Morphology-selected synthesis of copper ferrite via spray drying with excellent sodium storage properties

Ceramics International 45 (2019) 20796–20802

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Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Morphology-selected synthesis of copper ferrite via spray drying with excellent sodium storage properties

T

Huaicong Yana,1, Yiyong Zhanga,1, Yunxiao Wangb, Jiaming Liuc, Xue Lia,*, Yingjie Zhanga, Peng Donga,** a

National and Local Joint Engineering Laboratory for Lithium-ion Batteries and Materials Preparation Technology, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, PR China b Institute for Superconducting & Electronic Materials, Australian Institute for Innovative Materials University of Wollongong, Innovation Campus, Squaire Ways North Wollongong, NSW, 2500, Australia c School of Metallurgy and Chemistry Engineering, Jiangxi University of Science and Technology, Ganzhou, 341000, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Copper ferrite Sodium ion battery Morphology-selected Glycine

A series of morphology-selected copper ferrite (CuFe2O4) anode materials for sodium ion batteries were firstly prepared via a highly efficient spray drying method using glycine as the complex and accelerator. It was found that the glycine/nitrate (G/N) molar ratio has an important influence on the crystallinity, morphology and electrochemical properties of sodium storage. Compared with the other prepared materials, the G/N-0.3 (glycine-nitrate molar ratio of 0.3) exhibits the most excellent electrochemical performance. The excellent electrochemical properties can be attributed to the optimized molar ratio of glycine-nitrate, which induce a good crystallinity with a proper porous structure, thereby improving the cycle life of the battery.

1. Introduction

as anode materials of SIB, in addition, the discharge capacity after 150 cycles was 135 mAh g−1 at 50 mA g−1 [17]. Wu et al. synthesized rodlike CuFe2O4 through coprecipitation process, and firstly reported CuFe2O4 as negative electrode materials in SIBs, the first discharge capacity could reach 846 mAh g-1 [18]. Li et al. prepared crystal structure controlled CuFe2O4 as anode materials for SIBs by solution combustion synthesis method, and after 80 cycles, the reversible capacity could still retain as 331mAh g-1. In terms of modification [19], Wu et al. fabricated NiFe2O4/reduced graphene oxide (NFO/RGO) nanocomposites as electrode materials for SIBs by the one-step hydrothermal method, and the discharge capacity of NFO/RGO (20 wt%) after 50 cycles was still up to 450 mAh g-1 [20]. Liu et al. fabricated MgFe2O4@C as negative electrode materials with ultra-high electrochemical performance (the capacity retention is 90% after 4200 cycles) for SIBs by the feasible electrospinning technique [21]. He et al. fabricated CoFe2O4-polypyrrole nanotubes as negative electrode materials for SIBs by the simple hydrothermal method, and the reversible capacity was 400 mAh g-1 at 100 mA g-1 after 200 cycles [22]. Undiserablely, the methods used above all have obvious disadvantages. The template method is too expensive to be suitable for large-scale application. The solution combustion synthesis is always

In recent years, sodium-ion batteries (SIBs) are regarded as a valuable energy storage system because of their rich sodium resources and low cost. It has similar electrochemical properties to lithium-ion batteries(LIBs) [1-5]. However, compared with Li+ (radius 0.59 Å, mass 6.94 g mol-1), Na+ has larger radius (radius 1.02 Å) and heavier mass (mass 22.99 g mol-1). It is difficult to find such negative electrode materials that allow sodium ions to be rapidly and stably inserted/extracted [6]. In consequence, it is urgent to research a kind of negative materials with high specific capacity and high cycling performance [78]. As alternative anode materials, transition metal oxides (TMOs) have superior sodium storage properties [9-11]. Alcántara R et al. firstly applied TMO as the anode of SIBs [12]. Then they reported NiFe2O4 as the active electrode material in LIB for the first time, and found that the capacity of the first circle was around 900 mAh g-1 [13]. Among the TMOs, the iron-based metal oxide material is environmentally friendly with high electrochemical activity, and is considered to be negative electrode materials with high research value [14-16]. In terms of preparation, MgFe2O4 was prepared by template method

*

Corresponding author at: Corresponding author at: E-mail addresses: [email protected] (X. Li), [email protected] (P. Dong). 1 Y. Y. Zhang and H. C. Yan contributed equally tothis work and share first authorship. **

https://doi.org/10.1016/j.ceramint.2019.07.066 Received 1 June 2019; Received in revised form 3 July 2019; Accepted 6 July 2019 Available online 08 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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time-cost. The hydrothermal method at low temperatures usually results in poor crystallinity. The coprecipitation process can cause metal ions to precipitate imcompletely. The electrospinning technique can only form a spun nanowire/belt, which causes a lower tap density. To resolve the problems of the above methods, the CuFe2O4 powder is prepared via spray drying followed by sintering in this work. This method is facile, and can obtain particles with large specific surface area, high crystallinity, and good uniformity of composition. In this paper, we developed the preparation of CuFe2O4 materials for the first time using the spray drying method and applied it to the SIB anode materials, which is a simple process and can be morphologically selected. The morphology was controlled by changing the molar ratio of glycine-to-nitrate (G/N), then explored the optimal electrochemical performance. The morphology and electrochemical properties of CuFe2O4 with different G/N molar ratios were investigated as well. The results indicate that CuFe2O4 with the G/N molar ratio is 0.3 presents the best sodium storage capacity, and the discharge capacity is 864.4 mAh g-1 at 50 mA g-1, which is close to theoretical capacity (896.4 mAh g-1). Moreover, after 50 cycles the capacity can be maintained as 483.7 mAh g-1 with a coulombic efficiency of 98%.

scanning calorimetry (TG/DSC) and data recording via a NETZSCH STA 449F3 were used. The morphology of the materials was observed by using field emission scanning electron microscope (SEM, FEI Nova Nano SEM 450) and transition electron microscope (TEM; JEM-2100F, Japan). The specific surface area was tested by using ASAP 2460 Brunauer-Emmett-Teller (BET). The content percentage of Cu and Fe was analyzed by using Spectrometer Plasma 1000 inductively coupled plasma atomic spectrometer (ICP). 2.3. Electrochemical measurements

2. Experiment

The working electrode was made of CuFe2O4, acetylene black and CMC with the weight ratio was 70:20:10. The electrolyte composition was 1M NaClO4 and 5 vol% fluoroethylene carbonate (FEC) solution in 100 % anhydrous propylene carbonate (PC). The battery was tested in a LAND CT 2001A system and subjected to cycling and rate performance electrochemical testing at 0.01-3.0 V (vs. Na/Na+). Cyclic voltammetry (CV) tests was performed on a Metrohm Autolab PGSTAT302N electrochemical workstation in the voltage range 0.005-3 V (vs. Na/Na+) at 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) was tested by using a Metrohm Autolab PGSTAT302N electrochemical workstation with a test frequency range of 0.01 Hz to 100 kHz.

2.1. Sample synthesis

3. Results and conclusions

The schematic diagram of a CuFe2O4 preparation scheme is shown in Fig. 1. The CuFe2O4 synthesis process is completed in two steps. The first step is spray drying for the precursor, and the second step is the combustion synthesis. The specific process can be summarized as follows: firstly, 40 mmol of Cu(NO3)2·3H2O and 80 mmol of Fe (NO3)3·9H2O were dissolved in 280 mL deionized water, and then 36 mmol of glycine was mixed as a complexing agent and a flammable agent (glycine-nitrate molar ratio of 0.3) to the above solution. As a comparative study, the molar amount of glycine was set to 12 mmol and 67.2 mmol, respectively, according to the above procedure(denoted as G/N-0.1, G/N-0.3, and G/N-0.56, respectively). The mixed solution was heated at a constant temperature (30 °C, and stirred under constant magnetic force for 20 min until the glycine and nitrate were completely dissolved, then the completely mixed solution was spray-dried. The inlet temperature was 190 °C, at the same time, the feed rate of 65 mL/h at nebulizer pressure 2 MPa, until the solution was converted to a dry precursor powder. Finally, a crucible containing the precursor was placed in a box furnace at 800 °C in air atmosphere for 6 hours. Finally, CuFe2O4 with different morphologies were obtained.

Fig. 2 presents the XRD spectra of the prepared samples after calcination at 800 °C for 6 hours. It can be concluded from the figure that the peaks of the products obtained by calcination with different glycinenitrate ratios are basically similar, and the 2θ angular distribution of the curve peaks are about 18.3°, 29.9°, 30.5°, 34.7°, 35.9°, 37.1°, 41.8°, 43.8°, 53.9°, 57.0°, 57.8°, 62.1°, 63.6°, and 74.6°, which are related to (101), (112), (200) planes for CuO, and (103), (211), (202), (004), (220), (312), (303), (224), (400), (413) diffraction crystal planes are corresponding to (tetragonal) anti-spinel CuFe2O4 (JCPDS 34-0425). This shows that the obtained products are all CuFe2O4. At the same time, it can be seen that the G/N-0.1 has the weakest peak intensity and the lowest crystallinity. However, the G/N-0.3 material has the strongest peak intensity and the highest crystallinity. In addition, all the prepared samples have a weak heterogeneous peak near 38.7°, which is corresponding to the (111) plane of monoclinic CuO (JCPDS 80-1916). Sample G/N-0.1 has the strongest peak, indicating a higher CuO content. It can be seen that CuO increases with an increase in the content of glycine. The possible reason is that the Fe3+ complexation constant is logβ=20.19 ± 0.02, which is larger than the complexing constant of lgβ=8.08 for Cu2+, indicating Fe3+ is more stable with glycine complexing. Therefore, more Cu2+ are not effectively complexed and then agglomerated with glycine increases. They eventually form CuO in the process of calcination in air atmosphere [23-24]. TG-DSC measures the weight change and heat change of the sample during heating. Fig. 3 is the TG-DSC display curves of the three sample

2.2. Characterization The crystal structure of material was identified by X-ray diffractometer (XRD, Minflex 600). In order to ascertain the temperature and heat changes of the precursor, thermogravimetric differential

Fig. 1. The schematic diagram of a CuFe2O4 preparation scheme. 20797

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Fig. 2. XRD patterns of the as-prepared samples.

precursors. It shows that the thermal decomposition process of the precursor is more complicated, which can be divided into dehydration, self-propagating, and recrystallization stages. G/N-0.1 has a small endothermic peak at 109.4 °C at the beginning of the test with a drop in mass, indicating that dehydration began to occur. After that, a sharp exothermic peak appeared at 184.8 °C with a rapid decline in mass, and the heat flux reaches -2.59 mW mg-1, which represents that the selfpropagation reaction occurred at this temperature, and a large amount of C, H, O, N exhausts in the form of a gas. Then there is a small endothermic peak at 229.5 °C and the weight is still declining. The small endothermic peaks stem from the burning of the precursors, which indicates that the amount of glycine used as a fuel is too small to cause a sufficient reaction. G/N-0.1 has a slight exothermic peak, and the residual mass rate of the sample was maintained at about 60.6% and the sample continued to absorb heat, indicating that the sample is undergoing a recrystallization [25]. G/N-0.3 also has a smaller endothermic peak at the beginning of the test with a drop in quality, indicating that dehydration began at this point. A sharp exothermic peak begins to appear at 174.5 °C at the beginning of the test with a drop in quality, releasing more heat than that of G/N-0.1. The mass change of G/N-0.3 is small around 34.1%, and the heat flux reaches -6.88 mW mg-1, indicating that the sample is undergoing a recrystallization. Similarly, the G/N-0.56 test also has a small endothermic peak accompanied by a drop in mass, indicating a dehydration occurred here. After that, a sharp exothermic peak appeared at 184.8 °C with a rapid drop in mass, and the heat flux is -3.52 mW mg-1, which releases more heat than G/N0.1, but releases less heat than that of G/N-0.3. The reaction for G/N0.3 is more complete. The mass change of G/N-0.56 is around 24.7% and continues to absorb heat, indicating that it begins in the process of recrystallization, and the mass of G/N-0.56 is the smallest. The reaction is more dramatic with the increase of the amount of glycine, and remove a lot of elements C, H, O, and N in the form of a gas [19]. By comparing the prepared samples, the ratio of glycine to nitrate in G/N0.3 is more appropriate because the exothermic flow rate is the largest.

Therefore the reaction process of G/N-0.3 is the most sufficient. The particles morphology and grains size of sample were firstly obseved by SEM. Fig. 4 presents the SEM of G/N-0.1, G/N-0.3 and G/N0.56 of field emission. The particles of G/N-0.1 exhibit a concave spherical shape because the rapid cooling of the prepared precursor can cause the surface shrinking, thereby cause the spherical surface to be depressed. G/N-0.1 has no obvious agglomeration, the original particle size is 100~300 nm, and the secondary particle size of the spherical shape is 1~8 μm. The morphology of G/N-0.3 is porous, because the reaction is more intense as the number of glycine increases, and the spherical precursor is calcined to form a broken porous block structure with a particles size of around 100 nm to 300 nm. In addition, the morphology of G/N-0.56 is similar to that of G/N-0.3, showing a porous block structure with no obvious agglomeration. The particle size is about 100~500 nm. The prepared samples have different morphologies, which indicates that the adjustment of the amount of glycine can effectively control the morphology of CuFe2O4. G/N-0.1 can also maintain a spherical structure. While G/N-0.3 and 0.56 are porous blocks, indicating that the amount of glycine used in the reaction for G/ N-0.1 is slower and the gas release is insufficient. More glycine can be released more gas during the reaction and form a porous structure. Then these porous structure can help to alleviate the serious volume expansion caused by Na+ embedding, thereby improving the electrochemical properties of batteries. Test the BET specific surface area of CuFe2O4 by N2 adsorptiondesorption isotherm. As shown in Fig. 5, that all three samples are type IV adsorption-desorption isotherms, indicating that different glycinecontrolled CuFe2O4 morphology belongs to mesoporous materials. G/N0.3 shows the largest specific surface area (2.0489 m2 g-1), followed by G/N-0.56 (1.0203 m2 g-1), and the smallest specific surface area is G/N0.1 (0.4212 m2 g-1), because with the increase of glycine. The reaction is more sufficient, the release of more gas increases the specific surface area of the material. But the specific surface area of G/N-0.56 is reduced due to the agglomeration caused by excess glycine. The ratio of glycine to nitrate is directly affecting the specific surface area of the material, the rapid insertion and deintercalation of sodium ions during charge and discharge is due to the high specific surface area of the material. and increase the storage capacity of sodium ions [26]. Table 1 displays the ICP data of the prepared samples. All the molar ratio of the samples exceeds 0.5, indicating that the Cu element is excessive, which results in the impurity of CuO. The three groups of experiments all meet with the molecular weight ratio, but still, contain a small amount of Cu. G/N-0.3 has the lowest Cu content, Cu content only exceeds 2% of the normal ratio, followed by G/N-0.56 Cu content of more than 3%, while sample G/N-0.1 has the most Cu content, exceeding 5% of the normal ratio. The results are consistent with the above XRD data. We performed the TEM and HRTEM images refinement analysis on the G/N-0.3 with the best electrochemical performance analysis. Fig. 6a and Fig. 6c shows the grain size of G/N-0.3, this is consistent with SEM particle size analysis. Fig. 6b and Fig. 6d displays evident lattice fringes,

Fig. 3. TG-DSC curves of (a) G/N-0.1, (b) G/N-0.3 and (c) G/N-0.56 precursor. 20798

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Fig. 4. FESEM images of G/N-0.1 (a and b), G/N-0.3 (c and d) and G/N-0.56 (e and f). Table 1 ICP date of G/N-0.1, 0.3 and 0.56.

Cu(w%) Fe(w%) Cu/Fe(mol)

Fig. 5. Nitrogen adsorption-desorption isotherms of G/N-0.1, G/N-0.3 and G/ N-0.56 composites.

and the lattice spacings are 0.49 nm and 0.29 nm, indicating the (101) and (112) plane of CuFe2O4 (JCPDS card no.34−0425), respectively. From this, it can be judged that the material is mainly composed of tetragonal anti-spinel type CuFe2O4. Testing electrochemical performance by assembling materials into coin-type half-cells. Fig. 7 reveals the CV of three groups of samples. The first and subsequent cycles of the prepared samples are

G/N-0.1

G/N-0.3

G/N-0.56

6.46 10.32 0.55

6.12 10.57 0.51

7.07 11.74 0.53

significantly distinct, indicating an irreversible reaction in the first circle. The prepared samples have two redox peaks in all cycles, moreover in the initial cycle, the peak drop at 0.01 V. In addition, G/N0.1 has a cathode peak at 1.02 V in the first cycle, and the deoxidation peaks of G/N-0.3 and G/N-0.56 are at 0.97 V and 1.04 V, representing CuFe2O4 is reduced to metallic Cu and Fe, the corresponding reaction process is as shown in (1.1). There are two oxidation peaks during the charging of the next three cycles. The anodic peaks of G/N-0.1 are located near 0.70 V and 0.92 V, separately. The anodic summits of G/N0.3 are located near 0.74 V and 0.94 V, separately. The oxidation summit of G/N-0.56 is located near 0.72 V and 0.93 V, separately. The oxidation summit near 0.71 V is that Fe is oxidized to Fe2O3, and the oxidation peak near 0.93 V means that Cu is further oxidized to CuO. The corresponding reaction formula is as follows as the reaction (1.2). In the next cycle, the peak positions of the two pairs of redox summits in each circle are similar, showing the electrode structure is relatively stable and the electrochemical process is highly reversible. The redox peak of 0.70 V/0.01 V represents the redox reaction of Fe2O3/Fe. The corresponding reaction formula is (1.3). The 0.92 V/0.72 V redox peak corresponds to the redox reaction of CuO/Cu. The corresponding reaction formula is as shown in reaction (1.4). The combined reaction

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Fig. 6. TEM and HRTEM images of the G/N-0.3 (a, b, c and d).

formula is (1.5). By comparing the potential difference, the peak of G/ N-0.3 of the reduction peak at a larger potential is around 0.97 V, while the G/N-0.56 is near to 1.04 V, a relatively small potential difference indicates that G/N-0.3 has less polarization than that of G/N-0.56. And it can be explained that the charge and discharge process for G/N-0.3 is excellent in reversibility. The reaction process is as follows: CuFe2O4 + 8Na+ + 8e- → Cu + 2Fe + 4Na2O Cu + 2Fe + 4Na2O → CuO + Fe2O3 + 8Na Fe2O3 + 6 Na CuO + 2Na

+

+

+

+ 8e

(1.1) -

+ 6e ↔ 3Na2O + 2Fe -

+ 2e ↔ Na2O + Cu -

CuO + Fe2O3 + 8Na

+

(1.2) (1.3) (1.4)

+ 8e ↔ Cu + 2Fe + 4Na2O (1.5) -

Fig. 8 shows the charge and discharge densities at a current density of 50 mA g-1. The prepared sample has a first-round reversible capacity greater than 750 mAh g-1, especially, the maximum reversible capacity of G/N-0.3 in the first cycle is 864.4 mAh g-1. In addition, the discharge curve is around 1.15 V and 0.18 V. There are two discharge platforms that consistent well with the CV first cycle reduction peak. The first coulombic efficiencies of G/N-0.1, 0.3 and 0.56 are 64.88%, 66.05% and 64.98%, respectively. The charge and discharge of all samples after 15 cycles are 474.4 / 488.6 mAh g-1, 520.2 / 534.5 mAh g-1 and 484.6 / 499.2 mAh g-1, separately, wherein the capacity retention rates are 60.02%, 60.78%, and 61.86%, respectively. G/N-0.1 has the smallest

capacity retention rate and the lowest capacity after recycling, indicating that the polarization phenomenon is obvious, and the G/N0.56 capacity retention rate is the highest, indicating excellent stability. G/N-0.3 has the highest capacity and good stability. At the same time, the G/N-0.3 and the G/N-0.56 curve have a good overlap with small polarization, which indicates that the subsequent cycle can still have excellent reversible performance. The cycle performance at 50 mA g-1 is as shown in Fig. 9. It can be seen that the first coulombic efficiency of each group is about 65%, and the coulombic efficiency is above 93% from the second circle. Irreversible capacity may be mostly because of the formation of SEI film. The subsequent capacity of each group of samples continues to decrease, and the coulombic efficiency increases. The retention of capacity after 50 cycles maintained at about 98%, which is due to the continuous expansion and removal of the negative electrode by Na+, leading to the expansion and irreversible pulverization of the anode structure. The charge and discharge capacities of the prepared samples after 50 cycles are 449.4/458.3 mAh g-1, 477.4/483.7 mAh g-1 and 463.7/472.7 mAh g-1, with the coulombic efficiencies of 98.03%, 98.70% and 98.09%, respectively. It can be seen that G/N-0.3 has a higher capacity and coulombic efficiency, this good cycle performance may be due to the porous structure which increases the activation region of the electrode in contact with the electrolyte, and reduces the structural collapse caused by the cycle. Fig. 10 is the graph showing the rate capability of samples at various

Fig. 7. Cyclic voltammetry plots of the first four cycles for (a) G/N-0.1, (b) G/N-0.3 and (c) G/N-0.56. 20800

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Fig. 8. Charge-discharge plots of the 1st, 5th, 10th and 15th at the current of 50 mA g-1 for (a) G/N-0.1, (b) G/N-0.3 and (c) G/N-0.56.

Fig. 9. The cycling performance of G/N-0.1, 0.3 and 0.56 at a current density of 50 mA g-1 (0.01-3.0 V).

capacity of 60.0 mA g-1 at 2000 mA g-1. These indicate that G/N-0.3 has the best rate performance, the cause of this phenomenon may be better crystallinity and unique porous morphology. EIS was also carried out for the battery before and after the cycling test. Fig. 11 is the EIS impedance diagram with an equivalent circuit diagram after fitting. The impedance curves of the three sets of samples are flat semicircles in high-frequency region and the medium-frequency region and are linear lines in low-frequency region. The fitted curve and the actual test curve basically coincide, which proves that the equivalent circuit diagram of the fit is closed to the real situation. In the equivalent circuit, Rb represents the interface impedance between the electrode and the electrolyte, and Rct represents the charge transfer impedance. Table 2 shows the Rb and Rct fitting data values and the error values. The error values are all within 5%, indicating that the equivalent circuit diagram of the fitting is available. The values of the new battery Rb of the samples are 661.4 Ω, 603.3 Ω, and 637.1 Ω, respectively. After ten cycles, the Rb values of the three groups of samples becomes to 166.8 Ω, 150.5 Ω, and 192.9 Ω. In comparison, it is obvious that G/N-0.3 has the lowest Rb value, and the Rct for G/N-0.3 is still lower than the other two sets of sample Rct values after 10 cycles. The results show that G/N-0.3 has a better charge transfer capability and a smaller interface impedance advantage. 4. Conclusion

Fig. 10. Rate capabilities of G/N-0.1, 0.3 and 0.56 electrodes at various current densities from 100 to 2000 mA g-1.

current densities from 100 to 2000 mA g-1. In comparison, G/N-0.3 has the highest reversible capacity. The respectively correspond average capacities are 489.1, 431.4, 384.4, 340.1, 232.0, and 188.0 mAh g-1. When 100 mA g-1 is recovered, its average capacity immediately rises back to 404.3 mA g-1. However, G/N-0.1 and G/N-0.56 rate performance is significantly poorer than G/N-0.3, and the average capacity of G/N-0.1 decreases faster than G/N-0.56. G/N-0.1 has an average

In summary, using glycine as a complexing and flammable agent, a morphology-selected CuFe2O4 material with excellent properties can be efficiently synthesized via a spray drying method. Morphology and electrochemical performance of CuFe2O4 with different molar ratios of glycine-nitrate are also studied in detail. The results indicate that with the increase of glycine content, the morphology of the material changes from concave sphere to porous block. In the electrochemical test, G/N0.3 sample exhibits the best electrochemical performance, the discharge capacity is 483.7 mA g-1 after 50 cycles and the capacity retention rate is as high as 98% with lower charge transfer impedance and interface impedance. At the same time, it also shows an excellent rate performance (At 100 mA g-1 shows 489.1 mAh g-1, up to 2000 mA g-1 large current still has 188.0 mAh g-1. When 100 mA g-1 is recovered, its average capacity immediately recovered to 404.3 mA g-1). All of these due to the good crystallinity, small impurity content, sufficient calcination, and the special porous structure of G/N-0.3, which is favorable for alleviating the serious volume expansion caused by Na+ embedding, thereby improving the cycle life of the battery. These advantages make CuFe2O4 as a valuable SIBs anode materials, and the preparation method of CuFe2O4 material is simple and efficient, which can be applied to sythesize other sodium ion battery anode materials. Acknowledgment We gratefully acknowledge the National Natural Science Foundation of China (No. 51604132 and 51764029), the Provincial

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Fig. 11. Nyquist polts of fresh and 10th cycled electrode at open-cricuit potential, and equivalent circuit model for (a) G/N-0.1, (b) G/N-0.3, (c) G/N-0.56. Table 2 Simulated resistance values (Rb, Rct) and errors of G/N-0.1, 0.3 and 0.56.

Sample Sample Sample Sample Sample Sample

G/N-0.1(fresh cell) G/N-0.1(after 10 cycles) G/N-0.3(fresh cell) G/N-0.3(after 10 cycles) G/N-0.56(fresh cell) G/N-0.56(after 10 cycles)

Rb(Ω)

Rct(Ω)

Erros(%)

661.4 166.8 603.3 150.5 637.1 192.9

1.4 9.6 1.9 1.8 9.7 2.2

0.6 2.0 1.6 1.9 1.0 1.7

[10]

[11]

[12]

[13]

[14]

Natural Science Foundation of Yunnan (No. 2017FB085 and 2018FB087).

[15]

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