Graphene-Scaffolded Na3V2(PO4)3 Microsphere Cathode with High

Subscriber access provided by University of Colorado Boulder

Article 3

2

4

3

Graphene-Scaffolded NaV(PO) Microsphere Cathode with High Rate Capability and Cycling Stability for Sodium Ion Batteries Jiexin Zhang, Yongjin Fang, Lifen Xiao, Jiangfeng Qian, Yuliang Cao, Xinping Ai, and Hanxi Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16000 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 12, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

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

ACS Applied Materials & Interfaces

Graphene-Scaffolded Na3V2(PO4)3 Microsphere Cathode with High Rate Capability and Cycling Stability for Sodium Ion Batteries Jiexin Zhang,a Yongjin Fang,a Lifen Xiao, b Jiangfeng Qian,a Yuliang Cao,a* Xinping Ai,a and Hanxi Yang a a

Hubei Key Lab. of Electrochemical Power Sources, College of Chemistry and Molecular Sciences,

Wuhan University, Wuhan 430072, China. b

E-mail: [email protected] Tel: (+) 86-27-68754526

College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China.

Abstract： High voltage, high rate and cycling-stable cathodes are urgently needed for development of commercially viable sodium ion batteries (SIBs). Herein, we report a facile spray-drying method to synthesize graphene-scaffolded Na3V2(PO4)3 microspheres (NVP@rGO), in which nano-crystalline Na3V2(PO4)3 is embedded in graphene sheets to form porous microspheres. Benefiting from the highly conductive graphene framework and porous structure, the NVP@rGO material exhibits a high reversible capacity (115 mAh g-1 at 0.2 C), long-term cycle life (81 % of capacity retention up to 3000 cycles at 5 C) and excellent rate performance (44 mAh g-1 at 50 C). The electrochemical properties of a full Na-ion cell with the NVP@rGO cathode and Sb/C anode is also investigated. The present results suggest promising applications of the NVP@rGO material as a high performance cathode for sodium ion batteries. Keywords: Sodium ion batteries; Cathode; Na3V2(PO4)3/rGO microsphere; graphene; spray-drying synthesis. 1

ACS Paragon Plus Environment


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

Page 2 of 21

1. Introduction Sodium ion batteries have been considered as a promising alternative to Li-ion batteries for energy storage applications because to the low cost and abundant resources of sodium.1-3 However, it is hard to find suitable host materials for reversible Na ion insertion/removal since the larger size and sluggish diffusion kinetics of Na ion as compared to Li ion. In the past few years, many works have exhibited a variety of cathode host materials such as transition-metal oxides,4-9 phosphates,10-15 ferrocyanides16-17 have been explored as potential cathodes for SIBs, which show a relatively high operating voltage (~3.0 V).18 However, most of the reported cathode materials displayed either multiple charge/discharge steps or poor cycling stability, hindering battery applications. In contrast, NASICON-type Na3V2(PO4)3 appears to be an appealing candidate for Na-ion battery cathodes due to its high working voltage plateau, excellent rate capability and cycling stability.19-24 Yamaki firstly revealed reversible sodiation/desodiation behaviors of Na3V2(PO4)3 in 2010 but did not obtain satisfactory charge/discharge performances due to the inherent poor electronic conductivity of the phosphate framework.25 Later, Jian et al reported a carbon-coated Na3V2(PO4)3 material, which demonstrated improved Na-storage performance.20 Afterwards, many efforts have been devoted to improve the electrochemical Na-insertion performance of Na3V2(PO4)3 by nanoarchitecturing, carbon coating and nanocompositing with conductive matrix.26-30 Recently, Saravanan et al. revealed a porous Na3V2(PO4)3/C composite with a capacity retention of 50% over a long cycle life of 30000 cycles at 40 C.31 Fang et al. also reported a carbon fiber-wrapped Na3V2(PO4)3 as a Na-insertion cathode with a ultra-high rate capability (38 mAh g-1 at 500 C) and superior long-cycling lifespan (20000 cycles at 30 C).12 These results clearly demonstrate significant influence of carbon networks on the electrochemical performance of Na3V2(PO4)3/C composite. Therefore, building a better

2


Page 3 of 21

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


conductive matrix by more facile synthesis still remains a challenge for development of commercially viable Na3V2(PO4)3/C material. Graphene appears to be an ideal scaffolding material for building a three-dimensional (3D) conductive framework for high performance electrodes because of its superior electronic conductivity and structural flexibility.22, 32-34 Though several Na3V2(PO4)3/graphene composites have been reported,35-38 the graphene sheets were used only as a conductive additive for increasing the electronic conduction between the Na3V2(PO4)3 particles, but not used for constructing a flexible conductive scaffold to reinforce the electrode structure. Herein, we report a porous and microspherical graphene-scaffolded Na3V2(PO4)3 (denoted as NVP@rGO), where Na3V2(PO4)3 particles were embedded in a 3D interconnected graphene framework. Such an elaborate structural architecture provides not only an enhanced electronic conductivity between the Na3V2(PO4)3 particles, but also a flexible buffer to accommodate the structural variation during sodiation and desodiation reactions. As a result, the as prepared NVP@rGO electrode demonstrated a high Na storage capacity (115 mAh g-1 at 0.2 C) with superior cycling stability (81 % of capacity retention over 3000 cycles at 5 C) and high rate capability (44 mAh g-1 at 50 C), showing a promising application for sodium ion batteries.

2. Experimental 2.1 Sample preparation The NVP@rGO composites were synthesized by a facile spray-drying method described as follows. Stoichiometric NH4H2PO4, Na2CO3 and V2O5 first were dispersed in water. Oxalic acid was added into the mixture to reduce the V5+ ions to V3+ during stirring at 70 ºC. The amount of oxalic acid was

3



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

Page 4 of 21

set to 1:4 (V2O5: oxalic acid, molar ratio). After the solution became transparent, an appropriate Graphene oxide (GO) solution was added under vigorous stirring (5 mg mL-1, 98.5% purity, Sinocarbon Graphene Marketing Center, Shanxi, China). Then the slurry was spray-dried to form a solid NVP@GO precursor. The precursor was sintered at 850 ºC with a rate of 2 ºC min-1 in Ar atmosphere to form the NVP@graphene (denoted as NVP@rGO). The bare NVP was synthesized using the same process except that no GO was introduced. In order to investigate the scaffolded graphene microspheres, the Na3V2(PO4)3 nanoparticles were etched by HCl solution: 0.05 g of NVP@rGO nanoparticles were added in 10 ml of 4 M HCl in a 20 ml vial. The mixture was ultrasound for 5 minutes and stirred at 80 ºC for 6 hours. After centrifuged and washed with water and ethanol for several times, the 3D graphene network was obtained. Moreover, for the full cell, Sb/C anode was synthesized by a high-energy ball milling (HEBM) method in our previous work.39

2.2 Characterization Transmission electron microscope (TEM) and scanning electron microscope (SEM) images were conducted on a transmission electron microscope (JEM-2100FEF) and a scanning electron microscope (SEM, ZEISS Merlin Compact), respectively. The element dispersive spectroscopy (EDS) mapping analysis of the composites was determined by an energy dispersive spectrum (EDS) oxford detector (INCAPentalFETx3, Oxford Instruments). X-ray powder diffraction (XRD) patterns were carried out on a Shimadzu XRD-6000 diffractometer with Cu Kα. The diffraction data were recorded with a scan rate of 2° min-1 in the 2 θ range of 10 - 80°. The BET surface area was obtained by using a surface area and porosity analyzer (ASAP-2020 HD88). Raman spectroscopy was carried out by using a laser micro-Raman spectrometer (Renishaw in Via, Renishaw, 532 nm excitation

4


Page 5 of 21

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


wavelength). The carbon content was tested on CHNSO Vario EL cube (Elementar Analysen systeme GmbH, Germany).

2.3 Electrochemical measurements The NVP@rGO (or bare NVP) cathodes were prepared by using the slurry containing 70 wt% of NVP@rGO (or bare NVP), 20 wt% of acetylene black and 10 wt% of poly(vinyldifluoride) (PVdF) in N-methyl-2-pyrrolidone (NMP) solution to coat onto a Al foil. The average mass loading of the electrode was about 2 mg cm-2. Electrochemical characterization was tested by using 2016 coin cells. The Sb/C anodes were prepared by using the slurry containing 70 wt% of Sb/C, 20 wt% of acetylene black and 10 wt% of Polyacrylic acid (PAA, 25 wt%) to coat onto a Cu foil. The electrolyte was 1.0 mol L–1 NaClO4 dissolved in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 by vol.). For half cells, the Na thin disks were homemade as anodes. For Sb/C//NVP@rGO full cells, the mass ratio of the Na3V2(PO4)3/C cathode and Sb/C anode was designed to be close to 5.2:1, allowing for their reversible capacity and the excess of anode. All the cells were assembled in a glove box (water/oxygen content < 0.5 ppm) and tested at room temperature. The galvanostatic discharging–charging experiments were tested in the voltage range of 2.0 - 4.0 V by using a LAND battery analyzer (Wuhan Kingnuo Electronic Co., China). Cyclic voltammetric measurements were carried out on a CHI 600a electrochemical workstation (ChenHua Instruments Co., China) at a scan rate of 0.1 mV s−1. Electrochemical impedance spectra (EIS) measurements with frequency ranging from 10 mHz to 100 kHz are conducted on both bare NVP and NVP@rGO electrodes using the AutoLab PGSTAT 128N

(Eco Chemie, Netherlands). Before the EIS tests, the electrodes were

cycled for 1 cycle and then followed by a 4 h relaxation.

5



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

Page 6 of 21

3. Results and Discussion The NVP@rGO microspheres were synthesized by a facile spray-drying method as illustrated in Figure 1. For comparison, the graphene-free Na3V2(PO4)3 particles (denoted as NVP) were synthesized via a similar method without introduction of GO. Detailed synthetic procedures are described in Experimental Section.

Figure 1 Synthesis schematic of NVP@rGO microspheres.

Figure 2a shows the XRD patterns of the NVP and NVP@rGO samples. Both NVP and NVP@rGO samples display same diffraction peaks, which can be well indexed to a NASICON-type framework with R-3c space group, indicating the good crystallinity of the Na3V2(PO4)3 particles. The NASICON-type Na3V2(PO4)3 structure exhibits a 3-demension framework of VO6 octahedra sharing all of its corners with PO4 tetrahedra, which permits two independent sodium atoms to occupy the two different void sites: the M1 sites were located by Na1 ions (yellow) with six-fold coordination and the M2 sites were occupied by Na2 ions (green) forming eight-fold coordination. Only Na2 ions can be extracted during charging and discharging because of strong limitation of the Na ions at M1 sites by the surrounding oxygen atoms. So, a theoretical capacity of 117 mAh g-1 is obtained based on the reversible insertion/extraction of the two Na ions at M2 sites. The surface signals of the bare

6


Page 7 of 21

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


NVP and NVP@rGO can be investigated from their Raman spectra (Figure 2b). The pristine NVP sample shows a number of vibration bands around 1000 cm-1, corresponding to the stretching vibrations of PO43-.12, 40 Broad and weak Raman signals at 1356 cm-1 and 1605 cm-1 are assigned to the D (sp3-typed) and G (sp2-typed) bands owing to residual carbon.12, 20 However, the NVP@rGO sample exhibits higher intensities of D and G bands at 1356 cm-1 and 1605 cm-1 and weak peaks at 3170, 2928 and 2692 cm-1, reflecting the 2D’, D+G, and 2D modes of reduced graphene oxide, respectively.22 The failure to observe the characteristic modes of PO43- originating from the Na3V2(PO4)3 indicates a full screen of the PO4 signals by the tightly coated graphene sheets. The carbon amounts in the bare NVP and NVP@rGO were tested by elemental analysis to be 0.93% and 5.24%, respectively.

Figure 2. (a) X-ray diffraction patterns of the bare NVP and NVP@rGO. Inset is the crystal structure of Na3V2(PO4)3 with NASICON structure, with Na1 (yellow), Na2 (green), V (blue) and P (pink). (b) Raman spectra of NVP and NVP@rGO.

Figure 3a presents the morphology of the as-synthesized NVP@rGO powders by SEM images. As shown in Figure 3a, the NVP@rGO sample has a microspherical structure with a size distribution ranging from 5 to 18 µm. Higher magnified image in Figure 3b reveals that each spherical particle is composed of aggregated nanoparticles wrapped in the rGO network. TEM images further show the

7



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

Page 8 of 21

detailed nanostructure of the NVP@rGO sample. Figure 3c displays that a single NVP@rGO microsphere has a rough surface, indicating the coating of rGO. Figure 3d reveals that the graphene nanosheets were well interconnected among the Na3V2(PO4)3 nanoparticles and tightly cover the surface of each nanoparticles to form ~3 nm surface layer (Figure 3e). This highly conductive graphene coating has been suggested to play a key role for achieving high power capability,12, 34 which is different from amorphous carbon coating reported in previous literatures.20, 40-42 The high resolution TEM image showed a lattice spacing of 0.44 nm (Figure 3e), which matches well with the d-value of the (104) planes of the Na3V2(PO4)3 in the XRD spectrum (Figure 2a). Additionally, elemental mapping by EDS in Figure 3f-k shows homogenous distribution of Na, V, O, P and C in the NVP@rGO microspheres, further confirming a well-dispersed distribution of graphene in the composite.

8


Page 9 of 21

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


Figure 3. (a) and (b) SEM, (c) and (d) TEM, (e) HRTEM images of NVP@rGO; (f) typical SEM image of NVP@rGO and the corresponding elemental mapping of (g) sodium (purple), (h) vanadium (green), (i) phosphorus (yellow), (j) oxygen (red) and (k) carbon (blue).

The scaffolded graphene microspheres can also be visualized by removing the Na3V2(PO4)3 particles from the NVP@rGO microspheres with wet chemical etching of HCl solution. The SEM (Figure 4a and b) and TEM images (Figure 4c and d) clearly illustrated that the foam-like microspheres consist of three-dimension graphene sheets, similar to the shape of the NVP@rGO

9



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

Page 10 of 21

microspheres. The 3D porous graphene microsphere constitutes a highly conductive framework among the secondary Na3V2(PO4)3 nanoparticles, which can be considerably beneficial to the electrochemical properties of the NVP@rGO microspheres.

Figure 4. (a) and (b) SEM images; (c) and (d) TEM images of a three-dimension graphene microsphere.

Very differently, the bare NVP sample appeared just as irregular microparticles after spray-drying process (Figure S1a-c). This morphological difference between the NVP and NVP@rGO samples suggest that the GO plays a vital role in the formation of microsphere. On the one hand, the GO can act as a binder to bond the reactants by the functional group on the surface of GO so as to form spherical particles during spray-drying process. On the other hand, the graphene sheets coated on the surface of Na3V2(PO4)3 particles prevent the growth and aggregation of the particles during the post-calcination process. It can also be seen that bare NVP particles showed clear boundaries and large particle size (> 1 µm) (Figure S1c), suggesting the lack of the graphene

10


Page 11 of 21

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


isolation protection. Figure S1d shows very thin of amorphous residual carbon layer coated unhomogeneously on the surface of Na3V2(PO4)3 particles was also observed, which originated from the pyrolysis products of oxalic acid. The thin and unhomogeneous amorphous carbon layer is easy to be penetrated by the Raman laser, leading to the presence of the Raman signal of the PO43-, which is in accordance with the results of the Raman spectra (Figure 2b). The

surface area and

porosity of the samples were characterized

by nitrogen

adsorption-desorption isotherms (Figure S2). As expected, the NVP particles exhibited very low Brunauer-Emmett-Teller (BET) area of 1.58 m2 g-1, while the NVP@rGO sample gave a higher BET surface area of 31.28 m2 g-1 and its type-II isotherm with an H3 hysteresis loop indicates a typical mesoporous structure. It is because the 3D porous graphene scaffold provides sufficient surface areas and porosity, in favor of the electron transfer and the penetration of the electrolyte to improve the rate capability of the NVP@rGO sample. The electrochemical properties of the bare NVP and NVP@rGO composites were investigated by cyclic voltammetry and galvanostatic charge-discharge using 2032-type coin cells. Figure 5a displays the typical cyclic voltammograms (CV) of the NVP and NVP@rGO electrodes at a scanning rate of 0.1 mV s-1 in the potential region of 3.1-3.6 V (vs Na/Na+). A couple of symmetric redox peaks appears at 3.42/3.32 V for NVP@rGO, corresponding to the oxidation/reduction reactions of V3+↔V4+. In contrast, the bare NVP electrode shows broad redox peaks at 3.48/3.26 V with a larger potential gap, implying that the bare NVP electrode exhibits lower kinetic behavior owing to the lack of the conductive network compared to the NVP@rGO electrode. Figure 5b shows initial charge-discharge profiles of the bare NVP and NVP@rGO electrodes cycled in the voltage region of 2.0 - 4.0 V at 0.2 C rate (1 C refers to a current density of 117 mA g-1). The NVP@rGO

11



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

Page 12 of 21

electrode shows a flat voltage plateau around 3.3 V, which is characteristic of the two-phase Na insertion/extraction reactions. However the bare NVP electrode displayed sloping discharge/charge profiles with larger potential range. The initial specific discharge capacity of the NVP@rGO electrode is 115 mAh g-1, which is close to its theoretical capacity (117 mAh g-1) and much higher than that (90 mAh g-1) of the NVP electrode at the same rate. This difference on the potential gap and reversible capacity further proves the improved kinetics of the NVP@rGO by its graphene framework. The higher discharge voltage and larger reversible capacity observed from the NVP@rGO electrode are exclusively attributed to the highly conductive graphene matrix, which facilitates electronic transfer throughout the active particles and therefore promotes electrochemical utilization of the material for reversible Na+ intercalation reaction. Figure 5c compares the rate property of the NVP and NVP@rGO electrodes at various charge-discharge current densitis. The NVP@rGO electrode displays much higher reversible capacity than the NVP electrode at the same rates. The NVP@rGO electrode shows very high rate capability with discharge capacities of 115, 112, 108, 104, 97, 90, 78, 63 mAh g-1 at the rate of 0.2, 0.5, 1, 2, 5, 10, 20 and 30 C, respectively. Even at a high rate of 50 C (charging/discharging in 72 s), a reversible capacity of 44 mAh g-1 can be obtained. This excellent rate property is much better than the most of Na3V2(PO4)3/graphene materials reported in previous works.19-22, 28, 32, 37, 43-44 When the current rate turns back to 0.2 C, the NVP@rGO electrode can recover a capacity of 108 mAh g-1, almost 94% capacity retention compared with its initial capacity. For comparison, the bare NVP electrode shows continuous capacity decay as the charge/discharge rate increases and its reversible capacity drops almost to zero at current density of 20 C. The high rate capability and cyclability of the NVP@rGO electrode compared with the bare NVP electrode suggest that the graphene

12


Page 13 of 21

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


framework provides not only a fast electronic transport for improving the rate capability but also acts as a flexible matrix to release the structural stress during charging and discharging. Figure 5d displays the charge/discharge curves of the NVP@rGO electrode at various C-rates. Even up to a high rate of 30 C, the NVP@rGO electrode is capable to deliver ~70% of its reversible capacity at 0.2 C, exhibiting a strong rate capability. Besides, the NVP@rGO electrode also exhibits an ultra long-term cyclability. As shown in Figure 5e, the NVP@rGO electrode shows high capacity retention of 81% after 3000 cycles at 5 C rate. At moderate rates of 0.5 C and 1 C (Figure S3), this material also remains 88% and 91% of its initial capacity over 500 cycles, respectively. This stable cyclability combined with high voltage and high rate capability makes the NVP@rGO composite a competitive candidate for sodium ion batteries applications.

13



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

Page 14 of 21

Figure 5. (a) CV curves of the NVP and NVP@rGO electrodes at a scan rate of 0.1 mV s-1; (b) typical charge-dicharge curves of the NVP and NVP@rGO electrodes at 0.2 C rate; (c) rate properties of the NVP and NVP@rGO electrodes; (d) charge-discharge profiles of NVP@rGO electrode at different current rates; (e) cycling property of the NVP@rGO electrode at a 5 C rate over 3000 cycles in voltage range of 2.0 – 4.0 V.

To better understand the improved performance by graphene, electrochemical impedance spectroscopic (EIS) of the NVP and NVP@rGO electrodes were carried out after the first discharge (Figure 6). The the Nyquist plots show that both of the bare NVP and NVP@rGO electrodes have

14


Page 15 of 21

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


similar shapes, corresponding to a semicircle in the high frequency and a sloping line in the low frequency region. It suggests that the charge transfer and sodium ion diffusion control the electrochemical process in different frequency region.19, 33 The semicircle at the high frequency and inclined line at the low frequency assigns to the charge transfer impedance (Rct) on the electrode/electrolyte interface and the sodium-diffusion within the bulk phase of the electrode material, respectively. The kinetic results of the bare NVP and NVP@rGO electrodes were fitted based on the equivalent circuit (inset in Figure 6).19, 33 The fitting data were shown in Table S1. Obviously, the NVP@rGO electrode exhibits much lower Rct (622 Ω) than that (1055 Ω) of NVP electrode, suggesting a fast charge transfer kinetics for sodium ion intercalation

Figure 6. EIS of the bare NVP and NVP@rGO electrodes after 1st cycle at 0.2 C.

Inspired by the remarkable electrochemical property of the NVP@rGO, we used this cathode material to assemble a full Na ion battery using high-capacity and high-safety Sb/C anode, which has been revealed in our previous study as an excellent anode material with a high reversible capacity of ~ 600 mAh g-1 and an suitable low potential of ~0.8 V for sodium ion battery (Figure S4).39 , 45 For the full cell, the capacity is calculated based on the active mass of Na3V2(PO4)3/C cathode. The typical charge/discharge curves of the full cell at 0.1 C (11.7 mA g-1) are given in Figure 7a. As 15



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

Page 16 of 21

expected, the full cell showed delivered a reversible capacity of ca. 110 mAh g-1 and a voltage plateau of ~ 2.6 V. Therefore, the specific energy density of the Sb/C//NVP@rGO full cell based on only cathode and anode active materials can reach 242 Wh kg-1 at 0.1 C. In addition, the full cell also exhibits excellent rate capacity of 96, 63, 50 and 46 mAh g-1 at current rates of 0.2 C, 1 C, 5 C and 10 C, respectively. Even at a very high rate of 20 C (2.34 A g-1), the cell can also give a discharge capacity of 43 mAh g-1 and high power density of 4.56 kW kg-1 based on cathode and anode active mass, showing superior power capability (Figure 7b). When cycled at 0.5 C, the cell remains 78% of its initial capacity after 40 cycles (Figure S5), indicating a practical feasibility for the NVP@rGO cathode to construct high rate and long-cycling sodium ion batteries.

Figure 7. (a) Typical charge-discharge profiles of the Sb/C//Na3V2(PO4)3/C full cell at 0.1 C; (b) rate property of the Sb/C//Na3V2(PO4)3/C full cell.

4. Conclusions In summary, we reported a facile spray-drying method to synthesize a porous and microspherical

graphene-scaffolded

Na3V2(PO4)3

with

greatly

improved

electrochemical

performances. The NVP@rGO electrode exhibited a high Na storage capacity (115 mAh g-1 at 0.2 C) with long-term cycle life (81 % of capacity retention over 3000 cycles at 5 C) and high rate property (44 mAh g-1 at 50 C). In addition, a Sb/C//NVP@rGO full cell also exhibited high capacity (110 16


Page 17 of 21

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


mAh g-1) at 0.1 C rate based on cathode mass, high rate capability (43 mAh g-1, corresponding to a superior power density of 4.56 kW kg-1 at 20 C) and cycling stability (78% capacity retention over 40 cycles at 0.5 C). The improved electrochemical properties of the NVP@rGO microsphere originate from its high conductive and flexible graphene network, which not only suppress the aggregation of NVP nanoparticles but also accommodate the structural stress of the material during charging and discharging. Additionally, the electrode material with spherical structure also facilitates the improvement of the volumetric energy density and the preparation of the electrode for the practical applications. Therefore, the spray-drying method combined with the GO reported in this work may provide a facile and effective approach to enhance the applicability and the electrochemical performance of Na-storable materials for large-scale Na-ion batteries.

Associated content Supporting Information Available. SEM images, TEM image and HRTEM image of NVP, nitrogen adsorption/desorption isotherms of the NVP or NVP@rGO composites, cycling performance of the NVP@rGO electrode, typical discharge-charge profile of Sb/C composite, cycling performance of the Sb/C electrode and the Sb/C//NVP@rGO sodium ion battery.

Author Information Corresponding Authors *E-mail: [email protected];

Acknowledgements: We thank financial support by the National Key Research Program of China (No. 17



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

Page 18 of 21

2016YFB0901501), National Science Foundation of China (Nos. 21673165, 21333007 and 21273090), and Hubei National Funds for Distinguished Young Scholars (2014CFA038).

References 1.

Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014,

114 (23), 11636-11682. 2.

Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Sodium‐ion batteries. Adv. Funct. Mater. 2013, 23 (8), 947-958.

3.

Pan, H.; Hu, Y.-S.; Chen, L. Room-temperature stationary sodium-ion batteries for large-scale electric energy storage.

Energy Environ. Sci. 2013, 6 (8), 2338-2360. 4.

Yuan, D.; Liang, X.; Wu, L.; Cao, Y.; Ai, X.; Feng, J.; Yang, H. A honeycomb-layered Na3Ni2SbO6: a high-rate and

cycle-stable cathode for sodium-ion batteries. Adv. Mater. 2014, 26 (36), 6301-6. 5.

Cao, Y.; Xiao, L.; Wang, W.; Choi, D.; Nie, Z.; Yu, J.; Saraf, L. V.; Yang, Z.; Liu, J. Reversible Sodium Ion Insertion in

Single Crystalline Manganese Oxide Nanowires with Long Cycle Life. Adv. Mater. 2011, 23 (28), 3155-3160. 6.

Wang, Y.; Yu, X.; Xu, S.; Bai, J.; Xiao, R.; Hu, Y. S.; Li, H.; Yang, X. Q.; Chen, L.; Huang, X. A zero-strain layered metal

oxide as the negative electrode for long-life sodium-ion batteries. Nat. commun. 2013, 4, 2365. 7.

Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S.

P2-type Nax[Fe1/2Mn1/2]O2 made from earth-abundant elements for rechargeable Na batteries. Nat. Mater. 2012, 11 (6), 512-517. 8.

Kim, D.; Kang, S.-H.; Slater, M.; Rood, S.; Vaughey, J. T.; Karan, N.; Balasubramanian, M.; Johnson, C. S. Enabling

Sodium Batteries Using Lithium-Substituted Sodium Layered Transition Metal Oxide Cathodes. Adv. Energy Mater. 2011, 1 (3), 333-336. 9.

Billaud, J.; Clément, R. J.; Armstrong, A. R.; Canales-Vázquez, J.; Rozier, P.; Grey, C. P.; Bruce, P. G. β-NaMnO2: A

High-Performance Cathode for Sodium-Ion Batteries. J. Am. Chem. Soc. 2014, 136 (49), 17243-17248. 10. Fang, Y.; Xiao, L.; Qian, J.; Ai, X.; Yang, H.; Cao, Y. Mesoporous Amorphous FePO4 Nanospheres as High-Performance Cathode Material for Sodium-Ion Batteries. Nano lett. 2014, 14 (6), 3539-43. 11. Jian, Z.; Yuan, C.; Han, W.; Lu, X.; Gu, L.; Xi, X.; Hu, Y.-S.; Li, H.; Chen, W.; Chen, D.; Ikuhara, Y.; Chen, L. Atomic Structure and Kinetics of NASICON NaxV2(PO4)3 Cathode for Sodium-Ion Batteries. Adv. Funct. Mater. 2014, 24 (27), 4265-4272. 12. Fang, Y.; Xiao, L.; Ai, X.; Cao, Y.; Yang, H. Hierarchical Carbon Framework Wrapped Na3V2(PO4)3 as a Superior High-Rate and Extended Lifespan Cathode for Sodium-Ion Batteries. Adv. Mater. 2015, 27 (39), 5895-5900. 13. Zhu, C.; Song, K.; van Aken, P. A.; Maier, J.; Yu, Y. Carbon-Coated Na3V2(PO4)3 Embedded in Porous Carbon Matrix: An Ultrafast Na-Storage Cathode with the Potential of Outperforming Li Cathodes. Nano lett. 2014, 14 (4), 2175-2180. 14. Oh, S.-M.; Myung, S.-T.; Hassoun, J.; Scrosati, B.; Sun, Y.-K. Reversible NaFePO4 electrode for sodium secondary batteries. Electrochem. Commun. 2012, 22, 149-152. 15. Fang, Y.; Zhang, J.; Xiao, L.; Ai, X.; Cao, Y.; Yang, H. Phosphate Framework Electrode Materials for Sodium Ion Batteries. Adv. Sci. 2017, 4, 1600392. 16. Qian, J. F.; Zhou, M.; Cao, Y. L.; Ai, X. P.; Yang, H. X. Nanosized Na4Fe(CN)6/C Composite as a Low-Cost and High-Rate Cathode Material for Sodium-Ion Batteries. Adv. Energy Mater. 2012, 2 (4), 410-414. 17. Wang, L.; Lu, Y.; Liu, J.; Xu, M.; Cheng, J.; Zhang, D.; Goodenough, J. B. A superior low-cost cathode for a Na-ion battery. Angew. Chem. Int. Ed. Engl. 2013, 52 (7), 1964-1967. 18. Fang, Y.-J.; Chen, Z.-X.; Ai, X.-P.; Yang, H.-X.; Cao, Y.-L. Recent Developments in Cathode Materials for Na Ion Batteries. Acta Phys. -Chim. Sin. 2017, 33 (1), 211-241. 19. Duan, W.; Zhu, Z.; Li, H.; Hu, Z.; Zhang, K.; Cheng, F.; Chen, J. Na3V2(PO4)3@C core-shell nanocomposites for 18


Page 19 of 21

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


rechargeable sodium-ion batteries. J. Mater. Chem. A 2014, 2 (23), 8668-8675. 20. Jian, Z.; Zhao, L.; Pan, H.; Hu, Y.-S.; Li, H.; Chen, W.; Chen, L. Carbon coated Na3V2(PO4)3 as novel electrode material for sodium ion batteries. Electrochem. Commun. 2012, 14 (1), 86-89. 21. Li, H.; Bai, Y.; Wu, F.; Li, Y.; Wu, C. Budding willow branches shaped Na3V2(PO4)3/C nanofibers synthesized via an electrospinning technique and used as cathode material for sodium ion batteries. J. Power Sources 2015, 273, 784-792. 22. Jung, Y. H.; Lim, C. H.; Kim, D. K. Graphene-supported Na3V2(PO4)3 as a high rate cathode material for sodium-ion batteries. J. Mater. Chem. A 2013, 1 (37), 11350. 23. Jiang, Y.; Yang, Z.; Li, W.; Zeng, L.; Pan, F.; Wang, M.; Wei, X.; Hu, G.; Gu, L.; Yu, Y. Nanoconfined Carbon-Coated Na3V2(PO4)3 Particles in Mesoporous Carbon Enabling Ultralong Cycle Life for Sodium-Ion Batteries. Adv. Energy Mater. 2015, 5 (10), 201402104. 24. Song, W. X.; Hou, H. S.; Ji X. B. Progress in the Investigation and Application of Na3V2(PO4)3 for Electrochemical Energy Storage. Acta Phys. -Chim. Sin. 2017, 33(1), 103-129. 25. Plashnitsa, L. S.; Kobayashi, E.; Noguchi, Y.; Okada, S.; Yamaki, J.-i. Performance of NASICON Symmetric Cell with Ionic Liquid Electrolyte. J. Electrochem. Soc. 2010, 157 (4), A536. 26. Kang, J.; Baek, S.; Mathew, V.; Gim, J.; Song, J.; Park, H.; Chae, E.; Rai, A. K.; Kim, J., High rate performance of a Na3V2(PO4)3/C cathode prepared by pyro-synthesis for sodium-ion batteries. J. Mater. Chem. 2012, 22 (39), 20857-20860. 27. Yang, J.; Han, D.-W.; Jo, M. R.; Song, K.; Kim, Y.-I.; Chou, S.-L.; Liu, H.-K.; Kang, Y.-M. Na3V2(PO4)3 particles partly embedded in carbon nanofibers with superb kinetics for ultra-high power sodium ion batteries. J. Mater. Chem. A 2015, 3 (3), 1005-1009. 28. Li, S.; Dong, Y.; Xu, L.; Xu, X.; He, L.; Mai, L. Effect of carbon matrix dimensions on the electrochemical properties of Na3V2(PO4)3 nanograins for high-performance symmetric sodium-ion batteries. Adv. Mater. 2014, 26 (21), 3545-3553. 29. Lim, S. Y.; Kim, H.; Shakoor, R. A.; Jung, Y.; Choi, J. W. Electrochemical and Thermal Properties of NASICON Structured Na3V2(PO4)3 as a Sodium Rechargeable Battery Cathode: A Combined Experimental and Theoretical Study. J. Electrochem. Soc. 2012, 159 (9), A1393-A1397. 30. Mao, J.; Luo, C.; Gao, T.; Fan, X.; Wang, C. Scalable synthesis of Na3V2(PO4)3/C porous hollow spheres as a cathode for Na-ion batteries. J. Mater. Chem. A 2015, 3 (19), 10378-10385. 31. Saravanan, K.; Mason, C. W.; Rudola, A.; Wong, K. H.; Balaya, P. The First Report on Excellent Cycling Stability and Superior Rate Capability of Na3V2(PO4)3for Sodium Ion Batteries. Adv. Energy Mater. 2013, 3 (4), 444-450. 32. Zhang, W.; Liu, Y.; Chen, C.; Li, Z.; Huang, Y.; Hu, X. Flexible and Binder-Free Electrodes of Sb/rGO and Na3V2(PO4)3/rGO Nanocomposites for Sodium-Ion Batteries. Small 2015, 11 (31), 3822-3829. 33. Rui, X.; Sun, W.; Wu, C.; Yu, Y.; Yan, Q. An Advanced Sodium-Ion Battery Composed of Carbon Coated Na3V2(PO4)3 in a Porous Graphene Network. Adv. Mater. 2015, 27 (42), 6670-6676. 34. Fang, Y.; Xiao, L.; Qian, J.; Cao, Y.; Ai, X.; Huang, Y.; Yang, H. 3D Graphene Decorated NaTi2(PO4)(3) Microspheres as a Superior High-Rate and Ultracycle-Stable Anode Material for Sodium Ion Batteries. Adv. Energy Mater. 2016, 6 (19), 1502197. 35. Hu, Q.; Liao, J.-Y.; Zou, B.-K.; Wang, H.-Y.; Chen, C.-H. In situ catalytic formation of graphene decoration on Na3V2(PO4)3 particles for ultrafast and long-life sodium storage. J. Mater. Chem. A 2016, 4 (43), 16801-16804. 36. Rajagopalan, R.; Zhang, L.; Dou, S. X.; Liu, H. K. Tuned In Situ Growth of Nanolayered rGO on 3D Na3V2(PO4)3Matrices: A Step toward Long Lasting, High Power Na-Ion Batteries. Adv. Mater. Interfaces 2016, 3 (13), 1600007. 37. Tao, S.; Wang, X.; Cui, P.; Wang, Y.; Haleem, Y. A.; Wei, S.; Huang, W.; Song, L.; Chu, W. Fabrication of graphene-encapsulated Na3V2(PO4)3as high-performance cathode materials for sodium-ion batteries. RSC Adv. 2016, 6 (49), 43591-43597.

19



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

Page 20 of 21

38. Xu, Y.; Wei, Q.; Xu, C.; Li, Q.; An, Q.; Zhang, P.; Sheng, J.; Zhou, L.; Mai, L. Layer-by-Layer Na3V2(PO4)3Embedded in Reduced Graphene Oxide as Superior Rate and Ultralong-Life Sodium-Ion Battery Cathode. Adv. Energy Mater. 2016, 6 (14), 1600389. 39. Wu, L.; Pei, F.; Mao, R.; Wu, F.; Wu, Y.; Qian, J.; Cao, Y.; Ai, X.; Yang, H. SiC–Sb–C nanocomposites as high-capacity and cycling-stable anode for sodium-ion batteries. Electrochim. Acta 2013, 87, 41-45. 40. Jian, Z.; Han, W.; Lu, X.; Yang, H.; Hu, Y.-S.; Zhou, J.; Zhou, Z.; Li, J.; Chen, W.; Chen, D.; Chen, L. Superior Electrochemical Performance and Storage Mechanism of Na3V2(PO4)3Cathode for Room-Temperature Sodium-Ion Batteries. Adv. Energy Mater. 2013, 3 (2), 156-160. 41. Wang, D.; Chen, N.; Li, M.; Wang, C.; Ehrenberg, H.; Bie, X.; Wei, Y.; Chen, G.; Du, F. 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 2015, 3 (16), 8636-8642. 42. Wang, Q.; Zhao, B.; Zhang, S.; Gao, X.; Deng, C. Superior sodium intercalation of honeycomb-structured hierarchical porous Na3V2(PO4)3/C microballs prepared by a facile one-pot synthesis. J. Mater. Chem. A 2015, 3 (15), 7732-7740. 43. Chu, Z.; Yue, C. Graphene oxide wrapped Na3V2(PO4)3/C nanocomposite as superior cathode material for sodium-ion batteries. Ceram. Int. 2016, 42 (1), 820-827. 44. Fang, J.; Wang, S.; Li, Z.; Chen, H.; Xia, L.; Ding, L.; Wang, H. Porous Na3V2(PO4)3@C nanoparticles enwrapped in three-dimensional graphene for high performance sodium-ion batteries. J. Mater. Chem. A 2016, 4 (4), 1180-1185. 45. Wu, L.; Hu, X. H.; Qian, J. F.; Pei, F.; Wu, F. Y.; Mao, R. J.; Ai, X. P.; Yang, H. X.; Cao, Y. L. Sb-C nanofibers with long cycle life as an anode material for high-performance sodium-ion batteries. Energy Environ. Sci. 2014, 7 (1), 323-328.

20


Page 21 of 21

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


Table of Contents Graphic 270x189mm (96 x 96 DPI)


Graphene-Scaffolded Na3V2(PO4)3 Microsphere Cathode with High

Recommend Documents