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Anodes
High-Performance Sb/Sb2O3 Anode Materials Using a Polypyrrole Nanowire Network for Na-Ion Batteries Do-Hwan Nam,* Kyung-Sik Hong, Sung-Jin Lim, Min-Joong Kim, and Hyuk-Sang Kwon*
Three-dimensional
porous Sb/Sb2O3 anode materials are successfully fabricated using a simple electrodeposition method with a polypyrrole nanowire network. The Sb/Sb2O3–PPy electrode exhibits excellent cycle performance and outstanding rate capabilities; the charge capacity is sustained at 512.01 mAh g−1 over 100 cycles, and 56.7% of the charge capacity at a current density of 66 mA g−1 is retained at 3300 mA g−1. The improved electrochemical performance of the Sb/Sb2O3–PPy electrode is attributed not only to the use of a highly porous polypyrrole nanowire network as a substrate but also to the buffer effects of the Sb2O3 matrix on the volume expansion of Sb. Ex situ scanning electron microscopy observation confirms that the Sb/Sb2O3–PPy electrode sustains a strong bond between the nanodeposits and polypyrrole nanowires even after 100 cycles, which maintains good electrical contact of Sb/Sb2O3 with the current collector without loss of the active materials.
1. Introduction Recently, rechargeable Na-ion batteries have received increasing attention as a near-term alternative to Li-ion batteries for large-scale systems, such as grid storage devices, because of their low cost and relatively low redox potential (0.3 V higher than that of Li/Li+) and the high natural abundance of Na.[1–3] Accordingly, extensive research has been conducted to develop suitable electrode materials with a high specific capacity, low irreversible loss, high Coulombic efficiency, and long cycle life.[4–9] Despite its high specific capacity, pure Na metal is not an appropriate anode material for Na-ion batteries because the dendritic deposition of Na during charging poses a safety risk in addition to its reduced capacity and increased electrode impedance. Among the anode material candidates, Sb is one of the most promising due to its high theoretical capacity and relatively low reaction potential.[10,11] When assuming complete sodiation of Sb into
Dr. D.-H. Nam, K.-S. Hong, S.-J. Lim, Dr. M.-J. Kim, Prof. H.-S. Kwon Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology 291 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea E-mail: [email protected]; [email protected] DOI: 10.1002/smll.201500491 small 2015, DOI: 10.1002/smll.201500491
Na3Sb, the theoretical capacity of Sb is ≈660 mAh g−1. Nevertheless, pure Sb electrodes exhibit poor cyclability because of the structural and electrical failure of Sb that originates from the significant volume change (up to ≈390%) during sodiation/desodiation. Indeed, pure Sb powders reported a high initial capacity of 624 mAh g−1; however, their capacity rapidly decreased to less than 100 mAh g−1 after only 25 cycles.[12] Therefore, improvement of the cycle stability still faces major challenges. To overcome these issues, several strategies have been suggested. One approach introduces active or inactive phases to act as a buffer matrix against the volume expansion of Sb, such as Sb/C,[12] SnSb,[11,13,14] FeSb2,[15] Sb/Cu2Sb,[16] Cu2Sb,[17,18] SiC/Sb/C,[19] and Sn–Ge–Sb.[20] Xiao et al. suggested that multiple phases in the Sb-based composites can enhance the cycle stability and maintain electrical contact by self-supporting each other.[13] According to Wu et al.,[17] when Sb nanoparticles are uniformly dispersed in a SiC substrate and encapsulated by carbon, the cycle performance can be significantly improved. Thus, the synthesis of Sb-based composites has been demonstrated as an effective method for enhancing the cycle stability of electrodes with relatively high capacities. Another approach uses porous substrates that offer not only void space to accommodate the volumetric expansion of the Sb-based active materials but also enhanced bonding
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Scheme 1. Simplified description for fabricating the Sb/Sb2O3–PPy electrode by electrodeposition of Sb/Sb2O3 from potassium antimony tartrate bath with the use of the polypyrrole nanowire network as a substrate and its reaction mechanism during sodiation/desodiation.
between the active materials and the current collector.[21,22] When porous Cu foam was used as the substrate for an Sbbased anode in Na-ion batteries, the Cu foam accommodated the volumetric expansion of the active materials and effectively inhibited the delamination of the active materials, which considerably improved the cycle stability.[16] Herein, we demonstrated for the first time the unique and promising approach, an electrochemical synthesis of a three-dimensional porous Sb-based electrode with the use of a polypyrrole nanowire network. In a recent communication, we proposed a novel electrochemical method for synthesizing polypyrrole nanowires via a one-step process from an aqueous solution (Scheme 1).[23] The synthesized nanowires were directly deposited on the substrate as a thin film consisting of fine polypyrrole nanowires with a nanoporous and interconnected network structure. This open architecture is expected to be highly desirable in energy storage devices because of its excellent mass transfer and high specific surface area.[24–27] In particular, since such a polymerbased structure can better accommodate sodiation-induced stress and effectively suppress the mechanical degradation of alloying materials (e.g., pulverization or fracturing), the polypyrrole nanowire network may provide an effective solution to the problems of severe capacity fading of the electrode. In this study, an Sb/Sb2O3 electrode was electrochemically fabricated on the polypyrrole nanowire network, and its electrochemical performance and sodiation/ desodiation mechanism for the Na-ion battery anodes were investigated.
2. Results and Discussion According to Bryngelsson et al.[28] the electrodeposition mechanism of Sb from a potassium antimony tartrate bath is highly dependent on the presence of HCl in the solution. Specifically, the composition of the fabricated electrodeposits differs based on the pH value of the electrolyte as follows: the production of pure Sb is favored at pH 1.3 (in the presence of
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0.11 m HCl) and the production of mixtures of Sb and Sb2O3 are favored at pH 4–5 (in the absence of HCl). This unique deposition behavior is mainly attributed to the protonation of tartrate, which occurs with the reduction of Sb3+. In the presence of tartaric acid (basic formula: C4H6O6, structural formula: HOOC–CH(OH)–CH(OH)–COOH), Sb3+ exists in the dimer form of Sb2(C4H2O6)22− rather than in the hydrated form in the electrolyte. During Sb3+ reduction, the tartrate is liberated from the complex and is spontaneously restored to its protonated form (2(C4H2O6)4− + 4H+ → 2(C4H4O6)2−) because the protonated form is thermodynamically stable at pH values above 4.37.[29] The protonation of tartrate causes a rapid increases in the pH near the electrode. Consequently, Sb2O3 is generated on the electrode during the deposition of Sb according to reaction (2). Accordingly, under specific conditions, the mixture of Sb and Sb2O3 can be fabricated from a one-step of electrodeposition via reactions (1) and (2). In particular, this synthetic approach is advantageous because the composition ratio of Sb and Sb2O3 in the electrodeposits can be controlled by adjusting the current density, solution buffer capacity, electrolyte pH, and antimony tartrate concentration (Scheme 1). Sb 2 (C 4 H 2O 6 ) 22 − + 4H + + 6e − → 2Sb + 2(C 4 H 4O 6 ) 2 −
(1)
Sb 2 (C 4 H 2O 6 ) 22 − + 2OH − + H 2O → Sb 2O 3 + 2(C 4 H 4O 6 ) 2 − (2)
Figure 1a,b shows the morphologies of the polypyrrole nanowire network synthesized by cathodic electropolymerization at −0.6 VSCE for 15 min from an aqueous solution. As illustrated in Figure 1b, the network consists of fine nanowires with a high degree of interlocking and has a nanoporous and interconnected open structure. The nanowires are highly uniform throughout the substrate, with an apparent thickness of ≈100 nm and an average length of ≈2 µm. In this study, we used glycerol as a surface modifier
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54.54°) were well matched with the (012), (104), and (110) planes of a crystalline Sb phase (a rhombohedral structure of R-3m with lattice constants of a = 4.037 Å and c = 11.273 Å (PDF No. 01-071-1173)) and the (222), (400), (440), and (622) planes of a crystalline Sb2O3 phase (a cubic structure of Fd-3m with lattice constants of a = 11.1519 Å (PDF No. 01-071-0365)), respectively. One notable feature of the XRD patterns is that the peak intensities of the crystalline Sb2O3 phase increased compared with those previously reported for Sb/Sb2O3 nanoparticles. The XRD patterns of the Sb/Sb2O3 composites in the study of Bryngelsson et al.[29] only contain Sb peaks, with no crystalline Sb2O3 peaks. The authors demonstrated that the finding occurs because the small Sb2O3 nanoparticles that precipitate on the electrode are entirely covered and embedded in a matrix of crystalline Sb. Thus, the enhanced crystallinity of Sb2O3 in Figure 2a suggests that the crystal structures of the Sb/Sb2O3 nanodeposits in this study differ from the previous structure[28] despite the similar deposition conditions. To investigate the microstructure of the nanodeposit, further structural analyses were conducted using HRTEM. As depicted in Figure 3a, short-range ordered crystalline lattices within randomly oriFigure 1. SEM images of (a, b) the polypyrrole nanowire network and (c–e) the Sb/Sb2O3 nanodeposits electrodeposited on the polypyrrole nanowires. f) HRTEM image of the Sb/ ented domains were clearly observed in Sb2O3 nanodeposits on the polypyrrole nanowire (inset: scanning transmission electron the Sb/Sb2O3 nanodeposits. The distances microscopy (STEM) image of Sb/Sb2O3–PPy composites). between the two adjacent lattice planes were measured to be 0.215, 0.311, 0.354, for the polypyrrole nanowires to increase the amount of and 0.168 nm, which can be assigned to the {110}, {012}, {101} electrodeposits and to enhance the bond between the planes of rhombohedral Sb and the {622} planes of cubic polypyrrole nanowires and electrodeposits. After the electro- Sb2O3, respectively. This result confirms that the nanodeposits deposition of Sb/Sb2O3, small nanodeposits of 100–500 nm in are polycrystalline and consist of small Sb and Sb2O3 crystaldiameter were evenly generated inside and on the surface of lites, which is consistent with the XRD analysis. the polypyrrole nanowire network (Figure 1c). In addition, it Another notable observation from the HRTEM analysis is noteworthy that a considerable number of interconnected is that a mixture of Sb crystallites and Sb2O3 crystallites was pores was sustained in the nanowire network after the elec- embedded in an Sb2O3 matrix. Although the presence of trodeposition of Sb/Sb2O3 (Figure 1d). The high-magnifica- the Sb2O3 matrix is difficult to verify in Figure 3a, Figure 3b tion scanning electron microscopy (SEM) (Figure 1e) and clearly shows a long-range ordered crystal lattice at the interhigh-resolution transmission electron microscopy (HRTEM) face between the nanodeposits and polypyrrole nanowires. images (Figure 1f) demonstrate that the nanodeposits were The observed fringe spacing of the nanofibers was 0.256 nm, tightly attached to the polypyrrole nanowires. which was assigned to a d-spacing of 0.256 nm for the {331} From the energy dispersive X-ray spectroscopy (EDS) planes of Sb2O3. In addition, the lattice fringes of the {622} analysis shown in Figure 2a, Sb was successfully electrodepos- planes of Sb2O3 could be observed along the boundary ited on the polypyrrole nanowires. However, a large amount between the nanodeposits and polypyrrole nanowires in of O was observed in the EDS analysis for the electrodeposits, Figure 3a. The growth mechanisms of the Sb/Sb2O3 nanoindicating that the electrodeposits may not be solely com- deposits on the polypyrrole nanowires are not fully underposed of a pure Sb phase. As expected, the X-ray diffraction stood, but one possible explanation for this finding is that (XRD) pattern for the electrodeposits verifies the formation the oxygen within the structure of overoxidized polypyrof a mixture of an Sb and an Sb2O3 phase. The seven major role may act as nucleation sites that facilitate the deposipeaks in the diffraction pattern for the electrodeposits (i.e., tion of Sb2O3 on the polypyrrole nanowires. According to the peaks at 28.68°, 40.07°, 41.92°, 27.69°, 32.08°, 46.00°, and previous studies,[27,30] the polypyrrole nanowires synthesized small 2015, DOI: 10.1002/smll.201500491
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are not included in this paper). Therefore, it was assumed that the deposition and growth rate of Sb2O3 would be faster than Sb, which results in the incorporation of the Sb crystallites into the Sb2O3 matrix. The SEM, XRD, and HRTEM results clearly demonstrate that an Sb/Sb2O3 electrode with a high porosity and open structure was successfully fabricated using a simple electrodeposition method with the polypyrrole nanowire network as a substrate. Therefore, in this study, the electrode is called an Sb/Sb2O3–PPy electrode. The cycle performance and Coulombic efficiency of the Sb/Sb2O3–PPy electrode were examined at 66 mA g−1 over a voltage range of 0.001 to 1.5 V (vs Na/Na+). Based on the half-cell reaction, the insertion of Na+ into the Sb/Sb2O3–PPy electrode is called discharge and the extraction of Na+ from the electrode is called charge. As illustrated in Figure 4a, the Sb/ Sb2O3–PPy electrode exhibits an improved cycle performance with a relatively high Coulombic efficiency over other Sb-based composites.[13,16,18] The charge capacity was maintained at 512.01 mAh g−1 after 100 cycles, corresponding to 98.34% retention of the charge capacity relative to the maximum charge capacity of 520.64 mAh g−1 at the 19th cycle. The initial discharge and charge capacity were 611.03 and 466.81 mAh g−1, respectively, which corresponded to a Coulombic efficiency of 76.40%. This low Coulombic efficiency Figure 2. a) EDS spectrum and b) XRD pattern of the Sb/Sb2O3 electrodeposits on the during the 1st cycle was mainly attributed polypyrrole nanowire network. to the irreversible Coulombic losses that resulted from electrolyte decomposition by cathodic electropolymerization are partially overoxidized; on the surface of the polypyrrole nanowires and the Sb/Sb2O3 thus, a large amount of oxygen atoms exist on the surfaces of nanodeposits. In this experiment, fluoroethylene carbonate the polypyrrole nanowires. In this structure, the metal oxide (FEC) was introduced as an electrolyte additive to inhibit may be more easily deposited on the polypyrrole nanowires electrolyte decomposition by modifying the surface passivarelative to the pure metal. Indeed, although the electrodepo- tion layer.[31] However, the extensive decomposition of the sition of pure Sb and other metallic phases on the polypyr- electrolyte during the first several cycles was not effectively role nanowire network was attempted, it was difficult to suppressed due to the large surface area of the polypyrrole grow the electrodeposits directly on polypyrrole (these data nanowire network. In the following 20 cycles, the discharge capacity was reduced from 611.03 to 537.01 mAh g−1, whereas the charge capacity increased from 466.81 to 520.89 mAh g−1, and the corresponding Coulombic efficiency steadily increased from 76.40% to 97.00%. As presented in Figure 4b, the voltage profile of the Sb/Sb2O3–PPy electrode mainly consisted of two conjugated plateaus at ≈0.7 and 0.45 V during discharging and 0.8 and 0.9 V during charging. While the discharge plateau at 0.45 V became shorter over 50 cycles, the capacity of the plateau at 0.7 V was not reduced, despite the decreasing discharge capacity. This behavior indicates that the decrease in the discharge capacity with cycles Figure 3. HRTEM images of the Sb/Sb2O3 nanodeposits on the did not result from the loss of the active phase, but mainly resulted from the electrolyte decomposition below 0.4 V. In polypyrrole nanowire.
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Figure 4. a) Cycle performance and Coulombic efficiency and b) charge/discharge curves for Sb/Sb2O3–PPy at the current density of 66 mA g−1 and at a voltage range between 0.001 and 1.5 V (vs Na/Na+). c) Specific capacities and d) discharge/charge curves for Sb/Sb2O3–PPy over changing current densities from 66 to 3300 mA g−1 and back to 66 mA g−1 C at a voltage range between 0.001 and 1.5 V (vs Na/Na+).
addition, the charge profile of the Sb/Sb2O3–PPy electrode exhibited the same behavior between the 10th and 50th cycles, indicating that highly stable sodiation/desodiation reactions occur repeatedly for the Sb/Sb2O3–PPy electrode without changes in the reaction pathway. The Sb/Sb2O3–PPy electrode exhibits not only excellent cycle stability but also outstanding rate capabilities. As displayed in Figure 4c, the Sb/Sb2O3–PPy electrode showed a charge capacity of 527.84 mAh g−1 at a current density of 66 mA g−1 and retained a high reversible capacity of 299.46 mAh g−1 at 3300 mA g−1, which represents a 43.27% reduction. Evidently, the rate capability of Sb/Sb2O3–PPy is superior to the previously reported Sb-based composites,[12,13,16,18] demonstrating that the use of a highly porous polypyrrole nanowire network with a large surface area provides good electronic conductivity and facilitates the efficient transfer of Na+ from the solvent to the electrode. Although the discharge capacity rapidly decreased from 1320 to 3300 mA g−1, the high charge capacity of 525.18 mAh g−1 was restored when the current density returned to 66 mA g−1 from 3300 mA g−1 at the 41st cycle. The recovery of the capacity demonstrates that the structure of the Sb/Sb2O3–ppy is sustained and implies that obvious reductions in the capacity at high current densities may be attributed to decreases in the amount of inserted Na+ into Sb/Sb2O3. Figure 4d shows the increasing overpotentials of sodiation/desodiation as the current density increases from 66 to 3300 mA g−1. Because the Coulombic efficiencies were maintained above ≈99% at high current densities, the imperfect sodiation of Sb/Sb2O3 resulting from increasing overpotentials during discharge is a major cause of rapid reductions in the capacity. small 2015, DOI: 10.1002/smll.201500491
To better understand the electrochemical reactions of Sb/ Sb2O3–PPy with Na+, the phase changes of Sb/Sb2O3 during cycling were investigated using ex situ XRD. Figure 5 displays the XRD patterns for Sb/Sb2O3 when discharged to A, B, C, and D or when charged to E, F, and G during the 1st cycle (as marked in the voltage curve). After the formation step (A), the crystal structure of Sb/Sb2O3 did not changed with the initial structure in Figure 2b. At the end of the first discharge plateau (B), the diffraction peaks attributed to Sb nearly vanished. However, a new phase was not obtained. This result verifies that crystalline Sb was completely converted into an amorphous Na–Sb phase during the first discharge plateau. Following the discharge plateau, new peaks appeared at 33.54°, 34.33°, and 38.61°, which correspond to the hexagonal Na3Sb phase (JCPDS No. 00-065-2166). When completely sodiated (D), the phase structure of Na3Sb was maintained and the intensities of the corresponding peaks increased. During the following charge process, the peaks for Na3Sb disappeared at 0.80 V (E), and the diffraction peaks corresponding to crystalline Sb reappeared at 0.95 V (F). These results confirm that the crystalline Sb phase undergoes two phase transitions (Sb ↔ amorphous NaxSb ↔ crystalline Na3Sb) during sodiation and desodiation and that the sodiation process (A → D) and desodiation process (D → G) occurred reversibly. Another notable feature of the ex situ XRD analysis is that Sb2O3 remained as the crystalline phase during sodiation. When crystalline Sb was sequentially converted into an amorphous Na–Sb phase and a hexagonal Na3Sb phase, the peaks corresponding to Sb2O3 did not obviously change. Because Sb2O3 is a rechargeable anode material for Na-ion
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Figure 5. XRD patterns for Sb/Sb2O3–PPy at various stages during the 1st cycle.
batteries,[32,33] the remaining peaks of Sb2O3 could indicate that the Sb2O3 in Sb/Sb2O3 is not fully converted to Sb and Na2O. In the Na-ion battery system, Sb2O3 is reduced to the metallic Sb phase and Na2O (Sb2O3 + 6Na+ + 6e− ↔ 2Sb + 3Na2O). Next, the reversible alloying and dealloying of Na+ occur reversibly with the metallic Sb phase (Sb + 6Na+ + 6e− ↔ 2Na3Sb).[32] If the Sb2O3 phase is completely converted into metallic Sb and Na2O, the peaks corresponding to crystalline Sb2O3 should vanish before reaching a plateau at 0.7 V. Thus, only a small amount of Sb2O3 was assumed to react with Na+, and most of the Sb2O3 remained inactive. This is because the Sb2O3 matrix may have low reactivity with Na+ due to the strong bond between Sb2O3 and the polypyrrole nanowires. Figure 6 shows the morphological changes of Sb/Sb2O3– PPy after cycling, which were characterized by ex situ SEM. As illustrated in Figure 6a, the Sb/Sb2O3 nanodeposits exhibit a reversible volume expansion during one cycle, with no obvious particle cracking or pulverization. Particularly, the nanodeposits were tightly attached to the polypyrrole nanowires (Figure 6b). This observation implies that the relatively low volumetric changes occurred at the contact between Sb/Sb2O3 and the polypyrrole nanowires. Moreover, the inactive Sb2O3 phase buffered the volume changes of the Sb/Sb2O3 nanodeposits. Although the Sb/Sb2O3 nanodeposits did not sustain their initial “particle shape” after 100 cycles (as displayed in Figure 6c,d), it is noteworthy that the interconnected and highly porous structures were sustained. Thus, it can be deduced from these observations that
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the electrode consistently accommodates sodiation-induced stress and effectively suppresses the mechanical degradation of Sb/Sb2O3. Therefore, the outstanding cycle stability of the Sb/Sb2O3–PPy electrode is attributed not only to the highly porous structure of the electrode but also to the strong bond between the nanodeposits and the polypyrrole nanowires that originate from the low reactivity of the Sb2O3 matrix to Na+, which allows for good electrical contact with the current collector without loss of the active materials.
3. Conclusions In this study, 3D porous Sb/Sb2O3 anode materials are successfully fabricated using a simple electrodeposition method with a polypyrrole nanowire network as a substrate. The Sb/ Sb2O3–PPy electrode exhibited excellent cycle performance and outstanding rate capabilities. The outstanding electrochemical performance of the Sb/Sb2O3–PPy electrode is attributed not only to the use of a highly porous polypyrrole nanowire network as a substrate but also to the buffer effects of the Sb2O3 matrix on the volume expansion of Sb. Specifically, the Sb/Sb2O3–PPy electrode maintained a strong bond between the nanodeposits and the polypyrrole nanowires over 100 cycles, which allows for good electrical contact with the current collector without loss of active materials. Although further research is required to improve the initial Coulombic efficiency and rate capabilities, the results demonstrated that Sb/Sb2O3–PPy is one of the most promising
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crystal structures of the electrodeposits were investigated using XRD and HRTEM. To prepare the specimens for HRTEM, the dried Sb/ Sb2O3–polypyrrole was scraped from the substrates and dispersed in ethanol via sonication for 1 h. The electrochemical properties of Sb/ Sb2O3–PPy were investigated using Swageloktype cell that was assembled in an Ar-filled glove box. The cell consisted of an Sb/Sb2O3– PPy electrode sheet (0.67 mg cm−2) and a Na metal film without a separator.[6,9] The electrolyte was anhydrous propylene carbonate with 1 M NaClO4 and 2 vol% FEC. The charge/ discharge characteristics of the electrode were examined galvanostatically at a current density of 66 mA g−1 (0.1 C rate based on the theoretical capacity of Sb) and between 0.001 and 1.50 V (vs Na/Na+) after the formation step (sodiation/desodiation process at 30 mA g−1 for the activation). The rate capabilities of the electrode were evaluated at Figure 6. SEM images of Sb/Sb2O3–PPy a,b) after one cycle and c,d) after 100 cycles. various current densities of between 66 and 3300 mA g−1. To examine the morphology and sustainable anode materials for Na-ion batteries. Moreover, phase structure changes of the electrodes with cycling, ex situ SEM the improvement of the electrochemical performance of Sb/ observation and ex situ XRD analyses were performed on each Sb2O3–PPy suggests the feasibility of using the polypyrrole electrode. For ex situ SEM characterization, the cycled electrodes nanowire network as an effective substrate for recharge- were extracted from the Swagelok-type cells and rinsed with anhyable battery electrode materials. Therefore, we expect that drous dimethyl carbonate. For the ex situ XRD analysis, the cycled the synthetic approach suggested in this study will be readily samples were sealed with Kapton tape in an Ar-filled glove box.
applied as a facile and cost-effective process for fabricating diverse metal-oxide-polymer hybrid materials for use in various electrochemistry and nanotechnology fields.
Acknowledgements
4. Experimental Section The polypyrrole nanowire network was fabricated by cathodic polymerization from an aqueous solution containing 0.2 M NaNO3, 0.8 M HNO3, and 0.25 M pyrrole.[23] The electropolymerization of pyrrole was conducted using a three-electrode cell. A sheet of nodular Cu was used as the working electrode, a stainless steel plate precoated with polypyrrole was used as the counter electrode and a saturated calomel electrode (SCE, 0.241 V vs SHE(standard hydrogen electrode)) was used as the reference electrode. The polypyrrole nanowires were electropolymerized at a constant potential of −0.6 VSCE for 15 min while the solution was agitated with a magnetic stirrer at ≈800 rpm.[27] All of the experiments were conducted at room temperature (25 ± 0.5 °C). After rinsing with distilled water, the fabricated polypyrrole nanowire network was dried in a vacuum for 12 h. Then, the Sb/Sb2O3 was electrodeposited on the polypyrrole nanowire network at a potential of −1.4 VSCE for 2 min in an electrolytic bath containing 0.02 M K2Sb2(C4H2O6)2, 0.05 M KCl and 8 mL L−1 glycerol. In addition, the electrodeposition of Sb/Sb2O3 was performed using a standard three-electrode cell with a polypyrrole network as the working electrode, a Pt mesh as the counter electrode and a SCE (0.241 V vs SHE) as the reference electrode. The surface morphology and composition of the electrodes were examined using SEM and EDS, respectively. The small 2015, DOI: 10.1002/smll.201500491
This work was supported by POONGSAN and by the Center for Inorganic Photovoltaic Materials (Grant No. 2013-001796) grant funded by the Korea Government (Ministry of Sciency, ICT and Future Planning).
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[21] D. H. Nam, R. H. Kim, D. W. Han, H. S. Kwon, Electrochim. Acta 2012, 66, 126. [22] D.-H. Nam, R.-H. Kim, C.-L. Lee, H.-S. Kwon, J. Electrochem. Soc. 2012, 159, A1822. [23] D.-H. Nam, M.-J. Kim, S.-J. Lim, I.-S. Song, H.-S. Kwon, J. Mater. Chem. A 2013, 1, 8061. [24] D. Nan, Z.-H. Huang, R. Lv, L. Yang, J.-G. Wang, W. Shen, Y. Lin, X. Yu, L. Ye, H. Sun, F. Kang, J. Mater. Chem. A 2014, 2, 19678. [25] L. Li, A. Manthiram, Adv. Energy Mater. 2014, 4, 1301795. [26] L. Qie, W.-M. Chen, Z.-H. Wang, Q.-G. Shao, X. Li, L.-X. Yuan, X.-L. Hu, W.-X. Zhang, Y. H. Huang, Adv. Mater. 2012, 24, 2047. [27] D.-H. Nam, S.-J. Lim, M.-J. Kim, H.-S. Kwon, RSC Adv. 2014, 4, 10212. [28] H. Bryngelsson, J. Eskhult, L. Nyholm, M. Herrane, O. Alm, K. Edstrom, Chem. Mater. 2007, 19, 1170. [29] D. C. Harris, Quantitative Chemical Analysis, 6th ed., W. H. Freeman and Company, New York 2003. [30] D.-H. Nam, S.-J. Lim, M.-J. Kim, H.-S. Kwon, RSC Adv. 2013, 3, 16102. [31] S. Komaba, T. Ishikawa, N. Yabuuchi, W. Murata, A. Ito, Y. Ohsawa, ACS Appl. Mater. Interfaces 2011, 3, 4165. [32] K. Li, H. Liu, G. Wang, Arab J. Sci. Eng. 2014, 39, 6589. [33] M. Hu, Y. Jiang, W. Sun, H. Wang, C. Jin, M. Yan, ACS Appl. Mater. Interfaces 2014, 6, 19449. Received: February 16, 2015 Revised: February 17, 2015 Published online:
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Anodes
High-Performance Sb/Sb2O3 Anode Materials Using a Polypyrrole Nanowire Network for Na-Ion Batteries Do-Hwan Nam,* Kyung-Sik Hong, Sung-Jin Lim, Min-Joong Kim, and Hyuk-Sang Kwon*
Three-dimensional
porous Sb/Sb2O3 anode materials are successfully fabricated using a simple electrodeposition method with a polypyrrole nanowire network. The Sb/Sb2O3–PPy electrode exhibits excellent cycle performance and outstanding rate capabilities; the charge capacity is sustained at 512.01 mAh g−1 over 100 cycles, and 56.7% of the charge capacity at a current density of 66 mA g−1 is retained at 3300 mA g−1. The improved electrochemical performance of the Sb/Sb2O3–PPy electrode is attributed not only to the use of a highly porous polypyrrole nanowire network as a substrate but also to the buffer effects of the Sb2O3 matrix on the volume expansion of Sb. Ex situ scanning electron microscopy observation confirms that the Sb/Sb2O3–PPy electrode sustains a strong bond between the nanodeposits and polypyrrole nanowires even after 100 cycles, which maintains good electrical contact of Sb/Sb2O3 with the current collector without loss of the active materials.
1. Introduction Recently, rechargeable Na-ion batteries have received increasing attention as a near-term alternative to Li-ion batteries for large-scale systems, such as grid storage devices, because of their low cost and relatively low redox potential (0.3 V higher than that of Li/Li+) and the high natural abundance of Na.[1–3] Accordingly, extensive research has been conducted to develop suitable electrode materials with a high specific capacity, low irreversible loss, high Coulombic efficiency, and long cycle life.[4–9] Despite its high specific capacity, pure Na metal is not an appropriate anode material for Na-ion batteries because the dendritic deposition of Na during charging poses a safety risk in addition to its reduced capacity and increased electrode impedance. Among the anode material candidates, Sb is one of the most promising due to its high theoretical capacity and relatively low reaction potential.[10,11] When assuming complete sodiation of Sb into
Dr. D.-H. Nam, K.-S. Hong, S.-J. Lim, Dr. M.-J. Kim, Prof. H.-S. Kwon Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology 291 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea E-mail: [email protected]; [email protected] DOI: 10.1002/smll.201500491 small 2015, DOI: 10.1002/smll.201500491
Na3Sb, the theoretical capacity of Sb is ≈660 mAh g−1. Nevertheless, pure Sb electrodes exhibit poor cyclability because of the structural and electrical failure of Sb that originates from the significant volume change (up to ≈390%) during sodiation/desodiation. Indeed, pure Sb powders reported a high initial capacity of 624 mAh g−1; however, their capacity rapidly decreased to less than 100 mAh g−1 after only 25 cycles.[12] Therefore, improvement of the cycle stability still faces major challenges. To overcome these issues, several strategies have been suggested. One approach introduces active or inactive phases to act as a buffer matrix against the volume expansion of Sb, such as Sb/C,[12] SnSb,[11,13,14] FeSb2,[15] Sb/Cu2Sb,[16] Cu2Sb,[17,18] SiC/Sb/C,[19] and Sn–Ge–Sb.[20] Xiao et al. suggested that multiple phases in the Sb-based composites can enhance the cycle stability and maintain electrical contact by self-supporting each other.[13] According to Wu et al.,[17] when Sb nanoparticles are uniformly dispersed in a SiC substrate and encapsulated by carbon, the cycle performance can be significantly improved. Thus, the synthesis of Sb-based composites has been demonstrated as an effective method for enhancing the cycle stability of electrodes with relatively high capacities. Another approach uses porous substrates that offer not only void space to accommodate the volumetric expansion of the Sb-based active materials but also enhanced bonding
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Scheme 1. Simplified description for fabricating the Sb/Sb2O3–PPy electrode by electrodeposition of Sb/Sb2O3 from potassium antimony tartrate bath with the use of the polypyrrole nanowire network as a substrate and its reaction mechanism during sodiation/desodiation.
between the active materials and the current collector.[21,22] When porous Cu foam was used as the substrate for an Sbbased anode in Na-ion batteries, the Cu foam accommodated the volumetric expansion of the active materials and effectively inhibited the delamination of the active materials, which considerably improved the cycle stability.[16] Herein, we demonstrated for the first time the unique and promising approach, an electrochemical synthesis of a three-dimensional porous Sb-based electrode with the use of a polypyrrole nanowire network. In a recent communication, we proposed a novel electrochemical method for synthesizing polypyrrole nanowires via a one-step process from an aqueous solution (Scheme 1).[23] The synthesized nanowires were directly deposited on the substrate as a thin film consisting of fine polypyrrole nanowires with a nanoporous and interconnected network structure. This open architecture is expected to be highly desirable in energy storage devices because of its excellent mass transfer and high specific surface area.[24–27] In particular, since such a polymerbased structure can better accommodate sodiation-induced stress and effectively suppress the mechanical degradation of alloying materials (e.g., pulverization or fracturing), the polypyrrole nanowire network may provide an effective solution to the problems of severe capacity fading of the electrode. In this study, an Sb/Sb2O3 electrode was electrochemically fabricated on the polypyrrole nanowire network, and its electrochemical performance and sodiation/ desodiation mechanism for the Na-ion battery anodes were investigated.
2. Results and Discussion According to Bryngelsson et al.[28] the electrodeposition mechanism of Sb from a potassium antimony tartrate bath is highly dependent on the presence of HCl in the solution. Specifically, the composition of the fabricated electrodeposits differs based on the pH value of the electrolyte as follows: the production of pure Sb is favored at pH 1.3 (in the presence of
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0.11 m HCl) and the production of mixtures of Sb and Sb2O3 are favored at pH 4–5 (in the absence of HCl). This unique deposition behavior is mainly attributed to the protonation of tartrate, which occurs with the reduction of Sb3+. In the presence of tartaric acid (basic formula: C4H6O6, structural formula: HOOC–CH(OH)–CH(OH)–COOH), Sb3+ exists in the dimer form of Sb2(C4H2O6)22− rather than in the hydrated form in the electrolyte. During Sb3+ reduction, the tartrate is liberated from the complex and is spontaneously restored to its protonated form (2(C4H2O6)4− + 4H+ → 2(C4H4O6)2−) because the protonated form is thermodynamically stable at pH values above 4.37.[29] The protonation of tartrate causes a rapid increases in the pH near the electrode. Consequently, Sb2O3 is generated on the electrode during the deposition of Sb according to reaction (2). Accordingly, under specific conditions, the mixture of Sb and Sb2O3 can be fabricated from a one-step of electrodeposition via reactions (1) and (2). In particular, this synthetic approach is advantageous because the composition ratio of Sb and Sb2O3 in the electrodeposits can be controlled by adjusting the current density, solution buffer capacity, electrolyte pH, and antimony tartrate concentration (Scheme 1). Sb 2 (C 4 H 2O 6 ) 22 − + 4H + + 6e − → 2Sb + 2(C 4 H 4O 6 ) 2 −
(1)
Sb 2 (C 4 H 2O 6 ) 22 − + 2OH − + H 2O → Sb 2O 3 + 2(C 4 H 4O 6 ) 2 − (2)
Figure 1a,b shows the morphologies of the polypyrrole nanowire network synthesized by cathodic electropolymerization at −0.6 VSCE for 15 min from an aqueous solution. As illustrated in Figure 1b, the network consists of fine nanowires with a high degree of interlocking and has a nanoporous and interconnected open structure. The nanowires are highly uniform throughout the substrate, with an apparent thickness of ≈100 nm and an average length of ≈2 µm. In this study, we used glycerol as a surface modifier
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54.54°) were well matched with the (012), (104), and (110) planes of a crystalline Sb phase (a rhombohedral structure of R-3m with lattice constants of a = 4.037 Å and c = 11.273 Å (PDF No. 01-071-1173)) and the (222), (400), (440), and (622) planes of a crystalline Sb2O3 phase (a cubic structure of Fd-3m with lattice constants of a = 11.1519 Å (PDF No. 01-071-0365)), respectively. One notable feature of the XRD patterns is that the peak intensities of the crystalline Sb2O3 phase increased compared with those previously reported for Sb/Sb2O3 nanoparticles. The XRD patterns of the Sb/Sb2O3 composites in the study of Bryngelsson et al.[29] only contain Sb peaks, with no crystalline Sb2O3 peaks. The authors demonstrated that the finding occurs because the small Sb2O3 nanoparticles that precipitate on the electrode are entirely covered and embedded in a matrix of crystalline Sb. Thus, the enhanced crystallinity of Sb2O3 in Figure 2a suggests that the crystal structures of the Sb/Sb2O3 nanodeposits in this study differ from the previous structure[28] despite the similar deposition conditions. To investigate the microstructure of the nanodeposit, further structural analyses were conducted using HRTEM. As depicted in Figure 3a, short-range ordered crystalline lattices within randomly oriFigure 1. SEM images of (a, b) the polypyrrole nanowire network and (c–e) the Sb/Sb2O3 nanodeposits electrodeposited on the polypyrrole nanowires. f) HRTEM image of the Sb/ ented domains were clearly observed in Sb2O3 nanodeposits on the polypyrrole nanowire (inset: scanning transmission electron the Sb/Sb2O3 nanodeposits. The distances microscopy (STEM) image of Sb/Sb2O3–PPy composites). between the two adjacent lattice planes were measured to be 0.215, 0.311, 0.354, for the polypyrrole nanowires to increase the amount of and 0.168 nm, which can be assigned to the {110}, {012}, {101} electrodeposits and to enhance the bond between the planes of rhombohedral Sb and the {622} planes of cubic polypyrrole nanowires and electrodeposits. After the electro- Sb2O3, respectively. This result confirms that the nanodeposits deposition of Sb/Sb2O3, small nanodeposits of 100–500 nm in are polycrystalline and consist of small Sb and Sb2O3 crystaldiameter were evenly generated inside and on the surface of lites, which is consistent with the XRD analysis. the polypyrrole nanowire network (Figure 1c). In addition, it Another notable observation from the HRTEM analysis is noteworthy that a considerable number of interconnected is that a mixture of Sb crystallites and Sb2O3 crystallites was pores was sustained in the nanowire network after the elec- embedded in an Sb2O3 matrix. Although the presence of trodeposition of Sb/Sb2O3 (Figure 1d). The high-magnifica- the Sb2O3 matrix is difficult to verify in Figure 3a, Figure 3b tion scanning electron microscopy (SEM) (Figure 1e) and clearly shows a long-range ordered crystal lattice at the interhigh-resolution transmission electron microscopy (HRTEM) face between the nanodeposits and polypyrrole nanowires. images (Figure 1f) demonstrate that the nanodeposits were The observed fringe spacing of the nanofibers was 0.256 nm, tightly attached to the polypyrrole nanowires. which was assigned to a d-spacing of 0.256 nm for the {331} From the energy dispersive X-ray spectroscopy (EDS) planes of Sb2O3. In addition, the lattice fringes of the {622} analysis shown in Figure 2a, Sb was successfully electrodepos- planes of Sb2O3 could be observed along the boundary ited on the polypyrrole nanowires. However, a large amount between the nanodeposits and polypyrrole nanowires in of O was observed in the EDS analysis for the electrodeposits, Figure 3a. The growth mechanisms of the Sb/Sb2O3 nanoindicating that the electrodeposits may not be solely com- deposits on the polypyrrole nanowires are not fully underposed of a pure Sb phase. As expected, the X-ray diffraction stood, but one possible explanation for this finding is that (XRD) pattern for the electrodeposits verifies the formation the oxygen within the structure of overoxidized polypyrof a mixture of an Sb and an Sb2O3 phase. The seven major role may act as nucleation sites that facilitate the deposipeaks in the diffraction pattern for the electrodeposits (i.e., tion of Sb2O3 on the polypyrrole nanowires. According to the peaks at 28.68°, 40.07°, 41.92°, 27.69°, 32.08°, 46.00°, and previous studies,[27,30] the polypyrrole nanowires synthesized small 2015, DOI: 10.1002/smll.201500491
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are not included in this paper). Therefore, it was assumed that the deposition and growth rate of Sb2O3 would be faster than Sb, which results in the incorporation of the Sb crystallites into the Sb2O3 matrix. The SEM, XRD, and HRTEM results clearly demonstrate that an Sb/Sb2O3 electrode with a high porosity and open structure was successfully fabricated using a simple electrodeposition method with the polypyrrole nanowire network as a substrate. Therefore, in this study, the electrode is called an Sb/Sb2O3–PPy electrode. The cycle performance and Coulombic efficiency of the Sb/Sb2O3–PPy electrode were examined at 66 mA g−1 over a voltage range of 0.001 to 1.5 V (vs Na/Na+). Based on the half-cell reaction, the insertion of Na+ into the Sb/Sb2O3–PPy electrode is called discharge and the extraction of Na+ from the electrode is called charge. As illustrated in Figure 4a, the Sb/ Sb2O3–PPy electrode exhibits an improved cycle performance with a relatively high Coulombic efficiency over other Sb-based composites.[13,16,18] The charge capacity was maintained at 512.01 mAh g−1 after 100 cycles, corresponding to 98.34% retention of the charge capacity relative to the maximum charge capacity of 520.64 mAh g−1 at the 19th cycle. The initial discharge and charge capacity were 611.03 and 466.81 mAh g−1, respectively, which corresponded to a Coulombic efficiency of 76.40%. This low Coulombic efficiency Figure 2. a) EDS spectrum and b) XRD pattern of the Sb/Sb2O3 electrodeposits on the during the 1st cycle was mainly attributed polypyrrole nanowire network. to the irreversible Coulombic losses that resulted from electrolyte decomposition by cathodic electropolymerization are partially overoxidized; on the surface of the polypyrrole nanowires and the Sb/Sb2O3 thus, a large amount of oxygen atoms exist on the surfaces of nanodeposits. In this experiment, fluoroethylene carbonate the polypyrrole nanowires. In this structure, the metal oxide (FEC) was introduced as an electrolyte additive to inhibit may be more easily deposited on the polypyrrole nanowires electrolyte decomposition by modifying the surface passivarelative to the pure metal. Indeed, although the electrodepo- tion layer.[31] However, the extensive decomposition of the sition of pure Sb and other metallic phases on the polypyr- electrolyte during the first several cycles was not effectively role nanowire network was attempted, it was difficult to suppressed due to the large surface area of the polypyrrole grow the electrodeposits directly on polypyrrole (these data nanowire network. In the following 20 cycles, the discharge capacity was reduced from 611.03 to 537.01 mAh g−1, whereas the charge capacity increased from 466.81 to 520.89 mAh g−1, and the corresponding Coulombic efficiency steadily increased from 76.40% to 97.00%. As presented in Figure 4b, the voltage profile of the Sb/Sb2O3–PPy electrode mainly consisted of two conjugated plateaus at ≈0.7 and 0.45 V during discharging and 0.8 and 0.9 V during charging. While the discharge plateau at 0.45 V became shorter over 50 cycles, the capacity of the plateau at 0.7 V was not reduced, despite the decreasing discharge capacity. This behavior indicates that the decrease in the discharge capacity with cycles Figure 3. HRTEM images of the Sb/Sb2O3 nanodeposits on the did not result from the loss of the active phase, but mainly resulted from the electrolyte decomposition below 0.4 V. In polypyrrole nanowire.
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Figure 4. a) Cycle performance and Coulombic efficiency and b) charge/discharge curves for Sb/Sb2O3–PPy at the current density of 66 mA g−1 and at a voltage range between 0.001 and 1.5 V (vs Na/Na+). c) Specific capacities and d) discharge/charge curves for Sb/Sb2O3–PPy over changing current densities from 66 to 3300 mA g−1 and back to 66 mA g−1 C at a voltage range between 0.001 and 1.5 V (vs Na/Na+).
addition, the charge profile of the Sb/Sb2O3–PPy electrode exhibited the same behavior between the 10th and 50th cycles, indicating that highly stable sodiation/desodiation reactions occur repeatedly for the Sb/Sb2O3–PPy electrode without changes in the reaction pathway. The Sb/Sb2O3–PPy electrode exhibits not only excellent cycle stability but also outstanding rate capabilities. As displayed in Figure 4c, the Sb/Sb2O3–PPy electrode showed a charge capacity of 527.84 mAh g−1 at a current density of 66 mA g−1 and retained a high reversible capacity of 299.46 mAh g−1 at 3300 mA g−1, which represents a 43.27% reduction. Evidently, the rate capability of Sb/Sb2O3–PPy is superior to the previously reported Sb-based composites,[12,13,16,18] demonstrating that the use of a highly porous polypyrrole nanowire network with a large surface area provides good electronic conductivity and facilitates the efficient transfer of Na+ from the solvent to the electrode. Although the discharge capacity rapidly decreased from 1320 to 3300 mA g−1, the high charge capacity of 525.18 mAh g−1 was restored when the current density returned to 66 mA g−1 from 3300 mA g−1 at the 41st cycle. The recovery of the capacity demonstrates that the structure of the Sb/Sb2O3–ppy is sustained and implies that obvious reductions in the capacity at high current densities may be attributed to decreases in the amount of inserted Na+ into Sb/Sb2O3. Figure 4d shows the increasing overpotentials of sodiation/desodiation as the current density increases from 66 to 3300 mA g−1. Because the Coulombic efficiencies were maintained above ≈99% at high current densities, the imperfect sodiation of Sb/Sb2O3 resulting from increasing overpotentials during discharge is a major cause of rapid reductions in the capacity. small 2015, DOI: 10.1002/smll.201500491
To better understand the electrochemical reactions of Sb/ Sb2O3–PPy with Na+, the phase changes of Sb/Sb2O3 during cycling were investigated using ex situ XRD. Figure 5 displays the XRD patterns for Sb/Sb2O3 when discharged to A, B, C, and D or when charged to E, F, and G during the 1st cycle (as marked in the voltage curve). After the formation step (A), the crystal structure of Sb/Sb2O3 did not changed with the initial structure in Figure 2b. At the end of the first discharge plateau (B), the diffraction peaks attributed to Sb nearly vanished. However, a new phase was not obtained. This result verifies that crystalline Sb was completely converted into an amorphous Na–Sb phase during the first discharge plateau. Following the discharge plateau, new peaks appeared at 33.54°, 34.33°, and 38.61°, which correspond to the hexagonal Na3Sb phase (JCPDS No. 00-065-2166). When completely sodiated (D), the phase structure of Na3Sb was maintained and the intensities of the corresponding peaks increased. During the following charge process, the peaks for Na3Sb disappeared at 0.80 V (E), and the diffraction peaks corresponding to crystalline Sb reappeared at 0.95 V (F). These results confirm that the crystalline Sb phase undergoes two phase transitions (Sb ↔ amorphous NaxSb ↔ crystalline Na3Sb) during sodiation and desodiation and that the sodiation process (A → D) and desodiation process (D → G) occurred reversibly. Another notable feature of the ex situ XRD analysis is that Sb2O3 remained as the crystalline phase during sodiation. When crystalline Sb was sequentially converted into an amorphous Na–Sb phase and a hexagonal Na3Sb phase, the peaks corresponding to Sb2O3 did not obviously change. Because Sb2O3 is a rechargeable anode material for Na-ion
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Figure 5. XRD patterns for Sb/Sb2O3–PPy at various stages during the 1st cycle.
batteries,[32,33] the remaining peaks of Sb2O3 could indicate that the Sb2O3 in Sb/Sb2O3 is not fully converted to Sb and Na2O. In the Na-ion battery system, Sb2O3 is reduced to the metallic Sb phase and Na2O (Sb2O3 + 6Na+ + 6e− ↔ 2Sb + 3Na2O). Next, the reversible alloying and dealloying of Na+ occur reversibly with the metallic Sb phase (Sb + 6Na+ + 6e− ↔ 2Na3Sb).[32] If the Sb2O3 phase is completely converted into metallic Sb and Na2O, the peaks corresponding to crystalline Sb2O3 should vanish before reaching a plateau at 0.7 V. Thus, only a small amount of Sb2O3 was assumed to react with Na+, and most of the Sb2O3 remained inactive. This is because the Sb2O3 matrix may have low reactivity with Na+ due to the strong bond between Sb2O3 and the polypyrrole nanowires. Figure 6 shows the morphological changes of Sb/Sb2O3– PPy after cycling, which were characterized by ex situ SEM. As illustrated in Figure 6a, the Sb/Sb2O3 nanodeposits exhibit a reversible volume expansion during one cycle, with no obvious particle cracking or pulverization. Particularly, the nanodeposits were tightly attached to the polypyrrole nanowires (Figure 6b). This observation implies that the relatively low volumetric changes occurred at the contact between Sb/Sb2O3 and the polypyrrole nanowires. Moreover, the inactive Sb2O3 phase buffered the volume changes of the Sb/Sb2O3 nanodeposits. Although the Sb/Sb2O3 nanodeposits did not sustain their initial “particle shape” after 100 cycles (as displayed in Figure 6c,d), it is noteworthy that the interconnected and highly porous structures were sustained. Thus, it can be deduced from these observations that
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the electrode consistently accommodates sodiation-induced stress and effectively suppresses the mechanical degradation of Sb/Sb2O3. Therefore, the outstanding cycle stability of the Sb/Sb2O3–PPy electrode is attributed not only to the highly porous structure of the electrode but also to the strong bond between the nanodeposits and the polypyrrole nanowires that originate from the low reactivity of the Sb2O3 matrix to Na+, which allows for good electrical contact with the current collector without loss of the active materials.
3. Conclusions In this study, 3D porous Sb/Sb2O3 anode materials are successfully fabricated using a simple electrodeposition method with a polypyrrole nanowire network as a substrate. The Sb/ Sb2O3–PPy electrode exhibited excellent cycle performance and outstanding rate capabilities. The outstanding electrochemical performance of the Sb/Sb2O3–PPy electrode is attributed not only to the use of a highly porous polypyrrole nanowire network as a substrate but also to the buffer effects of the Sb2O3 matrix on the volume expansion of Sb. Specifically, the Sb/Sb2O3–PPy electrode maintained a strong bond between the nanodeposits and the polypyrrole nanowires over 100 cycles, which allows for good electrical contact with the current collector without loss of active materials. Although further research is required to improve the initial Coulombic efficiency and rate capabilities, the results demonstrated that Sb/Sb2O3–PPy is one of the most promising
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crystal structures of the electrodeposits were investigated using XRD and HRTEM. To prepare the specimens for HRTEM, the dried Sb/ Sb2O3–polypyrrole was scraped from the substrates and dispersed in ethanol via sonication for 1 h. The electrochemical properties of Sb/ Sb2O3–PPy were investigated using Swageloktype cell that was assembled in an Ar-filled glove box. The cell consisted of an Sb/Sb2O3– PPy electrode sheet (0.67 mg cm−2) and a Na metal film without a separator.[6,9] The electrolyte was anhydrous propylene carbonate with 1 M NaClO4 and 2 vol% FEC. The charge/ discharge characteristics of the electrode were examined galvanostatically at a current density of 66 mA g−1 (0.1 C rate based on the theoretical capacity of Sb) and between 0.001 and 1.50 V (vs Na/Na+) after the formation step (sodiation/desodiation process at 30 mA g−1 for the activation). The rate capabilities of the electrode were evaluated at Figure 6. SEM images of Sb/Sb2O3–PPy a,b) after one cycle and c,d) after 100 cycles. various current densities of between 66 and 3300 mA g−1. To examine the morphology and sustainable anode materials for Na-ion batteries. Moreover, phase structure changes of the electrodes with cycling, ex situ SEM the improvement of the electrochemical performance of Sb/ observation and ex situ XRD analyses were performed on each Sb2O3–PPy suggests the feasibility of using the polypyrrole electrode. For ex situ SEM characterization, the cycled electrodes nanowire network as an effective substrate for recharge- were extracted from the Swagelok-type cells and rinsed with anhyable battery electrode materials. Therefore, we expect that drous dimethyl carbonate. For the ex situ XRD analysis, the cycled the synthetic approach suggested in this study will be readily samples were sealed with Kapton tape in an Ar-filled glove box.
applied as a facile and cost-effective process for fabricating diverse metal-oxide-polymer hybrid materials for use in various electrochemistry and nanotechnology fields.
Acknowledgements
4. Experimental Section The polypyrrole nanowire network was fabricated by cathodic polymerization from an aqueous solution containing 0.2 M NaNO3, 0.8 M HNO3, and 0.25 M pyrrole.[23] The electropolymerization of pyrrole was conducted using a three-electrode cell. A sheet of nodular Cu was used as the working electrode, a stainless steel plate precoated with polypyrrole was used as the counter electrode and a saturated calomel electrode (SCE, 0.241 V vs SHE(standard hydrogen electrode)) was used as the reference electrode. The polypyrrole nanowires were electropolymerized at a constant potential of −0.6 VSCE for 15 min while the solution was agitated with a magnetic stirrer at ≈800 rpm.[27] All of the experiments were conducted at room temperature (25 ± 0.5 °C). After rinsing with distilled water, the fabricated polypyrrole nanowire network was dried in a vacuum for 12 h. Then, the Sb/Sb2O3 was electrodeposited on the polypyrrole nanowire network at a potential of −1.4 VSCE for 2 min in an electrolytic bath containing 0.02 M K2Sb2(C4H2O6)2, 0.05 M KCl and 8 mL L−1 glycerol. In addition, the electrodeposition of Sb/Sb2O3 was performed using a standard three-electrode cell with a polypyrrole network as the working electrode, a Pt mesh as the counter electrode and a SCE (0.241 V vs SHE) as the reference electrode. The surface morphology and composition of the electrodes were examined using SEM and EDS, respectively. The small 2015, DOI: 10.1002/smll.201500491
This work was supported by POONGSAN and by the Center for Inorganic Photovoltaic Materials (Grant No. 2013-001796) grant funded by the Korea Government (Ministry of Sciency, ICT and Future Planning).
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small 2015, DOI: 10.1002/smll.201500491
