Recently, energy storage has been extensively investigated to satisfy the demand of portable electronic devices, electric vehicles, and smart electronics. Among all the energy storage systems, lithium-ion batteries (LIBs) have been attracting considerable attention in scientific and industrial communities due to their unique characteristics in terms of high energy density and long cycling life [1-3]. In the recent years, transition metal oxides have been widely investigated as promising anode materials for high energy density lithium-ion batteries, which is mainly due to their higher theoretical Li-ion storage capacities than that of conventional graphite anodes [4-8]. Generally, the mechanisms of transition metal oxides working as anode materials can be summarized as the following equation: MO + 2Li+ + 2e-↔Li2O + M (M = Cu, Fe, Ni, Co, etc.) [9, 10]. During the above electrochemical conversion reactions, few significant problems such as structural pulverization and volumetric expansion/contraction may arise, which will lead to structure cracking and capacity fading of active materials [11-15].
Among the transition metal oxides, CuO is noteworthy to be explored as a promising anode material for LIBs applications, due to its high theoretical capacity (670 mAh/g), low-cost, easy synthesis and eco-friendliness. However, similar to the other transition metal oxide anodes, CuO also suffers very rapid capacity decay and poor cycling stability caused by huge and uneven volume variations (around 174%) during the lithium insertion/extraction process [16-20]. One possible approach to improve the electrochemical performance of CuO materials is to use well configured micro/nanostructure electrode materials. Micro/nanostructured materials bring unique advantages as electrode materials in lithium ion batteries such as shorter lithium diffusion pathways, larger electrode/electrolyte contact area, and facile lithium insertion/extraction as compared to the conventional bulk counterpart [21-26].
In recent years, CuO micro/nanostructures have been recognized as promising candidates due to their high surface-to-volume ratio and the synergistic effect between the nanoscale building blocks and micro-sized assemblies, which not only maintain the high activity of the CuO materials, but also provide effective structure to relieve the volume variation during Li-ion insertion/extraction process. Therefore, many efforts have been explored to prepared micro/nanostructures, including hydrothermal methods, electrospinning technique, wet chemical methods, templateassisted synthesis, and thermal oxidation process. Yang et al.  reported the preparation of CuO nanorods using a simple electrochemical etching and subsequent heat treatment method, which exhibited excellent rate capability and improved cyclability with a specific capacity of 740 mAh/g after 50 cycles at a current rate of 50 mA/g. Hu et al.  prepared dendrite-shaped CuO hollow micro/nanostructures by Kirkendall-effect-based approach. The fine and polycrystalline CuO hollow structures as anode materials for lithium ion batteries exhibit revisable capacity of 300 mAh/g after 50 cycles at a current rate of 0.5 C, and with the average coulombic efficiency of ~97%. Gao et al.  designed and prepared hierarchical CuO hollow micro/nanostructures by a tyrosine-assisted green strategy. The structures composed of CuO nanosheets self-organized into hollow micrometer-sized monoliths with hierarchical architecture. The hierarchical CuO hollow micro/nanostructures are used as anode materials in lithium ion batteries with a high discharge/charge capacity. Despites these research efforts, the fabrications of CuO micro/nanostructured under a facile and friendly condition are still a key challenge.
Herein, we report a facile and controllable approach to prepare peony-like CuO micro/nanostructures. The peony-like CuO micro/nanostructures with size of 3–5 μm were assembled by the units of CuO nanoplates. The surfactant (ethidene diamine) on the surface of CuO nanoplates was a key to induce their self assembly to form peony-like micro/nanostructures through hydrogen bonds and van der Waals forces. The phase and purity of the selfassembly of peony-like CuO micro/nanostructures were confirmed by the X-ray diffraction (XRD) pattern. Fig. 1 shows the XRD patterns of the self-assembly of peony-like CuO products, all diffraction peaks can be indexed to monoclinic CuO (JCPDS No. 41-0254). No impurities were detected, indicating the high purity of products. In addition, the sharp peaks indicated that the product is well crystallized.
The morphologies of peony-like CuO micro/nanostructures were characterized by scanning electron microscopy (SEM) and transmission electron microscope (TEM). Fig. 2a shows the overall morphologies of as-obtained products with low magnification. It can be seen that the samples are in large quality and mainly composed of uniform, flower-like architectures ranging from 3 μm to 5 μm in diameter. As increasing the magnification (Figs. 2b and c), it can be clearly observed that the surface of the individual CuO is quite rough and consists many nanoplates. These nanoplates look like petals, which were then attached to each other to form peony-like structure through orientational assembly. The sizes of the nanoplates were mostly 100 nm in thickness, and 1.5 μm in length. TEM images (Figs. 2d and e) further confirmed that asprepared CuO had flower-like architectures that were composed of numerous delicate nanoplates consistent with SEM observations. The HRTEM image of the selected area taken near edge of single CuO nanoplate is shown in Fig. 2f. A lattice spacing of 0.231 nm is recognized and can be ascribed to the (200) planes of CuO. To understand the porous feature and the specific surface areas of the peony-like CuO micro/nanostructures, Brunauer-Emmett-Teller (BET) gas-sorption measurement is applied (Fig. S1a in Supporting information). The nitrogen adsorption isotherm is a typical Ⅳ-type curve with a distinct hysteresis loop, indicating the existence of a mesoporous structure. The BET surface area is measured to be 23 m2/g. In addition, Barrett-Joyner-Halenda (BJH) pore size distribution of the peony-like CuO micro/nanostructures shows peaks centered at around 2–20 nm (Fig. S1b in Supporting information).
|Fig. 2. (a–c) SEM images of peony-like CuO micro/nanostructures, (d–f) TEM and HRTEM images of peony-like CuO micro/nanostructures.|
Understanding the growth mechanism of nanocrystals is critical to control shape and size of nanoparticles [30-34]. In order to understand the formation process of the peony-like CuO micro/nanostructures, systematic investigations were carried out with different reaction time at 85 ℃ (Fig. 3). Fig. 3a depicts the SEM image of the sample obtained after 5 min, a small amount of CuO nanoplates began to appear (Fig. 3a) with the average length of 1 μm (inset of Fig. 3a). With the reaction time up to 10 min, the number of CuO nanoplates was increased (Fig. 3b). When the reaction was carried out for 15 min, the CuO nanoplates obviously began to aggregate (Fig. 3c). High magnification SEM image (inset of Fig. 3c) shows that most CuO nanoplates were stacked with each other and form a multilayer structure. When the reaction time was extended to 30 min, all of CuO nanoplates were self-assembled into multilayer structures with size of about 2 μm (Fig. 3d). When the reaction time was prolonged to 40 min, almost all the products became preliminary flower-like architectures of size of about 3 μm (Fig. 3e). High magnification SEM image (inset of Fig. 3e) exhibits that these multiplayer nanoplates are oriented together and not stacked in random directions. Until the reaction time lasted 60 min, the size of three-dimensional structures increased gradually. The morphology became peony-like flowery micro/nanostructure with large size (Fig. 3f).
|Fig. 3. SEM images of as-prepared peony-like CuO micro/nanostructures for various reaction time durations at 85 ℃, (a) 5 min; (b) 10 min; (c) 15 min; (d) 30 min; (e) 40 min and (f) 60 min.|
To substantially understand the effect of ethidene diamine on the morphology of peony-like CuO micro/nanostructures, different amounts of ethidene diamine was used in the experiments. When no ethidene diamine was added to the system under the same reaction conditions, products obtained were not peony-like micro/nanostructures, only producing large amount of nanoplates (Fig. 4a). When the ethidene diamine increased to 0.1 mL, some of nanoplates is randomly gathered together (Fig. 4b). When the ethidene diamine added up to 0.2 mL, more quantities of CuO nanoplates are connected together with each other, but some of them are still dispersed as a single nanoplate in the sample (Fig. 4c). When 0.3 mL of ethidene diamine was used, it can be seen that almost all the CuO nanoplates stacked together to form multilayer architecture (Fig. 4d). When ethidene diamine increased from 0.3 mL to 0.35 mL, these multilayered structure of CuO nanoplates self-assembly to form a preliminary flower-like morphology, but the overall structure is cluttered (Fig. 4e). Until 0.4 mL of ethidene diamine was used, the overall morphology of a typical product composed of large-scale uniform, peony-like CuO micro/nanostructure was observed (Fig. 4f). The above result revealed that ethidene diamine has a great impact on the formation of the resultant peony-like CuO micro/nanostructures in the present study. During the initial stage of synthesis, amine group of ethidene diamine can coordinate with Cu ions to form complexes . With the growth of CuO nanocrystals, ethidene diamine can adsorb on its surfaces, and the upper and lower surfaces of the CuO nanoplates will be connect together by hydrogen bonding and van der Waals force, and finally assembled into peony-like flower architecture with multilayer structure.
|Fig. 4. SEM images of as-prepared peony-like CuO micro/nanostructures at 85 ℃ for different usage of ethidene diamine (a) 0 mL; (b) 0.1 mL; (c) 0.2 mL; (d) 0.3 mL; (e) 0.35 mL and (f) 0.4 mL.|
The electrochemical performance of the peony-like CuO micro/nanostructures electrodes was investigated by voltammetry (CV) and galvanostatic discharge-charge measurements, as illustrated in Fig. 5. For the CV measurements metallic lithium acts as both counter and reference electrodes. Fig. 5a depicts the first four CV of the peony-like CuO micro/nanostructure at a scan rate of 0.1 mV/s. In the first cycle, three reduction peaks are observed at 0.7 V, 0.92 V, 1.75 V (vs. Li+/Li) during the discharge cycle. These peaks correspond to a multi-step electrochemical reaction, including: (1) The reaction of a Cu Ⅱ1-x Cu ⅠxO1-x/2 solid solution with CuO phase, (2) The formation of Cu2O phase, (3) The decomposition of Cu2O into Cu and Li2O, which is consistent with discharge/charge curves with a multiple-plateau feature [36-38]. In the first cathodic sweep the cell showed a weaker broad peak ~2.4 V vs. Li, which corresponds to the decomposition of the organic layer and the partial oxidization of the copper to the copper oxides. The electrochemical reaction mechanism can be described as:
|Fig. 5. (a) CV curves of the peony-like CuO micro/nanostructures, (b) charge-discharge voltage profiles of the peony-like CuO micro/nanostructures, (c) capacity performance of the peony-like CuO micro/nanostructures and commercial CuO at a current rate of 0.1 C, (d) cycle performances of the peony-like CuO micro/nanostructures at different current rates.|
From the second cycle onward the anodic potentials are shifted toward higher voltages likely ~0.75 V, 1.16 V and 2.27 V vs. Li. Similarly, the oxidation potential also shifted to ~2.42 V vs. Li. In the subsequent cycles, the overlapping of cycling traces is noted during both cathodic and anodic sweeps, indicating a high degree of reversibility for the redox reaction and the excellent cycle stability of the peony-like micro/nanostructures. Fig. 5b shows the galvanostatic charge-discharge curves of the peony-like CuO mirco/nanostructures for the initial two cycles, 50th cycle, 100th cycle and the 200th cycle at 0.1 C. In the first cycle, the constant slope with several small plateaus indicates a multiphase transition between CuO and lithium. The initial charge and discharge capacities are found to be approximately 991 mAh/g and 366 mAh/g, respectively, resulting in an initial coulombic efficiency of ~37%. The initial irreversible capacity loss may be mainly ascribed to diverse irreversible processes such as interfacial lithium storage, inevitable formation of solid electrolyte interface (SEI layer) and organic conductive polymer, as well as the electrolyte decomposition, which are common for most anode materials [39, 40]. Remarkably, from the second cycle onwards, the peony-like CuO micro/nanostructures exhibit no obvious capacity decay, the specific capacity maintains a steady growth in the subsequent cycles. In addation, the discharge and charge plateaus occurring at potential ranges of 2.2–2.7V and 1.5–0.6V vs. Li+/Li agree with the typical CV profile. The cycling performance of the peony-like CuO micro/nanostructures electrodes was tested at a current density of 0.1C, as illustrated in Fig. 5c. It can be observed that the electrode shows good capacity retention performance, delivering a high reversible capacity of 456mAh/g after 200 cycles. The gradually increasing reversible capacity during cycling is a common phenomenon for metal-oxide anode materials, probably resulting from a gradual activation process of the metal-oxide electrodes as well as reversible reactions between metal particles and electrolytes. In comparison, the cycling performance of commercial CuO was investigated. The first discharge capacity of commercial CuO is 923mAh/g. However, these values quickly decrease to 307mAh/g after 10 cycles, 119mAh/g after 50 cycles, and 121mAh/g after 200 cycles, indicating poor capacity retention. Notably, the result reveals outstanding performance of our peonylike CuO micro/nanostructures when compared to the previous reported CuO anode materials at similar current densities (Table 1) [41-46]. Moreover, the SEM image of peony-like CuO micro/nanostructures after 10 cycles at 0.1C was shown in Fig. 6. Obviously, the shape integrity is still retained, which demonstrating peony-like micro/nanostructures can withstand the stress of volume changes and prevent pulverization during the dischargecharge cycles. At a high rate of 2C and 5C were measured, the specific charge capacity of peony-like CuO micro/nanostructures are 176mAh/g and 77mAh/g after 40 cycles, respectively (Fig. S2 in Supporting information). The relation of different dischargecharge rates with specific capacity for the peony-like CuO micro/nanostructures from 0.1C to 1C was investigated, and the results are shown in Fig. 5d. It can be observed that a discharge capacity of 399.1mAh/g is obtained at 0.1C after 20 cycles, and this value is slowly reduced to 346.4mAh/g, 316.4mAh/g, 279.3mAh/g, and 219.5mAh/g when the current rate is consecutively set at the levels of 0.2C, 0.3C, 0.5C, and 1C, respectively. In particular, when the current density is returned to 0.2C, the capacity almost returns back to the initial capacity, suggesting a good rate performance. The remarkably improved cycling stability and rate capability might be attributed to the peony-like CuO micro/nanostructure consisting of high active site 2D nanoplates, which could not only provide large electrode/electrolyte contact area but also shorten the Li ion diffusion path way, so that the lithiation reaction could take place more rapidly and efficiently. Moreover, the interval between the multilayer structures of CuO nanoplates electrode could provide enough interior space to accommodate the volume change during Li+ insertion and de-insertion process, which is beneficial to the structural integrity.
In summary, the peony-like CuO micro/nanostructures were fabricated by a facile hydrothermal approach. The effects of different reaction parameters on the morphologies of Cu nanosheets were studied. The surfactant of ethidene diamine on the surface of CuO nanoplates could induce these nanoplates selfassemble to form peony-like micro/nanostructures by hydrogen bonds and van der Waals forces. The novel peony-like CuO micro/nanostructures exhibit a high cycling stability and improved rate capability. The peony-like CuO micro/nanostructures electrodes show a high reversible capacity of 456mAh/g after 200 cycles, much higher than that of the commercial CuO nanocrystals at a current 0.1C. Meanwhile, the flower-like micro/nanostructure electrodes exhibit enhanced rate capability at current 0.1 –1C. The excellent electrochemical performance of peony-like CuO micro/nanostructures might be ascribed to the unique assembly structure, which provides large electrode/electrolyte contact area to accelerate the lithiation reaction. Also the interval between the multilayer structures of CuO nanoplates electrode could provide enough interior space to accommodate the volume change during Li+ insertion and de-insertion process. This synthetic strategy might also be extended to the synthesis other nanostructured transition-metal oxides for use in energy storage and conversion.Acknowledgments
This work was supported by the National Key Research and Development Program of China (No. 2016YFB0601100) and the Fundamental Research Funds for the Central Universities (No. FRFBD-16-008A).Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2017.09.064.
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