Macaroni‐Like Blue‐Gray Nb
 2
 O
 5
 Nanotubes for High‐Reversible Lithium‐Ion Storage

For increasing demands of high-performance energy storage system in electric vehicles and personal electronics, lithium-ion batteries (LIBs) have triggered intensively research interest because of high energy density and long-term cycle performance. Graphite, as a current commercial anode material, shows poor theoretical capacity (about 372mA h g ) that restricts the capacity of the whole device. Thus, it is highly desirable to search for low-cost anode materials with high specific capacity and long-term cyclability. In this regard, transitionmetal oxides, such as MnOm, MnO2, [10] Mn3O4, [11] Co3O4, [12,13] CuO, Fe3O4, [16,17] and Fe2O3, [18,19] sulfides like Co1 xS [20] and carbides have been extensively exploited because of their high rechargeable capacities. Among them, Nb2O5 is a potential material in energy storage like lithium-/sodium-ion batteries or supercapacitors, except for its applications in photocatalytic, electron transport layer in solar cell, and electronic-like electron field emitters. Nb2O5 shows so many admirable attributes including adjustable morphologies, controllable crystal type, and easy to synthesis that makes it be a kind of promising anode materials. Compared with industrialized carbon anode, Nb2O5 possesses better rate performance and power density. Moreover, high-performing silicon–carbon anode still suffer from the severe volume expansion, whereas the Nb2O5 presents a higher structural stability and minimal volume expansion (less than 5%). Furthermore, it has been applied in aluminum-ion battery system with an impressive specific capacity of 563mAh g , and showed an anomalously fast energy storage behavior in LIBs. Qu et al. reported flower-like T-Nb2O5 [38] and T-Nb2O5-based composite [39,40] for improved performance LIB anode, delivered specific capacity range from 178.2 to 332mAh g 1 after 100 cycles at 0.2 A g 1 current density. All these suggested the crystal structure of Nb2O5 may be a suitable host material for the high-rate insertion/extraction of metal ions. However, most of developed Nb2O5 release its merit in fast charging and discharging by composting with conductive substrate such as graphene and silver, or creating defect after the introduction of extraneous element such as nitrogen via fussy preparation method. In this work, blue-gray Nb2O5 (B-Nb2O5) nanotubes were prepared by a facile chemical vapor deposition (CVD)method from a single NbCl5 precursor followed by a further 4 h hydrogen annealing reduction at 600 C. It was different from the typical preparation methods that contained multiple raw materials and time-consuming reactions with hazardous hydrofluoric acid Dr. L. Wang, F. Huang, C. Li, Dr. Y. Liu, Prof. Z. Dai School of Chemistry and Materials Science Nanjing Normal University Nanjing 210023, P. R. China E-mail: daizhihuii@njnu.edu.cn


Introduction
For increasing demands of high-performance energy storage system in electric vehicles and personal electronics, lithium-ion batteries (LIBs) have triggered intensively research interest because of high energy density and long-term cycle performance. [1][2][3][4][5][6] Graphite, as a current commercial anode material, shows poor theoretical capacity (about 372 mA h g À1 ) that restricts the capacity of the whole device. [7] Thus, it is highly desirable to search for low-cost anode materials with high specific capacity and long-term cyclability. In this regard, transitionmetal oxides, such as MnOm, [8,9] MnO 2 , [10] Mn 3 O 4 , [11] Co 3 O 4 , [12,13] CuO, [14,15] Fe 3 O 4 , [16,17] and Fe 2 O 3 , [18,19] sulfides like Co 1Àx S [20] and carbides [21,22] have been extensively exploited because of their high rechargeable capacities. Among them, Nb 2 O 5 is a potential material in energy storage like lithium-/sodium-ion batteries or supercapacitors, [23][24][25][26][27][28][29][30] except for its applications in photocatalytic, [31] electron transport layer in solar cell, [32,33] and electronic-like electron field emitters. [34] Nb 2 O 5 shows so many admirable attributes including adjustable morphologies, controllable crystal type, and easy to synthesis that makes it be a kind of promising anode materials. Compared with industrialized carbon anode, Nb 2 O 5 possesses better rate performance and power density. [7] Moreover, high-performing silicon-carbon anode still suffer from the severe volume expansion, [35] whereas the Nb 2 O 5 presents a higher structural stability and minimal volume expansion (less than 5%). [36] Furthermore, it has been applied in aluminum-ion battery system with an impressive specific capacity of %563 mA h g À1 , [37] and showed an anomalously fast energy storage behavior in LIBs. [23,24] Qu et al. reported flower-like T-Nb 2 O 5 [38] and T-Nb 2 O 5 -based composite [39,40] for improved performance LIB anode, delivered specific capacity range from 178.2 to 332 mA h g À1 after 100 cycles at 0.2 A g À1 current density. All these suggested the crystal structure of Nb 2 O 5 may be a suitable host material for the high-rate insertion/extraction of metal ions. However, most of developed Nb 2 O 5 release its merit in fast charging and discharging by composting with conductive substrate such as graphene [41] and silver, [36] or creating defect after the introduction of extraneous element such as nitrogen [36] via fussy preparation method.
In this work, blue-gray Nb 2 O 5 (B-Nb 2 O 5 ) nanotubes were prepared by a facile chemical vapor deposition (CVD) method from a single NbCl 5 precursor followed by a further 4 h hydrogen annealing reduction at 600 C. It was different from the typical preparation methods that contained multiple raw materials and time-consuming reactions with hazardous hydrofluoric acid DOI: 10.1002/aesr.202100028 Due to the high reliability and high theoretical capacity, lithium-ion batteries (LIBs) have been widely studied in the world. Nevertheless, the existing LIB systems currently exhibit comparatively low capacities restricted by the anode materials. Herein, blue-gray Nb 2 O 5 (B-Nb 2 O 5 ) nanotubes are prepared which are rich in oxygen vacancy by a facile chemical vapor deposition (CVD) method and a further hydrogen annealing reduction as the anode material for LIBs, presenting a high discharge capacity of 375 mA h g À1 at 100 mA g À1 and a good rate performance up to 5 A g À1 with 126 mA h g À1 . The detailed ex situ X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) characterizations verified the highreversible process, which Li þ should insert into/extract from the (001) planes of Nb 2 O 5 crystal. Combined with a reversible PF 6 intercalation into/deintercalation from graphite cathode, a B-Nb 2 O 5 /graphite dual-ion cell can run about 50 cycles with the discharge capacity retention approaching 23 mA h g À1 at 100 mA g À1 . The importance of the modulation of morphology and vacancy in improving overall electrochemical performance is highlighted.
treatment. [42,43] The obtained Nb 2 O 5 showed a well-defined morphology and blue-gray color originating from the oxygen vacancy defect. [44] Working as an anode of LIBs, the Nb 2 O 5 -based battery delivered a high discharge capacity of about 300 mA h g À1 at the current density of 0.5 A g À1 with coulombic efficiency approaching 100% after 500 cycles based on the reversible insertion/extraction of Li þ into/from the (001) planes of Nb 2 O 5 crystal. In addition, a prototype Nb 2 O 5 /graphite dual-ion battery can be also operated normally, indicating a good versatility.

Result and Discussions
We prepared blue-gray oxygen vacancy Nb 2 O 5 (B-Nb 2 O 5 ) nanotubes by one-step CVD [37] and further hydrogen annealing reduction, as schematically shown in Figure 1a. Typically, anhydrous NbCl 5 powder was calcined and evaporated in a tube furnace at the temperature of 600 C for 4 h. As thus, white Nb 2 O 5 nanotubes (W-Nb 2 O 5 nanotubes) were obtained in the intine of the quartz tube of furnace. Then W-Nb 2 O 5 nanotubes were reduced by the hydrogen to obtain B-Nb 2 O 5 nanotubes ( Figure S1, Supporting Information). As control, commercial Nb 2 O 5 powder were also reduced at the same conditions (denote C-Nb 2 O 5 ). Electron microscopy was used to characterize the morphology. As shown in Figure 1b-d, scanning electron microscopy (SEM) images revealed that B-Nb 2 O 5 nanotubes were uniform nanotubes with diameter %150 nm and length range from 5 to 10 μm. The transmission electron microscopy (TEM) images suggested that the thickness of nanotubes was about 25 nm (Figure 1e,f ). The energy dispersive X-ray spectrometer (EDS) coupled with scanning transmission electron microscopy (STEM) was used to collect elemental distribution. Figure 1g shows that Nb and O elements were coexistent and uniformly distributed of in B-Nb 2 O 5 nanotubes. After anatomizing the EDS results shown in Figure S2, Supporting Information, we discovered that the oxygen content was lower compared with the theoretical value of Nb 2 O 5 . The calculated oxygen defects concentration of B-Nb 2 O 5 nanotubes was up to 9.83%. This value was the highest among W-Nb 2 O 5 nanotubes, C-Nb 2 O 5 , and commercial Nb 2 O 5 powder measured at the same parameter ( Figure S3 and S4, Supporting Information). Further structural and chemical probe characterizations are shown in Figure 2. Tilted to [010] crystal projection for crystal plane observation (detailed method in Figure S5, Supporting Information), high-resolution TEM (HRTEM) image ( Figure 2a and Figure S5, Supporting Information) of B-Nb 2 O 5 nanotubes proved that the preferred growth orientation was along the [001] direction. [37] Fast Fourier Transform (FFT, Figure 2b  , and NbO 2 were detected, indicating the high purity of the sample, and phase stability during the reduction process. [44] The XRD patterns of control samples are shown in Figure S6 and  Figure 2f ), which corresponded to the Nb─O stretching. [37] Xray photoelectron spectra (XPS) were collected to qualitatively and quantitatively investigate the surface elements, valence, and surface oxygen defect concentration of B-Nb 2 O 5 nanotubes. Figure 2g shows the high-resolution spectrum for Nb 3d and two characteristic peaks at 206.9 and 209.7 eV were ascribed to the Nb 3d 5/2 and Nb 3d 3/2 bands. The full width at half maximum (FWHM) of B-Nb 2 O 5 nanotubes is significantly larger than W-Nb 2 O 5 nanotubes and the FWHM of C-Nb 2 O 5 is significantly larger than commercial Nb 2 O 5 . These indicated that the content of Nb on the surface of B-Nb 2 O 5 and C-Nb 2 O 5 is increased after hydrogen reduction process, further proving the generation of surface O defects. In the scan region of the O 1s (Figure 2h), the peak at 529.8 eV was assigned to the O─Nb bonds. We discovered that the oxygen content was lower compared with the commercial Nb 2 O 5 . The calculated surface oxygen defect concentration of B-Nb 2 O 5 nanotubes was up to 13.1%. This value was the highest among W-Nb 2 O 5 nanotubes, C-Nb 2 O 5 , and commercial Nb 2 O 5 powder measured at the same condition (Table S1, Supporting Information). The crystal structure and morphology www.advancedsciencenews.com www.advenergysustres.com change little from thermal reduction process, but the color turn blue-gray and the O vacancy defects were markedly increased. In addition, after calcined to 800 C in air, the B-Nb 2 O 5 return to white color and weight increased ( Figure S7, Supporting Information). This is also in agreement with reported literature. [44] The electrochemical performance of B-Nb 2 O 5 nanotubes were evaluated by used as anode electrode for LIBs. Before the assembly of coin cell, we found anode consisting of B-Nb 2 O 5 nanotubes was more compact in contrast to commercial Nb 2 O 5 ( Figure S8, Supporting Information). Such electrode structure was beneficial to electron conducting and sturdiness during repeated cycling process. Figure 3a shows the cyclic voltammogram (CV) curves of B-Nb 2 O 5 nanotubes by combining with the metal Li as a counter electrode, which examined the electrochemical property at scan rate of 0.2 mV s À1 . From the first cathodic scan, a strong cathodic peak at 0.59 V was observed, which should be attributed to the formation of soild electrolyte interface (SEI). The cathodic peak around 0.75 V is assigned to organic salts (such as lithium ethylene carbonate) precipitates on the anode during the first few cycles, preventing further decomposition of the electrolyte. [45] After the first cycle, the dominant cathodic peak observed at 1.61 V and anodic peak at 1.94 V were corresponded to the process that Li þ intercalated into/deintercalated from B-Nb 2 O 5 nanotubes lattice. However, the commercial samples experienced some additional electrochemical processes except for the redox pair of 1.61/1.94 V ( Figure S9, Supporting Information), which may be caused by the side reactions between B-Nb 2 O 5 nanotubes anode and electrolyte. Correspondingly, typical galvanostatic discharge/charge curves for B-Nb 2 O 5 nanotubes were observed at 1.50-1.95 V (Figure 3b). The discharge slope of 1.50-1.95 V was ascribed to Li þ intercalation into Nb 2 O 5 lattice and the charging slope of 1.40-2.00 V was attributed to Li þ deintercalation from the intercalated Nb 2 O 5 lattice, in line with the results of CV tests. To testify the superiority of oxygen defect and nanotube morphology of B-Nb 2 O 5 in reducing the resistance of batteries, the electrochemical impedance spectroscopy (EIS) analysis of the B-Nb 2 O 5 nanotubes and control samples are shown in Figure 3c. The semicircles and sloping lines were attributed to the charge transfer resistance (R ct ) and Li þ diffusion resistance, respectively. [46] B-Nb 2 O 5 nanotube electrodes   (Figure 3d), suggesting a good rate performance. The discharge capacity of B-Nb 2 O 5 nanotubes was higher than that of W-Nb 2 O 5 nanotubes, and better than the discharge capacities of C-Nb 2 O 5 and commercial Nb 2 O 5 . This results certificated that both morphology and vacancy contributed a better rate performance, in accordance with the comparison of galvanostatic chargedischarge profiles from these four samples at the small current density of 0.1 A g À1 (Figure 3e). At the current density of 0.5 A g À1 , the capacity of commercial  Figure 3f ). B-Nb 2 O 5 nanotubes electrode still afforded about 300 mA h g À1 after 500 cycles, suggesting that the structure of B-Nb 2 O 5 nanotubes was beneficial to Li þ storage.
To study the cyclic performances at high current density, these four samples were assembled with Li metal to operate at 1 A g À1 current density. After 1500 cycles, residual capacity of the B-Nb 2 O 5 nanotubes was still over 200 mA h g À1 , whereas W-Nb 2 O 5 nanotubes, C-Nb 2 O 5 , and commercial Nb 2 O 5 were only about 165, 150, and 90 mA h g À1 , respectively (Figure 4a). Even after 2000 cycles, B-Nb 2 O 5 nanotubes electrode can run more than 2000 cycles with the capacity retention of 72.36% (165.2 mA h g À1 ). Li þ storage performance of B-Nb 2 O 5 nanotubes toward LIBs application was comparable with the previously reported results (Table S3, Supporting Information). For Li þ storage mechanism in Nb 2 O 5 , ex situ XRD was conducted for the lattice changes during the galvanostatic discharge-charge process. As shown in Figure 4b, the  as-assembled 2032-type LIBs half-cell consist of a Li foil, separator, B-Nb 2 O 5 nanotubes anode with copper current collector, and moderate electrolyte. The charge-discharge curves and points 1-8 were selected for ex situ XRD tests (Figure 4c,d). When the battery was fully discharged to 0.01 V (point 1), the evolution of the patterns revealed a shift of the (001) (Figure 4e), which might be ascribed to the Li species deposited during discharging process and dissolved during charging process. Li 1s peak (Figure 4f ) shifted to higher binding energy during charging process and reversed to opposite direction during discharging process, which can be attributed to Li þ reversible electrochemical redox reactions. The O 1s peak became complicated after first discharge to 0.01 V, suggested the formation of SEI layer that mainly contained Li 2 CO 3 and lithium alkyl carbonate from the O 1s peak (Figure 4g). Based on the excellent performances of B-Nb 2 O 5 nanotubes in LIBs half-cell tests, we assembled dual-ion batteries [48,49] with B-Nb 2 O 5 nanotubes as an anode material and graphite as a cathode (Figure 5a). During the charge processes, Li þ from www.advancedsciencenews.com www.advenergysustres.com the solution can be rapidly inserted into B-Nb 2 O 5 nanotubes anode, while PF 6 À quickly intercalated into the interlayers of graphite. As the cathode, graphite possessed high discharging platform (4.00-4.85 V). In situ Raman was implemented to probe the electrochemical mechanism of graphite ( Figure 5b). Charging to 5.5 V, E g peak and A 1g peak of PF 6 À diminished, and G band of graphite (1584 cm À1 ) vanished and split into a doublet, 1612 cm À1 for E 2g2 (i) and 1629 cm À1 for E 2g2 (b) upon PF 6 À intercalation (Figure 5c). [50] During discharging to 0 V, E g peak of PF 6 À reappeared due to the PF 6 À deintercalation from graphite interlayers (Figure 5d). Detailed Raman data are shown in Figure S10, Supporting Information. In consequence, B-Nb 2 O 5 /graphite battery experiences Li þ and PF 6 À (de)insertion during the charge-discharge process. This full cell exhibited clear discharge voltage plateaus in the ranges of 4.85-5.10 and 2.50-3.00 V after the first cycle activation (Figure 5e). The relatively high discharge voltage was promising in practical application. A specific capacity of %23 mA h g À1 (calculated by total mass of cathode and anode for this full cell, calculated energy density is 85.3 W h kg À1 at 371 W kg À1 ) remained after 50 cycles at the current density of 0.1 A g À1 (Figure 5f ), comparable to dual-graphite cells, [51,52] which can light a blue light-emitting diode (LED), demonstrating that the full-cell design might be promising in practical application.

Conclusion
In summary, we controlled prepared blue-gray oxygen vacancy defect Nb 2 O 5 nanotubes via a CVD method. As anode materials, the as-prepared B-Nb 2 O 5 nanotubes offered high specific capacity and remarkable cycling stability in LIBs (200 mA h g À1 over 1500 cycles at 1 A g À1 ), much superior to those of commercial Nb 2 O 5 , C-Nb 2 O 5 and W-Nb 2 O 5 nanotubes. Detailed ex situ XRD and XPS analyses at different reaction processes revealed that the lithiation mechanism involved a reversible Li þ insertion into/extraction from Nb 2 O 5 lattice. Combined with a reversible PF 6 À intercalation into/deintercalation from graphite cathode, at the current density of 0.1 A g À1 , B-Nb 2 O 5 /graphite dual-ion cell can provide a high capacity of %23 mA h g À1 after 50 cycles. This work demonstrated that the morphology and vacancy design of transitional metal oxides as advanced anode materials was a promising strategy for developing novel full-cell Li þ batteries.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.