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Hollow cobalt oxide nanoparticles embedded porous reduced graphene oxide anode for high performance lithium ion batteries
Journal Pre-proofs Full Length Article Hollow cobalt oxide nanoparticles embedded porous reduced graphene oxide anode for high performance lithium ion batteries Diben Wu, Yirui Ouyang, Wenlin Zhang, Zhuan Chen, Zhi Li, Shuo Wang, Fengqian Wang, Hongliang Li, Lian Ying Zhang PII: DOI: Reference:
S0169-4332(20)30067-2 https://doi.org/10.1016/j.apsusc.2020.145311 APSUSC 145311
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
22 October 2019 31 December 2019 6 January 2020
Please cite this article as: D. Wu, Y. Ouyang, W. Zhang, Z. Chen, Z. Li, S. Wang, F. Wang, H. Li, L.Y. Zhang, Hollow cobalt oxide nanoparticles embedded porous reduced graphene oxide anode for high performance lithium ion batteries, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145311
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© 2020 Published by Elsevier B.V.
Hollow cobalt oxide nanoparticles embedded porous reduced graphene oxide anode for high performance lithium ion batteries Diben Wua, Yirui Ouyanga, Wenlin Zhangb, Zhuan Chena, Zhi Lia, Shuo Wanga, Fengqian Wang,a Hongliang Lia and Lian Ying Zhanga,c* a Institute
of Materials for Energy and Environment, State Key Laboratory of Bio-Fibers and Eco-
Textiles, School of Materials Science and Engineering, Qingdao University, 266071, P. R. China. Corresponding author, Email: [email protected] (L. Y. Zhang) b
Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 400715, P. R. China.
c
CAS Key Laboratory of Low-Coal Conversion Science & Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, P.R. China.
Keywords: cobalt oxide; hollow structure; porous reduced graphene oxide; anode material; lithium ion batteries Abstract: Hollow cobalt oxide nanoparticles embedded porous reduced graphene oxide hybrids are constructed by means of a facile etching strategy and the Kirkendall effect. When examined as an anode for lithium-ion batteries, this unique structured composite exhibits remarkable lithium storage capability such as excellent rate capability, superior reversible specific capacity together with good cycling durability, highlighting the importance of embedding active materials on graphene sheets for maximum utilization of hollow structured cobalt oxide and porous reduced graphene oxide for energy devices.
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1. Introduction The development of high-performance advanced lithium-ion batteries (LIBs) is crucial for portable electronic devices and electric vehicles. [1-3] In comparison to their commonly used anode graphite, transition metal oxides are believed to be one of the most attractive candidates because of the large theoretical specific capacity. [4-6] It is well known that nanostructured materials with high specific surface area usually reflects rich active sites, [7] which is highly desirable to improve reversible capacities for LIBs.[4,8] Recently, hollow structures received much attention in many aspects of electrochemistry especially in LIBs due to their high electrochemical active area and alleviated inner lattice stress, thereby retaining the structural integrity of electrode materials and high capacity during the repetitive lithiation and delithiation processes.[9,10] The Kirkendall effect is a classical phenomenon in metallurgy, which normally refers to an unbalanced counter diffusion process through the interface of coupled species.[11] The synthetic strategy catches tremendous attention due to the produced hollow voids in metal oxides without an additional template removal process.[12,13] Thus, it has significant applications in the preparation of hollow metal oxide nanostructures.[14,15] Unfortunately, for most of metal oxide-based anodes including the hollow nanostructures, they usually face low rate capability and poor cycling stability owning to their low electronic conductivity. [16-18] It is well known that graphene has superior electronic conductivity and large specific surface area. Thus, an effective strategy is to couple metal oxide with graphene to enhance the overall electron conductivity.[9,19] Lian et al. prepared the composite of graphene wrapped hollow cobalt oxide (Co3O4) spheres, which showed enhanced rate performance and cycling stability in comparison to plain Co3O4.[20] Nevertheless, its rate capability and cycle life need further enhancement to meet the practical application for LIBs.
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Recently, Duan et al reported the use of holey-graphene supported niobia (Nb2O5) as an anode in LIBs system, which exhibited high rate capability compared to nonporous graphene supported Nb2O5.[21] They also demonstrated that porosity in holey-graphene is crucial for achieving optimized charge transport and high-rate energy storage. Nevertheless, the composite suffers from insufficient coupling effect because of the small fraction of Nb2O5 nanoparticles (NPs) bonding with graphene. Thus, the design and fabrication of hollow structured metal oxide NPs with large theoretical capacity coupled porous graphene with high contact area is highly desirable. Unfortunately, this kind of architecture has not been realized to date. We believed that the nanostructures with this unique architecture would be highly promising as an excellent anode for LIBs with high rate capability, reversible capacity as well as cycling durability. Herein, as a model study for other transition metal oxides, hollow Co3O4 NPs embedded porous reduced graphene oxide (H-Co3O4/P-RGO) nanocomposite was successfully constructed by using a facile etching strategy and the Kirkendall effect. One etched pore located around one nanoparticle (NP). When used as an anode for LIBs, it exhibits superior reversible capacity (1016 mA h g-1 after 200 cycles at 0.2 A g-1), excellent rate performance (810 mA h g-1 at 0.5 A g-1, 510 mA h g-1 at 10 A g-1) and good cycling durability. To demonstrate the advantages of hollow structured Co3O4 in H-Co3O4/P-RGO, Co3O4 NPs embedded porous reduced graphene oxide (Co3O4/P-RGO) composite was also prepared as a control sample.
2. Experimental 2.1 Preparation of H-Co3O4/P-RGO Graphene oxide (GO) used in this work was synthesized with modified hummer’s method. [22-25] 116.2 mg Co(NO3)2∙6H2O was mixed with 5.0 mL deionized water under sonification for 16 min, followed by adding 25.0 mL deionized water and 150 mg GO under strong stirring for further 16
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min. Then a certain amount of NH3∙H2O (~ 1.5 mL) was added into the above mixture until its pH value reaches around 8~9. Finally, the mixture was freeze-dried, followed by thermal treatment at 800 ℃ for 2 h in nitrogen and further at 280 ℃ for 3 h in air. The collected black powers were denoted as H-Co3O4/P-RGO. For comparison, Co3O4/P-RGO composite was also prepared via the similar procedures to prepare H-Co3O4/P-RGO except that the calcination time is 0.5 h at 800 ℃. 2.2 Material characterizations The phase structures of the samples were characterized using Rigaku Ultima IV X-ray diffractometer (XRD) with Cu-Ka radiation (λ=0.15418 nm). The loading of metal oxide in HCo3O4/P-RGO was measured in air (heat rate: 10 ℃ min-1) by the Mettler Toledo TGA-2 thermal gravimetric analyzer (TGA). Raman spectra were obtained from a Renishaw in Via Plus MicroRaman spectroscopy system with a 50 mW DPSS laser at 532 nm. Field emission scanning electron microscope (JEOL JSM-7800F) and transmission electron microscope (JEOL JEM-2100 Plus) are used to characterize the microstructure of the samples in this work. X-ray photoelectron spectroscopy (XPS) with PHI 5000 Versa Probe III was operated to analyze compositions and chemical states of samples. Nitrogen adsorption/desorption isotherms were measured using an Autosorb-IQ-MP/XR surface area and pore analyzer (Quantachrome), and the specific surface areas of H-Co3O4/P-RGO and Co3O4/P-RGO were calculated using the Brunauer-Emmett-Teller (BET) method. The pore size distribution of H-Co3O4/P-RGO and Co3O4/P-RGO were analyzed using Barrett-Joyner-Halenda (BJH) method. 2.3 Electrochemical measurements The lithium storage behavior of the as-prepared H-Co3O4/P-RGO or Co3O4/P-RGO was examined using CR2016 coin-type half cells. A mixed slurry of H-Co3O4/P-RGO or Co3O4/P-RGO composite, carbon black and polyvinylidene fluoride (PVDF) at a weight of 8:1:1 in N-metheyl-
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2-pyrrolidone (NMP) solvent was coated on copper foil followed by dried at 110 ℃ for 10 h in a vacuum oven to construct a working electrode. 1 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1, v/v) is used as electrolyte. The charge-discharge curves, rate performance and cycling stability of electrodes are tested on Neware battery testing system. The electrochemical workstation CHI 760 (Shanghai Chenhua) was used to measure the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) curves of assembled LIBs.
3. Results and discussion 3.1 Nanostructure and morphology The crystal structures of prepared H-Co3O4/P-RGO and Co3O4/P-RGO composites are firstly measured with XRD technique (Fig. 1a), and their peaks at 19.00°, 31.27°, 36.85°, 44.80°, 59.35°, and 65.23° correspond to (111), (220), (311), (400), (511) and (440) faces of cubic Co3O4 (JCPDS card no. 78-1970), respectively.[9,26] Additionally, XRD patterns of GO and RGO (the obtained GO was treated at the same calcination conditions to H-Co3O4/P-RGO) are also taken for comparison. Clearly, for GO, a sharp diffraction peak at 9.8° can be found, then the peak shifts to 26.4° ((0 0 2) plane) after the thermal treatment, which is closer to that of graphite (26.5°), indicating that GO was reduced successfully and corresponding products are RGO. The XPS measurements are performed to detect the compositions and chemical states of H-Co3O4/P-RGO (Fig. S1a). The three peaks at 289.3, 285.5, 284.6 eV in Fig. S1b are attributed to O-C=O, C-O and C-C, respectively.[27] Their small ratio of oxygen-containing groups indicates that GO is reduced to RGO successfully.[28,29] For the Co2p region in Fig. 1b, the two peaks at 780.2 and 795.2 eV could be assigned to the Co2p3/2 and Co2p1/2 spin-orbit peaks of Co3O4, respectively. Additionally, the calculated intensity ratio of Co2p3/2 and Co2p1/2 peaks is around 2:1 and their binding energy difference is about 15 eV, reflecting typical characteristics of the cubic Co3O4.[30]
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While for O1s, the four peaks at 529.7, 530.7, 531.8 and 533.3 eV correspond to Co-O, Co-O-C, Co-O/C=O and C-O-C/C-OH bonds, respectively (Fig. 1c).[31] The Co-O bonds in H-Co3O4/PRGO suggest a strong interfacial interaction between the etched holes on RGO and hollow structured Co3O4 NPs.[32] Fig. S2 shows that Co3O4/P-RGO has similar XPS spectra of C1s, Co2p and O1s in comparison to those of H-Co3O4/P-RGO, indicating the same compositions and chemical states between the above two composites. Raman spectra of H-Co3O4/P-RGO and Co3O4/P-RGO composites are also recorded as in Fig. 1d. The five peaks in the two composites correspond to the A1g (680 cm-1), F2g (613 cm-1), F2g (520 cm-1), Eg (482 cm-1), F2g (193 cm-1) modes of Co3O4, confirming the formation of Co3O4. The G band is a characteristic peak of graphite E2g, which depends on the vibration of the sp2 bond in the two-dimensional graphite hexagonal crystal. While the D band indicates the shortcoming and confusion of the hexagonal graphite layer.[33,34] The relative intensity ratio of D band to G band (ID/IG) relies on the type of the graphitic materials and reflects the degree of graphitization of measured carbon materials.[35,36] The calculated ID/IG value of H-Co3O4/P-RGO is 1.01, which is much higher than that of GO (0.78), RGO (0.84) and Co3O4/P-RGO (0.95). The high ID/IG value of H-Co3O4/P-RGO could be attributed to the fact that GO is reduced into RGO under high-temperature thermal treatment, and rich defects on RGO are produced accordingly.[28,37] The thermal treatment at 800℃ for 2 h is beneficial to generating more defects in comparison to that for 0.5 h. SEM images in Fig. 2a-b reveal that the Co3O4 NPs in H-Co3O4/P-RGO are uniformly embedded into RGO sheets, and one etched pore in RGO sheets located around one nanoparticle. Rich pores in P-RGO can be obviously found after removing H-Co3O4 (Fig. S3). TEM image in Fig. 2c confirms the results of SEM analysis. Interestingly, the Co3O4 NP in H-Co3O4/P-RGO depict a darker shell in comparison to its internal structure, demonstrating a hollow structure of
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the Co3O4 NP (Fig. 2d). The HAADF-STEM image in Fig. 2e and corresponding EDX elemental distribution of Co and O in the representative nanoparticle (Fig. 2f-g) confirm the hollow structure of Co3O4 in H-Co3O4/P-RGO. For H-Co3O4/P-RGO, its measured specific surface area is about 230 m2 g-1. The pore size distribution in Fig. 2h shows a series of mesopores with 4 nm and 10-20 nm, which concurs with the SEM and TEM images in Fig. 2a-c. TGA analysis in Fig. 2i shows that the Co3O4 loading in the H-Co3O4/P-RGO composite is about 74.10%. For Co3O4/P-RGO composite, Co3O4 NPs also have a uniform distribution on RGO sheets, and one etched pore can also be found around one nanoparticle (Fig. 3a-b). However, no obvious hollow structures could be found for the Co3O4 NPs in Co3O4/P-RGO composite. Fig. 3c shows that Co3O4/P-RGO possesses a series of mesopores centred at 4 nm and 10~20 nm in Co3O4/P-RGO and a specific surface area of around 184 m2 g-1, which is a little lower than that of H-Co3O4/P-RGO. The advantage of H-Co3O4/P-RGO result originates from the hollow structure of Co3O4 nanocrystals. The TGA analysis in Fig. 3d demonstrates that Co3O4 loading in Co3O4/P-RGO is about 73.88%, which is very close to that of H-Co3O4/P-RGO. Fig. S4a-b reveals that the Co(NO3)2 is transformed into Co/CoO NPs through the thermal decomposition process (800 ℃, 0.5 h) because C is a strong reductant in high temperatures. It has been discovered that metals and metal oxides could etch graphene to form nanoscale holes, and the carbon atoms on the graphene sheets are partially oxidized into carbon monoxide and/or dioxide. [38-41] Therefore, it is believed that the obtained Co/CoO NPs could also work as etching agents to oxidize their neighbouring carbon atoms on RGO sheets, and in-situ generate pores around the nanoparticles. To understand the formation mechanism of hollow structured Co3O4 NPs, the calcination time of Co(NO3)2 is extended to 2 h, and the morphology of formed Co/CoO NPs is taken and shown in Fig. S4c. Hollow structured Co/CoO NPs could be found clearly. The formation process of the hollow structure could be related
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to the nanoscale Kirkendall effect because the outward diffusion of Co is faster than the inward diffusion rate of oxygen.[40,42] After a further calcination process in air, the formed Co/CoO NPs are oxidized into Co3O4 NPs, while their hollow structure could be well maintained. 3.2 Electrochemical behaviors Fig. 4a exhibits the CV curves of H-Co3O4/P-RGO composite from 0.01 to 3.0 V. In the first cycle, the strong cathodic peak at 0.92 V in the discharge process attributes to the reduction of Cobased compound and solid electrolyte interface (SEI) formation.[26,43,44] While the main anodic peak at around 2.08 V corresponds to the oxidation reaction (delithiation) of Co forming Co3O4 and the decomposition of Li2O.[45,46] Its redox reaction between Li ions and Co3O4 can be illustrated in the following two steps: Co3O4 + 8Li → 3Co + 4Li2O and 3Co + 4Li2O → Co3O4 + 8Li.[47,48] In the subsequent cycles, two main prominent cathode peaks at 1.27 V and 0.95 V are ascribed to the reduction of Co3+ to Co2+ and Co2+ to Co0, respectively.[49,50] The positions of oxidation peaks could be well maintained, indicating a good reproducibility and high cycling stability of prepared H-Co3O4/P-RGO composite. Fig. 4b shows the comparison of 1st, 2nd, 50th and 100th charge and discharge curves for H-Co3O4/P-RGO. In the first discharge curve, the long voltage plateau at 1.1 V can be easily observed, which is well consistent with the typical characteristics of Co3O4 electrodes.[51] In the first cycle, the charge and discharge capacities are 813 and 1164 mA h g-1, respectively, and calculated corresponding coulombic efficiency is around 69.8%, which could be mainly due to the high specific surface area of H-Co3O4/P-RGO composite up to 230 m2 g-1.[52] It is worthy of a note that this value has certain advantages in comparison to previous reported Co3O4based hybrids such as MWCNTs/Co3O4 (69.3%)[53] and Co3O4@C (67%),[54] exhibiting excellent availability of H-Co3O4/RGO. In the second cycle, the charge and discharge capacities are around 797.8 and 855.7 mA h g-1, respectively, with a coulombic efficiency as high as 93.3%. The
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improved coulombic efficiency indicates the suppressed the irreversible reaction through formed SEI layer. Further, it is worthy of a note that an obvious plateau at ~2.0 V could be found in the 1st and 2nd charge voltage profile, while the plateau disappears after 50 cycles. This result should be related to the decreased crystal size of Co-based nanoparticles (Fig. S5b), which is in good agreement with the analysis in previous reports. [55,56] Fig. 4c shows the rate performance of H-Co3O4/RGO and Co3O4/P-RGO at various rates from 0.1 to 10.0 A g-1. For H-Co3O4/P-RGO, its discharge capacities are 840, 810, 760, 620 and 510 mA h g-1 at the rates of 0.1, 0.5, 1.0, 5.0 and 10.0 A g-1, respectively. While for Co3O4/P-RGO, its discharge capacities are only 580, 530, 490, 360, 260 and 90 mA h g-1 at the same current densities. Thus, the H-Co3O4/P-RGO has much better rate performance in comparison to Co3O4/P-RGO. It is worth noting that even at a high rate of 10.0 A g-1, H-Co3O4/P-RGO also exhibits a remarkable discharge capacity about 510 mA h g-1. When the current density of H-Co3O4/P-RGO is reduced back to 0.1 A g-1, its reversible capacity increases up to 930 mA h g-1, which is much higher than its original value (840 mA h g-1) and that of Co3O4/P-RGO at the same current density (620 mA h g-1), demonstrating an excellent rate performance and high reversible capacity of H-Co3O4/P-RGO. Fig. 4d describes the cycling performance of prepared H-Co3O4/P-RGO and Co3O4/P-RGO electrodes at the current density of 0.2 A g-1. For H-Co3O4/P-RGO, its initial discharge specific capacity is as high as 1164 mA h g-1, which decreases to 833 mA h g-1 after 4 cycles. Then the reversible capacity slightly increases with the cycling and reaches 1016 mA h g-1 even after 200 cycles. The slightly increased reversible capacity should be derived from the gradual activation of H-Co3O4/P-RGO electrode because the unique holes in RGO sheets and hollow structured Co3O4 NPs are beneficial to facilitate electrolyte diffusion and confine volume expansion, respectively. EIS analysis is performed before and after the cycling to demonstrate the resistance inside the
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battery. [57-60] The increased reversible capacity of H-Co3O4/P-RGO could be related to the reduced charge transfer resistance resulting from the gradual activation process and the formation of stable SEI layer (Fig. S6) [61,62]. Its calculated coulombic efficiency is close to 100%, highlighting the high reversibility of H-Co3O4/P-RGO electrode. While for Co3O4/P-RGO, its reversible capacity is only 645 mA h g-1 after 200 cycles, which is much lower than that of H-Co3O4/P-RGO. In comparison to the reported composites of Co3O4-graphene in Table 1, [20,37,63-70] the H-Co3O4/P-RGO electrode also has great advantages especially in long-time cycling performance and reversible capacity. These results confirm that the prepared H-Co3O4/P-RGO in this work possesses excellent rate performance, superior reversible capacity as well as high cycling durability.
Table 1. Comparison of electrochemical performance for different Co3O4 and graphene composites.
Anodes
Reversible capacity Current density
Cycle life
Ref
(mA h g-1)
(A g-1)
Mesoporous Co3O4@Graphene
785
1.0
200
Ref[20]
Co3O4-graphene
830
0.2
75
Ref[37]
Mesoporous Co3O4/Graphene
900
0.1
60
Ref[63]
Co3O4/graphene
800
0.2
45
Ref[64]
Co3O4/graphene
935
0.05
30
Ref[65]
Mesoporous Co3O4-graphene
820
0.1
35
Ref[66]
Co3O4-graphene
1036
0.1
50
Ref[67]
Co3O4/graphene
840
0.1
50
Ref[68]
Co3O4@graphene
740
0.2
60
Ref[69]
Co3O4 fibers/graphene
840
0.1
40
Ref[70]
H-Co3O4/P-RGO
1016
0.2
200
This work
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3.3 Mechanism discussion The mechanism of performance enhancement for prepared H-Co3O4/P-RGO composite was proposed and discussed (Fig. 5). RGO has high electrical conductivity, specific surface area and mechanical flexibility, and thus the fact that using RGO as a support for hollow Co3O4 NPs would provide fast electron transport.[23] The rich pores in RGO sheets would offer effective channels to reduce electrolyte diffusion distance for high rate capability, and work as Li ion storage sites for high reversible capacity.[20,66,71] The hollow structure of Co3O4 crystals almost maintained even after 50 cycles at a current density of 200 mA g-1 (Fig. S5). Thus, the high cycling stability of HCo3O4/P-RGO electrode could be contributed to the unique hollow structured Co3O4 NPs, which alleviate inner lattice stress and accommodate the volume expansion and contraction during repetitive charge/discharge cycles. Additionally, the SEM and TEM images in Fig. 2a-d show that the synthesized hollow structured Co3O4 NPs are embedded in porous RGO sheets, enabling a strong coupling effect between Co3O4 NPs and connected RGO sheet due to high fraction of surface atoms of Co3O4 NPs bonded with RGO sheets, which is highly desirable for high cycling life.[29] In comparison to normal Co3O4 NPs, the hollow structure of Co3O4 NPs for H-Co3O4/PRGO composite is believed to supply more Li ion storage sites for higher reversible capacity and more channels for faster mass transfer causing higher rate performance. Thus, the superior rateperformance of H-Co3O4/P-RGO composite is mainly ascribed to the synergetic effects between etched pores in RGO sheets and hollow structured Co3O4 NPs.
4. Conclusions In summary, hollow cobalt oxide embedded porous reduced graphene oxide nanocomposite was developed as an advanced anode for lithium ion batteries using a facile etching strategy and the Kirkendall effect. The composite is capable of effectively using the high electrical conductivity
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and specific surface area, reducing electrolyte diffusion distance of porous reduced graphene oxide as well as high reversible capacity, alleviating inner lattice stress, and enriching Li ion storage sites of hollow structured Co3O4 NPs. It shows superior rate performance (810 mA h g-1 at 0.5 A g-1, 510 mA h g-1 at 10 A g-1), high reversible capacity up to 1016 mA h g-1 (after 200 cycles at 0.2 A g-1) as well as great cycling stability. This work demonstrates that hollow structured metal oxide embedded porous reduced graphene oxide could not only effectively mitigate the capacity fading and performance degradation but also offer desirable rate performance, reversible capacity and cycling life, which may open up new strategies to construct other electrode materials for advanced lithium ion batteries.
Acknowledgments We gratefully acknowledge to the financial support from Key Research and Development Plan of Shandong Province (No.2018GGX102019) and Natural Science Foundation of Shandong Province (No. ZR2017BB022). CAS Key Laboratory of Low-Coal Conversion Science & Engineering, China (KLLCCSE-201706, SARI, CAS), China Postdoctoral Science Foundation, China (No. 2017 M612201) and Applied Basic Research Program of Qingdao (No. 18-2-2-5-jch) are also acknowledged.
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Figure captions Fig. 1. (a) XRD patterns of H-Co3O4/P-RGO, H-Co3O4/P-RGO, GO and RGO. XPS spectra of (b) Co2p and (c) O1s of H-Co3O4/P-RGO composite. (d) Comparison of Raman spectra of H-Co3O4/P-RGO, Co3O4/P-RGO, GO and RGO.
Fig. 2. (a-b) SEM and (c) TEM images of prepared H-Co3O4/P-RGO. (d) TEM image of one hollow Co3O4 NP. (e) HAADF-STEM image of prepared H-Co3O4/P-RGO and (f-g) EDX elemental distribution of Co
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and O in the representative hollow Co3O4 NP. (h) Nitrogen adsorption-desorption isotherm and corresponding pore size distribution (inset) of H-Co3O4/P-RGO. (i) TGA curve of the H-Co3O4/P-RGO.
Fig. 3. (a-b) TEM images, (c) nitrogen adsorption-desorption isotherm and corresponding pore size distribution (inset) of Co3O4/P-RGO. (d) TGA curve of the Co3O4/P-RGO.
Fig. 4. (a) CV curves of H-Co3O4/P-RGO at initial 6 cycles. Scan rate: 0.1 mV s-1; (b) Charge and discharge profiles of H-Co3O4/P-RGO at various cycles. Current density: 0.2 A g-1; The comparisons of (c) rate performance (current densities, from 0.1 to 10.0 A g-1), (d) cycling performance of H-Co3O4/P-RGO, Co3O4/P-RGO and corresponding coulombic efficiency over 200 cycles (current density: 0.2 A g-1).
Fig. 5. Schematic illustration of enhancement mechanism of prepared H-Co3O4/P-RGO composite and its application as an anode for LIBs.
Figures
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Fig. 1.
23
Fig. 2.
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Fig. 3.
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Fig. 4.
Fig. 5.
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Highlights
Hollow Co3O4 NPs embedded porous RGO composite is successfully prepared.
H-Co3O4/P-RGO is obtained using etching strategy and the Kirkendall effect.
H-Co3O4/P-RGO shows high rate capability, reversible capacity and cycle stability.
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Diben Wu: Conceptualization, Investigation, Writing-Original Draft Yirui Ouyang: Investigation Wenlin Zhang: Data Curation Zhuan Chen: Investigation Zhi Li: Investigation Shuo Wang: Visualization Fengqian Wang: Visualization Hongliang Li: Writing-Review & Editing Lian Ying Zhang: Conceptualization, Writing-Review & Editing, Supervision
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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S0169-4332(20)30067-2 https://doi.org/10.1016/j.apsusc.2020.145311 APSUSC 145311
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
22 October 2019 31 December 2019 6 January 2020
Please cite this article as: D. Wu, Y. Ouyang, W. Zhang, Z. Chen, Z. Li, S. Wang, F. Wang, H. Li, L.Y. Zhang, Hollow cobalt oxide nanoparticles embedded porous reduced graphene oxide anode for high performance lithium ion batteries, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145311
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Hollow cobalt oxide nanoparticles embedded porous reduced graphene oxide anode for high performance lithium ion batteries Diben Wua, Yirui Ouyanga, Wenlin Zhangb, Zhuan Chena, Zhi Lia, Shuo Wanga, Fengqian Wang,a Hongliang Lia and Lian Ying Zhanga,c* a Institute
of Materials for Energy and Environment, State Key Laboratory of Bio-Fibers and Eco-
Textiles, School of Materials Science and Engineering, Qingdao University, 266071, P. R. China. Corresponding author, Email: [email protected] (L. Y. Zhang) b
Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 400715, P. R. China.
c
CAS Key Laboratory of Low-Coal Conversion Science & Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, P.R. China.
Keywords: cobalt oxide; hollow structure; porous reduced graphene oxide; anode material; lithium ion batteries Abstract: Hollow cobalt oxide nanoparticles embedded porous reduced graphene oxide hybrids are constructed by means of a facile etching strategy and the Kirkendall effect. When examined as an anode for lithium-ion batteries, this unique structured composite exhibits remarkable lithium storage capability such as excellent rate capability, superior reversible specific capacity together with good cycling durability, highlighting the importance of embedding active materials on graphene sheets for maximum utilization of hollow structured cobalt oxide and porous reduced graphene oxide for energy devices.
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1. Introduction The development of high-performance advanced lithium-ion batteries (LIBs) is crucial for portable electronic devices and electric vehicles. [1-3] In comparison to their commonly used anode graphite, transition metal oxides are believed to be one of the most attractive candidates because of the large theoretical specific capacity. [4-6] It is well known that nanostructured materials with high specific surface area usually reflects rich active sites, [7] which is highly desirable to improve reversible capacities for LIBs.[4,8] Recently, hollow structures received much attention in many aspects of electrochemistry especially in LIBs due to their high electrochemical active area and alleviated inner lattice stress, thereby retaining the structural integrity of electrode materials and high capacity during the repetitive lithiation and delithiation processes.[9,10] The Kirkendall effect is a classical phenomenon in metallurgy, which normally refers to an unbalanced counter diffusion process through the interface of coupled species.[11] The synthetic strategy catches tremendous attention due to the produced hollow voids in metal oxides without an additional template removal process.[12,13] Thus, it has significant applications in the preparation of hollow metal oxide nanostructures.[14,15] Unfortunately, for most of metal oxide-based anodes including the hollow nanostructures, they usually face low rate capability and poor cycling stability owning to their low electronic conductivity. [16-18] It is well known that graphene has superior electronic conductivity and large specific surface area. Thus, an effective strategy is to couple metal oxide with graphene to enhance the overall electron conductivity.[9,19] Lian et al. prepared the composite of graphene wrapped hollow cobalt oxide (Co3O4) spheres, which showed enhanced rate performance and cycling stability in comparison to plain Co3O4.[20] Nevertheless, its rate capability and cycle life need further enhancement to meet the practical application for LIBs.
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Recently, Duan et al reported the use of holey-graphene supported niobia (Nb2O5) as an anode in LIBs system, which exhibited high rate capability compared to nonporous graphene supported Nb2O5.[21] They also demonstrated that porosity in holey-graphene is crucial for achieving optimized charge transport and high-rate energy storage. Nevertheless, the composite suffers from insufficient coupling effect because of the small fraction of Nb2O5 nanoparticles (NPs) bonding with graphene. Thus, the design and fabrication of hollow structured metal oxide NPs with large theoretical capacity coupled porous graphene with high contact area is highly desirable. Unfortunately, this kind of architecture has not been realized to date. We believed that the nanostructures with this unique architecture would be highly promising as an excellent anode for LIBs with high rate capability, reversible capacity as well as cycling durability. Herein, as a model study for other transition metal oxides, hollow Co3O4 NPs embedded porous reduced graphene oxide (H-Co3O4/P-RGO) nanocomposite was successfully constructed by using a facile etching strategy and the Kirkendall effect. One etched pore located around one nanoparticle (NP). When used as an anode for LIBs, it exhibits superior reversible capacity (1016 mA h g-1 after 200 cycles at 0.2 A g-1), excellent rate performance (810 mA h g-1 at 0.5 A g-1, 510 mA h g-1 at 10 A g-1) and good cycling durability. To demonstrate the advantages of hollow structured Co3O4 in H-Co3O4/P-RGO, Co3O4 NPs embedded porous reduced graphene oxide (Co3O4/P-RGO) composite was also prepared as a control sample.
2. Experimental 2.1 Preparation of H-Co3O4/P-RGO Graphene oxide (GO) used in this work was synthesized with modified hummer’s method. [22-25] 116.2 mg Co(NO3)2∙6H2O was mixed with 5.0 mL deionized water under sonification for 16 min, followed by adding 25.0 mL deionized water and 150 mg GO under strong stirring for further 16
3
min. Then a certain amount of NH3∙H2O (~ 1.5 mL) was added into the above mixture until its pH value reaches around 8~9. Finally, the mixture was freeze-dried, followed by thermal treatment at 800 ℃ for 2 h in nitrogen and further at 280 ℃ for 3 h in air. The collected black powers were denoted as H-Co3O4/P-RGO. For comparison, Co3O4/P-RGO composite was also prepared via the similar procedures to prepare H-Co3O4/P-RGO except that the calcination time is 0.5 h at 800 ℃. 2.2 Material characterizations The phase structures of the samples were characterized using Rigaku Ultima IV X-ray diffractometer (XRD) with Cu-Ka radiation (λ=0.15418 nm). The loading of metal oxide in HCo3O4/P-RGO was measured in air (heat rate: 10 ℃ min-1) by the Mettler Toledo TGA-2 thermal gravimetric analyzer (TGA). Raman spectra were obtained from a Renishaw in Via Plus MicroRaman spectroscopy system with a 50 mW DPSS laser at 532 nm. Field emission scanning electron microscope (JEOL JSM-7800F) and transmission electron microscope (JEOL JEM-2100 Plus) are used to characterize the microstructure of the samples in this work. X-ray photoelectron spectroscopy (XPS) with PHI 5000 Versa Probe III was operated to analyze compositions and chemical states of samples. Nitrogen adsorption/desorption isotherms were measured using an Autosorb-IQ-MP/XR surface area and pore analyzer (Quantachrome), and the specific surface areas of H-Co3O4/P-RGO and Co3O4/P-RGO were calculated using the Brunauer-Emmett-Teller (BET) method. The pore size distribution of H-Co3O4/P-RGO and Co3O4/P-RGO were analyzed using Barrett-Joyner-Halenda (BJH) method. 2.3 Electrochemical measurements The lithium storage behavior of the as-prepared H-Co3O4/P-RGO or Co3O4/P-RGO was examined using CR2016 coin-type half cells. A mixed slurry of H-Co3O4/P-RGO or Co3O4/P-RGO composite, carbon black and polyvinylidene fluoride (PVDF) at a weight of 8:1:1 in N-metheyl-
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2-pyrrolidone (NMP) solvent was coated on copper foil followed by dried at 110 ℃ for 10 h in a vacuum oven to construct a working electrode. 1 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1, v/v) is used as electrolyte. The charge-discharge curves, rate performance and cycling stability of electrodes are tested on Neware battery testing system. The electrochemical workstation CHI 760 (Shanghai Chenhua) was used to measure the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) curves of assembled LIBs.
3. Results and discussion 3.1 Nanostructure and morphology The crystal structures of prepared H-Co3O4/P-RGO and Co3O4/P-RGO composites are firstly measured with XRD technique (Fig. 1a), and their peaks at 19.00°, 31.27°, 36.85°, 44.80°, 59.35°, and 65.23° correspond to (111), (220), (311), (400), (511) and (440) faces of cubic Co3O4 (JCPDS card no. 78-1970), respectively.[9,26] Additionally, XRD patterns of GO and RGO (the obtained GO was treated at the same calcination conditions to H-Co3O4/P-RGO) are also taken for comparison. Clearly, for GO, a sharp diffraction peak at 9.8° can be found, then the peak shifts to 26.4° ((0 0 2) plane) after the thermal treatment, which is closer to that of graphite (26.5°), indicating that GO was reduced successfully and corresponding products are RGO. The XPS measurements are performed to detect the compositions and chemical states of H-Co3O4/P-RGO (Fig. S1a). The three peaks at 289.3, 285.5, 284.6 eV in Fig. S1b are attributed to O-C=O, C-O and C-C, respectively.[27] Their small ratio of oxygen-containing groups indicates that GO is reduced to RGO successfully.[28,29] For the Co2p region in Fig. 1b, the two peaks at 780.2 and 795.2 eV could be assigned to the Co2p3/2 and Co2p1/2 spin-orbit peaks of Co3O4, respectively. Additionally, the calculated intensity ratio of Co2p3/2 and Co2p1/2 peaks is around 2:1 and their binding energy difference is about 15 eV, reflecting typical characteristics of the cubic Co3O4.[30]
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While for O1s, the four peaks at 529.7, 530.7, 531.8 and 533.3 eV correspond to Co-O, Co-O-C, Co-O/C=O and C-O-C/C-OH bonds, respectively (Fig. 1c).[31] The Co-O bonds in H-Co3O4/PRGO suggest a strong interfacial interaction between the etched holes on RGO and hollow structured Co3O4 NPs.[32] Fig. S2 shows that Co3O4/P-RGO has similar XPS spectra of C1s, Co2p and O1s in comparison to those of H-Co3O4/P-RGO, indicating the same compositions and chemical states between the above two composites. Raman spectra of H-Co3O4/P-RGO and Co3O4/P-RGO composites are also recorded as in Fig. 1d. The five peaks in the two composites correspond to the A1g (680 cm-1), F2g (613 cm-1), F2g (520 cm-1), Eg (482 cm-1), F2g (193 cm-1) modes of Co3O4, confirming the formation of Co3O4. The G band is a characteristic peak of graphite E2g, which depends on the vibration of the sp2 bond in the two-dimensional graphite hexagonal crystal. While the D band indicates the shortcoming and confusion of the hexagonal graphite layer.[33,34] The relative intensity ratio of D band to G band (ID/IG) relies on the type of the graphitic materials and reflects the degree of graphitization of measured carbon materials.[35,36] The calculated ID/IG value of H-Co3O4/P-RGO is 1.01, which is much higher than that of GO (0.78), RGO (0.84) and Co3O4/P-RGO (0.95). The high ID/IG value of H-Co3O4/P-RGO could be attributed to the fact that GO is reduced into RGO under high-temperature thermal treatment, and rich defects on RGO are produced accordingly.[28,37] The thermal treatment at 800℃ for 2 h is beneficial to generating more defects in comparison to that for 0.5 h. SEM images in Fig. 2a-b reveal that the Co3O4 NPs in H-Co3O4/P-RGO are uniformly embedded into RGO sheets, and one etched pore in RGO sheets located around one nanoparticle. Rich pores in P-RGO can be obviously found after removing H-Co3O4 (Fig. S3). TEM image in Fig. 2c confirms the results of SEM analysis. Interestingly, the Co3O4 NP in H-Co3O4/P-RGO depict a darker shell in comparison to its internal structure, demonstrating a hollow structure of
6
the Co3O4 NP (Fig. 2d). The HAADF-STEM image in Fig. 2e and corresponding EDX elemental distribution of Co and O in the representative nanoparticle (Fig. 2f-g) confirm the hollow structure of Co3O4 in H-Co3O4/P-RGO. For H-Co3O4/P-RGO, its measured specific surface area is about 230 m2 g-1. The pore size distribution in Fig. 2h shows a series of mesopores with 4 nm and 10-20 nm, which concurs with the SEM and TEM images in Fig. 2a-c. TGA analysis in Fig. 2i shows that the Co3O4 loading in the H-Co3O4/P-RGO composite is about 74.10%. For Co3O4/P-RGO composite, Co3O4 NPs also have a uniform distribution on RGO sheets, and one etched pore can also be found around one nanoparticle (Fig. 3a-b). However, no obvious hollow structures could be found for the Co3O4 NPs in Co3O4/P-RGO composite. Fig. 3c shows that Co3O4/P-RGO possesses a series of mesopores centred at 4 nm and 10~20 nm in Co3O4/P-RGO and a specific surface area of around 184 m2 g-1, which is a little lower than that of H-Co3O4/P-RGO. The advantage of H-Co3O4/P-RGO result originates from the hollow structure of Co3O4 nanocrystals. The TGA analysis in Fig. 3d demonstrates that Co3O4 loading in Co3O4/P-RGO is about 73.88%, which is very close to that of H-Co3O4/P-RGO. Fig. S4a-b reveals that the Co(NO3)2 is transformed into Co/CoO NPs through the thermal decomposition process (800 ℃, 0.5 h) because C is a strong reductant in high temperatures. It has been discovered that metals and metal oxides could etch graphene to form nanoscale holes, and the carbon atoms on the graphene sheets are partially oxidized into carbon monoxide and/or dioxide. [38-41] Therefore, it is believed that the obtained Co/CoO NPs could also work as etching agents to oxidize their neighbouring carbon atoms on RGO sheets, and in-situ generate pores around the nanoparticles. To understand the formation mechanism of hollow structured Co3O4 NPs, the calcination time of Co(NO3)2 is extended to 2 h, and the morphology of formed Co/CoO NPs is taken and shown in Fig. S4c. Hollow structured Co/CoO NPs could be found clearly. The formation process of the hollow structure could be related
7
to the nanoscale Kirkendall effect because the outward diffusion of Co is faster than the inward diffusion rate of oxygen.[40,42] After a further calcination process in air, the formed Co/CoO NPs are oxidized into Co3O4 NPs, while their hollow structure could be well maintained. 3.2 Electrochemical behaviors Fig. 4a exhibits the CV curves of H-Co3O4/P-RGO composite from 0.01 to 3.0 V. In the first cycle, the strong cathodic peak at 0.92 V in the discharge process attributes to the reduction of Cobased compound and solid electrolyte interface (SEI) formation.[26,43,44] While the main anodic peak at around 2.08 V corresponds to the oxidation reaction (delithiation) of Co forming Co3O4 and the decomposition of Li2O.[45,46] Its redox reaction between Li ions and Co3O4 can be illustrated in the following two steps: Co3O4 + 8Li → 3Co + 4Li2O and 3Co + 4Li2O → Co3O4 + 8Li.[47,48] In the subsequent cycles, two main prominent cathode peaks at 1.27 V and 0.95 V are ascribed to the reduction of Co3+ to Co2+ and Co2+ to Co0, respectively.[49,50] The positions of oxidation peaks could be well maintained, indicating a good reproducibility and high cycling stability of prepared H-Co3O4/P-RGO composite. Fig. 4b shows the comparison of 1st, 2nd, 50th and 100th charge and discharge curves for H-Co3O4/P-RGO. In the first discharge curve, the long voltage plateau at 1.1 V can be easily observed, which is well consistent with the typical characteristics of Co3O4 electrodes.[51] In the first cycle, the charge and discharge capacities are 813 and 1164 mA h g-1, respectively, and calculated corresponding coulombic efficiency is around 69.8%, which could be mainly due to the high specific surface area of H-Co3O4/P-RGO composite up to 230 m2 g-1.[52] It is worthy of a note that this value has certain advantages in comparison to previous reported Co3O4based hybrids such as MWCNTs/Co3O4 (69.3%)[53] and Co3O4@C (67%),[54] exhibiting excellent availability of H-Co3O4/RGO. In the second cycle, the charge and discharge capacities are around 797.8 and 855.7 mA h g-1, respectively, with a coulombic efficiency as high as 93.3%. The
8
improved coulombic efficiency indicates the suppressed the irreversible reaction through formed SEI layer. Further, it is worthy of a note that an obvious plateau at ~2.0 V could be found in the 1st and 2nd charge voltage profile, while the plateau disappears after 50 cycles. This result should be related to the decreased crystal size of Co-based nanoparticles (Fig. S5b), which is in good agreement with the analysis in previous reports. [55,56] Fig. 4c shows the rate performance of H-Co3O4/RGO and Co3O4/P-RGO at various rates from 0.1 to 10.0 A g-1. For H-Co3O4/P-RGO, its discharge capacities are 840, 810, 760, 620 and 510 mA h g-1 at the rates of 0.1, 0.5, 1.0, 5.0 and 10.0 A g-1, respectively. While for Co3O4/P-RGO, its discharge capacities are only 580, 530, 490, 360, 260 and 90 mA h g-1 at the same current densities. Thus, the H-Co3O4/P-RGO has much better rate performance in comparison to Co3O4/P-RGO. It is worth noting that even at a high rate of 10.0 A g-1, H-Co3O4/P-RGO also exhibits a remarkable discharge capacity about 510 mA h g-1. When the current density of H-Co3O4/P-RGO is reduced back to 0.1 A g-1, its reversible capacity increases up to 930 mA h g-1, which is much higher than its original value (840 mA h g-1) and that of Co3O4/P-RGO at the same current density (620 mA h g-1), demonstrating an excellent rate performance and high reversible capacity of H-Co3O4/P-RGO. Fig. 4d describes the cycling performance of prepared H-Co3O4/P-RGO and Co3O4/P-RGO electrodes at the current density of 0.2 A g-1. For H-Co3O4/P-RGO, its initial discharge specific capacity is as high as 1164 mA h g-1, which decreases to 833 mA h g-1 after 4 cycles. Then the reversible capacity slightly increases with the cycling and reaches 1016 mA h g-1 even after 200 cycles. The slightly increased reversible capacity should be derived from the gradual activation of H-Co3O4/P-RGO electrode because the unique holes in RGO sheets and hollow structured Co3O4 NPs are beneficial to facilitate electrolyte diffusion and confine volume expansion, respectively. EIS analysis is performed before and after the cycling to demonstrate the resistance inside the
9
battery. [57-60] The increased reversible capacity of H-Co3O4/P-RGO could be related to the reduced charge transfer resistance resulting from the gradual activation process and the formation of stable SEI layer (Fig. S6) [61,62]. Its calculated coulombic efficiency is close to 100%, highlighting the high reversibility of H-Co3O4/P-RGO electrode. While for Co3O4/P-RGO, its reversible capacity is only 645 mA h g-1 after 200 cycles, which is much lower than that of H-Co3O4/P-RGO. In comparison to the reported composites of Co3O4-graphene in Table 1, [20,37,63-70] the H-Co3O4/P-RGO electrode also has great advantages especially in long-time cycling performance and reversible capacity. These results confirm that the prepared H-Co3O4/P-RGO in this work possesses excellent rate performance, superior reversible capacity as well as high cycling durability.
Table 1. Comparison of electrochemical performance for different Co3O4 and graphene composites.
Anodes
Reversible capacity Current density
Cycle life
Ref
(mA h g-1)
(A g-1)
Mesoporous Co3O4@Graphene
785
1.0
200
Ref[20]
Co3O4-graphene
830
0.2
75
Ref[37]
Mesoporous Co3O4/Graphene
900
0.1
60
Ref[63]
Co3O4/graphene
800
0.2
45
Ref[64]
Co3O4/graphene
935
0.05
30
Ref[65]
Mesoporous Co3O4-graphene
820
0.1
35
Ref[66]
Co3O4-graphene
1036
0.1
50
Ref[67]
Co3O4/graphene
840
0.1
50
Ref[68]
Co3O4@graphene
740
0.2
60
Ref[69]
Co3O4 fibers/graphene
840
0.1
40
Ref[70]
H-Co3O4/P-RGO
1016
0.2
200
This work
10
3.3 Mechanism discussion The mechanism of performance enhancement for prepared H-Co3O4/P-RGO composite was proposed and discussed (Fig. 5). RGO has high electrical conductivity, specific surface area and mechanical flexibility, and thus the fact that using RGO as a support for hollow Co3O4 NPs would provide fast electron transport.[23] The rich pores in RGO sheets would offer effective channels to reduce electrolyte diffusion distance for high rate capability, and work as Li ion storage sites for high reversible capacity.[20,66,71] The hollow structure of Co3O4 crystals almost maintained even after 50 cycles at a current density of 200 mA g-1 (Fig. S5). Thus, the high cycling stability of HCo3O4/P-RGO electrode could be contributed to the unique hollow structured Co3O4 NPs, which alleviate inner lattice stress and accommodate the volume expansion and contraction during repetitive charge/discharge cycles. Additionally, the SEM and TEM images in Fig. 2a-d show that the synthesized hollow structured Co3O4 NPs are embedded in porous RGO sheets, enabling a strong coupling effect between Co3O4 NPs and connected RGO sheet due to high fraction of surface atoms of Co3O4 NPs bonded with RGO sheets, which is highly desirable for high cycling life.[29] In comparison to normal Co3O4 NPs, the hollow structure of Co3O4 NPs for H-Co3O4/PRGO composite is believed to supply more Li ion storage sites for higher reversible capacity and more channels for faster mass transfer causing higher rate performance. Thus, the superior rateperformance of H-Co3O4/P-RGO composite is mainly ascribed to the synergetic effects between etched pores in RGO sheets and hollow structured Co3O4 NPs.
4. Conclusions In summary, hollow cobalt oxide embedded porous reduced graphene oxide nanocomposite was developed as an advanced anode for lithium ion batteries using a facile etching strategy and the Kirkendall effect. The composite is capable of effectively using the high electrical conductivity
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and specific surface area, reducing electrolyte diffusion distance of porous reduced graphene oxide as well as high reversible capacity, alleviating inner lattice stress, and enriching Li ion storage sites of hollow structured Co3O4 NPs. It shows superior rate performance (810 mA h g-1 at 0.5 A g-1, 510 mA h g-1 at 10 A g-1), high reversible capacity up to 1016 mA h g-1 (after 200 cycles at 0.2 A g-1) as well as great cycling stability. This work demonstrates that hollow structured metal oxide embedded porous reduced graphene oxide could not only effectively mitigate the capacity fading and performance degradation but also offer desirable rate performance, reversible capacity and cycling life, which may open up new strategies to construct other electrode materials for advanced lithium ion batteries.
Acknowledgments We gratefully acknowledge to the financial support from Key Research and Development Plan of Shandong Province (No.2018GGX102019) and Natural Science Foundation of Shandong Province (No. ZR2017BB022). CAS Key Laboratory of Low-Coal Conversion Science & Engineering, China (KLLCCSE-201706, SARI, CAS), China Postdoctoral Science Foundation, China (No. 2017 M612201) and Applied Basic Research Program of Qingdao (No. 18-2-2-5-jch) are also acknowledged.
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Figure captions Fig. 1. (a) XRD patterns of H-Co3O4/P-RGO, H-Co3O4/P-RGO, GO and RGO. XPS spectra of (b) Co2p and (c) O1s of H-Co3O4/P-RGO composite. (d) Comparison of Raman spectra of H-Co3O4/P-RGO, Co3O4/P-RGO, GO and RGO.
Fig. 2. (a-b) SEM and (c) TEM images of prepared H-Co3O4/P-RGO. (d) TEM image of one hollow Co3O4 NP. (e) HAADF-STEM image of prepared H-Co3O4/P-RGO and (f-g) EDX elemental distribution of Co
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and O in the representative hollow Co3O4 NP. (h) Nitrogen adsorption-desorption isotherm and corresponding pore size distribution (inset) of H-Co3O4/P-RGO. (i) TGA curve of the H-Co3O4/P-RGO.
Fig. 3. (a-b) TEM images, (c) nitrogen adsorption-desorption isotherm and corresponding pore size distribution (inset) of Co3O4/P-RGO. (d) TGA curve of the Co3O4/P-RGO.
Fig. 4. (a) CV curves of H-Co3O4/P-RGO at initial 6 cycles. Scan rate: 0.1 mV s-1; (b) Charge and discharge profiles of H-Co3O4/P-RGO at various cycles. Current density: 0.2 A g-1; The comparisons of (c) rate performance (current densities, from 0.1 to 10.0 A g-1), (d) cycling performance of H-Co3O4/P-RGO, Co3O4/P-RGO and corresponding coulombic efficiency over 200 cycles (current density: 0.2 A g-1).
Fig. 5. Schematic illustration of enhancement mechanism of prepared H-Co3O4/P-RGO composite and its application as an anode for LIBs.
Figures
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Fig. 1.
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Fig. 2.
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Fig. 3.
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Fig. 4.
Fig. 5.
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Highlights
Hollow Co3O4 NPs embedded porous RGO composite is successfully prepared.
H-Co3O4/P-RGO is obtained using etching strategy and the Kirkendall effect.
H-Co3O4/P-RGO shows high rate capability, reversible capacity and cycle stability.
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Diben Wu: Conceptualization, Investigation, Writing-Original Draft Yirui Ouyang: Investigation Wenlin Zhang: Data Curation Zhuan Chen: Investigation Zhi Li: Investigation Shuo Wang: Visualization Fengqian Wang: Visualization Hongliang Li: Writing-Review & Editing Lian Ying Zhang: Conceptualization, Writing-Review & Editing, Supervision
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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