Anchoring ZnO Nanoparticles in Nitrogen-Doped

materials Article

Anchoring ZnO Nanoparticles in Nitrogen-Doped Graphene Sheets as a High-Performance Anode Material for Lithium-Ion Batteries Guanghui Yuan 1 , Jiming Xiang 1 , Huafeng Jin 1 , Lizhou Wu 1 , Yanzi Jin 1 and Yan Zhao 2, * 1

2

*

School of Chemistry and Chemical Engineering, Ankang University, Ankang 725000, China; [email protected] (G.Y.); [email protected] (J.X.); [email protected] (H.J.); [email protected] (L.W.); [email protected] (Y.J.) Synergy Innovation Institute of GDUT, Heyuan 517000, China Correspondence: [email protected]

Received: 21 November 2017; Accepted: 5 January 2018; Published: 10 January 2018

Abstract: A novel binary nanocomposite, ZnO/nitrogen-doped graphene (ZnO/NG), is synthesized via a facile solution method. In this prepared ZnO/NG composite, highly-crystalline ZnO nanoparticles with a size of about 10 nm are anchored uniformly on the N-doped graphene nanosheets. Electrochemical properties of the ZnO/NG composite as anode materials are systematically investigated in lithium-ion batteries. Specifically, the ZnO/NG composite can maintain the reversible specific discharge capacity at 870 mAh g−1 after 200 cycles at 100 mA g−1 . Besides the enhanced electronic conductivity provided by interlaced N-doped graphene nanosheets, the excellent lithium storage properties of the ZnO/NG composite can be due to nanosized structure of ZnO particles, shortening the Li+ diffusion distance, increasing reaction sites, and buffering the ZnO volume change during the charge/discharge process. Keywords: anode material; graphene nanocomposite; lithium-ion battery; ZnO/nitrogen-doped; ZnO nanoparticles

1. Introduction Lithium-ion batteries (LIBs) are one of the most promising modern electrochemical devices for energy storage, due to their high voltage, high energy density, and long lifespan [1–4]. Graphite has been widely used as the anode material due to its exhibiting stable performance in commercial LIBs [5]. Whereas the theoretical capacity of graphite is only 372 mAh g−1 and it cannot meet the increasing power demands [6]. To address these issues, remarkable efforts have been made to develop various promising candidates for anode materials to replace graphite, such as transition metal oxides (MO, M = Mn, Fe, Co, Ni, Cu, Zn, etc.) [7–14]. Among the above metal oxides, ZnO anode stands out as a potential alternative anode due to its high theoretical capacity (978 mAh g−1 ), low cost, ease to preparation, and chemical stability [15–17]. However, pure ZnO generally exhibits low reversible capacity and severe capacity fading, such a result is mainly caused by its large volume variation during the Li-ion insertion/extraction processes [16]. A lot of effort has therefore been devoted to conquer the above-mentioned shortcomings and a series of methods have been performed to improve properties of ZnO electrode. These methods include (i) preparing ordered ZnO nanostructured materials [18–21]; (ii) compositing ZnO with carbon materials [5,15–17]; (iii) doping with other metal oxides [22–24]. These techniques can improve conductivity, promote the lithiation/delithiation process, or buffer volume changes to mitigate the pulverization of the active particles. Among these methods, the remarkable and effective strategy to overcome the low conductivity is to composite ZnO with high electro-conducting material such

Materials 2018, 11, 96; doi:10.3390/ma11010096

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Materials 2018, 11, 96 Materials 2017, 10, 96

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material such as porous carbon[25]. in the form of carboncarbon supporting [25]. Among as electro-conducting porous carbon in the form of carbon supporting Among various materials, graphene carbon materials, has material been recognized serviceable material because of its hasvarious been recognized as verygraphene serviceable becauseasofvery its excellent high thermal conductivity, excellent high thermal conductivity, electrical conductivity, and high specific surface area [26]. electrical conductivity, and high specific surface area [26]. Considerable efforts to achieve controllable Considerable efforts to achieve controllable graphene and ZnO/graphene composites have been graphene and ZnO/graphene composites have been extensively made by many different techniques, extensively made by many techniques, such asmethod plasma[28], synthesis [27],heterocoagulation homogenizing such as plasma synthesis [27],different homogenizing dispersion stepwise dispersion method [28], stepwise heterocoagulation method [29], freeze drying method [30] and so method [29], freeze drying method [30] and so on [31,32]. Although these composites demonstrated on [31,32]. Although these composites demonstrated enhanced electrochemical performance, it is enhanced electrochemical performance, it is still not completely satisfactory. To meet this challenge, still not completely satisfactory. To meet this challenge, nitrogen-doped (N-doped) graphene can nitrogen-doped (N-doped) graphene can increase its conductivity by raising the Fermi level towards increase its conductivity by raising the Fermi level towards the conduction band compared with the conduction band compared with pure graphene [33]. Moreover, N-doped graphene (NG) can pure graphene [33]. Moreover, N-doped graphene (NG) can offer more active nucleation sites, which offer more active nucleation sites, which effectively prevent the aggregation of nanoparticles and effectively prevent the aggregation of nanoparticles and strengthen the binding energy [34,35]. strengthen the binding energy [34,35]. compared graphene, NG is favorable with for the Therefore, compared to graphene, NGTherefore, is more favorable fortothe composition of more the nanosheets composition of the nanosheets with metal oxides. metal oxides. ToTo thethe best of of our knowledge, ZnO/N-doped graphene best our knowledge,there thereare arefew fewstudies studies on on synthesizing synthesizing ZnO/N-doped graphene composite. method toto synthesize synthesizenanostructured nanostructured composite.InInthis thiswork, work,we wereport report aa facile facile and and scalable scalable method ZnO/N-doped graphene (ZnO/NG) nanocomposite. The effect of N-doped graphene ZnO anode ZnO/N-doped graphene (ZnO/NG) nanocomposite. The effect of N-doped graphene onon ZnO anode performance areare systemically ZnO/NGnanocomposite nanocomposite can exhibit performance systemicallyinvestigated investigatedin inLIBs, LIBs, in in which which the ZnO/NG can exhibit superior cycling stability superior cycling stabilityand andrate ratecapability. capability.

2. Results and Discussion 2. Results and Discussion The crystal structure of of thethe obtained ZnO/NG The crystal structure obtained ZnO/NGcomposite compositeisisfirstly firstlyinvestigated investigatedby byXRD. XRD. Figure Figure 1a 1a exhibits the XRD patterns of ZnO, N-dopedgrapheme, grapheme,and and the the as-prepared composite. exhibits the XRD patterns of ZnO, N-doped as-preparedZnO/NG ZnO/NG composite. The mainpeaks peaksof of ZnO/NG ZnO/NG composite with thethe peaks of ZnO, whichwhich can becan indexed as ZnO as The main compositeagree agree with peaks of ZnO, be indexed with the lattice parameters of a = 0.325, b = 0.325, and c = 0.5207 nm (JCPDS No. 36-1451). In addition, ZnO with the lattice parameters of a = 0.325, b = 0.325, and c = 0.5207 nm (JCPDS No. 36-1451). ZnO/NG an obvious diffraction peakdiffraction at around 26°, which is attributed to the is In the addition, thecomposite ZnO/NGdisplays composite displays an obvious peak at around 26◦ , which (002) reflection of stacking layers of graphene. The average size of ZnO crystallite in ZnO/NG attributed to the (002) reflection of stacking layers of graphene. The average size of ZnO crystallite is calculated be 9.9 nm Theformula. XRD pattern of in composite ZnO/NG composite is to calculated to using be 9.9 the nmDebye using Scherrer’s the Debyeformula. Scherrer’s The XRD ZnO/NG indicates that small ZnO nanoparticles are successfully deposited on the surface of pattern of ZnO/NG indicates that small ZnO nanoparticles are successfully deposited on the surface of N-doped graphene sheets. The presence of N-doped graphene has no significant effect on the N-doped graphene sheets. The presence of N-doped graphene has no significant effect on the formation formation of ZnO nanoparticles. The results can also be proved by the following TEM data. The of ZnO nanoparticles. The results can also be proved by the following TEM data. The successful successful deposition of ZnO nanoparticles on the surface of N-doped graphene sheets can facilitate deposition of ZnO nanoparticles on the surface of N-doped graphene sheets can facilitate electron electron transportation and alleviate the volume expansion of ZnO during the discharge/charge transportation and alleviate the volume expansion of ZnO during the discharge/charge cycling, cycling, resulting in a drastic improvement of the electrochemical performance, which can be proved resulting a drastic electrochemical improvement ofmeasurements. the electrochemical performance, which can of be proved by the in following To determine the percentage ZnO in by thethe following electrochemical measurements. To determine themeasurement percentage ofwas ZnOcarried in the prepared prepared ZnO/NG composite, thermo gravimetric (TG) out underZnO/NG an air composite, thermo gravimetric (TG) measurement was carried out under an air atmosphere. As shown atmosphere. As shown in Figure 1b, the TGA curve of ZnO/NG has an obvious one-step weight loss in from Figurearound 1b, the400 TGA ZnO/NG has an obvious weight loss fromapproximately around 400 ◦ C. °C.curve Whenofthe temperature reaches 700one-step °C, the sample remains ◦ When the temperature reaches the sample remains approximately unchanged. Comparing to the700 TGAC, curve of pristine ZnO, the percentage ofunchanged. coating NGComparing in ZnO/NG to thecomposite TGA curve pristine ZnO, is of about 30.4 wt %. the percentage of coating NG in ZnO/NG composite is about 30.4 wt %.

Figure XRDpatterns; patterns,(b) (b)thermogravimetric thermogravimetric curves curves of composite. Figure 1. 1. (a)(a) XRD of ZnO ZnOand andZnO/NG ZnO/NG composite.


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To elucidate the valences of C, Zn, N elements and their bonds configuration in ZnO/NG To XPS elucidate the valences C, Zn, investigated. N elements and their2a bonds configuration in spectrums ZnO/NG of composite, measurements areoffurther Figure displays the survey composite, XPS measurements are further investigated. Figure 2a displays the survey spectrums of ZnO/NG and ZnO/G composite. Four elements (Zn, O, C, and N) can easily be detected through ZnO/NG and ZnO/G composite. Four elements (Zn, O, C, and N) can easily be detected through the the characteristic peaks of Zn2p, O1s, C1s, and N1s in the survey spectrums of ZnO/NG composite. characteristic peaks of Zn2p, O1s, C1s, and N1s in the survey spectrums of ZnO/NG composite. In In comparison, only the Zn, C, and O elements are identified through the survey spectrum of ZnO/G comparison, only the Zn, C, and O elements are identified through the survey spectrum of ZnO/G composite. TheThe result reveals that inthe theZnO/NG ZnO/NG composite synthesized composite. result reveals thatnitrogen nitrogendoping doping existed existed in composite synthesized by by our simple solution method. Figure 2b shows the high-resolution XPS spectra of Zn2p, which consists our simple solution method. Figure 2b shows the high-resolution XPS spectra of Zn2p, which of two strongofpeaks at 1045.5 eV and 1022.5eV eV,and identifying theidentifying Zn2p1/2 and spin-orbit peaks consists two strong peaks at 1045.5 1022.5 eV, the Zn2p3/2 Zn2p1/2 and Zn2p3/2 of ZnO, respectively. Zn2p1/2 −Zn2p3/2 energy separationenergy is around 23.0 eV. high-resolution spin-orbit peaks ofThe ZnO, respectively. The Zn2p1/2−Zn2p3/2 separation is The around 23.0 eV. scanThe of C1s is shown in Figure 2c.is shown The strongest at strongest 285.0 eV peak corresponds tocorresponds the graphite-like high-resolution scan of C1s in Figurepeak 2c. The at 285.0 eV to the graphite-like sp2 C (C−C and C=C bonds), indicating that most C atoms in the ZnO/NG sp2 C (C−C and C=C bonds), indicating that most C atoms in the ZnO/NG composite are ranged composite are ranged in the conjugated honeycomb lattice.at The peakeV located at 286.1 related to in the conjugated honeycomb lattice. The peak located 286.1 is related toeV theis C-N linkage, theisC-N linkage, which is originated the N atoms’ofincorporation of the ZnO/NG The composite. Thepeak which originated from the N atoms’from incorporation the ZnO/NG composite. weakest weakest peak at the 288.8 eV reflects thewhich C=O linkage, which indicates a little residual oxygen existed in at 288.8 eV reflects C=O linkage, indicates a little residual oxygen existed in the ZnO/NG the ZnO/NG composite. In addition, the special peak at 283.5 eV in the C1s spectrum can be assigned composite. In addition, the special peak at 283.5 eV in the C1s spectrum can be assigned to the Zn-O-C to the Zn-O-C bond [36]. The N1s spectrum of the ZnO/NG composite shown in Figure 2d can be bond [36]. The N1s spectrum of the ZnO/NG composite shown in Figure 2d can be attributable to attributable to the pyridinic N, pyrrolic N, and graphitic N atoms doped in graphene, according to the pyridinic N, pyrrolic N, and graphitic N atoms doped in graphene, according to the peaks at 398.3, the peaks at 398.3, 399.8, and 401.7 eV, respectively [37,38]. The N atoms doped in graphene have 399.8, and 401.7 eV, respectively [37,38]. The atoms doped in graphene have excellent donor excellent electron donor characteristics andNgood charge mobility in the graphene lattice,electron which can characteristics goodcarbon chargecatalytic mobilityactivity in the graphene lattice, which can effectively improve the carbon effectively and improve in electrochemical reactions [39]. Predicatively, catalytic activity in electrochemical reactions [39].enhance Predicatively, the ZnO/NG compositeofobtained ZnO/NG composite obtained in this work can the electrochemical performance anode in this material work can thebatteries. electrochemical performance of anode material in lithium-ion batteries. inenhance lithium-ion

Figure 2. Survey XPS spectra (a) of ZnO/NG and ZnO/G composites; XPS spectra of (b) Zn2p, (c) C1s, Figure 2. Survey XPS spectra (a) of ZnO/NG and ZnO/G composites; XPS spectra of (b) Zn2p, (c) C1s, and (d) N1s of ZnO/NG composite. and (d) N1s of ZnO/NG composite.

Figure 3 shows the scanning electron microscopy (SEM) and transmission electron microscopy Figureimages 3 shows the as-prepared scanning electron microscopy (SEM) and transmission microscopy (TEM) of the NG and ZnO/NG composite. The SEM image electron shows that small (TEM) images of the as-prepared NG and ZnO/NG composite. SEMN-doped image shows thatsurface, small ZnO ZnO nanoparticles were well supported homogeneously on theThe curved graphene

nanoparticles were well supported homogeneously on the curved N-doped graphene surface, as seen in Figure 3a. To further investigate the distribution and size of ZnO particles in ZnO/NG composite,


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the pristine NG and ZnO/NG composites are the characterized TEM. shows the TEM image as seen in Figure 3a. To further investigate distributionby and sizeFigure of ZnO3bparticles in ZnO/NG of the synthesized N-doped The stacking are layers of N-doped graphene an obvious composite, the pristine NGgraphene. and ZnO/NG composites characterized by TEM. Figureshow 3b shows the TEM image of the synthesized N-doped graphene. The stacking layers ofand N-doped show in wrinkled surface, which is beneficial for ZnO nanoparticles to deposit anchorgraphene on. As shown an obvious wrinkled TEM surface, which beneficial composite for ZnO nanoparticles tosmall deposit andnanoparticles anchor on. As are the high-magnification image ofisZnO/NG (Figure 3c), ZnO shown in the TEM image of ZnO/NG composite (Figure This 3c), result small strongly ZnO barely obvious andhigh-magnification agglomeration and the nanoparticles size are homogeneous. nanoparticles are barely obvious and agglomeration and the nanoparticles size are homogeneous. indicates that the presence of nitrogen plays an essential role in the formation of homogeneous ZnO. strongly indicates that the presence of nitrogen plays an essential role in the formation of The This ZnOresult nanoparticle diameter distribution obtained from TEM is determined to be about 10.0 nm homogeneous ZnO. The ZnO nanoparticle diameter distribution obtained from TEM is determined (Figure 3d). The lattice fringe of an interplane distance is measured to be 0.26 nm, which corresponds to be about 10.0 nm (Figure 3d). The lattice fringe of an interplane distance is measured to be 0.26 to the (002) plane of the ZnO crystals (insert in Figure 3d). The SEM and TEM images prove that the nm, which corresponds to the (002) plane of the ZnO crystals (insert in Figure 3d). The SEM and ZnO/NG nanocomposite is successfully by our facile method. In the prepared TEM images prove that the ZnO/NGsynthesized nanocomposite is successfully synthesized by our ZnO/NG facile nanocomposite, the ZnO particles with a size of about 10.0 nm are anchored uniformly on the wrinkled method. In the prepared ZnO/NG nanocomposite, the ZnO particles with a size of about 10.0 nm are and anchored twisted N-doped sheets.and twisted N-doped graphene sheets. uniformlygraphene on the wrinkled

Figure 3. (a) SEM image of the ZnO/NG composite, TEM images of (b) NG and (c,d) ZnO/NG composite.

Figure 3. (a) SEM image of the ZnO/NG composite; TEM images of (b) NG and (c,d) ZnO/NG composite.

To examine the effect of N-doped graphene, the electrochemical performance of ZnO/NG To examine the effect of N-doped graphene, theinvestigated. electrochemical performance of ZnO/NG nanocomposite in lithium-ion battery is systematically The cyclic voltammogarm (CV) profiles of ZnO/NG electrode in theisinitial three cycles are exhibited in cyclic Figurevoltammogarm 4a at 0.1 mV s−1(CV) nanocomposite in lithium-ion battery systematically investigated. The between 0–3.0 V. electrode In the firstincathodic scan, a strong peakinnearby V 0.1 canmV be observed profiles of ZnO/NG the initial three cyclesreduction are exhibited Figure0.15 4a at s−1 between in the ZnO/NG composite electrode. It could be assigned to the reaction of ZnO with Li into and 0–3.0 V. In the first cathodic scan, a strong reduction peak nearby 0.15 V can be observed in theZn ZnO/NG Li2O, the formation of Li-Zn alloy, and the solid electrolyte interphase (SEI) layer. In the subsequent composite electrode. It could be assigned to the reaction of ZnO with Li into Zn and Li2 O, the formation second and third cathodic scans, the reduction peaks nearby 0.41 and 0.60 V demonstrate the of Li-Zn alloy, and the solid electrolyte interphase (SEI) layer. In the subsequent second and third reversibility of the lithiation process of ZnO. The disappearance of the strong peak nearby 0.20 V cathodic scans, thethe reduction peaks nearby 0.41 and 0.60 V demonstrate thethree reversibility of theexhibit lithiation indicates that formation of the SEI layer is irreversible. The initial anodic scans process of ZnO. The disappearance of the strong peak nearby 0.20 V indicates that the formation of the consistency in shape. The oxidation peaks nearby 0.38, 0.54, and 0.70 V indicate the multi-step SEI layer is irreversible. Thealloy, initialwhile threethe anodic scans consistency shape. The oxidation decomposition of Li–Zn nearby 1.34exhibit V can be ascribed toin the decomposition of Li2peaks O and0.38, formation of ZnO the reaction between Li2decomposition O and Zn [18,20].ofOverall, the CVwhile curves show nearby 0.54, and 0.70with V indicate the multi-step Li–Zn alloy, the nearby consistency, indicating goodformation reversibility of the ZnO/NG electrode. 1.34 high V canreproducibility be ascribed to and the decomposition of Li2 O and of ZnO with the reaction between

Li2 O and Zn [18,20]. Overall, the CV curves show high reproducibility and consistency, indicating good reversibility of the ZnO/NG electrode. The intensity of reduction and oxidation peaks grow weaker


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with the scans progress, which may be because of the incomplete conversion between Zn and ZnO. The intensity of reduction and oxidation peaks grow weaker with the scans progress, which may be Figure 4b depicts the initial galvanostatic charge/discharge profiles of the ZnO/NG nanocomposite at because of the incomplete conversion between Zn and ZnO. Figure 4b depicts the initial a current density of 100 mA g−1 . As shown in Figure 4b, the plateau voltages of the charge/discharge galvanostatic charge/discharge profiles of the ZnO/NG nanocomposite at a current density of 100 profiles are in good with peakvoltages voltagesofin profiles in Figure 4a. are TheinZnO/NG −1. As mA g shownagreement in Figure 4b, thethe plateau theCV charge/discharge profiles good −1 in the first cycle. The coulombic nanocomposite delivers a large discharge capacity of 1894 mAh g agreement with the peak voltages in CV profiles in Figure 4a. The ZnO/NG nanocomposite delivers −1 in theby efficiency only 56.5%, which is mainly the irreversible extra discharge during a largeis discharge capacity of 1894 mAh gcaused first cycle. The coulombic efficiency iscapacity only 56.5%, which is mainly causedThe by the irreversible extraobserved discharge in capacity during the SEI layer formation. the SEI layer formation. long slope region the initial discharge disappears in the The long slope region observed in the initial discharge disappears in the following cycles. The following cycles. The ZnO/NG nanocomposite delivers discharge capacities of 1148 and 968 mAh g−1 ZnO/NG delivers dischargeThe capacities of 1148 and 968 mAhefficiencies g−1 in the second and in the secondnanocomposite and third cycles, respectively. corresponding coulombic have reached third cycles, respectively. The corresponding coulombic efficiencies have reached points of 79.9% points of 79.9% and 87.4%. After the first discharge process, the charge/discharge curves maintain and 87.4%. After the first discharge process, the charge/discharge curves maintain a similar size and a similar size and shape, indicating the ZnO/NG nanocomposite keeps a relatively steady state during shape, indicating the ZnO/NG nanocomposite keeps a relatively steady state during the the lithiation/delithiation processes. lithiation/delithiation processes.

Figure 4. (a) CV curves of the ZnO/NG composite electrode at a scan rate of 0.1 mV s−1 and Figure 4. (a) CV curves of the ZnO/NG composite electrode at a scan rate of 0.1 mV s−1 and (b) discharge/charge profiles of ZnO/NG anode at 100 mA g−1. (b) discharge/charge profiles of ZnO/NG anode at 100 mA g−1 .

The rate performance and cycle ability are important aspects for the application of ZnO/NG The rate performance and cycle ability are important aspects for the application ZnO/NG nanocomposite in lithium-ion batteries. Figure 5a shows the rate performances of the of ZnO/NG composite and counterpartsbatteries. (pristine ZnO and 5a ZnO/G composite) 100 mA g−1, respectively. The nanocomposite initslithium-ion Figure shows the rateatperformances of the ZnO/NG ZnO/G and composite is prepared (pristine by the same as preparing ZnO/N-doped just composite its counterparts ZnOprocedure and ZnO/G composite) at 100 mAgrapheme, g−1 , respectively. using graphene instead of N-doped graphene. So the influence of different experiment methods on The ZnO/G composite is prepared by the same procedure as preparing ZnO/N-doped grapheme, different electrochemical performances can be eliminated. As demonstrated in Figure 5a, the test just using graphene instead of N-doped graphene. So the influence of different experiment methods current increases stepwise, and the rate performances of the ZnO/NG and ZnO/G electrodes have on different electrochemical performances can be eliminated. As demonstrated in Figure 5a, the test been significantly improved comparing to the pristine ZnO electrode. In detail, for the ZnO/NG current increases stepwise, and the rate performances of the ZnO/NG and ZnO/G electrodes have been electrode, the reversible discharge capacities of 968, 687, 567, 404, and 271 mAh g−1 are achieved at significantly improved comparing to the pristine ZnO electrode. In detail, for the ZnO/NG electrode, current rates of 100, 200, 400, 800, and 1600 mA g−1, respectively. The further return of the discharge − 1 the reversible discharge capacities of 968,the 687, 567,reversible 404, anddischarge 271 mAhcapacity g areofachieved current rate to the initial 100 mA g−1 can recover stable nearly 680atmAh −1 , respectively. The further return of the discharge rate ratesg−1of 100, 200, 400, 800, and 1600 mA g −1 . For the ZnO/G electrode at different current densities of 100, 200, 400, 800, and 1600 mA g , the 1 can recover the stable reversible discharge capacity of nearly to the initial 100 mA g−capacities 680 mAh g−1 . reversible discharge of each period are 849, 521, 404, 282, and 166 mAh g−1, respectively. For While the ZnO/G at different densities of 100, 200, 400, 1600 for the electrode pristine ZnO electrode, current the reversible discharge capacities at 800, each and current ratemA areg−1 , − 1 −1 100~300 mAh g smaller than that of theperiod ZnO/Gare and ZnO/NG electrodes. cycle, respectively. abilities the reversible discharge capacities of each 849, 521, 404, 282, andLong-term 166 mAh g −1 are exhibited in Figure 5b. It can be easily of the ZnO/G, and anodes 100 mA gdischarge While for ZnO, the pristine ZnOZnO/NG electrode, the at reversible capacities at each current rate are seen mAh that both specific capacities capacity and retentions of theelectrodes. ZnO/G andLong-term ZnO/NG anodes are 100~300 g−1the smaller than that of and the ZnO/G ZnO/NG cycle abilities much better than that of the ZnO anode. During the three anodes, the ZnO/NG anode could − 1 of the ZnO, ZnO/G, and ZnO/NG anodes at 100 mA g are exhibited in Figure 5b. It can be easily highest reversible specific discharge capacity of 870 mAh g−1 at 100 mA g−1 after 200 seenmaintain that boththe the specific capacities and capacity retentions of the ZnO/G and ZnO/NG anodes are cycles. The ZnO/G and pristine ZnO anodes could only deliver the reversible specific discharge much better than that of the ZnO anode. During the three anodes, the ZnO/NG anode could maintain capacity of 493 and 318 mAh g−1 at 100 mA g−1 after 200 cycles, respectively. The coulombic the highest reversible specific discharge capacity of 870 mAh g−1 at 100 mA g−1 after 200 cycles. efficiencies of the three anodes are a little lower in the first several cycles, which could be inferred The from ZnO/G and pristineextra ZnOdischarge anodes could only deliver the layer reversible specific discharge the irreversible capacity during the SEI formation. After 50 cycles,capacity all the of

493 and 318 mAh g−1 at 100 mA g−1 after 200 cycles, respectively. The coulombic efficiencies of the three anodes are a little lower in the first several cycles, which could be inferred from the irreversible


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extra discharge capacity during the SEI layer formation. After 50 cycles, all the coulombic efficiencies efficiencies these 99%, anodes stabilize around 99%,retention. showing Detailed good capacity retention. of coulombic these anodes stabilize of around showing good capacity observations from Detailed observations from Figure 5 reveal that the reversibility and cycling stability of the ZnO/NG Figure 5 reveal that the reversibility and cycling stability of the ZnO/NG and ZnO/G anodes are better and ZnO/G areZnO better than These that ofenhanced the pristine ZnO anode. These enhanced may electrochemical than that of theanodes pristine anode. electrochemical performances be due to the performances may be due to the positive effect of the graphene or N-doped graphene additive, positive effect of the graphene or N-doped graphene additive, which are not only capable of providing which are not only capable of providing a high electronic conductivity and short Li+ diffusion a high electronic conductivity and short Li+ diffusion distances, but also serving as a stable carrier for distances, but also serving as a stable carrier for ultrafine ZnO particles anchored on it. Furthermore, ultrafine ZnO particles anchored on it. Furthermore, the ZnO/NG nanocomposite can deliver better the ZnO/NG nanocomposite can deliver better electrochemical performances than ZnO/G electrochemical performances than ZnO/G composite, which may be originated from the following composite, which may be originated from the following two aspects. Firstly, the N atoms doped in two aspects. Firstly, the N atoms doped in graphene have excellent electron donor characteristics and graphene have excellent electron donor characteristics and good charge mobility in graphene lattice, good charge in graphene can activity effectively carbon reactions catalytic activity which canmobility effectively improve lattice, carbon which catalytic in improve electrochemical [33,34]. in electrochemical [33,34]. Secondly, the binding energy and between ultrafine ZnO nanosheets particles and Secondly, the reactions binding energy between ultrafine ZnO particles N-doped graphene N-doped graphene nanosheets are improved due to the extra active sites provided by nitrogen doping. are improved due to the extra active sites provided by nitrogen doping.

Figure 5. (a) Rate capabilities, (b)cycle cycleabilities abilitiesofofZnO, ZnO,ZnO/G, ZnO/G, and g−g1.−1 . Figure 5. (a) Rate capabilities; (b) and ZnO/NG ZnO/NGanodes anodesatat100 100mA mA

Compared to previous work, the as-prepared ZnO/NG composite exhibits more excellent Compared toperformances, previous work, the in as-prepared ZnO/NG composite excellent electrochemical as listed Table 1. The large initial dischargeexhibits capacitymore delivered by electrochemical performances, as listed in Table 1. The large initial discharge capacity delivered ZnO/NG composite is mainly caused by the irreversible extra discharge capacity during the SEIby ZnO/NG composite is mainly caused by the irreversible extra dischargecomposite capacity during SEIthe layer layer formation. The significantly improved cycle ability of ZnO/NG benefitsthe from formation. The significantly cycle ability ZnO/NGgraphene. composite benefits fromgraphene the unique unique structure and the improved positive effect of the ofN-doped The N-doped structure and the of thethe N-doped graphene. N-doped grapheneand nanosheets can not nanosheets can positive not onlyeffect improve conductivity of theThe ZnO/NG composite provide more only improve the conductivity of the composite provide more active electrochemical active electrochemical sites, but theyZnO/NG can also alleviate theand volume expansion of ZnO during the discharge/charge cycling. It is the worth noting that N-doped graphene itselfdischarge/charge can be used as anode sites, but they can also alleviate volume expansion of ZnO during the cycling. material for lithium-ion batteries. During the discharge process of the ZnO/NG composite, some Li It is worth noting that N-doped graphene itself can be used as anode material for lithium-ion ions can react with ZnO to form Li-Zn alloy, other Li ions can intercalate into the stacking layers of batteries. During the discharge process of the ZnO/NG composite, some Li ions can react with N-doped graphene in theother meantime. the ZnO/NG composite lithium graphene storage ZnO to form Li-Zn alloy, Li ionsTherefore, can intercalate into the stackingshows layersbetter of N-doped capacities than N-doped graphene material. in the meantime. Therefore, the ZnO/NG composite shows better lithium storage capacities than

N-doped graphene material.

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Table 1. Comparison of electrochemical performances between the N-doped graphene and ZnO/NG composite. Table 1. Comparison of electrochemical performances between the N-doped graphene and Cycle Reversible ZnO/NG composite. Initial Discharge Sample Rate Ref. Capacity Number Capacity ZnO/NG 1894 mAh g−1 200 100 mA g−1 870 mAh g−1 this work Initial Discharge Reversible Ref. Cycle Number Rate Sample −1 Capacity Capacity ZnO@graphene 1050 mAh g 50 1C 460 mAh g−1 [28] Graphene-porous ZnO/NG 200 this work 1894 mAh g−1 100 mA g−1 870 mAh g−1 100 0.1 C 660 mAh g−1 [29] 1300 mAh g−1 −1 ZnO@graphene 50 1 C [28] 1050 mAh g 460 mAh g−1 carbon-ZnO −1 −1 Graphene-porous carbon-ZnO 100 0.1 C [29] 1300 mAh g 660 mAh g −1 −1 −1 ZnO–graphene 1450 mAh g −1 100 100 mA g 900 mAh g [30] ZnO–graphene 100 [30] 1450 mAh g 100 mA g−1 900 mAh g−1 N-doped graphene 1420 mAh g−1 −1 50 100 mA g−1 −1630 mAh g−1 −1 [39] N-doped graphene

1420 mAh g

50

100 mA g

630 mAh g

[39]

To confirm the positive effect of graphene or N-doped graphene additives on the conductivity confirm the positive effect of or N-doped graphene additives on the conductivity(EIS) and and To charge transfer behavior in graphene composites, electrochemical impedance spectroscopy charge transfer behavior in composites, electrochemical impedance spectroscopy (EIS) measurements measurements were investigated on the ZnO, ZnO/G, and ZnO/NG electrodes. The EIS data for the were investigated on the ZnO, ZnO/NG electrodes. for the three electrodes three electrodes measured afterZnO/G, the firstand cycle are shown in FigureThe 6a. EIS All data the impedance plots for the measured after the first cycle are shown in Figure 6a. All the impedance plots for the fully discharged fully discharged states are composed of two parts, a semicircle in high-to-medium frequency and a states are composed of two parts, semicircleisin high-to-medium and a straight line in straight line in low frequency. The asemicircle mainly attributed tofrequency the charge transfer impedance lowthe frequency. The semicircle mainly attributed to the charge the electrode. of electrode. The straight isline is mainly attributed to the transfer Warburgimpedance impedanceofreflecting the The straight line is mainly attributed to the Warburg impedance reflecting the solid-state diffusion of + solid-state diffusion of Li into the bulk of the active materials [40]. It can be easily seen from Figure + into the bulk of the active materials [40]. It can be easily seen from Figure 6a that the charge transfer Li 6a that the charge transfer impedances of the ZnO, ZnO/G, and ZnO/NG electrodes are nearly 325, impedances ZnO, ZnO/G, and ZnO/NG electrodes of areSEI nearly 325, 130, and 90 Ω, 130, and 90 of Ω,the respectively. Considering the formation is completed after firstrespectively. cycle, the Considering the formation of SEI is completed after first cycle, the impedances can be mainly impedances can be mainly attributed to the additive and the material structure. By adding attributed graphene to theN-doped additivegraphene, and the material structure. By adding graphene and N-doped charge and the charge transfer impedances of the ZnO/G andgraphene, ZnO/NG the electrodes transfer impedances of the ZnO/G and electrodes decrease graphene dramatically, indicating that decrease dramatically, indicating that ZnO/NG the graphene or N-doped additive plays a the graphene or N-doped graphene additive plays a significant role in improving the conductivity. significant role in improving the conductivity. The ZnO/NG electrodes exhibit the smallest charge The ZnO/NG electrodes smallest charge transfer impedance. The results could be due to transfer impedance. Theexhibit resultsthe could be due to the changes in the material conductivity and the changes in the material conductivity and morphology introduced by N-doped graphene sheets [35]. morphology introduced by N-doped graphene sheets [35]. The N-doped graphene can enhance the The N-doped canby enhance the charge transfer conditions by providing anshortening effective electron charge transfergraphene conditions providing an effective electron conduction path and the Li+ + conduction path and shortening the Li diffusion distance within the nanosized ZnO particles anchored diffusion distance within the nanosized ZnO particles anchored evenly on the N-doped graphene evenly on the N-doped sheets. In addition, the EISatdata of thestages ZnO/NG different sheets. In addition, the graphene EIS data of the ZnO/NG electrode different are electrode shown inat Figure 6b. stages are shown in Figure 6b. The diameter of semicircles in the high-to-medium frequency region The diameter of semicircles in the high-to-medium frequency region reduce gradually as the test reduce gradually as the the decrease test proceeds, indicating the decrease of the charge proceeds, indicating of the charge transfer impedances. The chargetransfer transferimpedances. impedance The charge transfer impedance of the new ZnO/NG electrode is as high as about 170the Ω, which is of the new ZnO/NG electrode is as high as about 170 Ω, which is mainly because effective mainly because the effective electron conduction andin SEI layerelectrode. are not formed in fresh electrode. electron conduction paths and SEI layer are notpaths formed fresh Low and stable charge Low and stable charge transfer impedances can be realized for the ZnO/NG electrode during repeated transfer impedances can be realized for the ZnO/NG electrode during repeated discharge/charge discharge/charge cycles, which is about Ωcycle and 80 Ω 200th after 1st cycle and 200th cycle, respectively. cycles, which is about 90 Ω and 80 Ω after901st and cycle, respectively. This phenomenon This phenomenon indicates that the of special structurecomposite of the ZnO/NG composite has beautiful rigidity indicates that the special structure the ZnO/NG has beautiful rigidity and stability to and stability to endure the repeated lithium insertion/extraction processes. endure the repeated lithium insertion/extraction processes.

Figure plots of of (a) (a) ZnO, ZnO, ZnO/G ZnO/G and Figure 6. 6. Nyquist Nyquist plots and ZnO/NG ZnO/NGelectrodes electrodes after after first first discharge/charge discharge/charge cycle, cycle; and (b) ZnO/NG electrode at different stages with the frequency region of 100 kHz to 0.01 Hz. and (b) ZnO/NG electrode at different stages with the frequency region of 100 kHz to 0.01 Hz.


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3. Materials and Methods 3. Materials andofMethods Synthesis nitrogen-doped graphene (NG): Firstly, graphene oxide (GO) was synthesized fromSynthesis natural flake graphite by a graphene modified (NG): Hummers’ [41]. Subsequently, 0.060 g GOfrom was of nitrogen-doped Firstly,method graphene oxide (GO) was synthesized dispersed into 100 mLbyethanol. After ultrasonication 2 h,Subsequently, 12 mL hydrazine and 8 mL of natural flake graphite a modified Hummers’ methodfor [41]. 0.060hydrate g GO was dispersed ammonia (25%) were added into the suspension in order. The mixture was stirred vigorously for 15 into 100 mL ethanol. After ultrasonication for 2 h, 12 mL hydrazine hydrate and 8 mL of ammonia min and then sealed into three 50 mL teflon-lined stainless steel autoclaves at 180 °C for 3 h. The (25%) were added into the suspension in order. The mixture was stirred vigorously for 15 min and ◦ blacksealed powder autoclaves collected and washed by ethanolatand then dried then intointhree 50 mL was teflon-lined stainless steel autoclaves 180distilled C for 3water h. Theand black powder in a vacuum oven at 80 °C overnight to obtain the NG product. in autoclaves was collected and washed by ethanol and distilled water and then dried in a vacuum of ZnO/NG composite: Firstly, 0.720 g Zinc acetate (Zn(CH3COO)2) and 0.020 g NG oven Synthesis at 80 ◦ C overnight to obtain the NG product. wereSynthesis dispersedofinZnO/NG 50 mL ethanol to form a homogeneous solution. A measure of 0.190 g lithium composite: Firstly, 0.720 g Zinc acetate (Zn(CH 3 COO)2 ) and 0.020 g NG hydroxide (LiOH) was dissolved in another 50 mL ethanol solution. Both solutions for were dispersed in 50 mL ethanol to form a homogeneous solution. A measure of were 0.190 stirred g lithium 2 h using(LiOH) a magnetic stirrer. inSubsequently, LiOH solution was addedwere dropwise to 2the hydroxide was dissolved another 50 mLthe ethanol solution. Both solutions stirred for h Zn(CH 3 COO) 2 solution under constant vigorous stirring. After stirring for 24 h at room using a magnetic stirrer. Subsequently, the LiOH solution was added dropwise to the Zn(CH3 COO)2 temperature, homogenously blackstirring suspension The blacka homogenously powders were solution underaconstant vigorousdispersed stirring. After for 24was h at obtained. room temperature, separatedblack via filtration andwas washed thoroughly bypowders deionized water and ethanol. Finally, black dispersed suspension obtained. The black were separated via filtration andthe washed ZnO/NG composite was obtained followed by freeze-drying in vacuum freeze drier and grinding in thoroughly by deionized water and ethanol. Finally, the black ZnO/NG composite was obtained agate mortar. For comparison, a ZnO/G sample with addition of graphene and a ZnO sample followed by freeze-drying in vacuum freeze drier and grinding in agate mortar. For comparison, addition were prepared as and wella ZnO by the samewithout procedure. The preparation of the awithout ZnO/G any sample with addition of graphene sample any addition were prepared as ZnO/NG composite is illustrated in Figure 7. well by the same procedure. The preparation of the ZnO/NG composite is illustrated in Figure 7.

Figure7.7. The The preparation preparationof ofthe theZnO/NG ZnO/NG composite. Figure

Powder X-ray Tokyo, Japan) patterns of the Powder X-ray diffraction diffraction (XRD, (XRD,SmartLab, SmartLab,Rigaku RigakuCorporation, Corporation, Tokyo, Japan) patterns of −1 samples were detected using Cu Cu Kα Kα radiation (λ (λ = 0.15418 nm) at at 0.02° 2θ 2θ range of −1 the the samples were detected using radiation = 0.15418 nm) 0.02s◦ sin in the range 10°–70°. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images ◦ ◦ of 10 –70 . Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained obtained on on aa Quanta Quanta 400 were 400 ESEM-FEG ESEM-FEG instrument instrument(FEI (FEICorporation, Corporation,Hillsboro, Hillsboro,OR, OR,USA) USA)and anda JEM-2100F instrument (JEOL Corporation, Akishima, Tokyo, Japan), respectively. X-ray a JEM-2100F instrument (JEOL Corporation, Akishima, Tokyo, Japan), respectively. X-ray photoelectron photoelectron(XPS, spectroscopy (XPS, PHI 5400 electron spectrometer) performed using spectroscopy PHI 5400 electron spectrometer) was performed using was unmonochromated Mg unmonochromated Mg Kα radiation (hν = 1253.6 eV) as the excitation source. NG content in Kα radiation (hν = 1253.6 eV) as the excitation source. NG content in ZnO/NG composite was ZnO/NG composite was estimated by thermogravimetric analysis (TGA, SDT Q600, TA estimated by thermogravimetric analysis (TGA, SDT Q600, TA Instruments, New Castle, DE, USA) in Instruments, New Castle, DE, USA) in◦ C/min) the temperature of 30–700 °C (10 °C/min) under air ◦ C (10 the temperature range of 30–700 under airrange atmosphere. atmosphere. The testing ZnO/NG, ZnO/G, and ZnO electrodes were prepared by mixing 80 wt % as-prepared testing 10 ZnO/NG, ZnO/G, andfluoride ZnO electrodes were prepared by mixingMilwaukee, 80 wt % activeThe materials, wt % polyvinylidene (PVDF, Sigma-Aldrich Corporation, as-prepared active 10 wt % polyvinylidene fluoride (PVDF, Sigma-Aldrich Corporation, Corporation, WI, USA), and 10 wtmaterials, % acetylene black in N-methylpyrrolidone (NMP, Sigma-Aldrich Milwaukee, WI, USA), and 10 wt % acetylene black in N-methylpyrrolidone (NMP, Sigma-Aldrich Milwaukee, WI, USA), and 10 wt % acetylene black in) solvent to form homogeneous slurries. Corporation, Milwaukee, WI, USA), 10 wt % acetylene black in) solvent to form The mixture slurries were coated ontoand nickel foams measuring 10 mm in diameter andhomogeneous then dried in slurries. The mixture slurries were coated onto nickel foams measuring 10 mm in diameter and then ◦ a vacuum oven at 120 C for 10 h. In order to acquire preferable contact between active materials dried in a vacuum oven at 120 °C for 10 h. In order to acquire preferable contact between active


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and nickel foams, these testing electrodes were pressed at 15 MPa for several minutes. The loading density of active material in each electrode was about 2 mg cm−2 . For testing the electrochemical performances of the testing electrodes, the ZnO/NG//Li batteries were assembled in coin-type (CR2025) cells in a glove box filled with argon, using pure lithium foil electrodes as the counter electrodes. The electrolyte was a mixed solvent of diethylene carbonate, dimethyl carbonate, and ethylene carbonate (1:1:1) containing of 1 M LiPF6 , and the separator was microporous polypropylene membrane (Celgard 2400). The testing ZnO/G and ZnO electrodes were prepared in the same way as the counterparts. The galvanostatic charge and discharge tests were carried out on a program-control battery system (Wuhan LAND Electronic Co., Ltd., Wuhan, China). Cyclic voltammetry (CV) tests were performed on a electrochemical workstation (CHI 600E) at 0.1 mV s−1 in 0–3.0 V. Electrochemical impedance spectroscopy (EIS) properties were measured in the frequency range of 0.01 Hz–100 kHz using the same CHI 600E electrochemical workstation. 4. Conclusions A novel ZnO/nitrogen-doped graphene (ZnO/NG) nanocomposite was synthesized via a facile solution method. In the prepared ZnO/NG nanocomposite, the ZnO particles with size of about 10.0 nm were anchored uniformly on the wrinkled and twisted N-doped graphene sheets. When used as anode materials, the ZnO/NG nanocomposite exhibited much better electrochemical abilities compared to pristine ZnO and the ZnO/graphene (ZnO/G) nanocomposite. The resulting ZnO/NG nanocomposite can maintain a reversible specific discharge capacity at 870 mAh g−1 after 200 cycles at 100 mA g−1 . The enhanced electrochemical performances of the ZnO/NG nanocomposite can be attributed to the N-doped graphene additive and the unique structure of the composite. The cross-linked N-doped graphene nanosheets in the ZnO/NG nanocomposite have outstandingly improved conductivity and high surface areas, which facilitate electron transportation and provide plenty of active sites for lithium ions, resulting in a drastic improvement of the rate performance. The ultrafine ZnO particles anchored evenly on N-doped graphene nanosheets can not only enlarge the electrolyte/ZnO contact area, but also alleviate the volume expansion of ZnO during the discharge/charge cycling, resulting in an extreme enhancement of the cycle stability. Acknowledgments: This work was supported by the Natural Science Fund of Education Department of Shaanxi Provincial Government (grant number 16JK1018); Natural Science Fund and Subject Merging Fund of Ankang University for high-level talents (Grant No. 2016AYQDZR05 and 2017AYJC01). Author Contributions: Yan Zhao and Guanghui Yuan conceived and designed the experiments. Guanghui Yuan and Jiming Xiang carried out the experiments. Huafeng Jin and Lizhou Wu analyzed the data. Yanzi Jin and Guanghui Yuan contributed in the drafting and revision of the manuscript. Yan Zhao and Guanghui Yuan supervised the work and finalized the manuscript. All authors read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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