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Electrosprayed graphene films decorated with bimetallic (zinc-iron) oxide for lithium-ion battery anodes
Accepted Manuscript Electrosprayed graphene films decorated with bimetallic (zinc-iron) oxide for lithiumion battery anodes Bhavana Joshi, Edmund Samuel, Min-Woo Kim, Karam Kim, Tae-Gun Kim, Mark T. Swihart, Woo Young Yoon, Sam S. Yoon PII:
S0925-8388(18)34723-6
DOI:
https://doi.org/10.1016/j.jallcom.2018.12.170
Reference:
JALCOM 48811
To appear in:
Journal of Alloys and Compounds
Received Date: 22 August 2018 Revised Date:
6 December 2018
Accepted Date: 12 December 2018
Please cite this article as: B. Joshi, E. Samuel, M.-W. Kim, K. Kim, T.-G. Kim, M.T. Swihart, W.Y. Yoon, S.S. Yoon, Electrosprayed graphene films decorated with bimetallic (zinc-iron) oxide for lithium-ion battery anodes, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2018.12.170. 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 proof before it is published in its final 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.
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Electrosprayed Graphene Films Decorated with Bimetallic (Zinc-Iron) Oxide for Lithium-Ion Battery Anodes
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Bhavana Joshia,†, Edmund Samuela,†, Min-Woo Kima, Karam Kima, Tae-Gun Kima, Mark T. Swihartb, Woo Young Yoonc,*, Sam S. Yoona,* a
School of Mechanical Engineering, Korea University, Seoul 136-713, Republic of Korea Department of Chemical & Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-4200, U.S.A. c Department of Materials Science & Engineering, Korea University, Seoul 02841, Republic of Korea
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Abstract
Bimetallic (zinc-iron) oxide with a pillar-like morphology is decorated over graphene films. The electrosprayed composite demonstrates good electrochemical performance as an anode material for lithium-ion batteries. Scanning and transmission electron microscopy images confirm the effective decoration of graphene sheets with bimetallic Zn-Fe oxide particles.
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The composite shows a high capacity of 1601 mAh·g-1 at 100 mA·g-1 owing to its unique layer-particle morphology. The anode material exhibits excellent capacity retention of 77% (~1235 mAh·g-1 at 100 mA·g-1) after 100 cycles, due to buffering of volumetric strain in the metal oxide by flexible conductive graphene sheets. Zinc forms an alloy with the lithium
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ions, and the graphene sheets prevent particle agglomeration, keeping transport distances within particles small. Thus, synthesizing a composite of graphene with Zn-Fe oxides via
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electrospray deposition is a promising method for preparing high-performance anode materials for lithium-ion batteries.
Keywords: ZnFe2O4, electrospray, LIB, anode, graphene *
Corresponding author: [email protected], [email protected]
†
These authors have contributed equally.
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1. Introduction The primary focus of research related to lithium-ion batteries (LIBs) is the development of high-voltage cathode and high-capacity anode materials. [1] Given the
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instability of organic electrolytes at higher voltages, significant efforts have been devoted to developing high-capacity anode materials [2]. As has been reported in several reviews, transition metal oxides (TMOs) have garnered a great deal of attention as anode materials
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because of their high lithiation capacity [3-5]. However, charge transfer in TMOs is hampered by their low conductivity. Furthermore, volume expansion/contraction in these
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materials leads to pulverization of the active oxide material, which in turn reduces the number of Li-ion hosting sites owing to particle agglomeration.[6, 7] These two factors limit the practical applicability of TMOs in LIBs. To prevent the agglomeration of a pulverized single TMO during lithiation/delithiation and to increase its electrochemical activity for
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enhanced lithium storage, a second TMO is combined with it. Using two TMOs (TTMOs) has been shown to be an effective solution [8]. TTMOs allow for more precise tailoring of the energy density as each TMO has its own characteristic working potential [9].
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Anodes based on two TMOs include those produced from spinel ferrite-based materials, such as CoFe2O4 [10], NiFe2O4 [11], MnFe2O4 [12], MgFe2O4 [13], and ZnFe2O4
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[9]. Among these, ferrites other than ZnFe2O4 do not form alloys with lithium. Thus, these materials have limited storage capacity. Another key advantage of using zinc and iron together is that they are both abundantly available and low cost. Furthermore, they are both environmentally benign. Zn increases the capacity of the anode material owing to its ability to form an alloy with Li ions (Li-Zn). On the other hand, iron oxide can be reduced to metallic Fe, which leads to the formation of Li2O during lithiation. Although the theoretical capacities of ZnO and Fe2O3 are 978 mAh·g-1 and 1007 mAh·g-1, respectively, they both
ACCEPTED MANUSCRIPT undergo volume expansion, owing to their low conductivities [14, 15]. Moreover, an advantageous blending of ‘conversion’ and ‘de-alloying–alloying’ reaction mechanisms occur in the bimetallic (Zn-Fe) oxide, providing a high theoretical capacity of ~1072 mAh·g-1 [16, 17]. Zinc ferrite structures for use in LIBs have been primarily synthesized by
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solvothermal and hydrothermal methods [9, 18]. Zhao et al. synthesized spherical 30–40 nm sized ZnFe2O4 particles by a hydrothermal method [19]. These particles showed an initial capacity of 1287 mAh·g-1 at a current density of 0.2 mA·cm-2. Qiao et al. reported ZnFe2O4
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nanofibers that showed a capacity of 1248 mAh·g-1 during the first cycle [20]. However, low reversible capacity, current rate capability, and capacity retention remain challenges for
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ZnFe2O4. Thus, forming carbon-based composites of ZnFe2O4 becomes promising, as carbonaceous material cushioning can effectively improve the electronic conductivity while buffering volume change [21, 22]. Furthermore, graphene sheets can help preserve the structure of the active material, resulting in enhanced cycling stability [23]. Based on this, Sui
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et al. fabricated a multiwalled carbon nanotube-ZnFe2O4 composite that showed a good reversible capacity [9]. Furthermore, a ZnFe2O4-carbon nanocomposite prepared by solgel/ball milling exhibited a capacity of 681 mAh·g-1 after 100 cycles; this is superior to that
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of pristine ZnFe2O4 [24-26]. Among various carbon materials such as carbon black (CB),
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carbon nanotubes (CNT), or graphene, the sheet type morphology of graphene is particularly advantageous, because it not only restricts agglomeration of the particles but also facilitates uniform distribution of the particles on the sheets. In this study, we enhanced the rate capability and cycling stability of ZnFe2O4 through
the addition of graphene. A composite was formed by electrostatic spray deposition (ESD), which is a non-vacuum and cost-effective technique for the deposition of nanomaterials [27, 28]. ESD allows for the tailoring of the deposit microstructure. For example, the deposited nano/submicron particles can be made to exhibit a pillar-like morphology by ensuring the
ACCEPTED MANUSCRIPT deposition of highly charged nanodroplets on the substrate [29]. This pillar-like morphology results in a high surface-to-bulk ratio, which, in turn assists in the accommodation of a large number of Li ions. It also results in mesoporosity, which helps alleviate the strain experienced by the anode due to repeated volume expansion/contraction that occurs during
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discharging/charging processes.
We used ESD to deposit active materials including Fe2O3, bimetallic (Zn-Fe) oxide (biMO), and graphene/bimetallic (Zn-Fe) oxide (G/biMO) directly onto 1 mm thick spacers,
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for use as LIB anodes. G/biMO exhibited impressively high stability and a capacity retention
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because of its pillar-like morphology and the synergistic effect of the well-dispersed TTMOs
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and graphene.
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2. Experimental Procedure 2.1 Chemical synthesis The starting materials, namely, zinc acetate dihydrate (ZnAc, Zn(CH3COO)2·2H2O,
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Oriental Chemical Industries), iron chloride hexahydrate (FeCl3·6H2O, Sigma Aldrich), polyvinylpyrrolidone (PVP, MW 40 kDa, Sigma Aldrich), potassium hydroxide (KOH, Sigma Aldrich), and graphene (N002-PDR, Angstrom Materials), were used as received.
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Ethanol (99.9% Duksan) and propylene glycol (PG, Duksan) were used as the solvents for the zinc and iron salts, respectively. To form the precursor solution, 0.022 g of graphene (~10
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weight percent relative to Zn and/or Fe salts) was first mixed in a water/ethanol solution (2:4 ratio). Next, 37.5 mM ZnAc and 75 mM FeCl3 were mixed with 2.2 ml of the graphene dispersion and stirred until they were well dissolved. The Zn/Fe ratio was kept at 1:2. Next, PG was added to the graphene/ZnAc-FeCl3 (G/ZnFe) solution, which was then subjected to
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ultrasonication for 90 min to ensure uniform dispersion of the graphene and of the metal atoms over the exfoliated graphene sheets. Then, 2.5 ml of KOH was added dropwise to initiate precipitation, and the solution was ultrasonicated for 10 min. PVP was added to the
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solution before further ultrasonication to maintain the dispersion state of the graphene sheets
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and bimetallic (Zn-Fe) oxide nanoparticles. The solution was then subjected to ultrasonication for another 20 min. These processes resulted in the growth of bimetallic oxide nanoparticles on the graphene sheets. A controlled synthesis of bimetallic oxide nanoparticle solution with no graphene, was similarly conducted using a PG/ethanol mixture. Similarly, a solution that contained only Fe was also prepared by excluding graphene and ZnAc. Hereafter, the samples with active material Fe2O3, bimetallic (Zn-Fe) oxide, and graphene/bimetallic (Zn-Fe) oxide composite will be referred to as FO, biMO, and G/biMO, respectively.
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2.2 Electrospray deposition The above-described precursor solutions were electrosprayed (Fig. 1) on precleaned stainless steel spacer disks with a thickness of 1 mm. The substrate temperature was
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maintained at 150 °C and an 11 kV voltage was applied between the nozzle and substrate placed 4 cm apart. The fabricated samples were annealed at 450 °C in two steps in an argon atmosphere. In the first step, the temperature was raised to 200 °C and kept constant for 2 h.
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Next, the temperature was increased from 200 to 450 °C and kept constant for 3 h. The heating rate was 3 °C min-1. The electrospray procedure is illustrated in Fig. 1a and shows the
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spraying of the precursor comprised of FeCl3, ZnAc, and graphene. Figure 1b compares the coin substrates with and without deposition. The inset SEM image shows the morphology of the sprayed film. Figure 1c depicts the lithiation mechanism between the sprayed G/ZnFe2O4
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film and Li-foil across the separator.
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Fig. 1. (a) Schematic diagram of the electrospray deposition with the synthesized precursor. (b) A snapshot of biMO (w/o graphene) and G/biMO (w/ graphene) electrodes deposited over
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a spacer. The SEM image is of G/biMO. (c) Schematic illustration of the lithiation
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mechanism between the sprayed G/biMO film and Li-foil across the separator.
2.3 Physicochemical characterization The annealed samples were characterized by X-ray diffraction (XRD) analysis (SmartLab, Rigaku) to evaluate their crystalline structures. Raman spectroscopy (Jasco, NRS3100) was performed to analyze the graphene films and their defects, as well as the bonding characteristics of Zn and FeO. Field-emission scanning electron microscopy (FESEM; S-
ACCEPTED MANUSCRIPT 5000, Hitachi, Ltd.) and transmission electron microscopy (TEM; JEM 2100F, JEOL Inc.) were used to study the surface morphologies of the samples. Energy-dispersive X-ray spectroscopy (EDX) along with SEM and TEM was used to determine the elemental concentrations and elemental maps, respectively, of the samples. X-ray photoelectron
determine the chemical states of the elements.
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2.4 Electrochemical characterization
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spectroscopy (XPS; Theta Probe Base System, Thermo Fisher Scientific Co.) was used to
The spacer disks with coatings (weight ~1.3 mg) were directly used as anodes in
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CR2032 coin-type half-cells along with a metallic Li sheet (16 mm in diameter) as the reference electrode. A Celgard 2400 (Celgard, Chungbuk, South Korea) polymer membrane used to electrically isolate the Li-foil and anode material. The liquid electrolyte used in the cells comprised 1 M LiPF6 in a solvent mixture of ethylene carbonate, dimethyl carbonate,
(R3-1)
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and ethyl methyl carbonate (1:1:1 by volume) (PuriEL, Soulbrain, Seongnam, South Korea). Approximately 0.3 mL of electrolyte was used to ensure good wetting of the
electrodes and separator. The electrochemical performance of the samples was measured
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based on their galvanostatic charge/discharge curves obtained for voltages of 0.005 – 3 V at
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25 °C using a WBCS3000 battery cycler system (WonATech, Seoul, South Korea). The cells were subjected to rate tests at current densities of 0.1, 0.2, 0.5, and 1 C, where C = 1000 mA·g-1. Impedance measurements were carried out using electrochemical impedance spectroscopy (EIS) for frequencies of 100 kHz to 0.01 Hz in the open-circuit mode utilizing a perturbation voltage of 5 mV. The results of the impedance measurements were used to determine the solution resistance (Rs) and charge-transfer resistance (Rct), among other parameters.
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3. Results and Discussion
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3.1 Material properties
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Fig. 2. (a) XRD patterns and (b) Raman spectra of various samples. The XRD pattern of the G/biMO sample deposited by ESD is shown in Fig. 2a. The
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pattern exhibits sharp peaks at diffraction angles (2θ) of 30°, 35.4°, 43.6°, 56.8°, 62.2°, 66.3°, and 74°, which correspond to the (220), (311), (400), (333), (440), (442), and (622) planes of ZnFe2O4, respectively. The XRD pattern matches well with that of ZnFe2O4 (JCPDS 821049[30]/22-1012[31]), confirming the formation of a well-crystallized cubic spinel structure. In contrast, hematite (α-Fe2O3) and maghemite (γ-Fe2O3) phases are present in the Fe2O3 sample, as evidenced by the peaks marked with “▼”and “●”. The peak at 28.2° corresponds to α-Fe2O3, while diffraction peaks at 33.3°, 35.8°, 40.8°, 43.8°, 50.3°, 54.3°,
ACCEPTED MANUSCRIPT 62°, 64°, 66°, 74°, and 75° represent γ-Fe2O3. These patterns match JCPDS cards 01-1053 (αFe2O3) and 39-1346 (γ-Fe2O3), respectively [32]. These Fe2O3 phases are also observed in the biMO and G/biMO samples. The estimated average sizes of the ZnFe2O4 particles based on the (311) plane using the Debye Scherrer equation are approximately 16 and 40 nm for the
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biMO and G/biMO samples, respectively. No graphene-related peaks are detected in the XRD pattern of G/biMO. We attribute their absence to the low concentration of graphene relative to Fe2O3 and ZnFe2O4, and the exfoliated nature of the graphene nanosheets. From
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the XRD pattern, we infer that biMO is a mixture of Fe2O3 and ZnFe2O4 phases.
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Figure 2b shows the Raman spectra of various samples. Broad peaks at 1343 and 1590 cm-1, corresponding to the D and G bands of graphene, respectively, confirm the incorporation of graphene in the G/biMO sample. The D band indicates the presence of defects owing to exfoliation and loading of ZnFe2O4 particles over the graphene sheets. The Fe2O3 and ZnFe2O4 samples exhibit peaks at wavenumbers in the range 100–800 cm-1. Peaks
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corresponding to seven phonon modes in the α/γ- Fe2O3 phases can be observed clearly in the spectrum of Fe2O3. As per group theory, these are two A1g modes (222 and 496 cm-1), five Eg
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modes (244, 290, 338, 410, and 611 cm-1), and one disorder Eu mode (656 cm-1) [33]. In contrast, peaks related to five active phonon modes can be observed in ZnFe2O4. In spinel
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ferrites, the peaks observed at wavenumbers greater than 600 cm-1 correspond to tetrahedral vibrations (AO4), while the lower-frequency modes are attributed to octahedral vibrations (BO6) [34]. The dominant peak at 1315 cm-1 in the Raman spectra of Fe2O3 and ZnFe2O4 is due to the second-order scattering processes of α-Fe2O3, in accordance with a report by Kumar et al. [35]. Finally, the appearance of peaks related to the α/γ-Fe2O3 phases in the Raman spectra is consistent with the XRD results.
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Fig. 3. (a) TEM image, (b) HRTEM image, (c) SAED pattern, and (d) elemental maps confirming the presence of C, Zn, Fe, and O in G/biMO.
TEM was used to analyze the morphology and microstructure of G/biMO in detail. The bright and contrasting areas in Fig. 3a represent the graphene sheets, while the dark spots
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are the biMO particles loaded on the graphene sheets. The biMO nanoparticles have a welldistributed morphology and dimensions, with sizes of 20–30 nm. The small ZnFe2O4 particles decrease the length of the Li diffusion path, and thus, enhance the electrochemical activity.
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The HRTEM image in Fig. 3b shows lattice fringes with d spacings of ~0.48, 0.29, and 0.25
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nm, corresponding to the (111), (220), and (311) planes of ZnFe2O4, respectively. The selected area electron diffraction (SAED) pattern in Fig. 3c contains six concentric rings, confirming the polycrystalline nature of γ-Fe2O3 and ZnFe2O4. The rings are formed because the randomly distributed nanoparticles result in the continuous angular distribution of the (hkl) planes, that is, the (221) plane of γ-Fe2O3 and the (222), (331), (333), (440), and (620) planes of ZnFe2O4. The SAED pattern is consistent with the XRD analysis results and confirm the cubic structures of Fe2O3 (JCPDS 39-1346) and ZnFe2O4 (JCPDS 82-1049). The composition of G/biMO is confirmed by the elemental maps of Zn, Fe, O, and C determined
ACCEPTED MANUSCRIPT by EDX. The different colored areas in Fig. 3d indicate the Zn-, Fe-, O-, and C-rich areas of
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the sample, confirming the formation of G/ZnFe2O4.
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Fig. 4. SEM images of (a) FO, (b) biMO, and (c) G/biMO. The insets show higher-resolution
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images of corresponding samples.
SEM images of FO, biMO, and G/biMO are shown in Fig. 4. The images of the Fe2O3 and biMO samples show a well-controlled deposition by ESD. Furthermore, a pillar-like morphology is observed in Fe2O3 and biMO (see Figs. 4a and 4b). In the absence of graphene, the ZnFe2O4 particles partially agglomerate owing to the formation of these pillars.
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The consistent formation of biMO nanoparticles over the graphene sheets can be seen clearly in Fig. 4c. The roughened graphene surface is indicative of exfoliation. Furthermore, the
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biMO particles deposited on these sheets significantly enhance the surface-to-bulk ratio and shorten the path length for lithium-ion diffusion. The graphene sheets act as cushions and
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prevent the agglomeration of the biMO nanoparticles, while the particles concurrently prevent the graphene sheets from restacking during lithiation/delithiation [36, 37]. Thus, the fused graphene sheets and biMO nanoparticles improve the electrochemical performance of the active material during long-term cycling (see Fig. 6b). The surface elements and their oxidation states in the G/biMO composite were analyzed using XPS. The survey scan (Fig. S1a) shows the presence of elemental Fe, Zn, O, and C. The core Zn2p spectrum in Fig. S1b shows two peaks at binding energies of 1021.4 and 1045.0 eV, attributed to Zn2p3/2 and Zn2p1/2, respectively, confirming that Zn is present
ACCEPTED MANUSCRIPT in the +2 oxidation state. Furthermore, the peaks at 710.9 and 724.8 eV in Fig. S1c are attributed to Fe2p3/2 and Fe2p1/2, respectively. The satellite peaks of Fe2p observed at binding energies of 718.6 and 733.0 eV are indicative of the Fe3+ state. The core O1s spectrum in Fig. S1d shows a broad asymmetric peak, which can be deconvoluted into four peaks with
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binding energies of 529.3, 530.15, 531.2, and 532.0 eV. The peaks at 529.3 and 530.15 eV are attributed to the lattice oxygen in Zn/Fe-O. The presence of other groups, such as the OH group, is confirmed by the peak at 531.2 eV, while the peak at 532.0 eV is related to defects
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due to oxygen deficiency in biMO. The deconvoluted C1s core spectrum (Fig. S2) from graphene contains peaks at 284.0, 285.0, 286.2, and 288.5 eV, which correspond to C-C (sp2),
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C-C (sp3), C-O, and C=O bonds, respectively. The contents of Zn-Fe and carbon in the composite G/biMO were estimated to be approximately 27.3 and 72.7 wt %, respectively, based on TGA as shown in Fig. S3 in the supporting information.
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3.2 Electrochemical performance
The electrochemical performance and specific capacity of the anode materials used in LIBs depend on the reactions that occur with Li ions. The crystallographic structures for
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Fe2O3 and ZnFe2O4 corresponding to the XRD data in Fig. 2a, are shown in Fig. S4. The ionic radius of Li is small. Hence, the insertion of Li-ions within the crystal structures of
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Fe2O3 and ZnFe2O4 initially causes minimal or no disorder in the structures. However, with an increase in the number density of the Li ions, due to the acceleration of the kinetics of the redox reaction with the continuous discharging/lithiation of the half-cells, Fe2O3 is converted into metallic Fe and Li2O. In contrast, ZnFe2O4 is converted into metallic Fe, ZnO, Li2O, and a LiZn alloy, in steps. Furthermore, due to the forced lithiation process, as well as the occurrence of the redox reaction at lower potentials, the unit cells of Fe2O3 and ZnFe2O4 expand, as shown in Figs. S4c and S4f, respectively. As this occurs, the pillar-like
ACCEPTED MANUSCRIPT morphology of the G/ZnFe2O4 results in buffering during the expansion process, allowing for the accommodation of a significant number of Li ions. During the lithiation of Fe2O3 (involving reversible and partially irreversible redox reactions), the oxide is converted into a metallic phase. A total of six Li ions are lithiated per
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Fe2O3 formula unit during the intercalation of Li and/or the conversion of the metal oxide during the electrochemical process. In this study, in order to further improve the capacity of the metal oxide, we synthesize the ternary composite ZnFe2O4. Because of the incorporation
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of zinc in the iron oxide, the number of lithiated Li ions increases from six to nine. The
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additional three Li ions reduce ZnO to metallic Zn and form a LiZn alloy. The redox reaction for the alloying process is a two-step chemical reaction that occurs during the lithiation stage, as explained below. The insertion of lithium begins with intercalation (reactions 1 and 2, Fig. S4e), followed by the conversion reaction (3), and alloying reaction (4) (see Fig. S4f and
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Schematic 1) [24, 38, 39]:
+ . + . ↔ . () . + . + . ↔ ()
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+ + → + + () + + ↔ ()
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+ ↔ + + ( )
+ ↔ + + ()
The reactions in (3), (5), and (6) show that ZnFe2O4 is reduced to ZnO and Fe2O3.
Hence, the subsequent lithiation/delithiation processes only involve reactions (4) and (5) in the case of ZnO and (6) in the case of Fe2O3. The phase-separation process is described in Scheme 1.
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Scheme 1. Intercalation of lithium ions in ZnFe2O4.
Fig. 5. Discharge/charge profiles and differential capacity plots of (a and c) Fe2O3 and (b and d) biMO. As described above, the lithiation/delithiation of the anode causes the active material to undergo multiple processes during the redox reactions. The dissociation of ZnFe2O4 into
ACCEPTED MANUSCRIPT metallic or alloy phases is analyzed in greater depth based on the discharge (lithiation)/charge (delithiation) profiles of the anodic materials. The discharge/charge profiles of Fe2O3 and ZnFe2O4 for the first and second cycle (N = 1 and 2) are presented in Fig. 5. The discharge capacities of Fe2O3 and ZnFe2O4 during the first cycle, determined from the capacity-voltage
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profile, are 1350 and 1484 mAh·g-1, respectively. Furthermore, as shown in Figs. 5a and 5b, the capacity-voltage profiles for the first discharge cycle show four distinct voltage regions between 0.005 and 3.0 V. In the first region, the voltage decreases from 3 to 1.4 V during the
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discharging process, resulting in a small change in the capacity, which is indicative of a small degree of intercalation of free lithium ions. However, owing to the presence of these free Li
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ions, the redox reaction is initiated. Thus, the active materials Fe2O3 and ZnFe2O4 transform into LixFe2O3 and LixZnFe2O4 (x ≈ 0.5), respectively, as indicated by the small increase in the capacity in region II (1.4 to 0.81 V). The primary transition/reduction of the active material occurs in region III (0.81 to 0.65 V) in the case of Fe2O3, as shown in Fig. 5a. The observed
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peak at 0.81 V in the differential capacity plot for N = 1 (see Fig. 5c) confirms the transition of Fe2O3 into a metallic Fe3+ phase. Region IV is shown in Fig. 5a and consists of a plateaulike region with a gradual fall in the voltage and a rise in the specific capacity [39, 40]. In this
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region, the metallic Fe3+ ions are reduced to Fe0. Furthermore, the electrolyte decomposes,
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forming a solid electrolyte interphase (SEI) layer owing to the partially irreversible redox reactions. The redox reaction leads to the formation of a complex polymeric gel. The specific capacity of biMO (1484 mAh·g-1) during the first lithiation stage is
greater than that of Fe2O3 (1350 mAh·g-1). In the second region (see Fig. 5b), a greater number of Li ions react with LixZnFe2O4, thus increasing x to 2 and completing the intercalation reaction (see Schematic 1; the intercalation of Li ions in ZnFe2O4). The lithiation results in the intercalation of nine Li ions during the redox reaction. This intercalation and the reduction of ZnFe2O4 result in an increase in the specific capacity over
ACCEPTED MANUSCRIPT the four stages marked in Fig. 5b. These four stages correspond to the electrochemical response of the differential capacity and are indicated by the four peaks observed at 1.53, 0.96, 0.79, and 0.11 V in Fig. 5d. The redox reaction (1) is described above and represents the first lithiation stage of ZnFe2O4. However, during the delithiation stage, ZnFe2O4 is not
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restored, probably because a LiZn alloy is formed, as shown in reaction (4). However, it has also been reported that ZnFe2O4 decomposes into ZnO and Fe2O3 after the first delithiation stage [24, 38, 39]. ZnO acts as a buffer, minimizing the agglomeration of the Fe2O3 particles
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during the repetitive charging/discharging that causes pulverization of the metal oxide (both ZnO and Fe2O3). The peak observed at 1.53 V in the differential capacity profile for N = 1 is
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absent in the profile for N = 2 and higher, as shown in Fig. 5d. This confirms that ZnFe2O4 is not restored. However, the shift of the peak at 0.79 to 0.96 V confirms the formation of Fe2O3. Furthermore, it we note that the discharge capacity of biMO (1484 mAh·g-1) during the initial cycle, in which the transition metal ions are reduced to the metallic state, leads to
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the formation of 4Li2O as per the formula (see reaction 3) and is higher than the theoretical capacity (~1072 mAh·g-1). However, the capacities of Fe2O3 and ZnFe2O4 after delithiation are 916 and 1095 mAh·g-1, respectively, with the active materials exhibiting coulombic
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efficiencies of 67% and 74%. The high irreversible capacity may be due to the formation of
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the SEI layer and solvent decomposition in the electrolyte [41]]. Concurrently, the Fe2O3 and ZnFe2O4 nanoparticles also convert into an amorphous phase in the presence of the electrolyte, as the cell potential approaches 0.005 V (versus Li). A similar phenomenon has been reported in the case of Li/CoFe2O4 cells [10].
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Fig. 6. Electrochemical characterization. (a) Rate capability for voltages of 0.005–3.0 V. (b)
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Long-term cycling capacity and coulombic efficiency at 100 mA·g-1 for various anode materials. (c) Discharge/charge profiles, and (d) differential capacity of G/biMO.
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It is essential to investigate the anode performance at lower to higher current densities to realize the current capability ability of the active material. Thus, all three
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electrodes were subjected to electrochemical testing at different current densities. Their performance is presented in Fig. 6a. Among the three electrodes, G/biMO exhibits superior first reversible discharge capacities, which are 1066, 815, 651, and 522 mAh·g-1, at current densities of 100, 200, 500, and 1000 mA·g-1, respectively. (R3-2) Note that these capacities are based on the total mass of G/biMO, including the graphene, which can contribute significantly to the total capacity. Furthermore, when the current density is changed from high to low, the G/ZnFe2O4 electrode exhibits better reversible performance, showing a
ACCEPTED MANUSCRIPT capacity of 1064 mAh·g-1 after 35 cycles. In contrast, the biMO and FO electrodes show much lower capacities of 768 and 482 mAh·g-1, respectively. The difference in the capacities can be ascribed to the good dispersion of the biMO nanoparticles over the graphene sheets, which prevents their aggregation and enhances rate capability. The G/biMO composite
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provides improved electrical conductivity due to graphene and two simultaneous reaction mechanisms, dealloying/alloying of LiZn and the Fe2O3/Li2O conversion reaction, supplemented by capacity provided by the graphene itself.
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The long-term cycling performances of the Fe2O3, biMO, and G/biMO electrodes
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were evaluated at a low current density of 100 mA·g-1 (see Fig. 6b), and at a high current density of 1000 mA·g-1 (see Fig. S5). As expected, the performance of the Fe2O3 electrode decreases after 20 cycles due to excessive capacity fading owing to the volume expansion and pulverization (see Fig. S6). Compared with the Fe2O3 electrode, biMO shows better
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performance. Bimetallic oxides generally serve as a good buffer because the two different metals react with lithium at different potentials. However, its capacity also slowly fades due to agglomeration of respective oxides of zinc and iron after several cycles. On the other hand,
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the G/biMO electrode does not show capacity fading, even after 100 cycles. The initial values at a higher current density were similar for Fe2O3, biMO, and G/biMO electrodes (982, 975
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and 993 mAh·g-1, respectively). These values are much smaller (see Fig. 6a and b) when compared with specific capacity measured at low current density (100 mA·g-1) due to fast diffusion of Li-ions in to the active material. The rapid insertion of Li-ions may have resulted in thinner SEI layer formation. The stable performance by biMO, and G/biMO even at a higher current rate was observed. However, the Fe2O3 electrode shows a continuous decrease in the specific capacity. Additionally, the saturated specific capacities for N ≥10 for the respective cases are consistent with rate capability measurements as shown in Fig. 6a. The Coulombic efficiency of G/biMO is ~99% after the first few cycles, as shown in Fig. 6b. The
ACCEPTED MANUSCRIPT high capacity and excellent Coulombic efficiency confirm that bi-metallic oxide based anodes are preferable to single-component (Fe2O3) anodes in terms of stability, as the latter inevitably exhibit capacity fading due to structural changes. Furthermore, the addition of graphene to biMO allows for improved electronic transport by shortening the diffusion path
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of the Li+ ions; this is also confirmed by the results of EIS shown in Fig. 7. A comparison of the reported capacities of biMO and its composites with that of G/biMO in Table 1 suggests that electrostatically sprayed G/biMO is a promising anode material for LIBs.
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The discharge/charge profile of the G/biMO half-cell is shown in Fig. 6c. As the lithiation process progresses under the open-circuit voltage, the anode shows a capacity of
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~200 mAh·g-1 (regions I and II). This is followed by a plateau at 0.8 V, resulting in an additional capacity of 600 mAh·g-1 (region III). With a successive decrease in the voltage, region IV shows a further increase in the capacity of 800 mAh·g-1, leading to the total capacity after the first lithiation stage of 1601 mAh·g-1. The first delithiation capacity, in this
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case, is 1250 mAh·g-1, corresponding to a Coulombic efficiency of 79%. Regions II and III for N = 2 are different, owing to the structural modification of the ZnFe2O4 phase. The
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differential capacity curves of G/biMO for N = 1 and 2, shown in Fig. 6d, exhibit some similarities. Thus, we conclude that because aggregation of the ZnO/Fe2O3 phases formed
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after the first cycle is prevented and the electronic conductivity is improved due to the presence of graphene, G/biMO has better rate capability and long-term stability as compared to biMO without graphene. The samples tested at 100 mA·g-1 for 100 cycles were subsequently imaged by SEM.
The low and high magnification SEM images presented in Fig. S6 shows the morphologies of the FO, biMO and G/biMO electrodes. All the samples show SEI layer coating and particle pulverization however, the pulverized particles are aggregated in the absence of graphene sheets. The SEI layer was little thick in FO samples whereas, a thinner SEI layer appeared in
ACCEPTED MANUSCRIPT biMO and G/biMO. The morphological characteristics observed by SEM after cycling are consistent with capacity loss as shown in Fig. 6b. This observation is consistent with the
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remarkable capacity retention of G/biMO at different current rate and long-term cycling tests.
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Fig. 7. (a) EIS curves for various samples. (b) Magnified versions of EIS curves. (c) Equivalent circuit and impedance values. (d) Schematic of the crystal of G/biMO composite
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showing a faster electron transfer owing to the presence of graphene. EIS measurements were performed on the half-cells over a range of frequencies from
100 kHz to 0.01 Hz; the resulting Nyquist plots (Z′ versus Z″) are shown in Fig. 7a. Magnified versions of the plots, given in Fig. 7b, show the charge-transfer impedances of the electrodes. The semicircle in the case of G/biMO is small and is followed by a steep curve corresponding to the Warburg impedance; these are indicative of quicker electronic transport and faster Li-ion diffusion kinetics, respectively. Conversely, for the case of Fe2O3 and
ACCEPTED MANUSCRIPT ZnFe2O4, the semicircle is larger and incomplete, and the Warburg impedance curve is less steep, indicating poor electronic transport and slower Li-ion diffusion kinetics, respectively. The data in Figs. 7a and 7b were fitted using the Randle’s equivalent circuit shown in Fig. 7c. The solution resistance (Rs) and charge-transfer resistance (Rct) of Fe2O3, biMO, and
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G/biMO, were determined from the fitted lines, as shown in Fig. 7c. The lower impedance values for the G/biMO composite sample confirm that the graphene sheets provide a shorter pathway for electron transfer. Additionally, from the highly inclined Warburg impedance
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region we infer better Li-ion diffusion; the agglomeration of metal oxide particles is suppressed due to the graphene sheets. A schematic of the rapid and unhindered electron
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transfer in the G/biMO composite crystal due to the graphene sheets (see horizontal electron flow) is shown in Fig. 7d. The dotted line across the oxide pathway shown in Fig. 7d represents the hindered electron transfer through Fe2O3 and/or biMO because of their lower conductivity, resulting in inadequate diffusion that also occurs for the FO and biMO samples
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(see impedance spectra). The EIS obtained after cycling is shown in Fig. S7, revealing that the formation of the SEI layer increases the charge transfer resistance. The obtained values of Rct for the FO, biMO, and G/biMO composites are 133, 205, and 269 Ω respectively. The
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increase in the impedance Rct is attributed to the decomposed solvent of the electrolyte and
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the electrode conductance due to the presence of the SEI layer [41]. The SEM images of materials after cycling, presented in supporting information Fig. S6, clearly show solid electrolyte interphase (SEI) layer formation in G/biMO.
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4. Conclusions In this study, we employed a facile, ambient pressure ESD method to fabricate a composite consisting of graphene sheets decorated with pillar-like uniformly-dispersed
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ZnFe2O4 nanoparticles, for use as a high-performance anode material in LIBs. The electrochemical characteristics of a half-cell LIB based on this G/ZnFe2O4 anode were indicative of significantly improved lithium storage, with the anode showing a high capacity,
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good rate capability, and long-term cycling stability compared with those of cells based on single- (Fe2O3) and bi- (ZnFe2O4) component anodes without graphene. The excellent long-
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term cycling performance of G/ZnFe2O4 was due to the presence of graphene sheets and Zn, which buffered the volumetric expansion and prevented the agglomeration of ZnO and Fe2O3 during lithiation/delithiation, resulting in high capacity retention. The superior characteristics of the G/ZnFe2O4 composite, which was fabricated by ESD, including its high cycling
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stability, reversible capacity, and enhanced Coulombic efficiency, suggest that it is highly suitable for use as an anode material in LIBs.
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Acknowledgement
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This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016M1A2A2936760) and NRF-2017R1A2B4005639
ACCEPTED MANUSCRIPT Table 1. Comparison of previously reported specific capacities of ZnFe2O4 and its composites, and that of G/ZnFe2O4
Electrode material
Synthesis method
Capacity (mAh·g-1) (Nth cycle) current rate ( mA·g-1)
ZnFe2O4/C
Hydrothermal
1100 (100) 50
ZnFe2O4
Hydrothermal
867 (80) 200
ZnFe2O4/graphene
Hydrothermal
1049 (100) 100
[43]
[9]
[18]
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Ref
[42]
Hydrothermal
1152 (50) 60
Hydrothermal
900 (50) 65
ZnFe2O4/C
Solvothermal
841 (30) 65
ZnFe2O4@C/graphene
Solvothermal
705 (180) 232
[45]
ZnO.Fe3O4
Molten salt
820 (50) 60
[46]
ZnFe2O4
Template sacrifice
824 (100) 200
[47]
ZnFe2O4
Thermal decomposition
983 (50) 100
[48]
ZnFe2O4
Polymer pyrolysis
833 (50) 116
[38]
ZnFe2O4
Electrospun
625 (50) 50
[20]
ZnFe2O4@C
Ball milling
681 (100) 71
[24]
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MWCNT/ZnFe2O4 ZnFe2O4
[39]
[44]
Solvothermal
814 (50) 100
[49]
ZnFe2O4
Urea combustion
615 (50) 60
[40]
ZnFe2O4/graphene
Urea combustion
721 (75) 100
ZnFe2O4/graphene
Electrospray
1235 (100) 100
[50] Present work
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ZnFe2O4 /graphene
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Highlights •
G/ZnFe2O4 as an anode for LIB, it exhibits 1235 mAh·g-1 after 100 cycles.
•
The excellent electrochemical performance, high capacity and cycling stability were
•
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achieved. Graphene decorated nanostructured ZnFe2O4 attributed to numerous active sites and interspaces.
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Facile electro-spraying of active material straightaway on stainless steel spacer.
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•
S0925-8388(18)34723-6
DOI:
https://doi.org/10.1016/j.jallcom.2018.12.170
Reference:
JALCOM 48811
To appear in:
Journal of Alloys and Compounds
Received Date: 22 August 2018 Revised Date:
6 December 2018
Accepted Date: 12 December 2018
Please cite this article as: B. Joshi, E. Samuel, M.-W. Kim, K. Kim, T.-G. Kim, M.T. Swihart, W.Y. Yoon, S.S. Yoon, Electrosprayed graphene films decorated with bimetallic (zinc-iron) oxide for lithium-ion battery anodes, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2018.12.170. 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 proof before it is published in its final 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.
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Electrosprayed Graphene Films Decorated with Bimetallic (Zinc-Iron) Oxide for Lithium-Ion Battery Anodes
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Bhavana Joshia,†, Edmund Samuela,†, Min-Woo Kima, Karam Kima, Tae-Gun Kima, Mark T. Swihartb, Woo Young Yoonc,*, Sam S. Yoona,* a
School of Mechanical Engineering, Korea University, Seoul 136-713, Republic of Korea Department of Chemical & Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-4200, U.S.A. c Department of Materials Science & Engineering, Korea University, Seoul 02841, Republic of Korea
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Abstract
Bimetallic (zinc-iron) oxide with a pillar-like morphology is decorated over graphene films. The electrosprayed composite demonstrates good electrochemical performance as an anode material for lithium-ion batteries. Scanning and transmission electron microscopy images confirm the effective decoration of graphene sheets with bimetallic Zn-Fe oxide particles.
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The composite shows a high capacity of 1601 mAh·g-1 at 100 mA·g-1 owing to its unique layer-particle morphology. The anode material exhibits excellent capacity retention of 77% (~1235 mAh·g-1 at 100 mA·g-1) after 100 cycles, due to buffering of volumetric strain in the metal oxide by flexible conductive graphene sheets. Zinc forms an alloy with the lithium
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ions, and the graphene sheets prevent particle agglomeration, keeping transport distances within particles small. Thus, synthesizing a composite of graphene with Zn-Fe oxides via
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electrospray deposition is a promising method for preparing high-performance anode materials for lithium-ion batteries.
Keywords: ZnFe2O4, electrospray, LIB, anode, graphene *
Corresponding author: [email protected], [email protected]
†
These authors have contributed equally.
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1. Introduction The primary focus of research related to lithium-ion batteries (LIBs) is the development of high-voltage cathode and high-capacity anode materials. [1] Given the
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instability of organic electrolytes at higher voltages, significant efforts have been devoted to developing high-capacity anode materials [2]. As has been reported in several reviews, transition metal oxides (TMOs) have garnered a great deal of attention as anode materials
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because of their high lithiation capacity [3-5]. However, charge transfer in TMOs is hampered by their low conductivity. Furthermore, volume expansion/contraction in these
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materials leads to pulverization of the active oxide material, which in turn reduces the number of Li-ion hosting sites owing to particle agglomeration.[6, 7] These two factors limit the practical applicability of TMOs in LIBs. To prevent the agglomeration of a pulverized single TMO during lithiation/delithiation and to increase its electrochemical activity for
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enhanced lithium storage, a second TMO is combined with it. Using two TMOs (TTMOs) has been shown to be an effective solution [8]. TTMOs allow for more precise tailoring of the energy density as each TMO has its own characteristic working potential [9].
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Anodes based on two TMOs include those produced from spinel ferrite-based materials, such as CoFe2O4 [10], NiFe2O4 [11], MnFe2O4 [12], MgFe2O4 [13], and ZnFe2O4
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[9]. Among these, ferrites other than ZnFe2O4 do not form alloys with lithium. Thus, these materials have limited storage capacity. Another key advantage of using zinc and iron together is that they are both abundantly available and low cost. Furthermore, they are both environmentally benign. Zn increases the capacity of the anode material owing to its ability to form an alloy with Li ions (Li-Zn). On the other hand, iron oxide can be reduced to metallic Fe, which leads to the formation of Li2O during lithiation. Although the theoretical capacities of ZnO and Fe2O3 are 978 mAh·g-1 and 1007 mAh·g-1, respectively, they both
ACCEPTED MANUSCRIPT undergo volume expansion, owing to their low conductivities [14, 15]. Moreover, an advantageous blending of ‘conversion’ and ‘de-alloying–alloying’ reaction mechanisms occur in the bimetallic (Zn-Fe) oxide, providing a high theoretical capacity of ~1072 mAh·g-1 [16, 17]. Zinc ferrite structures for use in LIBs have been primarily synthesized by
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solvothermal and hydrothermal methods [9, 18]. Zhao et al. synthesized spherical 30–40 nm sized ZnFe2O4 particles by a hydrothermal method [19]. These particles showed an initial capacity of 1287 mAh·g-1 at a current density of 0.2 mA·cm-2. Qiao et al. reported ZnFe2O4
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nanofibers that showed a capacity of 1248 mAh·g-1 during the first cycle [20]. However, low reversible capacity, current rate capability, and capacity retention remain challenges for
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ZnFe2O4. Thus, forming carbon-based composites of ZnFe2O4 becomes promising, as carbonaceous material cushioning can effectively improve the electronic conductivity while buffering volume change [21, 22]. Furthermore, graphene sheets can help preserve the structure of the active material, resulting in enhanced cycling stability [23]. Based on this, Sui
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et al. fabricated a multiwalled carbon nanotube-ZnFe2O4 composite that showed a good reversible capacity [9]. Furthermore, a ZnFe2O4-carbon nanocomposite prepared by solgel/ball milling exhibited a capacity of 681 mAh·g-1 after 100 cycles; this is superior to that
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of pristine ZnFe2O4 [24-26]. Among various carbon materials such as carbon black (CB),
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carbon nanotubes (CNT), or graphene, the sheet type morphology of graphene is particularly advantageous, because it not only restricts agglomeration of the particles but also facilitates uniform distribution of the particles on the sheets. In this study, we enhanced the rate capability and cycling stability of ZnFe2O4 through
the addition of graphene. A composite was formed by electrostatic spray deposition (ESD), which is a non-vacuum and cost-effective technique for the deposition of nanomaterials [27, 28]. ESD allows for the tailoring of the deposit microstructure. For example, the deposited nano/submicron particles can be made to exhibit a pillar-like morphology by ensuring the
ACCEPTED MANUSCRIPT deposition of highly charged nanodroplets on the substrate [29]. This pillar-like morphology results in a high surface-to-bulk ratio, which, in turn assists in the accommodation of a large number of Li ions. It also results in mesoporosity, which helps alleviate the strain experienced by the anode due to repeated volume expansion/contraction that occurs during
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discharging/charging processes.
We used ESD to deposit active materials including Fe2O3, bimetallic (Zn-Fe) oxide (biMO), and graphene/bimetallic (Zn-Fe) oxide (G/biMO) directly onto 1 mm thick spacers,
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for use as LIB anodes. G/biMO exhibited impressively high stability and a capacity retention
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because of its pillar-like morphology and the synergistic effect of the well-dispersed TTMOs
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and graphene.
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2. Experimental Procedure 2.1 Chemical synthesis The starting materials, namely, zinc acetate dihydrate (ZnAc, Zn(CH3COO)2·2H2O,
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Oriental Chemical Industries), iron chloride hexahydrate (FeCl3·6H2O, Sigma Aldrich), polyvinylpyrrolidone (PVP, MW 40 kDa, Sigma Aldrich), potassium hydroxide (KOH, Sigma Aldrich), and graphene (N002-PDR, Angstrom Materials), were used as received.
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Ethanol (99.9% Duksan) and propylene glycol (PG, Duksan) were used as the solvents for the zinc and iron salts, respectively. To form the precursor solution, 0.022 g of graphene (~10
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weight percent relative to Zn and/or Fe salts) was first mixed in a water/ethanol solution (2:4 ratio). Next, 37.5 mM ZnAc and 75 mM FeCl3 were mixed with 2.2 ml of the graphene dispersion and stirred until they were well dissolved. The Zn/Fe ratio was kept at 1:2. Next, PG was added to the graphene/ZnAc-FeCl3 (G/ZnFe) solution, which was then subjected to
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ultrasonication for 90 min to ensure uniform dispersion of the graphene and of the metal atoms over the exfoliated graphene sheets. Then, 2.5 ml of KOH was added dropwise to initiate precipitation, and the solution was ultrasonicated for 10 min. PVP was added to the
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solution before further ultrasonication to maintain the dispersion state of the graphene sheets
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and bimetallic (Zn-Fe) oxide nanoparticles. The solution was then subjected to ultrasonication for another 20 min. These processes resulted in the growth of bimetallic oxide nanoparticles on the graphene sheets. A controlled synthesis of bimetallic oxide nanoparticle solution with no graphene, was similarly conducted using a PG/ethanol mixture. Similarly, a solution that contained only Fe was also prepared by excluding graphene and ZnAc. Hereafter, the samples with active material Fe2O3, bimetallic (Zn-Fe) oxide, and graphene/bimetallic (Zn-Fe) oxide composite will be referred to as FO, biMO, and G/biMO, respectively.
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2.2 Electrospray deposition The above-described precursor solutions were electrosprayed (Fig. 1) on precleaned stainless steel spacer disks with a thickness of 1 mm. The substrate temperature was
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maintained at 150 °C and an 11 kV voltage was applied between the nozzle and substrate placed 4 cm apart. The fabricated samples were annealed at 450 °C in two steps in an argon atmosphere. In the first step, the temperature was raised to 200 °C and kept constant for 2 h.
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Next, the temperature was increased from 200 to 450 °C and kept constant for 3 h. The heating rate was 3 °C min-1. The electrospray procedure is illustrated in Fig. 1a and shows the
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spraying of the precursor comprised of FeCl3, ZnAc, and graphene. Figure 1b compares the coin substrates with and without deposition. The inset SEM image shows the morphology of the sprayed film. Figure 1c depicts the lithiation mechanism between the sprayed G/ZnFe2O4
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film and Li-foil across the separator.
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Fig. 1. (a) Schematic diagram of the electrospray deposition with the synthesized precursor. (b) A snapshot of biMO (w/o graphene) and G/biMO (w/ graphene) electrodes deposited over
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a spacer. The SEM image is of G/biMO. (c) Schematic illustration of the lithiation
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mechanism between the sprayed G/biMO film and Li-foil across the separator.
2.3 Physicochemical characterization The annealed samples were characterized by X-ray diffraction (XRD) analysis (SmartLab, Rigaku) to evaluate their crystalline structures. Raman spectroscopy (Jasco, NRS3100) was performed to analyze the graphene films and their defects, as well as the bonding characteristics of Zn and FeO. Field-emission scanning electron microscopy (FESEM; S-
ACCEPTED MANUSCRIPT 5000, Hitachi, Ltd.) and transmission electron microscopy (TEM; JEM 2100F, JEOL Inc.) were used to study the surface morphologies of the samples. Energy-dispersive X-ray spectroscopy (EDX) along with SEM and TEM was used to determine the elemental concentrations and elemental maps, respectively, of the samples. X-ray photoelectron
determine the chemical states of the elements.
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2.4 Electrochemical characterization
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spectroscopy (XPS; Theta Probe Base System, Thermo Fisher Scientific Co.) was used to
The spacer disks with coatings (weight ~1.3 mg) were directly used as anodes in
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CR2032 coin-type half-cells along with a metallic Li sheet (16 mm in diameter) as the reference electrode. A Celgard 2400 (Celgard, Chungbuk, South Korea) polymer membrane used to electrically isolate the Li-foil and anode material. The liquid electrolyte used in the cells comprised 1 M LiPF6 in a solvent mixture of ethylene carbonate, dimethyl carbonate,
(R3-1)
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and ethyl methyl carbonate (1:1:1 by volume) (PuriEL, Soulbrain, Seongnam, South Korea). Approximately 0.3 mL of electrolyte was used to ensure good wetting of the
electrodes and separator. The electrochemical performance of the samples was measured
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based on their galvanostatic charge/discharge curves obtained for voltages of 0.005 – 3 V at
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25 °C using a WBCS3000 battery cycler system (WonATech, Seoul, South Korea). The cells were subjected to rate tests at current densities of 0.1, 0.2, 0.5, and 1 C, where C = 1000 mA·g-1. Impedance measurements were carried out using electrochemical impedance spectroscopy (EIS) for frequencies of 100 kHz to 0.01 Hz in the open-circuit mode utilizing a perturbation voltage of 5 mV. The results of the impedance measurements were used to determine the solution resistance (Rs) and charge-transfer resistance (Rct), among other parameters.
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3. Results and Discussion
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3.1 Material properties
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Fig. 2. (a) XRD patterns and (b) Raman spectra of various samples. The XRD pattern of the G/biMO sample deposited by ESD is shown in Fig. 2a. The
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pattern exhibits sharp peaks at diffraction angles (2θ) of 30°, 35.4°, 43.6°, 56.8°, 62.2°, 66.3°, and 74°, which correspond to the (220), (311), (400), (333), (440), (442), and (622) planes of ZnFe2O4, respectively. The XRD pattern matches well with that of ZnFe2O4 (JCPDS 821049[30]/22-1012[31]), confirming the formation of a well-crystallized cubic spinel structure. In contrast, hematite (α-Fe2O3) and maghemite (γ-Fe2O3) phases are present in the Fe2O3 sample, as evidenced by the peaks marked with “▼”and “●”. The peak at 28.2° corresponds to α-Fe2O3, while diffraction peaks at 33.3°, 35.8°, 40.8°, 43.8°, 50.3°, 54.3°,
ACCEPTED MANUSCRIPT 62°, 64°, 66°, 74°, and 75° represent γ-Fe2O3. These patterns match JCPDS cards 01-1053 (αFe2O3) and 39-1346 (γ-Fe2O3), respectively [32]. These Fe2O3 phases are also observed in the biMO and G/biMO samples. The estimated average sizes of the ZnFe2O4 particles based on the (311) plane using the Debye Scherrer equation are approximately 16 and 40 nm for the
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biMO and G/biMO samples, respectively. No graphene-related peaks are detected in the XRD pattern of G/biMO. We attribute their absence to the low concentration of graphene relative to Fe2O3 and ZnFe2O4, and the exfoliated nature of the graphene nanosheets. From
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the XRD pattern, we infer that biMO is a mixture of Fe2O3 and ZnFe2O4 phases.
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Figure 2b shows the Raman spectra of various samples. Broad peaks at 1343 and 1590 cm-1, corresponding to the D and G bands of graphene, respectively, confirm the incorporation of graphene in the G/biMO sample. The D band indicates the presence of defects owing to exfoliation and loading of ZnFe2O4 particles over the graphene sheets. The Fe2O3 and ZnFe2O4 samples exhibit peaks at wavenumbers in the range 100–800 cm-1. Peaks
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corresponding to seven phonon modes in the α/γ- Fe2O3 phases can be observed clearly in the spectrum of Fe2O3. As per group theory, these are two A1g modes (222 and 496 cm-1), five Eg
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modes (244, 290, 338, 410, and 611 cm-1), and one disorder Eu mode (656 cm-1) [33]. In contrast, peaks related to five active phonon modes can be observed in ZnFe2O4. In spinel
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ferrites, the peaks observed at wavenumbers greater than 600 cm-1 correspond to tetrahedral vibrations (AO4), while the lower-frequency modes are attributed to octahedral vibrations (BO6) [34]. The dominant peak at 1315 cm-1 in the Raman spectra of Fe2O3 and ZnFe2O4 is due to the second-order scattering processes of α-Fe2O3, in accordance with a report by Kumar et al. [35]. Finally, the appearance of peaks related to the α/γ-Fe2O3 phases in the Raman spectra is consistent with the XRD results.
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Fig. 3. (a) TEM image, (b) HRTEM image, (c) SAED pattern, and (d) elemental maps confirming the presence of C, Zn, Fe, and O in G/biMO.
TEM was used to analyze the morphology and microstructure of G/biMO in detail. The bright and contrasting areas in Fig. 3a represent the graphene sheets, while the dark spots
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are the biMO particles loaded on the graphene sheets. The biMO nanoparticles have a welldistributed morphology and dimensions, with sizes of 20–30 nm. The small ZnFe2O4 particles decrease the length of the Li diffusion path, and thus, enhance the electrochemical activity.
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The HRTEM image in Fig. 3b shows lattice fringes with d spacings of ~0.48, 0.29, and 0.25
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nm, corresponding to the (111), (220), and (311) planes of ZnFe2O4, respectively. The selected area electron diffraction (SAED) pattern in Fig. 3c contains six concentric rings, confirming the polycrystalline nature of γ-Fe2O3 and ZnFe2O4. The rings are formed because the randomly distributed nanoparticles result in the continuous angular distribution of the (hkl) planes, that is, the (221) plane of γ-Fe2O3 and the (222), (331), (333), (440), and (620) planes of ZnFe2O4. The SAED pattern is consistent with the XRD analysis results and confirm the cubic structures of Fe2O3 (JCPDS 39-1346) and ZnFe2O4 (JCPDS 82-1049). The composition of G/biMO is confirmed by the elemental maps of Zn, Fe, O, and C determined
ACCEPTED MANUSCRIPT by EDX. The different colored areas in Fig. 3d indicate the Zn-, Fe-, O-, and C-rich areas of
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the sample, confirming the formation of G/ZnFe2O4.
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Fig. 4. SEM images of (a) FO, (b) biMO, and (c) G/biMO. The insets show higher-resolution
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images of corresponding samples.
SEM images of FO, biMO, and G/biMO are shown in Fig. 4. The images of the Fe2O3 and biMO samples show a well-controlled deposition by ESD. Furthermore, a pillar-like morphology is observed in Fe2O3 and biMO (see Figs. 4a and 4b). In the absence of graphene, the ZnFe2O4 particles partially agglomerate owing to the formation of these pillars.
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The consistent formation of biMO nanoparticles over the graphene sheets can be seen clearly in Fig. 4c. The roughened graphene surface is indicative of exfoliation. Furthermore, the
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biMO particles deposited on these sheets significantly enhance the surface-to-bulk ratio and shorten the path length for lithium-ion diffusion. The graphene sheets act as cushions and
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prevent the agglomeration of the biMO nanoparticles, while the particles concurrently prevent the graphene sheets from restacking during lithiation/delithiation [36, 37]. Thus, the fused graphene sheets and biMO nanoparticles improve the electrochemical performance of the active material during long-term cycling (see Fig. 6b). The surface elements and their oxidation states in the G/biMO composite were analyzed using XPS. The survey scan (Fig. S1a) shows the presence of elemental Fe, Zn, O, and C. The core Zn2p spectrum in Fig. S1b shows two peaks at binding energies of 1021.4 and 1045.0 eV, attributed to Zn2p3/2 and Zn2p1/2, respectively, confirming that Zn is present
ACCEPTED MANUSCRIPT in the +2 oxidation state. Furthermore, the peaks at 710.9 and 724.8 eV in Fig. S1c are attributed to Fe2p3/2 and Fe2p1/2, respectively. The satellite peaks of Fe2p observed at binding energies of 718.6 and 733.0 eV are indicative of the Fe3+ state. The core O1s spectrum in Fig. S1d shows a broad asymmetric peak, which can be deconvoluted into four peaks with
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binding energies of 529.3, 530.15, 531.2, and 532.0 eV. The peaks at 529.3 and 530.15 eV are attributed to the lattice oxygen in Zn/Fe-O. The presence of other groups, such as the OH group, is confirmed by the peak at 531.2 eV, while the peak at 532.0 eV is related to defects
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due to oxygen deficiency in biMO. The deconvoluted C1s core spectrum (Fig. S2) from graphene contains peaks at 284.0, 285.0, 286.2, and 288.5 eV, which correspond to C-C (sp2),
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C-C (sp3), C-O, and C=O bonds, respectively. The contents of Zn-Fe and carbon in the composite G/biMO were estimated to be approximately 27.3 and 72.7 wt %, respectively, based on TGA as shown in Fig. S3 in the supporting information.
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3.2 Electrochemical performance
The electrochemical performance and specific capacity of the anode materials used in LIBs depend on the reactions that occur with Li ions. The crystallographic structures for
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Fe2O3 and ZnFe2O4 corresponding to the XRD data in Fig. 2a, are shown in Fig. S4. The ionic radius of Li is small. Hence, the insertion of Li-ions within the crystal structures of
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Fe2O3 and ZnFe2O4 initially causes minimal or no disorder in the structures. However, with an increase in the number density of the Li ions, due to the acceleration of the kinetics of the redox reaction with the continuous discharging/lithiation of the half-cells, Fe2O3 is converted into metallic Fe and Li2O. In contrast, ZnFe2O4 is converted into metallic Fe, ZnO, Li2O, and a LiZn alloy, in steps. Furthermore, due to the forced lithiation process, as well as the occurrence of the redox reaction at lower potentials, the unit cells of Fe2O3 and ZnFe2O4 expand, as shown in Figs. S4c and S4f, respectively. As this occurs, the pillar-like
ACCEPTED MANUSCRIPT morphology of the G/ZnFe2O4 results in buffering during the expansion process, allowing for the accommodation of a significant number of Li ions. During the lithiation of Fe2O3 (involving reversible and partially irreversible redox reactions), the oxide is converted into a metallic phase. A total of six Li ions are lithiated per
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Fe2O3 formula unit during the intercalation of Li and/or the conversion of the metal oxide during the electrochemical process. In this study, in order to further improve the capacity of the metal oxide, we synthesize the ternary composite ZnFe2O4. Because of the incorporation
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of zinc in the iron oxide, the number of lithiated Li ions increases from six to nine. The
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additional three Li ions reduce ZnO to metallic Zn and form a LiZn alloy. The redox reaction for the alloying process is a two-step chemical reaction that occurs during the lithiation stage, as explained below. The insertion of lithium begins with intercalation (reactions 1 and 2, Fig. S4e), followed by the conversion reaction (3), and alloying reaction (4) (see Fig. S4f and
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Schematic 1) [24, 38, 39]:
+ . + . ↔ . () . + . + . ↔ ()
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+ + → + + () + + ↔ ()
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+ ↔ + + ( )
+ ↔ + + ()
The reactions in (3), (5), and (6) show that ZnFe2O4 is reduced to ZnO and Fe2O3.
Hence, the subsequent lithiation/delithiation processes only involve reactions (4) and (5) in the case of ZnO and (6) in the case of Fe2O3. The phase-separation process is described in Scheme 1.
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Scheme 1. Intercalation of lithium ions in ZnFe2O4.
Fig. 5. Discharge/charge profiles and differential capacity plots of (a and c) Fe2O3 and (b and d) biMO. As described above, the lithiation/delithiation of the anode causes the active material to undergo multiple processes during the redox reactions. The dissociation of ZnFe2O4 into
ACCEPTED MANUSCRIPT metallic or alloy phases is analyzed in greater depth based on the discharge (lithiation)/charge (delithiation) profiles of the anodic materials. The discharge/charge profiles of Fe2O3 and ZnFe2O4 for the first and second cycle (N = 1 and 2) are presented in Fig. 5. The discharge capacities of Fe2O3 and ZnFe2O4 during the first cycle, determined from the capacity-voltage
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profile, are 1350 and 1484 mAh·g-1, respectively. Furthermore, as shown in Figs. 5a and 5b, the capacity-voltage profiles for the first discharge cycle show four distinct voltage regions between 0.005 and 3.0 V. In the first region, the voltage decreases from 3 to 1.4 V during the
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discharging process, resulting in a small change in the capacity, which is indicative of a small degree of intercalation of free lithium ions. However, owing to the presence of these free Li
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ions, the redox reaction is initiated. Thus, the active materials Fe2O3 and ZnFe2O4 transform into LixFe2O3 and LixZnFe2O4 (x ≈ 0.5), respectively, as indicated by the small increase in the capacity in region II (1.4 to 0.81 V). The primary transition/reduction of the active material occurs in region III (0.81 to 0.65 V) in the case of Fe2O3, as shown in Fig. 5a. The observed
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peak at 0.81 V in the differential capacity plot for N = 1 (see Fig. 5c) confirms the transition of Fe2O3 into a metallic Fe3+ phase. Region IV is shown in Fig. 5a and consists of a plateaulike region with a gradual fall in the voltage and a rise in the specific capacity [39, 40]. In this
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region, the metallic Fe3+ ions are reduced to Fe0. Furthermore, the electrolyte decomposes,
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forming a solid electrolyte interphase (SEI) layer owing to the partially irreversible redox reactions. The redox reaction leads to the formation of a complex polymeric gel. The specific capacity of biMO (1484 mAh·g-1) during the first lithiation stage is
greater than that of Fe2O3 (1350 mAh·g-1). In the second region (see Fig. 5b), a greater number of Li ions react with LixZnFe2O4, thus increasing x to 2 and completing the intercalation reaction (see Schematic 1; the intercalation of Li ions in ZnFe2O4). The lithiation results in the intercalation of nine Li ions during the redox reaction. This intercalation and the reduction of ZnFe2O4 result in an increase in the specific capacity over
ACCEPTED MANUSCRIPT the four stages marked in Fig. 5b. These four stages correspond to the electrochemical response of the differential capacity and are indicated by the four peaks observed at 1.53, 0.96, 0.79, and 0.11 V in Fig. 5d. The redox reaction (1) is described above and represents the first lithiation stage of ZnFe2O4. However, during the delithiation stage, ZnFe2O4 is not
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restored, probably because a LiZn alloy is formed, as shown in reaction (4). However, it has also been reported that ZnFe2O4 decomposes into ZnO and Fe2O3 after the first delithiation stage [24, 38, 39]. ZnO acts as a buffer, minimizing the agglomeration of the Fe2O3 particles
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during the repetitive charging/discharging that causes pulverization of the metal oxide (both ZnO and Fe2O3). The peak observed at 1.53 V in the differential capacity profile for N = 1 is
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absent in the profile for N = 2 and higher, as shown in Fig. 5d. This confirms that ZnFe2O4 is not restored. However, the shift of the peak at 0.79 to 0.96 V confirms the formation of Fe2O3. Furthermore, it we note that the discharge capacity of biMO (1484 mAh·g-1) during the initial cycle, in which the transition metal ions are reduced to the metallic state, leads to
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the formation of 4Li2O as per the formula (see reaction 3) and is higher than the theoretical capacity (~1072 mAh·g-1). However, the capacities of Fe2O3 and ZnFe2O4 after delithiation are 916 and 1095 mAh·g-1, respectively, with the active materials exhibiting coulombic
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efficiencies of 67% and 74%. The high irreversible capacity may be due to the formation of
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the SEI layer and solvent decomposition in the electrolyte [41]]. Concurrently, the Fe2O3 and ZnFe2O4 nanoparticles also convert into an amorphous phase in the presence of the electrolyte, as the cell potential approaches 0.005 V (versus Li). A similar phenomenon has been reported in the case of Li/CoFe2O4 cells [10].
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Fig. 6. Electrochemical characterization. (a) Rate capability for voltages of 0.005–3.0 V. (b)
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Long-term cycling capacity and coulombic efficiency at 100 mA·g-1 for various anode materials. (c) Discharge/charge profiles, and (d) differential capacity of G/biMO.
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It is essential to investigate the anode performance at lower to higher current densities to realize the current capability ability of the active material. Thus, all three
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electrodes were subjected to electrochemical testing at different current densities. Their performance is presented in Fig. 6a. Among the three electrodes, G/biMO exhibits superior first reversible discharge capacities, which are 1066, 815, 651, and 522 mAh·g-1, at current densities of 100, 200, 500, and 1000 mA·g-1, respectively. (R3-2) Note that these capacities are based on the total mass of G/biMO, including the graphene, which can contribute significantly to the total capacity. Furthermore, when the current density is changed from high to low, the G/ZnFe2O4 electrode exhibits better reversible performance, showing a
ACCEPTED MANUSCRIPT capacity of 1064 mAh·g-1 after 35 cycles. In contrast, the biMO and FO electrodes show much lower capacities of 768 and 482 mAh·g-1, respectively. The difference in the capacities can be ascribed to the good dispersion of the biMO nanoparticles over the graphene sheets, which prevents their aggregation and enhances rate capability. The G/biMO composite
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provides improved electrical conductivity due to graphene and two simultaneous reaction mechanisms, dealloying/alloying of LiZn and the Fe2O3/Li2O conversion reaction, supplemented by capacity provided by the graphene itself.
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The long-term cycling performances of the Fe2O3, biMO, and G/biMO electrodes
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were evaluated at a low current density of 100 mA·g-1 (see Fig. 6b), and at a high current density of 1000 mA·g-1 (see Fig. S5). As expected, the performance of the Fe2O3 electrode decreases after 20 cycles due to excessive capacity fading owing to the volume expansion and pulverization (see Fig. S6). Compared with the Fe2O3 electrode, biMO shows better
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performance. Bimetallic oxides generally serve as a good buffer because the two different metals react with lithium at different potentials. However, its capacity also slowly fades due to agglomeration of respective oxides of zinc and iron after several cycles. On the other hand,
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the G/biMO electrode does not show capacity fading, even after 100 cycles. The initial values at a higher current density were similar for Fe2O3, biMO, and G/biMO electrodes (982, 975
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and 993 mAh·g-1, respectively). These values are much smaller (see Fig. 6a and b) when compared with specific capacity measured at low current density (100 mA·g-1) due to fast diffusion of Li-ions in to the active material. The rapid insertion of Li-ions may have resulted in thinner SEI layer formation. The stable performance by biMO, and G/biMO even at a higher current rate was observed. However, the Fe2O3 electrode shows a continuous decrease in the specific capacity. Additionally, the saturated specific capacities for N ≥10 for the respective cases are consistent with rate capability measurements as shown in Fig. 6a. The Coulombic efficiency of G/biMO is ~99% after the first few cycles, as shown in Fig. 6b. The
ACCEPTED MANUSCRIPT high capacity and excellent Coulombic efficiency confirm that bi-metallic oxide based anodes are preferable to single-component (Fe2O3) anodes in terms of stability, as the latter inevitably exhibit capacity fading due to structural changes. Furthermore, the addition of graphene to biMO allows for improved electronic transport by shortening the diffusion path
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of the Li+ ions; this is also confirmed by the results of EIS shown in Fig. 7. A comparison of the reported capacities of biMO and its composites with that of G/biMO in Table 1 suggests that electrostatically sprayed G/biMO is a promising anode material for LIBs.
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The discharge/charge profile of the G/biMO half-cell is shown in Fig. 6c. As the lithiation process progresses under the open-circuit voltage, the anode shows a capacity of
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~200 mAh·g-1 (regions I and II). This is followed by a plateau at 0.8 V, resulting in an additional capacity of 600 mAh·g-1 (region III). With a successive decrease in the voltage, region IV shows a further increase in the capacity of 800 mAh·g-1, leading to the total capacity after the first lithiation stage of 1601 mAh·g-1. The first delithiation capacity, in this
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case, is 1250 mAh·g-1, corresponding to a Coulombic efficiency of 79%. Regions II and III for N = 2 are different, owing to the structural modification of the ZnFe2O4 phase. The
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differential capacity curves of G/biMO for N = 1 and 2, shown in Fig. 6d, exhibit some similarities. Thus, we conclude that because aggregation of the ZnO/Fe2O3 phases formed
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after the first cycle is prevented and the electronic conductivity is improved due to the presence of graphene, G/biMO has better rate capability and long-term stability as compared to biMO without graphene. The samples tested at 100 mA·g-1 for 100 cycles were subsequently imaged by SEM.
The low and high magnification SEM images presented in Fig. S6 shows the morphologies of the FO, biMO and G/biMO electrodes. All the samples show SEI layer coating and particle pulverization however, the pulverized particles are aggregated in the absence of graphene sheets. The SEI layer was little thick in FO samples whereas, a thinner SEI layer appeared in
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remarkable capacity retention of G/biMO at different current rate and long-term cycling tests.
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Fig. 7. (a) EIS curves for various samples. (b) Magnified versions of EIS curves. (c) Equivalent circuit and impedance values. (d) Schematic of the crystal of G/biMO composite
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showing a faster electron transfer owing to the presence of graphene. EIS measurements were performed on the half-cells over a range of frequencies from
100 kHz to 0.01 Hz; the resulting Nyquist plots (Z′ versus Z″) are shown in Fig. 7a. Magnified versions of the plots, given in Fig. 7b, show the charge-transfer impedances of the electrodes. The semicircle in the case of G/biMO is small and is followed by a steep curve corresponding to the Warburg impedance; these are indicative of quicker electronic transport and faster Li-ion diffusion kinetics, respectively. Conversely, for the case of Fe2O3 and
ACCEPTED MANUSCRIPT ZnFe2O4, the semicircle is larger and incomplete, and the Warburg impedance curve is less steep, indicating poor electronic transport and slower Li-ion diffusion kinetics, respectively. The data in Figs. 7a and 7b were fitted using the Randle’s equivalent circuit shown in Fig. 7c. The solution resistance (Rs) and charge-transfer resistance (Rct) of Fe2O3, biMO, and
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G/biMO, were determined from the fitted lines, as shown in Fig. 7c. The lower impedance values for the G/biMO composite sample confirm that the graphene sheets provide a shorter pathway for electron transfer. Additionally, from the highly inclined Warburg impedance
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region we infer better Li-ion diffusion; the agglomeration of metal oxide particles is suppressed due to the graphene sheets. A schematic of the rapid and unhindered electron
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transfer in the G/biMO composite crystal due to the graphene sheets (see horizontal electron flow) is shown in Fig. 7d. The dotted line across the oxide pathway shown in Fig. 7d represents the hindered electron transfer through Fe2O3 and/or biMO because of their lower conductivity, resulting in inadequate diffusion that also occurs for the FO and biMO samples
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(see impedance spectra). The EIS obtained after cycling is shown in Fig. S7, revealing that the formation of the SEI layer increases the charge transfer resistance. The obtained values of Rct for the FO, biMO, and G/biMO composites are 133, 205, and 269 Ω respectively. The
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increase in the impedance Rct is attributed to the decomposed solvent of the electrolyte and
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the electrode conductance due to the presence of the SEI layer [41]. The SEM images of materials after cycling, presented in supporting information Fig. S6, clearly show solid electrolyte interphase (SEI) layer formation in G/biMO.
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4. Conclusions In this study, we employed a facile, ambient pressure ESD method to fabricate a composite consisting of graphene sheets decorated with pillar-like uniformly-dispersed
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ZnFe2O4 nanoparticles, for use as a high-performance anode material in LIBs. The electrochemical characteristics of a half-cell LIB based on this G/ZnFe2O4 anode were indicative of significantly improved lithium storage, with the anode showing a high capacity,
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good rate capability, and long-term cycling stability compared with those of cells based on single- (Fe2O3) and bi- (ZnFe2O4) component anodes without graphene. The excellent long-
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term cycling performance of G/ZnFe2O4 was due to the presence of graphene sheets and Zn, which buffered the volumetric expansion and prevented the agglomeration of ZnO and Fe2O3 during lithiation/delithiation, resulting in high capacity retention. The superior characteristics of the G/ZnFe2O4 composite, which was fabricated by ESD, including its high cycling
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stability, reversible capacity, and enhanced Coulombic efficiency, suggest that it is highly suitable for use as an anode material in LIBs.
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Acknowledgement
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This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016M1A2A2936760) and NRF-2017R1A2B4005639
ACCEPTED MANUSCRIPT Table 1. Comparison of previously reported specific capacities of ZnFe2O4 and its composites, and that of G/ZnFe2O4
Electrode material
Synthesis method
Capacity (mAh·g-1) (Nth cycle) current rate ( mA·g-1)
ZnFe2O4/C
Hydrothermal
1100 (100) 50
ZnFe2O4
Hydrothermal
867 (80) 200
ZnFe2O4/graphene
Hydrothermal
1049 (100) 100
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Ref
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Hydrothermal
1152 (50) 60
Hydrothermal
900 (50) 65
ZnFe2O4/C
Solvothermal
841 (30) 65
ZnFe2O4@C/graphene
Solvothermal
705 (180) 232
[45]
ZnO.Fe3O4
Molten salt
820 (50) 60
[46]
ZnFe2O4
Template sacrifice
824 (100) 200
[47]
ZnFe2O4
Thermal decomposition
983 (50) 100
[48]
ZnFe2O4
Polymer pyrolysis
833 (50) 116
[38]
ZnFe2O4
Electrospun
625 (50) 50
[20]
ZnFe2O4@C
Ball milling
681 (100) 71
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MWCNT/ZnFe2O4 ZnFe2O4
[39]
[44]
Solvothermal
814 (50) 100
[49]
ZnFe2O4
Urea combustion
615 (50) 60
[40]
ZnFe2O4/graphene
Urea combustion
721 (75) 100
ZnFe2O4/graphene
Electrospray
1235 (100) 100
[50] Present work
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ZnFe2O4 /graphene
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Highlights •
G/ZnFe2O4 as an anode for LIB, it exhibits 1235 mAh·g-1 after 100 cycles.
•
The excellent electrochemical performance, high capacity and cycling stability were
•
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achieved. Graphene decorated nanostructured ZnFe2O4 attributed to numerous active sites and interspaces.
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Facile electro-spraying of active material straightaway on stainless steel spacer.
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•