Electrochimica Acta 100 (2013) 140–146
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Co3 O4 /C nanocapsules with onion-like carbon shells as anode material for lithium ion batteries Xianguo Liu a,b,∗ , Siu Wing Or b,∗∗ , Chuangui Jin a , Yaohui Lv a , Weihuo Li a , Chao Feng a , Feng Xiao a , Yuping Sun c a
School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan 243002, PR China Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong c Center for Engineering Practice and Innovation Education, Anhui University of Technology, Ma’anshan 243002, PR China b
a r t i c l e
i n f o
Article history: Received 6 March 2013 Received in revised form 25 March 2013 Accepted 25 March 2013 Available online 8 April 2013 Keywords: Carbon layer Cobalt oxide Anode electrode Lithium ion batteries
a b s t r a c t The synthesis and characterization of core/shell-type Co3 O4 /C nanocapsules for application as anode material in lithium ion batteries are reported in this paper. The synthesis process involves the preparation of Co/C nanocapsules using a modiﬁed arc-discharge method and the annealing of the Co/C nanocapsules at 300 ◦ C for 2 h in air. The as-synthesized products show a spherical shape and a core/shell-type structure in which a Co3 O4 nanoparticle core of diameter 10–30 nm is encapsulated by an onion-like carbon shell of thickness approximately 1 nm. The Co/C nanocapsules can be stable below 130 ◦ C, and be oxidized above 205 ◦ C in air. The Co3 O4 /C nanocapsules deliver an initial discharge capacity of 1467.6 mAh g−1 at 0.5 C and maintain a high reversible capacity of 1026.9 mAh g−1 after 50 charge–discharge cycles, much higher than the Co3 O4 nanoparticles (471.5 mAh g−1 ). A postmortem analysis of the Co3 O4 and Co3 O4 /C anodes subjected to prolonged cycling reveals the existence of a lower degree of surface cracking and particle breakage in the Co3 O4 /C anode than the Co3 O4 anode. The improved electrochemical performance and structural stability in the Co3 O4 /C nanocapsules are attributed to the enhanced electrical conductivity and structural buffering provided by the onion-like carbon shell. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction The ever-growing needs for high capacity, high power, and large-scale applications of lithium ion batteries have prompted an increasing research effort to develop novel electrode materials with improved electrochemical performance and structural stability [1–3]. Among various metal oxides, Co3 O4 is regarded as a promising candidate for anode not only because it can store more than eight lithium atoms per formula unit but also because it possesses a higher reversible discharge capacity (∼890 mAh/g) than the commonly used graphite (<372 mAh/g) . Nevertheless, its large volume expansion upon lithium uptake brings about the pulverization and deterioration of Co3 O4 , and consequently rapid capacity decay and poor capacity retention . In addition, the unstable solid electrolyte interface (SEI) ﬁlm can be formed on the surface of Co3 O4 , which can leads to rapid capacity decay of
∗ Corresponding author at: School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan 243002, PR China. Tel.: +86 555 2311570; fax: +86 555 2311570. ∗∗ Corresponding author. E-mail addresses: [email protected]
(X. Liu), [email protected]
(S.W. Or). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.03.179
Co3 O4 and to severe safety problem for lithium ion batteries . In these respects, research attention in recent years has been paid to study nanostructured electrode in order to improve the electrochemical performance and structural stability. Basically, there are two superiorities of the electrodes with nanostructures: (1) their large surface area endows them with high capacity and a favorable response to high-rate cycling due to the dramatic increment of reaction sites and the interface between the active materials and the electrolyte, as the nanoscale ingredient makes lithium ion diffusion much easier by shorting the diffusion length effectively to grain size; (2) the interior space among the nanoscale structures allows the volume variation upon insertion/extraction of lithium ions to be better accommodated . To date, various Co3 O4 nanostructures such as nanoparticles, nanoﬁbers, nanorods, and nanotubes have been studied for electrode applications [7–10]. However, it is still a great challenge to simultaneously maintain a large reversible capacity, a high coulombic efﬁciency, a long cycling life, and a good rate capability in nanostructured Co3 O4 electrode materials. In this regard, the commonly used approach is to make nanocomposite materials (e.g., the inactive/active concept), particularly with carbon, where the function of carbon is twofold: providing a physical buffering layer for the large volume change (cushion effect) and increasing the electrical conductivity . For example, Co3 O4 –C and Co3 O4 –graphene nanocomposites with high
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capacity and improved cyclability were achieved as anode materials for lithium ion batteries [11,12]. More recently, core/shell-type nanocapsules consisting of an inner nanoparticle core encapsulated by an outer shell have been recognized as a good way to improve the cycling behavior and kinetics of lithium intercalation and deintercalation. Recently, a few reports have ever been made on the chemical synthesis of Co3 O4 /C nanocapsules for lithium ion battery applications [2,3,11]. Co3 O4 nanoparticles with the hollow carbon spheres were prepared by layer-by-layer coating, which processed the 500 mAh g−1 of the reversible capacity at a current density of 744 mA g−1 . The Co3 O4 –C core–shell nanowire array was synthesized by combining a facial hydrothermal synthesis and direct current magnetron sputtering, which delivers an initial discharge capacity 1330.8 mAh g−1 at 445 mA g−1 . Jayaprakash et al. reported the synthesis of amorphous carbon coated Co3 O4 nanospheres using a hydrothermal approach, exhibiting a lithium deinsertion capacity of 567 mAh g−1 at the end of 107 cycles of discharge and charge at a current rate of 445 mA g−1 . As discussed in our previous works on the microwave absorption of nanocapsules with carbon as shells, compared with the amorphous carbon shells, onion-like carbon shells can effectively improve the electrical conductivity [13,14]. In our present works, we design and elaborate a new type of Co3 O4 nanocapsules with onion-like carbon shells. The electrochemical performances of Co3 O4 nanocapsules with onion-like carbon shells as an anode for lithium ion batteries are investigated in detail. Co3 O4 nanocapsules with highly reversible capacity, excellent cycle performance and high safety may be attractive candidates for anode materials for lithium ion batteries. 2. Experimental 2.1. Synthesis A modiﬁed arc-discharge method, which has been described in detail elsewhere, was used to prepare Co/C nanocapsules [13,14]. In brief, bulk Co placed on a water-cooled copper 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 atmospheric pressure 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 Co3 O4 /C nanocapsules and Co3 O4 nanoparticles, the products prepared by the modiﬁed arc-discharge method were put on an Al2 O3 crucible and were annealed at 300 and 500 ◦ C for 2 h in a tubular furnace in still air, respectively. 2.2. Characterization 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 ˚ The transmiscurrent of 50 mA with Cu K␣ radiation ( = 1.5418 A). sion 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 thermal stability of the products synthesized after the arc-discharge process was investigated by TG–DTA measurements at a heating rate of 10 ◦ C min−1 in air. The electrochemical experiments were performed with standard R2032 type coin cells. The working electrodes were
Fig. 1. XRD patterns of the products synthesized after the arc-discharge process, annealing process at 300 ◦ C for 2 h in air, and annealing process at 500 ◦ C for 2 h in air.
prepared by mixing the Co3 O4 /C nanocapsules/Co3 O4 nanoparticles, carbon black and poly(vinyl diﬂuoride) (PVDF) at weight ratio of 80:10:10 and by pasting with pure Cu foil. A metallic lithium foil was used as the counter electrode, while 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DMC) (1:1 in volume) was employed as the electrolyte. The cells were assembled in an argon-ﬁlled glove box. The galvanostatic charge–discharge tests were carried out using a LAND battery program-control test system (Wuhan, China) in the potential range of 0–3.2 V at room temperature. The cyclic voltammetry (CV) test was implemented using an electrochemical workstation (Model 2273, Princeton Applied Research, USA) in the potential window of 0–3.2 V (vs. Li/Li+ ) at a scan rate of 0.1 mV s−1 . Electrochemical impedance spectroscopy (EIS) measurements were performed on this apparatus using a three-electrode cell with the metallic lithium foil as both the counter and reference electrodes over a frequency range of 100 kHz to 10 MHz at different charge–discharge stages by applying an AC signal of 5 mV. 3. Results and discussion Fig. 1 shows the XRD patterns of the products synthesized after the arc-discharge process, the annealing process at 300 ◦ C for 2 h in air, and the annealing process at 500 ◦ C for 2 h in air. It is seen that the XRD pattern of the products synthesized after the arc-discharge process match well with fcc-Co structure and show no peaks for the hcp-Co structure. The observation is coincident with the previous reports [15,16]. It is known that the hcp-Co phase can exist stably at low temperature, but the fcc-Co phase is a high-temperature phase and is stable only above 417 ◦ C. The reasons for the existence of the high-temperature fcc-Co phase at room temperature can be attributed to the higher surface energy of nanoscale particles and rapid cooling during the arc-discharge [15,16]. Moreover, Co-oxide peaks are not found from the XRD pattern, indicating that the products may be free from surface oxidation, due to the protective carbon shell. The XRD patterns of the products annealed at 300 and 500 ◦ C for 2 h in air can be indexed as single phase Co3 O4 with no evidence of the presence of other oxides such as CoO and Co2 O3 . This is because the Co3 O4 phase is more stable than the CoO and Co2 O3 phases below 925 ◦ C in air . With increasing the annealed temperature up to 500 ◦ C, there are a gradual increase intensity and a gradual contraction in diffraction peaks, reﬂecting the growth of the Co3 O4 phase. It is noted that no evidence for pure C is recorded for the shell of the products because of breaking down of the
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Fig. 2. TG–DTA curves for Co/C nanocapsules recorded in air with a heating rate of 10 ◦ C min−1 .
periodic boundary condition (translation symmetry) along the radial direction [13,14]. Fig. 2 shows the TG–DTA curves for the Co/C nanocapsules a heating rate of 10 ◦ C min−1 in air. Below 100 ◦ C, the occurrence of a weight loss of about 0.92 wt% can be ascribed to the release of the absorbed gases or moisture on the surface of the Co/C nanocapsules . For the DTA curve, the exothermal peaks at 205 and 250 ◦ C accompanied with a sharp weight gain can be attributed to a chemical reaction in which metallic Co is oxidized (ignoring the weight loss from the oxidation of carbon to CO2 ). At elevated temperature above 250 ◦ C, the weight loss is about 0.73 wt%, which may be explained by an active chemical reaction in which the residual carbon shell is oxidized in CO2 gas. The oxidation temperature of carbon shell in the present case, involving onion-like carbon shells on the surface of Co nanoparticles, was signiﬁcantly lower than those of carbon nanotubes (527–727 ◦ C) and bulk graphite crystals (846 ◦ C) . The endothermal peak at 910 ◦ C is a result of the phase transformation from a low-temperature Co3 O4 -type oxide to a high-temperature oxide of CoO-type oxide . From the TG analysis, the Co/C nanocapsules appear to be stable below 130 ◦ C in air. Fig. 3 illustrates the TEM and HRTEM images of the products synthesized after the arc-discharge process (Fig. 3(a) and (b)), the annealing process at 300 ◦ C for 2 h in air (Fig. 3(c) and (d)), and the annealing process at 500 ◦ C for 2 h in air (Fig. 3(e) and (f)). It is seen from the TEM images that the products, including the Co/C nanocapsules in Fig. 3(a), Co3 O4 /C nanocapsules in Fig. 3(c), and Co3 O4 nanoparticles in Fig. 3(e), show a spherical shape with different distributions of diameter. Speciﬁcally, the Co/C nanocapsules have a diameter of 10–40 nm, which is larger than the Co3 O4 /C nanocapsules of 10–30 nm and smaller than the Co3 O4 nanoparticles of 20–80 nm. The HRTEM images in Fig. 3(b) and (d) indicate that the Co/C and Co3 O4 /C nanocapsules own a core/shell-type structure consisting of an inner nanoparticle core encapsulated by an onion-like carbon shell. The lattice plane spacing of the coating shell is about 0.34 nm, corresponding to the (0 0 2) plane of graphite [13,14]. The shell of the Co/C nanocapsules is about 6 nm
thick (Fig. 3(b)), while that of the Co3 O4 /C nanocapsules is about 1 nm thick (Fig. 3(d)). The HRTEM image in Fig. 3(f) reveals no core/shell-type structure for the Co3 O4 nanoparticles. The measured interplanar distance of 0.234 nm in core can be assigned to the characteristic interplanar distance of (3 1 1) of Co3 O4 . The above phenomena can be explained as follows. The fcc-Co nanoparticles are oxidized to form the Co3 O4 nanoparticles during annealing. The volume change from Co to Co3 O4 leads to an increase in the size of the Co3 O4 nanoparticles. As shown in Fig. 3(b), there is a hollow structure between Co nanoparticles and the onion-like carbon shell and this hollow structure can accommodate the volume change of the Co nanoparticles during oxidation and that of the Co3 O4 nanoparticles during charge–discharge cycling . When the annealing temperature reaches 500 ◦ C, the onion-like carbon shell is almost depleted. The electrochemical performances of Co3 O4 nanocapsules/nanoparticles as anode materials for lithium ion batteries are investigated. Fig. 4 shows the galvanostatic discharge–charge curves for the ﬁrst cycle and the 30th cycle of the Co3 O4 /C nanocapsules and the Co3 O4 nanoparticles electrode measured at a current density of 0.5 C (1 C = 890 mAh g−1 ). The initial discharge capacities of the Co3 O4 /C nanocapsules and the Co3 O4 nanoparticles are 1467.6 and 1248.8 mAh g−1 , respectively. These values are much higher than the theoretical capacity of 890 mAh g−1 of pure Co3 O4 (calculated from the electrode reaction Co3 O4 + 8Li → 3Co + 4Li2 O) and can be descried by the formation of SEI ﬁlms on the surfaces of the Co3 O4 /C nanocapsules and the Co3 O4 nanoparticles and the intercalation of lithium into onion-like carbon shells during the discharge process [3,4,8,10]. The ﬁrst irreversible capacity loss of the Co3 O4 /C nanocapsules (12.2%) was less than that of the Co3 O4 nanoparticles (18.6%), suggesting the existence of a high ﬁrst coulombic efﬁciency of 87.8% in the Co3 O4 /C nanocapsules. The reversible capacity of the Co3 O4 /C nanocapsules is the 30th cycle is 1044.0 mAh g−1 , which is higher than the Co3 O4 nanoparticles of 621.4 mAh g−1 . It is clear that the Co3 O4 /C nanocapsules have much better cycling performance compared to the Co3 O4 nanoparticles. The improvement may be due to the suppression of the side reactions between the Co3 O4 nanoparticles and the electrolyte as well as the optimization of the SEI ﬁlm after onion-like carbon coating . These phenomena become more apparent after 30 cycles. For comparison purpose, Table 1 shows the comparison of the electrochemical properties between our Co3 O4 /C nanocapsules and Co3 O4 nanoparticles and the Co3 O4 /C and Co3 O4 nanomaterials reported in literatures. In our work, the onion-like carbon shell not only maintains the integrity of the electrodes but also decreases the polarization and enhances the capacity retention. The good interface afﬁnity between oxides and the carbon shell ensures structural stability during cycling, thereby giving better electrochemical performance of the Co3 O4 /C nanocapsules . Fig. 5 shows the cycling performance of the electrodes between 0 and 3.2 eV at 0.5 C. The Co3 O4 /C nanocapsules deliver an initial discharge capacity of 1467.6 mAh g−1 and exhibit a good capacity retention of 1026.9 mAh g−1 after 50 cycles without an obvious capacity fading. By contrast, the Co3 O4 nanoparticles decays
Table 1 Comparison of the electrochemical properties of Co3 O4 nanoparticles and Co3 O4 /C nanocapsules in this work with those of Co3 O4 and Co3 O4 /C reported in the literature. Samples
Current density (mA g−1 )
Initial capacity (mAh g−1 )
Co3 O4 Co3 O4 /C Co3 O4 –carbon spheres Co3 O4 –C nanowire Co3 O4 nanowire Co3 O4 superstructures Co3 O4 /C nanostructures
445 445 74 445 445 178 445
1248.8 1467.6 1068.2 1330.8 1262.0 1285 818.7
471.5 mAh g−1 after 50 cycles 1026.9 mAh g−1 after 50 cycles 802.3 mAh g−1 after 20 cycles 989.0 mAh g−1 after 50 cycles 490.5 mAh g−1 after 50 cycles 1385 mAh g−1 after 50 cycles 567 mAh g−1 after 107 cycles
This work This work     
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Fig. 3. TEM and HRTEM images of the products synthesized after (a and b) the arc-discharge process, (c and d) the annealing process at 300 ◦ C for 2 h in air, and (e and f) the annealing process at 500 ◦ C for 2 h in air.
quickly from the initial discharge capacity of 1248.8–471.5 mAh g−1 after 50 cycles. Obviously, the Co3 O4 nanocapsules electrode has better cycling performance. In order to further understand the effect of the onion-like carbon shell on the electrochemical performance of the Co3 O4 nanoparticles, cyclic voltammetry (CV) tests are carried out in this work, usually, a series of irreversible reactions during the ﬁrst discharge process such as the decomposition
of electrolyte and the formation of SEI ﬁlm, resulting in large initial irreversible capacity and low coulombic efﬁciency [3,20]. Except for the ﬁrst cycle, the following CV curves are similar, so the second cycle has been investigated. Fig. 6 shows the CV curves of Co3 O4 /C nanocapsules and Co3 O4 nanoparticles electrodes in the potential range of 0–3.2 V at a scanning rate of 0.5 mV s−1 in the second cycle. Both the electrodes have the similar shapes of CV
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Fig. 4. The charge–discharge curves of Co3 O4 nanocapsules/nanoparticles for the ﬁrst and 30th cycles.
Fig. 5. Cycling performance of Co3 O4 /C nanocapsules and Co3 O4 nanoparticles electrodes at 0.5 C.
curves. In our experiment, the one board anodic peak and two cathodic peaks can be observed. These peaks correspond to the extraction and insertion of lithium ions. The cathodic peak corresponds to a multistep electrochemical reaction, which is generally attributed to the reaction of Co (III) to Co (II) and metal Co [3,21]. In the anodic scan, one peak in the range of 1.9–2.5 V is recorded, which is ascribed to the reversible oxidation of metal Co to cobalt
Fig. 6. CV curves of Co3 O4 nanocapsules/nanoparticles for the second cycles. Scan rate: 0.5 mV s−1 , potential range: 0–3.2 V.
Fig. 7. Nyquist plots of Co3 O4 /C nanocapsules and Co3 O4 nanoparticles electrodes obtained by applying a sine wave with amplitude of 5.0 mV over frequency range 100 kHz to 0.01 Hz.
oxide . Obviously, the Co3 O4 /C nanocapsules electrode exhibit a smaller peak potential separation and a higher current density than the Co3 O4 nanoparticles because of the presence of a higher electrochemical activity and a larger reversibility during cycling . In order to understand why the Co3 O4 /C nanocapsules electrode exhibits such a superior electrochemical performance compared to Co3 O4 nanoparticles, AC impedance measurements are performed at about 2.7 V (vs. Li/Li+ ) after 30 cycles, as shown in Fig. 7. Nyquist plots show that both proﬁles exhibit a straight line and a semicircle in low- and high-frequency regions. The straight line in the low-frequency is attributed to the diffusion of the lithium ions into the active anode materials, which is the typical Warburg behavior . The semicircle in the high-frequency region represents charge-transfer resistance. The value of the diameter of the semicircle on Zre axis implies an approximate indication of the charge transfer resistance (Rct ). As seen in Fig. 7, it clearly shows that the charge transfer resistance of Co3 O4 /C is smaller than that of the Co3 O4 . Usually, if the charge transfer resistance is small, it indicates the facile charge transfer at the active material/electrolyte interface. The low charge transfer resistance is beneﬁt to enhance the electron kinetics in the electrode material and improves the electrochemical performance of the electrode material. So, the Co3 O4 /C nanocapsules can allow better penetration of electrolyte to enhance the electrochemical performance . Another excellent property associated with this Co3 O4 nanocapsules electrode is its high rate capability. Fig. 8 shows the rate capability of the Co3 O4 nanocapsules/nanoparticles electrode. The
Fig. 8. Discharge capacity of the Co3 O4 /C nanocapsules and Co3 O4 nanoparticles electrode at different discharge–charge rates (0.2 C, 0.5 C, 1 C and 2 C).
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Fig. 9. TEM images of (a) Co3 O4 nanoparticles and (b) Co3 O4 /C nanocapsules after 50 discharge–charge cycles.
cells are ﬁrst cycled with a rate of 0.2 C, and then the rate is increased to 0.5 C and 1 C, ﬁnally increases to 2 C. As can be seen, the initial discharge capacities of the Co3 O4 /C nanocapsules at rates of 0.2, 0.5, 1 and 2 C are 1671, 1326, 1139, and 925 mAh g−1 , respectively. The Co3 O4 nanoparticles exhibit comparatively smaller initial discharge capacities of 1525, 1123, 761, and 459 mAh g−1 . The Co3 O4 /C nanocapsules only have a little fading when the discharge rate is returned from 0.1 C to 2 C. This suggests that the Co3 O4 /C nanocapsules are promising negative electrodes for lithium ion batteries, which have the characteristics of high reversible capacity, good cyclability, and high rate capability. To understand the effect of the onion-like carbon shell on enhancement of the electrochemical performance of the Co3 O4 nanoparticles, the morphology and microstructure variation of both Co3 O4 nanoparticles and Co3 O4 /C nanocapsules are examined using TEM after 50 discharge/charge cycles, as shown in Fig. 9. It is apparent from the postmortem analysis that the morphology of the Co3 O4 nanoparticles (Fig. 9(a)), after 50 discharge/charge cycles, undergo drastic change, both in terms of shape and size with substantially broken nanoparticles readily apparent. In contrast, the postmortem analysis of the Co3 O4 /C nanocapsules (Fig. 9(b)) demonstrates that repeated insertion/deinsertion of Li+ ions into/from the metal oxide crystal lattice structure has minimal effect on the active particle size or shape. Remarkably, we do not observe a single broken nanocapsule after extended cycling of the material, which underscores the crucial role the onion-like carbon shells play in mechanically stabilizing the material during repeated lithium insertion and deinsertion reactions . The good electrochemical performance of the Co3 O4 nanocapsules is ascribed to the introduction of the onion-like carbon shell in the core–shell structured nanocapsules. In our work, the onionlike carbon shell on the Co3 O4 /C nanocapsules is believed to have three major roles. First, carbon itself is an electronic conductor, which ensures good electrical contact of Co3 O4 with the current collector and enhances the charge transfer/Li+ transport. With the full and uniform coating of carbon, electrons can easily reach all the positions where Li+ ion intercalation takes place. This feature is particularly helpful when the battery is cycled at high currents, and also offers much more conductive pathways, which are very helpful to the capacity, cyclability, and rate capability [23,24]. Second, the onion-like carbon shell essentially acts as a structural buffer to alleviate the volume variation of the Co3 O4 nanoparticles core . The Co3 O4 structure is easily to be destroyed during the
discharge–charge process accompanying a large volume change. For the Co3 O4 /C nanocapsules, the preservation of the structure during the Li+ insertion/extraction processes helps to maintain the electrical continuity. Third, the SEI ﬁlms formed on carbon shell are more uniform and stable than those formed on the Co3 O4 nanoparticles surface. Hence, the onion-like carbon shell is more favorable for structural stabilization in the Co3 O4 nanocapsules. 4. Conclusions We have designed and fabricated a new type of Co3 O4 /C nanocapsules, featuring a spherical Co3 O4 nanoparticle core of diameter 10–30 nm encapsulated by an onion-like carbon shell of thickness approximately 1 nm and studied their electrochemical performance and structural stability for anode in lithium ion batteries. The TG–DTA analysis has shown that the Co/C nanocapsules are stable below 130 ◦ C, and can be oxidized above 205 ◦ C, and can be transformed to CoO above 910 ◦ C. As shown in CV curves, the Co3 O4 nanocapsules electrode exhibit a smaller peak potential separation and higher current densities than the Co3 O4 nanoparticles, indicating a high electrochemical activity and an enhancement of reversibility during cycling. The Co3 O4 nanocapsules delivered an initial discharge capacity of 1467.6 mAh g−1 at 0.5 C and maintained a high reversible capacity of 1026.9 mAh g−1 after 50 cycles, much higher than the Co3 O4 nanoparticles (471.5 mAh g−1 ). AC impedance measurements show the charge transfer resistance of Co3 O4 nanocapsules is smaller than that of the Co3 O4 nanoparticles, indicating the facile charge transfer at the active material/electrolyte interface. Postmortem analysis of the Co3 O4 nanoparticles and the Co3 O4 /C nanocapsules conducted after extended cycling studies for 50 discharge/charge cycles shows that while the Co3 O4 /C nanocapsules retain its morphology, the Co3 O4 nanoparticles do not. As a result, the Co3 O4 /C nanocapsules exhibit remarkable lithium storage performance including highly reversible capacity, excellent cycle performance and rate capability. They thus shed light on the utility of the onion-like carbon shell to improve the electrochemical performance and safety of a variety of transition-metal oxide nanoparticles. Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant Nos. 51201002 and 51071001), the Research
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