Tellurium-Impregnated Porous Cobalt-Doped Carbon Polyhedra as

Jul 25, 2017 - To ensure sufficient space to host tellurium, the carbon polyhedron products are treated in HCl to partially remove cobalt particles. T...
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Tellurium-Impregnated Porous Cobalt-Doped Carbon Polyhedra as Superior Cathodes for Lithium−Tellurium Batteries Jiarui He,†,⊥ Weiqiang Lv,‡,⊥ Yuanfu Chen,*,† Kechun Wen,‡ Chen Xu,† Wanli Zhang,† Yanrong Li,† Wu Qin,§ and Weidong He*,†,‡,∥,^ †

State Key Laboratory of Electronic Thin Films and Integrated Devices and ‡School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P.R. China § National Engineering Laboratory for Biomass Power Generation Equipment, School of Renewable Energy Engineering, North China Electric Power University, Beijing 102206, P.R. China ∥ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China ^ Shenzhen Li-S Technology Co., Ltd., Shenzhen 518120, P.R. China S Supporting Information *

ABSTRACT: Lithium−tellurium (Li−Te) batteries are attractive for energy storage owing to their high theoretical volumetric capacity of 2621 mAh cm−3. In this work, highly nanoporous cobalt and nitrogen codoped carbon polyhedra (C−Co−N) derived from a metal−organic framework (MOF) is synthesized and employed as tellurium host for Li−Te batteries. The Te@C−Co−N cathode with a high Te loading of 77.2 wt % exhibits record-breaking electrochemical performances including an ultrahigh initial capacity of 2615.2 mAh cm−3 approaching the theoretical capacity of Te (2621 mAh cm−3), a superior cycling stability with a high capacity retention of 93.6%, a ∼99% Columbic efficiency after 800 cycles as well as rate capacities of 2160, 1327.6, and 894.8 mAh cm−3 at 4, 10, and 20 C, respectively. The redox chemistry of tellurium is revealed by in operando Raman spectroscopic analysis and density functional theory simulations. The results illustrate that the performances are attributed to the highly conductive C−Co−N matrix with an advantageous structure of abundant micropores, which provides highly efficient channels for electron transfer and ionic diffusion as well as sufficient surface area to efficiently host tellurium while mitigating polytelluride dissolution and suppressing volume expansion. KEYWORDS: tellurium, metal−organic frameworks, cathode, in-operando Raman spectroscopy, lithium−tellurium battery

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the electrolyte, which results in the serious active mass loss and shuttle effect.10−13 As the congener of sulfur, tellurium exhibits a similar (de)lithiation mechanism. The higher electronic conductivity of tellurium (2 × 102 S m−1) as compared to sulfur ensures

ighly efficient energy storage is the key to realizing highly efficient applications of electric vehicles and portable electronics.1−6 Lithium sulfur batteries are particularly attractive because of high theoretical specific capacity (1675 mAh g−1) and abundant resources of sulfur.7−9 Despite various advances in the scientific field, commercialization of Li−S batteries is still challenged by several issues, such as the insulating properties of sulfur (5 × 10−28 S m−1), large volume variation (∼79 vol %) during the charge/discharge operation, and the highly soluble intermediates (Li2Sn, n > 2) in © 2017 American Chemical Society

Received: May 3, 2017 Accepted: July 25, 2017 Published: July 25, 2017 8144

DOI: 10.1021/acsnano.7b03057 ACS Nano 2017, 11, 8144−8152

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Figure 1. (a) Schematic of the synthetic procedure of the Te@C−Co−N composite. SEM images of (b and c) C−Co−N and (d and e) Te@ C−Co−N composite. (f) Low-magnification and (g) corresponding high-resolution TEM images of Te@C−Co−N. (h) TEM image of the Te@C−Co−N and the corresponding elemental mapping of (i) carbon, (j) tellurium, (k) cobalt, and (l) nitrogen.

In addition, cobalt and nitrogen codoped in the matrix strengthens the interactions with both tellurium and polytellurides. The highly nanoporous and conductive C−Co−N architecture not only provides sufficient Te loading sites but also significantly enhances the electrode conductivity and effectively suppresses the volume expansion during the charge and discharge process. The electrochemical redox chemistry of tellurium on the electrolyte/matrix interface is investigated via in operando Raman spectroscopy and density functional theory (DFT) simulations, which facilitates the rational design of tellurium-based cathodes with high performances.

efficient utilization of tellurium when it is employed as cathode.14−16 The high theoretical of tellurium (2621 mAh cm−3) is also highly advantageous for practical applications since a high volumetric capacity allows for efficient battery packing as well as the rational design of portable devices and electric vehicles.14,17 With those merits, tellurium has been considered as a potential candidate to replace sulfur cathodes. Liu et al. reported pioneering work on tellurium/porous carbon (Te/C) as a cathode material for lithium tellurium (Li−Te) batteries, though the utilization of tellurium was not fully realized.14 Most recently, several strategies have been proposed to enhance the utilization of tellurium by using carbonaceous materials as conductive matrix.14−18 In particular, in our previous work, we constructed a three-dimensional rGO/ tellurium nanowire aerogel (3DGT). However, owing to the weak interaction of the physical adsorption and the open pore structure of 3DGT, the active materials show gradual loss during cycles. Indeed, it is still challenging to realize an electrode that efficiently confines tellurium within the conductive electrode framework with synergistic chemical and physical interactions at a molecular level to enhance the cathode conductivity and also suppress the Te volume expansion (ΔV = 104%) and the intermediate dissolution.19,20 In this study, porous carbon polyhedra codoped with cobalt nitrogen (C−Co−N), derived from metal−organic frameworks (MOF, ZIF-67 was used in this work), is employed as matrix to host tellurium in the framework. MOF with abundant nanoscale cavities and open channels ensures sufficient space for hosting tellurium.

RESULTS AND DISCUSSION The preparation process of the Te@C−Co−N is displayed in Figure 1a. Carbonization of ZIF-67 polyhedra is realized in a tube furnace under N2 flow at 900 °C for 5 h. Both the X-ray diffraction (XRD) pattern (Figure S1a) and high-resolution transmission electron microscopy (HR-TEM) image (Figure S1b) indicate that the cobalt ions in the frameworks were reduced to cobalt nanoparticles. To ensure sufficient space to host tellurium, the carbon polyhedron products are treated in HCl to partially remove cobalt particles. The content of cobalt particles can be easily controlled by adjusting the etching time and the concentration of HCl, as shown in Figure S2. Tellurium is impregnated into abundant pores of C−Co−N matrix via a simple tellurium-melt-diffusion method. The scanning electron microscopy (SEM) image shown in Figure 1b indicates that C− Co−N particles with a uniform rhombic dodecahedral shape and a size of ∼350 nm were synthesized. Highly porous structure of the C−Co−N polyhedra is observed from the high 8145

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Figure 2. (a) TGA curves and (b) XRD patterns of the C−Co−N and Te@C−Co−N composites. (c) Raman spectra of Te, C−Co−N, and Te@C−Co−N. (d) N2 adsorption/desorption isotherm and pore size distribution (inset) of C−Co−N and Te@C−Co−N composites.

Figure 3. (a) C 1s, (b) N 1s, and (c) Co 2p3/2 X-ray photoelectron spectroscopy (XPS) of the C−Co−N composite. (d) Calculated adsorption energy between Li2Te4 and C, C−N, and C−Co.

magnification SEM image in Figure 1c. Such abundant pores in C−Co−N not only provide sufficient surface area for loading tellurium but also suppress the volume expansion in the charge/discharge process. After impregnating tellurium into the pores of C−Co−N, the rhombic dodecahedral shape of C− Co−N is well retained and Te@ C−Co−N composites are synthesized. The surface of Te@C−Co−N appears to be smoother than that of C−Co−N, as shown in Figure 1d,e, which suggests tellurium was successfully impregnated into the pores of C−Co−N. Figure 1f shows a typical low-magnification

TEM image of the obtained Te@C−Co−N. The small nanoparticles are observed in Te@C−Co−N polyhedra. The HR-TEM image in Figure 1g shows the lattice fringe of the nanoparticles with an interplanar spacing of 0.32 nm, which matches well with that of the (101) plane of the tellurium. The 0.35 nm lattice spacing corresponds to the (002) plane of graphite. The 0.204 nm lattice spacing shown in Figure S1b corresponds to the (111) plane of cobalt. The elemental mapping images in Figure 1h−l show the distribution of carbon, tellurium, cobalt, and nitrogen in Te@C−Co−N. The 8146

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Figure 4. (a) CV curves of the Te@C−Co−N at a scan rate of 0.1 mV s−1 in the initial four cycles. (b) Discharge and charge curves of the Te@ C−Co−N cathode at a 0.2 C. (c) Cyclic performance of the Te@C−Co−N cathode at 0.2 C for 200 cycles. (d) Rate performance at various C rates for the Te@C−Co−N cathode. (e) Cycling stability of Te@C−Co−N cathode at 1 C for 800 cycles.

the C−Co−N, as shown in Figure 2d. The BET specific surface area of C−Co−N is calculated to be 291.1 m2 g−1. The poresize distribution in the inset of Figure 2d indicates that the average pore size in C−Co−N is ∼1.8 nm and corresponding specific area and pore volume are 291.1 m2 g−1 and 0.64 cm3 g−1, respectively. According to Table S1, the Te weight ratio in Te@C−Co−N can be calculated as 77.75 wt %, which is very close to 77.2 wt % calculated by the TGA measurement as shown in Figure 2a. Such results indicate that the high specific surface area and the abundant micropores in C−Co−N ensure sufficient adsorption of tellurium and polytellurides inside the C−Co−N framework. The coexistence of cobalt and nitrogen in C−Co−N is confirmed by the X-ray photoelectron spectroscopy (XPS). As shown in the C 1s spectrum in Figure 3a, the peaks of sp2 carbon, CN species, C−N bonds, and OC−O are observed at 284.8, 285.7, 287.6, and 289.6 eV, respectively. The N 1s spectrum in Figure 3b confirms the existence of nitrogen species in C−Co−N. The three characteristic peaks at 402.1, 400.8, and 398.8 eV are assigned to quaternary, pyrrolic, and pyridinic nitrogen in C−Co−N, respectively.8 The Co 2p spectrum shown in Figure 3c confirms the presence of cobalt in C−Co−N, in which the peaks of Co 2p1/2 and Co 2p3/2 are observed at 785.9 and 780.3 eV, respectively. The absorption energies between a typical polytelluride Li2Te4 and C, C−N

mapping images of tellurium, cobalt, and nitrogen match well with that of carbon, confirming the uniform distribution of tellurium, cobalt, and nitrogen in the framework of Te@C− Co−N. Te and C XPS depth profiles were acquired, as shown Figure S3. The contents of Te and C are homogeneously distributed from 0 to 270 nm in the depth direction, which confirms that Te is homogeneously loaded into the C−Co−N instead of accumulating on the surface. The tellurium content in Te@C−Co−N is determined by TGA under N2 atmosphere, as shown in Figure 2a. The weight loss of Te@C−Co−N between 400 and 700 °C corresponds to the evaporation of tellurium. As compared to that of C−Co−N, the content of tellurium in Te@C−Co−N is 77.2 wt %, which is higher than that of previous reports on Li−Te batteries.16,17 Figure 2b shows the XRD patterns of Te, C−Co−N, and Te@ C−Co−N. The diffraction peaks in Te@C−Co−N match well with those of tellurium (JCPDS no. 36-1452), indicating that tellurium is impregnated into Te@C−Co−N. Figure 2c shows the Raman spectra of Te, C−Co−N and Te@C−Co−N. Two characteristic peaks of tellurium at 120, 140 cm−1 correspond to the A1 bond-stretching mode and E bond-stretching mode, respectively. Peaks at 1350 and 1570 cm−1 correspond to the D band and G band of graphite, indicating the graphitic nature of C−Co−N. Brunauer−Emmett−Teller (BET) gas sorptometry measurements were conducted to examine the porous nature of 8147

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Figure 5. (a) CV curves of the Te@C−Co−N cathode in the initial cycle. (b) Contour plot of Raman spectra of the Te@C−Co−N electrode recorded during the initial cycle. (c) CV curves of the Te@C−Co−N cathode in the initial four cycles. (d) Contour plot of Raman spectra of the Te@C−Co−N electrode recorded during the initial four cycles.

in Table S1.14,16−20 The post-mortem SEM image of Te@C− Co−N after 200 cycles at 0.2 C (Figure S5) illustrates that the highly porous C−Co−N can effectively suppress the volume variation of tellurium during charges/discharges. The actual volume expansion of Te in the Te@C−Co−N sample is well below the theoretical value of 104%. The carbon derived from ZIF-8 (C−Zn−N) without cobalt particles was used as control sample in Figure 4c. The better cycling performance of Te@ C−Zn−N as compared with Te@C−Zn−N confirms that Co enhances the adsorption of polytellurides and improves the redox reaction kinetics. The high Columbic efficiency (∼99%) of Te@C−Co−N in LiNO3-additive-free electrolyte confirms the strong confinement to polytellurides by the nanoporous C−Co−N matrix. The rate performance of Te@C−Co−N was conducted under various C-rates from 0.2 to 20 C, as shown in Figure 4d. The Te@C−Co−N cathode exhibits a capacity of 2465.9 mAh cm−3 at 1 C, corresponding to 94.1% of the theoretical capacity of

and C−Co matrixes are calculated by DFT simulations and are given in Figure 3d. The results show that the absorption energy increases dramatically after N and Co are incorporated into the carbon matrix, indicating that the existence of cobalt and nitrogen in C−Co−N strengthens significantly the immobilization of tellurium and polytellurides. Thus, impregnating tellurium into C−Co−N to improve the electrochemical performance in Li−Te batteries is possible in principle. Detailed calculation models are found in Figure S4 (Supporting Information). The Te@C−Co−N cathode was tested at 0.2 C to characterize the cyclic performance, as shown in Figure 4c. The Te@C−Co−N cathode delivers a high initial capacity of 2615.2 mAh cm−3 (theoretical capacity: 2621 mAh cm−3). Even after 200 cycles, the capacity still remains at 2473.4 mAh cm−3, corresponding to a high capacity retention of 94.6%. Such a result is much higher than that of our previous report (capacity retention of 88%) and other tellurium based cathode, as shown 8148

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Figure 6. Calculated adsorption energy of lithium polytellurides species on C−Co−N surface: (a) C−Co−N and Li2Te; (b) C−Co−N and Li2Te2; (c) C−Co−N and Li2Te4; (d) C−Co−N and Li2Te6; (e) C−Co−N and Li2Te8; (f) C−Co−N and Te8. (g) Calculated adsorption energy between Li2Tex and C−Co−N and graphene. (h) EIS spectra of Te@C−Co−N fresh cell and those after 800th cycles.

retention of Te@C−Co−N under the high current density illustrates the mechanical stability of Te@C−Co−N. Noted that the loading of Te@C−Co−N is 77.2%, which is higher than that of 3DGT with a loading of 63.3% in the previous report.17 Such results indicate that the strain and stress in the charge/discharge process is efficiently relaxed by the hierarchically mesoporous and microporous structure of the MOFderived carbon.21 The pronounced cyclic performances can be attributed to the advantageous architecture as well as the cobalt and nitrogen codoped C−Co−N matrix. To reveal the tellurium redox chemistry on the electrolyte/ matrix interface, in operando Raman spectroscopy was employed, and the assignment of Raman peaks was assisted by the characteristic Raman features of telluride species from DFT simulations. The cyclic voltammetry (CV) plots of Te@ C−Co−N in the initial four cycles are conducted in the voltage between 1.0 and 3.0 V at a sweep rate of 0.1 mV s−1. In the meantime, the Raman spectra on the electrode/matrix interface were conducted continuously. The structure information on polytellurides is investigated. As shown in Figure S6, the

tellurium. Even at the ultrahigh C-rate of 20 C, the Te@C− Co−N delivers a capacity of 894.8 mAh cm−3. Intriguingly, the capacity of Te@C−Co−N restores to 2542.9 mAh cm−3 as the C-rate switches from 20 to 0.2 C, indicating the excellent structure stability of the Te@C−Co−N. The pronounced rate performance can be attributed to the highly conductive and porous C−Co−N for efficient transportations of electrons and ions during the fast charge/discharge process. Typically, there exists severe volume change of tellurium due to the high current density charge/discharge process.6,20 It remains challenging to realize a tellurium-based cathode with stable cyclic performances under a high current density. The cycling performance of Te@C−Co−N cathode under the Crate of 1 C was conducted to further demonstrate the advantages of Te@C−Co−N. As shown in Figure 4e, the Te@C−Co−N cathode delivers initial charge and discharge capacities of ∼2602.9 mAh cm−3 (417.1 mAh g−1) and ∼2484.4 mAh cm−3 (398.1 mAh g−1), respectively. Even after 800 cycles, the capacities of charge and discharge can still remain at 2353.6 and 2324.5 mAh cm−3, respectively. Such a high capacity 8149

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the cell still maintains a low impedance, which further confirms the stability of the Te@C−Co−N. In particular, Rct of Te@C− Co−N (57.1 Ω) is also smaller than that of our previous report of 3DGT (132.3 Ω) even after 800 cycles. Such a result confirms that the doped cobalt and nitrogen in C−Co−N facilitate the transportation of electrons and ions during the charge/discharge process.

difference in crystal structures for S, Se, and Te gives rise to the fundamental difference in properties of polysulfides and polytellurides. The optimized structures of both chainlike lithium polytellurides and cyclo lithium polytellurides are given in Figure S7. Only Li2Tex (x ≥ 4) owns two types of molecule configurations. As shown in Table S2, the calculated binding energy values of the cyclo polytellurides are slightly larger as compared with the chain-like polytellurides, indicating that cyclo polytellurides are more energetically stable. According to the in operando Raman spectra of the Te@C−Co−N cathode during the charges/discharges in Figure S8 and the DFTcalculated Raman spectra of polytellurides in Figure S9, the strong peaks at ∼110−150 cm−1 are attributed to solid crystalline Te and Li2Te. Peaks between 150 and 200 cm−1 are assigned to Li2Tex (x = 2−8). Te8 only has one peak at ∼160 cm−1, and thus, the existence of Te8 during charge/ discharge is also possible according to the Raman results. The weak peak at ∼280 cm−1 is assigned to Li2Te2 and cyclo Li2Te6, and the peak at ∼335 cm−1 is assigned to cyclo Li2Te3, Li2Te4, Li2Te5, and Li2Te8. Weak peaks of Li2Te2 and Li2Te3 can be observed at ∼400−415 cm−1. No Raman peaks are found at ∼450 cm−1, indicating the absence of chainlike Li2Tex (x ≥ 4). Figure 5a shows the initial cycle of CV of Te@C−Co−N, and the corresponding in operando Raman spectrum is shown in Figure 5b. According to Figure 5a,b, the reaction in the initial cathodic sweep is explained as follows: the tellurium converts to Li2Te8 and Li2Te6 at the beginning of the lithiation at the potential of 2.7 V.17 With the reaction of tellurium processing, Li2Te4 was formed at the potential of 2.3 V. At 1.75 V, Li2Te4 transforms to Li2Te3 and Li2Te2. Finally, the short-chain Li2Te3 and Li2Te2 convert to Li2Te. During the subsequent anodic scan, at the sharp anodic peak of 1.89 V, the intensity of the Raman shift around 120 cm−1 is still weak, which indicates Li2Te does not immediately convert to tellurium but converts to Li2Te2 and Li2Te3 at the sharp anodic peaks. The characteristic peaks of Li2Te8 and Li2Te6 can be clearly detected at 2.7 V. At the end of the reaction, the Li2Te8 and Li2Te6 transform to tellurium. To further investigate the reaction during the long-term cycling, the in operando Raman spectrum for the first four cycles were conducted as shown in Figure 5c,d. The sharp redox peaks and the Raman feature remain stable with CV circles, which suggests the superior electrochemical stability of Te@C−Co−N cathode. Such results agree well with galvanostatic charge/discharge profiles of Te@C−Co−N. The DFT simulation is employed to calculate the absorption energies of lithium polytellurides species on C−Co−N matrix, which further illustrates the strong immobilization of lithium polytellurides species for C−Co−N matrix, as shown in Figure 6a−g. The calculation results indicate that the C−Co−N matrix exhibits higher chemical adsorption energy to polytellurides compared to the graphene, which suggests that C−Co−N has excellent confinement for soluble polysulfides Li2Ten (n > 2). Such a result further gives the insight into the better cyclic performance than that of our previous report.17 The electrochemical impedance spectroscopy (EIS) of Te@C−Co−N before and after 800th cycles is shown in Figure 6h. Both plots display a semicircle at the high frequency assigned to the charge transfer resistance (Rct) of the electrode and the straight line in the low-frequency region corresponding to a semi-infinite Warburg diffusion process (Zw). In addition, the junction of the semicircle at the real axis in high frequency corresponds to the internal resistance (Rs). It is noted that even after 800 cycles,

CONCLUSIONS In summary, nanoporous Te@C−Co−N is prepared via a meltdiffusion method by incorporating tellurium into cobalt- and nitrogen-doped graphitic carbon as derived from metal−organic frameworks (MOFs). The obtained Te@C−Co−N with advantageous architecture remarkably immobilizes tellurium within the cathode structure through physical and chemical interactions at a molecular level. Benefiting from the synergistic interactions between C−Co−N and tellurium, the Te@C− Co−N composite delivers pronounced electrochemical performances. The cathode owns a reversible initial capacity close to the theoretical value, a high capacity retention even after 200 cycles, and a nearly 100% Coulumbic efficiency. In addition, the cathode exhibits excellent rate performances with a capacity of 894.8 mAh cm−3 even at ultrahigh C-rate of 10 C. Our work demonstrates that Te@C−Co−N composite structures are promising for high-performance lithium tellurium batteries. EXPERIMENTAL SECTION Synthesis of ZIF-67, ZIF-8, C−Co−N, and C−Zn−N. All chemicals and solvents were purchased from commercial sources and used without further purification. In a typical synthesis, 5.238 g of Co(NO3)2·6H2O and 3.955 g of 2-methylimidazole were dissolved in a 200 mL methanol solution. After 2-methylimidazole solution was slowly added into Co(NO3)2·6H2O solution under stirring, the resulting solution was kept at the room temperature for 20 h. The resulted precipitates were collected with centrifugation, washed with ethanol, and finally dried in vacuum at 60 °C. The ZIF-8 was obtained with a similar method by using Zn(NO3)2·6H2O. The as-prepared ZIF-67 and ZIF-8 polyhedrons were annealed in a tube furnace under a N2 flow at 900 °C for 5 h and a heating rate of 10 °C·min−1 to obtain carbonized MOF samples. Finally, the C−Co−N and C−Zn−N samples were obtained by treating black carbonized MOF samples in 2 M HCl for 12 h to partially remove Co and Zn particles, decrease mass density, and obtain extensive pores for hosting tellurium. Synthesis of Te@C−Co−N and Te@C−Zn−N. The obtained C− Co−N samples were mixed with pure tellurium (80 wt %). Then, the samples were placed in a porcelain container. The mixture was annealed 480 °C for 12 h under argon atmosphere, and then the Te@ C−Co−N composite was obtained. Te@C−Zn−N was prepared with a similar way by using C−Zn−N (Figure S10). Characterizations. The component ratio of the composite was determined by thermogravimetric analysis (TA Instruments TGAQ50). The structure of the obtained samples was characterized by Xray diffraction (XRD Rigaku D/MAX-rA diffractometer) using Cu Kα radiation. Raman spectra were acquired at room temperature with a 532 nm excitation laser (Renishaw InVia Reflex). The surface area analysis was conducted using BET theory (Quantachrome, nova 2000e). The morphology investigations were performed by SEM (JSM-7000F, JEOL) and TEM (Tecnai F20 at 200 kV) with an energy dispersive X-ray spectrometer. XPS and XPS depth profiles were acquired with a Kratos Analytical spectrometer at room temperature with monochromatic Al Kα (1486.6 eV) radiation. The depth profiles of the sample were obtained by Ar-ion etching and subsequent measurement. The etching rate of the ion beam is 1 nm s−1 for Te@ C−CoN. Electrochemical Measurements. The Te@C−Co−N cathode was prepared with a conventional slurry coating method. 80 wt % Te@ 8150

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ACS Nano C−Co−N 10 wt % carbon black (Super P Timcal) and 10 wt % polyvinylidene fluoride in N-methyl-2-pyrrolidone were mixed to form a homogeneous slurry. Then the slurry was spread onto an aluminum foil (15 μm, MTI Corp.) and dried at 60 °C in vacuum for 24 h. The tellurium mass loading in the electrode was 2 mg cm−2. The dried cathode was placed on a disk of 14 mm in diameter for assembling cells. Using lithium metal as anode, coin-type (CR2025) cells were assembled in an argon-filled MBraun glovebox with oxygen and water content below 0.5 ppm. Celgard 2400 was used as separator. 1.0 M lithium bis(trifluoromethanesulfonyl)imide in 1,3-dioxolane and 1,2dimethoxyethane (v/v = 1:1) (Zhangjiagang Guotai-Huarong New Chemical Materials Co., Ltd.) were used as electrolyte. Cyclic voltammetry and EIS were conducted by a CHI660D electrochemical workstation (CHI instrument). CV measurements were performed at a scan rate of 0.1 mV s−1 in the voltage range of 1.0 to 3 V. EIS tests were carried out at an open-circuit potential in the frequency range between 100 kHz and 0.01 Hz. Galvanostatic charge−discharge cycles were taken with a LAND CT2001A instrument (Wuhan Jinnuo Electronic Co. Ltd.) at different C rates between 1.0 and 3 V (vs Li+/ Li) at the room temperature. In this study, 1 C corresponds to a current density value of 420 mA g−1 and specific capacity values were calculated based on tellurium mass. In operando Raman spectra were collected simultaneously with CV tests, using a special designed stainless steel cell with a small quartz transparent window. A 785 nm excitation laser was employed with a cumulative time of 50 s in the Raman spectrum acquirement. Computational Methods. The atomic configurations and binding energies between MOF and polytellurides (Li2Tex, 2 ≤ x ≤ 8) were calculated using DFT within the Perdew−Berke−Ernzerh of generalized gradient approximation, as implemented in the Dmol3 package. The double numerical plus polarization basis sets with effective core potential were employed to express atomic potentials. Considering the large size of ZIF-67 cell with 368 atoms, as shown in Figure S4d, a segment of Co(2-Methylimidazole)4 containing 49 atoms (Figure S4e) was used to model its interaction with polytellurides according to the symmetry of ZIF-67. Different initial binding configurations between Co(2-methylimidazole)4 and polytellurides were simulated to obtain optimized binding configurations with the lowest energy. Self-consistent field calculations were carried out until the SCF tolerance was below 1 × 10−6.

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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/acsnano.7b03057. Computational calculations (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yuanfu Chen: 0000-0002-6561-1684 Weidong He: 0000-0001-8242-2888 Author Contributions ⊥

J.H. and W.L. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The research was supported by the National Natural Science Foundation of China (Grant Nos. 51372033 and 21403031), the National High Technology Research and Development Program of China (Grant No. 2015AA034202), and the 111 Project (Grant No. B13042). 8151

DOI: 10.1021/acsnano.7b03057 ACS Nano 2017, 11, 8144−8152

Article

ACS Nano Materials for Rechargeable Lithium-Tellurium Batteries. Adv. Energy Mater. 2015, 5, 1401999. (20) Seo, J.; Seong, G.; Park, C. Te/C Nanocomposites for Li-Te Secondary Batteries. Sci. Rep. 2015, 5, 7969. (21) Li, Z.; Yin, L. Nitrogen-Doped Mof-Derived Micropores Carbon as Immobilizer for Small Sulfur Molecules as a Cathode for Lithium Sulfur Batteries with Excellent Electrochemical Performance. ACS Appl. Mater. Interfaces 2015, 7, 4029−4038.

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DOI: 10.1021/acsnano.7b03057 ACS Nano 2017, 11, 8144−8152