Granular molybdenum dioxide precipitated on N-doped carbon nanorods with multistage architecture for ultralong-life sodium-ion batteries

Electrochimica Acta 325 (2019) 134903

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Granular molybdenum dioxide precipitated on N-doped carbon nanorods with multistage architecture for ultralong-life sodium-ion batteries Fanyan Zeng a, *, Leyan Yang a, Yang Pan b, **, Meng Xu a, Hongyan Liu a, Maohui Yu a, Manman Guo a, ***, Cailei Yuan a a

Jiangxi Key Laboratory of Nanomaterials and Sensors, Jiangxi Key Laboratory of Photoelectronics and Telecommunication, School of Physics, Communication and Electronics, Jiangxi Normal University, Nanchang, 330022, Jiangxi, China College of Life Science, Jiangxi Normal University, Nanchang 330022, Jiangxi, China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 July 2019 Received in revised form 27 August 2019 Accepted 16 September 2019 Available online 19 September 2019

Developing high-performance anode materials is a crucial research target of sodium-ion batteries (SIBs). Transition metal oxides (TMOs) have attracted great interest as potential anodes, but their applications are still hindered by slow reaction kinetics and large volume changes. Herein, Mo-aniline nanorods (MoANRs) are prepared as precursors by a simple self-polymerized method in acid condition. After the in-situ phase transformation during annealing, multistage composites (N-CNRs@g-MoO2) are formed, with Ndoped carbon nanorods (N-CNRs) converted from polymeric aniline ligands, on which granular molybdenum dioxide (g-MoO2) are uniformly precipitated and residual MoO2 nanodots are remained. As anode materials for SIBs, N-CNRs@g-MoO2 electrode is benefited from the shortened ion/electron diffusion length caused by steady g-MoO2 and residual nanodots, and the enhanced electrical conductivity and relieved volume changes introduced by N-CNRs and unique architecture. Thus, N-CNRs@g-MoO2 electrode delivers high discharge capacity (497.5 mAh g
1 at 0.05 A g
1), excellent rate performance and ultra-long cycling stability (165.6 mAh g
1 at 10.0 A g
1 after 12000 cycles), and 122% capacity retention is obtained at 1.0 A g
1 over 500 cycles even after the rate test. The significant enhancements in sodiumion storage are mainly attributed to the multistage architecture and synergistic advantages among MoO2 nanodots, N-CNRs and g-MoO2. These results indicate that the in-situ phase transformation route has great potential in constructing novel composites with unique architecture for high-performance SIBs. © 2019 Elsevier Ltd. All rights reserved.

Keywords: N-doped carbon nanorods Precipitates In-situ phase transformation Ultra-long cycling stability Sodium-ion batteries

1. Introduction To meet the growing energy demands, low carbon economy has become an inevitable trend for the sustainable development of human society. The innovation and efficiency of energy storage systems (ESSs) have captured worldwide attention in the past three decades [1e3]. Among various promising ESSs, lithium-ion batteries (LIBs) have been successfully applied in portable electronics and hybrid electric vehicles owing to their high energy densities, long cycling life and no memory effects. However, the wide

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (F. Zeng), [email protected] (Y. Pan), [email protected] (M. Guo). https://doi.org/10.1016/j.electacta.2019.134903 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

application of LIBs encounters great challenges because of the high cost and scarcity of lithium resources [4e6]. Recently, with similar reaction mechanism to LIBs, sodium-ion batteries (SIBs) have attracted intensive research interest in large-scale energy storage applications and presented overwhelming advantages in natural abundance and low cost. Since Naþ shows a larger radius (1.02 Å) and a heavier mass (22.99 g mol
1) than Liþ (radius 0.76 Å and mass 6.94 g mol
1), thus the key challenge faced by SIBs is to exploit suitable host materials for fast and stable sodium-ion storage [7,8]. Nevertheless, it has been found that many LIB anode materials are not suitable for accommodating Naþ because of the slow reaction kinetics and large volume changes during the insertion/extraction of Naþ, as well as the mismatching between Naþ and graphite [9e11]. Therefore, considerable efforts have been devoted toward exploring low-cost and high-performance anode materials for SIBs. Up to date, many kinds of potential anode materials for SIBs

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have been investigated, mainly including carbon materials [12e14], alloy materials [15,16], transition metal oxides/sulfides [17,18] and their composites [17e19]. Among various transition metal oxides, molybdenum dioxide (MoO2) is attracting significant attention as one promising candidate of high-performance anodes due to its high theoretical capacity, low electrical resistivity and affordable cost. Compared with the bulk counterparts, nanoscale MoO2 (nanoparticles and nanodots), with shortened ion/electron diffusion lengths and increased reaction kinetics, are considered as a preferred option to achieve enhanced anode performance [20,21]. Nevertheless, the practical applications of nanoscale MoO2 are still hindered by poor rate performance and inferior cycling stability caused by low electronic conductivity and large volume expansion. To address these problems, binding nanoscale MoO2 with carbon matrixes has become an effective strategy, which could construct many novel composites with unique architecture for highperformance SIBs. For instance, Feng et al. reported a powerful combined method to prepare the vertical graphene/MoO2 core/ shell arrays, which displayed high reversible capacity and stable cycling life for sodium-ion storage [17]. Qiu et al. fabricated the composites of ultrafine MoO2 nanoparticles in carbon matrix, which presented the excellent electrochemical performance for SIBs [22]. These results adequately demonstrate that carbon materials could effectively buffer volume changes and significantly improve electrical conductivity of nanoscale MoO2. Nitrogenous polymers are frequently-used carbon sources and nitrogen sources. In carbonizing process, the polymers could be broken down into carbon skeleton and lots of volatile species, resulting in the formation of N-doped carbon materials with various morphologies and abundant pores [23e25]. In addition, the polymers between organic ligands and metal ions, including metalorganic frameworks, are attracting great attention in electronics, batteries and electrocatalysis because of their tunable composition and diverse morphology [26,27]. Taking advantage of the periodic arrangement of organic ligands and metal ions, the polymeric organic ligands could be gradually decomposed into N-doped carbon materials and the metal ions are transformed into oxides by the in-situ phase transformation during calcining, in which N-doped carbon materials and metal oxides can be closely integrated through strong coupling interaction to form composites [28e30]. These composites used as anode materials would exhibit prominent synergistic effects, high electrical conductivity and excellent structural stability for energy storage. Furthermore, the incorporation of nitrogen-atoms into carbon lattice would introduce multiple advantages, such as enhancing mechanical stability, improving electrical properties and introducing defects or active sites, and further improve the electrochemical performance of carbon materials [31e34]. Hence, the polymers hold good prospects for acquiring high-performance anode materials for SIBs. In this work, Mo-aniline nanorods (Mo-ANRs) are prepared by a simple self-polymerized approach in acid condition. The nanorods are served as precursors of MoO2, nitrogen atoms and carbon materials. Under in-situ phase transformation during annealing, the aniline component is converted to the N-doped carbon nanorods (N-CNRs), and meanwhile, MoO2 nanodots are continuously precipitated, either growing into granular molybdenum dioxide on N-CNRs or remaining in N-CNRs, acquiring N-CNRs@g-MoO2 composites with multistage architecture. The uniform and steady g-MoO2 and MoO2 nanodots could not only obviously shorten the ion/electron diffusion length but also effectively buffer the volume changes during charge/discharge process, and N-CNRs work as the conductive substrates. Benefiting from the multistage architecture and synergistic advantages, N-CNRs@g-MoO2 composites as anode materials exhibit excellent electrochemical properties, which could be evidenced by high reversible capacity, excellent rate

performance, high capacity retention and ultra-long cycling stability. 2. Experimental sections 2.1. Preparation of N-CNRs@g-MoO2 composites All reagents were of analytical grade (Aladdin, China) and were used directly without further purification. Mo-aniline nanorods (Mo-ANRs) between MoO2þ 4 anions and aniline were prepared by a simple self-polymerized approach as reported in previous literatures [35,36]. In a typical synthesis, 4 g ammonium molybdate tetrahydrate ((NH4)6Mo7O24$4H2O) was fully dissolved in a 250 ml round-bottom flask containing 120 ml deionized water, and stirred for 30 min. Then 10 ml aniline was slowly added into the flask to form a settled solution with stirring for 30 min, and 20 ml 2.0 M HCl aqueous solution was dropwise added into above solution under stirring until white precipitates (Mo-ANRs) presented. After that, the flask was placed in an oil bath at 50  C with magnetic stirring for 12 h, and the Mo-ANRs were filtered, washed three times with distilled water, and dried under an oven at 80  C for 12 h. In the end, the Mo-ANRs were transferred into a tube furnace under argon (99.99%) at 500  C for 2 h to acquire N-CNRs@g-MoO2 composites. Moreover, MoOx-CNRs were prepared by the same process but the annealing process was carried out at 400  C for 2 h. 2.2. Materials characterization The crystal structure of the as-prepared samples was characterized by a X-ray diffraction (XRD, Bruker D8-Advanced) using Cu Ka radiation; the Raman spectra were recorded by a Raman spectroscopy (LABRAM-010); the Brunauer-Emmett-Teller (BET) and pore size distribution (PSD) were analyzed by a surface area analyzer at 77 K (Beckman Coulter SA-3100); the thermogravimetry (TG) and differential thermal analysis (DTA) were performed on a Netzsch STA 449F3 instrument in air at a heating rate of 10  C min
1; the surface morphologies were observed by a field-emission Scanning Electron Microscope (FE-SEM, ZEISS Merlin Compact); the microstructure was examined by a Transmission Electron Microscopy (TEM, JEOL JEM-2100); the elemental distribution was analyzed by an Energy Dispersive X-ray (EDX) Detector; the X-ray photoelectron spectroscopy (XPS) spectrum was tested by a Thermo Fisher Scientific spectrometer (Escalab 250Xi) with Al Ka X-ray source. 2.3. Electrochemical measurements The anode properties of the as-prepared samples for SIBs were evaluated by using CR2032-type coin cells. A homogeneous slurry of working electrodes was constituted by mixing active materials, acetylene black and poly (vinylidene fluoride) (PVDF) in a weight ratio of 8:1:1 using N-methyl-2-pyrrolidinone (NMP) as solvent. The slurry was spread on a copper foil (current collector) and dried at 100  C for 12 h, and the electrodes were cut into disks (ɸ12 mm) with a mass loading of 0.2e0.5 mg cm
2. The cells were assembled in an Ar-filled glove box with the moisture and oxygen below 0.5 ppm, with the glass fiber filter (Whatman GF/C) as separator, thin plates of pure sodium as reference/counter electrode, and 1 M NaClO4 in a mixture of ethylene carbonate and diethyl carbonate (1:1 Vol.%, EC: DEC) with 5.0% FEC as electrolyte. Cyclic voltammetry (CV) curves were performed using CHI660D electrochemical workstation between 0.005 and 3.0 V (vs. Naþ/Na) at the different scan rates, and electrochemical impedance spectroscopy (EIS) was operated on CHI660D in the frequency range of 100 kHz-10 mHz. Galvanostatic discharge-charge curves were acquired with a LAND

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CT2001A battery-test system at various current densities within the voltage window of 0.005e3.0 V.

3. Results and discussion The typical preparation process of granular molybdenum dioxide precipitated N-doped carbon nanorods (N-CNRs@g-MoO2) with abundant residual MoO2 nanodots are schematically summarized in Fig. 1. Briefly, Mo-amine chelates are first synthesized by a chelation reaction between MoO2þ 4 anions and aniline, and then the chelates are anisotropically grown into Mo-aniline nanorods (MoANRs) via the self-polymerization of aniline in acid condition [35,36]. As the precursors of MoO2, carbon materials and nitrogen atoms, Mo-ANRs are treated by a 2-h annealing process at 500  C. In the furnace, the polymeric aniline ligands are decomposed into NCNRs and volatile species, which pose enough pores and present constrictive widths in comparison with Mo-ANRs. Meanwhile, MoO2 nanodots are continuously precipitated from Mo-ANRs and grown into g-MoO2, and residual MoO2 nanodots are remained in N-CNRs, resulting in the formation of multistage architecture in NCNRs@g-MoO2 composites. The crystal structure of the as-prepared Mo-ANRs precursors, MoOx-CNRs and N-CNRs@g-MoO2 were confirmed by X-ray diffraction patterns. As shown in Fig. S1, the diffraction peaks of Mo-ANRs precursors could be well indexed to anilinium trimolybdate dehydrate (JCPDS no. 50e2402), and these strong peaks in the range of 5e15 suggest that Mo-amine chelates lead to the preferential growth in the one-dimensional (010) direction. For MoOx-CNRs, an obvious diffraction peak retained at 7.1 signifies the crystal structure with 1.26 nm d-values, which is peak shift of Mo-ANRs precursors at 6.5 . Although the crystal structure is remained, other diffraction peaks become weak or even disappear, indicating that the structure of precursors have been damaged and massive volatile species have been evaporated during calcining. The XRD pattern in Fig. 2a represents the crystal structure of NCNRs@g-MoO2. The characteristic diffraction peaks are well assigned to (-111), (
211), (
312), (031), (
102) and (231) of monoclinic MoO2 phase (JCPDS no. 32e0671), and the sharp and intense peaks indicate the high crystallinity. The characteristic peaks of carbon are not observed by the XRD pattern, but the amorphous carbon in N-CNRs@g-MoO2 composites can be identified by Raman spectrum in Fig. 2b. It is obvious that the spectrum shows typical characteristic bands of carbon at around 1358 cm
1 (D-band) and 1594 cm
1 (G-band), and the ID/IG ratio is calculated to be ~1.15, indicating the amorphous carbon in N-CNRs@g-MoO2 composites. Furthermore, the Raman spectrum displays the typical characteristic bands of MoO2, which present the vibrational modes of Mo]O stretching (989 and 816 cm
1), OeMoeO stretching

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(658 cm
1), OeMoeO bending (331 cm
1) and OeMoeO wagging (277 cm
1) [37,38]. Furthermore, N2 adsorption-desorption isotherms were obtained to analyze pore structure of Mo-ANRs precursors and NCNRs@g-MoO2 composites. As observed in Fig. S2a, Mo-ANRs precursors exhibit a weak type H4 hysteresis loop and a specific surface area of 13.7 m2g-1, and the gradual PSD curve in Fig. S2a (inset) implies the absence of mesopores. The isotherm and PSD curve of N-CNRs@g-MoO2 composites are displayed in Fig. 2c. An increased specific surface area (37.4 m2 g
1) is obtained compared with their precursors, which may be related to the formation of multistage architecture. It is noteworthy from Fig. 2c (inset) that N-CNRs@gMoO2 composites have centered PSDs in 3.6 nm and 27.2 nm, significantly different from Mo-ANRs precursors. The pore size in 3.6 nm could be attributed to the formation of microstructure in NCNRs after massive small-molecule volatilization, and the pore size in 27.2 nm could be concerned with the interval among the nanorods and their clusters. Thermogravimetric analyses of the as-prepared samples were carried out in air to evaluate the contents of Ndoped carbon and MoO2. From the curve of N-CNRs@g-MoO2 composites in Fig. 2d, the initial 8.8 wt.% mass lost below 180  C can be ascribed to the evaporation of adsorbed water, and the following obvious decrease in mass (15.4 wt.%) between 225  C and 450  C could be related to the oxidation of N-doped carbon, which is confirmed via the corresponding DTA curve at the peak of 411.3  C. The content of MoO2 in the composites is calculated to be 75.8 wt.%. In comparison, the curve of precursors is shown in Fig. S2b. The mass decrease about 22.5 wt.% may be attributed to the phase transformation of MoO2þ 4 anions to MoO2, the decomposition of polymeric aniline ligands to N-doped carbon and the oxidation of N-doped carbon, which are respectively verified by the corresponding DTA curve at the three peaks of 245.0, 334.6 and 411.3  C. These results in Fig. 2 suggest that N-CNRs@g-MoO2 composites with unique porous structure are constituted of amorphous Ndoped carbon (15.4 wt.%) and well-crystallized MoO2 (75.8 wt.%). The morphological details of Mo-ANRs precursors, MoOx-CNRs and N-CNRs@g-MoO2 composites were observed by FE-SEM. As displayed in the SEM images at different resolutions (Figs. S3ae3c), the Mo-ANRs precursors present a smooth and uniform rod-like morphology with 80e150 nm in width and several micrometers in length. After calcining at 400  C, the precursors are transformed into MoOx-CNRs (Figs. S3ee3f), whose widths are narrower than that of precursors owing to the constriction of nanorods and the evaporation of massive volatile species. And MoOx-CNRs are concentrated into clusters with a remarkable increase in length, possibly because the nanorods are cross-linked and integrated by an end-to-end approach on their surfaces. When the carbonized temperature is set at 500  C, the precursors are further transformed

Fig. 1. Schematic illustration for the preparation of N-CNRs@g-MoO2 composites with abundant residual MoO2 nanodots from Mo-ANRs precursors.

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Fig. 2. (a) XRD pattern, (b) Raman spectrum, (c) Nitrogen absorption-desorption isotherm and PSD curve (inset), and (d) TG and DTA curves of N-CNRs@g-MoO2 composites.

into N-CNRs@g-MoO2 composites, and their SEM images are displayed in Fig. 3. It can be observed that N-CNRs@g-MoO2 composites not only exhibit similar shapes to those of MoOx-CNRs, but

also possess abundant g-MoO2 on the surface of N-CNRs. At a high magnification (Fig. 3d), it is obvious that g-MoO2 are tightly anchored on the N-CNRs.

Fig. 3. SEM images of N-CNRs@g-MoO2 composites at different magnifications.

F. Zeng et al. / Electrochimica Acta 325 (2019) 134903

To illustrate the detailed microstructure of Mo-ANRs precursors and N-CNRs@g-MoO2 composites, TEM analyses were carried out. The images of Mo-ANRs precursors in Fig. S4 further verify the rodlike structure with 80e150 nm in width and several micrometers in length. For N-CNRs@g-MoO2 composites, Fig. 4a shows the low magnification TEM image, which exhibits that 20-60 nm-sized gMoO2 are tightly anchored on the surface of N-CNRs. The high resolution (HR) TEM image (Fig. 4b) further reveals that the nanoparticle on N-CNR is well crystallized, and the distance between two adjacent lattice fringes is 0.34 nm, which is consistent to the (
110) lattice plane of monoclinic MoO2. From the HR-TEM image of Fig. 4c, abundant MoO2 nanodots are observed in the body of N-CNRs. And obvious (
110) lattice plane of MoO2 nanodot is exhibited in Fig. 4d, indicating that not all nanodots are precipitated into nanoparticles. The selected area electron diffraction (SAED) pattern in Fig. 4e shows the distinct polycrystalline diffraction rings of N-CNRs@g-MoO2 composites, revealing the (
110), (
210) and (
312) crystal plane from inside to outside. The EDX spectroscopy results are displayed in Fig. 4f and g. The EDX spectrum (Fig. 4f) clearly confirms the presence of C, N, O and Mo elements in N-CNRs@g-MoO2 composites, and the HAADF-STEM image and corresponding element mappings (Fig. 4g) indicate that C, N, O and Mo elements are uniformly distributed in the composites. These results demonstrate that N-CNRs@g-MoO2 composites with the multistage architecture are consisted of MoO2 nanodots, N-CNRs and g-MoO2. The chemical composition and surface electronic state of NCNRs@g-MoO2 composites were analyzed by XPS measurement. Clear peaks of C 1s and N 1s can be seen from the survey spectrum

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displayed in Fig. 5a, which could be ascribed to the N-doped carbon materials, and Mo 3d, Mo 3p and O 1s are assigned to the MoO2 phase. The C 1s peak was further examined by the high-resolution (HR) XPS in Fig. 5b. The strong peaks at 384.6 and 385.3 eV are associated with the characteristic of the CeC and CeN bond, respectively, indicating that abundant nitrogen atoms are introduced into carbon nanorods, and the weak peak at 387.4 eV can be attributed to the residual oxygen-containing bonds CeO. The HR spectrum of N 1s peak (Fig. 5c) can be fitted into four peaks, among which the three peaks at 400.4, 398.7, 397.3 eV correspond to the quaternary, pyrrolic and pyridinic type N atoms, respectively, and the peak at 395.5 eV is related to MoeN bond formed by the reaction between MoO2 nanodots and nitrogen atoms [39,40]. In the O 1s spectrum (Fig. 5d), the peaks located at 530.7 and 532.4 eV are assigned to the oxygen (MoeO) in MoO2 and the residual oxygencontaining functional groups (CeO) in the N-CNRs, respectively. Fig. 5e shows the HR spectrum of Mo 3d peak, which can be divided into four peaks. Characteristic peaks at 229.6 and 232.8 eV correspond to the binding energies of Mo4þ 3d5/2 and Mo4þ 3d3/2, which are attributed to the oxidation state of the tetravalent Mo in MoO2. The other two peaks at 230.6 and 235.9 eV are related to Mo6þ 3d5/2 and Mo6þ 3d3/2, implying a slight surface oxidation on MoO2 [17,22,41]. Based on these results, it is evidenced that N-CNRs@ gMoO2 composites are composed of N-doped carbon materials and MoO2. Sodium-ion storage properties of N-CNRs@g-MoO2 electrode were characterized by the cyclic voltammetry and galvanostatic discharge-charge curves in the voltage range of 0.005e3.0 V (vs Naþ/Na). The first three CV and galvanostatic discharge-charge

Fig. 4. (a) TEM image, (bed) HR-TEM images, (e) SAED pattern, (f) EDX spectrum, and (g) HAADF- STEM image and corresponding element mapping results of N-CNRs@g-MoO2 composites.

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Fig. 5. XPS spectra of N-CNRs@g-MoO2 composites: (a) survey spectrum and HR spectrum of (b) C 1s, (c) N 1s, (d) O 1s, and (e) Mo 3d.

curves of MoOx-CNRs electrode are shown in Fig. S5. It is obvious that MoOx-CNRs electrode shows low CV currents (Fig. S5a) at the scan rate of 0.2 mV s
1, and displays a low reversible capacity of 19.3 mAh g
1 and a fast capacity loss at 0.05 A g
1 in Fig. S5b. These data could be attributed to the deficiency of stable crystal structure and the residual of organic components in MoOx-CNRs. As seen in Fig. 6a, CV currents of N-CNRs@g-MoO2 electrode are significantly improved at the same voltages and cycles as MoOx-CNRs electrode. Moreover, during the initial cathodic sweep, the broad reduction peak at around 0.95 V could be assigned to the inevitable formation of solid electrolyte interface (SEI) film due to the decomposition of electrolyte on electrode surface and the reversible sodiation reaction between MoO2 and Naþ to form metallic Mo and Na2O, and the sharp reduction peak between 0.005 and 0.35 V may be due to the sodiation of N-CNRs. In the subsequent anodic sweep, a broad peak between 0.59 and 2.05 V is observed, which is attributed to the desodiation reaction of Na2O and the oxidation of metallic Mo to MoO2 [17,20,22]. In the following scans, the disappeared reduction peak at 0.95 V indicates the formation of SEI layers, and the overlapped CV curves suggest that N-CNRs@g-MoO2 electrode acquires high electrochemical reversibility for SIBs. The representative initial three galvanostatic discharge-charge curves (Fig. 6b) of N-CNRs@g-MoO2 electrode were measured at 0.05 A g
1 in the voltage range of 0.005e3.0 V (vs. Naþ/Na). As seen in the first discharge curve, a voltage plateau is located at about 1.26 V, corresponding to the formation of SEI film on electrode surface and the reversible sodiation reaction between MoO2 and Naþ. In the subsequent charge curve, the plateau at around 0.90 V could be attributed to the desodiation of the electrode. The result is in good agreement with the CV curves in Fig. 6a. Furthermore, NCNRs@g-MoO2 electrode delivers a high initial discharge capacity of 775.7 mA h g
1 and a large reversible charge capacity of 448.6 mA h g
1 with an initial coulombic efficiency of 58.6%. The irreversible capacity loss is ascribed to the formation of SEI film as well as the decomposition of the electrolyte, which are similar to other anode materials based on transition metal oxides and carbon materials [17,21,22]. In the following two cycles, the efficiency

increases to near 100%, implying quick acquisition of electrode stability after the formation of SEI film. The rate capability of N-CNRs@g-MoO2 electrode was evaluated via the increasing current rates from 0.05 to 10.0 A g
1, for 10 cycles at each rate. As shown in Fig. 6c, the reversible capacities are stable with a slight declining trend in the first several cycles at various rates. After that, the current rates are returned from 10.0 to 0.1 A g
1, and then maintained in 1.0 A g
1 for 500 cycles. When the current rates are 0.05, 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 A g
1, the corresponding discharge capacities are 497.5, 472.3, 453.4, 402.5, 340.2, 270.3, 210.6 and 156.1 mAh g
1, respectively. When the current rates are decreased from 5.0 to 0.1 A g
1, the corresponding reversible capacities are still stable and increase regularly, implying the high repeatability of rate capability. After that, N-CNRs@gMoO2 electrode displays a gradual increasing trend in reversible capacity at 1.0 A g
1 for 500 cycles, and a high capacity retention of 122 % are obtained, which could be ascribed to the reversible growth of a polymeric gel-like film and the adequate activation of electrode material during the cycling process [42,43]. It is speculated that the multistage architecture and synergistic advantages of N-CNRs@g-MoO2 electrode could facilitate electron transport/ion diffusion and relieve volume changes for sodium-ion storage, leading to superior rate capability for SIBs. Long-term cycling performance of N-CNRs@g-MoO2 electrode (Fig. 7) was investigated under the constant current rates of 1.0, 2.0 A g
1 and 5.0, 10.0 A g
1 after activation and stabilization at a low current rate of 0.1 A g
1 for 10 cycles. Fig. 7a shows the cycling performance at 1.0 and 2.0 A g
1. It is obvious that the reversible capacities are respectively stabilized at 343.2 mAh g
1 for 1.0 A g
1 and 301.3 mAh g
1 for 2.0 A g
1 after 1800 and 3000 cycles with the coulombic efficiencies of nearly 100%. Importantly, the electrode displays ultra-long cycling stability and high capacity retention at high current rates presented in Fig. 7b. The reversible capacities are 218.2 mAh g
1 at 5.0 A g
1 after 5000 cycles and 165.6 mAh g
1 at 10.0 A g
1 after 12000 cycles with the coulombic efficiency of near 100%, respectively. It is worth noting that N-CNRs@g-MoO2 electrode exhibits a gradual increasing trend in the reversible capacities

F. Zeng et al. / Electrochimica Acta 325 (2019) 134903

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Fig. 6. (a) The first three CV curves at 0.2 mV s
1, (b) initial three galvanostatic discharge-charge curves at 0.05 A g
1 and (c) rate performance at various current densities of NCNRs@g-MoO2 electrode.

Fig. 7. Long-term cycling performance of N-CNRs@g-MoO2 electrode at (a) 1.0, 2.0 A g
1 and (b) 5.0, 10.0 A g
1.

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at the 1.0, 2.0 and 5.0 A g
1, implying the reversible growth of a polymeric gel-like film on the electrode [42,43]. Compared with other Mo-based materials, sodium-ion storage properties of NCNRs@g-MoO2 electrode are excellent (Table S1). As described above, the ultra-long cycling stability of N-CNRs@g-MoO2 electrode can be ascribed to the steady precipitated nanoparticles and residual nanodots in the conductive N-CNRs, which could provide stable architecture and relieve volume changes for sodium-ion storage. The electrochemical impedance spectroscopy (EIS) was implemented to analyze the reaction kinetics of N-CNRs@g-MoO2 electrode. Fig. S6 displays the Nyquist plots of electrode before and after cycles. In these curves, the semicircle in high-middle frequency region and the straight line in low frequency region are respectively assigned to charge-transfer resistance and Warburg impedance, and small Warburg impedance corresponds to the fast ion diffusion in solid phase [44,45]. Before cycling, N-CNRs@g-MoO2 electrode exhibits a satisfying electrochemical behavior. With the increase of cycle number, there is an obvious increase in diameter of highmedium frequency region, meaning the increasing charge-transfer resistance, which can be attributed to the continuous deposition of SEI film on the electrode surface. Moreover, after 3000 cycles, NCNRs@g-MoO2 electrode exhibits a larger slope than that of the electrode after 3 cycles in the low-frequency region, indicating the Warburg impedance is gradually decreased. This can be related to the gradual activation of electrode during charge/discharge process. These results suggest that N-CNRs@g-MoO2 composites can provide high electrochemical activities and excellent structural stability, and synergistically improve sodium-ion storage

performance. In order to better insight into the electrochemical reaction mechanism of N-CNRs@g-MoO2 electrode for sodium-ion storage, the CV curves were obtained at various sweep rates ranging from 0.2 to 1.6 mV s
1 and exhibited in Fig. 8a. These curves present similar shapes, indicating the high electrochemical stability. The pseudocapacitive contribution or diffusion behavior could be quantified by linear relationship between log i and log v at redox peaks, and the relationship is derived from the equation: i ¼ anb, where a and b are both variable parameters. The b-value between 0.5 and 1.0 is the slope of log i - log v plot. A b-value close to 0.5 signifies diffusion-controlled behavior, whereas a 1.0 b-value implies pseudocapacitive behavior [46e48]. Fig. 8b displays the log i log v plot of N-CNRs@g-MoO2 electrode, and the b values are calculated to be 0.75 for both cathodic (0.48 V) and anodic peak (0.75 V), implying the partial pseudocapacitive behavior of NCNRs@g-MoO2 electrode. The pseudocapacitive behavior is beneficial to promote the reaction kinetics at high rates for excellent rate capability [49,50]. Furthermore, the ratio between pseudocapacitive contribution and diffusion behavior can be quantitatively analyzed by the following equation [51,52]:

iðVÞ ¼ k1 v þ k2 v1=2 ; where k1 and k2 are constants for a given potential, and the k1v and k2v1/2 are related to the capacitive- and diffusion-controlled contribution, respectively. According the equation, it is calculated that 50.4% of the total capacity (Fig. 8c) is ascribed to the capacitive kinetics at 1.0 mVs
1 (shaded regions). As presented in Fig. 8d, the

Fig. 8. Kinetic analysis of N-CNRs@g-MoO2 electrode as anode for SIBs: (a) CV curves at various scan rates from 0.2 to 1.6 mV s
1, (b) linear relationship between log i and log v at the redox peaks, (c) capacitive ratio (shaded regions) at 1.0 mV s
1 and (d) capacitive contribution ratios at different scan rates.

F. Zeng et al. / Electrochimica Acta 325 (2019) 134903

calculated capacitive contributions increase gradually from 31.5 to 56.7% with the increasing scan rates from 0.2 to 1.6 mV s
1, implying that the pseudocapacitive behaviors dominate half of whole capacity at high sweep rates. These results could be ascribed to the multistage architecture and synergistic advantages of NCNRs@g-MoO2 electrode, which can provide more extrinsic capacitive sites on the electrode surface or near surface. Thus, the conclusion drawn is that the partial pseudocapacitive behavior plays an important role in enhancing rate capability. 4. Conclusions In conclusion, N-CNRs@g-MoO2 composites are fabricated via a simple self-polymerized method and a subsequent calcination process. The composites with multistage architecture are made up of MoO2 nanodots, N-CNRs and g-MoO2. As anode materials for SIBs, N-CNRs@g-MoO2 benefiting from the multistage architecture could effectively shorten ion/electron diffusion length, buffer volume changes and enhance electrical conductivity of electrode. These advantages could synergistically enhance the reversible capacity, rate capability and cycling stability. Importantly, N-CNRs@gMoO2 electrode exhibits ultra-long cycling stability and high capacity retention at high current rates. The reversible capacities are 218.2 mAh g
1 at 5.0 A g
1 after 5000 cycles and 165.6 mAh g
1 at 10.0 A g
1 after 12000 cycles, and a 122% capacity retention is obtained at 1.0 A g
1 over 500 cycles even after different cycles. These results are significant to inspire the design and fabrication of advanced electrode materials based on transition metal oxides and carbon materials for high-capacity, high-rate and long-life SIBs. Conflicts of interest The authors declare no competing financial interest. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant no. 51562010, 51762021, 61561026), Natural Science Foundation of Jiangxi Province of China (Grant No. 20192BAB213019, 20192ACB21009, 20161BAB216119). Science & Technology Project of Jiangxi Provincial Education Department (Grant No. GJJ160345, GJJ160303). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134903. References [1] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonz alez, T. Rojo, Na-ion batteries, recent advances and present challenges to become low cost energy storage systems, Energy Environ. Sci. 5 (2012) 5884e5901. [2] D. Larcher, J. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2014) 19e29. [3] L. Li, Z. Wu, S. Yuan, X. Zhang, Advances and challenges for flexible energy storage and conversion devices and systems, Energy Environ. Sci. 7 (2014) 2101e2122. [4] B. Scrosati, J. Hassoun, Y. Sun, Lithium-ion batteries. A look into the future, Energy Environ. Sci. 4 (2011) 3287e3295. [5] C. Jiang, E. Hosono, H. Zhou, Nanomaterials for lithium ion batteries, Nano Today 1 (2006) 28e33.  , P. Bruce, B. Scrosati, J. Tarascon, W. van Schalkwijk, Nanostructured [6] A.S. Arico materials for advanced energy conversion and storage devices, Nat. Mater. 4 (2005) 366e377. [7] J. Hwang, S. Myung, Y. Sun, Sodium-ion batteries: present and future, Chem. Soc. Rev. 46 (2017) 3529e3614. [8] S. Kim, D. Seo, X. Ma, G. Ceder, K. Kang, Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithium-ion batteries,

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