Formation of carbon-coated ZnFe₂O₄ nanowires and their highly reversible lithium storage properties

Abstract

In this paper, carbon-decorated ZnFe₂O₄ nanowires, having one-dimensional geometry with diameters of 70–150 nm and lengths of several micrometers, were prepared and used as a highly reversible lithium ion anode material. They can be obtained from calcination of glucose-coated ZnFe₂(C₂O₄)₃ nanowires, which were prepared in glucose containing microemulsion solutions. The physicochemical properties of carbon-coated ZnFe₂O₄ nanowires were investigated by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The carbon-coated ZnFe₂O₄ nanowires showed a substantially increased discharge capacity of ca. 1285.1 mA h g⁻¹ at the first cycle as compared with non-carbon-coated ZnFe₂O₄ nanowires (ca. 1024.3 mA h g⁻¹) and ZnFe₂O₄ nanoparticles (ca. 1148.7 mA h g⁻¹). Moreover, the discharge capacity of the carbon-coated ZnFe₂O₄ nanowires was maintained with no degradation even after 100 charge/discharge cycles. The high cycling durability, rate capability, and coulombic efficiency suggest that the carbon-coated ZnFe₂O₄ nanowires prepared here can be promising anode candidates for a highly reversible lithium storage electrode.

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Introduction

The increasing demand for high performance lithium ion batteries (LIBs) has triggered tremendous research efforts for the development of electrode materials with high lithium storage capacity, long-term cycling stability, and excellent rate capability.^1–3 In particular, 3d transition-metal oxides have attracted special attention as an alternative anode material to current commercial graphite due to their high lithium storage capacity, safety and relatively low cost.^1–5 Among them, iron oxides have been explored as potential anode materials because of their high specific capacity, low cost, and environmental friendliness.^6,7 However, long-term cycling stability of iron oxides still remains a challenge for the practical LIB application due to their poor electronic conductivity and large volume variations during repeated charge/discharge processes, which could lead to eventual cracking of electrode and rapid capacity fade.^8–10 Recently, several strategies were proposed to improve the electronic conductivity and cycling stability of iron oxide electrodes. For instance, to minimize Li⁺ diffusion pathways and mechanical stress during charge/discharge, various nanostructured iron oxides have been proposed, such as urchin-like spheres,⁶ nanorods,⁸ nanotubes,⁹ hollow spheres,¹¹ etc. Especially, one-dimensional (1D) nanostructured iron oxides can offer high specific capacity, improved cycling stability and rate capability, because they can make electron and Li⁺ transport faster by offering large electrode/electrolyte contact areas and efficient accommodation of strain energy without cracking or crumbling.^8,9,12 In addition, to enhance charge transfer rate and buffer the rapid volume changes during charge/discharge, hybridizations with another materials such as amorphous carbon,¹³ graphene,¹⁴ and carbon nanotube¹⁵ were also proposed. Among these hybridization methods, carbon decoration technique has been widely adopted for highly stable electrode materials because of its simple synthetic route.^13,16

Recently, besides simple iron oxides, spinel structured ternary iron oxides, such as CoFe₂O₄,^17–19 CuFe₂O₄,^20,21 NiFe₂O₄,^22,23 and ZnFe₂O₄^24–27 have attracted great interest for improving the capacity retention of iron-based oxide. ZnFe₂O₄ has been proposed as a promising candidate for a LIB anode because of its high theoretical specific capacity (1072 mA h g⁻¹), low toxicity, and abundance.^28–32 However, there are still some issues to be solved in ZnFe₂O₄ materials such as low electronic conductivity, and large volume variations during repeated cyclings.^33,34 In this regard, carbon-decorated 1D ZnFe₂O₄ nanowires (C@ZFO NWs) are prepared for the LIB electrodes, whose configuration could improve the charge transfer rate and cycling reversibility with following features: (i) carbon coated layers and 1D structure could improve the conductivity of the electrode system, which could enhance electron transfer and decrease ohmic losses.³⁵ (ii) Large surface-to-volume ratio of 1D C@ZFO NWs could enable electrolyte permeability to increase for faster Li⁺ diffusion.³⁶

Herein, we report a facile approach for the synthesis of C@ZFO NWs by a calcination of glucose-coated ZnFe₂(C₂O₄)₃ nanowires, which were prepared in glucose containing microemulsion system. To the best of our knowledge, this is the first time that 1D C@ZFO NWs have been applied to the LIB electrodes with highly reversible charge/discharge and high rate capability. The prepared C@ZFO NWs electrode shows an excellent initial discharge capacity (ca. 1285.1 mA h g⁻¹), long-term cycling stability, and high coulombic efficiency, as compared with non-carbon-coated ZnFe₂O₄ NWs (ZFO NWs) and ZnFe₂O₄ nanoparticles (ZFO NPs). From their unique architecture and good electrochemical performance, these C@ZFO NWs could be considered as promising electrode materials for next-generation lithium storage platforms.

Experimental

Synthesis of carbon-coated ZnFe₂O₄ nanowires

For the fabrication of C@ZFO NWs, 1.0 g of cetyltrimethylammonium bromide (≥99%, Aldrich) was dissolved in mixed solvent of 35 mL of cyclohexane (Kanto Chemical) and 1.5 mL of n-pentanol (≥99%, Aldrich), which was stirred for 30 min. After then, 2.0 mL of 1.0 M aqueous solution of oxalic acid (Junsei Chemical) was added into above solution which was stirred additionally for 1 h until it became transparent. The 1.25 mL of an aqueous solution containing 0.05 M of Zn(NO₃)₂·6H₂O (98%, Aldrich) and 0.1 M of FeSO₄·7H₂O (99.9%, Aldrich) was added to the above solution and was stirred for 6 h. Finally, 2.0 mL of 0.1 M glucose aqueous solution was added and stirred for 6 h. The final product was rinsed thoroughly with absolute acetone to remove residual salts, and subsequently dried in a convection oven at 70 °C. The resulting material was heat-treated in an Ar atmosphere at 450 °C for 2 h with a heating rate of 2 °C min⁻¹.

For comparison, non-carbon-coated ZFO NWs were prepared via a same procedure without addition of glucose aqueous solution. The ZnFe₂(C₂O₄)₃ NWs and ZFO NWs were obtained with different combinations of Zn:Fe precursors (see Fig. S1 in the ESI†). ZFO NPs were synthesized via a hydrothermal route. In a typical synthesis, 0.22 g of (CH₃COO)₂Zn·2H₂O (≥99%, Aldrich) and 0.35 g of (CH₃COO)₂Fe (≥99.99%, Aldrich) were dissolved in 50 mL of ethylene glycol. This solution was stirred for 30 min under atmospheric conditions and maintained at 160 °C for 12 h, followed by cooling to room temperature. Obtained material was rinsed thoroughly with distilled water and absolute ethanol and then dried in a convection oven at 70 °C.

Physicochemical characterization and electrochemical measurements

The shapes and morphology of the samples were investigated by using scanning electron microscopy (SEM; JEOL JSM-7500F). The X-ray diffraction (XRD) patterns were recorded with a Rigaku Rotalflex RU-200B diffractometer using a Cu-Kα (λ = 1.5418 Å) source with a Ni filter at 40 kV, 40 mA and a scan rate of 0.02° s⁻¹. Raman spectra (Renishaw Invia) were obtained using 514 nm Ar⁺ laser excitation with a power of 8.5 mW. The spectra were recorded in wave number range between 2000 and 100 cm⁻¹. The carbon content (wt%) in the obtained C@ZFO NWs was determined by thermogravimetric analysis (TGA; Shimadzu TGA-50) at a heating rate of 10 °C min⁻¹ in air. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations coupled with energy-dispersive X-ray spectrometry (EDX) were carried out (Tecnai G2 F30 S-Twin) at 300 kV. The surface area and pore size distribution of the sample were characterized by Brunauer–Emmett–Teller (BET) analysis using a N₂ adsorption on an ASAP 2020 (Micromeritics, USA). X-ray photoelectron spectroscopy (XPS; MultiLab 2000) analysis was conducted using a monochromic Al-Kα source (E = 1486.6 eV). Data processing of the XPS measurements was conducted with the XPSPEAK software program.

The C@ZFO NWs, carboxylmethyl cellulose binder, and carbon black were mixed in a weight ratio of 80 : 10 : 10. The prepared slurry was pasted onto a copper foil using a doctor blade method. The specific capacity of C@ZFO NWs was calculated based on their total mass. Prior to the battery cycling tests, the electrode was dried in a vacuum oven overnight at 120 °C. The electrolyte consisted of 1 M LiPF₆ in a 1 : 1 v/v mixture of ethylene carbonate and diethyl carbonate (Cheil Industries). Pure lithium foil was used as counter and reference electrode, and Cellgard 2400 was used as a separator film. The cell (CR2032 coin type) was assembled in an argon-filled glove box in which the moisture and oxygen concentrations were maintained below 1 ppm. The cells were aged for 24 h before the electrochemical measurements, and the cyclic voltammograms (CVs) were recorded at a scan rate of 0.05 mV s⁻¹ from 3.0 to 0.01 V with an AMETEK Solartron Analytical 1400. The assembled cells were also galvanostatically cycled at a current rate of 100 mA g⁻¹ between 0.01 and 3.0 V on a WBCS 3000 battery tester (WonA Tech). Galvanostatic intermittent titration technique (GITT) experiment was performed by charging/discharging the cells for 30 min at a current density of 100 mA g⁻¹ and relaxing for 1 h to reach quasi-equilibrium state and then repeating the process to the end of voltage limit after 10 cycles. The impedance response at the AC amplitude of 5 mV in the frequency range of 100 kHz to 10 mHz was measured using a Solartron 1260 frequency response analyzer after the first cycle.

Results and discussion

Fig. 1a shows schematic illustrations of the synthesis procedure for the C@ZFO NWs together with a hypothetical depiction of the Li⁺ storage process of as-prepared electrode system. In the first step, glucose molecule-coated ZnFe₂(C₂O₄)₃ NWs were prepared in the glucose-containing microemulsion solution. In the second step, the carbon layer was formed on the ZFO NWs through a calcination in Ar atmosphere, which allows both phase transformation of zinc iron oxalate and carbonization of glucose molecules for the formation of C@ZFO NWs. The conceptual Li⁺ storage process in the C@ZFO NWs electrode system is illustrated in Fig. 1a(iii), in which Li⁺ could migrate readily into the intervals between the C@ZFO NWs networks and then enter into the C@ZFO NWs material, which shortens the Li⁺ diffusion distance. Electrons could also be transported effectively in this architecture because both carbon layers and lD structure itself could allow easy electron transfer along the NWs axis direction.^37–39 Fig. 1b shows a SEM image of the glucose-coated ZnFe₂(C₂O₄)₃ NWs, which were used as a precursor for the synthesis of the C@ZFO NWs in this work.

Fig. 1 (a) Illustration of the formation procedures for the C@ZFO NWs together with schematic description of charging process of as-prepared electrode system; (i) glucose-coated ZnFe₂(C₂O₄)₃ NWs, (ii) C@ZFO NWs, and (iii) electrochemical charging process. SEM images of (b) glucose-coated ZnFe₂(C₂O₄)₃ NWs and (c) C@ZFO NWs.

The image shows the uniform 1D morphologies without agglomeration of other structures, indicating uniform formation of 1D glucose-coated ZnFe₂(C₂O₄)₃ NWs. The inset of Fig. 1b reveals the very smooth surface of glucose-coated ZnFe₂(C₂O₄)₃ NWs with diameters of 100–250 nm and lengths of several micrometers. The C@ZFO NWs were prepared through the calcination route at 450 °C in Ar atmosphere, as shown in Fig. 1c. As-prepared C@ZFO NWs maintained their original 1D structure from the precursor NWs after the calcination and had diameters of 70–150 nm and lengths of several micrometers. Note that the dimensions of C@ZFO NWs were reduced as compared to the glucose-coated ZnFe₂(C₂O₄)₃ NWs, because of the release of CO₂ from ZnFe₂(C₂O₄)₃ NWs during the calcination process.^40,41 Although the carbon layers cannot be recognized in Fig. 1c, the color change from brown of non-carbon-coated ZFO NWs to light-black of C@ZFO NWs could enable one to deduce the presence of carbon layers formed on the ZFO NWs.⁴²

The crystal structure of the as-prepared materials was examined with XRD measurements in Fig. 2a. The diffraction peaks of glucose-coated ZnFe₂(C₂O₄)₃ NWs can be indexed to the standard patterns of zinc iron oxalate (JCPDS 49-1079), where the diffraction peaks at 2θ of 22.7, 24.9, 30.1, 33.7, 47.2, and 50.7° can be ascribed to the (002), (111), (40-2), (11-3), (023), and (312) planes of the zinc iron oxalate, respectively. These peak positions are also consistent with earlier reports.^43–46 Note that the coated glucose did not affect the XRD patterns of ZnFe₂(C₂O₄)₃ NWs because the diffraction patterns of glucose-coated ZnFe₂(C₂O₄)₃ NWs are same as non-glucose-coated ZnFe₂(C₂O₄)₃ NWs (see Fig. S2a in the ESI†). After the calcination process, the XRD patterns indicate that the C@ZFO NWs exhibit the spinel phase of ZnFe₂O₄ from comparison with the standard material (JCPDS 22-1012). The diffraction peaks at 2θ of 30.0, 31.6, 35.3, 42.9, 56.6, 62.6, and 67.8° correspond to the planes (220), (200), (311), (400), (511), (440), and (442), respectively, of the polycrystalline ZnFe₂O₄ phase. The spinel ZnFe₂O₄ belongs to space group Fd3̄m (227) with lattice parameters of a = 8.441 Å.⁴⁵ The average crystallite size of the C@ZFO NWs was estimated to be approximately 6.8 nm, which can be calculated from Scherrer equation. According to the XRD patterns, the calcination route of glucose-coated ZnFe₂(C₂O₄)₃ NWs successfully transformed ZnFe₂(C₂O₄)₃ into pure spinel ZnFe₂O₄ phase, which is further confirmed by Raman, TEM, XPS measurements. This result demonstrates that the crystal structure of ZnFe₂O₄ was not changed after the carbon decoration on the ZFO NWs through calcination in Ar atmosphere.³⁶ The C@ZFO NWs did not show any XRD peaks from the carbon layers, indicating that the carbon layer could be formed on the ZFO NWs with low crystallinity or low content.⁴² Moreover, no notable peak shifts or intensity variations induced by secondary phases or impurities were detected, indicating high purity of the final products. Fig. 2b presents the Raman spectra of the C@ZFO NWs, which shows two strong peaks located at 1367.5 (D-band) and 1591.4 cm⁻¹ (G-band). These peaks confirm definitely the presence of carbon layers on the ZFO NWs. Furthermore, TGA analysis reveals that the carbon content in the C@ZFO NWs is about 2.4 wt% (see Fig. S3a in the ESI†). Because the carbon layer makes it difficult to distinguish the spectrum of the spinel ZFO in C@ZFO NWs owing to the attenuation of the Raman signal, the Raman spectra of ZFO NWs were further recorded at a low wave number range.^47,48 Inset of Fig. 2b shows the Raman spectrum of non-carbon-coated ZFO NWs in the 900 to 100 cm⁻¹ wave number range. The mode at above 600 cm⁻¹ corresponds to oxygen motion in tetrahedral AO₄ groups, therefore, the mode at 633.8 cm⁻¹ could be of A_1g symmetry.^49,50 The other low-frequency phonon modes like at 440.9 and 358.2 are due to vibrations of metal ions in octahedral BO₆ groups, which are attributed to the symmetric and anti-symmetric bending of oxygen in M–O bond.^51,52 These peak positions are in good agreement with those of previously reported ZnFe₂O₄ materials.^49–52

Fig. 2 (a) XRD patterns of the glucose-coated ZnFe₂(C₂O₄)₃ NWs and C@ZFO NWs. (b) Raman spectra of the C@ZFO NWs. Inset in panel (b) shows the Raman spectra of ZFO NWs in the 900 to 100 cm⁻¹ wave number range.

The detailed structural properties of the C@ZFO NWs were further examined by TEM analysis. As shown in Fig. 3a, the C@ZFO NWs are uniformly coated with amorphous carbon (approximately 10 nm in thickness), unlike non-carbon-coated ZFO NWs (see Fig. S4a in the ESI†). Furthermore, the image shows that the C@ZFO NWs consisted of many primary ZnFe₂O₄ nanoparticles with diameters of 5–20 nm which form the 1D NW morphology. From the presence of multiple diffraction rings in the selected area electron diffraction (SAED) pattern of the inset of Fig. 3a, the polycrystalline nature of the C@ZFO NWs was confirmed. These diffraction rings in the SAED correspond to the diffraction peaks of (220), (311), and (400) lattice planes of spinel ZnFe₂O₄ phases, which is in line with the d-spacing of XRD patterns. Lattice fringes with d spacings of 0.244, 0.254, and 0.49 nm were observed from HRTEM image (Fig. 3b), which is in accordance with the (222), (311), and (111) interplane spacings of the spinel ZnFe₂O₄ phase, respectively.

Fig. 3 (a) TEM and (b) HRTEM image of the C@ZFO NWs. The inset in panel (a) shows a corresponding SAED pattern. (c) Dark field TEM image with corresponding elemental mappings of C, Zn, Fe, and O for the C@ZFO NWs.

The image shows many tiny holes throughout the NWs, originating from the inter-nanoparticle voids, which could enhance the surface-to-volume ratio and thus ensure a high electrode/electrolyte contact area for efficient Li⁺ transport.^34,35 The 1D C@ZFO NWs show porous structure with a specific surface area of 120.1 m² g⁻¹ (see Fig. S5 in the ESI†) from the type IV BET isotherms. The average pore size of 7.5 nm was obtained using the Barrett–Joyner–Halenda (BJH) method. The elemental distribution in the C@ZFO NWs was examined by elemental mappings of the electron energy loss spectra. As depicted in Fig. 3c, the elements of C, Zn, Fe, and O are distributed uniformly across the C@ZFO NWs. Well-dispersed C element demonstrates the carbon layers were uniformly decorated onto the ZFO NWs. This carbon layer could improve the electrical connection in the electrode network system, which could improve the rate capability at a relatively high current density.³⁷

To further examine the oxidation state of C, Zn, and Fe elements in the C@ZFO NWs, XPS analysis was performed. The XPS survey spectra of the C@ZFO NWs are depicted in Fig. S3b (ESI†). As shown in Fig. 4a, the C 1s peaks can be resolved into three peak components: main peak at 284.5 eV is assigned to non-oxygenated carbon (C–C), whereas minor peaks at 285.9 and 288.4 eV can be attributed to oxidized carbons such as C–O and C=O, respectively.^42,53 In Fig. 4b, the peaks of 1044.4 and 1021.3 eV can be attributed to Zn 2p_1/2 and Zn 2p_3/2, respectively. The binding energy separation between two peaks is 23.1 eV, which is in line with previous reports.^29,54,55 Fig. 4c shows Fe 2p spectrum at the binding energies of 724.0 and 710.4 eV, which can be ascribed to Fe 2p_1/2 and Fe 2p_3/2, respectively. The binding energy difference between the Fe 2p_1/2 and Fe 2p_3/2 peaks is 13.6 eV, which is consistent with that of ZnFe₂O₄ materials.^28,29,55 As shown in Fig. 4d, the O 1s peak is deconvoluted into two peaks: one peak at 529.6 eV corresponds to the lattice oxygen in ZnFe₂O₄, while the other peak at 530.9 eV is ascribed to oxygen in carbonate species on the C@ZFO NWs,^37,56 which is in a good agreement with results of Fig. 4a. Consequently, the XPS spectra further demonstrate that the C@ZFO NWs are composed of ZnFe₂O₄ and carbon in this study. In addition, EDX spectra shown in Fig. S6 (ESI†) also indicate the presence of C, Zn, Fe, and O elements in the sample.

Fig. 4 XPS spectra of (a) C 1s, (b) Zn 2p, (c) Fe 2p, and (d) O 1s regions of the C@ZFO NWs.

The electrochemical properties of as-prepared C@ZFO NWs were investigated by CVs, galvanostatic charge/discharge, and GITT measurements. Fig. 5 shows the CV curves for the C@ZFO NWs electrode measured at a scan rate of 0.05 mV s⁻¹ over the potential range of 3.0–0.01 V (versus Li/Li⁺) whose CV profiles are similar to previously reported ZnFe₂O₄-based materials.^28–30 The following equations represent sequential Li⁺ storage reactions with the ZnFe₂O₄.^{28–30,32–34}

Fig. 5 CVs of C@ZFO NWs electrode between 3.0 and 0.01 V at a scan rate of 0.05 mV s⁻¹.

The first discharge:

ZnFe₂O₄ + 9Li⁺ + 9e⁻ → ZnLi + 2Fe + 4Li₂O (1)

Reversible reactions:

ZnO + 2Li⁺ + 2e⁻ ⇄ Zn + Li₂O (2)

Fe₂O₃ + 6Li⁺ + 6e⁻ ⇄ 2Fe + 3Li₂O (3)

Zn + Li⁺ + e⁻ ⇄ ZnLi (4)

As shown in CV profiles, there is a significant difference between the first and subsequent loops, suggesting that a different Li⁺ storage reaction takes place in the first scan compared with the following scans. In the first cathodic sweep, the CVs show a broad peak at approximately 0.65 V, which could be attributed to the reduction of Zn²⁺ and Fe³⁺ to Zn⁰ and Fe⁰, respectively, and an irreversible electrolyte decomposition to form solid electrolyte interphase (SEI) layers. Moreover, peaks near 0 V are ascribed to ZnLi alloy formation like eqn (1)^28–30 and a Li–C intercalation reaction.³⁷ In the first anodic sweep, the wide oxidation peaks near 0.1 V correspond to the Li⁺ de-alloying from the ZnLi alloy and de-intercalation from Li–C compound formed in the reductive sweep, according to eqn (4).^28–30,37 The peaks at approximately 1.55 V can be attributed to the oxidation of metallic Fe⁰ and Zn⁰ to Fe³⁺ and Zn²⁺ along with the decomposition of the Li₂O, as indicated in eqn (2) and (3).^32–34 In subsequent cycles, the cathodic peak shifts from 0.65 to 0.88 V, which could be indicative of a structural rearrangement in the subsequent cycles.⁵⁷ Following anodic peaks are very close to the first one, suggesting identical electrochemical reactions are involved for the anodic scans. More importantly, there is no notable variation of the oxidation/reduction peaks after the first cycle, indicating high reversibility and structural stability of the electrode reactions.

Fig. 6a shows the voltage profiles of the C@ZFO NWs electrode, which were obtained at a current density of 100 mA g⁻¹ in the potential window of 0.01 to 3.0 V (versus Li/Li⁺). During the first discharge, the voltage shows a pronounced plateau at approximately 0.8 V arising from the formation of metallic Zn⁰ and Fe⁰ in the Li₂O matrix and of SEI layers, which is consistent with the earlier CV profiles. Note that the C@ZFO NWs electrode shows a higher initial coulombic efficiency (82.1%) than that of non-carbon-coated ZFO NWs (76.1%) and ZFO NPs (55.6%) (Table S1 in ESI†). Because the outer carbon layer could reduce the side reactions with the electrolyte, the reversibility of the charge/discharge process is further enhanced for the C@ZFO NWs electrode with a high coulombic efficiency.⁴² Thereafter, the coulombic efficiency of the C@ZFO NWs electrode increases to 94.1% in the second cycle and then is stabilized until the end of the measurement (see Fig. S7a in the ESI†), indicating a high reversibility of the electrode reactions, which is also consistent with CV profiles. Fig. 6b shows the capacity-cycle number curves for the C@ZFO NWs electrode up to 100 cycles at a rate of 100 mA g⁻¹ in a potential range from 0.01 to 3.0 V at room temperature. The initial discharge capacity of the C@ZFO NWs was 1285.1 mA h g⁻¹, and a high discharge capacity of 1292.1 mA h g⁻¹ was maintained even after 100 cycles. Such a high discharge capacity could be ascribed to increased electrical conductivity in the presence of the outer carbon layer, by improving the kinetic properties of the materials. Furthermore, we observed that the C@ZFO NWs maintained their 1D configuration even after 100 charge/discharge cycles (see Fig. S7b in the ESI†), showing high structural durability of C@ZFO NWs. For comparison, we also tested the non-carbon-coated ZFO NWs and ZFO NPs under the identical test conditions. The C@ZFO NWs and ZFO NWs showed higher stability than ZFO NPs, indicating that the 1D NWs structure can alleviate the strain energy during repeated charge/discharge processes, compared with NPs structure. Since theoretical specific capacity of carbon (ca. 372 mA h g⁻¹) is smaller than that of ZnFe₂O₄ (ca. 1072 mA h g⁻¹), the presence of carbon on the ZFO NWs could reduce the Li⁺ storage capacity in terms of total mass of C@ZFO NWs compared with non-carbon-coated ZFO NW. Interestingly, however, the discharge capacity of C@ZFO NWs is higher than that of ZFO NWs, which could be explained that a greater quantity of ZnFe₂O₄ particles could participate in Li⁺ storage reactions effectively in the C@ZFO NWs electrode system, because the carbon layer could enhance the integration abilities of the each ZnFe₂O₄ particle or charge transfer through the 1D NWs.^36,37,42 Note that the discharge capacity of C@ZFO NWs and ZFO NWs appears to be increased gradually. It has been reported that transition-metal oxides, such as CoO, Co₃O₄, Fe₃O₄, and Mn₃O₄, often show a gradual capacity increase during charge/discharge cycles. It is known that this phenomenon is attributed to the reversible formation/dissolution of a polymeric gel-like film or reversible interfacial Li⁺ storage reactions.⁵⁷

Fig. 6 (a) Voltage profiles of the C@ZFO NWs electrode at a current density of 100 mA g⁻¹ between 0.01 and 3.0 V. Inset shows the plot of the differential capacity versus the voltage. (b) Capacity-cycle number curves of C@ZFO NWs, ZFO NWs and ZFO NPs in the potential window of 0.01–3.0 V. (c) Cycling performance of C@ZFO NWs, ZFO NWs and ZFO NPs at various current rates (100, 200, 400, 800, 1600, and 3200 mA g⁻¹) between 0.01 and 3.0 V. (d) Normalized capacity at each step by the average capacity values at a current density of 100 mA g⁻¹ in the first step.

Besides the high specific capacity and long-term cyclability, the good rate capability is also of great importance for high-power applications. We further investigated the rate performance of the C@ZFO NWs electrode at various current densities from 100 up to 3200 mA g⁻¹, as shown in Fig. 6c. The non-carbon-coated ZFO NWs and ZFO NPs electrode were also tested under identical conditions. For the current densities of 100, 200, 400, 800, 1600, and 3200 mA g⁻¹, the C@ZFO NWs delivered average discharge capacities of 997.1, 969.5, 972.4, 939.5, 862.3, and 753.1 mA h g⁻¹, respectively. On the other hand, the ZFO NWs delivered significantly decreased discharge capacities of 829.8, 601.2, 531.6, 508.1, 464.0, and 403.1 mA h g⁻¹, and the ZFO NPs delivered very small discharge capacities of 317.9, 118.4, 71.3, 36.4, 11.2, and 1.5 mA h g⁻¹, respectively. As shown in Fig. 6d, the normalized capacity of the C@ZFO NWs electrode (75.5%) is much higher than that of the ZFO NWs electrode (48.6%) and ZFO NPs electrode (0.5%) at a high current density of 3200 mA g⁻¹ (or 14.7 min per half-cycle). Consequently, as the charge/discharge rate increased, the capacity fade of the C@ZFO NWs was considerably decreased as compared with that of the ZFO NWs and ZFO NPs, indicating good Li⁺ storage reversibility and structural stability of C@ZFO NWs electrode system even at relatively high current densities.

To understand the reasons for the enhanced rate capability and cycling durability of the C@ZFO NWs with respect to the ZFO NWs electrode, total internal resistance was obtained from the GITT measurements after 10 cycles, as shown in Fig. 7a. The total internal resistance was calculated by the potential difference between closed-circuit voltage and quasi-open-circuit voltage divided by the input current.^58–60 The total internal resistance involves interface resistance, electronic resistance, and Li⁺ transfer resistance, etc.^61,62 As shown in Fig. 7b, the total internal resistance decreases during the lithiation, while it increases during the de-lithiation period. During the lithiation period, consecutive resistance decrease is related to volume expansion of C@ZFO NWs by Li⁺ storage reactions, which leads to an extension of the contact area between C@ZFO NWs and carbon additives. On the other hand, the increase of the internal resistance over the de-lithiation could result from loss of contact between C@ZFO NWs and carbon additives by contraction of the C@ZFO NWs. More importantly, the internal resistance of the C@ZFO NWs electrode is much smaller than that of the ZFO NWs electrode at both the lithiation and de-lithiation periods except the full lithiation regions, as shown in Fig. 7b, because a larger amount of Li₂O phase with low conductivity was formed in the C@ZFO NWs electrode compared with ZFO NWs electrode in the low potential regions. This result indicates the loss of conductive network is less serious in the C@ZFO NWs electrode compared to the ZFO NWs electrode. Accordingly, the C@ZFO NWs are able to show enhanced rate performance compared with ZFO NWs at rather high current densities. The enhanced kinetic properties of C@ZFO NWs electrode, as compared to the ZFO NWs electrode, were further elucidated by electrochemical impedance spectroscopy measurements in Fig. S8d (ESI†). Another issue of 3d transition-metal oxide-based electrodes is a high voltage hysteresis during charge/discharge cycles as compared with the intercalation electrodes, which could decrease a round-trip efficiency.^63,64 As shown in Fig. S8 (ESI†), the voltage hysteresis decreased from 0.81 V of ZFO NPs and 0.79 V of ZFO NWs to 0.77 V of C@ZFO NWs, which is based on the second charge/discharge profiles at a current density of 100 mA g⁻¹. This reduction of voltage hysteresis is a further indication of the enhanced Li⁺ transport kinetics in the C@ZFO NWs network electrode system.

Fig. 7 (a) GITT curves of C@ZFO NWs and ZFO NWs electrodes by charging/discharging for 30 min and relaxing for 1 h after 10th cycle. (b) Total internal resistance of C@ZFO NWs and ZFO NWs electrodes obtained from GITT measurements.

The results show that C@ZFO NWs electrode has improved properties like larger lithium storage capacity, higher cyclability, higher coulombic efficiency, and better rate performance, compared to non-carbon-decorated ZFO NWs and ZFO NPs electrode. As depicted in Fig. 1a(iii), this excellent performance of C@ZFO NWs electrode could be attributed to its unique structural features as follows: (i) carbon coated layers could improve the conductivity of the electrode system, which enhances electron transfer and decreases ohmic resistance. (ii) Large surface-to-volume ratio of 1D C@ZFO NWs in electrode system could enable electrolyte permeability to increase for faster Li⁺ transport. (iii) Porosity characteristics from the polycrystalline 1D NWs also could contribute to the improved electrochemical performance of the electrode, because they facilitate Li⁺ diffusion during repeated charge/discharge processes.

Conclusions

In conclusion, we have reported on carbon-coated ZnFe₂O₄ nanowires via a calcination of glucose-coated ZnFe₂(C₂O₄)₃ nanowires, which were prepared in glucose containing microemulsion solutions. When the carbon-coated ZnFe₂O₄ nanowires were used as LIBs anode, they showed a high initial discharge capacity (1285.1 mA h g⁻¹), high initial coulombic efficiency (82.1%), excellent cyclability, and high rate capability compared with non-carbon-coated ZnFe₂O₄ nanowires and ZnFe₂O₄ nanoparticles. The advanced lithium storage properties of carbon-coated ZnFe₂O₄ nanowires are related to their unique structural features such as carbon layers that improve the electron transport and interspacing formation in the nanowire network structure that facilitates Li⁺ transfer rate. Therefore, the C@ZFO NWs electrode, with high long-term durability and high energy density, could be a promising potential anode system for next-generation LIBs.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (no. 2013029776 (Mid-career Researcher Program)) and by the Global Frontier R&D Program (0420-20120126) on Center for Multiscale Energy System through NRF. We also appreciate the financial support by the Core Technology Development Program from the Research Institute for Solar and Sustainable Energies (RISE/GIST).

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Footnote

† Electronic supplementary information (ESI) available: Additional SEM, XRD, TGA, XPS, TEM, BET, EDX and electrochemical data. See DOI: 10.1039/c4ra02095b

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