Enhanced electrochemical performance of barium hexaferrite nanoplates by Zn²⁺ doping serving as anode materials

Abstract

Zn²⁺-doped barium hexaferrite (BaFe₁₂O₁₉) nanoplates with ca. 0.4–1.5 μm in diameter and a thickness of ca. 50 nm have been successfully fabricated through a facile hydrothermal approach, followed by an annealing process. The as-prepared Zn²⁺-doped BaFe₁₂O₁₉ nanoplates were first evaluated as anode materials for lithium-ion batteries (LIBs). Electrochemical tests demonstrated that the Zn²⁺-doped BaFe₁₂O₁₉ nanoplates with 2.7 mol% Zn²⁺ doping delivered a higher reversible capacity of 665.5 mA h g⁻¹ than that (441.5 mA h g⁻¹) of BaFe₁₂O₁₉ nanoplates after 250 cycles at a current density of 100 mA g⁻¹. The superior cycling performance witnessed in Zn²⁺-doped BaFe₁₂O₁₉ are attributed to the Zn²⁺ doping, which can efficiently enhance the electronic conductivity of BaFe₁₂O₁₉ as confirmed by the impedance measurements.

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1. Introduction

With increasing energy consumption and environmental deterioration, efficient and environmentally-friendly energy conversion and storage devices have attracted great attention in recent years. Rechargeable lithium-ion batteries (LIBs) have been considered as one of the most competitive energy storage devices due to their high energy density, long life-time, and the sustainable and renewable characteristics. The electrochemical performances of LIBs are largely dependent on their electrode materials. In the past few decades, graphite has been employed as the commercial anode material of LIBs due to its superior electrochemical and structural stability. However, the low theoretical capacity (372 mA h g⁻¹) and poor cycling performance of graphite at high current density have restricted its large-scale application in the ever-growing number of energy storage devices.¹ Since Tarascon and coworkers² firstly reported the conversion reaction mechanism of transition-metal oxides (TMOs) as the LIBs anode materials, researchers have been focusing on exploring alternative TMO anode materials for higher theoretical capacities than graphite.^3–6 However, most of the transition-metal oxides serving as anodes for LIBs usually suffer from fast capacity fading and poor cycling performance due to low electrical conductivity and large volume expansion/shrinkage during the lithiation/delithiation processes, which will lead to the destruction of the anode structure and rapid degrading of electrochemical performance. A variety of approaches have been employed to improve the reversible capacity and rate capability of transition-metal oxides. Among them, one efficient strategy is to design various nanostructures, such as nanoparticles,⁷ nanofibers⁸ and nanosheets,⁹ because nanostructured materials provide better stress–strain relief and maintain the structural integrity of anode materials during the repetitive lithium insertion/extraction processes, and hence enhancing the cycle stability of LIBs. In addition, nanostructured anodes can shorten the pathway for Li-ion diffusion and electron transport as well as provide more contact area with electrolyte, which may facilitate a higher Li-ion flux across the electrode/electrolyte interface leading to higher energy density.^10,11 Another strategy relies mainly on the electrical conductivity improvement of transition-metal oxides. The doping with semiconductor components has been employed to increase the electrical conductivity of electrode materials, which is beneficial to LIBs.^12–14 For example, the boron-doped TiO₂ anode exhibited a higher reversible capacity (166.9 mA h g⁻¹) than that (107.7 mA h g⁻¹) of TiO₂ sample at a current density of 1 C,¹⁵ attributing to the increase of interplanar spacing and the decrease of both surface and charge transfer resistance. The Mo⁶⁺ doped TiO₂ nanoparticles also displayed a significantly lower charge transfer resistance and higher reversible capacity (169.5 mA h g⁻¹ after 30 cycles).¹⁶ The Zn²⁺ doped LiFePO₄ cathode material exhibited better lithium-ion storage performance comparing with LiFePO₄ counterpart due to the expanding of lattice volume and the decrease of charge transfer resistance.¹⁷ The enhanced electrochemical performances were ascribed to the improvements of electron and Li-ion conductivity by the doping of heterogeneous elements.

Recently, nanostructured ternary ferrite (MFe₂O₄) anodes, such as ZnFe₂O₄,^18,19 CoFe₂O₄,²⁰ CuFe₂O₄ ²¹ and NiFe₂O₄ ²² have been investigated and exhibited good electrochemical performances due to their high theoretical specific capacities and the feasibility to adjust the working voltage by altering the content of two metal elements.^8,21 Barium hexaferrites (BaFe₁₂O₁₉), an important ferrite, is commonly utilized as magnetic material due to its relatively high Curie temperature, high coercive force and high magnetic anisotropy field, as well as its excellent chemical stability and corrosion resistivity.^23,24 However, to the best of our knowledge, no investigation has been presented on utilizing barium hexaferrites (BaFe₁₂O₁₉) nanoplates as LIBs anode so far.

In this work, the Zn²⁺-doped BaFe₁₂O₁₉ nanoplates were synthesized through a facile hydrothermal route followed by a calcination process. The nanoplates with the thickness of ca. 50 nm will shorten the pathway for Li-ion diffusion and provide more contact surface area between the active materials and electrolyte, and Zn²⁺ doping will increase the electronic conductivity of BaFe₁₂O₁₉. Comparing with BaFe₁₂O₁₉, the electrochemical measurements demonstrate that the as-prepared Zn²⁺-doped BaFe₁₂O₁₉ nanoplates exhibit superior cycling performances.

2. Experimental

2.1. Materials

Iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O), barium nitrate (Ba(NO₃)₂), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and sodium hydroxide (NaOH) were all purchased from Sinopharm Chemical Reagent Co., Ltd. Carbon black, Li foil and Celgard 2300 were provided by Hefei Kejing Material Technology Co., Ltd, China. Polyvinylidene fluoride (PVDF), LiPF₆ (dissolved in ethylene carbonate, dimethyl carbonate, and ethylene methyl carbonate with a volume ratio of 1 : 1 : 1) were purchased from Shenzhen Biyuan Technology Co., Ltd, China. All chemicals are of analytical grade and were used as received without further purification.

2.2. Fabrication of BaFe₁₂O₁₉ and Zn²⁺-doped BaFe₁₂O₁₉ nanoplates

The Zn²⁺-doped BaFe₁₂O₁₉ nanoplates were fabricated by a facile hydrothermal route. Specifically, Fe(NO₃)₃·9H₂O (8 mmol), Ba(NO₃)₂ (1 mmol) and Zn(NO₃)₂·6H₂O (0.25 mmol) were dissolved in 20 mL deionized water, followed by the dropwise addition of 20 mL NaOH (2.2 M) solution under magnetic stirring. The colloid solution was then transformed into a 100 mL Teflon-lined autoclave, and subsequently sealed and heated at 200 °C for 48 h in an oven. After reaction, the brown precipitate was centrifuged and washed using deionized water for several times, and then dried at 60 °C in air overnight. As the added amount of NaOH was excessive (the molar ratio of OH⁻ to all cations was 5 : 1) to allow all cations (Fe³⁺, Ba²⁺, Zn²⁺) be transformed into the precipitate. Finally, the Zn²⁺-doped BaFe₁₂O₁₉ nanoplates were obtained by annealing the brown precipitate at 900 °C for 2 h in air. The preparation procedure of BaFe₁₂O₁₉ nanoplates was the same as that of Zn²⁺-doped BaFe₁₂O₁₉ only without adding Zn(NO₃)₂·6H₂O in the first step.

2.3. Characterizations

The structure of products was determined by X-ray powder diffraction (XRD) on a Rigaku D/Max-RC X-ray diffractometer with Ni filtered Cu Kα radiation (λ = 0.1542 nm, V = 40 kV, I = 40 mA) in the range of 10–80° at a scanning rate of 4° min⁻¹. The morphologies of the samples were examined by using a JSM-6700F field emission scanning electron microscopy (FE-SEM) at an accelerating voltage of 20 kV and an electric current of 1.0 × 10⁻¹⁰ A, and a JEOL JEM-2100 high-resolution transmission electron microscopy (HR-TEM) with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was recorded on a Kratos Analytical spectrometer, using Al Kα (hν = 1486.6 eV) radiation as the excitation source, under a condition of anode voltage of 12 kV and an emission current of 10 mA.

2.4. Electrochemical measurements

To prepare the working electrode, the active material, carbon black, and polyvinylidene fluoride (PVDF) with a weight ratio of 8 : 1 : 1 were mixed in N-methyl-2-pyrrolidinone (NMP) to form a homogenous slurry, which was coated on a copper foil substrate, followed by drying in a vacuum oven at 120 °C for 12 h. The CR2025-type cells were assembled using Li foil as counter and reference electrode, Celgard 2300 as separator, and 1 M LiPF₆ (dissolved in ethylene carbonate, dimethyl carbonate, and ethylene methyl carbonate with a volume ratio of 1 : 1 : 1) as electrolyte. The assembly was performed in a glove box filled with argon atmosphere. The performance of the cells was evaluated galvanostatically in the voltage range from 0.02 to 3 V at certain current densities on a LAND CT2001A battery test system. Cyclic voltammogram (CV) was obtained on a IviumStat electrochemistry workstation with a scanning rate of 0.3 mV s⁻¹ and the potential vs. Li⁺/Li ranging from 0.01 to 3 V. The electrochemical impedance spectra were obtained on the same instrument with AC signal amplitude of 10 mV in the frequency range from 100 kHz to 0.01 Hz. The data were adopted to draw Nyquist plots using real part Z′ as X axis, and imaginary part Z′′ as Y axis.

3. Results and discussion

3.1. Characterizations of Zn²⁺-doped BaFe₁₂O₁₉ nanoplates

As shown in Fig. 1a, XRD pattern confirms that all diffraction peaks of the product synthesized by combining the facile hydrothermal approach with a calcination process match well with those of hexagonal-structured BaFe₁₂O₁₉ (JCPDS no. 39-1433), and no other peaks are detected, confirming the high purity of BaFe₁₂O₁₉. For comparison, Fig. 1b indicates that the diffraction pattern of Zn²⁺-doped BaFe₁₂O₁₉ is similar to that of BaFe₁₂O₁₉ sample. However, from the enlarged (114) peak (insert of Fig. 1), a slight peak shift to lower angle, which demonstrates the Zn²⁺ doping into crystalline lattice of BaFe₁₂O₁₉, is detected. The similar phenomenon has also been found in our previous research.²⁵ To further illustrate the variation of crystal structure, the diffraction data of (114) plane is used to calculate the lattice constant. With the Zn²⁺ doping, the calculated lattice distance between (114) planes by the Braggs law changes from 0.2623 to 0.2633 nm according to the peak shift angle.²⁶

Fig. 1 XRD pattern of standard BaFe₁₂O₁₉ (a) and Zn²⁺-doped BaFe₁₂O₁₉ (b). The inset is the highlighted (114) peak of BaFe₁₂O₁₉ and Zn²⁺-doped BaFe₁₂O₁₉.

The morphology of the products was examined by the field emission scanning electron microscope (FE-SEM). As shown in Fig. 2a, large-scale BaFe₁₂O₁₉ nanoplates with ca. 1.0–2.5 μm in diameter and the thickness of ca. 50 nm have been synthesized through a facile hydrothermal route followed by a calcination process. The high magnification image shows that the synthesized BaFe₁₂O₁₉ nanoplates have hexagonal structure (Fig. 2c and e). With the Zn²⁺ doping, there is no obvious variation on the morphology of the as-synthesized Zn²⁺-doped BaFe₁₂O₁₉ (Fig. 2b), but the size of nanoplates decreased to ca. 0.4–1.5 μm (Fig. 2d and f), which is smaller than that of BaFe₁₂O₁₉ nanoplates. The histogram of particle diameter distribution given in Fig. S1a† also showed that, although there was a wide diameter distribution, the Zn²⁺-doped BaFe₁₂O₁₉ sample had smaller average diameter than BaFe₁₂O₁₉. Meanwhile, the thickness of both samples had no obvious variation from the statistical size in Fig. S1b.† Previous investigations also confirm that the heterogeneous element doping will inhibit the growth of oxide grains.^16,27 The elemental maps of Zn²⁺-doped BaFe₁₂O₁₉ samples are shown in Fig. 3. From the selected area on one nanoplate in SEM image, a uniform distribution of Fe, Ba, Zn and O elements can be found, also suggesting the Zn²⁺ doping into the crystalline lattice of BaFe₁₂O₁₉. As the added amount of NaOH was much excessive and the XRD analysis showed that no other obvious diffraction peaks were detected, we can speculate that Zn²⁺ doping amount in the synthesized Zn²⁺-doped BaFe₁₂O₁₉ nanoplates is 2.7 mol% according to the addition molar ratio of reactants. Meanwhile, although the EDX analysis showed in Fig. S2 and Table S1 of ESI† is not accurate to measure the Zn²⁺ doping amount, the molar ratio of Zn²⁺ is 3.1 mol% calculated from the EDX data (Zn²⁺ : Ba²⁺ : Fe³⁺ = 0.55 : 1.66 : 15.22), which is close to 2.7 mol%, also suggesting the complete precipitation of Zn²⁺. In the TEM image of Zn²⁺-doped BaFe₁₂O₁₉ nanoplates (Fig. 4a), the diameter of nanoplates is ca. 0.4–1.0 μm, consistent with the SEM observation as shown in Fig. 2b, d and f. From the higher resolution image (Fig. 4b), the measured lattice spacing is 0.441 nm, corresponding to the (102) crystal plane of hexagonal BaFe₁₂O₁₉. The HR-TEM measurement reveals the single crystalline nature of the nanoplates.

Fig. 2 SEM images of BaFe₁₂O₁₉ (a, c and e) and Zn²⁺-doped BaFe₁₂O₁₉ (b, d and f) nanoplates.

Fig. 3 SEM and EDX mapping images (Fe, O, Ba, Zn) of Zn²⁺-doped BaFe₁₂O₁₉ nanoplates.

Fig. 4 HR-TEM images of Zn²⁺-doped BaFe₁₂O₁₉ nanoplates at different magnifications (a and b).

X-ray photoelectron spectroscopy (XPS) analysis was carried out to determine the composition and chemical bonding of BaFe₁₂O₁₉ and Zn²⁺-doped BaFe₁₂O₁₉ nanoplates. From the survey scan spectra (Fig. 5a), the peaks of Fe 2p, Ba 3d, and O 1s were detected in both samples indicating the existence of Fe, Ba, and O elements, while the Zn 2p peaks were only found in Zn²⁺-doped BaFe₁₂O₁₉ sample. The C 1s peak in both samples results from a small amount of carbon contamination due to the ambient exposure of samples, which is consistent with previous research.²⁸ Two peaks at the binding energy of 779.7 and 795.0 eV (Fig. 5b) are assigned to Ba 3d_5/2 and Ba 3d_3/2, respectively. Previous investigation indicated that the Ba 3d_5/2 and Ba 3d_3/2 XPS peaks of BaO were observed at the binding energy of 779.65 and 795.00 eV,²⁷ respectively, confirming that the Ba state in two samples is Ba²⁺. The Fe 2p_3/2 and Fe 2p_1/2 peaks are observed at 710.7 and 724.3 eV (Fig. 5c), respectively. The fingerprint shakeup satellite peak of Fe₂O₃ at ca. 719 eV is observed, suggesting that the valence of Fe in two samples is Fe³⁺.²⁹ As shown in Fig. 5d, the Zn 2p_3/2 and Zn 2p_1/2 peaks at 1021.2 and 1044.3 eV, which was only detected in the Zn²⁺-doped BaFe₁₂O₁₉ sample, further confirmed the Zn²⁺ doping into BaFe₁₂O₁₉ nanoplates.

Fig. 5 XPS spectra of (a) survey-scan, (b) Ba 3d, (c) Fe 2p, and (d) Zn 2p of BaFe₁₂O₁₉ and Zn²⁺-doped BaFe₁₂O₁₉ nanoplates.

3.2. Electrochemical performances of Zn²⁺-doped BaFe₁₂O₁₉ nanoplates

In order to evaluate the cycle performances of the electrodes, the discharge/charge measurements were evaluated by LAND CT2001A battery test system at a current density of 100 mA g⁻¹. Fig. 6a and b shows the selected discharge/charge curves of BaFe₁₂O₁₉ and Zn²⁺-doped BaFe₁₂O₁₉ nanoplates, respectively. In the cathodic reaction process, the first discharge curve of BaFe₁₂O₁₉ (Fig. 6a) reveals a decrease to 0.73 V, corresponding to the Li-ion insertion into BaFe₁₂O₁₉.³⁰ A well-defined potential plateau at ca. 0.73 V suggests the reduction of Ba²⁺ and Fe³⁺ to their metallic state and the formation of amorphous Li₂O, while the first charge curve shows no apparent plateau but a slope from 1.3 to 1.9 V, suggesting the oxidation of the metallic iron and barium into Ba²⁺ and Fe³⁺. When the first cycle is finished, the cell exhibits the initial discharge and charge capacities of 806.8 and 401.2 mA h g⁻¹, respectively, corresponding to the initial coulombic efficiency (the ratio of charge capacity to discharge capacity) of only 49.7%. The large capacity loss can be attributed to the lithium insertion into irreversible sites and the irreversible lithium loss due to the formation of solid electrolyte interface (SEI) layer during the first discharge/charge process.^31–33 For the Zn²⁺-doped BaFe₁₂O₁₉ sample, as shown in Fig. 6b, the potential/capacity profiles display the similar variation tendencies to those of pure BaFe₁₂O₁₉. In the first discharge curve, the initial discharge capacity is 885.1 mA h g⁻¹ with the voltage decreasing to 0.02 V. When the first charge was completed, a specific capacity of 525.0 mA h g⁻¹ was obtained, and the initial coulombic efficiency was 59.3%, which is obviously higher than that of BaFe₁₂O₁₉ sample (49.7%), indicating that Zn²⁺ doping not only enhanced the specific capacity of BaFe₁₂O₁₉ but also the initial coulombic efficiency probably due to the improvement of electronic conductivity. The CV profiles for the initial three cycles of BaFe₁₂O₁₉ and Zn²⁺-doped BaFe₁₂O₁₉ nanoplates are shown in Fig. 6c and d, respectively. As shown in the Fig. 6c, during the first cathodic scan, a wide peak from 0.3 to 0.75 V might be attributed to the reduction of Ba²⁺ and Fe³⁺ to their metallic state and the formation of amorphous Li₂O, the anodic peak at around 1.75 V can be attributed to the oxidation of the metallic iron and barium into Ba²⁺ and Fe³⁺.^34–36 Fig. 6d shows a similar profile, the cathodic peak is located at ca. 0.5 V in the first cycle and the anodic peak is at ca. 1.6 V, suggesting the similar oxidation and reduction processes. From the second cycle, the cathodic peak becomes weak and broad, and the cathodic and anodic peaks shift to higher voltage, which can be attributed to the polarization of the active material in the first cycle.^37,38

Fig. 6 Galvanostatic discharge/charge curves of the 1st, 2nd, 150th, and 250th cycle for (a) BaFe₁₂O₁₉ and (b) Zn²⁺-doped BaFe₁₂O₁₉ nanoplates. Cyclic voltammograms at a scanning rate of 0.3 mV s⁻¹ between 0.1 and 3.0 V for (c) BaFe₁₂O₁₉ and (d) Zn²⁺-doped BaFe₁₂O₁₉ nanoplates.

The cycling performances of both samples are depicted in Fig. 7. It can be found that the Zn²⁺-doped BaFe₁₂O₁₉ anode delivers the reversible capacity of 665.5 mA h g⁻¹ after 250 cycles at the current density of 100 mA g⁻¹. The specifical capacity decreases gradually to ca. 300 mA h g⁻¹ in the initial 12 cycles attributing to the extra consumption of lithium during the formation of SEI layer and the irreversible conversion reaction.^39,40 After 12 cycles, the reversible capacity of Zn²⁺-doped BaFe₁₂O₁₉ anode exhibited an increasing tendency with the cycling onward, even reached 665.5 mA h g⁻¹ after 250 cycles due to the gradual activation of anode made of metal oxide nanomaterials during the discharge/charge processes.^41–43 The coulombic efficiency of Zn²⁺-doped BaFe₁₂O₁₉ anode quickly reaches to 90% in the second cycle, and remains above 99% after 10 cycles. In comparison, the BaFe₁₂O₁₉ anode shows the similar variation tendency of reversible capacity with cycling and only delivers the reversible capacity of 441.5 mA h g⁻¹ after 250 cycles, which is much lower than that (665.5 mA h g⁻¹) of Zn²⁺-doped BaFe₁₂O₁₉ nanoplates. The higher capacity retention of Zn²⁺-doped BaFe₁₂O₁₉ nanoplates than that of BaFe₁₂O₁₉ at each cycle demonstrates that Zn²⁺ doping significantly improves the cycling performance and leads to higher reversible capacity.

Fig. 7 Cycling performance of BaFe₁₂O₁₉ and Zn²⁺-doped BaFe₁₂O₁₉ nanoplates at a current density of 100 mA g⁻¹, and columbic efficiency of Zn²⁺-doped BaFe₁₂O₁₉ nanoplates.

Fig. 8 shows the rate performance of BaFe₁₂O₁₉ and Zn²⁺-doped BaFe₁₂O₁₉ nanoplates evaluated at different current densities after 100 cycles. The Zn²⁺-doped BaFe₁₂O₁₉ product delivers reversible capacities of 325.2, 260.8, 162.9, 65.3 mA h g⁻¹ at the current densities of 200, 400, 800 and 1600 mA g⁻¹, respectively. When the current density is returned to 100 mA g⁻¹, the discharge capacity is ca. 380.4 mA h g⁻¹. The BaFe₁₂O₁₉ nanoplates only deliver the reversible capacities of 163.6, 110.2, 68.7, 33.3 mA h g⁻¹ at the current densities of 200, 400, 800 and 1600 mA g⁻¹, respectively. When the current density is returned to 100 mA g⁻¹, the discharge capacity is ca. 203.4 mA h g⁻¹. Apparently, the Zn²⁺ doped BaFe₁₂O₁₉ nanoplates exhibit much higher capacities than the sample without Zn²⁺ doping at the same current density, demonstrating that Zn²⁺ doping can effectively promote the rate performance.

Fig. 8 Discharge and charge capacities of Zn²⁺-doped BaFe₁₂O₁₉ and BaFe₁₂O₁₉ nanoplates at different cycling rates (from 100 to 1600 mA g⁻¹).

To further clarify the effects of Zn²⁺ doping on the electrochemical performance, the electrochemical impedance spectroscopy (EIS) measurement was conducted to reveal the lithium ion diffusion and electron transfer. As shown in Fig. 9, the Nyquist plots exhibit a semicircle in the high frequency region and an oblique line in the low frequency region. The diameter of semicircle is in direct proportion to the impedance, while the slope of the oblique line at low frequency corresponds to the Warburg impedance associated with the Li-ion diffusion.^44,45 Before the discharge/charge process, the measured resistance values for Zn²⁺-doped BaFe₁₂O₁₉ and BaFe₁₂O₁₉ anode are ca. 119 and 260 Ω, respectively (Fig. 9a). Apparently, the resistance value of Zn²⁺-doped BaFe₁₂O₁₉ is much smaller than that of BaFe₁₂O₁₉, indicating that Zn²⁺ doping indeed enhanced the electronic conductivity and charge transfer of BaFe₁₂O₁₉.^46,47 Moreover, the Zn²⁺-doped BaFe₁₂O₁₉ and BaFe₁₂O₁₉ anodes exhibit almost the same slope for the oblique lines at low frequency, suggesting that there are no obvious differences on the lithium ion diffusion in both samples before cycling. Fig. 9b shows the Nyquist plots of both cells measured after 250 cycling. The measured resistance value is 77 Ω for Zn²⁺-doped BaFe₁₂O₁₉ anode, which is lower than that of BaFe₁₂O₁₉ (137 Ω), attributing to the conductivity improvement caused by Zn²⁺ doping. It is worth noted that, comparing with the resistance values measured before discharge/charge process, both Zn²⁺-doped BaFe₁₂O₁₉ and BaFe₁₂O₁₉ anodes display the decrease of resistance values after 250 cycling, which results in the reversible capacity increase with cycling due to the gradual activation of the anode materials (Fig.7).⁴³ Combining the electrochemical performances with the structure characterization, the Zn²⁺-doped BaFe₁₂O₁₉ nanoplates exhibit higher reversible capacity (665.5 mA h g⁻¹) than BaFe₁₂O₁₉ nanoplates (441.5 mA h g⁻¹) after 250 cycles although there is no obvious variation on the morphology and structure of two products. The impedance measurements indicate that the Zn²⁺-doping is beneficial to increase the electronic conductivity of BaFe₁₂O₁₉, finally improving the cycling performance.

Fig. 9 Nyquist plots of BaFe₁₂O₁₉ and Zn²⁺-doped BaFe₁₂O₁₉ anodes (a) before discharge/charge process and (b) after 250 cycles.

4. Conclusions

In summary, BaFe₁₂O₁₉ and Zn²⁺-doped BaFe₁₂O₁₉ nanoplates have been synthesized by a facile approach. As anode material for Li-ion battery, the Zn²⁺-doped BaFe₁₂O₁₉ nanoplates exhibited more superior cycling performance comparing with BaFe₁₂O₁₉. The as-prepared nanoplates with the thickness of ca. 50 nm provide large contact surface area between the active material and electrolyte, and Zn²⁺ doping increases the electronic conductivity of BaFe₁₂O₁₉. Therefore, the researches on the Zn²⁺-doped BaFe₁₂O₁₉ nanoplates demonstrated an efficient way to enhance the electronic conductivity of metallic oxides, and thereby realize the improvement of electrochemical performances.

Acknowledgements

This work was supported by the Fundamental Research Funds of Shandong University (2015JC016). The authors also acknowledge the financial supports from the Doctoral Program of Higher Education of China (20130131110068), and Natural Science Fund for Distinguished Young Scholars of Shandong (JQ201312).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12127b

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