Carbon-Coated Na3V2(PO4)3 Anchored on Freestanding Graphite

Subscriber access provided by NORTHERN KENTUCKY UNIV STEELY

Article

Carbon Coated Na3V2(PO4)3 Anchored on Freestanding Graphite Foam for High Performance Sodium-Ion Cathode Xiongwu Zhong, Zhen-Zhong Yang, Yu Jiang, Weihan Li, Lin Gu, and Yan Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11873 • Publication Date (Web): 03 Nov 2016 Downloaded from http://pubs.acs.org on November 4, 2016

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 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

ACS Applied Materials & Interfaces

Carbon Coated Na3V2(PO4)3 Anchored on Freestanding Graphite Foam for High Performance Sodium-Ion Cathode

Xiongwu Zhong,a Zhenzhong Yang,d Yu Jiang,a Weihan Li,a Lin Gu,d,e Yan Yu*, a,b,c a

Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences

(CAS), Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui, 230026, China. E-mail: [email protected] b

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),

Nankai University, Tianjin 300071, China.

c

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui, 230026, China.

d

Beijing Laboratory for Electron Microscopy, Institute of Physics, Chinese Academy

of Sciences (CAS), Beijing, 100190, China. e

Collaborative Innovation Center of Quantum Matter , Beijing, 100190 , China.

Abstract Na3V2(PO4)3 (NVP) has been considered as a most promising cathode materials for sodium ion battery (SIBs). But NVP usually exhibits poor cycling stability and rate performance due to the low intrinsic electrical conductivity. Herein, we prepared carbon coated Na3V2(PO4)3 anchored on freestanding graphite foam (denoted as NVP@C-GF) as cathode for SIBs. The NVP@C-GF exhibits superior sodium-ion storage performance, including rate capability (56 mAhg-1 at 200C) and long cycle life (54 mAhg-1 at 100 C after 20000 cycles). The resulting NVP@C-GF inherits the advantages of 3D free-standing graphite that possesses high electrical conductivity and porous structure for electrolyte to soak in. Furthermore, carbon coated NVP particles anchored on the surface of GF not only accommodate the volume change of NVP during charge/discharge, but also reduce the diffusion distance of Na+ ion.

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

Keywords: sodium ion battery, cathode, graphite foam, Na3V2(PO4)3, freestanding electrode. Introduction Lithium-ion batteries (LIBs) have been considered as one of the most important technologies for energy storage in portable devices, electric vehicles (EVs), and hybrid electric vehicles (HEVs).1-2 However, there are two drawbacks of LIBs for large-scale application: the limited and unevenly distributed availability of lithium in earth. As LIBs alternatives, sodium-ion batteries (SIBs) has attracted increasing attention because of the abundant resources and low cost of Na.3-4 Although the storage mechanism of Na in electrode materials is similar to that of Li, most usually applied electrode materials for LIBs are not suitable for Na+ insertion due to the radius of Na+ is 55% larger than that of Li+.5 The larger ionic radius of Na results in the more sluggish of Na insertion and extraction in the crystal structure of cathode materials for SIBs, leading to poor rate performances.6 A number of efforts have been made to develop superior cathodic materials with large interstitial space to achieve rapid ion insertion and extraction, such as Na2V6O15, Na0.44MnO2, Na4Fe(CN)6 and Na3V2(PO4)2F3.7-8 Recently, Na ion superionic conductor (NASION)-structure compounds, such as Na3V2(PO4)3 (denoted as NVP), have been investigated as prospective cathode materials for SIB, which feature high ionic conductivity and large interstitial space that enable fast Na+ diffusivities.9-11 In addition, NVP possess high energy density of ~400 Whkg-1 (117 mAhg-1 × 3.4 V for the V3+/V4+ redox couple).9, 12 However, the poor electronic conductivity of NVP leads to poor electrochemical performance and prevents its practical application.13 The current strategies to improve the electronic conductivity of NVP are to reduce the particle size, coat a conductive layer on the surface of NVP and embed the NVP in carbon matrix.14-15 Among the various strategies, embedding the nanosized NVP in carbon matrix has been demonstrated as one of the most effective approaches.14 For example, Kang’s group reported NVP particles partly embedded in carbon nanofibers, delivering a reversible capacity of 89 mAhg-1 at 50 C.16 However, most of these reports usually require somewhat complex


Page 2 of 17

Page 3 of 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


chemical processes to fabricate the carbon matrix and the obtained carbon matrix is amorphous with unsatisfactory electronic conductivity.17

In addition, for a

conventional electrode preparation process, auxiliary additives (i.e. polymer binder, conductive agents) are needed to improve the mechanical integrity and electronic conductivity of the electrodes. But these additives have no capacity contribution to the electrode and definitely decrease overall performance when the total volume of the battery is taken into account. To construct ideal electrode architecture, it is better to design an electrode with a three-dimensional (3D) interconnected network allowing both efficient ion and electron transport.18 A variety of 3D structures have been designed for high performance battery, including 3D metal foam, inverse opal structures, and 3D porous graphite foam.19-20 To further increase the energy density and power density of the battery, it is ideal to use freestanding electrodes (metal current collector free) in which all the materials could participate in sodium/lithium storage.21 The free standing configuration can not only simplify cell packing by reducing inactive ingredients, but also improve the electrode performance. Carbon-based structures are popular to be used as free-standing current collectors for the electrode of battery because of their high conductivity, tunable dimensions, and low density.22-24 Many groups have reported that the use of free-standing, lightweight, and highly conductive 3D porous graphite foam (GF), loaded with electrochemically active nanostructures, as electrodes for lithium storage with high power rate and long lifespan.25-26 However, a similar structure of cathode material for SIB (such as NVP) has not been investigated. Herein, we prepared a binder-free film electrode by anchoring carbon coated NVP on 3D porous graphite foam (GF), denoted as NVP@C-GF hereafter. The NVP@C-GF electrode exhibits a reversible capacity, as high as 95 mAhg-1 and shows outstanding capacity retention 95% after 1000 cycles at 10 C, when tested for Na storage in the voltage window 2.9~3.8V (vs. Na+/Na). Moreover, the NVP@C-GF preserves 46% of its theoretical capacity when cycled at 100 C after 20000 cycles. Experimental Section



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 17

Synthesis of NVP@C-GF and NVP@C：In general, GF was prepared by the CVD method with Ni foam as a template. 0.91 g V2O5 was added into 30 ml of distilled water under continuous stirring, and then 1.89 g oxalic acid was added as reductive agent. The mixture was under vigorous stirring at 70 oC for 1h. The another solution was prepared by dissolving 1.725 g NH4H2PO4, 0.795 g Na2CO3 and 1.2 g glucose in 20 ml of distilled water. Subsequently, two solution was mixing together to yield the NVP@C precursor. Next, the NVP@C precursor was dipped into a piece of O2 plasma treated GF. The NVP@C-GF precursor was dried in a vaccum oven overnight following with sintering at 800 oC for 6h in an atmosphere of Ar to obtain NVP@C-GF. And NVP@C was prepared without the present of GF.

Characterization: X-ray diffraction (XRD) was collected by Rigaku TTR-||| using Cu Kα radiation. The TEM and SEM images of NVP@C-GF and NVP@C was obtained on a JEOL 4000EX transmission electron microscope (HRTEM) (JEOL, Tokyo, Japan) and a JSM-6700 field-emission scanning electron microscope (JEOL, Tokyo, Japan) operated at 5keV. Raman measurement was recorded on a DXR Raman microscope from Thermo Scientific with a laser wavelength of 532 nm. Element analysis was used to determine the amorphous carbon content of NVP@C-GF and NVP@C.

Electrochemical measurement: The NVP@C electrode was prepared by mixing NVP@C, carbon black and poly(vinylidene fluoride) (PVDF) with

a weight

ratio of 80 : 10: 10 onto a Al foil. The freestanding NVP@C-GF was directly used as electrodes and the mass area of NVP and amorphous carbon was about 0.9 mg cm-2. In sodium half batteries, the coin type half-cells (CR2032) were assembled in an argon-filled glovebox, and sodium metal was used as anode. The electrolyte was 1 M NaClO4 in propylene carbonate (PC), and separator was the glass fiber.


Page 5 of 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


Results and Discussion The schematic representation of fabrication process of the freestanding NVP@C-GF is shown Figure 1. The graphite foam (GF) was firstly prepared by CVD method using commercial Ni foam as a template. After removing the Ni frameworks by acid treatment, the freestanding GF remained the 3D interconnected structure that is a replica of Ni foam (see Figure S1). The GF possess a 3D interconnected network structure, showing a pore size of 50-200 µm. Then the solution of NVP precursor and carbon resource was dipped into GF that was treated with the O2 plasma before. Figure 2a and 2b show the scanning electron microscope (SEM) images of the GF film loaded with NVP precursor. The surface of GF was covered by the slurry of NVP precursor. Subsequently, the GF with NVP precursor slurry was annealed at high temperature to get NVP@C-GF. Figure 2c shows the SEM image of the NVP@C-GF. The NVP particles not only disperse on the surface of GF but also adhere to the GF matrix. Figure 2d shows that the NVP particles aggregate and tend to be trapped in the fold positions of the GF matrix. For comparison, we also prepared the GF supported NVP (denoted as NVP-GF) sample by similar process except that glucose was not added. For NVP-GF sample, the NVP (Figure S2) shows a particle size of ~1 um and most of the particles agglomerate. Obviously, after introduction of glucose, the size of NVP in NVP@C-GF (Figure 2e) decrease to ~200 nm, which is attributed to the carbon coating. Carbon coating on the surface of NVP is one of the most effective ways to prevent the undesirable growth of NVP particles during the post heat treatment process.9

HRTEM image of

NVP@C-GF confirms the highly crystallite NVP particle coated by an amorphous carbon layer. The carbon layer could anchor the NVP particles on the GF matrix, offering seamless contact between the GF matrix and NVP particles. Figure 3a shows the X-ray diffraction (XRD) patterns for the as-prepared NVP@C-GF. The peaks at 27o, 44o, and 54o were indexed to graphite carbon, while all other peaks indexed to the NASICON structured NVP with a R3c space group (rhombohedral structure).12,

27

Furthermore, Raman spectrum was employed to

characterize the as-prepared NVP@C-GF and bare GF, respectively. As shown in



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 3b, two distinct peaks are attributed to the D band at ca. 1341 cm-1 and the G band at ca. 1600 cm-1, respectively. The D band is attributed to a series of structural defects and the G band is observed for all graphitic structures because of the first-order scattering of the E2g mode.28 The D/G intensity ratio (ID/IG) could provide the gauge for the amount of structural defects. Both of ID /IG were very low, certifying GF as a superior current substrate with high conductivity. Besides, the ratio of D and G bands of NVP@C-GF is larger than that of bare GF due to the existence of amorphous carbon. And element analysis results confirm that the amorphous carbon content of NVP@C-GF is 12.1 %. To investigate the important role of 3D GF in the NVP@C-GF, we chose the carbon coated NVP particles (NVP@C) for comparison. The NVP@C composites were prepared by pyrolysis of the same NVP@C precursor without GF. As shown in Figure S3a, the overview SEM image shows an irregular morphology of NVP@C, which consists of microsized particles. Figure S3b shows the high-magnification SEM image of NVP@C. The primary particle is less than 400 nm in size and agglomerates to form larger secondary particles. The electrochemical performance of the NVP@C-GF and NVP@C for SIB cathodes was evaluated in a potential range of 2.7-3.8 V versus Na+/Na.14 Figure 4a displayed the galvanostatic charge/discharge voltage profiles of the NVP@C-GF and NVP@C cathodes at a current rate of 1 C (1 C = 110 mAhg-1). Both of them exhibit typical voltage profiles of a NVP cathode, displaying flat plateaus around 3.4 V that corresponds to the characteristic V4+/V3+ redox couple.27, 29 The NVP@C-GF delivers similar charge capacities of about 106 mAhg-1, which was slightly higher than that of the NVP@C (103 mAhg-1). Obviously, the gap between charge and discharge of the NVP@C-GF electrode (0.05 V) is narrower than that of NVP@C electrode (0.1 V), indicating the shortened diffusion length for Na+ and enhanced redox kinetics.30 The NVP@C-GF and NVP@C shows initial coulombic efficiencies (ICE) of 95% and 84%, respectively. The improved ICE in NVP@C-GF maybe due to the GF matrix that helps to prevent the formation of a solid electrolyte interface (SEI) on the surface of NVP. Figure 4b shows the long cyclability of the NVP@C-GF and NVP@C at 1C. The NVP@C-GF shows a slightly improved cyclability and delivers a reversible


Page 6 of 17

Page 7 of 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


capacity of 106 mAhg-1 after 100 cycles. The capacity retentions for the NVP@C-GF and the NVP@C are 96.6 % and 90.8 % after 100 cycles, respectively. The improved cyclability of the NVP@C-GF maybe attributed to the 3D GF matrix that forms an electronic wiring pathway along the NVP particles and enhance their reversibility.27 In this paper, the capacity is calculated based on the mass of NVP because GF and amorphous carbon delivers ignorable capacity in this voltage range. Figure 4c compares the rate capability of the NVP@C-GF and the NVP@C. The NVP@C-GF exhibits only about 2% enhanced rate performance compared with that of NVP@C at low current densities (1C & 2C), resulting from the decreased sodium ion diffusion length and increased surface area of the NVP@C-GF. Note that, the NVP@C-GF shows a much improved rate capability compared to the NVP@C at high current densities (from 5C to 200C). The NVP@C-GF delivers reversible capacities of 97, 95, 92, 88, 83, and 56 mAhg-1 at 5C, 10C, 20C, 50C, 100C, and 200C, respectively. In case of cycled at 20C, the NVP@C-GF delivers 45 times larger capacity than that of NVP@C, which is attributed to 3D interconnected GF matrix that enable fast electron transport. Moreover, after 80 cycles at different current densities, the specific capacity of NVP@C-GF could recover 106 mAhg-1 when the C rate was tuned back to 1 C, demonstrating the superior reversibility of sodium storage in NVP@C-GF. What is more, we compare the electrochemical performance of NVP@C-G with other NVP based cathode materials in Table S1. Our NVP@C-G exhibits more superior performance. Figure 4d shows the long cycle life of the NVP@C-GF at 10C and 50C. The NVP@C-GF electrode exhibits outstanding electrochemical performance in the long-term cycling test with high current rate. As shown in Figure 4d, it can deliver reversible capacity of 93 and 80 mAhg-1 over 1000 cycles in current rate of 10 and 50 C, respectively. The coulombic efficiency of NVP@C-GF in current rate of 10 C is nearly 100%, which is a little higher than that in 50 C (~98%). More importantly, even cycled for 20000 times in an ultrahigh current rate of 100 C, the NVP@C-GF electrode can still demonstrate outstanding cycling stability in which 76 % of the initial capacity can be maintained (Figure 5). The inset of Figure 5 shows the



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

polarization voltage of ~0.48 V for the 1st cycle and ~0.68 V for the 20000th cycle, displaying the high cycling stability of NVP@C-GF. The excellent electrochemical performance of the NVP@C-GF electrode is believed to result from special structure: i) The interconnected carbon 3D GFs framework functions as a robust and flexible buffer to accommodate volume changes results from the repeated Na ion insertion/extraction. ii) The combination of 3D GFs matrix and carbon coating on every NVP surface shows a high electrical conductivity of the overall electrode, thus enabling superior rate performance; ii) The unique 3D interconnecting formworks possesses high contact surface area between electrode materials and electrolyte, offering fast ions transfer; iii) carbon coated NVP particles anchored on the surface of GF not only improve the electronic conductivity of NVP, but also reduce the diffusion distance of Na+ ion.

Conclusion In summary, we have developed a simple process to fabricate freestanding carbon coated Na3V2(PO4)3 anchored on freestanding graphite foam. We have demonstrated that an excellent rate performance of NVP@C-GF for sodium ion batteries. The reversible capacity of the NVP@C-GF (92 mAhg-1) is much higher that of NVP@C (2 mAhg-1) at the rate of 20 C. It exhibits long-term cyclability and delivers a very stable reversible capacity of 54 mAhg-1 at 100 C after 20000 cycles. The 3D GFs matrix and carbon shell on NVP can provide ultrahigh electric conductivity and offer large contact area between electrode materials and electrolyte, thus enabling ultrafast diffusion of Na+ ion and electron. Our results suggest GF as a superior current collector of high-power electrode for energy storage applications.

Author Information Corresponding Author E-mail: [email protected]


Page 8 of 17

Page 9 of 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


Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51622210, No. 21373195, No. 51522212 and No. 51421002), the Fundamental Research Funds for the Central Universities (WK340000004), and the Collaborative Innovation Center of Suzhou Nano Science and Technology.

ASSOCIATED CONTENT Supporting Information Available: the supplementary characterization of as-prepared materials. This information is available free of charge via the internet at http://pubs.acs.org.

References:

1. Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652-657. 2. Yu, Y.; Gu, L.; Lang, X. Y.; Zhu, C. B.; Fujita, T.; Chen, M. W.; Maier, J. Li Storage in 3D Nanoporous Au-Supported Nanocrystalline Tin. Adv. Mater. 2011, 23, 2443-2447. 3. Zeng, L.; Wei, X.; Wang, J.; Jiang, Y.; Li, W.; Yu, Y. Flexible One-Dimensional Carbon-Selenium Composite Nanofibers with Superior Electrochemical Performance for Li-Se/Na-Se Batteries. J. Power Sources 2015, 281, 461-469. 4. Kim, S. W.; Seo, D. H.; Ma, X. H.; Ceder, G.; Kang, K. Electrode Materials for Rechargeable Sodium-Ion Batteries: Potential Alternatives to Current Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 710-721. 5. Pan, H.; Hu, Y. S.; Chen, L. Room-Temperature Stationary Sodium-Ion Batteriesfor Large-Scale Electric Energy Storage. Energy Environ. Sci. 2013, 6, 2338-2360. 6. Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-Gonzalez, J.; Rojo, T. Na-Ion Batteries, Recent Advances and Present Challengesto Become Low Cost Energy Storage Systems. Energy Environ. Sci. 2012, 5, 5884-5901. 7. Qi, Y. R.; Mu, L. Q.; Zhao, J. M.; Hu, Y. S.; Liu, H. Z.; Dai, S. Superior Na-Storage Performance of Low-Temperature-Synthesized Na3(VO1-xPO4)2F1+2x (0

Carbon-Coated Na3V2(PO4)3 Anchored on Freestanding Graphite

Recommend Documents