Graphene-Embedded Co3O4 Rose-Spheres for Enhanced

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Graphene-embedded Co3O4 rose-spheres for enhanced performance in lithium ion batteries Mingjun Jing, Minjie Zhou, Gangyong Li, Zhengu Chen, Wen-Yuan Xu, Xiaobo Chen, and Zhaohui Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16396 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 4, 2017

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ACS Applied Materials & Interfaces

Graphene-embedded Co3O4 rose-spheres for enhanced performance in lithium ion batteries Mingjun Jinga, Minjie Zhoua,b, Gangyong Lia, Zhengu Chena, Wenyuan Xua, Xiaobo Chenb,*，Zhaohui Houa,b,*

a

School of Chemistry and Chemical Engineering, Hunan Institute of Science and

Technology, Yueyang 414006, China. b

Department of Chemistry, University of Missouri – Kansas City, Kansas City,

Missouri, 64110, USA.

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ABSTRACT: Co3O4 has been widely studied as a promising candidate as an anode material for lithium ion batteries. However, the huge volume change and structural strain associated with the Li+ insertion and extraction process leads to the pulverization and deterioration of the electrode, resulting in a poor performance in lithium ion batteries. In this paper, Co3O4 rose-spheres obtained via hydrothermal technique are successfully embedded in graphene through an electrostatic self-assembly process. Graphene-embedded Co3O4 rose-spheres (G-Co3O4) show a high reversible capacity, a good cyclic performance and an excellent rate capability, e.g., a stable capacity of 1110.8 mAh g−1 at 90 mA g-1 (0.1 C), and a reversible capacity of 462.3 mAh g-1 at 1800 mA g-1 (2 C), benefitted from the novel architecture of graphene-embedded Co3O4 rose-spheres. This work has demonstrated a feasible strategy to improve the performance of Co3O4 for lithium-ion battery application.

KEYWORDS:

Co3O4

rose-spheres,

graphene,

graphene-embedded

Co3O4

rose-spheres, electrostatic self-assembly, lithium ion batteries


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1. INTRODUCTION Lithium ion batteries (LIBs) have been regarded as the most popular energy storage devices for portable electronic devices and electric/hybrid vehicles, overcoming the usage of toxic materials and avoiding the memory effect in traditional batteries (e.g., nickel-cadmium batteries, nickel-metal hydride batteries, lead-acid batteries, etc).1-2 In 2000, transition metal oxides (MOx) were considered as high capacity candidate material for anode in LIBs by Tarascon et al.3 A series of MOx, for example Co3O4,4-6 NiO,7 CuO,8 MnO2,9 Fe3O4,10 have been intensively studied in LIBs. Among them, Co3O4 can be considered as a hopeful candidate material for anode because it can theoretically accommodate up to eight lithium atoms per formula unit, corresponding to a high theoretical capacity of 890 mAh g−1.5, 11 Its appropriate discharge plateau (1.0 V vs. Li/Li+) can avoid the production of lithium dendrite, improving the safety of LIBs.11 Unfortunately, the main drawback of Co3O4 as commercial anode material is the huge volume change and structural strain with the Li+ insertion and extraction process, which leads to the pulverization and deterioration of the electrode.12 Its electrochemical performance largely depends on its morphology and structure. Various nanostructures and microstructures, such as nanosheets,13 nanowires,14 hollow microspheres,15

flower-like

structure,16

nanofibers,17

porous

structures,2

,

nano-octahedra,18 have been adopted to improve its performance. Among these structures, three-dimensional (3D) flower-like structure can accommodate the large volume changes and at the same time allow a better penetration of electrolyte into the 3 ACS Paragon Plus Environment


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electrode materials,19-20 bringing in better performance. For example, the 3D nanoflowers prepared by Lou et al showed a good cyclic retention rate.21 However, it is still a difficult challenge to obtain outstanding reversible capacity, cycling life and rate of Co3O4 electrode. A carbonaceous matrix, such as carbon aerogel,22 carbon nanotube23 and graphene,17, 24-25 has been employed to not only alleviate volume change, but also increase electrical conductivity of Co3O4. Among them, graphene has attracted more attention due to its large theoretical surface area (2675 m2 g-1) and high electrical conductivity (64 mS cm-1).26-30 For example, a Co3O4@graphene composite obtained by chemical deposition method showed a cyclic capacity of 740 mAh g-1 after 60 cycles at a current rate of 200 mA g-1.5 Mesoporous Co3O4/graphene membrane displayed much better rate ability and cycling stability than those of pure Co3O4, which exhibited the large capacity of 800 mA h g−1 at 100 mA g-1.31 However, much various morphologies Co3O4 nanomaterials were usually anchored on the surface of graphene

nanosheets

and

thus

modest

relief

in

the

volume

change.

Graphene-encapsulated structure particularly could be a promising for relieving volume expansion. In this work, graphene-embedded Co3O4 rose-spheres (G-Co3O4) are prepared via a simple electrostatic self-assembly process. The G-Co3O4 displays a superior lithium storage performance over bare Co3O4 rosespheres, due to the good accommodation of the volume change and the improved conductivity for lithium-ion storage. 4 ACS Paragon Plus Environment

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Compared with other techniques of Co3O4-based materials, this electrostatic self-assembly approach possesses low process temperature and simple processing steps. Furthermore, the morphological structure of the pure as-obtained Co3O4 could not be changed during the the composite process. First, pure Co3O4 rose-spheres are first

prepared

with

a

hydrothermal

approach

and

then

modified

with

3-aminopropyltrimethoxysilane (APS) to obtain negative charges on the surface. The negative charged graphene oxide nanosheets are then attracted to the surface of each petals, fill the space between petals, and wrap the outside of the modified Co3O4 rosespheres. Finally, the graphene oxide nanosheets are reduced by hydrazine into conductive graphene nanosheets.

At the end, Co3O4 rose-spheres are completely

embedded in the conductive graphene nanosheets. In this case, each petal in the Co3O4 rosespheres is in intimate contact with the conductive graphene nanosheets. So it is believed that the electrical conductivity of these graphene-Co3O4 rosespheres (GCo3O4) is much better than bare Co3O4 rosespheres, while still maintaining the benefit for volume expansion from the rosesphere structure. As a result, the G-Co3O4 displays a superior lithium storage performance of 1110.8 mAh g−1 at 90 mA g-1 (0.1 C), compared to 425.1 mAh g−1 of the bare Co3O4 rosespheres after 30 cycles. Thus, this work has demonstrated a feasible strategy to enhance the performance of Co3O4 for LIBs application.



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Scheme 1. Schematic of the electrostatic self-assembly process of G-Co3O4 rosespheres. 2. EXPERIMENTAL SECTION Co3O4 rosespheres were prepared through hydrothermal method. In a typical procedure, 2 g of Co(CH3COO)2·4H2O and 0.75 g of polyvinylpyrrolidone (PVP) were dissolved in 120 mL EG/H2O mixed solvent (67% EG) and stirring for 30 min. Then, the homogeneous solution was transferred into a 150 mL Teflon-lined stainless steel autoclave and heated in an oven at 230 oC for 24 h. After being cooled to room temperature, the precipitate was collected via centrifugation and washed with deionized water. Then, the precursor was dried at 60 oC and calcined at 350 oC for 3 h in air. Finally the Co3O4 rosespheres were obtained. To further study the formation of Co3O4 rosespheres, the samples at different reaction time were also obtained. Graphene oxide was first obtained by a Hummers’ method from graphite flakes (Alfa Aesar, 99.8%).32 As-prepared Co3O4 rosespheres were modified with organic 6 ACS Paragon Plus Environment

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groups to be charged positively. The details are as follows: 0.5 g of as-obtained Co3O4 was dispersed in 50 mL toluene, and then 0.5 mL 3-aminopropyltrimethoxysilane (APS) was added under ultrasonic treatment. After refluxing for 24 h under argon atmosphere, the APS-modified Co3O4 were obtained. Graphene-encapsulated Co3O4 rosespheres (G-Co3O4) was fabricated via a simple self-assembly process (Scheme 1). In brief, the modified Co3O4 rosespheres with positive charge could be self-assembled with the negative charged GO to form G-Co3O4 composite. The as-prepared APS-modified Co3O4 rosespheres and GO with mass ratio 4:1 were dispersed in deionized water, respectively. And then the values of pH were adjusted to 3 utilizing hydrochloric acid aqueous solution (1 mol L-1). The two solutions were further mixed together under stirring for 2 h. Finally, hydrazine (35 wt %) was added to the above suspension. G-Co3O4 composite was obtained through centrifugation and washed using deionized H2O. The phase composition and structure of the as-prepared samples were characterized by X-ray diffraction (XRD, D/max-Ultima IV). The chemical structure of

APS-Co3O4

sample

was

carried

out

by

Fourier

transform

infrared

spectrophotometer (FT-IR, AVTATAR, 370) with KBr as a reference. The morphologies of materials could be observed via utilizing scanning electron microscope (SEM, Hitachi S-4800). The surface elemental compositions were investigated by X-ray photoelectron spectroscopy (XPS, K-Alpha 1063). The structure of APS-Co3O4 was also characterized with energy dispersive X-ray (EDX) analysis. The thermogravimetric analysis (TGA, DTA7300) was carried out from 7 ACS Paragon Plus Environment


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room temperature to 800 oC under flowing air at a heating rate of 10 oC min-1. Specific surface area was measured by the Bruauer-Emmett-Teller (BET) method by the analyzer (ASAP2020 HT88). The electrochemical experiments were performed utilizing 2025-type coin cells, which were assembled in an argon-filled glove box(H2O < 0.5 ppm, O2 < 0.5 ppm). The active materials (80 wt %), polyvinylidene fluoride (PVDF, 10 wt %) and conductive carbon black (10 wt %) were mixed into N-methyl-2-pyrrolidinone (NMP) to form slurry which was coated onto pure Cu foil and subsequently dried at 80 oC for 10 h in vacuum. The foil was cutted to several disks to form working electrodes. The 2025-type coin cells were assembled utilizing LiPF6 (1 mol L-1) as electrolyte in ethylene carbonate /dimethyl carbonate (1:1, v/v). Cyclic voltammograms (CV) curves (0.005-3 V, 5mV s-1) were recorded on a CHI 660B electrochemical workstation. Galvanostatic discharge-charged cycles with a voltage window of 0.005-3 V ( vs Li/Li+ ) were recorded on a LAND battery tester.

3. RESULTS AND DISCUSSION Co3O4 rosespheres were obtained by reacting cobalt acetate in the presence of polyvinylpyrrolidone in an ethylene glycol/water solvent at 230 °C for 24 h. The formation process of the Co3O4 rosespheres was studied with scanning electron microscopy (SEM). Figure 1a - d show their morphological evolution at different reaction time. First, small Co3O4 nanosheets were formed after 2h reaction (Figure 1a). As the reaction proceeded to 6h, these nanosheets grew in size and many layers of 8 ACS Paragon Plus Environment

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nanosheets were attached to each other, forming curved stacks of nanosheets, as shown in Figure 1b. The stacking of nanosheets might be due to the van de Walls interaction between the nanosheets due to large surface exposed, and the curving might be due to the lattice tension or surface interaction.33 As the reaction time further increased to 12 h, more nanosheets were stacked and curved together as shown in Figure 1c. Some rosespheres were almost formed, while some were partially formed with the center missing. Thus, it was likely once a stack of nanosheets were formed and curved, more nanosheets were attracted to the concave center to complete the formation of the rosesphere. This was likely due to the lowering the surface energy by the continuous stacking and curving of the nanosheets. As the reaction time increased to 24 h, complete Co3O4 rosespheres were formed when the concave part was completely filled by the curved nanosheets shown in Figure 1d. The diameter of the Co3O4 nanosphere was in the ranging of 15-30 µm (Figure 1d and Figure S1), and the thickness of each petals was around 10 nm (Figure 2a and 2b).



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Figure 1. SEM images of Co3O4 after a series of hydrothermal time: (a) 2 h, (b) 6 h, (c) 12 h and (d) 24 h. In order to effectively attached graphene oxide nanosheets to Co3O4 rosespheres, we modified the surface of the Co3O4 rosespheres with APS. The FTIR spectra and EDX of APS-modified Co3O4 rosespheres were shown in Figure S2. In Figure S2a, the bands centered at 3444.0 cm-1 and 1627.8 cm-1 were attributed to the O-H stretching modes of interlayer water molecules and the bending mode of water molecules, respectively.34 Two strong distinctive bands at 655.0 and 557.2 cm-1 were attributed to the Co-O stretching.35 The weak band centered at 2917.4 cm-1 is related to the C-H stretching.36 The three bands at 1400.6, 1035.1 and 971.2 cm-1 were assigned to N-H, Si-O-Co and (Si-O)n stretching,37-38 respectively. As shown in Figure 10 ACS Paragon Plus Environment

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S2b, the APS-modified Co3O4 rose-spheres are mainly consisted of Co and O elements with a little of Si and C elements. Combined with the analysis of FTIR, all these results confirmed that the Co3O4 rosespheres could be successfully modified by APS. Compared to the Co3O4 rosespheres with well separated individual petals (Figure 2a and 2b), the space between petals in the G-Co3O4 rosespheres was almost completely filled with graphene and the surface was also almost completely covered with graphene (Figure 2c and 2d), although the size of the G-Co3O4 rosesphere was similar to that of the Co3O4 rosespheres. This indicated that the packing of graphene sheets to the surface of Co3O4 was very efficient, allowing intimate contact between the graphene and the Co3O4 with a good electrical conductivity for charge transport. Furthermore, the structure of G-Co3O4 rosesphere could be confirmed by EDX mapping analysis, which is shown in Figure. S3. Co and O are homogenously distributed in the G-Co3O4 composite, which could illustrate the globular structure G-Co3O4 again. C element is mainly distributed at the marginal division of the Co3O4 sphere, and small amount of C element is distributed at an interlayer. All above results confirm that the Co3O4 rose-spheres are well embedded in the conductive graphene nanosheets.



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Figure 2. SEM images of Co3O4 (a, b) and G-Co3O4 rosespheres (c, d). The crystal phase structures of pure Co3O4 and G-Co3O4 rosespheres were confirmed with XRD measurement. Their diffraction peaks matched well crystallized Co3O4 with a face-centered cubic structure (JCPDS No.42-1467), as shown in Figure S4. After wrapped with graphene, additional small (002) diffraction peak appeared at 2θ of 23° from the graphene sheets (Figure S5).39 Meanwhile, the positions of diffraction peaks of Co3O4 did not apparently change, suggesting that the structure of the Co3O4 did not change in the G-Co3O4. Thermogravimetric analysis (Figure S6) indicated that 17.8 wt% of graphene were efficiently attached to the Co3O4 rosespheres.

As shown in Figure S6, the

weight loss from 25 to 300 °C was attributed to the release of adsorbed water and coordinated water on the surface and trapped inside,40 the following weight loss from 12 ACS Paragon Plus Environment

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300 to 500 °C was mainly owing to the oxidization of graphene (17.8 wt%). The surface chemical composition of the G-Co3O4 rosespheres was analyzed with XPS. In the Co 2p XPS spectrum as shown in Figure 3a, the two peaks at 780.2 and 795.6 eV were related to 2p3/2 and 2p1/2 of Co2+ ions in Co3O4, respectively.24, 28 Moreover, Co 2p XPS spectra displays another two fitted peaks: 2p3/2 satellites (786.1 eV) and 2p1/2 satellite (803.0 eV), which shows peaks characteristic for a mixture of Co2+/Co3+ as present in Co3O4.39 It is noting that the shakeup intensity around 786 eV indicates more Co2+ species are existence on the surface of Co3O4. This might be mainly due to the slight surface reduction of Co3O4 during the reduction process of graphene via hydrazine. In the O 1s spectrum (Figure 3b), the peak at 532.4 eV was from C-OH or C-O-C groups, while the peak at 531.4 eV was ascribed to C=O groups,41 and the peak at 529.8 eV verified the presence of Co-O species.42 The N2 absorption/desorption isotherms shown in Figure 3c-d indicated that both Co3O4 and G-Co3O4 had typical IV features. The BET specific surface areas were 48 and 87 m2 g-1 for Co3O4 and G-Co3O4 rosespheres, respectively. The increased surface area was attributed to the existence of graphene shell, and possible secondary pores can formed between Co3O4 and graphene sheets.41 This would improve ion adsorption to the electrode in the batteries.



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Figure 3. (a) Co 2p and (b) O1s core-level XPS spectra of G-Co3O4 rosespheres, and nitrogen adsorption/desorption isotherms of (c) Co3O4 and (d) G-Co3O4 rosespheres. The electrochemical property of as-obtained G-Co3O4 rosespheres for LIBs was firstly evaluated via Cyclic voltammetry (CV). Figure 4 shows the CV curves obtained at a scanning rate of 0.1 mV s-1. The lithium storage mechanism of Co3O4 is described via the electrochemical conversion reaction as follows:43 Co3 O4 +xLi ↔ Lix Co3 O4

(1)

Lix Co3 O4 +൫8-x൯Li ↔4 Li2 O+3Co

(2)

Co3 O4 +8Li + 8݁ ି ↔ 4 Li2 O+3Co

(3)

In the first cycle, a weak shoulder peak at 1.1 V was associated with the formation of intermediate LixCo3O4 (eqn (1)), and the main cathodic peak observed at 0.70 V was attributed to the reduction of Co-based species to metallic Co (eqn (2) and 14 ACS Paragon Plus Environment

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(3)) and the formation of solid electrolyte interphase (SEI) film.6, 18 The peak at about 2.1 V on anodic scan curve was connected with the oxidation of metallic Co into Co-based oxides, agreeing well with other Co-based materials.28, 43 Moreover, the weak broad oxidation peak about 0.30 V was due to the lithium extraction from graphene shells.24 In the second cycle, the peak intensity dropped significantly, indicating irreversible reactions occurred in the first cycle. That might be mainly owing to SEI layer formation and the electrolyte decomposition. In the third cycle, a pair of redox peak (0.9/2.0 V) corresponded to the redox couple of Co3O4/Co.33 The peak location and area integration after the 2nd cycle did not apparently change, suggesting a good reversibility and stability.

Figure 4. CV curves of G-Co3O4 rosespheres at 0.1 mV s−1 with three cycles. Figure 5a and b displayed the discharge-charge curves of Co3O4 and G-Co3O4 rosespheres at various cycles measured at 90 mA g−1 (0.1 C) with the potential range from 0.005 to 3 V (vs Li/Li+). In the first discharge step, Co3O4 and G-Co3O4 both had a similar voltage curve with a long voltage plateau at approximately 1.0 V. As shown 15 ACS Paragon Plus Environment


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in Figure 5a, as-prepared pure Co3O4 rosespheres had an initial discharge and charge capacity of 1635.1 and 1112.1 mAh g−1 at 90 mA g−1(0.1C), respectively. However, only 486.9 mAh g-1 reversible capacity was retained after 30 cycles. In contrast, the G-Co3O4 rosespheres had an initial discharge and charge capacity of 1727.9 and 984.5 mAh g−1, respectively. And a high reversible capacity 1110.8 mAh g−1 was still retained after 30 cycles. Apparently, G-Co3O4 rosespheres had a much better reversibility than Co3O4 rosespheres. The initial capacity loss was likely due to the possible irreversible processes such as electrolyte decomposition and inevitable formation of SEI films.5 The initial lower coulombic efficiency of G-Co3O4 rosespheres was likely from the irreversible lithium insertion and extraction from graphene.17

Figure 5. Charge-discharge curves of pure Co3O4 (a) and G-Co3O4 composite (b) at 90 mA g−1 (0.1C). As shown in Figure 6a, G-Co3O4 had a much better and more stable cycling performance than Co3O4. It quickly reached a stable capacity after the first cycle and its stable reversible discharge capacity was 1110.8 mAh g−1 at the 30th cycle. In comparison, the discharge capacity of either Co3O4 rosespheres or graphene sheets 16 ACS Paragon Plus Environment

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dropped continuously with the increase of the cycle number. At the 30th cycle, the graphene only had a capacity of 139.8 mAh g−1 and Co3O4 rosespheres only had 486.9 mAh g−1. The enhanced cycling stability and capacity of G-Co3O4 rosespheres was mainly due to the addition of graphene, likely from the enhanced conductivity and the unique architecture for better accommodation for volume expansion. The reversible capacity of G-Co3O4 was much higher than theoretical capacity of Co3O4 (890 mAh g−1). These large excess capacities was likely from the lithium storage in the interconnected mesopores via an electric double layer capacitive mechanism or the larger electrochemical active surface area of graphene.22 The cyclic charge/discharge capacity and the columbic efficiency of the G-Co3O4 rosespheres were shown in Figure 6b. The reversible capacity of G-Co3O4 was still up to 990.8 mAh g−1 at 0.1C after 100 charge/discharge cycles. The capacity increased lightly at early cycles, likely due to the formation of a gel-like polymeric layer within rosesphere structure.44 Coulombic efficiencies of G-Co3O4 increased to above 95% after the eight cycles, although the first coulombic efficiency of was only 56.9 %. Moreover, the cycled G-Co3O4 electrode material has been further analyzed using SEM, which is shown in Figure S7. It can be seen that the whole G-Co3O4 rose-spheres construction could still maintain after 100 cycles, suggesting the structural stability of the composite. Additionally, the cycling stability of G-Co3O4 at 0.2 C was further measured in Figure S8. The capacity of G-Co3O4 was still maintained 655.9 mAh g−1 at 0.2 C after 200 cycles. After 10 cycles, the Coulombic efficiencies of G-Co3O4 all are above 95%. The rate performance was shown in 17 ACS Paragon Plus Environment


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Figure 6c. At a rate of 0.2 C, the G-Co3O4 rosespheres had a reversible capacity of 913.8 mAh g−1, and 462.3 mAh g-1 even at 2 C. When the rate was change back from 5C to 0.2 C, the specific capacity recovered to 859.9 mAh g−1. This demonstrated the robust rate performance of the G-Co3O4 rosespheres. Taking Co3O4 based material reported for comparison (Table S1), the as-obtained G-Co3O4 exhibits good cycling stability and rate capability. Those excellent electrochemical performances of G-Co3O4 rosespheres were mainly attributed to the unique rosesphere architecture filled with and covered by the graphene sheets. This structure had mesopores to increase the electrolyte/electrode contact area, allowing large Li+ adsorption on the surface and shortening path length for Li+ transport, and meanwhile accommodated the volume change during electrochemical reaction.2 Meanwhile, the intimate contact of the graphene with the petals of the G-Co3O4 rosespheres provided a large-area highly conductive medium for electron transfer. The continuous graphene sheets might also limit the cracking of electrode.


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Figure 6. (a) Cycling performances of Co3O4 and G-Co3O4 rosespheres (graphene is for comparison), (b) Cycling stability and coulombic efficiency of G-Co3O4 rosespheres at 90 mA g−1 (0.1 C), and (c) Capacity rate performance of G-Co3O4 rosespheres.



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4. CONCLUSIONS In a summary, G-Co3O4 rosespheres with Co3O4 rosesphere core and graphene filler/cover is synthesized with a facile electrostatic self-assembly approach. These G-Co3O4 rosespheres display excellent capacity and rate performance as an anode materials for lithium-ion storage, with a stable reversible capacity of 1110.8 mAh g−1 at the current of 90 mA g−1 (0.1C) after 30 cycles and 462.3 mAh g-1 even at 1800 mA g-1 (2 C). The excellent performance is likely due to the enhanced charge conductivity and adsorption, along with the large tolerance for volume changes in the charge/discharge process of LIBs.


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ASSOCIATED CONTENT

— Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

SEM of pure Co3O4 rose-spheres; FTIR spectrum and EDX of APS-modified Co3O4; energy dispersive X-ray spectrometry mapping analysis of G-Co3O4; XRD patterns of samples; TGA curve of G-Co3O4; SEM image of G-Co3O4 electrode material after 100 cycles; Cycling stability of G-Co3O4 at 0.2 C; Table of the comparison of cycling stability and rate capacity retention between this work and the previous reports. AUTHOR INFORMATION Corresponding Author * Email address: [email protected]; [email protected].

ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (No. 51272075, No. 51372080 and No. 51238002). Z. H. and M. Zhou thank the China Scholarship Council for financial support for oversee research. X. C. acknowledges the support from the College of Arts and Sciences, University of Missouri − Kansas City. REFERENCES 1.

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Table Of Contents (TOC) graphic


Graphene-Embedded Co3O4 Rose-Spheres for Enhanced

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