Hard Carbon Wrapped Na3V2(PO4)3@C Porous Composite

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Hard Carbon Wrapped Na3V2(PO4)3@C Porous Composite Extending Cycling Lifespan for Sodium-Ion Batteries Lei Chen, Yanming Zhao, Shenghong Liu, and Long Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14006 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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ACS Applied Materials & Interfaces

Hard Carbon Wrapped Na3V2(PO4)3@C Porous Composite Extending Cycling Lifespan for Sodium-Ion Batteries

Lei Chen1, Yanming Zhao*1,2, Shenghong Liu1, and Long Zhao1 1

School of Physics, South China University of Technology, Guangzhou, 510640, P.

R. China 2

School of Material Science and Engineering, South China University of Technology,

Guangzhou, 510640, P. R. China Abstract Although the NASICON-type of Na3V2(PO4)3 is regarded as a potential cathode candidate for advanced sodium ion batteries (SIBs), it behaves undesirable rate performance and low cyclability, which are resulted from its poor electronic conductivity. Here, we utilized conductive polyaniline (PANI) grown in situ to obtain the hard carbon coated porous Na3V2(PO4)3@C composite (NVP@C@HC) with a typically simple and effective sol-gel process. Based on the restriction of double carbon layers, the NVP size decreases distinctly, which can curtail sodium ion diffusion distance and enhance the electronic conductivity. As expected, the product displays good discharge capacity (111.6 mA h g-1 at 1 C), outstanding rate capacity (60.4 mA h g-1 at 50 C) and remarkable cycling stability (63.3 mA h g-1 with retention of 83.3 % at 40 C over 3000 cycles). Also, it performs a long-term cycling capacity of 58.5 mA h g-1 exceeding 15000 cycles at 20 C-rate (with the capacity-loss of 0.24 % per cycle).

ACS Paragon Plus Environment


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KEYWORDS: Polyaniline; Sodium-ion batteries; Na3V2(PO4)3; Hard carbon; Cathodes; Dual carbon-coating. Introduction Researches on highly effective electrical energy storage systems (EESs) have received tremendous attention because of the exploitation of fossil resources and the increasing environmental issues in modern society1-3. Among these energy storage systems, secondary batteries have gained enormous interest for large-scale energy storage equipment. Compared with other rechargeable batteries, lithium-ion batteries (LIBs) with high power/energy density and long lifetime have been very successful in electric vehicles and consumer electronics over recent years4-8. However, with the widespread application of LIBs, there is a bottleneck of the high cost and finite resources of lithium9. Therefore, it has become urgent to seek new kinds of energy storage devices that could complement the existing lithium-ion batteries10.

As the most promising alternatives for LIBs, sodium-ion batteries (SIBs) are advantageous for the next-generation large-scale applications owing to the wide abundance and the low cost of sodium-containing precursors11,12. As a result of the larger radius of Na (0.98 Å) compared with Li (0.69 Å) and higher equivalent weight than that of Li result in the difficulty of rapid Na+ intercalation/extraction and great volume change during charging/discharging reactions, which finally behave the unsatisfactory cycling stability and rate capability13-15. Therefore, it is desirable to develop suitable electrode materials capable of excellent electrochemical performance in SIBs. Currently, many electrode materials, such as NaxMO2 (M = Co16, Fe17, Mn18,


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V19, Cr20, etc.), phosphate-based NaMPO4 (M = Fe21, Co22, Ni23, Mn24) and fluorophosphates25 as cathodes, Na2C8H4O426, Na2Ti3O727, metal oxides28 and carbonaceous materials29 as anodes of SIBs have been investigated widely. NASICON-type materials, in particular sodium vanadium phosphate (Na3V2(PO4)3, denoted as NVP), have stood out from the aforementioned materials because of their superionic conductor framework, large channels for the transfer of Na ions, as well as moderate operating potential (for NVP, ~3.4 V) and good thermal stability30-32. Nevertheless, the pure NVP manifests poor electric conductivity, which significantly restricts the electrochemical performance, particularly the high-rate performance and long-cycling stability at high current density, for NVP applied in large-energy storage systems and high-power applications.

In response, kinds of strategies have been implemented to enhance sodium storage performances of Na3V2(PO4)3 cathodes. For instance: i). downsizing NVP particles to shorten the lengths of Na ion diffusion and increase the surface area during the sodiation/desodiation process33. The high phase formation temperature of NVP, however, seriously inhibit the particle size. ii). the conductive carbon-coated NVP grains to improve the electronic conductivity34-37. Additionally, Wang group and Yu group have reported that nitrogen-doped NVP/C and nitrogen, sulfur-co-doped Na3V2(PO4)3/3D porous carbon display excellent cycle life, respectively38,39. Compared with other types of carbon, hard carbon (HC) with randomly oriented graphitic layers and short-range ordered structure in plane can act as the superior conductive intermediary for charge transmission40. In recent years, a number of



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micro/nanostructured hard carbons have been exploited as anodes for energy storage, due to the kinetically favorable for the transportation of ion and electron41. Further, hard carbon coated active material composites have been also already proved to enhance the storage performance for lithium batteries, effectively42,43.

Here, we presented a simple synthetic method to constrain the Na3V2(PO4)3@C in hard carbon (the source obtained from pyrolytic polyaniline) (denoted as NVP@C@HC) to improve the electrochemical properties. The NVP precursor were dispersed uniformly and tightly anchored on polyaniline surfaces within the help of chelating interactions of citric acid and vanadate. Subsequent high-temperature calcination generated NVP@C gains fixed in hard carbon, denoted as NVP@C@HC. As expected, the double carbon-coated porous NVP@C@HC composite exhibited an excellent rate capability (60.4 mA h g-1 at 50 C), a long cyclability (with retention of 83.3% at 40 C after 3000 cycles). The outstanding electrochemical behavior can be ascribed to a series of causes: i) the double carbon-coating layer enhances the surface conductivity and prevents NVP grain reunion during cycling44,45; ii) the double carbon network suppresses growth of NVP particles, which shortens the length of electronic transport46,47; iii) the carbon layer accommodates the volume change during charge-discharge48.

Results and discussion As schematically illustrated in Figure 1, a simple sol-gel method was firstly proposed to in-situ fabricate the porous NVP@C@HC cathode materials. Wherein, the two


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carbon sources could be prepared by the pyrolysis of polyaniline (PANI) and critic acid, respectively. For comparison, carbon-coated Na3V2(PO4)3 composite (denoted as NVP@C) and Na3V2(PO4)3 composite embedded in hard carbon (denoted as NVP@HC) were obtained, respectively (for details see experimental section).

Figure 1. Schematic illustration of the sol-gel synthesis of NVP@C@HC composite, and the starting PANI were mixed uniformly as hard carbon source.

The X-ray diffraction (XRD) patterns of the three samples were tested and presented in Figure 2a. All diffraction peaks are well-matched with the rhombohedral R-3c space group, and which are in accordance with the calculated pattern for NASICON-type Na3V2(PO4)3 structure (JCPDS No. 53-0018) without any impurities49. Furthermore, there are no features related to hard carbon between NVP@C@HC and NVP@C, indicating that hard carbon had no influence upon crystal patterns of NVP. For the purpose of confirming the existence of carbon, Raman spectra were also tested and recorded to investigate the product compositions, as reveals in Figure 2b. Two obvious peaks are detected at around 1350 (belongs to D band, disordered carbon) and 1600 cm-1 (belongs to G band, graphitic carbon) in all of three samples, respectively. The intensity ratios (ID/IG) are 0.90, 0.93 and 0.94 for the



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NVP@C, NVP@HC and NVP@C@HC, respectively. No doubt, the ID/IG values of both NVP@HC and NVP@C@HC are higher than that of NVP@C, demonstrating the high degree of graphitization and high conductivity of carbon, which are caused by the addition of hard carbon50. To further check influence of carbon in the compositions, the thermogravimetric (TG) curves of three Na3V2(PO4)3 composites are depicted in Figure 2c. The slight weight loss of NVP composites from 50 °C to 300 °C are ascribed to the evaporation of adsorbed water. The rapid mass loss from 350 °C to 500 °C illustrate the carbon contents of 5.61 %, 14.50 %, and 16.49 % in mass for NVP@C, NVP@HC, and NVP@C@HC, respectively. It is noteworthy that the TG curves of NVP composites turn up after approximately 550 °C, owing to the oxidation of V3+ to V4+ and V5+51. Figure 2d shows that the nitrogen adsorption-desorption

isotherms

of

NVP@C@HC

composite.

The

double

carbon-coating NVP exhibits a type IV isotherm, suggesting the mesoporous characteristics. As Figure 2d shown, the NVP@C@HC sample has a BET surface area of 113.6 m2 g-1, which clearly surpasses these of NVP@C (60.2 m2 g-1, Fig. S1) and NVP@HC (7.1 m2 g-1, Fig. S2), respectively. Moreover, the pore size distribution was calculated through the Barrett-Joyner-Halenda (BJH) model (inset in Figure 2d). The distribution curve of NVP@C@HC composite displays the little pore size peak at about 8.46 nm, compared with these of NVP@C (9.67 nm, inset in Fig. S1) and NVP@HC composite (14.22 nm, inset in Fig. S2), which demonstrates double carbon layer could suppress the increase of NVP particles in annealing43. Hence, the exaltation of electrochemical behavior should be related with the larger surface area,


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which could facilitate the ultra-fast diffusion of Na ions and offer easier penetration of electrolyte48, 52.

Figure 2. (a) XRD patterns of NVP@C, NVP@HC and NVP@C@HC, (b) Raman spectrum of NVP@C, NVP@C@HC and NVP@HC, (c) TG curves of NVP@C, NVP@HC and NVP@C@HC; (d) N2 absorption-desorption isotherm of NVP@C@HC and the pore-size distribution curves in the inset.

The structure and morphology of the NVP@C and NVP@HC composites were firstly displayed by using scanning electron microscopy (SEM). The NVP@C particles have the irregular morphology and consist of the micron-sized particles, which are randomly distributed with average sizes of 2-3µm (Fig. S3, in Supporting Information). Fig. S4 shows that the NVP embedding in the hard carbon composites are obtained, and the average particle sizes of NVP@HC decrease distinctly. These



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results demonstrate that the generated hard carbon under the high calcination temperature could effectively restrict the NVP particles growth. The detailed morphological and microstructure of NVP@C@HC were further explored by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) measurements (Figure 3a-3d). A TEM image (Figure 3b) reveals the particle sizes of NVP@C@HC, with an irregular morphology, range from 500 to 800 nm. The TEM image with high magnification in Figure 3c shows the NVP particles are encapsulated in the carbon matrix. Fig. S5 and Figure 3d reveal the existence of porous amorphous carbon layer around 6 nm in thickness. The thin layer is beneficial to suppress the particles growth and enhance electronic and ionic conductivity. Furthermore, the HRTEM image in Figure 3d also exhibits distinguishable parallel fringe with interlayer spacing of 0.378 nm, corresponding to the (113) plane of NVP, which is in agreement with XRD results. In addition, EDX mapping analysis reveals that the elements of carbon (C, Figure 4a), nitrogen (N, Figure 4b), sodium (Na, Figure 4c), phosphorus (P, Figure 4d), vanadium (V, Figure 4e) and oxygen (O, Figure 4f) are uniformly dispersed in the double carbon-coated composite. To further verify the presence of N element in the coating carbon layer, the XPS spectrum of N 1s was tested and split into three types (Fig. S6), which are corresponding to the quaternary (403.6

eV),

pyridonic

(398.8

eV),

and

pyridinic

respectively38,39,53,56.


nitrogen

(396.8

eV),

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Figure 3. (a) SEM images; (b) TEM images; (c and d) HRTEM images of the NVP@C@HC.

Figure 4. SEM images, and corresponding EDX mapping of C, N, Na, V, P, O elements in the NVP@C@HC composites (a-f).

Electrochemical properties of the three samples were evaluated in a half-cell using



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sodium metal as the counter electrode. Figure 5a initially displays the representative cyclic voltammetry (CV) cycles of NVP@C@HC composite at the rate of 0.5 mV s-1 in the voltage window of 2.0-4.0 V vs. Na/Na+. A sharp oxidation peak at a voltage of 3.61 V is observed, representing the extraction of sodium ions during anodic scan. However, this single peak splits as two clear reduction peaks centered at 3.27 and 3.05 V during discharging process. The two well-defined peaks as a result of the V4+/V3+ redox reaction might be attributed to the transfer from Na(1) to Na(2) triggered by the local redox environment rearrangement51. Similar phenomenon is also generally reported in other NASICON-type Na3M2(PO4)3 (M=Sc, Cr, Fe) structured frameworks53. The peak located at 3.05 V shifts towards 3.27 V by degrees and disappears completely after a few cycles. Interestingly, this transition did not affect the charge/discharge capacity and the areas of reduction peaks kept almost no change. Figure 5b shows the different charge/discharge cures of the NVP@C@HC electrode at a current density of 1 C (1 C=117 mA g-1). From these curves, two apparent different voltage platforms are found in the first few discharge profiles and the lower plateau wears off progressively with cycles increasing, which is in accordance with results of CV curves. Besides, the first three cyclic voltammogram plots of hard carbon at 0.2 mV s-1 within the potential range from 0.005 to 4.0 V (vs. Na/Na+) were evaluated, as shows in Fig. S7 (Supporting Information). There are no obvious oxidation/reduction peaks from 2.0 to 4.0 V, meaning that the redox reaction of hard carbon occurs below 2.0 V. Thus, the reversible capacities of hard carbon in the NVP@C@HC composite are insignificant during charge/discharge process 29, 44.


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Figure 5. (a) The representative CV plots of as-prepared NVP@C@HC at a scanning of 0.5 mV s-1; (b) Galvanostatic charge/discharge profiles of NVP@C@HC at 1 C for 100 cycles.

To verify our assumption that the dual carbon-coated NVP was really beneficial to improve the battery performance, the electrochemical performance of NVP@C@HC were plotted, compared with those of NVP@HC and NVP@C, in Figure 6. Charge/discharge galvanostatic curves of the three samples at a current density of 1 C in the potential window between 2.0 and 4.0 V, as shown in Figure 6a. All the three cathodes exhibits a flat voltage plateau at ~3.4 V, corresponding to the redox pair of V3+/V4+. Obviously, the NVP@C@HC electrode delivers a higher reversible capacity of 111.6 mA h g-1, in comparison with NVP@C (101.7 mA h g-1) and NVP@HC (89.0 mA h g-1), respectively, which suggests the double carbon-coating could enhance the Na+ insertion/extraction reversibility. Additionally, the potential gap (insert in Figure 6a) of the NVP@C@HC electrode is narrowest compared with the two other electrodes, indicating decreased electrochemical polarization and high electronic conductivity. The cyclability of three carbon-coated composites at a current density of 1 C was demonstrated in Figure 6b. The initial capacity of NVP@C@HC cathode is 111.6 mA h g-1 for discharge and retains a reversible capacity of 106.3 mA h g-1 at the



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200th cycle with retention higher than 95%. In contrast, the reversible capacities of NVP@HC and NVP@C cathodes are 69.4 and 94.4 mA h g-1 after 200 cycles under the same current rate, respectively. Besides the negligible capacity fading, the corresponding CE of NVP@C@HC is almost reached to 100 %, indicating good cycling performance.

In order to confirm the influence of dual carbon-coating layer on electrochemical behavior in the NVP@C@HC composite, rate and long-cycling performance were tested. As depicts in Figure 6c, the discharge/charge plots of NVP@C@HC are obtained at diverse rates from 1 to 50 C over a voltage window of 2.0-4.0 V. Apparently, along with current rates increasing, the voltage gap of the NVP@C@HC cathode is expanded gradually. But it still exhibits a distinct voltage platform under every current density, suggesting the superior dynamic electrochemical stability. Furthermore, more evidence on the increase of the polarization of charge-discharge curves could be proved in the differential capacity plots (Fig. S8) at different current densities, in which NVP@C@HC behaved a small voltage difference of 68 mV at 1C, while a pair of redox peaks for NVP@C@HC dirtied to back and forth along with the different current rates. When the rate at 50C, it has the largest voltage gas up to 669.69 mV. Figure 6d illustrates a comparison of rate capability among the three cathodes. The NVP@HC electrode has the discharge capacities of 87.0, 76.5, 68.2, 57.9, 45.6 and 32.3 mA h g-1 when current densities are 1, 2, 5, 10, 15, 20 C, respectively. Whereas, the NVP@C displays an initial reversible capacity of 100.2 mA h g-1 at 1C, subsequently it drops rapidly with the current density increasing from


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2 to 10 C. When cycled at 20 C, NVP@C exhibits a negligible capacity ~0.3 mA h g-1, indicating that high rate performance is not feasible with this NVP@C electrode. Unlike the case of NVP@HC and NVP@C, the reversible capacities of NVP@C@HC remain 112.3, 110.4, 106.9, 102.9, 98.7, 94.8, 84.8 and 73.8 mA h g-1 at 1, 2, 5, 10, 15, 20, 30 and 40 C-rates, respectively. More importantly, when reached at 50 C (5.85 A g-1, 1 C = 117 mA g-1), NVP@C@HC is still able to maintain a surprisingly high capacity of 61.0 mA h g-1 with a capacity retention of 54.3 %, demonstrating an excellent stability of the structure. Remarkably, the reversible capacity can be restored to its initial value of 112.1 mA h g-1 even after 70 cycles at 1 C, which indicates that the super structure could adapt the volume change during charging/discharging. Figure 6e illustrates a comparison of rate capabilities of other previously reported similar NVP cathodes for SIBs38,

44, 50, 54, S9

. To our best

knowledge, the porous NVP@C@HC electrode in this work only take a simple and low-cost production technique to achieve excellent rate capability (As seen in Table S2). Based on the above analysis, electrochemical impedance spectroscopy (EIS) measurements were carried out to prove the improved electrochemical behavior of NVP@C@HC composite. Figure 6f displays the Nyquist plots of the three cathodes in fresh assembled cells with equivalent circuit inset. All of the Nyquist impedance plots consist of a high frequency semicircle circle and a low frequency slope line, which represent the charge-transfer resistance (Rct) and the Warburg resistance (Zw), respectively. As was revealed, the NVP@C@HC composite exhibits the lowest charge transfer resistance (Rct=335 Ω) in comparison to NVP@C (Rct=352 Ω) and



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NVP@HC (Rct=445 Ω), indicating that the double carbon coating could improve electronic conductivity of the NVP, which accounted for the outstanding electrochemical performance. Additionally, the sodium ion diffusion coefficient (DNa+) is a key index to appraise the kinetics activity in an electrochemical reaction, and it could be expressed and calculated as55:

where R is the gas constant, T is the absolute temperature, F is Faraday constant, A is attributed to area of electrode surface, C is concentration of Na+ ion, n is the number of electrons, and σw is the Warburg impedance coefficient, which can be gained by calculating the relationship between real impedance (Z') and the reciprocal square-root of frequency (ω-1/2), from the eq (2). Fig. S9 shows the plot of the relationship between Z' and ω-1/2 at low frequency region for the three samples. And the specific values of slope (σw) and DNa+ for all samples are calculated and placed in Table S1. According to the calculated results, the DNa+ value of NVP@C@HC is 1.54×10-13 cm2 s-1, which is higher than these of NVP@C (1.38×10-13 cm2 s-1) and NVP@HC (3.74×10-14 cm2 s-1), respectively. It is mainly due to the fact that the double carbon-coating with improved specific area offers easy electrolyte penetration and shortens the distance of Na ion diffusion, which is consistent with the previous rate performance56.


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Figure 6. (a) The first charge/discharge curves of the NVP@C, NVP@HC, and NVP@C@HC at 1 C; (b) Cycling stability of the NVP@C, NVP@HC, and NVP@C@HC at 1 C for 200 cycles; (c) Charge and discharge profiles of NVP@C@HC from 1 to 50 C; (d) Capacities of all three samples at different C-rates; (e) Comparison of rate performance of NVP@C to the recently results in other literature; (f) The Nyquist plots of the three electrodes with equivalent circuit inset.

Additionally, the ultra-long cycle behaviors of three samples were measured. Figure 7a initially compares the long-term cycle properties of NVP@C@HC with those of NVP@HC and NVP@C composites during 500 cycles at 5 C. NVP@C@HC



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achieves a reversible capacity of 104.9 mA h g-1 (with retention of 90.4 % after 500 cycles), which presents higher reversible capacity than those of NVP@C and NVP@HC composites (29.2 and 56.5 mA h g-1 over 500 cycles, respectively). The ultra-high cyclability of NVP@C@HC was also investigated at 40 and 20 C, respectively. As shows in Figure 7b, great cycling capacities of the NVP@C@HC could still be obtained at a higher rate of up to 40 C. The specific discharge capacity retains at 63.4 mA h g-1, with a capacity retention of 83.4 % after 1000 cycles. Even after 3000 cycles, it still keeps 82 % of the initial discharge capacity with a loss-rate of 0.42 % per cycle (Figure 7b). Furthermore, the NVP@C@HC electrode also delivers excellent discharge capacities of 66.8 and 58.5 mA h g-1 exceeding 10000 and 15000 cycles at 20 C-rate (2.34 A g-1, 1 C = 117 mA g-1), corresponding to 70.7 % and 61.9 % of capacity retention, respectively. Noticeably, the coulombic efficiency (CE) of the NVP@C@HC could maintain nearly 100 % throughout the cycling (Figure 7c). Finally, the ex-SEM images and XRD patterns of NVPC@C@HC composite after 300 and 3000 cycles at 20 C were given to show the stability of electrode after superior high-rate cycles (Fig. S10 and S11, respectively). Obviously, XRD patterns of the three electrodes at the different cycles display the almost same diffraction patterns, reflecting the NVP@C@HC with a stable crystal structure57. The long-cycling stability could be owing to the special dual carbon-coated structure, which could suppress particle agglomeration, enhance the surface conductivity, shorten the length of electronic transport, and adapt the strain during charge-discharge process.


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Figure 7. (a) Long cycles of the NVP @C, NVP@HC and the NVP@C@HC electrodes for 500 cycles at 5 C; (b and c) Long life performance and CE of NVP@C@HC for 3000 cycles at 40 C and 15000 cycles at 20 C, respectively.

Conclusions In summary, the double carbon-coating porous NVP (denoted as NVP@C@HC) composite was successfully fabricated with NVP@C particles embedding uniformly in a continuous carbon matrix through a facile sol-gel process. When tested as cathode for SIBs, NVP@C@HC reveals a good discharge capacity of 111.6 mA h g-1 at 1C (retention of 95.3 % after 200 cycles), high-rate performance (61.0 mA h g-1 at 50 C), and outstanding long-term cyclability (58.5 mA h g-1 with retention of 61.9 % at 20 C after 15000 cycles). The fact is that the double-coated strategy could effectively enhance the electronic conductivity of NVP. Because of the restriction of dual carbon layers, the agglomeration of NVP grains are suppressed during the calcination process,



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which could effectively buffer large volume strains58. Moreover, the unique structure can also achieve fast sodium ion diffusion, which improves the cycling stability and rate performance distinctly. Therefore, we consider that the simple and cost-effective method would be helpful to extend to other carbon-coated composite materials which need increases in electronic conductivity.

Experimental Section Preparation of Polyaniline (PANI) Polyaniline (PANI) was synthesized according to the literature reported29: 2.0 mL of aniline (AR, Shanghai Macklin Biochemical Technology Co., Ltd) was added at once with vigorous stirring in 100 mL of 1.5 mol L-1 HCl. It was stirred for another 10 minutes while the reaction mixture turned into a yellowish aniline solution. 4.56 g of ammonium

persulphate

((NH4)2S2O8,

AR,

Shanghai

Macklin

Biochemical

Technology Co., Ltd) was dissolved in deionized water (20 mL) and then quickly added to the aniline solution. The mixture was stirred at room temperature for overnight. The resulting PANI solid was repeatedly washed by distilled water and ethanol, finally dried at 80 °C for overnight.

Preparation of NVP@C@HC The NVP@C@HC sample was facilely prepared via a simple sol-gel method, and Sodium hydroxide (NaOH, AR, Sinopharm Chemical Reagent Co., Ltd), ammonium dihydrogen phosphate (NH4H2PO4, AR, Guangzhou Chemical Reagent Factory), ammonium vanadate (NH4VO3, AR, Tianjin Fuchen Fine Chemical Res. Inst.), citric


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acid (AR, Aladdin Reagent Co., Ltd.) and polyaniline (PANI) were used as initial materials. Before preparing the NVP@C@HC material, the aqueous PANI suspension was initially sonicated for 10 min. Then citric acid was completely dissolved into the suspension and stirred magnetically for 0.5 h at room temperature. Next, NaOH, NH4VO3 and NH4H2PO4 at the stoichiometric ratio (the rate of the mole is Na: V: P = 3: 2: 3) were dissolved in the suspension in sequence under continuous stirring. Then, the mixture was heated at 80 °C and suspension turned into a uniform mixing. The stirring was kept for several hours until the mixture turned into a gel, which was dried in a vacuum oven at 80 °C for 12 hours. The xerogel was ground in the agate mortar and calcined at 350 °C for 4 h under the Ar/H2 (V/V=95:5) atmosphere at a heating rate of 3 °C. At last, the precursor powder was annealed at 800 °C for 8 h under the Ar/H2 (V/V=95:5) atmosphere to yield the final product. For comparison, the NVP@C was synthesized in the same process as mentioned above without adding polyaniline, and the NVP@HC was prepared without adding citric acid. Material characterization The sample product was characterized using powder XRD in the 2θ range from 10 to 80o by using a Bruker D8 advance diffractometer, with Cu Kα radiation at a scanning step of 0.02. Raman spectrum was measured at room temperature using on a LanRanHR (HORIBA Scientific, France) spectrometer. The thermal properties of the sample was collected by a STA-449-F3 Thermal Analyzer (NETZSCH, Germany) with a heating rate of 10 °C/min in the temperature range from 25 °C to 800 °C under air atmosphere flow. SEM images of these composites were obtained by Nova



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NanoSEM 430 (FEI, USA). The energy-dispersive X-ray (EDX) analysis was performed using the EDX system attached to the electron microscopy. Transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) were performed to further analyze the morphology and structure of products. The N2 adsorption-desorption measurements of the samples were performed on ASAP 2460 Surface Area and Porosimetry instrument (Micromeritics, USA). Electrochemical characterization Electrochemical performances were assembled by using 2032 coin-type simulated cells in an argon-filled glove box. Sodium metal was used as counter and reference electrode. The electrodes mixed with active material (80 wt %), carbon black (10 wt %) as a conductive additive, and polyvinylidene difluoride (PVDF, 10 wt %) as the binder scattered in N-methylpyrrolidone (NMP). Then, the slurry was coated uniformly using a doctor-blade on an aluminum foil with roughly 2.0 mg cm-2 mass loading. The electrodes were dried at 120 °C under vacuum overnight. 1 M NaClO4 in ethyl carbonate (EC) and dimethyl carbonate (DMC) with a volumetric ratio of 1:1 with 5 wt % additive FEC (Fluoroethylene carbonate) was used as the electrolyte, and a glass fiber separator (Whatman, GF/B) was used as the separator. All the mass of active materials were calculated behind removing the total carbon weight. Galvanostatic charge/discharge measurements were conducted on an automatic battery tester system (Land, China) in the potential range of 2.0-4.0 V (vs. Na+/Na) under various current densities. Cyclic voltammetry (CV) measurements and electrochemical impedance electrochemical impedance were performed on an


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AUTOLAB PGSTAT302N (Metrohm, Netherlands), respectively. CV tests were carried out within the range of 2.0 to 4.0 V (vs. Na/Na+) at 0.1 mV s-1. EIS were recorded at the frequency range 100 KHz to 0.01Hz with alternating-current voltage of 5 mV. ASSOCIATED CONTENT

Supporting Information. BET data and SEM photographs of Na3V2(PO4)3@C and Na3V2(PO4)3@HC, XPS and HRTEM of Na3V2(PO4)3@C@HC, ex-suit SEM images and XRD of Na3V2(PO4)3@C@HC, cyclic voltammograms for hard carbon. Comparison of Na3V2(PO4)3@C for SIBs with previous reported studies. The Supporting Information is available free of charge on the http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author

*Corresponding author. Tel.: +86-20-87111963. E-mail address: [email protected].

Competing interests The authors declare no competing financial interests. Acknowledgements This work was funded by NSFC Grant (No. 51672086 & 51372089 ) supported through NSFC Committee of China, the Foundation (No. 2017B030308005) supported through the Science and Technology Bureau of Guangdong Government, and the Fundamental Research Funds for the Central Universities (No. 2014ZB0014).



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Hard Carbon Wrapped Na3V2(PO4)3@C Porous Composite

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