J. Cent. South Univ. (2020) 27: 3625-3636

DOI: https://doi.org/10.1007/s11771-020-4570-0

Synthesis of a novel hexagonal porous TT-Nb₂O₅ via solid state reaction for high-performance lithium ion battery anodes

ZHOU Yu(周玉), LIU Ke(刘柯), ZHOU Yue(周悦), NI Jia-hua(倪佳华), DOU Ai-chun(窦爱春),SU Ming-ru(苏明如), LIU Yun-jian(刘云建)

School of Materials Science and Technology, Jiangsu University, Zhenjiang 212013, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Hexagonal porous Nb₂O₅ was synthesized for the first time via a facile solid-state reaction. The structure and electrochemical properties have been optimized through tuning heating temperature. X-ray diffraction results indicate that pseudo hexagonal Nb₂O₅ (TT-Nb₂O₅) and orthorhombic Nb₂O₅ have been synthesized at different temperatures. Hexagonal sheet and porous structure of Nb₂O₅ were characterized by scanning electron microscopy and N₂-adsorption-desorption isotherms. The as-prepared TT-Nb₂O₅ (heated at 600 °C) shows the best performance with a remarkable charge capacity of 178 mA·h/g at 0.2C, which is higher than that of T-Nb₂O₅. Even at 20C, TT-Nb₂O₅ offers unprecedented rate capability up to 86 mA·h/g. The high rate capacity is due to pseudocapacitive Li⁺ intercalation mechanism of TT-Nb₂O₅. The reported results demonstrate that Nb₂O₅ with good crystal structure and high specific surface area is a powerful composite design for high-rate and safe anode materials.

Key words: lithium-ion battery; Nb₂O₅; hexagonal porous sheet; electrochemical performance

Cite this article as: ZHOU Yu, LIU Ke, ZHOU Yue, NI Jia-hua, DOU Ai-chun, SU Ming-ru, LIU Yun-jian. Synthesis of a novel hexagonal porous TT-Nb₂O₅ via solid state reaction for high-performance lithium ion battery anodes [J]. Journal of Central South University, 2020, 27(12): 3625-3636. DOI: https://doi.org/10.1007/s11771-020-4570-0.

1 Introduction

Lithium ion batteries (LIBs) have been widely applied for electric vehicles (EV) and portable electronic products due to their high energy density and long cycle life [1-6]. However, the low insertion voltage and poor rate performance of commercial graphite anode will cause formation of Li dendrite at high current density, which is difficult to meet the growing demands for EV with high security [7-9]. Among all the anode materials, Li₄Ti₅O₁₂ has been considered successful anode for its suitable working voltage and excellent cycle performance [10-12]. Operating voltage between 1.0 and 2.0 V can suppress the generation of SEI layer and lithium dendrites. But the maximum capacity of Li₄Ti₅O₁₂ is only 175 mA·h/g. Nb₂O₅ attracts particular attention because of its higher theoretical specific capacity (~200 mA·h/g) and excellent rate performance at suitable operating voltage [13, 14].

Nb₂O₅ has been synthesized with many crystal structures by controlling annealing temperature, such as pseudo hexagonal Nb₂O₅ (TT-Nb₂O₅), orthorhombic Nb₂O₅ (T-Nb₂O₅), tetragonal Nb₂O₅ (M-Nb₂O₅), and monoclinic Nb₂O₅ (H-Nb₂O₅) [15]. Nb₂O₅ with different crystal structures shows different lithium storage performances. In previous reports [16], T-Nb₂O₅ exhibits the best development of nanomaterials, TT-Nb₂O₅ with high specific surface area can also show the high capacity and rate performance [17, 18]. It was found that the rapid pseudocapacitive charge storage mechanism plays an essential role in retaining the energy density and rate performance of TT-Nb₂O₅ [19]. Therefore, the electrochemical performance of Nb₂O₅ is not only determined by the crystal structures, but also influenced by the specific surface area. Nb₂O₅ with different nanostructure or porous structure has been prepared for LIBs [20-22]. However, there is no systematic research on the electrochemical performance of Nb₂O₅ with different crystal structures and specific surface area. Moreover, the traditional solvothermal or hydrothermal methods are too complex to precisely control the synthesis process [23, 24]. Hence, it is necessary to synthesize Nb₂O₅ with different crystal structures and specific surface area by a practical method.

In this paper, the micron-sized porous hexagonal sheets TT-Nb₂O₅ and T-Nb₂O₅ were first synthesized with hexagonal NbSe₂ as rare material by facile solid-state reaction. The specific surface area of TT-Nb₂O₅ sheets was higher than that of T-Nb₂O₅, which could deliver higher capacity and better cycling performance at large current density (86 mA·h/g at 20C). Furthermore, high ratio of the intercalation pesudocapacitance of TT-Nb₂O₅ has been demonstrated as a dominant contribution to the excellent rate capability.

2 Material and methods

2.1 Synthesis

The porous flake Nb₂O₅ was synthesized through a solid-state reaction and the synthesis schematic process is shown in Figure 1. The starting materials niobium and selenium powders were weighed according to the molar ratio (Se excess 4%). After ball-milling for 8 h, the mixture was conveyed to a crucible and calcined at 700 °C for 2 h in Ar atmosphere for the preparation of the hexagonal NbSe₂ sheet. Finally, the porous flake Nb₂O₅ was obtained by annealing of as-prepared solid product at 400, 500, 600 and 700 °C for 2 h in air, and the as-prepared Nb₂O₅ samples were marked as NbO₄, NbO₅, NbO₆ and NbO₇, respectively.

2.2 Characterization

The crystal structure of as-prepared Nb₂O₅ was characterized by X-ray powder diffraction (XRD) on the Ultima IV with Cu K_α radiation in the 2θ range of 10°-70°. The morphology and the element mapping of the samples were collected on FEI Nova Nano 450 SEM. Transmission electron microscopy (TEM) images were performed with FEI Tecnai F20 with acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) data were also collected by PHI-5000 versaprobe for further analysis.

Figure 1 Synthesis schematic process of hexagonal porous sheet Nb₂O₅

2.3 Electrochemistry testing

The porous sheet Nb₂O₅ was ground with acetylene black and polyvinylidene fluoride in a mass ratio of 7:2:1 and then dispersed into 1-methyl-2-pyrrolidinone to mix evenly. The slurry was passed onto the copper foils. After being dried in vacuum at 80 °C for 24 h, the electrode was punched into disks with diameter of 12 mm. The mass loading of the active materials was about 1.2 mg/cm². CR2025 cells were assembled in the argon-filled glove box. 1 mol/L LiPF₆ in diethyl carbonate (DMC)/ethylene carbonate (EC) (1:1 volume ratio) was used as the electrolyte. Celgard 2500 polypropylene separator was selected as the separator and the pure lithium metal was the counterpart electrode. The galvanostatic discharge/ charge of the half cells was examined by NEWWARE battery testing system at 25 °C in the voltage range of 1-3 V (vs Li⁺/Li). The cyclic voltammetry (CV) tests and electrochemical impedance spectroscopy (EIS) results were collected on CHI660D electrochemical workstation.

3 Results and discussion

3.1 Structure and morphology characterization

Figure 2(a) shows the XRD patterns of the Nb₂O₅ calcined at various temperatures ranging from 400 to 700 °C. The peaks of NbO₄, NbO₅ and NbO₆ accord well with the standard data of pseudohexagonal structure Nb₂O₅ (TT-Nb₂O₅, PDF#28-0317). The XRD peaks (001), (100) and (101) with the increase of calcination temperature become more and more stronger, which indicates that the higher crystalline nature of samples [25]. When the temperature reaches 700 °C, the diffraction peaks of NbO₇ sample could be indexed to the typical orthorhombic Nb₂O₅ (T-Nb₂O₅, PDF#27-1003). When the annealing temperature reaches 700 °C, the crystal structure of Nb₂O₅ has changed from TT-Nb₂O₅ to T-Nb₂O₅. The crystallite size of the Nb₂O₅ sheets was calculated by the Scherrer equation. The crystallite sizes of the NbO₄, NbO₅, NbO₆ and NbO₇ were 28.6, 35.3, 38.4 and 52.7 nm, respectively. Low calcination temperature causes small crystal size and poor crystallinity, which weakens intensity peaks of NbO₄ and NbO₅ samples [26]. The intensity of the peak for (001) was stronger than the (100) peak demonstrating that the samples oriented-grow at high temperature.

The XPS spectrum of NbO₆ and the components of Nb 3d and O 1s are shown in Figures 2(b-d). Figure 2(c) exhibits that the binding energies at 207.4 and 210.2 eV are consistent with the binding energy for the standard Nb⁵⁺ in Nb₂O₅ [27]. As shown in Figure 2(d), the peak appearing at 529.6 eV reveals that the valence of O is -2 in NbO₆, and the peak centered at about 533 eV is associated with the surface oxygen [28]. XPS characterization results indicate that the Nb⁴⁺ in NbSe₂ is converted to Nb⁵⁺ during calcination, corresponding to the formation of Nb₂O₅.

The morphologies and nanostructures of the NbSe₂ and Nb₂O₅ sheets are observed by HRSEM. As shown in Figure 3, all the samples show hexagonal sheet structure. The raw material NbSe₂ (Figures 3(i), (j)) shows good hexagonal sheet and flat surface morphology. The pores of Nb₂O₅ sheets is due to the volatilization of selenium during calcination in air. The reaction equation can be expressed as:

4NbSe₂+5O₂=2Nb₂O₅+8Se                 (1)

As temperature increases, the faster speed of volatilization leads to more pores on the sample surface. When temperature reaches 600 °C, small cracks appear on the sample surface. Figure 3(k) and Figure 3(l) display mapping images of NbO₆ and NbO₇. The single color Nb, O and the mixing color images display homogeneous element distribution in the samples. There is no selenium residue in the samples, suggesting that the as-prepared samples are pure Nb₂O₅. The NbO₆ (Figure 4(a)) and NbO₇ (Figure 4(b)) show regular hexagonal sheet, which are in good agreement with SEM results. Furthermore, the lattice fringes of NbO₆ and NbO₇ can be measured from the HRTEM images, indicating the presence of a well-defined crystal structure. Figures 4(c) and (d) show clear lattice fringes with d-spacing of 0.32 nm for NbO₆ and 0.38 nm for NbO₇, which are well matched to the interplanar spacings of (100) plane of TT-Nb₂O₅ (PDF#28-0317) and (001) plane of T-Nb₂O₅ (PDF#27-1003), respectively [29].

Figure 2 XRD patterns of Nb₂O₅ calcined at different temperatures (a) and XPS spectrum of wide survey of NbO₆ (b), Nb 3d spectrum (c) and O 1s spectrum (d)

Figure 3 SEM images of NbO₄ (a, b), NbO₅ (c, d), NbO₆ (e, f), NbO₇ (g, h), NbSe₂ (i, j) and EDS elemental mapping images of NbO₆ and NbO₇ (k, l)

Figure 5 displays the N₂-adsorption-desorption isotherms of all samples and the results are listed in Table 1. NbO₄ exhibits the largest specific surface area of 26.043 m²/g. As the calcination temperature raises, the mean pore diameter of Nb₂O₅ sheets gradually increases, leading to a decrease of mesopores, which in turn leads to the decrease of specific surface area. The mean pore diameter of Nb₂O₅ sheets (Table 1) indicates that mesopores (2-50 nm) are predominantly present in Nb₂O₅sheets. Figure 5(e) indicates the schematic diagram of mesopore mechanism. The electrolyte can penetrate into mesoporous and facilitate the diffusion of lithium ions.

Figure 4 TEM and HRTEM images of NbO₆ (a, c) and NbO₇ (b, d)

Table 1 Specific surface area and average pore diameter

3.2 Electrochemical characterization

Figure 6 shows the initial two cyclic voltammetry (CV) curves of the NbO₄, NbO₅, NbO₆ and NbO₇. All the anodic peaks appear at about 1.8 V, which arise from the variation from Nb³⁺ to Nb⁵⁺ during Li-ion de-insertion [30]. For NbO₄, NbO₅ and NbO₆, the broad cathodic peak appearing at about 1.75 V can be observed, corresponding to the reduction of Nb⁵⁺ after lithiation. Compared with the first and second CV curves, the cathodic and anodic peaks almost overlapped, indicating the good reversibility of Li storage properties. In the negative scan process for NbO₇, two reduction peaks are located at around 1.8 and 1.5 V, corresponding to the reaction of Nb⁵⁺/Nb⁴⁺ and Nb⁴⁺/Nb³⁺ [31, 32].

The initial charge and discharge profiles at 0.2C are plotted in Figure 7(a). The NbO₄ delivers a high discharge capacity of 251 mA·h/g, but a low charge capacity of 126 mA·h/g. With the increase of calcining temperature, the charge capacity of NbO₅, NbO₆ and NbO₇ increases to 134, 178 and 168 mA·h/g, respectively. NbO₆ sample shows the highest charge capacity. Initial coulombic efficiencies of NbO₄, NbO₅, NbO₆ and NbO₇ are 50%, 57%, 68%, 83%, respectively. The low initial coulombic efficiency is highly related to the poor reversibility of the conversion reaction [33]. Figures 7(b) and (c) present the cycling performance of four samples at current density of 0.2C and 1C, respectively. NbO₆ electrode delivers the highest charge capacity of 178 mA·h/g after 50 cycles at 0.2C, with capacity retention rate of 100%. Contrastively, NbO₄, NbO₅ and NbO₇ exhibit charge capacity of 115, 117 and 162 mA·h/g, corresponding to capacity retention ratios of 88%, 91% and 95%, respectively. At current density of 1C, the charge capacity of NbO₆ is 158 mA·h/g after 200 cycles, with a high capacity retention rate of 95% (Figure 7(c)). The excellent cycle performance of NbO₆ can be attributed to its high crystallinity (TT-Nb₂O₅). Although the NbO₇ owns another crystal structure (T-Nb₂O₅), it exhibits greater charge capacity (168 mA·h/g) and cycle stability (capacity retention of 92% over 200 cycles at 1C) than NbO₄ and NbO₅ samples. Figure 7(d) compares the rate capability of all four electrodes at the rate of 0.2C to 20C. NbO₆ still exhibited the highest capacities at various rates. When the current density increased to 20C, the charge capacity of NbO₆ decreased to 86 mA·h/g. Ultra-stable rate characteristics of the NbO₆ sample in this work were compared with other Nb₂O₅ materials in previously reported (Figure 7(e)). The results suggest that rate performance of NbO₆ in this work was outstanding.

Figure 5 N₂-adsorption-desorption isotherms of NbO₄ (a), NbO₅ (b), NbO₆ (c), NbO₇ (d) and schematic diagram of mesopore mechanism (e)

The mechanism of the better performance of TT-Nb₂O₅ (NbO₆) was studied based on pseudocapacitive effect. CV tests were carried out at various scan rates to quantitatively determine the pseudocapacitive effect. As depicted in Figures 8(a) and (b), all the CV curves show similar shapes with almost no peak separations during anodic and cathodic process. Therefore, it can be indicated that the obtained materials present well capacity retention at higher sweep rates. The bump shape of the CV curves exhibited typical pseudocapacitive Li⁺ intercalation mechanism [34]. The electrochemical kinetics data can be analyzed by the CV curves following a power law [35]:

i=av^b                                   (2)

where i is the peak current of each CV curve, v is the sweep rate and a, b are adjustable values. When b is 0.5, it means a diffusion-controlled process ascribed to Li⁺ intercalation and transportation. When b is 1, it represents that the current is surface- controlled [36]. In order to determine the b-value, the power law can be transferred as:

lgi=blgv+lga                             (3)

By plotting lgi vs lgv corresponding to anodic and cathodic peak current from 0.1 to 5 mV/s, the b-value can be obtained from the slope after linear fitting. As seen, the b-values of anodic and cathodic peaks of NbO₆ and NbO₇ are 0.97/1.09 and 0.79/0.90, respectively, showing that the current is almost surface-controlled. In order to determine the respective current contribution from capacitive effect and diffusion-controlled process, equation (2) can be rewritten as [37]:

i=k₁v+k₂v^1/2                           (4)

where k₁v is capacitive current and k₂v^1/2 is diffusion current. Both k₁ and k₂ can be determined from the slope and intercept of the linear relationship of i/v^1/2 and v^1/2. The CV curves of NbO₆ and NbO₇ at 5 mV/s are shown in Figures 8(c) and (d), in which the capacitive current (k₁v, blue area) is separated from the total current. In addition, the charge storage contributed by capacitive current and diffusion current at other scan rates is calculated as shown in Figures 8(e) and (f). The ratio of capacitive contribution for NbO₆ increases from 54% to 89%, which is higher than that of NbO₇ (from 37% to 80%). The greater b value for NbO₆ (0.97) than NbO₇ (0.79) demonstrates the faster surface-controlled kinetics. The higher ratio of capacitive contribution responses for better rate performance.

Figure 6 Cyclic voltammograms recorded for NbO₄ (a), NbO₅ (b), NbO₆ (c), NbO₇ (d) at a scan rate of 0.1 mV/s in voltage range of 1.0-3.0 V

Figure 7 Initial charge-discharge curves at 0.2C (a), cycling performance (b, c), rate performance (d) and comparison on electrochemical performance of all Nb₂O₅- based anode materials (e)

EIS measurement was conducted to further investigate the electrochemical kinetics of Nb₂O₅ electrodes (Figure 9). The fresh cells were kept at 1.6 V after 2 cycles at 0.2C. Nyquist plots of all four samples consist of arcs at high frequency and straight lines at low frequency, which is corresponding to the charge-transfer resistance (R_ct) at the electrode/electrolyte interface and the Warburg impedance (Z_w) arising from the Li⁺ diffusion in the bulk lattice, respectively [38, 39]. The electrolyte resistance (R_s) can be obtained from the intercept of the semicircle on the X-axis. The equivalent circuit based on above is inset in Figure 9 [40, 41]. As listed in Table 2, the fitted R_ct values are 130.8, 109.9, 65.3 and 65.6 Ω for NbO₄, NbO₅, NbO₆ and NbO₇ electrodes, respectively. The lowest R_ct of NbO₆ means the lowest charge transfer resistance and better electrical conductivity, leading to its higher capacity and cycling performance.

Figure 8 CV curves of NbO₆ (a) and NbO₇ (b) at various sweep rates (v) from 0.1 to 5.0 mV/s (Insets: b values determined by the power law (lgi=blgv+lga); Separation of the surface capacitive current (k₁v, shadow area) and diffusion currents (k₂ v^1/2, blank area) at 5.0 mV/s of NbO₆ (c), NbO₇ (d) and pseudocapacitive contribution ratio in the total intercalated charge as a function of sweep rates during CV processes for NbO₆ (e) and NbO₇ (f)

Table 2 Impedance parameters of equipment

Figure 9 Nyquist plots and equivalent circuit of all four electrodes

4 Conclusions

In summary, a novel hexagonal porous TT-Nb₂O₅ sheet has been successfully synthesized for the first time by facile solid stated reaction for LIBs anode. Reaction temperature has important influence on the structure and electrochemical performance of Nb₂O₅. Nb₂O₅ with high crystallization and small specific surface area was prepared at high temperature. TT-Nb₂O₅ synthesized at 600 °C exhibits the best electrochemical performance. A reversible capacity of 178 mA·h/g can be retained after 50 cycles at 0.2C. When cycling at 20C, the charge capacity of TT-Nb₂O₅ can still deliver 86 mA·h/g. The largest capacity and lower charge transfer resistance of TT-Nb₂O₅ were due to its favorable crystal structure. High ratio of the intercalation pesudocapacitance makes the TT-Nb₂O₅ electrode have more excellent rate performance than T-Nb₂O₅. The present results emphasize that Nb₂O₅ with high crystallization and large specific surface area is a very promising anode material.

Contributors

LIU Yun-jian provided the concept and edited the draft of manuscript. ZHOU Yu conducted the literature review, and wrote the first draft of the manuscript, and replied to reviewers’ comments. LIU Ke, ZHOU Yue and NI Jia-hua provided the measured dataand analyzed the measured data. DOU Ai-chun and SU Ming-ru provided resources and validation. All authors revised the final version.

Conflict of interest

ZHOU Yu, LIU Ke, ZHOU Yue, NI Jia-hua, DOU Ai-chun, SU Ming-ru, LIU Yun-jian declare that they have no conflict of interest.

References

[1] GOODENOUGH J, PARK K. The Li-ion rechargeable battery: A perspective [J]. Journal of the American Chemical Society, 2013, 135: 1167-1176. DOI: 10.1021/ja3091438.

[2] GUAN Pei-yuan, ZHOU Lu, YU Zhen-lu, SUN Yuan-dong, LIU Yun-jian, WU Fei-xiang, JIANG Yi-feng, CHU De-wei. Recent progress of surface coating on cathode materials for high performance lithium-ion batteries [J]. Journal of Energy Chemistry, 2020, 43: 220-235. DOI: 10.1016/j.jechem.2019. 08.022.

[3] ZHENG Jun-chao, YANG Zhuo, DAI Alvin, TANG Lin-bo, WEI Han-xin, LI Yun-jiao, HE Zhen-jiang, LU jun. Boosting cell performance of LiNi_0.8Co_0.15Al_0.05O₂ via surface structure design [J]. Small, 2019, 15(50): 1904854. DOI: 10.1002/smll.201904854.

[4] QU Xing-yu, YU Zhen-lu, RUAN Ding-shan, DOU Ai-chun, SU Ming-ru, ZHOU Yu, LIU Yun-jian, CHU De-wei, Enhanced electrochemical performance of Ni-rich cathode materials with Li_1.3Al_0.3Ti_1.7(PO₄)₃ coating [J]. ACS Sustainable Chemistry & Engineering, 2020, 8(15): 5819-5830. DOI: 10.1021/acssuschemeng.9b05539.

[5] ZHOU Zhi-wei, LUO Zi-yan, HE Zhen-jiang, ZHENG Jun-chao, LI Yun-jiao. A novel hollow porous structure designed for Na_0.44Mn_2/3Co_1/6Ni_1/6O₂ cathode material of sodium-ion batteries [J]. Journal of Power Sources, 2020, 479: 228788. DOI: 10.1016/j.jpowsour.2020.228788.

[6] WU Ling, HU Yong, ZHANG Xiao-ping, LIU Jie-qun, ZHU Xing, ZHONG Sheng-kui. Synthesis of carbon-coated Na₂MnPO₄F hollow spheres as a potential cathode material for Na-ion batteries [J]. Journal of Power Sources, 2018, 374: 40-47. DOI: 10.1016/j.jpowsour.2017.11.029.

[7] LIU Yi-jian, GUO Hao, ZHANG Bao-hua, WEN Gong-yu, VAJTAI R, WU Ling, AJAYAN P, WANG Liang. Sustainable synthesis of N-doped hollow porous carbon spheres via a spray-drying method for lithium-sulfur storage with ultralong cycle life [J]. Batteries & Supercaps, 2020, DOI: 10.1002/batt.202000143.

[8] WANG Gang, CHEN Yuan-hua, CHEN Chao, KANG Xiong-wu, YANG Cheng-hao, WANG Fei, LIU Yong, XIONG Xun-hui. Self-stabilized and strong adhesive supramolecular polymer protective layer enables ultrahigh rate and large capacity lithium metal anode [J]. Angewandte Chemie Inthernational Edition, 2020, 59: 2055-2060. DOI: 10.1002/anie.201913351.

[9] ZHENG Sheng-quan, DOU Ai-chun, SU Ming-ru, LIU Yun-jian. Influence of Nb doping on electrochemical performance of nanostructured cation disordered Li_1+x/100Ni_1/2-x/100Ti_1/2-x/100Nb_x/100O₂ composites cathode for Li-ion batteries [J]. Journal of Nanoscience and Nanotechnology, 2020, 20(1): 452-459. DOI: 10.1166/jnn.2020.16884.

[10] FANG Rui, MIAO Chang, MOU Hao-yi, XIAO Wei. Facile synthesis of Si@TiO₂@rGO composite with sandwich-like nanostructure as superior performance anodes for lithium ion batteries [J]. Journal of Alloys and Compounds, 2020, 818: 152884. DOI: 10.1016/j.jallcom.2019.152884.

[11] WU Ling, ZHENG Jie, WANG Liang, XIONG Xun-hui, SHAO Yan-yan, WANG Gang, WANG Jeng-han, ZHONG Sheng-kui, WU Ming-hong. PPy-encapsulated SnS₂ nanosheets stabilized by defects on a TiO₂ support as a durable anode material for lithium-ion batteries [J]. Angew Chem Int Ed Engl, 2018, 58(3): 811-815. DOI: 10.1002/ange.201811784.

[12] YU Zhen-lu, QU Xing-yu, WAN Tao, DOU Ai-chun, ZHOU Yu, PENG Xiao-qi, SU Ming-ru, LIU Yun-jian, CHU De-wei. Synthesis and mechanism of high structural stability of nickel-rich cathode materials by adjusting Li-excess [J]. ACS Appl Mater Interfaces, 2020, 12(36): 40393-40403. DOI: 10.1021/acsami.0c12541.

[13] AUGUSTYN V, COME J, LOWE M, KIM J, TABERNA P, TOLERT S, ABRUNA H, SIMON P, DUNN B. High-rate electrochemical energy storage through Li⁺ intercalation pseudocapacitance [J]. Nature Matrials, 2013, 12: 518-522. DOI: 10.1038/nmat3601.

[14] YAN Li-tao, RUI Xian-hong, CHEN Gen, XU Wei-chuan, ZOU Gui-fu, LUO Hong-mei. Recent advances in nanostructured Nb-based oxides for electrochemical energy storage [J]. Nanoscale, 2016, 8: 8443-8465. DOI: 10.1039/ C6NR01340F.

[15] KODAMA R, TERADA Y, NAKAI I, KOMABA S, KUMAGAI N. Electrochemical and in situ XAFS-XRD investigation of Nb₂O₅ for rechargeable lithium batteries [J]. Journal of the Electrochemical Society, 2006, 153: 583-588. DOI: 10.1149/1.2163788.

[16] WANG Xu, YAN Chao-yi, YAN Jian, SUMBOJA A, LEE P. Orthorhombic niobium oxide nanowires for next generation hybrid supercapacitor device [J]. Nano Energy, 2015, 11: 765-772. DOI: 10.1016/j.nanoen.2014.11.020.

[17] LIU Xiao-di, LIU Guang-yin, CHEN Hao, MA Jian-min, ZHANG Rui-xue. Facile synthesis of Nb₂O₅ nanobelts assembled from nanorods and their applications in lithium ion batteries [J]. Journal of Physics and Chemistry of Solids, 2017, 111: 8-11. DOI: 10.1016/j.jpcs.2017.07.007.

[18] WANG Hong-xun, ZHANG Bao, ZENG Xian-qing, YAN Li-jing, ZHENG Jun-chao, LING Min, HOU Yang, LU Ying-ying, LIANG Cheng-du. 3D porous carbon nanofibers with CeO₂-decorated as cathode matrix for high performance lithium-sulfur batteries [J]. Journal of Power Sources, 2020, 473: 228588. DOI: 10.1016/j.jpowsour.2020.228588.

[19] CHEONG J, JUNG J, YOUN D, KIM C, YU S, CHO S, YOON K, KIM I. Mesoporous orthorhombic Nb₂O₅ nanofibers as pseudocapacitive electrodes with ultra-stable Li storage characteristics [J]. Journal of Power Sources, 2017, 360: 434-442. DOI: 10.1016/j.jpowsour.2017.06.030.

[20] CHEONG J, YOUN D, KIM C, JUNG J, OGATA A, JIN G. Ag-coated one-dimensional orthorhombic Nb₂O₅ fibers as high performance electrodes for lithium storage [J]. Electrochimica Acta, 2018, 269: 388-396. DOI: 10.1016/ j.electacta.2018.03.028.

[21] LIU Guang-yin, JIN Bo, BAO Ke-yan, XIE Hai-quan, GUO Jia-li, JI Xiao-guang, ZHANG Rui-xue, JIANG Qing. Facile synthesis of porous Nb₂O₅ microspheres as anodes for lithium-ion batteries [J]. International Journal of Hydrogen Energy, 2017, 42: 6065-6071. DOI: 10.1016/j.ijhydene.2016. 11.126.

[22] LOU Shuai-feng, CHENG Xin-qun, WANG Long, GAO Jin-long, LI Qin, MA Yun-lin, GAO Yun-zhi, ZUO Peng-jian, DU Chun-yu, YIN Ge-ping. High-rate capability of three-dimensionally ordered macroporous T-Nb₂O₅ through Li⁺ intercalation pseudocapacitance [J]. Journal of Power Sources, 2017, 361: 80-86. DOI: 10.1016/j.jpowsour.2017. 06.023.

[23] LU Shi-jie, WANG Zhi-teng, ZHANG Xia-hui, HE Zhen-jiang, TONG Hui, LI Yun-jiao, ZHENG Jun-chao. In situ-formed hollow cobalt sulfide wrapped by reduced graphene oxide as an anode for high-performance lithium-ion batteries [J]. ACS Appl Mater Interfaces, 2020, 12(2): 2671-2678. DOI: 10.1021/acsami.9b18931.

[24] HUO Yi-feng, QIN Ning, LIAO Cheng-zhu, FENG Hui-fen, GU Ying-ying, CHENG Hua. Hydrothermal synthesis and energy storage performance of ultrafine Ce₂Sn₂O₇ nanocubes [J]. Journal of Central South University, 2019, 26(6): 1416-1425. DOI: 10.1007/s11771-019-4097-4.

[25] LIU Yu-bin, LIN Li-wei, ZHANG Wei-feng, WEI Ming-deng. Heterogeneous TiO₂@Nb₂O₅ composite as a high-performance anode for lithium-ion batteries [J]. Scientific Reports, 2017, 7: 7204. DOI: 10.1038/s41598-017- 07562-5.

[26] LIU Xiao-di, DUAN Xiao-chuan, PENG Peng, ZHENG Wen-jun. Hydrothermal synthesis of copper selenides with controllable phases and morphologies from an ionic liquid precursor [J]. Nanoscale, 2011, 3: 5090-5095. DOI: 10.1039/C1NR10833F.

[27] CAO Dun-ping, YAO Zhen-guo, LIU Jian-jun, ZHANG Jin-cang, LI Chi-lin. H-Nb₂O₅ wired by tetragonal tungsten bronze related domains as high-rate anode for Li-ion batteries [J]. Energy Storage Materials, 2018, 11: 152-160. DOI: 10.1016/j.ensm.2017.10.005.

[28] VIET A, REDDY M, JOSE R, CHOWDARI B，RAMAKRISHNA S. Nb₂O₅ photoelectrodes for dye-sensitized solar cells: Choice of the polymorph [J]. The Journal of Physical Chemistry C, 2010, 114(49): 21795-21800. DOI: 10.1021/jp106515k.

[29] YUN Si-ning, SI Yi-ming, SHI Jing, ZHANG Tai-hong, HOU Yu-zhi, LIU Hang, MENG Sheng, HAGFELDT A. Electronic structures and catalytic activities of niobium oxides as electrocatalysts in liquid-junction photovoltaic devices [J]. Solar RRL, 2020, 4(3): 1900430. DOI: 10.1002/ solr.201900430.

[30] KIM K, KIM M, CHA P, KANG S, KIM J. Structural modification of self-organized nanoporous niobium oxide via hydrogen treatment [J]. Chemistry of Materials, 2016, 28(5): 1453-1461. DOI: 10.1021/acs.chemmater.5b04845.

[31] KONG Ling-ping, ZHANG Chuan-fang, WANG Ji-tong, QIAO Wen-ming, LING Li-cheng, LONG Dong-hui. Free-standing T-Nb₂O₅/graphene composite papers with ultrahigh gravimetric/volumetric capacitance for Li-ion intercalation pseudocapacitor [J]. ACS Nano, 2015, 9(11): 11200-11208. DOI: 10.1021/acsnano.5b04737.

[32] YANG Chao, YU Shu, LIN Chun-fu, LV Fan, WU Shun-qing, YANG Yong, WANG Wei, ZHU Zi-zhong, LI Jian-bao, WANG Ning, GUO Shao-jun. Cr_0.5Nb_24.5O₆₂ nanowires with high electronic conductivity for high-rate and long-life lithium-ion storage [J]. ACS Nano, 2017, 11(4): 4217-4224. DOI: 10.1021/acsnano.7b01163.

[33] HE Han-na, SUN Dan, TANG You-gen, WANG Hai-yan, SHAO Min-hua. Understanding and improving the initial Coulombic efficiency of high-capacity anode materials for practical sodium ion batteries [J]. Energy Storage Materials, 2019, 23: 233-251. DOI: 10.1016/j.ensm.2019.05.008.

[34] LUKATSKAYA M, DUNN B, GOGOTSI Y. Multidimensional materials and device architectures for future hybrid energy storage [J]. Nature Communications, 2016, 7: 12647. DOI: 10.1038/ncomms12647.

[35] RAUDA I, AUGUSTYN V, DUNN B, TOLBERT S. Enhancing pseudocapacitive charge storage in polymer templated mesoporous materials [J]. Accounts of Chemical Research, 2013, 46(5): 1113-1124. DOI: 10.1021/ ar300167h.

[36] WANG Ru-tao, LANG Jun-wei, ZHANG Peng, LIN Zong-yuan, YAN Xing-bin. Fast and large lithium storage in 3D porous VN nanowires-graphene composite as a superior anode toward high performance hybrid supercapacitors [J]. Advanced Functional Materials, 2015, 25(15): 2270-2278. DOI: 10.1002/adfm.201404472.

[37] LI Guang-chao, YIN Zhou-lan, GUO Hua-jun, WANG Zhi-xing, YAN Guo-chun, YANG Zhe-wei, LIU Yong, JI Xiao-bo, WANG Jie-xi. Metalorganic quantum dots and their graphene-like derivative porous graphitic carbon for advanced lithium-ion hybrid supercapacitor [J]. Advanced Energy Materials, 2019, 9(2): 1802878. DOI: 10.1002/aenm.201802878.

[38] YANG Shu-qi, WANG Peng-bo, WEI Han-xin, TANG Lin-bo, ZHANG Xia-hui, HE Zhen-jiang, LI Yun-jiao, TONG Hui, ZHENG Jun-chao. Li₄V₂Mn(PO₄)₄ stablized Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ cathode materials for lithium ion batteries [J]. Nano Energy, 2019, 63: 103889. DOI: 10.1016/ j.nanoen.2019.103889.

[39] TAN Yu-ming, CHEN Xian-hong, ZHU Yi-rong, CHEN Li-juan. Synthesis of spherical tremella-like Sb₂O₃ structures derived from metal-organic framworks and its lithium storage properties [J]. Journal of Central South University, 2019, 26: 1469-1480. DOI: 10.1007/s11771-019-4103-x.

[40] ZHOU Xiao-zhong, LU He-jie, TANG Xing-chang, ZENG Ya-ping, YU Xin. Facile synthesis of Sb@Sb₂O₃/reduced graphene oxide composite with superior Lithium-storage performance [J]. Journal of Central South University, 2019, 26(6): 1493-1502. DOI: 10.1007/s11771-019-4105-8.

[41] FANG Rui, XIAO Wei, MIAO Chang, MEI Ping, YAN Xue-ming, ZHANG Yan, JIANG Yu. Improved lithium storage performance of pomegranate-like Si@NC/rGO composite anodes by facile in-situ nitrogen doped carbon coating and freeze drying processes [J]. Journal of Alloys and Compounds, 2020, 834: 155230. DOI: 10.1016/j.jallcom. 2020.155230.

(Edited by HE Yun-bin)

中文导读

六方多孔TT-Nb₂O₅微米片的固相制备与储锂性能

摘要：本文通过固相反应首次合成了一种六方多孔Nb₂O₅材料。通过优化热处理温度调节Nb₂O₅材料的结构和电化学性能。XRD结果表明，在不同温度下可以合成伪六方晶系与正交晶系Nb₂O₅；SEM结果表明，所合成的Nb₂O₅材料具有正六边形貌与多孔结构。将该材料用作锂离子电池负极材料时，在600 ℃制备的伪六方结构Nb₂O₅(TT-Nb₂O₅)表现出最优的电化学性能。0.2C电流密度下，其可逆比容量为178 mA·h/g；当电流密度提高到20C时，可逆比容量仍有86 mA·h/g。优异的电化学性能得益于Nb₂O₅的赝电容式锂离子嵌入机制，通过晶体结构与比表面积调控显著提高Nb₂O₅的综合电化学性能。

关键词：锂离子电池；五氧化二铌；六方多孔；电化学性能

Foundation item: Projects(51974137, 51774150) supported by the National Natural Science Foundation of China; Project(2020M671361) supported by China Postdoctoral Science Foundation

Received date: 2020-06-02; Accepted date: 2020-09-14

Corresponding author: LIU Yun-jian, PhD, Professor; Tel/Fax: +86-511-88790190; E-mail: lyjian122331@163.com; ORCID: https:// orcid.org/0000-0002-8729-1351