Electrochemical Properties and Redox Mechanism of Na2Ni0.4Co0.6

Nov 28, 2017 - Nickel-based ferrocyanides have attracted wide interest as cathode for aqueous sodium-ion batteries in fields of sustainable energies s...
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Article Cite This: J. Phys. Chem. C 2017, 121, 27805−27812

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Electrochemical Properties and Redox Mechanism of Na2Ni0.4Co0.6[Fe(CN)6] Nanocrystallites as High-Capacity Cathode for Aqueous Sodium-Ion Batteries Wanfeng Li, Fang Zhang, Xingde Xiang,* and Xiucheng Zhang Department of Chemistry and Chemical Engineering, College of Science, Northeast Forestry University, Harbin 150040, China S Supporting Information *

ABSTRACT: Nickel-based ferrocyanides have attracted wide interest as cathode for aqueous sodium-ion batteries in fields of sustainable energies such as wind and solar, owing to excellent cycling stability and high-rate capability. However, they suffer from low specific capacities (∼60 mAh g−1). Herein, Na2Ni0.4Co0.6[Fe(CN)6] nanocrystallites are reported for the first time as high-capacity cathode for aqueous sodium-ion batteries. Its electrochemical properties and redox mechanism have been understood by combining the X-ray diffraction technique, Fouriertransform infrared spectroscopy, cyclic voltammetry, electrochemical impedance microscopy, and charge/discharge measurements. It is revealed that the material undergoes a reversible three-step single-phase reaction mechanism during Na extraction through sequential electrochemical oxidation of nitrogen-coordinated Co2+ ions and carboncoordinated Fe2+ ions and achieves superior electrochemical performance with a high reversible capacity of 85 mAh g−1 at 0.5 C, an average operating potential of 0.62 V (vs Ag/AgCl), and a high capacity retention of 90% after 100 cycles. The combination of high specific energy and good cycling performance enables the Na2Ni0.4Co0.6[Fe(CN)6] material exhibiting promising application for high-performance aqueous sodium-ion batteries. stably cycled with a reversible capacity of about 60 mAh g−1, a rate capacity of about 40 mAh g−1 at 41.7 C, and a capacity retention of 98% after 1000 cycles. As presented by Wu et al.,41 NiHCF in neutral Na2SO4 electrolyte can deliver discharge capacities of 65 mAh g−1 at 1 C and 63 mAh g−1 at 5 C, with slight capacity decay after 500 cycles. As exhibited by Li et al.,43 NiHCF in high-concentration NaClO4 electrolyte can provide a reversible capacity of 63.1 mAh g−1 at 10 C and achieve a capacity retention of 96.3% after 1000 cycles. However, the NiHCF materials suffer from low specific capacities (∼60 mAh g−1) caused by the single-electron reaction mechanism through the carbon-coordinated Fe3+/Fe2+ couple. Fortunately, introducing electrochemically active transition-metals (such as Fe, V, Co, etc.) into nitrogen-coordinated “Ni” sites can achieve much improved specific capacities of Prussian blue analogues due to the multiple single-electron reaction mechanism.34,36,44−47 For instance, high-quality iron ferrocyanides offer an impressive capacity of ∼100 mAh g−1 at 10 C based on the redox reactions of high-spin Fe3+/Fe2+ and low-spin Fe3+/Fe2+ couples.45 Vanadium ferrocyanides provide a discharge capacity of 91 mAh g−1 at the current density of 110 mA g−1 through electrochemical reactions of V4+/V3+ and Fe3+/Fe2+ couples.44 To the best of our knowledge, cobalt ferrocyanides can reach a

1. INTRODUCTION Sustainable energies such as wind power and solar radiation are strongly desired as replacer of fossil fuels for healthy life and modern industries, owing to their wide availability and environmental friendliness. However, their practical application is hindered by the intermittency and instability caused by the climate, weather, and regional factors. Accordingly, diverse rechargeable technologies such as alkali-ion batteries,1−3 sulfur batteries,4−6 and air batteries7−9 are being studied for largescale stationary storage of those sustainable energies. Among those technologies, aqueous sodium-ion batteries (SIBs) attract particular attention due to earth abundance of sodium resources and incombustibility of aqueous electrolytes as well as reversibility of Na-intercalation chemistry.10−12 To date, lots of elements,13−16 oxides,17−23 sulfides,24−26 phosphides,27−29 phosphates,30−33 and ferrocyanides34−38 have exhibited electrochemical activity as electrode materials for SIBs. However, most of them cannot be used for aqueous SIBs owing to the chemical instability in aqueous solutions or unmatched working potentials (not within the electrochemical window of water). It is necessary to develop suitable electrode materials for aqueous SIBs. Nickel-based ferrocyanides with the general formula of AxNi[Fe(CN)6] (A = K, Na, 1 ≤ x ≤ 2, NiHCF) reveal promising application for aqueous SIBs due to excellent cycling performance and high-rate capability.39−43 As reported by Wessells et al.,39 NiHCF in acidic NaNO3 electrolyte can be © 2017 American Chemical Society

Received: August 9, 2017 Revised: November 16, 2017 Published: November 28, 2017 27805

DOI: 10.1021/acs.jpcc.7b07920 J. Phys. Chem. C 2017, 121, 27805−27812

Article

The Journal of Physical Chemistry C high specific capacity of ∼130 mAh g−1 and a high average working potential of ∼0.5 V (vs Ag/AgCl), but suffering from insufficient cycling stability. In order to achieve combined performance of long-cycling NiHCF and high-capacity cobalt ferrocyanides, nickel/cobalt-based ferrocyanides are worthy of being studied. In order to develop high-performance nickel/cobalt-based ferrocyanide composition for aqueous SIBs, in this work, Na2Ni1−xCox[Fe(CN)6] (0 ≤ x ≤ 1) series have been synthesized with a room-temperature coprecipitation method and then comparatively investigated by galvanostatic charge/ discharge measurements. Furthermore, the electrochemical properties and the reaction mechanism of the optimized composition (Na2Ni0.4Co0.6[Fe(CN)6]) have been importantly analyzed by using the X-ray diffraction technique, Fouriertransform infrared spectroscopy, cyclic voltammetry, and electrochemical impedance microscopy. Experimental results reveal that it is a promising cathode for aqueous SIBs due to the advantageous features of high specific capacity, high working potential, and good cycling stability.

sh−1. The charge/discharge tests were done on a Land Test System (CT2001A). Electrochemical impedance spectroscopy was conducted on an electrochemical working station (VERSASTAT4) by applying a perturbation of 10 mV in the frequency range of 10 kHz to 10 mHz. In this work, 1 C is defined as 70 mA g−1.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Structure. The synthesis of Na2Ni0.4Co0.6[Fe(CN)6] material was accomplished by using a room-temperature precipitation method with NiCl2, CoCl2, and Na4[Fe(CN)6] as start materials. In brief, a precipitation reaction of 0.4NiCl2 + 0.6CoCl2 + Na4[Fe(CN)6] → Na2Ni0.4Co0.6[Fe(CN)6] + 2NaCl was performed by slowly adding the mixed solution of NiCl2 and CoCl2 into Na4[Fe(CN)6] solution under vigorous stirring at room temperature. An aging protocol was subsequently done by resting the precipitation-containing solution for 24 h. As similar procedures, other Na2Ni1−xCox[Fe(CN)6] (0 ≤ x ≤ 1) compositions can be also obtained by altering the molar ratio of NiCl2 and CoCl2. Crystal structure of the as-synthesized Na2Ni0.4Co0.6[Fe(CN)6] material has been first confirmed by using X-ray diffraction (XRD). As shown in Figure 1, all XRD reflections

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. Na2Ni0.4Co0.6[Fe(CN)6] was synthesized with a simple room-temperature precipitation method. In particular, solution A was prepared by dissolving stoichiometric amounts of NiCl2·6H2O (0.476 g, 99.9%, Aladdin) and CoCl2·2H2O (0.715 g, 99.9%, Aladdin) into 50 mL of distilled water; solution B was prepared by adding the required amount of Na4Fe(CN)6·10H2O (2.423 g, 99.9%, Aladdin) into 50 mL of distilled water. Under vigorous stirring at room temperature, solution A was slowly dropped into solution B to perform a precipitation reaction of 0.4NiCl2 + 0.6CoCl2 + Na4[Fe(CN)6]→ Na2Ni0.4Co0.6[Fe(CN)6] + 2NaCl. After 6 h, an aging process was done by resting the reaction solution at room temperature for 24 h. The desired product was finally obtained by separating, washing, and drying the precipitation. As similar procedures, other Na2Ni1−xCox[Fe(CN)6] (0 ≤ x ≤ 1) compositions can be also produced by altering the molar ratio of NiCl2 and CoCl2. 2.2. Physical Characterization. The crystal structure of the materials was confirmed by using X-ray diffraction (X’Pert Powder) with a Cu Kα radiation source, and the XRD pattern was recorded in the range of 10°−70°. The crystallite size (D) was estimated according to Debye−Scherrer equation: D = 0.89λ/(B cosθ), where λ is the wavelength of X-ray (0.154056 nm), B is the peak (200) width at half-height, and θ is the scattering angle corresponding to the peak (200). The microstructural changes of the material during Na extraction process were identified by ex situ technologies including X-ray diffraction and Fourier-transform infrared spectroscopy (FTIR, Spectrum 400). Before the ex situ measurements, the samples with various states of charge were prepared by charging/ discharging at the current rate of 0.5 C. 2.3. Electrochemical Measurements. The working electrode was fabricated by mixing 70 wt % active material, 10 wt % binder (PVDF), and 20 wt % conductive agent (Super P carbon) and brushing the mixture onto a titanium foil. Threeelectrode system was assembled by using the working electrode, Ag/AgCl (saturated KCl solution) reference electrode, desodiated Na2Ni[Fe(CN)6] (NiHCF) counter electrode, and 6 M NaClO4 electrolyte. The cyclic voltammetry was carried out on electrochemical workstation (CHI 600E) in the potential range of 0−1.1 V at the scanning rate of 0.1 mV

Figure 1. XRD pattern and crystal structure (inset) of Na2Ni0.4Co0.6[Fe(CN)6] material.

can be well indexed with space group of Fm-3m, suggesting a typical face-centered-cubic structure of Prussian blue analogues (the inset of Figure 1).48,49 In the structure, Co and Ni are surrounded with 6-fold nitrogen atoms, while Fe is coordinated with six carbon atoms. Bridging of MN6 (M= Co, Ni) octahedra and FeC6 octahedra by CN groups leads to a three-dimensional open framework containing large interstitial sites that allow for fast Na+ diffusion. The scattering peaks around 17.1°, 24.2°, 34.7°, and 38.9° are respectively assigned to (200), (220), (400), and (420) planes. The lattice parameters are refined to be a = b = c = 10.3311 Å, which are located between those of Na2Ni[Fe(CN)6] (10.2640 Å) and Na2Co[Fe(CN)6] (10.4387 Å) (Figure S1). Hence, it can be considered as a solid solution of Na2Ni[Fe(CN)6] and Na2Co[Fe(CN)6]. Furthermore, the crystallite size of the sample is estimated according to Debye−Scherrer equation to be 12.8 nm, suggesting the successful synthesis of nanostructured Na2Ni0.4Co0.6[Fe(CN)6] crystallites. Nanosizing would favor fast reaction kinetics of diffusion-controlled 27806

DOI: 10.1021/acs.jpcc.7b07920 J. Phys. Chem. C 2017, 121, 27805−27812

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The Journal of Physical Chemistry C

Figure 2. Electrochemical properties of the Na2Ni0.4Co0.6[Fe(CN)6] material: (a) charge/discharge profiles at the current rate of 0.5 C, (b) rate capability at various current rates, and (c) cycling performance at the current rate of 1 C.

Figure 3. Cyclic voltammograms of the Na2Ni1−xCox[Fe(CN)6] (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) samples at a scanning rate of 0.1 mV s−1.

assigned to insertion of Na+ ions into the lattice, accompanied by reduction of Co3+ and Fe3+ ions. Different from those of Na2Ni[Fe(CN)6] and Na2Co[Fe(CN)6] samples (Figure S2), the profiles are composed of several potential slopes, without any plateaus. More importantly, it delivers a stable high capacity of about 85 mAh g−1 and a high working potential of 0.62 V, achieving a high specific energy of 121 Wh kg−1 when coupled with the NaTi2(PO4)3 anode. This is much higher than those of previously reported Na2Ni[Fe(CN)6] (87 Wh kg−1),41 Na2VTi(PO4)3 (68 Wh kg−1),50 Na3MnTi(PO4)3 (82 Wh kg−1),11 and Na0.60Mn0.56Ti0.34O2 (76 Wh kg−1).51 In addition, it reaches

reactions owing to shortening of ion-diffusion pathway and enlarging of ion-diffusion interface. 3.2. Charge/Discharge Performance. Charge/discharge properties of the Na2Ni0.4Co0.6[Fe(CN)6] material have been investigated on a three-electrode system with desodiated NiHCF counter electrode and Ag/AgCl (saturated KCl solution) reference electrode. Figure 2a shows the charge/ discharge profiles of the material at a current rate of 0.5 C in the potential range of 0−1.1 V. The charge profile corresponds to extraction of Na+ ions from the lattice, with electrochemical oxidation of Co2+ and Fe2+ ions, while the discharge curves are 27807

DOI: 10.1021/acs.jpcc.7b07920 J. Phys. Chem. C 2017, 121, 27805−27812

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The Journal of Physical Chemistry C Table 1. Electrochemical Performance of the Materials Reported in This Work and the Literature material 52

Na0.44MnO2 NaFePO453 Na2VTi(PO4)350 Na3MnTi(PO4)311 Na2Ni[Fe(CN)6]41 Na2Cu[Fe(CN)6]54 Na0.66Mn0.66Ti0.34O251 Fe[Fe(CN)6]47 Na0.4(VO)3[Fe(CN)6]246 Na2Ni0.4Co0.6[Fe(CN)6] a

specific capacity (mAh g−1)

working potential (V vs Ag/AgCl)

specific energya (Wh kg−1)

45 73 56 58.4 65 59 76 125 91 85

0.41 0.03 0.42 0.6 0.48 0.61 0.2 0.2 0.85 0.62

54 61 68 82 83 83 76 125 150 121

capacity retention 96% 92% 92% 98% 93% 93% 84% 83% 60% 90% 95%

after after after after after after after after after after after

1000 cycles at 4 C 20 cycles at 0.2 C 500 cycles at 5 C 100 cycles at 1 C 500 cycles at 5 C 500 cycles at 5 C 300 cycles at 2 C 500 cycles at 10 C 250 cycles at 1.2 C 100 cycles at 1 C 100 cycles at 5 C

Specific energy is calculated by adopting NaTi2(PO4)3 (−0.8 V vs Ag/AgCl) as reference anode.

high Coulombic efficiencies of 95.4% at the first cycle and almost 100% after the first cycle, showing excellent charge/ discharge reversibility. Figure 2b shows the reversible capacities of the Na2Ni0.4Co0.6[Fe(CN)6] material at various current rates. It exhibits stable reversible capacities of 80 mAh g−1 at 1 C, 73 mAh g−1 at 2 C, 60 mAh g−1 at 5 C, and 44 mAh g−1 at 10 C. The capacities at 1, 2, 5, and 10 C are respectively 94.1%, 85.9%, 70.6%, and 51.8% of the capacity at 0.5 C, indicating superior high-rate capability. Figure 3c plots the capacity variation upon extended cycles at the current rate of 1 C. It is found that the capacity retention is 98.6% after 20 cycles, 92.8% after 50 cycles, and 90.1% after 100 cycles. The capacity fading per cycle is less than 0.1%, indicative of excellent cycling performance. In addition, the high capacity retention can be also achieved when the high current rate of 5 C is applied (Figure S3). It should be mentioned that the material shows relatively low Coulombic efficiencies at the lower current rate due to the side reactions with electrolytes. With an aim to exhibit the superiority of the Na2Ni0.4Co0.6[Fe(CN)6] material, Figures S2 and S4 comparatively present the electrochemical properties of the Na2Ni1−xCox[Fe(CN)6] (0 ≤ x ≤ 1). As observed, with increasing the Co substitution, the reversible capacity increases owing to the contribution of electrochemically active Co3+/Co2+ couple, while the cycling performance decreases owing to the large volume change induced by redox reaction of the Co3+/Co2+ couple. Clearly, Na2Ni0.4Co0.6[Fe(CN)6] provides significantly higher specific capacity than Na2Ni[Fe(CN)6] (64 mAh g−1) and much better cycling performance than Na2Co[Fe(CN)6] (76.3% capacity retention after 100 cycles at 1 C), showing the combination of high reversible capacity and good cycling stability. Furthermore, the overall performance has been compared with those of previous reported cathode materials, as shown in Table 1. Obviously, the Na2Ni0.4Co0.6[Fe(CN)6] material shows superior electrochemical performance with combined advantages of high specific energy and long-term cycling stability. In order to understand the superior electrochemical performance, the reaction kinetics of the material has been investigated by using galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). The GITT curves (Figure S5) were obtained by alternately charging/discharging 10 min at 0.5 C and resting 30 min. The potential drop during the resting period represents the electrochemical polarization of Naintercalation electrodes. As seen, the material shows a small potential drop of 10−20 mV, signifying the fast reaction kinetics. As exhibited in Figure S6, the EIS spectrum is

composed of one semicircle-like region at high−medium frequencies and one line-like region at low frequencies. The semicircle-like region corresponds to the charge-transfer reaction, while the line-like region is assigned to the diffusion of Na+ ions in material bulk. The charge-transfer impedance is estimated to be ∼40 Ω. The apparent coefficient of Na+ diffusion is calculated to be ∼10−11 cm−2 s−1. The fast reaction kinetics featured with low charge-transfer impedance and favorable ionic diffusivity can be ascribed to the unique openframework structure and nanostructured crystallites. 3.3. Electrochemical Reaction Mechanism. For the sake of pursuing the electrochemical reaction mechanism, the redox behavior of the Na2Ni0.4Co0.6[Fe(CN)6] material has been investigated with cycling voltammetry. Figure 3 shows the cyclic voltammograms tested at the scanning rate of 0.1 mV s−1. There are two couples of sharp oxidation/reduction peaks (∼0.44 and ∼0.67 V) and one couple of broaden peaks centered at ∼0.90 V, suggesting that the material undergoes a three-step redox reaction mechanism. In order to specify the affiliation of those redox reactions, Figure 3 comparatively exhibits cyclic voltammograms of Na2Ni1−xCox[Fe(CN)6] (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) samples. Clearly, the material without Co substitution (x = 0) shows a couple of strong redox peaks in a narrow potential range of 0.35−0.65 V, which are ascribed to the electrochemical reaction of carbon-coordinated Fe3+/Fe2+ couple.55 In contrast, the sample with full Co substitution (x = 1.0) exhibits two couples of redox peaks respectively located at 0.46 and 0.97 V. The one at lower potential correspond to the oxidation/reduction of the nitrogen-coordinated Co3+/Co2+ couple, while the other one at higher potential is assigned to the redox reaction of the carbon-coordinated Fe3+/Fe2+ couple.34 The upshifted potential of Fe3+/Fe2+ redox peaks relative to that in Na2Ni[Fe(CN)6] is attributed to the hybridization effect of Co−NC− Fe, which lowers the energy level of the Fe3+/Fe2+ redox center.56−58 Interestingly, Co substitution significantly changes the redox behavior of the Na2Ni1−xCox[Fe(CN)6] (0 ≤ x ≤ 1) material, which defers from those of Na2Ni[Fe(CN)6] and Na2Co[Fe(CN)6] samples. With increasing the Co content from x = 0 to x = 0.2, the material show double couples of sharp redox peaks around 0.6 V. Since the Co content is low, the redox currents should be mainly attributed to the oxidation/reduction of Fe3+/Fe2+ couple with nickel-rich MN6 neighbors (M = Ni, Co). With increasing the Co content from x = 0.2 to x = 0.6, the intensity of redox peaks centered at 0.45 V increases owing to the enhanced redox reaction of the nitrogen-coordinated Co3+/Co2+ couple. Meanwhile, the redox 27808

DOI: 10.1021/acs.jpcc.7b07920 J. Phys. Chem. C 2017, 121, 27805−27812

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The Journal of Physical Chemistry C

Figure 4. Ex situ FTIR spectra of (a) Na2Ni[Fe(CN)6], (b) Na2Co[Fe(CN)6], and (c) Na2Ni0.4Co0.6[Fe(CN)6] samples with various states of charge.

potential of Fe3+/Fe2+ couple positively shifts due to the enhanced hybridization effect of Co−NC−Fe. Further increasing Co content from x = 0.6 to x = 1.0 leads to gradual appearance of an additional redox peaks (∼0.9 V) due to the increased content of Fe3+/Fe2+ couple surrounded with cobaltrich MN6 environment and gradual disappearance of the redox peaks (∼0.6 V) due to the decreased content of Fe3+/Fe2+ couple surrounded with nickel-rich MN6 environment. Based on above discussion, the electrochemical reaction of the Na2Ni0.4Co0.6[Fe(CN)6] material can be accordingly understood as sequential redox reactions of the nitrogen-coordinated Co3+/Co2+ couple and carbon-coordinated Fe3+/Fe2+ couple. In order to further support the electrochemical reaction mechanism, Figure 4 comparatively presents the ex situ FTIR spectra of the Na2Ni[Fe(CN)6], Na2Co[Fe(CN)6], and Na2Ni0.4Co0.6[Fe(CN)6] samples with various states of charge. The characteristic absorption peak centered at ∼2090 cm−1 is assigned to the stretching vibration of the cyanide group (C N), which is sensitive to the electronic structure of the transition-metals (M = Ni, Co, etc.) in the Fe−CN−M section.59−61 Clearly, the Na extraction does not leads to any observable shift of the cyanide peak for the Na2Ni[Fe(CN)6] sample. In contrast, the peak for Na2Co[Fe(CN)6] reveals a significant blue-shift from 2080 to 2117 cm−1 below 0.8 V and stands at 2117 cm −1 above 0.8 V. Previous reports demonstrated that the Na-extraction reaction of the Na2Ni[Fe(CN)6] material is fully attributed to the electrochemical oxidation of the carbon-coordinated Fe3+/Fe2+ couple,55 while the Na-extraction reaction of the Na2Co[Fe(CN)6] material occurs though the electrochemical oxidation of the nitrogencoordinated Co3+/Co2+ couple below 0.8 V and carboncoordinated Fe3+/Fe2+ couple above 0.8 V.34 Hence, it can be concluded that the redox reaction of carbon-coordinated Fe3+/ Fe2+ couple has no distinct effect on the peak position of the cyanide, while the redox reaction of nitrogen-coordinated Co3+/Co2+ couple would cause a significant blue-shift of the

peak. As observed from Figure 3a, the target Na2Ni0.4Co0.6[Fe(CN)6] sample shows a significant blue-shift below 0.48 V and unchanged position of the cyanide peak. This indicated that the Na2Ni0.4Co0.6[Fe(CN)6] material undergoes the first oxidation of nitrogen-coordinated Co3+/Co2+ couple and subsequent oxidation of carbon-coordinated Fe3+/Fe2+ couple during charge process, consistent with the results from cyclic voltammetry. Furthermore, the phase evolution mechanism of the Na2Ni0.4Co0.6[Fe(CN)6] material during electrochemical reactions has been investigated by employing the ex situ XRD technique. Figure 5 exhibits the XRD patterns of the material at various states of charge and discharge. As observed, the charge in the range 0.4−0.6 V leads to positive shift of XRD peaks without formation of any additional new peaks, suggesting the

Figure 5. Ex situ XRD patterns of the Na2Ni0.4Co0.6[Fe(CN)6] material at various states of charge and discharge. 27809

DOI: 10.1021/acs.jpcc.7b07920 J. Phys. Chem. C 2017, 121, 27805−27812

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The Journal of Physical Chemistry C single-phase evolution mechanism induced by extraction of Na+ ions and oxidation of nitrogen-coordinated Co2+ ions. The shift is attributed to the lattice expansion caused by the change of larger high-spin Co2+ ions into smaller low-spin Co3+ ions.57 Subsequently, there is no observable movement of XRD peaks during charge from 0.6 to 1.1 V. This is because of the ignorable change of the lattice volume during the oxidation process of carbon-coordinated Fe 2+ ions. The inverse phenomenon can be found during subsequent discharges, suggesting highly reversible phase-evolution process.

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4. CONCLUSIONS Na2Ni0.4Co0.6[Fe(CN)6] nanocrystallites have been reported as a new high-capacity cathode material for aqueous sodium-ion batteries. Combined characterization of X-ray diffraction, Fourier-transform infrared spectroscopy, cyclic voltammetry, electrochemical impedance microscopy, and charge/discharge measurements demonstrates that the material undergoes a reversible three-step single-phase reaction mechanism through oxidation/reduction of the nitrogen-coordinated Co3+/Co2+ couple and carbon-coordinated Fe3+/Fe2+ couple. It exhibits promising application as cathode material for high-energy aqueous sodium-ion batteries owing to the superior overall electrochemical performance including high specific capacity of 85 mAh g−1, high working potential of 0.62 V (vs Ag/AgCl), good high-rate capability (44 mAh g−1 at 10 C), and excellent cycling stability (90% capacity retention after 100 cycles at 1 C).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07920. XRD patterns of Na2NixCo1−x[Fe(CN)6] (0 ⩽ x ⩽ 1) samples; charge/discharge curves of Na2NixCo1−x[Fe(CN)6] (0 ⩽ x ⩽ 1) samples; cycling performance of Na2Ni0.4Co0.6[Fe(CN)6] at 5 C; cycling performance of Na2NixCo1−x[Fe(CN)6] (0 ⩽ x ⩽ 1) samples at 1 C; GITT curve of Na2Ni0.4Co0.6[Fe(CN)6] sample; EIS spectrum of Na2Ni0.4Ni0.6[Fe(CN)6] sample (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.X.). ORCID

Xingde Xiang: 0000-0002-5342-2835 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Fundamental Research Funds for the Central Universities (No. 2572017CB30).

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REFERENCES

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DOI: 10.1021/acs.jpcc.7b07920 J. Phys. Chem. C 2017, 121, 27805−27812