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Comparative Study of Li and Na Electrochemical Reactions with Iron Oxide Nanowires
Electrochimica Acta 118 (2014) 143–149
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Comparative Study of Li and Na Electrochemical Reactions with Iron Oxide Nanowires Bo Huang, Kaiping Tai, Mingou Zhang, Yiran Xiao, Shen J. Dillon ∗ Department of Materials Science and Engineering, University of Illinois Urbana-Champaign, Urbana, IL, United States
a r t i c l e
i n f o
Article history: Received 10 September 2013 Received in revised form 2 December 2013 Accepted 3 December 2013 Available online 17 December 2013 Keywords: lithium battery sodium battery iron oxide nanowire conversion reaction TEM investigation.
a b s t r a c t This study emphasizes optimization of Fe2 O3 and Fe3 O4 nanowire conversion electrodes by directlygrowing them on current collectors, preparing them as single crystals, and coating their surfaces with conductive carbon coatings. The systems with the least polarization during Li-ion cycling are then tested as electrodes for Na-ion chemistry. Precipitation of nanograined material during the first cycle reduces the polarization associated with Li insertion upon subsequent cycles. After the first cycle, delithiation primarily contributes to polarization associated with the conversion reaction with lithiation occurring close to the equilibrium potential. The initial reduction reaction does not proceed to completion for Na chemistries. Electron microscopy reveals significant Na insertion that occurs along with the formation of defect networks. However, the results indicate that an insufficient amount is present to form critical nuclei necessary to induce the conversion reaction. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Conversion reactions possess the potential to greatly enhance the energy density of electrochemical storage technologies. Many conversion electrodes exhibit capacities in the range of 500-1000 mAhg−1 and can achieve good cycle life.[1] Metal oxide based conversion reactions with Li typically proceed as follows [2]: Mx O + 2Li+ + 2e− ↔ Li2 O + xM(M = Fe, Ni, Cu, Co, etc.)
(1)
High capacity high voltage conversion cathodes based on metal fluorides show particular promise in improving Li-ion energy density.[3] Conversion reactions typically suffer from hysteresis that limits their round trip efficiency.[1] Since conversion reactions do not require intercalation, ideally they should be well suited to perform well as electrodes for Na-ion reactions. Developing high capacity Na-ion batteries is desirable because of Na’s relative abundance and low cost. However, limited success has been achieved in applying conversion reaction electrodes to Na-ion systems.[4] This work investigates the rate limiting processes associated with Li-ion and Na-ion cycling of model iron oxides (Fe2 O3 and Fe3 O4 ) conversion reaction electrode materials. Iron oxides are interesting conversion reaction anodes due to the high capacity (1007 mAh/g for Fe2 O3 and 900 mAh/g for Fe3 O4 [5,6]), abundance, low toxicity, and low cost. Similarly, iron fluorides could serve as ideal next-generation Li-ion cathodes [3].
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S.J. Dillon). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.12.007
Unfortunately, poor electronic conductivity and short characteristic diffusion lengths at room temperature handicaps iron oxides and limit their commercial applicability. Therefore, various approaches to nanostructuring have been married with carbon coating strategies to achieve reasonable performance [7–15]. Nanostructures grown directly on current collector are favored to reduce contact resistance and achieve optimal charge transport[16]. FeS2 , Ni3 S2 and NiCo2 O4 have been shown to function as conversion reaction electrodes for Na-ion batteries.[4,17–20] These reactions result in nanocrystalline metal surrounded by Na2 O or Na2 S. It has been suggested by some that iron oxides do not function as Na-ion conversion electrodes.[4] Na insertion was observed in Fe3 O4 and Fe2 O3 nanoparticles ranging from 10-400 nm with increased capacity resulting from particle size reduction [21]. However, no conversion reaction was observed. Recent work demonstrates Na conversion reaction with Fe3 O4 in the particle size range from 4 to 10 nm [22]. However, much of the capacity achieved was in the range where Na intercalates into carbon, which was present in large amounts. The primary evidence for the reaction is the observation of Na2 O in electron diffraction and a single Fe ring that overlaps with Fe3 O4 . The Na2 O could possibly form upon exposure of the sample to air while loading it into the transmission electron microscope. However, the overall capacity does exceed what would be expected from the carbon alone. Hollow ␥Fe2 O3 nanoparticles with abundant cation vacancies have recently been reported to be promising electrode materials for Na-ion batteries in terms of the both capacity and cycle life [23]. The various results motivate further investigation into Na conversion reactions with Fe-based oxides.
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Fig. 1. SEM micrographs, TEM micrographs, and selected area electron diffraction (SAED) patterns of as-prepared nanowires: ␣-Fe2 O3 (a-c), ␣-Fe2 O3 -C (d-f), Fe3 O4 -C (g-i) and Fe3 O4 (j-l).
In this work, we grow ␣-Fe2 O3 and Fe3 O4 single crystal nanowires with and without carbon coating directly on current collectors. The systems are optimized for Li-ion cycling and subsequently characterized as Na-ion hosts. 2. Experimental 2.1. Preparation of ˛-Fe2 O3 single crystal nanowires without carbon coating 220 m diameter (99.99%, Goodfellow) 10 cm long iron wire was cleaned in dilute hydrochloric acid (2% in volume) and rinsed with acetone, alcohol, and deionized water. AC power (∼4 W at 60 Hz) was then applied for ∼10 mins to promote surface oxidation during Joule heating [24]. Nanowire growth proceeds by rapid cation diffusion that emerges from grain boundaries of a thicker underlying oxide.
2.3. Preparation of Fe3 O4 single crystal nanowires with carbon coating [25] 220 m diameter iron wire (99.99%, Goodfellow) was first cleaned as described above. It was the pre-oxidized at 250 ◦ C for 0.5 h on the hotplate in ambient conditions. The samples were subsequently annealed at 550 ◦ C in a crucible also containing pure Cu. Annealing was performed in Ar that flowed through toluene prior to entering the furnace. During annealing Cu deposits on the Fe substrate and catalyzes the growth of Fe3 O4 nanowires. This process does not result in a thick underlying oxide typical of the Fe2 O3 growth process. 2.4. Preparation of Fe3 O4 single crystal nanowires without carbon coating Reactive ion etching in 5% oxygen and 95% argon for 40 sec was used to remove the carbon coating from the Fe3 O4 nanowires prepared as described above.
2.2. Preparation of ˛-Fe2 O3 single crystal nanowires with carbon coating
2.5. Electrochemical testing
The resulting ␣-Fe2 O3 single crystal nanowires were heated to 500 ◦ C for 5 h in a tube furnace containing Ar that had flowed through toluene. Decomposition of the toluene produces a carbon coating on the nanowires.
The nanowires were tested in a vial cell within a dry Ar-filled glovebox (Mbraun Labstar). The iron wire substrate served as the electrode current collector. This was cycled against either a metallic lithium or sodium counter electrode in ethylene carbonate (EC)
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Fig. 2. Cyclic voltammagrams of nanowires cycled against Li metal at scan rates of 500 V/s: ␣-Fe2 O3 (a), ␣-Fe2 O3 -C (b), Fe3 O4 (c) and Fe3 O4 -C (d).
dimethyl carbonate (DMC) (1:1 by volume) 1 M LiClO4 or NaClO4 electrolyte, respectively. The electrochemical tests were carried out using a computer-controlled potentiostat/galvanostat (SP200, Biologic Co.). Cyclic voltammetry (CV) was performed at a scan rate of 500 V/s. In order to promote a more complete reaction during Na-ion cycling, the samples were also maintained at a constant potential of 0.01 V (vs. Na/Na+ ) for 10 h, after the potential sweeps. The rate performance of Fe3 O4 nanowires was tested at various current densities (0.2 mAh/cm2 -1.6 mAh/cm2 ). After testing, all of the samples were washed by propylene carbonate (PC) and acetone, and then dried in the glove box. 2.6. Charaterization The pristine and reacted nanowires were characterized by scanning electron microscopy, SEM (JEOL-6060LV), transmission electron microscopy, TEM (JEOL-2010Lab6 and JEOL-2010Cryo), and Energy-dispersive X-ray spectroscopy (EDS) in the scanning transmission electron microscopy (JEOL-2010F EF-FEG).
and k) function as catalysts during the synthesis at low oxygen partial pressure [25]. According to each of the selected area electron diffraction (SAED) patterns (Fig. 1c, f, i and l), the prepared nanowires are single crystalline. Fig. 2 shows cyclic voltammetry performed on each sample at a scan rate of 500 V/s between 0.25 and 2.5 V vs. Li/Li+ . The first cathodic peak in each sample occurs at 0.75 V. The initial cathodic peaks shift to 0.85-0.9 V in the second cycle. This value is associated with the reversible electrochemical potential for Fe2 O3 and Fe3 O4 [26,27]. During subsequent cycles some increase in cathodic polarization is observed. The carbon coating provides limited improvement in the polarization associated with Fe2 O3 , but has no observable impact on the cathodic polarization in Fe3 O4 . After the first cycle, the single crystal nanowires are converted to polycrystalline nanowires [28]. The excess cathodic polarization observed in the first cycle likely results from the conversion of a single crystal nanowire to a polycrystalline nanowire.
3. Results and discussion 3.1. Cycling Iron Oxide Nanowires Against Li Fig. 1 shows SEM and TEM micrographs of the pristine ␣Fe2 O3 , ␣-Fe2 O3 -C, Fe3 O4 -C, and Fe3 O4 single crystal nanowires. A high density of relatively long nanowires (2∼10 m for Fe2 O3 and 5∼20 m for Fe3 O4 ) resulted from the different growth processes. The Fe3 O4 -C nanowires were present in the highest density and were the longest nanowires. The ␣-Fe2 O3 , ␣-Fe2 O3 -C nanowires are needle shaped, while the Fe3 O4 nanowires are relatively uniform in diameter. Fig. 1e indicates a ∼3 nm carbon layer uniformly surrounds the coated nanowires. The copper nanoparticles terminating the carbon-coated Fe3 O4 and Fe3 O4 nanowires (Fig. 1h
Fig. 3. Rate performance, columbic efficiency, and cycle life of Fe3 O4 -C nanowire array cycle against Li at various current densities.
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Fig. 4. Cyclic voltammagrams of ␣-Fe2 O3 -C (a) and Fe3 O4 -C (b) nanowires cycled against Na metal at scan rates of 500 V/s.
All of the anodic peaks are much broader in width than the cathodic peaks, indicating sluggish oxidation during delithiation. The peak separation (Ep = Ecath - Eanod ) is smaller than values 1 and 1.6 V reported elsewhere [15,29]. This may result from two factors; high surface area nanostructures can limit diffusional path lengths and the reduced contact resistance, associated with directly growing the electrodes on the current collectors [16], optimizing charge transport.
It is well known that carbonous coatings often enhance electron transport, suppress polarization, and extend cycle life. Relative to the pristine ␣-Fe2 O3 , carbon-coated ␣-Fe2 O3 nanowires exhibit lower hysteresis and improved stability during multiple cycles. Uncoated and carbon-coated Fe3 O4 perform similarly to one another and the ␣-Fe2 O3 -C. The larger current density in the Fe3 O4 results from the larger nanowire density. The intrinsic electronic conductivity of Fe3 O4 ( ∼ 10−2 S/m) significantly exceeds that of
Fig. 5. Bright field (BF) images, dark field (DF) images, and SAED patterns of nanowires after the first half of cyclic voltammetry: Li-reacted ␣-Fe2 O3 -C (a-c) and Fe3 O4 -C (d-f), and Na-reacted ␣-Fe2 O3 -C (g-i) and Fe3 O4 -C (j-l).
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Fig. 6. BF image of pristine Fe3 O4 -C nanowire (a), BF image (b), SAED pattern (c) and HRTEM image (d) of the Na-reacted Fe3 O4 -C nanowire discharged at 0.01 V (vs. Na/Na+ ) for 10 h.
␣-Fe2 O3 ( ∼ 10−8 S/m) at room temperature [30,31]. This difference accounts for the difference in polarization between the two materials. It also accounts for the lack of improvement of Fe3 O4 by carbon coating. Overall the CV results indicate that Fe3 O4 -C nanowires exhibit the best performance in terms of hysteresis and cycling stability. The capacity retention, rate capability, and columbic efficiency of
these electrodes under galvanostatic cycling against Li was characterized as shown in Fig. 3. The columbic efficiency increases from 60% to 96% in the first 5 cycles. The possible sources of irreversible capacity include: solid electrolyte interface (SEI) formation on the large surface area of nanowire array, which may also be observed in Fig. 5d, lithium insertion into irreversible sites, or side reactions with absorbed and impurity species.[16] Taberna
Fig. 7. BF image (a) and EDS maps (b-d) of Na-reacted Fe3 O4 -C nanowire discharged at 0.01 V (vs. Na/Na+ ) for 10 h.
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et al. electrochemically deposited Fe3 O4 nano-particles on copper nanowire arrays, achieving a large areal capacity of 0.35 mAh/cm2 [5]. Our direct nanowire growth improves the areal energy density to 1.1 mAh/cm2 , with similar electrochemical performance. We note that at the lowest rate (∼C/5) that these nanowires approximately achieve the theoretical capacity. 3.2. Cycling Iron Oxide Nanowires Against Na The conversion electrode systems optimized for lithium ion chemistry described above is extended to sodium ion systems. Fig. 4 shows CV for ␣-Fe2 O3 -C and Fe3 O4 -C nanowires electrodes cycled against sodium in the range of 0.2-2.2 V (vs. Na/Na+ ), which was utilized in order to keep absolute potentials consistent with the Li experiments. The cathodic current density for Na insertion during the first cycle is one order of magnitude smaller than for Li insertion. No appreciable anodic current is measured, and the cathodic current degrades upon subsequent cycling. Limited Na is inserted into the nanowires and almost none is extracted anodically. The decay in the cathodic peak suggests that the electrode progressively saturates with Na during the CV cycles. Testing the Fe substrate alone resulted in a current response an order of magnitude lower than when the nanowires are present. This is consistent with the fact that there is negligible solubility and no intermediate phases in the Na-Fe system. We also tested the samples in propylene carbonate 1 M NaClO4 to compare with ref. [23], and observed a similar response to that in Fig. 4. All the nanowires were investigated by transmission electron microscopy (TEM) after the first half cycle of potential sweep as shown in Fig. 5. The morphology, phase, and structure are characterized by bright field (BF) imaging, dark field (DF) imaging and selected area electron diffraction (SAED), respectively. Prior investigation of the mechanism for the conversion reaction in ␣-Fe2 O3 nanowires [28] indicates that reduction of Fe2 O3 is completed and convert to BCC Fe occurs by 0.6 V. The results in Fig. 5 indicate that ␣-Fe2 O3 and Fe3 O4 completely reduce to BCC Fe. The size of the Fe nanocrystals ranges from 5 to 25 nm. No appreciable SEI is observed on the nanowires cycled against Na. These nanowires remain single crystalline, but display a significant amount of BF diffraction contrast relative to the initial nanowires. In attempt to complete Na insertion into the Fe3 O4 -C nanowires, they were cycled to 0.01 V vs. Na/Na+ and held for 10 h. The contrast of BF image demonstrates the occurrence of a large concentration of dislocations within the nanowire (Fig. 6b), which are absent in the pristine Fe3 O4 -C (Fig. 6a). The SAED pattern (Fig. 6c) demonstrates that the Fe3 O4 remains single crystalline. No reduced Fe appears in the SAED or high-resolution images (Fig. 6d). Additional evidence is that the 0.30 nm d-space of [220] measured from HRTEM images, matches the standard value of d-space for Fe3 O4 (XRD-PDF 01-0786086). Fig. 7 shows EDS maps of Na, Fe, and O in the Fe3 O4 nanowires, discharged to 0.01 V vs. Na/Na+ for 10 h. The results suggest that a small amount of Na intercalates into the nanowire. The uniform intensity should not result from SEI, which would cause the Na to be concentrated at the surface. Additionally, SEI is not observed at the surface by high-resolution imaging (Fig. 6d). Overall, the results indicate that Na diffuses into the nanowires and induces significant dislocation production, but does not initiate the conversion reaction. This Na is subsequently trapped in the nanowire and cannot diffuse out during the cathodic sweep. This differs significantly from reactions in the Li system, where the anodic reaction occurs near the theoretical potential and the cathodic reaction is chemically reversible with some degree of hysteresis. The significant discrepancies likely arise from the differences in ionic radii between Li (76 pm) and Na (102 pm) [1]. A larger hysteresis was observed for Li insertion during the first cycle, which is attributed
to the conversion of the single crystalline nanowires to polycrystalline nanowires that have grain boundary pathways for enhanced transport and heterogeneous nucleation. It should be noted that we also first cycled nanowire samples against Li to induce the conversion reaction that results in polycrystalline nanowires, and then attempted to cycle these nanowires versus Na. However, the approach did not appreciably affect the Na insertion reaction. Damage accumulation in Fe2 O3 and Fe3 O4 nanowires due to strain induced by Na insertion is consistent with results for Li insertion in SnO2 where the large strain induces dislocation production and amorphization [32]. The effect is attributed to the large in-plane misfit associated with Li insertion. Na insertion into Fe2 O3 or Fe3 O4 likely introduces large shear stresses that both induce damage and ultimately suppress the conversion reaction by not allowing the Na to reach a critical concentration necessary to nucleate Na2 O and reduced Fe. Hariharan et al. recently investigated Na insertion into nanocrystalline Fe3 O4 , in the size range of 4-10 nm. Anodic currents were attributed to Na insertion following the conversion reaction [22]: Fe3 O4 + 8e + 8Na+ ↔ 3Fe0 + 4Na2 O
(2)
Their results suggest that all Fe2+/3+ can be reduced to Fe0 during the discharge. SAED in this study indicates the formation of Na2 O with only a weak ring associated with Fe, which overlaps with Fe3 O4 . The capacity achieved is half of that associated with Li, indicating that the Fe2+/3+ should only be partially reduced. Additionally, some of the capacity is likely associated with intercalation into carbon. However, carbon cannot account for all of the observed capacity indicating that the conversion reaction must proceed to some degree. This fine (4-10 nm) powder must be small enough that the volumetric strain associated with the initial Na intercalation does not hinder the accumulation of sufficient Na to overcome the barrier associated with nucleating Fe and Na2 O. The nanowires in this study are not sufficiently small to allow the reaction to proceed to this step. Extraction of Li from Fe2 O3 and Fe3 O4 has a larger associated polarization than insertion. The same problem may also plague the Na conversion reaction, even in the case of fine nanoparticles. Such irreversibility may account for the relatively low columbic efficiency and rapid capacity fade associated with the Na conversion reaction in Fe3 O4 [22].
4. Conclusions The electrochemical response of ␣-Fe2 O3 , ␣-Fe2 O3 -C, Fe3 O4 , and Fe3 O4 -C single crystal nanowire conversion anodes was characterized during cycling against Li and Na. ␣-Fe2 O3 -C and Fe3 O4 -C nanowires exhibited the best performance in terms of capacity, reversibility, and cyclability versus Li. While Na could be partially inserted into the nanowires, the conversion reaction could not be induced and the reaction was not chemically reversible. The accumulation of damage during insertion suggests that the reaction is strain limited. The work supports earlier results that indicate Na insertion is possible and that at smaller particle sizes it may be possible to induce the conversion reaction.
Acknowledgements The authors are grateful for funding provided by the U.S. Department of Energy, Basic Energy Sciences (Contract No. DESC0006509). The research was carried out in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Comparative Study of Li and Na Electrochemical Reactions with Iron Oxide Nanowires Bo Huang, Kaiping Tai, Mingou Zhang, Yiran Xiao, Shen J. Dillon ∗ Department of Materials Science and Engineering, University of Illinois Urbana-Champaign, Urbana, IL, United States
a r t i c l e
i n f o
Article history: Received 10 September 2013 Received in revised form 2 December 2013 Accepted 3 December 2013 Available online 17 December 2013 Keywords: lithium battery sodium battery iron oxide nanowire conversion reaction TEM investigation.
a b s t r a c t This study emphasizes optimization of Fe2 O3 and Fe3 O4 nanowire conversion electrodes by directlygrowing them on current collectors, preparing them as single crystals, and coating their surfaces with conductive carbon coatings. The systems with the least polarization during Li-ion cycling are then tested as electrodes for Na-ion chemistry. Precipitation of nanograined material during the first cycle reduces the polarization associated with Li insertion upon subsequent cycles. After the first cycle, delithiation primarily contributes to polarization associated with the conversion reaction with lithiation occurring close to the equilibrium potential. The initial reduction reaction does not proceed to completion for Na chemistries. Electron microscopy reveals significant Na insertion that occurs along with the formation of defect networks. However, the results indicate that an insufficient amount is present to form critical nuclei necessary to induce the conversion reaction. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Conversion reactions possess the potential to greatly enhance the energy density of electrochemical storage technologies. Many conversion electrodes exhibit capacities in the range of 500-1000 mAhg−1 and can achieve good cycle life.[1] Metal oxide based conversion reactions with Li typically proceed as follows [2]: Mx O + 2Li+ + 2e− ↔ Li2 O + xM(M = Fe, Ni, Cu, Co, etc.)
(1)
High capacity high voltage conversion cathodes based on metal fluorides show particular promise in improving Li-ion energy density.[3] Conversion reactions typically suffer from hysteresis that limits their round trip efficiency.[1] Since conversion reactions do not require intercalation, ideally they should be well suited to perform well as electrodes for Na-ion reactions. Developing high capacity Na-ion batteries is desirable because of Na’s relative abundance and low cost. However, limited success has been achieved in applying conversion reaction electrodes to Na-ion systems.[4] This work investigates the rate limiting processes associated with Li-ion and Na-ion cycling of model iron oxides (Fe2 O3 and Fe3 O4 ) conversion reaction electrode materials. Iron oxides are interesting conversion reaction anodes due to the high capacity (1007 mAh/g for Fe2 O3 and 900 mAh/g for Fe3 O4 [5,6]), abundance, low toxicity, and low cost. Similarly, iron fluorides could serve as ideal next-generation Li-ion cathodes [3].
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S.J. Dillon). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.12.007
Unfortunately, poor electronic conductivity and short characteristic diffusion lengths at room temperature handicaps iron oxides and limit their commercial applicability. Therefore, various approaches to nanostructuring have been married with carbon coating strategies to achieve reasonable performance [7–15]. Nanostructures grown directly on current collector are favored to reduce contact resistance and achieve optimal charge transport[16]. FeS2 , Ni3 S2 and NiCo2 O4 have been shown to function as conversion reaction electrodes for Na-ion batteries.[4,17–20] These reactions result in nanocrystalline metal surrounded by Na2 O or Na2 S. It has been suggested by some that iron oxides do not function as Na-ion conversion electrodes.[4] Na insertion was observed in Fe3 O4 and Fe2 O3 nanoparticles ranging from 10-400 nm with increased capacity resulting from particle size reduction [21]. However, no conversion reaction was observed. Recent work demonstrates Na conversion reaction with Fe3 O4 in the particle size range from 4 to 10 nm [22]. However, much of the capacity achieved was in the range where Na intercalates into carbon, which was present in large amounts. The primary evidence for the reaction is the observation of Na2 O in electron diffraction and a single Fe ring that overlaps with Fe3 O4 . The Na2 O could possibly form upon exposure of the sample to air while loading it into the transmission electron microscope. However, the overall capacity does exceed what would be expected from the carbon alone. Hollow ␥Fe2 O3 nanoparticles with abundant cation vacancies have recently been reported to be promising electrode materials for Na-ion batteries in terms of the both capacity and cycle life [23]. The various results motivate further investigation into Na conversion reactions with Fe-based oxides.
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Fig. 1. SEM micrographs, TEM micrographs, and selected area electron diffraction (SAED) patterns of as-prepared nanowires: ␣-Fe2 O3 (a-c), ␣-Fe2 O3 -C (d-f), Fe3 O4 -C (g-i) and Fe3 O4 (j-l).
In this work, we grow ␣-Fe2 O3 and Fe3 O4 single crystal nanowires with and without carbon coating directly on current collectors. The systems are optimized for Li-ion cycling and subsequently characterized as Na-ion hosts. 2. Experimental 2.1. Preparation of ˛-Fe2 O3 single crystal nanowires without carbon coating 220 m diameter (99.99%, Goodfellow) 10 cm long iron wire was cleaned in dilute hydrochloric acid (2% in volume) and rinsed with acetone, alcohol, and deionized water. AC power (∼4 W at 60 Hz) was then applied for ∼10 mins to promote surface oxidation during Joule heating [24]. Nanowire growth proceeds by rapid cation diffusion that emerges from grain boundaries of a thicker underlying oxide.
2.3. Preparation of Fe3 O4 single crystal nanowires with carbon coating [25] 220 m diameter iron wire (99.99%, Goodfellow) was first cleaned as described above. It was the pre-oxidized at 250 ◦ C for 0.5 h on the hotplate in ambient conditions. The samples were subsequently annealed at 550 ◦ C in a crucible also containing pure Cu. Annealing was performed in Ar that flowed through toluene prior to entering the furnace. During annealing Cu deposits on the Fe substrate and catalyzes the growth of Fe3 O4 nanowires. This process does not result in a thick underlying oxide typical of the Fe2 O3 growth process. 2.4. Preparation of Fe3 O4 single crystal nanowires without carbon coating Reactive ion etching in 5% oxygen and 95% argon for 40 sec was used to remove the carbon coating from the Fe3 O4 nanowires prepared as described above.
2.2. Preparation of ˛-Fe2 O3 single crystal nanowires with carbon coating
2.5. Electrochemical testing
The resulting ␣-Fe2 O3 single crystal nanowires were heated to 500 ◦ C for 5 h in a tube furnace containing Ar that had flowed through toluene. Decomposition of the toluene produces a carbon coating on the nanowires.
The nanowires were tested in a vial cell within a dry Ar-filled glovebox (Mbraun Labstar). The iron wire substrate served as the electrode current collector. This was cycled against either a metallic lithium or sodium counter electrode in ethylene carbonate (EC)
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Fig. 2. Cyclic voltammagrams of nanowires cycled against Li metal at scan rates of 500 V/s: ␣-Fe2 O3 (a), ␣-Fe2 O3 -C (b), Fe3 O4 (c) and Fe3 O4 -C (d).
dimethyl carbonate (DMC) (1:1 by volume) 1 M LiClO4 or NaClO4 electrolyte, respectively. The electrochemical tests were carried out using a computer-controlled potentiostat/galvanostat (SP200, Biologic Co.). Cyclic voltammetry (CV) was performed at a scan rate of 500 V/s. In order to promote a more complete reaction during Na-ion cycling, the samples were also maintained at a constant potential of 0.01 V (vs. Na/Na+ ) for 10 h, after the potential sweeps. The rate performance of Fe3 O4 nanowires was tested at various current densities (0.2 mAh/cm2 -1.6 mAh/cm2 ). After testing, all of the samples were washed by propylene carbonate (PC) and acetone, and then dried in the glove box. 2.6. Charaterization The pristine and reacted nanowires were characterized by scanning electron microscopy, SEM (JEOL-6060LV), transmission electron microscopy, TEM (JEOL-2010Lab6 and JEOL-2010Cryo), and Energy-dispersive X-ray spectroscopy (EDS) in the scanning transmission electron microscopy (JEOL-2010F EF-FEG).
and k) function as catalysts during the synthesis at low oxygen partial pressure [25]. According to each of the selected area electron diffraction (SAED) patterns (Fig. 1c, f, i and l), the prepared nanowires are single crystalline. Fig. 2 shows cyclic voltammetry performed on each sample at a scan rate of 500 V/s between 0.25 and 2.5 V vs. Li/Li+ . The first cathodic peak in each sample occurs at 0.75 V. The initial cathodic peaks shift to 0.85-0.9 V in the second cycle. This value is associated with the reversible electrochemical potential for Fe2 O3 and Fe3 O4 [26,27]. During subsequent cycles some increase in cathodic polarization is observed. The carbon coating provides limited improvement in the polarization associated with Fe2 O3 , but has no observable impact on the cathodic polarization in Fe3 O4 . After the first cycle, the single crystal nanowires are converted to polycrystalline nanowires [28]. The excess cathodic polarization observed in the first cycle likely results from the conversion of a single crystal nanowire to a polycrystalline nanowire.
3. Results and discussion 3.1. Cycling Iron Oxide Nanowires Against Li Fig. 1 shows SEM and TEM micrographs of the pristine ␣Fe2 O3 , ␣-Fe2 O3 -C, Fe3 O4 -C, and Fe3 O4 single crystal nanowires. A high density of relatively long nanowires (2∼10 m for Fe2 O3 and 5∼20 m for Fe3 O4 ) resulted from the different growth processes. The Fe3 O4 -C nanowires were present in the highest density and were the longest nanowires. The ␣-Fe2 O3 , ␣-Fe2 O3 -C nanowires are needle shaped, while the Fe3 O4 nanowires are relatively uniform in diameter. Fig. 1e indicates a ∼3 nm carbon layer uniformly surrounds the coated nanowires. The copper nanoparticles terminating the carbon-coated Fe3 O4 and Fe3 O4 nanowires (Fig. 1h
Fig. 3. Rate performance, columbic efficiency, and cycle life of Fe3 O4 -C nanowire array cycle against Li at various current densities.
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Fig. 4. Cyclic voltammagrams of ␣-Fe2 O3 -C (a) and Fe3 O4 -C (b) nanowires cycled against Na metal at scan rates of 500 V/s.
All of the anodic peaks are much broader in width than the cathodic peaks, indicating sluggish oxidation during delithiation. The peak separation (Ep = Ecath - Eanod ) is smaller than values 1 and 1.6 V reported elsewhere [15,29]. This may result from two factors; high surface area nanostructures can limit diffusional path lengths and the reduced contact resistance, associated with directly growing the electrodes on the current collectors [16], optimizing charge transport.
It is well known that carbonous coatings often enhance electron transport, suppress polarization, and extend cycle life. Relative to the pristine ␣-Fe2 O3 , carbon-coated ␣-Fe2 O3 nanowires exhibit lower hysteresis and improved stability during multiple cycles. Uncoated and carbon-coated Fe3 O4 perform similarly to one another and the ␣-Fe2 O3 -C. The larger current density in the Fe3 O4 results from the larger nanowire density. The intrinsic electronic conductivity of Fe3 O4 ( ∼ 10−2 S/m) significantly exceeds that of
Fig. 5. Bright field (BF) images, dark field (DF) images, and SAED patterns of nanowires after the first half of cyclic voltammetry: Li-reacted ␣-Fe2 O3 -C (a-c) and Fe3 O4 -C (d-f), and Na-reacted ␣-Fe2 O3 -C (g-i) and Fe3 O4 -C (j-l).
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Fig. 6. BF image of pristine Fe3 O4 -C nanowire (a), BF image (b), SAED pattern (c) and HRTEM image (d) of the Na-reacted Fe3 O4 -C nanowire discharged at 0.01 V (vs. Na/Na+ ) for 10 h.
␣-Fe2 O3 ( ∼ 10−8 S/m) at room temperature [30,31]. This difference accounts for the difference in polarization between the two materials. It also accounts for the lack of improvement of Fe3 O4 by carbon coating. Overall the CV results indicate that Fe3 O4 -C nanowires exhibit the best performance in terms of hysteresis and cycling stability. The capacity retention, rate capability, and columbic efficiency of
these electrodes under galvanostatic cycling against Li was characterized as shown in Fig. 3. The columbic efficiency increases from 60% to 96% in the first 5 cycles. The possible sources of irreversible capacity include: solid electrolyte interface (SEI) formation on the large surface area of nanowire array, which may also be observed in Fig. 5d, lithium insertion into irreversible sites, or side reactions with absorbed and impurity species.[16] Taberna
Fig. 7. BF image (a) and EDS maps (b-d) of Na-reacted Fe3 O4 -C nanowire discharged at 0.01 V (vs. Na/Na+ ) for 10 h.
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et al. electrochemically deposited Fe3 O4 nano-particles on copper nanowire arrays, achieving a large areal capacity of 0.35 mAh/cm2 [5]. Our direct nanowire growth improves the areal energy density to 1.1 mAh/cm2 , with similar electrochemical performance. We note that at the lowest rate (∼C/5) that these nanowires approximately achieve the theoretical capacity. 3.2. Cycling Iron Oxide Nanowires Against Na The conversion electrode systems optimized for lithium ion chemistry described above is extended to sodium ion systems. Fig. 4 shows CV for ␣-Fe2 O3 -C and Fe3 O4 -C nanowires electrodes cycled against sodium in the range of 0.2-2.2 V (vs. Na/Na+ ), which was utilized in order to keep absolute potentials consistent with the Li experiments. The cathodic current density for Na insertion during the first cycle is one order of magnitude smaller than for Li insertion. No appreciable anodic current is measured, and the cathodic current degrades upon subsequent cycling. Limited Na is inserted into the nanowires and almost none is extracted anodically. The decay in the cathodic peak suggests that the electrode progressively saturates with Na during the CV cycles. Testing the Fe substrate alone resulted in a current response an order of magnitude lower than when the nanowires are present. This is consistent with the fact that there is negligible solubility and no intermediate phases in the Na-Fe system. We also tested the samples in propylene carbonate 1 M NaClO4 to compare with ref. [23], and observed a similar response to that in Fig. 4. All the nanowires were investigated by transmission electron microscopy (TEM) after the first half cycle of potential sweep as shown in Fig. 5. The morphology, phase, and structure are characterized by bright field (BF) imaging, dark field (DF) imaging and selected area electron diffraction (SAED), respectively. Prior investigation of the mechanism for the conversion reaction in ␣-Fe2 O3 nanowires [28] indicates that reduction of Fe2 O3 is completed and convert to BCC Fe occurs by 0.6 V. The results in Fig. 5 indicate that ␣-Fe2 O3 and Fe3 O4 completely reduce to BCC Fe. The size of the Fe nanocrystals ranges from 5 to 25 nm. No appreciable SEI is observed on the nanowires cycled against Na. These nanowires remain single crystalline, but display a significant amount of BF diffraction contrast relative to the initial nanowires. In attempt to complete Na insertion into the Fe3 O4 -C nanowires, they were cycled to 0.01 V vs. Na/Na+ and held for 10 h. The contrast of BF image demonstrates the occurrence of a large concentration of dislocations within the nanowire (Fig. 6b), which are absent in the pristine Fe3 O4 -C (Fig. 6a). The SAED pattern (Fig. 6c) demonstrates that the Fe3 O4 remains single crystalline. No reduced Fe appears in the SAED or high-resolution images (Fig. 6d). Additional evidence is that the 0.30 nm d-space of [220] measured from HRTEM images, matches the standard value of d-space for Fe3 O4 (XRD-PDF 01-0786086). Fig. 7 shows EDS maps of Na, Fe, and O in the Fe3 O4 nanowires, discharged to 0.01 V vs. Na/Na+ for 10 h. The results suggest that a small amount of Na intercalates into the nanowire. The uniform intensity should not result from SEI, which would cause the Na to be concentrated at the surface. Additionally, SEI is not observed at the surface by high-resolution imaging (Fig. 6d). Overall, the results indicate that Na diffuses into the nanowires and induces significant dislocation production, but does not initiate the conversion reaction. This Na is subsequently trapped in the nanowire and cannot diffuse out during the cathodic sweep. This differs significantly from reactions in the Li system, where the anodic reaction occurs near the theoretical potential and the cathodic reaction is chemically reversible with some degree of hysteresis. The significant discrepancies likely arise from the differences in ionic radii between Li (76 pm) and Na (102 pm) [1]. A larger hysteresis was observed for Li insertion during the first cycle, which is attributed
to the conversion of the single crystalline nanowires to polycrystalline nanowires that have grain boundary pathways for enhanced transport and heterogeneous nucleation. It should be noted that we also first cycled nanowire samples against Li to induce the conversion reaction that results in polycrystalline nanowires, and then attempted to cycle these nanowires versus Na. However, the approach did not appreciably affect the Na insertion reaction. Damage accumulation in Fe2 O3 and Fe3 O4 nanowires due to strain induced by Na insertion is consistent with results for Li insertion in SnO2 where the large strain induces dislocation production and amorphization [32]. The effect is attributed to the large in-plane misfit associated with Li insertion. Na insertion into Fe2 O3 or Fe3 O4 likely introduces large shear stresses that both induce damage and ultimately suppress the conversion reaction by not allowing the Na to reach a critical concentration necessary to nucleate Na2 O and reduced Fe. Hariharan et al. recently investigated Na insertion into nanocrystalline Fe3 O4 , in the size range of 4-10 nm. Anodic currents were attributed to Na insertion following the conversion reaction [22]: Fe3 O4 + 8e + 8Na+ ↔ 3Fe0 + 4Na2 O
(2)
Their results suggest that all Fe2+/3+ can be reduced to Fe0 during the discharge. SAED in this study indicates the formation of Na2 O with only a weak ring associated with Fe, which overlaps with Fe3 O4 . The capacity achieved is half of that associated with Li, indicating that the Fe2+/3+ should only be partially reduced. Additionally, some of the capacity is likely associated with intercalation into carbon. However, carbon cannot account for all of the observed capacity indicating that the conversion reaction must proceed to some degree. This fine (4-10 nm) powder must be small enough that the volumetric strain associated with the initial Na intercalation does not hinder the accumulation of sufficient Na to overcome the barrier associated with nucleating Fe and Na2 O. The nanowires in this study are not sufficiently small to allow the reaction to proceed to this step. Extraction of Li from Fe2 O3 and Fe3 O4 has a larger associated polarization than insertion. The same problem may also plague the Na conversion reaction, even in the case of fine nanoparticles. Such irreversibility may account for the relatively low columbic efficiency and rapid capacity fade associated with the Na conversion reaction in Fe3 O4 [22].
4. Conclusions The electrochemical response of ␣-Fe2 O3 , ␣-Fe2 O3 -C, Fe3 O4 , and Fe3 O4 -C single crystal nanowire conversion anodes was characterized during cycling against Li and Na. ␣-Fe2 O3 -C and Fe3 O4 -C nanowires exhibited the best performance in terms of capacity, reversibility, and cyclability versus Li. While Na could be partially inserted into the nanowires, the conversion reaction could not be induced and the reaction was not chemically reversible. The accumulation of damage during insertion suggests that the reaction is strain limited. The work supports earlier results that indicate Na insertion is possible and that at smaller particle sizes it may be possible to induce the conversion reaction.
Acknowledgements The authors are grateful for funding provided by the U.S. Department of Energy, Basic Energy Sciences (Contract No. DESC0006509). The research was carried out in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois.
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