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onion-like carbon nanocapsules as improved anode material for lithium-ion batteries
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 7511–7518 www.elsevier.com/locate/ceramint
Core/shell-structured nickel cobaltite/onion-like carbon nanocapsules as improved anode material for lithium-ion batteries Xianguo Liua,b,n, Caiyun Cuia,b, Niandu Wua,b, Siu Wing Orb,nn, Nannan Bia,b b
a School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, PR China Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
Received 30 January 2015; received in revised form 11 February 2015; accepted 12 February 2015 Available online 20 February 2015
Abstract Core/shell-structured nanocapsules consisting of a nickel cobaltite (NiCo2O4) nanoparticle core encapsulated in an onion-like carbon (C) shell are synthesized by arc-discharge and air-annealing methods. Void spaces between NiCo2O4 core and the carbon shell are observed in the NiCo2O4/C nanocapsules. Lithium-ion batteries fabricated using the nanocapsules as the anode material exhibit enhanced initial coulombic efficiency of 82.3% and specific capacity of 1197.2 mA h/g after 300 cycles at 0.2 A g 1 current density. Varying the rate of charge/discharge current from 0.2 to 4 A/g does not show negative effects on the recycling stability of the nanocapsules and a recoverable specific capacity as high as 1270.4 mA h/g is obtained. The introduction of the onion-like C shell and the presence of the void spaces are found to increase the contact areas between the electrolyte and the nanocapsules for improved electrolyte diffusion, to enhance the electronic conductivity and ionic mobility of the NiCo2O4 nanoparticle cores, and to accommodate the change in volume during the lithium-ion insertion/extraction process. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Core–shell nanocapsules; Onion-like Carbon shells; Nickel cobaltite; Anode; Lithium-ion batteries
1. Introduction Development of rechargeable energy storage devices with high capacity, high rate capability, and long cycling life is the key to the developing sustainable and clean energy sources such as wind and solar energy sources [1]. Lithium ion batteries (LIBs) are considered as one of the most promising rechargeable energy storage devices because of their higher energy density and longer cycle life compared to conventional rechargeable batteries [1–5]. The low theoretical specific capacity (372 mA h/g) in graphite, the prevailing commercial anode material, is far from meeting the requirements for high energy/power density. Nanostructured transition metal oxides n Corresponding author at: School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, PR China. Tel./fax: þ86 555 2311570. nn Corresponding author. E-mail addresses: [email protected] (X. Liu), [email protected] (S.W. Or).
http://dx.doi.org/10.1016/j.ceramint.2015.02.073 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
have been considered as a possible alternative anode material for LIBs due to their higher theoretical specific capacity (500– 1000 mA h/g) and reversible conversion mechanism for lithium storage. The low electrical conductivity in conjunction with the pulverization problems induced by volume expansion/ contraction during the lithium-ion insertion/extraction process significantly reduce the performance and lifetime of nanostructured transition metal oxide-based anodes. In the past decade, extensive efforts have been made to replace single transition metal oxide anode with other inexpensive and environmentally friendly metal oxides [6–8]. As an abundant multiple oxidation state ternary metal oxide [6], nickel cobaltite (NiCo2O4) is considered as a very promising electrode material for supercapacitor owing to its high electronic conductivity, low diffusion resistance to protons/cations, and high electrolyte penetration [7]. In addition, NiCo2O4 is capable of delivering a theoretical specific capacity of 890 mA h g 1. These features are favorable to develop high-performance electrode materials [6–9]. Today, there are only a few reports on the application of NiCo2O4 as an anode
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material for LIBs. These include: 1) the synthesis of threedimensional (3D) hierarchical porous flower-like NiCo2O4 using a facile hydrothermal approach followed by calcination in air to give a reversible specific capacity of 939 mA h g 1 at 100 mA g 1 and to preserve the high capacity after 60 cycles [10]; 2) the growth of mesoporous NiCo2O4 nanowires on carbon textiles substrates to exhibit a reversible capacity of 1012 mA h g 1 at a 0.5 A g 1 and a capability of 854 mA h g 1 after 100 cycles [11]; 3) the preparation of monodisperse NiCo2O4 mesoporous microspheres by a facile solvothermal method followed by pyrolysis of the Ni0.33Co0.67CO3 precursor to increase capacitance and cycling stability [12]; and 4) the fabrication of highly porous NiCo2O4 nanoflakes and nanobelts by a hydrothermal technique followed by calcination of the NiCo2O4 precursors to exhibit high specific capacities of 1033 and 1056 mA h g 1, good cycling stability, and high rate capability [8].In all these works, lithiation-induced volume expansion results in the fracture and aggregation of anode materials. Various composite materials with oxides, Si and C, including core/shell structures and nanocomposites, are employed to prevent nanoparticles from pulverization and to improve the electrochemical performance [13–15]. In particular, C has been extensively studied because it is a cheap, abundant, and lowtoxic source to enhance the conductivity and stability of anode materials [16]. The core/shell-type nanostructure, named by nanocapsules, is identified as a good way to markedly improve the cycling behavior and kinetics of lithium intercalation and de-intercalation in composites [17]. For example, onion-like Ccoated NiO, Co3O4 and CuO nanocapsules demonstrate superior electrochemical performances [18–20]. The onion-like C shell acts as a barrier to prevent aggregation of transition metal oxides and provides a void space for volume changes. Therefore, the development of core/shell-structured nanocapsules with NiCo2O4 nanoparticles as the core and onion-like C as the shell is imperative to new generation anode materials in highperformance LIBs. In this paper, core/shell-structured NiCo2O4/onion-like C nanocapsules have been prepared by a modified arc-discharge method followed by an annealing process at 100 1C for 2 h in air. The electrochemical performance of NiCo2O4/onion-like carbon nanocapsules as an anode for LIBs is investigated. 2. Experiments 2.1. Synthesis of NiCo2O4/onion-like C nanocapsules A modified arc-discharge process and an air-annealing process, which have been described in details elsewhere, were used to prepare NiCo2O4/onion-like C nanocapsules [18–21]. Metallic powders of Ni and Co of 99.9% purity with an average size of 10 μm were mixed thoughtfully for the preparation of targets in which the molar ratio of Ni/Co was set to 1:2 in accordance with the Ni–Co binary phase diagram and their evaporation pressures. Elemental powders were compacted into a cylinder shape with a diameter of 20 mm under a pressure of about 20 MPa. In the modified arc-discharge process, the compressed Ni–Co powder placed on a water-cooled carbon crucible was employed as the anode, while the cathode was a carbon needle. After the arc-
discharge chamber was evacuated, 1.6 104 Pa pure argon, 0.4 104 Pa hydrogen, and 40 ml liquid ethanol were introduced into the chamber. The arc-discharge current was maintained at 80 A for 0.5 h. The partial pressure of ethanol was found to increase with the time, and the pressure of the chamber could reach 1 atm at 0.5 h because the decomposition of ethanol and the expansion of the gas both increased with increasing temperature. The products were collected from the depositions formed on the top of the chamber after passivation for 8 h in argon. To prepare the NiCo2O4/onion-like C nanocapsules and NiCo2O4 nanoparticles, the products prepared by the modified arc-discharge process were put on an Al2O3 crucible and were annealed at 100 and 300 1C for 2 h in a tubular furnace in still air, respectively. 2.2. Characterization of NiCo2O4/onion-like C nanocapsules The composition and phase purity of the products were analyzed by an X-ray diffraction (XRD) technique at a voltage of 40 kV and a current of 50 mA with Cu Kα radiation (λ=1.5418 Å). The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a JEOL JEM-2010 transmission electron microscope at an acceleration voltage of 200 kV. The oxidation behavior was investigated by thermal gravimetric analysis (TGA) in air atmosphere at a heating rate of 5 1C min 1 from 50 to 400 1C. Raman spectroscopy was used to estimate the bond structure of the graphite shells. 2.3. Electrochemical measurements of NiCo2O4/onion-like C nanocapsules The electrochemical measurements were performed under ambient temperature using standard R2032-type coin cells with lithium as both the counter electrode and the reference electrode. The working electrodes were prepared by mixing the NiCo2O4/ onion-like C nanocapsules, conductivity agent (acetylene black), and poly(vinyl difluoride) (PVDF) at a weight ratio of 50:30:20 and by pasting with pure Cu foil. 1 M LiPF6 in ethylene carbonate (EC)–diethyl carbonate (DEC) (1:1 in volume) was employed as the electrolyte. The cells were assembled in an argon-filled glove box with both the moisture and the oxygen content below 1 ppm. Galvanostatic charge–discharge was carried out using a LAND battery program-control test system (Wuhan, China) in the potential range of 0.01–3.0 V at a setting current rate. The cyclic voltammetry (CV) test was implemented using an electrochemical workstation (Model 2273, Princeton Applied Research, USA). Electrochemical impedance spectroscopy (EIS) measurements were performed on this apparatus over a frequency range of 0.01 Hz–0.1 MHz at different charge–discharge stages. 3. Results and discussion The phase purity and crystalline structure of the sample were detected by XRD. As shown in Fig. 1, all of these diffraction peaks can be assigned to (111), (220), (311), (222), (400), (422), (511), (440), and (531) crystal planes and indexed as the spinel crystalline structured NiCo2O4 (JCPDS no. 73-1702). It should be noted that
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there are no evidences of pure C, suggesting its low graphitization degree, if any [22]. C atoms are usually favorable to form shells on nanoparticles in an onion-like structure. Since C is on the shell of the products, it is also difficult to detect its XRD pattern because of the breakdown of the periodic boundary condition (translation symmetry) along the radial direction [21]. For the TGA curve in the inset (a) of Fig. 1, the amount of C is estimated to be 9.8% in NiCo2O4/onion-like C nanocapsules. When the temperature reaches 298 1C, the onion-like C shell disappears. The inset (b) in Fig. 1
Fig. 1. XRD pattern of the sample obtained by annealing NiCo2/onion-like C nanocapsules at 100 1C for 2 h in air. The inset (a) is the TGA curve. The inset (b) is the Raman spectra.
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presents typical Raman spectra of NiCo2O4/onion-like C nanocapsules in the 1100–1700 cm 1 range. Glassy C exhibits two broad peaks around 1326 cm 1 (labeled as D peak for “disordered C”) and 1563 cm 1(labeled as G peak for “graphite carbon”). The Dpeak shifts to a lower frequency compared to the bulk diamond (1332 cm 1), because of the phonon-confinement effects caused by serious curvature and uneven distribution of the graphite-atom planes [21]. A lot of lattice defects are also confirmed to exist in the graphite layers. Moreover, the G peak is located at a lower frequency of 1563 cm 1 due to the graphite phase of the C atoms [21]. The morphology and microstructure of the as-synthesized NiCo2O4/onion-like C nanocapsules and NiCo2O4 nanoparticles were examined with TEM. From the TEM images shown in Fig. 2(a) and (c), the overall morphology of NiCo2O4/onionlike C nanocapsules and NiCo2O4 nanoparticles exhibit the sphere-like shape. The average size of the NiCo2O4/onion-like C nanocapsules is estimated to be 42.4 nm, which is smaller than that (56.5 nm) of the NiCo2O4 nanoparticles. As shown in Fig. 2(b), the NiCo2O4/onion-like C nanocapsules own a clear core/shell-type structure, in which the NiCo2O4 nanoparticles cores are encapsulated by the onion-like C shells. The inset of Fig. 2(b) corresponds to the frame in Fig. 2(b). The lattice plane spacing of the onion-like shells is 0.34 nm, corresponding to the (002) plane of graphite. Nevertheless, a mass of lattice imperfections can be seen in the C layers as a
Fig. 2. TEM images of (a) NiCo2O4/onion-like C nanocapsules and (c) NiCo2O4 nanoparticles; HRTEM images of (b) NiCo2O4/onion-like C nanocapsules and (d) NiCo2O4 nanoparticles. The inset is the magnified part of the frame shown in (b).
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consequence of the serious bending and collapsing of the graphite atom layer. This is similar to the NiO/C, CuO/C and Co3O4/C nanocapsules with onion-like C shells [18–20]. It is worthy to note that there is a hollow structure between the NiCo2O4 nanoparticles cores and the onion-like C shells. This hollow structure can provide enough void spaces to accommodate the volume change of the NiCo2O4 nanoparticles during the charge/discharge process. The formation mechanism of the void can be explained by the formation process of core/shell structured nanocapsules by a modified arc discharge technique. When carbon atoms were absorbed on the surface of NiCo nanoparticles during the arc discharge process, some gas, such as H2 and Ar, were simultaneously coated into the core/shell structured nanocapsules. After the passivation process of nanocapsules, the absorbed gas escaped from the defective onion-like C shells. Then the void space between onion-like C shells and NiCo nanoparticles were formed. In Fig. 2(d), the HRTEM image observation shows that the NiCo2O4 nanoparticles do not have the core/shell-type structure since the onion-like C shells were evaporated during the air-annealing process. The measured interplanar distance of 0.25 nm in the NiCo2O4 nanoparticles can be assigned to the characteristic interplanar distance of (311) of NiCo2O4. To evaluate the electrochemical properties of the NiCo2O4/ onion-like C nanocapsules for use as anode in LIBs, CV curves, galvanostatic discharge–charge profiles, and cycling measurements were conducted at room temperature in the voltage window of 0.01–3.0 V. For comparison, the electrochemical properties of the
NiCo2O4 nanoparticles were also investigated under the same conditions. Fig. 3(a) shows the CV curves of the NiCo2O4/onionlike C nanocapsules for the first four cycles at a scan rate of 0.1 mV s 1. For the first cathodic scan, an intensive reduction peak centered at 0.84 V can be clearly observed and explained by the reduction of Ni2 þ and Co3 þ to metallic Ni and Co (Eq. (1)), respectively. The broad peak centered at 1.24 V can be attributed to the destruction of the crystal structure and is easily distinguishable from the other cycles [23]. The minor peak at 0.63 V can be described by the formation of a solid electrolyte interface (SEI) film. In the first anodic oxidation process, the two anodic peaks at around 1.7 and 2.2 V can be related to the oxidation of Ni0 to Ni2 þ (Eq. (2)) and Co0 to Co3 þ (Eq. (3) and Eq. (4)) [8,10,12]. In the subsequent cycles, the positive shift of both the reduction and the oxidation peaks may be due to the Li þ insertion and drastic Li-driven structural or textual modifications during the first lithiation process caused by some activation processes [24–26]. Interestingly, the subsequent CV curves exhibit good reproducibility with almost the same peak intensity and integrated area of the cathdic/anodic peak, suggesting a high reversibility of lithium storage. As shown in Fig. 3(b), both the reduction and the oxidation peaks of NiCo2O4 nanoparticles shift to higher potential than the NiCo2O4/onion-like C nanocapsules in the first cycle. The observation can be explained by the higher electronic conductivity of the onion-like C shells in the nanocapsules, which is beneficial to the diffusion of lithium ions [27]. Nevertheless, both the peak intensity and integral areas decrease significantly during the subsequent cycles, indicating severe capacity fading. On the basis
Fig. 3. First four consecutive CV curves of (a) NiCo2O4/onion-like C nanocapsules and (b) NiCo2O4 nanoparticles at a scan rate of 0.1 mV s 1 in the voltage range of 0.01–3.0 V, and galvanostatic charge–discharge voltage profiles at a current density of 200 mA g 1 for the selected cycles of (c) NiCo2O4/onion-like C nanocapsules and (d) NiCo2O4 nanoparticles.
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of the CV tests, together with the storage mechanisms of NiO and CoO reported previously, the whole electrochemical process can be described as follows [8–12,23]: NiCo2O4 þ 8Li þ þ 8e -Ni þ 2Co þ 4Li2O
(1)
Ni þ Li2O2NiO þ 2Li þ þ 2e
(2)
Co þ Li2O2CoO þ 2Li þ þ 2e
(3) þ
CoO þ 1/3Li2O21/3Co3O4 þ 2/3Li þ 2/3e
(4)
Fig. 3(c) shows the charge–discharge voltage profiles of the NiCo2O4/onion-like C nanocapsules electrode for the selected cycles at a current density of 200 mA g 1. It is clear that there are two plateaus around 1.7 and 2.2 V in the charge process, in agreement with the anode peaks in the CV curves. The consequent charge curves show an increase in potential associated with polarization and related to ion transfer during the charge–discharge process [27]. In the first discharge process, both the NiCo2O4/onion-like C nanocapsules and NiCo2O4 nanoparticles electrodes exhibit wide and steady potential plateaus at around 0.9 V, followed by a gradual decrease in voltage. There plateau in the discharge process indicates the reduction of Ni2 þ and Co2 þ [27]. The potential plateaus in the subsequent discharge curves shift to higher voltage than the first cycle. The phenomenon can be explained by Eq. (1), in which NiCo2O4 irreversibly reacts with Li þ [8,12,28]. The initial discharge capacities are 1709.2 mA h g 1 ( 15.4 mol of Li). The initial extra capacity at the first discharge can be attributed to the formation of a SEI film. The fact that the initial charge capacity of 1407.5 mA h g 1 exceeds 8 mol of Li that can be theoretically delivered in light of the conversion reaction (Eq. (1)) [10] may be explained by the reversible formation/dissolution of a polymeric gel-like film as a result of the decomposition of the electrolyte [10,12,29]. The irreversible capacity loss in the first cycle indicates the formation of SEI layer that cannot fully decompose during the first charge. The NiCo2O4/onion-like C nanocapsules deliver a capacity retention ratio of 82.3%. The increased capacity retention ratios of 93.6%, 97.8% and 98.7% are found in the 2nd, 50th and 100th cycles, respectively. In addition, a large deviation in potential exists between the charge and discharge curves, due to the ion transfer-related
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polarization during the cycling process [10,29,30]. The charge–discharge curves of NiCo2O4 nanoparticles were also tested for comparison (Fig. 3(d)). The NiCo2O4 nanoparticles exhibit the initial discharge and charge capacities of 1649.8 and 1159.6 mA h g 1, respectively. The values are lower than those of the NiCo2O4/onion-like C nanocapsules, implying that the onion-like C shells and the void spaces can significantly improve the lithium storage. Moreover, the NiCo2O4 nanoparticles deliver an initial coulomb efficiency of 70.3%, which is lower than that (82.3%) of the NiCo2O4/onion-like C nanocapsules. In the subsequent cycles, the discharge capacity of NiCo2O4 nanoparticles electrodes decrease markedly, reflecting the electrochemical unstable nature of NiCo2O4 nanoparticles anode during the cycling. Fig. 4(a) presents the cycling performance and the coulombic efficiency of both NiCo2O4/onion-like C nanocapsule and NiCo2O4 nanoparticle anodes at a current density of 0.2 A g 1. Apparently, NiCo2O4/onion-like C nanocapsules demonstrate a much better cyclic retention than the NiCo2O4 nanoparticles. After a long cycling period of 300 cycles, the reversible capacity of the NiCo2O4/onion-like C nanocapsules can remain at high discharge capacity of 1197.2 mA h g 1 at 0.2 A g 1, and its coulombic efficiency maintains consistently at 98.6%. By contrast, the discharge capacity of the NiCo2O4 nanoparticles drops to 438.4 mA h g 1 after 300 cycles. The poor cycle life of the NiCo2O4 nanoparticles is mainly due to the larger volume change and the electronic accumulation of the NiCo2O4 nanoparticles [31]. It is noted that there is a gradually increasing trend in capacity after 100 cycles. After that, the cycling performance of the NiCo2O4/onion-like C nanocapsules becomes very stable. This, again, provides a valid proof of the significance of the synergistic effect provided by the onion-like C shell, and is a general observation for transition metal oxides. There are three possible causes. First, the surface area of the electrode will increase during the pulverization process, leading to more active sites for lithium storage. The pulverized particles are supposed to be still attached to the C shell. Second, Ni and Co nanoparticles generating form the initial stage will increase the overall conductivity of the electrode, improve the charge transfer kinetics, and increase the capacity in the following cycles. Third, the decomposition of the electrolyte will lead to the formation of the organic polymeric/gel-like SEI layer on the electrode surface, thus
Fig. 4. (a) Cycling performance and coulombic efficiency at 200 mA g 1 and (b) cycling performance at various current rates of NiCo2O4/onion-like C nanocapsules electrode and NiCo2O4 nanoparticles electrode.
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Fig. 5. (a) Nyquist polts of NiCo2O4/onion-like C nanocapsules electrode and NiCo2O4 nanoparticles electrode at open-circuit voltage after two cycles. The CPE is the constant phase angle element, which is used to replace the capacitance (C) for convenience to fitting and (b) Nyquist spectra of NiCo2O4/onion-like C nanocapsules for the selected cycles. The enlargement shows high frequency region.
improving the mechanical cohesion of the active materials without hindering the ion transfer and providing excess lithium and ion storage sites, especially in the low potential region [32]. For the NiCo2O4 nanoparticles, there is no observable increment in capacity during cycling because C shells are not available for keeping aggregation during cycling. Low initial coulomb efficiency and unstable cycle performance are the main obstacles to the application of high capacity anode materials. When NiCo2O4 nanoparticles are exposed to the electrolyte, an unstable SEI and a continuous consumption of lithium can lead to the above major problems. In order to improve cycle performance of the NiCo2O4 nanoparticles, composite materials with large surface areas are often designed at the expense of inducing great consumption of lithium ions in the first cycle and a low initial coulomb efficiency [33]. The NiCo2O4 nanoparticles in our NiCo2O4/ onion-like C nanocapsules were properly coated by the onionlike C shells. The onion-like C shells can accommodate volume change during the charge–discharge process. Besides, the inner defects in the onion-like C shell and the void spaces between the C shells and the NiCo2O4 nanoparticles can simultaneously avoid the formation of SEI on their surface, thus increasing the initial coulomb efficiency. High rate capability is an important parameter for anode materials in LIBs to reduce the discharge–charge time required in practical applications [10]. To better understand the electrochemical behavior of the electrodes, the rate capability of both the NiCo2O4/onion-like C nanocapsules and NiCo2O4 nanoparticles electrodes is also evaluated at different current densities and shown in Fig. 4(b). The electrode of NiCo2O4/ onion-like C nanocapsule shows superior rate capability with only minimal capacity fading when the charge rate increases from 0.2 to 4 A g 1 compared to the NiCo2O4 nanoparticles electrode. It can even maintain the discharge capacities of 1268.1, 1182.3, 1084.1, 995.2, and 868.6 mA h g 1 after 10 cycles at a current density of 0.2, 0.4, 0.8, 1.6 and 4 A g 1, respectively. Moreover, upon altering the current density back to a low current density of 0.2 A g 1, a discharge capacity as high as 1270.4 mA h g 1 can be recoverable, which is slightly
higher than the initial capacity after 10 cycles. The phenomena indicate that NiCo2O4/onion-like C nanocapsules have great potential as a high-rate anode material in LIBs. By contrast, only 72.2% of the initial capacity after 10 cycles can be regained by the NiCo2O4 nanoparticles electrode. It is generally known that the interfacial charge-transfer process and Li þ diffusion are the keys for cycling stability and rate performance [10]. In order to obtain an insight into the transport kinetics of the electrochemical reaction process at open-circuit condition, EIS analysis was performed. Nyquist dispersions are shown in Fig. 5. The Nyquist plots consist of a semicircle at high frequencies and a sloping line at low frequencies. The semicircle can be referred to the charge transfer resistance on electrode/electrolyte interface, while the sloping line can be assigned to the Li þ diffusion through electrolyte and active material [8–10,4]. The intercepts on the real axis can be considered as the combined resistance of the ionic resistance of the electrolyte, the intrinsic resistance of the active materials, and the contact resistance at the active material/current collector interface [34]. The electrochemical impedance spectra displayed a smaller half-cycle curve and a lower slope of the straight line for the NiCo2O4/onion-like C nanocapsules compared to the NiCo2O4 nanoparticles. This should be derived from the improved electron conductivity and ionic mobility of the NiCo2O4/onion-like C nanocapsules. The charge-transfer resistance Rct of the NiCo2O4/onion-like C nanocapsules electrode is 20 Ω, which is significantly lower than that of NiCo2O4 nanoparticles electrode (Rct ¼ 35 Ω). This suggests that onion-like C shells not only greatly boost the electronic conductivity but also enable much easier chargetransfer at the electrode/electrolyte interfrace of the NiCo2O4/ onion-like C nanocapsules (compared with the NiCo2O4 nanoparticles) [23]. One can observe in the inset of Fig. 5 (b), from 2 to 80 cycles, there is a slight increase in Rct estimated from the depressed semicircles, indicating a relatively expedient diffusion of Li þ and electrons as the cycle number increased [9]. These results also demonstrate the stability of the NiCo2O4/onion-like C nanocapsules, which ensures better capacity retention and is a further indication of
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Fig. 6. TEM images of (a) NiCo2O4/onion-like C nanocapsules and (b) NiCo2O4 nanoparticles after 300 discharge–charge cycles.
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of 1270.4 mA h g 1 has been recoverable upon altering the current density back to a low current density of 0.2 A g 1. The remarkably electrochemical properties of the NiCo2O4/onion-like C nanocapsules anode in terms of high reversible capacity, high initial coulomb efficiency, excellent cycling performance, and high rate capability have been found to originate from the synergistic contribution from unique architecture, including the onion-like C shells, NiCo2O4 nanoparticles, and void spaces between the shells and the cores. The simple preparation process and excellent electrochemical performances render NiCo2O4/onion-like C nanocapsules to be an attractive anode material for LIBs. Acknowledgment
the advantages of core/shell-type structure. After 120 cycles, the value of Rct decrease slightly compared to the 80th cycled state. This impedance evolution can well match the cycling electrochemical performance as shown in Fig. 4(a). To understand the effect of discharge/charge on the onion-like carbon shell, the morphology and microstructure of both NiCo2O4/ onion-like C nanocapsules and NiCo2O4 nanoparticles were examined by TEM technique after 300 discharge/charge cycles and are shown in Fig. 6. The postmortem analysis of the NiCo2O4/ onion-like C nanocapsules (Fig. 6(a)) demonstrates that repeated insertion/deinsertion of Li þ ions into/from the oxide crystal lattice structure has a minimal effect on the active particle shape. By contrast, NiCo2O4 nanopartilces (Fig. 6(b)), after 300 discharge/ charge cycles, undergo a drastic change. The results further confirm that the onion-like C shells play an important role in mechanically stabilizing the material during repeated lithium insertion and deinsertion reactions [18,19]. The unique microstructure, including the onion-like C shells, NiCo2O4 nanoparticles, and void spaces between the shells and the cores result in the remarkably electrochemical properties in terms of high discharge capacity, high initial coulomb efficiency, excellent cycling performance, and rate capability in the NiCo2O4/onion-like C nanocapsule electrode. These features can be understood from the following aspects: first, the onion-like C shell improves electronic conductivity and ionic mobility of the NiCo2O4 nanoparticles. Second, void spaces provide high surface area for Li þ intercalation and structural flexibility for volume change. Third, the inner defects in the onion-like C shells and the void spaces can simultaneously avoid the formation of SEI on their surface, thus increasing the initial coulomb efficiency. 4. Conclusion NiCo2O4/onion-like C core/shell-structured nanocapsules composed of NiCo2O4 nanoparticles cores and onion-like C shells have been successfully synthesized by a modified arc discharge process followed by an air-annealing process. LIBs with the NiCo2O4/onion-like C nanocapsules as the anode have demonstrated a high discharge capacity of 1197.2 mA hg 1 after 300 cycles at 200 mA g 1, a high initial Coumbic efficiency of 82.3%, and a discharge capacity up to 868.6 mA h g 1 even after 10 cycles at a current density as high as 4 A g 1. A capacity
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 7511–7518 www.elsevier.com/locate/ceramint
Core/shell-structured nickel cobaltite/onion-like carbon nanocapsules as improved anode material for lithium-ion batteries Xianguo Liua,b,n, Caiyun Cuia,b, Niandu Wua,b, Siu Wing Orb,nn, Nannan Bia,b b
a School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, PR China Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
Received 30 January 2015; received in revised form 11 February 2015; accepted 12 February 2015 Available online 20 February 2015
Abstract Core/shell-structured nanocapsules consisting of a nickel cobaltite (NiCo2O4) nanoparticle core encapsulated in an onion-like carbon (C) shell are synthesized by arc-discharge and air-annealing methods. Void spaces between NiCo2O4 core and the carbon shell are observed in the NiCo2O4/C nanocapsules. Lithium-ion batteries fabricated using the nanocapsules as the anode material exhibit enhanced initial coulombic efficiency of 82.3% and specific capacity of 1197.2 mA h/g after 300 cycles at 0.2 A g 1 current density. Varying the rate of charge/discharge current from 0.2 to 4 A/g does not show negative effects on the recycling stability of the nanocapsules and a recoverable specific capacity as high as 1270.4 mA h/g is obtained. The introduction of the onion-like C shell and the presence of the void spaces are found to increase the contact areas between the electrolyte and the nanocapsules for improved electrolyte diffusion, to enhance the electronic conductivity and ionic mobility of the NiCo2O4 nanoparticle cores, and to accommodate the change in volume during the lithium-ion insertion/extraction process. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Core–shell nanocapsules; Onion-like Carbon shells; Nickel cobaltite; Anode; Lithium-ion batteries
1. Introduction Development of rechargeable energy storage devices with high capacity, high rate capability, and long cycling life is the key to the developing sustainable and clean energy sources such as wind and solar energy sources [1]. Lithium ion batteries (LIBs) are considered as one of the most promising rechargeable energy storage devices because of their higher energy density and longer cycle life compared to conventional rechargeable batteries [1–5]. The low theoretical specific capacity (372 mA h/g) in graphite, the prevailing commercial anode material, is far from meeting the requirements for high energy/power density. Nanostructured transition metal oxides n Corresponding author at: School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, PR China. Tel./fax: þ86 555 2311570. nn Corresponding author. E-mail addresses: [email protected] (X. Liu), [email protected] (S.W. Or).
http://dx.doi.org/10.1016/j.ceramint.2015.02.073 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
have been considered as a possible alternative anode material for LIBs due to their higher theoretical specific capacity (500– 1000 mA h/g) and reversible conversion mechanism for lithium storage. The low electrical conductivity in conjunction with the pulverization problems induced by volume expansion/ contraction during the lithium-ion insertion/extraction process significantly reduce the performance and lifetime of nanostructured transition metal oxide-based anodes. In the past decade, extensive efforts have been made to replace single transition metal oxide anode with other inexpensive and environmentally friendly metal oxides [6–8]. As an abundant multiple oxidation state ternary metal oxide [6], nickel cobaltite (NiCo2O4) is considered as a very promising electrode material for supercapacitor owing to its high electronic conductivity, low diffusion resistance to protons/cations, and high electrolyte penetration [7]. In addition, NiCo2O4 is capable of delivering a theoretical specific capacity of 890 mA h g 1. These features are favorable to develop high-performance electrode materials [6–9]. Today, there are only a few reports on the application of NiCo2O4 as an anode
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material for LIBs. These include: 1) the synthesis of threedimensional (3D) hierarchical porous flower-like NiCo2O4 using a facile hydrothermal approach followed by calcination in air to give a reversible specific capacity of 939 mA h g 1 at 100 mA g 1 and to preserve the high capacity after 60 cycles [10]; 2) the growth of mesoporous NiCo2O4 nanowires on carbon textiles substrates to exhibit a reversible capacity of 1012 mA h g 1 at a 0.5 A g 1 and a capability of 854 mA h g 1 after 100 cycles [11]; 3) the preparation of monodisperse NiCo2O4 mesoporous microspheres by a facile solvothermal method followed by pyrolysis of the Ni0.33Co0.67CO3 precursor to increase capacitance and cycling stability [12]; and 4) the fabrication of highly porous NiCo2O4 nanoflakes and nanobelts by a hydrothermal technique followed by calcination of the NiCo2O4 precursors to exhibit high specific capacities of 1033 and 1056 mA h g 1, good cycling stability, and high rate capability [8].In all these works, lithiation-induced volume expansion results in the fracture and aggregation of anode materials. Various composite materials with oxides, Si and C, including core/shell structures and nanocomposites, are employed to prevent nanoparticles from pulverization and to improve the electrochemical performance [13–15]. In particular, C has been extensively studied because it is a cheap, abundant, and lowtoxic source to enhance the conductivity and stability of anode materials [16]. The core/shell-type nanostructure, named by nanocapsules, is identified as a good way to markedly improve the cycling behavior and kinetics of lithium intercalation and de-intercalation in composites [17]. For example, onion-like Ccoated NiO, Co3O4 and CuO nanocapsules demonstrate superior electrochemical performances [18–20]. The onion-like C shell acts as a barrier to prevent aggregation of transition metal oxides and provides a void space for volume changes. Therefore, the development of core/shell-structured nanocapsules with NiCo2O4 nanoparticles as the core and onion-like C as the shell is imperative to new generation anode materials in highperformance LIBs. In this paper, core/shell-structured NiCo2O4/onion-like C nanocapsules have been prepared by a modified arc-discharge method followed by an annealing process at 100 1C for 2 h in air. The electrochemical performance of NiCo2O4/onion-like carbon nanocapsules as an anode for LIBs is investigated. 2. Experiments 2.1. Synthesis of NiCo2O4/onion-like C nanocapsules A modified arc-discharge process and an air-annealing process, which have been described in details elsewhere, were used to prepare NiCo2O4/onion-like C nanocapsules [18–21]. Metallic powders of Ni and Co of 99.9% purity with an average size of 10 μm were mixed thoughtfully for the preparation of targets in which the molar ratio of Ni/Co was set to 1:2 in accordance with the Ni–Co binary phase diagram and their evaporation pressures. Elemental powders were compacted into a cylinder shape with a diameter of 20 mm under a pressure of about 20 MPa. In the modified arc-discharge process, the compressed Ni–Co powder placed on a water-cooled carbon crucible was employed as the anode, while the cathode was a carbon needle. After the arc-
discharge chamber was evacuated, 1.6 104 Pa pure argon, 0.4 104 Pa hydrogen, and 40 ml liquid ethanol were introduced into the chamber. The arc-discharge current was maintained at 80 A for 0.5 h. The partial pressure of ethanol was found to increase with the time, and the pressure of the chamber could reach 1 atm at 0.5 h because the decomposition of ethanol and the expansion of the gas both increased with increasing temperature. The products were collected from the depositions formed on the top of the chamber after passivation for 8 h in argon. To prepare the NiCo2O4/onion-like C nanocapsules and NiCo2O4 nanoparticles, the products prepared by the modified arc-discharge process were put on an Al2O3 crucible and were annealed at 100 and 300 1C for 2 h in a tubular furnace in still air, respectively. 2.2. Characterization of NiCo2O4/onion-like C nanocapsules The composition and phase purity of the products were analyzed by an X-ray diffraction (XRD) technique at a voltage of 40 kV and a current of 50 mA with Cu Kα radiation (λ=1.5418 Å). The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a JEOL JEM-2010 transmission electron microscope at an acceleration voltage of 200 kV. The oxidation behavior was investigated by thermal gravimetric analysis (TGA) in air atmosphere at a heating rate of 5 1C min 1 from 50 to 400 1C. Raman spectroscopy was used to estimate the bond structure of the graphite shells. 2.3. Electrochemical measurements of NiCo2O4/onion-like C nanocapsules The electrochemical measurements were performed under ambient temperature using standard R2032-type coin cells with lithium as both the counter electrode and the reference electrode. The working electrodes were prepared by mixing the NiCo2O4/ onion-like C nanocapsules, conductivity agent (acetylene black), and poly(vinyl difluoride) (PVDF) at a weight ratio of 50:30:20 and by pasting with pure Cu foil. 1 M LiPF6 in ethylene carbonate (EC)–diethyl carbonate (DEC) (1:1 in volume) was employed as the electrolyte. The cells were assembled in an argon-filled glove box with both the moisture and the oxygen content below 1 ppm. Galvanostatic charge–discharge was carried out using a LAND battery program-control test system (Wuhan, China) in the potential range of 0.01–3.0 V at a setting current rate. The cyclic voltammetry (CV) test was implemented using an electrochemical workstation (Model 2273, Princeton Applied Research, USA). Electrochemical impedance spectroscopy (EIS) measurements were performed on this apparatus over a frequency range of 0.01 Hz–0.1 MHz at different charge–discharge stages. 3. Results and discussion The phase purity and crystalline structure of the sample were detected by XRD. As shown in Fig. 1, all of these diffraction peaks can be assigned to (111), (220), (311), (222), (400), (422), (511), (440), and (531) crystal planes and indexed as the spinel crystalline structured NiCo2O4 (JCPDS no. 73-1702). It should be noted that
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there are no evidences of pure C, suggesting its low graphitization degree, if any [22]. C atoms are usually favorable to form shells on nanoparticles in an onion-like structure. Since C is on the shell of the products, it is also difficult to detect its XRD pattern because of the breakdown of the periodic boundary condition (translation symmetry) along the radial direction [21]. For the TGA curve in the inset (a) of Fig. 1, the amount of C is estimated to be 9.8% in NiCo2O4/onion-like C nanocapsules. When the temperature reaches 298 1C, the onion-like C shell disappears. The inset (b) in Fig. 1
Fig. 1. XRD pattern of the sample obtained by annealing NiCo2/onion-like C nanocapsules at 100 1C for 2 h in air. The inset (a) is the TGA curve. The inset (b) is the Raman spectra.
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presents typical Raman spectra of NiCo2O4/onion-like C nanocapsules in the 1100–1700 cm 1 range. Glassy C exhibits two broad peaks around 1326 cm 1 (labeled as D peak for “disordered C”) and 1563 cm 1(labeled as G peak for “graphite carbon”). The Dpeak shifts to a lower frequency compared to the bulk diamond (1332 cm 1), because of the phonon-confinement effects caused by serious curvature and uneven distribution of the graphite-atom planes [21]. A lot of lattice defects are also confirmed to exist in the graphite layers. Moreover, the G peak is located at a lower frequency of 1563 cm 1 due to the graphite phase of the C atoms [21]. The morphology and microstructure of the as-synthesized NiCo2O4/onion-like C nanocapsules and NiCo2O4 nanoparticles were examined with TEM. From the TEM images shown in Fig. 2(a) and (c), the overall morphology of NiCo2O4/onionlike C nanocapsules and NiCo2O4 nanoparticles exhibit the sphere-like shape. The average size of the NiCo2O4/onion-like C nanocapsules is estimated to be 42.4 nm, which is smaller than that (56.5 nm) of the NiCo2O4 nanoparticles. As shown in Fig. 2(b), the NiCo2O4/onion-like C nanocapsules own a clear core/shell-type structure, in which the NiCo2O4 nanoparticles cores are encapsulated by the onion-like C shells. The inset of Fig. 2(b) corresponds to the frame in Fig. 2(b). The lattice plane spacing of the onion-like shells is 0.34 nm, corresponding to the (002) plane of graphite. Nevertheless, a mass of lattice imperfections can be seen in the C layers as a
Fig. 2. TEM images of (a) NiCo2O4/onion-like C nanocapsules and (c) NiCo2O4 nanoparticles; HRTEM images of (b) NiCo2O4/onion-like C nanocapsules and (d) NiCo2O4 nanoparticles. The inset is the magnified part of the frame shown in (b).
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consequence of the serious bending and collapsing of the graphite atom layer. This is similar to the NiO/C, CuO/C and Co3O4/C nanocapsules with onion-like C shells [18–20]. It is worthy to note that there is a hollow structure between the NiCo2O4 nanoparticles cores and the onion-like C shells. This hollow structure can provide enough void spaces to accommodate the volume change of the NiCo2O4 nanoparticles during the charge/discharge process. The formation mechanism of the void can be explained by the formation process of core/shell structured nanocapsules by a modified arc discharge technique. When carbon atoms were absorbed on the surface of NiCo nanoparticles during the arc discharge process, some gas, such as H2 and Ar, were simultaneously coated into the core/shell structured nanocapsules. After the passivation process of nanocapsules, the absorbed gas escaped from the defective onion-like C shells. Then the void space between onion-like C shells and NiCo nanoparticles were formed. In Fig. 2(d), the HRTEM image observation shows that the NiCo2O4 nanoparticles do not have the core/shell-type structure since the onion-like C shells were evaporated during the air-annealing process. The measured interplanar distance of 0.25 nm in the NiCo2O4 nanoparticles can be assigned to the characteristic interplanar distance of (311) of NiCo2O4. To evaluate the electrochemical properties of the NiCo2O4/ onion-like C nanocapsules for use as anode in LIBs, CV curves, galvanostatic discharge–charge profiles, and cycling measurements were conducted at room temperature in the voltage window of 0.01–3.0 V. For comparison, the electrochemical properties of the
NiCo2O4 nanoparticles were also investigated under the same conditions. Fig. 3(a) shows the CV curves of the NiCo2O4/onionlike C nanocapsules for the first four cycles at a scan rate of 0.1 mV s 1. For the first cathodic scan, an intensive reduction peak centered at 0.84 V can be clearly observed and explained by the reduction of Ni2 þ and Co3 þ to metallic Ni and Co (Eq. (1)), respectively. The broad peak centered at 1.24 V can be attributed to the destruction of the crystal structure and is easily distinguishable from the other cycles [23]. The minor peak at 0.63 V can be described by the formation of a solid electrolyte interface (SEI) film. In the first anodic oxidation process, the two anodic peaks at around 1.7 and 2.2 V can be related to the oxidation of Ni0 to Ni2 þ (Eq. (2)) and Co0 to Co3 þ (Eq. (3) and Eq. (4)) [8,10,12]. In the subsequent cycles, the positive shift of both the reduction and the oxidation peaks may be due to the Li þ insertion and drastic Li-driven structural or textual modifications during the first lithiation process caused by some activation processes [24–26]. Interestingly, the subsequent CV curves exhibit good reproducibility with almost the same peak intensity and integrated area of the cathdic/anodic peak, suggesting a high reversibility of lithium storage. As shown in Fig. 3(b), both the reduction and the oxidation peaks of NiCo2O4 nanoparticles shift to higher potential than the NiCo2O4/onion-like C nanocapsules in the first cycle. The observation can be explained by the higher electronic conductivity of the onion-like C shells in the nanocapsules, which is beneficial to the diffusion of lithium ions [27]. Nevertheless, both the peak intensity and integral areas decrease significantly during the subsequent cycles, indicating severe capacity fading. On the basis
Fig. 3. First four consecutive CV curves of (a) NiCo2O4/onion-like C nanocapsules and (b) NiCo2O4 nanoparticles at a scan rate of 0.1 mV s 1 in the voltage range of 0.01–3.0 V, and galvanostatic charge–discharge voltage profiles at a current density of 200 mA g 1 for the selected cycles of (c) NiCo2O4/onion-like C nanocapsules and (d) NiCo2O4 nanoparticles.
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of the CV tests, together with the storage mechanisms of NiO and CoO reported previously, the whole electrochemical process can be described as follows [8–12,23]: NiCo2O4 þ 8Li þ þ 8e -Ni þ 2Co þ 4Li2O
(1)
Ni þ Li2O2NiO þ 2Li þ þ 2e
(2)
Co þ Li2O2CoO þ 2Li þ þ 2e
(3) þ
CoO þ 1/3Li2O21/3Co3O4 þ 2/3Li þ 2/3e
(4)
Fig. 3(c) shows the charge–discharge voltage profiles of the NiCo2O4/onion-like C nanocapsules electrode for the selected cycles at a current density of 200 mA g 1. It is clear that there are two plateaus around 1.7 and 2.2 V in the charge process, in agreement with the anode peaks in the CV curves. The consequent charge curves show an increase in potential associated with polarization and related to ion transfer during the charge–discharge process [27]. In the first discharge process, both the NiCo2O4/onion-like C nanocapsules and NiCo2O4 nanoparticles electrodes exhibit wide and steady potential plateaus at around 0.9 V, followed by a gradual decrease in voltage. There plateau in the discharge process indicates the reduction of Ni2 þ and Co2 þ [27]. The potential plateaus in the subsequent discharge curves shift to higher voltage than the first cycle. The phenomenon can be explained by Eq. (1), in which NiCo2O4 irreversibly reacts with Li þ [8,12,28]. The initial discharge capacities are 1709.2 mA h g 1 ( 15.4 mol of Li). The initial extra capacity at the first discharge can be attributed to the formation of a SEI film. The fact that the initial charge capacity of 1407.5 mA h g 1 exceeds 8 mol of Li that can be theoretically delivered in light of the conversion reaction (Eq. (1)) [10] may be explained by the reversible formation/dissolution of a polymeric gel-like film as a result of the decomposition of the electrolyte [10,12,29]. The irreversible capacity loss in the first cycle indicates the formation of SEI layer that cannot fully decompose during the first charge. The NiCo2O4/onion-like C nanocapsules deliver a capacity retention ratio of 82.3%. The increased capacity retention ratios of 93.6%, 97.8% and 98.7% are found in the 2nd, 50th and 100th cycles, respectively. In addition, a large deviation in potential exists between the charge and discharge curves, due to the ion transfer-related
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polarization during the cycling process [10,29,30]. The charge–discharge curves of NiCo2O4 nanoparticles were also tested for comparison (Fig. 3(d)). The NiCo2O4 nanoparticles exhibit the initial discharge and charge capacities of 1649.8 and 1159.6 mA h g 1, respectively. The values are lower than those of the NiCo2O4/onion-like C nanocapsules, implying that the onion-like C shells and the void spaces can significantly improve the lithium storage. Moreover, the NiCo2O4 nanoparticles deliver an initial coulomb efficiency of 70.3%, which is lower than that (82.3%) of the NiCo2O4/onion-like C nanocapsules. In the subsequent cycles, the discharge capacity of NiCo2O4 nanoparticles electrodes decrease markedly, reflecting the electrochemical unstable nature of NiCo2O4 nanoparticles anode during the cycling. Fig. 4(a) presents the cycling performance and the coulombic efficiency of both NiCo2O4/onion-like C nanocapsule and NiCo2O4 nanoparticle anodes at a current density of 0.2 A g 1. Apparently, NiCo2O4/onion-like C nanocapsules demonstrate a much better cyclic retention than the NiCo2O4 nanoparticles. After a long cycling period of 300 cycles, the reversible capacity of the NiCo2O4/onion-like C nanocapsules can remain at high discharge capacity of 1197.2 mA h g 1 at 0.2 A g 1, and its coulombic efficiency maintains consistently at 98.6%. By contrast, the discharge capacity of the NiCo2O4 nanoparticles drops to 438.4 mA h g 1 after 300 cycles. The poor cycle life of the NiCo2O4 nanoparticles is mainly due to the larger volume change and the electronic accumulation of the NiCo2O4 nanoparticles [31]. It is noted that there is a gradually increasing trend in capacity after 100 cycles. After that, the cycling performance of the NiCo2O4/onion-like C nanocapsules becomes very stable. This, again, provides a valid proof of the significance of the synergistic effect provided by the onion-like C shell, and is a general observation for transition metal oxides. There are three possible causes. First, the surface area of the electrode will increase during the pulverization process, leading to more active sites for lithium storage. The pulverized particles are supposed to be still attached to the C shell. Second, Ni and Co nanoparticles generating form the initial stage will increase the overall conductivity of the electrode, improve the charge transfer kinetics, and increase the capacity in the following cycles. Third, the decomposition of the electrolyte will lead to the formation of the organic polymeric/gel-like SEI layer on the electrode surface, thus
Fig. 4. (a) Cycling performance and coulombic efficiency at 200 mA g 1 and (b) cycling performance at various current rates of NiCo2O4/onion-like C nanocapsules electrode and NiCo2O4 nanoparticles electrode.
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Fig. 5. (a) Nyquist polts of NiCo2O4/onion-like C nanocapsules electrode and NiCo2O4 nanoparticles electrode at open-circuit voltage after two cycles. The CPE is the constant phase angle element, which is used to replace the capacitance (C) for convenience to fitting and (b) Nyquist spectra of NiCo2O4/onion-like C nanocapsules for the selected cycles. The enlargement shows high frequency region.
improving the mechanical cohesion of the active materials without hindering the ion transfer and providing excess lithium and ion storage sites, especially in the low potential region [32]. For the NiCo2O4 nanoparticles, there is no observable increment in capacity during cycling because C shells are not available for keeping aggregation during cycling. Low initial coulomb efficiency and unstable cycle performance are the main obstacles to the application of high capacity anode materials. When NiCo2O4 nanoparticles are exposed to the electrolyte, an unstable SEI and a continuous consumption of lithium can lead to the above major problems. In order to improve cycle performance of the NiCo2O4 nanoparticles, composite materials with large surface areas are often designed at the expense of inducing great consumption of lithium ions in the first cycle and a low initial coulomb efficiency [33]. The NiCo2O4 nanoparticles in our NiCo2O4/ onion-like C nanocapsules were properly coated by the onionlike C shells. The onion-like C shells can accommodate volume change during the charge–discharge process. Besides, the inner defects in the onion-like C shell and the void spaces between the C shells and the NiCo2O4 nanoparticles can simultaneously avoid the formation of SEI on their surface, thus increasing the initial coulomb efficiency. High rate capability is an important parameter for anode materials in LIBs to reduce the discharge–charge time required in practical applications [10]. To better understand the electrochemical behavior of the electrodes, the rate capability of both the NiCo2O4/onion-like C nanocapsules and NiCo2O4 nanoparticles electrodes is also evaluated at different current densities and shown in Fig. 4(b). The electrode of NiCo2O4/ onion-like C nanocapsule shows superior rate capability with only minimal capacity fading when the charge rate increases from 0.2 to 4 A g 1 compared to the NiCo2O4 nanoparticles electrode. It can even maintain the discharge capacities of 1268.1, 1182.3, 1084.1, 995.2, and 868.6 mA h g 1 after 10 cycles at a current density of 0.2, 0.4, 0.8, 1.6 and 4 A g 1, respectively. Moreover, upon altering the current density back to a low current density of 0.2 A g 1, a discharge capacity as high as 1270.4 mA h g 1 can be recoverable, which is slightly
higher than the initial capacity after 10 cycles. The phenomena indicate that NiCo2O4/onion-like C nanocapsules have great potential as a high-rate anode material in LIBs. By contrast, only 72.2% of the initial capacity after 10 cycles can be regained by the NiCo2O4 nanoparticles electrode. It is generally known that the interfacial charge-transfer process and Li þ diffusion are the keys for cycling stability and rate performance [10]. In order to obtain an insight into the transport kinetics of the electrochemical reaction process at open-circuit condition, EIS analysis was performed. Nyquist dispersions are shown in Fig. 5. The Nyquist plots consist of a semicircle at high frequencies and a sloping line at low frequencies. The semicircle can be referred to the charge transfer resistance on electrode/electrolyte interface, while the sloping line can be assigned to the Li þ diffusion through electrolyte and active material [8–10,4]. The intercepts on the real axis can be considered as the combined resistance of the ionic resistance of the electrolyte, the intrinsic resistance of the active materials, and the contact resistance at the active material/current collector interface [34]. The electrochemical impedance spectra displayed a smaller half-cycle curve and a lower slope of the straight line for the NiCo2O4/onion-like C nanocapsules compared to the NiCo2O4 nanoparticles. This should be derived from the improved electron conductivity and ionic mobility of the NiCo2O4/onion-like C nanocapsules. The charge-transfer resistance Rct of the NiCo2O4/onion-like C nanocapsules electrode is 20 Ω, which is significantly lower than that of NiCo2O4 nanoparticles electrode (Rct ¼ 35 Ω). This suggests that onion-like C shells not only greatly boost the electronic conductivity but also enable much easier chargetransfer at the electrode/electrolyte interfrace of the NiCo2O4/ onion-like C nanocapsules (compared with the NiCo2O4 nanoparticles) [23]. One can observe in the inset of Fig. 5 (b), from 2 to 80 cycles, there is a slight increase in Rct estimated from the depressed semicircles, indicating a relatively expedient diffusion of Li þ and electrons as the cycle number increased [9]. These results also demonstrate the stability of the NiCo2O4/onion-like C nanocapsules, which ensures better capacity retention and is a further indication of
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Fig. 6. TEM images of (a) NiCo2O4/onion-like C nanocapsules and (b) NiCo2O4 nanoparticles after 300 discharge–charge cycles.
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of 1270.4 mA h g 1 has been recoverable upon altering the current density back to a low current density of 0.2 A g 1. The remarkably electrochemical properties of the NiCo2O4/onion-like C nanocapsules anode in terms of high reversible capacity, high initial coulomb efficiency, excellent cycling performance, and high rate capability have been found to originate from the synergistic contribution from unique architecture, including the onion-like C shells, NiCo2O4 nanoparticles, and void spaces between the shells and the cores. The simple preparation process and excellent electrochemical performances render NiCo2O4/onion-like C nanocapsules to be an attractive anode material for LIBs. Acknowledgment
the advantages of core/shell-type structure. After 120 cycles, the value of Rct decrease slightly compared to the 80th cycled state. This impedance evolution can well match the cycling electrochemical performance as shown in Fig. 4(a). To understand the effect of discharge/charge on the onion-like carbon shell, the morphology and microstructure of both NiCo2O4/ onion-like C nanocapsules and NiCo2O4 nanoparticles were examined by TEM technique after 300 discharge/charge cycles and are shown in Fig. 6. The postmortem analysis of the NiCo2O4/ onion-like C nanocapsules (Fig. 6(a)) demonstrates that repeated insertion/deinsertion of Li þ ions into/from the oxide crystal lattice structure has a minimal effect on the active particle shape. By contrast, NiCo2O4 nanopartilces (Fig. 6(b)), after 300 discharge/ charge cycles, undergo a drastic change. The results further confirm that the onion-like C shells play an important role in mechanically stabilizing the material during repeated lithium insertion and deinsertion reactions [18,19]. The unique microstructure, including the onion-like C shells, NiCo2O4 nanoparticles, and void spaces between the shells and the cores result in the remarkably electrochemical properties in terms of high discharge capacity, high initial coulomb efficiency, excellent cycling performance, and rate capability in the NiCo2O4/onion-like C nanocapsule electrode. These features can be understood from the following aspects: first, the onion-like C shell improves electronic conductivity and ionic mobility of the NiCo2O4 nanoparticles. Second, void spaces provide high surface area for Li þ intercalation and structural flexibility for volume change. Third, the inner defects in the onion-like C shells and the void spaces can simultaneously avoid the formation of SEI on their surface, thus increasing the initial coulomb efficiency. 4. Conclusion NiCo2O4/onion-like C core/shell-structured nanocapsules composed of NiCo2O4 nanoparticles cores and onion-like C shells have been successfully synthesized by a modified arc discharge process followed by an air-annealing process. LIBs with the NiCo2O4/onion-like C nanocapsules as the anode have demonstrated a high discharge capacity of 1197.2 mA hg 1 after 300 cycles at 200 mA g 1, a high initial Coumbic efficiency of 82.3%, and a discharge capacity up to 868.6 mA h g 1 even after 10 cycles at a current density as high as 4 A g 1. A capacity
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