Transition-Metal-Doped Zinc Oxide Nanoparticles as a New Lithium

Dec 4, 2013 - Transition-Metal-Doped Zinc Oxide Nanoparticles as a New Lithium-Ion Anode Material ... in terms of fossil fuels by powering electric ve...
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Transition-Metal-Doped Zinc Oxide Nanoparticles as a New LithiumIon Anode Material Dominic Bresser,† Franziska Mueller,† Martin Fiedler,‡ Steffen Krueger,† Richard Kloepsch,† Dietmar Baither,‡ Martin Winter,† Elie Paillard,† and Stefano Passerini*,† †

Institute of Physical Chemistry & MEET Battery Research Centre, University of Muenster, Corrensstrasse 28/30 & 46, 48149 Muenster, Germany ‡ Institute of Materials Physics, University of Muenster, Wilhelm-Klemm-Str. 10, 48149 Muenster, Germany

ABSTRACT: Herein, we present a new synthesis method for transition-metal-doped zinc oxide nanoparticles utilized and characterized for the first time as anode material for lithium-ion batteries. In fact, the introduction of a transition metal (for instance, iron or cobalt) into the zinc oxide lattice results in an advanced performance with reversible lithium storage capacities exceeding 900 mAh g−1, i.e., more than twice that of graphite. In situ XRD analysis reveals the electrochemical reduction of the wurtzite structure and the reversible formation of a LiZn alloy. The additional application of a carbon coating of such nanoparticles enables further improvement in terms of capacity retention and high rate (dis)charge capability. Moreover, the newly developed, simple, and environmentally friendly synthesis of these n-type doped nanoparticles is considered to be also applicable to other transition metals, presumably showing comparable electrochemical performances. KEYWORDS: zinc oxide, iron and cobalt doping, carbon coating, lithium-ion anode, battery rapid capacity fading and reduced cycle life.5−7 For such reasons, zinc metal has attracted only little interest as a possible lithium-ion anode candidate. The approach of using ZnO to generate a dispersion of metallic zinc in an electrochemically formed inactive matrix of Li2O as proposed, for instance, for tin,8,9 resulted in even more severe capacity fading and low specific capacities.10−15 However, it has been reported that the initially formed metallic zinc nanograins are capable of partially enabling the reversible formation of Li2O,15,16 as shown for cobalt, iron, and nickel-based oxides,17 indicating a hybrid reaction mechanism of ZnO with lithium possibly related to its unique characteristics as post-transition metal. Accordingly, previous studies on metallic nickel-coated ZnO particles,18 Ni3ZnC0.7−ZnO composites,19 and nanostructured composite films based on ZnO and NiO, CoO, or Fe2O320−23 showed some improvement of the electrochemical performance of ZnO-based lithium-ion anodes.

1. INTRODUCTION Modern society must, in fact, find new solutions to save, generate, and store power to meet the energy demands of the future. Within this energy trilogy, electrochemical energy storage devices, particularly lithium-based batteries, will certainly play a central role, as they allow energy savings in terms of fossil fuels by powering electric vehicles and might promote the expansion of renewable energy generation by providing highly efficient energy storage devices.1−3 Nevertheless, to achieve this desirable goal, today’s batteries still need further improvement, in terms of safety as well as energy and power density. Real breakthroughs, however, can only be achieved by identifying and developing new electrode materials rather than optimizing already existing ones. Regarding the anode side, current research mainly focuses on the development of either silicon or tin-based anodes, because of their high theoretical capacities of ∼4200 and 990 mAh g−1, respectively.4−6 In contrast, zinc, despite its relatively low cost and natural abundance compared to tin, offers a specific capacity of only 410 mAh g−1, while, at the same time, also suffering from rather large volume variations upon (de)lithiation, resulting in a © 2013 American Chemical Society

Received: October 18, 2013 Revised: December 2, 2013 Published: December 4, 2013 4977

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Figure 1. (a) X-ray diffraction (XRD) patterns of pure, Co-doped, and Fe-doped ZnO nanoparticles. The reference JCPDS File Card No. 01-0716424 for pure ZnO is given in the bottom. (b−d) SEM images of pure ZnO nanoparticles (panel (b)), Co-doped ZnO nanoparticles (panel (c)), and Fe-doped ZnO nanoparticles (panel (d)). h under air at 450 °C for Zn0.9Fe0.1O and at 400 °C for Zn0.9Co0.1O (heating rate ≈ 3 °C min−1). Carbon-coated Zn0.9Fe0.1O nanoparticles were prepared by dissolving 0.9 g of sucrose in 3.5 mL of deionized water and then a subsequent addition of 1.0 g of Zn0.9Fe0.1O under continuous stirring. The obtained mixture was homogenized using planetary ball-milling (Vario-Planetary Mill Pulverisette 4, Fritsch) set at 800 rpm for 1.5 h and subsequently dried at 80 °C overnight. The dry composite was then thermally treated at 500 °C for 4 h in a tubular furnace (Model R50/250/12, Nabertherm) under argon atmosphere. The weight ratio of the residual carbon was determined by TGA under O2 (Model Q5000, TA Instruments). 2.2. Morphological and Structural Characterization. The obtained samples were characterized by performing high-resolution scanning electron microscopy (HRSEM) (ZEISS Auriga microscope) and powder XRD analysis (Bruker D8 Avance; Cu Kα radiation, λ = 0.154 nm). The homogeneity of the carbon coating on Zn0.9Fe0.1O nanoparticles was confirmed by means of Raman spectroscopy (SENTERRA Raman spectrometer, Bruker Optics), equipped with a 785-nm laser and an output power of 10 mW. High-resolution transmission electron microscopy (HRTEM) analysis was carried out on samples deposited on a copper grid coated by a holey carbon film using a ZEISS Libra 200FE operating at an accelerating voltage of 200 kV. 2.3. Electrochemical Characterization. Electrodes were prepared, comprising 75 wt % of active material, 20 wt % of conductive carbon (Super C65, TIMCAL), and 5 wt % of sodium carboxymethyl cellulose (CMC) (WALOCEL CRT 2000 PPA 12, Dow Wolff Cellulosics). For the electrode preparation, CMC was dissolved in deionized water to obtain a 1.25 wt % solution. Finally, Super C65 and the corresponding nanopowder were added and the resulting mixture was dispersed by ball-milling for 2 h. The resulting slurry was then cast on dendritic copper foil (Schlenk) using a laboratory doctor blade (wet film thickness = 120 μm). Subsequently, disk electrodes (⌀ = 12 mm) were punched and dried for 12 h at 120 °C under vacuum. The active material mass loading was 1.7−2.1 mg cm−2. Electrochemical studies were carried out in three-electrode Swagelok-type cells, with lithium metal foils (Rockwood Lithium, battery-grade) as counter and

Herein, we will show that a real enhancement of the electrochemical properties of zinc oxide can only be realized by mixing transition metals (for instance, iron or cobalt) with zinc on the atomic level rather than on the microscale or nanoscale, i.e., replacing Zn by transition metals (for instance, Co or Fe) within the wurtzite lattice. So far, such transition-metal-doped zinc oxides have gathered significant interest only in the field of spintronics.24,25 However, to the best of our knowledge, they have never been considered as reversible lithium storage hosts. Following a new, environmentally benign, sucrose-assisted, wet chemical synthesis, n-type transition-metal (TM)-doped zinc oxide nanoparticles were prepared, having the general formula Zn0.9TM0.1O (TM = Fe or Co). The doping of zinc oxide results in a substantially improved electrochemical performance compared to pristine zinc oxide when utilized as lithium-ion anode. Further improvement is realized by additionally confining the doped ZnO particles within a thin carbon coating. Electrodes based on these composite active materials show greatly improved cycling stability and offer reversible specific capacities of more than 900 mAh g−1, accompanied by excellent rate capabilities.

2. MATERIALS AND METHODS 2.1. Materials Synthesis. Pure ZnO, as well as TM-doped ZnO nanoparticles were synthesized by dissolving stoichiometric amounts of the precursorszinc(II) gluconate hydrate (ABCR), iron gluconate dihyrate (Aldrich), and cobalt(II) gluconate hydrate (ABCR)in deionized water, resulting in a total metal ion concentration of 0.2 M. The obtained solution was added dropwise under vigorous magnetic stirring to a second aqueous solution comprising 1.2 M of sucrose (Acros Organics). After stirring the solution for additional 15 min at room temperature (RT), the water was evaporated at ∼160 °C, while the sucrose started to be thermally decomposed. The resulting solid precursor was dried at 300 °C, subsequently grinded and calcined for 3 4978

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Figure 2. Potential profiles recorded during (a) the first lithiation and (c) delithiation of Zn0.9Fe0.1O nanoparticles. The corresponding XRD patterns obtained by simultaneous XRD analysis are illustrated in the corresponding waterfall diagrams (b and d). For clarity reasons, only the 2θ range from 30° to 45° and only every second XRD scan is presented. Applied specific current: 0.05 A g−1. Duration of each XRD scan, including the initial rest step: 30 min. separated by two sheets of Whatman glass fiber drenched with 500 μL of 1 M LiPF6 in EC:DEC (3:7 vol). Subsequently, the cell was allowed to rest for 10 h, prior to the galvanostatic cycling, carried out using a VSP potentiostat/galvanostat (BioLogic), applying a specific current of 0.05 A g−1 and setting the cut-off potentials to 0.01 and 3.0 V (i.e., fully discharging and charging the electrode within ∼30 and 18 h, respectively). Simultaneous XRD analysis was performed in a 2θ range from 25 to 80° with a step size of 0.02758° and a time per step of 0.65 s. Accordingly, every scan lasted exactly 30 min, including an initial rest step of 419 s.

reference electrodes. The cells were assembled in an MBraun glovebox with oxygen and water contents below 0.5 ppm. A stack of polypropylene fleeces (Freudenberg FS 2190) drenched with a 1 M solution of LiPF6 in a 3:7 volume mixture of ethylene carbonate and diethyl carbonate (EC:DEC, 3:7 vol) was used as separator. All potential values given in this manuscript refer to the Li/Li+ couple. For galvanostatic cycling, a Maccor Battery Tester 4300 was used. Cyclic voltammetry experiments were carried out using a VMP3 potentiostat (BioLogic). All electrochemical studies were performed at 20 °C ± 2 °C in a potential range extending from 0.01 V to 3.0 V. Note that for the calculation of the specific capacity the mass of the carbon coating for Zn0.9Fe0.1O nanoparticles was included. 2.4. In Situ XRD Analysis. In situ XRD analysis of Zn0.9Fe0.1O nanoparticles upon galvanostatic (de)lithiation was carried out using a self-designed in situ cell.26 The electrode was prepared by dissolving 0.01 g of CMC in deionized H2O, adding 0.065 g of Zn0.9Fe0.1O and 0.025 g of Super C65.The mixture was dispersed by means of ballmilling and cast onto a beryllium (Be) disk (thickness of 250 μm, Brush Wellman), acting as current collector and “window” for the Xray beam. The coated Be window was dried at 80 °C for 30 min and at 40 °C under vacuum overnight. Metallic lithium foil was used as counter and reference electrode. Counter and working electrode were

3. RESULTS AND DISCUSSION 3.1. Synthesis, Morphological, and Structural Characterization. Pure and transition-metal (TM)-doped ZnO nanoparticles were synthesized using the corresponding gluconate salts dissolved in an aqueous solution containing sucrose, acting as a particle growth inhibitor. The sucrose was removed by thermal treatment under air at rather low temperatures (400−450 °C). This newly developed synthesis is particularly advantageous as no toxic nitrogen-containing gases are generated. In addition, no adjustment of the pH value 4979

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mechanism proposed for ZnO, the following electrochemical reactions can be proposed for Zn0.9Fe0.1O:

or any further processing steps were required to obtain airstable, phase-pure nanoparticles as confirmed by scanning electron microscopy (SEM) and powder X-ray diffraction (XRD) analysis (Figure 1). The successful introduction of Fe and Co into the ZnO lattice − tetrahedral divalent cobalt, iron, and zinc have rather similar ionic radii (0.58, 0.63, and 0.60 Å, respectively27,28) − results in a net color change of the obtained nanopowder, from white (pure ZnO) to orange and green for Fe-doped and Co-doped ZnO, respectively. The diffraction patterns of the three materials match that of the zinc oxide wurtzite structure (JCPDS File Card No. 01-071-6424, hexagonal symmetry and P63mc space group), without any phase impurity. Moreover, preliminary results, obtained by Xray absorption near edge structure (XANES) and extended Xray absorption fine structure (EXAFS) revealed the ionic character of the transition metal and the substitution of zinc by iron and cobalt in the wurtzite structure. Nevertheless, these results are beyond the scope of this manuscript and will be published in near future elsewhere. Regarding the XRD patterns, however, the reflections broadening and intensity decrease (Figure 1a) indicate a reduction of the crystallite size in the order ZnO > Zn0.9Co0.1O > Zn0.9Fe0.1O. This is in good agreement with the observed particle size of the three samples in the SEM images (Figures 1b−d). In addition, it has already been reported that the substitution of Zn by Co29,30 or Fe31,32 results in a decreased crystallite size, which is generally related to a reduced surface energy and thus a less distinctive particle growth upon thermal treatment. This effect, particularly evident for Fe-doped ZnO, is also in good agreement with the change in the preferred crystallographic direction reported by Mari ́ et al.33 For all three samples, however, the obtainment of small spherical nanoparticles is attributed to the presence of sucrose acting as particle growth inhibitor, like the polyester network known from the Pechini process.34 The rather mild conditions upon the subsequent thermal treatment for sucrose removal do not favor significant particle growth or agglomeration. Since Fedoped ZnO is certainly more interesting than Co-doped ZnO from an economical and environmental point of view as well as for health reasons,31,32 we focused the following electrochemical characterization on the comparison of such a material with pure ZnO. 3.2. In Situ XRD Analysis of Zn0.9Fe0.1O. The electrochemical lithiation of ZnO is considered to occur according to a two-step mechanism, involving the initial reduction of zinc oxide to metallic zinc accompanied by the formation of amorphous Li2O (eq 1), followed by the alloying of zinc with up to one lithium (eq 2), resulting in an overall specific capacity of 987 mAh g−1.5,13,15 ZnO + 2Li+ + 2e− → Zn 0 + Li 2O ≙ 658 mAh g −1

(1)

Zn 0 + Li+ + e− → LiZn

(2)

≙ 329 mAh g −1 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐

Zn 0.9Fe0.1O + 2Li+ + 2e− → 0.9Zn 0 + 0.1Fe 0 + Li 2O

≙ 666 mAh g −1 (3)

0.9Zn 0 + 0.9Li+ + 0.9e− → 0.9LiZn ≙ 300 mAh g −1 (4) ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 966 mAh g −1

To confirm this reaction mechanism, in situ XRD analysis was performed coupled to galvanostatic lithiation (discharge, Figure 2a) and delithiation (charge, Figure 2c). The corresponding series of consecutive X-ray diffraction (XRD) scans for the lithiation and delithiation are presented in Figures 2b and 2d, respectively. As seen in Figure 2a, the electrode potential initially decreases rather rapidly before showing two sequential, but distinct, plateaus appearing within 0.8 and 0.7 V and at ∼0.5 V, which are followed by a fairly gentle potential decrease down to the lower cut-off potential (0.01 V). Considering the simultaneously performed XRD investigation (Figure 2b), three different structural changes can be observed, which are easily assigned to the different features in the potential profile. The marked decrease in intensity of the (100), (002), and (101) reflections of Zn0.9Fe0.1O, occurring at 31.6°, 34.3°, and 36.1°, respectively, is certainly the most evident feature associated with the conversion of Zn0.9Fe0.1O to Zn, Fe and Li2O (region A, scans 6−36). However, it is reasonable to assume that, prior to this, lithium occupies interstitial sites in the wurtzite structure,35−38 resulting in very minor changes in the (002) reflection (up to scan 6). This process, which results in the gradual decrease of the electrode potential prior to the first voltage plateau observed in Figure 2a, is presumably accompanied by initial electrolyte decomposition. This aspect certainly deserves more-detailed investigation, which is, however, beyond the scope of this paper. Several scans later, a new feature appears in the 2θ range from 40° to 45°, indicating the formation of a new phase (region B, scan 11 and the following scans). Although such a reflection is very broad, indicating either low crystallinity or very small crystallite size, its careful analysis reveals a maximum intensity at ∼42.7°, which matches the most intense reflection of one of the few reported LiZn alloy structures (Li0.105Zn0.895, JCPDS File Card No. 01-071-9525). From the coexistence of the reflections belonging to Zn0.9Fe0.1O and Li0.105Zn0.895, it is obvious that the initially reduced zinc starts to alloy with lithium in competition with the formation of Li2O. Thus, the full lithiation of Zn0.9Fe0.1O is not a strictly subsequent process, since the alloying of Zn occurs together with the reduction of Zn0.9Fe0.1O to Zn0, Fe0, and Li2O (A+B). After scan 43, a progressive shift of the intensity maximum toward lower 2θ values indicates an increase of the lattice parameters, which is, in turn, caused by the ongoing increase of the lithium content in the alloy.39 Finally, the maximum intensity is observed at 41.0°, in good agreement with the formation of LiZn phase (C) (JCPDS File Card No. 03-065-3016). Neither reflections related to the presence of metallic iron, as the formed crystals are presumably smaller than the coherence length of the XRD, nor reflections related to Li2O, which is considered to be largely amorphous,26,40 are observed. Nevertheless, the presence of these two compounds has been very recently revealed by transmission electron microscopy (TEM), selected area

987 mAh g −1

For clarity reasons, the following equations consider only the previously described two-step-like occurring electrochemical reactions with lithium. However, it should be pointed out that the given specific capacity values in eqs 1−4 always refer to the overall mass of the initial oxide. To the best of our knowledge, this theoretical specific capacity has never been reported so far, because of the low reversibility of Li2O formation.15,16 However, according to the 4980

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Figure 3. Performance of (a) pure ZnO and (c) Zn0.9Fe0.1O electrodes subjected to continuous galvanostatic charge−discharge cycles at increasing specific currents. Panels (b) and (d) show the corresponding potential profiles for selected cycles (2, 4, 6, 8, and 10) applying a specific current of 0.048 A g−1.

associated to the structural and morphological changes taking place upon (de)lithiation.16,42 It is evident from the corresponding potential profiles (Figure 3b) that both processesthe formation of Li2O (eq 1) at potentials between 1.0 and 0.5 V and the (de)alloying step at lower potentials5,43,44 (eq 2)are not fully reversible (indicated by the upper and lower gray arrow, respectively). However, the electrochemical performance is significantly improved by the substitution of zinc by iron within the wurtzite lattice (Figure 3c). The reversible specific capacity obtained for the first cycle is, as for the in situ XRD experiment, ∼900 mAh g−1 and stabilizes at ∼735, 675, and 590 mAh g−1 for applied specific currents of 0.048, 0.095, and 0.19 A g−1, respectively. It is thus proposed that the Fe-doping (n-type) does not only have a beneficial effect on the electronic conductivity, as recently reported for Al-doped ZnO,45 but furthermore enhances the reversible formation of Li2O. In addition, a possibly reduced dissolution of Zn cations31 might have an additional advantageous effect on the electrochemical performance and cycling stability. However, although a specific capacity of ∼650 mAh g−1 was retained after cycling at elevated currents, its value continuously fades upon further cycling. Moreover, some initial capacity fading is still observed, apparently caused by an increasing internal resistance upon lithiation (indicated by the gray arrow in Figure 3d). 3.4. Carbon-Coated Zn0.9Fe0.1O. In a further attempt to improve the material performance, carbon-coated Zn0.9Fe0.1O nanoparticles were prepared using a rather simple processing based on sucrose as a carbon precursor optimized in previous work.26,46 The amount of remaining carbon was determined, by means of thermogravimetric analysis (TGA), to be ∼18.5 wt %.

electron diffraction (SAED), and Mössbauer spectroscopy studies for a similar active material (MnFe2O4).41 For the subsequent delithiation, the potential profile (Figure 2 c) can be divided into four different sections. Initially, the potential increases rather smoothly (D) and is accompanied by a slight shift of the intensity maximum to higher 2θ values, up to 41.5° at scan 64 (Figure 2d), which is in good agreement with a decrease of the lattice parameters, because of a decreasing content of lithium in the alloy.39 The following steeper potential increase and the concurrent further shift of the intensity maximum to ∼42.6° is again in good agreement with the reversible formation of only partially lithiated zinc alloy according to JCPDS File Card No. 01-071-9525 (E). After this, the intensity of this broad reflection is continuously decreasing (F). Finally, a new very broad reflection in the 2θ range from 31° to 36° appears (G), which, however, could not be clearly assigned to a specific phase. Further investigation by complementary methods might help identifying possibly formed phases. However, this is beyond the scope of the present work. 3.3. Galvanostatic Cycling: Comparison of ZnO and Zn0.9Fe0.1O. The reversibility of the lithiation process in Fedoped ZnO, in terms of specific capacity, was further investigated by electrochemical techniques and compared with that of pure ZnO synthesized and made into electrodes using the same processing. Electrodes based on this latter material show a reversible capacity of ∼687 mAh g−1 in the first cycle, which is, however, rapidly fading upon subsequent charge−discharge cycles before stabilizing at ∼260 mAh g−1 for an applied specific current of 0.095 A g−1 (Figure 3a). This initial rapid capacity decay upon continuous cycling is 4981

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Figure 4. (a, b) SEM images at different magnifications and (c) Raman spectra of carbon-coated Zn0.9Fe0.1O. (d) Comparison of the XRD patterns of Fe-doped ZnO before and after carbon coating. The reference for pure ZnO (JCPDS File Card No. 01-071-6424) is shown at the bottom. (e−g) HRTEM images of carbon-coated Zn0.9Fe0.1O: (e) an overview of the composite structure, showing the percolating carbonaceous network; (f) highmagnification view of the region marked by the black frame in panel (e); and (g) HRTEM image of a single particle. Inset in panel (g) shows the corresponding Fourier transform.

resolution scanning electron microscopy (HRSEM) (see Figures 4a and 4b) and Raman spectroscopy (Figure 4c). In fact, neither severe particle agglomeration nor significant

Subsequent morphological and structural characterization of the carbon-coated Zn0.9Fe0.1O nanoparticles showed a highly homogeneous carbon distribution, as revealed by high4982

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Figure 5. (a) Performance of carbon-coated Zn0.9Fe0.1O-based electrode subjected to constant current galvanostatic cycling. (b) Applied specific current of 0.024 A g−1 and 0.048 A g−1 for the first cycle and the subsequent cycles, respectively; corresponding potential profiles for cycles 2−30 are also given. (c) Galvanostatic cycling at elevated currents. (d) Corresponding potential profiles for selected cycles (10, 20, 30, 40, 50, 60, 70, 80, and 90) at elevated applied currents.

rows are well-recognizable in the particle shown in Figure 4g. The particle was imaged in the [011] direction and selected lattice planes are indexed in the corresponding Fourier transformed pattern (see inset in Figure 4g), showing lattice spacings in good agreement with those obtained by XRD analysis. The electrochemical characterization of carbon-coated Zn0.9Fe0.1O nanoparticles reveals that the reversible capacity within the first cycle is slightly reduced to ∼840 mAh g−1, which is expected as the carbon coating weight is also considered as active material (Figure 5). However, a highly reversible lithium uptake and release is observed with a specific capacity of around 800 mAh g−1 for an applied specific current of 0.048 A g−1 (Figure 5a). The excellent cycling stability is confirmed by the corresponding potential profiles for cycles 2− 30, showing almost no changes upon cycling (Figure 5b). The beneficial effect of the carbon coating as well as the establishment of the carbonaceous percolating network (interconnecting the nanoparticles) is additionally revealed by the substantially enhanced rate capability. The electrode offers specific capacities of ∼720, 670, 580, 475, and 360 mAh g−1 for applied currents of 0.095, 0.19, 0.48, 0.95, and 1.9 A g−1, respectively (Figure 5c). These values are almost twice that of graphite (372 mAh g−1), the state-of-the-art anode material in lithium-ion batteries, which is especially remarkable considering the high currents used. At extremely high current (4.75 and 9.5 A g−1) the capacity decreases to ∼195 and 70 mAh g−1, respectively. Indeed, from the corresponding potential profiles (Figure 5d) the appearance of a rather large ohmic drop for

particle growth upon the subsequent thermal treatment was observed (see Figures 4a and 4b). Moreover, only the bands corresponding to partially disordered graphitic carbon were observed in the Raman spectra, i.e., at lower wavenumbers the D- and G-band appearing at ∼1350 cm−1 and 1580−1590 cm−1, respectively, and at higher wavenumbers, the 2D, D+G, and 2G bands.47,48 In order to confirm the crystalline integrity of the Zn0.9Fe0.1O nanoparticles, XRD analysis was performed (see Figure 4d). Beside a slightly increased crystallinity resulting from the additional thermal treatment, indicated by a more distinct split of the (002) and the (101) reflection at ∼34.4° and 36.2°, respectively, no significant changes in the XRD pattern could be detected. Further morphological and structural characterization was carried out by means of highresolution transmission electron microscopy (HRTEM), revealing the generation of a carbonaceous percolating network interconnecting the carbon-coated spherical Zn0.9Fe0.1O nanoparticles, having an average particle diameter of ∼10−15 nm (Figure 4e). Magnifying the selected region in Figure 4e, marked by a black frame, shows a narrow rim surrounding the crystalline particles (Figure 4f), indicating the amorphous carbon layer on the nanocrystals surface. The fringes, corresponding to the {100} lattice planes of Zn0.9Fe0.1O, are clearly visible (marked black, determined to 2.81 Å), further confirming the crystallinity and structural integrity of the nanoparticles after application of the carbon coating. Moreover, although the clusters tend to react under the electron beam, the lattice structure of the hexagonal Zn0.9Fe0.1O could be wellanalyzed by means of the HRTEM images. Different atomic 4983

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applied currents higher than 1.9 A g−1, accompanied by a less characteristic shape of the potential profile, indicating that the initial lithium uptake/release mechanism is kinetically limited for such high currents. Nevertheless, considering the ultrahigh currents, these results are still in the same range as some of the best results reported so far for high-power, nanostructured Li4Ti5O12.49,50 Generally, however, the remarkable improvement of the electrochemical performance in terms of cycling stability, high rate capability, and capacity retention after applying elevated currents by simply carbon coating the Zn0.9Fe0.1O nanoparticles indicates an enhanced electronic conductivity as well as improved mechanical electrode integrity, because of the volume change buffering. 3.5. Electrochemical Characterization of Zn0.9Co0.1O. Eventually, in order to show that the substantial improvement in terms of electrochemical performance of ZnO-based lithiumion anode materials is not singularly limited to Fe-doped ZnO, we also investigated the electrochemical performance of Zn0.9Co0.1O nanoparticles. According to the proposed reaction mechanism for Fe-doped ZnO, Zn0.9Co0.1O is able to reversibly store up to 2.9 lithium, offering a theoretical specific capacity of ∼962 mAh g−1, which is slightly lower than for Zn0.9Fe0.1O, because of the higher atomic weight of cobalt, relative to iron. Indeed, Co-doped ZnO nanoparticles show a highly stable cycling performance, offering an initial reversible specific capacity of ∼970 mAh g−1 and ∼920 mAh g−1 upon subsequent cycling at an applied specific current of 0.048 A g−1 (Figure 6).

the transition metal within the ZnO lattice. Fe-doped ZnO nanoparticles were electrochemically investigated by means of in situ X-ray diffraction (XRD) and the results show that the alloying of lithium and zinc overlaps the reduction of iron and zinc, which occurs to a large extent in parallel rather than consecutively. More remarkably, however, the doping of ZnO nanoparticles by iron and cobalt substantially enhances the electrochemical performance of these materials, rendering them highly promising for application in advanced lithium-ion anodes. While cobalt doping appears even more effective than iron doping, iron is certainly more interesting from an environmental and economic point of view. In fact, coating Fe-doped ZnO nanoparticles with carbon revealed a superior electrochemical performance in terms of cycling stability, high rate performance, and capacity retention. Generally, we believe that the sucrose-assisted synthesis presented herein is equally applicable to introduce other functional transition-metal atoms (for instance, nickel or copper) into the ZnO structure, offering comparable electrochemical performances.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the European Commission within the ORION Project (No. 229036) under the Seventh Framework Programme (7th FWP) is gratefully acknowledged. Furthermore, the authors would like to thank Mr. Vassilios Siozios for performing the thermogravimetry (TG) analyses.

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REFERENCES

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Figure 6. Zn0.9Co0.1O-based electrode subjected to galvanostatic constant current cycling.

The slight excess in reversible capacity observed in the first cycle with respect to the theoretical capacity is presumably related to the capacity contributed by the comprised conductive carbon. Nonetheless, the capacity values are generally higher for Co-doped ZnO relatively to Zn0.9Fe0.1O, which might be related to the intrinsically superior ability of cobalt to enable the reversible formation of Li2O.17 However, the results further support the proposed reaction mechanism and underline the importance of finely dispersing such functional elements as cobalt and iron at the atomic level within the ZnO lattice rather than “mixing” them on a microscale or nanoscale.

4. CONCLUSIONS A new synthesis of transition-metal-doped zinc oxide nanoparticles was presented. The structural and morphological characterization revealed the successful replacement of Zn by 4984

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