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MWCNT composites as anode materials for high areal capacity lithium ion batteries
Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries Junmin Xu a,b, Yongfei Liu c, Lei He a, Changjin Zhang a,n, Yuheng Zhang a a
High Magnetic Field Laboratory, Chinese Academy of Sciences and University of Science and Technology of China, Hefei 230031, China Department of Physics and Engineering and Key Laboratory of Material Physics, Zhengzhou University, Zhengzhou, Henan 450052, PR China c Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 November 2015 Received in revised form 22 April 2016 Accepted 22 April 2016
CuO mesocrystal entangled with multi-wall carbon nanotube (MWCNT) composites are synthesized through a facile scalable precipitation and a followed oriented aggregation process. When evaluated as anode materials for lithium ion batteries, the CuO-MWCNT composites exhibit high areal capacity and good cycling stability (1.11 mA h cm 2 after 400 cycles at the current density of 0.39 mA cm 2). The excellent electrochemical performance can be ascribed to the synergy effect of the unique structure of defect-rich CuO mesocrystals and the flexible conductive MWCNTs. The assembled architecture of CuO mesocrystals can favor the Li-ion transport and accommodate the volume change effectively, as well as possess the structural and chemical stability of bulk materials, while the entangled MWCNTs can maintain the structural and electrical integrity of the electrode during the cycles. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Mesocrystal Multi-wall carbon nanotube Anode Lithium ion batteries
1. Introduction Oriented attachment, as a nonclassical crystal growth mechanism, involves the self-assembly of nanoparticles, crystallographic reorganization within the assemblies, and further conversion to mesoscopically structured crystals, namely, mesocrystals [1,2]. Mesocrystals, as a new class of nanostructure solid materials, can be composed of several to thousands of primary nanocrystals with common crystallographic orientation, but without coherent and crystalline material linking them, which can be regarded as mosaic-like quasi-monocrystals with abundant defects. Mesocrystals usually exhibit unique physical/chemical properties, which are derived from the collective interactions of individual nanoparticles as well as the defects [3]. Such superstructures would possess the structural and chemical stability of bulk materials while exploiting the beneficial properties associated with primary nanocrystals and their large reactive surface area, which attracts increasing attention in the fields of catalysis, energy storage and conversion, et al. [4–6]. Transition metal oxide (TMO) is one kind of promising anode materials for the next generation of lithium ion batteries (LIBs) due to their high theoretical capacity, safety performance, and environmental benignity [7–9]. However, the intrinsic low electrical conductivity of TMOs and the drastic volume change during n
Corresponding author. E-mail address: zhangcj@hmfl.ac.cn (C. Zhang).
the lithiation/delithiation process hinder their practical applications in high energy and high power devices. Constructing conductive carbon material (including amorphous carbon, carbon nanotube, and graphene) and TMO composites is a common approach to solve these problems [10]. Another typical strategy is to synthesize nano-sized TMOs with unique morphology, since nanostructured materials can increase the specific surface area and thus facilitate the lithiation/delithiation process [11]. Despite the great development of nanomaterial anodes, the issues of the low tap density, high processing costs, and potential nano-toxicity effects associated with nanomaterial synthesis and processing are still up in the air [6]. In this context, mesocrystals may be the most suitable candidate to solve the above obstacles. The defects within the mesocrystals are active sites for the electrochemical lithiation/ delithiation processes. When used as anodes for LIBs, the inherent uniform porosity and defects in mesocrystals not only can favor the fast Li-ion transport and electrochemical reactions exceedingly [12,13], but also can accommodate the concurrent volume change effectively [14]. On the other hand, as nanoparticle assemblies, mesocrystals have much larger size than nanoparticles, which lead to a convenient and non-hazardous processing. Especially, the high tap density of the micron-sized mesocrystals shows great advantage in fabricating the electrodes with high energy density and power density per unit volume or area [15,16], which is of significance for applications in portable electronic microdevices and thin-film LIBs [17]. However, the rational designing and fabricating the mesocrystalline materials aiming to TMO anodes is still infrequently reported.
http://dx.doi.org/10.1016/j.ceramint.2016.04.129 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Xu, et al., Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.129i
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Herein, for the first time, we fabricated the CuO mesocrystal composites incorporated with multi-wall carbon nanotubes (MWCNTs). When tested as anodes for LIBs, the CuO mesocrystal/ MWCNT composites present high areal capacity and cycling stability. A reversible discharge capacity as high as 1.11 mA h cm 2 (equivalent to a specific capacity of ∼572 mA h g 1) can be delivered after 400 cycles at the current density of 0.39 mA cm 2 (∼200 mA g 1). The main reason for the excellent electrochemical performances of the CuO mesocrystal/MWCNT composites are analyzed and discussed.
the working electrodes (the specific weight of active material loaded in each disk electrode is about 1.947 mg/cm2). The coin cells were assembled in an Ar-filled glove box, using lithium foil as the counter electrode and Celgard 2325 as the separator. The electrolyte consisted of 1 M LiPF6 solution in ethylene carbonate/ dimethyl carbonate/diethyl carbonate (1:1:1 by volume). The charge/discharge measurement, cyclic voltammetry (CV) measurement, and electrochemical impedance spectroscopy (EIS) for the cells were conducted on the Neware BTS TC53 battery test system and Zahner Zennium electrochemistry work station, respectively.
2. Experimental 3. Results and discussion 2.1. Synthesis of CuO mesocrystal/MWCNTs composites All the reagents were of analytical grade and used without further purification. Typical procedures for the synthesis of CuOMWCNT composites were as follows: Firstly, 0.65 g of MWCNTs and 9.8 g of Cu(NO3)2 3H2O were dispersed/dissolved in 1 L of deionized water under supersonic treatment for 30 min Then 240 mL of NH3 H2O solution (1.5 M) was added into the above mixed suspension under constant stirring. Next, 70 mL of NaOH solution (10 M) was added dropwise into the as-prepared suspension to form a blue precipitate. The suspension was further heated at 80 °C for 24 h to obtain the final product, CuO mesocrystal/MWCNTs composites. For comparison, CuO mesocrystal materials were also prepared in the absence of MWCNTs by a similar procedure. 2.2. Characterizations The as-prepared products were characterized using powder X-ray diffractometry (XRD, X’Pert Pro diffractometer with a Cu Kα radiation), scanning electron microscopy (SEM, Sirion 200) and transmission electron microscopy (TEM, JEOL-2010) equipped with an energy dispersive spectroscopy (EDS) detector. Thermal gravimetric analysis (TGA) was carried out using a Perkin-Elmer Pyris 1 Thermal Gravimetric Analyzer with a heating rate of 10 °C/ min from 50 °C to 700 °C under air flow. 2.3. Electrochemical measurement Electrochemical performances of the products were tested using CR2032 coin cells. Electrodes were prepared by coating the slurry consisting of active material, polyvinylidene fluoride (PVDF), and acetylene black (8:1:1 by weight) onto a copper foil. Then the copper foil was cut into round disks with diameter of 12 mm as
Fig. 1a shows the XRD patterns of the obtained CuO precipitates and CuO-MWCNT composites. The diffraction peaks of the pristine CuO can be perfectly indexed to monoclinic phase CuO (JCPDS card No. 45-0937), while the CuO-MWCNT composites show an additional faint diffraction peak at 26°, which can be attributed to the (002) plane of the MWCNTs. TGA results (Fig. 1b) show that the CuO-MWCNT composites consist of 83 wt% CuO and 17 wt% MWCNT. The morphological features of the products were examined by scanning electron microscopy (SEM). The obtained CuO displays micro-sized flower-like structure composed of intersecting nanosheets (Fig. 2a). By comparison, CuO architectures entangled by three-dimensional (3D) networks are formed in the addition of MWCNTs (Fig. 2b). The transmission electron microscopy (TEM) image reveals that the CuO sheets have two different morphologies: regular sheet with a smooth surface and bundle-like assemblies with jagged surface (the inset of Fig. 2c), indicating that the bundlelike assemblies are the intermediates for the final CuO sheets. Furthermore, the high resolution TEM (HRTEM) investigation reveals that the assemblies have a common crystallographic orientation with some discontinuous areas marked by white circles in Fig. 2c, which implies that the as-prepared CuO are mesocrystals and the oriented attachment is the potential growth mechanism. The lattice spacing of 0.28 nm corresponds to the (011) plane of CuO. Meanwhile, the corresponding SAED pattern (Fig. 2d) also shows a typical mesocrystal characteristic with elongated diffraction spots, which is due to the misorientations deviating from perfect oriented attachment within the assemblies. Fig. 2e and f display the typical TEM images of the CuO-MWCNT composites. From the TEM images, the coexistence of CuO and MWCNTs can be clearly observed and the lattice spacings are corresponding to the (011) plane of CuO and the (002) of carbon nanotubes, respectively, demonstrating MWCNTs are indeed incorporated into CuO mesocrystals to form CuOMWCNT composites.
Fig. 1. XRD patterns (a) and TG curves (b) for the CuO and CuO-MWCNT composites.
Please cite this article as: J. Xu, et al., Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.129i
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Fig. 2. SEM images of (a) CuO and (b) CuO-MWCNT composites; (c) HRTEM image of a CuO mesocrystal and the corresponding (d) SAED pattern; (e) and (f) TEM images of CuO-MWCNT composites.
Cyclic voltammetry (CV) measurements were first performed on the Li/CuO-MWCNT cells between 0 and 3.0 V at a scan rate of 0.2 mV s 1. As shown in Fig. 3a, three peaks at ∼1.58, 0.74, and 0.59 V in the first cathodic scan can be ascribed to the formation of the solid solution LixCuO, the formation of Cu2O, and the further reduction into Cu° and Li2O, respectively [18–20]. During the first charge process, two broad anodic peaks at ∼1.55 and 2.52 V are observed, corresponding to the oxidative reactions of Cu0 to Cu þ and Cu þ to Cu2 þ , respectively [18]. After the first cycle, the reduction peaks shift toward higher voltages at ∼1.13 and 0.69 V, respectively. Especially, the CV curves for the 2nd-4th cycles are similar in shape, indicating a more reversible electrode reaction. Fig. 3(b) presents the representative charge/discharge curves of the CuO-MWCNT composites at a current density of 0.195 mA cm 2 (∼0.15 C, 1 C ¼670 mA g 1) ranging from 0 to 3.0 V. In the first discharge process, three obvious sloping potential
ranges (2.0–1.23, 1.16–0.88, and 0.78–0 V vs Li þ /Li, respectively) can be observed corresponding to the three steps of the reduction of CuO. By contrast, all the slopes and plateaus become narrow during the subsequent discharging processes. The electrode delivers a high discharge areal capacity of 2.06 mA h cm 2 (equivalent to a specific capacity of 1060 mA h g 1) in the first cycle. The corresponding initial charge areal capacity is 1.23 mA h cm 2 (∼633 mA h g 1) in the first Li extraction process, leading to an irreversible capacity loss of ∼0.83 mA h cm 2 (∼427 mA h g 1) and a low coulombic efficiency of 60%, which should be ascribed to the inevitable formation of solid electrolyte interphase (SEI) layer on the surface of the active materials [7,8]. In the following charge/ discharge processes, all the profiles overlap perfectly, highly in accordance with the above CV analyses. The cycling performances of the CuO-MWCNT composites and the pristine CuO sheets are shown in Fig. 3(c) at an areal current
Please cite this article as: J. Xu, et al., Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.129i
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Fig. 3. (a) CV curves and (b) charge-discharge profiles for the CuO-MWCNT electrode; (c) Cycling performances of the CuO, CuO-MWCNT and MWCNT electrodes; (d) Rate performances of the CuO and CuO-MWCNT electrodes. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
density of 0.390 mA cm 2(∼0.3 C, 1 C ¼ 670 mA g 1) between 3.0 and 0.01 V. The initial discharge specific capacities are 1.98 and 1.95 mA h cm 2 for the CuO-MWCNT and CuO anodes, respectively; the corresponding specific capacities, ∼1019 and ∼881 mA h g 1, are both larger than the theoretical specific capacity of CuO (670 mA h g 1). The obvious capacity fading can be observed in the subsequent cycles, which should be closely related to the side reactions and irreversible structure transformation [7,18,21]. However, the electrodes exhibit a gradually increased areal/specific capacity during the next charge-discharge cycles, which can be mainly attributed to the reversible growth and dissolution of the electrochemical active polymeric gel-like film by the kinetically activated electrolyte degradation. Noteworthily, the areal capacities of the CuO-MWCNT electrode are always higher than those of CuO electrode in the whole cycling testing, which should be ascribed to robust microstructure and fine electric contacts contributed from the flexible and conductive MWCNTs. The CuO-MWCNT composite anode shows good cycling performance and even delivers a high reversible capacity of 1.11 mA h cm 2 (∼572 mA h g 1) after 400 cycles at 0.39 mA cm 2, which is 1.8 times higher than that of the pristine CuO anode with a capacity of 0.62 mA h cm 2 (∼282 mA h g 1). In addition, the cycling performance of the individual MWCNT electrode is also examined for comparison, as shown by green curves with circle markers in Fig. 3. One can find that the specific discharge capacities of the individual MWCNT electrode decrease gradually during the initial several cycles and then stabilize at ∼0.274 mA h cm 2 (∼230 mA h g 1). This is lower than the reversible capacities of the CuO electrode, indicating that the capacity enhancement of the CuO-MWCNT composite electrode is due to the synergistic effect of the CuO nanosheet and MWCNTs.
Fig. 3d shows the rate capability of the pristine CuO and CuOMWCNT electrodes at different current densities. One can see that a discharge capacity of 1.03 mA h cm 2 (∼528 mA h g 1) is obtained at 0.195 mA cm 2 (∼0.15 C, 1 C ¼670 mA g 1) after 12 cycles for the CuO-MWCNT electrode. This capacity slowly reduces to 0.69, 0.39, and 0.25 mA h cm 2 when the current rate consecutively changes to 0.39, 0.65, and 1.3 mA cm 2, respectively, and then recover to 0.69 mA h cm 2 at the density of 0.195 mA cm 2, exhibiting improved rate performance as compared to the pristine CuO anode. Fig. 4a shows the Nyquist plots for the CuO and CuO-MWCNT electrodes at fresh cells, and CuO-MWCNT electrode after 400 cycles. The three profiles share the common feature of a small high-frequency depressed semicircle and a large medium-frequency depressed semicircle followed by a linear tail in the lowfrequency region, which correspond to the surface film impedance, the charge-transfer impedance, and the Warburg impedance, respectively [10]. Apparently, the diameter of the semicircle for CuO-MWCNT electrode in the medium-frequency region is smaller than that of the CuO electrode. The result indicates that the CuO-MWCNT composite electrode possesses lower chargetransfer impedances, which can result in faster ion transport during the electrochemical lithium insertion/extraction reaction and better rate performance. After 400 charge-discharge cycles, the diameters of the semicircle for the CuO-MWCNT electrode in the high and medium frequency regions significantly decrease, as compared to those of the fresh cell (the inset of Fig. 4a). In other words, much smaller contact and charge-transfer impedances are obtained after 400 cycles, indicating the electrochemical activation and improved kinetics of the reaction upon cycling. Herein, the electrochemical activation occurs along with an irreversible
Please cite this article as: J. Xu, et al., Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.129i
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ions, and favor the formation of a robust electrode structure. Meanwhile, the porous mesocrystalline structure can also accommodate the volume change generated during the lithiation/delithiation process, maintaining the structural and electrical integrity of the electrodes.
4. Conclusions In summary, CuO mesocrystal/MWCNT composites have been fabricated through a facile precipitation method and followed by an oriented attachment process. Such defect-rich architectures can shorten the diffusion pathway of Li-ions and electrons, and buffer the volume changes during the cyclic charge-discharge processes effectively, resulting in good Li-ion storage performance and structure stability. The CuO-MWCNT electrode delivers a high areal capacity of 1.11 mA h cm 2 (∼572 mA h g 1) over 400 cycles at the current density of 0.39 mA cm 2 (∼200 mA g 1). Our results suggest that the composite composed of mesocrystal and carbon materials can be used as promising anode materials for the applications in high energy and high power LIBs.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 11174290, U1232142, U1532267), and the Scientific Research Grant of Hefei Science Center of Chinese Academy of Sciences (Grant No. 2015SRG-HSC025). Fig. 4. (a) Nyquist plots (Z′ vs. –Z′′) of the CuO and CuO-MWCNT electrodes at fresh cells, and CuO-MWCNT electrode after 400 cycles over the frequency range from 100 kHz to 0.01 Hz; (b) TEM image of the CuO-MWCNT electrodes after 400 cycles at 0.390 mA cm 2 (∼ 0.3 C, 1 C ¼ 670 mA g 1).
structure evolution of the electrode materials. During the structure evolution process, the particle sizes of the electrode materials gradually become smaller and smaller due to the electrochemical mill effect [22]. And the surfaces of fine particles generate the stable SEI film, which leads to a decrease in the reversible capacity of the as-prepared electrode during the initial many cycles. Subsequently, the decrease of the particle sizes will facilitate the reversible reaction of the electrode so that the reversible capacity of the as-prepared electrode gradually recovers. By using TEM investigation, one can see that the defect-rich sheet-like CuO mesocrystals have transformed to wormhole-like porous structure (as shown in Fig. 4b) after 400 cycles, which confirms the existence of the structure evolution. Such porous structure can further favor the Li-ion transport and Li-ion insertion/extraction reaction, and thus give rise to the decrease of the impedances, highly agreement with the above discussions. This CuO-MWCNT composites, composed of CuO mesocrystals and flexible MWCNT networks, also show higher areal/specific capacity and cycling performance compared to commercial planar Liion batteries (0.113 mA h cm 2) [23,24] and many previous reports, such as Ni/SnOx/C hybrid nanostructured arrays (first discharge capacity is 0.95 mA h cm 2 and decreases to 0.47 mA h cm 2 after 100 cycles at a current density of 0.4 mA cm 2) [25], SnO2/α-Fe2O3 nanotube array (0.344 mA h cm 2 at 0.3 mA cm 2) [26], CuO nanofibers (426 mA h g 1 at 0.15 C) [20], CuO/graphene composite (423 mA h g 1 at 0.1 C) [27], and the 3D macroporous CuO (390 mA h g 1 at 0.1 C) [28]. The improved electrochemical performance for the CuO-MWCNT electrode might be attributed to the defect-rich micro/nanostructure of CuO mesocrystals, the conductive 3D MWNT networks, and their synergic effects, which could shorten the path lengths for the transport of electrons and lithium
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Please cite this article as: J. Xu, et al., Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.129i
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Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries Junmin Xu a,b, Yongfei Liu c, Lei He a, Changjin Zhang a,n, Yuheng Zhang a a
High Magnetic Field Laboratory, Chinese Academy of Sciences and University of Science and Technology of China, Hefei 230031, China Department of Physics and Engineering and Key Laboratory of Material Physics, Zhengzhou University, Zhengzhou, Henan 450052, PR China c Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 November 2015 Received in revised form 22 April 2016 Accepted 22 April 2016
CuO mesocrystal entangled with multi-wall carbon nanotube (MWCNT) composites are synthesized through a facile scalable precipitation and a followed oriented aggregation process. When evaluated as anode materials for lithium ion batteries, the CuO-MWCNT composites exhibit high areal capacity and good cycling stability (1.11 mA h cm 2 after 400 cycles at the current density of 0.39 mA cm 2). The excellent electrochemical performance can be ascribed to the synergy effect of the unique structure of defect-rich CuO mesocrystals and the flexible conductive MWCNTs. The assembled architecture of CuO mesocrystals can favor the Li-ion transport and accommodate the volume change effectively, as well as possess the structural and chemical stability of bulk materials, while the entangled MWCNTs can maintain the structural and electrical integrity of the electrode during the cycles. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Mesocrystal Multi-wall carbon nanotube Anode Lithium ion batteries
1. Introduction Oriented attachment, as a nonclassical crystal growth mechanism, involves the self-assembly of nanoparticles, crystallographic reorganization within the assemblies, and further conversion to mesoscopically structured crystals, namely, mesocrystals [1,2]. Mesocrystals, as a new class of nanostructure solid materials, can be composed of several to thousands of primary nanocrystals with common crystallographic orientation, but without coherent and crystalline material linking them, which can be regarded as mosaic-like quasi-monocrystals with abundant defects. Mesocrystals usually exhibit unique physical/chemical properties, which are derived from the collective interactions of individual nanoparticles as well as the defects [3]. Such superstructures would possess the structural and chemical stability of bulk materials while exploiting the beneficial properties associated with primary nanocrystals and their large reactive surface area, which attracts increasing attention in the fields of catalysis, energy storage and conversion, et al. [4–6]. Transition metal oxide (TMO) is one kind of promising anode materials for the next generation of lithium ion batteries (LIBs) due to their high theoretical capacity, safety performance, and environmental benignity [7–9]. However, the intrinsic low electrical conductivity of TMOs and the drastic volume change during n
Corresponding author. E-mail address: zhangcj@hmfl.ac.cn (C. Zhang).
the lithiation/delithiation process hinder their practical applications in high energy and high power devices. Constructing conductive carbon material (including amorphous carbon, carbon nanotube, and graphene) and TMO composites is a common approach to solve these problems [10]. Another typical strategy is to synthesize nano-sized TMOs with unique morphology, since nanostructured materials can increase the specific surface area and thus facilitate the lithiation/delithiation process [11]. Despite the great development of nanomaterial anodes, the issues of the low tap density, high processing costs, and potential nano-toxicity effects associated with nanomaterial synthesis and processing are still up in the air [6]. In this context, mesocrystals may be the most suitable candidate to solve the above obstacles. The defects within the mesocrystals are active sites for the electrochemical lithiation/ delithiation processes. When used as anodes for LIBs, the inherent uniform porosity and defects in mesocrystals not only can favor the fast Li-ion transport and electrochemical reactions exceedingly [12,13], but also can accommodate the concurrent volume change effectively [14]. On the other hand, as nanoparticle assemblies, mesocrystals have much larger size than nanoparticles, which lead to a convenient and non-hazardous processing. Especially, the high tap density of the micron-sized mesocrystals shows great advantage in fabricating the electrodes with high energy density and power density per unit volume or area [15,16], which is of significance for applications in portable electronic microdevices and thin-film LIBs [17]. However, the rational designing and fabricating the mesocrystalline materials aiming to TMO anodes is still infrequently reported.
http://dx.doi.org/10.1016/j.ceramint.2016.04.129 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Xu, et al., Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.129i
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Herein, for the first time, we fabricated the CuO mesocrystal composites incorporated with multi-wall carbon nanotubes (MWCNTs). When tested as anodes for LIBs, the CuO mesocrystal/ MWCNT composites present high areal capacity and cycling stability. A reversible discharge capacity as high as 1.11 mA h cm 2 (equivalent to a specific capacity of ∼572 mA h g 1) can be delivered after 400 cycles at the current density of 0.39 mA cm 2 (∼200 mA g 1). The main reason for the excellent electrochemical performances of the CuO mesocrystal/MWCNT composites are analyzed and discussed.
the working electrodes (the specific weight of active material loaded in each disk electrode is about 1.947 mg/cm2). The coin cells were assembled in an Ar-filled glove box, using lithium foil as the counter electrode and Celgard 2325 as the separator. The electrolyte consisted of 1 M LiPF6 solution in ethylene carbonate/ dimethyl carbonate/diethyl carbonate (1:1:1 by volume). The charge/discharge measurement, cyclic voltammetry (CV) measurement, and electrochemical impedance spectroscopy (EIS) for the cells were conducted on the Neware BTS TC53 battery test system and Zahner Zennium electrochemistry work station, respectively.
2. Experimental 3. Results and discussion 2.1. Synthesis of CuO mesocrystal/MWCNTs composites All the reagents were of analytical grade and used without further purification. Typical procedures for the synthesis of CuOMWCNT composites were as follows: Firstly, 0.65 g of MWCNTs and 9.8 g of Cu(NO3)2 3H2O were dispersed/dissolved in 1 L of deionized water under supersonic treatment for 30 min Then 240 mL of NH3 H2O solution (1.5 M) was added into the above mixed suspension under constant stirring. Next, 70 mL of NaOH solution (10 M) was added dropwise into the as-prepared suspension to form a blue precipitate. The suspension was further heated at 80 °C for 24 h to obtain the final product, CuO mesocrystal/MWCNTs composites. For comparison, CuO mesocrystal materials were also prepared in the absence of MWCNTs by a similar procedure. 2.2. Characterizations The as-prepared products were characterized using powder X-ray diffractometry (XRD, X’Pert Pro diffractometer with a Cu Kα radiation), scanning electron microscopy (SEM, Sirion 200) and transmission electron microscopy (TEM, JEOL-2010) equipped with an energy dispersive spectroscopy (EDS) detector. Thermal gravimetric analysis (TGA) was carried out using a Perkin-Elmer Pyris 1 Thermal Gravimetric Analyzer with a heating rate of 10 °C/ min from 50 °C to 700 °C under air flow. 2.3. Electrochemical measurement Electrochemical performances of the products were tested using CR2032 coin cells. Electrodes were prepared by coating the slurry consisting of active material, polyvinylidene fluoride (PVDF), and acetylene black (8:1:1 by weight) onto a copper foil. Then the copper foil was cut into round disks with diameter of 12 mm as
Fig. 1a shows the XRD patterns of the obtained CuO precipitates and CuO-MWCNT composites. The diffraction peaks of the pristine CuO can be perfectly indexed to monoclinic phase CuO (JCPDS card No. 45-0937), while the CuO-MWCNT composites show an additional faint diffraction peak at 26°, which can be attributed to the (002) plane of the MWCNTs. TGA results (Fig. 1b) show that the CuO-MWCNT composites consist of 83 wt% CuO and 17 wt% MWCNT. The morphological features of the products were examined by scanning electron microscopy (SEM). The obtained CuO displays micro-sized flower-like structure composed of intersecting nanosheets (Fig. 2a). By comparison, CuO architectures entangled by three-dimensional (3D) networks are formed in the addition of MWCNTs (Fig. 2b). The transmission electron microscopy (TEM) image reveals that the CuO sheets have two different morphologies: regular sheet with a smooth surface and bundle-like assemblies with jagged surface (the inset of Fig. 2c), indicating that the bundlelike assemblies are the intermediates for the final CuO sheets. Furthermore, the high resolution TEM (HRTEM) investigation reveals that the assemblies have a common crystallographic orientation with some discontinuous areas marked by white circles in Fig. 2c, which implies that the as-prepared CuO are mesocrystals and the oriented attachment is the potential growth mechanism. The lattice spacing of 0.28 nm corresponds to the (011) plane of CuO. Meanwhile, the corresponding SAED pattern (Fig. 2d) also shows a typical mesocrystal characteristic with elongated diffraction spots, which is due to the misorientations deviating from perfect oriented attachment within the assemblies. Fig. 2e and f display the typical TEM images of the CuO-MWCNT composites. From the TEM images, the coexistence of CuO and MWCNTs can be clearly observed and the lattice spacings are corresponding to the (011) plane of CuO and the (002) of carbon nanotubes, respectively, demonstrating MWCNTs are indeed incorporated into CuO mesocrystals to form CuOMWCNT composites.
Fig. 1. XRD patterns (a) and TG curves (b) for the CuO and CuO-MWCNT composites.
Please cite this article as: J. Xu, et al., Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.129i
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Fig. 2. SEM images of (a) CuO and (b) CuO-MWCNT composites; (c) HRTEM image of a CuO mesocrystal and the corresponding (d) SAED pattern; (e) and (f) TEM images of CuO-MWCNT composites.
Cyclic voltammetry (CV) measurements were first performed on the Li/CuO-MWCNT cells between 0 and 3.0 V at a scan rate of 0.2 mV s 1. As shown in Fig. 3a, three peaks at ∼1.58, 0.74, and 0.59 V in the first cathodic scan can be ascribed to the formation of the solid solution LixCuO, the formation of Cu2O, and the further reduction into Cu° and Li2O, respectively [18–20]. During the first charge process, two broad anodic peaks at ∼1.55 and 2.52 V are observed, corresponding to the oxidative reactions of Cu0 to Cu þ and Cu þ to Cu2 þ , respectively [18]. After the first cycle, the reduction peaks shift toward higher voltages at ∼1.13 and 0.69 V, respectively. Especially, the CV curves for the 2nd-4th cycles are similar in shape, indicating a more reversible electrode reaction. Fig. 3(b) presents the representative charge/discharge curves of the CuO-MWCNT composites at a current density of 0.195 mA cm 2 (∼0.15 C, 1 C ¼670 mA g 1) ranging from 0 to 3.0 V. In the first discharge process, three obvious sloping potential
ranges (2.0–1.23, 1.16–0.88, and 0.78–0 V vs Li þ /Li, respectively) can be observed corresponding to the three steps of the reduction of CuO. By contrast, all the slopes and plateaus become narrow during the subsequent discharging processes. The electrode delivers a high discharge areal capacity of 2.06 mA h cm 2 (equivalent to a specific capacity of 1060 mA h g 1) in the first cycle. The corresponding initial charge areal capacity is 1.23 mA h cm 2 (∼633 mA h g 1) in the first Li extraction process, leading to an irreversible capacity loss of ∼0.83 mA h cm 2 (∼427 mA h g 1) and a low coulombic efficiency of 60%, which should be ascribed to the inevitable formation of solid electrolyte interphase (SEI) layer on the surface of the active materials [7,8]. In the following charge/ discharge processes, all the profiles overlap perfectly, highly in accordance with the above CV analyses. The cycling performances of the CuO-MWCNT composites and the pristine CuO sheets are shown in Fig. 3(c) at an areal current
Please cite this article as: J. Xu, et al., Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.129i
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Fig. 3. (a) CV curves and (b) charge-discharge profiles for the CuO-MWCNT electrode; (c) Cycling performances of the CuO, CuO-MWCNT and MWCNT electrodes; (d) Rate performances of the CuO and CuO-MWCNT electrodes. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
density of 0.390 mA cm 2(∼0.3 C, 1 C ¼ 670 mA g 1) between 3.0 and 0.01 V. The initial discharge specific capacities are 1.98 and 1.95 mA h cm 2 for the CuO-MWCNT and CuO anodes, respectively; the corresponding specific capacities, ∼1019 and ∼881 mA h g 1, are both larger than the theoretical specific capacity of CuO (670 mA h g 1). The obvious capacity fading can be observed in the subsequent cycles, which should be closely related to the side reactions and irreversible structure transformation [7,18,21]. However, the electrodes exhibit a gradually increased areal/specific capacity during the next charge-discharge cycles, which can be mainly attributed to the reversible growth and dissolution of the electrochemical active polymeric gel-like film by the kinetically activated electrolyte degradation. Noteworthily, the areal capacities of the CuO-MWCNT electrode are always higher than those of CuO electrode in the whole cycling testing, which should be ascribed to robust microstructure and fine electric contacts contributed from the flexible and conductive MWCNTs. The CuO-MWCNT composite anode shows good cycling performance and even delivers a high reversible capacity of 1.11 mA h cm 2 (∼572 mA h g 1) after 400 cycles at 0.39 mA cm 2, which is 1.8 times higher than that of the pristine CuO anode with a capacity of 0.62 mA h cm 2 (∼282 mA h g 1). In addition, the cycling performance of the individual MWCNT electrode is also examined for comparison, as shown by green curves with circle markers in Fig. 3. One can find that the specific discharge capacities of the individual MWCNT electrode decrease gradually during the initial several cycles and then stabilize at ∼0.274 mA h cm 2 (∼230 mA h g 1). This is lower than the reversible capacities of the CuO electrode, indicating that the capacity enhancement of the CuO-MWCNT composite electrode is due to the synergistic effect of the CuO nanosheet and MWCNTs.
Fig. 3d shows the rate capability of the pristine CuO and CuOMWCNT electrodes at different current densities. One can see that a discharge capacity of 1.03 mA h cm 2 (∼528 mA h g 1) is obtained at 0.195 mA cm 2 (∼0.15 C, 1 C ¼670 mA g 1) after 12 cycles for the CuO-MWCNT electrode. This capacity slowly reduces to 0.69, 0.39, and 0.25 mA h cm 2 when the current rate consecutively changes to 0.39, 0.65, and 1.3 mA cm 2, respectively, and then recover to 0.69 mA h cm 2 at the density of 0.195 mA cm 2, exhibiting improved rate performance as compared to the pristine CuO anode. Fig. 4a shows the Nyquist plots for the CuO and CuO-MWCNT electrodes at fresh cells, and CuO-MWCNT electrode after 400 cycles. The three profiles share the common feature of a small high-frequency depressed semicircle and a large medium-frequency depressed semicircle followed by a linear tail in the lowfrequency region, which correspond to the surface film impedance, the charge-transfer impedance, and the Warburg impedance, respectively [10]. Apparently, the diameter of the semicircle for CuO-MWCNT electrode in the medium-frequency region is smaller than that of the CuO electrode. The result indicates that the CuO-MWCNT composite electrode possesses lower chargetransfer impedances, which can result in faster ion transport during the electrochemical lithium insertion/extraction reaction and better rate performance. After 400 charge-discharge cycles, the diameters of the semicircle for the CuO-MWCNT electrode in the high and medium frequency regions significantly decrease, as compared to those of the fresh cell (the inset of Fig. 4a). In other words, much smaller contact and charge-transfer impedances are obtained after 400 cycles, indicating the electrochemical activation and improved kinetics of the reaction upon cycling. Herein, the electrochemical activation occurs along with an irreversible
Please cite this article as: J. Xu, et al., Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.129i
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ions, and favor the formation of a robust electrode structure. Meanwhile, the porous mesocrystalline structure can also accommodate the volume change generated during the lithiation/delithiation process, maintaining the structural and electrical integrity of the electrodes.
4. Conclusions In summary, CuO mesocrystal/MWCNT composites have been fabricated through a facile precipitation method and followed by an oriented attachment process. Such defect-rich architectures can shorten the diffusion pathway of Li-ions and electrons, and buffer the volume changes during the cyclic charge-discharge processes effectively, resulting in good Li-ion storage performance and structure stability. The CuO-MWCNT electrode delivers a high areal capacity of 1.11 mA h cm 2 (∼572 mA h g 1) over 400 cycles at the current density of 0.39 mA cm 2 (∼200 mA g 1). Our results suggest that the composite composed of mesocrystal and carbon materials can be used as promising anode materials for the applications in high energy and high power LIBs.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 11174290, U1232142, U1532267), and the Scientific Research Grant of Hefei Science Center of Chinese Academy of Sciences (Grant No. 2015SRG-HSC025). Fig. 4. (a) Nyquist plots (Z′ vs. –Z′′) of the CuO and CuO-MWCNT electrodes at fresh cells, and CuO-MWCNT electrode after 400 cycles over the frequency range from 100 kHz to 0.01 Hz; (b) TEM image of the CuO-MWCNT electrodes after 400 cycles at 0.390 mA cm 2 (∼ 0.3 C, 1 C ¼ 670 mA g 1).
structure evolution of the electrode materials. During the structure evolution process, the particle sizes of the electrode materials gradually become smaller and smaller due to the electrochemical mill effect [22]. And the surfaces of fine particles generate the stable SEI film, which leads to a decrease in the reversible capacity of the as-prepared electrode during the initial many cycles. Subsequently, the decrease of the particle sizes will facilitate the reversible reaction of the electrode so that the reversible capacity of the as-prepared electrode gradually recovers. By using TEM investigation, one can see that the defect-rich sheet-like CuO mesocrystals have transformed to wormhole-like porous structure (as shown in Fig. 4b) after 400 cycles, which confirms the existence of the structure evolution. Such porous structure can further favor the Li-ion transport and Li-ion insertion/extraction reaction, and thus give rise to the decrease of the impedances, highly agreement with the above discussions. This CuO-MWCNT composites, composed of CuO mesocrystals and flexible MWCNT networks, also show higher areal/specific capacity and cycling performance compared to commercial planar Liion batteries (0.113 mA h cm 2) [23,24] and many previous reports, such as Ni/SnOx/C hybrid nanostructured arrays (first discharge capacity is 0.95 mA h cm 2 and decreases to 0.47 mA h cm 2 after 100 cycles at a current density of 0.4 mA cm 2) [25], SnO2/α-Fe2O3 nanotube array (0.344 mA h cm 2 at 0.3 mA cm 2) [26], CuO nanofibers (426 mA h g 1 at 0.15 C) [20], CuO/graphene composite (423 mA h g 1 at 0.1 C) [27], and the 3D macroporous CuO (390 mA h g 1 at 0.1 C) [28]. The improved electrochemical performance for the CuO-MWCNT electrode might be attributed to the defect-rich micro/nanostructure of CuO mesocrystals, the conductive 3D MWNT networks, and their synergic effects, which could shorten the path lengths for the transport of electrons and lithium
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Please cite this article as: J. Xu, et al., Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.129i