Vanadium Oxide Thin Film Formation on Graphene Oxide by

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Vanadium oxide thin film formation on graphene oxide by microexplosive decomposition of ammonium peroxovanadate and its application as sodium ion battery anode Alexey A. Mikhaylov, Alexander G. Medvedev, Dmitry A. Grishanov, Sergey Sladkevich, Jenny Gun, Petr V. Prikhodchenko, Zhichuan J. Xu, Arun Nagasubramanian, Madhavi Srinivasan, and Ovadia Lev Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00035 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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Vanadium oxide thin film formation on graphene oxide by microexplosive decomposition of ammonium peroxovanadate and its application as sodium ion battery anode Alexey A. Mikhaylov,[a, b, c] Alexander G. Medvedev,[a] Dmitry A. Grishanov,[a] Sergey. Sladkevich,[b] Jenny Gun,[b] Petr V. Prikhodchenko,*[a] Zhichuan J. Xu,[c] Arun Nagasubramanian,[d] Madhavi Srinivasan,[c, d] and Ovadia Lev*[b, c] [a]

Kurnakov Institute of General and Inorganic Chemistry Russian Academy of Sciences

Leninskii prosp. 31, Moscow 119991 (Russia) E-mail: [email protected] [b]

The Casali Center of Applied Chemistry, The Institute of Chemistry, The Hebrew University

of Jerusalem Edmond J. Safra Campus, Jerusalem 91904 (Israel) E-mail: [email protected] [c]

Singapore-HUJ Alliance for Research and Enterprise, NEW-CREATE Phase II Campus for

Research Excellence and Technological Enterprise (CREATE) 1 CREATE Way, Singapore 138602 (Singapore) [d]

Energy Research Institute@NTU Nanyang Technological University 50 Nanyang Avenue,

Singapore 639798 (Singapore).

ACS Paragon Plus Environment

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KEYWORDS V3O7 Peroxovanadate Anode Microexplosion Na-ion battery Hydrogen peroxide.

Abstract

Formation of vanadium oxide nanofilm - coated graphene oxide (GO) is achieved by thermally induced explosive disintegration of a microcrystalline ammonium peroxovanadate-GO composite. GO sheets isolate the microcrystalline grains and capture and contain the microexplosion products, resulting in the deposition of the nanoscale products on the GO. Thermal treatment of the supported nanofilm yields a sequence of nanocrystalline phases of vanadium oxide (V3O7, VO2) as a function of temperature. This is the first demonstration of microexplosive disintegration of a crystalline peroxo compound to yield a nanocoating. The large number of recently reported peroxide – rich crystalline materials suggests that the process can be a useful general route for nanofilm formation. The V3O7@GO composite product was tested as a sodium ion battery anode and showed high charge capacity at high rate chargedischarge cycling (150 mAh g-1 at 3000 mAg-1 vs 300 mAh g-1 at 100 mAg-1) due to the nanomorphology of the vanadium oxide.

Introduction Peroxo compounds are increasingly used as a means to control the shape and structure of advanced materials 1–4. The environmental footprint of peroxide is small compared to other ligands, its decomposition yields water and oxygen and no waste, and its removal does not require a solvent rinse. Zero dimension (quantum dots), 1-d (rods and tubes), 2-D (nanosheets


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and films) and high surface area 3-D morphologies can be nanostructured using peroxide chemistry 2. Recently, decomposition of intercalated peroxide was reported to produce porous magnesium-aluminium layered double hydroxides 5. High surface area mesoporous barium stannate was produced from peroxo stannate and BaCO3 followed by mild heat treatment with peroxide decomposition. Lanthanum catalyzed thermal decomposition of peroxostannate yields a useful precursor for the formation of a uniform film of lanthanum doped barium stannate with high electron mobility 3. The chemistry of peroxovanadate is very rich. Thus, for example, 14 different peroxo complexes of V(V) containing only peroxo, hydroxo and oxo ligands were reported in a recent review 6. Microporous vanadium pentoxide was obtained by reaction of vanadium oxide with hydrogen peroxide 7. In this article, we present a new way to restructure supported peroxovanadate by explosive disintegration, and we show that the vanadium products of the explosion are contained by the GO sheets, yielding a graphene oxide - vanadium oxide composite. As a demonstrative application, we report the use of the material for a sodium ion battery anode (NIB), a challenging application, which requires a very short diffusion path in the solid. Despite the contemporary dominance of rechargeable lithium ion batteries (LIBs) in the fields of electric grids and electric vehicles, the development of an alternative type of rechargeable battery attracts scientific attention. The high cost of lithium and the geographically limited abundance of lithium ores motivate a search for alterative batteries and battery materials. The sodium ion battery is emerging as one such alternative route despite its notable drawbacks: Na+ has a higher reduction potential than lithium; elemental sodium reacts more violently with water; and its atomic size is larger. Therefore, with the exception of hard carbon 8, sodium does not intercalate readily into graphitic materials and the sodiation and desodiation involve a large volume change,


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which causes electrode pulverization and loss of electric contact after prolonged charge – discharge cycling. Recently, the oxides of vanadium received increased attention 9–18, probably due to their varied morphologies and redox states which can be tailored by modern nanotechnology. However, vanadium oxide electrodes suffer from two major disadvantages, low electric conductivity (e.g. 10-2-10-3 Ω-1 cm- 1 for V2O5 11) and low lithium mobility (10-12 – 10-13 cm2 s-1 for V2O5) 11,16. These drawbacks can be largely compensated by synthesis of nanomorphologies and carbon supporting nanostructures to decrease the lithium diffusion pathway and provide enhanced electric conductivity. Reduced graphene oxide (rGO) – vanadium oxide composites attracted considerable attention due to the thin cross section of a few layers of rGO, its high conductivity and the elasticity of the GO, which can maintain contact with the active material even after deformation 12,13,15,19–21. Until now, vanadium pentoxide has proven to provide superior NIB and LIB anodes compared to other forms vanadium oxides 20–24.Recently, Sun et al 20 showed a remarkable 900 mAh g-1 charge capacity by atomic layer deposited vanadium pentoxide on GO. Wang et al., 16 provided an example of a self-assembled, layered vanadium pentoxide - graphene oxide by a solvothermal approach (20 h in DMF at 200°C), and a reversible capacity of a lithium ion anode of 1010 mAh g-1 was demonstrated at 500 mAg-1. The more challenging NIB anode applications of VOx exhibit lower charge capacity. 13,14,17 Experimental Section Preparation of ammonium peroxovanadate precursor. 720 mg of V2O5 were introduced into a mixture of 10 ml of DIW and 8 ml of ammonium hydroxide, NH4OH, and then 18 ml of 50% H2O2 were added under stirring. The resulting solution was stirred for an additional 5 min. The process was conducted in an ice-bath.


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Preparation of graphene oxide coated peroxovanadate NH4VOOH-GO-RT 1.5% graphene oxide aqueous dispersion was obtained by the modified Hummer’s method as described in 25. 15 g of 1.5% GO dispersion were introduced into 25 mL of H2O and then 10 mL of 50% H2O2 were added under stirring. The dispersion was left in an ultrasonic bath for 5 min. The precursor solution of ammonium peroxovanadate was introduced into the GO dispersion under continuous stirring and left under stirring for 5 min. Then 600 ml of ethanol-diethyl ether (1:1) mixture were successively added slowly under fast stirring. The product was separated by filtration and washed 3 times by ethanol-diethyl ether (1:1) mixture and dried in a vacuum desiccator. Heat treatment of the NH4VOOH-GO-RT powder was carried out in a tube under Ar flow (380°C) and vacuum (500°C). The sample VOx-rGO-380 was heated to the indicated temperature (380°C) at a 15 °C min-1 rate, and the temperature was then maintained for 5 min. The sample VOx-rGO-500 was heated to the indicated temperature (500°C) at a 1 °C min-1 rate, and the temperature was then maintained for 2 hours. Electrochemical evaluation Each of the different sodium intercalation materials was mixed with acetylene black and carboxymethylcellulose sodium salt (CMC, Sigma-Aldrich) in a weight ratio of 70:15:15 with deionized water as the medium to form a slurry. The slurry was then coated on roughened copper foil as a current collector using a doctor blade. The electrode was then dried at 80°C and pressed in a roll press. The electrodes were cut into 16 mm discs and further dried at 110 °C for 4 h under vacuum before being introduced into an argon-filled glove box. 1 M NaClO4 in ethylene


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carbonate/diethyl carbonate (1:1 by volume) was used as electrolytes for NIB. 2% v/v fluorinated ethylene carbonate (FEC) was added to the electrolyte mixtures to improve the solid electrolyte interface (SEI) stability. The coin cells were assembled in an Ar-filled glove box with concentrations of moisture and oxygen below 0.1 ppm. The cells were then tested with a battery tester between 0 to 2.5 V vs. Na/Na+. Cyclic voltammetry (CV) profiles were taken at a scan rate of 0.1 mV s-1 between 0.005 V and 2.5 V vs. Na/Na+. Rate performance was measured by testing the cell at current rates of 100 - 2000 mAg-1. Results The essential features of thin film formation of vanadium oxide@GO by the microexplosive decomposition of large crystallites can be readily followed in the set of electron microscope images taken as a function of temperature (Figures 1 and 2) and by the corresponding XRD diffractograms (Figure 3). In addition, in order to glean insight into the underlying process leading to the disintegration of the large crystallites and the formation of the thin film of vanadium oxide, we carried out a DSC study (Figure 4). A schematic diagram of the two main steps of the transformation is depicted in Scheme 1.


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Scheme 1. Vanadium oxide thin film formation on graphene oxide by microexplosive decomposition of ammonium peroxovanadate.

Figure 1. Electron microscope images of different forms of vanadium oxide and peroxovanadate at different temperatures. SEM imaging of (a) NH4VOOH-GO-RT, (d) VOx-rGO-380 and corresponding EDX mapping. Figure 1 shows the initial deposit of ammonium peroxovanadate at room temperature on GO. NH4VOOH-GO-RT comprises a large, submicron crystalline material. The STEM image (Figure 2b) and the secondary electron SEM image (Figure 2a) of the room temperature material assess this observation by showing a larger section with several crystallites and illuminate the difference between the GO support and the bright crystallites. The initial crystalline material in NH4VOOH-GO-RT was identified as monoclinic (NH4)3(VO(O2)2)2(OH)(H2O) (PDF #01-0743594). A comparison of the carbon (Figure 1b) and vanadium elemental densities (Figure 1c)


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shows that they follow opposite tendencies, the high vanadium regions coincide with the low carbon densities, and they both correspond to the crystalline site in Figure 1a, which is reasonable since the crystallites are large and they overlay the GO. It can also be noted, that the

Figure 2. Electron microscope images of different forms of vanadium oxide and peroxovanadate at different temperatures. Secondary electron SEM imaging (a) and STEM image (b) of NH4VOOH-GO-RT at room temperature. Secondary electron images of VOx-rGO-380 and VOxrGO-500 are presented in frames (c) and (e), respectively along with the relevant TEM micrographs, frames (d) and (f). vanadium signal at the GO sites that are not covered by large crystals is the same as the background vanadium signal, showing that there are no vanadium species on GO besides the large crystals. The room temperature composite would be unattractive for electrochemistry, since the lithium and sodium diffusion distance within the crystallites is very large.


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Thermal treatment starts the decomposition of the (NH4)3((VO(O2)2)2(OH)(H2O) at 90°C (Figure 4) with a sharp mass loss of 26.5 (as indicated in the TGA curve in Figure S3 of the Supporting Information) to yield a uniform amorphous material with a small signal of ammonium vanadate, NH4VO3 which can be identified based on the XRD diffractograms taken at 25°C and 150°C (Figure 3). The corresponding SEM image presented in Figure 1d shows that the large crystals disappeared after heating. Frames b, e and c, f in Figure 1 show elemental EDX mapping of carbon and vanadium respectively. After heat treatment, at 150°C, there is qualitative correspondence between the signals of carbon and vanadium (contrary to the room temperature element distributions), which can be explained by uniform coating of the GO by a thin film. The difference in the local signals can then be attributed to corrugations of the graphene oxide and difference in the local thickness of the composite. The graphene fraction in the electrodes can be estimated based the TGA curve in Figure S3 to be less than 7%.


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Figure 3. X-ray diffractograms of GO-supported peroxovanadate before (a) and after (b-d) thermal treatment at indicated temperatures.


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Figure 4. DSC curve of GO-supported peroxovanadate. A multistep exothermic heat flow starting at 90°C and ending at 130°C can be observed (Figure 4) with a very sharp peak at 113°C. There was a net release of 465 J/g by the peroxide decomposition. The size reduction takes place during the exothermal (microexplosive) transformation of the peroxovanadate to the vanadate and the amorphous phase due to the release of gaseous ammonia, oxygen and water. At T=380°C, the material denoted VOx-rGO-380 is obtained. It is comprised of small, approximately 5nm crystals (Figure 2d) and larger nanorod crystals that can be seen in the TEM micrograph of Figure S2. The material is identified as anhydrous V3O7 crystals with calculated crystal size of 57 nm by Scherrer equation based on XRD diffraction. These correspond to the reflections from the rods which out mask the diffraction peaks of the smaller crystals.


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Figure 5. V 2p (left frame) and C 1s (right frame) XPS spectra of GO-supported peroxovanadate before (a,d) and after heat treatment at 380°C (b,e) and 500°C (c,f). After heat treatment to 500°C, VOx-rGO-500 is formed, and its diffractogram shows that the material was reduced to vanadium dioxide. This material is practically indistinguishable from bare GO by SEM imaging alone (Figure 2e). However, the vanadium oxide film can be readily observed by TEM (Figure 2f) showing that the GO is uniformly coated by nanocrystallites. The

Vanadium Oxide Thin Film Formation on Graphene Oxide by

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