rGO Composites with Superior

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High pseudocapacitance in FeOOH/rGO composites with superior performance for high rate anode in Li-ion battery Hui Qi, Liyun Cao, Jiayin Li, Jianfeng Huang, Zhanwei Xu, Yayi Cheng, Xingang Kong, and Kazumichi Yanagisawa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11840 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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ACS Applied Materials & Interfaces

High pseudocapacitance in FeOOH/rGO composites with superior performance for high rate anode in Li-ion battery Hui Qi1,2, Liyun Cao1, Jiayin Li1*, Jianfeng Huang1*, Zhanwei Xu1, Yayi Cheng1,2, Xingang Kong1, Kazumichi Yanagisawa2 1 School of Material and Science and Engineering, Shaanxi University of Science & Technology, Xi’an, China, 710021 2 Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Kochi, Japan, 780-8520

*Corresponding author: [email protected] (Jiayin Li), [email protected] (Jianfeng Huang).

Abstract Capacitive storage has been considered as one type of Li-ion storage with fast faradic surface redox reactions to offer high power density for electrochemical applications. But it is often limited by low extent of energy contribution during charge/discharge process, providing insufficient influences to total capacity of Li-ion storage in electrodes. In this work, we demonstrate a predominated pseudocapacitance storage (Contributes 82% of the total capacity) from an in-situ pulverization process of FeOOH rods on rGO (reduced graphene oxide) sheets for the first time. Such high extent of pseudocapacitive storage in FeOOH/rGO electrode achieves high energy density with superior cycling performance over 200 cycles at different current densities (1135 mAh/g at 1 A/g and 783 mAh/g at 5 A/g). It is further revealed the in-situ pulverization process is essential for the high pseudocapacitance in this electrode. Because it not only produces porous structure for high exposure of tiny FeOOH crystallites to electrolyte, but also maintains stable electrochemical contact during ultrahigh rate charge transfer with high energy density in the battery. The utilization of in-situ pulverization in Fe-based anode to realize predominated pseudocapacitance with high energy density may inspire future design of electrode structure in Li-ion batteries. Keywords: Pseudocapacitance; In-situ pulverization; FeOOH; Reduced graphene oxide; Li-ion batteries

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1. Introduction

Transition metal oxides (TMOs) have been receiving much interest as they could provide excellent performance in many energy-related fields, including water-splitting 1, supercapacitors and lithium ion batteries (LIBs)

3-9

2

etc. Particularly, iron oxide materials (Fe2O3, Fe3O4) are

considered to be one kind of the most promising candidates for the next generation of anode material in LIBs because of their natural abundance, low cost, nontoxic and high theoretical capacity

10-17

.

Besides, FeOOH was firstly reported by Amine et al, suggesting it as a new anode in LIB with superior performance of cycling capacity 18. Xu et al further reveal its theoretical capacity is as high as 905 mAh/g 19. These reports triggered many following works to further explore excellent cycling capacity and rate performance of FeOOH

20-27

, exhibiting surprisingly high capacity in LIBs.

Whereas the above works still left an intriguing issue worth further exploration: High content of conductive graphite was often added during the preparation of pure FeOOH electrode

19, 21, 23

. This

high addition of graphite suggests pure FeOOH is still in great demand for improvement of charge conductivity to support its high rate performance in LIBs. The above-mentioned issue may find a solution from the support of graphene-based matrix system. Because numerous works have proved active-material/graphene (reduced graphene oxide (rGO)) composites could provide much support to enhance the charge conductivity and ion diffusion of Li-ion in electrodes28,

29, 30-33

. Similarly, FeOOH benefits from the graphene-based materials

composites system, presenting enhanced performances of FeOOH/rGO composites25, 34. However, few works are reported to specifically explain how the graphene-based matrix could affect FeOOH during the charge/discharge process, which is significant for promoting high electrochemical activity of this composites system to pursue higher rate performances. Therefore, more detailed effects of


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graphene-based materials on FeOOH still need further exploration to understand the specific electrochemical mechanism at high charge/discharge rate of LIBs, which may also be illuminating for their practical application in future requirements of LIBs. In this work, FeOOH rods were homogeneously anchored on reduced graphene oxide sheets by a facile hydrothermal technique. These FeOOH rods/rGO composites exhibit remarkable electrochemical performance when tested as an anode in LIBs, providing a high reversible capacity of 1443 mAh/g at 0.2 A/g and even 783 mAh/g at 5A/g after 200 cycles. To further understand these excellent performances, an in-situ pulverization of FeOOH crystallites on the rGO sheets was found to induce a surface-controlled pseudocapacitive behavior in the electrode system. For the first time, this behavior is revealed to perform as a dominated charge storage mechanism of the FeOOH/rGO electrode, leading to high capacity and fast charge transfer rate performance in LIB. It is believed this work could further promote the structural design of future composite anode systems, as well as the utilization of surface-controlled storage mechanism with excellent performance of electrode material in LIBs. 2 Experimental 2.1 Synthesis of FeOOH/reduced graphene oxides (FeOOH/rGO) composites Graphene oxide was synthesized from graphite powder using a modified Hummer's method. In a typical procedure, 40 mg of graphene oxide (GO) was dispersed in 50 mL of deionized (DI) water under sonication for 2 h to form a homogenous suspension. Afterwards, 10 mmol (1.98 g) of FeCl2· 4H2O was added directly into the suspension under stirring for 60 min. This mixed suspension was transferred into a 100 mL Teflon-lined autoclave, sealed and maintained at 90 °C for 12 h. And then cooled down to room temperature naturally. The precipitates were separated by centrifugation



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and washed with water several times and freeze-dried to obtain the final products. The bare FeOOH was synthesized under the same condition without the addition of GO for comparison. 2.2 Characterizations The crystallographic phase of as-prepared samples was characterized by X-ray diffraction on an Ultima IV X-ray diffractometer with Cu Karadiation (λ=0.15406 nm) at a scanning rate of 8° min-1 (Rigaku, Japan), in the 2θ range from 10° to 70°. Particle size and morphology were observed using a field emission scanning electron microscope (FESEM, S-4800), transmission electron microscopy (TEM,

JEM-3010)

and

high-resolution

transmission

electron

microscopy

(HRTEM).

Thermogravimetric Analysis (TGA) was performed to analyze the weight ratio of samples at a heating rate of 10 °C min-1 in air from room temperature to 630 °C. X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) was utilized to study the surface chemistry of the samples. Fourier transform infrared (FT-IR) measurements were carried out on a Bruker Vector-22 infrared spectrophotometer from KBr pellets in the range of 550-4000 cm-1. 2.3 Electrochemical tests The electrochemical measurements were evaluated using coin-type half cells assembled in an argon-filled glove box. Li sheets served as the counter electrode and reference electrodes. The working electrodes contained 80 wt% active materials, 10 wt% Super P and 10 wt% carboxymethyl cellulose (CMC) to form a slurry in deionized water. Then the electrode was prepared by uniformly painted on a copper foil with a thickness of 15 µm followed by drying in a vacuum oven at 80 °C for 24 h. The mass loading of the electrode is 1.0-1.2 mg/cm2. The electrolyte was a solution of 1.0 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed solvent with a v/v ratio of 1:1. The galvanostatical charge-discharge measurements were performed at different current


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densities with a cut-off voltage range from 0.01 V to 3.0 V (vs. Li/Li+). The charge/discharge capacities were calculated based on the weight of active materials in the electrodes. Cyclic voltammetry (CV) measurements were carried out on a CHI instrument (CHI660). The AC Electrochemical impedance spectroscopy (EIS) tests were also carried out using the CHI660 electrochemical workstation with an amplification voltage of 10 mV over a frequency range from 100 kHz to 0.01 Hz. The Electrochemical properties of all electrodes were characterized at room temperature. In order to investigate the structural and morphological changes of electrodes, the cells were disassembled after 50 cycles. The cycled electrodes were washed with dimethyl carbonate (DMC) several times in a glove-box to remove the electrolyte before being used for ex-situ TEM analysis. 3 Results and discussion 3.1 Structure and Morphology. Figure 1a displays typical XRD patterns of the as-synthesized samples with and without rGO. These two samples exhibit similar patterns with all diffraction peaks that are indexed to β-FeOOH structure (JCPDS card NO.75-1594). TGA results in Figure 1b further shows the bare FeOOH presents only one mass loss of 14.9 wt% between the temperature of 100~400 °C, corresponding to the theoretical weight loss from thermal decomposition (10%) in FeOOH. On the other hand, the FeOOH/rGO composites shows two major weight loss in its TGA result, which are identified as the theoretical thermal decomposition of FeOOH and the oxidation loss of graphene under 400~600 °C. A further calculation of these weight loss in FeOOH/rGO sample reveals rGO holds 18.6% of the specific content in the composites (81.4% for FeOOH). Figure S1 further shows the Raman spectra of the FeOOH/rGO composites and GO. Both of these two samples present the disordered D band



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and ordered G band of carbon materials at ~1342 cm-1 and ~1578 cm-1, respectively

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35, 36

. The

intensity ratio of these two bands (ID/IG) in FeOOH/rGO is obviously higher than that of GO, indicating the growth of FeOOH particle may disorder the carbon structure of rGO sheets 11, 35.

Figure 1 (a) XRD patterns of the FeOOH and FeOOH/rGO, (b) TG curves of the FeOOH and FeOOH/rGO

SEM and TEM images of the bare FeOOH and FeOOH/rGO composites were taken to find their differences in morphology. Figure 2a, b shows the as-prepared bare FeOOH samples which exhibit rod-like structures with a length of 800~900 nm. While the FeOOH/rGO composites presents smaller rods of ~200 nm growing directly on the rGO sheets (marked by arrows in Figure 2c), indicating the surface of the graphene sheets serves as crystallization sites to induce the growth of FeOOH rods-like structure (Figure 2d). In Figure 2e the TEM image of FeOOH/rGO composites presents a uniformly distributed morphology on the rGO sheet. The marked space lattice in Figure 2f further indicates these rods correspond to the crystal structure of β-FeOOH, which agrees well with the above XRD results. The connection between FeOOH rods and rGO sheets in FeOOH/rGO composites is identified by XPS. Figure 3a presents the typical survey spectrum for FeOOH/rGO. The spectrum clearly shows that Fe, O, and C elements can be detected in the composite. The high-resolution Fe 2p spectrum of the composite is shown in the inset of Figure 3a. This spectrum exhibits two distinct


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Figure 2 SEM images of (a) (b) bare FeOOH, (c) (d) FeOOH/rGO composites TEM images of (e) (f) FeOOH/rGO composites

peaks and a weak peak at 711.4, 724.9 eV and 719.9 eV, respectively. These peaks are referred to the characteristic peaks of Fe 2p3/2, Fe 2p1/2 and the shake-up satellite peak of Fe 2p3/2, which is consistent with other previously reported work

4, 26, 37

. In Figure 3b, the C1s spectrum can be

integrated into three peaks at 284.6 eV, 285.9 eV and 287.7 eV, which are associated with C-C, C-O, and C=O/O–C=O bonds, respectively 4, 26. Furthermore, the C-O bonds may indicate C-O-H, C-O-C and C-O-Fe structure between in the FeOOH/rGO composites. Therefore, the O1s spectrum is



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Figure 3 (a) Survey XPS spectrum and Fe 2p XPS spectrum (inset) of FeOOH/rGO composites (b) C 1s XPS spectra (c) O1s XPS spectra of FeOOH/rGO composites and (d) FTIR spectra of the FeOOH/rGO and GO

investigated to explore the existence of Fe-O-C bonds in the composites. For the composites, the O 1s spectra are fitted into three peaks, which can be assigned to Fe-O (530.2 eV), Fe-O-C (531.6 eV), Fe-OH (532.6 eV) groups, respectively 4, 5. The presence of Fe-O-C illustrates the linkage of FeOOH with graphene. FT-IR spectra are employed to further explore the reduction of GO and the specific connection between FeOOH and rGO in the composites. All of the FTIR peaks of both GO and FeOOH/rGO composites are indexed according to their wavenumbers, as is shown in Figure 3d. Generally, the broadening peaks at 3700 cm-1 are attributed to the stretching vibration –OH in H2O, which may result from the adsorbed water in the samples 38. The wide band at 3154 cm-1 results from stretching vibration of O-H in FeOOH

38-40

. The characteristic peaks for GO at 1725, 1570 and 1220 cm-1 are


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related to the vibration of C=O, C=C, and C–O–H, respectively

39, 40

. Two typical bands at 895 and

798 cm-1 can be ascribed to Fe-OH bending vibration in FeOOH. The peak at 630 cm-1 originates from the Fe-O(H) stretching vibration 38. Comparing the indexed peaks in both samples, it can be found that the FeOOH/rGO presents a decreased intensity of C=O peak than that in GO, indicating the GO sheets have been reduced to rGO to some extent in the FeOOH/rGO composites. Furthermore, a new peak around 1000 cm-1 is found in FeOOH/rGO sample. This peak is attributed to the vibration of C–O bond in Fe–O–C structure 4, 5, 38

, suggesting the formation of chemical bonds of FeOOH on the surface of rGO sheets. Therefore,

concluding from the above results indicates that the addition of Fe2+ into GO suspension leads to the reduction of C=O on the surface of GO, chemically forming many Fe-O-C bonds between FeOOH and GO. These chemical bonds may further affect the electrochemical activity of FeOOH/rGO composites during their charge/discharge process in LIB. Cyclic voltammetry (CV) was employed to investigate the lithiation/delithiation behavior of bare FeOOH and FeOOH/rGO composites. As can be seen from Figure 4a, the first negative scan is characterized by three peaks centered at 1.45, 1.25, 0.52 V, indicating a multiple-step process of lithiation between lithium and FeOOH/rGO composites. The first peak at 1.45 V is corresponding to FeOOH to form LixFeOOH (x

rGO Composites with Superior

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