Intercalation of Sodium Ions into Hollow Iron Oxide Nanoparticles

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Intercalation of Sodium Ions into Hollow Iron Oxide Nanoparticles Bonil Koo,†,* Soma Chattopadhyay,⊥ Tomohiro Shibata,⊥ Vitali B. Prakapenka,∥ Christopher S. Johnson,‡ Tijana Rajh,† and Elena V. Shevchenko†,* †

Center for Nanoscale Materials and ‡Chemical Sciences and Engineering, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ∥ Center for Advanced Radiation Sources, University of Chicago, 5901 South Ellis Avenue, Chicago, Illinois 60637, United States ⊥ CSRRI-IIT, MRCAT, APS, Argonne National Laboratory, and Physics Department, Illinois Institute of Technology, 3300 South Federal Street, Chicago, Illinois 60616, United States S Supporting Information *

ABSTRACT: Cation vacancies in hollow γ-Fe2O3 nanoparticles are utilized for efficient sodium ion transport. As a result, fast rechargeable cathodes can be assembled from Earth-abundant elements such as iron oxide and sodium. We monitored in situ structural and electronic transformations of hollow iron oxide nanoparticles by synchrotron X-ray adsorption and diffraction techniques. Our results revealed that the cation vacancies in hollow γ-Fe2O3 nanoparticles can serve as hosts for sodium ions in high voltage range (4.0−1.1 V), allowing utilization of γ-Fe2O3 nanoparticles as a cathode material with high capacity (up to 189 mAh/g), excellent Coulombic efficiency (99.0%), good capacity retention, and superior rate performance (up to 99 mAh/g at 3000 mA/g (50 C)). The appearance of the capacity at high voltage in iron oxide that is a typical anode and the fact that this capacity is comparable with the capacities observed in typical cathodes emphasize the importance of the proper understanding of the structure− properties correlation. In addition to that, encapsulation of hollow γ-Fe2O3 nanoparticles between two layers of carbon nanotubes allows fabrication of lightweight, binder-free, flexible, and stable electrodes. We also discuss the effect of electrolyte salts such as NaClO4 and NaPF6 on the Coulombic efficiency at different cycling rates. KEYWORDS: sodium ion battery, fast rechargeable cathode, hollow nanoparticle, iron oxide, cation vacancy, in situ study

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INTRODUCTION Nanometer-to-micrometer sized hollow particles with controlled size, shell thickness, porosity, and composition1−3 are promising candidates for application in biomedicine,4−6 catalysis,7−12 and design of gas sensors.13,14 Effective transport of atoms, ions, or molecules through the thin shells of hollow nanoparticles (NPs) also makes them attractive candidates for energy storage.15−22 For example, in Li+ ion batteries, anodes made from hollow nanostructures (e.g., TiO2,15 Co3O4,17 SnO2,18 CuO,20 and α-Fe2O321) demonstrated higher capacities and cyclabilities as compared to anodes assembled from analogous solid nano-/microparticles. Also, hollow structures of lithiated layered oxides such as LiNi0.5Mn1.5O422 and LiMn2O419 being used as cathode materials showed better rate performance. Better electrode performance of hollow NPs based electrodes was attributed to efficient Li+ transport through the thin shells. In addition to that, the hollow γ-Fe2O3 NPs synthesized via Kirkendall effect were shown to serve both as cathode and anode for Li+ ion batteries with superior capacity retention and rate performance.16 New strategies toward replacing lithium ion batteries with energy storage systems based on more Earth-abundant elements are needed because of limited lithium sources.23 © XXXX American Chemical Society

Sodium ion batteries are promising candidates for large-scale energy storage systems owing to the abundance and low cost of sodium. However, it has been difficult to find appropriate Na+ electrode materials since the large size of the sodium ion leads to rather limited intercalation/extrusion into/out of active materials. As a result, materials efficiently working with lithium ions either do not work or show very limited activity with sodium ions. In order to solve this issue, layered24−27 and tunnel-type oxides28 and phosphates29−32 have been proposed for batteries based on Na+ ions. Bulk materials demonstrated relatively low capacity and cyclability ( discharge capacity) of the initial cycles obtained at the slow (30 mA/g) rate. Coulombic efficiency vs cycle number was plotted and shown in Figure S8a, Supporting Information. In order to figure out the origin of the anomalous capacity we performed rate study on the same electrode. Thus the hollow NP/CNT electrode was tested under a standard condition (1.0 D

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Figure 5. (a) Capacity versus cycle numbers for the hollow NP/CNT composite electrode operated at different current rates (300 mA/g and 30 mA/g) in the voltage range of 4.0−1.1 V with 1.0 M NaClO4 in PC as the electrolyte. (b) Voltage profiles were plotted for the 13th, 15th, 18th, and 20th cycles (the rate of cycling for 13th and 20th was 300 mA/g and 30 mA/g for 15th and 18th, respectively).

Figure 6. Rate study for four different electrode/electrolyte systems: (a) pure hollow NP, (b) solid NP/CNT, and (c) pure CNT electrodes with 1.0 M NaClO4 in PC as the electrolyte and (d) pure CNT electrode with 1.0 M NaPF6 in PC as the electrolyte. Red rectangles indicate the anomalous Coulombic efficiency.

that the side reaction occurs at the surface of CNTs. Indeed the control experiments with electrodes made from CNTs only confirmed this assumption. Thus significant anomalous Coulombic efficiency has been observed during the cycling of CNTs between 4.0 and 1.1 V at slow rate (Figure 6c). In our experiments we used NaClO4 dissolved in PC as electrolyte. This electrolyte was previously reported to be stable below 4.0 V.61−63 However, our experimental results indicate that CNTs catalyze the oxidative decomposition of PC that was known to occur at significantly higher voltage. The trace of NaCl detected in synchrotron XRD patterns (Figure 3a) on the electrode after slow rate charging confirms the reduction of NaClO4. However, the oxidative decomposition of PC is slow, and it seems not to affect the electrode performance at fast cycling such as 300 mA/g and 3000 mA/g. This is similar to the irreversible electrolyte decomposition that occurred only at slow scan rate in graphite intercalation compounds.61,64 To date there are two of the most common Na+ ion electrolyte salts such as NaClO4 and NaPF6. In order to improve the

M NaClO4 in PC with Na metal as a counter electrode) at fast rate (300 mA/g) and slow rate (30 mA/g), alternately (Figure 5). We observe that the initial slow rate cycling had significantly larger charging capacity than discharging capacity and the irreversibility at slow rate was reduced after cycles, which is consistent in the feature shown in Figure 4a. These data indicated that the process leading to the anomalous Coulombic efficiency is kinetically controlled and does not take place at fast rate. The next question is where exactly this side process takes place. We conducted controlled experiments with electrodes made from (i) hollow γ-Fe2O3 NPs (no CNTs) only, (ii) solid γ-Fe2O3 NPs sealed between two layers of CNTs, and (iii) CNTs only. In the case of cathodes assembled from hollow γFe2O3 NPs by filtering colloidal suspension of NPs through the separator membrane we observed significantly lower capacity; however, no anomalous Coulombic efficiency has been observed (Figure 6a). On the other hand, composite solid γFe2O3 NP/CNT cathode showed anomalous Coulombic efficiency at slow cycling rate (30 mA/g) (Figure 6b) indicating E

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Figure 7. (a) Capacity versus cycle numbers and (b) voltage profiles (13th, 15th, 18th, and 20th cycles) for the hollow NP/CNT composite electrode operated at different current rates (300 mA/g and 30 mA/g) in the voltage range of 4.0−1.1 V with 1.0 M NaPF6 in PC as the electrolyte.

rates. Although the Na+ ion battery cells with use of NaPF6 showed slightly less capacity during extensive cycles (108 mAh/ g at 300 mA/g and 63 mAh/g at 3000 mA/g at 200th cycle) as compared with the NaClO4 case, these defect-based electrodes still show capacities comparable with the capacities demonstrated by typical Na+ ion cathode materials.23,65,66

performance of the electrode at slow rate, we performed the electrochemical cycling of CNT-based electrodes in NaPF6 that in analogy to LiPF6 can be less oxidative.61 It is worth mentioning that no anomalous Coulombic efficiency has been observed for CNT-based electrodes being cycled in NaPF6/PC electrolyte (Figure 6d). Cycling of the hollow γ-Fe2O3 NP/ CNT electrode at different rates in NaPF6/PC electrolyte also demonstrate no anomalous Coulombic efficiency at slow (30 mA/g) rate; however, the capacities at all rates were slightly lower than ones achieved in NaClO4/PC electrolyte (Figure 7a). Also, no extra capacity not associated with Na+ ions intercalation has been observed (Figure 7b) as it was observed for the same type of samples cycled under identical conditions in NaClO4/PC electrolyte (Figure 5b). Note that significant increase in fast rate capacity after slow rate cycles was observed both in NaClO4/PC and NaPF6/PC electrolytes. Thus, 70 mAh/g at the 13th cycle evolved up to 94 mAh/g at the 20th cycle in the case of NaPF6/PC, and 99 mAh/g at the 13th cycle went up to 140 mAh/g at the 20th cycle in case of NaClO4/PC. This can be explained as effective crystal organization provided during the slow rate cycling toward making an efficient ionic pathway.16,33,60 Figure 8 summarizes long-term performance of the Na+ ion cathode made from hollow γ-Fe2O3 NP/CNT in NaPF6/PC electrolyte tested in the high voltage range (4.0−1.1 V) with different

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CONCLUSIONS This study shows that hollow γ-Fe2O3 NPs with a large amount of iron vacancies can serve as a host material for the fast Na+ intercalation at high voltage. Hollow γ-Fe2O3 NPs encapsulated between CNTs demonstrated high capacity (189 mAh/g) and excellent Coulombic efficiency (99.0%) and, for the first time, showed superior rate performance during extensive cycling as cathodes with Na+ ions. Thus capacity of 99 mAh/g was achieved at 3000 mA/g (50 C) after more than 500 cycles. Crystal structure is preserved until all vacancies are filled and particle morphology is also maintained, verifying the stability of the hollow NPs. Surprisingly, the capacity of the cell with the Na+ ions was only 2% and 30% lower as compared with Li+ ion cells at slow and high cycling rates, respectively. Our work emphasizes that the new morphology of nanostructures can be an important key to improve Na+ battery performance. Our defect-based cathode operates at relatively low voltage (∼2 V vs metallic Na) that limits the choice of the anode material in design of the full cell. However, the appearance of the capacity at high voltage in the material that is a typical anode and the fact that this capacity is comparable with the capacities observed in typical cathodes emphasize the importance of the proper understanding of the structure−properties correlation. Our hollow γ-Fe2O3 NP-based cathode does not have a sodium ion source and thus requires Na+-containing anode material in a full cell. Another approach that can address this issue is a chemical treatment of our hollow NPs with the precursor of sodium ions.67 The use of CNTs as a conductive matrix and sealer of NPs that serve as active electrode materials allows achieving very high performance of NP-based electrode. Side reactions catalyzed by the CNTs in NaClO4/PC electrolyte at slow rate can be avoided by switching of the electrolyte to NaPF6/ PC in which hollow γ-Fe2O3 NPs are still very active for Na+ ions transport.

Figure 8. Capacity versus cycle numbers for the hollow γ-Fe2O3 NP/ CNT composite electrodes combined with NaPF6 in PC as the electrolyte operated in the voltage range of 4.0−1.1 V at different rates (30, 300, and 3000 mA/g). Electrodes showed good capacity retention during extensive cycles (200 cycles). F

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We believe that further compositional optimization of the defect-enriched active materials via doping34 or synthesis of typical cathode materials24−32 with high concentration of structural defects is a very promising research direction to design fast rechargeable, high voltage batteries with high capacities.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of core/shell and hollow NPs, XRD and XANES/ EXAFS data on hollow γ-Fe2O3 NPs, capacity retention and TEM images after Li+ and Na+ conversion reactions, voltage profile of CNTs, LCF of XANES spectra, and Coulombic efficiency and capacity vs cycle number. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.K.); [email protected] (E.V.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Dr. Sanja Tepavcevic for fruitful discussions about electrochemical data analysis. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The X-ray diffraction work was performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR-0622171) and Department of Energy - Geosciences (DE-FG02-94ER14466). MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. The use of the Advanced Photon Source at ANL was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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