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Surface modification of Na3V2(PO4)3 by nitrogen and sulfur dualdoped carbon layer with advanced sodium storage property Xinghui Liang, Xing Ou, Fenghua Zheng, Qichang Pan, Xunhui Xiong, Renzong Hu, Chenghao Yang, and Meilin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00818 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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ACS Applied Materials & Interfaces

Surface Modification of Na3V2(PO4)3 by Nitrogen and Sulfur Dual-Doped Carbon Layer with Advanced Sodium Storage Property Xinghui Liang,a,§ Xing Ou,a,§ Fenghua Zheng,a Qichang Pan,a Xunhui Xiong,a Renzong Hu,b Chenghao Yang,a, ∗ Meilin Liua,c a

Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, P. R. China b Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials, School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, China c School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, GA30332-0245, USA ABSTRACT: Nitrogen and sulfur dual-doped carbon layer wrapped Na3V2(PO4)3 nanoparticles (NVP@NSC) have been successfully fabricated by a facile solid-state method. In this hierarchical structure, the Na3V2(PO4)3 nanoparticles are well dispersed and closely coated by nitrogen and sulfur dual-doped carbon layer, constructing an effective and interconnected conducting network to reduce the internal resistance. Furthermore, the uniform coating layers alleviate the agglomeration of Na3V2(PO4)3, as well as mitigate the side reaction between electrode and electrolyte. Due to the excellent electron transfer mutually enhancing sodium diffusion for this extraordinary structure, the NVP@NSC composite delivers an impressive discharge capacity of 113.0 mAh g-1 at 1C, and shows a capacity retention of 82.1% after 5000 cycles at an ultrahigh rate of 50C, suggesting the remarkable rate capability and long cyclicity. Surprisingly, a reversible capacity of 91.1 mAh g-1 is maintained after 1000 cycles at 5C under the elevated temperature of 55°C. The approach of nitrogen and sulfur dual-doped carbon coated Na3V2(PO4)3 provides an effective and promising strategy to enhance the ultrahigh rate and ultralong life property of cathode, which can be used for large-scale commercial production in sodium ion batteries. KEYWORDS: Sodium ion batteries, cathode, Na3V2(PO4)3, nitrogen and sulfur dual-doping, rate property. ∗

Corresponding author. Tel.: +86-803-39381203; E-mail: [email protected] (C. Yang).

§

These authors contributed equally. 1

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1. INTRODUCTION With the everlasting demand for sustainable and efficient energy storage utilization, lithium ion batteries (LIBs) have aroused considerable interests owing to their high energy density and long cycle stability.1,2 Nevertheless, due to the limiting of possible lithium shortage and unevenly distribution lithium resource, as well as the increasing price for large-scale implementation of Li chemicals for LIBs, it is imperative to exploit a novel type of energy storage to partially replace LIBs.3,4 Hence, sodium ion batteries (SIBs) have obtained abundant researchers’ attentions recently and are considered as a promising candidate for near future storage system, which exhibit similar chemical property to LIBs, and possess widespread and available sodium resource with low price.5,6 Currently, extensively attempts have been made and investigated to enhance sodium storage property of cathodes, including layered transition metal oxide,7 prussian blue analogues,8 sodium polyanionic compounds,9 and organic composite.10 Among them, the polyanion-based compounds are regarded as a suitable cathode due to structure diversity and thermally stability. Particularly, the Na3V2(PO4)3 (NVP) is intensively pursued to be a promising candidate, which owns unique and open NASICON superionic conductor framework with high reversible capacity and excellent structure stability.11,12 Meanwhile, NVP could exhibit high and flat voltage plateau at about 3.4 V, correlated with the redox couples V4+/V3+ with negligibly voltage polarization, resulting in high energy density when assembled NVP as sodium ion full batteries.13 Nevertheless, bare NVP exhibits poor intrinsic electronic conductivity and decreased mobility at phase boundaries, which is originated from the isolation between VO6 octahedra and PO4 tetrahedral in the rhombohedral structure.14 Similar to LiFePO4,15 low electrical conductivity of NVP exerts a detrimental influence on electrochemical performance, presenting low practical capacity and inferior rate capability. 2


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To overcome the issues, numerous methods have been employed, including nonosizing structure design and applying carbon coating. Specifically, morphology construction is an effective approach to improve the reaction kinetics by shorting the Na+ ion transport path and offering more channels for Na+ ion intercalation, Mai's group16 have synthesized a novel 3D NVP nanofiber with a thickness of 20-40 nm, which could deliver an reversible capacity of 110 mAh g-1 at 10C with 95.9% retention after 1000 cycles, demonstrating impressive electrochemical performance. Meanwhile, the highly efficient and most common strategy is to apply electron conductor of carbonaceous material to improve the conductivity of NVP. Klee et al.17 exhibited a carbon coated NVP composite fabricated by an oleic acid-based surfactant-assisted method, which delivers a reversible capacity of 76.6 mAh g-1 at 40C, and maintains 105.3 and 96.7 mAh g-1 at rates of 5C and 10C after 100 cycles, respectively. Although the reversible capacity and cycle stability are increased in a certain degree, the rate capability, especially at ultrahigh current rate for long-term cycling and tough condition of elevated temperature still requires to be strengthened. Currently, it is convinced that non-metallic heteroatom doping can enhance the electrical conductivity and electrochemical property of carbonaceous material. For instance, nitrogen doping on carbon layer is an effective way to fabricate numerous active defects so that improving ion diffusion compared with the conventional carbon coating.18,19 Meanwhile, sulfur-doped carbon-contained material displays significant improvements on electrochemical behavior, owing to sulfur-doping would clearly enlarge the defect degree of carbonaceous materials, which is inclined to occupy the edge and defective sites of frame structure.20,21 According to previous reports,22,23 the multiple heteroatom doping on coating carbon material exerts a positive effect on electrochemical properties of host material, which provides rapid ion diffusion and fast electron pathway, resulting in 3



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the high rate capability and long cycle stability. In this work, the nitrogen and sulfur co-doped carbon-coating modification technique is initially used to modify the NVP cathode for SIBs synthesized by a facile solid-state method, in which urea and benzyl disulfide used as the nitrogen and sulfur source, respectively. The inducement effect between chemical dopants and the geometric lattice defects in uniform carbon layer could efficiently mitigate the particle agglomeration and greatly facilitate the Na+ ion diffusion and electron transport, so that enhancing the electrochemical activities of NVP electrode. As expected, the resulting composite demonstrate low intrinsic resistances, thus fairly outstanding electrochemical property on account of the ultrahigh rate capability and ultralong cycle life under normal conditions or elevated temperature. 2. EXPERIMENTAL SECTION 2.1. Materials preparation The bare Na3V2(PO4)3, carbon-coated Na3V2(PO4)3 and nitrogen and sulfur dual-doped Na3V2(PO4)3 composite, denoted as NVP, NVP@C and NVP@NSC, respectively, were prepared via one-step solid-state synthesis. Citric acid, glucose, urea and benzyl disulfide were serve as reductive agent, carbon, nitrogen and sulfur source, respectively. In a typical approach of NVP@NSC, the precursors of analytical reagents NH4H2PO4, NH4VO3, Na2CO3, glucose and benzyl disulfide were weighed in the mole ratio of 3:2:1.5:0.15:0.03, and grounded by ball milling (with a speed of 500 r/min) for 10 hours in acetone solution. Then the mixture was dried at 80 °C and sealed into the middle of tubular furnace and a boat of urea was placed in the upstream. The material was presintered at 350 °C for 4 h, the intermediate powders were reground and sintered for 12 h at the temperatures of 700 °C under hydrogen/nitrogen mixture atmosphere (H2:N2=8:92) to yield final 4


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NVP@NSC material. To further clarify the modification impact of the nitrogen and sulfur dual-doped carbon layer, the preparation of NVP@C was similar to that of NVP@NSC process in the absence of urea and benzyl disulfide, while the synthesis of bare NVP was employed the same procedure but without the addition of urea, benzyl disulfide and glucose, respectively. 2.2. Materials characterization Thermo gravimetric (TG) analysis was measured at the temperature from room temperature to 700 °C with an elevated rate of 10 °C min-1 by the SDT Q600 apparatus. The X-ray diffraction (XRD) tests using Cu Kα radiation were performed on a Bruker D8 Advance diffractometer. The data was then analysed by the Rietveld structure refinement program on TOPAS 4.0. The in situ XRD analysis was conducted to investigated the structure evolution and reaction mechanism, as similar as reported literature.24 The Raman spectra were obtained by Jobin-Yvon LabRAM HR-800 spectrometer. X-ray photoelectron spectrometer (XPS) was conducted on Kratos Model XSAM800. The morphology was observed by a JSM-7600F field-emission scanning electron microscopy (FESEM) and a Tecnai G2 F20 S-TWIN transmission electron microscope (TEM) couple with a Bruker AXS energy dispersive spectroscopy (EDS). The light element content in the composite was confirmed by using chemical analysis by CHNS/O measurement (Vario EL, Elementar). 2.3. Electrochemical measurements Electrochemical characterizations were performed on CR2032 coin-type cell and assembled in a pure argon-filled glove box. Normal loading of cathode mass was in the range of 3.0-4.0 mg cm-2. The working electrodes were fabricated by mixing active material (NVP) with carbon black and polyvinylidene fluoride (weight ratio of 8:1:1) in N-methyl-2-pyrrolidone (NMP) solvent, while it was fabricated to a 12 mm diameter disk. The separator was glass microfiber membrane (Whatman) 5



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and the electrolyte was 1M NaClO4 dissolved in an ethylene carbonate (EC) and propylene carbonate (PC) mixture solution (1:1, volume ratio) with 5 vol.% FEC additive, respectively. It should be noted that a two-electrode cell system is employed with sodium metal tablet used as the reference electrode and counter electrode for electrochemical measurements, and all samples are mass normalized before tests. The sodiation-desodiation cycling is galvanostatically performed at various rates with voltages window of 2.0-4.0 V (versus Na/Na+) at room temperature (25°C) and elevated temperature (55 °C), while the specific capacity was evaluated based on the active materials. Cyclic voltammetric was carried out in the potential window of 2.0-4.0 V by conducting a CHI 660E electrochemical workstation. EIS were recorded by using IM6 (Zahner) electrochemical station with 5 mV amplitude in the frequency range of 100 KHz-0.01 Hz. 3. RESULTS AND DISCUSSION The detailed synthesis, and electron transport as well as Na+ ion diffusion of NVP@NSC electrode are schematic illustrated in Figure 1. It is advantage to construct an intimate carbon three dimensional carbon layer network architecture, which can mitigate the aggregation of NVP particles and enhance the adhesion between NVP nanoparticles and carbon layers. More importantly, the decomposition outcomes of benzyl disulfide and urea would react with carbonization product during this dissociation process, forming the nitrogen and sulfur dual-doping carbon layers with abundant defects.25 As a consequence, it is hypothesized that the surface modification of NVP nanoparticles by nitrogen and sulfur dual-doping carbon layer can exhibit superior electrical conductivity and excellent electrochemical performance, on account of the synergistic effect between nitrogen and sulfur doping: Firstly, dual-doping easily tailors the electric property and rises to a unique electron distribution, owing to that sulfur atoms share similar electro negativity with carbonaceous materials 6


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and nitrogen atoms generate tremendous extrinsic defects.26 Secondly, based on the theoretical calculations, it is believed the band gap of carbonaceous materials is expanded and the electronic property is greatly improved.27

Figure 1 Schematic illustration of the fabricated process of NVP@NSC composites. To confirm the as-prepared samples structure, the XRD patterns and the corresponding Rietveld refinement analysis of NVP, NVP@C and NVP@NSC are illustrated in Figure 2a and Figure S1. For sample NVP@NSC, all diffraction peaks are well-indexed to the rhombohedral structure with the R3c space group (JCPDS No. 53-0018), which is in accordance with the previous literature.28,29 Moreover, there are no impurity peaks in this pattern, which manifests the high crystallinity and proves that dual-doping carbon layer has no adverse impact on the structure of Na3V2(PO4)3. Due to low content and amorphous phase of carbon with nitrogen-sulfur dual-doping, no significant diffraction peak is ascribed to the carbon in NVP@NSC.

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Figure 2. XRD (a), Raman spectrum (b), TGA curves (c), and XPS spectra core level of S1s (d), N1s (e) and C1s (f) for NVP@NSC composite. To further investigate the carbon in all samples, Raman spectra were performed as depicted in Figure 2b. For NVP@NSC, it is demonstrated that the Raman spectra can be deconvoluted into several broad signal peaks by curve fitting, as shown in Figure S2. Five typical characteristic peaks located at 1164, 1348, 1506, 1587 and 1617 cm-1, corresponding to the D4, D1, D3, G, and D2 bands, respectively,30 while the G, D2, and D3 peaks are strongly overlapped. Besides, the intensity ratio of ID1/(IG +ID1+ID2) indicates to the amount of defects and disorder in carbon materials.30,31 It is worth noting that the NVP@NSC possesses the larger value of ID1/(IG +ID1+ID2) (0.666) than that of 8


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NVP@C (0.654), suggesting a relatively higher degree of defectiveness during the thermal treatment, which can prove the efficient nitrogen and sulfur doping into the carbon and thus result in more defect on the surface. To confirm the carbon content of these NVP composites, the thermal gravimetric analysis (TGA) was conducted in air atmosphere and calculated as shown in Figure 2c. Normally, due to the Na3V2(PO4)3 will react with oxygen in the high temperature,13 which is confirmed by ex situ XRD analysis of the oxidized product of Na3V2(PO4)3 at high temperature, as presented in Figure S3, so that the total weight change for bare NVP is 4.6% increment. Therefore, based on the composites consisted of bare NVP and carbon layer, the carbon contents of NVP@C and NVP@NSC are 3.25 wt.% and 3.53 wt.%, respectively. Meanwhile, the carbon, nitrogen and sulfur contents in NVP@NSC composite further measured by elemental analysis of CHNS test (Table S2) are 3.96%, 0.73% and 0.15%, respectively, which are in consistent with the TGA result. Additionally, the carbon content for the bare NVP and NVP@C samples are 0.32% and 3.47%, respectively. The surface chemical information of NVP@NSC sample was further investigated by XPS analysis. The V 2p core level (Figure S4) is fitted to two major peak with a binding energy of 516.5 and 523.2 eV, related to the V 2p3/2 and V 2p1/2, respectively, which are in agreement with the data of V3+ in Na3V2(PO4)3 electrode.32,33 The deconvolution of S 2p (Figure 2d) exhibits three peaks centered at 163.2, 165.4 and 164.1 eV, respectively. The former two peaks can be ascribed to the S-C-S covalent bond of thiophene-S with the 2p3/2 and 2p1/2, which validate the effective sulfur doping into the lattice of carbon layer,20,34 while the latter peak is related to the S-O group. Furthermore, the XPS spectra of N 1s (Figure 2e) is deconvoluted into three peaks at 397.8, 400.2 and 401.6 eV, which are originated from the pyridinic N, pyrrolic N and quaternary N, respectively, 9



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suggesting the presence of N element in the coating carbon layer.35,36 The C 1s spectrum (Figure 2f) is resolved into three peaks at 284.3, 285.1 and 287.2 eV, assigned to sp2 C-C, C-O/C-N/C-S, and C=O bonds, respectively. The XPS peaks related to the C-N and C-S bond confirm that a small part of carbon atoms are substituted by nitrogen and sulfur atoms in the NVP@NSC matrix, while S and N element are covalently bonding to the carbon network.37,38 According to the first-principle calculation results, these functional groups are supposed to improve the binding energy between nonpolar carbon atoms and polar Na3V2(PO4)3 particles. Furthermore, this type of hereroatom doping provides more active defects and sites, which can facilitate the Na+ diffusion and benefit for Na+ storage, so that significantly improving the rate capability and cycling stability of Na3V2(PO4)3 cathode.39,40 The microstructure and morphology of NVP, NVP@C and NVP@NSC samples were confirmed by SEM and TEM measurements. The bare NVP sample displays regular shape with size distribution of about 900 nm (Figure S5), while the TEM images reveal that most of bare NVP particles owns smooth surface. According to Figure S6, it is found that NVP@C sample has similar morphology with particle sizes of approximately 600-800 nm, which is wrapped by a thin amorphous carbon layer from carbonization of glucose. It can enhance particle-to-particle electronic contact, so that decrease the resistance among the particle interfaces. As shown in Figure 3a and b, the NVP@NSC sample has irregular particles with relatively smaller size of 400-700 nm from SEM characterization. Compared with NVP@C, the NVP@NSC exhibits smaller particle size after incorporation of exotic atom into the carbon, which can effectively alleviate the agglomeration during the heat process. It is ascribed to that N and S doping can provide more active defects and sites as nucleation on the carbon layer surface and suppression the crystalline growth of Na3V2(PO4)3 10


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particles.19,35 While the fine architecture of NVP@NSC is further observed by TEM as revealed in Figure 3c. The Na3V2(PO4)3 particles are embedded into the thin carbon network and form the three dimension porous structure, which is beneficial for facilitating Na+ diffusion and improving the electronic conductivity.

Figure 3. (a, b) SEM images, (c) TEM, (d) HRTEM and (e) EDS elemental mapping of NVP@NSC composites. The HRTEM image also shows that the integrated carbon layer with a 5-7 nm thickness uniformly coat on the Na3V2(PO4)3 nanoparticles (Figure 3d), displaying a typical core-shell structure of carbon coating. Meanwhile, it can be clearly observed a lattice fringe with d-spacing of 11



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0.283 nm, ascribed to the (211) plane of the rhombohedral Na3V2(PO4)3. The corresponding selected area electron diffraction (SAED) image offers a set of sharp spots, which are attributed to (211), (300) and (226) planes of the Na3V2(PO4)3 phase and confirmed with good crystallinity. To further verify the element distribution of NVP@NSC, energy dispersive X-ray (EDX) mapping was conducted. Due to the low content of N and S, the weak signal of N and S elements are obtained (Figure 3e). It is still clearly seen that V, P, C, S and N elements share homogenous distribution, demonstrating that the uniform doping of nitrogen and sulfur in carbon layers efficiently wrap on the surfaces of NVP. Both XPS and EDX strongly confirm the existence of nitrogen and sulfur in the NVP@NSC composite. To acquire the information for the reaction mechanism of as-prepared Na3V2(PO4)3 cathode, in situ XRD test was conducted on a special in situ cell with a wide 2θ range (20-40°). The contour plots of in situ XRD patterns are obtained at different states during initial charging and discharging process within the potential window of 2.0-4.0 V at rate of 0.1 A g-1, as presented in Figure 4a. It is noted that the different phase compositions are color-relevant, while red and blue represent lowest and highest intensity of diffraction peaks, respectively, and yellow indicate the variation of main phase in this pattern. Figure 4b displays the corresponding XRD patterns and piled sequentially. The galvanostatic charge-discharge profile during the initial cycle is set on the left to provide specific states of charge or discharge. The constant unchanged peaks located at 26.5° and 38.5° are ascribed to carbon substrate and beryllium oxide.41 While the main peak at 20.2°, 23.8°, 28.8°, 32.1° and 35.7° are related to the (104), (113), (024), (116) and (300) planes of Na3V2(PO4)3 at the state of open circuit voltage (OCV), which have been marked in Figure 4. The whole process of charging and discharging can be divided into four stages. In stage I from OCV to 3.4 V, the intensity of 12


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Na3V2(PO4)3 peaks are gradually weakened and shift to higher 2θ, while some small peaks nearby are appeared, which correspond to the Na+ extraction from the host Na3V2(PO4)3 and the initial formation of NaV2(PO4)3. It is regarded as coexistence of two phases during this process. In stage II during charging until 4.0 V, the peaks of Na3V2(PO4)3 are totally disappeared and the intensity of NaV2(PO4)3 is enhanced, indicating that the NaV2(PO4)3 reigns over this region and it is in consistent well with the previous literature.42 After discharging in stage III, the intensity of NaV2(PO4)3 peaks decreases with appearing and strengthening peaks of Na3V2(PO4)3. When discharging to 2.0 V in stage IV, all peaks are attributed to Na3V2(PO4)3 without any impurities, suggesting the completely phase transformation.

Figure 4. (a) Contour plots of in situ XRD results of NVP@NSC electrode against the voltage profile during the initial cycle at cut-off voltage of 2.0-4.0 V. (b) Corresponding in situ XRD patterns of NVP@NSC cathode. 13



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Compared with the XRD patterns procured before and after the initial cycle, there is little significant variations observed about peak positions. This result indeed suggests that the Na3V2(PO4)3 is two-phase reaction mechanism, and it is maintained the structural stability and integrity in spite of small volume changes.16,43 However, it also indicates that the capacity fading and poor rate property of Na3V2(PO4)3 electrode may be attributed to the high electrode resistance and low reaction kinetic, which lead to the tough (de)intercalation of the Na+ ion into Na3V2(PO4)3.44 Specifically, the fully desodiated phase of NaV2(PO4)3 with some degree of disordering, exhibits a disorder-to-order transition during the first sodiation, which will change the unit cell size and alter the ion channel. It has detrimental effect on the kinetics of the electrochemical reaction, especially the rate property. This phenomenon is usually observed for other similar polyanionic materials, such as LiFePO445 and Li3V2(PO4)3.46 To address this issue, the method of the nitrogen and sulfur dual-doped carbon in situ wrapping Na3V2(PO4)3 is adopted to enhances the electronic conductivity and reaction kinetic. Firstly, the electrochemistry behavior of NVP, NVP@C and NVP@NSC were conducted and evaluated by CV measurements between 2.0 and 4.0 V at 0.1 mV s-1 as illustrated in Figure 5a. The CV tests of all samples show similar profiles with a couple of oxidation peak and corresponding reduction peak, which are located at about 3.5 V and 3.4 V, respectively, representing the typical sodium extraction and insertion process with the V4+/V3+ redox couple. It is confirmed that the difference between the oxidation and reduction peak indicates the degree of electrochemical polarization.47 It is noted that the redox peaks of NVP@NSC were centered at 3.44 and 3.30 V, providing a potential difference of 0.14 V, which is lower than that of NVP and NVP@C. The lower difference between the redox peaks for NVP@NSC is attributed to the higher electrical conductivity, offering the lower electrochemical 14


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polarization and higher interfacial stability. The rate ability is a vital purpose to evaluate the electrochemical performance, and the galvanostatic sodiation and desodiation process of the electrodes were investigated at current rate from 0.1C to 200C as shown in Figure 5b and Figure S9. It is clearly observed the initial discharge capacities of NVP and NVP@C are 107.0 and 112.0 mAh g-1 at 0.1C (Figure S9), respectively, while the NVP@NSC demonstrates the highest discharge capacity of 113.2 mAh g-1 (Figure 5b). Additionally, the NVP@NSC exhibits a more smooth voltage platform and a narrower voltage gap between charge and discharge profile, which is in consistence with the CV curves. Furthermore, the coulombic efficiency of NVP and NVP@C are 93.5% and 98.2%, respectively, while the NVP@NSC possesses the optimal coulombic efficiency of 98.4%, indicating that effective nitrogen and sulfur dual-doped can not only decrease the electrode polarization, but enhance the coulombic efficiency of bulk Na3V2(PO4)3. When the current rates are increased gradually, the reversible capacities of all samples decline due to the unavoidable electrochemical polarization. Compared with the relatively poor electrochemical performance of NVP and NVP@C, the NVP@NSC delivers the highest capacity and better rate capability with a discharge capacity of 113.2, 113.0, 111.0, 110.5, 108.0, 105.0, 95.0 and 87.5 mAh g-1 at 0.1, 1, 2, 5, 10, 20, 50 and 100C, respectively (Figure 5c). Surprisingly, even at ultrahigh rate of 200C within 17 seconds per cycle, the discharge capacity of 77.6 mAh g-1 is achieved and the distinct voltage plateau is still observed for NVP@NSC, which is equivalent to 68.6% of the reversible capacity at 0.1C, indicating the best capacity retention and lowest polarization with increased current rates. Remarkably, the NVP@NSC electrode can recover to 112.6 mAh g-1 as the current rate is turned back to 1C again, presenting rarely capacity decay after 100 cycles. However, it is noted that there are significant fading for NVP and NVP@C, 15



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demonstrating the inferior rate property.

Figure 5. Cyclic voltammograms curves (a) and rate capability of NVP, NVP@C and NVP@NSC (b); galvanostatic charge-discharge profile of NVP@NSC from 0.1C to 200C (c). Rate property of the as-synthesized NVP@NSC at various current densities in comparison with references (d). Cycling performances of NVP@NSC at the rate of 20C, 50C, and 100C (e). To further demonstrate the superior performance of NVP@NSC, the long-term cycling measurements were carried out and evaluated at 20C, 50C and 100C, respectively. As illustrated in Figure 5e and Figure S10-S11, the initially discharge capacity of NVP, NVP@C and the NVP@NSC is 43.3, 82.2, and 102.5 mAh g-1 at 20C rate, respectively, which decrease to 19.4, 67.2, and 99.4 16


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mAh g-1 after 2000 cycles, with a corresponding capacity fading of 47.9%, 84.8%, 97.0%. Furthermore, the discharge capacity of NVP@NSC still reaches 78.2 mAh g-1 at rate of 50C with merely 17.9% capacity fading after 5000 cycles. Especially, when tested at as high as 100C, the NVP@NSC still delivers 60.4 mAh g-1 after 5000 cycles with an amazing capacity retention of 71.5%. However, under the identical test parameters, bare NVP can not stand the high-rate and long-term cycling, while the capacity retention of NVP@C shows comparatively low of only 22.4%. Compared with other previous works regarding Na3V2(PO4)3 cathode,48-54 the rate performance of NVP@NSC, especially at high current rate of 100C and 200C, exhibits highest reversible capacity and obvious superiority as listed in Figure 5d and Table S3. The electrochemical performance of the Na3V2(PO4)3 electrode with different coating materials were also investigated at harsh testing conditions of elevated temperature. As shown in Figure 6a. When tested in the temperature of 55 °C at 1C rate, the initial discharge capacity of NVP@NSC is 115.2 mAh g-1, and slightly higher than that of room temperature, suggesting improved electrochemical kinetics at higher temperatures. After 200 cycles, the reversible capacity is still as large as 105.8 mAh g-1 with retention of 91.8% (Figure 6b). Moreover, superior capacity is obtained for NVP@NSC electrode, again reflecting the polarization of voltage difference. Specifically, the NVP@NSC maintains the working voltage of 3.2 V after 500 cycles at 5C, which stands for higher specific energy at this extreme condition (Figure 6c). In contrast, NVP@C and NVP gets larger polarization and more distinctive difference within increasing cycles. In addition, NVP@C and NVP exhibit obvious capacity losses to 56.4 and 14.5 mAh g-1, with a capacity retention only 50.1% and 24.7% after 1000 cycles at 5C rate, respectively (Figure 6d). It is worthy to note that the NVP@NSC delivers the highest reversible capacity of 91.1 mAh g-1, holding capacity retention of 80% and 17



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indicating amazing electrochemical performance. In brief, the high temperature of 55 °C will unavoidably augment the structural instability and considerably increase the reconstruction SEI layer accumulated at the electrode surface, which enlarge the kinetic barrier for the reversible reaction and lead to serious capacity fading. However, it is proved that the nitrogen and sulfur co-doping carbon layers increase the electron transport and prevent the unwanted side reaction between bulk materials with electrolyte.

Figure 6. Electrodes tested at elevated temperature of 55 °C: the initial charge-discharge curves (a) and cycling capability (b) of NVP, NVP@C and NVP@NSC at 1C. Charge-discharge curves in various cycles (c) and Rate performance (d) of the NVP@NSC at 5 C. As aforementioned, the outstanding rate performance of NVP@NSC benefits from exquisite architecture that the nitrogen and sulfur dual-doped carbon in situ wrapping Na3V2(PO4)3 nanoparticles. Particularly, the unique interconnected carbon layers with large surface area are appropriate substrates for growth of the Na3V2(PO4)3 nanoparticles and suppressing the aggregation 18


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significantly. They not only facilitate Na+ ion diffusion and provide efficient electron transport path, but also offer effective active sites to react with the electrolyte. Moreover, as an efficient method for enhancing chemical reactivity of coating carbon layers, nitrogen and sulfur co-doped can induce a few topological defects and form a disordered structure, which effectively tailor ion transport behavior and improve sodium absorption ability. Meanwhile, nano-sized Na3V2(PO4)3 particle and sufficient interlayer distance guarantee the Na+ ion diffusion and enhance electron transport, which further results in the ultrahigh rate capability and ultralong cycle ability of the NVP@NSC composite. To obtain intuitional insight into the reaction kinetic and influence of nitrogen and sulfur dual-doping on charge transfer, electrochemical impedance spectra (EIS) were carried out and investigated. Figure 7 clearly illustrates the EIS measurements of NVP, NVP@C and NVP@NSC at open circuit voltage before cycling and fully desodiated state after various cyclings. Each curve is comprised of a straight inclined line and a semicircle in low frequency and high-middle frequency area, respectively, which are well fitted by using equivalent circuit model as shown in Figure 7e.55,56 In this model, Rs relates to the Ohmic resistance of solvent resistance and Rct stands for charge-transfer resistance. While the symbol of CPE and Zw are ascribed to the double layer capacitance and Warburg impedance regarded to Na+ ion diffusion in the Na3V2(PO4)3 nanoparticles, respectively. It is observed that the obtained value of Rct for NVP@NSC is 34.0 Ω (Figure 7c), which is clearly smallest among three samples as presented in Table S4, suggesting the increased charge transfer speed and rapid Na+ ions diffusion through the electrode-electrolyte interface. It is noted that the sodium ion diffusion coefficient (DNa) is a key index to appraise the kinetics activity in an electrochemical reaction, and it is estimated and calculated by the following Equation 19



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(1)57 DNa = R2T2 /2A2n4F4C2σw2

(1)

Z' = Rs + Rct + σwω-1/2

(2)

Figure 7. The recorded impedance spectra of NVP (a), NVP@C (b) and NVP@NSC (c). The relationship between Z' and ω-1/2 in low frequency region (d). Equivalent circuit used for fitting the experimental EIS data (e). Linear fitting of the current peak of CV curves against the square root of scan rate (f). Where R stands for gas constant, T is ascribed to absolute temperature, F is related to Faraday constant, A is attributed to area of electrode surface, C is concentration of Na+ ion, σw is the Warburg 20


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impedance coefficient, which can be obtained by calculating the relationship between plot of real impedance (Z') and lower angular frequency (ω-1/2) according to Equation (2). The specific values of slope (σw) for all samples are calculated and listed in Figure 7d.58 As presented in Table S4, the sodium ion diffusion coefficients of NVP, NVP@C and NVP@NSC are 8.85×10-13, 4.04×10-11 and 2.18×10-10 cm2 s-1, respectively (Figure 7d). It is obviously shown that NVP@NSC exhibits the highest value of sodium diffusion coefficient, which is in consistent with the previous rate performance (Figure 5 and Figure 6). When tested at fully desodiation state after various cycles at 1C, there is an additional weak semicircle in the middle-frequency of EIS spectra, which is related to the resistance (RSEI) of the solid electrolyte interface (SEI) (Figure 7e), indicating the very small SEI film on this material. After 100 cycling, the value of Rct for NVP increases from 101.9 Ω to 328.6 Ω (Figure 7a), while for NVP@C enhances from 66.2 Ω to 200.2 Ω (Figure 7b). Nevertheless, the NVP@NSC is only 66.8 Ω after 100 cycles (Figure 7c), indicating the lowest resistance values. Because a thick SEI film is a major concern for the large IR drops and low rate capability, therefore, the formation of unacceptable SEI film as well as the electrode/electrolyte interfacial reaction have been suppressed and mitigated by coating a nitrogen and sulfur co-doping carbon layer. Additionally, the sodium diffusion property was further investigated by the CV measurements with multistep sweeping rate. According to the Randles-Sevcik Equation as presented in Figure 7f and Figure S13,59,60 the reaction kinetics of NVP@NSC is clearly more favored than that of NVP and NVP@C, confirming higher sodium ion diffusion coefficient of NVP@NSC once again, which is in agreement with conclusion obtained by the EIS method. The above results indicate that the electrical conductivity and ionic transport of Na3V2(PO4)3 can be greatly enhanced by well wrapped carbon 21



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layers with an appropriate amount of nitrogen and sulfur co-doping, in accordance with the aforementioned outstanding electrochemical performance. To further evaluate the practical characteristics of Na3V2(PO4)3 cathode, the full sodium ion batteries were fabricated by applying NVP@NSC as the cathode and hard carbon as the anode, as illustrated in Figure 8a. During the desodiation process, Na+ ions are extracted from the cathode Na3V2(PO4)3, migrated through the electrolyte and intercalated into the hard carbon. On discharging, Na-ions and electrons transport reversely. As shown in Figure S14, when tested in half-cell assembled by using sodium tablet as counterpart anode, the hard carbon electrode delivers the initial sodiation and desodiation capacity of 387.4 and 171.9 mAh g-1 at a rate of 100 mA g-1, respectively, with coulombic efficiency of only 44.4%. After 300 charge-discharge cycles, it exhibits a reversible capacity of 134.8 mAh g-1, demonstrating relatively stable property. Owing the low coulombic efficiency of hard carbon, the electrochemical performance of Na3V2(PO4)3 will be severely affected by the hard carbon, which is hardly assessed with full battery objectively and comprehensively. Therefore, different from other methods of full battery assembling, the anode hard carbon was pre-sodiated so as to reduce its initial irreversible capacity. It is important to control the cathode and anode capacity ratio to approximately 1.1, so that keeping electrode system balance. It is obviously seen that the sodiation and desodiation capacities of full battery are 99.1 and 98.5 mAh g-1 at 100 mA g-1 during the second cycle, corresponding to a coulombic efficiency of 99.4% (Figure 8b). For the long-term cycling of the full battery, it exhibits a discharge capacity of 66.1 mAh g-1 with a capacity retention of 67.1% after 100 cycles, suggesting an impressive rate capability and cycling stability (Figure 8c). Furthermore, because the full cell is performed between 2.0 and 3.8 V, it is noted that the energy density for NVP@NSC is 329.0 Wh kg-1 (Figure 8d), which is higher than other reported 22


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sodium-ion batteries. The outstanding electrochemical properties of NVP@NSC are ascribed to the effective wrapping of conductive nitrogen and sulfur dual-doping. Hence, as-prepared NVP@NSC is a promising cathode candidate for sodium ion battery applications with long cycle life and advanced energy storage.

Figure 8. (a) Schematic illustration of symmetric full sodium battery using NVP@NSC and hard carbon as positive and negative electrodes. (b) Typical charge-discharge profiles of NVP@NSC/hard carbon full sodium battery for the initial ten cycles at 1C. Cycling performance (c) and energy density (d) of NVP@NSC-hard carbon full sodium battery for 100 cycles at 1C. 4. CONCLUSION The NVP@NSC composite has been successfully synthesized by a simple and productive strategy. Well-dispersed NVP nanoparticles are intimate wrapped by the nitrogen and sulfur co-doped carbon layer, forming an interconnected conductive network structure, which can effectively enhance the electronic transport. Furthermore, the nitrogen and sulfur co-doped carbon provides numerous defects site to shorten Na+ diffusion paths, resulting in the ideal electrochemical reaction kinetics. Significantly, the resultant NVP@NSC composite exhibits an incredible sodium 23



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storage performance, especially the ultrahigh rate capability (77.6 mAh g-1 at a high rate of 200C) and ultralong cycling life (a high capacity retention of 82.1% after 5000 cycles at 50C), which holds promising as cathode material for SIBs. In addition, the NVP@NSC exhibits outstanding property even at high temperature of 55 °C, owing to the dual-doping with multiple electrochemical active sites. In terms of its facile and practical, the approach can be employed to enhance the conductivity of other cathode materials and provided a new insight to apply in large-scale industrial production. ASSOCIATED CONTENT Supporting information Additional details of the Rietveld refinement of the XRD patterns, the SEM and TEM images, cycling capability, CV curves at different scan rates for bare NVP and NVP@C. The electrochemical performance of hard carbon. Tables for comparison of rate capability with others reported in the literatures and results of electrochemical impedance. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(C.H. Yang). Email: [email protected].

Author Contributions §These authors (X. Liang and X. Ou) contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS 24


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We gratefully acknowledge the financial support from Natural Science Foundation of China (51402109), Project of Public Interest Research and Capacity Building of Guangdong Province (2014A010106007), Pearl River S&T Nova Program of Guangzhou (201506010030), Guangdong Innovative and Entrepreneurial Research Team Program (No.2014ZT05N200) and Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2016A030306010). REFERENCES (1) Larcher, D.; Tarascon, J. Towards Greener and More Sustainable Batteries for Electrical Energy Storage. Nat. Chem. 2015, 7, 19-29. (2) Ding, X.; Liu, X.; Huang, Y.; Zhang, X.; Zhao, Q.; Xiang, X.; Li, G.; He, P.; Wen, Z.; Li, J.; Huang, Y. Enhanced Electrochemical Performance Promoted by Monolayer Graphene and Void Space in Silicon Composite Anode Materials. Nano Energy 2016, 27, 647-657. (3) Fan, X.; Mao, J.; Zhu, Y.; Luo, C.; Suo, L.; Gao, T.; Han, F.; Liou, S.C.; Wang, C. Superior Stable Self-Healing SnP3 Anode for Sodium-Ion Batteries. Adv. Energy Mater. 2015, 5, 201500174. (4) Han, M. H.; Gonzalo, E.; Singh, G.; Rojo, T. A Comprehensive Review of Sodium Layered Oxides: Powerful Cathodes for Na-Ion Batteries. Energy Environ. Sci. 2015, 8, 81-102. (5) Langrock, A.; Xu, Y.; Liu, Y.; Ehrman, S.; Manivannan, A.; Wang, C. Carbon Coated Hollow Na2FePO4F Spheres for Na-Ion Battery Cathodes. J. Power Sources 2013, 223, 62-67. (6) Xiong, X.; Wang, G.; Lin, Y.; Wang, Y.; Ou, X.; Zheng, F.; Yang, C.; Wang, J. H.; Liu, M. Enhancing Sodium Ion Battery Performance by Strongly Binding Nanostructured Sb2S3 on Sulfur-Doped Graphene Sheets. ACS Nano 2016, 10, 10953-10959. (7) Wang, Y.; Xiao, R.; Hu, Y. S.; Avdeev, M.; Chen, L. P2-Na0.6[Cr0.6Ti0.4]O2 Cation-Disordered Electrode for High-Rate Symmetric Rechargeable Sodium-Ion Batteries. Nat. Commun. 2015, 6, 25



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Surface Modification of Na - ACS Publications - American

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