Thin Solid Films 690 (2019) 137522
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CuO nanowire arrays synthesized at room temperature as a highperformance anode material for Li/Na-ion batteries ⁎
Ying Sua,1, Ting Liub,1, Pei Zhangb, , Peng Zhengb,
T
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a
Key laboratory of Auxiliary Chemistry and Technology for Chemical Industry, Ministry of Education, Shaanxi University of Science and Technology, Xian 710021 Shaanxi, People's Republic of China b School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xian 710021 Shaanxi, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Anode Copper oxide Nanowire arrays Lithium-ion battery Sodium-ion battery
CuO nanowire arrays with thin nanowires and large interspaces grown on copper foil are prepared by a one-step room temperature solution approach without subsequent annealing. As the anodes for Li/Na-ion batteries, the large empty spaces between the nanowires enable Li/Na ions to diffuse into the electrode quickly. Due to the narrow diameter of the nanowire, the volume could expand in a timely manner, and the large interspace between the nanowires could accommodate the expansion effectively; thus, an undamaged framework is maintained. The one-dimensional nanowire and the tight contact between the nanowire and substrate favor electron delivery. A high discharge-charge capacity (i.e., at 100 mA g−1 it was 760 mAh g−1 for the Li-ion batteries and 351 mAh g−1 for the Na-ion batteries), excellent rate performance and long cycling life are obtained. This onestep synthesis route at room temperature is straightforward and industrially scalable. These factors, combined with the excellent electrochemical performance, make CuO nanowire arrays a promising anode for Li/Na-ion batteries.
1. Introduction Due to their high energy, high power density and long cycling life, lithium-ion batteries (LIBs) have been successfully used in portable electronics as energy conversion devices since 1990 [1–6]. Due to the increased demand for (hybrid) electric vehicles and smart grids, sodium-ion batteries (SIBs) have attracted intense attention due to the widespread distribution and the low cost of sodium compared with lithium [7–9]. SIBs are also based on the rocking-chair battery principle, which is similar to that of LIBs. However, the large radius of Na ions requires a more stable electrode structure. As the electrode uptakes more ions, the electrode expands. The accumulated stress makes the electrode framework collapse. Subsequently, the inserted ions decrease, and the capacity and cycling stability decrease. To circumvent this problem, various morphologies of electrodes have been explored. For example, multishelled hollow microspheres of a-Fe2O3 and Co3O4 were prepared by Wang et al. [10,11] and displayed excellent capacity and stability, as the thin shells could buffer the mechanical stresses that accompany volume changes during de/lithiation. Yolk-shell and pomegranate-like clusters were synthesized by Cui et al. [12–15] in which the empty spaces within the cluster structure can accommodate a sulfur
volume change, thus obtaining excellent electrochemical performance. In addition, nanoarrays are preferred structures because of the following characteristics: (1) electrons can be quickly transferred into the substrate by single dimension of the nanowires, (2) the large volume expansion can be accommodated by the empty spaces between the nanowires, and (3) ions can be quickly diffused into the nanowires through empty spaces. Cupric oxide (CuO) is regarded as a promising anode because of its high specific capacity (674 mAh g−1) and low cost [16–18]. Diverse morphologies of CuO anodes have been investigated, including nanorods, sheets, spheres and wires [19–23]. The nanowire arrays grown on copper foil have also been investigated. In addition to the abovementioned advantages, copper foil is the current collector of the batteries; thus, the prepared CuO nanowire arrays grown on copper foil can be used directly as the anode without any additives. Currently, there are two approaches to prepare the CuO: (1) thermally oxidizing copper substrates [24–26] and (2) solution synthesis followed by annealing [27]. Annealing is employed in both methods and would make the wires thick and compact. When the wires become thick, more stresses are generated as large ions are inserted, and the empty spaces between the nanowires become small, which is a disadvantage for the
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Corresponding authors. E-mail addresses:
[email protected] (P. Zhang),
[email protected] (P. Zheng). 1 These authors worked equally. https://doi.org/10.1016/j.tsf.2019.137522 Received 16 August 2018; Received in revised form 25 August 2019; Accepted 25 August 2019 Available online 26 August 2019 0040-6090/ © 2019 Published by Elsevier B.V.
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Fig. 1. (a) XRD patterns of the standard Cu sample (black line), the standard CuO sample (blue line), and CuO nanowire arrays grown on Cu foil (red line); (b) HRTEM image of the nanowire, where the inset is the corresponding SAED pattern; (c–e) FE-SEM images of CuO nanowire arrays, where the inset of Fig. 1e is the optical image of synthesized CuO; and (f) EDX spectrum of CuO nanowire arrays. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
additional annealing treatment of the CuO nanowire arrays, the wires become thick and compact, and the performance decreases.
stability of the electrode because it has a negative effect on the performance. In this paper, a one-step solution synthesis of CuO nanowire arrays grown on copper foil without subsequent annealing is described. The produced CuO nanowires have a diameter of 8 nm, and the empty spaces between the nanowires are sufficiently large. With the merits of the abovementioned structure, the nanowire arrays as the anodes display a high capacity and good stability for both LIBs and SIBs. We found that the synthetic process is sensitive to the reaction temperature. When reducing the reaction temperature from 25 °C to 0 °C, the product changes from black CuO nanowires into blue Cu(OH)2 nanowires. Upon
2. Experimental 2.1. Synthesis All the chemicals were of analytical grade and used as-purchased without further purification. Copper foil with a thickness of 20 μm was supplied by Huizhou United Copper Foils Ltd. Co. (P. R. China) and was used as the substrate. Hydrochloric acid, sodium hydroxide, and 2
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Fig. 2. (a, b) SEM images of Cu(OH)2 nanobundles, where the inset of Fig. 1b is the corresponding optical image; (c, d) FE-SEM images of the sample synthesized between the temperatures of 5–15 °C, where the inset of Fig. 1d is the corresponding optical image; and (e, f) FE-SEM images of annealed CuO nanowire arrays.
the color of the surface changed from black to golden, and the surface CuO was removed. The active mass of CuO was obtained from the weight change. The average mass loading of CuO on the Cu foil was ca. 1.5 mg cm−2.
ammonium hydroxide were supplied by Beijing Chemical Works. The copper foil was washed with hydrochloric acid for 5 min and subsequently washed with deionized water several times to remove surface impurities and blow-dried by compressed air. For the synthesis of the CuO nanowire arrays, a round copper foil with a diameter of 16 mm was immersed into a solution containing 400 mL water, 1 mL ammonia (28%) and 80 mg sodium hydroxide. Then, the solution was sealed and reacted for 96 h at 25 °C. For the synthesis of the Cu(OH)2 nanowire arrays, 0 °C was used as the reaction temperature during the process and the other conditions were unchanged. To determine the mass of the CuO, the synthesized CuO films (1 cm2) were washed with 0.1 M HCl according to the reaction of CuO + 2HCl = CuCl2 + H2O. After ultrasonicating for several seconds,
2.2. Characterization X-ray powder diffraction (XRD) analysis was carried out with a Rigaku D/MAX2200PC using Cu Kα radiation. Field emission scanning electron microscopy (FE-SEM) images were collected on a Rigaku S4800 microscope equipped with an EDX (Thermo scientific NSS). High-resolution transmission electron microscopy (HRTEM) images were observed using an FEI Tecnai F20 microscope. Copper grids were 3
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Fig. 3. LIB performance with CuO nanowire arrays: (a) CV curves at a scan rate of 0.1 mV s−1, (b) galvanostatic discharge-charge profiles at 100 mA g−1, (c, d) rate performance, and (e) cycling performance at 0.3 A g−1 and the corresponding Coulombic efficiency.
galvanostatically charged and discharged on a Newaresles battery test system (BTS) (Shenzhen, China) with a cut-off voltage range of 0.01–3.0 V (vs. Li/Li+) and 0.01–3.0 V (vs. Na/Na+). Cyclic voltammetry (CV) was conducted on a CHI 660E electrochemical workstation.
used instead of copper foil for the HRTEM analysis, whereas the other conditions remained unchanged.
2.3. Electrochemical measurements
3. Results and discussion
CuO nanowire arrays were used directly as an electrode for lithium/ sodium half-cells without additional conductive agents and polymeric binders. The electrochemical properties of the electrodes were monitored by assembling them into coin cells (type CR2032) in an argonfilled glove box with water and oxygen contents of less than 0.5 ppm. For the lithium half-cells, lithium foil served as the counter electrode and polypropylene (PP) film (Celgard 2500) as the separator. The electrolyte was made of LiPF6 (1 M) and a mixture of EC (ethylene carbonate):DEC (diethyl carbonate) in a volume ratio of 1:1. The sodium half-cell was assembled in an analogous way. Metallic Na was used as the counter electrode, and glass fiber (GF/A, Whatman) was used as the separator. The electrolyte was made of NaPF6 (1 M) and a mixture of EC/DEC in a volume ratio of 1:1. The cells were
Fig. 1 shows the structural characteristics of the CuO nanowire arrays synthetized by the one-step solution maintained at 25 °C without any additional annealing. The phase structure was investigated by XRD, as shown in Fig. 1a. The resulting black sample is indexed to CuO (JCPDS 45–0937), and two weak peaks at 2θ = 35° and 39° are attributed to the crystal faces of (002) and (200). The other strong peaks are characteristic of the Cu foil substrate. The HRTEM in Fig. 1b shows obvious lattice fringes for the nanowire, which indicates a highly crystalline film, and the interplanar distances of ca. 0.231 and 0.253 nm coincide well with the interlayer spacing of (200) and (002) of cubic CuO. The inset of Fig. 1b is the selected area electron diffraction (SAED) 4
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Fig. 4. SIB performance with CuO nanowire arrays: (a) CV curves at a scan rate of 0.1 mV s−1, (b) galvanostatic discharge-charge profiles at 20 mA g−1, (c, d) rate performance, and (e) cycling performance at 0.1 A g−1.
foil changed directly into black CuO without appearing blue. The structure characteristic of the black one was analyzed above. We conjecture that the increased temperature plays a key role in the instantaneous oxidation of [Cu(NH3)n]2+ into CuO. To confirm the key role of the temperature, experiments were carried out at various temperatures with the other conditions unchanged. At 0 °C, the pure blue color of the Cu(OH)2 nanobundles (inset of Fig. 2b) was obtained. The nanobundles are composed of a nanobelt with a diameter of 300 nm, and their growth orientation had a little effect on the Cu foil (Fig. 2a and b). At temperatures between 5 and 15 °C, the mixed black and blue colors appeared simultaneously (inset of Fig. 2d). The black color is ascribed to the existence of CuO nanowires, and the blue color is attributed to the formation of Cu(OH)2 nanobundles. These morphologies are observed in Fig. 2c and d. The blue color of the nanobundles appearing below 20 °C agrees with the Cu(OH)2 mechanism. Therefore, the color variation of the sample at 5–20 °C supports the impact of the temperature. To obtain good crystallinity of CuO nanowire arrays, 25 °C was chosen. When the black CuO nanowire arrays were annealed at 200 °C, although the degree of crystallinity becomes strong, the nanowires become thick, the empty space among the wires becomes small,
pattern, which demonstrates that the as-prepared CuO nanowires are single crystalline. The highly crystalline but weak intensity peaks from the XRD imply there was a minor amount of CuO grown on the substrate. Fig. 1c–e shows a typical FE-SEM of the synthesized CuO nanowire arrays. The nanowire has a diameter of 8 nm, and the large empty spaces among the wires can be clearly observed. These nanowires have a uniform distribution and present identical growing orientations. In the inset of Fig. 1e, the black sample with CuO nanowire arrays could be bent and could not be scraped from the substrate by knife, which indicates the strong connection between the wires and the substrate, which is beneficial for electron transport. Fig. 1f shows the energy-dispersive X-ray spectroscopy (EDX) measured from the area of Fig. 1e, where the atom mole ratio of oxygen:copper approaches 1:1, which also confirms that the composition of the synthetized sample is CuO. Sodium hydroxide, ammonia, water and Cu foil were chosen as the reactants, which are usually used to prepare Cu(OH)2 nanowire arrays. The detailed formation mechanism of blue Cu(OH)2 can be seen in the references [16,28,29]. However, we found that when setting the reaction temperature of these mixtures at 25 °C, the golden color of the Cu 5
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Fig. 5. (a) Galvanostatic discharge-charge profiles and (b) cycling performance at 20 mA g−1 for annealed CuO nanowire arrays; (c, d) SEM images of annealed CuO nanowire arrays after cycling test for LIBs and SIBs, respectively; (e, f) FESEM images of CuO nanowire arrays for LIBs and SIBs, respectively.
described as follows:
and large nanorods arise simultaneously, as shown Fig. 2e and f. Such features are not advantageous for LIB/SIB anode performance. The produced black CuO nanowire arrays were evaluated for the electrochemical performance of the LIBs/SIBs using coin cells (2032) with lithium/sodium foil as the counter electrode. Fig. 3 shows the LIB performance of the half cells. Cyclic voltammetry (CV) (Fig. 3a) technology can reveal the (de)intercalation mechanism of the ion. The second and third cycle curves overlap closely, indicating good cycle stability. The area difference between the first and last two cycles is the reason for the formation of a solid-electrolyte interphase layer (SEI) and electrolyte decomposition [30]. Three cathodic current peaks (0.84, 1.32 and 2.17 V) and two anodic peaks (1.38 and 2.53 V) appear in the same position as the last two curves, which are attributed to the Li-ion insertion and extraction of the nanowire array electrode. The detailed mechanism of the three cathodic current peaks is attributed to the production of a Cu1-xIICuxIO1-x/2 intermediate, the Cu2O phase, and the final reduction to Cu and Li2O, respectively; the two anodic peaks are ascribed to the successive formation of Cu2O and CuO, as suggested previously [31,32]. Thus, the overall electrochemical reaction can be
CuO + 2 Li+ + 2e−—Cu + Li2 O Fig. 3b displays the discharge-charge profiles at a current density of 100 mA g−1. The 1st, 2nd and 10th discharge capacities arrive at 760, 640 and 638 mAh g−1, respectively, and the first Coulombic efficiency is 88.6%. At the end of the 10th cycle, the discharge curve overlapped the 2nd cycle, and these profiles are consistent with the CV curves. The high capacity resulted from the small nanowires having extensive and prompt contact with the Li ions that diffused from the electrolyte via large voids among the wires. The large interspace also contributes to the excellent rate performance. Fig. 3b and c show the rate performance of the CuO electrode. At current densities of 0.3, 0.5, 1 and 2 A g−1, the discharge capacity reached 498, 325, 267 and 166 mAh g−1, respectively. When shifted to 0.1 A g−1, the capacity of 639 mAh g−1 is recovered. As the anode for the LIBs, the CuO nanowire arrays also demonstrate outstanding cycling stability. As shown in Fig. 3e, during a hundred cycles at 0.3 A g−1, the capacity is maintained as high as 490 mAh g−1, and the Coulombic efficiency approaches 100%. The long6
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The large surface area of the nanowires allows electrons and ions to easily transfer to the electrode. As more ions insert into the nanowire, it can release the expanded volume in a timely manner, and the large interspace between the nanowires can accommodate the expanded volume effectively; thus, the structural integrity and good long-term cycling life are obtained. The solution synthesis is a straightforward process and is possible to produce on a large scale, which indicates that CuO nanowire arrays are a promising electrode material.
term stability was due to the thin nanowires, which could release the expansion volume quickly and prevent the loss of structural integrity and collapse as more ions were inserted into the electrode. To obtain a high capacity and excellent cycling stability for the SIBs, much harsher requirements for the electrodes are needed than for the LIBs. With the same amount of ions inserted into the electrode, more volume expansion and accumulated stress would be generated from Na ions than from Li ions due to the larger radius of a Na ion (1.06 Å) than that of a Li ion (0.76 Å). If the expanded volume could not be released in a timely manner, the accumulated stress would cause the loss of structural integrity; thus, the capacity and cycling stability would be reduced. Most electrodes suitable for LIBs are not suitable for SIBs [33,34]. To this point, the electrode comprised of CuO nanowire arrays with thin nano-wires and large interspaces also show excellent performance for SIBs, as shown in Fig. 4. The SIBs have an opposite reaction as that of LIBs. From the first four CV curves (Fig. 4a), four cathodic current peaks (0.25, 0.78, 1.33 and 2.11 V) and three anodic peaks (0.84, 1.6 and 2.5 V) could be observed. The intensity of the peaks for the SIBs is weaker than that for LIBs, which indicates the slow reaction kinetics for the SIBs compared with that for LIBs [35]. Therefore, there are extra peaks compared with those of the LIBs, which are ascribed to the intermediates during the multistep electrochemical redox process [36]. Fig. 4b shows the discharge-charge curves for the 1st, 2nd and 10th cycles, and the discharge capacities are 774, 523 and 516 mAh g−1 at 20 mA g−1, respectively. The first Coulombic efficiency is 60%, and the loss capacity is the same as mentioned previously for the LIBs. Fig. 4c and d present the rates, which delivered capacities of 520, 420, 351 and 238 mAh g−1 at 20, 50, 100 and 200 mA g−1, respectively. When the current density is shifted to 20 mA g−1 again, the capacity recovers to 516 mAh g−1. The CuO nanowire arrays also demonstrate a good cycling life (Fig. 4e). During 100 cycles at 100 mA g−1, the electrode maintained a capacity of 352 mAh g−1. The nanowire is narrow, and the spaces between the wires are large enough to accommodate the expanded volume; thus, excellent cycling stability occurred. The capacity is reduced due to the more sluggish reaction kinetics of SIBs compared with those of LIBs. To further contrast the excellent performance of the thin nanowires with large interspaces, we tested the annealed CuO nanowire arrays with thick wires and narrow voids as the anode for SIBs. At a galvanostatic current of 20 mA g−1, the discharge capacities are 327, 228 and 150 mAh g−1 for the 1st, 2nd and 40th cycles, respectively, which are much lower than those of unannealed CuO nanowire arrays (Fig. 5a). The cycle stability has a large decrease at the end of the 40th cycle, and a capacity of 67.7% is retained compared with that of the 2nd cycle (Fig. 5b). It is suggested that the poor performance of the annealed sample results from the compacted wire and damaged structure. The morphological changes upon Li/Na ion insertion and extraction are shown in Fig. 5c and d, respectively. As the anode for the LIBs, the nanowires could be clearly observed (Fig. 5c), while for the SIBs, the wires were fused together, and the voids disappeared (Fig. 5d) for the annealed sample. Such a situation does not occur for the unannealed sample. The interspace is observed for the anode for the LIBs and SIBs, and compared with the LIBs (Fig. 5e), the voids decrease for the SIBs (Fig. 5f). These morphological changes can also support the excellent structural performance of the unannealed CuO nanowire arrays.
Declaration of Competing Interest The authors declare no competing financial interests. Acknowledgements The authors acknowledge the financial support from Natural Science Basic Research Plan in Shaanxi Province of China (No.2018JQ5071), the scientific research starting foundation of Shaanxi university of science and technology (No.BJ16-14) and Special Scientific Research Project of Shaanxi Education Department (No.18JK0118). References [1] J.B. Goodenough, K.-S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176. [2] K.-X. Wang, X.-H. Li, J.-S. Chen, Surface and interface engineering of electrode materials for lithium-ion batteries, Adv. Mater. 27 (2015) 527–545. [3] W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang, J.-G. Zhang, Lithium metal anodes for rechargeable batteries, Energy Environ. Sci. 7 (2014) 513–537. [4] P. Zheng, T. Liu, Y. Su, L. Zhang, S. Guo, TiO2 nanotubes wrapped with reduced graphene oxide as a high-performance anode material for lithium-ion batteries, Sci. Rep. 6 (2016) 36580. [5] J. Cao, S. Guo, R. Yan, C. Zhang, J. Guo, P. Zheng, Carbon-coated single-crystalline LiMn2O4 nanowires synthesized by high-temperature solid-state reaction with high capacity for Li-ion battery, J. Alloys Compd. 741 (2018) 1–6. [6] P. Zheng, T. Liu, J. Cao, J. Song, X. Wang, J. Huo, C. Zhang, S. Guo, S nano-layer deposited on the surface of 3D carbon for high cycle-stability Li–S batteries, J. Alloys Compd. 737 (2018) 1–7. [7] L.P. Wang, L. Yu, X. Wang, M. Srinivasan, Z.J. Xu, Recent developments in electrode materials for sodium-ion batteries, J. Mater. Chem. A 3 (2015) 9353–9378. [8] N. Chen, Q. Pan, Versatile fabrication of ultralight magnetic foams and application for oil–water separation, ACS Nano 7 (2013) 6875–6883. [9] S.-W. Kim, D.-H. Seo, X. Ma, G. Ceder, K. Kang, Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithium-ion batteries, Adv. Energy Mater. 2 (2012) 710–721. [10] S. Xu, C.M. Hessel, H. Ren, R. Yu, Q. Jin, M. Yang, H. Zhao, D. Wang, [small alpha]Fe2O3 multi-shelled hollow microspheres for lithium ion battery anodes with superior capacity and charge retention, Energy Environ. Sci. 7 (2014) 632–637. [11] J. Wang, N. Yang, H. Tang, Z. Dong, Q. Jin, M. Yang, D. Kisailus, H. Zhao, Z. Tang, D. Wang, Accurate control of multishelled Co3O4 hollow microspheres as highperformance anode materials in lithium-ion batteries, Angew. Chem. Int. Ed. 52 (2013) 6417–6420. [12] Z. Wei Seh, W. Li, J.J. Cha, G. Zheng, Y. Yang, M.T. McDowell, P.-C. Hsu, Y. Cui, Sulphur–TiO2 yolk–shell nanoarchitecture with internal void space for long-cycle lithium–sulphur batteries, Nat. Commun. 4 (2013) 1331. [13] N. Liu, H. Wu, M.T. McDowell, Y. Yao, C. Wang, Y. Cui, A yolk-shell design for stabilized and scalable Li-ion battery alloy anodes, Nano Lett. 12 (2012) 3315–3321. [14] W. Li, Z. Liang, Z. Lu, H. Yao, Z.W. Seh, K. Yan, G. Zheng, Y. Cui, A sulfur cathode with pomegranate-like cluster structure, Adv. Energy Mater. 5 (2015) 1500211. [15] N. Liu, Z. Lu, J. Zhao, M.T. McDowell, H.-W. Lee, W. Zhao, Y. Cui, A pomegranateinspired nanoscale design for large-volume-change lithium battery anodes, Nat. Nanotechnol. 9 (2014) 187–192. [16] L. Wang, H. Gong, C. Wang, D. Wang, K. Tang, Y. Qian, Facile synthesis of novel tunable highly porous CuO nanorods for high rate lithium battery anodes with realized long cycle life and high reversible capacity, Nanoscale 4 (2012) 6850–6855. [17] Y. Zhang, M. Xu, F. Wang, X. Song, Y. Wang, S. Yang, CuO necklace: controlled synthesis of a metal oxide and carbon nanotube heterostructure for enhanced lithium storage performance, J. Phys. Chem. C 117 (2013) 12346–12351. [18] L. Wang, W. Cheng, H. Gong, C. Wang, D. Wang, K. Tang, Y. Qian, Facile synthesis of nanocrystalline-assembled bundle-like CuO nanostructure with high rate capacities and enhanced cycling stability as an anode material for lithium-ion batteries, J. Mater. Chem. 22 (2012) 11297–11302. [19] A.S. Zoolfakar, R.A. Rani, A.J. Morfa, A.P. O'Mullane, K. Kalantar-zadeh, Nanostructured copper oxide semiconductors: a perspective on materials, synthesis methods and applications, J. Mater. Chem. C 2 (2014) 5247–5270.
4. Conclusions In summary, this work demonstrates that CuO nanowire arrays with thin nanowires and large interspaces grown on copper foil can be prepared by a one-step room temperature solution approach. As the anode for Li/Na-ion batteries, the nanowires deliver a high capacity of 639 mAh g−1 at 100 mA g−1 and 520 mAh g−1 at 20 mA g−1. In addition, an excellent rate performance and outstanding cycle stability are also obtained. Such excellent performance results from the structure. 7
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