www.MaterialsViews.com
Lithium Ion Batteries
Facile Synthesis of Ultrasmall CoS2 Nanoparticles within Thin N-Doped Porous Carbon Shell for High Performance Lithium-Ion Batteries Qingfei Wang, Ruqiang Zou,* Wei Xia, Jin Ma, Bin Qiu, Asif Mahmood, Ruo Zhao, Yangyuchen Yang, Dingguo Xia, and Qiang Xu
Cobalt sulfide (CoS2) is considered one of the most promising alternative anode materials for high-performance lithium-ion batteries (LIBs) by virtue of its remarkable electrical conductivity, high theoretical capacity, and low cost. However, it suffers from a poor cycling stability and low rate capability because of its volume expansion and dissolution of the polysulfide intermediates in the organic electrolytes during the battery charge/discharge process. In this study, a novel porous carbon/CoS2 composite is prepared by using nano metal–organic framework (MOF) templates for high-preformance LIBs. The as-made ultrasmall CoS2 (15 nm) nanoparticles in N-rich carbon exhibit promising lithium storage properties with negligible loss of capacity at high charge/discharge rate. At a current density of 100 mA g−1, a capacity of 560 mA h g−1 is maintained after 50 cycles. Even at a current density as high as 2500 mA g−1, a reversible capacity of 410 mA h g−1 is obtained. The excellent and highly stable battery performance should be attributed to the synergism of the ultrasmall CoS2 particles and the thin N-rich porous carbon shells derieved from nanosized MOF precusors.
1. Introduction Lithium-ion batteries (LIBs) are creating great changes in modern life, and have been widely used in portable electronic devices due to their high energy density and long cycle life. However, their performance is still far from meeting the requirements of the mass, growing electric vehicles’ market.[1] Graphite is the prevailing commercial anode material and has nearly approached its theoretical limit (372 mA h g−1).[2] Among the available alternative anode materials, cobalt
Q. Wang, Prof. R. Zou, W. Xia, J. Ma, B. Qiu, A. Mahmood, R. Zhao, Y. Yang, Prof. D. Xia Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871, China E-mail: [email protected] Prof. Q. Xu National Institute of Advanced Industrial Science and Technology (AIST) Ikeda, Osaka 563-8577, Japan DOI: 10.1002/smll.201403579 small 2015, DOI: 10.1002/smll.201403579
sulfide (CoS2) has attracted much attention due to its remarkable electrical conductivity and high theoretical capacity.[3] However, its practical application suffers from several problems. For example, the pulverization problem caused by the huge volume change during charge/discharge process could cause a rapid capacity fading during cycling. This problem is further aggravated by the dissolution of the polysulfide intermediates in the organic electrolytes. Moreover, the poor rate performance of CoS2 also limits its practical application. So far, considerable efforts have been devoted for better cycling stability and higher rate performance. Previous works have shown that reducing the particle size of the anode material into nano-range could weaken the mechanical stress generated during the charge/discharge process, and consequently inhibit the pulverization problem.[4] In addition, the short diffusion path of lithium ions in the nanoparticles would be helpful to the rate performance. Besides reducing the particle size, using porous carbons[5] or graphene[6] as a host structure for the CoS2 could offer spare space for the volume expansion and prevent the pulverization effectively. More importantly, the host structure could adsorb and trap the polysulfide intermediates, consequently help to extend
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
1
full papers www.MaterialsViews.com
the cycle life of the materials. Notwithstanding these advantages, the rational design and controllable synthesis of nanostructured CoS2 composite still face great challenges. Metal–organic frameworks (MOFs), as a new category of porous materials, have been widely used in areas like gas storage,[7] catalysis,[8] biomedicine,[9] and electrochemistry.[10] Recently, there is an increasing tendency to use MOFs as precursor to synthesize nanostructured functional materials, like porous carbon[11] and porous carbon/metal or metal oxide composites.[12] Under properly controlled carbonization condition, perfect nanostructures could be obtained with ultrasmall nanoparticles uniformly dispersed in porous carbon matrix. Moreover, by choosing proper MOFs with N-rich organic ligands, the incorporation of nitrogen species in resultant carbon matrix could be easily achieved, which could enhance the energy storage capacity.[13] Several recent works have revealed the significant advantage of using this strategy to prepare ideal nanostructured composite as electrode materials for LIBs.[14] Herein, we expanded the application of this strategy to synthesize porous carbon/cobalt sulfide composite (C/CoS2) for the first time. ZIF-67, which has a sodalite (SOD) topological structure with Co (II) metal ions and N-containing methylimidazole ligand, was chosen as the MOF precursor.[15] After a simple low temperature postvulcanizing step, we got ultrasmall CoS2 nanoparticles finely dispersed in Scheme 1. Schematic illustration of the synthesis process of ultrasmall CoS2 nanoparticles N-rich porous carbon matrix. Compared in N-rich carbon. with the other methods, this strategy is quite convenient. Moreover, different from the previous work using bulk crystals, we chose ZIF-67 of highly uniform rhombic dodecahedron particles with an nanocrystals (NanoZIF-67) as the precursor, which reduced average size of 300 nm (Figure 1b). Pyrolysis of NanoZIF-67 at 650 °C under Ar atmosthe particle size of the carbon matrix to several hundred nanometers. Such ideal nanostructure endues the composite phere gave the C/Co composite, denoted as NC/Co-650. with excellent battery performance. Additionally, by tai- PXRD patterns of NC/Co-650 show that all the diffraction loring the sizes of CoS2 particles and carbon matrices, we peaks can be assigned to metallic cobalt (Figure 1a). The studied the relationship between the particle size and battery low peak intensities and broad peak indicate the nanosized performance. feature of cobalt particles. From the transmission electron microscopy (TEM) image (Figure 1c), it is clearly observed that NC/Co-650 retains the pristine rhombic dodecahedron structure, and the black dots are uniformly distributed in 2. Results and Discussion the carbon matrix. High-resolution TEM (HRTEM) image Synthesis of the C/CoS2 composite is illustrated in Scheme 1. further reveals that the grain size of the Co nanoparticles is The NanoZIF-67 were well-controlled synthesized according ≈10 nm (Figure 1d). The lattice fringes of Co can be found to our previous research.[16] The sample purity was confirmed clearly in Figure S1, Supporting Information. Notedly, the by the powder X-ray diffraction (PXRD) patterns, as shown organic ligands were transferred into N-rich porous carbon in Figure 1a. Scanning electron microscopy (SEM) images matrix during the carbonization process, while the Co2+ was clearly show that the NanoZIF-67 precursor is composed reduced to Co nanoparticle with average grain size of ≈10 nm
2 www.small-journal.com
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201403579
www.MaterialsViews.com
8.53 wt% N species. High-resolution X-ray photoelectron spectrometer (XPS) provides insight into the nitrogen species. As shown in Figure S2, Supporting Information, the spectra can be deconvoluted into three peaks, which correspond to the pyridinic (398.9 eV), pyrrolic (400.4 eV), and graphitic (401.3 eV) nitrogen species, respectively.[17] The porous structure of the amorphous carbon matrix is further investigated by measuring the nitrogen adsorption isotherm at 77 K (Figure S3, Supporting Information). The pore size distribution calculated using the quenched solid density functional theory (QSDFT) equilibrium model reveals a hierarchical porosity of the carbon matrix (Figure S3 inset, Supporting Information). The content of CoS2 in the composite could be estimated by the inductive coupled plasma (ICP) analysis. According to the result, the Co content reaches 24.27 wt%, equivalent to 50.69 wt% CoS2 in the composite. As mentioned above, the application of cobalt sulfide in LIBs is plagued by pulverization of electrode, dissolution of intermediates, and poor rate performance. Figure 1. a) Comparison of PXRD patterns of NanoZIF-67 with the simulated one from single Given that the ideal nanostructure and crystal structures (upper), and NC/Co-650 with metallic Co (JCPDS Card No. 15-0806) (lower). high nitrogen content of NC/CoS2-650 b) SEM images of NanoZIF-67. c) Transmission electron microscopy (TEM). d) High-resolution may help to overcome these challenges, TEM (HRTEM) images of NC/Co-650. we examined its potential application as anode material in LIBs. highly dispersed in the rhombic dodecahedron carbon Figure 3a shows the typical galvanostatic charge/dismatrix. charge curves for NC/CoS2-650 in different cycles between Due to the high reactivity of the Co nanoparticles in NC/ 0.1 and 3.0 V at a current density of 100 mA g−1. It should Co-650, the C/CoS2 composite was synthesized by a simple be noted that all the current densities and specific capacities reaction of NC/Co-650 with sulfur at 300 °C in a sealed glass here are calculated based on the total mass of the composite tube. The final product was denoted as NC/CoS2-650. Figure 2a to avoid the overestimate (Figure S4, Supporting Informareveals that the NC/CoS2-650 maintains the ideal nanostruc- tion). As shown in Figure 3a, the first discharge curve is difture of its precursor, and the nanoparticles are well-dispersed ferent from the subsequent cycles with three plateaus during in the carbon matrix. The typical size of the small particles is the lithiation process. In the first cycle, NC/CoS2-650 disaround 15 nm according to the HRTEM image (Figure 2b). plays a high discharge capacity of 1100 mA h g−1. The large The in situ transformation of Co into CoS2 is revealed by initial capacity may benefit from the full utilization of CoS2 the lattice fringes with a spacing of 0.276 nm for the [200] nanoparticles and the irreversible formation of a solid–elecplane of CoS2 in Figure 2b. Furthermore, the HRTEM trolyte interphase (SEI) on the surface of the composite. image shows the amorphous nature of the obtained carbon During the second cycle, the discharge capacity decreases matrices. The highly distributed structure is also proven to 701 mA h g−1 with a corresponding charge capacity of by the uniform distribution of C, Co, and S elements from 670 mA h g−1, leading to a high Coulombic efficiency (CE) energy dispersive spectrometer (EDS) mapping (Figure 2c). of 96%. This high CE indicates the good reversibility of the Figure 2d presents the PXRD pattern for NC/CoS2-650, in lithiation/delithiation processes. Different from the first cycle, which most of the peaks correspond to CoS2 (JCPDS Card the subsequent discharge curves display two plateaus in the No. 41-1471). Similar to the C/Co precursor, CoS2 exhibits potential ranges of 1.8–1.6 and 1.5–1.3 V, which correspond PXRD peak broadening due to the small size. The amor- to the insertion of a small amount of lithium and displacephous nature of the obtained carbon matrix is also supported ment reaction of Li2S + Co, respectively.[18] The charge/disby the Raman spectrum of NC/CoS2-650 (Figure 2e), in which charge behavior of the composite was further investigated the two broad bands at 1338 and 1590 cm−1 are assigned to by the cyclic voltammetry (CV) experiment with a scan rate typical D and G bands of amorphous carbon, respectively. of 0.2 mV s−1 in the potential range of 0.1–3.0 V. As shown Elemental analysis illustrates that NC/CoS2-650 possesses in Figure 3b, three reduction peaks centered at 1.3, 0.9, and small 2015, DOI: 10.1002/smll.201403579
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
3
full papers www.MaterialsViews.com
current densities of 100, 200, 1000, and 2500 mA g−1, respectively. This result is remarkable, compared with the previously reported CoS2 electrodes.[3,18,19] Moreover, after changing the current density back to 100 mA g−1, the capacity of NC/CoS2-650 reverted to the original values, implying its good reversibility. The excellent battery performance of NC/CoS2-650 originates from the ideal nanostructure of the composite. The good cycling stability of NC/CoS2-650 should be attributed to the following aspects. First, the inside CoS2 nanoparticles have better adaptability to the strain arising from the lithiation/delithiation processes, and could avoid the pulverization problem effectively. Second, the carbon matrix is able to confine the CoS2 nanoparticles in small cages, which offer sufficient space for the volume expansion of CoS2 nanoparticles during discharge/charge process, thus keeping the structural integrity. TEM images of the electrode after 50 charge/ discharge cycles at 100 and 2500 mA g−1 clearly show that the composite still maintains its original structure (Figure 4), further verifying the stability of the unique structure and the generation of the long cycle stability.[20] Finally, the N-rich hierarchical porous carbon not only offers electrolyte pathways to the inside CoS2 particles, but also adsorbs the polysulfide Figure 2. View of a) TEM images, b) HRTEM images, c) energy dispersive spectrometer (EDS) intermide, which consequently help to mapping, d) PXRD patterns, and e) Raman spectrum of NC/CoS2-650. extend the cycle life of the material. The superior rate capability of NC/CoS2-650 0.7 V are observed in the first cathodic sweep. However, should be attributed to the large surface area for increased from the second cycle onward, only two distinct peaks cen- Li-ion flux and the shorter path lengths for electronic and tered at 1.3 and 1.7 V are observed, which are in good agree- ionic transport, provided by the CoS2 nanoparticles and small ment with the plateaus in the galvanostatic discharge curves. carbon matrices.[21] Most importantly, the hierarchical porous As mentioned above, the peaks should be originated from structure of the carbon shells derived from nanosized MOF the decomposition of CoS2 into Li2S and Co. In the anodic precursors fascinates the diffusion of lithium ions and elecsweep, oxidation peaks between 2.0 and 2.3 V are observed, trolyte through the carbon matrix. To further understand the excellent battery performance which are related to the formation of CoS2. Different from the previous literature,[3,18] the peaks are stable after first of NC/CoS2-650 and gain a deep insight into the relationship cycle, implying the good cycling stability of NC/CoS2-650. between the particle size and battery performance, other two The cycle performances of NC/CoS2-650 at different cur- C/CoS2 composites were prepared for comparison. The corrent densities are shown in Figure 3c. NC/CoS2-650 exhibits responding PXRD patterns of the samples are provided in quite good cycling stability at a current density of 100 mA g−1. Figures S5–S7, Supporting Information. After 50 cycles, the sample maintains a reversible capacity of C/CoS2 composite with large carbon matrix (denoted about 560 mA h g−1. More importantly, the good cycle sta- as BC/CoS2-650, in which “B” represents its bulk carbon bility is maintained at high current density. It is shown that matrix) was synthesized by replacing NanoZIF-67 precurat 2500 mA g−1 the composite still gives a reversible capacity sors with bulk crystals. Even though the size of carbon matrix of about 410 mA h g−1 after 50 cycles. The high rate perfor- increased, the inside CoS2 particles are still as small as that in mance of NC/CoS2-650 is further demonstrated by cycling the NC/CoS2-650 (Figure S8a,b, Supporting Information). Raman battery at various current densities from 100 to 2500 mA g−1 and nitrogen sorption experiments were conducted to charas shown in Figure 3d. The capacity drops moderately acterize the structure of BC/CoS2-650 (Figures S9 and S10, with the increase of current density. NC/CoS2-650 delivers Supporting Information). Elemental analysis reveals that BC/ reversible capacities of 710, 570, 490, and 340 mA h g−1 at CoS2-650 has a close nitrogen content of 9%. High-resolution
4 www.small-journal.com
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201403579
www.MaterialsViews.com
Figure 3. View of a) voltage profile, b) cyclic voltammograms, c) cycle-life performance at 100 and 2500 mA g−1, and d) charge/discharge capacity at various current densities of NC/CoS2-650.
N 1s XPS spectrum was collected to obtain insight into the nitrogen species (Figure S11, Supporting Information). Comparing the cycling performance of NC/CoS2-650 and BC/ CoS2-650 at 500 mA g−1 between 0.1 and 3.0 V (Figure 5a), similar stabilities could be obtained, but the average capacity of BC/CoS2-650 is lower than that of NC/CoS2-650. Given that BC/CoS2-650 has the similar ultrasmall CoS2 particles, the difference should be originated from the particle size and porous structure of the carbon matrix. The thick carbon matrix of BC/CoS2-650 may hinder the transfer of lithium ions into its core side, leading to insufficient utilization of all the CoS2 nanoparticles in the composite. According to the pore size distribution results (Figure S10 inset, Supporting Information), the lack of larger mesopores in BC/CoS2-650 will aggravate the condition, thus resulting in the low specific capacity. Another C/CoS2 composite with larger CoS2 particles was prepared by increasing the carbonization temperature to 1000 °C (denoted as NC/CoS2-1000). During the high temperature carbonization process at 1000 °C, the cobalt particles aggregated and formed to large particles of CoS2 without uniform shape in the final C/CoS2 composite (Figure S8c,d, Supporting Information). Moreover, the morphology of the rhombic dodecahedron carbon matrix was also changed by the high temperature processing. Besides small 2015, DOI: 10.1002/smll.201403579
the morphology of the carbon matrix, the nitrogen content of NC/CoS2-1000 decreases sharply compared with NC/ CoS2-650 (Table S1, Supporting Information), due to the loss of pyridinic and pyrrolic nitrogen atoms (Figure S12, Supporting Information). As shown in Figure 5a, the capacity of NC/CoS2-1000 falls dramatically after the initial several cycles. Given that NC/CoS2 has similar hierarchical porosity to NC/CoS2-650 (Figure S10 inset, Supporting Information), the larger particle size of CoS2 and lower nitrogen content in NC/CoS2-1000 should be the vital reasons for the worse performance. The huge volume change of large CoS2 particles may cause the protective carbon matrix to crack and fracture.[22] Without the protection, some small pieces from the decomposed large CoS2 particles flee into the electrolyte, resulting in “dead materials.” Furthermore, the low content of nitrogen may weaken the adsorption ability of the carbon host toward the polysulfide.[23] Figure 5b shows the Nyquist plots for NC/CoS2-650 and NC/CoS2-1000. The impedance associated with the charge transfer resistance in NC/CoS21000 is much lower than that in NC/CoS2-650, leading to the conclusion that the battery performance mainly depends on the particle size of CoS2 and the nitrogen content of the carbon matrix, as well as the thickness of the carbon shell. Consequently, the comparisons have illustrated the superiority of the ideal nanostructure of NC/CoS2-650.
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
5
full papers www.MaterialsViews.com
NanoZIF-67: 1.43 g of Co(NO3)2·6H2O and 3.24 g of 2-methylimidazole were dissolved in 100 mL methanol, respectively. The two solutions were mixed at room temperature under magnetic stirring for 15 min, and stood for 24 h. Solid product was separated by centrifugation and washed with methanol for three times, followed by vacuum drying at 150 °C for 8 h. Synthesis of C/Co Composite: The asprepared ZIF precursors were carbonized at 650 and 1000 °C, respectively. Usually, the carbonization process was kept for 1 h under Ar atmosphere with a heating rate of 5 °C min−1 and cooled down to room temperature naturally. The products were denoted as NC/Co-650, BC/Co-650, and NC/Co-1000 in which “N” or “B” indicate their “nano” or “bulk” crystal precursor, and the number represents their carbonization temperature. Synthesis of C/CoS2 Composite: The C/Co composite and sulfur were grounded together with a mass ration of 2:1, and sealed in a glass tube. Then the tube was heated at 300 °C for one day. The denotations of the final products were similar with their C/Co precursors, except that “Co” was changed into “CoS2.” Figure 4. TEM images of NC/CoS2-650 after 50 charge/discharge cycles at a,b) 100 and c,d) Materials Characterizations: PXRD pat−1 2500 mA g , respectively. terns were recorded on a Bruker D8 Advanced diffractometer using Cu Kα radiation (λ = 1.54050 Å) and operating at 40 kV and 3. Conclusion 100 mA. SEM images were obtained on a Hitachi S-4800 electron In summary, we employed a nanoMOF-derived synthesis microscope and equipped with a Bruker Quantax EDS. Elemental strategy to prepare C/CoS2 composite with ultrasmall CoS2 analysis was performed on a Vario EL Elemental Analyzer. TEM nanoparticles finely embedded in thin N-rich porous carbon. images were taken on a FEI Tecnai F20 microscope. XPS measureThe composite exhibits promising Li storage properties. At a ments were performed on a Kratos Axis Ultra Imaging Photoeleccurrent density of 100 mA g−1, a capacity of 560 mA h g−1 tron Spectrometer using monochromatic Al Kα line (1486.7 eV). was maintained after 50 cycles. Increasing the current den- Raman spectra were recorded on a RENISHAW Raman spectrum sity to 2500 mA g−1, a reversible capacity of 410 mA h g−1 equipment. To determine the quantity of cobalt in the sample, was still obtained. Moreover, by tailoring the particle size a Leeman prodigy inductively coupled plasma optical emission of the CoS2 and carbon matrix, we found that the excellent spectrometer (ICP-OES) was used. N2 adsorption isotherms were battery performance originated from the small size of the measured on an Autosorb-iQ automatic volumetric instrument CoS2 particles and the carbon matrix. The novel and facile within the pressure range 0–1 atm at 77 K. Electrochemical Measurements: The as-prepared C/CoS2 synthesis introduced here could be expanded to prepare other ultrasmall metal sulfide particles. These results may composites were mixed with acetylene black, and polyvinylidene help in the development of new concepts for the design fluoride (PVDF) binder at a weight ratio of 70:20:10 in N-methyland synthesis of electrode materials for next generation pyrrolidone (NMP) to form a slurry. The working electrodes were prepared by coating the slurry onto stainless steel foils and dried in LIBs. vacuum at 80 °C for 24 h. The half cells were assembled in a glove box filled with high pure Ar, using lithium metal as the counter electrode, and a glass fiber (GF/D) from Whatman as the sepa4. Experimental Section rator. The electrolyte was 1 mol L−1 LiPF6 in 1:1 ethylene carbonate Synthesis of ZIF-67: BulkZIF-67: 3.32 g of Co(Ac)2·4H2O and (EC)/dimethyl carbonate (DMC). The cells were cycled between 3.28 g of 2-methylimidazole were dissolved in 100 mL methanol. 3.0 and 0.1 V on a Neware battery instrument at room temperaThe mixed solution was sealed in a 125 mL teflon-lined steel auto- ture. The specific capacity was calculated on the total mass of the clave, and heated at 120 °C for three days. The as-prepared crys- composite. The CV measurements were conducted with a Zahner tals were filtered out and washed by methanol, finally dried under Zennium Electrochemical Workstation in the potential range 0.1–3.0 V at a scan rate of 0.2 mV s−1. The electrochemical vacuum at 150 °C for 8 h.
6 www.small-journal.com
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201403579
www.MaterialsViews.com
of Education Program for New Century Excellent Talents of China (NCET-11-0027).
Figure 5. a) Cycle-life performances of NC/CoS2-650, NC/CoS2-1000, and BC/CoS2-650 at 500 mA g−1 and b) Nyquist plots of NC/CoS2-650 and NC/CoS2-1000.
impedance spectra were measured using the same instrument with the frequency range from 0.01 to 100 000 Hz with a sine wave with amplitude of 5.0 mV.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
[1] a) J. B. Goodenough, K.-S. Park, J. Am. Chem. Soc. 2013, 135, 1167; b) J. B. Goodenough, Y. Kim, Chem. Mater. 2010, 22, 587. [2] J. Yue, X. Zhao, D. Xia, Electrochem. Commun. 2012, 18, 44. [3] Y. Wang, J. Wu, Y. Tang, X. Lv, C. Yang, M. Qin, F. Huang, X. Li, X. Zhang, ACS Appl. Mater. Interfaces 2012, 4, 4246. [4] a) J. Wang, S. H. Ng, G. X. Wang, J. Chen, L. Zhao, Y. Chen, H. K. Liu, J. Power Sources 2006, 159, 287; b) P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J. M. Tarascon, Nature (London) 2000, 407, 496; c) P. G. Bruce, B. Scrosati, J.-M. Tarascon, Angew. Chem. Int. Ed. 2008, 47, 2930. [5] W. Shi, J. Zhu, X. Rui, X. Cao, C. Chen, H. Zhang, H. H. Hng, Q. Yan, ACS Appl. Mater. Interfaces 2012, 4, 2999. [6] a) J. Xie, S. Liu, G. Cao, T. Zhu, X. Zhao, Nano Energy 2013, 2, 49; b) B. Qiu, X. Zhao, D. Xia, J. Alloy Compd. 2013, 579, 372; c) N. Mahmood, C. Zhang, J. Jiang, F. Liu, Y. Hou, Chem. Eur. J. 2013, 19, 5183; d) G. Huang, T. Chen, Z. Wang, K. Chang, W. Chen, J. Power Sources 2013, 235, 122; e) Q. Su, J. Xie, J. Zhang, Y. Zhong, G. Du, B. Xu, ACS Appl. Mater. Interfaces 2014, 6, 3016. [7] a) Y. S. Bae, R. Q. Snurr, Angew. Chem. Int. Ed. 2011, 50, 11586; b) J. R. Li, R. J. Kuppler, H. C. Zhou, Chem. Soc. Rev. 2009, 38, 1477. [8] M. Yoon, R. Srirambalaji, K. Kim, Chem. Rev. 2012, 112, 1196. [9] J. Della Rocca, D. M. Liu, W. B. Lin, Acc. Chem. Res. 2011, 44, 957. [10] S.-L. Li, Q. Xu, Energy Environ. Sci. 2013, 6, 1656. [11] B. Liu, H. Shioyama, T. Akita, Q. Xu, J. Am. Chem. Soc. 2008, 130, 5390. [12] J.-K. Sun, Q. Xu, Energy Environ. Sci. 2014, 7, 2071. [13] Q. F. Wang, W. Xia, W. H. Guo, L. An, D. G. Xia, R. Q. Zou, Chem. Asian J. 2013, 8, 1879. [14] a) X. Xu, R. Cao, S. Jeong, J. Cho, Nano Lett. 2012, 12, 4988; b) S. J. Yang, S. Nam, T. Kim, J. H. Im, H. Jung, J. H. Kang, S. Wi, B. Park, C. R. Park, J. Am. Chem. Soc. 2013, 135, 7394. [15] R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O’Keeffe, O. M. Yaghi, Science 2008, 319, 939. [16] W. Xia, J. Zhu, W. Guo, L. An, D. Xia, R. Zou, J. Mater. Chem. A 2014, 2, 11606. [17] J. Pels, F. Kapteijn, J. Moulijn, Q. Zhu, K. Thomas, Carbon 1995, 33, 1641. [18] Q. Wang, L. Jiao, Y. Han, H. Du, W. Peng, Q. Huan, D. Song, Y. Si, Y. Wang, H. Yuan, J. Phys. Chem. C 2011, 115, 8300. [19] J. M. Yan, H. Z. Huang, J. Zhang, Z. J. Liu, Y. Yang, J. Power Sources 2005, 146, 264. [20] Z. Zhu, S. Wang, J. Du, Q. Jin, T. Zhang, F. Cheng, J. Chen, Nano Lett. 2014, 14, 153. [21] Y. Kim, J.-H. Lee, S. Cho, Y. Kwon, I. In, J. Lee, N.-H. You, E. Reichmanis, H. Ko, K.-T. Lee, ACS Nano 2014, 8, 6701. [22] Z. Wei Seh, W. Li, J. J. Cha, G. Zheng, Y. Yang, M. T. McDowell, P. C. Hsu, Y. Cui, Nat. Commun. 2013, 4, 1331. [23] F. Sun, J. Wang, H. Chen, W. Li, W. Qiao, D. Long, L. Ling, ACS Appl. Mater. Interfaces 2013, 5, 5630.
This work is supported by National Natural Science Foundation of China (11175006, 51322205, and 21371014), the Ministry
small 2015, DOI: 10.1002/smll.201403579
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: December 2, 2014 Revised: December 29, 2014 Published online:
www.small-journal.com
7
Lithium Ion Batteries
Facile Synthesis of Ultrasmall CoS2 Nanoparticles within Thin N-Doped Porous Carbon Shell for High Performance Lithium-Ion Batteries Qingfei Wang, Ruqiang Zou,* Wei Xia, Jin Ma, Bin Qiu, Asif Mahmood, Ruo Zhao, Yangyuchen Yang, Dingguo Xia, and Qiang Xu
Cobalt sulfide (CoS2) is considered one of the most promising alternative anode materials for high-performance lithium-ion batteries (LIBs) by virtue of its remarkable electrical conductivity, high theoretical capacity, and low cost. However, it suffers from a poor cycling stability and low rate capability because of its volume expansion and dissolution of the polysulfide intermediates in the organic electrolytes during the battery charge/discharge process. In this study, a novel porous carbon/CoS2 composite is prepared by using nano metal–organic framework (MOF) templates for high-preformance LIBs. The as-made ultrasmall CoS2 (15 nm) nanoparticles in N-rich carbon exhibit promising lithium storage properties with negligible loss of capacity at high charge/discharge rate. At a current density of 100 mA g−1, a capacity of 560 mA h g−1 is maintained after 50 cycles. Even at a current density as high as 2500 mA g−1, a reversible capacity of 410 mA h g−1 is obtained. The excellent and highly stable battery performance should be attributed to the synergism of the ultrasmall CoS2 particles and the thin N-rich porous carbon shells derieved from nanosized MOF precusors.
1. Introduction Lithium-ion batteries (LIBs) are creating great changes in modern life, and have been widely used in portable electronic devices due to their high energy density and long cycle life. However, their performance is still far from meeting the requirements of the mass, growing electric vehicles’ market.[1] Graphite is the prevailing commercial anode material and has nearly approached its theoretical limit (372 mA h g−1).[2] Among the available alternative anode materials, cobalt
Q. Wang, Prof. R. Zou, W. Xia, J. Ma, B. Qiu, A. Mahmood, R. Zhao, Y. Yang, Prof. D. Xia Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871, China E-mail: [email protected] Prof. Q. Xu National Institute of Advanced Industrial Science and Technology (AIST) Ikeda, Osaka 563-8577, Japan DOI: 10.1002/smll.201403579 small 2015, DOI: 10.1002/smll.201403579
sulfide (CoS2) has attracted much attention due to its remarkable electrical conductivity and high theoretical capacity.[3] However, its practical application suffers from several problems. For example, the pulverization problem caused by the huge volume change during charge/discharge process could cause a rapid capacity fading during cycling. This problem is further aggravated by the dissolution of the polysulfide intermediates in the organic electrolytes. Moreover, the poor rate performance of CoS2 also limits its practical application. So far, considerable efforts have been devoted for better cycling stability and higher rate performance. Previous works have shown that reducing the particle size of the anode material into nano-range could weaken the mechanical stress generated during the charge/discharge process, and consequently inhibit the pulverization problem.[4] In addition, the short diffusion path of lithium ions in the nanoparticles would be helpful to the rate performance. Besides reducing the particle size, using porous carbons[5] or graphene[6] as a host structure for the CoS2 could offer spare space for the volume expansion and prevent the pulverization effectively. More importantly, the host structure could adsorb and trap the polysulfide intermediates, consequently help to extend
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
1
full papers www.MaterialsViews.com
the cycle life of the materials. Notwithstanding these advantages, the rational design and controllable synthesis of nanostructured CoS2 composite still face great challenges. Metal–organic frameworks (MOFs), as a new category of porous materials, have been widely used in areas like gas storage,[7] catalysis,[8] biomedicine,[9] and electrochemistry.[10] Recently, there is an increasing tendency to use MOFs as precursor to synthesize nanostructured functional materials, like porous carbon[11] and porous carbon/metal or metal oxide composites.[12] Under properly controlled carbonization condition, perfect nanostructures could be obtained with ultrasmall nanoparticles uniformly dispersed in porous carbon matrix. Moreover, by choosing proper MOFs with N-rich organic ligands, the incorporation of nitrogen species in resultant carbon matrix could be easily achieved, which could enhance the energy storage capacity.[13] Several recent works have revealed the significant advantage of using this strategy to prepare ideal nanostructured composite as electrode materials for LIBs.[14] Herein, we expanded the application of this strategy to synthesize porous carbon/cobalt sulfide composite (C/CoS2) for the first time. ZIF-67, which has a sodalite (SOD) topological structure with Co (II) metal ions and N-containing methylimidazole ligand, was chosen as the MOF precursor.[15] After a simple low temperature postvulcanizing step, we got ultrasmall CoS2 nanoparticles finely dispersed in Scheme 1. Schematic illustration of the synthesis process of ultrasmall CoS2 nanoparticles N-rich porous carbon matrix. Compared in N-rich carbon. with the other methods, this strategy is quite convenient. Moreover, different from the previous work using bulk crystals, we chose ZIF-67 of highly uniform rhombic dodecahedron particles with an nanocrystals (NanoZIF-67) as the precursor, which reduced average size of 300 nm (Figure 1b). Pyrolysis of NanoZIF-67 at 650 °C under Ar atmosthe particle size of the carbon matrix to several hundred nanometers. Such ideal nanostructure endues the composite phere gave the C/Co composite, denoted as NC/Co-650. with excellent battery performance. Additionally, by tai- PXRD patterns of NC/Co-650 show that all the diffraction loring the sizes of CoS2 particles and carbon matrices, we peaks can be assigned to metallic cobalt (Figure 1a). The studied the relationship between the particle size and battery low peak intensities and broad peak indicate the nanosized performance. feature of cobalt particles. From the transmission electron microscopy (TEM) image (Figure 1c), it is clearly observed that NC/Co-650 retains the pristine rhombic dodecahedron structure, and the black dots are uniformly distributed in 2. Results and Discussion the carbon matrix. High-resolution TEM (HRTEM) image Synthesis of the C/CoS2 composite is illustrated in Scheme 1. further reveals that the grain size of the Co nanoparticles is The NanoZIF-67 were well-controlled synthesized according ≈10 nm (Figure 1d). The lattice fringes of Co can be found to our previous research.[16] The sample purity was confirmed clearly in Figure S1, Supporting Information. Notedly, the by the powder X-ray diffraction (PXRD) patterns, as shown organic ligands were transferred into N-rich porous carbon in Figure 1a. Scanning electron microscopy (SEM) images matrix during the carbonization process, while the Co2+ was clearly show that the NanoZIF-67 precursor is composed reduced to Co nanoparticle with average grain size of ≈10 nm
2 www.small-journal.com
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201403579
www.MaterialsViews.com
8.53 wt% N species. High-resolution X-ray photoelectron spectrometer (XPS) provides insight into the nitrogen species. As shown in Figure S2, Supporting Information, the spectra can be deconvoluted into three peaks, which correspond to the pyridinic (398.9 eV), pyrrolic (400.4 eV), and graphitic (401.3 eV) nitrogen species, respectively.[17] The porous structure of the amorphous carbon matrix is further investigated by measuring the nitrogen adsorption isotherm at 77 K (Figure S3, Supporting Information). The pore size distribution calculated using the quenched solid density functional theory (QSDFT) equilibrium model reveals a hierarchical porosity of the carbon matrix (Figure S3 inset, Supporting Information). The content of CoS2 in the composite could be estimated by the inductive coupled plasma (ICP) analysis. According to the result, the Co content reaches 24.27 wt%, equivalent to 50.69 wt% CoS2 in the composite. As mentioned above, the application of cobalt sulfide in LIBs is plagued by pulverization of electrode, dissolution of intermediates, and poor rate performance. Figure 1. a) Comparison of PXRD patterns of NanoZIF-67 with the simulated one from single Given that the ideal nanostructure and crystal structures (upper), and NC/Co-650 with metallic Co (JCPDS Card No. 15-0806) (lower). high nitrogen content of NC/CoS2-650 b) SEM images of NanoZIF-67. c) Transmission electron microscopy (TEM). d) High-resolution may help to overcome these challenges, TEM (HRTEM) images of NC/Co-650. we examined its potential application as anode material in LIBs. highly dispersed in the rhombic dodecahedron carbon Figure 3a shows the typical galvanostatic charge/dismatrix. charge curves for NC/CoS2-650 in different cycles between Due to the high reactivity of the Co nanoparticles in NC/ 0.1 and 3.0 V at a current density of 100 mA g−1. It should Co-650, the C/CoS2 composite was synthesized by a simple be noted that all the current densities and specific capacities reaction of NC/Co-650 with sulfur at 300 °C in a sealed glass here are calculated based on the total mass of the composite tube. The final product was denoted as NC/CoS2-650. Figure 2a to avoid the overestimate (Figure S4, Supporting Informareveals that the NC/CoS2-650 maintains the ideal nanostruc- tion). As shown in Figure 3a, the first discharge curve is difture of its precursor, and the nanoparticles are well-dispersed ferent from the subsequent cycles with three plateaus during in the carbon matrix. The typical size of the small particles is the lithiation process. In the first cycle, NC/CoS2-650 disaround 15 nm according to the HRTEM image (Figure 2b). plays a high discharge capacity of 1100 mA h g−1. The large The in situ transformation of Co into CoS2 is revealed by initial capacity may benefit from the full utilization of CoS2 the lattice fringes with a spacing of 0.276 nm for the [200] nanoparticles and the irreversible formation of a solid–elecplane of CoS2 in Figure 2b. Furthermore, the HRTEM trolyte interphase (SEI) on the surface of the composite. image shows the amorphous nature of the obtained carbon During the second cycle, the discharge capacity decreases matrices. The highly distributed structure is also proven to 701 mA h g−1 with a corresponding charge capacity of by the uniform distribution of C, Co, and S elements from 670 mA h g−1, leading to a high Coulombic efficiency (CE) energy dispersive spectrometer (EDS) mapping (Figure 2c). of 96%. This high CE indicates the good reversibility of the Figure 2d presents the PXRD pattern for NC/CoS2-650, in lithiation/delithiation processes. Different from the first cycle, which most of the peaks correspond to CoS2 (JCPDS Card the subsequent discharge curves display two plateaus in the No. 41-1471). Similar to the C/Co precursor, CoS2 exhibits potential ranges of 1.8–1.6 and 1.5–1.3 V, which correspond PXRD peak broadening due to the small size. The amor- to the insertion of a small amount of lithium and displacephous nature of the obtained carbon matrix is also supported ment reaction of Li2S + Co, respectively.[18] The charge/disby the Raman spectrum of NC/CoS2-650 (Figure 2e), in which charge behavior of the composite was further investigated the two broad bands at 1338 and 1590 cm−1 are assigned to by the cyclic voltammetry (CV) experiment with a scan rate typical D and G bands of amorphous carbon, respectively. of 0.2 mV s−1 in the potential range of 0.1–3.0 V. As shown Elemental analysis illustrates that NC/CoS2-650 possesses in Figure 3b, three reduction peaks centered at 1.3, 0.9, and small 2015, DOI: 10.1002/smll.201403579
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
3
full papers www.MaterialsViews.com
current densities of 100, 200, 1000, and 2500 mA g−1, respectively. This result is remarkable, compared with the previously reported CoS2 electrodes.[3,18,19] Moreover, after changing the current density back to 100 mA g−1, the capacity of NC/CoS2-650 reverted to the original values, implying its good reversibility. The excellent battery performance of NC/CoS2-650 originates from the ideal nanostructure of the composite. The good cycling stability of NC/CoS2-650 should be attributed to the following aspects. First, the inside CoS2 nanoparticles have better adaptability to the strain arising from the lithiation/delithiation processes, and could avoid the pulverization problem effectively. Second, the carbon matrix is able to confine the CoS2 nanoparticles in small cages, which offer sufficient space for the volume expansion of CoS2 nanoparticles during discharge/charge process, thus keeping the structural integrity. TEM images of the electrode after 50 charge/ discharge cycles at 100 and 2500 mA g−1 clearly show that the composite still maintains its original structure (Figure 4), further verifying the stability of the unique structure and the generation of the long cycle stability.[20] Finally, the N-rich hierarchical porous carbon not only offers electrolyte pathways to the inside CoS2 particles, but also adsorbs the polysulfide Figure 2. View of a) TEM images, b) HRTEM images, c) energy dispersive spectrometer (EDS) intermide, which consequently help to mapping, d) PXRD patterns, and e) Raman spectrum of NC/CoS2-650. extend the cycle life of the material. The superior rate capability of NC/CoS2-650 0.7 V are observed in the first cathodic sweep. However, should be attributed to the large surface area for increased from the second cycle onward, only two distinct peaks cen- Li-ion flux and the shorter path lengths for electronic and tered at 1.3 and 1.7 V are observed, which are in good agree- ionic transport, provided by the CoS2 nanoparticles and small ment with the plateaus in the galvanostatic discharge curves. carbon matrices.[21] Most importantly, the hierarchical porous As mentioned above, the peaks should be originated from structure of the carbon shells derived from nanosized MOF the decomposition of CoS2 into Li2S and Co. In the anodic precursors fascinates the diffusion of lithium ions and elecsweep, oxidation peaks between 2.0 and 2.3 V are observed, trolyte through the carbon matrix. To further understand the excellent battery performance which are related to the formation of CoS2. Different from the previous literature,[3,18] the peaks are stable after first of NC/CoS2-650 and gain a deep insight into the relationship cycle, implying the good cycling stability of NC/CoS2-650. between the particle size and battery performance, other two The cycle performances of NC/CoS2-650 at different cur- C/CoS2 composites were prepared for comparison. The corrent densities are shown in Figure 3c. NC/CoS2-650 exhibits responding PXRD patterns of the samples are provided in quite good cycling stability at a current density of 100 mA g−1. Figures S5–S7, Supporting Information. After 50 cycles, the sample maintains a reversible capacity of C/CoS2 composite with large carbon matrix (denoted about 560 mA h g−1. More importantly, the good cycle sta- as BC/CoS2-650, in which “B” represents its bulk carbon bility is maintained at high current density. It is shown that matrix) was synthesized by replacing NanoZIF-67 precurat 2500 mA g−1 the composite still gives a reversible capacity sors with bulk crystals. Even though the size of carbon matrix of about 410 mA h g−1 after 50 cycles. The high rate perfor- increased, the inside CoS2 particles are still as small as that in mance of NC/CoS2-650 is further demonstrated by cycling the NC/CoS2-650 (Figure S8a,b, Supporting Information). Raman battery at various current densities from 100 to 2500 mA g−1 and nitrogen sorption experiments were conducted to charas shown in Figure 3d. The capacity drops moderately acterize the structure of BC/CoS2-650 (Figures S9 and S10, with the increase of current density. NC/CoS2-650 delivers Supporting Information). Elemental analysis reveals that BC/ reversible capacities of 710, 570, 490, and 340 mA h g−1 at CoS2-650 has a close nitrogen content of 9%. High-resolution
4 www.small-journal.com
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201403579
www.MaterialsViews.com
Figure 3. View of a) voltage profile, b) cyclic voltammograms, c) cycle-life performance at 100 and 2500 mA g−1, and d) charge/discharge capacity at various current densities of NC/CoS2-650.
N 1s XPS spectrum was collected to obtain insight into the nitrogen species (Figure S11, Supporting Information). Comparing the cycling performance of NC/CoS2-650 and BC/ CoS2-650 at 500 mA g−1 between 0.1 and 3.0 V (Figure 5a), similar stabilities could be obtained, but the average capacity of BC/CoS2-650 is lower than that of NC/CoS2-650. Given that BC/CoS2-650 has the similar ultrasmall CoS2 particles, the difference should be originated from the particle size and porous structure of the carbon matrix. The thick carbon matrix of BC/CoS2-650 may hinder the transfer of lithium ions into its core side, leading to insufficient utilization of all the CoS2 nanoparticles in the composite. According to the pore size distribution results (Figure S10 inset, Supporting Information), the lack of larger mesopores in BC/CoS2-650 will aggravate the condition, thus resulting in the low specific capacity. Another C/CoS2 composite with larger CoS2 particles was prepared by increasing the carbonization temperature to 1000 °C (denoted as NC/CoS2-1000). During the high temperature carbonization process at 1000 °C, the cobalt particles aggregated and formed to large particles of CoS2 without uniform shape in the final C/CoS2 composite (Figure S8c,d, Supporting Information). Moreover, the morphology of the rhombic dodecahedron carbon matrix was also changed by the high temperature processing. Besides small 2015, DOI: 10.1002/smll.201403579
the morphology of the carbon matrix, the nitrogen content of NC/CoS2-1000 decreases sharply compared with NC/ CoS2-650 (Table S1, Supporting Information), due to the loss of pyridinic and pyrrolic nitrogen atoms (Figure S12, Supporting Information). As shown in Figure 5a, the capacity of NC/CoS2-1000 falls dramatically after the initial several cycles. Given that NC/CoS2 has similar hierarchical porosity to NC/CoS2-650 (Figure S10 inset, Supporting Information), the larger particle size of CoS2 and lower nitrogen content in NC/CoS2-1000 should be the vital reasons for the worse performance. The huge volume change of large CoS2 particles may cause the protective carbon matrix to crack and fracture.[22] Without the protection, some small pieces from the decomposed large CoS2 particles flee into the electrolyte, resulting in “dead materials.” Furthermore, the low content of nitrogen may weaken the adsorption ability of the carbon host toward the polysulfide.[23] Figure 5b shows the Nyquist plots for NC/CoS2-650 and NC/CoS2-1000. The impedance associated with the charge transfer resistance in NC/CoS21000 is much lower than that in NC/CoS2-650, leading to the conclusion that the battery performance mainly depends on the particle size of CoS2 and the nitrogen content of the carbon matrix, as well as the thickness of the carbon shell. Consequently, the comparisons have illustrated the superiority of the ideal nanostructure of NC/CoS2-650.
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
5
full papers www.MaterialsViews.com
NanoZIF-67: 1.43 g of Co(NO3)2·6H2O and 3.24 g of 2-methylimidazole were dissolved in 100 mL methanol, respectively. The two solutions were mixed at room temperature under magnetic stirring for 15 min, and stood for 24 h. Solid product was separated by centrifugation and washed with methanol for three times, followed by vacuum drying at 150 °C for 8 h. Synthesis of C/Co Composite: The asprepared ZIF precursors were carbonized at 650 and 1000 °C, respectively. Usually, the carbonization process was kept for 1 h under Ar atmosphere with a heating rate of 5 °C min−1 and cooled down to room temperature naturally. The products were denoted as NC/Co-650, BC/Co-650, and NC/Co-1000 in which “N” or “B” indicate their “nano” or “bulk” crystal precursor, and the number represents their carbonization temperature. Synthesis of C/CoS2 Composite: The C/Co composite and sulfur were grounded together with a mass ration of 2:1, and sealed in a glass tube. Then the tube was heated at 300 °C for one day. The denotations of the final products were similar with their C/Co precursors, except that “Co” was changed into “CoS2.” Figure 4. TEM images of NC/CoS2-650 after 50 charge/discharge cycles at a,b) 100 and c,d) Materials Characterizations: PXRD pat−1 2500 mA g , respectively. terns were recorded on a Bruker D8 Advanced diffractometer using Cu Kα radiation (λ = 1.54050 Å) and operating at 40 kV and 3. Conclusion 100 mA. SEM images were obtained on a Hitachi S-4800 electron In summary, we employed a nanoMOF-derived synthesis microscope and equipped with a Bruker Quantax EDS. Elemental strategy to prepare C/CoS2 composite with ultrasmall CoS2 analysis was performed on a Vario EL Elemental Analyzer. TEM nanoparticles finely embedded in thin N-rich porous carbon. images were taken on a FEI Tecnai F20 microscope. XPS measureThe composite exhibits promising Li storage properties. At a ments were performed on a Kratos Axis Ultra Imaging Photoeleccurrent density of 100 mA g−1, a capacity of 560 mA h g−1 tron Spectrometer using monochromatic Al Kα line (1486.7 eV). was maintained after 50 cycles. Increasing the current den- Raman spectra were recorded on a RENISHAW Raman spectrum sity to 2500 mA g−1, a reversible capacity of 410 mA h g−1 equipment. To determine the quantity of cobalt in the sample, was still obtained. Moreover, by tailoring the particle size a Leeman prodigy inductively coupled plasma optical emission of the CoS2 and carbon matrix, we found that the excellent spectrometer (ICP-OES) was used. N2 adsorption isotherms were battery performance originated from the small size of the measured on an Autosorb-iQ automatic volumetric instrument CoS2 particles and the carbon matrix. The novel and facile within the pressure range 0–1 atm at 77 K. Electrochemical Measurements: The as-prepared C/CoS2 synthesis introduced here could be expanded to prepare other ultrasmall metal sulfide particles. These results may composites were mixed with acetylene black, and polyvinylidene help in the development of new concepts for the design fluoride (PVDF) binder at a weight ratio of 70:20:10 in N-methyland synthesis of electrode materials for next generation pyrrolidone (NMP) to form a slurry. The working electrodes were prepared by coating the slurry onto stainless steel foils and dried in LIBs. vacuum at 80 °C for 24 h. The half cells were assembled in a glove box filled with high pure Ar, using lithium metal as the counter electrode, and a glass fiber (GF/D) from Whatman as the sepa4. Experimental Section rator. The electrolyte was 1 mol L−1 LiPF6 in 1:1 ethylene carbonate Synthesis of ZIF-67: BulkZIF-67: 3.32 g of Co(Ac)2·4H2O and (EC)/dimethyl carbonate (DMC). The cells were cycled between 3.28 g of 2-methylimidazole were dissolved in 100 mL methanol. 3.0 and 0.1 V on a Neware battery instrument at room temperaThe mixed solution was sealed in a 125 mL teflon-lined steel auto- ture. The specific capacity was calculated on the total mass of the clave, and heated at 120 °C for three days. The as-prepared crys- composite. The CV measurements were conducted with a Zahner tals were filtered out and washed by methanol, finally dried under Zennium Electrochemical Workstation in the potential range 0.1–3.0 V at a scan rate of 0.2 mV s−1. The electrochemical vacuum at 150 °C for 8 h.
6 www.small-journal.com
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, DOI: 10.1002/smll.201403579
www.MaterialsViews.com
of Education Program for New Century Excellent Talents of China (NCET-11-0027).
Figure 5. a) Cycle-life performances of NC/CoS2-650, NC/CoS2-1000, and BC/CoS2-650 at 500 mA g−1 and b) Nyquist plots of NC/CoS2-650 and NC/CoS2-1000.
impedance spectra were measured using the same instrument with the frequency range from 0.01 to 100 000 Hz with a sine wave with amplitude of 5.0 mV.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
[1] a) J. B. Goodenough, K.-S. Park, J. Am. Chem. Soc. 2013, 135, 1167; b) J. B. Goodenough, Y. Kim, Chem. Mater. 2010, 22, 587. [2] J. Yue, X. Zhao, D. Xia, Electrochem. Commun. 2012, 18, 44. [3] Y. Wang, J. Wu, Y. Tang, X. Lv, C. Yang, M. Qin, F. Huang, X. Li, X. Zhang, ACS Appl. Mater. Interfaces 2012, 4, 4246. [4] a) J. Wang, S. H. Ng, G. X. Wang, J. Chen, L. Zhao, Y. Chen, H. K. Liu, J. Power Sources 2006, 159, 287; b) P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J. M. Tarascon, Nature (London) 2000, 407, 496; c) P. G. Bruce, B. Scrosati, J.-M. Tarascon, Angew. Chem. Int. Ed. 2008, 47, 2930. [5] W. Shi, J. Zhu, X. Rui, X. Cao, C. Chen, H. Zhang, H. H. Hng, Q. Yan, ACS Appl. Mater. Interfaces 2012, 4, 2999. [6] a) J. Xie, S. Liu, G. Cao, T. Zhu, X. Zhao, Nano Energy 2013, 2, 49; b) B. Qiu, X. Zhao, D. Xia, J. Alloy Compd. 2013, 579, 372; c) N. Mahmood, C. Zhang, J. Jiang, F. Liu, Y. Hou, Chem. Eur. J. 2013, 19, 5183; d) G. Huang, T. Chen, Z. Wang, K. Chang, W. Chen, J. Power Sources 2013, 235, 122; e) Q. Su, J. Xie, J. Zhang, Y. Zhong, G. Du, B. Xu, ACS Appl. Mater. Interfaces 2014, 6, 3016. [7] a) Y. S. Bae, R. Q. Snurr, Angew. Chem. Int. Ed. 2011, 50, 11586; b) J. R. Li, R. J. Kuppler, H. C. Zhou, Chem. Soc. Rev. 2009, 38, 1477. [8] M. Yoon, R. Srirambalaji, K. Kim, Chem. Rev. 2012, 112, 1196. [9] J. Della Rocca, D. M. Liu, W. B. Lin, Acc. Chem. Res. 2011, 44, 957. [10] S.-L. Li, Q. Xu, Energy Environ. Sci. 2013, 6, 1656. [11] B. Liu, H. Shioyama, T. Akita, Q. Xu, J. Am. Chem. Soc. 2008, 130, 5390. [12] J.-K. Sun, Q. Xu, Energy Environ. Sci. 2014, 7, 2071. [13] Q. F. Wang, W. Xia, W. H. Guo, L. An, D. G. Xia, R. Q. Zou, Chem. Asian J. 2013, 8, 1879. [14] a) X. Xu, R. Cao, S. Jeong, J. Cho, Nano Lett. 2012, 12, 4988; b) S. J. Yang, S. Nam, T. Kim, J. H. Im, H. Jung, J. H. Kang, S. Wi, B. Park, C. R. Park, J. Am. Chem. Soc. 2013, 135, 7394. [15] R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O’Keeffe, O. M. Yaghi, Science 2008, 319, 939. [16] W. Xia, J. Zhu, W. Guo, L. An, D. Xia, R. Zou, J. Mater. Chem. A 2014, 2, 11606. [17] J. Pels, F. Kapteijn, J. Moulijn, Q. Zhu, K. Thomas, Carbon 1995, 33, 1641. [18] Q. Wang, L. Jiao, Y. Han, H. Du, W. Peng, Q. Huan, D. Song, Y. Si, Y. Wang, H. Yuan, J. Phys. Chem. C 2011, 115, 8300. [19] J. M. Yan, H. Z. Huang, J. Zhang, Z. J. Liu, Y. Yang, J. Power Sources 2005, 146, 264. [20] Z. Zhu, S. Wang, J. Du, Q. Jin, T. Zhang, F. Cheng, J. Chen, Nano Lett. 2014, 14, 153. [21] Y. Kim, J.-H. Lee, S. Cho, Y. Kwon, I. In, J. Lee, N.-H. You, E. Reichmanis, H. Ko, K.-T. Lee, ACS Nano 2014, 8, 6701. [22] Z. Wei Seh, W. Li, J. J. Cha, G. Zheng, Y. Yang, M. T. McDowell, P. C. Hsu, Y. Cui, Nat. Commun. 2013, 4, 1331. [23] F. Sun, J. Wang, H. Chen, W. Li, W. Qiao, D. Long, L. Ling, ACS Appl. Mater. Interfaces 2013, 5, 5630.
This work is supported by National Natural Science Foundation of China (11175006, 51322205, and 21371014), the Ministry
small 2015, DOI: 10.1002/smll.201403579
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: December 2, 2014 Revised: December 29, 2014 Published online:
www.small-journal.com
7
