Self-assembled 3D NixCo3-xO4 pseudocube superstructure as potential anode material for Li-Ion batteries

Self-assembled 3D NixCo3-xO4 pseudocube superstructure as potential anode material for Li-Ion batteries

Journal of Alloys and Compounds 814 (2020) 152319 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:/...

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Journal of Alloys and Compounds 814 (2020) 152319

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage:

Self-assembled 3D NixCo3-xO4 pseudocube superstructure as potential anode material for Li-Ion batteries Qi Yang a, *, Jianyin Wang a, Xiaobing Lou a, Zhenhua Chen b, Bingwen Hu a, ** a State Key Laboratory of Precision Spectroscopy, Shanghai Key Laboratory of Magnetic Resonance, Institute of Functional Materials, School of Physics and Materials Science, East China Normal University, Shanghai, 200062, China b Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, 201210, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 March 2019 Received in revised form 27 August 2019 Accepted 16 September 2019 Available online 16 September 2019

Self-assembled 3D NixCo3-xO4 pseudocube superstructure was successfully synthesized through a facile solvothermal strategy followed by a calcination process. Structure analyses shows that the mesoscopic 3D pseudocube superstructures are stacked by mesoporous nanoflakes with tiny nanoparticles as basic unit, as an anode material for Li-Ion Batteries, such a hierarchical structure can greatly buffer the volume expansion and effectively shorten lithium ion diffusion distance. Comparing with the 1D NiCo2O4 nanorods and 2D Co3O4 nanoflakes synthesized via changing the synthesis parameters, it conclude that the nickel leach process plays a key role in formation of this unique morphology and a mechanism is proposed. As expected, the 3D NixCo3-xO4 pseudocube exhibits the most impressive lithium storage performance, delivering a high reversible capacity of 1225 mA h g1 after 200 cycles at 200 mA g1, and even at high current of 4000 mA g1, it still delivers a specific capacity of 645 mA h g1. These results suggest a promising application for advanced energy storage units. © 2019 Elsevier B.V. All rights reserved.

Keywords: Pseudocube superstructure NixCo3-xO4 Leach Li-ion battery Electrochemical performance

1. Introduction Look toward to the over-growing demands for electronic devices, ranging from large-scale energy storage systems to daily needs of portable electronics devices, the exploration of lithium ion batteries (LIBs) with high power, high energy, and long-term cyclability is an urgent task [1e3]. Based on the fact that the traditional anode material graphite only exhibits a capacity of 372 mA h g1, far from our anticipations, striving for superior anode materials is a significant direction in achieving the above expectations [4,5]. Transition metal oxides (TMOs) electrodes have been regarded as one type of promising alternative anode material due to their noticeably high theoretical capacities (>600 mA h g1) [6e8]. Especially for binary metal oxides, it always displays synergic effects of multiple metal species [9e11]. Among various binary metal oxides, NiCo2O4 is widely studied due to its good electronic conductivity, low diffusion resistance, and easy electrolyte penetration

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Yang), [email protected] (B. Hu). 0925-8388/© 2019 Elsevier B.V. All rights reserved.

properties [12e14]. However, the practical application of this anode material is lagged by their poor rate capability and cycling stability, which are caused by the devastating huge volume changes during charge/discharge processes and aggregation of particles, leading to slow kinetics of electrochemical conversion reaction during cycling progresses [15e17]. Specifically, the elaborate designing of nanoscale architectures has been developed to reconcile the above inherent contradictions [18]. Numerous researchers have tried to fabricate various nanostructures, such as zero-dimensional (0D) nanoparticles [19], onedimensional (1D) nanowires [20,21], two-dimensional (2D) nanoflakes [22e24], and three-dimensional (3D) hollow nanospheres [25,26]. Generally, the low-dimensional (0D, 1D, 2D) nanostructures always exhibit large specific surface areas, abundant mass-transportation channels, full contact with the electrolytes, and short ions and electrons diffusion distance, and these feature are crucial for improving the rate performances. The 3D structures are favorable for buffering the volume expansion, since the stress derived from the abrupt structural change can be released in all directions [27]. From this perspective, designing 3D interconnected structures with well-developed mass transportation channels and low-dimensional nanostructure as basic unit is a potential solution to enhance the stability and high rate ability simultaneously.


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Recently, significant efforts have been devoted to fabricating many types of 3D-structure materials with complex interior structures as anodes for LIBs. For instance, Lou's group have reported several kinds of NiCo2O4 hollow spheres and boxes [28]. Zhu et al. synthesized 3D mesoporous network of NiCo2O4 by introducing a 3D N-doped carbon network as the sacrificed template route [29]. However, due to the restriction of surface energy minimization principles, multiple steps were often involved in the synthesis method or the main products were simple shell structures, therefore, fabricating the 3D interconnected networks of NiCo2O4 is still a great challenge. Herein, a mesoscopic 3D NixCo3-xO4 pseudocube superstructure was successfully synthesized via a facile solvothermal strategy, as schematically illustrated in Scheme 1. With reaction time lasting, the nanorod-morphology NiCo(OH)2CO3 precursors gradually converted to binary NiCoCO3 pseudocube superstructure, simultaneously, nickel element was leached by the high concentration of NH3$H2O. In this procedure, self-assembly, self-leveling, selfperforation and self-delamination were involved, endowing the precursor with a laminar and porous 3D feature. After calcination, a porous 3D pseudocube-superstructure NixCo3-xO4 was obtained. Comparing with the 1D NiCo2O4 nanorod and 2D Co3O4 nanoflake synthesized via changing the synthesis parameters, the Ni-doping and urea concentration effect on the phase and morphology were illuminated. Additionally, the XRD, SEM, TEM, N2-sorption, XPS, and XAS techniques were used to clarified the structure, morphology, pore distribution, and electronic property of the above three samples. When applied as an anode for Li-ion batteries, the pseudocube-superstructure NixCo3-xO4 exhibits excellent structural stability far exceeding the performance of 1D NiCo2O4 nanorod and 2D Co3O4 nanoflake. High reversible capacity of 1225 mA h g1 after 200 cycles at 200 mA g1 and excellent rate performance of 645 mA h g1 at 4000 mA g1 were demonstrated. 2. Experimental details 2.1. Chemicals All the chemicals used herein are of analytical grade purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Materials synthesis The NixCo3-xO4 pseudocube superstructure was prepared via a coprecipitation method with the aids of hydrothermal technique. 2.67 mmol Ni(NO3)2$6H2O, 5.33 mmol of Co(NO3)2$6H2O, and 160 mmol of urea were dissolved in 70 mL mixture solution of deionized water and ethylene glycol with deionized water/ethylene glycol volume ratio of 20/50. After vigorous stirring for 1 h, the homogeneous solution was transferred into a 100 mL Teflon-lined

stainless-steel autoclave and held at 150  C for 8 h. After cooling down to room-temperature, the as obtained precipitates were filtered, washed with deionized water several times until neutral PH value and then dried at 70  C for 24 h. Finally, black powders of mesosocopic 3D NixCo3-xO4 pseudocube superstructure were obtained after calcination at 300  C for 6 h in air with a heating rate 1  C/min, and marked as M-NCO. For comparison, the sample Co3O4 was prepared by substituting Ni(NO3)2$6H2O with equal amount of Co(NO3)2$6H2O through the above-mentioned procedures, and the nano-scale product mark as NeNCO was synthesized via changing the amount of urea to 40 mmol in the same way. 2.3. Materials characterizations The bulk Ni/Co concentrations were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) with a Perkin-Elmer 3300DV emission spectrometer. The X-ray diffraction (XRD) analyses were performed on a Rigaku Ultima IV X-ray Diffractometer with Cu Ka radiation (l ¼ 1.5418 Å) operated at 35 kV and 25 mA with a scan step of 0.02 . Thermogravimetric analysis (TGA) curves were performed on a STA 449 F3 Jupiter® simultaneous thermo-analyzer under air atmosphere from 25  C to 600  C at a heating rate 10  C min1. Scanning electron microscope (SEM) images were conducted on a Hitachi S-4800 instrument. Before test, the samples were mounted on aluminum stubs, and sputtered with gold. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were conducted on a Philips Tecnai F20 instrument operating at 200 kV. For the corresponding element mapping acquisition, the energy dispersive spectroscopy (EDS) was applied to measured samples under scanning transmission electrons microscopy (STEM) mode on the same TEM machine. X-ray photoelectron spectroscopy (XPS) analyses were recorded on an AXIS Ultra DLD spectrometer (SHIMADZU Ltd., Japan) with 250 W of Al Ka radiation. The C 1s line at 284.8 eV was used to calibrate the binding energies. Soft X-ray absorption spectroscopy (sXAS) was performed at National Synchrotron Radiation Laboratory (NSRL, BL12B-a: MCD). Total electron yield (TEY) spectra at O K-edge were recorded by detecting the electric current caused by excited electrons from the sample. The Brunauer-Emmett-Teller (BET) surface areas and BarrettJoynerHalenda (BJH) pore size distributions measurements of samples were conducted on a volumetric adsorption apparatus (ASAP 2020 M, Micromeritics) at 77 K using liquid N2. 2.4. Electrochemical measurements The electrochemical performance of the as-synthesized M-NCO, NeNCO, and Co3O4 products as anode materials were investigated by using CR2032 half-cells, which fabricated in an argon-filled

Scheme 1. Schematic diagram of the formation process of NixCo3-xO4 (M-NCO).

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glove box with both H2O and O2 contents below 0.1 ppm. The working electrode was prepared by pasting a mixed slurry that consisted of 70 wt% active material, 20 wt% Super-P carbon black, and 10 wt% carboxymethyl cellulose (CMC) (deionized water as solvent) onto a copper foil substrate, followed by drying under vacuum at 110  C for 12 h. The mass loading of active material on each electrode is in the range of 1.0e1.2 mg cm2. Lithium pellets (diameter of 14.0 mm) were used as the counter/reference electrode and the separator is Celgard 2325 membrane (diameter of 19.0 mm). The electrolyte used in the cells was 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/ EMC/DMC, 1:1:1, v/v) containing 5 vol % fluoroethylene carbonate (FEC). Galvanostatic discharge-charge was cycled in the voltage range of 0.01e3.0 V (vs. Liþ/Li) at various current rates on a LAND 2001A battery testing system. Cyclic voltammetry (CV) testing was carried out on an electrochemical workstation (CHI660e) at a scan rate of 0.2 mV s1 in the voltage range of 0.01e3 V versus Li/Liþ.

3. Results and discussion 3.1. Morphology and structure The crystal structure and morphology of as-obtained products are characterized firstly. Fig. 1 exhibits the X-ray diffraction (XRD) patterns. For Co3O4 sample, a series of distinct diffraction peaks are observed, which can be unambiguously indexed to the cubic spinel structure of Co3O4 (JCPDS card no. 65e3103, space group Fd3m (227)). After doping with Ni, the spinel crystal structure was maintained, and no impurity phase like NiO, Ni2O3, NiCo(OH)CO3, and NiCoCO3 were observed for NeNCO (1D NiCo2O4) and M-NCO (3D NixCo3-xO4). TGA curves (Fig. S1, Supporting Information) also show no obvious weight loss over the whole temperature range for the three as-obtained products, confirming that the precursors were completely transformed into metal oxides after calcination. The broadening of the peaks in the XRD patterns indicate that the products exhibit nanoscale dimensions (15.2 nm, 10.6 nm, 14.1 nm for Co3O4, NeNCO, M-NCO, respectively, in Table 1), as will be discussed later. Additionally, bulk concentrations of Ni and Co were determined by ICP-AES and shown in Table 1. It is interesting to see that the Ni/Co concentration for NeNCO (22 wt %, 53 wt %) is well consistent with it designed, indicating the formation of pure NiCo2O4, while M-NCO (8 wt %, 68 wt %) gives obviously lower Ni concentration. Moreover, lattice constants (a) that calculated from the results of XRD show a monotonic increasing on lattice constants with Ni concentrations: Co3O4 (8.0596 Å) < M-NCO (8.0989 Å) < NeNCO (8.1199 Å). Among them the lattice parameters

Fig. 1. XRD patterns of the as-obtained Co3O4, NeNCO, and M-NCO products.


of the as-obtained Co3O4 and NeNCO products are nearly equal to the theoretical value of Co3O4 (8.056 Å, JCPDS card no. 65e3103) and NiCo2O4 (8.114 Å, JCPDS, PDF no. 73e1702), respectively, confirming the formation of Co3O4 and NiCo2O4 in phase pure form. Compared with NeNCO, the lattice constant of M-NCO is slightly decreased, because Ni site was partially substituted by Co. The morphology of as-obtained products was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2, pure Co3O4 exhibits randomly arranged nanoflake morphology (Fig. 2a, d), NeNCO shows nanorod morphology with about 500 nm in length and 30 nm in diameter (Fig. 2b,e), while M-NCO shows obviously different 3D pseudocube superstructure with sizes in the range of 10e30 mm, furthermore, the pseudocube is constructed by 2D nanoflakes aggregation (Fig. 2c,f; Fig. S2, Supporting Information). In addition, the surface of pseudocube was found to be rough and porous. Although many types of NixCo3-xO4 superstructures have been reported in current researches [30,31], this superstructure of mesoscopic pseudocube was rarely reported. Since the XRD results show that all the products have nanoscale dimensions, transmission electron microscopy (TEM) technique was used to reveal the details. Fig. 3 shows that both the nanorod morphology NeNCO and the pseudocube superstructure morphology M-NCO are composed by tiny nanoparticles, except that the nanoparticles stacked more closely for M-NCO product. Furthermore, EDS elemental mapping images obtained from the corresponding HAADF-STEM data (Fig. 3c, f) and SEM images (Fig. S3, Supporting Information) confirm the homogeneous distribution of Ni and Co atoms in the N-NCO and M-CNO products. As we known, the structures with abundant grain boundaries would augment the contact interface between the electrode and electrolyte, serving to facilitate the electron and lithium ions transport. Therefore, the special 3D pseudocube-structure M-NCO with tiny nanoparticles as basic unit was expected to show excellent electrochemical performance. Several control experiments were conducted to reveal the formation process of M-NCO product and the effect of urea concentration. The precursors were collected sequentially after 2 h, 4 h, and 8 h of hydrothermal reactions, and their typical XRD patterns and SEM images are presented in Fig. 4 and Fig. 5. The diffraction pattern of precursor after 2 h reaction is matched well with that of the reported binary NiCo(OH)2CO3 [32], and shows uniform nanorod morphology similar with NeNCO samples. After 4 h reaction, the superstructure of pseudocube was basically formed, and only a few amounts of nanorods exist on the surface of pseudocubes, indicating that the pseudocubes were obtained by self-assembly of nanorods. The 4 h reaction precursor shows smooth surface and nearly no pore structures. It is also very interesting to see that the crystal structure is completely transformed and the 2q values match well with the standard XRD data of CoCO3 (JCPDS card no. 11e0692), indicating that the self-assembled precursors are transformed to binary NiCoCO3 due to the effect of absolutely excess amount of CO2 3 . After 8 h reaction, the 3D pseudocubes stacked by porous 2D NiCoCO3 nanoflakes were completely formed and the self-delamination process can probably be ascribed to the assistance of ethylene glycol. An interpretation has been proposed that the ethylene glycol molecules could act as soft templates to form interconnected layered structure by hydroxyl groups intercalating into neighboring layers [33,34]. Furthermore, ICP-AES results show that the weight ratio of Ni/Co concentrations for 2 h, 4 h, 8 h reaction precursors are 19/45, 4.6/44.4, 3.7/50, respectively, indicating Ni was leached with the increasing of the reaction time. Combined with the fact that Co3O4 product synthesized in the same condition shows obviously different nanoflake morphology, we deduce that Ni was leached in the process of forming pseudocube


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Table 1 Structure parameters. Sample

Ni(wt. %)a

Co(wt. %)a


a (Å)c

Surface area (m2 g1)d

Pore volume (cm3 g1)d



e 22 8

e 53 68

15.2 10.6 14.1

8.0596 8.1199 8.0989

79 116 98

0.21 0.50 0.20

Nanoflake Nanorod pseudocube

a b c d

Determined by ICP-AES. Calculated from XRD patterns by Scherrer equation. Calculated from the XRD patterns by least-squares estimation. Calculated from nitrogen adsorption/desorption isotherm using BET methods.

Fig. 2. SEM images of the as-obtained (a, d) Co3O4, (b, e) NeNCO,and (c, f) M-NCO products.

Fig. 3. TEM images of the as-obtained (a, b) NeNCO and (d, e) M-NCO products. EDS elemental mapping images of (c) NeNCO and (f) M-NCO from the corresponding HAADF-STEM data.

superstructure due to the absolute excess amount of urea following the below equations:

COðNH2 Þ2 þ H2 O ¼ CO2 þ 2NH3


 2þ þ CO3 2 þ 4H2 O NiCO3 þ 4NH3 ,H2 O ¼ NiðNH3 Þ4


After heating the pseudocube superstructure precursor in the air, carbonate precursors convert into metal oxides nanoparticles and retain the layered 3D-pseudocube structure. The schematic illustration of the probable formation process of M-NCO product was shown in Scheme 1. The mesoporous architecture of M-NCO can also be confirmed by N2-sorption results in Fig. 6a. A typical type-Ⅳ isotherm with a mixed H3 and H1 type hysteresis loop

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Fig. 4. XRD patterns of the NieCo-based precursors after 2 h, 4 h, and 8 h hydrothermal reactions.

starting from P/P0 ¼ 0.5 demonstrates the presence of interlayer pores. The Co3O4 and NeNCO products show different adsorption isotherm curve (Figs. S4a and b, Supporting Information). In order to reveal the existence of regular interlayer pores, BJH method was used to analyze the pore size distributions of the three products. As displayed in Fig. 6b, M-NCO product shows a high-intensity size distribution (centered at 33 nm) at mesoporous region. While, for the disordered nanoflakes and nanorods morphology, the Co3O4 and NeNCO show wider pore size distribution at both mesoporous and macroporous regions centered 39 and 22 nm, respectively. The BET surface area values of Co3O4, NeNCO and M-NCO are calculated to be 79, 116 and 98 m2 g1 with pore volume of 0.21, 0.50 and 0.20 cm3 g1. The moderate surface area and pore volume of MNCO correspond well with the SEM results in Fig. 2c and may decrease the side reactions with the electrolyte, leading to higher cycling stability. 3.2. Electronic property The electronic properties of the as-obtained products are characterized by X-ray photoelectron (XPS) and synchrotron-based soft X-ray spectroscopy (sXAS) measurements. The survey spectra (Fig. 7a) show the existence of Co, O in Co3O4 and Co, Ni, O in NeNCO, M-NCO products, respectively, without any other impurities. Table 2 shows that the surface Ni/Co atomic ratios of NeNCO and M-NCO are 40/60 and 18/82, respectively, which is well consistent with the ICP results. By using a Gaussian fitting method, the Co2p spectra (Fig. 7b) are best fitted considering two spin-orbit doublets characteristic of Co2þ and Co3þ and two couple of shakeup satellites. The Ni 2p spectra (Fig. 7c) can be also best fitted into Ni2þ

Fig. 6. (a) N2-sorption isotherm of M-NCO product. (b) Pore size distribution of the asobtained Co3O4, NeNCO, and M-NCO products.

and Ni3þ and two shakeup satellites. Interestingly, for NeNCO and M-NCO products, the ratios of Ni2þ/Ni3þ are exactly the same, but the ratios of Co2þ/Co3þ are obviously changed and more Co2þ species exist on the M-NCO product (Table 2). Concerning the atomic structure and charge arrangement in spinel NixCo3-xO4, there is still controversy in literature [35]. Ni3þ was known to locate in octahedral sites of the spinel structure in coexistence with the Ni2þ species. Octahedrally located cobalt and tetrahedrally located cobalt is not as straightforward. For the 3D M-NCO sample the leach process made Co rearrangement in the structure, and further affected the oxidation state distributions. The O 1s regions show three oxygen contributions (Fig. 7d). Specifically, the main peak at ~529.8 eV agrees with the lattice O2 (Olat), and the third peak at ~532.9 eV can be clearly attributed to multiplicity of physi- and chemi-sorbed water at or near the surface (Oads). While the second peak at ~531.2 eV is complicated, although

Fig. 5. SEM images of the NieCo-based precursors collected sequentially after (a) 2 h, (b) 4 h, and (c) 8 h hydrothermal reactions.


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Fig. 7. (a) XPS surveys, high-resolution (b) Co 2p, (c) Ni 2p, and (d) O 1s XPS spectra of the as-obtained Co3O4, NeNCO, and M-NCO products.

Table 2 Detailed elements content and their valence determined by XPS results in Fig. 7. Sample

Ni(at. %)

Co(at. %)





e 40 18

e 60 82

33/67 36/64 53/47

65/26/8 40/49/9 50/37/12

e 84/16 84/16

the low level of hydroxyl contaminants or defects cannot be discounted, most of intensity is reported to be intrinsic to the NiCo2O4 surface (Osur). The ~531.2 eV peak is characteristic of all prepared products, but the intensity varies drastically, from 40 to 70% of the

main peak intensity, especially, it increased with the increasing amount of nickel content. The O K-edge XAS result in Fig. 8 confirmed that this trend was caused by the intrinsic of the NiCo2O4 surface [36]. In general, the pre-edge region between 525 and 532 eV corresponds to the excitations from an O 1s orbital to hybridized O 2p-Ni/Co 3d orbitals, while the higher main peaks at 541 eV stand for the excitations to hybridized O 2p-Ni/Co 4sp orbitals, which generally has awider bandwidth and therefore is largely similar for Ni and Co [37]. The pre-edge region is very sensitive to the local environment evolution around oxygen ions, and it is enlarged in Fig. 8b. It is clearly observed that with the increment of nickel element, the peak at 527.5 eV ascribed to O 2p-

Fig. 8. (a) O K-edges sXAS spectra of the as-obtained Co3O4, NeNCO, and M-NCO products. (b) Magnified view of pre-edge region in (a).

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Fig. 9. (a) Cyclic voltammogram for the first five cycles of M-NCO at a scan rate of 0.2 mV s1. (b) Galvanostatic chargeedischarge profiles of M-NCO at 200 mA g1. (c) Cyclic voltammogram of M-NCO at different scan rates. (d) Linear relationship of peak currents versus the square root of scan rate for the as-prepared Co3O4, NeNCO, and M-NCO electrodes.

Ni 3d orbitals is gradually appeared and enhanced, while the peak at 529 eV ascribed to O 2p-Co 3d orbitals is slightly shift to lower energy due to the increase extent of hybridization. All of the above analyses reveal the changes in the electronic and local structures for the three Co3O4, NeNCO, and M-NCO products, which can usually lead to different electrochemical performances [38].

3.3. Electrochemical performance The Li-ion storage behavior of the M-NCO was initially investigated using cyclic voltammetry (CV). Fig. 9a displays representative CV curves of M-NCO for the first five cycles at a scan rate of 0.2 mV s1 within the voltage window of 0.01 and 3.0 V (vs. Li/Liþ). The CV curves display a trend that is in good agreement with the discharge-charge voltage profiles at a current density of 200 mA g1 in Fig. 9b. In the first cycle, an irreversible cathodic peak is observed at 0.8 V, corresponding to the reduction of NixCo3xO4 to metallic Ni and Co with the formation of Li2O. The following anodic sweep shows two oxidation peaks centered at 1.4 V and 2.1 V, which can be attributed to the oxidation of metallic Ni and Co to NiOx and CoOx [39]. In the subsequent cycles, the reduction peak shifts to a higher potential at 1.0 V and both the anodic and the cathodic peaks overlap very well, indicating its good electrochemical reversibility. Compared with M-NCO, the CV curves of NeNCO and Co3O4 show some little different potential peaks, which might be caused by the different amount of Ni content in the samples (Fig. S5, Supporting Information). Meanwhile, cyclic voltammograms at different scanning rates were also collected and shown in Fig. 9c. The peak currents exhibit a linear relationship with n1/2 as plotted in Fig. 9d. The M-NCO electrode shows moderate Liþ diffusion coefficient compared with Co3O4 and NeNCO. Considering the size of 3D pseudocube-superstructure M-NCO is about 10e30 mm, the comparable Liþ diffusion coefficient with 2D

nanoflake Co3O4 and 1D nanorod NeNCO is quite amazing, which will benefit the high-rate performance. High-rate cycling performance anode materials hold great promise for actual applications in LIBs due to the increasing demand for high-power applications, and metal oxide electrodes always suffer more severe stress impact, showing pulverization phenomenon during high-rate operation. Therefore, the cycling behaviors of Co3O4, NeNCO, and M-NCO electrodes were first evaluated at a current density of 1000 mA g1, as displayed in Fig. 10a. The M-NCO electrode exhibits superior reversibility and cyclability, and a reversible capacity as high as 797 mAh g1 is retained after 300 galvanostatic cycles, with coulombic efficiency (CE) approaching 100%. By contrast, in the case of Co3O4 and NeNCO electrodes, the capacities rapidly decline to a very low level, especially NeNCO electrodes. Furthermore, M-NCO electrode also shows ultrastable cycling capability at a small current density of 200 mA g1. From Fig. 10b, we can observe that, after a slightly capacity growth in the initial cycles, it gets stable and shows a capacity of 1225 mA h g1 after 200 cycles, which is superior to that mentioned in the previous literatures (Table S1). Ex-situ SEM image of M-NCO electrode after 300th cycles at a current density of 1000 mA g1 confirms the excellent cycling stability (Fig. 10d). It shows that the architecture of M-NCO sample has undergone negligible changes and displayed its original morphology, indicating its excellent structural stability. The rate performances of M-NCO electrodes were evaluated via a multistep galvanostatic strategy, as illustrated in Fig. 10c and Fig. S6. The electrodes shows average charge capacities of ~1060, ~1035, ~955, ~820, and ~645 mA h g1 at the corresponding current densities of 200, 500, 1000, 2000 and 4000 mA g1. Even at the high current density of 4000 mA g1, it still delivered a capacity of 645 mA g1, which is much higher than the theoretical specific capacity of graphite. When the current rate decreases again to


Q. Yang et al. / Journal of Alloys and Compounds 814 (2020) 152319

Fig. 10. (a) Cycling performance and coulombic efficiency of M-NCO at 1000 mA g1; Cycling performances of Co3O4 and NeNCO are also plotted for comparation. (b) Cycling performance of M-NCO at 200 mA g1. (c) Rate performance of M-NCO. (d) SEM micrograph of M-NCO electrode after 300 cycles at 1000 mA g1.

200 mA g1, an ultrahigh reversible capacity of 1079 mAh g1 is still achieved after 200 deep cycles, which was even slightly higher than the initial capacity at the same current rate. Additionally, electrochemical impedance spectroscopy (EIS) analysis of the NixCo3-xO4 electrode at different cycles provided further evidence of the superior electrode reaction kinetics and stability of the battery (Fig. S7). The two Nyquist curves were similar. Specifically, in the spectra, the intercepts of the curves on the real axis are 7.5 and 3.8 U for the 1st and 100th cycles, respectively, implying very low internal resistance (Re) of the NixCo3-xO4 electrode [2,40]. The highfrequency arc is indicative of solid electrolyte interface (SEI) film resistance (RSEI), while that at the medium frequency region is attributed to the charge-transfer resistance (Rct) at the electrode/ electrolyte interface [2]. The linear region in low-frequency region corresponds to Warburg diffusion resistance (Zw) in the solid electrode materials [2]. The RSEI (about 20e30 U) and Rct (<10 U) were relatively small and changed slightly with cycling, implying the formation of thin and stable SEI on the surface of the M-NCO electrode and easy transfer of ions and electrons at the electrolyteelectrode interface after the repeated cycle [40]. Besides, the little variation of the slope of the steep straight line at a low frequency indicates fast and stable solid-state Liþ diffusion [40]. The above results unambiguously highlight the favorable mass and charge transport kinetics in the battery. The integrated electrochemical curves indicate that the M-NCO electrodes achieved the simultaneous optimization of multiple constraints (capacity, cycling stability, and rate capability) as an anode material. In a word, the mesoscopic 3D pseudocube superstructures stacked by 2D nanoflake with 0D nanoparticle as basic unit can effectively alleviate the volume expansion and improving the electrochemical performance. NixCo3-xO4 electrodes with this special 3D structure should be a very promising anode for LIBs.

4. Conclusion In conclusion, we have demonstrated a facile strategy to synthesize a special 3D NixCo3-xO4 pseudocube superstructure, which was stacked by 2D nanoflake with tiny 0D nanoparticle as basic unit. With unique morphology, pore distribution, electronic property as well as local structure, the as-prepared NixCo3-xO4 was applied as lithium-ion battery anode, exhibiting the most impressive lithium storage performance. High reversible capacity of 1225 mA h g1 after 200 cycles at 200 mA g1 and excellent rate performance of 645 mA h g1 at 4000 mA g1 were both demonstrated. Contrast with 1D NiCo2O4 nanorods and 2D Co3O4 nanoflakes which only attained limited cycling abilities by using similar synthetic method, the NixCo3-xO4 possesses several critical merits: (1) 3D hierarchical superstructure consist by 0D nanoparticles and 2D nanoflakes will facilitate the diffusion of lithium ions and buffer the volume expansion during cycles; (2) the well-formed mesoporous feature and modest specific surface areas should be a good compromise in dealing with the contradiction of alleviating the side reactions and keeping the well contact of electrode/electrolyte; (3) the nickel leach process in the forming NixCo3-xO4 product makes Ni sites partially be substituted by Co element, which produces a synergistic effect to harvest novel electronic properties for more potential applications. Acknowledgements This work is supported by National Science Foundation of China (NSFC) (grant nos. 21802044 and 11505280), Fundamental Research Funds for Central Universities, National Key Basic Research Program of China (2013CB921800), and National High Technology Research and Development Program of China

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(2014AA123401). We also acknowledge the support from National Synchrotron Radiation Laboratory (NSRL) at Hefei for the sXAS experiments.


Appendix A. Supplementary data


Supplementary data to this article can be found online at



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