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High Anodic Performance of Co 1,3,5-Benzenetricarboxylate Coordination Polymers for Li-Ion Battery Chao Li,† Xiaobing Lou,† Ming Shen,† Xiaoshi Hu,† Zhi Guo,‡ Yong Wang,*,‡ Bingwen Hu,*,† and Qun Chen† †
School of Physics and Materials Science, Shanghai Key Laboratory of Magnetic Resonance, Engineering Research Center for Nanophotonics & Advanced Instrument (Ministry of Education), Institute of Functinal Materials, East China Normal University, Shanghai 200062, P. R. China ‡ Shanghai Synchrotron Radiation Facility (SSRF), Shanghai 201204, P. R. China S Supporting Information *
ABSTRACT: We report the designed synthesis of Co 1,3,5-benzenetricarboxylate coordination polymers (CPs) via a straightforward hydrothermal method, in which three kinds of reaction solvents are selected to form CPs with various morphologies and dimensions. When tested as anode materials in Li-ion battery, the cycling stabilities of the three CoBTC CPs at a current density of 100 mA g−1 have not evident difference; however, the reversible capacities are widely divergent when the current density is increased to 2 A g−1. The optimized product CoBTC-EtOH maintains a reversible capacity of 473 mAh g−1 at a rate of 2 A g−1 after 500 galvanostatic charging/discharging cycles while retaining a nearly 100% Coulombic efficiency. The hollow microspherical morphology, accessible specific area, and the absence of coordination solvent of CoBTC-EtOH might be responsible for such difference. Furthermore, the ex situ soft Xray absorption spectroscopy studies of CoBTC-EtOH under different states-of-charge suggest that the Co ions remain in the Co2+ state during the charging/discharging process. Therefore, Li ions are inserted to the organic moiety (including the carboxylate groups and the benzene ring) of CoBTC without the direct engagement of Co ions during electrochemical cycling. KEYWORDS: Co, benzenetricarboxylate, coordination polymers, shaped-controlled synthesis, anode, Li-ion battery
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storage,23,24 and proton conductivity.25,26 In particular, applications as electrical storage materials in state-of-the-art LIBs, electrochemical capacitors (ECs),27,28 Li−O2 batteries,29 and Li−S batteries.30−32 In 2006, Chen et al. first reported Zn4O(1,3,5-benzenetribenzoate)2 (MOF-177) as anode for LIBs, the electrochemical performance of MOF-177 used for Li-ion storage was not appreciable since the cycling stability is limited.33 Since then, more experimental discoveries of MOFs or CPs anode materials have been reported. The Mn-(tfbdc)(4,4′-bpy)(H2O)2 (MnLCP) anode exhibited a reversible Li-ion storage capacity of ∼390 mAh g−1 after 50 cycles through conversion reaction.34 Co2(OH)2BDC (BDC = 1,4-benzenedicarboxylate) presented a high reversible capacity of about 650 mAh g−1 as well as an impressive cyclic stability at a current density of 50 mA g−1 after 100 cycles. 35 More recently, 2,6-Naph(COOLi)2, 36 NiMe4bpz,37 Zn(IM)1.5(abIM)0.5 (IM = imidazole, abIM = 2aminobenzimidazole),38 Co 2,5-furandicarboxylate,39 Mn 2,5-
INTRODUCTION Ever-increasing energy demands and depleting of fossil-fuel resources around the world have galvanized numerous attempts to develop sustainable energy alternatives, including both renewable energy technologies and sustainable energy-based appliances.1−3 Among the currently operated electrical energy storage (EES) devices, lithium-ion rechargeable batteries (LIBs) are actively being developed for advanced electric vehicles and grid-scale energy storage due to their high energy density (>180 Wh kg−1), great longevity (2−3 years), and environmentally benignity.4−8 It is well-known that commercial graphite anode material has a relatively small theoretical specific capacity of 372 mAh g−1, there is therefore a compelling research attention in the development of novel anode materials with higher capacity as well as long-term cyclic stability, such as transition metal oxides,9−12 silicon,13−15 and amorphous phosphorus.16 Metal− organic frameworks (MOFs) or coordination polymers (CPs) are microporous or mesoporous materials composed of one-, two-, or three-dimensional networks resulting from the covalently bonding of metal ion nodes and bridging ligands.17,18 MOFs always have well-defined crystalline structure while CPs not. Based on features of rich surface chemistry, tunable pore size, and structural versatility, MOFs/CPs have advanced the fields of chemical sensing,19,20 separation catalysis,21,22 gas © XXXX American Chemical Society
Special Issue: New Materials and Approaches for Beyond Li-ion Batteries Received: March 25, 2016 Accepted: May 4, 2016
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DOI: 10.1021/acsami.6b03648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces thiophenedicarboxylate40 and Mn 1,3,5-benzenetricarboxylate41 with considerable Li-ion storage performance have also been reported. Despite their considerable advantages, MOFs or CPs are plagued with problems that have hindered their widespread practical realization in LIBs. There are several pivotal issues: (1) MOFs/CPs are often brittle and may disintegrate during cycling; (2) the electrical conductivity of MOFs/CPs is usually too small, thus yielding a low electrochemical activity; (3) the influence of the level of porosity in the framework remains elusive. A series of Co(II)-1,3,5-benzenetricarboxylate (referred as CoBTC) CPs with different topologies have been synthesized through different processes; however, none of these examples has been applied in energy storage systems previously.42−45 In the present study, we report the designed synthesis of CoBTC CPs via an expedient hydrothermal method and investigate their electrochemical performances as anode materials in Li-ion coin cells. The three CoBTC CPs, formed from three kinds of reaction solvents, have different morphologies and dimensions. The cycling stabilities of the three CoBTC CPs in LIBs at low current density have not significant difference; however, the reversible capacities are widely divergent when the current density is increased from 100 mA g−1 to 2 A g−1. The optimized product CoBTC-EtOH of these CoBTC CPs maintains a reversible capacity of 473 mAh g−1 at a rate of 2 A g−1 after 500 galvanostatic charging/discharging cycles while retaining a nearly 100% Coulombic efficiency. To the best of our knowledge, this should be the best LIB performance among MOFs and CPsbased anode materials at such a high rate (2 A g−1). Besides, the lithiation/delithiation process of this CoBTC material was investigated by ex situ soft X-ray absorption spectroscopy (sXAS) technique. It is observed that the Co L-edges spectra under different states-of-charge (SOC) do not exhibit discernible changes in energy position, suggesting that the Co-ions in this material remain in the Co2+ state during charging/discharging process. Therefore, Li ions are inserted to the organic moiety (including the carboxylate groups and the benzene ring) of CoBTC without the direct engagement of Co ions during electrochemical cycling.
Scheme 1. Schematic Illustration of the Preparation Process of CoBTC CPs through a Straightforward Hydrothermal Method, in Which EtOH, DMF, and EtOH/DMF (1:1,vol%) Are Selected To Form Three CoBTC CPs with Various Morphologies and Dimensionsa
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When reacting in EtOH, the product CoBTC-EtOH appears as monodispersed hollow microspheres with varied diameters between 3 and 8 um. In contrast, the CoBTC obtained in pure DMF (CoBTCDMF) appears as a two-dimensional layered architecture, the CoBTC synthesized in a mixed solvent of EtOH and DMF (CoBTC-DMF/ EtOH) shows a more apparent laminar morphology with walls composed of nanorods of 50-300 nm in length and ∼50 nm in width.
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RESULTS AND DISCUSSION Characterizations of CoBTC CPs. The carboxylate functional groups of BTC3− can serve as nucleation sites for Co2+ and subsequent CoBTC crystals growth. Besides, the solvent applied to hydrothermal synthesis plays a key role, and the coordinated solvent molecules occluding inside the pores of MOFs may have detrimental effect on reversible Li ion insertion/extraction. Here in this work, we selected three kinds of reaction solvents to investigate the effect of solvents on the LIBs performance, as illustrated in Scheme 1. Detailed structure analyses of these CoBTC materials were performed by various techniques. Figure 1 presents the scanning electron microscope (SEM) images of the as-prepared three CoBTC materials. Obviously, the morphology and dimension of the products are strongly dependent on the reaction solvents. When reacting in absolute ethanol (EtOH), the product (referred as CoBTC-EtOH) is composed of monodispersed microspheres with varied diameters between 3 and 8 um, as shown in Figure 1a. Under higher magnifications (Figure 1b), we clearly observe that these microspheres have relatively coarse surface. By contrast, the CoBTC obtained in pure N,N-dimethylformamide (referred as CoBTC-DMF) appears as lamellar crystals, and the crystals have a two-dimensional layered architecture which contains irregular walls (Figure 1c, d). The CoBTC synthesized in a mixed solvent
Figure 1. SEM micrographs of the as-prepared (a, b) CoBTC-EtOH, (c, d) CoBTC-DMF, and (e, f) CoBTC-DMF/EtOH at low and high magnifications.
of EtOH and DMF (referred as CoBTC-DMF/EtOH) shows a more apparent laminar morphology (Figure 1e). The magnified SEM images also indicate that these walls are composed of nanorods of 50−300 nm in length and ∼50 nm in width (Figure 1f). Transmission electron microscopy (TEM) examination (Figure 2a) clearly showed the existence of a macroporous cavity (>500 nm) in each CoBTC-EtOH microsphere. Comparatively, the uniform-contrast TEM images of CoBTC-DMF and CoBTC-DMF/EtOH in Figure S1a, b clearly demonstrate their solid and dense nature without discernible porosities. On B
DOI: 10.1021/acsami.6b03648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 2. (a) TEM micrograph and the corresponding SAED pattern of CoBTC-EtOH. (b) EDS elemental mapping images of CoBTC-EtOH from selected region. (c) XPRD patterns of CoBTC-EtOH. (d) FT-IR spectra of CoBTC-EtOH and 1,3,5-H3BTC.
Figure 3. (a) Raman spectra of CoBTC-EtOH and 1,3,5-H3BTC. (b) TGA curve of CoBTC-EtOH under N2 atmosphere. (c) High-resolution Co 2p XPS spectrum of CoBTC-EtOH. (d) Nitrogen adsorption−desorption isotherms and the corresponding BJH pore size distribution of CoBTC-EtOH.
the basis of the above observations and analyses, the shapecontrolled synthesis of CoBTC materials is realizable by controlling the reaction solvents. Figure 2c presents the X-ray powder diffraction (XRPD) patterns of CoBTC-EtOH, in which the as-prepared CoBTCEtOH only show two well-defined diffraction peaks at 2θ = 8.6
and 10.4°. The weak property of the other peaks implies that only a handful of crystals are existed in CoBTC-EtOH while the majority is in the amorphous form. Hence, it is difficult to determine the exact crystal structure of CoBTC-EtOH; however, the strong peaks at 2θ = 8.6 and 10.4° are almost superposed with the (110) and (111) planes of the Co2(BTC)Cl(DEF)3 (DEF = C
DOI: 10.1021/acsami.6b03648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. (a) Cycling performances of CoBTC-EtOH, CoBTC-DMF, and CoBTC-DMF/EtOH at a current density of 100 mA g−1. (b) Rate performance of CoBTC-EtOH at different current densities from 100 to 2000 mA g−1. (c) Rate performances of CoBTC-DMF and CoBTC-DMF/ EtOH at different current densities from 100 to 2000 mA g−1. (d) Cycling performances of CoBTC-EtOH, CoBTC-DMF, and CoBTC-DMF/EtOH at a high current density of 2 A g−1. A relatively low rate of 100 mA g−1 was applied for the initial two cycles to activate the electrodes.
1445−1370 cm−1 can be assigned to the symmetric stretching vibrations of the carboxylate groups.41,47 The absence of the sharp absorption band from the vibrations of hydroxide (∼3600 cm−1) indicate that EtOH molecules are not incorporated in CoBTC-EtOH.35 As well, the peak shifts in the Raman spectra (Figure 3a) indicate an altered chemical environment around the organic ligands, and the Raman peaks at 470 and 684 cm−1 could be assigned to ν(Co−O) and δ(COO)ip (Raman active), respectively.47,48 These analyses demonstrate that Co-ions have coordinated with the organic ligands successfully, similar analyses can be done with CoBTC-DMF and CoBTC-DMF/ EtOH (Figure S3b). The thermal behavior of CoBTC-EtOH was investigated using thermogravimetric analysis (TGA). As depicted in Figure 3b, CoBTC-EtOH shows that weight loss begins at ∼386 °C, above which the gasification of the BTC ligands takes place with subsequent decomposition of the CoBTC-EtOH skeleton. After the complete breakdown of BTC ligands at ca. 426 °C, the remanent materials are then converted to Co3O4. It should be highlighted here that the thermal behaviors of CoBTC-DMF and CoBTC-DMF/EtOH are much different from CoBTC-EtOH, as can be seen in Figure S3c. Figure 3c exhibits the high-resolution Co 2p XPS spectrum for CoBTC-EtOH, it is observed that the characteristic Co 2p1/2 peak and its satellite peak are located at 797.40 and 802.13 eV, respectively, while the characteristic Co 2p3/2 peak and its satellite peak are located at 781.49 and 786.38 eV, respectively, which demonstrates the existence of Co2+ from
N,N′-diethylformamid) crystal structure (Figure S2, Space group: P213, Cell: a = 14.6518(17) Å, b = 14.6518(17) Å, c = 14.6518(17) Å, β = 90°),44 so the crystal structure of CoBTCEtOH should be similar to Co2(BTC)Cl(DEF)3. By contrast, CoBTC-DMF and CoBTC-DMF/EtOH show peaks located at nearly the same angles, and the peaks are sharp and strong, suggesting a high crystallinity (Figure S3a), which is also in accordance with the selected area electron diffraction (SAED) analyses (Figure S1a, b). It has been reported before that the crystallinity of MOFs relys on the amount of coordination solvent molecules blocked inside the pores.46 The X-ray photoelectron spectroscopy (XPS) surveys in Figure S4 detect expected N element in CoBTC-DMF and CoBTC-DMF/EtOH, suggesing the exsistence of coordinated DMF in them, so the better crystallinity of CoBTC-DMF and CoBTC-DMF/EtOH can be attributed to the coordinated DMF molecules inside their pores. Figure 2d presents the Fourier transform infrared spectroscopy (FT-IR) spectrum of CoBTC-EtOH, and the homologous spectrum for benzene-1,3,5-tricarboxylic acid (1,3,5-H3BTC) is also displayed for comparison. The complete deprotonation of 1,3,5-H3BTC upon reaction with Co2+ is attested by the disappearance of the characteristic peaks of the nonionized carboxyl groups (νCO, 536 cm−1; νCO, 1721 cm−1; νOH, 3082 cm−1). The new bands arise in the regions of 1630−1564 cm−1 can be assigned to the asymmetric stretching vibrations of the carboxylate groups, while the new peaks arise in the regions of D
DOI: 10.1021/acsami.6b03648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
voltage range of 0.01−3.0 V versus Li/Li+. As summarized in Figure 4b, a durable and steady rate capability at various charge/ discharge rates is observed for CoBTC-EtOH. When cycled at 100 mA g−1, 200 mA g−1, 400 mA g−1,600 mA g−1, 1 A g−1, and 2 A g−1, the CoBTC-EtOH electrode delivers an average charge capacity of 845, 712, 597, 573, 580, and 584 mAh g−1, respectively. The reversible capacity is almost invariable when the current density is increased from 600 mA g−1 to 2 A g−1. These tests demonstrated the superb rate capability of CoBTCEtOH. More encouragingly, when the current density was recovered to 100 mA g−1, an average charge capacity of 1004 mAh g−1 can be resumed and sustained up to the 90th cycle without apparent decay, demonstrating the flexibility of the CoBTC-EtOH skeleton in response to precipitate current changes. The slight augment of the specific capacity might be ascribed to the reversible growth of an electrochemical gel-like polymer layer.58−60 In comparison, the reversible capacities of CoBTC-DMF and CoBTC-DMF/EtOH under relatively low current densities (
