Observation of ZrNb14O37 Nanowires as a Lithium Container via In

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22429−22438

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Observation of ZrNb14O37 Nanowires as a Lithium Container via In Situ and Ex Situ Techniques for High-Performance Lithium-Ion Batteries Yuhang Li,† Runtian Zheng,† Haoxiang Yu, Xing Cheng, Tingting Liu, Na Peng, Jundong Zhang, Miao Shui, and Jie Shu*

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Faculty of Materials Science and Chemical Engineering, Ningbo University, No. 818 Fenghua Road, Ningbo 315211, Zhejiang Province, People’s Republic of China S Supporting Information *

ABSTRACT: Based on the high theoretical capacity and relatively high safety voltage, niobium-based oxides are regarded as promising intercalation-type electrode materials for advanced lithium-ion batteries (LIBs). Here, ZrNb14O37 nanowires are fabricated via a facile electrospinning method, presenting a nanoparticle-in-nanowire architecture. As an anode for LIBs, the as-fabricated ZrNb14O37 nanowires maintain a capacity of 244.9 mA h g−1 at 100 mA g−1 and present excellent cycling capability (0.026% of capacity fading per cycle during 1000 cycles) as well as outstanding rate performance. In situ X-ray diffraction measurement is conducted to understand the fundamental reaction mechanism during the lithiation/delithiation process. The ex situ observations, including X-ray photoelectron spectroscopy and transmission electron microscopy, are further performed to provide more lines of evidence of the reaction mechanism. Moreover, the excellent electrochemical performance of the full cell constructed using ZrNb14O37 nanowires and LiCoO2 suggests that ZrNb14O37 nanowires are a promising anode material. This work sheds new light on understanding the lithium storage mechanism and may open new opportunities to develop new anode materials for LIBs. KEYWORDS: anode material, ZrNb14O37 nanowires, in situ XRD, ex situ observation, full cell performance.8−17 Early in 1983, Cava et al. have researched lithium insertion in niobium-based oxides with the Wadsley− Roth structure.18 This work did not catch much attention until TiNb2O7 was proposed as a lithium container by Goodenough group, which releases a high Li uptake capacity of ∼285 mA g−1 at a current rate of 0.1 C from 1.0 to 2.5 V.12 Afterward, many works focus on the lithium behaviors of various ratios of titanium niobium oxides (Ti−Nb−O), including TiNb6O17,19,20 Ti2Nb10O29,9,21 and TiNb24O62.4,10,22 Up to date, various members of the niobium-based oxide family have also been studied extensively and reported as anode materials, such as iron-niobium oxides (Fe−Nb−O)15,23,24 and tungstenniobium oxides (W−Nb−O).25−27 In spite of the fact that these niobium-based oxides exhibit high theoretical capacities and relatively high safety voltage, they suffer from relatively low practical capacities, poor long cycling performance, and low conductivity. In order to alleviate these problems, metal ion doping,28,29 conductive-phase compositing,30,31 introduction of oxygen vacancies,9,32−34 and nanostructure strategies15,23 are

1. INTRODUCTION The conventional graphite is extensively applied in commercial lithium-ion batteries (LIBs) because of its advantages of lowcost and excellent conductivity and dominates the LIB market for portable electronic devices since its first commercialization by Sony in 1990s.1,2 However, possible safety hazard may hinder graphite’s further applications in power LIBs because of the probable deposition formation of lithium dendrites at high rates when the lithium insertion voltage closes to 0.1 V versus Li/Li+.3,4 As an alternative anode material, Li4Ti5O12 has a high working voltage (1.55 V vs Li/Li+), avoiding the growth of lithium dendrites on the surface. In particular, Li4Ti5O12 is a zero-strain material with almost negligible volume change during repeating cycles and exhibits excellent cycling performance. However, the low theoretical capacity (175 mA h g−1) and poor electrical conductivity of Li4Ti5O12 severely restrict its long-calendar applications.5−7 Therefore, the current major anode materials (graphite and Li4Ti5O12) fail to meet the demand of high-performance LIBs, motivating researchers to search for the next-generation anode materials. In the recent years, niobium-based oxides (M−Nb−O, M = V, Ti, Cr, Fe, etc.) are considered as promising electrode materials because of their high capacity, safety voltage, and rate © 2019 American Chemical Society

Received: April 3, 2019 Accepted: May 29, 2019 Published: May 29, 2019 22429

DOI: 10.1021/acsami.9b05841 ACS Appl. Mater. Interfaces 2019, 11, 22429−22438

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of the synthesis of ZrNb14O37 nanowires. (b) Crystal structural model of ZrNb14O37. (c) XRD and (d) Rietveld refinement pattern of ZrNb14O37 nanowires.

with a nanoparticle-in-nanowire architecture by electrospinning technology. When employed as an anode for LIBs, the ZrNb14O37 nanowires exhibit excellent electrochemical performance. The ZrNb14O37 nanowires provide a high capacity of 244.9 mA h g−1 at 100 mA g−1. They exhibit excellent cycling stability with 0.026% of capacity loss per cycle after 1000 cycles and the rate capability reaches 168.2 mA h g−1 at 700 mA h g−1. Then, in situ XRD measurement is performed to elucidate the reaction mechanism of ZrNb14O37 nanowires toward lithium storage during the lithiation/delithiation process. Ex situ observation techniques [X-ray photoelectron spectroscopy (XPS), TEM, and high-resolution TEM (HRTEM)] are further carried out to provide more details of the insertion/ extraction mechanism. In addition, a full cell using ZrNb14O37 nanowires as the anode and LiCoO2 as the cathode (noted as ZrNb14O37//LiCoO2) is assembled to evaluate its practical application.

employed, which prove to be effective methods to enhance their electrochemical performance. In 2017, Yang and his coworker preliminarily applied porous nanostructure zirconium niobium oxide (ZrNb24O62) as a lithium container, which delivers excellent electrochemical performance.6 Although ZrNb24O62 exhibits interesting electrochemical characteristics, no other works have been done to conduct zirconium niobium oxides as anodes for LIBs and unveil the reaction thermodynamics. Understanding the structural evolution and reaction mechanism is critical for designing new anode materials and guiding for optimization on electrochemical performance. In situ characterization techniques, such as in situ transmission electron microscopy (TEM),35 in situ scanning electron microscopy (SEM), 36,37 and in situ X-ray diffraction (XRD), 3,11 are certainly powerful techniques to help researchers to analyze the battery process.38 Ou et al. studied the electrochemical reaction mechanisms of V5S8 by coupling in situ XRD and ex situ TEM characterization during the initial cycle process.39 On the basis of the results, they proposed the fundamental reaction mechanisms and clarified the high capacity mechanisms. Lee et al. analyzed the structural collapse mechanism of SnO2/Fe2O 3/RGO via multiple in situ characterization techniques (e.g., in situ electrochemical impedance spectroscopy, TEM, and XRD).40 Chen et al. combined in situ XRD analysis and first-principles calculation to study FeF3·0.33H2O as an anode, demonstrating the mechanism of lithium ion insertion during the cycle process.41 The qualitative and quantitative information from the in situ characterization techniques provides more evidence on the fundamental mechanisms. These characterization techniques help to confirm the high capacity,39,42 capacity degradation,43 and electrochemical reaction mechanisms.44,45 Thus, the application of these advanced in situ characterization techniques will expedite the process of exploring more appropriate electrode materials. Here, we report a new zirconium niobium oxide (ZrNb14O37) as an anode for LIBs. The large theoretical uptake capacity of ZrNb14O37 is as high as 378.3 mA h g−1 because of high Nb5+ content (65.6 wt %), which is higher than that of other niobium-based materials, such as KNb5O13 (65.28%), 13,14 K 6 Nb 10.8 O 11 (19.31%), 14 and PNb 9 PO 25 (64.42%).46 We design and prepare ZrNb14O37 nanowires

2. EXPERIMENTAL SECTION The bulk ZrNb14O37 powder and ZrNb14O37 nanowires were obtained via a traditional solid-state reaction method and a facile electrospinning method, respectively. As illustrated in Figure 1a, the precursor fibers were prepared by the electrospinning technique. Then, the precursor fibers were calcined at 950 °C to prepare ZrNb14O37 nanowires. The crystalline structure and morphological character of the as-prepared ZrNb14O37 nanowires were analyzed and the details are given in the Supporting Information. The electrochemical performance, including galvanostatic charge/discharge, cyclic voltammetry, long-term cycling stability, and rate performance, was evaluated via assembling CR2032-type cells. In situ XRD and ex situ measurements were carried out to understand the fundamental mechanism. More details are described in the Supporting Information.

3. RESULTS AND DISCUSSION The electrospinning technique is a facile method to prepare nanostructured materials (e.g., nanotubes, nanowires, and nanorods), which are extensively used as energy storage materials.47 As illustrated in Figure 1a, the precursor fibers are prepared via an electrospinning technique and the ZrNb14O37 nanowires are obtained after calcination (see details in the Supporting Information). The phase purity of ZrNb14O37 nanowires is confirmed by XRD measurements. The crystal structure is exhibited in Figure 1b and the obtained XRD patterns are illustrated in Figure 1c (ZrNb14O37 nanowires) 22430


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Figure 2. (a−c) SEM images, (d,e) TEM images, (f,g) HRTEM images, and (h) SAED image of ZrNb14O37 nanowires; (i−m) EDS mappings of Zr, Nb, and O in ZrNb14O37 nanowires.

Figure 3. (a) CV curves at 0.1 mV s−1 and (b) charge/discharge curves at 100 mA g−1 for ZrNb14O37 nanowires. (c) Rate performance of ZrNb14O37 nanowires. (d) Comparison of rate performance between ZrNb14O37 nanowires with other anode materials. (e) Long-term cycling of ZrNb14O37 nanowires.

as a = 20.94964 (197), b = 3.82223 (30), and c = 19.30021 (167) Å (Rp = 0.085, Rwp = 0.078, and χ2 = 2.83).The unit-cell volume of ZrNb14O37 is 1457.045 (0.218) Å3, larger than other niobium-based materials (e.g., KNb5O13 and K6Nb10.8O3014). Based on the same ReO3-type structure, the large unit-cell volume may provide larger diffusion paths. The ZrNb14O37

and Figure S1 (bulk ZrNb14O37). The characteristic diffraction peaks can match with monoclinic ZrNb14O37 (JCPDS no. 230453) without obvious diffraction peaks of impurity such as ZrO2 and Nb2O5. Figure 1d displays the Rietveld refinement pattern of ZrNb14O37 nanowires and the resulting data are summarized in Table S1. The lattice parameters are calculated 22431


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Figure 4. (a) CV curves at different scan rates of the ZrNb14O37 nanowires electrode. (b) Relationship between peak current and sweep rate. (c) Capacitive contribution at 1.0 mV s−1. (d) Contribution ratios from capacitive at various scan rates from 0.1 to 1 mV s−1.

has a high crystalline nature, in which the diffraction dots of (004), (601), and (−5−12) planes of ZrNb14O37 are found. The corresponding energy-dispersive system (EDS) elemental mapping images for ZrNb14O37 nanowires (Figure 2i,j) are displayed in Figure 2k−m, which present homogeneous dispersion of Zr, Nb, and O elements throughout the nanotube-in-nanowire architecture, confirming the formation of ZrNb14O37 nanowires. On the basis of the above results, it can be expected that ZrNb14O37 nanowires are a promising anode candidate for LIBs with enhanced electrochemical performance because of their nanoparticle-in-nanowire architecture. Figure 3 presents the lithium storage performance of the asobtained ZrNb14O37 nanowires. To understand the lithium storage progress of ZrNb14O37 nanowires, cyclic voltammogram (CV) measurement is conducted as scanned at 0.1 mV s−1 at the voltage of 1−3 V, which prevents the formation of solid electrolyte interphase films.50 As displayed in Figure 3a, the initial CV plot is slightly different from the following cycles. This phenomenon probably originates from the irreversible lithiation behavior in the initial cycle, resulting in the initial Coulombic efficiency lower than 100%. The oxidation peak located at 1.27 V is associated with Nb4+/Nb3+ redox couples, whereas the corresponding reduction peak is not obvious in the CV plots. A similar phenomenon associated with the Nb4+/ Nb3+ redox couples can be observed in other niobium-based oxides.4,28 A pair of sharp redox appearing at 1.82/1.51 V is attributed to the redox reaction of Nb5+/Nb4+ redox couple. In addition, another pair of peaks located at 2.08/1.96 V is also related to the activity of Nb5+/Nb4+ couple. The galvanostatic charge/discharge is carried out at a constant current density of 100 mA g−1. The charge/discharge plateaus appear at 1.82 and 1.51 V (Figure 3b), respectively, which are in agreement with the results of CVs in Figure 3a. The initial charge/discharge capacities can reach 244.9 and 283.1 mA h g−1 with a Coulombic efficiency of 86.51%, respectively. Rate-capability tests of ZrNb14O37 nanowires are carried out and the resulting curves are shown Figure 3c. The ZrNb14O37 nanowires anode shows reversible capacities of 236.4, 218.2, 207.2, 198.1, 190.9,

exhibits Wadsley−Roth shear structure. Observed from the detail crystal structure model of ZrNb14O37 displayed in Figures 1b and S2 (view along the x-, y-, and z-axes), the structural framework is constructed by 3 × 5 × ∞ blocks of corner- and edge-sharing MO6 (M = Zr, Ti) octahedra. Zr4+ and Nb5+ ions are disordered in the octahedral sites with a molar ratio of 1:14. Compared with TiNb2O7 (3 × 3 × ∞)22 and Ti2Nb10O29 (3 × 4 × ∞),21,48 ZrNb14O37 has a more open framework, which can be beneficial for the insertion/extraction of lithium ions. The morphological features and structures of ZrNb14O37 nanowires are observed by SEM, TEM, and HRTEM (more details are described in the Supporting Information). The topview SEM image (Figure 2a) shows that nanowires align in random orientation. A closer observation in Figure 2b illustrates that ZrNb14O37 nanowires present a typical nanoparticle-in-nanowire architecture and the precursor fibers display a smooth surface before calcination (Figure S3). It can be observed from the high-magnification SEM image (Figure 2c) that the resulting products are composed of wellconnected nanoparticles with a diameter of approximately 120 nm. To further characterize the detailed morphologies and crystal structures of ZrNb14O37 nanowires, TEM observation are carried out. Clearly, from the observation in Figure 2d,e, the as-prepared samples show a nanoparticle-in-nanowire structure with a diameter of 80−130 nm, which is consistent with the SEM investigation. Polyvinylpyrrolidone plays a significant role in the construction of such a nanoparticle-innanowire structure, which is similar to the observation in the previous report on V2O5 nanowires.49 Compared with bulk ZrNb14O37 (Figure S4), the ZrNb14O37 nanowires can provide larger contacting areas between electrode materials and electrolyte and shorter diffusion paths for lithium ions and electrons. The distance between two adjacent planes (d value) is measured to be 0.377 nm (Figure 2f), corresponding to the (−111) plane of ZrNb14O37 nanowires. Also, the interplanar spacing of 0.279 nm corresponds to the (−5−12) plane of ZrNb14O37 nanowires (Figure 2g). The selected area electron diffraction (SAED) pattern (Figure 2h) shows that the sample 22432


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Figure 5. (a) Schematic construction of in situ XRD system; (b) in situ XRD patterns of ZrNb14O37 nanowires during the initial cycle; (c,d) change of the (−111), (111), (512), and (513) reflections during cycling; and (e) evolution of lattice parameters during the initial lithiation/delithiation process.

181.8, and 168.2 mA h g−1 at current densities of 100, 200, 300, 400, 500, 600, and 700 mA g−1, respectively. When the current density decreases from 700 to 100 mA g−1, it still can maintain a capacity of 232.0 mA h g−1. Compared with other niobium-based anodes (WNb12O33,27 Nb2O5,51 Nb18W16O93,52 CrNb49O124,53 VNb9O25,54 and BiNbO455), the as-prepared ZrNb14O37 nanowires present higher rate capacities (Figure 3d). Figures 3e and S5 illustrate the long-term performance of ZrNb14O37 nanowires and bulk, respectively. The ZrNb14O37 nanowires can afford a reversible capacity of 181.5 mA h g−1 with 0.026% of capacity fading per cycle upon 1000 cycles, while the bulk only retains a final capacity of 85.5 mA h g−1. Therefore, compared with the bulk ZrNb14O37, the ZrNb14O37 nanowires deliver a higher reversible capacity, better cycling stability, and outstanding rate performance (Figures S5−S7). The excellent electrochemical performance is attributed to the unique nanotube-in-nanowire construction and structure stability (described in detail in the next section). This construction provides larger contacting areas for electrochemical reactions and shorter transportation paths for lithium ions and electrons, promoting a remarkable electrochemical performance. In order to give compressive insights into the asprepared products, we compare theoretical capacity, specific capacity, initial efficiency, and platform voltage of ZrNb14O37 nanowires with those of other intercalation-type anodes52,56−59 in previous reports and present the results in Figure S8. The ZrNb14O37 nanowires have a theoretical capacity of 378.3 mA h g−1, lower than that of GeNb18O47 (386.4 mA h g−1)57 and TiNb6O17 (397.1 mA h g−1).58 However, it presents a higher

specific capacity (Figure S8). The initial Coulombic efficiency reaches 86.51%, which is better than most of other intercalation-type anodes. The excellent performance may result from the nanosized architecture and stabilization of the structure (discussion in the following section). These suggest that the as-spun ZrNb14O37 nanowires are considerable competitive advantages over other anode materials for LIBs. CVs are measured at various scan rates from 0.1 to 1.0 mV s−1 to evaluate the electrochemical kinetics of ZrNb14O37 nanowires. All of the cathodic and anodic peaks during the lithiation/delithiation reactions clearly grow with increasing scan rates, demonstrating a high activity and good kinetics of ZrNb14O37 nanowires (Figure 4a). The current (i) obeys the following power-law relationship with the sweep rate (v): i = avb,60 where both a and b are the discretionary constants. The b-value is between 0.5 and 1.0. In particular, the b-value of 0.5 indicates a completely diffusion-controlled process, whereas 1.0 represents a total capacitive-controlled process. As shown in Figure 4b, the b values for cathodic and anodic current peaks are 0.702 and 0.748, respectively, suggesting that the electrochemical process is controlled by pseudocapacitance. Based on the equation i(V)/v1/2 = k1v1/2 + k2,61 the current response at a fixed scan rate could be quantitatively divided into capacitive-controlled process (k1v) and diffusion-controlled processes (k2v1/2). As a result, the typical CV curves at 1.0 mV s−1 are displayed in Figure 4c. The capacitive contribution ratio is calculated to be 86.72%. As depicted in Figure 4d, the percentage of capacitive contribution gradually increases from 56.78 to 86.72% with the scan rate increasing 22433


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Figure 6. (a) Charge/discharge curves for XPS study. (b) Nb 3d and (c) Zr 3d XPS spectra of ZrNb14O37 nanowires at different lithiated/ delithiated states (I: pristine state; II: discharge to 1 V; III: charge to 3 V). The ex situ TEM and HRTEM images of the ZrNb14O37 nanowires at different states for (d,e) pristine sample, (f,g) discharge to 1 V, and (h,i) charge to 3 V.

from 0.1 to 1.0 mV s−1. In order to better understand the electrochemical kinetic process, the diffusion coefficient of the lithium ions (DLi+) is calculated based on the CVs at various scan rates in Figure 4a. Figure S9 presents cathodic and anodic peak currents (ip) versus the square root of scanning rates plots for the electrodes. DLi+ can be calculated based on classical Randles−Sevcik equation (eq S1)50,62 and the slope value obtained from Figure S9. The DLi+ of ZrNb14O37 nanowire is calculated to be 4.63 × 10−14 cm2 s−1. It is higher than that of some reported niobium-based materials, such as Ti2Nb10O29 (1.55 × 10−15 cm2 s−1),28 TiNb6O17 (4.28 × 10−14 cm2 s−1),20 MoNb12O33 (4 × 10−14 cm2 s−1),63 and TiNb2O7 (1.05 × 10−15 cm2 s−1).8 However, it is lower than that of GaNb49O124 (7.92 × 10−14 cm2 s−1)64 and AlNb11O29 (1.12 × 10−13 cm2 s−1)65 in previous reports. To grasp insight into the fundamental mechanism, the in situ XRD technique is employed to monitor the detail structural evolution of ZrNb14O37 in real time during the lithium insertion and extraction process. Figure 5a schematically illustrates an in situ XRD system for investigation, which is composed of XRD equipment and a LAND CT2001A battery tester (see more in Supporting Information). A homemade in situ cell is developed with Be window for characterization, which is described in detail in precious reports.66,67 The in situ XRD data are obtained during the first cycle at 35 mA g−1 in the voltage window of 1−3 V with 2θ of selected regions ranging from 20° to 50°, and the corresponding XRD patterns are depicted in Figures 5b−d and S10. Before cycling, several main peaks are located at 23.25°, 24.46°, and 31.75°, which are assigned to the (−111), (111), and (114) crystallographic planes of ZrNb14O37. Taken the (−111) reflection as a reference, the phase evolution of ZrNb14O37 can be described. With the in situ cell discharged to 1.61 V, the (−111) peak slowly moves to a lower angle and decreases in intensity, indicating the lithium ion insertion into ZrNb14O37 and the formation of intermediate LixZrNb14O37 (“x” is derived from the Li-storage capacity based on the eq S2). Upon further discharging to 1.55 V, this reflection gradually disappears and a new reflection at 22.4° generates, which continually shifts to lower angles and increases in intensity after a full discharge to 1 V. When the in situ cell is fully charged to 3 V, this characteristic peak nearly migrates back to original Bragg position and re-obtains the relative intensity, suggesting a high structure stability and reversibility.

According to the main peak behaviors from in situ XRD patterns, the discharge process can be divided into three different states by the reaction mechanism.11,21 The state I (3− 1.61 V) is a solid-solution reaction region, with 5.25 lithium ion insertion into per formula unit of ZrNb14O37. In this state, the obtained capacity is 70.9 mA h g−1, which can be assigned to the formation of intermediate Li5.25ZrNb14O37. With the voltage from 1.61 V slowly decreasing to 1.55 V (state II), a two-phase coexistence reaction takes place and extra 2.87 lithium ions insert into the octahedral framework structure, suggesting the phase transformation from Li5.25ZrNb14O37 to Li8.12ZrNb14O37. In the state III (1.55−1 V), the second solidsolution reaction occurs and the electrode provides a capacity of 173.4 mA h g−1, indicating that 12.84 lithium ions insert in the per formula unit and Li20.96ZrNb14O37 phase forms. In summary, the discharge process is a complex reaction consisting of two solid-solution reactions and a two-phase coexistence reaction. Figure 5e illustrates the evolution of lattice parameters of a, b, c and V during the initial lithiation/ delithiation process. The original lattice parameters (a, b, and c) of ZrNb14O37 nanowires are 20.94964 (197), 3.82223 (30), and 19.30021 (167) Å before cycle, respectively. The volume is calculated as 1457.045 (218) Å3. With the lithium ion insertion into the ZrNb14O37, the value of a decreases while the values of b and c increase, leading to the unit-cell volume expanding to 1581.72 Å3. With the lithium ion extraction from the framework, the lattice parameter presents an expansion along the x-axis and a simultaneous contraction along the y- and zaxis, resulting in a decreasing in unit-cell volume. In the lithiation process, the whole volume expansion of ZrNb14O37 is 8.56%. This value is lower than the 17.5% of TiNb24O6210 and close to the TiNb2O7 (7.22%)68 and W5Nb16O55 (∼8.3%),25 indicating the good cycling stability of ZrNb14O37. To confirm the results of in situ XRD, a series of ex situ measurements are conducted to provide more lines of evidence about the reaction mechanism under different lithiation/ delithiation states. The XPS, TEM, and HRTEM observations are carried out to analyze products at different states of the charge/discharge process. Figures 6b,c and S11 present the XPS spectra of ZrNb14O37 nanowires. As depicted in Figure 6b(I), two peaks of the pristine sample are located at around 209.88 and 207.11 eV, corresponding to Nb 3d3/2 and Nb 3d5/2, respectively. Figure 6c(I) illustrates a pair of peaks at 184.57 and 182.21 eV, which should be attributed to Zr 3d3/2, 22434


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Figure 7. (a) Schematic illustration of the ZrNb14O37//LiCoO2 full cell. (b) Typical charge/discharge curves of initial three cycles of the ZrNb14O37//LiCoO2 full cell (the inset shows the CV profile for the initial cycle at a sweeping rate of 0.1 mV s−1). (c) Cycling performance of the full cell at 100 mA g−1. (d) Rate performance of the full cell.

situ XRD investigation, confirming the high structural reversibility of ZrNb14O37. The ZrNb14O37 nanowires demonstrate outstanding lithium storage properties, including a high capacity, high safety voltage, excellent rate performance, and long cycling performance. The excellent electrochemical performance motivates us to construct a full cell to evaluate the feasibility of ZrNb14O37 nanowires as an anode in possible applications. As presented in schematic illustration (Figure 7a), commercially available LiCoO2 is selected as a cathode to couple with the ZrNb14O37 nanowire anode. To achieve good electrochemical performance, the mass ratio of LiCoO2 cathode to ZrNb14O37 nanowires anode is fixed at 1.8:1. The LiCoO2 cathode is charged to 4.2 V and the ZrNb14O37 anode is discharged to 1 V in the half cell for half a cycle in order to pre-lithiate the electrode. Then, the half cells are opened and the electrodes are taken out for full cell assembly. The full cell delivers a capacity of 103.3 mA h g−1 at 100 mA g−1 with a Coulombic efficiency of 98.16%, and the initial three cycles almost overlap with each other (Figure 7b). In order to further investigate the electrochemical performance of the full cell system, CV measurement is carried out. A peak at 2.34 V during the anodic scan is assigned to the lithium ion extraction from the LiCoO2 cathode and the reduction of the Nb element in the ZrNb14O37 anode. In the cathodic scan, a reversible peak at 2.17 V is observed, which is related to re-intercalation of lithium ions in the LiCoO2 cathode. Figure 7c illustrates the long-life performance of the full cell. The full cell can retain a capacity of 84.0 mA h g−1 with a capacity retention of 81.3% after 100 cycles. The rate performance of the full cell at various current rates was also evaluated (Figure 7d). The full cell can achieve a high capacity of 99.56 mA h g−1 at the current density of 100 mA g−1. It still can deliver a capacity of 71.92 mA h g−1 at the current density of 700 m A g−1. With the current density back to 100 mA g−1, it still can retain a capacity of 87.72 mA h g−1, suggesting the good rate capability of the full cell. It suggests that the full cell has good cycling performance during the lithiation/delithiation process. For demonstration purposes, after fully charged, the full cell can be

and Zr 3d5/2, respectively. When the cell is deeply discharged to 1 V, an obvious Li 1s peak occurs at 56.36 eV (Figure S11b(II)), as a result of the continual lithium ion insertion reaction. The characteristic peaks of Nb4+ can be obviously observed at 208.51 and 205.58 V in Figure 6b(II), indicating the reduction from Nb5+ to Nb4+. Additionally, the existence of featured peak of Nb3+ (204.20 eV) in the lithiated product is associated with partial transformation from Nb4+ to Nb3+. The redox couples (Nb5+/Nb4+ and Nb4+/Nb3+) are similar to those in the previous reports of this process in other niobiumbased oxides, such as Cr0.5Nb24.5O62,69 Mg2Nb34O87,70 and TiNb2O7.71 However, as shown in Figure 6c, there is no obvious change in the Zr 3d peaks in the lithiation process, which indicates that all the Zr4+ ions in the sample do not take part in the reduction reaction during the discharging process.6 At the fully charged state of 3 V, the peak for Li 1 s still can be observed at 55.56 eV with weaker signals (Figure S11b(III)), suggesting few distributions of lithium atoms in the octahedral framework structure of ZrNb14O37. This accounts for the initial Coulombic efficiency is lower than 100%, which can be found in previous work on niobium-based compounds.16,72,73 With the extraction of lithium ions, the featured peak of Nb5+ recovers its original position during the delithiation process (Figure 6b(III)). From the state II to III, the Nb3+ peak decreases and finally disappears. However, a weak peak for Nb4+ still can be observed in Figure 6c(III). This phenomenon further implies that the extraction of lithium ions is incomplete. Figure 6d−i illustrates the structural details of ZrNb14O37 during the lithiation/delithiation process from ex situ TEM and HRTEM observation. At the discharge state of 1 V, the interplanar spacing of 0.283 nm (Figure 6g) is matched to the (−5−12) plane of ZrNb14O37. It is larger than the standard value as a consequence of lithium ion insertion. At the fully charged state of 3.0 V, the interlayer distance for the (−5−12) plane of ZrNb14O37 (Figure 6i) is examined to 0.279 nm on account of lithium ion extraction from the octahedral framework structure, suggesting that the ZrNb14O37 has a high structural stability during the charge/discharge process. The HRTEM patterns are consistent with the result from in 22435



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used to light up commercial light-emitting diode bulbs (Figure S12), indicating its potential application as the LIB anode.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05841. Detailed experimental data including synthesis, characterization techniques, and electrochemical evaluations; diffusion coefficient calculation; theoretical capacity calculation; XRD pattern; crystal structure; SEM images; cycle performance; charge/discharge curves; rate performance; in situ XRD patterns; XPS spectra and possible applications of ZrNb14O37 nanowires and bulk ZrNb14O37; and refined structure parameters, atomic fractional coordinates, and displacement factors of ZrNb14O37 nanowires (PDF)

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REFERENCES

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4. CONCLUSIONS In conclusion, for the first time, ZrNb14O37 nanowires with a nanoparticle-in-nanowire architecture are fabricated via a simple electrospinning technology. As the anode material for LIBs, the obtained ZrNb14O37 nanowires exhibit excellent lithium storage performance. The ZrNb14O37 nanowires have a safety voltage platform and deliver a high capacity of 244.9 mA h g−1. In addition, it exhibits high rate capability and excellent cycle performance with 0.026% of capacity fading per cycle upon 1000 cycles. The excellent electrochemical performance may arise from the nanostructure construction and stabilization of the structure. The nanostructure construction provides larger contacting areas and shorter transportation paths for lithium ions and electrons, enhancing the electrochemical performance. The framework of ZrNb14O37 maintains the structural stability with small volume changes (8.56%) during the insertion and extraction process, which is evidenced and demonstrated by the in situ XRD investigation. Moreover, the lithium storage reaction mechanisms are unveiled by in situ XRD investigation, which are further confirmed by ex situ XPS and TEM characterizations. In addition, coupled with the commercial LiCoO2 as the cathode in the full cell, the ZrNb14O37 nanowires enable the full cell to exhibit excellent electrochemical performance. This work provides a useful guidance for exploration of anode materials for high-performance LIBs.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jie Shu: 0000-0002-2326-6157 Author Contributions †

Y.L. and R.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is sponsored by NSAF (U1830106) and K .C. Wong Magna Fund from Ningbo University. 22436


Research Article

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Observation of ZrNb14O37 Nanowires as a Lithium Container via In

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