Nitrogen-Doped Graphene Framework for Enhanced Anodic Performance in Lithium Ion Batteries

Accepted Manuscript Title: Surfactant-Assisted Hydrothermal Synthesis of Cobalt Oxide/Nitrogen-Doped Graphene Framework for Enhanced Anodic Performance in Lithium Ion Batteries Author: Xia Xing Ruili Liu Shaoqing Liu Suo Xiao Yi Xu Chi Wang Dongqing Wu PII: DOI: Reference:

S0013-4686(16)30371-1 http://dx.doi.org/doi:10.1016/j.electacta.2016.02.096 EA 26713

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

7-10-2015 13-2-2016 15-2-2016

Please cite this article as: Xia Xing, Ruili Liu, Shaoqing Liu, Suo Xiao, Yi Xu, Chi Wang, Dongqing Wu, Surfactant-Assisted Hydrothermal Synthesis of Cobalt Oxide/NitrogenDoped Graphene Framework for Enhanced Anodic Performance in Lithium Ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.02.096 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Surfactant-Assisted

Hydrothermal

Oxide/Nitrogen-Doped

Graphene

Synthesis Framework

of for

Cobalt Enhanced

Anodic Performance in Lithium Ion Batteries Xia Xinga, Ruili Liub,*, Shaoqing Liu a, Suo Xiao a, Yi Xu a, Chi Wang c, and Dongqing Wuc,* a

Department of Chemical Engineering,School of Environment and Chemical Engineering,

Shanghai University, Shanghai 200444, People's Republic of China b

National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic

Engineering, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China c

School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai

200240, People's Republic of China

*Corresponding author. E-mail: [email protected] [email protected] (Prof. D. Wu)

(Prof.

R.

Liu);

Abstract In this work, the composites of nitrogen-doped graphene framework and Co3O4 nanoparticles with adjustable morphologies (NG/Co3O4) were fabricated via a surfactant-assisted hydrothermal route for first time. Three different surfactants including triblock copolymer F127, cetyltrimethyl ammonium bromide and sodium dodecyl sulfate are involved in the hybrid-assembly of graphene oxide, o-phthalonitrile and cobalt acetate in water/ethanol. Among the obtained samples, the one using F127 (NG/Co3O4-F127) manifests the most homogeneous distribution of Co3O4 NPs with the size of ~ 15 nm in the macropore-walls formed by NG. As the anode material in lithium ion battery (LIB), NG/Co3O4-F127 exhibits excellent 1

electrochemical performance, which is superior to the other composites and most of the previously reported Co3O4 based anode materials in LIBs.

Keywords: surfactant; graphene; cobalt oxide; lithium-ion battery

1. Introduction In the last decades, nanomaterials have received tremendous interests due to the unique properties derived from the nano-scaled sizes and potential applications in energy, sensing, catalysis, and so on [1-4]. Since the dimensions and structures of nanomaterials have profound impacts on their performances, numerous fabrication strategies have been developed to produce nanomaterials with desired morphologies. Among the reported methods, surfactants have been widely used as the templates to direct the formation of nanomaterials with defined structures because the hydrophobic and hydrophilic domains created by the self-assembly of these amphiphilic molecules can provide confined spaces for the in-situ growth of various functional nanomaterials [5, 6]. As an important semiconductor material, cobalt oxide (Co3O4) has been extensively investigated as the electrode materials in lithium ion batteries (LIBs) and supercapacitors as well as the catalyst in fuel cells owing to its excellent electrochemical activities in these devices [7-9]. In order to tune the electrochemical behaviors, Co3O4 based materials with various nanostructures like nanowires [10], nanotubes [11, 12], nanorods [13], nanosheets [14, 15], and nanoboxes [16] have been 2

prepared by different approaches. However, the instinct low conductivity of Co3O4 nanoparticles (NPs) together with their huge volume variation during the charge/discharge process still hinder their practical applications in energy devices even though their morphology can be well controlled [17, 18]. As an alternative solution, the combination of Co3O4 NPs with nitrogen-doped graphene (NG) can effectively enhance the electrochemical performance of the resulting composites since NG can serve as an ideal platform with high conductivity, high surface area and good mechanical stability to strongly couple with Co3O4 NPs [19]. Compared with graphene, NG is believed to have better electronic conductivity. Additionally, nitrogen atoms in NG could increases the surface wettability towards electrolyte. More importantly, NG has a large number of surface defects that further enhance lithium intercalation properties and lead to an increased accommodation behavior for lithium. Nevertheless, Co3O4 NPs utilized in these composites are generally prepared directly from the hydrolysis of cobalt salts [20]. Little attention has been paid to the control over the morphologies of Co3O4 NPs in the composites of NG and Co3O4 NPs. In this work, we for the first time report a surfactant-assisted hydrothermal route towards the composites of macroporous NG framework and Co3O4 NPs with adjustable morphologies (NG/Co3O4). During the fabrication process, three surfactants with different structural features including triblock copolymer Pluronic F127, cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) are involved in the hybrid-assembly of graphene oxide (GO), o-phthalonitrile and cobalt acetate in water/ethanol. The amphiphilic nature of the surfactant both enables 3

the closed combination of the different components with varied solubility in the mixed solvent and renders the formation of Co3O4 NPs with defined structures [21, 22]. The three surfactants indeed show different effects on the morphologies of the obtained NG/Co3O4 composites. Among the as-made samples, the one using non-ionic surfactant F127 (NG/Co3O4-F127) manifests the most homogeneous distribution of Co3O4 NPs with the size of ~ 15 nm in the macropore-walls formed by NG. As the anode material in LIBs, NG/Co3O4-F127 exhibits excellent electrochemical performance with a highly stable capacity of 1328 mAh g-1 at a current density of 100 mA g-1 for 200 cycles. Even at an ultrafast charging rate of 5A g-1, a decent capacity of 500 mAh g-1 still can be achieved by NG/Co3O4-F127, which is superior to the other composites and most of the previously reported Co3O4 based LIB anode materials [5, 23-25].

2. Experimental

2.1. Chemicals F127, CTAB were purchased from Sigma-Aldrich Corp. o-Phthalonitrile was obtained from Aladdin Crop. The other chemicals used in this work were purchased from Shanghai Chemical Crop. All the reagents were of analytical grade and used as received without further purification.

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2.2. Materials synthesis Synthesized by modified Hummer’s method [26], the suspension of graphene oxide (GO, 10 mg mL-1, 75 ml) was first diluted with ethanol to 5 mg mL-1 (150 ml). Next, o-phthalonitrile (750 mg), cobalt (II) acetate (370 mg) and surfactant (Triblock copolymer Pluronic F127 (Mw = 12600, PEO106PPO70PEO106), cetyltrimethyl ammonium bromide (CTAB) or Sodium dodecyl sulfate (SDS), 150 mg) was added to the GO dispersion. The mixture was dispersed uniformly by ultrasonication for 2 h and then hydrothermally treated in a Teflon-lined autoclave (200ml) at 180 oC for 24 h to produce the hydrogel-like mixture of o-phthalonitrile, cobalt oxide and graphene. After washing with ultrapure water to remove ethanol and freeze-drying process, the hydrogel was thermally treated at 800 oC in nitrogen for 2 h and then at 400 oC in air for 20 min to produce nitrogen-doped graphene framework loaded with cobalt oxide (NG/Co3O4). According to the different surfactants used in the fabrication process, the resulting composites were named as NG/Co3O4-F127, NG/Co3O4-CTAB, and NG/Co3O4-SDS,

respectively.

In

controlled

experiment,

the

composite

of

nitrogen-doped graphene framework and Co3O4 was fabricated without the assistance of surfactant and denoted as NG/Co3O4.

2.3. Material characterization: Transmission electron microscopy (TEM) images were acquired using JEM-2010F at operating voltage of 200 kV. The sample was dispersed in ultrapure water and dropped on carbon-coated copper grid. Scanning electron microscopy (SEM) 5

measurements were performed on a JSM-6700F scanning electron microscope. X-ray diffraction (XRD) patterns were carried out on a 3KW D/MAX2200V X-ray diffraction using Cu Kα radiation (40 kV, 40 mA). Nitrogen physisorption measurements were taken with ASAP 2010 M+C apparatus. Thermogravimetric analysis (TGA) was measured on NETZSCH STA 409 PG/PC instrument in air. X-ray photoelectron spectroscopy (XPS) experiments were investigated on AXIS UltraDLD system from Kratos with Al Kα radiation as X-ray source for radiation and the XPS data are analyzed by Thermo Avantage.

2.4. Electrochemical Measurements: The performance of the samples as the anode in lithium ion battery was evaluated with CR 2016 coin cells. The working electrodes were prepared by mixing the 80 wt% of active material, 10 wt% of the acetylene black, and 10 wt% of the binder (PVDF) dissolved in N-methyl-2-pyrrolidone (NMP). Then the mixture were coated onto a copper foil, followed by drying in vacuum at 80℃ for 18h . The active material loading in these electrodes is about 1 mg cm-2. Pure Lithium foil was used as counter electrode. The electrolyte was composed of a solution of 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DEC) (1:1 v/v). LAND 2001A system within a cutoff voltage range of 0.005 -3.0 V. Cyclic voltammetry (CV) was performed on a CHI660D electrochemical workstation at a scan rate of 0.1 mV s−1 in a potential range of 0.00-3.00V (vs. Li+/Li). AC impedance spectrum measurements were carried out on the CHI660D electrochemical workstation in the frequency range from 0.01 Hz to 6

1000 KHz at open circuit potential.

3. Results and discussion 3.1. Structure and morphology The fabrication process of NG/Co3O4 is shown in Fig. 1. Firstly, o-phthalonitrile, cobalt (II) acetate and surfactant (F127, CTAB or SDS) were mixed with GO in a binary solvent system of ethanol and water (v/v = 1:1). During this process, surfactant molecules could form micelles with the hydrophobic parts inside and the hydrophilic parts outside, which thus have good compatibility with the oxygen containing functional groups on GO [27]. On the other hand, Co2+ ions can also bind on the oxygen rich parts of GO via the Coulombic interactions between them [28]. The following hydrothermal treatment can lead to the formation of Co(OH) NPs and the reduction of GO. In this step, the size of Co(OH)2 could be confined by the micelles of the surfactants during the hydrolysis process. Moreover, the addition of surfactant molecules can help the dispersion of hydrophobic o-phthalonitrile on the resulting composites [29, 30]. After the thermal treatment, Co(OH)2 can be converted to Co3O4 NPs and the decomposition of o-phthalonitrile enable the doping of nitrogen atoms in the aromatic framework of graphene. As the result, the NG/Co3O4 composites can be obtained as black monoliths (Fig. S1a inset). According to the different surfactants used in the fabrication process, the obtained composites were named as NG/Co3O4-SDS, NG/Co3O4-CTAB, and NG/Co3O4-F127, respectively. In

7

controlled experiment, the composite of nitrogen-doped graphene framework and Co3O4 was fabricated without the assistance of surfactant and denoted as NG/Co3O4.

NG/Co3O4, NG/Co3O4-SDS, NG/Co3O4-CTAB, and NG/Co3O4-F127 are first characterized by scanning electron microscopy equipped with energy dispersive X-ray analysis (SEM/EDX). As exemplified by NG/Co3O4-F127 (Fig. S1a), these monolithic composites contain interconnected macroporous frameworks with the pore size ranging from 5 -20 μm. More importantly, the Co3O4 NPs in the samples using surfactants are uniformly loaded without obvious agglomeration (Fig. 2a, 2b, and 2c). In contrast, the serious aggregation of Co3O4 NPs can be observed on the macropore-wall of NG/Co3O4 (Fig. 2d), implying the important role of the surfactant molecules in controlling the growth of Co3O4 NPs. Additionally, elemental mapping images of NG/Co3O4-F127 further disclose that C, N, and Co atoms (Fig. S2) are homogeneously distributed in the sample, proving the successful nitrogen doping in graphene.

The transmission electron microscopy (TEM) images of the four composites further illustrate the morphologies of the Co3O4 NPs in them (Fig. 3). As shown in Fig. 3a, owing to the strong aggregation, Co3O4 NPs with the varied diameters ranging from 30 to 200 nm can be found in NG/Co3O4, which is in accordance with the observation in the SEM images. In the case of NG/Co3O4-CTAB (Fig. 3b) and NG/Co3O4-SDS (Fig. 3c), the sizes of Co3O4 NPs are around 30 nm, which are much small than those 8

in NG/Co3O4. However, the some large particles (~ 100 nm) with uneven shapes still can be observed in both composites. On the contrary, NG/Co3O4-F127 manifests the uniform decoration of quasi-spherical Co3O4 NPs of ~ 15 nm on its surface (Fig. 3d). Among the surfactants, F127 is nonionic block polymer and only provides hydrogen bonds and hydrophilic interactions to interact with the other components, which thus can regulate the formation of Co(OH)2 NPs with narrow size distribution on the surface of GO [31]. In contrast, both cationic CTAB and ionic SDS cannot effectively avoid the aggregation of the Co3O4 NPs in the composites, implying that the ionic surfactants are not favorable for the morphology control of the cobalt oxide NPs in this work. The HRTEM images of the Co3O4 NPs in Fig. S3 give two sets of lattice fringes of about 0.284, 0.243 and 0.203 nm, corresponding with (220), (311) and (400) crystal planes of Co3O4 [9]. The microstructures of the four samples were then characterized with X-ray diffraction (XRD, Fig. S4a). In accordance with the HRTEM results, the XRD patterns of all the composites contain prominent peaks with 2θ values of 19, 31.2, 36.8, 44.8, 59.3 and 65.2o, which can be assigned to the (111), (220), (311), (400), (511) and (440) reflections of Co3O4 (JCPDS no. 42-1467), confirming the formation of Co3O4 NPs in the composites. Additionally, the diffraction locating at ~26 o can be can be indexed to the (002) diffraction plane of graphite, which should be derived from the packing of the reduced graphene oxide in the aerogels. The porosity of the NG/Co3O4 composites was further measured by N2 adsorption/desorption measurements. In their N2 adsorption/desorption isotherms (Fig. S4b), all the samples 9

show typical type IV curves with a combination of H2 and H4 hysteresis loop at relative pressures (P/P0) of 0.4-1.0 [32]. The Brunauer-Emmethet-Teller (BET) specific surface area of NG/Co3O4-CTAB, NG/Co3O4-SDS, and NG/Co3O4-F127 are calculated as 214, 218 and 230 m2 g-1, respectively. Although the three surfactants have different influence on the morphologies of Co3O4 NPs, they don’t show evidently effect on the surface areas of the resulting composites. In contrast, the BET surface area of NG/Co3O4 is only 129 m2 g-1, which might be due to the strong aggregation of Co3O4 NPs. The relatively large surface area is beneficial for electrolyte diffusion to active sites with less resistance and the volume change of Co3O4 NPs during the charge/discharge process, which will thus facilitate the applications of the composites in LIBs. Additionally, thermogravimetric analysis (TGA) was executed to exam the carbon contents of the composites (Fig. S4c). Based on the weight loss of the samples over 800 oC, the carbon contents of NG/Co3O4, NG/Co3O4-CTAB, NG/Co3O4-SDS, NG/Co3O4-F127 are ~82.2, 83.9, 84.3 and 84.2 wt%, respectively. The composition of NG/Co3O4-F127 was further investigated by X-ray photoelectron spectroscopy (XPS). As shown in Fig. S5a, four peaks at 780.8, 532.5, 398.4 and 284.7 eV are attributed to Co 2p, O 1s, N 1s and C1s, respectively. The C 1s spectra of NG/Co3O4-F127 can be fitted with four individual peaks (Fig. S5b) at 284.7 eV, 285.2 eV, 286.2 eV and 287.9 eV representing C=C (the sp2-hybridized graphite-like carbon), C-C (sp3-hybridized diamond-like carbon), C-C and C=O. In the high-resolution Co 2p spectra of NG/Co3O4-F127 (Fig. S5c), the two major peaks at 10

780.2 and 795.6 eV can be attributed to the Co2p 3/2 and Co2p 1/2，which are in good agreement with the spin orbit peaks of Co3O4 [33]. On the other hand, high-resolution N 1s spectra of NG/Co3O4-F127 (Fig. S5d) can be fitted to three N configurations with 22% pyridinic N (398.4 eV), 25% pyrrolic N (400.0 eV) and 53% graphitic N (401.3 eV), which has proven to favor the electron transfer from N to the adjacent C atoms and reduce the adsorption energy of lithium [34]. Additionally, calculated from the XPS results, the contents of cobalt and nitrogen atoms in NG/Co3O4-F127 are ~1.53 and 1.04 wt%, respectively.

3.2. Electrochemical performance To evaluate the electrochemical activity of the four samples as the anode materials in LIBs, their cyclic voltammetry (CV) curves are recorded at a scanning rate of 0.1 mV s-1 over 0.00 - 3.00 V. As illustrated by Fig. 4a and 4b, all the composites exhibit very similar reduction and oxidation peaks in their CV curves. In the case of NG/Co3O4-F127, the reduction peaks at 0.6 and 0.8 V in the first cathodic scan should be due to the formation of a solid electrolyte interphase (SEI) film over the electrode [35,36]. The subsequent cycle of NG/Co3O4-F127, the reduction peak is shifted positively to at around 1.1 V, which are generally attributed to the reduction processes to CoO and metallic Co, respectively [36]. The reversible reaction occurring with lithium can be described by the following reactions (Eq 1) [24, 37]: Co3O4 + 8Li ↔ 4Li2 O + 3Co

(Eq 1)

In the subsequent cycles, the main reduction and anodic both showed very little 11

modification. The peak intensity and integral areas are nearly identical, suggesting the good reversibility of lithium insertion and extraction [37]. The cycling performance of NG-Co3O4, NG/Co3O4-SDS, NG/Co3O4-CTAB, and NG/Co3O4-F127 were evaluated by galvanostatic charge-discharge at a current density of 100 mAh g-1 (Fig. 4c). NG/Co3O4-F127 presents extremely large initial discharge and charge capacities of 1994 and 994 mAh g-1 with a Coulombic efficiency (CE) of ~ 50 %. The irreversibility of the initial capacity may be ascribed to the formation of the SEI layer and the irreversible conversion of Co3O4 to Co and Li2O, as indicted by the CV curve of the first cycle. The pre-doping lithium metal in the electrodes could be one option to improve the initial CE [38-40]. At the 200th cycle, NG/Co3O4-F127 still retains 67 % of the initial discharge capacity of 1328 mAh g-1. In contrast, the capacities of NG-Co3O4, NG/Co3O4-SDS, and NG/Co3O4-CTAB fade to 462, 511 and 618 mAh g-1 at 50th cycles, respectively. Remarkably, NG/Co3O4-F127 manifests a continuous increase of the capacity during the cycling performance test, which can be assigned to the delayed wetting process of the electrolytes into the active Co3O4 NPs in the porous NG framework. The rate performance of the composites is further studied at various current densities in the range of 0.1 ~ 5 A g−1 (Fig. 4d). Among the samples, NG/Co3O4-F127 manifests the highest capabilities at all the current densities. In particular, when the current density reached to 5 A g-1, its specific capacity is kept as 500 mAh g-1, which is much higher than those of NG/Co3O4 (228 mAh g-1), NG/Co3O4-SDS (252 mAh g-1) and NG/Co3O4-CTAB (363 mAh g-1). 12

In order to achieve more understanding on the difference of the four composites, their electrochemical impedance spectroscopy (EIS) spectra were recorded (Fig. S6a). And the internal resistances of the samples are calculated with the equivalent circuit model shown in Fig. S6b. The high-frequency semicircle in the EIS spectra corresponds to the constant phase element of the SEI film (CPE1) and contact resistance (Rf). The semicircle in the medium-frequency region is assigned to the charge-transfer impedance (Rct) and double-layer capacitance (CPE2). A straight sloping line at the low frequency end is from the Warburg impedance (ZW) and intercalation capacitance (Cint) [41, 42]. Obviously, NG/Co3O4-F127 possesses more depressed semicircles at high frequencies. According to the equivalent circuit, the contact resistance (Rf) and charge-discharge resistance (Rct) for NG/Co3O4-F127 are 20 and 78 Ω, which are significantly lower than those of NG/Co3O4 (59 and 361 Ω), NG/Co3O4-SDS (59 and 270 Ω) and NG/Co3O4-CATB (46 and 222 Ω). Compared with the other three composites and previously reported LIB anode based Co3O4, the improved electrochemical performance of NG/Co3O4-F127 could be attributed to following reasons. Firstly, the 3D NG network substantially facilitates the diffusion of the electrolyte into the electrode materials, thus decreasing the inner resistance of LIB electrodes [43]. Secondly, the homogeneous loading of Co3O4 NPs on NG can effectively avoid the aggregation of graphene sheets and enhance the conductivity of the resulting composites. Additionally, nitrogen doped graphene in the composite has a large number of surface defects that further enhance lithium intercalation properties and lead to an increased accommodation behavior for lithium. 13

More importantly, the uniform Co3O4 NPs with small sizes (~ 15 nm) in NG/Co3O4-F127 allow the sufficient exposure of the active sites for lithium storages, which thus improves the capacity of the composite. On the other hand, the aggregation and pulverization of Co3O4 NPs caused by the volume variation in the charge/discharge process can also be restrained for the Co3O4 NPs with smaller diameters, and the cycling stability of the NG/Co3O4-F127 based electrode can thus be enhanced [33].

4. Conclusion We have fabricated the composites of macroporous NG framework and Co3O4 NPs with adjustable morphologies via a surfactant-assisted hydrothermal route. It was found that the utilization of non-ionic surfactant F127 led to the formation of uniform Co3O4 NPs having a diameter of ~ 15 nm on the surface of macroporous NG scaffold, which subsequently enables the resulting NG/Co3O4-F127 to present an outstanding electrochemical performance as the LIB anode material. The surfactant involved fabrication protocol for the composites of semiconductor NPs and graphene can be further applied to construct other high performance electrode materials with appealing potentials in energy storage devices such as supercapacitors, sodium ion batteries, and fuel cells.

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Acknowledgement This research was financially supported by 973 Program of China (2013CB328804 and 2014CB239701), National Natural Science Foundation of China (61235007, 61575121, 21572132 and 21372155), Professor of Special Appointment at Shanghai Institutions of Higher Learning (Eastern Scholar), and Aeronautical Science Foundation of China (2015ZF57016) and 863 High-Tech Program (2013AA013402). We also acknowledge instrument analysis center of Shanghai Jiao Tong University for material characterization.

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Ultra-small

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Fig. 1. Schematic illustration of the synthesis procedure for the NG/Co3O4 composites with different surfactants.

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Fig. 2. SEM images of a) NG/Co3O4-F127; b) NG/Co3O4-SDS; c) NG/Co3O4-CTAB; d) NG/Co3O4.

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Fig. 3. TEM images of the NG/Co3O4 composites: a) NG/Co3O4; b) NG/Co3O4-CTAB; c) NG/Co3O4-SDS; d) NG/Co3O4-F127, and inset of d) shows HRTEM images of NG/Co3O4-F127.

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Fig. 4. Electrochemical characterization of the NG/Co3O4 composites: a) cyclic voltammetry profiles of NG/Co3O4-F127 for first two cycles; and b) cyclic voltammetry profiles of NG/Co3O4, NG/Co3O4-SDS, NG/Co3O4-CTAB, and NG/Co3O4-F127 at scanning rate of 0.1mV/s between 0.00-3.00V; c) the cycling performance of NG-Co3O4, NG/Co3O4-SDS, NG/Co3O4-CTAB, and NG/Co3O4-F127 at a current density of 100 mA g-1; and d) the rate capabilities of NG-Co3O4, NG/Co3O4-SDS, NG/Co3O4-CTAB, and NG/Co3O4-F127 at different current densities (0.1 - 5 A).

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