Jarosite Nanosheets Fabricated via Room-Temperature Synthesis as

Apr 7, 2015 - Max Planck Institute for Solid State Research, Heisenbergstrasse 1, ... Capacity retentions of above 90% are both obtained after 50 cycl...
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Jarosite Nanosheets Fabricated via Room-Temperature Synthesis as Cathode Materials for High-Rate Lithium Ion Batteries Yuan-Li Ding,‡ Yuren Wen,§ Peter A. van Aken,§ Joachim Maier,‡ and Yan Yu*,†,‡ †

Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, Anhui, People’s Republic of China ‡ Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany § Max Planck Institute for Intelligent Systems, Heisenbergstrasse 3, 70569 Stuttgart, Germany S Supporting Information *

ABSTRACT: Two-dimensional (2D) nanostructures of earth-abundant jarosite and their analogues were fabricated for the first time by a facile template-assisted redox coprecipitation method at room temperature. When evaluated as cathode materials for lithium ion batteries (LIBs), the asprepared 2D materials deliver high capacities and good rate capability and cycling performance. As for jarosite KFe3(SO4)2(OH)6 nanosheets (KNSs), the reversible capacities of 117, 114, and 75 mAh g−1 were achieved at 0.2, 1, and 10 C, respectively, 4−13 times higher than those of bulk sample. Capacity retentions of above 90% are both obtained after 50 cycles at 2 and 10 C. Such findings show that 2D jarosite nanostructures would be promising cathode materials for next-generation LIBs.

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INTRODUCTION The discovery of new electrode materials is a key to realizing safe, green, and efficient electrochemical energy storage systems, such as solar cells,1,2 fuel cells,3,4 supercapacitors,5−7 and lithium ion batteries (LIBs).8−13 As for LIBs, worldwide efforts have been devoted to developing more economical and environmentally friendly cathode hosts with more stable crystalline frameworks to replace commercial layered LiCoO2, which is associated with a safety issue (overcharge), high cost, and environmental hazard.14 In an intensive search for alternative materials, polyanionic (“XO4”, X = P, S, Si, W, Mo, ...) compounds have been super choices owing to their remarkable electrochemical and thermal stability, and, more importantly, larger open interstitial space that can accommodate the intercalation of small cations, such as Li+, thus making them more suitable as electrode materials for LIBs.14−16 In recent years, a great effort has been directed toward the development of Fe-based polyanionic hosts due to their earthabundant resources, low cost, and environmental benignity as well as safe and sustainable production and operation, such as LiFePO4,15 Li2FeP2O7,17 FePO4,18 Li2FeSiO4,19 LiFeBO3,20 Fe2(SO4)3,21 and FeSO4OH,22 etc. Among these systems, Fe-based sulfate was relatively less studied since the discovery of Li+ insertion behaviors in the NASICON type Fe2(SO4)3 framework by Manthiram and Goodenough,21 which has been recently demonstrated to be a promising cathode material for sodium intercalation.23 In recent years, Fe-based sulfates, such as Li2FeSO4,24 krohnkitetype Na2Fe(SO4)2·2H2O,25 alluaudite-type Na2Fe2(SO4)3,26 and fluorosulfates (LiMSO4F, M = Fe, Co, Ni)27,28 have © 2015 American Chemical Society

been demonstrated as interesting hosts for Li or Na intercalation. Generally, natural minerals contain either F− or OH− anions. Compared to fluorosulfates, hydroxysulfates would be more attractive for large-scale application owing to their environmental benignity and lower cost. For that reason, hydroxysulfates have been recently paid more and more attention to; such as, layered structure LiMSO4OH (M = Fe, Co), has been reported as promising cathode for LIBs.29,30 Very recently, a new hydroxysulfate (NaFe3(SO4)2(OH)6, Najarosite), one of the jarosite family compounds, has also been demonstrated to be an attractive host for lithium intercalation with a reversible capacity of about 110 mAh g−1 and 2.8 V vs Li+/Li at a low current rate.31 Jarosite is a natural mineral to be found in acidic and sulfate-rich environments with a general formula of AM3(SO4)2(OH)6, where A = K+, Na+, and NH4+ and M = Fe3+, Cr3+, V3+, Ga3+, Al3+, and In3+.32 Jarosite KFe3(SO4)2(OH)6 (K-jarosite) is intensively investigated for its spin chirality on a two-dimensional (2D) geometrically frustrated lattice and unique magnetic properties.33 Figure 1a displays that K-jarosite crystals are rhombohedral and of R3̅m symmetry with lattice constants a = b = 7.30 Å and c = 17.09 Å at room temperature.32 The structure of K-jarosite consists of linear tetrahedral−octahedral−tetrahedral (T-O-T) sheets. Potassium ions are located in 12-fold coordination between the T-O-T sheets. In spite of the presence of K+ or Na+ between the T-O-T sheets, small cations (such as Li+) can be Received: March 5, 2015 Revised: April 7, 2015 Published: April 7, 2015 3143

DOI: 10.1021/acs.chemmater.5b00849 Chem. Mater. 2015, 27, 3143−3149

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Figure 1. (a) Crystal structure of KFe3(SO4)2(OH)6. The structure is composed of FeO6 octahedra (green) bonded to SO4 tetrahedra (orange), making a tetrahedral−octahedral−tetrahedral (T-O-T) sheet-like structure. Hydrogen atoms are omitted for clarity. Potassium ions (gray white) are located in 12-fold coordination between the TO-T sheets. Iron, sulfur, and oxygen atoms are displayed in blue, yellow, and red, respectively. (b) Schematic diagram of lithium intercalation/deintercalation of KFe3(SO4)2(OH)6. Lithium ions are shown in light blue.

Figure 2. (a) Schematic illustration of jarosite nanosheets (KNSs, NNSs) prepared by a template-assisted redox co-precipitation method at room temperature. (b) Schematic illustration of formation mechanism of KNSs or NNSs based on a combination of redox reaction and co-precipitation reaction.

δ-MnO2 probably involve two steps. First, Fe2+ ions lose electrons to δ-MnO2 and are simultaneously oxidized to Fe3+ ions (eq 1), which adsorb on the surface of δ-MnO 2 nanosheets. Then, Fe3+ will further react with SO42− and K+ or Na+ by a co-precipitation reaction (eqs 4 and 5), as illustrated in Figure 2.

intercalated between these layers (Figure 1b).31 As such, K- or Na-jarosite can theoretically accept three Li+ and three electrons per formula under the premise of undergoing an intercalation mechanism accompanied by the reduction of Fe3+ to Fe2+, corresponding to a theoretical capacity of 160 or 166 mAh g−1, making it suitable as a potential cathode for LIBs. 2D electrode materials have attracted great attention as wellsuited structures for lithium storage, which can provide fast electron/ion transport in the thickness dimension, but also guarantee structural integrity owing to the sub-micrometer- or micrometer-sized in-plane dimensions.34,35 In addition, the nanosheet structure can effectively accommodate volume changes resulting from repeated Li+ insertion/extraction. To date, there have been substantial research efforts to fabricate various 2D electrode materials for LIBs since 2D structures play an important role in the determination of the rate capability and cycling performance because of more effective Li+ diffusion into the nanoparticles and self-accommodating buffering of 2D nanostructures.36,37 However, reported 2D nanostructures are mainly limited in the fields of metal,38 alloy,39 metal oxides,40,41 metal chalcogenides,42 metal hydroxides,43 and simplex polyanionic compounds44 by a liquid exfoliation method or by a hydrothermal or solvothermal route. The fabrication of 2D nanoarchitectures with complex polyanionc component (such as the jarosite family of compounds) through a facile synthesis procedure remains a great challenge. In this work, we report for the first time 2D nanostructures of the jarosite family of compounds such as K-jarosite nanosheets (KNSs), and their analogues (Na-jarosite nanosheets, NNSs) by a facile template-assisted redox coprecipitation strategy at room temperature. As illustrated in Figure 2, δ-MnO2 nanosheets were used as both oxidant and self-sacrificing template. The actual reactions between Fe2+ and

step 1: MnO2 + 2Fe2 + + 4H+ → Mn 2 + + 2Fe3 + + 2H 2O

(1)

half-reaction: MnO2 + 4H+ + 2e− → Mn 2 + + 2H 2O

E ⊖ = 1.23 V

(2)

half-reaction: Fe3 + + e− → Fe2 +

(3)

E ⊖ = 0.77 V

step 2: K+ + 3Fe3 + + 2SO4 2 − + 6H 2O → KFe3(SO4 )2 (OH)6 + 6H+

(4)

Na + + 3Fe3 + + 2SO4 2 − + 6H 2O → NaFe3(SO4 )2 (OH)6 + 6H+

(5)

or Compared to previously reported jarosite by elevatedtemperature or hydrothermal processes,45−47 our strategy is simple, green, cost-effective, and up-scalable, and thus promising for mass production. When evaluated as cathode for LIBs, the as-prepared KNSs and NNSs exhibit attractive lithium storage performance in terms of high capacity and rate capability and stable cyclability, making them as potentially earth-abundant hosts in next-generation LIBs.

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MATERIALS AND METHODS

Materials Synthesis. First, δ-MnO2 nanosheets are synthesized based on ref 48. In the typical synthesis, an aqueous solution of KMnO4 (0.02 M, 150 mL) was mixed with ethyl acetate (40 mL, 99.8%) and then refluxed at 85 °C until the pink color of KMnO4 solution disappears. The resulting colloidal product was separated 3144

DOI: 10.1021/acs.chemmater.5b00849 Chem. Mater. 2015, 27, 3143−3149

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Chemistry of Materials

Figure 3. (a) TEM image of δ-MnO2 nanosheets, (b) TEM and (c) SEM images of KNSs, (d) magnified view taken from the boxed area in c, (e) high-magnification TEM and (f) HRTEM images of KNSs, (g) SEM image of NNSs, and (h) magnified view taken from the boxed area in g.

Figure 4. XRD patterns of (a) KNSs and (b) NNSs (asterisk denotes the presence of a little amount of KFe3(SO4)2(OH)6 phase) and (c) XPS spectra and (d) Fe 2p XPS spectra of KNSs. from the upper part of ethyl acetate by a separating funnel. This brown colloid suspension was used for the syntheses of KNSs and NNSs. For the synthesis of KNSs, FeSO4·7H2O (1.2552 g) and KNO3 (0.1526 g) were dissolved into deionized water (30 mL) and then dropwise added into the preceding δ-MnO2 nanosheets suspension (50 mL) containing ethanol (40 mL) under magnetic stirring at room temperature. After around 10 min, the orange suspension was obtained. After aging 12 h at room temperature, the obtained suspension was centrifuged and washed by deionized water and ethanol at least three times. Finally, the orange slurry was dried in vacuum at room temperature. Similar to KNSs, NNSs were also prepared by the same procedure except using NaCl as sodium sources. For comparison, bulk K-jarosite was also prepared based on the reported ref 47. In the typical synthesis, KNO3 (3.0 g) and Fe2(SO4)3 (3.5 g) were dissolved into 0.01 M H2SO4 aqueous solution (100 mL) under magnetic stirring. Then, the previously mixed solution was stirred at 90 °C for 3 h, then centrifuged, and washed by deionized

water and ethanol at least three times. Finally, the light orange slurry was dried at 100 °C. Characterization of Materials. The crystalline structures of the products were examined by X-ray diffraction (XRD, PANalytical X’pert Pro using Cu Kα radiation). The morphology of the product was observed by field-emission scanning electron microscopy (SEM, Zeiss Merlin, 1 kV), transmission electron microscopy (TEM, Zeiss 912 Omega, 120 kV), and high-resolution TEM (HRTEM, JEOL 4000 FX, 400 kV). The Brunauer−Emmett−Teller (BET) measurement was carried out in the Quantachrome Instruments. The inductively coupled plasma−atomic emission spectrometry (ICP-AES) was performed using Spectro analytical instruments. X-ray photoelectron spectroscopy (XPS) spectra were recorded with an Axis Ultra Instrument (Kratos Analytical Ltd., Manchester, U.K.). Electrochemical Characterization. The electrochemical properties of the as-prepared products were evaluated using CR2032 cointype cells with lithium metal as the negative electrode. The slurry was prepared by mixing active materials, carbon black (super P), and 3145

DOI: 10.1021/acs.chemmater.5b00849 Chem. Mater. 2015, 27, 3143−3149

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Figure 5. (a) CV curves of KNSs electrode at a scan rate of 0.1 mV s−1 between 1.6 and 4.0 V, (b) galvanostatic charge and discharge curves for KNSs electrode at a current rate of 0.1 C, (c) rate capabilities of KNSs, NNSs, and bulk K-jarosite electrodes from 0.2 to 10 C, and (d) cycling performance of KNSs electrode at 2 and 10 C. The solid and empty circle symbols represent the discharge and charge capacities, respectively. sodium carboxymethyl cellulose (CMC) in a weight ratio of active material:super P:CMC = 70:20:10 in deionized water. The blended slurry was then cast onto aluminum or titanium foil and dried at 60 °C overnight in a vacuum oven. The mass loading of each electrode is 1.2−1.5 mg cm−2. Cell assembly was carried out in an Ar-filled glovebox. Polypropylene membrane (Celgard 2400) was used as separator. The electrolyte solution was 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 in volume). Galvanostatic charge/discharge tests were carried out at room temperature using a BTS battery tester (Neware BTS7.0). The cutoff voltage window is 1.6−4.0 V. Electrochemical impedance spectroscopy (EIS) of the cell was measured in the frequency range from 100 kHz to 10 mHz on a Voltalab 80 electrochemical workstation. The voltage perturbation is 5 mV.

rhombohedral jarosite KFe3(SO4)2(OH)6 phase (JCPDS No. 71-1777). The sample was further investigated by XPS, indicating the presence of K, Fe, S, and O, as shown in Figure 4c. In the high-resolution XPS spectrum of Fe 2p (Figure 4d), two distinct peaks are identified at binding energy of 724.4 eV for Fe 2p1/2 and 711.0 eV for Fe 2p3/2, with a small satellite peak at 719.1 eV between them, which is the characteristic peak of Fe3+.49 The Mn 2p characteristic peaks at 642.9 and 654.4 eV are not found,48,50 suggesting the complete depletion of δMnO2. ICP-AES results also show that the weight ratio of K:Fe:S in KNSs is 7.78:31.4:12.7, implying a possible potassium-rich phase of K1.06Fe3(SO4)2.11(OH)5.84 with slight sulfate excess and hydroxide deficiency. To investigate the generality of the fabrication of 2D jarosite nanostructures by a template-assisted redox coprecipitation route, NNSs are also successfully prepared, as shown in Figure 3g,h. Most of the XRD diffraction peaks of NNSs are indexed to Na-jarosite phase NaFe3(SO4)2(OH)6 (JCPDS No. 36-0425) except the presence of a little amount K-jarosite phase indicated by asterisk in Figure 4b, which is due to the use of δ-MnO2 containing a little amount of K. This is confirmed by ICP result (the weight ratio of K:Mn of δ-MnO2 is 5.75:54.90), revealing a possible K0.14MnO2 phase. KNSs as new intercalation materials for LIBs are examined by using cyclic voltammogram (CV) and galvanostatic charge/ discharge tests at a cutoff voltage window of 1.6−4.0 V. For comparison, microsized bulk K-jarosite was also prepared (Figure S2, Supporting Information). CV profiles display two pairs of redox peaks at 2.09/1.85 V and 2.87/2.42 V (Figure 5a), which is similar to lithium insertion/extraction behaviors of previous reported Na-jarosite.31 Such features in the CV curves are in good agreement with the corresponding sloped charge/ discharge plateaus in Figure 5b except that the first discharge curve shows a relatively flat plateau around 2.3 V, which is also

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RESULTS AND DISCUSSION The morphology of the product was first examined by SEM and TEM. As shown in Figure 3b−d, the product clearly shows 2D nanostructures with thicknesses of around 13 nm. Twodimensional nanostructures of δ-MnO2 are well-preserved after the template-assisted redox co-precipitation reaction except for a little increase of the thickness (∼13 nm) of KNSs compared to δ-MnO2 nanosheets (∼8.5 nm, Figure 3a and Figure S1, Supporting Information), which is probably due to the redox dissolution of Mn and simultaneous coprecipitation of K-jarosite or Na-jarosite on both sides of δMnO2 nanosheets. The surface of KNSs is rather rough in contrast to δ-MnO2 nanosheets. The obtained KNSs possess a specific surface area of 71 m2 g−1. As displayed in the highmagnification TEM image (Figure 3e), KNSs consist of welldefined nanobuilding blocks of 5−10 nm. The lattice fringes, assigned to the (024) interplane spacing of rhombohedral Kjarosite phase, are identified in the HRTEM image (Figure 3f), revealing a relatively good crystallinity of the product prepared at room temperature. As further confirmed by XRD pattern (Figure 4a), all of the diffraction peaks are well-indexed to the 3146

DOI: 10.1021/acs.chemmater.5b00849 Chem. Mater. 2015, 27, 3143−3149

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Figure 6. (a) EIS for KNSs and bulk K-jarosite electrodes after three charge/discharge cycles and (b) plot of the real part of impedance as a function of the inverse square root of angular frequency in the Warburg region for KNSs electrode.

Figure 7. Various characterizations for KNSs electrode after 100 cycles at 2 C: (a) SEM image, (b) high-magnification SEM image, (c) XRD pattern (broad peak at around 20° in c is due to Kapton foil background), and (d) XPS and (e) Fe 2p XPS spectra. (f) Schematic illustration of lithium intercalation/deintercalation for KNSs or NNSs electrode.

clearly observed from the sharp reduction peak at 2.3 V in the first CV curves (Figure S3, Supporting Information). The first discharge process is probably attributed to a biphasic mechanism while the subsequent sloping profiles are suggested to a solid-solution-like process.31 Apart from the first discharge process, subsequent charge/discharge profiles deliver similar features with two sloped plateaus at 2.87 and 2.09 V. The initial discharge and charge capacities are 135 mAh g−1 (2.5 Li uptake) and 111 mAh g−1 (2.1 Li removal) at a current rate of 0.1 C (1 C = 160 mA g−1), respectively, revealing a reversible insertion/extraction of around 2.1 Li per formula. As for NNSs, similar electrochemical behaviors are also observed in Figure S4a (Supporting Information). Although 2D nanostructures of jarosite deliver relatively low voltage plateaus compared to LiFePO4 (the theoretical capacity, 170 mAh g−1), such compounds possess comparable theoretical capacities of 160 or 166 mAh g−1. Importantly, the jarosite compounds can be prepared by a facile and low-cost route without requiring any high-temperature calcination, which is suitable for large-scale application. The obtained high reversible capacity for KNSs electrode encourages us to further examine their rate capability and cycling performance. As expected, KNSs electrode also delivers superior rate capability. On increasing current rate from 0.2 to

0.5, 1, 2, 5, and 10 C, the reversible capacities of 117, 118, 114, 105, 89, and 75 mAh g−1 were obtained, respectively, 4−13 times higher than those of bulk sample (Figure 5c). After continuous cycling with increasing current rates, a specific capacity of 115 mAh g−1 can be well-recovered at 0.2 C, which is about 98% retention of the specific capacity in the second cycle, confirming superior Li+ insertion/extraction reversibility in the KNSs electrode. Similar to KNSs, NNSs electrode also exhibits superior rate capability (Figure 5c). Compared to previous reported amorphous FePO4 nanosheets,18 crystalline K-/Na-jarosite nanosheets show relatively low capacities at a low current rate. When cycled at a high C rate of 10 C, KNSs and NNSs exhibit reversible capacities of 75−80 mAh g−1, which is comparable with 2D FePO4 mesoporous structures (89 mAh g−1).18 Besides high rate capability, KNSs exhibit good cycling stability as well. After 50 cycles at high current rates of 2 and 10 C, the capacity retentions of above 90% are both retained (Figure 5d). As for NNSs, a capacity retention of 84% based on the fourth cycle can be obtained after 100 cycles at 1 C as well, as shown in Figure S4b (Supporting Information). We believe that good lithium storage performance presented by KNSs or NNSs is attributed to the unique 2D nanoarchitectures, which provide fast Li+ diffusion kinetics compared to bulk materials. As the performance of the 3147

DOI: 10.1021/acs.chemmater.5b00849 Chem. Mater. 2015, 27, 3143−3149

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Chemistry of Materials electrodes depends on the simultaneous transport of Li+ and electrons to redox active interfaces, reducing the dimensions of the electrode structure significantly decreases the diffusion time by the equation: τ = L2/D, where τ is the mean diffusion time of Li+ and L and D are the diffusion length and coefficient of Li+, respectively. As showed in Figure S5 (Supporting Information), the mean diffusion time of lithium ions in the nanosheet structures is about 5 orders of magnitude lower than that of the bulk material (2−4 μm). To examine fast Li+ dynamics for 2D nanostructures of jarosite, we carried out EIS tests for KNSs and bulk K-jarosite electrodes after three charge/discharge cycles. As shown in Figure 6a, the Nyquist plots of the two electrodes show similar shapes, with a semicircle appearing in the high-frequency domain and a straight line in the low-frequency region, corresponding to the charge transfer process and mass transfer of lithium ions, respectively.51 Obviously, the diameter of the semicircle for KNSs electrode in the high-medium frequency region is at least two times smaller than that of bulk counterpart, revealing lower charge transfer resistances and higher electrochemical activity of KNSs electrode. The low-frequency Warburg impedance corresponding mainly to the Li+ diffusion in the electrode can be used to determine Li+ diffusion coefficient in the compound. Based on the model proposed by Ho et al.,52 the diffusion coefficient (DLi+) of Li+ for KNSs electrode can be calculated by the following equation: D Li

+

2 1 ⎡⎛ VM ⎞ δE ⎤ ⎢ ⎥ = ⎜ ⎟ 2 ⎢⎣⎝ AFσ W ⎠ δx ⎥⎦

contact area to facilitate Li+ insertion/extraction in contrast to the bulk counterpart, enabling fast Li+ diffusion, as shown in Figure 7f. (iii) The nanosheets with the lateral submicrometric/ micrometric dimensions not only effectively maintain good structure integrity but also simultaneously buffer the structural strain generated by volume variation during the repeated Li+ insertion/extraction process and counteract electrode pulverization, as illustrated in Figure 7f.

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CONCLUSION In summary, we prepared for the first time 2D nanostructures of jarosite and its analogues via a facile template-assisted redox co-precipitation approach at room temperature and examined their electrochemical activity as attractive cathode hosts for LIBs. The as-prepared 2D materials exhibit good rate capability and cycling performance. Considering the earth-abundant resources, low-cost fabrication, and attractive lithium storage performance, KNSs and NNSs show great potential as cathode candidates for LIBs.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images and XRD patterns of δ-MnO2 nanosheets and bulk K-jarosite, the CV profile, galvanostatic charge and discharge profiles, cycling performance of NNSs, and the calculation of Li+ diffusion time in nanosheet and bulk electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

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(6)

where VM is molar volume of the compound, and F is the Faraday constant (96,486 C mol−1). A is the contact area between compound and electrolyte (here the geometric area of the electrode, 0.785 cm2, is used for simplicity), and σW is the Warburg coefficient, which can be obtained from the slope of Z′ vs ω−1/2 plots (ω, angular frequency) in the Warburg region (Figure 6a). As shown in Figure 6b, the σW value is 47.75. δE/ δx can be determined from the third galvanostatic discharge/ charge profiles (Figure 5b).53 Based on eq 6, the DLi+ value could be calculated to be 7.31 × 10−10 cm2s−1. To investigate the structural and morphology stability of the as-prepared nanosheet electrode, we performed SEM, XRD, and XPS characterization for KNSs electrode after 100 cycles at 2 C. As displayed in Figure 7a,b, 2D morphology can still be well-retained, revealing superior structural integrity of the obtained KNSs upon cycling. As displayed in Figure 7c, the XRD peaks of the KNSs electrode after 100 cycles are indexed to rhombohedral jarosite KFe3(SO4)2(OH)6 phase (JCPDS No. 71-1777), suggesting good structural stability upon repeated Li+ intercalation/deintercalation. Moreover, KNSs electrode was also examined by XPS, showing the presence of K, Fe, S, and O (Figure 7d). In the high-resolution XPS spectrum of Fe 2p (Figure 7e), two peaks that are assigned to Fe 2p1/2 (∼724.2 eV) and Fe 2p3/2 (∼711.5 eV) were identified as well as a satellite peak at ∼719.0 eV, revealing that element Fe mainly exists in the form of Fe3+.49 These results further confirm morphology and structural stability of the as-prepared 2D jarosite nanostructures upon cycling. The good lithium storage performance of KNSs and NNSs electrodes could be ascribed to the following aspects. (i) The nanobuilding blocks in the whole 2D nanoassembly provide short distance for Li+ diffusion and electron transport, which is very crucial to the rate capability. (ii) The ultrathin nanostructures offer a sufficient

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to Mr. Mitsuharu Konuma for XPS measurement, Ms. A. Fuchs for BET analysis, and Dr. H. Hoier for XRD measurements. This work was financially supported by the Alexander von Humboldt Foundation (Sofja Kovalevskaja award), the National Natural Science Foundation of China (Grant Nos. 21171015 and 21373195), the “Recruitment Program of Global Experts”, the program for New Century Excellent Talents in University (Grant NCET12-0515), the Fundamental Research Funds for the Central Universities (Grants WK2060140014 and WK2060140016), the Collaborative Innovation Center of Suzhou Nano Science and Technology, and the Max Planck Society, as well as the European Union Seventh Framework Programme (FP7/2007− 2013) under Grant Agreement No. 312483 (ESTEEM2).

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REFERENCES

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DOI: 10.1021/acs.chemmater.5b00849 Chem. Mater. 2015, 27, 3143−3149

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DOI: 10.1021/acs.chemmater.5b00849 Chem. Mater. 2015, 27, 3143−3149