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NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries
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NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries Ruixue Sun a, Yezhen Zhang a, Yufeng Tang a, Yabei Li a, Shujiang Ding b,*, Xiaodi Liu a,** a
College of Chemistry and Pharmacy Engineering, Nanyang Normal University, Nanyang 473061, China Department of Applied Chemistry, School of Science, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
b
article info
abstract
Article history:
NiCoO2 nanosheets grown on nitrogen-doped porous carbon spheres (NiCoO2@N-PCs) have
Received 6 February 2017
been synthesized via a facile approach using gelatin nanospheres (GNSs) as the template,
Received in revised form
carbon and nitrogen sources. Due to the synergistic effect between the NiCoO2 nanosheets
18 March 2017
and N-PCs, the NiCoO2@N-PCs composite exhibits an ultrahigh discharge capacity of
Accepted 21 March 2017
978 mAh g1 at a current density of 200 mA g1 with minimal capacity loss even after 80
Available online xxx
cycles. The superior properties of NiCoO2@N-PCs illustrate that amorphous carbon matrix could significantly improve the electrochemical performance of high-capacity metal oxide
Keywords:
anode nanomaterials. Findings from this study suggest that these GNSs may be used to
NiCoO2
synthesize functional metal oxides, including MnO2, Fe2O3, CoO and NiO@N-PCs
Nanosheets
nanostructures.
N-doped
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Porous carbon Lithium-ion batteries
Introduction Lithium-ion batteries (LIBs) have attracted tremendous attention because of their light weight, high energy density, long cycle life, high efficiency, and relatively minimal impacts on environment [1e3]. It is central to develop low-cost electrode materials with high energy capacity for LIBs. Transition metal oxides (TMOs) are promising electrodes for new LIBs [4e13]. For example, ternary TMOs with two different metal
cations in a single crystal structure (e.g., NiCoO2 and ZnMn2O4) have received increased attention due to their complex chemical composition and synergic effect between the two different metal cations [14e18]. However, the application of TMOs-based electrodes has been confined by their low capacity retention and significant volume changes resulting from alloying/dealloying of metal ions (e.g., Ni2þ, Co2þ) with lithium. To overcome these limitations, several methods have been applied. For example, fabrication of nanostructured TMOs including nanosheets [19] and hollow spheres [20],
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Ding), [email protected] (X. Liu). http://dx.doi.org/10.1016/j.ijhydene.2017.03.165 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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porous flower-like structure [21,22], and TMOs-based composites have been reported [23e26]. Among these strategies, carbonaceous NiCo2O4 hybrid nanocomposites have proven particularly effective. Except for a few reports [27], the second type of ternary nickel cobaltite (NiCoO2) has not been well studied, especially in relation to LIBs. Carbonaceous materials, especially nanostructured porous carbon materials, can supply a large electrode/electrolyte interface for contacting with the electrolyte. This allows for reduced transport time for Liþ ions and provides strain relaxation during battery operation. Furthermore, carbonaceous materials offer high lithiation capability and excellent cycling stability compared with bulk electrodes. Reports show that doping carbon materials with heteroatoms (e.g., N and B) could optimize the electrochemical performance of the electrodes [28e32]. For example, N-doped porous carbon (N-PC) has been considered as an efficient anode material for LIBs owing to its enhanced lithium storage reaction kinetics [5,18,33e39]. Various methods have been proposed to prepare nitrogen-doped carbon materials such as thermal annealing [37], electrospinning technology [40], and carbonization of Nrich precursors [41]. For example, N-PCs vegetable-sponges were produced through a nitrogen-doped process with HNO3, which showed a reversible capacity of 870 mAh g1 at a current density of 0.5 A g1 after 300 cycles [42]. Recently, direct carbonization of porous structure biomaterials has been used to form N-PCs materials [43,44]. These biopolymers possess unique chemistry properties and can bind metal cations either through adsorption onto the raw biomass or within crosslinked gels [45]. Gelatin is a kind of peptides with partial hydrolysis of triple helix structures, which is extracted from various animal bones and skin. Due to its low cost, biocompatibility and high levels of carbon content, gelatin is hoped to be an advantageous precursor for producing N-PCs materials [46]. In this study, the design and facile synthesis of NiCoO2 nanosheets grown on N-doped porous carbon nanospheres (NiCoO2@N-PCs) are reported (Scheme 1). Uniform gelatin nanospheres (GNSs) were prepared according to inverse emulsion method and used as conformal templates to grow NiCoO2 nanosheets. NiCo-precursor@GNSs composite was fabricated on the basis of in situ growth of NiCo-precursor on the GNSs. Sintered in nitrogen, the GNSs were fully carbonized to produce N-PCs cores and the NiCo-precursor nanosheets were then transformed into highly crystallized NiCoO2 shells. Finally, the hierarchical NiCoO2 nanosheets grown on the NPCs surface were obtained.
Experiment Material synthesis Synthesis of gelatin nanospheres(GNSs). All reagents purchased from SigmaeAldrich were analytical grade, and used without further purification. GNSs were prepared in the light of a method reported formerly [47]. Briefly, 1.2 g gelatin was dissolved in 25 mL distilled water at 50 C under rigorous stirring. During stirring, 0.2 mL HCl (4.5 mol L1) was added dropwise to the gelatin solution and then stirring for 3 min. Acetone (48 mL) was then added dropwise. After that, the solution was chilled to room temperature, 0.2 mL glutaraldehyde was added to initiate crosslinking reaction for 24 h. The product was then washed with ethanol for three times. The template was then stored in ethanol for further use. Synthesis of NiCo-precursor@GNSs. In a typical synthesis, 50 mg GNSs were dispersed into 40 mL solution consisting of 5.0 104 mol Co(NO3)2$6H2O, 2.6 104 mol Ni(NO3)2$6H2O, 2.5 104 mol hexamethylenetetramine (HMT), and 2.8 102 mol Na-Citrate. After sonicating for 20 min, the solution was removed into a 100 mL round-bottomed flask and heated at 90 C for 7 h under magnetic stirring. After cooling to room temperature naturally, the block was collected by centrifugation, washed with ethanol for three times, and dried at 80 C under ambient conditions overnight to obtain the NiCo-precursor@GNSs [48]. After heating under nitrogen at 400 C for 2 h, the crystalline NiCoO2 nanosheets@N-PCs composite was obtained. For comparison, pure NiCoO2 was also prepared under identical conditions without addition of GNSs template.
Characterization Morphological information was collected using field-emission scanning electron microscopy (FESEM; JEOL, JSM-7000F) and transmission electron microscopy (TEM; JEOL, JEM-2100). The Raman spectra were collected using a spectrometer with backscattering geometry (l ¼ 514 nm; Horiba Jobin Yvon, HR 800). The specific surface area and pore size distribution of the product were determined using a BrunauereEmmetteTeller analyzer (BET; Autosorb-iQ, Quantachrome Instruments U.S.) at 77 K. Information on the crystallography of samples was obtained by powder X-ray diffraction (XRD; Shimadzu, Lab X XRD-6000). The thermogravimetric analysis (TGA; MettlerToledo TGA 1) was conducted under air flow and a steadily
Scheme 1 e Schematic synthesis of the hybrid NiCoO2@N-PCs composite. Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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increasing temperature (room temperature up to 800 C at a rate of 10 C min1). The chemical states of the product was determined using X-ray photoelectron spectroscopy (XPS) measurement performed on an Axis Ultra, Kratos (UK) at monochromatic Al Ka radiation (150 W, 15 kV and 1486.6 eV).
Electrochemical measurements Electrochemical tests were conducted under room temperature using two-electrode CR2025 coin cells. During these measurements, lithium served as both the counter and reference electrode. The working electrode was comprised of active materials (NiCoO2@N-PCs, NiCoO2, N-PCs), carbon black and poly-vinylidenedifluoride (PVDF) at a weight ratio of 7:2:1 with a background electrolyte of 1.0 M LiPF6. The loading amount of the active material was controlled to about 1.5 mg cm2. The cells were assembled in an Ar-filled glovebox in which the humidity and oxygen concentration were maintained below 1.0 ppm. Cyclic voltammetry (CV) was performed using an electrochemical workstation (CHI 660D). Chargeedischarge experiments were performed using a battery tester (NEWARE).
Results and discussion Morphology and structural analysis The typical morphology of the as-prepared GNSs template and NiCo-precursor@GNSs composite is shown in Fig. 1. The FESEM and TEM images of the GNSs indicate the template is spherical and its surface is smooth (Fig. 1A and B). Nearly every GNS was covered by the NiCo-precursor ultrathin nanosheets (Fig. 1C), which could be ascribed to the mass of e NH3 and eOH on the surface of the GNS well-organized with
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the activity of HMT and Na-Citrate during synthesis. As shown in TEM images (Fig. 1D), the uniform laminated and upright NiCo-precursor nanostructures surround the GNSs. After annealing at 400 C for 2 h under nitrogen, the NiCoprecursor nanosheets were readily alter to crystallized NiCoO2 and the GNSs entirely carbonized into N-PCs. Fig. 2A and B shows that the NiCoO2@N-PCs composite maintained the sphere with the diameter of approximately 200 nm and hierarchical structures, which demonstrates high structural stability. The as-obtained NiCoO2 nanosheets were found to be slightly shrunk compared to the NiCo-precursor nanosheets as a result of the thermal conversion. In addition, the hierarchical and porous nanostructures (Fig. 2A and B) may provide additional storage for lithium ions, suppress changes in volume of anode materials, and reduce the diffusion length of lithium ions. HRTEM images (Fig. 2C) of the NiCoO2 nanosheets indicate that the sample formed as a crystal lattice with a space of 0.24 nm, relating to the (111) plane of the cubic NiCoO2 phase. As shown by the selected-area electron diffraction (SAED) pattern (Fig. 2D), five intense rings indexed to the (222), (200), (220), (111), (311) planes of NiCoO2 nanosheets were presented. Fig. S1 shows the energy-dispersive Xray (EDX) spectrum of NiCoO2@N-PCs under SEM observation, and it can be seen that the atomic ratio of Ni and Co is about 1:1. The pore structure of the NiCoO2@N-PCs composite was also evaluated using a N2 adsorption desorption curve and pore size distribution curve (Fig. 3A). The specific surface area of the product was 230 m2 g1, which is considerably larger than values previously reported for porous NiCoO2 nanomaterials [27]. These differences may be due to the high specific surface area of the N-PCs obtained after carbonizing the GNSs. The size of the mesopores was ca. 18 nm, suggesting that aggregation of NiCoO2 nanoparticles occurred within the sheet-like subunits. To understand the crystalline
Fig. 1 e (A) SEM image and (B) TEM image of GNSs template; (C) SEM image and (D) TEM image of NiCo-precursor@GNSs composite. Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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Fig. 2 e SEM images (A and B), HRTEM image (C) and SAED pattern (D) of NiCoO2@N-PCs composite.
Fig. 3 e (A) N2 adsorptionedesorption isotherms (Inset of A shows the pore-size distribution calculated from the desorption branch), (B) XRD patterns, (C) Raman spectra of N-PCs(I) and NiCoO2@N-PCs composite(II), (D) TGA curve of NiCoO2@N-PCs composite.
Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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structure of the NiCoO2@N-PCs composite, the XRD pattern of the composite (Fig. 3B) was analyzed. All the XRD peaks obtained from the annealed sample could be vested in the spinel NiCoO2 phase of JCPDS no. 10-0188. In addition, Raman spectra may also be used to prospect the properties of N-PCs and NiCoO2@N-PCs. Two manifest peaks (ca.1331 cm1, ca.1607 cm1) were noticed relating to the D band and the G band (Fig. 3C), which are typical of the disorder coordination of carbon and the ordered crystalline graphite, respectively. The D and G bands of the NiCoO2@N-PCs composite were located at similar positions to those of the N-PCs, and the ratio of ID/IG becomes large, which due to the redox reaction between the ordering carbon and NiCoO2 [49]. The TGA data shows the thermal decomposition of NiCoO2@N-PCs composite hybrid nanostructures in air (Fig. 3D), along with weight loss at various stages. At the first stage (up to 200 C), the weight loss could be ascribed to the release of chemisorbed and occluded water in the NiCoO2@N-PCs composite by heating. The most significant weight loss occurred between 300 and 800 C, likely as a result of oxidative decomposition of carbon. According to the TGA result, the carbon content in the NiCoO2@N-PCs is 14.54%, and the composite exhibited a total weight loss of 28.65%. Both the composition of elements and the oxidation state of the NiCoO2@N-PCs composite were then characterized by XPS (Fig. 4A), which show that the material was primarily comprised of cobalt (Co2p), nickel (Ni2p), nitrogen (N1s), carbon (C1s) and oxygen (O1s) core level peaks. All peaks were
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classified according to their binding energy. As shown in Fig. 4B, the presence of two spin orbit couplets due to Ni2þ and Ni3þ, and the other two shakeup asteroids were suggested by the Ni2p spectra. Similarly, the two spin orbit couplets due to Co2þ and Co3þ, and two shakeup asteroids were shown by the Co2p spectra (Fig. 4C). The redox pairs Ni2þ/Ni3þ and Co2þ/Co3þ provide sufficient active sites, which could contribute to the high electrochemical performance of NiCoO2 [50]. Nitrogen functional groups were determined from the N1s XPS spectra, which may be divided into several peaks, including 400.2 eV (Fig. 4D), homologous with pyridine and pyridinic -N-oxide [43,51]. NiCoO2 nanosheets grown on the nitrogen-rich carbon are thought to promote lithium insertion.
Electrochemical performance Coin-type cell configuration was used to assess the lithium storage properties of NiCoO2@N-PCs composite and to compare their performance with that of the pure NiCoO2 and N-PCs under identical conditions. Fig. 5A shows the CV of the NiCoO2@N-PCs composite for the first, second and fifth cycles. For the first cathodic scan, a peak at ca. 0.33 V was identified, which can be attributed to the reduction of Ni2þ and Co2þ to metallic Ni and Co, respectively [52]. The reduction peaks were found to shift to 0.79 V in subsequent cycles. Two anodic peaks at ca. 1.57 and 2.25 V may be attributed to the oxidation of Ni0 to Ni2þ, Co0 to Co2þ and Co3þ, respectively. The lithium extraction reactions are proposed as follows:
Fig. 4 e XPS spectra of (A) survey spectrum, (B) Ni2p, (C) Co2p, (D) N1s for NiCoO2@N-PCs composite(A color version of this figure can be viewed online). Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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Fig. 5 e (A) Representative CVs at a scan rate of 0.5 mV s¡1 of NiCoO2@N-PCs composite, (B) chargeedischarge voltage profiles at a current density of 200 mA g¡1 of NiCoO2@N-PCs composite, (C) cycling performance of amorphous N-PCs(I), pure NiCoO2(II) and NiCoO2@N-PCs composite(III) at a current density of 200 mA g¡1, (D) rate performance of NiCoO2@N-PCs composite at different current densities.
þ
NiCoO2 þ4Li þ4e /Ni þ Co þ 2Li2 O þ
Ni þ Li2 O4NiO þ 2Li þ 2e þ
Co þ Li2 O4CoO þ 2Li þ 2e þ
3CoO þ Li2 O4Co3 O4 þ2Li þ 2e
(1) (2) (3) (4)
Lithium storage properties of NiCoO2@N-PCs composite were evaluated by galvanostatic discharge charge cycling at a current density of 200 mA g1 are shown in Fig. 5B. Two
voltage plateaus appeared at ca. 1.6 and 2.25 V during the first charge process for the NiCoO2@N-PCs, which is consistent with the CV results. The initial discharge and charge capacities are 1266 and 892 mAh g1, respectively, which were higher than the theoretical values (718 mAh g1 for NiO and 892 mAh g1 for Co3O4) [36,53] due to the formation of an solidelectrolyte interface (SEI) film and additional storage of Liþ ions in the defects or the pores of NiCoO2@N-PCs. The cycling performances of the NiCoO2@N-PCs composite, pure NiCoO2, and as-prepared N-PCs at a current density of 200 mA g1 are shown in Fig. 5C. Over 80 cycles, the discharge capacities of the pure NiCoO2 decrease substantially due to the NiCoO2
Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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nanostructures. The cycling performance of the as-prepared N-PCs was relatively stable. However, its discharge capacity is low, which may be attributed to the mesoporous nanostructures and low electrical conductivity. After a slight fluctuation during the first 30 cycles, the cycling performance of the NiCoO2@N-PCs composite becomes extremely stable, indicating a strong synergistic effect between the NiCoO2 nanosheets and N-PCs, and coulombic efficiency is 98.2%. Moreover, the cycling stability of NiCoO2@N-PCs composite was superior to that of previously reported NiCo2O4 based materials [19,22]. The rate performances of the NiCoO2@N-PCs composite, pure NiCoO2, and N-PCs are shown in Fig. 4D. At current densities of 200, 400, 600, and 800 mA g1, the capacities of NiCoO2@N-PCs composite are 978, 725, 714, and 693 mAh g1, respectively. When the current density returns to 200 mA g1, the NiCoO2@N-PCs composite still delivers a capacity of 909 mAh g1, only 7% loss after 60 cycles corresponding to the second cycle. Similarly, the pure NiCoO2 and amorphous N-PCs both display inferior rate capabilities compared to the NiCoO2@N-PCs composite. The superior lithium storage performance of the NiCoO2@N-PCs composite may be explained as follows: (i) compared to materials comprised of a single constituent, such as NiO and CoO, the ternary metal oxide NiCoO2 possesses considerably higher electrical conductivity; (ii) the higher specific surface area of NiCoO2@N-PCs composite can offer much more sites for Liþ storage; (iii) the amorphous N-PCs can strengthen electrical conductivity, reduce agglomeration, and buffer the large change in the volume of the NiCoO2 nanosheets (i.e., synergistic effects).
Conclusions NiCoO2@N-PCs composite was successfully synthesized as high-efficient anode material for LIBs. The as-prepared experimental product exhibits superior discharge capacity of 978 mAh g1 at a current density of 200 mA g1. It displays minimal capacity loss after 80 cycles due to the architecture of the NiCoO2@N-PCs composite and synergy between the NiCoO2 nanosheets and N-doped PCs. In addition, the amorphous N-PCs obtained from the relatively low-cost and simple synthesis of the GNSs allow for a considerable improvement in the electrochemical property of the transition metal oxide nanostructures. Taken in concert, findings from this study show that this material may have expected applications in a wide range of energy storage systems.
Acknowledgement This research was supported partially by the Fundamental Research Funds for the Central Universities (xjj2015119), the National Natural Science Foundation of China (Grant No. 21303131, U1304506 and 21501101), Technological Project of Henan Province (162102310115), the Financial Support from the Foundation of Henan Educational Committee (16B150012, 15A150022), Special Foundation of the Nanyang Normal University (70356).
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Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.165.
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Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries Ruixue Sun a, Yezhen Zhang a, Yufeng Tang a, Yabei Li a, Shujiang Ding b,*, Xiaodi Liu a,** a
College of Chemistry and Pharmacy Engineering, Nanyang Normal University, Nanyang 473061, China Department of Applied Chemistry, School of Science, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
b
article info
abstract
Article history:
NiCoO2 nanosheets grown on nitrogen-doped porous carbon spheres (NiCoO2@N-PCs) have
Received 6 February 2017
been synthesized via a facile approach using gelatin nanospheres (GNSs) as the template,
Received in revised form
carbon and nitrogen sources. Due to the synergistic effect between the NiCoO2 nanosheets
18 March 2017
and N-PCs, the NiCoO2@N-PCs composite exhibits an ultrahigh discharge capacity of
Accepted 21 March 2017
978 mAh g1 at a current density of 200 mA g1 with minimal capacity loss even after 80
Available online xxx
cycles. The superior properties of NiCoO2@N-PCs illustrate that amorphous carbon matrix could significantly improve the electrochemical performance of high-capacity metal oxide
Keywords:
anode nanomaterials. Findings from this study suggest that these GNSs may be used to
NiCoO2
synthesize functional metal oxides, including MnO2, Fe2O3, CoO and NiO@N-PCs
Nanosheets
nanostructures.
N-doped
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Porous carbon Lithium-ion batteries
Introduction Lithium-ion batteries (LIBs) have attracted tremendous attention because of their light weight, high energy density, long cycle life, high efficiency, and relatively minimal impacts on environment [1e3]. It is central to develop low-cost electrode materials with high energy capacity for LIBs. Transition metal oxides (TMOs) are promising electrodes for new LIBs [4e13]. For example, ternary TMOs with two different metal
cations in a single crystal structure (e.g., NiCoO2 and ZnMn2O4) have received increased attention due to their complex chemical composition and synergic effect between the two different metal cations [14e18]. However, the application of TMOs-based electrodes has been confined by their low capacity retention and significant volume changes resulting from alloying/dealloying of metal ions (e.g., Ni2þ, Co2þ) with lithium. To overcome these limitations, several methods have been applied. For example, fabrication of nanostructured TMOs including nanosheets [19] and hollow spheres [20],
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Ding), [email protected] (X. Liu). http://dx.doi.org/10.1016/j.ijhydene.2017.03.165 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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porous flower-like structure [21,22], and TMOs-based composites have been reported [23e26]. Among these strategies, carbonaceous NiCo2O4 hybrid nanocomposites have proven particularly effective. Except for a few reports [27], the second type of ternary nickel cobaltite (NiCoO2) has not been well studied, especially in relation to LIBs. Carbonaceous materials, especially nanostructured porous carbon materials, can supply a large electrode/electrolyte interface for contacting with the electrolyte. This allows for reduced transport time for Liþ ions and provides strain relaxation during battery operation. Furthermore, carbonaceous materials offer high lithiation capability and excellent cycling stability compared with bulk electrodes. Reports show that doping carbon materials with heteroatoms (e.g., N and B) could optimize the electrochemical performance of the electrodes [28e32]. For example, N-doped porous carbon (N-PC) has been considered as an efficient anode material for LIBs owing to its enhanced lithium storage reaction kinetics [5,18,33e39]. Various methods have been proposed to prepare nitrogen-doped carbon materials such as thermal annealing [37], electrospinning technology [40], and carbonization of Nrich precursors [41]. For example, N-PCs vegetable-sponges were produced through a nitrogen-doped process with HNO3, which showed a reversible capacity of 870 mAh g1 at a current density of 0.5 A g1 after 300 cycles [42]. Recently, direct carbonization of porous structure biomaterials has been used to form N-PCs materials [43,44]. These biopolymers possess unique chemistry properties and can bind metal cations either through adsorption onto the raw biomass or within crosslinked gels [45]. Gelatin is a kind of peptides with partial hydrolysis of triple helix structures, which is extracted from various animal bones and skin. Due to its low cost, biocompatibility and high levels of carbon content, gelatin is hoped to be an advantageous precursor for producing N-PCs materials [46]. In this study, the design and facile synthesis of NiCoO2 nanosheets grown on N-doped porous carbon nanospheres (NiCoO2@N-PCs) are reported (Scheme 1). Uniform gelatin nanospheres (GNSs) were prepared according to inverse emulsion method and used as conformal templates to grow NiCoO2 nanosheets. NiCo-precursor@GNSs composite was fabricated on the basis of in situ growth of NiCo-precursor on the GNSs. Sintered in nitrogen, the GNSs were fully carbonized to produce N-PCs cores and the NiCo-precursor nanosheets were then transformed into highly crystallized NiCoO2 shells. Finally, the hierarchical NiCoO2 nanosheets grown on the NPCs surface were obtained.
Experiment Material synthesis Synthesis of gelatin nanospheres(GNSs). All reagents purchased from SigmaeAldrich were analytical grade, and used without further purification. GNSs were prepared in the light of a method reported formerly [47]. Briefly, 1.2 g gelatin was dissolved in 25 mL distilled water at 50 C under rigorous stirring. During stirring, 0.2 mL HCl (4.5 mol L1) was added dropwise to the gelatin solution and then stirring for 3 min. Acetone (48 mL) was then added dropwise. After that, the solution was chilled to room temperature, 0.2 mL glutaraldehyde was added to initiate crosslinking reaction for 24 h. The product was then washed with ethanol for three times. The template was then stored in ethanol for further use. Synthesis of NiCo-precursor@GNSs. In a typical synthesis, 50 mg GNSs were dispersed into 40 mL solution consisting of 5.0 104 mol Co(NO3)2$6H2O, 2.6 104 mol Ni(NO3)2$6H2O, 2.5 104 mol hexamethylenetetramine (HMT), and 2.8 102 mol Na-Citrate. After sonicating for 20 min, the solution was removed into a 100 mL round-bottomed flask and heated at 90 C for 7 h under magnetic stirring. After cooling to room temperature naturally, the block was collected by centrifugation, washed with ethanol for three times, and dried at 80 C under ambient conditions overnight to obtain the NiCo-precursor@GNSs [48]. After heating under nitrogen at 400 C for 2 h, the crystalline NiCoO2 nanosheets@N-PCs composite was obtained. For comparison, pure NiCoO2 was also prepared under identical conditions without addition of GNSs template.
Characterization Morphological information was collected using field-emission scanning electron microscopy (FESEM; JEOL, JSM-7000F) and transmission electron microscopy (TEM; JEOL, JEM-2100). The Raman spectra were collected using a spectrometer with backscattering geometry (l ¼ 514 nm; Horiba Jobin Yvon, HR 800). The specific surface area and pore size distribution of the product were determined using a BrunauereEmmetteTeller analyzer (BET; Autosorb-iQ, Quantachrome Instruments U.S.) at 77 K. Information on the crystallography of samples was obtained by powder X-ray diffraction (XRD; Shimadzu, Lab X XRD-6000). The thermogravimetric analysis (TGA; MettlerToledo TGA 1) was conducted under air flow and a steadily
Scheme 1 e Schematic synthesis of the hybrid NiCoO2@N-PCs composite. Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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increasing temperature (room temperature up to 800 C at a rate of 10 C min1). The chemical states of the product was determined using X-ray photoelectron spectroscopy (XPS) measurement performed on an Axis Ultra, Kratos (UK) at monochromatic Al Ka radiation (150 W, 15 kV and 1486.6 eV).
Electrochemical measurements Electrochemical tests were conducted under room temperature using two-electrode CR2025 coin cells. During these measurements, lithium served as both the counter and reference electrode. The working electrode was comprised of active materials (NiCoO2@N-PCs, NiCoO2, N-PCs), carbon black and poly-vinylidenedifluoride (PVDF) at a weight ratio of 7:2:1 with a background electrolyte of 1.0 M LiPF6. The loading amount of the active material was controlled to about 1.5 mg cm2. The cells were assembled in an Ar-filled glovebox in which the humidity and oxygen concentration were maintained below 1.0 ppm. Cyclic voltammetry (CV) was performed using an electrochemical workstation (CHI 660D). Chargeedischarge experiments were performed using a battery tester (NEWARE).
Results and discussion Morphology and structural analysis The typical morphology of the as-prepared GNSs template and NiCo-precursor@GNSs composite is shown in Fig. 1. The FESEM and TEM images of the GNSs indicate the template is spherical and its surface is smooth (Fig. 1A and B). Nearly every GNS was covered by the NiCo-precursor ultrathin nanosheets (Fig. 1C), which could be ascribed to the mass of e NH3 and eOH on the surface of the GNS well-organized with
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the activity of HMT and Na-Citrate during synthesis. As shown in TEM images (Fig. 1D), the uniform laminated and upright NiCo-precursor nanostructures surround the GNSs. After annealing at 400 C for 2 h under nitrogen, the NiCoprecursor nanosheets were readily alter to crystallized NiCoO2 and the GNSs entirely carbonized into N-PCs. Fig. 2A and B shows that the NiCoO2@N-PCs composite maintained the sphere with the diameter of approximately 200 nm and hierarchical structures, which demonstrates high structural stability. The as-obtained NiCoO2 nanosheets were found to be slightly shrunk compared to the NiCo-precursor nanosheets as a result of the thermal conversion. In addition, the hierarchical and porous nanostructures (Fig. 2A and B) may provide additional storage for lithium ions, suppress changes in volume of anode materials, and reduce the diffusion length of lithium ions. HRTEM images (Fig. 2C) of the NiCoO2 nanosheets indicate that the sample formed as a crystal lattice with a space of 0.24 nm, relating to the (111) plane of the cubic NiCoO2 phase. As shown by the selected-area electron diffraction (SAED) pattern (Fig. 2D), five intense rings indexed to the (222), (200), (220), (111), (311) planes of NiCoO2 nanosheets were presented. Fig. S1 shows the energy-dispersive Xray (EDX) spectrum of NiCoO2@N-PCs under SEM observation, and it can be seen that the atomic ratio of Ni and Co is about 1:1. The pore structure of the NiCoO2@N-PCs composite was also evaluated using a N2 adsorption desorption curve and pore size distribution curve (Fig. 3A). The specific surface area of the product was 230 m2 g1, which is considerably larger than values previously reported for porous NiCoO2 nanomaterials [27]. These differences may be due to the high specific surface area of the N-PCs obtained after carbonizing the GNSs. The size of the mesopores was ca. 18 nm, suggesting that aggregation of NiCoO2 nanoparticles occurred within the sheet-like subunits. To understand the crystalline
Fig. 1 e (A) SEM image and (B) TEM image of GNSs template; (C) SEM image and (D) TEM image of NiCo-precursor@GNSs composite. Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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Fig. 2 e SEM images (A and B), HRTEM image (C) and SAED pattern (D) of NiCoO2@N-PCs composite.
Fig. 3 e (A) N2 adsorptionedesorption isotherms (Inset of A shows the pore-size distribution calculated from the desorption branch), (B) XRD patterns, (C) Raman spectra of N-PCs(I) and NiCoO2@N-PCs composite(II), (D) TGA curve of NiCoO2@N-PCs composite.
Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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structure of the NiCoO2@N-PCs composite, the XRD pattern of the composite (Fig. 3B) was analyzed. All the XRD peaks obtained from the annealed sample could be vested in the spinel NiCoO2 phase of JCPDS no. 10-0188. In addition, Raman spectra may also be used to prospect the properties of N-PCs and NiCoO2@N-PCs. Two manifest peaks (ca.1331 cm1, ca.1607 cm1) were noticed relating to the D band and the G band (Fig. 3C), which are typical of the disorder coordination of carbon and the ordered crystalline graphite, respectively. The D and G bands of the NiCoO2@N-PCs composite were located at similar positions to those of the N-PCs, and the ratio of ID/IG becomes large, which due to the redox reaction between the ordering carbon and NiCoO2 [49]. The TGA data shows the thermal decomposition of NiCoO2@N-PCs composite hybrid nanostructures in air (Fig. 3D), along with weight loss at various stages. At the first stage (up to 200 C), the weight loss could be ascribed to the release of chemisorbed and occluded water in the NiCoO2@N-PCs composite by heating. The most significant weight loss occurred between 300 and 800 C, likely as a result of oxidative decomposition of carbon. According to the TGA result, the carbon content in the NiCoO2@N-PCs is 14.54%, and the composite exhibited a total weight loss of 28.65%. Both the composition of elements and the oxidation state of the NiCoO2@N-PCs composite were then characterized by XPS (Fig. 4A), which show that the material was primarily comprised of cobalt (Co2p), nickel (Ni2p), nitrogen (N1s), carbon (C1s) and oxygen (O1s) core level peaks. All peaks were
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classified according to their binding energy. As shown in Fig. 4B, the presence of two spin orbit couplets due to Ni2þ and Ni3þ, and the other two shakeup asteroids were suggested by the Ni2p spectra. Similarly, the two spin orbit couplets due to Co2þ and Co3þ, and two shakeup asteroids were shown by the Co2p spectra (Fig. 4C). The redox pairs Ni2þ/Ni3þ and Co2þ/Co3þ provide sufficient active sites, which could contribute to the high electrochemical performance of NiCoO2 [50]. Nitrogen functional groups were determined from the N1s XPS spectra, which may be divided into several peaks, including 400.2 eV (Fig. 4D), homologous with pyridine and pyridinic -N-oxide [43,51]. NiCoO2 nanosheets grown on the nitrogen-rich carbon are thought to promote lithium insertion.
Electrochemical performance Coin-type cell configuration was used to assess the lithium storage properties of NiCoO2@N-PCs composite and to compare their performance with that of the pure NiCoO2 and N-PCs under identical conditions. Fig. 5A shows the CV of the NiCoO2@N-PCs composite for the first, second and fifth cycles. For the first cathodic scan, a peak at ca. 0.33 V was identified, which can be attributed to the reduction of Ni2þ and Co2þ to metallic Ni and Co, respectively [52]. The reduction peaks were found to shift to 0.79 V in subsequent cycles. Two anodic peaks at ca. 1.57 and 2.25 V may be attributed to the oxidation of Ni0 to Ni2þ, Co0 to Co2þ and Co3þ, respectively. The lithium extraction reactions are proposed as follows:
Fig. 4 e XPS spectra of (A) survey spectrum, (B) Ni2p, (C) Co2p, (D) N1s for NiCoO2@N-PCs composite(A color version of this figure can be viewed online). Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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Fig. 5 e (A) Representative CVs at a scan rate of 0.5 mV s¡1 of NiCoO2@N-PCs composite, (B) chargeedischarge voltage profiles at a current density of 200 mA g¡1 of NiCoO2@N-PCs composite, (C) cycling performance of amorphous N-PCs(I), pure NiCoO2(II) and NiCoO2@N-PCs composite(III) at a current density of 200 mA g¡1, (D) rate performance of NiCoO2@N-PCs composite at different current densities.
þ
NiCoO2 þ4Li þ4e /Ni þ Co þ 2Li2 O þ
Ni þ Li2 O4NiO þ 2Li þ 2e þ
Co þ Li2 O4CoO þ 2Li þ 2e þ
3CoO þ Li2 O4Co3 O4 þ2Li þ 2e
(1) (2) (3) (4)
Lithium storage properties of NiCoO2@N-PCs composite were evaluated by galvanostatic discharge charge cycling at a current density of 200 mA g1 are shown in Fig. 5B. Two
voltage plateaus appeared at ca. 1.6 and 2.25 V during the first charge process for the NiCoO2@N-PCs, which is consistent with the CV results. The initial discharge and charge capacities are 1266 and 892 mAh g1, respectively, which were higher than the theoretical values (718 mAh g1 for NiO and 892 mAh g1 for Co3O4) [36,53] due to the formation of an solidelectrolyte interface (SEI) film and additional storage of Liþ ions in the defects or the pores of NiCoO2@N-PCs. The cycling performances of the NiCoO2@N-PCs composite, pure NiCoO2, and as-prepared N-PCs at a current density of 200 mA g1 are shown in Fig. 5C. Over 80 cycles, the discharge capacities of the pure NiCoO2 decrease substantially due to the NiCoO2
Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165
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nanostructures. The cycling performance of the as-prepared N-PCs was relatively stable. However, its discharge capacity is low, which may be attributed to the mesoporous nanostructures and low electrical conductivity. After a slight fluctuation during the first 30 cycles, the cycling performance of the NiCoO2@N-PCs composite becomes extremely stable, indicating a strong synergistic effect between the NiCoO2 nanosheets and N-PCs, and coulombic efficiency is 98.2%. Moreover, the cycling stability of NiCoO2@N-PCs composite was superior to that of previously reported NiCo2O4 based materials [19,22]. The rate performances of the NiCoO2@N-PCs composite, pure NiCoO2, and N-PCs are shown in Fig. 4D. At current densities of 200, 400, 600, and 800 mA g1, the capacities of NiCoO2@N-PCs composite are 978, 725, 714, and 693 mAh g1, respectively. When the current density returns to 200 mA g1, the NiCoO2@N-PCs composite still delivers a capacity of 909 mAh g1, only 7% loss after 60 cycles corresponding to the second cycle. Similarly, the pure NiCoO2 and amorphous N-PCs both display inferior rate capabilities compared to the NiCoO2@N-PCs composite. The superior lithium storage performance of the NiCoO2@N-PCs composite may be explained as follows: (i) compared to materials comprised of a single constituent, such as NiO and CoO, the ternary metal oxide NiCoO2 possesses considerably higher electrical conductivity; (ii) the higher specific surface area of NiCoO2@N-PCs composite can offer much more sites for Liþ storage; (iii) the amorphous N-PCs can strengthen electrical conductivity, reduce agglomeration, and buffer the large change in the volume of the NiCoO2 nanosheets (i.e., synergistic effects).
Conclusions NiCoO2@N-PCs composite was successfully synthesized as high-efficient anode material for LIBs. The as-prepared experimental product exhibits superior discharge capacity of 978 mAh g1 at a current density of 200 mA g1. It displays minimal capacity loss after 80 cycles due to the architecture of the NiCoO2@N-PCs composite and synergy between the NiCoO2 nanosheets and N-doped PCs. In addition, the amorphous N-PCs obtained from the relatively low-cost and simple synthesis of the GNSs allow for a considerable improvement in the electrochemical property of the transition metal oxide nanostructures. Taken in concert, findings from this study show that this material may have expected applications in a wide range of energy storage systems.
Acknowledgement This research was supported partially by the Fundamental Research Funds for the Central Universities (xjj2015119), the National Natural Science Foundation of China (Grant No. 21303131, U1304506 and 21501101), Technological Project of Henan Province (162102310115), the Financial Support from the Foundation of Henan Educational Committee (16B150012, 15A150022), Special Foundation of the Nanyang Normal University (70356).
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Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.165.
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Please cite this article in press as: Sun R, et al., NiCoO2 nanosheets grown on nitrogen-doped porous carbon sphere as a high-performance anode material for lithium-ion batteries, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.165