Superior Sodium Storage of Carbon-coated NaV6O15 Nanotubes

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Energy, Environmental, and Catalysis Applications

Superior Sodium Storage of Carbon-coated NaV6O15 Nanotubes Cathode: Pseudocapacitance vs. Intercalation Xuexia Song, Jicheng Li, Zhaohui Li, Qizhen Xiao, Gangtie Lei, Zhongliang Hu, Yanhuai Ding, Hirbod Maleki Kheimeh Sari, and Xifei Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20494 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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ACS Applied Materials & Interfaces

Graphic abstract

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Superior Sodium Storage of Carbon-coated NaV6O15 Nanotubes Cathode: Pseudocapacitance vs. Intercalation Xuexia Songa,c,d,1, Jicheng Lia,1, Zhaohui Lia*, Qizhen Xiaoa, Gangtie Leia, Zhongliang Hub*, Yanhuai Dinge, Hirbod Maleki Kheimeh Saric,d, Xifei Lic,d* a

Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of

Education, College of Chemistry, Xiangtan University, Hunan 411105, P.R. China b

College of Metallurgy and Material Engineering, Hunan University of Technology, Hunan 412007, P.R. China c

Institute of Advanced Electrochemical Energy & School of Materials Science and

Engineering, Xi'an University of Technology, Xi'an, Shaanxi 710048, P.R. China d

Shaanxi International Joint Research Centre of Surface Technology for Energy Storage Materials, Xi'an, Shaanxi 710048, China e

Institute of Fundamental Mechanics and Materials Engineering, Xiangtan University, Hunan 411105, P.R. China

ABSTRACT To realize the effect of Na+ pseudocapacitance on sodium storage of cathode materials, clewlike carboncoated sodium vanadium bronze (NaV6O15) nanotubes (NaVBNT@C) were synthesized via a facile combined solgel/hydrothermal method. The resultant NaVBNT@C delivers high reversible capacities of 209 and 105 mAh g1 at the rates of 0.1C and 10C, respectively. Notably, at the higher rate of 5C (1250 mA g1), it can retain 94% of the initial capacity after 3000 cycles. It was found that the outstanding rate performance and longterm cycling life of NaVBNT@C is primarily due to the Na+ pseudocapacitance. Our study reveals that the design of Na+ pseudocapacitance is beneficial for harvesting superior performance of NaV6O15 cathode material in sodiumion batteries. KEYWORDS: vanadium bronze; sodiumion batteries; nanotube; pseudocapacitance; hydrothermal

1. Introduction 1 ACS Paragon Plus Environment

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Among the reported transition metal oxides (TMOs), layered V2O5 cathode material can deliver a theoretical capacity of 441 mAh g1 within the potential range of 1.54.0 V (vs. Li+/Li) in lithiumion batteries (LIBs).14 However, the unstable structure, weak conductivity and low ionic diffusion coefficient of V2O5 cause a sharp decrease in capacity upon cycling. In our previous study, the Mgdoped V2O5 nanoparticles embedded in carbon matrix presented an excellent electrochemical performance for LIBs5 because the doped Mg2+ ions pillared the layered structure to form a threedimensional (3D) structure.68 Nonetheless, the layered structure preintercalated by smaller cations such as Mg2+, Fe2+, Mn2+, Al3+ and Sn4+ does not favor de/intercalation of the large Na+ ions. Notably, chemical preintercalation of Na+ would significantly improve the sodiumstorage performance of the layered vanadium oxides. When 1/3 moles of Na+ ions are preintercalated per V2O5 unit, a sodium vanadium bronze NaV6O15 (i.e. βNa0.33V2O5) with twodimensional (2D) V4O12 layers connected by VO5 and edgesharing oxygen atoms can be achieved. The preintercalated Na+ ions between the V4O12 layers act as “pillars” to stabilize the structure upon insertion/extraction of Li+/Na+ ions. Although NaV6O15 has revealed high performance when used as the cathode in LIBs,9,10 its cyclability and rate capability are still unsatisfactory as the cathode in SIBs.1118 Thus, the carbon was coated on the surface of the NaV6O15 nanomaterials in order to further improve its electrical conductivity. Due to the synergistic effect of NaV6O15 and carbon coating, the NaV6O15@C composite materials with novel nanostructure would exhibit higher specific capacity and intensive cycle stability during cycling.The surface coating of electrode particles with a highly conductive substance (such as carbon) can increases the conductivity and facilitate the electron transportation in the electrode. Moreover, carbon acts as a bridge to connect the particles, as well as the protective shell layer to alleviate the volume change of electrodes, prevent the vanadium dissolution, and alleviate the aggregation of the particles during the charge and discharge processes, ensuring better cycleability during longterm charge/discharge.5,6,19 It is well known that nanomaterials possess shorter diffusion pathway and more intercalation sites than bulk materials. In particular, the electrode materials present a pseudocapacitive storage behavior during sodiation/desodiation processes when they have a structure consisted of nanoparticles integrated with carbon matrix.20 The resultant 2 ACS Paragon Plus Environment


pseudocapacitance is relative to a faradaic redox reaction that happens on/near the surface of the electrode materials; In consequence, the charge storage mechanisms are different from those in traditional batteries because they are not confined by the ion diffusion into the bulk material.21 In addition, pseudocapacitance can be divided into two types: intrinsic pseudocapacitance (such as RuO2, MnO2, Nb2O5, MoO3 and polyaniline) and extrinsic one (such as LiCoO2, TiO2 and V2O5). Compared with the diffusioncontrolled process, intercalation pseudocapacitance shows obvious advantages for batteries because it can occur within the bulk of electrode materials, providing batteries with faster charging/discharging property by reducing the solid state diffusion distance of sodium ions within the electrodes. Hence, it can lead to highpower and faster charging/discharging rates together with longterm cycling stability, and accordingly delay the degradation of electrode material.2124 For instance, Dunn and coworkers25 research showed great prospect towards highrate electrodes for LIBs propelled by an intercalation pseudocapacitive mechanism. Chen26, inspired by this, proved that intercalation pseudocapacitance predominates the charge storage progress in GTiO2 for SIB anodes, which shows excellent rate performance and longterm stability. In brief, the pseudocapacitancedominated contribution guarantees much faster and more stable Na storage performance.23 Unfortunately, all previously reported intercalation pseudocapacitances are focused on the capacitors,2730 and less attention has been drawn to the pseudocapacitance contribution in batteries. In our work, carboncoated NaV6O15 nanotubes with open ends are used as the cathode material for SIBs. The nanotubes with high crystallinity are intertwined with quasi spheres, and the carbon layer is homogeneously coated on the surface (Scheme 1). Owing to the opened terminates, the electrolyte could penetrate into the Nadoped nanotubes, rendering Na+ ions able to intercalate into the inner surface of nanotubes rapidly; while the coated carbon layer facilitates electrons passing through the outer surface of the nanotubes. As a result, the intercalationbased electrochemical reaction can occur rapidly. Moreover, excessive Na+ ions in nanotube and electrons in carbon layer simultaneously might lead to double layer capacitor at the interface, devoting additional capacity for the composite materials. More importantly, we demonstrate ultrafast, highly reversible, and cyclestable sodium storage, boosted by a pseudocapacitance contribution from the carboncoated NaV6O15 nanotubes. 3 ACS Paragon Plus Environment

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Indeed, developing a highly pseudocapacitive contribution for highperformance Naion batteries may be particularly significant for development of the nextgeneration electrochemical energy storage devices.

2.

Experimental

2.1 Synthesis of the carbon-coated Na-VBNTs Vanadium oxide (VOx) nanotubes were prepared according to the reported method.3135,41 Firstly, 1.82 g （0.01 mol）of V2O5 powder (AR., Xilong Chemical Reagent Company, China) was added slowly to 50 ml of H2O2 solution (30%, AR., Sinopharm Chemical Reagent Co. Ltd ), and stirred for 2h to form a sol. Subsequently, 7.42 g (0.04 mol) of dodecylamine (C12H25NH2, A.R., SigmaAldrich) was added to the sol, and agitated for 5h. After that, the sol was transferred to a Teflonlined stainless steel autoclave, and heated at 200 o

C for five days. The precipitates were obtained after centrifugation, and then washed with

distilled water and ethanol for three times. About 1.26 g of amineintercalated VOx powders was obtained after the precipitates were dried at 80 oC overnight under vacuum. For comparison, a part of VOx powders was separated and calcined at 450 oC at a ramping rate of 2 oC min1 in air for 3h to obtain the V2O5 nanotubes, designated as VONT. The calcination temperature was determined from the thermogravimetric analysis of the precursors (Figure S1a, supporting information). Secondly, 2.13 g (0.015 mol) of Na2SO4 and 0.91 g (0.005 mol) of the obtained VOx powder were dispersed in 50 ml of distilled water/ethanol (1:4 in volume) solution, and stirred for 24h. The mixture was then filtered, and the residue was washed with water and ethanol. When the washed residue was dried at 80 oC under vacuum for 24h, the Nadoped precursors were obtained, and then heated from room temperature to 450 oC at a heating rate of 2 oC min1, and held in air for 3h. After being cooled in muffle furnace, the sodium vanadium bronze nanotubes were acquired, designated as the NaVBNT. Finally, 0.50 g of NaVBNT powder was dispersed in a solution of 0.15 g sucrose in 10 ml deionized water and stirred for 1h, then heated at 80 oC until the water evaporated almost. After that, the dried powders were annealed at 400 oC for 0.5h in a tube furnace under argon atmosphere to get the carboncoated sodium vanadium bronze nanotubes, designated as the 4 ACS Paragon Plus Environment


NaVBNT@C. The mass percent of carbon was determined to be about 7.2% in the composite material through the TG analysis (Figure S1b) when the product was fired in air at a ramping rate of 10 oC min1 from 30 to 550 oC.

2.2 Materials characterization Thermogravimetric analysis (TGA) was measured using the TASDTQ 600 thermal analysis system (Texas Instruments, Inc., New Castle, DE) at a ramping rate of 10 oC min1. Scanning electron microscopy (SEM, JEOL JSM6510A) and transmission electron microscopy (TEM, JEOL JEM2010) were applied to observe the morphology of the sample. Xray diffraction (XRD) patterns were performed by a powder diffractometer (Bruker D8 Advanced) with a Ni ﬁlter at a sweep rate of 2° min1 in the 2θ range of 5°65° (Cu Kα, (λ = 0.154 nm) source). The nitrogen adsorptiondesorption isotherm was characterized using TriStar II 3020 instrument (Micromeritics Instrument Corp.), and the specific surface area was characterized by the BrunauerEmmettTeller (BET) method. Na and V contents in the samples were conducted by inductively coupled plasma atomic emission spectrometry (ICPAES, Profile spectrometer, Leeman Laboratories). Raman spectrum was collected using a RenishawInVia Raman Microscope. The valence state of the V element was analyzed by Xray photoelectron spectroscopy (XPS, Thermo Escalab 250XI).

2.3 Electrochemical characterization NaVBNT@C electrodes were prepared through mixing the asprepared NaVBNT@C powder, acetylene black (Super P) and PVDF (Knyar 2700) binder with the mass ratio of 80:10:10 at first. Nmethyl2pyrrolidone (NMP) was then added to the mixture and agitated to form slurry. The slurry was cast on aluminum foil and dried at 120 oC under vacuum overnight. Finally, the dried plate was punched into the discs with a diameter of 12 mm to creat the NaVBNT@C electrodes. The VONT and NaVBNT electrodes were fabricated similar to the NaVBNT@C electrodes. About 3.0 mg of the electroactive materials were loaded on each disc electrode. The tap densities were measured to 1.140±0.022, 1.161±0.018 and 1.025±0.020 g cm3 for the VONT, NaVBNT and NaVBNT@C samples, respectively. 5 ACS Paragon Plus Environment

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Cyclic voltammetry was conducted using the CHI660E electrochemical work station at various sweep rates over the voltage window of 4.01.5 V (vs. Na+/Na) using a twoelectrode system, in which the cathode acts as the working electrode and sodium plate as the counter and the reference electrodes. The cathode and Na electrode were separated by glass microfiber filter (GF/A, Whatman), immersed in the electrolyte 1 M NaClO4 in ethyl carbonate (EC)/dimethyl carbonate (DMC) (1:1 in weight), and sealed in CR2025type button cell. Electrochemical impedance spectroscopy (EIS) measurement was carried out by the same electrochemical workstation. Galvanostatic chargedischarge measurements were performed on the CT3008W Neware battery tester using the CR2025 cointype cells.

3.

Results and discussion

3.1 Morphology of the sample SEM images of the VONT, NaVBNT and NaVBNT@C samples are shown in Figure S2. It is found that the NaVBNT sample reveals the morphology of clewlike quasi spheres with a diameter range of 25 μm (Figure S2c), which is similar to that of VONT sample (Figure S2a,b). These spheres are constructed with onedimensional (1D) nanomaterials (Figure S2d) that are nanotubes in fact, as shown in Figure 1f. NaVBNT@C particles (Figure S2f) are larger than NaVBNT particles (Figure S2e), and some ultrafine particles are observed on their surfaces (Figure S2f). In addition, the porous structure of NaVBNT@C (Figure S2f) is different from that of NaVBNT (Figure S2d). The reason is that the carbon component generated from sucrose coated the nanotubes and filled the pores and made the clews integrate into larger particles as well. Evidently, the NaVBNT nanotubes display a length range of 0.3~2 μm (Figure 1a) and an outer diameter range of 15~30 nm (Figure 1b). Open ends of the nanotubes marked with yellow arrows in Figure 1b, are beneficial for electrolyte penetration and ions pass through the nanotubes. From the highresolution TEM image (HRTEM, Figure 1c), the inner diameter and wall thickness of the nanotube are observed to be about 5 nm and 8 nm, respectively. The distance between two adjacent nanoscrolls is about 0.45 nm, which is lower than that of the reported NaVOx nanotubes without calcination (0.92 nm).41 This might be due to decomposition of the intercalated amine template at high temperature. One can see that the 6 ACS Paragon Plus Environment


nanoscrolls exhibit some defects compared to those of VOx nanotubes without calcination.3640 In addition, NaVBNT displays a polycrystalline structure, which can be verified by the selected area electron diffraction (SAED) pattern (Figure 1d). Moreover, NaVBNT@C morphology appears to be consisted of adherent clewslike spheres (Figure 1e), with openended nanotubes (Figure 1e,f marked with yellow arrows). It is observable that NaVBNT@C still retains the nanotube morphology (Figure 1g) as well as the polycrystalline structure (Figure 1h) after repeated annealing at 400 oC under the inert atmosphere. A coating layer around 1 nm in thickness is noticeable on surface of the nanotube (Figure 1g), which is determined to be amorphous (Figure 1g, marked with white areas). This coating layer is confirmed to be carbon component that generated from the pyrolysis of sucrose by comparing the Energydispersive Xray spectroscopy (EDS) spectra of NaVBNT (Figure S3a) and NaVBNT@C (Figure S3b), which come from the selected area in Figure 1a and 1e, respectively. The coexistence of Na, V and O elements in the sample indicates that the product may be a kind of vanadium bonze doped with sodium (Figure S3a). According to the ICPAES analysis, the precise molar ratio of Na/V is determined to be 0.167 and 0.159 in NaVBNT and NaVBNT@C, respectively, both close to that in the chemical formula of NaV6O15(0.163).

3.2 Porous structure of the samples To understand the porous structures of the resultant samples, N2 adsorption/desorption tests were conducted. As shown in Figure S5, all three isotherms accord with the typeIV behavior with a H3hysteresis loop, which illustrated a mesoporous structure. The BET specific surface areas are 11.9, 12.5 and 15.7 m2 g1 for VONT, NaVBNT and NaVBNT@C, respectively. Different from other samples, NaVBNT@C presents a little steep profile over the p/p0 values ranged from 0 to 0.1, indicating a microporous structure (Figure S5e). The pore size distribution (PSD) plots of three samples are calculated based on the BarrettJoynerHalenda (BJH) procedure, and are shown in their respective N2 adsorption/desorption isothermal figures. It is found that three peaks centered at 3.3, 19.0 and 40.0 nm (Figure S5d) for NaVBNT are similar to those of VONT (3.2, 18.8 and 40.4 nm, Figure S5b), implying that the porous structure is retained after repeated firings at high temperatures. In the case of NaVBNT@C, a number of micropores (centered at 1.3 nm) 7 ACS Paragon Plus Environment

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coexist, and the mesopores become smaller (17.2 and 34.3 nm) (Figure S5f). It is worth noting that the inner diameter of nanotube hardly changes for this carboncoated sample, suggesting the carbon layer is completely deposited onto the surface of nanotubes. Among the pores, the smaller pores originate from the penetrating nanotube (Figure 1c and g) while the larger pores stem from the cavities surrounded by the intertwined nanotubes (Figure S2d and f). The latter could serve as a reservoir to store the electrolyte within bulk particles, and the former could provide the channels for ion transportation.19 Such a unique architecture, schematically depicted in Figure 3i, is beneficial to accelerating the electrochemical reaction due to fast transportation of both Na+ ions and electrons. Besides, side reactions like the decomposition of the electrolyte components on the surface of electroactive particles would be suppressed to some extent since the BET surface area of the micrometer quasispheres is lower than that of nanomaterials. Moreover, the micronsize spheres are much more easily casted on the current collector compared with the nanotubes. Because nanoparticles with huge specific surface energies are thermodynamic unstable, they extensively aggregate into irregular particles to low their surface energy. On the other hand, the nanotubes were difficultly dispersed homogeneously in the casting slurry even if they could retain the nanoscale morphology. 3.3 Crystal structure of the samples Figure 2a exhibits the XRD pattern of the NaVBNT sample. All characteristic peaks could be indexed to the monoclinic phase sodium vanadium bronze (NaV6O15). According to the retrieved refinement by GSAS II software,42 the fitted XRD pattern (Rp=6.58%, χ2=3.2) agrees well with that of the standard card (JCPDS no. 770146).13 The refined lattice parameters of NaVBNT are a=10.077 Å, b=3.601 Å, c=15.458 Å and β=109.38o. This suggests the asprepared sample has a relatively pure phase with a space group of A2/m (12). Indeed, no impurities were detected in the sample and its high degree of crystallinity wellmatched the SAED pattern (Figure 1d). Additionally, its crystal structure model (Figure 2b) indicates that the sample exhibits a 3Dtunnel structure along the [010] direction, which is constructed of VO5 pyramids and VO6 octahedra with three kinds of vanadium sites, labeled as V1, V2 and V3. This results in zigzag double chains composed of edgesharing (V1)O6 and cornersharing (V2)O6 distorted octahedra as same as edgesharing (V3)O5 squaredbase 8 ACS Paragon Plus Environment


pyramids.44,45 Na+ ions are aligned in the 3D tunnels indeed. Figure 2c displays the XRD patterns of the the three samples. It is found that the diffraction peaks of VONT match those of the standard card (JCPDS no. 411426). NaVBNT@C exhibits the similar characteristic peaks as those of NaVBNT except for a less intensity. Owing to amorphous nature and little amount, the diffraction peaks that belong to carbon are not detected. Raman spectra of the three samples are displayed in Figure 2d. One can see that twelve vibration peaks locate at 126, 151, 221, 271, 323, 388, 440, 516, 553, 693, 993 and 1011 cm1 for NaVBNT. According to those peaks of βNa0.33V2O543,46,47 and βCa0.33V2O5,48 the higherfrequency modes can be ascribed to the stretching vibrations of the different VO bonds in (V1)O6, (V2)O6 and (V3)O5 polyhedra. The band at 1011 cm1 represents V2O6 stretching vibration while the one at 993 cm1 can be ascribed to V1O4 stretching vibration. The Raman bands at 693 and 440 cm1 correspond to the asymmetric V1O2V1 stretching vibration and V1O5V3 bending vibration, respectively. The peak at 553 cm1 is attributed to V1O3 stretching vibration while the one at 516 cm1 stems from V3O7 stretching vibration. The Raman bands at the lower frequencies below 400 cm1 might be relative to those ladder modes of the V atoms and NaO bonds.49,50 The Raman spectrum is in agreement with the molecular structure of NaV6O15 presented in Figure 2b. After coating with carbon, two additional vibration peaks located at 1353 and 1590 cm1 appear in the Raman spectrum of the NaVBNT@C sample. of which, the former is related to the presence of disordered carbon structures (Dband) whereas the latter is bound up with the vibration of sp2hybridized carbon in the graphite crystallites (Gband).51 The intensity ratio of Dband to Gband (ID/IG) is about 1.1, indicative of amorphous carbon. VONT displays ten vibration peaks at 98, 142, 193, 281, 298, 405, 478, 524, 696 and 993 cm1, similar to the reported Raman spectra.5,6 Figure 2e and 2f show the high resolution XPS spectra of V2p signal of NaVBNT and NaVBNT@C, respectively. As it appears in Figure 2e, two peaks centered at 517.6 and 525.0 eV belong to the binding energy of V2p3/2 and V2p1/2, respectively. After the XPS spectra are fitted using XPS peak 4.1 software, these two peaks are split correspondingly into two pairs of peaks related to V4+ and V5+ valences. Clearly, as for NaVBNT, the peaks at 517.4 eV and 524.8 eV are ascribed to V5+2p3/2 and V5+2p1/2, respectively,52 while those at 516.4 eV and 523.5 eV with fewer intensity can be ascribed to V4+2p3/2 and V4+2p1/2. 9 ACS Paragon Plus Environment

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NaVBNT@C displays similar peaks to those of NaVBNT: 517.5 eV (V5+2p3/2)/516.9 (V4+2p3/2) and 525.1 eV (V5+2p1/2)/523.8 eV (V4+2p1/2). The fitted areas from the V5+ and V4+ components can be utilized to estimate the content of V4+ in the compound. The atomic ratio of V4+/V5+ is calculated to be 0.202 for NaVBNT and 0.231 for NaVBNT@C, matching the ratio (0.200) of the given chemical formula NaV4+1V5+5O15. The amount of V4+ component in NaVBNT@C is more than that of the NaVBNT because the former performed a pyrolysis of sucrose in Ar atmosphere. The peaks related to nitrogen element are not observed in the XPS survey spectrum of NaVBNT (Figure S4), indicating the organic amine template has been almost removed from the crystals. 3.4 Battery performance of the samples Figure 3 displays the galvanostatic chargedischarge curves of VONT (a), NaVBNT (b) and NaVBNT@C (c) at 0.1C rate within the potential range of 1.54.0 V for different cycles and the corresponding cycling performances (Figure 3e). It is found that the initial discharge profiles for NaVBNT and NaVBNT@C are different from that of VONT, which displays a lower voltage plateau than those subsequent ones. This indicates that the Nadoped electrodes possess higher polarization than that of the VONT electrode during the first discharge process. The initial discharge capacities are 152, 132 and 168 mAh g1 for the VONT, NaVBNT and NaVBNT@C electrodes, respectively. With continuous cycling, the discharge capacity of the VONT electrode significantly degrades(Figure 3e); whereas those of the NaVBNT and NaVBNT@C electrodes increase and reach the maximum values of 188 (after ten cycles, Figure 3e) and 209 (after five cycles, Figure 3e) mAh g1, respectively. It is noticeable that the VONT electrode has a poor cycling performance compared to the Nadoped samples. It is observed that the NaVBNT@C electrode delivers a higher capacity than the NaVBNT electrode, even the NaVBNT@C electrode offered the maximum discharge capacity of 209, 198, 185, 160, 141, 130, 125, and 105 mAh g1 at the rates of 0.1, 0.2, 0.5, 1, 2, 4, 5, and 10C (Figure 3d), respectively, although the former possesses a less amount of NaV6O15 and a larger amount of V4+ component in the formula than the latter. To explain this phenomenon, a “job sharing” mechanism might be applied.53 As is depicted in Figure 3i, the NaV6O15 nanotubes are coated with nanocarbon layers, leading to a “Janus” interface in the composite material. Na+ ions intercalate into the inner surface of nanotubes while the 10 ACS Paragon Plus Environment


electrons pass through the nanocarbon layer (outer surface of the nanotubes). Owing to the short diffusion distance of Na+ into the nanotube (l NaVBNT (0.12 S cm1) > VONT (0.006 S cm1), which are determined by the fourprobe method. On the other hand, one can see, in Figure S6a, the charge transfer resistance of the NaVBNT@C is lower than those of the other samples. It is due that carbon coating can improve the electrical conductivity of materials.It is noted that the CV curves appear as a rectangle over the potential ranges of 1.52.0 V and 3.54.0 V, similar to those of with pseudocapacitive behaviors.63 This result confirms the linear charge and discharge plots during the latter stages. It has been widely that the electrode materials based on intercalation reaction would exhibit capacitive behavior in a large part if they are produced in form of nanoscale particles.64 Owing to higher surface area and shorter diffusion pathway, nanomaterials capable of cationic intercalating would undergo a serious redox reaction at the surface or in the nearsurface regions. This leads to a faradic capacitance, socalled “intercalation pseudocapacitance”,65 which contributes to the whole delivered capacity of the electrode delivered.6669 The contribution of this pseudocapacitance can be characterized by analyzing the CV data on the basis of the relationship between measured current (i) and sweep rate (v) (Eq. (1)).

i  av b (1) where a and b are adjustable constants. When the value of b approaches 0.5, the current is contributed by ion diffusion; when the value of b is close to 1.0, the current is dominated by capacitive behavior.20 Figure 4b displays the CV profiles at various scan rates for the NaVBNT@C. From these curves, the plot of log(i) versus log(v) can be drawn (Figure 4df). The slope of the fitting line equals to the value of b. The plots of log(i) versus log(v) for VONT and NaVBNT are shown in Figure 4d and 4e, respectively. It is found that the b values for all samples are larger than 0.5 in both anodic and cathodic directions, implying that diffusion and capacitive controls occur during Na+ de/intercalation process. As a result, the current can be divided into two parts: capacitive current and iondiffusion current (Eq. (2)). 13 ACS Paragon Plus Environment

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i (V )  k1v  k2 v1/2

(2)

In the equation, k1v and k2v1/2 come from the capacitive contribution and iondiffusion contribution, respectively. The constants of k1 and k2 can be obtained from the i(V)/v1/2 versus v1/2 plot at a given potential (Figure S8). Thus, we can quantify the fraction of the two types of currents at specific potentials. Figure 4c presents the contribution of the intercalation pseudocapacitance estimated from the CV curve at 0.5 mV s1 based on Eq (2) for NaVBNT@C. It is found that the capacitive contribution (Figure 6c, violet area) forms a large constitution of the stored charges, agreeing well with those calculated b values (Figure 4i). Regarding the analysis of exsitu XRD patterns (Figure 5a), the crystallographic phase in NaVBNT@C hardly changes in the NaVBNT@C during sodiation/desodiation process. Accordingly, the capacitive behavior for the NaVBNT could be regarded as intercalationtype pseudocapacitance. Figure 4g, 4h and 4i display the total capacities for the VONT, NaVBNT and NaVBNT@ electrodes at various sweep rates, respectively. It is found that VONT possesses the highest percentage of diffusion contribution to total capacity among the three samples whereas the NaVBNT@C reveals the highest percentage of capacitive contribution. In addition, each sample presents an increased percentage in capacitive contribution with increasing the sweep rate. The results suggest that the pseudocapacitancedominated storage accounts for a large proportion of the total capacity, particularly for more conductive materials at higher sweep rates, which is in agreement with the reports.6971 Therefore, the pseudocapacitive behavior plays an important role in storing charge for NaVBNT because the rate of faradic redox reaction at the surface of nanotubes is considerably faster than that of ionic diffusion into the electrode. 3.6 Structural analysis of Na-VBNT@C at different charge states Figure 5a presents the exsitu XRD patterns of the NaVBNT@C electrode at different chargedischarge states. The detailed lattice parameters form the XRD patterns are summarized in Table 1. Notably, during the discharge (Naintercalation) process, the lattice parameter along the c-axis continuously rises up while those along the a- and b-axes hardly change, resulting in expansion of unit cells. Indeed, during the charge (Nadeintercalation) process, the lattice parameter along the c-axis reduces, leading to contraction of unit cells. At 14 ACS Paragon Plus Environment


the fully charged state, the core nanotubes presented similar lattice parameters to those of the fresh one (Table 1), indicating that Na+ ions can be reversibly inserted into and extracted from the obtained nanotubes. It is also observable that the diffraction peaks related to the (111), (104), (213) and (317) planes moved to lower diffraction angles during discharge course, but shifted towards higher diffraction angles during charge process (Figure 5b), which is consistent with the structural change tendency of lattice parameters. This phenomenon of lattice expansion and shrinkage could be called a “lattice breathing”,7,72,73 which allows the core nanotubes to keep their crystal structure during de/intercalation of Na+ ions, confirming the Naintercalation pseudocapacitive behaviors for the samples. Interestingly, at the fully charged state after 100 cycles, the core nanotubes owned a structure close to that at the 2.3 V discharged state, implying that a small amount of Na+ ions might be trapped in the crystal. These entrapped Na+ ions would reduce the delivered capacity of the core nanotubes because they occupy the intercalating sites in the crystal, but they could serve as “pillars” to stable the structure. The pillar effect from the intercalated Na+ ions can be conspicuously observed from the HRTEM images of the core nanotubes sample before cycling (Figure S9a) and after 100 cycles (Figure S9b). We can see that the core nanotubes still present distinct crystalline structure, although some disorders appear inside the crystals after cycling. As a consequence, NaVBNT@C possesses a superior cycling performance compared to the other vanadiumbased nanomaterials (Table S1, supporting information.). Figure 6 shows SEM images of the surface of NaVBNT@C electrode after 1000 cycles when run at 5C rate. One can see that although a few of cracks appear at the surface of the electrode after cycling (Figure 6a), some spherelike particles still maintain the morphology of nanotubeintertwined clews (Figure 6b), similar to the original one (Figure S2e). This result implies that the NaVBNT@C quasispheres could accommodate the volume change of NaV6O15 crystallites during the cycling process. It is reasonably concluded that Naintercalation pseudocapacitance could endow the NaVBNT@C electrode with outstanding rate capability and longterm cyclability because the faradaic redox reactions on/near the surface of nanotubes cannot destroy the crystal structure and hierarchical architecture. 15 ACS Paragon Plus Environment

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4. Conclusions Clewlike sodium vanadium bronze/carbon (NaVBNT@C) composites composed of carboncoated NaV6O15 nanotubes have been successfully prepared by a hydrothermal method using an organic amine template. The NaVBNT@C sample showed quasispherical morphology with a hierarchical architecture, rendering the electroactive NaVBNT to own outstanding rate capability and relatively long life span when used as the cathode for sodiumion batteries. NaVBNT@C could deliver an initial capacity of 168 mAh g1, and reach the maximum capacity of 209 mAh g1 at 0.1C rate. By the analysis of CV measurements, an intercalation pseudocapacitance could be employed to explain its chargestorage mechanism. Owing to this mechanism, it revealed an excellent rate capability, and delivered a high capacity up to 105 mAh g1 at 10C rate (2500 mA g1). Benefitting from the hierarchical architecture, the sample could retain 94% of the initial capacity after 3000 cycles when operated at 5C rate. The results suggest that onedimensional carboncoated nanotubes with open ends can facilitate not only Na+ ions diffusion but also electrons transportation. Moreover, the intertwined clews could alleviate the volume change of crystallites during the cycling process. This work demonstrates that the hierarchical carboncoated NaV6O15 nanotubebased quasispheres would be developed to a promising candidate as cathode material for sodiumion batteries. The sluggish Nadiffusion kinetics and largevolume change of crystallite, which considerably limit the wide application of insertiontype cathode materials in SIBs, would be improved significantly by introducing intercalation pseudocapacitive behaviour to cathode materials through the rational designing of particles morphology.

ASSOCIATED CONTENT Supporting information Thermogravimetric (TG) and differential TG (DTG) curves of the Nadoped precursors and the NaVBNT@C in air atmosphere at a ramping rate of 10 oC min1, SEM images of the VONT sample. SEM images of the NaVBNT and NaVBNT@C samples, EDX spectra of the NaVBNT and NaVBNT@C samples, XPS survey spectrum of the NaVBNT sample, Nitrogen adsorptiondesorption isotherms of the VONT, NaVBNT and NaVBNT@C 16 ACS Paragon Plus Environment


samples, and the corresponding pore size distribution plots, Nyquist curves and the corresponding Zre versus ω1/2 plots of the VONT, NaVBNT and NaVBNT@C samples, Cyclic voltammograms of for the first three cycles at a sweep rate of 0.1 mV s1 over the potential range of 1.54.0 V (vs. Na+/Na), Galvanostatic chargedischarge curves at 0.1C rate for various cycles, cyclic performance at the 5C rate, and Rate capability of the coatingcarbon electrode within the 1.54.0 V range. The coatingcarbon electrodes were prepared by mixing the carbon that generated from the pyrolysis of sucrose under Ar atmosphere with acetylene black (Super P) and PVDF (Knyar 2700) binder (mass ratio of 80:10:10) in Nmethyl2pyrrolidone (NMP) to form slurry, cast on aluminum foil and dried at 120 oC under vacuum overnight, and punched into the discs with a diameter of 12 mm, The i(V)/v1/2 versus v1/2 plots of the NaVBNT@C sample at various potentials, HRTEM images of the NaVBNT@C sample before cycling and after 100 cycles, Comparison of the electrochemical properties of the NaVBNT@C and other vanadiumbased nanomaterials for Naion batteries over the voltage range of 1.54.0 V.

ACKNOWLEDGEMENTS The authors greatly appreciate the financial support from National Natural Science Foundation of China (grant numbers 21376069, 21576075, 51672189), Hunan Provincial Natural Science Foundation of China (grant numbers 2018JJ2386, 2018JJ2393), National Key Research and Development Program of China (2018YFB0105900), and Xi'an Science and Technology Project of China (201805037YD15CG21(20)).

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Scheme 1. Schematic illustration of the formation process of the NaVBNT@C sample.


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Figure 1. TEM (a), HRTEM images (b, c) and SAED pattern (d) of the NaVBNT sample. TEM (e), HRTEM images (f, g) and SAED patterns (h) of the NaVBNT@C sample.



Figure 2. XRD pattern and Retrieved refinement result (a), crystal structure model (b) of the NaVBNT. XRD patterns (c) and Raman spectra (d) of the VONT, NaVBNT, NaVBNT@C samples. Highresolution V2p XPS spectra of the NaVBNT (e) and NaVBNT@C (f) samples.


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Figure 3. Galvanostatic chargedischarge curves of the VONT (a), NaVBNT (b) and NaVBNT@C (c) electrodes at 0.1C rate (25 mA g1) for various cycles. The voltages profiles at the maximum capacity when cycled at different rate from 0.1C to 10C(d). Cycling performances of the VONT, NaVBNT and NaVBNT@C electrodes at 0.1C rate (25 mA g1) (e). Rate capabilities of the VONT, NaVBNT and NaVBNT@C samples (f,g). Cycling performances of the NaVBNT and NaVBNT@C electrodes at 5C rate (h). Schematic illustration of the morphology of the NaVBNT@C sample (i).All chargedischarge tests were carried out at room temperature over the voltage range of 1.54.0 V using the assembled buttontype cells.



Figure 4. Cyclic voltammograms of the VONT, NaVBNT and NaVBNT@C electrodes at a sweep rate of 0.1 mV s1 (a), and the NaVBNT@C electrode at different sweep rates ranged from 0.1 to 0.5 mV s1 (b). Capacitive contribution of the NaVBNT@C electrode estimated from the CV curve at the sweep rate of 0.5 mV s1 (c). Log (i)~log(v) plots of the VONT (d), NaVBNT (e) and NaVBNT@C (f) electrodes. Estimated values of capacitive contribution of the VONT (g), NaVBNT (h) and NaVBNT@C (i) electrodes at various sweep rates.


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Figure 5. Exsitu XRD patterns of the NaVBNT@C electrode at the different charge states in the first cycle and after 100 cycles (a), and the corresponding enlarged XRD patterns belonging to the (111), (104), (213) and (317) planes (b).



Figure 6. Surface SEM images with the magnifications of 200 (a) and 10,000 (b) for the NaVBNT@C electrode after cycled at 5C rate for 1000 times.


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Table 1. Lattice parameters refined from XRD patterns of the NaVBNT@C electrode during the first cycle and after 100 cycles. Electrode state

Cell parameter a (Å)

b (Å)

c (Å)

β (o)

V (Å3)

ρ(g cm-3)

Fresh

10.102(2)

3.624(7)

15.456(7)

109.49(3)

533.57

3.5391

Discharged to 2.3 V

10.078(3)

3.617(4)

15.487(2)

110.77(8)

527.81

3.5777

Discharged to 1.5 V

10.084(4)

3.619(2)

15.758(4)

110.98(6)

536.97

3.5166

Charged to 2.6 V

10.021(3)

3.657(6)

15.571(1)

110.22(6)

533.04

3.5492

Charged to 3.1 V

10.098(8)

3.614(0)

15.507(8)

109.90(7)

532.26

3.5478

Charged to 4.0 V

10.124(3)

3.617(6)

15.458(1)

109.79(0)

531.66

3.5411

10.051(3)

3.622(2)

15.502(3)

110.50(4)

528.34

3.5741

After 100 cycles at fully charged


Superior Sodium Storage of Carbon-coated NaV6O15 Nanotubes

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