Co3O4 nanocomposites as high-performance anode for lithium-ion batteries

Journal of Colloid and Interface Science xxx (xxxx) xxx

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Porous N-doped carbon nanoflakes supported hybridized SnO2/Co3O4 nanocomposites as high-performance anode for lithium-ion batteries Jinkai Wang a, Hongkang Wang a,⇑, Tianhao Yao a, Ting Liu a, Yapeng Tian b, Chao Li c, Fang Li a, Lingjie Meng c, Yonghong Cheng a a State Key Lab of Electrical Insulation and Power Equipment, Center of Nanomaterials for Renewable Energy (CNRE), School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, China b Key Laboratory of the Ministry of Education, School of Electronic & Information Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China c Xi’an Key Laboratory of Sustainable Energy Material Chemistry, School of Science, and Instrument Analysis Center, Xi’an Jiaotong University, Xi’an 710049, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 27 August 2019 Revised 23 October 2019 Accepted 24 October 2019 Available online xxxx Keywords: Lithium ion batteries N-doped carbon nanoflakes SnO2 Co3O4 Atomic hybridization

a b s t r a c t Alloy-/conversion-type metal oxides usually exhibit high theoretical lithium storage capacities but suffer from the large volume change induced electrode pulverization and the poor electric conductivity, which limit their practical applications. Hybrid/mixed metal oxides with different working mechanisms/potentials can display advantageous synergistic enhancement effect if delicate structure engineering is performed. Herein, atomically hybridized SnO2/Co3O4 nanocomposites with amorphous nature are successfully cast onto the porous N-doped carbon (denoted as NC) nanoflakes through facile pyrolysis of the tin (II) 2-ethylhexanoate (C16H30O4Sn) and cobalt (II) 2-ethylhexanoate (C16H30O4Co) mixture within NC nanoflakes in air at 300 °C for 1 h. The Sn/Co atomic ratio and the loading amount of SnO2/ Co3O4 can be readily controlled, whose effect on lithium storage are investigated as anodes for lithium ion batteries (LIBs). Notably, SnO2/Co3O4@NC (RSn/Co = 1.25) nanoflakes exhibit the most excellent lithium storage properties, delivering a reversible capacity of 1450.3 mA h g 1 after 300 cycles at 200 mA g 1, which is much higher than that of the single metal oxide SnO2@NC and Co3O4@NC electrodes. Ó 2019 Elsevier Inc. All rights reserved.

⇑ Corresponding author. E-mail address: [email protected] (H. Wang). https://doi.org/10.1016/j.jcis.2019.10.096 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Please cite this article as: J. Wang, H. Wang, T. Yao et al., Porous N-doped carbon nanoflakes supported hybridized SnO2/Co3O4 nanocomposites as highperformance anode for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.096

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J. Wang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

1. Introduction

2. Experimental section

Lithium-ion batteries (LIBs) have been widely used in portable electronic devices and electric vehicles due to their environmental friendliness, high specific capacities and long lifespan [1–4]. However, the commercial graphite anode cannot satisfy the increasing demands for higher energy densities owing to its lower theoretical capacity of 372 mA h g 1 [5–7]. Therefore, exploration of alternative anode materials with higher lithium storage capacity and better cycle stability has become a major concern for developing the next-generation LIBs. Among various candidate anodes, metal oxides of SnO2 and Co3O4 have been widely studied because of their excellent physicochemical properties, higher lithium storage capacities and natural abundance with low cost. As a typical LIB anode, SnO2 has a high theoretical capacity of ~780 mA h g 1 which exhibits a two-step reaction mechanism: the generally recognized ‘‘irreversible” reduction of SnO2 into metallic Sn (SnO2 + 4Li+ + 4e ? Sn + 2Li2O) and the reversible alloying-dealloying process between Sn and LixSn (Sn + xLi+ + xe M LixSn (0  x  4.4)) [8–10]. Differently, Co3O4 is a conversion-type LIB anode, whose reaction mechanism is based on the reversible redox reaction of Co3O4 + 8Li+ + 8e M 3Co + 4Li2O, exhibiting a high theoretical capacity of ~890 mA h g 1 [11–13]. However, metal oxides as anode materials for LIBs usually suffer from a severe pulverization problem, namely, the large volume expansion/contraction upon lithium insertion/extraction, which causes the electrode destruction and finally the fast capacity fading upon cycling [14–16]. In order to overcome the above drawbacks, hybridizing metal oxides with different working potentials and mechanisms has been suggested as an effective strategy, as the step-wise lithium insertion/extraction of different components can function as buffering matrix for each other, thus alleviating the volume change induced electrode pulverization. Recently, it has been reported that various SnO2/Co3O4 hybrid composites have showed synergistically enhanced lithium storage performance through bringing additional multiple electrochemical active sites and shortening the lithium diffusion path [17,18]. For example, Park et al. reported the fabrication of Janus-structured mutually doped SnO2/Co3O4 composite, delivering a reversible capacity of 1058.7 mA h g 1 at 1 A g 1 for 1000 cycles [18]. Kim et al. synthesized SnO2@Co3O4 hollow nanospheres through the template-based sol-gel coating method, and the hybrid exhibited an extraordinary reversible capacity of 962 mA h g 1 at 100 mA g 1 after 100 cycles [19]. Zhao et al. successfully prepared the three-dimensional graphene foams encapsulated hollow SnO2@Co3O4 spheres via hydrothermal method using spherical SiO2 particles as templates, which delivered a reversible specific capacity of 815.2 mA h g 1 after 100 cycles at 200 mA g 1 [20]. It’s well known that metal oxides also suffer from the poor electrical conductivity, thus complexing SnO2/Co3O4 hybrid with highly conductive carbonaceous materials has also been received considerable attentions [19,20]. Carbonaceous materials (such as graphene and carbon nanofibers) especially with N-doping can bring additional lithium storage capacity, enhance the electrical conductivity and buffer the volume changes of the electrodes [21–23]. Herein, we successfully designed and fabricated porous N-doped carbon (NC) nanoflakes with interconnected nanocages via a combined strategy of chemical vapor deposition (CVD) and template synthesis. Besides, different from the complicated hydrothermal method, we successfully cast amorphous SnO2/ Co3O4 nanocomposites with controlled composition engineering on the NC through a facile pyrolysis synthesis, namely, annealing C16H30O4Sn/C16H30CoO4@NC mixture in air at 300 °C for 1 h. Interestingly, the as-prepared SnO2/Co3O4@NC demonstrated superior lithium storage properties when used as an anode for LIBs.

2.1. Materials synthesis 2.1.1. Synthesis of N-doped carbon (denoted as NC) nanoflakes NC nanoflakes were synthesized by a combined method of chemical vapor deposition (CVD) and template synthesis, according to our previous report but with some modification (Scheme S1) [24]. In a typical synthesis, 2 g flake-like magnesium carbonate basic pentahydrate ((MgCO3)4
Mg(OH)2
5H2O) was placed in a tube furnace and heated at 900 °C for 30 min under Ar (100 sccm) flow with a heating rate of 10 °C min 1, and pyridine (C5H5N), which severed as nitrogen-carbon source, was introduced by the Ar flow during the heating process. When temperature was high enough, (MgCO3)4
Mg(OH)2
5H2O started to decompose into MgO and the pyridine carbonized forming N-doped carbon shells on the surface of MgO (denoted as MgO@NC). Afterwards, the MgO template was removed by etching with diluted HCl solution, thus the NC nanoflake were finally obtained after washing thoroughly with distilled water and drying at 60 °C overnight. 2.1.2. Synthesis of NC nanoflake supported SnO2/Co3O4 composites (SnO2/Co3O4@NC) Firstly, 20 mg NC nanoflakes were dispersed in 5 mL ethanol under ultrasonic for 1 h. Then, 100 mg the mixture of tin (II) 2-ethylhexanoate (C16H30O4Sn, Aldrich) and cobalt (II) 2ethylhexanoate (C16H30CoO4) with Sn/Co atomic ratio of 1/1 were added into the above suspension under continuous ultrasonic irradiation for another 0.5 h. Then the mixture was dried at 60 °C in order to completely evaporate the ethanol. Afterwards, the resultant C16H30O4Sn/C16H30CoO4@NC mixture was annealed in air at 300 °C for 1 h, thus the SnO2/Co3O4@NC with conformal flakemorphology was finally obtained. The effect of the Sn/Co atomic ratio (RSn/Co) on the structure and the lithium storage properties were also investigated, and the products were denoted as SnO2/ [email protected], SnO2/Co3O4@NC-1 and SnO2/Co3O4@NC-2, where the numbers refer to the theoretical values, calculated on the base of the introduced amounts of C16H30O4Sn/C16H30CoO4. Note that the real RSn/Co values were slightly deviated from the theoretical ones, which were summarized in Table S1 and Fig. S1. For comparison, SnO2@NC and Co3O4@NC were also prepared under the same condition but with sole presence of C16H30O4Sn or C16H30CoO4. 2.2. Materials characterization The crystal structures were measured on X-ray diffraction (XRD, Bruker D2 Phaser X-ray diffractometer) with Cu Ka radiation (k = 1.5418 Å). The microstructures and morphologies were characterized by scanning electron microscopy (SEM, Quanta250F FEI) and transmission electron microscopy (TEM) using JEOL2100 and JEM-F200 TEM operated at 200 kV. Raman spectra were acquired by a Renishaw Raman RE01 scope. Thermogravimetric analyses (TGA) were performed on a Mettler thermal analysis TGA/DSC system in air. X-ray photoelectron spectroscopy (XPS) was carried out by using the Thermo Fisher XPS instrument (ESCALAB Xi+). Brunauere-Emmette-Teller (BET) method was used to determine the specific surface area and the pore structure, using a Quantachrome Surface Area Analyzer (Autosorb iQ-MP) to obtain the nitrogen sorption isotherms at 77 K. 2.3. Electrochemical measurements For the fabrication of working electrodes for LIBs, a paste containing 80 wt% active materials, 10 wt% acetylene black and 10 wt% polyacrylic acid (PAA, Sigma-Aldrich) was prepared by

Please cite this article as: J. Wang, H. Wang, T. Yao et al., Porous N-doped carbon nanoflakes supported hybridized SnO2/Co3O4 nanocomposites as highperformance anode for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.096

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grinding the mixture using distilled water as solvente. Afterwards, the paste was pasted onto Cu foil and then dried at 70 °C overnight in a vacuum oven. The active material in each electrode was about 1.0 ± 0.2 mg. Coin-type cells (CR2025) were assembled in an Arfilled glove box with H2O and O2 contents less than 1.0 ppm, in which lithium foil was used as the counter electrode, and the separator was a Celgard 2400 microporous membrane. The electrolyte is 1.0 M LiPF6 in ethylene carbonate/dimethyl carbonate (1/1 in volume). Cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) were performed on the electrochemical station (Autolab PGSTAT 302 N). Galvanostatic discharge/charge tests were measured on the NEWARE battery test system in the potential window of 0.01–3.0 V (vs. Li/Li+). EIS measurement is tested with the frequency ranging from 100 kHz to 0.01 Hz with a 5 mV AC amplitude applied.

3. Results and discussion Note that tin (II) 2-ethylhexanoate (C16H30O4Sn) and cobalt (II) 2-ethylhexanoate (C16H30O4Co) are similar in chemical formula as well as structure, which both can be dissolved in ethanol and mixed in atomic scale. As schematically shown in Fig. 1, with the evaporation of the ethanol and the presence of the NC nanoflakes, the Sn/Co precursors can be readily deposited on the surface of the NC nanoflakes, leading to the uniform distribution of the Sn/Co precursors in atomic scale. After annealing the Sn/Co/NC precursors, the atomically hybridized SnO2/Co3O4 nanocomposites can be readily cast on the NC nanoflakes. More importantly, the SnO2 and Co3O4 contents can be easily adjusted by controlling the introduction amount of the tin (II) or cobalt (II) 2-ethylhexanoate, and the relative contents of SnO2 and Co3O4 in the SnO2/Co3O4@NC nanocomposites play an important role on their lithium storage properties (Figs. S2–7). Fig. 2a compares the XRD patterns of the as-prepared nanocomposites with different Sn/Co atomic ratios. All the diffraction peaks located at 26.6°, 33.8° and 51.8° for the SnO2@NC nanocomposites can be indexed to the (1 1 0), (1 0 1) and (2 1 1) planes of the tetragonal SnO2 phase (JCPDS No. 41–1445). With introduction and increasing the Co content in the composites, the peaks belonging to the SnO2 phase become minor, while the peaks assigned to the Co3O4 gradually appear. For the Co3O4@NC, all the diffraction peaks can be attributed to the cubic Co3O4 phase (JCPDS No. 431003). Thermogravimetric analysis (TGA) was also performed to

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determine the carbon content in the composites, and Fig. 2b compares the TGA curves of the as-prepared products. As expected, the carbon contents in all the products are approximately 43 wt%, which can be readily adjusted by controlling the introduction amount of the Sn/Co precursors with easiness on engineering the atomic ratio of Sn/Co. Fig. 2c displays the Raman spectra of all the products, and two broad peaks at 1351 and 1583 cm 1 are all observed, which can be assigned to the amorphous/defective carbon species (D-peak) and the ordered graphitic carbon species (G-peak), respectively. Moreover, the ID/IG ratios for the SnO2@NC, SnO2/Co3O4@NC-2, SnO2/Co3O4@NC-1, SnO2/[email protected] and Co3O4@NC are almost the same as about 1.06, indicating that the disordered N-doped carbon nanoflakes are only functioned as a support and would not be affected by the pyrolysis of Sn/Co precursors when annealing in air. Differently, four peaks located at 190.1, 467.8, 510.7 and 668.8 cm 1 become more and more obvious with the increasing Co content, which correspond to the F12g, Eg, F22g and A1g modes of Co3O4, respectively [25]. The N2 adsorption-desorption isotherms of the NC nanoflakes and SnO2/Co3O4@NC-1 products are shown to investigate the specific surface area and the pore structure. As shown in Fig. 2d, the NC and SnO2/Co3O4@NC-1 products both exhibit the typical type-IV curves [26], indicating the presences of large numbers of mesopores, and the specific surface areas for the NC and SnO2/Co3O4@NC-1 products can be calculated as 1177.5 and 610.1 m2 g 1, respectively, according to the Brunauer Emmett Teller (BET) method. In addition, the pore size distribution plots reveal that both the NC and SnO2/Co3O4@NC-1 products possess mesopores (2–50 nm), and the decreased pore volume of SnO2/Co3O4@NC-1 suggests the occupation of the pores by the introduced SnO2/Co3O4 mixture. The morphological and microstructural characteristics of the NC and SnO2/Co3O4@NC-1 nanoflakes were revealed by SEM and TEM analyses. Fig. 3a shows the SEM image of the NC nanoflakes, which exhibits the flake-like morphology, and the TEM/HRTEM images reveal that the NC nanoflakes consist of interconnected nanocages with sizes of several to tens of nanometers (Fig. 3b, c). After annealing with NC nanoflakes with C16H30O4Sn/C16H30O4Co in air, the asobtained SnO2/Co3O4@NC-1 product still shows well preserved flake-like morphology, and no irregular aggregates can be observed within the nanoflakes (Fig. 3d). Apparently, no difference in morphology can be observed at low-magnified TEM image (Fig. 3e). However, HRTEM image clearly shows the appearance of nanoparticulate aggregates which are homogenously distributed on the NC

Fig. 1. Schematic illustration of the synthesis of hybridized SnO2/Co3O4 nanocomposites on the porous N-doped carbon nanoflakes.

Please cite this article as: J. Wang, H. Wang, T. Yao et al., Porous N-doped carbon nanoflakes supported hybridized SnO2/Co3O4 nanocomposites as highperformance anode for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.096

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Fig. 2. (a) XRD patterns, (b) TGA curves and (c) Raman Spectra of (Ⅰ) SnO2@NC, (Ⅱ) SnO2/Co3O4@NC-2, (III) SnO2/Co3O4@NC-1, (Ⅳ) SnO2/[email protected] and (Ⅴ) Co3O4@NC nanocomposites. (d) Nitrogen sorption isotherms with inset showing the pore size distributions of the NC nanoflakes and SnO2/Co3O4@NC-1.

Fig. 3. (a) SEM, (b) TEM and (c) HRTEM images of the NC nanoflakes. (d) SEM, (e) TEM and (f) HRTEM images of the SnO2/Co3O4@NC-1 nanoflakes. (g) Dark-field TEM image of a single SnO2/Co3O4@NC-1 nanoflake and the corresponding elemental maps of Sn (h), Co (i), O (j), N (k) and C (l).

Please cite this article as: J. Wang, H. Wang, T. Yao et al., Porous N-doped carbon nanoflakes supported hybridized SnO2/Co3O4 nanocomposites as highperformance anode for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.096

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nanoflakes (Fig. 3f). Interestingly, no apparent nanocrystallites with lattice fringes can be observed, indicating that both SnO2 and Co3O4 are poorly crystalline with amorphous nature, as Sn/ Co precursors are mixed in atomic scale which may result in the mutual doping in each other with interrupted lattice arrangement. In order to further certify the coexistence of SnO2 and Co3O4 on the NC nanoflakes, the elemental distributions were investigated by the elemental EDS maps (Fig. 3g–l). Apparently, the Sn, Co, N and O elements are uniformly distributed within the carbon matrices for the SnO2/Co3O4@NC-1, which further confirms that the SnO2 and Co3O4 are loaded on the NC nanoflakes. In order to gain the in-depth insight into the compositions and the chemical states of the SnO2/Co3O4@NC-1 nanocomposite, X-ray photoelectron spectroscopy (XPS) was measured and shown in Fig. 4. In the survey XPS spectrum (Fig. 4a), Sn, Co, O, C and N elements are all well detected and present in the composite. Fig. 4b compares the Sn 3d XPS spectra in the SnO2@NC and SnO2/Co3O4@NC-1, revealing the introduction of Co3O4 results in the shift of Sn 3d spectrum to the lower binding energy. The two characteristic peaks at 497.3 and 495.8 eV in the SnO2@NC can be correspondingly assigned to the Sn 3d5/2 and Sn 3d3/2 of SnO2 with the highest oxidation state of Sn4+ [27–29]. However, these two characteristic peaks in SnO2/Co3O4@NC-1 shift to 486.7 and 495.1 eV correspondingly, indicating the partial reduction of Sn4+ to Sn2+. The introduction of Co atom in the SnO2 lattice (namely, doping) would cause the charge imbalance, which reduces the electron density around the Sn atoms. According to the polarization theory [30], a small amount of Sn2+ is thus generated, and the surrounding electron density cloud would increase, resulting in a decrease of binding energy [31,32]. Fig. 4c shows the Co 2p XPS spectrum, in which

(a)

two major peaks locate at 795.8 and 780.1 eV can be ascribed to the Co 2p1/2 and 2p3/2, respectively. Besides, two shoulder peaks at 801.1 and 785.3 eV correspond to the satellite peaks, indicating the presence of Co2+ and Co3+ in Co3O4 [25]. Fig. 4d show the N 1 s XPS spectrum in the SnO2/Co3O4@NC-1, which arises from the Ndoped carbon nanoflakes, indicating the successful N-doping during the carbonization of pyridine (C5H5N). The N 1s spectrum can be deconvoluted into three peaks located at 402.7, 399.5 and 397.2 eV, which can be ascribed to graphitic N, pyrrolic N, and pyridinic N, respectively [33]. The electrochemical properties of the SnO2@NC, SnO2/Co3O4@NC-2, SnO2/Co3O4@NC-1, SnO2/[email protected] and Co3O4@NC electrodes were investigated as anodes for LIBs in half-cell using lithium foil as counter/reference electrode. Fig. 5a shows the cyclic voltammetry (CV) curves of the SnO2/Co3O4@NC-1 electrode for the initial 5 cycles in 0.01–3.0 V at a scan rate of 0.2 mV s 1. In the first cathodic scan, the broad reduction peaks in the range of 2.0–0.3 V can be associated with multiple electrochemical reactions, including the reductions of the SnO2 and Co3O4 into metallic Sn and Co (SnO2 + 4Li+ + 4e ? Sn + 2Li2O, Co3O4 + 8Li+ + 8e ? 3Co + 4Li2O), and the irreversible formation of the solid electrolyte interface (SEI) film [18,24,25]. Moreover, the peaks in the range of 0.01–0.65 V at the first cathodic scan can be observed, which is usually ascribed to the reversible Li-Sn alloying process (Sn + xLi+ + xe ? LixSn (0 < x  4.4)) and the interfacial Li storage in the N-doped carbon nanomaterials [17,34,35]. For the first anodic scan, the oxidation peak at around 0.6 V is associated with the dealloying reaction of LixSn to metallic Sn [36]. Besides, the other two peaks at 1.35 V and 2.10 V correspond to the reversible oxidation of the metallic Sn into SnO and SnO2, respectively, along with

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Fig. 4. (a) Survey XPS spectrum of the SnO2/Co3O4@NC-1 nanocomposites and its corresponding high resolution XPS spectra of (b) Sn 3d, (c) Co 2p and (d) N1s. Sn 3d XPS spectrum of SnO2@NC is also shown in (b).

Please cite this article as: J. Wang, H. Wang, T. Yao et al., Porous N-doped carbon nanoflakes supported hybridized SnO2/Co3O4 nanocomposites as highperformance anode for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.096

J. Wang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

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Fig. 5. Electrochemical properties of the SnO2@NC, SnO2/Co3O4@NC-2, SnO2/Co3O4@NC-1, SnO2/[email protected] and Co3O4@NC electrodes. (a) Cyclic voltammogram (CV) curves of the SnO2/Co3O4@NC-1 electrode for the initial 5 cycles at 0.2 mV s 1 in 0.01–3.0 V. (b, c) Galvanostatic discharge-charge profiles of the SnO2/Co3O4@NC-1 electrode (b) at 200 mA g 1 and (c) at different current densities, respectively. (d) Rate performance of all the electrodes at different current densities. (e) Cycle performance for all electrodes with the corresponding Coulombic efficiency (CE) of the SnO2/Co3O4@NC-1 electrode at 200 mA g 1. (f) CV curves of the SnO2/Co3O4@NC-1 electrode at different scan rates and (g) the corresponding logarithm peak current versus logarithm scan rate plots. (h) Nyquist plots of the SnO2/Co3O4@NC-1 electrode before cycling and after 300 cycles.

the reversible conversion reaction of Co and Li2O into Co3O4 [34,37,38]. It is worth noting that, the relative current density of the anodic peak at 1.35 V is usually lower than that at 0.6 V in SnO2-based anodes [24,39], the reverse peak intensity suggests the reversible formation between Li2O and SnO2 owing to the catalytic effect of the in situ generated transition metal Co nanograins, which thus leads to the greatly improved theoretical capacity of SnO2 (~1490 mA h g 1) [40–42]. From the second cycle onward, the cathodic curves become different with greatly decreased intensity, indicating the presence of the irreversible reactions owing to the structural destruction/reorganization and the formation of SEI film in the first cathodic scan. The peak at about 1.13 V can be attributed to the reversible reduction of oxidized Co into metallic Co [43], while the broad reduction peak at around 0.3 V can be ascribed to the reversible alloying process of metallic Sn with Li ions [44]. Remarkably, the CV curves overlap very well in the subsequent cycles, indicating the superior reversibility and good cycle stability of the Li+ insertion/extraction processes upon cycling. Fig. 5b compares the galvanostatic discharge-charge profiles of the SnO2/Co3O4@NC-1 electrode, which delivers initial specific discharge/charge capacities of 1321.8/805.4 mA h g 1 at 200 mA g 1,

with an initial Coulombic efficiency (ICE) of 60.9%. The irreversible capacity loss of the SnO2/Co3O4@NC-1 electrode could be mainly attributed to the formation of the SEI film, arising from the decomposition of electrolyte during the first discharge process [45]. Additionally, the Coulombic efficiencies gradually rise up to closely 100% for the following cycles. Moreover, the discharge-charge profiles of the SnO2/Co3O4@NC-1 electrode at different current densities are shown in Fig. 5c, which show similar profiles with increasing current density from 100 to even 2000 mA g 1. Even though the capacities decrease with increase of current densities, the Coulombic efficiencies reach approximately 100%, suggesting the good reversibility and high-rate capability. The rate performances of the SnO2@NC, SnO2/Co3O4@NC-2, SnO2/Co3O4@NC-1, SnO2/[email protected] and Co3O4@NC electrodes at different current densities ranging from 100 to 2000 mA g 1 is shown in Fig. 5d. The SnO2/Co3O4@NC-1 electrode delivers discharge capacities of 859.8, 802.9, 702.8, 539.1 and 406.1 mA h g 1 each after 10 cycles at current densities of 100, 200, 500, 1000 and 2000 mA g 1, respectively, which are higher than that of the other SnO2/[email protected], SnO2/Co3O4@NC-2, SnO2@NC and Co3O4@NC electrodes. When the current density goes back to 100 mA g 1, the SnO2/Co3O4@NC-1 electrode can still deliver a high reversible

Please cite this article as: J. Wang, H. Wang, T. Yao et al., Porous N-doped carbon nanoflakes supported hybridized SnO2/Co3O4 nanocomposites as highperformance anode for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.096

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Fig. 6. (a) SEM, (b) HRTEM (with inset of ED pattern) and (c) dark-field TEM images of the charged SnO2/Co3O4@NC-1 electrode after 300 cycles at 200 mA g Elemental maps of N (d), Sn (e), Co (f), O (g) and C (h) elements, corresponding to the dark-field TEM image in (c).

capacity of 939.7 mA h g 1 after another 10 cycles. In order to compare the rate performances of the electrodes, Table S2 summarized the capacity retention rates at different current densities at certain cycle numbers. Besides, Fig. 5e displays the cycle performances of the SnO2/Co3O4@NC-2, SnO2/Co3O4@NC-2, SnO2/Co3O4@NC-1, SnO2/[email protected] and Co3O4@NC electrodes at 200 mA g 1. The SnO2/Co3O4@NC-1 electrode delivers a discharge capacity of 1450.3 mA h g 1 even after 300 cycles, which is much higher than that of the other electrodes of SnO2/[email protected] (935.8 mA h g 1 after 160 cycles), SnO2/Co3O4@NC-2 (1079.2 mA h g 1 after 160 cycles), SnO2/Co3O4@NC-2 (814.4 mA h g 1 after 125 cycles) and Co3O4@NC (624.5 mA h g 1 after 135 cycles). Notably, a common phenomenon is observed that the capacities gradually increase in the following cycles, and this phenomenon widely happens in the transition metal compound based anodes [25,36,46–48]. This can be ascribed to the formation of a gel-like film resulting from decomposition of the electrolyte at low voltages under the catalytic effect of the in situ generated transition metal nanoparticles, and the gel-like film could bring additional pseudocapacitive lithium storage capacity [46]. In order to better understand the electrochemical kinetics of the hybrid SnO2/Co3O4 electrode, stepwise CV measurement of the SnO2/Co3O4@NC-1 electrode was performed at different scan rates varying from 0.2 to 2.0 mV s 1 (Fig. 5f). According to the equation: log(i) = log(a) + b log(v), in which i is peak current and v is scan rate, the cathodic/anodic peak currents in the CV curves increase with increasing the scan rates, indicating the presence of a pseudocapacitive behavior [25,49]. The b value is the key factor

1

. (d-h)

for measuring the solvated ions storage mechanism: pseudocapacitive-controlled process (b = 1, the capacity is independent of scan rates) or diffusion-controlled process (b = 0.5) [50–52]. The corresponding logarithm peak current versus logarithm scan rate plots are shown in Fig. 5g, which display linear relationship. The fitted slopes (namely, the b values) of the four peaks can be calculated as 0.83, 0.62, 0.77 and 0.83, respectively, indicating the mixed lithium storage mechanism in the current hybrid electrode. Specially, the presence of the pseudocapacitive process contributes to the high rate capability of the electrode materials, which can be associated with the novel hybrid structure of the SnO2/Co3O4. The hybridization of Sn-Co-O in atomic scale and their casting on the N-doped carbon nanoflakes in the surface region with large surface area and short diffusion length is greatly beneficial for fast lithium storage. Moreover, in order to further study the charge transfer resistance of the SnO2/Co3O4@NC-1 electrode, the Nyquist plots are shown in Fig. 5h. The SnO2/Co3O4@NC-1 electrode exhibits a charge transfer resistance (Rct) of 52 X before cycling, which even decreases to 20 X after 300 cycles, suggesting the fast charge transfer owing to the presence of the N-doped carbon nanoflakes. In addition, ex situ SEM/TEM analyses were also performed to study the structure evolution upon repeated lithiation/delithiation. Fig. 6a show the SEM image of the charged SnO2/Co3O4@NC-1 electrode after 300 cycles at 200 mA g 1, in which the nanoflake-like morphology is still well preserved, indicating the robustness of the electrode upon cycling. Furthermore, TEM/HRTEM images were also taken and shown in Fig. 6b-c, in which no large aggregated particles can be observed, and there is no apparent diffraction rings

Please cite this article as: J. Wang, H. Wang, T. Yao et al., Porous N-doped carbon nanoflakes supported hybridized SnO2/Co3O4 nanocomposites as highperformance anode for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.096

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in the electron diffraction (ED) pattern, indicating the amorphous nature of the cycle electrode. As shown in the EDS maps, the Sn, Co, O and N elements are well overlapped and uniformly distributed within the carbon matrix (Fig. 6c–h), suggesting the effective suppression of the grain coarsening and electrode pulverization. Moreover, the ex situ XPS analysis also confirmed the reversible reaction of SnO2 + 4Li+ + 4e M Sn + 2Li2O in the SnO2/Co3O4@NC-1 electrode (Figure S8). As compared with previous reported SnO2 and/or Co3O4 based anodes for LIBs (Table S3) [53–62], the currently reported SnO2/Co3O4@NC-1 electrode displays excellent lithium storage performance, which can be attributed to the novel structure characteristics of the hybrid electrode on the following aspects. First, the atomic-scale mixing of SnO2 and Co3O4 with amorphous nature demonstrated synergistic enhancement effect, which not only brings additional active sites for lithium storage and shortens the lithium diffusion distance, but also buffers the volume changes upon cycling, as SnO2 and Co3O4 exhibit different lithium storage mechanism and different working potential. Moreover, the ‘‘irreversible” formation of Li2O associated with SnO2 in the first discharge process can be catalyzed to be ‘‘reversible” owing to the presence of transition metal Co nanograins which are in situ generated within the SnO2-Co3O4 matrices, thus the theoretical capacity of SnO2 can be greatly improved (~1490 mA h g 1). Secondly, the N-doped nanoflakes with good conductivity and flexibility can not only enhance the electrode conductivity and buffer the volume changes, but also bring additional lithium storage capacity. These promising advantages are well integrated into the current material design, which finally contributed to the superior electrochemical properties of the SnO2/Co3O4@NC electrode. 4. Conclusions In summary, we have demonstrated a facile strategy to cast amorphous SnO2/Co3O4 hybrid (mixed in atomic scale) into porous N-doped carbon (NC) nanoflakes, in which NC nanoflakes were fabricated via a combined strategy of chemical vapor deposition (CVD) and template method, while the SnO2/Co3O4 hybrid with tunable Sn/Co molar ratio and loading amount can be easily deposited onto NC nanoflakes through facile pyrolysis of C16H30O4Sn/C16H30O4Co@NC mixture in air at 300 °C for 1 h. When examined as anodes for LIBs, the SnO2/Co3O4@NC-1 (RSn/Co = 1) nanoflake electrode exhibited superior lithium storage properties, delivering a reversible discharge capacity of 1450.3 mA h g 1 after 300 cycles at 200 mA g 1, which was much higher than that of the other counterpart electrodes (SnO2/Co3O4@NC (RSn/Co = 1, 0.5; SnO2@NC; Co3O4@NC). The superior electrochemical performance of SnO2/Co3O4@NC could be attributed to the novel structural characteristics: i) the atomically mixed amorphous SnO2 and Co3O4 anodes displayed synergistic enhancement effect, which can not only bring additional lithium active sites for lithium storage and shorten the lithium diffusion distance, but also alleviate the volume changes upon cycling owing to the stepwise lithium storage behaviors; ii) the N-doped carbon nanoflakes, serving as a conductive matrix, greatly improved the electrode conductivity and efficiently buffered the volumetric changes during lithiation/delithiation processes. More importantly, we developed a useful strategy to design hybrid metal oxides with different electrochemically active components, which is promising for developing advanced electrode materials for next-generation energy storage devices. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements This work was supported by the Natural Science Basis Research Plan in Shaanxi Province of China (No. 2018JM5085), and the Key Laboratory Construction Program of Xi’an Municipal Bureau of Science and Technology (201805056ZD7CG40). H.W. appreciates the support of the Tang Scholar Program from the Cyrus Tang Foundation. We thank Mr Chuansheng Ma and Ms Yanzhu Dai at International Center for Dielectric Research (ICDR) of Xi’an Jiaotong University, for the help with TEM/SEM measurements.

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Please cite this article as: J. Wang, H. Wang, T. Yao et al., Porous N-doped carbon nanoflakes supported hybridized SnO2/Co3O4 nanocomposites as highperformance anode for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.10.096