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Facile synthesis of N-doped carbon-coated nickel oxide nanoparticles embedded in N-doped carbon sheets for reversible lithium storage
Journal of Alloys and Compounds 745 (2018) 147e154
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Facile synthesis of N-doped carbon-coated nickel oxide nanoparticles embedded in N-doped carbon sheets for reversible lithium storage Zhiqing Jia a, b, Yingbin Tan a, Jiyang Sun a, b, Yongzhe Wang a, Zhonghui Cui a, *, Xiangxin Guo a, c, ** a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b University of Chinese Academy of Sciences, Beijing 100039, China c College of Physics, Qingdao University, Qingdao 266071, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 January 2018 Received in revised form 12 February 2018 Accepted 13 February 2018 Available online 22 February 2018
In this work, we present a facile strategy for the synthesis of well-structured transition metal oxides based composites by the decomposition of nitrate and polyvinylpyrrolidone (PVP) followed by simple oxidation at a low temperature and using them as high performance materials for reversible lithium storage. As proofs of this simple strategy, three kinds of TMOs, namely NiO, Fe2O3 and Co3O4, based composites have been prepared with this simple strategy. The morphology and structure analyses indicate that TMOs nanoparticles with a size of 10 nm are tightly coated by the in situ formed graphenelike carbon shell and evenly dispersed in the microsized nitrogen-doped carbon sheets (C@TMOs@NCSs) derived from the PVP. As lithium storage materials, the as-prepared C@NiO@NCSs composites show excellent electrochemical performances in terms of cycle stability (651 mAh g1 after 1500 cycles at 1 A g1 and 518 mAh g1 after 1400 cycles at 2 A g1) and rate capability (400 mAh g1 at 4 A g1). Such superior electrochemical performance is attributed the well-structured composites with features of tight carbon coating layer, evenly dispersed TMOs nanoparticles and robust carbon supports. © 2018 Elsevier B.V. All rights reserved.
Keywords: Reversible lithium storage Transition metal oxides Composites Cycle stability and rate capability
1. Introduction Lithium-ion batteries (LIBs) have been widely used in portable electronics and electric vehicles, owing to their high energy density and long lifespan [1e5]. Currently, one of the main research directions is searching for advanced electrode materials with enhanced lithium storage ability to improve the battery energy density [6e9]. Transition metal oxides (TMOs) have attracted considerable attention as their ability to uptake more than one lithium per transition metal, giving high theoretical specific capacities [10e17]. For example, the NiO and CuO can offer a high theoretical capacity of 718 and 674 mAh g1, respectively [17e21]. Despite TMOs anodes of high theoretical storage capacity, the fully
* Corresponding author. ** Corresponding author. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. E-mail addresses: [email protected] (Z. Cui), [email protected] (X. Guo). https://doi.org/10.1016/j.jallcom.2018.02.167 0925-8388/© 2018 Elsevier B.V. All rights reserved.
achieving of their theoretical capacity still faces challenges [15,22]: First, the TMOs usually have an intrinsic low electron conductivity, leading to poor reactivity and reaction kinetics upon lithium [23]. Second, these materials suffer from drastic volume expansion during the Liþ insertion-extraction processes, resulting in the pulverization of electroactive particles and then the fast capacity fading [19]. Finally, it is challenging to form a stable solid electrolyte interface (SEI) layer as the repeated volume change associated with repeated lithium insertion-extraction, leading to the limited cyclability and low Coulombic efficiency [24,25]. In response, tremendous efforts have been performed to optimize the electrochemical performance of TMOs anodes by enhancing their electrical conductivity and mitigating the adverse influences associated with large volume change. An effective strategy is to construct active/inactive composites with nanosized TMOs particles being evenly encapsulated into a conductive matrix (e.g., graphene, graphene oxides, carbon nanotubes and conductive polymers) [26e32]. In such design, carbon networks can enhance the electrical conductivity of the resulted hybrid materials, leading to the fasted electron transfer, the accelerated ionic diffusion across
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the electrolyte-electrode interface and the shortened diffusion pathway by confining the particles growth of TMOs during high temperature synthesis [33]. Simultaneously, the problem of volume expansion of the active material is tackled due to the physical buffer effect of the carbon matrix, leading to the prolonged lifetime [34]. Our recent study evidenced that well-designed nanocomposites with monodispersed carbon-coated electroactive particles can alleviate the large volume change upon cycling and provide excellent cycle stability and rate capability with high reversible capacity [35]. To synthesize the active/inactive composites, hydrothermal and solvothermal followed by calcining the resulted precursors at high temperature are the commonly used methods [15,19,23,36]. For example, Li et al. reported the high capacity composites of self-assembled porous CuO nanospheres decorated on reduced graphene oxide synthesized by solvothermal method [17]. Multi-yolk-shell copper oxide@carbon octahedral as high stability anode of LIBs has been synthesized by hydrothermal method [36]. And Mou et al. synthesized layer-cake-like Co3O4/C composites with hydrothermal technique [14]. In addition to the commonly used hydrothermal or solvothermal technique, developing of simple method with ability in large-scale production is greatly encouraged. Herein, we report a facile strategy for the synthesis of TMOs based composites and using them as high-performance materials for reversible lithium storage. In the as-prepared composites, the in situ formed nitrogen-doped graphene-like carbon coated transition metal oxides nanoparticles evenly embed in the nitrogen-doped carbon sheets (C@TMOs@NCSs). The nitrogen-doped carbon derives from the decomposition of polyvinylpyrrolidone (PVP). This sample procedure can be expanded to synthesize different kinds of TMOs based composites: three kinds of TMOs, namely NiO, Fe2O3 and Co3O4, based composites have been synthesized. As anode materials for LIBs, the obtained composites show excellent reversible lithium storage ability and rate capability, due to the in situ formed N-doped carbon coating layer and carbon matrix significantly increasing the conductivity of the abstained TMOs composites and alleviating the pulverization of TMOs during repeated lithium insertion and extraction. 2. Experimental 2.1. Chemicals Copper foil (25 mm), Polyvinylpyrrolidone (PVP) (MW ¼ 55000, AR), Ni(NO3)2$6H2O (98%), Co(NO3)2$6H2O (97.7%) and Fe(NO3)3$9H2O (98%) were purchased from Alfa Aesar. All of the chemicals were used directly as received. 2.2. Synthesis of the C@TMOs@NCSs composites Ni(NO3)2$6H2O (0.05 g) and PVP (0.1 g) were dissolved in 2 mL of ethanol by ultrasound for 5 min at room temperature. After forming homogeneous solution, it was evenly coated on the Cu foil substrate (20 cm 20 cm squares) by tape casting. Then the obtained Ni(NO3)2$6H2O-PVP/Cu film was heated to 100 C for 15 min to remove the ethanol solvent. The dried film was rolled up into a tube with the dimeter of 1 inch and then the obtained tube was wrapped by using a new copper strip. After that, the wrapped tube was annealed at 850 C for 30 min under the H2 (50 sccm) and Ar (500 sccm) flow to get the composite of C@Ni@NCSs, with the total pressure < 50 Torr. After fast cooled down to room temperature by opening the heating furnace without changing the gas flow, the obtained C@Ni@NCSs composites were scraped from Cu foil using a plastic knife. Finally, the C@NiO@NCSs composites were prepared by directly oxidized the as-synthesized C@Ni@NCSs powders in air
at 300 C for 60 min, with a ramp-rate of 5 C min1. C@Fe2O3@NCSs and C@Co3O4@NCSs were synthesized by following the same procedures with the use of their corresponding nitrate precursor. 2.3. Synthesis of NCSs The NCSs were obtained by removing the metal from C@Ni@NCSs composites by acid etching (2 M HCl for 24 h). 2.4. Synthesis of the NiO nanopartials (NiO NPs) The NiO NPs were obtained by directly heating the assynthesized C@NiO@NCSs powders in air to 600 C and holding for 60 min to remove the carbon species. 2.5. Materials characterization Powder XRD was conducted on a Bruker D2 Phase X-ray diffractometer using Cu Ka radiation at a scanning rate of 4 min1 in the 2q range of 10 e70 . SEM observation was carried out on a field emission scanning electron microscope (FEI, Magellan 400) operated at 20 kV. The transmission electron microscopy, highresolution TEM, and elemental mappings were collected on a JEOL-2100 instrument. Raman spectra was recorded in a JY HR-800 Raman spectrometer (Horiba Jobin Yvon) with an excitation laser beam wavelength of 633 nm. The contents of C were determined using a NETZSCH STA 449C thermogravimetry analyzer (Diamond PE) under an air atmosphere with a heating rate of 10 C min1 from room temperature to 600 C. The surface compositions of the composites were analyzed by X-ray photoelectron spectroscopy (XPS, ESCAlab-250) with an Al anode source. Nitrogen adsorptiondesorption isotherms was measured on a Micrometitics Tristar 3000 system. 2.6. Electrochemical measurements The working electrodes were prepared by coating the slurry of the as-prepared C@NiO@NCSs composites, Super P, and poly(vinyl difluoride) (5 wt%) binder in a weight ratio of 85:5:10 onto the Cu foil, which were then dried in a vacuum oven at 80 C overnight. The active material loading of each electrode was about 1 mg cm2. The CR2032-type coin cells were assembled in a high-purity argonfilled glovebox (M-Braun) using 1 M LiPF6 dissolved in a 1:1 (wt%) mixture of ethylene carbonate and diethyl carbonate with 1 wt% additive of vinyl carbonate as the electrolytes and Glass-fiber (Whatman, GF/A) as the separators. The cells were galvanostatically cycled at room temperature using a program-controlled test system (LAND CT2001A). The lithium storage capacity is calculated based on the total weight of the C@NiO@NCSs composites. The cyclic voltammetry (CV) measurements were carried out on an Autolab Electrochemical Workstation (PGSTAT302N) in the potential range of 0.001e3.0 V at a scan rate of 0.1 mV s1. The abovementioned measurements were all carried out at room temperature (25 C). 3. Results and discussion The synthesis of the composites of nitrogen-doped carbon coated nickel oxide nanoparticles embedded in nitrogen-doped carbon sheets (C@NiO@NCSs) is schematically illustrated in Scheme 1. First, the nitrate precursor slurry was evenly coated on the Cu foil substrate by tape casting. In the second step, the polyvinylpyrrolidone (PVP) carbonized into the nitrogen-doped carbon, and the metal nitrates decomposed into metal oxides. Part of the in
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Scheme 1. The schematic illustration of the fabrication process of the C@NiO@NCSs composites.
situ formed nitrogen-doped carbon act as reduce agents and promote the reduction of the nickel oxide derived from the decomposition of nitrates forming carbon coated nickel nanocomposites (carbothermal reduction), and the other act as the carbon matrix (i.e., nitrogen-doped carbon sheets, NCSs) depositing on the Cu foil. It is worth noting that the gas released from the decomposition of PVP introduces many voids in the NCSs. Finally, the C@NiO@NCSs nanocomposites were obtained by directly oxidizing the obtained C@Ni@NCSs in air at 300 C. In a typical synthesis, more than 1.5-g of such composites can be prepared, as evidenced by the optical image shown in Fig. S2. The morphology of the C@Ni@NCSs obtained in the second step was characterized by SEM and TEM. As shown in Fig. 1a and b, the C@Ni@NCSs composites consist of large carbon sheets in size of 2e5 mm with obvious macropores, which can facilitate the penetration of liquid electrolytes. The TEM images (Fig. 1c and d) show that Ni nanoparticles with an average size of ~10 nm are uniformly dispersed within the matrix of NCSs. Besides, many spherical nanovoids are visible in the matrix of NCSs, which can facilitate the penetration of the electrolytes and wetting of the large electroactive particles, guaranteeing fast lithium transportation. The crystalline structure of the C@Ni@NCSs was analyzed by X-ray diffraction (XRD). The peaks centered at 44.5 , 51.9 can be assigned to the metallic nickel (JCPDS No. 04-0850) [37]. Furthermore, Fig. 2a also evidences that some Cu nanoparticles have been incorporated into the obtained composites [30], which mainly come from the reduction of naturally formed CuO on the surface of Cu foil during the carbothermal reduction process. In addition to the peaks from the Ni and Cu nanoparticles, the XRD pattern shows a broad peak centered at 26 (Fig. 2a), which can be assigned to the nitrogendoped carbon derived from the decomposition of PVP. Raman spectra of the C@Ni@NCSs composites (Fig. 2b) shows four obvious bands, namely, D band at 1352 cm1, G band at 1602 cm1, second order 2D band at 2674 cm1 and DþG band at 2897 cm1 [36,38]. The appearance of D band indicates the formation of structural defects, which originate from the nitrogen-doping in the carbon sheets. The intensity ratio of the G band to the D band (IG/ID) is
calculated to be 0.92, indicating the low crystallinity of the NCSs [36]. The second order 2D and DþG bands are characteristic of the formation of the few-layer graphene [38]. After heating in air at 300 C for 1 h, the C@Ni@NCSs composites were oxidized into the C@NiO@NCSs composites. The microstructure of the resulted C@NiO@NCSs composites was characterized by SEM and TEM, as shown in Fig. 3. The SEM image shows that the C@NiO@NCSs composites completely replicate the architecture of the C@Ni@NCSs (Fig. 3e, f and Fig. S1). The TEM indicates that the NiO nanoparticles with an average size of 10 nm are evenly embedded in the NCSs matrix (Fig. 3aec). The high-resolution TEM (HRTEM) shows clear lattice fringes with an interlayer spacing of 0.21 nm, which is corresponding to the (200) planes of NiO [39]. From the HRTEM image, we also can see clearly that the NiO nanoparticles are tightly wrapped by a few-layer graphene-like carbon shell, which agrees with the appearance of second order 2D band and DþG band in Raman (Fig. 2b). The formation of this graphene-like carbon shell is further evidenced by the TEM image of NCSs (Fig. S5), which results from the formation of metal nanocrystals like Ni that in turn accelerates the graphitization of PVP under high temperature [40]. Formation of such graphene-like carbon shells is beneficial to the conductivity improvement of the resulted TMOs composites and the alleviation of their volume expansion during repeated lithiation and delithiation. The structure and chemical state of the obtained C@NiO@NCSs composites were analyzed by XRD and XPS, respectively. The XRD patterns (Fig. 2a and S6) indicate that the Ni nanoparticles have been oxidized into the face-centered-cubic NiO phase (JCPDS No. 78-0423) (Cu nanoparticles being oxidized into the monoclinic CuO phase, JCPDS No. 48-1548) [17,36,39]. Fig. 4aec shows the highresolution XPS spectrum of Ni 2p, Cu 2p and N 1s, respectively. The Ni 2p spectrum shows the characteristic 2p 1/2 peak (872.9 eV) and the shakeup satellite peak (880.7 eV), while the 2p 3/2 peak and the corresponding shakeup satellite peak appears at 855.8 and 861.9 eV, respectively [41]. The Cu 2p spectrum presents a main 2p 3/2 peak (934.3 eV) and two shakeup satellites peaks (944.1 and 941.4 eV), which indicates the existence of Cu2þ from the CuO phase
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Fig. 1. The morphology and structure of the C@Ni@NCSs composites. (aeb) SEM, and (ced) TEM.
Fig. 2. (a) XRD patterns and (b) Raman spectrum of the C@NiO@NCSs composites.
[21,42]. The high-resolution N1s spectrum reveals the existence of the graphitic N (401.2 eV) and the pyridinic N (399.0 eV) in the NCSs matrix [43,44]. The N content in the composites is calculated to be 2.3 wt%. It is believed that the feature of N-doping can significantly increase the electronic conductivity of the resulted carbon and introduce structural defects, resulting in an enhanced electrochemical performance [45]. The carbon content in the C@NiO@NCSs composites was determined by the thermogravimetric analysis (TGA), which is 33.7 wt% (Fig. S3). The porosity of the C@NiO@NCSs composites was measured by the N2 adsorptiondesorption isotherms (Fig. 4d), showing a characteristic of the combined type I/IV isotherms from P/P0 ¼ 0.5 to 1 which is attributed to the inner cavity formed by the decomposition of PVP [46]. The BET surface area of the C@NiO@NCSs composites is calculated to be 118 m2 g1, and the pore size distribution shown in Fig. S4 indicates that the C@NiO@NCSs composites are of porous structures with micropores and mesopores, which significantly favor the electrolytes penetration and then fast lithium ion transport. In addition to the successful preparation of the C@NiO@NCSs composites, Figs. S7eS8 evidence that this method can be
expanded to prepare other TMOs based composites by just changing the nitrate precursors. For example, Fe2O3 and Co3O4 based composites with similar morphology of C@NiO@NCSs have been synthesized by using iron nitrate and cobalt nitrate, respectively. It can be see clearly that the metal oxide nanoparticles confirmed by XRD (Fig. S7) are evenly dispersed in the carbon matrix (Fig. S8). As evidenced above, the C@NiO@NCSs composites combine several types of structural advantages, such as N-doping, evenly dispersed nanoparticles with graphene-like coating layer and robust carbon sheets supports, all of which are proven with positive effects on improving the lithium storage ability of electrodes. To understand the lithiation-delithiation behavior of this composites, we first carried out the cyclic voltammetry (CV) test within the voltage range of 0.001e3 V versus Li/Liþ. Fig. 5a displays the CV curves of the first-three circles under a scan rate of 0.1 mV s1. Three reduction peaks with the center of 0.82, 1.24 and 1.59 V are visible in the first cathodic scan, which are corresponding to the lithium insertion into the NiO (and the small amount of CuO impurity) to form the mixture of nanosized metal (i.e., Ni and Cu) and
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Fig. 3. The morphology and structure of the C@NiO@NCSs composites. (aec) TEM, (d) HRTEM and (eef) SEM.
Fig. 4. (aec) XPS spectra of Ni 2p, Cu 2p and N 1s, respectively. And (d) Nitrogen adsorption-desorption isotherms of the C@NiO@NCSs composites.
Li2O matrix as well as the formation of solid electrolyte interphase (SEI) films, respectively [47,48]. For the subsequent anodic scan, two oxidation peaks centering at 1.30 V and 2.40 V are ascribed to the oxidation of nanosized Ni and Cu into NiO and CuO, respectively [47,48]. The overall lithium insertion-extraction reactions of the NiO with small amount of CuO impurity can be described in the
following equation: NiO þ 2Liþ þ 2e 4 Ni þ Li2O
(1)
CuO þ 2Liþ þ 2e 4 Cu þ Li2O
(2)
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Fig. 5. The electrochemical performance of the C@NiO@NCSs composites. (a) CV curves for the first three circles at a scan rate of 0.1 mV S1; (b) Charge-discharge curves of selected cycles cycled at 1 A g1; (c) Rate performance; (d) Nyquist plots and corresponding simulation results for the first, 20th and 500th cycles and the inset is the equivalent circuit used for plot fitting (Re: external resistance, Rct: charge transfer resistance, CPE1: constant phase element, and Zw: Warburg impedance); (e) The long-term cycling performance and corresponding Coulombic efficiencies of the NiO nanoparticles, NCSs at 1 A g1 and the C@NiO@NCSs electrodes at 1 A g1 and 2 A g1.
Since the second cycle, the major reduction peaks shifts to 1.48 and 0.85 V due to the decrease of particle size and the rearrangement of electrode structures during the first cycle, which is commonly observed in the conversion electrodes. In addition, we can find that all the peaks become overlapped, suggesting the excellently reversible electrochemical behavior of the C@NiO@NCSs composites upon lithium insertion and extraction. The typical charge-discharge curves of the C@NiO@NCSs electrode of the selected cycles at 1 A g1 between 0.001 and 3.0 V are presented in Fig. 5b. It can be seen clearly that the voltage plateaus are well consistent with the peak positions observed in the CV profiles. The initial Coulombic efficiency is 63.2% and the initial irreversible capacity loss mainly associates with the electrolyte decomposition to form the stable solid electrolyte interface (SEI) films [49,50]. This phenomenon has been widely considered as the main reason causing the irreversible capacity loss in the first several cycles [51,52]. After the formation of stable SEI films in the first several cycles, the capacity for C@NiO@NCSs remains stable and the Coulombic efficiency is significantly increased to ~99.5%. For example, the capacity after 200 cycles stabilizes at ~580 mAh g1. Fig. 5c demonstrates the rate performance and the
corresponding Coulombic efficiencies of the C@NiO@NCSs electrodes at various current densities increasing from 0.5 to 4 A g1. This material delivers an initial discharge capacity of 1219 mAh g1 at the current density of 0.5 A g1 and a high reversible capacity of 400 mAh g1 at a high current rate of 4 A g1, showing a good rate capability. Apparently, as the current decreases from 4 to 0.5 A g1, the discharge capacities of the electrode recover to its initial values, suggesting the excellent reversibility and stability of the C@NiO@NCSs composites. An outstanding long-term cycling performance is a critical factor for LIBs anode materials. Fig. 5e shows the long-term cycling performance of the C@NiO@NCSs at the current density of 1 and 2 A g1. It can be seen clearly that such designed composites electrode demonstrates an excellent cycle stability, with a high capacity of 651 mAh g1 after 1500 cycles at 1 A g1. The Coulombic efficiency is also rapidly approaching to a stable value of approximately 100% after the first few cycles, indicating an excellent reversibility and stability. The excellent cycle performance of the C@NiO@NCSs composites is further evidenced by the high capacity of 518 mAh g1 remaining after 1400 cycles at a high current density of 2 A g1. Such excellent cycle stability is ascribed to the
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well-formed graphene-like carbon coating layer and robust carbon sheet supports in the C@NiO@NCSs composites, which is evidenced by the poor cycle performance of the bare NiO nanoparticles without these features. For example, the bare NiO nanoparticles shows the similar discharge capacity with that of the C@NiO@NCSs during the first cycle, but the capacity decays quickly to only 150 mAh g1 after 500 cycles, which is mainly due to the pulverization of the electroactive particles causing the loss of electronic wiring with the current collector. To confirm this, the morphology and structural stability of the C@NiO@NCSs electrode was analyzed by TEM. It can be seen clearly that the morphology of NiO nanoparticles and the even distribution of them in carbon matrix are well reserved for the C@NiO@NCSs electrode after 20 cycles (Fig. S9a). The HRTEM further shows that the NiO nanoparticles are tightly wrapped by graphene-like carbon shell without visible cracks (Fig. 9Sb). This clearly evidences that the as-synthesized C@NiO@NCSs composites has excellent stability upon repeated lithiation/delithiation. The contribution of NCSs to the reversible capacity of the C@NiO@NCSs composites is evaluated at the same conditions, which stabilizes at 145 mAh g1 after 700 cycles. This value is comparable to the capacity calculated from the theoretical capacity of graphite (372 mAh g1) and the content of carbon in the C@NiO@NCSs composites (i.e., 372 33.7% ¼ 125 mAh g1). Furthermore, the excellent electrochemical performance of the C@NiO@NCSs composites is supported by the comparison with the performance of the previously reported Ni and/or Cu-based conversion anodes, as shown in Table S1. In the long-term cycling, a capacity fading at the initial several cycles (e.g., from 688 to 520 mAh g1 within the initial 20 cycles) followed by a capacity increasing (e.g., increases to 710 mAh g1 over the next 550 cycles) is observed in the C@NiO@NCSs composites, which is a commonly observed in the conversion anodes [14,36,37,42,46]. According to the literature, the initial capacity fading of the conversion anodes is ascribed to the consumption of electrolytes forming stable SEI layers and the poor reactivity of pristine lager electrode particles [37,53]. After initial cycling, the pristine electrode particles have been refined into nanoparticles due to the repeated conversion reaction, which significantly improves the lithium insertion-extraction kinetics and the reactivity of electrodes. Fig. 5d shows the Nyquist plots collected in selected cycles, which give direct evidence of the improvement in reaction kinetics during cycling and are fitted according to previous report [54]. For the first cycle, the charge-transfer resistance (Rct) of the C@NiO@NCSs composites is fitted to be 158 U. After repeated lithium insertion-extraction, Rct remains almost stable during the initial 20 cycles and decreases to only 76 U after 500 cycles. This is a strong revelation of the activation process appeared in the C@NiO@NCSs composites and the facilitating of charge-transfer during repetitive cycling. The above results demonstrate that the C@NiO@NCSs composites electrode exhibit impressive electrochemical performance in terms of reversible capacity, cyclability and rate capability. Such excellent electrochemical storage performances achieved here are mainly originated from the well-structured TMOs-based composites itself and the advantages involved in the facile synthesis approach. Firstly, the synthesized C@NiO@NCSs composites have a moderate specific surface area, which provides a favourable environment for electrolyte access and thus facilitates the fast Liþ diffusion from the electrolyte to the TMOs. Secondly, the in situ formed graphene-like carbon coating layer on the surface of TMOs particles can provide flexible buffer to accommodate large volume expansion of TMOs during Liþ insertion-extraction and connected pathways for fast electron transfer. Thirdly, the N-doping feature increases the electronic conductivity of carbon sheets and provides more additional Li storage active sites. Finally yet importantly, the
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robust NCSs matrix provides high mechanical stability and maintain the integrity of the electrode. With the advantages of all these features, the C@NiO@NCSs composites presents superior performance as lithium storage materials. 4. Conclusions In summary, we have demonstrated a facile strategy for the synthesis of high performance TMOs based lithium storage materials. NiO, Fe2O3 and Co3O4 based composites have been synthesized with this simple procedure. The structure and morphology characterizations reveal that the TMOs nanoparticles are tightly wrapped by a graphene-like carbon shell and are evenly dispersed in the in situ formed nitrogen-doped carbon matrix carbonized from polyvinylpyrrolidone, guaranteeing a superior structure stability against long-term cycling. As lithium storage materials, such synthesized composites demonstrate excellent long-term cycle stability (651 mA h g1 after 1500 cycles at 1 A g1) and high-rate capability (400 mAh g1 at 4 A g1). These results demonstrate that this simple strategy is easy and effective way to prepare transition metal oxides based high-performance lithium storage materials. Acknowledgements This work is supported by the National Key Basic Research Program of China (No. 2014CB921004), NSFC (51325206 and 51402339) and China Postdoctoral Science Foundation (2015LH0026 and 2015M581667). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.02.167. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Facile synthesis of N-doped carbon-coated nickel oxide nanoparticles embedded in N-doped carbon sheets for reversible lithium storage Zhiqing Jia a, b, Yingbin Tan a, Jiyang Sun a, b, Yongzhe Wang a, Zhonghui Cui a, *, Xiangxin Guo a, c, ** a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b University of Chinese Academy of Sciences, Beijing 100039, China c College of Physics, Qingdao University, Qingdao 266071, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 January 2018 Received in revised form 12 February 2018 Accepted 13 February 2018 Available online 22 February 2018
In this work, we present a facile strategy for the synthesis of well-structured transition metal oxides based composites by the decomposition of nitrate and polyvinylpyrrolidone (PVP) followed by simple oxidation at a low temperature and using them as high performance materials for reversible lithium storage. As proofs of this simple strategy, three kinds of TMOs, namely NiO, Fe2O3 and Co3O4, based composites have been prepared with this simple strategy. The morphology and structure analyses indicate that TMOs nanoparticles with a size of 10 nm are tightly coated by the in situ formed graphenelike carbon shell and evenly dispersed in the microsized nitrogen-doped carbon sheets (C@TMOs@NCSs) derived from the PVP. As lithium storage materials, the as-prepared C@NiO@NCSs composites show excellent electrochemical performances in terms of cycle stability (651 mAh g1 after 1500 cycles at 1 A g1 and 518 mAh g1 after 1400 cycles at 2 A g1) and rate capability (400 mAh g1 at 4 A g1). Such superior electrochemical performance is attributed the well-structured composites with features of tight carbon coating layer, evenly dispersed TMOs nanoparticles and robust carbon supports. © 2018 Elsevier B.V. All rights reserved.
Keywords: Reversible lithium storage Transition metal oxides Composites Cycle stability and rate capability
1. Introduction Lithium-ion batteries (LIBs) have been widely used in portable electronics and electric vehicles, owing to their high energy density and long lifespan [1e5]. Currently, one of the main research directions is searching for advanced electrode materials with enhanced lithium storage ability to improve the battery energy density [6e9]. Transition metal oxides (TMOs) have attracted considerable attention as their ability to uptake more than one lithium per transition metal, giving high theoretical specific capacities [10e17]. For example, the NiO and CuO can offer a high theoretical capacity of 718 and 674 mAh g1, respectively [17e21]. Despite TMOs anodes of high theoretical storage capacity, the fully
* Corresponding author. ** Corresponding author. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. E-mail addresses: [email protected] (Z. Cui), [email protected] (X. Guo). https://doi.org/10.1016/j.jallcom.2018.02.167 0925-8388/© 2018 Elsevier B.V. All rights reserved.
achieving of their theoretical capacity still faces challenges [15,22]: First, the TMOs usually have an intrinsic low electron conductivity, leading to poor reactivity and reaction kinetics upon lithium [23]. Second, these materials suffer from drastic volume expansion during the Liþ insertion-extraction processes, resulting in the pulverization of electroactive particles and then the fast capacity fading [19]. Finally, it is challenging to form a stable solid electrolyte interface (SEI) layer as the repeated volume change associated with repeated lithium insertion-extraction, leading to the limited cyclability and low Coulombic efficiency [24,25]. In response, tremendous efforts have been performed to optimize the electrochemical performance of TMOs anodes by enhancing their electrical conductivity and mitigating the adverse influences associated with large volume change. An effective strategy is to construct active/inactive composites with nanosized TMOs particles being evenly encapsulated into a conductive matrix (e.g., graphene, graphene oxides, carbon nanotubes and conductive polymers) [26e32]. In such design, carbon networks can enhance the electrical conductivity of the resulted hybrid materials, leading to the fasted electron transfer, the accelerated ionic diffusion across
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the electrolyte-electrode interface and the shortened diffusion pathway by confining the particles growth of TMOs during high temperature synthesis [33]. Simultaneously, the problem of volume expansion of the active material is tackled due to the physical buffer effect of the carbon matrix, leading to the prolonged lifetime [34]. Our recent study evidenced that well-designed nanocomposites with monodispersed carbon-coated electroactive particles can alleviate the large volume change upon cycling and provide excellent cycle stability and rate capability with high reversible capacity [35]. To synthesize the active/inactive composites, hydrothermal and solvothermal followed by calcining the resulted precursors at high temperature are the commonly used methods [15,19,23,36]. For example, Li et al. reported the high capacity composites of self-assembled porous CuO nanospheres decorated on reduced graphene oxide synthesized by solvothermal method [17]. Multi-yolk-shell copper oxide@carbon octahedral as high stability anode of LIBs has been synthesized by hydrothermal method [36]. And Mou et al. synthesized layer-cake-like Co3O4/C composites with hydrothermal technique [14]. In addition to the commonly used hydrothermal or solvothermal technique, developing of simple method with ability in large-scale production is greatly encouraged. Herein, we report a facile strategy for the synthesis of TMOs based composites and using them as high-performance materials for reversible lithium storage. In the as-prepared composites, the in situ formed nitrogen-doped graphene-like carbon coated transition metal oxides nanoparticles evenly embed in the nitrogen-doped carbon sheets (C@TMOs@NCSs). The nitrogen-doped carbon derives from the decomposition of polyvinylpyrrolidone (PVP). This sample procedure can be expanded to synthesize different kinds of TMOs based composites: three kinds of TMOs, namely NiO, Fe2O3 and Co3O4, based composites have been synthesized. As anode materials for LIBs, the obtained composites show excellent reversible lithium storage ability and rate capability, due to the in situ formed N-doped carbon coating layer and carbon matrix significantly increasing the conductivity of the abstained TMOs composites and alleviating the pulverization of TMOs during repeated lithium insertion and extraction. 2. Experimental 2.1. Chemicals Copper foil (25 mm), Polyvinylpyrrolidone (PVP) (MW ¼ 55000, AR), Ni(NO3)2$6H2O (98%), Co(NO3)2$6H2O (97.7%) and Fe(NO3)3$9H2O (98%) were purchased from Alfa Aesar. All of the chemicals were used directly as received. 2.2. Synthesis of the C@TMOs@NCSs composites Ni(NO3)2$6H2O (0.05 g) and PVP (0.1 g) were dissolved in 2 mL of ethanol by ultrasound for 5 min at room temperature. After forming homogeneous solution, it was evenly coated on the Cu foil substrate (20 cm 20 cm squares) by tape casting. Then the obtained Ni(NO3)2$6H2O-PVP/Cu film was heated to 100 C for 15 min to remove the ethanol solvent. The dried film was rolled up into a tube with the dimeter of 1 inch and then the obtained tube was wrapped by using a new copper strip. After that, the wrapped tube was annealed at 850 C for 30 min under the H2 (50 sccm) and Ar (500 sccm) flow to get the composite of C@Ni@NCSs, with the total pressure < 50 Torr. After fast cooled down to room temperature by opening the heating furnace without changing the gas flow, the obtained C@Ni@NCSs composites were scraped from Cu foil using a plastic knife. Finally, the C@NiO@NCSs composites were prepared by directly oxidized the as-synthesized C@Ni@NCSs powders in air
at 300 C for 60 min, with a ramp-rate of 5 C min1. C@Fe2O3@NCSs and C@Co3O4@NCSs were synthesized by following the same procedures with the use of their corresponding nitrate precursor. 2.3. Synthesis of NCSs The NCSs were obtained by removing the metal from C@Ni@NCSs composites by acid etching (2 M HCl for 24 h). 2.4. Synthesis of the NiO nanopartials (NiO NPs) The NiO NPs were obtained by directly heating the assynthesized C@NiO@NCSs powders in air to 600 C and holding for 60 min to remove the carbon species. 2.5. Materials characterization Powder XRD was conducted on a Bruker D2 Phase X-ray diffractometer using Cu Ka radiation at a scanning rate of 4 min1 in the 2q range of 10 e70 . SEM observation was carried out on a field emission scanning electron microscope (FEI, Magellan 400) operated at 20 kV. The transmission electron microscopy, highresolution TEM, and elemental mappings were collected on a JEOL-2100 instrument. Raman spectra was recorded in a JY HR-800 Raman spectrometer (Horiba Jobin Yvon) with an excitation laser beam wavelength of 633 nm. The contents of C were determined using a NETZSCH STA 449C thermogravimetry analyzer (Diamond PE) under an air atmosphere with a heating rate of 10 C min1 from room temperature to 600 C. The surface compositions of the composites were analyzed by X-ray photoelectron spectroscopy (XPS, ESCAlab-250) with an Al anode source. Nitrogen adsorptiondesorption isotherms was measured on a Micrometitics Tristar 3000 system. 2.6. Electrochemical measurements The working electrodes were prepared by coating the slurry of the as-prepared C@NiO@NCSs composites, Super P, and poly(vinyl difluoride) (5 wt%) binder in a weight ratio of 85:5:10 onto the Cu foil, which were then dried in a vacuum oven at 80 C overnight. The active material loading of each electrode was about 1 mg cm2. The CR2032-type coin cells were assembled in a high-purity argonfilled glovebox (M-Braun) using 1 M LiPF6 dissolved in a 1:1 (wt%) mixture of ethylene carbonate and diethyl carbonate with 1 wt% additive of vinyl carbonate as the electrolytes and Glass-fiber (Whatman, GF/A) as the separators. The cells were galvanostatically cycled at room temperature using a program-controlled test system (LAND CT2001A). The lithium storage capacity is calculated based on the total weight of the C@NiO@NCSs composites. The cyclic voltammetry (CV) measurements were carried out on an Autolab Electrochemical Workstation (PGSTAT302N) in the potential range of 0.001e3.0 V at a scan rate of 0.1 mV s1. The abovementioned measurements were all carried out at room temperature (25 C). 3. Results and discussion The synthesis of the composites of nitrogen-doped carbon coated nickel oxide nanoparticles embedded in nitrogen-doped carbon sheets (C@NiO@NCSs) is schematically illustrated in Scheme 1. First, the nitrate precursor slurry was evenly coated on the Cu foil substrate by tape casting. In the second step, the polyvinylpyrrolidone (PVP) carbonized into the nitrogen-doped carbon, and the metal nitrates decomposed into metal oxides. Part of the in
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Scheme 1. The schematic illustration of the fabrication process of the C@NiO@NCSs composites.
situ formed nitrogen-doped carbon act as reduce agents and promote the reduction of the nickel oxide derived from the decomposition of nitrates forming carbon coated nickel nanocomposites (carbothermal reduction), and the other act as the carbon matrix (i.e., nitrogen-doped carbon sheets, NCSs) depositing on the Cu foil. It is worth noting that the gas released from the decomposition of PVP introduces many voids in the NCSs. Finally, the C@NiO@NCSs nanocomposites were obtained by directly oxidizing the obtained C@Ni@NCSs in air at 300 C. In a typical synthesis, more than 1.5-g of such composites can be prepared, as evidenced by the optical image shown in Fig. S2. The morphology of the C@Ni@NCSs obtained in the second step was characterized by SEM and TEM. As shown in Fig. 1a and b, the C@Ni@NCSs composites consist of large carbon sheets in size of 2e5 mm with obvious macropores, which can facilitate the penetration of liquid electrolytes. The TEM images (Fig. 1c and d) show that Ni nanoparticles with an average size of ~10 nm are uniformly dispersed within the matrix of NCSs. Besides, many spherical nanovoids are visible in the matrix of NCSs, which can facilitate the penetration of the electrolytes and wetting of the large electroactive particles, guaranteeing fast lithium transportation. The crystalline structure of the C@Ni@NCSs was analyzed by X-ray diffraction (XRD). The peaks centered at 44.5 , 51.9 can be assigned to the metallic nickel (JCPDS No. 04-0850) [37]. Furthermore, Fig. 2a also evidences that some Cu nanoparticles have been incorporated into the obtained composites [30], which mainly come from the reduction of naturally formed CuO on the surface of Cu foil during the carbothermal reduction process. In addition to the peaks from the Ni and Cu nanoparticles, the XRD pattern shows a broad peak centered at 26 (Fig. 2a), which can be assigned to the nitrogendoped carbon derived from the decomposition of PVP. Raman spectra of the C@Ni@NCSs composites (Fig. 2b) shows four obvious bands, namely, D band at 1352 cm1, G band at 1602 cm1, second order 2D band at 2674 cm1 and DþG band at 2897 cm1 [36,38]. The appearance of D band indicates the formation of structural defects, which originate from the nitrogen-doping in the carbon sheets. The intensity ratio of the G band to the D band (IG/ID) is
calculated to be 0.92, indicating the low crystallinity of the NCSs [36]. The second order 2D and DþG bands are characteristic of the formation of the few-layer graphene [38]. After heating in air at 300 C for 1 h, the C@Ni@NCSs composites were oxidized into the C@NiO@NCSs composites. The microstructure of the resulted C@NiO@NCSs composites was characterized by SEM and TEM, as shown in Fig. 3. The SEM image shows that the C@NiO@NCSs composites completely replicate the architecture of the C@Ni@NCSs (Fig. 3e, f and Fig. S1). The TEM indicates that the NiO nanoparticles with an average size of 10 nm are evenly embedded in the NCSs matrix (Fig. 3aec). The high-resolution TEM (HRTEM) shows clear lattice fringes with an interlayer spacing of 0.21 nm, which is corresponding to the (200) planes of NiO [39]. From the HRTEM image, we also can see clearly that the NiO nanoparticles are tightly wrapped by a few-layer graphene-like carbon shell, which agrees with the appearance of second order 2D band and DþG band in Raman (Fig. 2b). The formation of this graphene-like carbon shell is further evidenced by the TEM image of NCSs (Fig. S5), which results from the formation of metal nanocrystals like Ni that in turn accelerates the graphitization of PVP under high temperature [40]. Formation of such graphene-like carbon shells is beneficial to the conductivity improvement of the resulted TMOs composites and the alleviation of their volume expansion during repeated lithiation and delithiation. The structure and chemical state of the obtained C@NiO@NCSs composites were analyzed by XRD and XPS, respectively. The XRD patterns (Fig. 2a and S6) indicate that the Ni nanoparticles have been oxidized into the face-centered-cubic NiO phase (JCPDS No. 78-0423) (Cu nanoparticles being oxidized into the monoclinic CuO phase, JCPDS No. 48-1548) [17,36,39]. Fig. 4aec shows the highresolution XPS spectrum of Ni 2p, Cu 2p and N 1s, respectively. The Ni 2p spectrum shows the characteristic 2p 1/2 peak (872.9 eV) and the shakeup satellite peak (880.7 eV), while the 2p 3/2 peak and the corresponding shakeup satellite peak appears at 855.8 and 861.9 eV, respectively [41]. The Cu 2p spectrum presents a main 2p 3/2 peak (934.3 eV) and two shakeup satellites peaks (944.1 and 941.4 eV), which indicates the existence of Cu2þ from the CuO phase
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Fig. 1. The morphology and structure of the C@Ni@NCSs composites. (aeb) SEM, and (ced) TEM.
Fig. 2. (a) XRD patterns and (b) Raman spectrum of the C@NiO@NCSs composites.
[21,42]. The high-resolution N1s spectrum reveals the existence of the graphitic N (401.2 eV) and the pyridinic N (399.0 eV) in the NCSs matrix [43,44]. The N content in the composites is calculated to be 2.3 wt%. It is believed that the feature of N-doping can significantly increase the electronic conductivity of the resulted carbon and introduce structural defects, resulting in an enhanced electrochemical performance [45]. The carbon content in the C@NiO@NCSs composites was determined by the thermogravimetric analysis (TGA), which is 33.7 wt% (Fig. S3). The porosity of the C@NiO@NCSs composites was measured by the N2 adsorptiondesorption isotherms (Fig. 4d), showing a characteristic of the combined type I/IV isotherms from P/P0 ¼ 0.5 to 1 which is attributed to the inner cavity formed by the decomposition of PVP [46]. The BET surface area of the C@NiO@NCSs composites is calculated to be 118 m2 g1, and the pore size distribution shown in Fig. S4 indicates that the C@NiO@NCSs composites are of porous structures with micropores and mesopores, which significantly favor the electrolytes penetration and then fast lithium ion transport. In addition to the successful preparation of the C@NiO@NCSs composites, Figs. S7eS8 evidence that this method can be
expanded to prepare other TMOs based composites by just changing the nitrate precursors. For example, Fe2O3 and Co3O4 based composites with similar morphology of C@NiO@NCSs have been synthesized by using iron nitrate and cobalt nitrate, respectively. It can be see clearly that the metal oxide nanoparticles confirmed by XRD (Fig. S7) are evenly dispersed in the carbon matrix (Fig. S8). As evidenced above, the C@NiO@NCSs composites combine several types of structural advantages, such as N-doping, evenly dispersed nanoparticles with graphene-like coating layer and robust carbon sheets supports, all of which are proven with positive effects on improving the lithium storage ability of electrodes. To understand the lithiation-delithiation behavior of this composites, we first carried out the cyclic voltammetry (CV) test within the voltage range of 0.001e3 V versus Li/Liþ. Fig. 5a displays the CV curves of the first-three circles under a scan rate of 0.1 mV s1. Three reduction peaks with the center of 0.82, 1.24 and 1.59 V are visible in the first cathodic scan, which are corresponding to the lithium insertion into the NiO (and the small amount of CuO impurity) to form the mixture of nanosized metal (i.e., Ni and Cu) and
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Fig. 3. The morphology and structure of the C@NiO@NCSs composites. (aec) TEM, (d) HRTEM and (eef) SEM.
Fig. 4. (aec) XPS spectra of Ni 2p, Cu 2p and N 1s, respectively. And (d) Nitrogen adsorption-desorption isotherms of the C@NiO@NCSs composites.
Li2O matrix as well as the formation of solid electrolyte interphase (SEI) films, respectively [47,48]. For the subsequent anodic scan, two oxidation peaks centering at 1.30 V and 2.40 V are ascribed to the oxidation of nanosized Ni and Cu into NiO and CuO, respectively [47,48]. The overall lithium insertion-extraction reactions of the NiO with small amount of CuO impurity can be described in the
following equation: NiO þ 2Liþ þ 2e 4 Ni þ Li2O
(1)
CuO þ 2Liþ þ 2e 4 Cu þ Li2O
(2)
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Fig. 5. The electrochemical performance of the C@NiO@NCSs composites. (a) CV curves for the first three circles at a scan rate of 0.1 mV S1; (b) Charge-discharge curves of selected cycles cycled at 1 A g1; (c) Rate performance; (d) Nyquist plots and corresponding simulation results for the first, 20th and 500th cycles and the inset is the equivalent circuit used for plot fitting (Re: external resistance, Rct: charge transfer resistance, CPE1: constant phase element, and Zw: Warburg impedance); (e) The long-term cycling performance and corresponding Coulombic efficiencies of the NiO nanoparticles, NCSs at 1 A g1 and the C@NiO@NCSs electrodes at 1 A g1 and 2 A g1.
Since the second cycle, the major reduction peaks shifts to 1.48 and 0.85 V due to the decrease of particle size and the rearrangement of electrode structures during the first cycle, which is commonly observed in the conversion electrodes. In addition, we can find that all the peaks become overlapped, suggesting the excellently reversible electrochemical behavior of the C@NiO@NCSs composites upon lithium insertion and extraction. The typical charge-discharge curves of the C@NiO@NCSs electrode of the selected cycles at 1 A g1 between 0.001 and 3.0 V are presented in Fig. 5b. It can be seen clearly that the voltage plateaus are well consistent with the peak positions observed in the CV profiles. The initial Coulombic efficiency is 63.2% and the initial irreversible capacity loss mainly associates with the electrolyte decomposition to form the stable solid electrolyte interface (SEI) films [49,50]. This phenomenon has been widely considered as the main reason causing the irreversible capacity loss in the first several cycles [51,52]. After the formation of stable SEI films in the first several cycles, the capacity for C@NiO@NCSs remains stable and the Coulombic efficiency is significantly increased to ~99.5%. For example, the capacity after 200 cycles stabilizes at ~580 mAh g1. Fig. 5c demonstrates the rate performance and the
corresponding Coulombic efficiencies of the C@NiO@NCSs electrodes at various current densities increasing from 0.5 to 4 A g1. This material delivers an initial discharge capacity of 1219 mAh g1 at the current density of 0.5 A g1 and a high reversible capacity of 400 mAh g1 at a high current rate of 4 A g1, showing a good rate capability. Apparently, as the current decreases from 4 to 0.5 A g1, the discharge capacities of the electrode recover to its initial values, suggesting the excellent reversibility and stability of the C@NiO@NCSs composites. An outstanding long-term cycling performance is a critical factor for LIBs anode materials. Fig. 5e shows the long-term cycling performance of the C@NiO@NCSs at the current density of 1 and 2 A g1. It can be seen clearly that such designed composites electrode demonstrates an excellent cycle stability, with a high capacity of 651 mAh g1 after 1500 cycles at 1 A g1. The Coulombic efficiency is also rapidly approaching to a stable value of approximately 100% after the first few cycles, indicating an excellent reversibility and stability. The excellent cycle performance of the C@NiO@NCSs composites is further evidenced by the high capacity of 518 mAh g1 remaining after 1400 cycles at a high current density of 2 A g1. Such excellent cycle stability is ascribed to the
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well-formed graphene-like carbon coating layer and robust carbon sheet supports in the C@NiO@NCSs composites, which is evidenced by the poor cycle performance of the bare NiO nanoparticles without these features. For example, the bare NiO nanoparticles shows the similar discharge capacity with that of the C@NiO@NCSs during the first cycle, but the capacity decays quickly to only 150 mAh g1 after 500 cycles, which is mainly due to the pulverization of the electroactive particles causing the loss of electronic wiring with the current collector. To confirm this, the morphology and structural stability of the C@NiO@NCSs electrode was analyzed by TEM. It can be seen clearly that the morphology of NiO nanoparticles and the even distribution of them in carbon matrix are well reserved for the C@NiO@NCSs electrode after 20 cycles (Fig. S9a). The HRTEM further shows that the NiO nanoparticles are tightly wrapped by graphene-like carbon shell without visible cracks (Fig. 9Sb). This clearly evidences that the as-synthesized C@NiO@NCSs composites has excellent stability upon repeated lithiation/delithiation. The contribution of NCSs to the reversible capacity of the C@NiO@NCSs composites is evaluated at the same conditions, which stabilizes at 145 mAh g1 after 700 cycles. This value is comparable to the capacity calculated from the theoretical capacity of graphite (372 mAh g1) and the content of carbon in the C@NiO@NCSs composites (i.e., 372 33.7% ¼ 125 mAh g1). Furthermore, the excellent electrochemical performance of the C@NiO@NCSs composites is supported by the comparison with the performance of the previously reported Ni and/or Cu-based conversion anodes, as shown in Table S1. In the long-term cycling, a capacity fading at the initial several cycles (e.g., from 688 to 520 mAh g1 within the initial 20 cycles) followed by a capacity increasing (e.g., increases to 710 mAh g1 over the next 550 cycles) is observed in the C@NiO@NCSs composites, which is a commonly observed in the conversion anodes [14,36,37,42,46]. According to the literature, the initial capacity fading of the conversion anodes is ascribed to the consumption of electrolytes forming stable SEI layers and the poor reactivity of pristine lager electrode particles [37,53]. After initial cycling, the pristine electrode particles have been refined into nanoparticles due to the repeated conversion reaction, which significantly improves the lithium insertion-extraction kinetics and the reactivity of electrodes. Fig. 5d shows the Nyquist plots collected in selected cycles, which give direct evidence of the improvement in reaction kinetics during cycling and are fitted according to previous report [54]. For the first cycle, the charge-transfer resistance (Rct) of the C@NiO@NCSs composites is fitted to be 158 U. After repeated lithium insertion-extraction, Rct remains almost stable during the initial 20 cycles and decreases to only 76 U after 500 cycles. This is a strong revelation of the activation process appeared in the C@NiO@NCSs composites and the facilitating of charge-transfer during repetitive cycling. The above results demonstrate that the C@NiO@NCSs composites electrode exhibit impressive electrochemical performance in terms of reversible capacity, cyclability and rate capability. Such excellent electrochemical storage performances achieved here are mainly originated from the well-structured TMOs-based composites itself and the advantages involved in the facile synthesis approach. Firstly, the synthesized C@NiO@NCSs composites have a moderate specific surface area, which provides a favourable environment for electrolyte access and thus facilitates the fast Liþ diffusion from the electrolyte to the TMOs. Secondly, the in situ formed graphene-like carbon coating layer on the surface of TMOs particles can provide flexible buffer to accommodate large volume expansion of TMOs during Liþ insertion-extraction and connected pathways for fast electron transfer. Thirdly, the N-doping feature increases the electronic conductivity of carbon sheets and provides more additional Li storage active sites. Finally yet importantly, the
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robust NCSs matrix provides high mechanical stability and maintain the integrity of the electrode. With the advantages of all these features, the C@NiO@NCSs composites presents superior performance as lithium storage materials. 4. Conclusions In summary, we have demonstrated a facile strategy for the synthesis of high performance TMOs based lithium storage materials. NiO, Fe2O3 and Co3O4 based composites have been synthesized with this simple procedure. The structure and morphology characterizations reveal that the TMOs nanoparticles are tightly wrapped by a graphene-like carbon shell and are evenly dispersed in the in situ formed nitrogen-doped carbon matrix carbonized from polyvinylpyrrolidone, guaranteeing a superior structure stability against long-term cycling. As lithium storage materials, such synthesized composites demonstrate excellent long-term cycle stability (651 mA h g1 after 1500 cycles at 1 A g1) and high-rate capability (400 mAh g1 at 4 A g1). These results demonstrate that this simple strategy is easy and effective way to prepare transition metal oxides based high-performance lithium storage materials. Acknowledgements This work is supported by the National Key Basic Research Program of China (No. 2014CB921004), NSFC (51325206 and 51402339) and China Postdoctoral Science Foundation (2015LH0026 and 2015M581667). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.02.167. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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