Enhanced sodium storage property of sodium vanadium phosphate via simultaneous carbon coating and Nb5+ doping

Journal Pre-proofs Enhanced Sodium Storage Property of Sodium Vanadium Phosphate via Simultaneous Carbon Coating and Nb5+ Doping Xiaohong Liu, Guilin Feng, Zhenguo Wu, Zuguang Yang, Shan Yang, Xiaodong Guo, Shuaihua Zhang, Xingtao Xu, Benhe Zhong, Yusuke Yamauchi PII: DOI: Reference:

S1385-8947(19)33368-6 https://doi.org/10.1016/j.cej.2019.123953 CEJ 123953

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

15 November 2019 22 December 2019 25 December 2019

Please cite this article as: X. Liu, G. Feng, Z. Wu, Z. Yang, S. Yang, X. Guo, S. Zhang, X. Xu, B. Zhong, Y. Yamauchi, Enhanced Sodium Storage Property of Sodium Vanadium Phosphate via Simultaneous Carbon Coating and Nb5+ Doping, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123953

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Enhanced Sodium Storage Property of Sodium Vanadium Phosphate via Simultaneous Carbon Coating and Nb5+ Doping Xiaohong Liu,a,b Guilin Feng,c Zhenguo Wu,a* Zuguang Yang,a Shan Yang,a Xiaodong Guo,a Shuaihua Zhang,b,d Xingtao Xu,b* Benhe Zhong,a and Yusuke Yamauchib,e

a

School of Chemical Engineering, Sichuan University, Chengdu 610065, China

b

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

c

Research Institute for Electronic Science (RIES), Hokkaido University, N20W10, Sapporo City 001-0020, Japan

d

Department of Chemistry, College of Science, Hebei Agricultural University, Baoding 071001, Hebei, PR China

e

School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia

E-mails: [email protected] (Z. G. Wu); [email protected] (X. T. Xu)

1

Abstract Featuring favorable ion transfer and high thermal stability, NASICON-structured Na3V2(PO4)3 has been regarded as a promising cathode candidate for sodium-ion batteries. However, this material might be impeded by inferior rate capability owing to its disappointing electron conductivity. To address this issue, a combined technique of carbon coating and Nb5+ doping was carried out for the first time. On one hand, the coated carbon nano-shell could construct an electron-conductive network and buffer the volume stain. On the other hand, the introduction of Nb5+ into the Na3V2(PO4)3 crystal could regulate the relevant crystal parameters and create more vacancies, further facilitating the transfer of sodium ions. As a result, the optimized Nb-doped Na3V2(PO4)3@C material achieved an excellent performance of 81.6 mA h g-1 at 50 C and a high-capacity retention ratio of 80.8% even after 1,600 cycles. This work not only highlights the significance of carbon coating and Nb5+ doping, but also shows promising opportunities in potential cathode alternatives for sodium-ion batteries. Keywords Na3V2(PO4)3, Nb5+ doping, Carbon coating, Electron conductivity, Sodium-ion batteries

2

1. Introduction With the aggravating deterioration of the environment and increasing concerns regarding the energy crisis, pursuing advanced energy storage techniques for reproducible energy utilization is more urgent than ever before.1,

2

Over the past three decades, lithium-ion batteries have been successfully commercialized;

however, with increasing demand for lithium-ion batteries, limited lithium resources restrict their further application. Of particular note, sodium-ion batteries (SIBs) have been identified as the next potential large-scale energy storage devices to replace lithium-ion batteries due to their low cost and rich reserves.3, 4Although

great progress in SIBs has been attained in recent years, there remains a huge issue of developing

suitable host materials since the ionic radius (1.02 Å) of Na+ is larger than that of Li+.5, 6 Among various potential cathode electrode materials for SIBs, the NASICON-structured Na3V2(PO4)3 (NVP) has prompted much interest, considering its three-dimensional channels for higher ionic conductivity, excellent thermal and moisture stability, and decent theoretical capacity (117.6 mA h g-1), as well as a satisfactory potential plateau (3.4 V vs. Na/Na+).7–9 However, the intrinsic electron conductivity of the NVP originating from its crystal structure always leads to a poor rate of performance and inferior cycling stability, which severely hinders its practical application.10,

11

To date, the enormous efforts have contributed to

facilitating electron conductivity by the designing of multifarious NVP/carbon composites.8,

12, 13

For

instance, Wang et al. constructed honeycomb-like hierarchical porous NVP/C microballs by facile one-pot synthesis, achieving 93.6% of the initial capacity after 200 cycles at 1 C.12 Moreover, Zhou et al. required a NVP nanostructure by employing ELP16 proteins as a bio-template, which delivered 51 mA h g-1 at 200 C.13 In addition, a carbon matrix with different dimensions was explored by Mai et al., who claimed that NVP nanograins dispersed in acetylene carbon nanospheres would show a higher capacity of 117.5 mA h g−1 at 0.5 C and better cycling stability of 96.4% capacity retention at a rate of 5 C over 200 cycles.14 Despite the excellent electrochemical performance of the NVP material obtained by surface carbon coating, the intrinsic electron conductivity of NVP could not realize great improvement without bulk structure modification.8,12,14 To better manage this bottleneck, tremendous effort has been given to a combination of carbon coating and alien ion doping, such as Ga3+,15 Ca2+,16 Mg2+,17 Ti4+,18 Cu2+,19 and Mo6+,20 which has been proven to be an efficient strategy for enhancing the electrochemical property of NVP. In particular, high-valence Mo6+ doping in NVP could induce more vacancies, thus, enhancing the electron conductivity of NVP and further improving the rate performance.20 Coincidentally, similar perspective has been proposed by Sung-Yoon Chung et al.—that the lattice electronic conductivity of LiFePO4 can be increased by a factor of more than 108 relative to the pure endmember by selective doping with supervalent cations.21 Meanwhile, combined with other opinions about doping in polyanion materials,22, 23

it could be assumed that introducing a suitable high-valence element could create more vacancies and

boost its intrinsic electron conductivity.24, 25 As far as we know, there have been no reports about the replacement of V with pentavalent ions in the analysis of NVP materials. The influences of the partial replacement of pentavalent ions in the V site still are 3

unknown and deserve investigating. Nb, which belongs to same periodic element group as V, has familiar chemical properties, and could form a similar NASICON-structured composite,26 is selected as a doping element to modify the NVP materials. In this paper, different doping contents, accompanied with a synchronous carbon coating, were designed and prepared for investigating the substitution effects of Nb on the crystal structure, morphological changes, and electrochemical characteristics for NVP. As a result, our Nb-doped NVP@C displayed a performance of 81.6 mA h g-1 at 50 C and maintained 80.8% of its original capacity after 1600 cycles at 50 C, which verifies that moderate Nb substitution and carbon coating could promote the rate capability and cycling stability of NVP materials. 2. Experimental sections 2.1. Synthesis All chemicals were purchased from Chengdu Kelong Chemical Reagent Factory and used without further purification. Several types of NVP@C cathode materials with different Nb contents were synthesized via a sol-gel route. Briefly, citric acid, which acted as carbon sources and a complexing agent, was first dissolved in 50 mL of deionized water, followed by the successive addition of NH4VO3, Nb2O5, Na2CO3, and NH4H2PO4 into the above solution while stirring at 60 ℃ for 4 h. The mixture solution was subsequently heated at 90 ℃ to evaporate water until a gel was formed. The gel was then transferred to a 90 ℃ oven overnight and ground into powder before sintering. Afterward, the precursor was first heated under Ar flow at 350 ℃ for 4 h and then 800 ℃ for 8 h with a heating speed of 5 ℃ min-1 to harvest the final products. The final products, containing different Nb contents (see Table S1), were denoted as Nb-0, Nb-0.1, Nb-0.2, and Nb-0.3, respectively. 2.2. Characterization The material compositions of the samples were identified by an ANJIELUN ICPOES730 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). The phase compositions of samples were confirmed by X-ray Diffraction (XRD, Bruker SMART X2S) with Cu K radiation and Fourier Transform Infrared Spectra (FTIR Spectra, Spotlight 400/400N) with the KBr method. Carbon information was measured by carbon-sulfur analyzer (TL851-6D) and Raman spectra (LEISHAONI InVia Reflex). The valence states of all elements in the materials were tested by X-ray photoelectron spectroscopy (XPS, MOSAIFEI ESCALAB 250Xi). Morphological observations were conducted by using a Hitachi S-4800 Field Emission Scanning Electron Microscope (SEM) and a FEI Talos F200S Transmission Electron Microscope (TEM). The electron conductivity of all samples was studied using the four-point probe method after the powder samples were pressured into dense wafers. The Brunauer-Emmett-Teller (BET) surface areas were explored by the nitrogen adsorption desorption isotherms at 77 K. The electrochemical performances of all samples were conducted in a coin cell CR2025 on a Neware Battery Testing System with constant current charging and discharging in a potential window of 2.3−3.9 V 4

at 25 ℃. Cathodes were prepared by coating well-mixed slurries of 80 wt.% active material, 13 wt.% acetylene black, and 7 wt.% polyvinylidene fluoride binder onto an aluminum foil current collector, and then dried in a vacuum oven at 120 ℃ for 12 h. Afterward, the batteries were configured in an Ar-filled glove box with metallic sodium as the anode, Whatman GF/D as the separator, and a 1 M NaClO4 solution in a solvent mixture of dimethyl carbonate and ethylene carbonate (EC), polycarbonate (PC), and fluoroethylene carbonate (FEC) with a volumetric ratio of 49:49:2 as the electrolyte. Electrochemical Impedance Spectroscopy (EIS) tests were monitored with a frequency window of 0.1 Hz to 100 kHz with a fresh cell on open-circuit voltage, and cyclic voltammograms (CV) curves were recorded with a CHI 650b electrochemical work station at different sweep rates (0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 mV s-1). 3. Results and discussion To evaluate the doping content of Nb-doped NVP materials, ICP-OES was first conducted. As shown in Table S1 of the Supporting Information, all kinds of elements in each sample are consistent with the designed values, indicating the successful replacement of Nb at V sites. The crystal structure of pristine NVP is displayed in Fig. S1. Two VO6 octahedra corner three PO4 tetrahedra by sharing O atoms forming basic lantern units, and Nb would substitute for the V site, forming NbO6 octahedra in the Nb-doped NVP crystal structure.27 The crystal-phase structures of the prepared materials were confirmed by XRD measurements. As illustrated in Fig. 1a, all diffraction peaks could be well indexed to the rhombohedral NVP phase with the R-3c space group (JCPDS No. 53-0018),28,

29

and no other peaks corresponding to

purities could be detected for Nb-0, Nb-0.1, Nb-0.2, and Nb-0.3 samples, suggesting that the small substitution of Nb had little apparent influence on the crystal phase of the NVP, and the coated carbon is disordered or too small to be observed.30 XRD refinements were carried out to further investigate the effect of Nb doping on the crystal texture (Fig. S2), and the corresponding reliability factors (i.e., Rp and Rwp) indicate the values below 10 %, suggesting that the refinement is receivable. Moreover, as shown in Fig. 1b, the cell parameters of a and c are in agreement with previous reports.31, 32 With the increase of Nb content in Nb-doped NVP materials, both a and c values and cell volume are gradually decreasing, which might be ascribed to the smaller ionic radius of Nb5+ (0.640 Å) as compared to V3+ (0.645 Å).22, 33 The carbon content in all samples was analyzed by carbon sulfur analyzer (Table S1). It was found that the carbon content of Nb-0, Nb-0.1, Nb-0.2, and Nb-0.3 samples were 4.28 wt.%, 4.46 wt.%, 4.35 wt.%, and 4.63 wt.%, respectively. Raman spectra of all samples, as illustrated in Fig. 1c, show that signatures located at approximately 1350 cm-1 and 1580 cm-1 are attributed, respectively, to the D-band (amorphous carbon) and the G-band (graphitized carbon),34 and the slight peaks in the scope of 400–600 cm-1 could be indexed to the existence of PO4 or VO6. The relative intensity ratio of the D-band to the G-band (ID/IG) is a vital value to reveal the degree of graphitization of carbon,35 and the values of ID/IG are 1.04, 0.99, 0.99, and 0.98 for Nb-0, Nb-0.1, Nb-0.2, and Nb-0.3, respectively. Obviously, the ID/IG values are gradually decreasing as the content of the Nb element increases, suggesting the higher degree of graphitization and improved electron 5

conductivity of the sample with higher Nb content, which is possibly due to the catalytic graphitization of the Nb element.36 Additionally, FTIR spectra of all Nb-doped NVP materials were observed for further study of the phase composition. As displayed in Fig. 1d, similar peaks are found, and no obvious change could be surveyed in these samples, indicating possession of the pure phase. The characteristic band at 1600 cm-1 could reflect the C=O stretching vibration in the carbon material, and the peaks at 1180, 1080, and 580 cm-1 could correspond to the different vibration modes of PO4 tetrahedra, while the peaks at 640 and 560 cm-1 represent the vibration of the V-O bond.37 In addition, the obvious signatures of Nb-0.2 could reflect an enhanced bond strength in the NVP phase, illustrating better structural stability of Nb-0.2.32

Fig. 1. (a) XRD pattern, (b) cell parameters and volumes, (c) Raman spectra, and (d) FTIR spectra of Nb-0, Nb-0.1, Nb-0.2, and Nb-0.3 samples. Morphological evolution was uncovered by SEM and TEM observations. In SEM images of the NVP and Nb-doped NVP materials (Fig. 2a–d), all samples show that irregular morphology and smaller particle size could be obtained with low Nb content, possibly attributed to the pulverization function of the Nb element. Nitrogen adsorption-desorption isotherms of all samples, as displayed in Fig. S3, show that the specific surface area of Nb-0, Nb-0.1, Nb-0.2, and Nb-0.3 samples are calculated to be 18.91, 28.2, 34.62, and 8.04 m2 g-1, respectively. The largest specific surface area of Nb-0.2 could be favorable for electrolyte infiltration and faster sodium-ion diffusion. Energy dispersive X-ray spectroscopy (EDS) mapping images (Fig. 2e) show that all elements of Na, V, Nb, P and O are uniformly distributed in Nb-0.2 sample. TEM images of the pristine NVP sample (Fig. 2f and g) show that NVP particles are in serious aggregation with large black regions detected corresponding to NVP crystals, and the surface is surrounded by an uneven carbon layer in the range of a few and dozens of nanometers. The residue carbon materials interconnecting NVP particles 6

would serve as a conductive network for fast electron transfer.38, 39 After Nb doping, as shown in Fig. 2h, many smaller NVP particles are surrounded by a homogenous amorphous carbon layer, and the residue carbons are interconnected to form a conductive network. Lattice spacings of 0.357, 0.363, and 0.436 nm, as shown in Fig. 2i, can be indexed to the (202), (006), and (110) planes of rhombohedral NVP, respectively, indicating the high crystallinity of NVP.30, 40

Fig. 2. SEM images of (a) Nb-0, (b) Nb-0.1, (c) Nb-0.2, and (d) Nb-0.3; (e) EDS mapping of Nb-0.2; TEM images of (f,g) Nb-0 and (h,i) Nb-0.2. XPS measurement was carried out to analyze the valence properties of C, V, and Nb elements on the surface of all samples, as shown in Figs. 3 and S4. The high-resolution C1s spectra of all samples are deconvoluted into four configurations at 288.5, 287.1, 285.9, and 284.7 eV (Figs. 3a,b and S4a,b), referring to the O=C-O, C=O, C-O, and C-C bonds, respectively,41 and the corresponding ratios of various compositions are listed in Table S2. Among them, the existing C-C is responsible for electron conductivity. With an increase of Nb content, the Nb-doped NVP samples achieve a higher C-C content (Table S2), consistent with the Raman spectra results (Fig. 1c), indicating better electron conductivity. For a qualitative analysis of the increasing electron conductivity, the four-point probe method was applied, and the measured electron conductivities of Nb-0, Nb-0.1, Nb-0.2, and Nb-0.3 were 4.54×10-3, 6.29×10-3, 1.72×10-2, and 5.08×10-2 S cm-1, respectively. With the increase of Nb content in the NVP materials, it was found that the electron conductivity had been obviously enhanced, which further confirms our former conclusion. For V 2p spectra in Figs. 3c,d, two pairs of peaks, representing the existence of V3+ and V4+, could be separated at 516.2 and 523.1 eV for Nb-0, and 517.4 and 524.1 eV for Nb-0.2, respectively.41 For better comparison, the ratio of V3+/V4+ was calculated by the respective peak areas (Figs. 3c,d and S4c,d; Tables S2 and S3). The ratios of V3+/V4+ in Nb-0, Nb-0.1, Nb-0.2, and Nb-0.3 samples are 48.3%, 52.8%, 55.2%, and 55.2%, respectively, indicating that with more Nb element introduced, more V3+ rather than V4+ could be obtained, which may be originated from a balance of the valence and the conservation of the charge.16 Nb 3d spectra were further tested to identify the valence state of the Nb element (Figs. 3e,f and S4e,f); with the increase of 7

Nb content, more apparent Nb signals could be collected. Two broad peaks could be segregated at 207.5 and 210.4 eV, indexed to Nb 3d5/2 and Nb 3d3/2, respectively, reflecting that the valence state of Nb element is +5.42

Fig. 3. XPS spectra of (a, b) C 1s, (c, d) V 2p, and (e, f) Nb 3d for (a, c, e) Nb-0 and (b, d, f) Nb-0.2 samples. The electrochemical properties of all samples were measured in a potential range from 2.3−3.9 V, based on the theoretical capacity of 117.6 mA h g-1 at 1 C and the total mass of NVP and carbon compound. As shown in Fig. 4a, all samples except for Nb-0.3 show similar capacities at a low current density of 0.1 C. As compared with the pristine NVP sample, the Nb-doped samples show a lower capacity at the current density ranging from 0.5–10 C, possibly due to the fact that Nb serves as an inert element in a specific potential window and does not provide capacity.43 With a further increase of current density, higher capacities could be achieved for Nb-0.1 (92.3 mA h g-1 at 20 C and 88.2 mA h g-1 at 30 C) and Nb-0.2 (89.4 mA h g-1 at 20 C and 85.9 mA h g-1 at 30 C), which are obviously higher than those of pristine NVP. Particularly, Nb-0.2 8

exhibited a higher capacity of 81.6 mA h g-1 as compared to Nb-0.1 (76.3 mA h g-1) at a high-current density of 50 C. When the current density returned to 1C, Nb-0, Nb-0.1, and Nb-0.2 samples exhibited nearly the same capacity, expressing a favorable reversibility. However, the Nb-0.3 sample with a higher Nb content displayed a lower capacity, owing to the excessive inserted Nb element. In addition, the rate performance of Nb-doped materials with appropriate contents is superior to that of pristine NVP, demonstrating that Nb substitution is an efficient strategy to enhance the rate capability of NVPs. Furthermore, the charging/discharging curves for the representative sample of Nb-0.2 are displayed in Fig. 4b, and the curves for the other samples are shown in Fig. S5a–c for comparison. All curves show a similar voltage plateau at 3.4 V, corresponding to a V3+/V4+ redox couple.44 As shown in Fig. S4d, the discharging platform will transfer to the low voltages with the increase of current density, rooting in the worsened cell polarization. The discharging platform of all samples (Fig. S5d) shows that all samples possess the same discharging platform at a low current density, while the potential of Nb-0.2 decreases relatively slower than other samples with the increase of current density, suggesting a lower degree of polarization.45 In order to verify the effect of Nb5+ substitution on the cycle stability of the NVP@C electrodes, the cycling performance was investigated for the as-prepared NVP@C samples with various Nb contents at two different current densities (1 and 5 C). As displayed in Fig. 4c and d, the original capacity of the pristine NVP@C is 101.4 mA h g-1, and a high capacity of 83.4 mA h g-1 could be maintained after 500 cycles. As with rate performance, Nb-0.1 and Nb-0.2 show tiny capacity fading and better cycling performance. The initial capacities of Nb-0.1, Nb-0.2, and Nb-0.3 are 101.4, 101.4, and 80.2 mA h g-1, respectively. After 500 cycles, the final capacities are 86.8, 95.4, and 79.1 mA h g-1 for Nb-0.1, Nb-0.2, and Nb-0.3, corresponding to capacity retentions of 67.9%, 91.4%, and 98.6%, respectively. Furthermore, at a high current density of 5 C, all samples show similar cycling performances with those at 1 C. Nb-0 expresses the highest original capacity but suffers from fast capacity fading. Nb-0.3 shows the lowest capacity and the most stable cycling performance. Both Nb-0.1 and Nb-0.2 show improved cycling performance, but Nb-0.2 exhibits better capacity retention as compared to Nb-0.1. These experimental results demonstrate that improved cycling stability of the NVP@C samples could be obtained with more Nb doping. As shown in Fig. 4e, even at 20 and 50 C, Nb-0.2 still shows an excellent cycling stability. The original capacity at 20 C is 88.72 mA h g-1, and 78.1% of the discharged capacity could be retained after 1600 cycles. At 50 C, a capacity of 56.6 mA h g-1 could be retained after 1600 cycles, corresponding to a capacity retention of 80.8% (initial capacity is 69.99 mA h g-1) and capacity fading rate of 0.011% per cycle.

9

Fig. 4. (a) Rate capability of Nb-x (x = 0, 0.1, 0.2, and 0.4); (b) charging/discharging curves of the Nb-0.2 sample; cycling stability at (c) 1 C and (d) 5 C of Nb-x (x = 0, 0.1, 0.2, and 0.4); (e) cycling behavior of the Nb-0.2 sample at 20 C and 50 C. To further confirm the contribution of Nb substitution in the NVP@C materials, the dynamic characteristics of Nb-0 and Nb-0.2 samples were first explored by the CV curves at various sweep rates. As shown in Fig. 5a and b, a couple of peaks corresponding to the V3+/V4+ redox couple appear at ~ 3.4 V at 0.1 mV s-1.46 With a further increase of sweep rates, anodic peaks gradually shift to the lower voltages, while cathodic peaks gradually move to the higher voltages. Anodic peaks contain two small peaks at low sweep rates, which may be indexed to the adjustment of sodium ions at Na1 and Na2 sites in the NVP crystals (Fig. S1).47 The dependency relation between peak current (Ip) and the square root of the sweep rate is shown in Fig. 5c, and the linear relationship of Ip and υ1/2 indicates that the electrode reaction would tend to be a diffusion-controlled process.48 The Na ions diffusion coefficient (DNa+) in the cell can be obtained from the slope of the linear relation using the Randles-Sevcik equation (Ip = 2.69×105n3/2ACDNa+1/2υ1/2).48, 49 Based on this equation, the DNa+ is positively correlated with the slope of Ip and υ1/2. Thus, it could be predicted that the DNa+ of Nb-0.2 is higher than that of the pristine sample, indicating that Nb doping effectively improves 10

the diffusion coefficient of sodium ions. Furthermore, b values in the equation (I = aυb) calculated from the plot of the positive log(I) vs. log(υ) in Fig. 5d are 0.5289 and 0.6874 for Nb-0 and Nb-0.2, respectively. Generally, the electrode reaction favors diffusion-controlled behavior when b is close to 0.5, while the electrode reaction tends to be surface-controlled, as b is next to 1.50 This means that Nb-0.2 has a higher contribution from surface capacitive effects than from diffusion behaviors. In detail, according to the equation I(V)/υ1/2 = kυ1/2+k1, the proportion of diffusion and surface contribution could be identified. Especially, with the augmentation of the scan rate, the capacitive contribution of Nb-0 and Nb-0.2 increases, and the diffusion contribution is decreased,51 as assumed in Fig. 5e and f. Nb-0.2 always has a higher capacitive contribution than the Nb-0 sample, which may be ascribed to the smaller particle sizes of Nb-0.2. In addition, Nyquist plots combined with a fitting circuit model of fresh cells are displayed in Fig. 5g. All cells exhibit a slope in the low-frequency range and a depressed circle in the high-frequency region, which is consistent with a previous report.48 Actually, the depressed circle in the high-frequency region is correlated with Rct, which represents the electron transfer barrier. The relevant fitting results are summarized in Table S4. The Nb-doped NVP materials have smaller Rct values than un-doped NVP. Combined with the equation DNa+ = R2T2/(2An4F4C2σ2) and Z΄ = RD+RL+σω−1/2,52 DNa+ values obtained by the plot of Z΄ and ω−1/2 in low frequencies (Fig. 5h) are 8.63×10-16, 4.59×10-15, 5.38×10-15, and 2.82×10-15 cm2 s-1 for Nb-0, Nb-0.1, Nb-0.2, and Nb-0.3, respectively. As compared with previous reports as summarized in Table S5, Nb-doped NVP materials also exert a favorable rate capability and better long-term cycling stability, originating from their enhanced electron conductivity, smaller particle sizes, and structural stabilization induced by the incorporation of Nb. High-valent Nb5+ doping could cause charge allocation and crystal defects, and more vacancies could be obtained, which would be beneficial for electron transfer. Our design provides a unique view of alien ion doping in polyanion materials.

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Fig. 5. CV curves of (a) Nb-0 and (b) Nb-0.2 at different sweep rates; plots and fitting results of (c) Ip vs. v1/2, and (d) log(Ip) vs. log(v) for Nb-0 and Nb-0.2; percentages of diffusion-controlled contribution and capacitive contribution for (e) Nb-0 and (f) Nb-0.2 at 3.4 V under different sweep rates; (g) Nyquist plots and (h) relationship of Z’ and ω-1/2 of all samples.

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4. Conclusions In summary, a range of Nb5+-doped NVP@C (x = 0, 0.1, 0.2, 0.3) cathode materials were successfully constructed by a sol-gel method for the first time. The experimental results demonstrated that the electron conductivity of NVP is greatly enhanced by Nb5+ doping, generating a fantastic enhancement of the rate performance and cycling behavior. For optimized Nb-0.2 sample, the specific capacity decreased from 101.4 mA h g-1 to 81.6 mA h g-1 when the current densities increased from 0.1 C to 50 C, indicating a preferable rate performance. What’s more, Nb-0.2 sample exerts a stable cycle behavior over 1600 cycles at 50 C with a capacity fading rate of 0.011% per cycle. These positive results powerfully support the contention that high-valent metal ion substitution is a valid method of facilitating the high rate and stable cycling stability of NASICON structured cathode materials by creating more vacancies and regulating the crystal structure. Therefore, the improved intrinsic electron conductivity and structure stabilization induced by supervalent element replacement may indicate a promising method for achieving other polyanion cathode materials. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (201878195, 201506133, 201805198), the Youth Foundation of Sichuan University (No. 2017SCU04A08), the National Key Research and Development of China (grant No. 2017YFB0307504 and 2016YFD0200404), and the research Foundation for the Postdoctoral Program of Sichuan University (Nos. 2017SCU12018 and 2018SCU12045). The authors also are thankful for the support of the China Scholarship Council, as well as the National Institute for Material Science. This work was also supported by Australian Research Council (ARC) Future Fellowship (FT150100479). This work was performed in part at the Queensland node of the Australian National Fabrication Facility Queensland Node (ANFF-Q), a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for Australia’s researchers. What’s more, many thanks are owed to Dr. Shuaihua Zhang for critically reading this paper. Notes The authors declare that they have no competing financial interests.

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References [1]

Z. Wei, B. Ding, H. Dou, J. Gascon, X. Kong, Y. Xiong, B. Cai, R. Zhang, Y. Zhou, M. Long, J. Miao, Y. Dou, D.Yuan, J. Ma, 2020 roadmap on pore materials for energy and environmental applications, Chinese Chem. Lett. (2019) DOI: 10.1016/j.cclet.2019.11.022.

[2]

B. Xu, S. Qi, M. Jin, X. Cai, L. Lai, Z. Sun, X. Han, Z. Lin, H. Shao, P. Peng, Z. Xiang, J. E. Elshof, R. Tan, C. Liu, Z. Zhang, X. Duan, J. Ma, 2020 roadmap on two-dimensional materials for energy storage and conversion, Chinese Chem. Lett. (2019), DOI: 10.1016/j.cclet.2019.10.028.

[3]

J. Li, J. Li, D. Yan, S. Hou, X. Xu, T. Lu, Y. Yao, W. Mai, L. Pan, Design of pomegranate-like clusters with NiS2 nanoparticles anchored on nitrogen-doped porous carbon for improved sodium ion storage performance, J. Mater. Chem. A 6(15) (2018) 6595-6605.

[4]

M. Wu, B. Xu, Y. Zhang, S. Qi, W. Ni, J. Hua, J. Ma, Perspectives in emerging bismuth electrochemistry, Chem. Eng. J. 381 (2020) 122558.

[5]

D. Bin, F. Wang, A.G. Tamirat, L. Suo, Y. Wang, C. Wang, Y. Xia, Progress in aqueous rechargeable sodium-ion batteries, Adv. Energy Mater. 8(17) (2018) 1703008.

[6]

P. Barpanda, L. Lander, S.-i. Nishimura, A. Yamada, Polyanionic insertion materials for sodium-ion batteries, Adv. Energy Mater. 8(17) (2018) 1703055.

[7]

Z. Jian, Y.‐S. Hu, X. Ji, and W. Chen, NASICON-structured materials for energy storage, Adv. Mater. 29(20) (2016) 1601925.

[8]

X. Rui, W. Sun, C. Wu, Y. Yu, Q. Yan, An advanced sodium-ion battery composed of carbon coated Na3V2(PO4)3 in a porous graphene network, Adv. Mater. 27(42) (2015) 6670-6676.

[9]

Z. Jian, C. Yuan, W. Han, X. Lu, L. Gu, X. Xi, Y.-S. Hu, H. Li, W. Chen, D. Chen, Y. Ikuhara, L. Chen, Atomic structure and kinetics of NASICON NaxV2(PO4)3 cathode for sodium-ion batteries, Adv. Funct. Mater. 24(27) (2014) 4265-4272.

[10] S. Li, Y. Dong, L. Xu, X. Xu, L. He, L. Mai, Effect of carbon matrix dimensions on the electrochemical properties of Na3V2(PO4)3 nanograins for high-performance symmetric sodium-ion batteries, Adv. Mater. 26(21) (2014) 3545-3553. [11] C. Zhu, P. Kopold, P.A. van Aken, J. Maier, Y. Yu, High power-high energy sodium battery based on threefold interpenetrating network, Adv. Mater. 28(12) (2016) 2409-2416. [12] Q. Wang, B. Zhao, S. Zhang, X. Gao, C. Deng, Superior sodium intercalation of honeycomb-structured hierarchical porous Na3V2(PO4)3/C microballs prepared by a facile one-pot synthesis, J. Mater. Chem. A 3 (2015) 7732-7740. [13] Y. Zhou, X. Rui, W. Sun, Z. Xu, Y. Zhou, W. Jern Ng, Q. Yan, E. Fong, Biochemistry-enabled 3D foams for ultrafast battery cathodes, ACS Nano 9 (2015) 4628-4635. [14] Y. Xu, Q. Wei, C. Xu, Q. Li, Q. An, P. Zhang, J. Sheng, L. Zhou, L. Mai, Layer-by-layer Na3V2(PO4)3 embedded in reduced graphene oxide as superior rate and ultralong-life sodium-ion battery cathode, Adv. Energy Mater. 6 (2016) 1600389. 14

[15] Q. Hu, J.-Y. Liao, X.-D. He, S. Wang, L.-N. Xiao, X. Ding, C.-H. Chen, In situ catalytic formation of graphene-like graphitic layer decoration on Na3V2−xGax(PO4)3 (0 ≤ x ≤ 0.6) for ultrafast and high energy sodium storage, J. Mater. Chem. A 7 (2019) 4660-4667. [16] L. Zhao, H. zhao, Z. Du, J. Wang, X. Long, Z. Li. and K. Świerczek, Delicate lattice modulation enables superior Na storage performance of Na3V2(PO4)3 as both an anode and cathode material for sodium-ion batteries: understanding the role of calcium substitution for vanadium, J. Mater. Chem. A 7 (2019), 9807-9814. [17] H. Li, H. Tang, C. Ma, Y. Bai, J. Alvarado, B. Radhakrishnan, S.P. Ong, F. Wua, Y.S. Meng, C. Wu, Understanding the electrochemical mechanisms induced by gradient Mg2+ distribution of Na-rich Na3+xV2–xMgx(PO4)3/C for sodium ion batteries, Chem. Mater. 208 (2018) 2498-2505. [18] Y. Huang, X. Li, J. Wang, L. Miao, C. Li, J. Han, Y. Huang, Superior Na-ion storage achieved by Ti substitution in Na3V2(PO4)3, Energy Storage Mater. 15 (2018) 108-115. [19] D. Zhang, P. Feng, B. Xu, Z. Li, J. Qiao, J. Zhou, C. Chang, High rate performance of Na3V2-xCux(PO4)3/C cathodes for sodium ion batteries, J. Electrochem. Soc. 164 (2017) 3563-3569. [20] X. Li, Y. Huang, J. Wang, L. Miao, Y. Li, Y. Liu, Y. Qiu, C. Fang, J. Han, Y. Huang, High valence Mo-doped Na3V2(PO4)3/C as a high rate and stable cycle-life cathode for sodium battery, J. Mater. Chem. A 6 (2018) 1390-1396. [21] S.Y. Chung, J.T. Bloking, Y.M. Chiang, Electronically conductive phospho-olivines as lithium storage electrodes, Nat. Mater. 1 (2002) 123-128. [22] P. Zhang, Y. Wang, M. Lin, D. Zhang, X. Ren, Q. Yuan, Doping effect of Nb5+ on the microstructure and defects of LiFePO4, J. Electrochem. Soc. 159 (2012) A402-A409. [23] D. Zhang, J. Wang, K. Dong, A. Hao, First principles investigation on the elastic and electronic properties of Mn, Co, Nb, Mo doped LiFePO4, Comp. Mater. Sci. 155 (2018) 410-415. [24] Y. Wang, Z.-s. Feng, J.-j. Chen, C. Zhang, X. Jin, J. Hu, First principles study on electronic properties and occupancy sites of molybdenum doped into LiFePO4, Solid State Commun. 152 (2012) 1577-1580. [25] C.-A. Fisher, V.-M. Prieto and M.-S. Islam. Lithium battery materials LiMPO4 (M = Mn, Fe, Co, and Ni): insights into defect association, transport mechanisms, and doping behavior, Chem. Mater. 20(2008) 5907–5915. [26] L.-L. Zhang, X. Zhang, Y.-M. Sun, W. Luo, X.-L. Hu, X.-J. Wu, Y.-H. Huang, Improved electrochemical performance in Li3V2(PO4)3 promoted by niobium-incorporation, J. Electrochem. Soc. 158 (2011) 924. [27] W. Shen, H. Li, C. Wang, Z. Li, Q. Xu, H. Liu, Y. Wang, Improved electrochemical performance of the Na3V2(PO4)3 cathode by B-doping of the carbon coating layer for sodium-ion batteries, J. Mater. Chem. A 3 (2015) 15190-15201. [28] Q. Zheng, W. Liu, X. Li, H. Zhang, K. Feng, H. Zhang, Facile construction of nanoscale laminated Na3V2(PO4)3 for a high-performance sodium ion battery cathode, J. Mater. Chem. A 4 (2016) 15

19170-19178. [29] L. Guangqiang, J. Danlu, W. Hui, L. Xinzheng, Z. Honghai, J. Yang, Glucose-assisted synthesis of Na3V2(PO4)3/C composite as an electrode material for high-performance sodium-ion batteries, J. Power Sources 265 (2014) 325-334. [30] X. Liang, X. Ou, F. Zheng, Q. Pan, X. Xiong, R. Hu, C. Yang, M. Liu, Surface modification of Na3V2(PO4)3 by nitrogen and sulfur dual-doped carbon layer with advanced sodium storage property, ACS Appl. Mater. Interfaces 9 (2017) 13151-13162. [31] D. Wang, N. Chen, M. Li, C. Wang, H. Ehrenberg, X. Bie, Y. Wei, G. Chen, F. Du, Na3V2(PO4)3/C composite as the intercalation-type anode material for sodium-ion batteries with superior rate capability and long-cycle life, J. Mater. Chem. A 3 (2015) 8636-8642. [32] E. Wang, W. Xiang, R. Rajagopalan, Z. Wu, J. Yang, M. Chen, B. Zhong, S.X. Dou, S. Chou, X. Guo, Y.-M. Kang, Construction of 3D pomegranate-like Na3V2(PO4)3/conducting carbon composites for high-power sodium-ion batteries, J. Mater. Chem. A 5 (2017) 9833-9841. [33] A.I. Abutaha, S.R. Sarath Kumar, A. Mehdizadeh Dehkordi, T.M. Tritt, H.N. Alshareef, Doping site dependent thermoelectric properties of epitaxial strontium titanate thin films, J. Mater. Chem. C 2 (2014) 9712-9719. [34] C. Wang, D. Du, M. Song, Y. Wang, F. Li, A high‐power Na3V2(PO4)3‐Bi sodium‐ion full battery in a wide temperature range, Adv. Energy Mater. 9 (2019) 1900022. [35] G. Feng, X. Liu, Y. Liu, Z. Wu, Y. Chen, X. Guo, B. Zhong, W. Xiang, J. Li, Trapping polysulfides by chemical adsorption barrier of LixLayTiO3 for enhanced performance in lithium-sulfur batteries, Electrochim. Acta 283 (2018) 894-903. [36] W. Weisweiler, N. Subramanian, B. Terwiesch, Catalytic influence of metal melts on the graphitization of monolithic glasslike carbon, Carbon 9 (1971) 755-761. [37] X. Liu, G. Feng, E. Wang, H. Chen, Z. Wu, W. Xiang, Y. Zhong, Y. Chen, X. Guo, B. Zhong, Insight into preparation of Fe-Doped Na3V2(PO4)3@C from aspects of particle morphology design, crystal structure modulation, and carbon graphitization regulation, ACS Appl. Mater. Interfaces 11 (2019) 12421-12430. [38] H. Jin, M. Liu, E. Uchaker, J. Dong, Q. Zhang, S. Hou, J. Li, G. Cao, Nanoporous carbon leading to the high performance of a Na3V2O2(PO4)2F@carbon/graphene cathode in a sodium ion battery, CrystEngComm 19 (2017) 4287-4293. [39] Q. Liu, D. Wang, X. Yang, N. Chen, C. Wang, X. Bie, Y. Wei, G. Chen, F. Du, Carbon-coated Na3V2(PO4)2F3 nanoparticles embedded in a mesoporous carbon matrix as a potential cathode material for sodium-ion batteries with superior rate capability and long-term cycle life, J. Mater. Chem. A 3 (2015) 21478-21485. [40] H. Chen, B. Zhang, X. Wang, P. Dong, H. Tong, J.C. Zheng, W. Yu, J. Zhang, CNT-decorated Na3V2(PO4)3 microspheres as a high-rate and cycle-stable cathode material for sodium ion batteries, 16

ACS Appl. Mater. Interfaces 10 (2018) 3590-3595. [41] E. Wang, M. Chen, X. Liu, Y. Liu, H. Guo, Z. Wu, W. Xiang, B. Zhong, X. Guo, S. Chou, S.X. Dou, Organic cross‐linker enabling a 3D porous skeleton–supported Na3V2(PO4)3/carbon composite for high power sodium‐ion battery cathode, Small Methods 3 (2018) 1800169. [42] Q. Cheng, J. Liang, N. Lin, C. Guo, Y. Zhu, Y. Qian, Porous TiNb2O7 nanospheres as ultralong-life and high-power anodes for lithium-ion batteries, Electrochim. Acta 176 (2015) 456-462. [43] L. Yan, G. Chen, S. Sarker, S. Richins, H. Wang, W. Xu, X. Rui, H. Luo, Ultrafine Nb2O5 nanocrystal coating on reduced graphene oxide as anode material for high performance sodium ion battery, ACS Appl. Mater. Interfaces 8 (2016) 22213-22219. [44] S. Tao, P. Cui, W. Huang, Z. Yu, X. Wang, S. Wei, D. Liu, L. Song, W. Chu, Sol-gel design strategy for embedded Na3V2(PO4)3 particles into carbon matrices for high-performance sodium-ion batteries, Carbon 96 (2016) 1028-1033. [45] Z. Jian, W. Han, X. Lu, H. Yang, Y.-S. Hu, J. Zhou, Z. Zhou, J. Li, W. Chen, D. Chen, L. Chen, Superior electrochemical performance and storage mechanism of Na3V2(PO4)3 cathode for room-temperature sodium-ion batteries, Adv. Energy Mater. 3 (2013) 156-160. [46] Y. Zhao, X. Cao, G. Fang, Y. Wang, H. Yang, S. Liang, A. Pan, G. Cao, Hierarchically carbon-coated Na3V2(PO4)3 nanoflakes for high-rate capability and ultralong cycle-life sodium ion batteries, Chem. Eng. J. 339 (2018) 162-169. [47] Y. Jiang, Y. Yao, J. Shi, L. Zeng, L. Gu, Y. Yu, One-dimensional Na3V2(PO4)3/C nanowires as cathode materials for long-life and high rate Na-ion batteries, ChemNanoMat 2 (2016) 726-731. [48] K. Kretschmer, B. Sun, J. Zhang, X. Xie, H. Liu, G. Wang, 3D interconnected carbon fiber network-enabled ultralong life Na3V2(PO4)3@carbon paper cathode for sodium-ion batteries, Small 13 (2017) 1603318. [49] B.K. Lesel, J.S. Ko, B. Dunn, S.H. Tolbert, Mesoporous LixMn2O4 thin film cathodes for lithium-ion pseudocapacitors, ACS Nano 10 (2016) 7572-7581. [50] D. Xu, C. Chen, J. Xie, B. Zhang, L. Miao, J. Cai, Y. Huang, L. Zhang, A hierarchical N/S-codoped carbon anode fabricated facilely from cellulose/polyaniline microspheres for high-performance sodium-ion batteries, Adv. Energy Mater. 6 (2016) 1501929. [51] T. Chen, Z. Wu, W. Xiang, E. Wang, T. Chen, X. Guo, Y. Chen, B. Zhong, Cauliflower-like MnO@C/N composites with multiscale, expanded hierarchical ordered structures as electrode materials for Lithium- and Sodium-ion batteries, Electrochim. Acta 246 (2017) 931-940. [52] J. L. Li, W. Qin, J. P. Xie, R. Lin, Z. L. Wang, L. K. Pan, W. J. Mai, Rational design of MoS2-reduced graphene oxide sponges as free-standing anodes for sodium-ion batteries, Chem. Eng. J. 332, (2018) 260-266.

17

Graphical Abstract Na 1

Fast electron conduction

Na 2 PO4

y NbO6 z VO6

x

Normal

e-

e- e

Nb doping Reassignment

18

Faster

1. Carbon coating and Nb5+ substitution were simultaneously applied to modify Na3V2(PO4)3 composite. 2. Nb5+ doping could effectively enhance the electron conductivity of the materials. 3. The optimized material exhibits a high specific capacity and good cycling stability.

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