation/deintercalation of 3DOM FePO4 make it promote capacity over cycles. As a result, the rechargeable Li−O2 batteries with 3DOM FePO4 cathodes exhibit outstanding electrochemical performance, including high specific capacity, good rate performance, and excellent cycling stability (300 cycles at a fixed capacity of 1000 mAh g−1). If isolated from O2, the Li−O2 cell can be also served as a rechargeable Li-FePO4 cell, exhibiting an eminent cycle capability (100 cycles at a current density of 20 mA g−1).
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solution (acetone:tetrahydrofuran = 1:1 in volume) to remove PS spheres templates. After that, the prepared powders were calcined in a muffle furnace with a heating rate of 1 °C/min from room temperature to 400 °C, and the temperature was maintained for 12 h. The target products were yellow powders called 3DOM FePO4. Material Characterization. X-ray diffraction (XRD) measurements were performed on a Bruker D8 Focus power X-ray diffractometer with Cu Kα radiation to characterize 3DOM FePO4. The surface morphology and microstructure of obtained samples were observed with field emission scanning electron microscopy (SEM) using a Hitachi S-4800 microscope and transmission electron microscopy (TEM) on a JEOL JEM-2100 F microscope (Japan) operated at 200 kV. Elemental mapping of 3DOM FePO4 was performed using energy-dispersive X-ray (EDX) equipped in TEM. Fourier transform infrared (FT-IR) spectra were performed on a Nicolet 6700 spectrometer. X-ray spectra (XPS) were carried out on a XSAM800 Ultra spectrometer. 3DOM FePO4 cathodes with different discharge/charge stages were examined by ex situ SEM, TEM, XRD, FT-IR and XPS, respectively. Electrochemical Measurement. Preparation of O2 Catalytic Cathode. To prepare 3DOM FePO4 cathode, 45 wt % 3DOM FePO4, 45 wt % Ketjenblack (KB), and 10 wt % polyvinylidene fluoride binder (PVDF) were mixed in N-methyl-2-pyrrolidone (NMP). The resulting slurry was pasted on a current collector (TGP-H-060 carbon paper, Torray). The mass loading of 3DOM FePO4 cathode is about 1 mg cm−2, and bare KB-based electrodes were prepared with the same method for comparison. Fabrication and Electrochemical Measurements for Li−O2 Batteries. The battery assembly was operated in an argon-filled glovebox. O2 catalytic electrode and Li metal were separated by a separator dipping with TEGDME-(1M) LiTFSI electrolyte. The Li anode/separator/3DOM FePO4 cathode was then sealed into a Swagelok cell with a hole (∼0.8 cm−2) placed on cathode side. Land cycler (Wuhan Land Electronic Co. Ltd.) was employed for electrochemical measurements of Li−O2 cells in a pure/dry oxygenfilled glovebox. A quadrupole mass spectrometer (NETZSCH QMS 403 C) with leak inlet was connected to a customized Swagelok cell assembly for differential electrochemical mass spectrometry (DEMS) measurement. All of the specific capacities and current densities were calculated on the basis of the mass loading of carbon (KB).
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RESULTS AND DISCUSSION 3DOM FePO4 are synthesized by PS colloidal crystal templates. Field emission scanning electron microscopy (SEM) images of PS spheres templates with different magnifications are shown in Figure S1. These PS particles show ordered spheres arrays with a diameter of about 250 nm. After that, the obtained 3DOM FePO4 is successfully synthesized by replicating the void structure of the PS spheres templates. The typical SEM images of synthesized 3DOM FePO4 are shown in Figure 1a,b with different magnifications. The average diameter of the pore sizes is about 250 nm, which is almost consistent with prepared PS spheres. It can be further detected from Figure 1c,d that the structure of synthesized 3DOM FePO4 is in agreement with the SEM observation. The X-ray diffraction (XRD) pattern of prepared 3DOM FePO4 powder presents no apparently diffraction peaks (Figure S2), indicating that the 3DOM FePO4 is a completely amorphous character. Moreover, energydispersive X-ray (EDX) mapping is also utilized to further confirm the components of obtained 3DOM FePO4. Transmission electron microscopy (TEM) image of the sample and EDX mapping of elements Fe, P, and O are shown in Figure 1e−h. In particular, the Fe, P, and O elements are uniformly distributed (Figure 1f−h), proving the presence of amorphous 3DOM FePO4. The obtained 3DOM FePO4 materials are employed as cathodes for rechargeable Li−O2 cells to test their electro-
EXPERIMENTAL SECTION
Material Synthesis. PS spheres templates were synthesized by emulsion polymerization,32 previously. In the preparation of 3DOM FePO4, PS spheres templates were immersed in 0.3 M Fe(NO3)3 solution (H2O: C2H5OH = 3:1 in volume) for 6 h. Then, the PS spheres with adsorbed Fe(NO3)3 were immersed in 0.3 M NH4H2PO4 solution (H2O: C2H5OH = 3:1 in volume) for 6 h. By the reaction between Fe(NO3)3 and NH4H2PO4, the generated precipitation of FePO4 filled the void of PS spheres template. The obtained sample was washed by deionized (DI) water and then soaked in a mixed 31639
DOI: 10.1021/acsami.6b10115 ACS Appl. Mater. Interfaces 2016, 8, 31638−31645
Research Article
ACS Applied Materials & Interfaces
O2 cells with KB cathode is inferior to that of 3DOM FePO4 cathode, reflecting poor rate performance and low capacity retention. The result reveals that obtained 3DOM FePO4 is a high-efficiency catalyst for Li−O2 batteries. The superior electrocatalytic performance of 3DOM FePO4 cathodes can be attributed to the 3DOM structure, which can promote the reversible formation and decomposition of Li2O2, as well as improve the diffusion of oxygen and electrolyte in the discharge/charge processes.15,16,24,25 Figure 2c presents the initial 5 cycles within a cutoff potential window from 2.2 to 4.5 V at 500 mA g−1. The Li−O2 cell with the 3DOM FePO4 cathode displays an initial discharge capacity of 6021 mAh g−1, which is much higher than that of KB cathode (4370 mAh g−1 for the first cycle, Figure S3c), and the load curves are reproducible on subsequent cycles with a slight decrease, indicating remarkable catalytic activity of the 3DOM FePO4 cathode. Unfortunately, the discharge capacity reduces to 4815 mAh g−1 at the fifth cycle, with serious overpotential and limited charge capacity. In addition, Figure 2d reveals that the Coulombic efficiency of Li−O2 cell using 3DOM FePO4 cathode for initial 5 cycles (>75% after the fifth cycle), which is also superior to that of KB cathode (70% after the fifth cycle, Figure S 3d). The results suggest that 3DOM FePO4 as cathode materials plays a critical role to enhance the specific capacity and Coulombic efficiency. However, it also should be noted that the 3DOM FePO4-based Li−O2 batteries still display poor cycle performance at full discharge/charge depth, leading to a huge energy loss, which is similar to previous reports.9−16,19−25,33−36 With respect to the problems above, most reported nonaqueous Li−O2 cells were investigated with capacity-limited cycle method.3−16,19−26,33,35−38 Figure 3a presents the cycling performance of 3DOM FePO4-based Li−O2 cell with limited
Figure 1. (a,b) SEM images and (c,d) TEM images of 3DOM FePO4 with different magnifications. (e) TEM image of 3DOM FePO4 (bottom left) and (f−h) the corresponding EDX mapping images of Fe, P, and O elements.
catalytic behaviors. Li−O2 cells with KB cathodes are also tested in the same electrochemical measurements as a comparison. Figure 2a,b presents the first discharge/charge performance of Li−O2 cells with 3DOM FePO4 cathodes at different applied current densities. As expected, discharge/ charge capacities correlate to the current rate (Figure 2a). The Li−O2 cell with 3DOM FePO4 cathode delivers a specific discharge capacity of 9923 mAh g−1 at a current density of 100 mA g−1. At higher current densities, the specific capacity decreases to 9060 mAh g−1 at 200 mA g−1 and 6021 mAh g−1 at 500 mA g−1, and the corresponding capacity retention is 91.3 and 60.7%, respectively (Figure 2b). In comparison, KB cathodes (Figure S3a and b) are also measured at different applied current densities, whereas, the rate performance of Li−
Figure 2. Discharge/charge curves (a) and capacity retention capability (b) of Li−O2 cells using 3DOM FePO4 cathodes at different applied current densities. (c) The discharge/charge curves of the Li−O2 cells with 3DOM FePO4 cathodes for the initial 5 cycles at a current density of 500 mA g−1 with the voltage window of 2.2−4.5 V and (d) corresponding Coulombic efficiency over the initial 5 cycles. 31640
DOI: 10.1021/acsami.6b10115 ACS Appl. Mater. Interfaces 2016, 8, 31638−31645
Research Article
ACS Applied Materials & Interfaces
aggravating undesired reactions through the charge process.6−10,33,35,36 To clarify the correlation between prominent performance of 3DOM FePO4-based Li−O2 cells and their 3DOM structure, the morphologies of 3DOM FePO4 cathodes at different discharge/recharge stages are collected by ex situ SEM and TEM technologies. Figure 4a presents the SEM image of
Figure 4. Various morphologies of 3DOM FePO4 cathodes at different states: SEM images at (a) pristine, (b) discharged, and (c) recharged state, respectively; TEM images at (d) pristine, (e) discharged, and (f) recharged state, respectively. Figure 3. Discharge/charge curves of Li−O2 cells with (a) 3DOM FePO4 cathode and (b) KB cathode during different cycles with a fixed capacity of 1000 mAh g−1. Current density: 250 mA g−1.
pristine 3DOM FePO4 cathode, where 3DOM FePO4 is evenly covered with KB and PVDF. After discharge, it can be obviously observed that a large amount of discharge products are formed on the surface of 3DOM FePO4 cathodes (Figure 4b). As shown in Figure 4c, the discharge products disappear on the recharged cathode, demonstrating the reversible formation and decomposition of discharge products during cycles. In addition, TEM technology is also carried out to further confirm the deposition site of discharge products. Compared with the pristine 3DOM FePO4 cathode (Figure 4d), 3DOM FePO4 cathodes are fully occupied with discharged deposits after discharge (Figure 4e). These results indicate that the framework of 3DOM FePO4 cathodes can accommodate buffer space efficiently for discharge products to deposit. Moreover, the unique framework of synthesized 3DOM FePO4 appears again on recharged cathode (Figure 4f), proving the decomposition of discharge products. However, SEM and TEM investigation are not enough to clarify the discharge/charge species on the 3DOM FePO4 cathodes. Ex situ XRD and Fourier transform-infrared spectroscopy (FT-IR) technologies are utilized to investigate 3DOM FePO4 cathodes on pristine, discharged, and recharged stages during the first cycle. The characteristic peaks related to Li2O2 appear at the end of discharge and subsequently vanish after recharge (Figure 5a). This result suggests that the formation/decomposition of Li2O2 is reversible over the first cycle. XRD is sufficient to characterize Li2O2 crystal, whereas less-crystalline compounds formed by discharge/charge cycles cannot be characterized by XRD alone. FT-IR technology is also used to detect the products for Li−O2 batteries using 3DOM FePO4 cathodes during cycles. As shown in Figure 5b, the characteristic peaks of Li2O2 in FT-IR spectra can be observed in discharged 3DOM FePO4 cathode, whereas the peaks disappeared at the end of recharge. The results from ex situ FT-IR spectra demonstrate that Li2O2 acts as an overwhelming discharge product and vanishes after recharge. In addition, peaks in addition to Li2O2 may be assigned to overlap of the bands from Li2CO3 and other byproducts on discharge stage. This undesired side product could be generated
discharge/charge depth, and a long cycle life can be observed for 300 cycles at 250 mA g−1 with a fixed capacity of 1000 mAh g−1. Moreover, the terminal voltages of discharged 3DOM FePO4 cathode in the Li−O2 cell are higher than 2.2 V after 300 cycles, indicating that the perfect cycling stability of this cell can be significantly improved by the help of the 3DOM FePO4. Such long cycle life is much better than those of previously reported carbon, transition metal oxides, and noble metals.3−10,12−26,33,35−38 For comparison, the Li−O2 cell with KB cathode is also measured in the same condition, and the discharge voltages of KB-based electrode degraded to
