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RGO composite as an anode material for lithium-ion batteries
Author’s Accepted Manuscript Fabrication of NiO-ZnO/RGO composite as an anode material for lithium-ion batteries Liang Ma, Xian-Yinan Pei, Dong-Chuan Mo, ShuShen Lyu, Yuan-Xiang Fu www.elsevier.com/locate/ceri
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S0272-8842(18)32507-0 https://doi.org/10.1016/j.ceramint.2018.09.044 CERI19443
To appear in: Ceramics International Received date: 25 July 2018 Revised date: 4 September 2018 Accepted date: 5 September 2018 Cite this article as: Liang Ma, Xian-Yinan Pei, Dong-Chuan Mo, Shu-Shen Lyu and Yuan-Xiang Fu, Fabrication of NiO-ZnO/RGO composite as an anode material for lithium-ion batteries, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of NiO-ZnO/RGO composite as an anode material for lithium-ion batteries Liang Ma, Xian-Yinan Pei, Dong-Chuan Mo, Shu-Shen Lyu, Yuan-Xiang Fu* School of Chemical Engineering and Technology, Sun Yat-sen University Guangzhou 510275, P. R. China. * E-mail: [email protected]
Abstract： NiO-ZnO/RGO composite was obtained by the annealing of an Ni (OH)2-Zn (OH)2/RGO precursor, which has been fabricated by in situ ultrasonic agitation. Moreover, the NiO-ZnO nanoflakes are evenly distributed on the RGO sheets based on the scanning electron microscope (SEM) and transmission electron microscope (TEM) characterization results. When the NiO-ZnO/RGO composite was used as an anode material in lithium-ion batteries (LIBs), the electrodes exhibited a high reversible capacity of 1017 mA h/g at a current density of 100 mA/g after 200 cycles and a specific capacity of 458 mA h/g at 500 mA/g even after 400 cycles. The electrode even reached a capacity of 185 mA h/g at a current density of 2000 mA/g. The excellent electrochemical properties of the NiO-ZnO/RGO composite might be attributable to the NiO-ZnO nanoflakes offering rich electrochemical reaction sites and shortening the diffusion length for lithium ion (Li+), as well as the RGO sheets improving the transfer rates of Li+ and electron during the charge-discharge process.
Keywords: NiO-ZnO/RGO composite, porous, anode material, lithium-ion batteries.
1． Introduction Owing to their high reversible capacity, good rate capability, and long cycle life, lithium-ion batteries (LIBs) have been widely used as power sources for portable electronics and electric vehicles [1]. However, the increasing demand for high-performance electronic devices and hybrid electric vehicles has promoted much
research aimed at exploiting LIBs with a high power density and better long-term cycling performance [2]. Development of high-performance anode materials is considered to be an effective strategy [3]. A fact largely arising as a result to achieve this end, commercial graphite has a relatively low theoretical capacity of 372 mA h/g [4]. In recent years, transition metal oxides [5-13], especially nickel-based oxides, have led scholars to investigate such new materials as promising anode candidates in terms of their high theoretical capacity, good cycling performance, high recharge rates, environmentally friendly nature, and low-cost [14-21]. Moreover, binary nickel-based oxides
(e.g.,
CoO/NiO,
Fe3O4/NiO,
CuO/NiO,
ZnO/NiO,
SnO2/NiO,
and
NiO/NiFe2O4) displayed even better lithium storage capacity than that of pure NiO because of the synergetic effect found during the charge-discharge process in early reports [14-17, 22, 23]. Thus, binary NiO based oxides offer better practical application potential in LIBs as anode materials. Unfortunately, there are still some drawbacks in the application of such NiO materials as anodes in LIBs. These drawbacks can be especially linked to their poor electronic conductivity and structural changes during repeated cycling, resulting in poor rate capability and rapid capacity fading, respectively [24, 25]. Thus, to obtain high performances from binary NiO anode materials during the charge-discharge process, it is crucial that the electrodes maintain their good conductivity and low volume change. Owing to its high conductivity, superior thermal conductivity, and strength [26-29], graphene has aroused much interest among scholars as a two-dimensional carbon material. Various metal oxide/graphene composites have been fabricated as anode materials for LIBs, and such composites exhibited excellent comprehensive electrochemical properties compared with those of pure metal oxides [13, 30-32]. Taking into consideration the lithium storage capacity of binary nickel-based oxides compared to that of NiO [17, 30, 33, 34], fabricating binary nickel-based oxide/graphene hybrid materials is an effective strategy with which to obtain anode materials for LIBs. Meanwhile, to the best of our knowledge, until now there have been few reports of any anode material integrating the advantages of NiO-ZnO and graphene as anode materials for LIBs [34].
NiO-ZnO/RGO composite was fabricated by the annealing of nickel hydroxide– zinc hydroxide/GO precursor (Ni (OH)2-Zn (OH)2/GO), which was prepared by in situ ultrasonic agitation. As expected, the NiO-ZnO/RGO displays a high reversible capacity of 1017 mA h/g at a current density of 100 mA/g after 200 cycles, and a reversible capacity of 458 mA h/g is obtained at a current density of 500 mA/g even after 400 cycles when used as an anode material for a LIB. This result is largely attributable to the synergetic effects of NiO-ZnO and graphene sheets.
2. Experimental work 2.1 Fabrication of GO Graphene oxide (GO) was produced using an improved Hummers method with some adjustments [35, 36]. In brief, 2 g natural flake graphite was added into a solution of 180 ml H2SO4 with 20 ml H3PO4, and then 9 g KMnO4 was slowly added under stirring. The temperature of the mixture was kept at 50 °C for 2 h by use of a thermostatically-controlled water bath. Afterward, 200 ml deionized water and 4 ml 30 wt% H2O2 solution were successively dropped into the aforementioned solution. GO powder was then collected after centrifugation, washed several times with 30% HCl aqueous solution, ethanol, then deionized water, and freeze-dried.
2.2 Fabrication of NiO-ZnO/RGO The NiO-ZnO/RGO composite was synthesized by in situ ultrasonic agitation. GO powder (60 mg), and 60 mg polyvinyl pyrrolidone (PVP) were added into 60 ml deionized water under ultrasonication for 45 min. Subsequently, 1.45 g Ni (NO3)2·6H2O and 1.48g Zn (NO3)2·6H2O were successively added into the GO solution under constant stirring to form a homogeneous solution. Then, 6 ml of 25% aqueous ammonia was added into the as-prepared solution with mechanical stirring and ultrasonic treatment for 2 h. The Ni (OH)2-Zn (OH)2/GO precursor was obtained after being washed with distilled water several times, and then freeze-dried for 12 h in a lyophilizer. The final NiO-ZnO/RGO composite was produced by calcination of the Ni (OH)2-Zn (OH)2/GO precursor at 350 °C for 4 h in a nitrogen atmosphere (N2), and the
tap density of the NiO-ZnO/RGO tested is about 1.4 g/cm3. For comparison, the NiO and NiO/RGO were also prepared by the same procedure, while the NiO-ZnO was prepared without the GO sheets. 2.3 Characterization The samples were characterized by field-emission scanning electron microscopy (SEM) on a FEI FEI-Q400F; transmission electron microscope (TEM) on a JEOL JEM-2010 HR operating at 200 kV; powder X-ray diffraction (XRD) on a Rigaku D-MAX 2200 VPC X-ray diffractometer with Cu Ka radiation; and thermogravimetric (TG) analysis on a Netzsch 209 F3 Tarsus thermal analysis (TGA) apparatus from room temperature to 850 °C. X-ray photoelectron spectroscopy (XPS) measurements were -ray source, and N2 adsorption/desorption isotherms were obtained by a Micromeritics ASAP 2460 at a temperature of 77 K.
2.4 Electrochemical measurements The electrochemical performances of all samples were measured using CR-2032-type coin-type half cells. The working electrodes were prepared by 75 wt % active material, 15 wt% conductive carbon black additive, and 10 wt% polyvinylidene fluoride (PVDF). In the presence of a certain amount of N-methyl-2-pyrrolidone (NMP), these materials were uniformly mixed to produce a homogeneous slurry. The slurry was then painted on a copper foil current collector and dried at 120 °C for 10 h in a vacuum oven. The cells were assembled in an argon-filled glove box using the as-prepared working electrodes (~1.5 mg/cm2), a lithium foil as the counter-electrode, a Celgard 2400 membrane as the separator, and a mixture of 1M LiPF6 with ethylene carbonate (EC) and dimethyl carbonate (DME) (1:1, volume ratio) as the electrolyte. Galvanostatic charge/discharge tests were performed on the LAND CT2001A battery test system in the range of 0.01 to 3.0 V (v. Li/Li+) at different rates at room temperature. Cyclic voltammetric (CV) and electrochemical impedance spectrum (EIS) measurements were undertaken using an electrochemical workstation (Chenhua,
CHI660E) with a scan rate of 0.1 mV/s between 0.01 and 3 V (v. Li/Li+). 3. Results and Discussion The preparation process of the NiO-ZnO/RGO is illustrated in Fig. 1, Detailed experimental steps are described in the section covering fabrication of NiO-ZnO/RGO. The XRD pattern of the Ni (OH)2-Zn (OH)2/GO precursor shows the diffraction peaks of Ni (OH)2 (weak) and Zn (OH)2 [37, 38]. Moreover, a weak XRD pattern of GO also was observed at 2θ≈9.8° in Fig. S1a† [35, 36]. However, the precursor exhibited a typical sheet-like morphology in both the low- and high-magnification SEM images shown in Fig. S2†.
Fig. 1 Schematic showing the preparation of NiO-ZnO/RGO composite
NiO-ZnO/RGO composite was obtained by the annealing of Ni (OH)2-Zn (OH)2/GO in inert gas. The X-ray diffraction (XRD) pattern of NiO-ZnO/RGO composite displays some apparent diffraction peaks of NiO (JCPDS no. 65-2901) [39], as well as the ZnO sample (JCPDS no. 80-0075) [40] in Fig. 2a. Furthermore, a weak broad diffraction peak was observed at about 26° that could be attributed to the disorderedly stacked graphene sheets in the pattern [41, 42]. The NiO and NiO/RGO samples also exhibited all of the characteristic peaks of NiO (Fig. S3†). An SEM image of the NiO-ZnO/RGO composite (Fig. 2b) shows that the product is composed of many thin flakes with no naked graphene sheets, indicating the uniform deposition of NiO-ZnO. The magnified image (Fig. 2c) shows the presence of many curly flakes and a small number of discrete particles, which results from the loosely-packed NiO-ZnO formed on the RGO surface and some mixed NiO-ZnO particles. As shown in the transmission electron microscopy (TEM) images (Fig. 2d), there was NiO-ZnO/RGO composite present with its typical
single, or overlapping, flake morphology covering large areas of the sample, and the NiO-ZnO and RGO were seen to be well-combined (Fig. 2e). These data were consistent with the SEM results. The NiO sample displays flake-like morphology in Fig. S4†, and Fig. S5† shows an NiO-ZnO sample with a curled-sheet structure. As shown in Fig. S6 † , the NiO/RGO sample exhibits some round, curly flakes. The high-resolution transmission electron microscopy (HRTEM) image shows ZnO and NiO on the RGO sheet. The lattice fringes of NiO with an interplanar distance of 0.15 nm, and 0.28 nm for ZnO were also evidenced. The selected area electron diffraction (SAED) pattern (top right-hand corner of Fig. 2f) exhibits well-resolved diffraction rings, which can be indexed to the (100), (002), and (110) planes of ZnO, as well as the (111) and (220) planes of NiO [43, 44], suggesting the presence of ZnO and NiO on the RGO sheets. Fig. 2g shows the nitrogen adsorption–desorption isotherms of the NiO-ZnO/RGO composite. The sample displays a type-IV isotherm for a porous structure and the specific surface area (BET) was 110.49 m2/g. The pore size distribution was estimated using the Barrett-Joyner-Halenda (BJH) analysis method, and the pore size distribution ranged from 30 to 80 nm (Fig. 2g, insert). Thermogravimetric analysis (TGA) curves of NiO and NiO-ZnO/RGO composite in air from room temperature to 850 °C are shown in Fig. 2h. It can be observed that there had been a loss of approximately 2 % of the mass of the sample at 200 °C, which is attributable to the removal of adsorbed water and adsorbent substances. However, a mass loss of about 7.71 % occurred from 200 to 850 °C, which was due to the decomposition of NiO. For this NiO-ZnO/RGO composite, the observed mass loss of about 10 % also arose from the removal of the adsorbed water and adsorbent substances because of the existence of the RGO sheets. There was a total mass loss of about 14.2 % over the temperature range from 200 °C to 850 °C, which can be ascribed to the decomposition of the NiO and RGO sheets.
Fig. 2(a) The XRD patterns of the NiO-ZnO and NiO-ZnO/RGO; (b) low- and (c) high-magnification SEM images of NiO-ZnO/RGO; (d) TEM images and (e) HRTEM image of NiO-ZnO/RGO; (f) N2 adsorption/desorption isotherms and the corresponding pore size distribution of NiO-ZnO/RGO; and (g) the TGA curves of NiO-ZnO and NiO-ZnO/RGO samples.
Fig. 3(a) EDS spectrum and mapping images of NiO-ZnO/RGO composite; (b) XPS spectrum of NiO-ZnO/RGO composite’ high-resolution XPS spectra of (c) N 1s, (d) O1s, (e) Zn 2p, and (f) Ni 2p of NiO-ZnO/RGO composite.
Fig. 3a shows the energy dispersive spectrometer (EDS) spectrum of NiO-ZnO/RGO composite, indicating that there are certain amounts of Ni, Zn, O, and C elements in the sample, and the elemental mapping of the NiO-ZnO/RGO composite revealed the uniform distribution of Ni, Zn, O, and C species in the as-prepared product (bottom of Fig. 3a). Fig. 3b shows a typical X-ray photoelectron spectroscopy (XPS) spectrum from a NiO-ZnO/RGO composite specimen where four elements (Ni, Zn, O, and C) were distinguished [45, 46]. Fig. 3c shows the high-resolution XPS spectra of C1s – there are three peaks at 284.8, 286.1, and 288.7 eV, corresponding to C-C, C-O, and O=C-O, respectively [47]. For the high-resolution XPS spectra of O1s in Fig. 3d, two peaks located at 529.2 and 531.2 eV can be assigned to the O of NiO and ZnO, respectively [48, 49]. As shown in Fig. 3e, two strong peaks at 1020.6 eV and 1043.7 eV were observed in the high-resolution Zn 2p spectrum, which can respectively be attributed to the electron orbits of Zn 2p3/2 and Zn 2p1/2 [50]. Fig. 3f shows the high-resolution XPS spectrum of Ni 2p – two peaks were located at 854.7 eV and 860.6, which were assigned to Ni 2p3/2. The peaks at 872.3 eV and 879.6 eV correspond to Ni 2p1/2 [51, 52].
Fig. 4(a) Cyclic voltammetry (CV) curves of NiO-ZnO/RGO electrode at a scan rate of 0.1 mV/s; (b) galvanostatic charge-discharge profiles of NiO-ZnO/RGO at a current density of 100 mA/g; (c) comparison of cycling performance of NiO, NiO/ZnO, NiO/RGO, and (d) NiO-ZnO/RGO composite at current densities of 100 mA/g and 500 mA/g; (e) rate capability of the four samples at various current densities; and (f)
Nyquist plots of NiO-ZnO and NiO-ZnO/RGO fresh electrodes at 3.0 V (v. Li+/Li).
To evaluate the electrochemical performance of the NiO-ZnO/RGO composite as an anode material for LIBs, we examined the first five CV curves of the as-prepared product electrode in the voltage range of 0.01 to 3 V with a scan rate of 0.1 mV/s. Fig. 4a shows the CV curves of the NiO-ZnO/RGO composite electrode. In the first cathodic sweep process, a long voltage plateau at 0.75 V and 0.52 V respectively correspond to the reduction of ZnO and NiO, accompanied by the formation of Li 2O and a solid electrolyte interphase (SEI) [34, 53]. In the first anodic scan, the two broad peaks at approximately 2.35 V and 1.52 V were mainly attributed to the formation of NiO from Ni [54], while the peak located at approximately 1.76 V corresponds to the Zn to Zn2+ transition [13]. The curves were aligned to a certain extent during subsequent cycles, indicating that the electrode had good stability [55]. Fig. 4b shows the discharge and charge voltage profiles of the NiO-ZnO/RGO electrode over four cycles (the initial, second, 10th, and 50th cycles) with the voltage ranging from 0.01 to 3 V at a current density of 100 mA/g. The electrode delivers a relatively high reversible capacity during the initial cycle, and then the capacity only changes a little over subsequent cycles. A long voltage plateau for NiO-ZnO/RGO was observed from 0.95 to 0.7 V, which might be attributed to a lithiation reaction of NiO-ZnO composite (NiO + 2Li+ + 2e- → Li2O + Ni, and ZnO + 2Li+ + 2e- → Li2O + Zn) and the partial formation of a solid-electrolyte interface (SEI) film. The discharge curve has two slope plateaus at around 1.7 V and 2.2 V, which corresponds to the lithiation extraction processes (Li2O + Zn → ZnO + 2Li+ + 2e- and Li2O + Zn → ZnO +2Li+ + 2e-), and the formation of NiO and ZnO [56]. Meanwhile, the discharge capacity in the first, second, 10th, and 50th cycles reached 1393, 1006, 720, and 778 mA h/g, where the coulombic efficiencies were 66.3, 81.8, 94.1, and 98.1%, respectively. Fig. 4c shows the cyclic performance and coulombic efficiency for the NiO-ZnO/RGO composite electrode tested at a current density of 100 mA/g. The cyclic performances of the NiO, NiO/RGO, and NiO-ZnO electrodes are also presented for comparison. It can be observed that the reversible capacity of the NiO-ZnO/RGO electrode degraded to 690
mA h/g after 16 cycles, and then increased to 1017 mA h/g after 200 cycles. This behaviour revealed a competitive reversible capacity in comparison with the reported ZnO-NiO hybrid materials reported elsewhere [16, 34, 44, 57, 58]. The coulombic efficiency of the first charge-discharge cycle was approximately 66.33%, which would be ascribed to the generation of SEI film [55], as well as the decomposition of electrolyte [34]. Subsequently, the coulombic efficiency of the electrode can be retained above 98% after the 15th cycle was higher than NiO-ZnO hybrid [59]. Considering the theoretical capacities of pure NiO (718 mA h/g) and ZnO (978 mA h/g), the calculated theoretical capacity for the NiO-ZnO/RGO composite (with a mass ratio of NiO: ZnO equal to 1:1 and approximately 7 wt% RGO) should be 833 mA h/g, which is about 184 mA h/g less than that the test value of 1017 mA h/g. Compared to the early results for NiO, ZnO, and NiO-ZnO [16, 54, 60], the enhanced capacity of NiO-ZnO/RGO can be ascribed to the reversible formation of a polymeric gel-like film, which is present in most anode materials, whereas ZnO can increase the material viscosity to improve adhesion between the active material layer and the current collector [61]. More importantly, RGO sheets can prevent the aggregation of NiO-ZnO acting as a barrier, and can help to accommodate the volume changes in NiO-ZnO during lithium cycling. Thus, it is suggested that the high reversible capacity of the NiO-ZnO/RGO sample can be attributed to the synergistic effects of the NiO-ZnO and RGO sheets [60, 62-64]. Furthermore, the effect also existed in other binary transition metal oxide hybrid materials [22, 32]. It was observed that the discharge capacity of the NiO/RGO sample decreased to 457 mA h/g after 100 cycles, which was less than half that of the NiO-ZnO/RGO electrode. In addition, the NiO and NiO-ZnO electrodes exhibited significant degradation after the 25th cycle with a discharge capacity of 212 mA h/g at 180 cycles and 247 mA h/g at 150 cycles, respectively. Remarkably, the fluctuation change of cycling performance might be attributed to stability of electrode structure caused by volume change of NiO and ZnO[58]and electrolyte degradation[47]during the lithium cycling process. Fig. 4d shows an NiO-ZnO/RGO electrode capacity of approximately 458 mA h/g at a current density of 500 mA/g after 400 cycles. In contrast, the NiO/RGO electrode
maintained its capacity of 320 mA h/g, the NiO electrode had a capacity of 157 mA h/g, and that of the NiO-ZnO electrode decreased to 206 mA h/g. The charge-discharge cycling performances of the three NiO-ZnO/RGO sample electrodes with different temperature (NiO-ZnO/RGO-300, NiO-ZnO/RGO-350, and NiO-ZnO/RGO-400) under a current density of 100 mA/g after 30 cycles are compared in Fig.S7. The NiO-ZnO/RGO-350 sample maintains a higher reversible capacity of 814 mA h/g than 701 mA h/g of the NiO-ZnO/RGO-300 electrode and 427 mA h/g of the NiO-ZnO/RGO-400 electrode, indicating that 350ºC is the ideal treatment temperature, it is consistent with the TGA result. The same conclusion also appears on the 500 mA/g measurement result (after 200 cycles in Fig.S8†). As shown in Fig. 4e, the NiO-ZnO/RGO electrode had a capacity of 841, 604, 421, 310, and 182 mA h/g at current densities of 100 mA/g, 200 mA/g, 500 mA/g, 1 A/g, and 2 A/g, respectively. When the current density was decreased to 100 mA/g, the NiO-ZnO/RGO electrode recovered its reversible capacity of 702 mA h/g after 70 cycles. By contrast, NiO and NiO-ZnO electrodes suffered faster capacity degradation than the NiO-ZnO/RGO electrode at the same current density. For example, NiO and NiO-ZnO had capacities of less than 80 mA h/g at 2 A/g, and could not regain their initial capacities when the current density reverted to 100 mA/g. It is worth noting that the NiO/RGO electrode exhibited a similar reversible capacity to that of the NiO-ZnO/RGO electrode at 2 A/g, which could have arisen as a result of the positive effect of graphene on NiO flakes. The electrochemical impedance spectra (EIS) of the NiO-ZnO/RGO and bare NiO-ZnO fresh electrodes over the frequency range from 100 kHz to 0.01 Hz were investigated with the results being shown in Fig. 4f. The diameter of the semicircle for the NiO-ZnO/RGO electrode was smaller than that of the NiO-ZnO electrode. Accordingly, the NiO-ZnO/RGO electrode possessed a lower charge transfer resistance in comparison with the NiO-ZnO electrode, and the reduction in this resistance can be ascribed to the existence of RGO sheets. Furthermore, a simple equivalent circuit model was applied to fit the AC impedance spectra (insert, Fig. 4f). The simulated values of SEI film resistance Rs and charge transfer resistance Rct for the NiO-ZnO/RGO electrode were 19.1 and 216 Ω, respectively, both much less than those
of NiO-ZnO (1662 and 1894 Ω). This result was consistent with the cyclic performance and rate capability results. The high reversible capacity and good rate capability of the as-prepared NiO-ZnO/RGO electrode may be attributed to the synergistic effect of the NiO-ZnO and the RGO sheets. It can be observed that the conductive nature and sheet structure of active material in the previous electrode (Fig. 5a) differed from that of the NiO-ZnO and RGO sheets. These were evenly distributed after 400 charge-discharge cycles at a current density of 500 mA/g (Fig. 5b), from which it can be concluded that graphene stabilised the electrode structure and thus enhanced the cycling performance. The SEM result can also be proved by examining the TEM image (Fig. 5c). The NiO-ZnO composite, RGO sheet, and thin SEI film can be clearly seen in the HRTEM image (Fig. 5d). The above results indicated that the NiO-ZnO/RGO sample delivered good lithium storage capacity when used as an anode material in a LIB, which can be attributed to the synergistic effects of the NiO-ZnO nanoflakes offering a richness of reaction sites. Further, the RGO sheets improved the electrical conductivity of the active materials and stabilised the electrode structure.
Fig. 5 SEM images of NiO-ZnO/RGO electrode (a) before and (b) after 400 charge-discharge cycles, (c) Low-magnification TEM image and (d) HRTEM image of
the NiO-ZnO/RGO electrode after 400 charge-discharge cycles (red circle, Fig. 5c). 4. Conclusions In summary, we have demonstrated that NiO-ZnO/RGO composite electrodes displayed excellent electrochemical performance in terms of reversible capacity (1017 mA h/g after 200 cycles at a current density of 100 mA/g), rate capability and long cycle life (about 475 mA h/g after 400 cycles at a current density of 500 mA/g). The outstanding performance of the as-prepared NiO-ZnO/RGO composite electrodes for Li+ storage could be attributed to the synergistic effects of the NiO-ZnO nanoflakes and RGO sheets. The NiO-ZnO nanoflakes offered a richness of electrochemical reaction sites and shortened the diffusion path of Li+, whereas the RGO sheets could improve the mechanical stability and conductivity of the electrode during lithium cycling. Designing binary or ternary oxides and RGO sheets composite electrodes via an in situ ultrasonic agitation approach might achieve higher lithium-storage properties in future studies inspired by the findings within this work.
Acknowledgements We thank the Fundamental Research Funds for the Central Universities (Grant No. 17lgpy68), Pearl River S&T Nova Program of Guangzhou (Grant No.201710010043) and National Natural Science Foundation of China (Grant No. 51676212) for financial support.
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PII: DOI: Reference:
S0272-8842(18)32507-0 https://doi.org/10.1016/j.ceramint.2018.09.044 CERI19443
To appear in: Ceramics International Received date: 25 July 2018 Revised date: 4 September 2018 Accepted date: 5 September 2018 Cite this article as: Liang Ma, Xian-Yinan Pei, Dong-Chuan Mo, Shu-Shen Lyu and Yuan-Xiang Fu, Fabrication of NiO-ZnO/RGO composite as an anode material for lithium-ion batteries, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of NiO-ZnO/RGO composite as an anode material for lithium-ion batteries Liang Ma, Xian-Yinan Pei, Dong-Chuan Mo, Shu-Shen Lyu, Yuan-Xiang Fu* School of Chemical Engineering and Technology, Sun Yat-sen University Guangzhou 510275, P. R. China. * E-mail: [email protected]
Abstract： NiO-ZnO/RGO composite was obtained by the annealing of an Ni (OH)2-Zn (OH)2/RGO precursor, which has been fabricated by in situ ultrasonic agitation. Moreover, the NiO-ZnO nanoflakes are evenly distributed on the RGO sheets based on the scanning electron microscope (SEM) and transmission electron microscope (TEM) characterization results. When the NiO-ZnO/RGO composite was used as an anode material in lithium-ion batteries (LIBs), the electrodes exhibited a high reversible capacity of 1017 mA h/g at a current density of 100 mA/g after 200 cycles and a specific capacity of 458 mA h/g at 500 mA/g even after 400 cycles. The electrode even reached a capacity of 185 mA h/g at a current density of 2000 mA/g. The excellent electrochemical properties of the NiO-ZnO/RGO composite might be attributable to the NiO-ZnO nanoflakes offering rich electrochemical reaction sites and shortening the diffusion length for lithium ion (Li+), as well as the RGO sheets improving the transfer rates of Li+ and electron during the charge-discharge process.
Keywords: NiO-ZnO/RGO composite, porous, anode material, lithium-ion batteries.
1． Introduction Owing to their high reversible capacity, good rate capability, and long cycle life, lithium-ion batteries (LIBs) have been widely used as power sources for portable electronics and electric vehicles [1]. However, the increasing demand for high-performance electronic devices and hybrid electric vehicles has promoted much
research aimed at exploiting LIBs with a high power density and better long-term cycling performance [2]. Development of high-performance anode materials is considered to be an effective strategy [3]. A fact largely arising as a result to achieve this end, commercial graphite has a relatively low theoretical capacity of 372 mA h/g [4]. In recent years, transition metal oxides [5-13], especially nickel-based oxides, have led scholars to investigate such new materials as promising anode candidates in terms of their high theoretical capacity, good cycling performance, high recharge rates, environmentally friendly nature, and low-cost [14-21]. Moreover, binary nickel-based oxides
(e.g.,
CoO/NiO,
Fe3O4/NiO,
CuO/NiO,
ZnO/NiO,
SnO2/NiO,
and
NiO/NiFe2O4) displayed even better lithium storage capacity than that of pure NiO because of the synergetic effect found during the charge-discharge process in early reports [14-17, 22, 23]. Thus, binary NiO based oxides offer better practical application potential in LIBs as anode materials. Unfortunately, there are still some drawbacks in the application of such NiO materials as anodes in LIBs. These drawbacks can be especially linked to their poor electronic conductivity and structural changes during repeated cycling, resulting in poor rate capability and rapid capacity fading, respectively [24, 25]. Thus, to obtain high performances from binary NiO anode materials during the charge-discharge process, it is crucial that the electrodes maintain their good conductivity and low volume change. Owing to its high conductivity, superior thermal conductivity, and strength [26-29], graphene has aroused much interest among scholars as a two-dimensional carbon material. Various metal oxide/graphene composites have been fabricated as anode materials for LIBs, and such composites exhibited excellent comprehensive electrochemical properties compared with those of pure metal oxides [13, 30-32]. Taking into consideration the lithium storage capacity of binary nickel-based oxides compared to that of NiO [17, 30, 33, 34], fabricating binary nickel-based oxide/graphene hybrid materials is an effective strategy with which to obtain anode materials for LIBs. Meanwhile, to the best of our knowledge, until now there have been few reports of any anode material integrating the advantages of NiO-ZnO and graphene as anode materials for LIBs [34].
NiO-ZnO/RGO composite was fabricated by the annealing of nickel hydroxide– zinc hydroxide/GO precursor (Ni (OH)2-Zn (OH)2/GO), which was prepared by in situ ultrasonic agitation. As expected, the NiO-ZnO/RGO displays a high reversible capacity of 1017 mA h/g at a current density of 100 mA/g after 200 cycles, and a reversible capacity of 458 mA h/g is obtained at a current density of 500 mA/g even after 400 cycles when used as an anode material for a LIB. This result is largely attributable to the synergetic effects of NiO-ZnO and graphene sheets.
2. Experimental work 2.1 Fabrication of GO Graphene oxide (GO) was produced using an improved Hummers method with some adjustments [35, 36]. In brief, 2 g natural flake graphite was added into a solution of 180 ml H2SO4 with 20 ml H3PO4, and then 9 g KMnO4 was slowly added under stirring. The temperature of the mixture was kept at 50 °C for 2 h by use of a thermostatically-controlled water bath. Afterward, 200 ml deionized water and 4 ml 30 wt% H2O2 solution were successively dropped into the aforementioned solution. GO powder was then collected after centrifugation, washed several times with 30% HCl aqueous solution, ethanol, then deionized water, and freeze-dried.
2.2 Fabrication of NiO-ZnO/RGO The NiO-ZnO/RGO composite was synthesized by in situ ultrasonic agitation. GO powder (60 mg), and 60 mg polyvinyl pyrrolidone (PVP) were added into 60 ml deionized water under ultrasonication for 45 min. Subsequently, 1.45 g Ni (NO3)2·6H2O and 1.48g Zn (NO3)2·6H2O were successively added into the GO solution under constant stirring to form a homogeneous solution. Then, 6 ml of 25% aqueous ammonia was added into the as-prepared solution with mechanical stirring and ultrasonic treatment for 2 h. The Ni (OH)2-Zn (OH)2/GO precursor was obtained after being washed with distilled water several times, and then freeze-dried for 12 h in a lyophilizer. The final NiO-ZnO/RGO composite was produced by calcination of the Ni (OH)2-Zn (OH)2/GO precursor at 350 °C for 4 h in a nitrogen atmosphere (N2), and the
tap density of the NiO-ZnO/RGO tested is about 1.4 g/cm3. For comparison, the NiO and NiO/RGO were also prepared by the same procedure, while the NiO-ZnO was prepared without the GO sheets. 2.3 Characterization The samples were characterized by field-emission scanning electron microscopy (SEM) on a FEI FEI-Q400F; transmission electron microscope (TEM) on a JEOL JEM-2010 HR operating at 200 kV; powder X-ray diffraction (XRD) on a Rigaku D-MAX 2200 VPC X-ray diffractometer with Cu Ka radiation; and thermogravimetric (TG) analysis on a Netzsch 209 F3 Tarsus thermal analysis (TGA) apparatus from room temperature to 850 °C. X-ray photoelectron spectroscopy (XPS) measurements were -ray source, and N2 adsorption/desorption isotherms were obtained by a Micromeritics ASAP 2460 at a temperature of 77 K.
2.4 Electrochemical measurements The electrochemical performances of all samples were measured using CR-2032-type coin-type half cells. The working electrodes were prepared by 75 wt % active material, 15 wt% conductive carbon black additive, and 10 wt% polyvinylidene fluoride (PVDF). In the presence of a certain amount of N-methyl-2-pyrrolidone (NMP), these materials were uniformly mixed to produce a homogeneous slurry. The slurry was then painted on a copper foil current collector and dried at 120 °C for 10 h in a vacuum oven. The cells were assembled in an argon-filled glove box using the as-prepared working electrodes (~1.5 mg/cm2), a lithium foil as the counter-electrode, a Celgard 2400 membrane as the separator, and a mixture of 1M LiPF6 with ethylene carbonate (EC) and dimethyl carbonate (DME) (1:1, volume ratio) as the electrolyte. Galvanostatic charge/discharge tests were performed on the LAND CT2001A battery test system in the range of 0.01 to 3.0 V (v. Li/Li+) at different rates at room temperature. Cyclic voltammetric (CV) and electrochemical impedance spectrum (EIS) measurements were undertaken using an electrochemical workstation (Chenhua,
CHI660E) with a scan rate of 0.1 mV/s between 0.01 and 3 V (v. Li/Li+). 3. Results and Discussion The preparation process of the NiO-ZnO/RGO is illustrated in Fig. 1, Detailed experimental steps are described in the section covering fabrication of NiO-ZnO/RGO. The XRD pattern of the Ni (OH)2-Zn (OH)2/GO precursor shows the diffraction peaks of Ni (OH)2 (weak) and Zn (OH)2 [37, 38]. Moreover, a weak XRD pattern of GO also was observed at 2θ≈9.8° in Fig. S1a† [35, 36]. However, the precursor exhibited a typical sheet-like morphology in both the low- and high-magnification SEM images shown in Fig. S2†.
Fig. 1 Schematic showing the preparation of NiO-ZnO/RGO composite
NiO-ZnO/RGO composite was obtained by the annealing of Ni (OH)2-Zn (OH)2/GO in inert gas. The X-ray diffraction (XRD) pattern of NiO-ZnO/RGO composite displays some apparent diffraction peaks of NiO (JCPDS no. 65-2901) [39], as well as the ZnO sample (JCPDS no. 80-0075) [40] in Fig. 2a. Furthermore, a weak broad diffraction peak was observed at about 26° that could be attributed to the disorderedly stacked graphene sheets in the pattern [41, 42]. The NiO and NiO/RGO samples also exhibited all of the characteristic peaks of NiO (Fig. S3†). An SEM image of the NiO-ZnO/RGO composite (Fig. 2b) shows that the product is composed of many thin flakes with no naked graphene sheets, indicating the uniform deposition of NiO-ZnO. The magnified image (Fig. 2c) shows the presence of many curly flakes and a small number of discrete particles, which results from the loosely-packed NiO-ZnO formed on the RGO surface and some mixed NiO-ZnO particles. As shown in the transmission electron microscopy (TEM) images (Fig. 2d), there was NiO-ZnO/RGO composite present with its typical
single, or overlapping, flake morphology covering large areas of the sample, and the NiO-ZnO and RGO were seen to be well-combined (Fig. 2e). These data were consistent with the SEM results. The NiO sample displays flake-like morphology in Fig. S4†, and Fig. S5† shows an NiO-ZnO sample with a curled-sheet structure. As shown in Fig. S6 † , the NiO/RGO sample exhibits some round, curly flakes. The high-resolution transmission electron microscopy (HRTEM) image shows ZnO and NiO on the RGO sheet. The lattice fringes of NiO with an interplanar distance of 0.15 nm, and 0.28 nm for ZnO were also evidenced. The selected area electron diffraction (SAED) pattern (top right-hand corner of Fig. 2f) exhibits well-resolved diffraction rings, which can be indexed to the (100), (002), and (110) planes of ZnO, as well as the (111) and (220) planes of NiO [43, 44], suggesting the presence of ZnO and NiO on the RGO sheets. Fig. 2g shows the nitrogen adsorption–desorption isotherms of the NiO-ZnO/RGO composite. The sample displays a type-IV isotherm for a porous structure and the specific surface area (BET) was 110.49 m2/g. The pore size distribution was estimated using the Barrett-Joyner-Halenda (BJH) analysis method, and the pore size distribution ranged from 30 to 80 nm (Fig. 2g, insert). Thermogravimetric analysis (TGA) curves of NiO and NiO-ZnO/RGO composite in air from room temperature to 850 °C are shown in Fig. 2h. It can be observed that there had been a loss of approximately 2 % of the mass of the sample at 200 °C, which is attributable to the removal of adsorbed water and adsorbent substances. However, a mass loss of about 7.71 % occurred from 200 to 850 °C, which was due to the decomposition of NiO. For this NiO-ZnO/RGO composite, the observed mass loss of about 10 % also arose from the removal of the adsorbed water and adsorbent substances because of the existence of the RGO sheets. There was a total mass loss of about 14.2 % over the temperature range from 200 °C to 850 °C, which can be ascribed to the decomposition of the NiO and RGO sheets.
Fig. 2(a) The XRD patterns of the NiO-ZnO and NiO-ZnO/RGO; (b) low- and (c) high-magnification SEM images of NiO-ZnO/RGO; (d) TEM images and (e) HRTEM image of NiO-ZnO/RGO; (f) N2 adsorption/desorption isotherms and the corresponding pore size distribution of NiO-ZnO/RGO; and (g) the TGA curves of NiO-ZnO and NiO-ZnO/RGO samples.
Fig. 3(a) EDS spectrum and mapping images of NiO-ZnO/RGO composite; (b) XPS spectrum of NiO-ZnO/RGO composite’ high-resolution XPS spectra of (c) N 1s, (d) O1s, (e) Zn 2p, and (f) Ni 2p of NiO-ZnO/RGO composite.
Fig. 3a shows the energy dispersive spectrometer (EDS) spectrum of NiO-ZnO/RGO composite, indicating that there are certain amounts of Ni, Zn, O, and C elements in the sample, and the elemental mapping of the NiO-ZnO/RGO composite revealed the uniform distribution of Ni, Zn, O, and C species in the as-prepared product (bottom of Fig. 3a). Fig. 3b shows a typical X-ray photoelectron spectroscopy (XPS) spectrum from a NiO-ZnO/RGO composite specimen where four elements (Ni, Zn, O, and C) were distinguished [45, 46]. Fig. 3c shows the high-resolution XPS spectra of C1s – there are three peaks at 284.8, 286.1, and 288.7 eV, corresponding to C-C, C-O, and O=C-O, respectively [47]. For the high-resolution XPS spectra of O1s in Fig. 3d, two peaks located at 529.2 and 531.2 eV can be assigned to the O of NiO and ZnO, respectively [48, 49]. As shown in Fig. 3e, two strong peaks at 1020.6 eV and 1043.7 eV were observed in the high-resolution Zn 2p spectrum, which can respectively be attributed to the electron orbits of Zn 2p3/2 and Zn 2p1/2 [50]. Fig. 3f shows the high-resolution XPS spectrum of Ni 2p – two peaks were located at 854.7 eV and 860.6, which were assigned to Ni 2p3/2. The peaks at 872.3 eV and 879.6 eV correspond to Ni 2p1/2 [51, 52].
Fig. 4(a) Cyclic voltammetry (CV) curves of NiO-ZnO/RGO electrode at a scan rate of 0.1 mV/s; (b) galvanostatic charge-discharge profiles of NiO-ZnO/RGO at a current density of 100 mA/g; (c) comparison of cycling performance of NiO, NiO/ZnO, NiO/RGO, and (d) NiO-ZnO/RGO composite at current densities of 100 mA/g and 500 mA/g; (e) rate capability of the four samples at various current densities; and (f)
Nyquist plots of NiO-ZnO and NiO-ZnO/RGO fresh electrodes at 3.0 V (v. Li+/Li).
To evaluate the electrochemical performance of the NiO-ZnO/RGO composite as an anode material for LIBs, we examined the first five CV curves of the as-prepared product electrode in the voltage range of 0.01 to 3 V with a scan rate of 0.1 mV/s. Fig. 4a shows the CV curves of the NiO-ZnO/RGO composite electrode. In the first cathodic sweep process, a long voltage plateau at 0.75 V and 0.52 V respectively correspond to the reduction of ZnO and NiO, accompanied by the formation of Li 2O and a solid electrolyte interphase (SEI) [34, 53]. In the first anodic scan, the two broad peaks at approximately 2.35 V and 1.52 V were mainly attributed to the formation of NiO from Ni [54], while the peak located at approximately 1.76 V corresponds to the Zn to Zn2+ transition [13]. The curves were aligned to a certain extent during subsequent cycles, indicating that the electrode had good stability [55]. Fig. 4b shows the discharge and charge voltage profiles of the NiO-ZnO/RGO electrode over four cycles (the initial, second, 10th, and 50th cycles) with the voltage ranging from 0.01 to 3 V at a current density of 100 mA/g. The electrode delivers a relatively high reversible capacity during the initial cycle, and then the capacity only changes a little over subsequent cycles. A long voltage plateau for NiO-ZnO/RGO was observed from 0.95 to 0.7 V, which might be attributed to a lithiation reaction of NiO-ZnO composite (NiO + 2Li+ + 2e- → Li2O + Ni, and ZnO + 2Li+ + 2e- → Li2O + Zn) and the partial formation of a solid-electrolyte interface (SEI) film. The discharge curve has two slope plateaus at around 1.7 V and 2.2 V, which corresponds to the lithiation extraction processes (Li2O + Zn → ZnO + 2Li+ + 2e- and Li2O + Zn → ZnO +2Li+ + 2e-), and the formation of NiO and ZnO [56]. Meanwhile, the discharge capacity in the first, second, 10th, and 50th cycles reached 1393, 1006, 720, and 778 mA h/g, where the coulombic efficiencies were 66.3, 81.8, 94.1, and 98.1%, respectively. Fig. 4c shows the cyclic performance and coulombic efficiency for the NiO-ZnO/RGO composite electrode tested at a current density of 100 mA/g. The cyclic performances of the NiO, NiO/RGO, and NiO-ZnO electrodes are also presented for comparison. It can be observed that the reversible capacity of the NiO-ZnO/RGO electrode degraded to 690
mA h/g after 16 cycles, and then increased to 1017 mA h/g after 200 cycles. This behaviour revealed a competitive reversible capacity in comparison with the reported ZnO-NiO hybrid materials reported elsewhere [16, 34, 44, 57, 58]. The coulombic efficiency of the first charge-discharge cycle was approximately 66.33%, which would be ascribed to the generation of SEI film [55], as well as the decomposition of electrolyte [34]. Subsequently, the coulombic efficiency of the electrode can be retained above 98% after the 15th cycle was higher than NiO-ZnO hybrid [59]. Considering the theoretical capacities of pure NiO (718 mA h/g) and ZnO (978 mA h/g), the calculated theoretical capacity for the NiO-ZnO/RGO composite (with a mass ratio of NiO: ZnO equal to 1:1 and approximately 7 wt% RGO) should be 833 mA h/g, which is about 184 mA h/g less than that the test value of 1017 mA h/g. Compared to the early results for NiO, ZnO, and NiO-ZnO [16, 54, 60], the enhanced capacity of NiO-ZnO/RGO can be ascribed to the reversible formation of a polymeric gel-like film, which is present in most anode materials, whereas ZnO can increase the material viscosity to improve adhesion between the active material layer and the current collector [61]. More importantly, RGO sheets can prevent the aggregation of NiO-ZnO acting as a barrier, and can help to accommodate the volume changes in NiO-ZnO during lithium cycling. Thus, it is suggested that the high reversible capacity of the NiO-ZnO/RGO sample can be attributed to the synergistic effects of the NiO-ZnO and RGO sheets [60, 62-64]. Furthermore, the effect also existed in other binary transition metal oxide hybrid materials [22, 32]. It was observed that the discharge capacity of the NiO/RGO sample decreased to 457 mA h/g after 100 cycles, which was less than half that of the NiO-ZnO/RGO electrode. In addition, the NiO and NiO-ZnO electrodes exhibited significant degradation after the 25th cycle with a discharge capacity of 212 mA h/g at 180 cycles and 247 mA h/g at 150 cycles, respectively. Remarkably, the fluctuation change of cycling performance might be attributed to stability of electrode structure caused by volume change of NiO and ZnO[58]and electrolyte degradation[47]during the lithium cycling process. Fig. 4d shows an NiO-ZnO/RGO electrode capacity of approximately 458 mA h/g at a current density of 500 mA/g after 400 cycles. In contrast, the NiO/RGO electrode
maintained its capacity of 320 mA h/g, the NiO electrode had a capacity of 157 mA h/g, and that of the NiO-ZnO electrode decreased to 206 mA h/g. The charge-discharge cycling performances of the three NiO-ZnO/RGO sample electrodes with different temperature (NiO-ZnO/RGO-300, NiO-ZnO/RGO-350, and NiO-ZnO/RGO-400) under a current density of 100 mA/g after 30 cycles are compared in Fig.S7. The NiO-ZnO/RGO-350 sample maintains a higher reversible capacity of 814 mA h/g than 701 mA h/g of the NiO-ZnO/RGO-300 electrode and 427 mA h/g of the NiO-ZnO/RGO-400 electrode, indicating that 350ºC is the ideal treatment temperature, it is consistent with the TGA result. The same conclusion also appears on the 500 mA/g measurement result (after 200 cycles in Fig.S8†). As shown in Fig. 4e, the NiO-ZnO/RGO electrode had a capacity of 841, 604, 421, 310, and 182 mA h/g at current densities of 100 mA/g, 200 mA/g, 500 mA/g, 1 A/g, and 2 A/g, respectively. When the current density was decreased to 100 mA/g, the NiO-ZnO/RGO electrode recovered its reversible capacity of 702 mA h/g after 70 cycles. By contrast, NiO and NiO-ZnO electrodes suffered faster capacity degradation than the NiO-ZnO/RGO electrode at the same current density. For example, NiO and NiO-ZnO had capacities of less than 80 mA h/g at 2 A/g, and could not regain their initial capacities when the current density reverted to 100 mA/g. It is worth noting that the NiO/RGO electrode exhibited a similar reversible capacity to that of the NiO-ZnO/RGO electrode at 2 A/g, which could have arisen as a result of the positive effect of graphene on NiO flakes. The electrochemical impedance spectra (EIS) of the NiO-ZnO/RGO and bare NiO-ZnO fresh electrodes over the frequency range from 100 kHz to 0.01 Hz were investigated with the results being shown in Fig. 4f. The diameter of the semicircle for the NiO-ZnO/RGO electrode was smaller than that of the NiO-ZnO electrode. Accordingly, the NiO-ZnO/RGO electrode possessed a lower charge transfer resistance in comparison with the NiO-ZnO electrode, and the reduction in this resistance can be ascribed to the existence of RGO sheets. Furthermore, a simple equivalent circuit model was applied to fit the AC impedance spectra (insert, Fig. 4f). The simulated values of SEI film resistance Rs and charge transfer resistance Rct for the NiO-ZnO/RGO electrode were 19.1 and 216 Ω, respectively, both much less than those
of NiO-ZnO (1662 and 1894 Ω). This result was consistent with the cyclic performance and rate capability results. The high reversible capacity and good rate capability of the as-prepared NiO-ZnO/RGO electrode may be attributed to the synergistic effect of the NiO-ZnO and the RGO sheets. It can be observed that the conductive nature and sheet structure of active material in the previous electrode (Fig. 5a) differed from that of the NiO-ZnO and RGO sheets. These were evenly distributed after 400 charge-discharge cycles at a current density of 500 mA/g (Fig. 5b), from which it can be concluded that graphene stabilised the electrode structure and thus enhanced the cycling performance. The SEM result can also be proved by examining the TEM image (Fig. 5c). The NiO-ZnO composite, RGO sheet, and thin SEI film can be clearly seen in the HRTEM image (Fig. 5d). The above results indicated that the NiO-ZnO/RGO sample delivered good lithium storage capacity when used as an anode material in a LIB, which can be attributed to the synergistic effects of the NiO-ZnO nanoflakes offering a richness of reaction sites. Further, the RGO sheets improved the electrical conductivity of the active materials and stabilised the electrode structure.
Fig. 5 SEM images of NiO-ZnO/RGO electrode (a) before and (b) after 400 charge-discharge cycles, (c) Low-magnification TEM image and (d) HRTEM image of
the NiO-ZnO/RGO electrode after 400 charge-discharge cycles (red circle, Fig. 5c). 4. Conclusions In summary, we have demonstrated that NiO-ZnO/RGO composite electrodes displayed excellent electrochemical performance in terms of reversible capacity (1017 mA h/g after 200 cycles at a current density of 100 mA/g), rate capability and long cycle life (about 475 mA h/g after 400 cycles at a current density of 500 mA/g). The outstanding performance of the as-prepared NiO-ZnO/RGO composite electrodes for Li+ storage could be attributed to the synergistic effects of the NiO-ZnO nanoflakes and RGO sheets. The NiO-ZnO nanoflakes offered a richness of electrochemical reaction sites and shortened the diffusion path of Li+, whereas the RGO sheets could improve the mechanical stability and conductivity of the electrode during lithium cycling. Designing binary or ternary oxides and RGO sheets composite electrodes via an in situ ultrasonic agitation approach might achieve higher lithium-storage properties in future studies inspired by the findings within this work.
Acknowledgements We thank the Fundamental Research Funds for the Central Universities (Grant No. 17lgpy68), Pearl River S&T Nova Program of Guangzhou (Grant No.201710010043) and National Natural Science Foundation of China (Grant No. 51676212) for financial support.
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