Applied Surface Science 463 (2019) 986–993
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Binding ZnO nanorods in reduced graphene oxide via facile electrochemical method for Na-ion battery ⁎
Mingjun Jinga, , Fangyi Lia, Mengjie Chena, Fengliang Longc, Tianjing Wua,b,
College of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang 414006, China College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China c College of Automotive Engineering, Hunan Industry Polytechnic, Changsha 410007, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO nanorods/reduced graphene oxide CeOeZn bridge bond Sodium-ion battery Electrochemical dispersion
Alternating voltage electrochemical dispersion, as a green and facile electrochemical technique, has been successfully developed to design ZnO nanorods/reduced graphene oxide (ZnO/RGO) composite. The morphology of ZnO can be induced to form rod-like structure in this composite via the introduction of GO in the electrolyte solution. Meanwhile, GO can be reduced to RGO during the electrochemical process without adding extra reducing agent, which can enhance the electrical conductivity of ZnO/RGO composite and accommodate the volume change of ZnO. Moreover, ZnO nanorods are well embedded on the surface of RGO with the present of CeOeZn bridge bonds. This CeOeZn formed in the electrochemical preparation can be helpful to contact ZnO nanorods and RGO, which accelerate electron transport rate and enhance structural stability of ZnO/RGO electrode. As-prepared ZnO/RGO composite as an anode for sodium-ion batteries (SIBs) displays much higher capacity, rate performance and cycling behavior than those of pure ZnO material, especially shows ultra-long cycle life (91.9% capacity retention at 500 mA g−1 after 1000 cycles).
1. Introduction Sodium-ion batteries (SIBs) have gained tremendous attention, which is attributed to their potential low cost and the similar electrochemical working principles as lithium-ion batteries (LIBs) [1–3]. Increasing numerous negative electrode materials have been investigated for SIBs, such as carbon material, p-block elements (phosphorus/ phosphide, metals and alloys), metal oxides, metal sulﬁdes, metal selenides, and so on [4–7]. Transition metal oxides based on conversion reaction mechanism are considered as promising electrode materials with large-scale energy storage [7,8]. ZnO as a typical metal oxide has the merits of abundance in nature, low cost, environmental benignity and excellent electrochemical properties, which has been proven to be one of talented anode candidates for LIBs and SIBs [9,10]. Noting that swelling and shrinking of metal oxide particles associated with Na+ insertion and extraction can induce generate tremendous microstructural damage of electrodes, which subsequently lead to rapidly capacity fading [11,12]. Hence, the pure ZnO material usually exhibits poor cycling stability and rate behavior. Three strategies have been widely proposed to solve afore mentioned problems. One is to reduce the particle size, which can shorten the diﬀusion path of Na+. Due to the larger diameter of sodium ion
(1.02 Å vs Li 0.67 Å), this approach is more important for sodium storage [13–15]. The second way is to design unique morphologic structures, such as hierarchical porous microspheres, nanoﬁbers, ﬂowers, hollow microspheres, nanosheets and so on, which can eﬀectively ameliorate the large volume variation [16–20]. The third is to construct composite materials, typically introducing various carbon-conducting agents (including graphene, carbon nanotube, porous carbon and so on) [21–25]. Especially, graphene has been proved to be a talented carbon agent because of its large speciﬁc surface area and high electronic conductivity, which can eﬀectively improve the electron transport and remit the volume change. Based on the previous studies of other metal oxides [26–29], ZnO/graphene composite might become a promising candidate anode for SIBs. Moreover, previous reports have pointed out the “synergistic effects” among various composite materials attributed to their strong “interfacial interaction” [30,31]. Guo et al reported that the SneNeC and SneOeC bonds between SnO2 nanocrystals and N-RGO can be conductive to the high and stable electrochemical performance for LIBs . Li et al. studied the oxygen bridges (CeOeNi) between NiO nanosheets and graphene, which could facilitate fast electronic transmission and improve the reversible lithium storage . Additionally, the chemical bonding can be also considered as a potentially inspirable
Corresponding authors. E-mail addresses: [email protected]
(M. Jing), [email protected]
https://doi.org/10.1016/j.apsusc.2018.09.038 Received 19 July 2018; Received in revised form 4 September 2018; Accepted 5 September 2018 Available online 06 September 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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2.2. Material characterization
strategy for SIBs, such as P-O-C in P-CNT hybrid , Sb-N-C in Sb-CNC material , Mo-O-C in [email protected]
@C composite and so on . This synergistic eﬀect from chemical bonding can be eﬀectively to enhance structural stability and accelerate electron transport rate. Herein, designing ZnO/Carbon hybrid with strong interfacial interaction become a promising method to obtain outstanding electrochemical performance for SIBs. Alternating voltage electrochemical dispersion, as a new, green, simple, and high productivity electrochemical method, has been successfully adopted to obtain various kinds of metal oxide nanocrystals (NiO, CuO, TiO2, Mn3O4, etc) [35–37]. This approach exhibits facile processing steps under room temperature without extra reducing or oxidizing reagents, which is mainly based on metal dispergation under alternating current . In our previous report, we found that C dots containing oxygenic groups (CeO, C]O) are beneﬁt to capture Mn3O4 nanocrystals during the alternating voltage electrochemical process . So, it might be a talented technique to bind ZnO on graphene through alternating voltage electrochemical dispersion with utilizing graphene oxide (GO) as carbon source. In this paper, ZnO nanorods/reduced graphene oxide (ZnO/RGO) composite has been successfully prepared utilizing alternating voltage through in-situ introducing GO in NaCl electrolyte solution. Interesting note that GO has been reduced into RGO during the preparation process without adding any reducing reagent. And we found that ZnO nanorods are attached strongly to RGO via CeOeZn bridge bonds. Moreover, the pure ZnO products at various alternating voltages were further studied to explore the electrochemical formation mechanism of ZnO and ZnO/ RGO samples. Compared with the pure ZnO, ZnO/RGO hybrid as anode for SIBs displays higher speciﬁc capacity, rate behavior and cycling stability for its mesoporous structure and oxygen bridges.
X-ray diﬀraction (XRD) was used to illustrate the crystal patterns of samples on a Rigaku D/max X-ray diﬀractometer at 0.1° 2θ s−1 with Cu Kα radiation. Scanning electron microscopy (SEM, JSM-6510LV), transmission electron microscopy (TEM, JEOL JEM-2100F) and highresolution transmission electron microscopy (HRTEM, JEOL JEM2100F) have been utilized to investigate the morphologies of samples. Fourier transform infrared (FT–IR) spectra were tested on FT–IR spectrophotometer (AVTATAR, 370) with KBr as a reference. Raman spectra of samples were measured on the Thermo Scientiﬁc DXR Raman instrument (USA) working at 15 mW with 532 nm excitation. Also, the carbon content of composite based on thermogravimetric analysis (TGA) was obtained on a thermal analysis instrument (NETZSCH, STA449F3) from room temperature to 800 °C in air with a heating rate of 10 °C min−1. Based on Brunauer-Emmett-Teller (BET) multipoint method and Barret-Joyner-Halenda (BJH) model, the speciﬁc surface area and pore size distribution were determined by nitrogen physisorption at 77 K (BET, Micromeritics, ASAP 2020), respectively. The surface chemical composition of sample was analysed through the X-ray photoelectron spectroscopy (XPS, ESCALAB 250 spectrometer). 2.3. Electrochemical measurements The electrodes were prepared with the composition of active material, super P and carboxymethyl cellulose (weight ratios of 70:15:15). The detailed process is reformulated as follows. Carboxymethyl cellulose of 15 mg was ﬁrstly dissolved in deionized water with stirring for 24 h. And then the mixture contained active materials and super P were added into carboxymethyl cellulose solution to form homogeneous slurry. Finally, the electrodes were obtained through drying in a vacuum oven at 100 °C for 12 h. The coin half-cells (CR2016) were assembled in an Ar-ﬁlled glove box, utilizing metallic sodium foils as counter electrodes, porous polypropylene ﬁlm as separator, and NaClO4 in ethylene carbonate and propylene carbonate system (with a volume ratio of 1:1) as electrolyte solution. Cyclic voltammetry (CV) curves were tested at a scanning rate of 0.2 mV s−1 from 0.01 to 3.0 V (vs Na+/Na) on MULTI AUTOLAB M204 (MAC90086). Galvanostatic discharge/charge ﬁles were further conducted on Land CT2001A battery cycler. Electrochemical impedance measurements (EIS) with the frequency range from 100 kHz to 0.01 Hz were investigated on CHI 660B electrochemical working station at the open–circuit voltages.
2. Experimental section 2.1. Preparation of materials Synthesis of pure ZnO: The electrochemical cell was composed of two Zn wires electrodes (purity 99.9%, diameter of 0.5 mm) and 3 M NaCl as electrolyte solution. Then, alternating voltage (5 V, with the conversion frequency speed of 50 Hz) was applied to the two Zn electrodes, utilizing a YK-BP81005 regulator transformer. Accordingly, ﬂocculent substances started to diﬀuse away from the surface of both Zn electrodes. After 5 h alternating current action, ZnO white powder material was obtained through rinsing with distilled water, and then dried at 100 °C for 12 h in a vacuum. Aiming to declare the electrochemical formation of ZnO, the inﬂuence of diﬀerent voltage (3 V and 8 V) were also discussed under the same experimental conditions except the value of alternating voltage. Synthesis of ZnO/RGO composite: Firstly, the graphene oxide (GO) was conveniently prepared by an improved Hummers’ method . Then, the as-prepared GO was dispersed in 3 M NaCl water solution. Two Zn wires electrodes were immersed in this electrolyte solution with 5 V alternating voltage. After the similar process of pure ZnO, black ZnO/RGO sample was also successfully prepared.
3. Results and discussion 3.1. The characterization of structure, morphology and composition The powder diﬀraction peaks of as-obtained GO, ZnO and ZnO/RGO are displayed in Fig. 1a. All the diﬀraction peaks at 31.5°, 34.3°, 36.1°, 47.4°, 56.5°, 62.7°, 67.8°, 68.7°, 72.4°, and 76.7° of the samples refer to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2), (2 0 1), (0 0 4) and (2 0 2) planes of hexagonal ZnO phase (JCPDS Card no. 36-1451),
Fig. 1. (a) XRD patterns of GO, ZnO and ZnO/RGO samples. (b) and (c) SEM images of ZnO and ZnO/RGO samples, respectively. 987
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Fig. 2. (a) and (c) TEM and HRTEM images of as-obtained ZnO. (b) and (d) TEM and HRTEM images of ZnO/RGO composite.
Fig. 3. (a) FT–IR spectra of GO and ZnO/RGO. (b) Raman spectra of GO and ZnO/RGO. (c) TGA curve of ZnO/RGO. (d) Nitrogen adsorption and desorption isotherm of ZnO/RGO (the inset is BJH pore size distribution curve).
during the electrochemical process . In addition, the average sizes of ZnO in pure ZnO and ZnO/RGO samples are around 36.5 nm and 37.2 nm, respectively, based on the FWHM of (1 0 1) diﬀraction peaks via Scherrer equation [37,39]. Fig. 1b and c show the SEM images of pure ZnO and ZnO/RGO composite. The pure ZnO sample is composed of nanorods and irregular nanoparticles. While, the ZnO structure is mainly one-dimensional nanorod in the as-prepared ZnO/RGO composite, and ZnO nanorods are well dispersed on the surface of RGO
respectively. No other diﬀraction peaks appear in the ZnO sample, illustrating a high pure ZnO has been got without impurities. Compared with the ﬁles of ZnO and ZnO/RGO, the diﬀraction peak positions of ZnO/RGO composite are in accordance with pure ZnO, indicating the crystal phase of ZnO in composite has not been changed with the introduction of GO. It is noting that the peak at 11.5° (corresponding to the typical diﬀraction of GO) has not been distinguished in ZnO/RGO composite, which might be mainly attributed to the reduction of GO 988
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Fig. 4. (a) XRD patterns of samples obtained with the alternating voltage at 3 V, 5 V and 8 V. SEM images of products obtained at 3 V (b), 5 V (c) and 8 V (d). (e) Schematic illustration for the electrochemical preparation of pure ZnO and ZnO/RGO samples.
about 3432 cm−1 is attributed to the O-H stretching modes of H-bound OH groups and the interlayer water molecules . And the peak at 1631 cm−1 is allocated to the bending mode of water molecules . The peak at 1733 cm−1 in GO sample is due to the vibrational bond from C]O groups . It is worth noting that this peak almost disappeared in the ZnO/RGO composite, indicating that the GO has been turned into RGO via the process of electrochemistry. The CeO stretching vibration is also identiﬁed in samples, corresponding to the peak at 1077 cm−1 . Noting that this peak at 1077 cm−1 in ZnO/ RGO composite is obviously stronger than that in GO, illustrating CeOeZn bond could be formed between ZnO and RGO. Raman analysis of GO and ZnO/RGO composite has been carried out, which is presented in Fig. 3b. The D and G bands of GO at 1351 and 1596 cm−1 are due to the defects and disordering atomic arrangements of sp3 carbon and the plane vibration of sp2 carbon atoms of 2D layer . The ID/IG intensities of GO and ZnO/RGO samples are 0.79 and 0.90, respectively. This increase indicates the reduction of GO to RGO, agreeing well with the FT-IR results. Moreover, the peaks positions of D and G bands in ZnO/RGO composite are obviously shifted, which might be mainly attributed to the strong interaction and better combination of ZnO on the surface of RGO [38,42].
nanosheets. The morphology structures of as-obtained samples were further investigated by TEM and HRTEM, which are presented in Fig. 2. Both nanorods and irregular nanoparticles (with the average size of ∼38 nm) exist in the pure ZnO sample (Fig. 2a). Moverover, the lattice image of pure ZnO is shown in Fig. 2c. The lattice fringe of 0.251 nm is linked to (1 0 1) face of ZnO crystal, agreeing well with the results of XRD and SEM. As shown in Fig. 2b, numerous of nanorods with the average length of ∼50 nm and diameter of ∼20 nm are well dispersed on the surface of RGO. ZnO nanorods, with the lattice fringes of 0.251 nm and 0.166 nm corresponding to the (1 0 1) and (1 1 0) faces of ZnO, could be clearly conﬁrmed via HRTEM image in Fig. 2d. Also, the thin layer RGO marked with blue circle1 line exists in ZnO/RGO composite, illustrating the composite is composed of ZnO and RGO. The FT–IR spectra of GO and ZnO/RGO samples are shown in Fig. 3a. A strong distinctive peak at 464 cm−1 in ZnO/RGO composite is assigned to the characteristic of Zn-O stretching . The wide peak at
1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
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Fig. 4a states the crystal patterns of samples that are prepared at 3 V, 5 V and 8 V. It can be seen that pure ZnO material can be both got with alternate voltage at 5 V and 8 V. While the product is composed of ZnO and Zn(OH)2 with alternate voltage at 3 V. With the increase of alternate voltage, the size of product become large. Based on the above results and previous report , the electrochemical mechanism of pure ZnO and ZnO/RGO samples have been shown in Fig. 4e. It can be proposed that the Zn atoms on the surface of Zn electrode could be ﬁrstly oxidized to high valence Zn oxide (ZnOx) ﬁlm during anodic process. Then, under a negative half cycle stage, high valence ZnOx ﬁlm could be reduced into Zn(OH)x, and further transformed into stable ZnO nanocrystals. Finally, the ZnO nanocrystals are swept away from the electrode surface and transformed ZnO particles or nanorods via imperfect oriented attachment mechanism. In addition, GO sample can be as a reductant to promote the reduction process of ZnOx ﬁlm, and also could be beneﬁting to capture the as-formed ZnO nanocrystals. The formed ZnOx ﬁlm could be easily tend to form stable ZnO nanocrystals with addition of GO in the NaCl solution. Finally, the ZnO nanocrystals self-assembly grow into ZnO nanorods, which can be well embedded on the surface of RGO. Moreover, numerous of oxygen-containing functional groups of GO can be reduced in the strong alternating electric ﬁeld. Hence, it is easy to conclude that CeOeZn bond can be formed during the preparation of this composite. The surface chemical status of ZnO/RGO composite have been measured by XPS, which is presented in Fig. 5. In Fig. 5a, two peaks at 1021.9 eV and 1044.9 eV are assigned to Zn 2p3/2 and Zn 2p1/2, respectively, suggesting the presence of Zn2+ . The O 1s spectrum is shown in Fig. 5b, which contains four ﬁtted peaks. The major peak at 530.7 eV is indexed to ZneO bond . While two weak peaks at 533.4 eV and 535.3 eV are assigned to the CeO and OeH from the surface functional groups of RGO, respectively . It is stressing that the peak at 532.1 eV corresponds to CeOeZn and C]O, conﬁrming again the existence of strong interaction between ZnO and RGO [30,34]. Also, the C 1s spectrum can be decomposed into four ﬁtted peaks, as stated in Fig. 5c. A leading peak at 284.6 eV is attributed to C]C/CeC, while the peak at 282. 6 eV is assigned to CeOH/CeOZn . Moreover, the weak peaks at 287.4 eV for C]O and 289.0 eV for OeC]O are from the oxygen-containing functional groups, illustrating the GO sample has been almost reduced to graphene [28,34]. Based on the analysis of FT-IR, Raman and XPS, we can conclude that C-O-Zn bond has been formed between ZnO and RGO in the composite. This strong interaction might be helpful to improve the structural stability and the electron transport rate of ZnO/RGO, likely to some other MOx/ Carbon materials contained the similar model of interactions [30,47]. Therefore, the designed ZnO/RGO composite with strong interaction and mesoporous structure could display satisfactory electrochemical performance for SIBs. Fig. 5. XPS high-resolution spectrum (a) Zn 2p, (b) O 1s and (c) C 1s of ZnO/ RGO.
3.2. The electrochemical performances of samples
Fig. 3c shows the TGA curve of as-prepared ZnO/RGO material from 25 to 800 °C under air. The mass loss from 25 to 350 °C is mainly owing to the losing coordinated and adsorbed water . Then, the RGO began to disappear. After 700 °C, the weight is invariant. Hence, the weight content of ZnO in ZnO/RGO composite has been determined to be around 85.1 wt%. In addition, the speciﬁc surface area of ZnO/RGO sample is ascertained to 65.2 m2 g−1 based on the N2 adsorption/desorption isotherm in Fig. 3d. The inset of Fig. 3d states the pore distribution is mainly centred at 3.2 nm according to Barrett-Joyner-Halenda (BJH) pore size distribution data. This mesoporous structure might be beneﬁcial for accelerating the rate of ion transmission [13,43]. To clearly indicate the formation of ZnO from Zn electrode, the products obtained at various alternate voltage also have been investigated. Their XRD and SEM results are presented in Fig. 4a–d.
The electrochemical activities of ZnO and ZnO/RGO as anodes for SIBs were evaluated via using half-cells. Fig. 6a displays CV curves of ZnO/RGO, operated with the potential range of 0.01–3.0 V (vs Na+/ Na) at a scanning rate of 0.2 mV s−1. In the ﬁrst cathodic scan, two broad peaks at around 1.2 V and below 0.75 V are related to the reduction of ZnO → Zn, the alloying of Zn → NaZnx, and the formation of a solid electrolyte interphase (SEI) layer . In the following anodic scan, a broaden peak from 0.75 to 1.75 V is ascribed to the NaZnx → Zn and Zn → ZnO. In the second CV cycle, it shows a wide reduction peak at about 0.72 V, which is diﬀerent from the ﬁrst cathodic scan because of the irreversible phase transformation. Compared with the CV curves of ZnO previous reported, we ﬁnd that the redox peaks of ZnO/RGO become obviously broaden. This phenomenon also appeared in other composite, especially with interfacial interaction. The broaden phenomenon might be mainly attributed to the strong binding interaction through CeOeZn between ZnO and RGO. The third CV ﬁle and the 990
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Fig. 6. (a) CV ﬁles of the ﬁrst three cycles at a scanning rate of 0.2 mV s−1 for ZnO/RGO. (b) Charge-discharge curves of the ﬁrst two cycles at a current density of 100 mA g−1 for ZnO/RGO. (c) Rate performance of ZnO and ZnO/RGO electrodes at various current densities. (d) The discharge speciﬁc capacities of ZnO and ZnO/RGO at various current densities of 100, 200, 400, 800 and 1600 mA g−1.
Fig. 7. (a) Cycling behavior of ZnO and ZnO/RGO electrodes at 100 mA g−1. (b) Nyquist plots for ZnO and ZnO/RGO electrodes. (c) Cycling stability of ZnO/RGO at a high current density of 500 mA g−1. (d) Schematic diagram of electron hopping from RGO to ZnO.
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second ﬁle are overlapped well, which indicate that the ZnO/RGO electrode shows good electrochemical reversibility. Furthermore, the charge/discharge curves of this composite were further measured at a current density of 100 mA g−1, as shown in Fig. 6b. The ﬁrst discharge capacities of as-prepared ZnO/RGO is 376.4 mAh g−1. While the second discharge capacity become obviously lower, which is mainly attributed to the volume expansion and the formation of SEI ﬁlm. The rate performances of ZnO and ZnO/RGO samples are presented in Fig. 6c and d. Fig. 6c displays that the ﬁrst coulombic eﬃciencies are 48.6% and 55.0% for ZnO and ZnO/RGO at a current density of 100 mA g−1, respectively. This result suggests that ZnO/RGO electrode presents much better electrochemical reversibility than pure ZnO electrode in SIBs application. In Fig. 6d, the reversible discharge capacities of ZnO/RGO composite are 195.8, 159.2, 134.3, 114.2 and 98.7 mAh g−1 at varies current densities of 100, 200, 400, 800 and 1600 mA g−1, respectively. While, discharge capacities of ZnO electrode are 121.4, 89.1, 66.8, 47.6 and 31.6 mAh g−1 at varies current densities of 100, 200, 400, 800 and 1600 mA g−1, respectively. Compared to the pure ZnO, the rate capability of ZnO/RGO electrode is much superior. As shown in Fig. 6c, the speciﬁc capacity of ZnO/RGO electrode can return to 189.5 mAh g−1, when the current density goes back to 100 mA g−1. All these results indicate that ZnO/RGO composite with porous structure and strong interfacial interaction shows good electrochemical reversibility and excellent rate behaviour. The cycling stabilities of ZnO and ZnO/RGO electrodes have been evaluated at 100 mA g−1, and the results are shown in Fig. 7a. After 150 cycles, the discharge capacities of ZnO and ZnO/RGO are 86.8 and 188.6 mAh g−1, respectively. Obviously, the ZnO/RGO electrode displays much higher speciﬁc capacity and cycling stability than those of ZnO electrode. To further illustrate the distinct electrochemical properties, EIS analysis conducted on ZnO and ZnO/RGO electrodes is shown in Fig. 7b. The Nyquist plots of electrodes were carried out at their open-circuit voltage after 150 cycles at 100 mA g−1. The semicircle shape is mainly related to charge-transfer resistance (Rct) . The Rct of ZnO/RGO is much smaller than that of ZnO, indicating the charge transfer process can be obviously boosted during electrode electrochemical reactions with the incorporation of RGO. It is stressing that the low-frequency slope angle of ZnO/RGO electrode is about 78.5°, higher than that of ZnO electrode 76.8°. The steeper low-frequency tail illustrates better sodium ion diﬀusion rate in the electrode materials. Hence, the ZnO/RGO composite exhibits higher electrical conductivity, sodium ion diﬀusion and charge-transfer process, which contribute to possess outstanding high-rate performance and cycling stability compared to pure ZnO sample. Moreover, the cycling stability is further evaluated at 500 mA g−1, which is displayed in Fig. 7c. It is stressing that the about 91.1% of the capacity retention has been kept after 1000 cycles at 500 mA g−1. And the coulombic eﬃciency of is almost above 98.2%, illustrating splendid electrochemical reversibility of ZnO/RGO electrode. All these better electrochemical performances of ZnO/RGO electrode for SIBs might be mainly attributed to the microstructure and strong interaction in this composite, especially the synergistic eﬀects between ZnO and RGO (as shown in Fig. 7d). In this ZnO/RGO composite, one-dimensional ZnO nanorods and mesoporous structure can promote the rates of electron transfer and ionic diﬀusion. Also, the as-formed RGO of this composite can be favorable to increase the electrical conductivity and relieve the large volume expansion during the cyclical processes. More importantly, the interaction between ZnO and RGO through CeOeZn bridge bonds could be beneﬁcial to facilitate fast electronic transmission, overcome the aggregation of ZnO nanoparticles during the preparation and intercalation/deintercalation, and improve the reversible sodium storage. Meaningfully, the binding in this metal oxide/graphene composite might be extended to many other composite materials to pursue optimal performance for SIBs or other energy storage ﬁelds.
4. Conclusions In summary, alternating voltage electrochemical dispersion has been successfully developed to prepare ZnO/RGO composite at room temperature. It is interesting to note that the ZnO nanocrystals are easy to grow into one-dimenstional nanorods with the existence of GO in electrolyte solution. Meanwhile, the GO can be reduced to RGO during the electrochemical process without adding extra reducing agent. It is stressing that a numerous of CeOeZn bridge bonds between ZnO and RGO are formed in this composite utilizing the alternating voltage electrochemical method. The synergistic eﬀects in ZnO/RGO composite from CeOeZn bridge bonds between ZnO and RGO is beneﬁcial to facilitate electron hopping from RGO to ZnO and stabilize the structure of material. The designed ZnO/RGO composite as anode for SIBs displays higher capacity and rate behavior than those of pure ZnO sample. Even after 1000 cycles, the capacity retention can maintain 91.1% at 500 mA g−1. Owing to its above unique features, ZnO/RGO composite might become a talented electrode material candidate for high performance SIBs. Remarkably, electrochemical dispersion technique utilizing alternating voltage is a green and simple approach that can be further stretched to much more metal oxide/graphene materials. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21701044, 21601057, 51772092), Natural Science Foundation of Hunan Province China (Grant No. 2017JJ3097), Research Foundation of Education Bureau of Hunan Province, China (Grant no. 17A086), and National Training Programs of Innovation and Entrepreneurship for University Students of China (201710543004). References  V. Palomares, M. Casas-Cabanas, E. Castillo-Martínez, M.H. Han, T. Rojo, Update on Na-based battery materials. A growing research path, Energy Environ. Sci. 6 (2013) 2312–2337.  A. Firouzi, R. Qiao, S. Motallebi, C.W. Valencia, H.S. Israel, M. Fujimoto, L.A. Wray, Y.-D. Chuang, W. Yang, C.D. Wessells, Monovalent manganese based anodes and cosolvent electrolyte for stable low-cost high-rate sodium-ion batteries, Nat. Commun. 9 (2018) 861.  J.Z. Guo, Y. Yang, D.S. Liu, X.L. Wu, B.H. Hou, W.L. Pang, K.C. Huang, J.P. Zhang, Z.M. Su, A practicable Li/Na-ion hybrid full battery assembled by a high-voltage cathode and commercial graphite anode: superior energy storage performance and working mechanism, Adv. Energy Mater. (2018) 1702504.  H. Hou, C.E. Banks, M. Jing, Y. Zhang, X. Ji, Carbon quantum dots and their derivative 3D porous carbon frameworks for sodium-ion batteries with ultralong cycle life, Adv. Mater. 27 (2015) 7861–7866.  J. Qian, X. Wu, Y. Cao, X. Ai, H. Yang, High capacity and rate capability of amorphous phosphorus for sodium ion batteries, Angew. Chem. Int. Ed. 52 (2013) 4633–4636.  T. Wu, C. Zhang, H. Hou, P. Ge, G. Zou, W. Xu, S. Li, Z. Huang, T. Guo, M. Jing, X. Ji, Dual functions of potassium antimony(III)-tartrate in tuning antimony/carbon composites for long-life Na-ion batteries, Adv. Funct. Mater. (2018) 1705744.  Y. Zhao, L.P. Wang, M.T. Sougrati, Z. Feng, Y. Leconte, A. Fisher, M. Srinivasan, Z. Xu, A review on design strategies for carbon based metal oxides and sulﬁdes nanocomposites for high performance Li and Na ion battery anodes, Adv. Energy Mater. 7 (2017) 1601424.  Y. Zhang, C.W. Foster, C.E. Banks, L. Shao, H. Hou, G. Zou, J. Chen, Z. Huang, X. Ji, Graphene-rich wrapped petal-like rutile TiO2 tuned by carbon dots for high-performance sodium storage, Adv. Mater. 28 (2016) 9391–9399.  C. Wang, Y. Gong, B. Liu, K. Fu, Y. Yao, E. Hitz, Y. Li, J. Dai, S. Xu, W. Luo, E.D. Wachsman, L. Hu, Conformal, nanoscale ZnO surface modiﬁcation of garnetbased solid-state electrolyte for lithium metal anodes, Nano Lett. 17 (2017) 565–571.  G.H. An, D.Y. Lee, H.J. Ahn, Tunneled mesoporous carbon nanoﬁbers with embedded ZnO nanoparticles for ultrafast lithium storage, ACS Appl. Mater. Interfaces 9 (2017) 12478–12485.  Y. Jiang, M. Hu, D. Zhang, T. Yuan, W. Sun, B. Xu, M. Yan, Transition metal oxides for high performance sodium ion battery anodes, Nano Energy 5 (2014) 60–66.  T. Li, A. Qin, L. Yang, C. Jie, Q. Wang, D. Zhang, H. Yang, In situ grown Fe2O3 single crystallites on reduced graphene oxide nanosheets as high performance conversion anode for sodium-ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 19900–19907.  H. Hou, L. Shao, Y. Zhang, G. Zou, J. Chen, X. Ji, Large-area carbon nanosheets doped with phosphorus: a high-performance anode material for sodium-ion batteries, Adv. Sci. 4 (2017) 1600243.
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