Preparation of Zn₃Nb₂O₈ anode material for high-performance lithium/sodium-ion batteries

Abstract

Niobium-based oxides (M-Nb-O) as promising lithium/sodium-ion storage anode materials have attracted much attention. More types of niobium-based oxides are prepared in order to provide more candidates for anode materials. Herein, Zn₃Nb₂O₈ as a novel intercalation-type anode material has been reported for the first time. Arborescent Zn₃Nb₂O₈ particles (Zn₃Nb₂O₈-A) and stump-like Zn₃Nb₂O₈ particles (Zn₃Nb₂O₈-B) have been prepared by solid-state and solvothermal methods, respectively. Benefiting from the microsized stump-like structure and the exposure of the (110) facet, Zn₃Nb₂O₈-B delivers superior long-term cycling stability with a 139.6% capacity retention (291.8 mA h g⁻¹) over 650 cycles at 0.5 A g⁻¹ and a large reversible specific capacity of 91.4 mA h g⁻¹ at 4.0 A g⁻¹ in lithium-ion batteries. Furthermore, the Zn₃Nb₂O₈-B electrode exhibits outstanding cycling stability (100.1 mA h g⁻¹ with 94.5% capacity retention after 400 cycles at 0.5 A g⁻¹) in sodium-ion batteries. The excellent electrochemical performance of the stump-like Zn₃Nb₂O₈-B materials can be attributed to the exposure of the (110) facet, enlarged interlayer spacing, small charge transfer resistance, and high pseudocapacitive contribution. Therefore, Zn₃Nb₂O₈-B has great application prospects as an anode material for lithium/sodium-ion storage.

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1 Introduction

The development of electronics, electric vehicles, and smart grids has created a huge demand for lithium-ion batteries (LIBs) with high energy density, long cycle life, and low cost.^1–3 As an important part of LIBs, anode materials should have the characteristics of high specific capacity and excellent cycle performance. Although the existing intercalation material technology has matured, the problem of slow diffusion kinetics of Li⁺ in graphite cannot be avoided, which limits the development and application of LIBs in the field of fast charging. Therefore, anode materials with the advantages of high rate and long life are an effective method to solve the above problems.

In recent years, niobium-based oxides have been found to exhibit stable Wadsley–Roth phases with shear-type crystal structures, fast ion intercalation and deintercalation, excellent rate capability, and high operating voltages (above 1.0 V). Therefore, niobium-based oxides have become very promising candidates for electrochemical energy storage materials. The M in binary niobium-based oxides (M-Nb-O) generally includes elements such as Sn,⁴ Fe,⁵ Zn,^6,7 Ti,^8,9 W,^10,11 Mo,^12,13 Cr,^14,15 Ge,¹⁶ Ga,¹⁷ V,¹⁸ Mg,¹⁹ P²⁰ or Ni,^21,22 which can provide more redox electron pairs and make the overall structure of niobium-based materials have higher energy density and theoretical capacity. Therefore, the application of M-Nb-O type anode materials in LIBs has recently attracted extensive attention.

Zinc has the advantages of light weight, relatively cheap price and the large ionic radius of Zn²⁺ (0.740 Å), which can provide a wider transport channel for Li⁺ and improve the diffusion rate of Li⁺. Therefore, zinc niobium oxide has great advantages as an energy storage material. At present, there are few reports on zinc niobium oxide as an electrode material, mainly involving ZnNb₂O₆,⁶ Zn₂Nb₃₄O₈₇.⁷ For example, bulk-ZnNb₂O₆ materials were successfully prepared by solid-state reaction and showed stable electrochemical performance in LIBs (specific capacity of 181.9 mA h g⁻¹ after 100 cycles at 0.1 A g⁻¹).⁶ Zhu's group successfully prepared Zn₂Nb₃₄O₈₇ blocks by a solid-state reaction method and used them as anode materials for LIBs, which showed excellent cycling performance (216.5 mA h g⁻¹ after 100 cycles at 0.4 A g⁻¹).⁷ It can be seen that the different proportions of Zn, Nb and O elements will affect the electrochemical properties of Zn–niobium oxide anode materials. Moreover, the Zn₃Nb₂O₈ materials and their applications as anode materials in lithium/sodium-ion batteries have yet to be investigated. Based on the above research results and related literature research, we have not found any literature on Zn₃Nb₂O₈ as an anode material and its application in lithium/sodium-ion batteries.

Herein, we prepared two Zn₃Nb₂O₈ materials with distinct morphologies through different methods and the Zn₃Nb₂O₈ was used as the electrode material in lithium/sodium-ion batteries for the first time. Arborescent Zn₃Nb₂O₈ particles (Zn₃Nb₂O₈-A) with a size ranging from 300 to 2300 nm were fabricated via a conventional solid-state reaction (Fig. 1a), and stump-like Zn₃Nb₂O₈ particles (Zn₃Nb₂O₈-B) with a size ranging from 250 to 1700 nm were fabricated through a solvothermal method (Fig. 1b). The morphological characteristics, crystalline structure, chemical composition and electrochemical performance of these two Zn₃Nb₂O₈ materials were systematically investigated. Compared with Zn₃Nb₂O₈-A, Zn₃Nb₂O₈-B exhibited good rate performance (reversible capacity of 91.4 mA h g⁻¹ at 4.0 A g⁻¹) and excellent cycling stability (291.8 mA h g⁻¹ after 650 cycles at 0.5 A g⁻¹). Therefore, Zn₃Nb₂O₈-B is expected to be a promising electrode material in the future.

Fig. 1 Schematic illustration of Zn₃Nb₂O₈ materials synthesized by (a) solid-state reaction and (b) solvothermal reaction.

2 Experimental

2.1 Material preparation

According to the traditional solid-phase synthesis method,^23,24 3.0 mmol of zinc oxide (ZnO, 99.99%, Macklin) and 1.0 mmol of niobium pentoxide (Nb₂O₅, 99.9%, Aladdin) were added to absolute ethanol, and mixed uniformly under the action of a ball mill (Fig. 1a). The mixed solution after ball milling was dried in a blast drying oven at 80 °C for 12 h to obtain a white powdery precursor. Finally, the precursor was calcined at a heating rate of 5 or 10 °C min⁻¹ for 2 hours in an air or argon atmosphere at 700–1200 °C to obtain zinc–niobium oxide, and the obtained product was named Zn₃Nb₂O₈-A.

3.0 mmol of zinc acetate (C₄H₆O₄Zn·2H₂O, 98%, Aladdin) and 2.0 mmol of niobium oxalate (C₁₀H₅NbO₂₀, 98%, Macklin) were added to a mixed solution of 40 mL of H₂O and 20 mL of C₂H₅OH, and stirred vigorously for 2 h (Fig. 1b). The uniformly stirred mixed solution was transferred to a Teflon reactor, and then placed in a blast oven at 180 °C for 24 h. The mixed solution was washed with deionized water and absolute ethanol for several times, dried in a blast drying oven at 80 °C for 12 h, and calcined at high temperature to obtain Zn₃Nb₂O₈-B.

2.2 Material characterization

The phase and crystal structure of the as-synthesized materials were analyzed by X-ray diffractometer (XRD, D/MAX-2200). The element information of the as-prepared materials was recorded by X-ray photoelectron spectrometry (XPS, Thermo Scientific K-Alpha). The morphologies of the materials were observed by scanning electron microscope (SEM, Hitachi SU-70) equipped with energy dispersive spectroscopy (EDS). The microstructures of the as-prepared materials were characterized by the transmission electron microscope (TEM, JEOL JEM-2100F).

2.3 Electrochemical measurements

The working electrode consists of 70 wt% active material (Zn₃Nb₂O₈), 20 wt% conductive agent Super P, 10 wt% polyvinylidenefluoride (PVDF), and appropriate amount of N-methyl-2-pyrrolidone (NMP). Then, the uniformly mixed slurry was coated on copper foil and dried in a vacuum oven at 80 °C for 12 h. The electrodes were assembled into coin-shaped cells (CR2032) using lithium foil as the counter electrode and Whatman glassfiber as the separator. For lithium-ion batteries, the electrolyte was composed of 1.0 mol L⁻¹ LiPF₆ solution in a mixture (1 : 1 : 1, in vol%) of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). 1.0 mol L⁻¹ NaPF₆ in diglyme was used as electrolyte for sodium-ion batteries. The LAND testing system (CT2001A) was used for constant current (dis)charge test and galvanostatic intermittent titration technique (GITT) measurement in the voltage range of 0.01–3.00 V. Electrochemical workstation (CHI 660E) was used to measure cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

3 Results and discussion

Firstly, the optimal sintering temperature of Zn₃Nb₂O₈ materials was investigated. Take Zn₃Nb₂O₈-A materials for example, as shown in Fig. S1a,† XRD patterns of the zinc niobate materials that were calcined at 700–1200 °C for 2 h in air. The products are ZnNb₂O₆ phase (no. JCPDS 76-1827) and unreacted ZnO precursor in the calcination temperature range from 700 to 850 °C. It is worth mentioning that a strong diffraction peak appears at 30.21°, which is consistent with the characteristic peak of ZnNb₂O₆. When the calcination temperature reaches 900 °C, the diffraction peak at 30.21° becomes weaker, but a weaker Zn₃Nb₂O₈ diffraction peak appears at 32.72°. When the calcination temperature rises to 950 °C, all diffraction peaks are completely consistent with the diffraction peaks of monoclinic Zn₃Nb₂O₈ (no. JCPDS 79-1164). When the calcination temperature continues to rise to 1000 or 1200 °C, the characteristic peak at 32.72° disappears, and a strong diffraction peak of ZnNb₂O₆ appears at 30.21°. In conclusion, the optimum calcination temperature of Zn₃Nb₂O₈ is preliminarily determined to be 950 °C. In addition, the effect of precursors in different calcination atmospheres (air or argon) and heating rates on the synthesis of Zn₃Nb₂O₈ was investigated, and the XRD patterns of the products are shown in Fig. S1b.† It can be seen that the XRD patterns of the three samples are consistent with the diffraction peaks of monoclinic Zn₃Nb₂O₈ (no. JCPDS 79-1164), indicating the successful synthesis of Zn₃Nb₂O₈ materials. The ideal calcination setting for Zn₃Nb₂O₈ material, however, is calcination to 950 °C at a heating rate of 10 °C min⁻¹ under an air atmosphere, taking time and cost into consideration. Based on the above optimal sintering conditions, the XRD spectra of the as-prepared Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B are shown in Fig. 2a. It can be seen that the diffraction peaks of both samples correspond to the diffraction peaks of monoclinic Zn₃Nb₂O₈, indicating the successful preparation of Zn₃Nb₂O₈ materials. Fig. 2b presents the crystal structure of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B, which has a corundum-like monoclinic cell structure (a = 19.093 Å, b = 5.927 Å and c = 5.22 Å), and the space group is C2/c. ZnO₆ and NbO₆ have shear-type ReO₃ octahedral structures. There are corner and edge sharing between octahedra to form quadrilateral channels. In addition, the distance between O1 and O3 is 4.31 Å, and the distance between O2 and O4 is 2.94 Å, which can allow the normal insertion and extraction of metal-ion (Li⁺ and Na⁺).

Fig. 2 (a) XRD pattern of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B calcined at 950 °C at 10 °C min⁻¹ in air. (b) Crystal structure of Zn₃Nb₂O₈, (c) XPS survey spectra of Zn₃Nb₂O₈-B; XPS spectra of Zn₃Nb₂O₈-B for (d) Zn 2p, (e) Nb 3d, (f) O 1s.

X-ray photoelectron spectroscopy (XPS) was applied to investigate the element composition and valence state of the material surface. Fig. 2c shows the XPS survey spectra of Zn₃Nb₂O₈-B materials, in which the zinc, niobium, oxygen, carbon elements were detected. In the Zn 2p spectra (Fig. 2d), the XPS peaks located at 1021.8 and 1044.9 eV can be attributed to the Zn 2p_3/2 and Zn 2p_1/2 binding energies of Zn²⁺, respectively.⁷ Fig. 2e shows two strong characteristic peaks centered at 207.0 eV and 209.7 eV in the high resolution Nb 3d XPS spectrum, consistent with 3d_5/2 and 3d_1/2, which confirms the presence of Nb⁵⁺.^25–27 The O 1s spectrum (Fig. 2f) can be spilt into three peaks at 530.3, 531.8, and 533.0 eV, which were related to the Zn–O/Nb–O, C–O, and C=O, respectively.²⁸ In the C 1s high-resolution spectrum of Zn₃Nb₂O₈-B (Fig. S2†), the peaks at 286.2, 285.3, and 284.8 eV are attributed to C=O, C–O, and C–C groups, respectively.¹¹ In addition, the XPS spectra of Zn₃Nb₂O₈-A are shown in Fig. S3.† These results demonstrate that the valence states of Zn, Nb and O elements in both samples are Zn²⁺, Nb⁵⁺ and O²⁻, respectively.

In order to explore the effect of preparation method on the morphology of Zn₃Nb₂O₈, scanning electron microscope (SEM) and elemental mapping characterizations were carried out. Fig. 3a and b shows the SEM images of Zn₃Nb₂O₈-A sample, it can be seen that these particles are smooth, with arborescent particle sizes ranging from 300 to 2300 nm. However, Zn₃Nb₂O₈-B sample shows a different morphology. As shown in Fig. 3e and f, SEM images clearly show that the Zn₃Nb₂O₈-B particles are rough, stump-like, no-aggregating, and the particle size ranges from 250 to 1700 nm. Meanwhile, Fig. 3i demonstrates the energy dispersive spectroscopy (EDS) mapping images of Zn₃Nb₂O₈-B, which further confirmed the homogeneous distribution of elements (Zn, Nb, O) in the Zn₃Nb₂O₈-B, indicating the Zn₃Nb₂O₈ has been successfully synthesized. In addition, the EDS mapping images of Zn₃Nb₂O₈-A are presented in Fig. S4,† the uniform homogeneous distribution of elements (Zn, Nb, O) in the Zn₃Nb₂O₈-A also proved its successful preparation. The microstructures of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B were confirmed by transmission electron microscope (TEM). As shown in Fig. 3c and d, there are distinct fringes with lattice spacing of about 0.44 nm, corresponding to the (310) facet in Zn₃Nb₂O₈-A. In addition, the distances of lattice fringes are measured to be 0.57 nm, corresponding to the (110) facet in Zn₃Nb₂O₈-B. Different crystal facets in Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B were observed by TEM, indicating that there may be more (110) facet exposure in Zn₃Nb₂O₈-B and more (310) facet exposure in Zn₃Nb₂O₈-A. Therefore, the area ratio of exposed (110)/(310) facets in Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B are roughly calculated based on these XRD pattern in Fig. 2a. The approximate ratios of (110)/(310) facets in Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B are 34.2% and 41.2%, respectively. XRD and TEM results show that the area ratio of exposed (110)/(310) facets in Zn₃Nb₂O₈-A is lower than that in Zn₃Nb₂O₈-B. The lattice spacing of (110) facet is larger than that of (310) facet, so Zn₃Nb₂O₈-B material may be more conducive to the rapid insertion and extraction of metal-ions (Li⁺ and Na⁺).

Fig. 3 (a and b) SEM images, (c) TEM image and (d) HRTEM image of Zn₃Nb₂O₈-A; (e and f) SEM images, (g) TEM image, (h) HRTEM image and (i) EDS mapping images of Zn₃Nb₂O₈-B.

In order to explore the effect of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B samples on the electrochemical performance, Zn₃Nb₂O₈-A/Li and Zn₃Nb₂O₈-B/Li half-cells for lithium-ion storage were assembled (Fig. 4a). Fig. 4b shows the cyclic voltammetry (CV) curves of Zn₃Nb₂O₈-B in the voltage range of 0.01–3.00 V at a scan rate of 0.1 mV s⁻¹. In the cathode process, the broad peaks at the potential of 0.1–0.5 V and 0.5–1.0 V are mainly due to the formation of irreversible SEI and LixZn₃Nb₂O₈ products (Zn₃Nb₂O₈ + xLi⁺ + xe⁻ ↔ Li_xZn₃Nb₂O₈).^6,20 In the first anodic process, there is a broad peak at 1.0–1.5 V, which is basically consistent with the subsequent cycle, indicating that the Zn₃Nb₂O₈-B material has a good reversibility performance. Additionally, Fig. S5† demonstrates that the CV curves of Zn₃Nb₂O₈-A are similar to those of Zn₃Nb₂O₈-B.

Fig. 4 (a) Schematic illustration of Zn₃Nb₂O₈//Li lithium-ion cell. (b) CV curves of Zn₃Nb₂O₈-B at the scan rate of 0.1 mV s⁻¹; (c) Nyquist plots of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B; (d) rate performances of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B at different current densities; (e) galvanostatic discharge and charge curves of Zn₃Nb₂O₈-B at different current densities from 0.05 to 4.0 A g⁻¹. (f) Cycling performances of Zn₃Nb₂O₈ at a current density of 0.2 A g⁻¹; (g) discharge and charge curves of Zn₃Nb₂O₈-B at a current density of 0.5 A g⁻¹; (h) cycling performances of Zn₃Nb₂O₈ at a current density of 0.5 A g⁻¹. (i) Comparison of cycling stability between Zn₃Nb₂O₈-B with other Nb-based anode materials.

The Nyquist plots of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B electrode materials show similar curves (Fig. 4c). The charge transfer resistance value of Zn₃Nb₂O₈-B is 75 Ω, which is lower than that of Zn₃Nb₂O₈-A (230 Ω), indicating that the (110)-exposed Zn₃Nb₂O₈-B provided better Li ion/electron transfer kinetics than (310)-exposed Zn₃Nb₂O₈-A. It is well known that different surface atoms are arranged according to different crystal facets.²⁹ This result indicating that the movement of Li ion/electron through the (110) facet is more easily than through the (310) facet of Zn₃Nb₂O₈. The rate performance of Zn₃Nb₂O₈ electrode was tested at various current densities, as shown in Fig. 4d. The reversible capacities of the Zn₃Nb₂O₈-B electrode are 371.8, 282.1, 247.9, 199.1, 161.8, 121.3, 102.1 and 91.4 mA h g⁻¹ at current densities of 0.05, 0.10, 0.20, 0.50, 1.00, 2.00, 3.00 and 4.00 A g⁻¹, respectively. The specific capacity returned to 308.1 mA h g⁻¹ when the current density was changed back to 0.05 A g⁻¹, revealing the electrode's exceptional structural stability throughout a broad current range. However, the reversible capacities of the Zn₃Nb₂O₈-A electrode are relatively low, which are 274.0, 205.9, 170.8, 117.8, 77.8, 53.4, 44.4, 40.1 and 231.3 mA h g⁻¹ at current densities of 0.05, 0.10, 0.20, 0.50, 1.00, 2.00, 3.00, 4.00 and 0.05 A g⁻¹, respectively. The corresponding galvanostatic discharge and charge curves of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B at different current densities are shown in Fig. 4e and S6,† it can be seen that the capacity is mainly contributed by the slope segment. In addition, the cycling performance was also investigated at a current density of 0.2 A g⁻¹ (Fig. 4f). The Zn₃Nb₂O₈-B electrode had a high specific capacity of 341.7 mA h g⁻¹ after 115 cycles. While the Zn₃Nb₂O₈-A electrode had a relative low specific capacity of 295.2 mA h g⁻¹ after 115 cycles. The partial discharge and charge curves of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B electrodes are shown in Fig. S7.† Increasing the current density to 0.5 A g⁻¹, the long-cycling stability of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B electrodes were investigated (Fig. 4h). After 650 cycles, the discharge capacities of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B are 291.8 and 127.3 mA h g⁻¹, respectively, and the corresponding discharge–charge curves are presented in Fig. 4g and S7c.† Obviously, the overall capacity of the electrode material decreases first and then increases during cycling. The initial capacity attenuation is mainly attributed to the fracture and exfoliate of the electrode material during the Li⁺ insertion and extraction processes. The subsequent capacity increase is ascribed to the rapid Li-diffusion kinetics in the SEI layer. This similar behavior has been reported in the previous literature.^6,30–32. Compared with Zn₃Nb₂O₈-A materials, Zn₃Nb₂O₈-B materials show excellent cycling and rate performance, which may be due to the exposure of (110) facets in Zn₃Nb₂O₈-B materials.^33–35 More importantly, as shown in Fig. 4i and Table S1,† the cycling stability of Zn₃Nb₂O₈-B was particularly better than that of other Nb-based anode materials (e.g. Nb₂O₅, K₆Nb_10.8O₃₀, ZnNb₂O₆, Bi₅Nb₃O₁₅, VNb₉O₂₅, CrNb₄₉O₁₂₄, TiNb₂O₇, WNb₁₂O₃₃ and GaNb₁₈O₄₇).

In order to further explore the mechanism for excellent electrochemical performance of Zn₃Nb₂O₈-B material. The reaction kinetics and charge storage mechanism of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B were analyzed using CV tests with scanning rates in the range of 0.1–10.0 mV s⁻¹ (Fig. S8a† and 5a). The relationship between the peak current i (A) and the scanning rate v (mV s⁻¹) follows the following equation: i = av^b. Diffusion-controlled processes are indicated by a b-value of 0.5, whereas surface capacitance-controlled processes are indicated by a b-value of 1.³⁶ The calculated b-values of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B are shown in Fig. S8b† and 5b, respectively. The b-values of anodic (oxidation) and cathodic (reduction) peaks for Zn₃Nb₂O₈-B electrode were calculated to be 0.77 and 0.95. While the Zn₃Nb₂O₈-A showed the b-values are 0.69 and 0.78 for the anodic and cathodic (reduction) peaks, respectively. The results indicating that the charge storage mechanism for Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B is a combination of surface capacitance-controlled process and diffusion-controlled behavior. However, Zn₃Nb₂O₈-B displays a greater b-values than Zn₃Nb₂O₈-A and exhibits a pseudo-capacitive tendency. This behavior could enable the fast charge transport.⁷ A typical CV curves at 2.0 mV s⁻¹ are shown in Fig. S9,† the pseudo-capacitive capacity contribution of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B at different sweep rates are calculated as shown in Fig. 5c and S8d,† respectively. The results demonstrate that the pseudo-capacitance contribution of both electrodes are obviously improving with sweep rates rising, but the capacitance contribution of Zn₃Nb₂O₈-B electrode is significantly higher than that of Zn₃Nb₂O₈-A electrode at the same sweep rate, indicating that the Zn₃Nb₂O₈-B electrode has fast electron/ion transport kinetics, which eventually makes it have good long-term stability.

Fig. 5 (a) CV curves of Zn₃Nb₂O₈-B from 0.1 to 10.0 mV s⁻¹; (b) plots of log(i) vs. log(v) of the Zn₃Nb₂O₈-B/Li cell; (c) contribution ratios of diffusion- and capacitive-controlled capacities at different scan rates. (d) GITT curves of Zn₃Nb₂O₈-B at 0.01–3.00 V.

In addition, the diffusion coefficient of Li⁺ in the electrode material is measured by GITT and can be calculated according to the following formula:³⁷

where τ is the relaxation time, n_B is the number of moles, V_M is the molar volume of the electrode material, S is the electrode/electrolyte contact area, ΔE_s is the voltage change caused by the pulse, and ΔE_t is the constant current (dis)charging voltage changes.

The GITT curves and corresponding D values of Zn₃Nb₂O₈-B during the charge–discharge process are shown in Fig. 5d, and the average value of log(D) during the (de)intercalation of Li⁺ is ∼1 × 10⁻¹². The results of EIS and GITT together confirm that Li⁺ can diffuse rapidly in the Zn₃Nb₂O₈-B electrode material. Therefore, the Zn₃Nb₂O₈-B electrode can effectively reduce the charge transfer resistance and accelerate Li⁺ migration, resulting in excellent rate performance.

We further investigated the application of Zn₃Nb₂O₈-B in sodium-ion batteries, the electrodes were assembled into coin-shaped cells using sodium foil as counter electrode. The rate behaviors of the Zn₃Nb₂O₈-B anode throughout a current density range of 0.05 to 4.0 A g⁻¹ are shown in Fig. 6a. Zn₃Nb₂O₈-B demonstrated good rate performance in comparison to Zn₃Nb₂O₈-A (Fig. 6c), producing reversible capacities of 212.2, 154.9, 121.9, 92.1, 72.0, 55.1, 52.1, and 49.4 mA h g⁻¹ at current densities of 0.05, 0.1, 0.2, 0.5, 1, 2, 3, and 4 A g⁻¹, respectively. Then, when the current density was decreased to 0.05 A g⁻¹, the specific capacity of the Zn₃Nb₂O₈-B anode recovered to 172.9 mA h g⁻¹, demonstrating the good kinetic process. The corresponding discharge and charge curves of Zn₃Nb₂O₈-B at different current densities are displayed in Fig. 6b. As a comparison of Zn₃Nb₂O₈-A electrode, the rate performance and discharge–charge curves at different current densities are shown in Fig. S10a and b.† In addition, Fig. 6d displays the long cycle stability of Zn₃Nb₂O₈-B electrode at 0.5 A g⁻¹, the discharge capacity of Zn₃Nb₂O₈-B is 100.1 mA h g⁻¹ with 94.5% capacity retention after 400 cycles. While the Zn₃Nb₂O₈-A electrode has a relative low specific capacity of 50.8 mA h g⁻¹ with 61.3% capacity retention after 400 cycles (Fig. S10c†). Compared with Zn₃Nb₂O₈-A electrode, the better electrochemical performance of Zn₃Nb₂O₈-B electrode may be related to the exposed (110) crystal planes, which enables more sodium-ions to be (de)intercalated. As shown in Fig. S11,† the EIS demonstrated that the impedance of Zn₃Nb₂O₈-A (12.7 Ω) is significantly higher than that of Zn₃Nb₂O₈-B (10.7 Ω), which indicated that Zn₃Nb₂O₈-B has a fast charge transfer rate. Based on the above tests and analysis show that Zn₃Nb₂O₈-B can be used as an excellent negative electrode for LIBs/SIBs.

Fig. 6 (a) Rate performances of Zn₃Nb₂O₈-B at different current densities for SIBs; (b) galvanostatic discharge and charge curves of Zn₃Nb₂O₈-B at different current densities from 0.05 to 3.0 A g⁻¹. (c) Comparison of specific capacities of Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B electrodes at different current densities. (d) Cycling performances of Zn₃Nb₂O₈-B at a current density of 0.5 A g⁻¹; (e) discharge and charge curves of Zn₃Nb₂O₈-B at a current density of 0.5 A g⁻¹.

4 Conclusions

In summary, we have successfully designed Zn₃Nb₂O₈-A and Zn₃Nb₂O₈-B anode materials via solid-state reaction and solvothermal methods, respectively. SEM results show that Zn₃Nb₂O₈-A particles were arborescent and smooth, while the Zn₃Nb₂O₈-B particles were stump-like and rough. XRD and TEM results show that the area ratio of exposed (110)/(310) facets in Zn₃Nb₂O₈-A is lower than that in Zn₃Nb₂O₈-B, the (110) facet is more conducive to the rapid insertion and extraction of metal-ions (Li⁺ and Na⁺) than (310) facet. Compared with the Zn₃Nb₂O₈-A anode material, the Zn₃Nb₂O₈-B anode material exhibits superior long-cycle cyclability and better rate performance. It displays a high reversible specific capacity of 291.8 mA h g⁻¹ at the current density of 0.5 A g⁻¹ after 650 cycles and a large reversible specific capacity of 91.4 mA h g⁻¹ at 4.0 A g⁻¹ in Li-ion batteries. In addition, Zn₃Nb₂O₈-B electrode exhibits excellent reversible specific capacity of 100.1 mA h g⁻¹ at the current density of 0.5 A g⁻¹ after 400 cycles in sodium-ion batteries. This work provides an effective strategy for the application in high performance lithium/sodium energy storage.

Data availability

Data are available on request from the authors.

Author contributions

Xuemin Yin: experimental work, writing the original draft, and reviewing the draft. Shuling Cheng: conceptualization, experimental work, data analysis. Yuyang Zhang: conceptualization, formal analysis. Chencheng Liu: conceptualization.

Conflicts of interest

The authors declare no competing interest.

Acknowledgements

The authors are grateful for the financial support from the Science Research Project of Hebei Education Department (BJK2024063), Open Foundation of Hebei Key Laboratory of Green Development of Rock and Mineral Materials (RM202305).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra03616f

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