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Hierarchical porous arrays of mesoporous Co3O4 nanosheets grown on graphene skin for high-rate and high-capacity energy storage
Journal Pre-proof Hierarchical porous arrays of mesoporous Co3O4 nanosheets grown on graphene skin for high-rate and high-capacity energy storage Kuan Wu, Bijiang Geng, Chen Zhang, Wenwen Shen, Dewen Yang, Zhen Li, Zuobao Yang, Dengyu Pan PII:
S0925-8388(19)34542-6
DOI:
https://doi.org/10.1016/j.jallcom.2019.153296
Reference:
JALCOM 153296
To appear in:
Journal of Alloys and Compounds
Received Date: 21 May 2019 Revised Date:
14 November 2019
Accepted Date: 5 December 2019
Please cite this article as: K. Wu, B. Geng, C. Zhang, W. Shen, D. Yang, Z. Li, Z. Yang, D. Pan, Hierarchical porous arrays of mesoporous Co3O4 nanosheets grown on graphene skin for high-rate and high-capacity energy storage, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2019.153296. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contributions Section Dengyu Pan, Zhen Li, and Zuobao Yang supervised the project, conceived and designed all the experiments. Kuan Wu and Bijiang Geng realized the material synthesis. Chen Zhang and Wenwen Shen performed experiments. All authors discussed the results and analyzed the data.
Hierarchical porous arrays of mesoporous Co3O4 nanosheets grown on graphene skin for high-rate and high-capacity energy storage Kuan Wu a,1, Bijiang Geng b,1, Chen Zhang b,1, Wenwen Shen b,1, Dewen Yang b, Zhen Li a,*, Zuobao Yang c,*, and Dengyu Pan b,* a
Shanghai Applied Radiation Institute, School of Environmental and Chemical Engineering,
Shanghai University, Shanghai 200444 P. R. China b
Department of Chemical Engineering, School of Environmental and Chemical Engineering,
Shanghai University, Shanghai 200444 P. R. China c
Institute of Materials, Ningbo University of Technology Ningbo, 315016 P. R. China
1
These authors contributed equally to this work.
* Corresponding author E-mail: [email protected] (Prof. Li), E-mail: [email protected] (Prof. Yang), E-mail: [email protected] (Prof. Pan)
Abstract The rate capability of an anode material is limited by slow Li-ion diffusion dynamics within the bulk, which leads to slow charging rates and low power densities in Li-ion batteries. To address this issue, we fabricated novel 3D hierarchical porous oxide arrays composed of mesoporous Co3O4 nanosheets grown on a thin layer of reduced graphene oxide “skin” as a stable buffering and conducting layer. The porous oxide-graphene hybrid anode shows the outstanding rate capability (~1400 mAh g-1 at 2.0 A g-1) among all oxide anode materials, high cycling stability and high retention of 84.5% over 200 cycles. Besides, the anode material in sodium-ion batteries also delivers a high capacity of 757 mAh g-1 and a high retention of 89.7% over 400 cycles. The high electrochemical performances are mainly due to mesoporous Co3O4 nanosheets provide abundant accessible sites for electrolyte diffusion and intercalation of Li+/Na+ ions into the active phases while the graphene coating dramatically boosts the overall Li/Na storage performances of the Co3O4 nanosheets by enhancing the binding to and electrical contact with the current collector. Kinetic analysis reveals that the rational integration of battery-type and capacitor-type electrochemical energy storage in the same anode material (the distinct 3D hierarchical porous structure) enables to offer excellent rate capability. This novel graphene-metal oxide nanopore array could be applied in high-performance Li-ion and Na-ion batteries. Keywords: cobalt oxide, nanopore arrays, Li-ion batteries, Na-ion batteries, excellent performance
1. Introduction Li ion batteries (LIBs) have higher energy storage densities compared with high-power-density supercapacitors [1-3]. However, the Li-ion energy storage in an anode material is limited by slow diffusion dynamics within the bulk, leading to poorer rate capacities, lower power densities and slower charge/discharge rates than those of supercapacitors [4]. Recently, sodium ion batteries (SIBs) own the potential for serving as an alternative to LIBs due to the wide availability and low cost of Na mineral salts. However, SIBs encounter much slower diffusion dynamics owing to the larger ion radius [5-12]. To alleviate technological challenges in the adoption of electric vehicles, grid-scale batteries, and power-intensive devices, high-rate transition metal oxide (TMO) anode materials such as Li4Ti5O12 [13] and niobium tungsten oxides [14] have been sought, but their rapid charge/discharge ability compromises on low specific capacities at high rates (for Li4Ti5O12, 148 mAh g-1 at 20 C (~3.4 A g-1); for niobium tungsten oxides, 128 mAh g-1 at 20 C (~3.0 A g-1). To overcome the low-capacity limitation of high-rate TMOs, highly electrochemically active TMO anode materials have been widely studied owing to their much higher theoretical specific capacities (e.g. Co3O4, 890 mAh g-1) [3]. However, TMO-based high-activity anode materials usually suffer from fast capacity fading at high rates and after more cycling times owing to large volume changes and poor reaction kinetics during intercalation/deintercalation [15]. Similar issues also exist in other high-capacity anode materials such as Si [16]. To address the rate and cycling problems in high-activity anode materials, various nanostructures of Co3O4 have been developed, such as nanowires [17], nanosheets [18,19], nanoflowers [20], and porous nanospheres [21]. In addition to these, nanoarrays of nanopores, nanowires or nanosheets directly grown on current collectors have drawn special attention [22, 23], because they have a higher ability to buffer large volume changes, a faster ion diffusion pathway along the gaps within the arrays, and no need of non-active binding agents [22]. However, the arrays could be exfoliated from the current collector after continued cycling owing to the lack of stable binding to the current collector, leading to rapid capacity fading during charge/discharge [23]. In this work, we report high-rate and high-capacity energy storage in a novel 3D porous hierarchical Co3O4 anode structure. The hierarchical hybrid anode was composed of mesoporous Co3O4 nanosheet enclosed nanopore arrays (NPA) grown on surface-treated Ni foam (NF), whose internal surface of large pores was coated with a thin layer “skin” of reduced graphene oxide (rGO) as a stable buffering and conducting layer [24-26]. We found that the 3D porous composite mainly exhibits the electrochemical characteristics of the
Co3O4 component while the graphene coating dramatically boosts the overall Li storage performances of the NPA by enhancing the binding to and electrical contact with the current collector. The composite anode offers the outstanding rate capacities: the discharge specific capacities of 1490, 1475, 1463, 1428 and 1399 mAh g-1 at 0.1, 0.5, 1.0, 1.5 and 2.0 A g-1, respectively (there is only a 6% capacity loss after the current density is increased by 20 times). The charge/discharge curves show long intercalation/deintercalation plateaus characteristic of Li ion storage, while kinetic analysis further reveals an unusual ultrafast surface capacitive characteristic. This hybrid energy storage mechanism could bridge the gap between LIBs/SIBs and supercapacitors, and combine their advantages in the same anode material, such as high energy density, high rate, and high cycling stability, to meet the crucial requirements in developing electric vehicles and advanced energy-storage systems.
2. Experimental Section 2.1 Materials All materials and chemicals were purchased commercially and used as received. Ni foam was purchased from Shanghai XiaoYuan Company. Other materials were purchased from Aladdin. 2.2 Synthesis of rGO/Ni foam GO was synthesized from graphite powder based on the modified hummer’s method [26]. Ni foam was cleaned by hydrochloric acid, acetone, and ethyl alcohol, and then immersed into 2.5 mg ml-1 GO suspension for several minutes. Then the GO loaded Ni foam (GO/Ni) was dried at 60 was annealed in Ar at 600
(2
for 30 min. Finally, GO/Ni foam
min-1) for 2 h to synthesize rGO/Ni foam (rGO/NF). The loading mass of
the rGO was about 0.48 mg cm-2. 2.3 Synthesis of porous Co3O4 NPA/Ni foam Electrochemical deposition of Co(OH)2 NPA on the Ni foam was performed in a three-electrode system composed of saturated calomel electrode (SCE) as the reference electrode and a Pt wire as the counter electrode. The deposition was operated in 0.1 M Co(NO3)2 aqueous solution at the potential between -1.2 to 1.4 V (vs. SCE) at a scan rate of 50 mV s-1 for 2 times. After deposition, the electrode was dried at 60
for 30
min. Co3O4 NPA/Ni foam (Co3O4 NPA/NF) was finally gained by annealing Co(OH)2 NPA/Ni foam at 400 for 2 h (2
min-1). The loading mass of the Co3O4 NPA was about 0.71 mg cm-2.
2.4 Synthesis of porous Co3O4 NPA/rGO/NF The preparation of Co3O4 NPA/rGO/NF was the same as synthesis of porous Co3O4 NPA/NF except
substituting GO/NF for Ni foam and annealing at 600
. The loading mass of the active materials was about
1.23 mg cm-2.
2.5 Electrochemical Measurements The Li/Na-storage performance of Co3O4 NPA/rGO/NF was analyzed in a half cell configuration using lithium/Sodium metal foil as the anode. The electrodes without any further treatment were directly assembled into CR2032-type coin cells in an Ar filled glove-box. The counter electrode, separator, and electrolyte are Li/Na foil, Celgard 2400/glass microfiber, 1 M LiPF6 [EC (ethylene carbonate) and DMC (dimethyl carbonate)]/NaPF6 [EC, PC (propylene carbonate) and FEC (fluoroethylene carbonate)] as the electrolyte. A multichannel battery testing system (LAND CT2001A) was used to measure Galvanostatic charge-discharge performance. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using an Autolab CHI660E. All the measurements were carried out at room temperature. 2.6 Structural and Morphological Characterization X-ray diffraction (XRD) patterns were recorded on a Rigaku 18 KW D/max-2550 with a Cu Kα radiation. Field emission scanning electron microscopy (FESEM) images were collected using a JEOL JEM-2100F with an acceleration voltage of 200 kV. Energy dispersive spectroscopy (EDS) was recorded by using Oxford EDS IE250. XPS analysis was carried out on a Kratos Axis Ultra DLD X-ray photoelectron spectrometer. Raman-shift spectra was recorded on a Renishaw in plus laser Raman spectrometer. The specific surface area of Co3O4 NPA/rGO/NF was calculated by a Brunauer-Emmett-Teller analysis of nitrogen adsorption.
3. Results and discussion
Scheme 1 a) The structural design of Co3O4 NPA/rGO/NF as a superior anode material. b) Illustration of the fabrication procedure for Co3O4 NPA with rGO-coated NF plate.
Scheme 1a shows the structural design of Co3O4 NPA/rGO/NF as a superior anode material, where the high-rate ion diffusion and electron transport pathways are mediated by the structural units of the unique architecture. In detail, reduced graphene oxide sheets (rGO) on the current collector Ni foam (NF) are used as conductive layer constituting electron-transport networks. The hierarchical porous network of Co3O4 nanopore arrays (NPA) with a large internal surface area forms an ultrafast ion transport pathway that mediates the ultrafast ion reaction dynamics. Scheme 1b demonstrates the overall synthetic procedure for the designed anode material. First, the internal surface of the porous NF substrate (Fig. 1a) was uniformly coated with rGO sheets by dip-coating NF in a GO dispersion, followed by thermal reduction at 600 oC. After the surface coating, the yellow NF became black, and a thin-layer graphene “skin” with many wrinkles was uniformly coated on the whole internal surface (Fig. 1b). Next, we prepared Co(OH)2 NPA standing on the graphene “skin” through a two-cycle electrodeposition process in a three-electrode system (Scheme 1b). Finally, we produced Co3O4 NPA on the rGO/NF plate by annealing the Co(OH)2 NPA at 600 oC (Scheme 1b). For comparison, Co3O4 NPA directly grown on the NF plate (Co3O4 NPA/NF) without rGO coating were prepared as previously reported [27], and we also verified the morphology of Co3O4 NPA/NF. (Fig. S1, Supporting information).
Fig. 1. a) SEM image of pristine NF. b) SEM image of rGO/NF. c) SEM image of Co(OH)2 NPA/rGO/NF without annealing. d) High resolution SEM image of Co(OH)2 NPA/rGO/NF without annealing. e) Low resolution SEM image of Co3O4 NPA/rGO/NF. f) High resolution SEM image of Co3O4 NPA/rGO/NF. g) Magnified view of the high-resolution SEM image of Co3O4NPA/rGO/NF. h) Cross-sectional SEM image of Co3O4NPA/rGO/NF. i-l) Co, O, C and overlapping EDS maps of Co3O4NPA/rGO/NF. The SEM image in Fig. 1c shows that a large area of homogeneous Co(OH)2 NPA on the rGO/NF was produced for further thermal treatment to prepare Co3O4 NPA/rGO/NF. The magnified view (Fig. 1d) displays that the Co(OH)2 NPA was composed of irregular large pores (pore diameter: 100 ~ 300 nm), on whose interior surface no smaller mesopores were observed. By annealing at 600 oC, the Co(OH)2 NPA on the rGO/NF was transformed into large-area Co3O4 NPA/rGO/NF with retained large pores of the same sizes as those before the treatment (Fig. 1e). The magnified view (Fig. 1f, 1g) further shows that the extended Co3O4
nanopore arrays were composed of mesoporous Co3O4 nanosheets with pore diameters smaller than 50 nm, thus forming a 3D hierarchical pore network interlinked within the oxide anode. Besides, a cross-sectional scanning electron microscopy (SEM) image of the prepared Co3O4 film presents an optimized Co3O4 film with a thickness of around 400 nm and a thin layer of rGO skin on the NF (Fig. 1h). We tried to increase the Co3O4 film thickness by increasing the electrodeposition cycling number, but the pores were blocked after 4 cycles, which instead influences the electrochemical performance (Fig. S2, S3, Supporting information). Energy dispersive spectroscopy (EDS) mapping presents the O and Co element distribution in the Co3O4 NPA as well as the C element in the underneath graphene layer (Fig. 1i-k). As designed in the Co3O4 NPA/rGO/NF (Scheme 1a), the formed two structural units, the 3D hierarchical pore network of the active Co3O4 film and the thin rGO skin as the conducting and buffering layer, can play a crucial and synergetic role in boosting the ultrafast ion reaction dynamics. X-ray diffraction (XRD) measurements were performed to characterize the samples of Co3O4 NPA/rGO/NF and Ni foam, as shown in Fig. S4a. Except for the three strong metal Ni peaks at 44.4°, 51.8°, and 76.3°, diffraction peaks at 29.2°, 37.2°, 59.7°, and 63.9° are indexed to Co3O4 with a face-center cubic phase, corresponding to (220), (311), (511), and (440), respectively (JCPDS 42-1467). The weak and broad (002) peak of rGO cannot be observed in the composite sample. Raman spectra were employed to further verify the Co3O4 and rGO compositions in the sample (Fig. S4b, Supporting information). The Co3O4 Raman peaks are located at 675, 500, 430 and 180 cm-1, corresponding to A1g, F2g, Eg and E2g phonon modes, respectively [28]. The D and G bands of rGO were also observed, confirming the existence of the rGO coating. To determine the valence states of the elements in the porous Co3O4 NPA/rGO/NF, the X-ray photoelectron spectroscopy (XPS) was employed. As depicted in Fig. S5a, the XPS spectrum shows the presence of Co, C, O and Ni elements. In the Co 2p spectrum (Fig. S5b, Supporting information), 2p3/2 and 2p1/2 signals are located at around 780 and 795 eV, respectively [28]. The fitted peaks of Co 2p are ascribed to the two oxidation states of Co3+ (779.5 eV) and Co2+ (780.8 eV), confirming the presence of Co3O4 [29]. The C 1s spectrum of Co3O4 NPA/rGO/NF (Fig. S5c, Supporting Information) can contain four dominant peaks at 284.8, 285.7, 288.3, and 290.7 eV, corresponding to conjugated C-C/C=C, C-O, C=O, and π-π* bonds, respectively [2, 17, 37]. The presence of Co3O4 is further confirmed by the O 1s signal (Fig. S5d, Supporting information), which is fitted to four peaks named as O1 to O4. The O1 peak at 529 eV is ascribed to the cobalt-oxygen bonds, and the highest-binding energy peak O4 at about 532 eV can be attributed to oxygen vacancies on the surface of Co3O4. Other signals O2 and O3 can be from remaining oxygen-containing groups (such as -OH,
-COOH) in reduced GO. According to previous report [29], the presence of oxygen vacancies also can enhance the conductivity of Co3O4. The nitrogen adsorption-desorption isotherms were performed to investigate the specific surface area and the porous structure characteristics of Co3O4 NPA/rGO/NF. The isotherms show a typical type IV isotherm with a distinct hysteresis loop, indicating the existence of mesoporous structure [3]. The specific surface area of Co3O4 NPA/rGO/NF was estimated to be 42.6 m2 g-1. Moreover, the pore size distributions of Co3O4 NPA/rGO/NF obtained using the Barrett-Joyner-Halenda (BJH) method (inset of Fig. S6) show pore distribution is mainly in the 6-12 nm. Such a mesoporous structure with a high specific surface area allows for the volume expansion of Co3O4 without mechanical constraints, benefiting fast diffusion of ions during the discharge/charge process. The electrochemical performance of the Co3O4 NPA/rGO/NF electrode was first evaluated in a Li ion half-cell. Fig. 2a shows the cyclic voltammogram (CV) curves of Co3O4 NPA/rGO/NF measured at 0.1 mV s-1 from 0 to 3.0 V for the first three cycles. The first discharge cycle presents an irreversible cathodic peak, which corresponds to the lithiation reaction of Co3O4 and irreversible decomposition of electrolyte because of the formation of SEI film (eq.1). The slope below 0.5 V is ascribed to the formation of a solid electrolyte interphase (SEI) film. An anodic peak is observed at 2.13 V, which is ascribed to the delithiation reaction (eq.2). Between the 2nd and 3rd cycles, the overlapping of curves indicated excellent reversibility and the main cathodic peaks are shifted to 1.2 V owing to the polarization of the electrode during the 1st cycle, which corresponding to CoO to Co (eq.3) [30]. Co3O4+8Li++8e- →3Co+4Li2O (1) Co+Li2O→CoO+2Li++2e-
(2)
CoO+2Li++2e-→Co+Li2O
(3)
Fig. 2b shows the charge and discharge curves of Co3O4 NPA/rGO/NF and its references including Co3O4 NPA/NF and rGO/NF. During the first cycle, the Co3O4 NPA/rGO/NF electrode delivers a high discharging capacity of 1500 mAh g-1 and a high charging capacity of 1412 mAh g-1 at 0.1 A g-1, with a Coulombic efficiency of 94%. In contrast, for rGO/NF, the discharging and charging specific capacities are only 810 and 790 mAh g-1 respectively, and for Co3O4 NPA/NF without the rGO layer, the capacities are lower (567 and 556 mAh g-1). The charge/discharge curve comparison between the three samples suggests that the high electrochemical capacity of the Co3O4/graphene composite is largely ascribed to the contribution of the Co3O4 component, because there are a long charging plateaus at around 1.2 V and a long discharging plateaus at
around 2.0 V characteristic of the Co3O4 active component [31]. The rate capacity of Co3O4 NPA/rGO/NF was further evaluated. Fig. 2c shows the charge and discharge curves of Co3O4 NPA/rGO/NF at the current densities changed from 0.1 to 2.0 A g-1. The discharge specific capacity at 0.1, 0.2, 0.5, 1.0, 1.5 and 2.0 A g-1 is 1490, 1480, 1475, 1463, 1428 and 1399 mAh g-1, respectively, and there is only a 6% capacity loss after the current density is increased by 20 times. In Fig. 2d, the rate capacity of the three samples is compared, where the current density was set at 0.1, 0.2, 0.5, 1.0, 1.5, 2.0 A g-1, and then back to 0.1 A g-1. For Co3O4 NPA/rGO/NF, the rate capacity is maintained around 1400 mAh g-1, though the current density is varied greatly, demonstrating excellent rate capacity performance. In contrast, Co3O4 NPA/NF and rGO/NF deliver rather lower rate capacity. Additionally, as shown in Fig. S7, higher current densities above 2.0 A g-1 were further evaluated. It is worth noting that Co3O4 NPA/rGO/NF still displayed a specific capacity of 400 mAh g-1 at 20 A g-1, which further indicting the high-rate energy storage of Co3O4 NPA/rGO/NF. The capacity contribution of rGO at different rates was added in Fig. S7. When compared with previously reported other oxide metal anode materials, including Co3O4 Nanowire arrays [32], CoO/graphene sheets [33], TiO2-C/MnO2 Nanowire arrays [34], Co3O4 nanosheets/Ni foam [27], Co3O4/carbon fiber [35], Co3O4 nanopaiticle/rGO [36], CoO hollow cube/rGO [37], SnO2 nanosheets [38] and NiO/rGO [39], the Co3O4 NPA/rGO/NF electrode exhibits much higher capacities at different rates (Fig. 2e). For Co3O4 nanosheets/Ni foam with similar active oxide composition (Co3O4) and similar current collector (Ni foam) to the sample Co3O4 NPA/rGO/NF reported here, for example, the specific capacity of the former at 1800 mA g-1 was only 450 mAh g-1 [27], while that of the latter at 2000 mA g-1 is increased by more than 3 times (1399 mAh g-1). This sharp contrast indicates that the design of the 3D hierarchical pore network and the rGO conducting and buffering layer in oxide-based anodes is of crucial importance for greatly improving their rate capacity and cycling stability.
Fig. 2 a) First three cyclic voltammogram curves of the Co3O4 NPA/rGO/NF electrode for Li-ion batteries at a scan rate of 0.1 mV s −1 in the range of 0.01−3.00 V. b) Galvanostatic discharge/charge curves of the Co3O4 NPA/rGO/NF, rGO/Ni foam and Co3O4 NPA/NF for the first cycle at 0.1 A g-1. c) Galvanostatic discharge/charge curves of the Co3O4 NPA/rGO/NF at rates of 0.1 to 2.0 A g-1. d) Rate capability of the rGO/NF, Co3O4 NPA/NF and Co3O4 NPA /rGO/NF between 0.1 A g-1 and 2.0 A g-1. e) Comparison of the rate capability of Co3O4 NPA/rGO/NF and related oxide anode materials for LIBs. (Ref.32: Co3O4 nanowire arrays; Ref.33: CoO/graphene sheets; Ref.34: TiO2-C/MnO2 Nanowire arrays; Ref.27: Co3O4 nanosheets/Ni foam; Ref.35: Co3O4/carbon fiber; Ref.36: Co3O4 nanopaiticle/rGO; Ref.37: CoO hollow cube/rGO; Ref.38: SnO2 nanosheets; Ref.39: NiO/rGO). f) Galvanostatic discharge/charge curves of the Co3O4 NPA/rGO/NF at 1.0 A g-1 for 200 cycles. g) Cycle stability of the Co3O4 NPA/rGO/NF at 1.0 A g-1 for 200 cycles. The cycling stability of Co3O4 NPA/rGO/NF was also tested at 1.0 and 5.0 A g-1. At a lower current density of 1.0 A, the discharge capacity drops slowly to 1198 mAh g-1 after 200 cycles (Fig. 2f), and a high retention of 84.5% for the 200-cycle test (Fig. 2g). The capacity of Co3O4 NPA/rGO/NF delivers about 700 mAh g-1
even at a high rate of 5.0 A g-1 and retains nearly 80 % after 100 cycles (Fig. S8, Supporting information). Furthermore, the array construction of the Co3O4 NPA/rGO/NF after 200 cycles can still be retained (Fig. S9, Supporting information), indicating the good structural stability. The high cycling stability of the Co3O4 NPA/rGO/NF electrode is ascribed to the 3D hierarchical pore network to buffer the large volume changes as well as the rGO buffering layer in oxide-based anodes to keep good electrical contanct with the current collector during fast discharge and charge processes.
Fig. 3 a) CV curves of Co3O4 NPA/rGO/NF at different scan rates from 0.1 to 1.0 mV s-1. b) b values plotted against battery potential of Co3O4 NPA/rGO/NF for oxidation scans. The inset is the current response plotted against scan rates of Co3O4 NPA/rGO/NF at various potentials. c) Capacitive contribution to the oxidation process at the scan rates of 1.0 mV s-1 marked by the shaded region. d) Capacitive contributions of Co3O4 NPA/rGO/NF at various scan rates (0.1, 0.2, 0.3, 0.7, 1.0 mV s-1). To understand the outstanding electrochemical performance of Co3O4 NPA/rGO/NF, the CV behavior was performed by a kinetic analysis. Fig. 3a shows the CV curves tested at different scan rates from 0.1 to 1.0 mV s-1, displaying gradual broadening of peaks with increasing scan rate. The total storage process can be described as two components, including the process of surface-induced (or non-faradic) capacitance and the diffusion-controlled (or faradaic) intercalation/deintercalation process. According to the equation (1) [40],
Where i is current response, v is potential, b and a are both variable parameters. b was determined by the slope of log i versus log v. As b ≈ 0.5, the current response is under controlled by a diffusion mechanism; as b
≈ 1, capacitive processes control the current response. As depicted in Fig. 3b, the fitting lines of log (i) and log (v) with different potentials of the oxidation processes are plotted. A high surface-induced capacitive contribution for the Co3O4 NPA/rGO/NF electrode can be displayed by a series of high b values (0.80, 0.98). The total capacitive contribution at different scan rates can be further determined by differentiating the fraction of capacitor-like (k1v) and diffusion-controlled (k2v1/2) currents according to the equation (2) [41]: V
/
As revealed in Fig. 3c, capacitive contribution at scan rate of 1 mV s-1 leads to nearly 86% fraction of the total current in the oxidation process, and the capacitive contribution is rising with increasing scan rates (Fig. 3d). Due to the capacitive effect, a large number of Li ions tend to be reserved on the interior surface of the electrode, avoiding Li-insertion-induced structural collapse and thus generating the excellent rate performance.
Fig. 4 a) Nyquist plots of Co3O4 NPA/rGO/NF and Co3O4 NPA/NF. b) Equivalent circuit model to fit the Nyquist plots. c) Values of Re and Rct acquired by fitting data. Electrochemical impedance spectroscopy (EIS) was also used to investigate the electrochemical reaction dynamics of Co3O4 NPA/rGO/NF and the reference sample Co3O4 NPA/NF. Their Nyquist plots are composed of a single depressed semicircle in the high-medium frequency region and an inclined line at low frequency (Fig. 4a). The intercept of the semicircle at the z' axis reflects the resistance of electrolyte diffusion (Re), and the diameter Rct corresponds to the charge-transfer resistance within Co3O4 NPA. The Re and Rct parameters
are determined by fitting the Nyquist plots with the equivalent circuit model (Fig. 4b). Rct is much lower for the Co3O4 NPA/rGO/NF electrode (47 Ω) compared to the Co3O4 NPA/NF (151 Ω) (Fig. 4c), indicating that Co3O4 NPA/rGO/NF has faster charge-transfer dynamics. It is also noted that the low-frequency tail of Co3O4 NPA/rGO/NF is nearly perpendicular to the z' axis (86° of the slope angle), extremely similar to the low-frequency response of complete electric double layer supercapacitors [42, 43], while the low-frequency tail of Co3O4 NPA/NF inclines at only 68° (Fig. 4a). This contrast not only reflects the higher lithium ion conductivity, faster charge-transfer dynamics, and the more capacitive feature of Co3O4 NPA/NF, but displays the characteristic of pseudo-capacitance,as verified CV of the Co3O4 NPA/rGO/NF electrode for supercapacitor (Fig. S10, Supporting information). The small charge-transfer resistance and nearly perpendicular low-frequency tail consistently point at the extraordinary electrochemical characteristics of the unique 3D porous hierarchical anode structure, including a high electrical conductivity associated with a fast graphene-mediated electronic pathway from the Ni current collector to the Co3O4 active layer, and a high surface-mediated Li ion conductivity closely related to the hierarchical porous structure. Because of these characteristics, the Co3O4 NPA/rGO/NF exhibits high-rate performance. For example, Wu et.al. reported the high capacity and rate capability of mesoporous Co3O4 nanowire arrays as anodes in Li-ion batteries, with a 60% capacity retained when the rate was increased by 20 times and a specific capacity of 240 mAh g-1 at a rate of 5500 mA g−1 after 20 cycles [32]. In contrast, for our Co3O4 NPA/rGO/NF, the retained capacity was as high as 94% after the current density was increased by 20 times, and the specific capacity is 550 mAh g-1 at a rate of 5000 mA g−1 after 100 cycles. Although they have similar mesoporous structures and the same Co3O4 composition, the coating of the current collectors with a soft graphene skin may play a crucial role in further improving the rate capability by stabilizing the interface between the mesoporous layer and the current collectors, increasing the electrical transport of the anodes, and buffering the large volume changes during fast discharge and charge processes.
Fig. 5 a) Rate capability of the Co3O4 NPA/rGO/NF as an anode in SIBs. b) Cyclic stability of the Co3O4 NPA/rGO/NF at 0.1 A g-1 for 400 cycles. c) Comparison of the rate capability of Co3O4 NPA/rGO/NF and related Co3O4 anode materials for SIBs (Ref.43: Mesoporous Co3O4 sheets/ 3D graphene; Ref.44: Co3O4; Ref.45: Co3O4@N-doped carbon; Ref.46: Co3O4 spheres/ carbon tubes). d) Capacitive contribution to the oxidation process at the scan rates of 1.0 mV s-1 marked by the shaded region. e) relationship of the scan rate used to evaluate the contribution of the capacitive reaction. The electrochemical energy storage performance of the Co3O4 NPA/rGO/NF electrode was also investigated for SIBs. The CV behavior of Co3O4 NPA/rGO/NF anode was recorded, as shown in Fig. S11a. The rate capability of the Co3O4 NPA/rGO/NF electrode was tested between 0.1 and 2.0 A g-1, which presents average capacity of 760, 700, 647, 513, 435 and 397 mAh g-1 at different rates (Fig. 5a). A high reversible capacity of 700 mAh g-1 at the current density of 0.1 A g-1 and a high retention of 89.7% after 400 cycles (Fig. 5b). Notably, the lower specific capacities for SIBs than for LIBs are expected due to the larger ion radius of Na than that of Li [5, 6, 12]. However, compared with other Co3O4 electrodes [44-47], such as Mesoporous Co3O4 sheets/ 3D graphene [44], Co3O4 [45], Co3O4@N-doped carbon [46], Co3O4/carbon tubes [47]. the Co3O4 NPA/rGO/NF anode material shows higher capacity and rate capability (Fig. 5c). To understand the outstanding rate performance of the Co3O4 NPA/rGO/NF anode in SIBs, its capacitive behavior was characterized by CV tests (Fig. S11b, Supporting information). As shown in Fig. S11c, a series of b values at different voltages than 0.6 are obtained, suggesting a capacitive contribution for the Co3O4
NPA/rGO/NF electrode for SIBs. Moreover, based on CV curves, the proportion of non-faradaic reaction shares nearly 83% during the electrochemical process at a scan rate of 1.0 mV s-1 (Fig. 5d). The corresponding non-faradaic attribution can also be calculated based on CV curves from 0.1 to 2.0 mV s-1(Fig. 5e). The high capacitive contribution is mainly because of the unique structure of Co3O4 NPA/rGO/NF electrode, i.e. The Co3O4 nanosheet porous arrays with the hierarchical mesoporous structure, which not only provide abundant electrochemically active sites and but also accelerate e-/ion transfer and shorten ion diffusion length, contributing to achieve high cycling stability and rate capability. Based on the above systematical studies on the electrochemical energy storage properties of the unique Co3O4 NPA/rGO/NF anode structure for both LIBs and SIBs, we can attribute the ultrahigh electrochemical performances, including the high rate capability and long cycling stability, to the several factors: (1) The Co3O4 nanosheet porous arrays with the hierarchical macroporous and mesoporous structure are beneficial to the transport of ions and could provide abundant electrochemically active sites for ion storage (Scheme 1a). (2) As a stable buffering and conducting layer, the rGO skin can enhance the binding of the NPA to and keep electrical contact with the current collector, thus ensuring the stability of the Co3O4 NPA/rGO/NF electrode during the rapid de/lithiation or de/sodiation. (3) The integrated units of vertically oriented Co3O4 nanosheets and graphene skin can strengthen the structural stability and prevent aggregating of the active materials during cycling, which also reduces side reactions in absence of conductive agent and binder. (4) The great capacitive effect also enhances the electrochemical performance of the Co3O4 NPA/rGO/NF in terms of an ultrahigh rate capability and long cycling life.
4. Conclusions In summary, we have developed a novel 3D porous hierarchical anode structure that combines the high energy storage densities and high power densities for LIBs and SIBs. To demonstrate the capacitive Li/Na ion storage conception in active oxide materials, we fabricated Co3O4 nanopore arrays by annealing Co(OH)2 nanosheet arrays. Upon annealing at 600 oC, an extended array of Co3O4 nanopores with pore diameters of 100-300 nm was formed via the interlinkage of ultrathin mesoporous Co3O4 nanosheets with fine pore diameter of <50 nm. To further improve the electronic transport pathway between the Co3O4 nanopore arrays and the Ni foam collector, a stable thin layer of reduced graphene oxide “skin” was uniformly deposited on the surface of macropores within the Ni foam. The composite electrode not only exhibits excellent rate capability with only a 6% loss (1490 to 1399 mAh g-1 at 0.1 to 2.0 A g-1), enhanced cycling stability, and high
retention rate (84.5% over 200 cycles at 1.0 A g-1) for Li ion batteries, but also displays an impressive capacity of 757 mAh g-1 at 100 mA g-1 after 400 cycles for Na ion batteries. Both CV and ESI measurements show that ion intercalation/deintercalation processes are dominated by an ultrafast supercapacitor-like storage behavior ascribed to the 3D porous Li-ion transport network and the graphene-mediated electronic transport pathway. This novel porous material could be employed in Li-ion batteries, sodium ion batteries, supercapacitors, and their hybrid devices called lithium-ion capacitors, which combine the advantages of both Li-ion batteries and supercapacitors.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 11774216) and the Science and Technology Commission of Shanghai Municipality (No. 16ZR1412100).
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1. 3D hierarchical porous oxide arrays composed of mesoporous Co3O4 nanosheets grown on a thin layer of reduced graphene oxide “skin” as a stable buffering and conducting layer. 2. The 3D porous composite mainly exhibits the graphene coating dramatically boosts the overall Li storage performances of the NPA by enhancing the binding to and electrical contact with the current collector. 3. The porous oxide-graphene hybrid anode shows the highest rate capability (~1400 mAh g-1 at 2.0 A g-1) among all oxide anode materials, high cycling stability over 200 cycles, and approximately 100% Coulombic efficiency. 4. The anode material in sodium-ion batteries also delivers a high capacity of 757 mAh g-1 with nearly 100% Coulombic efficiency over 400 cycles.
Conflict of Interest The authors declare no conflict of interest.
S0925-8388(19)34542-6
DOI:
https://doi.org/10.1016/j.jallcom.2019.153296
Reference:
JALCOM 153296
To appear in:
Journal of Alloys and Compounds
Received Date: 21 May 2019 Revised Date:
14 November 2019
Accepted Date: 5 December 2019
Please cite this article as: K. Wu, B. Geng, C. Zhang, W. Shen, D. Yang, Z. Li, Z. Yang, D. Pan, Hierarchical porous arrays of mesoporous Co3O4 nanosheets grown on graphene skin for high-rate and high-capacity energy storage, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2019.153296. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contributions Section Dengyu Pan, Zhen Li, and Zuobao Yang supervised the project, conceived and designed all the experiments. Kuan Wu and Bijiang Geng realized the material synthesis. Chen Zhang and Wenwen Shen performed experiments. All authors discussed the results and analyzed the data.
Hierarchical porous arrays of mesoporous Co3O4 nanosheets grown on graphene skin for high-rate and high-capacity energy storage Kuan Wu a,1, Bijiang Geng b,1, Chen Zhang b,1, Wenwen Shen b,1, Dewen Yang b, Zhen Li a,*, Zuobao Yang c,*, and Dengyu Pan b,* a
Shanghai Applied Radiation Institute, School of Environmental and Chemical Engineering,
Shanghai University, Shanghai 200444 P. R. China b
Department of Chemical Engineering, School of Environmental and Chemical Engineering,
Shanghai University, Shanghai 200444 P. R. China c
Institute of Materials, Ningbo University of Technology Ningbo, 315016 P. R. China
1
These authors contributed equally to this work.
* Corresponding author E-mail: [email protected] (Prof. Li), E-mail: [email protected] (Prof. Yang), E-mail: [email protected] (Prof. Pan)
Abstract The rate capability of an anode material is limited by slow Li-ion diffusion dynamics within the bulk, which leads to slow charging rates and low power densities in Li-ion batteries. To address this issue, we fabricated novel 3D hierarchical porous oxide arrays composed of mesoporous Co3O4 nanosheets grown on a thin layer of reduced graphene oxide “skin” as a stable buffering and conducting layer. The porous oxide-graphene hybrid anode shows the outstanding rate capability (~1400 mAh g-1 at 2.0 A g-1) among all oxide anode materials, high cycling stability and high retention of 84.5% over 200 cycles. Besides, the anode material in sodium-ion batteries also delivers a high capacity of 757 mAh g-1 and a high retention of 89.7% over 400 cycles. The high electrochemical performances are mainly due to mesoporous Co3O4 nanosheets provide abundant accessible sites for electrolyte diffusion and intercalation of Li+/Na+ ions into the active phases while the graphene coating dramatically boosts the overall Li/Na storage performances of the Co3O4 nanosheets by enhancing the binding to and electrical contact with the current collector. Kinetic analysis reveals that the rational integration of battery-type and capacitor-type electrochemical energy storage in the same anode material (the distinct 3D hierarchical porous structure) enables to offer excellent rate capability. This novel graphene-metal oxide nanopore array could be applied in high-performance Li-ion and Na-ion batteries. Keywords: cobalt oxide, nanopore arrays, Li-ion batteries, Na-ion batteries, excellent performance
1. Introduction Li ion batteries (LIBs) have higher energy storage densities compared with high-power-density supercapacitors [1-3]. However, the Li-ion energy storage in an anode material is limited by slow diffusion dynamics within the bulk, leading to poorer rate capacities, lower power densities and slower charge/discharge rates than those of supercapacitors [4]. Recently, sodium ion batteries (SIBs) own the potential for serving as an alternative to LIBs due to the wide availability and low cost of Na mineral salts. However, SIBs encounter much slower diffusion dynamics owing to the larger ion radius [5-12]. To alleviate technological challenges in the adoption of electric vehicles, grid-scale batteries, and power-intensive devices, high-rate transition metal oxide (TMO) anode materials such as Li4Ti5O12 [13] and niobium tungsten oxides [14] have been sought, but their rapid charge/discharge ability compromises on low specific capacities at high rates (for Li4Ti5O12, 148 mAh g-1 at 20 C (~3.4 A g-1); for niobium tungsten oxides, 128 mAh g-1 at 20 C (~3.0 A g-1). To overcome the low-capacity limitation of high-rate TMOs, highly electrochemically active TMO anode materials have been widely studied owing to their much higher theoretical specific capacities (e.g. Co3O4, 890 mAh g-1) [3]. However, TMO-based high-activity anode materials usually suffer from fast capacity fading at high rates and after more cycling times owing to large volume changes and poor reaction kinetics during intercalation/deintercalation [15]. Similar issues also exist in other high-capacity anode materials such as Si [16]. To address the rate and cycling problems in high-activity anode materials, various nanostructures of Co3O4 have been developed, such as nanowires [17], nanosheets [18,19], nanoflowers [20], and porous nanospheres [21]. In addition to these, nanoarrays of nanopores, nanowires or nanosheets directly grown on current collectors have drawn special attention [22, 23], because they have a higher ability to buffer large volume changes, a faster ion diffusion pathway along the gaps within the arrays, and no need of non-active binding agents [22]. However, the arrays could be exfoliated from the current collector after continued cycling owing to the lack of stable binding to the current collector, leading to rapid capacity fading during charge/discharge [23]. In this work, we report high-rate and high-capacity energy storage in a novel 3D porous hierarchical Co3O4 anode structure. The hierarchical hybrid anode was composed of mesoporous Co3O4 nanosheet enclosed nanopore arrays (NPA) grown on surface-treated Ni foam (NF), whose internal surface of large pores was coated with a thin layer “skin” of reduced graphene oxide (rGO) as a stable buffering and conducting layer [24-26]. We found that the 3D porous composite mainly exhibits the electrochemical characteristics of the
Co3O4 component while the graphene coating dramatically boosts the overall Li storage performances of the NPA by enhancing the binding to and electrical contact with the current collector. The composite anode offers the outstanding rate capacities: the discharge specific capacities of 1490, 1475, 1463, 1428 and 1399 mAh g-1 at 0.1, 0.5, 1.0, 1.5 and 2.0 A g-1, respectively (there is only a 6% capacity loss after the current density is increased by 20 times). The charge/discharge curves show long intercalation/deintercalation plateaus characteristic of Li ion storage, while kinetic analysis further reveals an unusual ultrafast surface capacitive characteristic. This hybrid energy storage mechanism could bridge the gap between LIBs/SIBs and supercapacitors, and combine their advantages in the same anode material, such as high energy density, high rate, and high cycling stability, to meet the crucial requirements in developing electric vehicles and advanced energy-storage systems.
2. Experimental Section 2.1 Materials All materials and chemicals were purchased commercially and used as received. Ni foam was purchased from Shanghai XiaoYuan Company. Other materials were purchased from Aladdin. 2.2 Synthesis of rGO/Ni foam GO was synthesized from graphite powder based on the modified hummer’s method [26]. Ni foam was cleaned by hydrochloric acid, acetone, and ethyl alcohol, and then immersed into 2.5 mg ml-1 GO suspension for several minutes. Then the GO loaded Ni foam (GO/Ni) was dried at 60 was annealed in Ar at 600
(2
for 30 min. Finally, GO/Ni foam
min-1) for 2 h to synthesize rGO/Ni foam (rGO/NF). The loading mass of
the rGO was about 0.48 mg cm-2. 2.3 Synthesis of porous Co3O4 NPA/Ni foam Electrochemical deposition of Co(OH)2 NPA on the Ni foam was performed in a three-electrode system composed of saturated calomel electrode (SCE) as the reference electrode and a Pt wire as the counter electrode. The deposition was operated in 0.1 M Co(NO3)2 aqueous solution at the potential between -1.2 to 1.4 V (vs. SCE) at a scan rate of 50 mV s-1 for 2 times. After deposition, the electrode was dried at 60
for 30
min. Co3O4 NPA/Ni foam (Co3O4 NPA/NF) was finally gained by annealing Co(OH)2 NPA/Ni foam at 400 for 2 h (2
min-1). The loading mass of the Co3O4 NPA was about 0.71 mg cm-2.
2.4 Synthesis of porous Co3O4 NPA/rGO/NF The preparation of Co3O4 NPA/rGO/NF was the same as synthesis of porous Co3O4 NPA/NF except
substituting GO/NF for Ni foam and annealing at 600
. The loading mass of the active materials was about
1.23 mg cm-2.
2.5 Electrochemical Measurements The Li/Na-storage performance of Co3O4 NPA/rGO/NF was analyzed in a half cell configuration using lithium/Sodium metal foil as the anode. The electrodes without any further treatment were directly assembled into CR2032-type coin cells in an Ar filled glove-box. The counter electrode, separator, and electrolyte are Li/Na foil, Celgard 2400/glass microfiber, 1 M LiPF6 [EC (ethylene carbonate) and DMC (dimethyl carbonate)]/NaPF6 [EC, PC (propylene carbonate) and FEC (fluoroethylene carbonate)] as the electrolyte. A multichannel battery testing system (LAND CT2001A) was used to measure Galvanostatic charge-discharge performance. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using an Autolab CHI660E. All the measurements were carried out at room temperature. 2.6 Structural and Morphological Characterization X-ray diffraction (XRD) patterns were recorded on a Rigaku 18 KW D/max-2550 with a Cu Kα radiation. Field emission scanning electron microscopy (FESEM) images were collected using a JEOL JEM-2100F with an acceleration voltage of 200 kV. Energy dispersive spectroscopy (EDS) was recorded by using Oxford EDS IE250. XPS analysis was carried out on a Kratos Axis Ultra DLD X-ray photoelectron spectrometer. Raman-shift spectra was recorded on a Renishaw in plus laser Raman spectrometer. The specific surface area of Co3O4 NPA/rGO/NF was calculated by a Brunauer-Emmett-Teller analysis of nitrogen adsorption.
3. Results and discussion
Scheme 1 a) The structural design of Co3O4 NPA/rGO/NF as a superior anode material. b) Illustration of the fabrication procedure for Co3O4 NPA with rGO-coated NF plate.
Scheme 1a shows the structural design of Co3O4 NPA/rGO/NF as a superior anode material, where the high-rate ion diffusion and electron transport pathways are mediated by the structural units of the unique architecture. In detail, reduced graphene oxide sheets (rGO) on the current collector Ni foam (NF) are used as conductive layer constituting electron-transport networks. The hierarchical porous network of Co3O4 nanopore arrays (NPA) with a large internal surface area forms an ultrafast ion transport pathway that mediates the ultrafast ion reaction dynamics. Scheme 1b demonstrates the overall synthetic procedure for the designed anode material. First, the internal surface of the porous NF substrate (Fig. 1a) was uniformly coated with rGO sheets by dip-coating NF in a GO dispersion, followed by thermal reduction at 600 oC. After the surface coating, the yellow NF became black, and a thin-layer graphene “skin” with many wrinkles was uniformly coated on the whole internal surface (Fig. 1b). Next, we prepared Co(OH)2 NPA standing on the graphene “skin” through a two-cycle electrodeposition process in a three-electrode system (Scheme 1b). Finally, we produced Co3O4 NPA on the rGO/NF plate by annealing the Co(OH)2 NPA at 600 oC (Scheme 1b). For comparison, Co3O4 NPA directly grown on the NF plate (Co3O4 NPA/NF) without rGO coating were prepared as previously reported [27], and we also verified the morphology of Co3O4 NPA/NF. (Fig. S1, Supporting information).
Fig. 1. a) SEM image of pristine NF. b) SEM image of rGO/NF. c) SEM image of Co(OH)2 NPA/rGO/NF without annealing. d) High resolution SEM image of Co(OH)2 NPA/rGO/NF without annealing. e) Low resolution SEM image of Co3O4 NPA/rGO/NF. f) High resolution SEM image of Co3O4 NPA/rGO/NF. g) Magnified view of the high-resolution SEM image of Co3O4NPA/rGO/NF. h) Cross-sectional SEM image of Co3O4NPA/rGO/NF. i-l) Co, O, C and overlapping EDS maps of Co3O4NPA/rGO/NF. The SEM image in Fig. 1c shows that a large area of homogeneous Co(OH)2 NPA on the rGO/NF was produced for further thermal treatment to prepare Co3O4 NPA/rGO/NF. The magnified view (Fig. 1d) displays that the Co(OH)2 NPA was composed of irregular large pores (pore diameter: 100 ~ 300 nm), on whose interior surface no smaller mesopores were observed. By annealing at 600 oC, the Co(OH)2 NPA on the rGO/NF was transformed into large-area Co3O4 NPA/rGO/NF with retained large pores of the same sizes as those before the treatment (Fig. 1e). The magnified view (Fig. 1f, 1g) further shows that the extended Co3O4
nanopore arrays were composed of mesoporous Co3O4 nanosheets with pore diameters smaller than 50 nm, thus forming a 3D hierarchical pore network interlinked within the oxide anode. Besides, a cross-sectional scanning electron microscopy (SEM) image of the prepared Co3O4 film presents an optimized Co3O4 film with a thickness of around 400 nm and a thin layer of rGO skin on the NF (Fig. 1h). We tried to increase the Co3O4 film thickness by increasing the electrodeposition cycling number, but the pores were blocked after 4 cycles, which instead influences the electrochemical performance (Fig. S2, S3, Supporting information). Energy dispersive spectroscopy (EDS) mapping presents the O and Co element distribution in the Co3O4 NPA as well as the C element in the underneath graphene layer (Fig. 1i-k). As designed in the Co3O4 NPA/rGO/NF (Scheme 1a), the formed two structural units, the 3D hierarchical pore network of the active Co3O4 film and the thin rGO skin as the conducting and buffering layer, can play a crucial and synergetic role in boosting the ultrafast ion reaction dynamics. X-ray diffraction (XRD) measurements were performed to characterize the samples of Co3O4 NPA/rGO/NF and Ni foam, as shown in Fig. S4a. Except for the three strong metal Ni peaks at 44.4°, 51.8°, and 76.3°, diffraction peaks at 29.2°, 37.2°, 59.7°, and 63.9° are indexed to Co3O4 with a face-center cubic phase, corresponding to (220), (311), (511), and (440), respectively (JCPDS 42-1467). The weak and broad (002) peak of rGO cannot be observed in the composite sample. Raman spectra were employed to further verify the Co3O4 and rGO compositions in the sample (Fig. S4b, Supporting information). The Co3O4 Raman peaks are located at 675, 500, 430 and 180 cm-1, corresponding to A1g, F2g, Eg and E2g phonon modes, respectively [28]. The D and G bands of rGO were also observed, confirming the existence of the rGO coating. To determine the valence states of the elements in the porous Co3O4 NPA/rGO/NF, the X-ray photoelectron spectroscopy (XPS) was employed. As depicted in Fig. S5a, the XPS spectrum shows the presence of Co, C, O and Ni elements. In the Co 2p spectrum (Fig. S5b, Supporting information), 2p3/2 and 2p1/2 signals are located at around 780 and 795 eV, respectively [28]. The fitted peaks of Co 2p are ascribed to the two oxidation states of Co3+ (779.5 eV) and Co2+ (780.8 eV), confirming the presence of Co3O4 [29]. The C 1s spectrum of Co3O4 NPA/rGO/NF (Fig. S5c, Supporting Information) can contain four dominant peaks at 284.8, 285.7, 288.3, and 290.7 eV, corresponding to conjugated C-C/C=C, C-O, C=O, and π-π* bonds, respectively [2, 17, 37]. The presence of Co3O4 is further confirmed by the O 1s signal (Fig. S5d, Supporting information), which is fitted to four peaks named as O1 to O4. The O1 peak at 529 eV is ascribed to the cobalt-oxygen bonds, and the highest-binding energy peak O4 at about 532 eV can be attributed to oxygen vacancies on the surface of Co3O4. Other signals O2 and O3 can be from remaining oxygen-containing groups (such as -OH,
-COOH) in reduced GO. According to previous report [29], the presence of oxygen vacancies also can enhance the conductivity of Co3O4. The nitrogen adsorption-desorption isotherms were performed to investigate the specific surface area and the porous structure characteristics of Co3O4 NPA/rGO/NF. The isotherms show a typical type IV isotherm with a distinct hysteresis loop, indicating the existence of mesoporous structure [3]. The specific surface area of Co3O4 NPA/rGO/NF was estimated to be 42.6 m2 g-1. Moreover, the pore size distributions of Co3O4 NPA/rGO/NF obtained using the Barrett-Joyner-Halenda (BJH) method (inset of Fig. S6) show pore distribution is mainly in the 6-12 nm. Such a mesoporous structure with a high specific surface area allows for the volume expansion of Co3O4 without mechanical constraints, benefiting fast diffusion of ions during the discharge/charge process. The electrochemical performance of the Co3O4 NPA/rGO/NF electrode was first evaluated in a Li ion half-cell. Fig. 2a shows the cyclic voltammogram (CV) curves of Co3O4 NPA/rGO/NF measured at 0.1 mV s-1 from 0 to 3.0 V for the first three cycles. The first discharge cycle presents an irreversible cathodic peak, which corresponds to the lithiation reaction of Co3O4 and irreversible decomposition of electrolyte because of the formation of SEI film (eq.1). The slope below 0.5 V is ascribed to the formation of a solid electrolyte interphase (SEI) film. An anodic peak is observed at 2.13 V, which is ascribed to the delithiation reaction (eq.2). Between the 2nd and 3rd cycles, the overlapping of curves indicated excellent reversibility and the main cathodic peaks are shifted to 1.2 V owing to the polarization of the electrode during the 1st cycle, which corresponding to CoO to Co (eq.3) [30]. Co3O4+8Li++8e- →3Co+4Li2O (1) Co+Li2O→CoO+2Li++2e-
(2)
CoO+2Li++2e-→Co+Li2O
(3)
Fig. 2b shows the charge and discharge curves of Co3O4 NPA/rGO/NF and its references including Co3O4 NPA/NF and rGO/NF. During the first cycle, the Co3O4 NPA/rGO/NF electrode delivers a high discharging capacity of 1500 mAh g-1 and a high charging capacity of 1412 mAh g-1 at 0.1 A g-1, with a Coulombic efficiency of 94%. In contrast, for rGO/NF, the discharging and charging specific capacities are only 810 and 790 mAh g-1 respectively, and for Co3O4 NPA/NF without the rGO layer, the capacities are lower (567 and 556 mAh g-1). The charge/discharge curve comparison between the three samples suggests that the high electrochemical capacity of the Co3O4/graphene composite is largely ascribed to the contribution of the Co3O4 component, because there are a long charging plateaus at around 1.2 V and a long discharging plateaus at
around 2.0 V characteristic of the Co3O4 active component [31]. The rate capacity of Co3O4 NPA/rGO/NF was further evaluated. Fig. 2c shows the charge and discharge curves of Co3O4 NPA/rGO/NF at the current densities changed from 0.1 to 2.0 A g-1. The discharge specific capacity at 0.1, 0.2, 0.5, 1.0, 1.5 and 2.0 A g-1 is 1490, 1480, 1475, 1463, 1428 and 1399 mAh g-1, respectively, and there is only a 6% capacity loss after the current density is increased by 20 times. In Fig. 2d, the rate capacity of the three samples is compared, where the current density was set at 0.1, 0.2, 0.5, 1.0, 1.5, 2.0 A g-1, and then back to 0.1 A g-1. For Co3O4 NPA/rGO/NF, the rate capacity is maintained around 1400 mAh g-1, though the current density is varied greatly, demonstrating excellent rate capacity performance. In contrast, Co3O4 NPA/NF and rGO/NF deliver rather lower rate capacity. Additionally, as shown in Fig. S7, higher current densities above 2.0 A g-1 were further evaluated. It is worth noting that Co3O4 NPA/rGO/NF still displayed a specific capacity of 400 mAh g-1 at 20 A g-1, which further indicting the high-rate energy storage of Co3O4 NPA/rGO/NF. The capacity contribution of rGO at different rates was added in Fig. S7. When compared with previously reported other oxide metal anode materials, including Co3O4 Nanowire arrays [32], CoO/graphene sheets [33], TiO2-C/MnO2 Nanowire arrays [34], Co3O4 nanosheets/Ni foam [27], Co3O4/carbon fiber [35], Co3O4 nanopaiticle/rGO [36], CoO hollow cube/rGO [37], SnO2 nanosheets [38] and NiO/rGO [39], the Co3O4 NPA/rGO/NF electrode exhibits much higher capacities at different rates (Fig. 2e). For Co3O4 nanosheets/Ni foam with similar active oxide composition (Co3O4) and similar current collector (Ni foam) to the sample Co3O4 NPA/rGO/NF reported here, for example, the specific capacity of the former at 1800 mA g-1 was only 450 mAh g-1 [27], while that of the latter at 2000 mA g-1 is increased by more than 3 times (1399 mAh g-1). This sharp contrast indicates that the design of the 3D hierarchical pore network and the rGO conducting and buffering layer in oxide-based anodes is of crucial importance for greatly improving their rate capacity and cycling stability.
Fig. 2 a) First three cyclic voltammogram curves of the Co3O4 NPA/rGO/NF electrode for Li-ion batteries at a scan rate of 0.1 mV s −1 in the range of 0.01−3.00 V. b) Galvanostatic discharge/charge curves of the Co3O4 NPA/rGO/NF, rGO/Ni foam and Co3O4 NPA/NF for the first cycle at 0.1 A g-1. c) Galvanostatic discharge/charge curves of the Co3O4 NPA/rGO/NF at rates of 0.1 to 2.0 A g-1. d) Rate capability of the rGO/NF, Co3O4 NPA/NF and Co3O4 NPA /rGO/NF between 0.1 A g-1 and 2.0 A g-1. e) Comparison of the rate capability of Co3O4 NPA/rGO/NF and related oxide anode materials for LIBs. (Ref.32: Co3O4 nanowire arrays; Ref.33: CoO/graphene sheets; Ref.34: TiO2-C/MnO2 Nanowire arrays; Ref.27: Co3O4 nanosheets/Ni foam; Ref.35: Co3O4/carbon fiber; Ref.36: Co3O4 nanopaiticle/rGO; Ref.37: CoO hollow cube/rGO; Ref.38: SnO2 nanosheets; Ref.39: NiO/rGO). f) Galvanostatic discharge/charge curves of the Co3O4 NPA/rGO/NF at 1.0 A g-1 for 200 cycles. g) Cycle stability of the Co3O4 NPA/rGO/NF at 1.0 A g-1 for 200 cycles. The cycling stability of Co3O4 NPA/rGO/NF was also tested at 1.0 and 5.0 A g-1. At a lower current density of 1.0 A, the discharge capacity drops slowly to 1198 mAh g-1 after 200 cycles (Fig. 2f), and a high retention of 84.5% for the 200-cycle test (Fig. 2g). The capacity of Co3O4 NPA/rGO/NF delivers about 700 mAh g-1
even at a high rate of 5.0 A g-1 and retains nearly 80 % after 100 cycles (Fig. S8, Supporting information). Furthermore, the array construction of the Co3O4 NPA/rGO/NF after 200 cycles can still be retained (Fig. S9, Supporting information), indicating the good structural stability. The high cycling stability of the Co3O4 NPA/rGO/NF electrode is ascribed to the 3D hierarchical pore network to buffer the large volume changes as well as the rGO buffering layer in oxide-based anodes to keep good electrical contanct with the current collector during fast discharge and charge processes.
Fig. 3 a) CV curves of Co3O4 NPA/rGO/NF at different scan rates from 0.1 to 1.0 mV s-1. b) b values plotted against battery potential of Co3O4 NPA/rGO/NF for oxidation scans. The inset is the current response plotted against scan rates of Co3O4 NPA/rGO/NF at various potentials. c) Capacitive contribution to the oxidation process at the scan rates of 1.0 mV s-1 marked by the shaded region. d) Capacitive contributions of Co3O4 NPA/rGO/NF at various scan rates (0.1, 0.2, 0.3, 0.7, 1.0 mV s-1). To understand the outstanding electrochemical performance of Co3O4 NPA/rGO/NF, the CV behavior was performed by a kinetic analysis. Fig. 3a shows the CV curves tested at different scan rates from 0.1 to 1.0 mV s-1, displaying gradual broadening of peaks with increasing scan rate. The total storage process can be described as two components, including the process of surface-induced (or non-faradic) capacitance and the diffusion-controlled (or faradaic) intercalation/deintercalation process. According to the equation (1) [40],
Where i is current response, v is potential, b and a are both variable parameters. b was determined by the slope of log i versus log v. As b ≈ 0.5, the current response is under controlled by a diffusion mechanism; as b
≈ 1, capacitive processes control the current response. As depicted in Fig. 3b, the fitting lines of log (i) and log (v) with different potentials of the oxidation processes are plotted. A high surface-induced capacitive contribution for the Co3O4 NPA/rGO/NF electrode can be displayed by a series of high b values (0.80, 0.98). The total capacitive contribution at different scan rates can be further determined by differentiating the fraction of capacitor-like (k1v) and diffusion-controlled (k2v1/2) currents according to the equation (2) [41]: V
/
As revealed in Fig. 3c, capacitive contribution at scan rate of 1 mV s-1 leads to nearly 86% fraction of the total current in the oxidation process, and the capacitive contribution is rising with increasing scan rates (Fig. 3d). Due to the capacitive effect, a large number of Li ions tend to be reserved on the interior surface of the electrode, avoiding Li-insertion-induced structural collapse and thus generating the excellent rate performance.
Fig. 4 a) Nyquist plots of Co3O4 NPA/rGO/NF and Co3O4 NPA/NF. b) Equivalent circuit model to fit the Nyquist plots. c) Values of Re and Rct acquired by fitting data. Electrochemical impedance spectroscopy (EIS) was also used to investigate the electrochemical reaction dynamics of Co3O4 NPA/rGO/NF and the reference sample Co3O4 NPA/NF. Their Nyquist plots are composed of a single depressed semicircle in the high-medium frequency region and an inclined line at low frequency (Fig. 4a). The intercept of the semicircle at the z' axis reflects the resistance of electrolyte diffusion (Re), and the diameter Rct corresponds to the charge-transfer resistance within Co3O4 NPA. The Re and Rct parameters
are determined by fitting the Nyquist plots with the equivalent circuit model (Fig. 4b). Rct is much lower for the Co3O4 NPA/rGO/NF electrode (47 Ω) compared to the Co3O4 NPA/NF (151 Ω) (Fig. 4c), indicating that Co3O4 NPA/rGO/NF has faster charge-transfer dynamics. It is also noted that the low-frequency tail of Co3O4 NPA/rGO/NF is nearly perpendicular to the z' axis (86° of the slope angle), extremely similar to the low-frequency response of complete electric double layer supercapacitors [42, 43], while the low-frequency tail of Co3O4 NPA/NF inclines at only 68° (Fig. 4a). This contrast not only reflects the higher lithium ion conductivity, faster charge-transfer dynamics, and the more capacitive feature of Co3O4 NPA/NF, but displays the characteristic of pseudo-capacitance,as verified CV of the Co3O4 NPA/rGO/NF electrode for supercapacitor (Fig. S10, Supporting information). The small charge-transfer resistance and nearly perpendicular low-frequency tail consistently point at the extraordinary electrochemical characteristics of the unique 3D porous hierarchical anode structure, including a high electrical conductivity associated with a fast graphene-mediated electronic pathway from the Ni current collector to the Co3O4 active layer, and a high surface-mediated Li ion conductivity closely related to the hierarchical porous structure. Because of these characteristics, the Co3O4 NPA/rGO/NF exhibits high-rate performance. For example, Wu et.al. reported the high capacity and rate capability of mesoporous Co3O4 nanowire arrays as anodes in Li-ion batteries, with a 60% capacity retained when the rate was increased by 20 times and a specific capacity of 240 mAh g-1 at a rate of 5500 mA g−1 after 20 cycles [32]. In contrast, for our Co3O4 NPA/rGO/NF, the retained capacity was as high as 94% after the current density was increased by 20 times, and the specific capacity is 550 mAh g-1 at a rate of 5000 mA g−1 after 100 cycles. Although they have similar mesoporous structures and the same Co3O4 composition, the coating of the current collectors with a soft graphene skin may play a crucial role in further improving the rate capability by stabilizing the interface between the mesoporous layer and the current collectors, increasing the electrical transport of the anodes, and buffering the large volume changes during fast discharge and charge processes.
Fig. 5 a) Rate capability of the Co3O4 NPA/rGO/NF as an anode in SIBs. b) Cyclic stability of the Co3O4 NPA/rGO/NF at 0.1 A g-1 for 400 cycles. c) Comparison of the rate capability of Co3O4 NPA/rGO/NF and related Co3O4 anode materials for SIBs (Ref.43: Mesoporous Co3O4 sheets/ 3D graphene; Ref.44: Co3O4; Ref.45: Co3O4@N-doped carbon; Ref.46: Co3O4 spheres/ carbon tubes). d) Capacitive contribution to the oxidation process at the scan rates of 1.0 mV s-1 marked by the shaded region. e) relationship of the scan rate used to evaluate the contribution of the capacitive reaction. The electrochemical energy storage performance of the Co3O4 NPA/rGO/NF electrode was also investigated for SIBs. The CV behavior of Co3O4 NPA/rGO/NF anode was recorded, as shown in Fig. S11a. The rate capability of the Co3O4 NPA/rGO/NF electrode was tested between 0.1 and 2.0 A g-1, which presents average capacity of 760, 700, 647, 513, 435 and 397 mAh g-1 at different rates (Fig. 5a). A high reversible capacity of 700 mAh g-1 at the current density of 0.1 A g-1 and a high retention of 89.7% after 400 cycles (Fig. 5b). Notably, the lower specific capacities for SIBs than for LIBs are expected due to the larger ion radius of Na than that of Li [5, 6, 12]. However, compared with other Co3O4 electrodes [44-47], such as Mesoporous Co3O4 sheets/ 3D graphene [44], Co3O4 [45], Co3O4@N-doped carbon [46], Co3O4/carbon tubes [47]. the Co3O4 NPA/rGO/NF anode material shows higher capacity and rate capability (Fig. 5c). To understand the outstanding rate performance of the Co3O4 NPA/rGO/NF anode in SIBs, its capacitive behavior was characterized by CV tests (Fig. S11b, Supporting information). As shown in Fig. S11c, a series of b values at different voltages than 0.6 are obtained, suggesting a capacitive contribution for the Co3O4
NPA/rGO/NF electrode for SIBs. Moreover, based on CV curves, the proportion of non-faradaic reaction shares nearly 83% during the electrochemical process at a scan rate of 1.0 mV s-1 (Fig. 5d). The corresponding non-faradaic attribution can also be calculated based on CV curves from 0.1 to 2.0 mV s-1(Fig. 5e). The high capacitive contribution is mainly because of the unique structure of Co3O4 NPA/rGO/NF electrode, i.e. The Co3O4 nanosheet porous arrays with the hierarchical mesoporous structure, which not only provide abundant electrochemically active sites and but also accelerate e-/ion transfer and shorten ion diffusion length, contributing to achieve high cycling stability and rate capability. Based on the above systematical studies on the electrochemical energy storage properties of the unique Co3O4 NPA/rGO/NF anode structure for both LIBs and SIBs, we can attribute the ultrahigh electrochemical performances, including the high rate capability and long cycling stability, to the several factors: (1) The Co3O4 nanosheet porous arrays with the hierarchical macroporous and mesoporous structure are beneficial to the transport of ions and could provide abundant electrochemically active sites for ion storage (Scheme 1a). (2) As a stable buffering and conducting layer, the rGO skin can enhance the binding of the NPA to and keep electrical contact with the current collector, thus ensuring the stability of the Co3O4 NPA/rGO/NF electrode during the rapid de/lithiation or de/sodiation. (3) The integrated units of vertically oriented Co3O4 nanosheets and graphene skin can strengthen the structural stability and prevent aggregating of the active materials during cycling, which also reduces side reactions in absence of conductive agent and binder. (4) The great capacitive effect also enhances the electrochemical performance of the Co3O4 NPA/rGO/NF in terms of an ultrahigh rate capability and long cycling life.
4. Conclusions In summary, we have developed a novel 3D porous hierarchical anode structure that combines the high energy storage densities and high power densities for LIBs and SIBs. To demonstrate the capacitive Li/Na ion storage conception in active oxide materials, we fabricated Co3O4 nanopore arrays by annealing Co(OH)2 nanosheet arrays. Upon annealing at 600 oC, an extended array of Co3O4 nanopores with pore diameters of 100-300 nm was formed via the interlinkage of ultrathin mesoporous Co3O4 nanosheets with fine pore diameter of <50 nm. To further improve the electronic transport pathway between the Co3O4 nanopore arrays and the Ni foam collector, a stable thin layer of reduced graphene oxide “skin” was uniformly deposited on the surface of macropores within the Ni foam. The composite electrode not only exhibits excellent rate capability with only a 6% loss (1490 to 1399 mAh g-1 at 0.1 to 2.0 A g-1), enhanced cycling stability, and high
retention rate (84.5% over 200 cycles at 1.0 A g-1) for Li ion batteries, but also displays an impressive capacity of 757 mAh g-1 at 100 mA g-1 after 400 cycles for Na ion batteries. Both CV and ESI measurements show that ion intercalation/deintercalation processes are dominated by an ultrafast supercapacitor-like storage behavior ascribed to the 3D porous Li-ion transport network and the graphene-mediated electronic transport pathway. This novel porous material could be employed in Li-ion batteries, sodium ion batteries, supercapacitors, and their hybrid devices called lithium-ion capacitors, which combine the advantages of both Li-ion batteries and supercapacitors.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 11774216) and the Science and Technology Commission of Shanghai Municipality (No. 16ZR1412100).
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1. 3D hierarchical porous oxide arrays composed of mesoporous Co3O4 nanosheets grown on a thin layer of reduced graphene oxide “skin” as a stable buffering and conducting layer. 2. The 3D porous composite mainly exhibits the graphene coating dramatically boosts the overall Li storage performances of the NPA by enhancing the binding to and electrical contact with the current collector. 3. The porous oxide-graphene hybrid anode shows the highest rate capability (~1400 mAh g-1 at 2.0 A g-1) among all oxide anode materials, high cycling stability over 200 cycles, and approximately 100% Coulombic efficiency. 4. The anode material in sodium-ion batteries also delivers a high capacity of 757 mAh g-1 with nearly 100% Coulombic efficiency over 400 cycles.
Conflict of Interest The authors declare no conflict of interest.