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Highly Crystallized Na2CoFe(CN)6 with Suppressed Lattice Defects as Superior Cathode Material for Sodium-Ion Batteries Xianyong Wu,† Chenghao Wu,† Congxiao Wei,† Ling Hu,† Jiangfeng Qian,*,† Yuliang Cao,† Xinping Ai,† Jiulin Wang,‡ and Hanxi Yang*,† †
College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
‡
S Supporting Information *
ABSTRACT: Prussian blue and its analogues have received particular attention as superior cathodes for Na-ion batteries due to their potential 2-Na storage capacity (∼170 mAh g−1) and low cost. However, most of the Prussian blue compounds obtained from the conventional synthetic routes contain large amounts of Fe(CN)6 vacancies and coordinated water molecules, which leads to the collapse of cyano-bridged framework and serious deterioration of their Na-storage ability. Herein, we propose a facile citrate-assisted controlled crystallization method to obtain low-defect Prussian blue lattice with greatly improved Na-storage capacity and cycling stability. As an example, the as-prepared Na2CoFe(CN)6 nanocrystals demonstrate a reversible 2-Na storage reaction with a high specific capacity of 150 mAh g−1 and a ∼ 90% capacity retention over 200 cycles, possibly serving as a low cost and high performance cathode for Na-ion batteries. In particular, the synthetic strategy described in this work may be extended to other coordination-framework materials for a wide range of energy conversion and storage applications. KEYWORDS: sodium-ion batteries, Prussian blue framework, lattice defects, controlled crystallization method, sodium cobalt hexacyanoferrate
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INTRODUCTION With fast-growing demand for the integration of renewable energy sources such as solar and wind into the electric grids, large-scale electric energy storage (EES) technologies are urgently desired to suppress their inherent intermittency.1,2 Among various types of rechargeable batteries, Li-ion batteries have been considered as most appealing candidate for grid-scale electric energy storage due to their high energy density and long cycle life; however, their limited lithium reserves on Earth and high cost might hinder their large-scale application.3−5 In pursuit of affordable electric storage batteries for EES applications, room-temperature Na-ion batteries appear to be an attractive alternative to their lithium counterparts because of the natural abundance and low cost of Na resources.6,7 To enable Na-ion technology, great efforts have been devoted to the development of suitable Na-insertion cathodes with not only adequate electrochemical capacity but also easy preparation. Until now, numerous host frameworks have been investigated as Na-storage cathodes, such as layered oxides, tunneled oxides, polyanion-type compounds and so on.8−13 Nevertheless, the strong binding of Na+ ions in close-packed oxides makes them kinetically frustrated in the intercalation reaction,8−10 while the heavy molar weight of large polyanions (XO4)n− (X = P, S, As, Si, etc.) restricts the reversible capacity to less than 120 mAh g−1.11−13 Recently, Prussian blue analogues Na2M[Fe(CN)6] (M = Mn, Fe, Co, Ni, Cu, etc.) have received particular attention as alternative Na-storage cathodes, due to their three-dimensional open frameworks, © XXXX American Chemical Society
potential two-electron redox capacity (up to 170 mAh/g with 2 Na+ storage) and fast intercalation kinetics.14−17 Generally, Prussian blue analogues can be easily synthesized via a simple coprecipitation reaction of transition-metal Mn+ cations and hexacyanoferrate Fe(CN)63−/4− anions.18−21 For instance, Goodenough et al. first introduced a KMFe(CN)6 family (M = Mn, Fe, Co, Ni, Cu, Zn) as nonaqueous sodium ion cathodes early in 2012.22 However, all these materials have reversible capacities less than 100 mAh g−1 and low Coulombic efficiencies of 60−80%, which are insufficient for practical battery applications. The latest research disclosed that the electrochemical performance of PB is closely related to its intrinsic crystal structure.23−29 It is known that during the conventional rapid precipitation process, large amounts of Fe(CN)6 vacancies occupied by coordinated water will exist in the crystal framework,30,31 leading to an actual chemical formula of Na2−xM[Fe(CN)6]1−y·□y·zH2O, in which □ stands for the Fe(CN)6 vacancies; 0 < x < 2; 0 < y < 1, as illustrated in Scheme 1. The presence of Fe(CN)6 vacancies will severely deteriorate the battery performance for the following reasons: (1) the increase of Fe(CN)6 vacancies will reduce the Na content in the lattice due to charge balance and introduce more water molecules into PB framework,30,31 thus decreasing the initial charge capacity and causing more side reactions brought Received: December 24, 2015 Accepted: February 5, 2016
A
DOI: 10.1021/acsami.5b12620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Therefore, it is of great importance to control the crystallization process so as to fabricate low-defect and welldefined PB frameworks and thus boost their ion storage capability. For instance, our group23,24 and Guo et al.25,26 reported on a slow chemical precipitation method to fabricate low-defect FeFe(CN)6 and Na0.61Fe[Fe(CN)6]0.94 nanocrystals that can realize a stable 2Na-storage reaction for hundreds of cycles with a high Coulombic efficiency of ∼100%. Cui’s group32 recently succeeded in crystallizing almost perfectly stoichiometric Na1.96Mn[Mn(CN)6]0.99 □0.01 2 H2O by using a large excess of sodium in solution during the synthesis, and achieved a surprisingly high capacity of 209 mAh g−1, nicely proving the effectiveness of minimizing the fraction of vacancy sites for advancing PB chemistry. Herein, we propose a facile citrate-assisted controlled crystallization method to synthesize low-defect and highly crystallized Na2CoFe(CN)6 nanocrystals. UV−vis spectrum confirms the chelating effects of citrate with Co ions, while in situ UV−vis spectral experiment proves the significantly decreased crystallization kinetics. Benefiting from its suppressed Fe(CN)6 defects and high crystalline, this material demonstrates highly reversible 2-Na storage reactions with a high specific capacity of 150 mAh g−1 and strong cycle stability with ∼90% capacity retention over 200 cycles. Such a synthetic method to obtain high-quality PB frameworks could be extended to a wide range of coordination-framework materials for electric energy conversion and storage applications.
Scheme 1. Schematic Crystal Structures of Prussian Blue Frameworks
a
An intact Na2MII[FeII(CN)6] framework without structural defects. The M ions are 6-fold coordinated to the nitrogen atoms of the CN ligands, whereas Fe ions are octahedrally neighbored with the carbon atoms of the CN ligands, forming a three-dimensional polymeric framework with large interstitial spaces. The guests (Na+ ions and zeolitic water molecules) occupy the interstitial “A” sites at the center of every eight sub-cubes. Zeolitic water molecules are omitted for clarity. bAn ideally defective NaMII[FeII(CN)6]0.75•□0.25 framework with 25% Fe(CN)6 vacancies existing in each unit cell. In this case, The M ions neighboring to vacancies are coordinated by water molecules whose total number in the unit is six. A real Prussian blue product would fall in between these two ideal crystal structures depending on the synthesis route.
about by H2O decomposition;22,26 (2) the crystal water molecules may compete with Na+ ions to accommodate the interstitial spaces,23,26 therefore blocking off the Na+ insertion reactions and decreasing the capacity utilization of PB compounds; (3) the randomly distributed Fe(CN)6 vacancies makes the PB frameworks fragile and vulnerable to collapse during the Na+ insertion/extraction reactions,23−27 thus leading to the structural instability and poor cycle life.
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RESULTS AND DISCUSSION In general, traditional synthesis of Prussian blue compounds Na2M[Fe(CN)6] is based on a direct precipitation reaction of M2+ cations and [Fe(CN)6]4− anions in aqueous solution, as shown in Figure 1a (I). Due to the extremely small solubility
Figure 1. (a) Schematic illustration of the conventional coprecipitation method (I) and the citrate-assisted controlled crystallization process (II) for the synthesis of Na2CoFe(CN)6; (b) and (c) UV−vis absorption spectra of citrate, Co2+, citrate−Co2+, Na4Fe(CN)6 and Na2CoFe(CN)6; (d) In situ UV−vis spectral monitoring of the crystallization kinetics of Na2CoFe(CN)6 synthesized with or without the addition of citrate ions as slow-release chelator. The absorption intensity is recorded at 370 nm, which is characteristic of Na2CoFe(CN)6. The inset is a magnified picture of the initial nucleation stage. B
DOI: 10.1021/acsami.5b12620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. Physical properties of the as-prepared Na2CoFe(CN)6 sample: (a) XRD patterns; (b) crystal structure; (c) SEM image; and (d and e) energy dispersive spectroscopy mapping images.
product constant of Na2Co[Fe(CN)6], Ksp = 1.8 × 10−15,33 the nucleation and growth stage occurs immediately and simultaneously, thus leading to irregularly shaped and randomly aggregated nanoparticles (∼50 nm) with large amounts of Fe(CN)6 defects and coordinated water in the lattice (Scheme 1b). To obtain a more well-defined crystal lattice (Scheme 1a), we adopted a citrate-chelating method to control the crystallization process as illustrated in Figure 1a (II). The Co2+ ions are first coordinated with citrate ions to form cobaltcitrate chelate and then coprecipitated with hexacyanoferrate ions in aqueous solution. Due to the very strong complex ability of citrate ions (Co2+·Citrate3−, log K1 = 12.5),33,34 the cobaltcitrate chelate acts as a reservoir to slowly release Co2+ ions, which will react with hexacyanoferrate ions to form initial nucleuses. The negatively charged citrate ions may even absorb on these nucleuses surfaces, thus suppressing the growth rate and inhibiting crystal aggregation. Eventually, the gradual and sustained release of Co ions will enable these nucleuses to grow into well-shaped and monodispersed Prussian blue nanocubes (∼600 nm) with greatly suppressed Fe(CN)6 defects and structural imperfection. UV−vis spectrometry was carried out to reveal the interactions between citrate ions and Co ions. As can be seen in Figure 1b, the blank Co2+ solution showed a maximal absorption at 510 nm wavelength, while pure citrate ions solution nearly had no absorption signal in this area. After the addition of citrate ions into Co2+ solution, the maximal absorption peak slightly shifted to lower a wavelength at 506 nm with obviously increased absorption intensity, indicating the coordination effect of citrate with Co ions.34 In situ UV−vis spectroscopy was used to monitor the crystallization kinetics of Na2CoFe(CN)6 with or without the presence of citrate as chelating agent. The absorption intensity at 370 nm wavelength was recorded as a function of reaction time, which reflects the change in the relative content of Na2CoFe(CN)6 product as shown in Figure 1c. As shown in Figure 1d, once the Co2+ and Fe(CN)64− ions were mixed together in the absence of citrate
ions, the absorption intensity reached its maximal value within a very short time (less than 10 s) and then remained unchanged, indicating the ultrafast crystallization process in a time scale of seconds. On the contrary, in the presence of citrate ions, the absorption intensity preserved at a very low value during 0−100 s, which could be ascribed to the initial nucleation stage. Then, it increased incrementally from 100 to 1000 s, which might correspond to the subsequent crystal growth process. Even after 3000 s, the intensity was still lower than that of control experiment. These results suggested that the crystallization speed could be significantly slowed down by the addition of citrate ions as slow-release chelator, and thus a highly crystallized Na2CoFe(CN)6 sample with reduced structural defects can be obtained. It is also found that the developed synthesis method is general and can be extended to prepare other Prussian blue analogues (e.g., Na2NiFe(CN)6 and Na2CuFe(CN)6) with high crystallinity and suppressed structural defects. On the basis of the ICP-AES spectroscopy and elemental analysis (Table S1), we can express the precise chemical composition of the as-synthesized Na2CoFe(CN)6 material as Na1.85Co[Fe(CN)6]0.99·□0.01·1.9H2O, whereas the control sample got in the absence of citrate ions is Na1.32Co[Fe(CN)6]0.88·□0.12·3.6H2O. The significant suppression of Fe(CN)6 defects from conventional 12% to 1% (Table S1) enables more Na+ ions (1.85 Na+) to enter into the lattice for charge counterbalance,35 which is favorable for the improvement of initial charge capacity. The inhibition of Fe(CN)6 vacancies can be also reflected by the reduction of water content in the lattice. As shown in Figure S1, Thermogravimetric analysis (TGA) revealed only 10% weight loss in the temperature interval 200−250 °C, corresponding to 1.9 H2O per Na2CoFe(CN)6 unit. Besides, this low-defect material demonstrates greatly improved thermal stability than that of control sample (Figure S1), suggesting a more stable and welldefined crystal structure. The infrared spectrum of the Na2CoFe(CN)6 sample given in Figure S2 shows a singlet C
DOI: 10.1021/acsami.5b12620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces peak at 2080 cm−1, which can be attributed to the stretching vibration of Fe2+-CN-Co2+, agreeing well with its chemical formula.36 Figure 2 shows the crystalline structure and morphological features of the Na2CoFe(CN)6 material. All the X-ray diffraction lines shown in Figure 2a are sharp and strong, indicating high crystallinity of the Na2CoFe(CN)6 sample. Careful comparison with the standard diffraction pattern of Prussian blue Fe4[Fe(CN)6]3 (JCPDS No. 52-1907) demonstrated a rhombohedral distortion from traditional facecentered cubic structure,37,38 as seen by its (220), (420), (440), and (620) diffraction peaks splitting into almost equal intensities. Very recently, similar phase transition has been also observed in other Prussian blue frameworks, such as Na1.72Mn[Fe(CN)6]0.9935 and Na1.92FeFe(CN)6,39 where large accommodation of Na+ ions (in our case, 1.85 Na+) causes an expansion of elementary cell and a decrease in symmetry. In contrast, the control Na2CoFe(CN)6 sample with insufficient Na+ content (only 1.32 Na+) in its lattice shows a typical facecentered cubic structure, further confirming such Na+-induced phase transformation phenomenon (Figure S3). The rhombohedral structure can be clearly visualized in Figure 2b. The SEM image in Figure 2c reveals the Na2CoFe(CN)6 sample appears as well-dispersed, uniformly sized and regularly shaped nanocubes with a narrow size distribution of ∼600 nm, which probably benefited from the as-mentioned controlled crystallization mechanism. Energy dispersive spectroscopy (EDS) mapping images in Figure 2d,e confirm the very uniform dispersion of Na, Co, Fe, C, N, and O element in the bulk Na2CoFe(CN)6 sample, further suggesting its high purity and crystal integrity. Electrochemical Na-storage performance of the Na2CoFe(CN)6 electrodes were characterized by cyclic voltammetry (CV) and galvanostatic charge−discharge tests. As displayed in Figure 3a, the main CV curves demonstrate two pairs of sharp and symmetric oxidation/redox peaks at 3.42/3.12 V and 3.89/ 3.73 V, indicating the 2-Na reaction mechanism of the Na2CoFe(CN)6 electrode. Taking into account the difference of coordination environment of Fe and Co atoms, the CV
peaks at lower and higher potentials can be attributed to the nitrogen-coordinated Co2+/Co3+ couple and carbon-coordinated Fe2+/Fe3+ couple, respectively, as will be confirmed by XPS tests in subsequent context. Surprisingly, the Fe2+/Fe3+ redox couple (∼3.8 V, Eredox = (Eanodic + Ecathodic)/2) in Co−Fe Prussian blue framework shows a significant deviation from the standard electrode potential (3.07 V vs Na+/Na) of Fe(CN)64−/Fe(CN)63−,40 and is even higher than that of Co2+/ Co3+ couple, which can been interpreted as a strong chargespin−lattice coupling effect in the low-defect Na2CoFe(CN)6 framework.38,41 Obviously, this coupling effect is beneficial for the Na2CoFe(CN)6 electrode to raise its working potential. Figure 3b shows the charge−discharge profiles of the asprepared Na2CoFe(CN)6 electrode between 2.0 and 4.1 V at a current density of 10 mA g−1. In accord with the CV curves, the Na2CoFe(CN)6 electrode exhibits two distinguishable potential plateaus at 3.8 and 3.2 V, corresponding to the successive redox reactions of the Fe2+/Fe3+ and Co2+/Co3+ couples, respectively. The high Na+ content (1.85) in Na2CoFe(CN)6 lattice is reflected in a charge capacity of 153 mAh g−1 in the first cycle, with initial Coulombic efficiency as high as 98%. On the basis of its chemical formula, the specific discharge capacity (150 mAh g−1) corresponds to a 95% utilization of its theoretical capacity (158 mAh g−1 for Na1.85Co[Fe(CN)6]0.99·1.9H2O). Such high capacity utilization and high Coulombic efficiency are most likely resulted from its perfect framework with suppressed Fe(CN)6 defects, which provides sufficient interstitial sites for reversible Na insertion. To understand the influence of Fe(CN)6 vacancies on the electrochemical performance of PB framework, we compared the typical charge/discharge profiles of the two Na2CoFe(CN)6 electrodes synthesized with and without the assistance of citrate ions. As shown in Figure S4, both Na2CoFe(CN)6 electrodes exhibit two distinct discharge plateaus at 3.8 and 3.2 V respectively, indicating a very similar Na-insertion reaction mechanism. However, careful comparison of their Na-storage properties demonstrates that the control sample exhibits greater voltage polarization (215 mV) between charge and discharge plateaus, delivers much lower discharge capacity (115 mAh g−1) and demonstrates lower Coulombic efficiency (∼95%) than that of as-prepared sample (144 mV, 150 mAh g−1, and ∼99%). By calculating the capacity contributions of Fe and Co redox centers, we found that the capacity difference mainly originates from the capacity utilization of Fe redox center (28 vs 60 mAh g−1) at higher reaction potential, as shown in Figure S4b, and this should be closely related to the amount of Fe(CN)6 vacancies in the PB lattice. It has been estimated that the Fe(CN)6 vacancies may impede the e− and Na+ transportation along the Fe-CN-Co framework, and increase the polarization between charge/ discharge plateaus.23 Besides, the coordinated water molecules will compete with Na+ to occupy the interstitial sites, thus blocking off the Na+ insertion reaction. 26 Hence, the suppression of Fe(CN)6 vacancies can greatly enhance the Na-storage ability of Fe atoms and decrease the polarization between charge and discharge. The high capacity and high redox potential of the asprepared Na2CoFe(CN)6 electrode contributes to an energy density as high as 510 Wh kg−1, which is comparable to that of cathodes for state-of-art Li-ion battery, such as, LiMn2O4 (120 mAh g−1, 480 Wh kg−1) and LiCoO2 (137 mAh g−1, 550 Wh kg−1).2,42 Furthermore, the discharge capacity above 3.0 V comprises ∼93% of its whole Na-insertion capacity, offering great benefit for practical implementation of Na-ion batteries.
Figure 3. Electrochemical performance of the Na2CoFe(CN)6 electrode: (a) typical CV curve measured at a scan rate of 0.1 mV s−1; (b) charge−discharge profiles at a current density of 10 mA g−1; (c) rate performance; and (d) long-term cycle performance at current density of 100 mA g−1. D
DOI: 10.1021/acsami.5b12620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. Na storage mechanism of the Na2CoFe(CN)6 electrode: (a) A typical charge/discharge profile of the Na2CoFe(CN)6 electrode at different depths: (point a) initial state +2.0 V, (point b) charged to +3.4 V, (point c) charged to +4.1 V, (point d) discharged to +3.4 V, and (point e) discharged to +2.0 V; (b) ex situ XRD patterns at selected states.
initio band calculation by Moritomo et al.38,41 Once reversed to discharge, the XRD patterns of Na2CoFe(CN)6 electrode restored its initial state, indicating the reversible structural transformation from cubic phase to rhombohedral one. On the whole, the Na2CoFe(CN)6 electrode undergoes a reversible 2Na reaction mechanism: rhombohedral Na2CoIIFeII(CN)6 ↔ cubic NaCoIIIFeII(CN)6 ↔ cubic CoIIIFeIII(CN)6.43
In the subsequent cycles (Figure 3b), its redox capacity remains stable at ∼150 mAh g−1, indicating the structural stability of FeCN-Co framework during charge and discharge process. In addition to its high capacity and high redox potential, the as prepared Na2CoFe(CN)6 electrode also demonstrates good rate capability and long-term cyclability. As shown in Figure 3c, the reversible capacity is 148, 135, 128, 88, and 60 at the current density of 20, 50, 100, 200, and 500 mA g−1, respectively. This good rate capability is mainly due to its three-dimension open framework structure with large interstitial space for fast Na+ insertion/extraction reactions. The long-term cycle performance is evaluated at a moderate current density of 100 mA g−1 as given in Figure 3d. The reversible capacity decreases slightly from 128 to 114 mAh g−1 over 200 cycles, corresponding to a capacity retention of ∼90%. In strong contrast, the control Na2CoFe(CN)6 sample synthesized via traditional coprecipitation method shows an enormous performance degradation with only 30% of its initial capacity retained after 200 cycles. Therefore, the superior cycle stability of as-prepared Na2CoFe(CN)6 definitely results from its highly crystallized and low-defect lattice, which facilitates highly reversible 2-Na storage reactions. The structural (XRD) and morphological features (SEM) of Na2CoFe(CN)6 electrode could be well preserved after 200 cycles (Figure S5), further confirming its stable and robust framework during the repeated Na insertion and extraction process. To clarify the phase transition and redox-active site during the 2-Na reaction process, ex situ XRD tests (Figure 4) and XPS analyses (Figure S6) were carried out at different charge/ discharge states. When this Na2CoFe(CN)6 electrode was charged from +2.0 V state to +3.4 V (Figure 4a), the (220) and (220) XRD doublets merged into a single peak at ∼25°, indicating the phase transition from rhombohedral lattice to a cubic one. As mentioned above, this phase transformation phenomenon is probably induced by the Na+ extraction from the lattice to form a Na-deficient framework. At the same time, XPS test (Figure S6) confirmed the valence change of Co ions from +2 to +3, while Fe ions remained at +2 state. When charged from +3.4 V to a terminal +4.1 V, the whole XRD lines remained unchanged, suggesting no phase change at deep charge. This stage was related to the oxidization of Fe ions from +2 to +3 state, as revealed by XPS analysis. As mentioned above, such positive shift of Fe(CN)64−/Fe(CN)63− couple from its standard electrode potential (3.07 V vs Na+/Na)40 is quite anomalous, and is most likely derived from the hybridization effect between low-spin Co3+ and low-spin Fe2+ atoms along cyanide-bridged ligands as already confirmed by ab
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CONCLUSION In summary, we developed a facile citrate-assisted controlled crystallization method to fabricate low-defect and well-defined Na2CoFe(CN)6 frameworks. In situ UV−vis spectral experiment proves the significantly decreased crystallization kinetics in the presence of citrate ions as the slow-release chelating agent. Owing to its high crystallinity and suppressed Fe(CN)6 defects, the as-prepared Na2CoFe(CN)6 material exhibits highly reversible 2-Na reactions with high capacity of 150 mAh g−1 and superior long-term cyclability of ∼90% over 200 cycles, showing appealing promise as Na-rich high-capacity cathode for Na ions applications. Moreover, this synthetic strategy may be extended to other coordination-framework materials for a wide range of energy conversion and storage applications.
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EXPERIMENTAL SECTION
Synthesis of Na 2 CoFe(CN) 6 samples. The as-prepared Na2CoFe(CN)6 sample was synthesized by a citrate-assisted controlled crystallization method at room temperature. Typically, 5 mmol CoCl2 and 5 mmol trisodium citrate were dissolved into 50 mL deionized water to form a 0.1 mol L−1 citrate3−−Co2+ chelate solution. Then, this solution (50 mL) and 0.1 mol L−1 Na4Fe(CN)6 solution (50 mL) were simultaneously added dropwise to 100 mL of deionized water for their coprecipitation reaction. The reaction solution was stirred for 12 h after complete addition. The resulting precipitates were filtered, washed with water and ethanol several times, and finally dried under vacuum at 60 °C for 24 h. As comparison, the control Na2CoFe(CN)6 samples were prepared similarly except for the addition of citrate ions. Physical Characterization. The chemical compositions of two Na2CoFe(CN)6 samples (Na2−xCo[Fe(CN)6]1−y•□y, □ stands for the Fe(CN)6 vacancy, 0 < y < 1) were determined by ICP-AES calibration of Na, Co, and Fe contents and elemental analysis (Elementar Analysen systeme GmbH, Germany) of C and N elements. The water contents in the Na2CoFe(CN)6 materials were characterized by thermogravimetric measurement (TA Instruments, New Castle, DE) in nitrogen atmosphere at a scan rate of 10 °C/min. The lattice structures of the samples were characterized by XRD technique on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation. The crystal structure was drawn by Diamond software. The morphologies of these samples were observed on Scanning Electron Microscope (Sirion 2000, FEI). FT-IR spectra were recorded on a NICOLET E
DOI: 10.1021/acsami.5b12620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces AVATAR 360 FT-IR spectrometer with KBr pellets. UV−vis spectra were recorded using a UV-3100 UV−vis−NIR recording spectrophotometer made by SHIMADZU Company. The in situ crystallization process was measured under the kinetics mode of UV−vis spectrophotometer, and the characteristic emission wavelength was set to be 370 nm. Upon mixing the two reaction solutions into the quartz absorption cell, the cell was immediately transferred into the sample holder of the spectrophotometer. The X-ray photoelectron spectra of the Na2CoFe(CN)6 samples were measured on a Thermo Scientific spectrometer (ESCLAB 250 Xi) equipped with an Al Kα achromatic X-ray source (1486.68 eV). The binding energies of all the elements were calibrated with respect to the carbon (284.6 eV). Electrochemical Measurement. The electrochemical measurements of the Na2CoFe(CN)6 electrodes were examined by 2016 type coin cells using Na2CoFe(CN)6 cathode as a working electrode and a Na disk as reference and counter electrode. The working electrode was prepared by rolling a mixture of 70 wt % as-prepared materials, 20 wt % conducting graphite, and 10 wt % polytetrafluoroethylene into a thin film with a thickness of ∼100 um. The mass loading of the active material within the film was about 4 mg cm−2. The electrolyte was 1.0 mol L−1 NaClO4 in ethylene carbonate/diethyl carbonate (EC/DEC, 50:50% vol) solution. All the cells were assembled in a glovebox with water/oxygen content lower than 1 ppm and tested at room temperature. The galvanostatic charge−discharge tests were performed on a LAND cycler (Wuhan Kingnuo Electronic Co., China). Cyclic voltammetric measurements were carried out at a scanning rate of 0.1 mV s−1 using a CHI 660a electrochemical workstation (ChenHua Instruments Co., China).
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(6) Qian, J.; Xiong, Y.; Cao, Y.; Ai, X.; Yang, H. Synergistic Nastorage Reactions in Sn4P3 as A High Capacity and Cycle-stable Anode of Na-ion Batteries. Nano Lett. 2014, 14, 1865−1869. (7) Zhu, Y.; Han, X.; Xu, Y.; Liu, Y.; Zheng, S.; Xu, K.; Hu, L.; Wang, C. Electrospun Sb/C Fibers for a Stable and Fast Sodium-Ion Battery Anode. ACS Nano 2013, 7, 6378−6386. (8) Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. P2-Type Nax[Fe1/2Mn1/2]O2 Made from Earth-Abundant Elements for Rechargeable Na batteries. Nat. Mater. 2012, 11, 512−517. (9) Yuan, D.; He, W.; Pei, F.; Wu, F.; Wu, Y.; Qian, J.; Cao, Y.; Ai, X.; Yang, H. Synthesis and Electrochemical Behaviors of Layered Na0.67[Mn0.65Co0.2Ni0.15]O2 Microflakes as a Stable Cathode Material for Sodium-Ion Batteries. J. Mater. Chem. A 2013, 1, 3895−3899. (10) Cao, Y.; Xiao, L.; Wang, W.; Choi, D.; Nie, Z.; Yu, J.; Saraf, L. V.; Yang, Z.; Liu, J. Reversible Sodium Ion Insertion in Single Crystalline Manganese Oxide Nanowires with Long Cycle Life. Adv. Mater. 2011, 23, 3155−3160. (11) Saravanan, K.; Mason, C. W.; Rudola, A.; Wong, K. H.; Balaya, P. The First Report on Excellent Cycling Stability and Superior Rate Capability of Na3V2(PO4)3 for Sodium Ion Batteries. Adv. Energy Mater. 2013, 3, 444−450. (12) Park, Y. U.; Seo, D. H.; Kwon, H. S.; Kim, B.; Kim, J.; Kim, H.; Kim, I.; Yoo, H. I.; Kang, K. A New High-Energy Cathode for a NaIon Battery with Ultrahigh Stability. J. Am. Chem. Soc. 2013, 135, 13870−13878. (13) Wu, X.; Cao, Y.; Ai, X.; Qian, J.; Yang, H. A Low-Cost and Environmentally Benign Aqueous Rechargeable Sodium-Ion Battery Based on NaTi2(PO4)3−Na2NiFe(CN)6 Intercalation Chemistry. Electrochem. Commun. 2013, 31, 145−148. (14) Yue, Y.; Binder, A. J.; Guo, B.; Zhang, Z.; Qiao, Z. A.; Tian, C.; Dai, S. Mesoporous Prussian Blue Analogues: Template-Free Synthesis and Sodium-Ion Battery Applications. Angew. Chem., Int. Ed. 2014, 53, 3134−3137. (15) Okubo, M.; Li, C. H.; Talham, D. R. High Rate Sodium Ion Insertion into Core-Shell Nanoparticles of Prussian Blue Analogues. Chem. Commun. 2014, 50, 1353−1355. (16) Lee, H.; Kim, Y. I.; Park, J. K.; Choi, J. W. Sodium Zinc Hexacyanoferrate with a Well-Defined Open Framework as a Positive Electrode for Sodium Ion Batteries. Chem. Commun. 2012, 48, 8416− 8418. (17) Wu, X.; Sun, M.; Guo, S.; Qian, J.; Liu, Y.; Cao, Y.; Ai, X.; Yang, H. Vacancy-Free Prussian Blue Nanocrystals with High Capacity and Superior Cyclability for Aqueous Sodium-Ion Batteries. ChemNanoMat 2015, 1, 188−193. (18) Sun, H.; Sun, H.; Wang, W.; Jiao, H.; Jiao, S. Fe4[Fe(CN)6]3: A Cathode Material for Sodium-Ion Batteries. RSC Adv. 2014, 4, 42991− 42995. (19) Zhang, L.; Chen, L.; Zhou, X.; Liu, Z. Towards High-Voltage Aqueous Metal-Ion Batteries Beyond 1.5 V: The Zinc/Zinc Hexacyanoferrate System. Adv. Energy Mater. 2014, 5, 1400930. (20) Wessells, C. D.; Peddada, S. V.; Huggins, R. A.; Cui, Y. Nickel Hexacyanoferrate Nanoparticle Electrodes for Aqueous Sodium and Potassium Ion Batteries. Nano Lett. 2011, 11, 5421−5425. (21) Wessells, C. D.; McDowell, M. T.; Peddada, S. V.; Pasta, M.; Huggins, R. A.; Cui, Y. Tunable Reaction Potentials in Open Framework Nanoparticle Battery Electrodes for Grid-Scale Energy Storage. ACS Nano 2012, 6, 1688−1694. (22) Lu, Y.; Wang, L.; Cheng, J.; Goodenough, J. B. Prussian Blue: A New Framework of Electrode Materials for Sodium Batteries. Chem. Commun. 2012, 48, 6544−6546. (23) Wu, X.; Deng, W.; Qian, J.; Cao, Y.; Ai, X.; Yang, H. SingleCrystal FeFe(CN)6 Nanoparticles: A High Capacity and High Rate Cathode for Na-Ion Batteries. J. Mater. Chem. A 2013, 1, 10130− 10134. (24) Wu, X.; Luo, Y.; Sun, M.; Qian, J.; Cao, Y.; Ai, X.; Yang, H. Low-Defect Prussian Blue Nanocubes as High Capacity and Long Life Cathodes for Aqueous Na-Ion Batteries. Nano Energy 2015, 13, 117− 123.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b12620. Chemical compositions, TG curves, infrared spectra, XRD patterns, SEM images, electrochemical properties of these two Na2CoFe(CN)6 materials. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] *E-mail:
[email protected]. Funding
We gratefully thank the National Natural Science Foundation of China (Grant no. 21333007 and 21303125) and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130141120007) for financial support. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. (2) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652−657. (3) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; CarreteroGonzález, J.; Rojo, T. Na-Ion Batteries, Recent Advances and Present Challenges to Become Low Cost Energy Storage Systems. Energy Environ. Sci. 2012, 5, 5884−5901. (4) Pan, H.; Hu, Y.-S.; Chen, L. Room-Temperature Stationary Sodium-Ion Batteries for Large-Scale Electric Energy Storage. Energy Environ. Sci. 2013, 6, 2338−2360. (5) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114, 11636− 11682. F
DOI: 10.1021/acsami.5b12620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces (25) You, Y.; Yu, X.; Yin, Y.; Nam, K.-W.; Guo, Y.-G. Sodium Iron Hexacyanoferrate with High Na Content as a Na-Rich Cathode Material for Na-Ion Batteries. Nano Res. 2015, 8, 117−128. (26) You, Y.; Wu, X.-L.; Yin, Y.-X.; Guo, Y.-G. High-Quality Prussian Blue Crystals as Superior Cathode Materials for Room-Temperature Sodium-Ion Batteries. Energy Environ. Sci. 2014, 7, 1643−1647. (27) Asakura, D.; Okubo, M.; Mizuno, Y.; Kudo, T.; Zhou, H.; Ikedo, K.; Mizokawa, T.; Okazawa, A.; Kojima, N. Fabrication of a CyanideBridged Coordination Polymer Electrode for Enhanced Electrochemical Ion Storage Ability. J. Phys. Chem. C 2012, 116, 8364−8369. (28) Yang, D.; Xu, J.; Liao, X.-Z.; He, Y.-S.; Liu, H.; Ma, Z.-F. Structure Optimization of Prussian Blue Analogue Cathode Materials for Advanced Sodium Ion Batteries. Chem. Commun. 2014, 50, 13377−13380. (29) Xie, M.; Xu, M.; Huang, Y.; Chen, R.; Zhang, X.; Li, L.; Wu, F. Na2NixCo1‑xFe(CN)6: A Class of Prussian Blue Analogs with Transition Metal Elements as Cathode Materials for Sodium Ion Batteries. Electrochem. Commun. 2015, 59, 91−94. (30) Imanishi, N.; Morikawa, T.; Kondo, J.; Takeda, Y.; Yamamoto, O.; Kinugasa, N.; Yamagishi, T. Lithium Intercalation Behavior into Iron Cyanide Complex as Positive Electrode of Lithium Secondary Battery. J. Power Sources 1999, 79, 215−219. (31) Pramudita, J. C.; Schmid, S.; Godfrey, T.; Whittle, T.; Alam, M.; Hanley, T.; Brand, H. E.; Sharma, N. Sodium Uptake in Cell Construction and Subsequent in Operando Electrode Behaviour of Prussian Blue Analogues, Fe[Fe(CN)6]1‑x·yH2O and FeCo(CN)6. Phys. Chem. Chem. Phys. 2014, 16, 24178−24187. (32) Lee, H. W.; Wang, R. Y.; Pasta, M.; Woo Lee, S.; Liu, N.; Cui, Y. Manganese Hexacyanomanganate Open Framework as a HighCapacity Positive Electrode Material for Sodium-Ion Batteries. Nat. Commun. 2014, 5, 5280. (33) Dean, J. A. Lange’s Handbook of Chemistry. McGraw-Hill: New York, 1985. (34) Matzapetakis, M.; Dakanali, M.; Raptopoulou, C.; Tangoulis, V.; Terzis, A.; Moon, N.; Giapintzakis, J.; Salifoglou, A. Synthesis, Spectroscopic, and Structural Characterization of the First Aqueous Cobalt (II)-Citrate Complex: Toward a Potentially Bioavailable Form of Cobalt in Biologically Relevant Fluids. JBIC, J. Biol. Inorg. Chem. 2000, 5, 469−474. (35) Wang, L.; Lu, Y.; Liu, J.; Xu, M.; Cheng, J.; Zhang, D.; Goodenough, J. B. A Superior Low-Cost Cathode for a Na-Ion Battery. Angew. Chem., Int. Ed. 2013, 52, 1964−1967. (36) Liu, Y.; Qiao, Y.; Zhang, W.; Li, Z.; Ji, X.; Miao, L.; Yuan, L.; Hu, X.; Huang, Y. Sodium Storage in Na-Rich NaxFeFe(CN)6 Nanocubes. Nano Energy 2015, 12, 386−393. (37) Matsuda, T.; Kim, J.; Moritomo, Y. Symmetry Switch of Cobalt Ferrocyanide Framework by Alkaline Cation Exchange. J. Am. Chem. Soc. 2010, 132, 12206−12207. (38) Moritomo, Y.; Igarashi, K.; Matsuda, T.; Kim, J. DopingInduced Structural Phase Transition in Na1.6‑xCo[Fe(CN)6]0.9·2.9H2O. J. Phys. Soc. Jpn. 2009, 78, 074602. (39) Wang, L.; Song, J.; Qiao, R.; Wray, L. A.; Hossain, M. A.; Chuang, Y. D.; Yang, W.; Lu, Y.; Evans, D.; Lee, J. J.; Vail, S.; Zhao, X.; Nishijima, M.; Kakimoto, S.; Goodenough, J. B. Rhombohedral Prussian White as Cathode for Rechargeable Sodium-Ion Batteries. J. Am. Chem. Soc. 2015, 137, 2548−2554. (40) Qian, J. F.; Zhou, M.; Cao, Y. L.; Ai, X. P.; Yang, H. X. Nanosized Na4Fe(CN)6/C Composite as a Low-Cost and High-Rate Cathode Material for Sodium-Ion Batteries. Adv. Energy Mater. 2012, 2, 410−414. (41) Takachi, M.; Matsuda, T.; Moritomo, Y. Cobalt Hexacyanoferrate as Cathode Material for Na+ Secondary Battery. Appl. Phys. Express 2013, 6, 025802. (42) Li, W.-J.; Chou, S.-L.; Wang, J.-Z.; Wang, J.-L.; Gu, Q.-F.; Liu, H.-K.; Dou, S.-X. Multifunctional Conducing Polymer Coated Na1+xMnFe(CN)6 Cathode for Sodium-Ion Batteries with Superior Performance Via a Facile and One-Step Chemistry Approach. Nano Energy 2015, 13, 200−207.
(43) Song, J.; Wang, L.; Lu, Y.; Liu, J.; Guo, B.; Xiao, P.; Lee, J. J.; Yang, X. Q.; Henkelman, G.; Goodenough, J. B. Removal of Interstitial H2O in Hexacyanometallates for a Superior Cathode of a Sodium-Ion Battery. J. Am. Chem. Soc. 2015, 137, 2658−2664.
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DOI: 10.1021/acsami.5b12620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
