Lithium-Ion-Battery Anode Materials with Improved Capacity from a

Aug 22, 2016 - A cadmium metal−organic framework (MOF) with a high capacity as an anode for a lithium-ion battery was synthesized under solvothermal...
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Lithium-Ion-Battery Anode Materials with Improved Capacity from a Metal−Organic Framework Xiao-Ming Lin,*,†,‡ Ji-Liang Niu,† Jia Lin,† Lei-Ming Wei,† Lei Hu,† Gang Zhang,‡ and Yue-Peng Cai*,† †

Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, School of Chemistry and Environment, South China Normal University, Guangzhou 510006, P. R. China ‡ State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China S Supporting Information *

storage by thermal decomposition.8 For instance, Zheng et al.9 prepared a nitrogen-doped porous carbon derived from MOF and found excellent lithium anodic performance. Bu et al.10 and Xu et al.11 have reported Co3O4 through thermoanalysis of MOFs, showing excellent electrochemical performance as anode materials. Inspired by these studies, herein, we reported a cadmiumbased MOF (Cd-MOF) with exceptional thermal stability that showed reversible lithium storage. Moreover, pyrolysis of this MOF gave an anode material with enhanced capacity and cyclic stability. Crystals of Cd-MOF were obtained by the hydrothermal reaction of CdCl2 and 1,1′,1″-(1,3,5-triazine-2,4,6triyl)tripiperidine-4-carboxylic acid (H3TTPCA) in a mixture solvent of dimethylformamide and H2O at 100 °C for 3 days. The as-prepared Cd-MOF crystal was determined and structurally characterized by single-crystal X-ray diffraction (XRD) analysis as the formula {[Cd(HTTPCA)]·2H2O}n. This MOF crystallizes in the monoclinic crystal system with a space group of C2. The asymmetric unit contains one Cd2+ ion, one incompletely deprotonated HTTPCA2− ligand, and two lattice H 2O molecules. As shown in Figure 1, each HTTPCA2− ligand coordinates six Cd2+ ions exclusively using its carboxylate groups in three different bridging modes, viz., μ1-η1, μ2-η1:η1, and μ3η1:η1:η1, satisfying an octahedral geometry around CdII, situated

ABSTRACT: We present a porous metal−organic framework (MOF) with remarkable thermal stability that exhibits a discharge capacity of 300 mAh g−1 as an anode material for a lithium-ion battery. Pyrolysis of the obtained MOF gives an anode material with improved capacity (741 mAh g−1) and superior cyclic stability.

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ithium-ion batteries (LIBs) with long-term and high storage capacities, as a promising power source, have a wide range of applications in portable electronic devices because of their small volume, light weight, environmental benignity, and lack of “memory effect”.1 Graphite, as the most commonly used commercial anode material with a theoretical capacity of 372 mAh g−1, cannot meet the growing demands for energy storage for LIBs.2 Currently, metals or metal oxides with high energy density have been widely explored as anode materials for lithium storage. However, the huge volume change and large voltage hysteresis that occur in these materials lead to a dramatic falloff in capacity during the lithium insertion and extraction process.3 Thus, tremendous research efforts have been made to develop novel anode materials with improved lithium storage capacity.4 Metal−organic frameworks (MOFs), as a type of crystalline porous materials with tunable pore structure and large surface area, have emerged in recent years as promising electrodes in LIBs.5 The inner pores of MOFs can facilitate the fast transport of lithium ions and electrons. Previous researches have demonstrated that MOFs could be directly used as electrode materials for lithium storage.6 Férey et al.5a first reported a mixedvalence cathode material, MIL-53 [Fe III (OH) 0.8 F 0.2 (O2CC6H4CO2)] that shows a reversible electrochemical reaction but with a low capacity (75 mAh g−1). Following this idea, many attempts on exploring novel electrode materials based on MOFs have been carried out, and several examples, such as [Zn(IM)1.5(abIM)0.5],6a Zn4O(1,3,5-benzenetribenzoate)2,6c and Zn3(HCOO)6,6e have been investigated as anodic electrode materials for LIBs.6 However, the capacity performance needs to be further improved for these MOF materials. Recently, Mahanty and coauthors presented a MOF [Mn-BTC] with a high lithium insertion capacity of 694 mAh g−1, which is the highest capacity value reported for those MOFs as anode materials up to now.7 On the other hand, MOFs have been proven to be effective templates or precursors to prepare porous carbons and metal oxides with outstanding electrochemical properties for lithium © XXXX American Chemical Society

Figure 1. (a) Representation of the coordination environment of the HTTPCA2− ligand. All hydrogen atoms were omitted for clarity (except for the deprotonated one). (b) 1D chain by carboxyl bridges in the title compound, showing six-coordination for the Cd2+ ion. (c) 3D framework with two types of 1D channels along the b axis. Received: May 7, 2016

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DOI: 10.1021/acs.inorgchem.6b01123 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

initial irreversible capacity. Figure 2b shows the galvanostatic charge/discharge profiles for the Cd(HTTPCA) electrode. The first and second discharge specific capacities are 710 and 435 mAh g−1, respectively, corresponding to a capacity loss of 38%. This irreversible lithium loss can be ascribed to the formation of a solid electrolyte interface (SEI) film and incomplete conversion reaction, which is common for most anode materials.14 However, the Cd(HTTPCA) electrode exhibited reversible reactions for stable lithium storage from the second cycle, similar to the CV curves. The electrode maintained a capacity of about 302 mAh g−1 after 100 cycles. Indeed, the XRD pattern measured after 100 charge/discharge cycles showed a profile identical with that of the original, confirming that the integrity of the MOF framework was maintained during the test of lithium storage (Figure S6). The Coulombic efficiency remained above 98% after the 10th cycle, demonstrating excellent reversibility during the processes (Figure 2c). At current densities of 0.1, 0.2, 0.5, and 1 A g−1, the 10th cycle discharge capacities of the Cd(HTTPCA) composite electrode were 355, 268, 230, and 200 mAh g−1, respectively (Figure 2d), suggesting a good rate performance. The capacity at a high current density of 1 A g−1 was higher than that of [Cu2(C8H4O4)4]n15 or Zn(IM)1.5(abIM)0.5.6a In addition, utilization of {[Cd(HTTPCA)]·2H2O}n as a sacrificial template for the preparation of a porous carbon material was also carried out. The as-made {[Cd(HTTPCA)]· 2H2O}n was calcined at 700, 800, and 900 °C in a nitrogen atmosphere, and then the resulting samples were washed by hydrofluoric acid and dried under vacuum to obtain nitrogendoped porous carbon materials (denoted as NC700, NC800, and NC900). Figure 3a shows the PXRD patterns of nitrogen-doped

in a coordination sphere composed of six oxygen atoms from six different HTTPCA2− ligands. The adjacent Cd2+ ions are bridged by carboxylate groups in the cadmium chain with a Cd···Cd distance of 3.807(4) Å. Importantly, each chain serves as a secondary building unit and is further connected by HTTPCA2− ligands into a 3D framework, containing two types of channels with different dimensions of ∼6.3 × 5.4 and ∼4.5 × 4.4 Å2 (excluding van der Waals radii), respectively. Both channels are filled with guest H2O molecules. The total potential solventaccessible volume of open channels is about 14.5%, as calculated by the PLATON program.12 The morphology of as-prepared CdMOF was investigated by scanning electron microscopy (SEM), as shown in Figure S1, which exhibited a large quantity of microcrystals with rectangular block shapes. IR spectral analysis distinctly showed the strong adsorption band around 3448 cm−1 attributed to the ν(OH) vibration of guest H2O molecules. The presence of the absorption at about 1718 cm−1 in the IR spectrum indicated that the carboxylic groups of the ligand are incompletely deprotonated (Figure S2). Thermogravimetric analysis (TGA) showed that the first weight loss (5.83%) between 100 and 150 °C was ascribed to the removal of free H2O molecules (calcd 5.90%). Subsequently, an abrupt weight loss above 350 °C occurred, indicative of the complete decomposition of the whole framework (Figure S3). The powder XRD (PXRD) patterns below 300 °C remained essentially the same as the corresponding simulated results, demonstrating that the framework remained stable up to this temperature after removal of the H2O molecules (Figure S4). Nitrogen adsorption/ desorption isotherms were carried out to evaluate the pore characteristics and exhibited a typical type I curve (Figure S5). The Brunauer−Emmett−Teller surface area is 821 m2 g−1 with a total pore volume of 0.50 cm3 g−1. Such high thermal stability and specific surface area are advantageous for reversible Li+ transfer and storage.13 The electrochemical performance of Cd(HTTPCA) for lithium storage was evaluated by cyclic voltammetry (CV) in the voltage range 0.1−3.0 V versus Li+/Li (Figure 2a). During the first anodic scan, a broad oxidation peak around 1.94 V was observed, while three cathodic reduction peaks at about 1.04, 0.56, and 0.28 V disappear from the second cycle, indicative of an

Figure 3. (a) PXRD patterns of carbon materials obtained at different temperatures. (b) Raman spectra. (c) XPS spectra. All carbon samples represent carbon, nitrogen, and oxygen atoms, respectively. (d) Nitrogen adsorption/desorption isotherms.

carbon particles at the above temperatures, where all of the diffraction peaks were located at ∼23° and ∼44°, corresponding to the (002) and (101) diffraction planes of the carbon phase, which is characteristic of disordered carbon materials. The PXRD patterns indicated that pure porous carbon was obtained without CdO (boiling point, 549 °C) or Cd (boiling point, 765 °C). The chemical composition and purity were further confirmed using X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS). It clearly indicated that NC800 mainly contained carbon and nitrogen elements (Figure S7). The

Figure 2. (a) CV curves of the Cd(HTTPCA) electrode at 0.1 mV s−1. (b) Galvanostatic charge/discharge voltage profiles. (c) Cyclic stability with a Coulombic efficiency at 100 mA g−1. (d) Rate performance at different discharge currents. B

DOI: 10.1021/acs.inorgchem.6b01123 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

100th cycle at a current density of 100 mA g−1. The Coulombic efficiency approached 99% after a few initial cycles, indicating that the obtained carbon material was stable and reversible during the charge/discharge texts (Figure 4c). In contrast, the NC800 electrode provided a better capacity performance than the Cd(HTTPCA) electrode. The remarkable cyclic stability and capacity might be attributed to the existence of an abundance of microtube structures in the nitrogen-doped carbon-based material, which acted as areservoirs for Li+ storage.17 In addition, the amount of nitrogen-rich composition to some extent can further improve the lithium storage capacity.6a,9 More importantly, a better symmetric pattern was observed compared to that of the synthesized Cd(HTTPCA) electrode measured at different current rates. The specific capacity could recover to almost the same value after the high rate measurements (Figure 4d). In conclusion, a porous Cd-MOF with excellent thermal stability was synthesized under the solvothermal method, which shows good performance in LIBs with stable cycling behavior and rate capability. Pyrolysis of this MOF led to nitrogen-doped porous carbon particles with sizes of several micrometers. Such microtube structures facilitate the transport of electrons and lithium ions and exhibit improved performance in LIBs with excellent cyclic stability.

absence of the other signal in EDS confirmed that the cadmium element had been removed completely. The O peak (0.53 keV) probably arose from the oxygen adsorbed in the nitrogen-doped carbon particles.16 Figure 3b shows the Raman spectra of the obtained carbon materials. Obviously, two broad peaks can be observed, also known as the D and G bands, respectively. The D band is attributed to disordered carbon or the presence of defects in the graphitic structures, while the G band is the typical feature of C−C bond vibrations. The ID/IG values of NC700, NC800, and NC900 were 0.86, 0.92, and 0.90, respectively, which indicated the disordered structure. The XPS spectra verified the existence of carbon, nitrogen, and oxygen elements in all three samples. Obviously, the carbon content decreased when the temperature increased from 700 to 900 °C, while the atomic contents of nitrogen were 4.34, 5.97, and 5.54 atom %, respectively (Figure 3c). Nitrogen adsorption/desorption isotherms are depicted in Figure 3d. All of the isotherms presented the typical type IV behavior. The specific surface areas were 504, 654, and 606 m2 g−1, and the total pore volumes were 0.60, 0.84, and 0.77 cm3 g−1, respectively. Figure S8 shows the cycling performance of the carbon samples as anode materials at 100 mA g−1. The specific capacity of NC800 (741 mAh g−1) was higher than those of NC700 (362 mAh g−1) and NC900 (543 mAh g−1). SEM was employed to explore the morphologies and structures of 3D porous carbon materials, as shown in Figure S9. It shows a porous microtube shape with a diameter of about 8 μm at 800 °C. We could clearly see that the microtube shape was gradually formed at 700 °C. When the temperature was raised to 900 °C, the microtube structures were more or less destroyed because of the higher temperature. The morphology of NC800 provided more favorable pathways for lithium-ion transport. Figure 4a shows the first three cyclic voltammograms of the NC800 material under similar experimental conditions. Notably,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01123. Experimental section, crystallographic details, SEM image, IR spectra, TGA curve, PXRD patterns, and nitrogen adsorption/desorption isotherms of Cd-MOF, and EDS and SEM images of carbon materials derived from CdMOF (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected];. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the National Natural Science Foundation of P. R. China (Grants 21471061 and 21401059), Applid Science and Technology Planning Project of Guangdong Province (Project 2015B010135009), Science and Technology Program of Guangzhou (Grant 2014J4100051), and Open Project of State Key Laboratory of Supramolecular Structure and Materials (Project sklssm201614).

Figure 4. (a) CV curves of a carbon NC800 electrode at 0.1 mV s−1. (b) Galvanostatic charge/discharge profiles at 100 mA g−1. (c) Cycle behaviors of the LIBs with a porous NC800 electrode at a current density of 100 mA g−1. (d) Rate performance at different discharge currents.

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a cathodic peak at 0.69 V was observed during the discharge. However, the CV curves of the subsequent cycles were different from those of the first cycle, indicative of the occurrence of irreversible reactions because of the formation of a SEI film. From the second cycle onward, the almost overlapped CV curves indicated superior reversibility for lithium storage of the NC800 particles. The specific capacity remained at 741 mAh g−1 after the

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DOI: 10.1021/acs.inorgchem.6b01123 Inorg. Chem. XXXX, XXX, XXX−XXX