Materials Today Energy 12 (2019) 53e61
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Metalorganic framework derived porous hollow ternary sulﬁde as robust anode material for sodium ion batteries Dongwei Cao a, Weidong Fan a, Wenpei Kang a, *, Yuyu Wang a, Kaian Sun b, Jinchong Zhao b, Zhenyu Xiao a, Daofeng Sun a, ** a b
School of Materials Science and Engineering, College of Science, China University of Petroleum (East China), Qingdao 266580, PR China State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2018 Accepted 30 December 2018
Multiple-metal compound materials have been demonstrated as high performance anode materials for energy storage. In this work, porous hollow polyhedron structured multicomponent sulﬁdes and Ndoped carbon-based composite (MnS-(ZnCo)S/N-C) is prepared through a gas-solid reaction using Mn, Zn, Co-based ternary metalorganic frameworks as precursor. As sodium ion batteries anode, MnS(ZnCo)S/N-C exhibits impressive electrochemical performance, especially for the cycling performance at high current. At a current density of 2.0 A g1, it delivers a capacity of 316 mAh g1 within 400 cycles and a high capacity retention (92.9%) compared with that of the third cycle. Furthermore, the composite electrode maintains a capacity retention of 40% when the current raises from 0.1 to 10.0 A g1 step by step. As a robust sodium storage host, this composite anode beneﬁts from porous hollow polyhedron structures composed of primary nanoparticles, N doped C coating on primary particles, and multicomponent sulﬁdes. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Multicomponent sulﬁde Long-term cycling performance Metal-organic frameworks Sodium-ion battery
1. Introduction Along with the increase of global population, the problem of energy shortage has emerged these years. It is of great signiﬁcance to exploit clean energy, and construct smart grid, for which novel energy storage system plays an important role. Sodium ion batteries (SIBs), have become competitive candidates for large-scale energy storage system compared with lithium ion batteries (LIBs) because of the low cost. But several issues still need to be overcome for SIBs before applied in practice [1e3]. Among the numerous factors, anode materials play a key role on electrochemical performance improvement [4e7]. Unfortunately, many highperformance anode materials that suitable for LIBs cannot be applied directly in SIBs because of the larger diameter of sodium ions (Liþ: 0.76 Å; Naþ: 1.02 Å). In recent years, exploitation of suitable anode materials with higher capacity, durability and rate capacity has proved to be a primary mission in the ﬁeld of SIBs [8,9].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected]
(D. Cao), [email protected]
(W. Fan), [email protected]
(W. Kang), [email protected]
(Y. Wang), [email protected]
(K. Sun), [email protected]
(J. Zhao), [email protected]
(Z. Xiao), [email protected]
(D. Sun). https://doi.org/10.1016/j.mtener.2018.12.013 2468-6069/© 2019 Elsevier Ltd. All rights reserved.
Metal sulﬁdes with nanostructures based on conversion reaction mechanism have attracted our entensive attention as an anode materials for SIBs, due to their higher capacities for transferring multiple electrons per metal center, suitable redox voltage range, and higher intrinsic conductivity compared to metal oxides [10e12]. In order to achieve an enhanced performance, designing metal sulﬁde with carbon-based material as nano-composites have been demonstrated to be effective approaches. On the other hand, for the LIBs, multicomponent metal compounds have been successfully applied as high-performance anode material based on the fact of buffering the volume change by separating into single metal compounds during cycling [13e15]. Considering the success and the advantages of multicomponent metal compounds in LIBs, great efforts have been made to explore multicomponent metal sulﬁdes as anode for SIBs [16e19]. Kim et al. reported a yolk-shell SnS-MnS2 composite anode for SIBs, which exhibited high capacity of 396 mAh g1 for the 100th cycle . Ni3Co6S8-RGO solid-solution nanostructures was also synthesized by Kang‘s group, and delivered high and stable capacities of 504 and 498 mAh g1 in the 2nd and 100th cycle at 0.5 A g1, respectively . The synergistic effect of the multicomponent in the composite can enhance their electrochemical performance. Considering the effective approaches for the anode performance enhancement, including carbon modiﬁcation and nano engineering,
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metal-organic frameworks (MOFs) have been considered as ideal precursors for the preparation of sulﬁde anodes for energy storage. This is because the distinctive advantage of MOFs, where they can be easily designed by incorporating various functional species of metal ions/clusters and organic linkers [20e22]. Moreover, delicate design of MOFs could be used to construct multicomponent metal compounds with designable structures and compositions [23e28]. These MOFs derived materials might exhibit multiple structure- and composition-dependent merits [21,29,30]. Speciﬁcally, the in-situ carbon transformed from the organic linkers, generally composed of heteroatoms-doping, can enhance the electronic conductivity and increase the active sites, offering potential solutions to key challenges in energy storage [31e34]. Liu et al. reported the hollow Ni3S2/Co9S8/N-doped carbon composite derived from a binary MOF, exhibiting a reversible speciﬁc capacity of 419.9 mAh g1 at a current density of 0.1 A g1 after 100 cycles . Yin's group reported [email protected]
core-double shell polyhedron as an anode for SIBs, which shows a signiﬁcant electrochemical performance with stable cycling stability . Carbon coated bimetallic sulﬁde hollow nanocube composite was obtained through sulﬁdation of prussian blue precursors wrapped by PDA . This composite electrode exhibited a capacity of 500 mAh g1 at a current density of 50 mA g1, and retained a capacity of more than 100 mAh g1 at high current density of 5.0 A g1 as an anode for SIBs. Hence, MOFs derived multicomponent sulﬁdes can present enhanced electrochemical performances as anode materials for SIBs. Herein, a stable Mn, Zn, Co-based ternary MOF (MnZnCo-TATB) was synthesized through a facile solvothermal reaction, after calcination, the MOF precursors were transformed into multicomponent sulﬁdes and N-doped carbon-based composite (MnS-(ZnCo) S/N-C) for the ﬁrst time. This composite possessed a porous hollow polyhedron structure inherited from the MnZnCo-TATB precursor. The MnS-(ZnCo)S/N-C were further demonstrated as a highperformance anode material in SIBs. A high capacity of 393 mAh g1 can be obtained after 400 cycles at the current density of 1.0 A g1, exhibiting an excellent cycle stability. The unique porous hollow structure composed of primary nanoparticles can not only provide large surface area but also can tolerate the volume expansion during charge/discharge process. Furthermore, the synergistic effect of in-situ N-doped carbon and the hybridization of multicomponent sulphide in the composite promote the conductivity and the activity of the materials. Our work provides a common method to develop advanced sodium host materials through designing MOFs template with desired structures and compositions. 2. Experimental 2.1. Synthesis of MnZnCo-TATB MOF The MnZnCo-TATB MOFs were synthesized through a modiﬁed procedure based on previous report . 2,4,6-tris(4carboxyphenyl)-1,3,5-triazine (H3TATB), (0.30 g, 6.81 104 mol), Co(NO3)2$6H2O (0.25 g, 8.59 104 mol), Mn(NO3)2 (0.15 mL, 8.59 104 mol) and Zn(NO3)2$6H2O (0.20 g, 6.72 104 mol) were dissolved into 30 mL N, N-dimethylformamide (DMF) to form a homogeneous solution. Then the solution was transferred into a 50 mL Teﬂon-lined stainless steel autoclave and maintained at 120 C for 2 days. After cooling to room temperature, the precipitate was centrifuged and washed with DMF for several times, and dried in vacuum at 80 C for 24 h. 2.2. Preparation of MnS-(ZnCo)S/N-C In a typical synthesis, the obtained pale-pink MnZnCo-TATB powder was sulfurated through a gas-solid reaction with sublimed
sulfur (mass ratio of 1:1) at high temperature. The MnZnCo-TATB was ﬁrstly put in a ceramic crucible with sublimed sulfur powder at the upstream in the tube furnace. The furnace was then annealed at 600 C with a rate of 1 C min1 and kept for 2 h with a Ar/H2 ﬂow gas at ﬂow rate of 10 ml min1. After thermal treatment, black MnS-(ZnCo)S/N-C powder was obtained. 2.3. Preparation of Mn3O4-(ZnCo)O/N-C For comparison, Mn3O4-(ZnCo)O/N-C composite was also synthesized via a two-step method. The obtained ZnMnCo-MOF was calcined at 600 C for 2 h with a heating rate of 1 C min1 in a Ar/ H2 atmosphere, and followed by a calcination at 400 C for 1 day in air. Black Mn3O4-(ZnCo)O/N-C powder was obtained. 2.4. Material characterization and performance measurement X-ray powder diffractions (XRD) of the samples were measured on a Bruker AXS D8 Advance with Cu Ka (l ¼ 1.5418 Å, 40.0 kV, 30.0 mA) radiation. Thermogravimetric analysis (TGA) was per€olos (TG-MS) in an air formed with Hengjiu HCT-1-QMS 403 D Ae atmosphere (10 C min1). X-ray photoelectron spectrometer (XPS, Thermo ESCALAB 250) was used for the analysis of the elements and their oxidation states of the samples. Scanning electron microscope (Philips XL30 FEG SEM) and transmission electron microscope (JEOL JEM-2100 TEM) were used to collect morphology and structure information. Raman spectra were collected via a HORIBA Evolution Raman scope using 532 nm incident wavelengths. The IR spectras were measured on Bruker VERTEX-70 spectrometer. The N2 adsorption-desorption isothermal curves at 77 K, were collected on Micro ASAP2020 to calculate the BrunauerEmmett-Teller (BET) surface area of the samples. The inductive coupled plasma emission spectrometer (ICP) test was carried out on an Optima 8000 ICP-OES after the samples calcined at 600 C in air and dissolved into HNO3. 2.5. Electrochemical measurements In order to prepare the working electrodes, the as-prepared sample was mixed with carbon black and carboxyl methyl cellulose (wt 8% CMC solution, using H2O as solvent) with a weight ratio of 60:20:20, forming a slurry. Then, the slurry was coated on copper foil and dried at 80 C in vacuum for 12 h. Discs with size of 12 mm were cut, using as working electrodes, giving a mass loading of 1.0e1.5 mg of the active material. NaClO4 (1.0 mol L1) dissolved into propylene carbonate with 10% (in volume) ﬂuoroethylene carbonate (FEC) was used as electrolyte. The coin cells were assembled in a glovebox with Ar atmosphere to measure the electrochemical performance. The Neware-5 V 10 mA system (Shenzhen Xinwei) was used to measure the performance of galvanostatic cycling at constant temperature of 25 C. Cyclic voltammetry (CV) curves were measured on the CHI 660E electrochemical workstation (0.0e3.0 V) at scan rate of 0.05 mV s1. Gamry 30115 electrochemical workstation was used to measure the electrochemical impedance spectroscopy (EIS) with frequencies of 0.1 MHze10 MHz. 3. Results and discussion Three isostructural MOFs with a diamondoid structure have been synthesized using 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine (H3TATB) as ligand, and Co2þ, Mn2þ or Zn2þ as metal sites, respectively . Encouraged by the success of constructing H3TATB-based MOFs, in this work, Mn2þ, Zn2þ, Co2þ-based ternary MOFs (MnZnCo-TATB) were synthesized using H3TATB as ligand
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through a modiﬁed solvothermal process. Inspired by the advantages of porous hollow Co3O4/N-C polyhedron derived from the Cobased MOF ([Co6O(TATB)4]$(H3Oþ)2$Py)  and multicomponent metal sulﬁdes/carbon composites as competitive anode materials for SIBs, porpous hollow MnS-(ZnCo)S/N-C polyhedron composite was prepared through a simple gas-solid reaction in a Ar/H2 atmosphere, using MnZnCo-TATB and S powder as precursor and sulfur source, respectively. The preparation process of the MnS(ZnCo)S/N-C polyhedron composite is illustrated in Fig. 1. The MnZnCo-TATB precursor exhibited a polyhedron morphology with a size of several microns, as shown in Fig. S1. The XRD pattern indicated the formation of MnZnCo-TATB with a 3D framework in an orthorhombic space group of Fd-3, as shown in Fig. S2. After a gas-solid reaction with S powder at 600 C, MnZnCoTATB was transformed into sulﬁdes. The sulﬁde product shows polyhedron structures inherited from the MOF precursor, which are well dispersed and a bit nonuniform as shown in the SEM images (Fig. 2a). A broken polyhedron can prove the hollow nature (Fig. 2b). And the magniﬁed SEM images indicated that the micropolyhedrons are composed of small primary particles with nano size (inset in Fig. 2b), resulting in the mesoporous structure. The XRD was measured to determine the phase composition of MnS-(ZnCo)S/N-C. As seen in Fig. 3a, the XRD diffraction patterns of the sulﬁdes can be indexed as a mixture of sulﬁdes including MnS (cubic, #06-0518) and ZnS (hexagonal, #79-2204). Co-based sulﬁde XRD pattern was not indexed. The element Co was considered to be doped in ZnS to form a solid solution-type phase (ZnCo)S, based on the peaks in the XRD shift to a lower angle side compared with standard XRD parten of ZnS, which is attributed to the ionic radius of the Co2þ ions (0.79 Å) is larger than that of the Zn2þ ions (0.75 Å) [37,38]. Therefore, the multicomponent sulﬁdes was denoted as
MnS-(ZnCo)S/N-C. Furthermore, the ratio of Co, Mn and Zn in the sulﬁdes was different to that of the addition into the MOF preparation system. This may be due to the different combination abilities between Co2þ, Mn2þ and Zn2þ ions and the ligands of TATB during the ternary MOFs formation process. The molar ratio of Co, Mn and Zn in the multicomponent sulﬁdes was estimated to be 1:6.03:6.19 based on the ICP results, as shown in Table S1. The sample was further measured by Raman as shown in Fig. 3b. The two main peaks positioned at 1350 and 1586 cm1 are corresponding to the D (disorder) band and G (graphitic) band of carbon, respectively, conﬁrming the existence of carbon [39,40]. The existence of carbon is attributed to the in-situ transformation from the organic linkers in MOFs during the gas-solid reaction process. In addition, the value of the intensity ratio between D and G bands (ID/ IG) was estimated to be 0.96, suggesting the low crystallinity of the carbon in the composite. Thermogravimetric analysis - mass spectrometry (TGA-MS) curves of the sample was shown in Fig. 3c. The composite experienced three decomposition processes. The ﬁrst step was below 400 C, and the corresponding weight loss should be ascribed to the loss of adsorbed water and a small amount of sulfur loading into the pores of the porous sulﬁdes. The second step was between 400 and 590 C, the weight loss is due to the decomposition of carbon and oxidation of sulﬁde in the composite based on the single CO2 and SO2 in the MS. The third weight loss was above 680 C, attributed to the oxidation of the other sulﬁde in the composite. Two SO2 singles were detected in the MS, indicating the different thermal stability of MnS and (ZnCo)S in the composite. Based on the total weight loss (46.8%) in the TGA curve and the ICP result, the carbon content in the MnS-(ZnCo)S/N-C composite can be estimated to be 25.5%, which is calculated in detail in the supporting information. BET analysis is used to
Fig. 1. Schematic illustration of the MnS-(ZnCo)S/N-C hollow polyhedron fabrication procedure.
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Fig. 2. SEM images of the MnS-(ZnCo)S/N-C composite.
estimate the speciﬁc surface area of MnZnCo-TATB MOFs and MnS(ZnCo)S/N-C, as shown in Fig. 3d. The surface area of MnZnCo-TATB was estimated to be as high as 693 m2 g1, which decreased to 60 m2 g1 for MnS-(ZnCo)S/N-C. This can be attributed to the microstructure collapse of MOFs during heat-treatment. The microstructure of the MnS-(ZnCo)S/N-C composite was additionally measured using TEM as shown in Fig. 4. The intergranular space among the black dot can be seen in Fig. 4a and b, further conﬁrming the microporous structures of MnS-(ZnCo)S/N-C composite interconnected by small primary particles. The area marked by red dashed line in the high resolution transmission electron microscopy (HRTEM) images as shown in Fig. 4c can be ascribed to the amorphous carbon surrounding the crystalline particles. In the HRTEM images, the lattice fringes with crystalline spaces of 0.331, 0.313, and 0.298 nm corresponded to the (100), (002) and (101) planes of hexagonal (ZnCo)S solid-solution phase, respectively, while the lattice fringes with a spacing of 0.186 nm are indexed as (220) plane of the cubic MnS phase. The diffraction rings corresponding to (ZnCo)S and MnS planes in the selected area electron diffraction (SAED) as shown in Fig. 4d prove their
polycrystalline properties. The HRTEM and SAED analysis further prove the co-existence of MnS and (ZnCo)S in the MnS-(ZnCo)S/N-C composite. The surface information and chemical states of the multicomponent MnS-(ZnCo)S/N-C composite were measured by XPS as shown in Fig. 5. In the survey spectrum (Fig. 5a), six elements including Co, Mn, Zn, S, C, N can be found. For the Zn 2p core level XPS spectrum (Fig. 5b), the peaks at 1021.7 and 1044.5 eV correspond to Zn 2p3/2 and Zn 2p1/2, respectively, proving the presence of Zn2þ in the MnS-(ZnCo)S/N-C composite. As shown in Fig. 5c, the Mn spectrum was located at 641.3 eV and 653.5 eV with a spinenergy separation of 12.2 eV, corresponding to the characteristic peaks of Mn 2p3/2 and Mn 2p1/2, respectively [41e43], and two satellite peaks were also observed. As shown in Fig. 5d, the characteristic peaks of Co2þ2p1/2 and Co2þ2p3/2 are located at 797.7 eV and 780.2 eV, and another two peaks at 778.2 eV and 795.4 eV should be originated from Co3þ2p3/2 and Co3þ2p1/2, respectively . Two satellite peaks at 801.7 and 781.5 eV can also be observed. The spectrum in Fig. 5e has two distinct peaks of 161.3 and 162.4 eV for S 2p3/2 and S 2p1/2, showing the existence of S2 ions [41,45]. As
Fig. 3. (a) XRD patterns (b) Raman spectra and (c) TGA-MS curves for the as-prepared MnS-(ZnCo)S/N-C composite; (d) Comparison of N2 adsorption-desorption isotherms curves for MnS-(ZnCo)S/N-C and MnZnCo-TATB.
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Fig. 4. (a, b) TEM images, (c) HR TEM image and (d) SAED patterns of the MnS-(ZnCo)S/N-C composite.
shown in Fig. 5f, C 1s spectrum can be ﬁtted into four peaks, the strongest peak at 284.5 eV is attributed to the C-C bonds in the composite, while the following peak at 285.2 eV can be attributed to the presence of C-N bonding. The remaining two peaks at 286.1 eV and 288.3 eV can be assigned to a trace amount of carboxyl in the material. And the peaks in N 1s spectrum (Fig. 5g), locating at 398.6 eV and 400.1 eV, can be attributed to pyridine and pyrrole N species, respectively , indicating the formation of N-doped C. This was further proved by FTIR (Fig. 5h), giving two peaks at ~1615 and ~1380 cm1, which correspond to C]N and CN bond, indicating the existence of N-doped C (N-C) derived from the ligands of TATB [46e48]. The sodium storage capacity of the MnS-(ZnCo)S/N-C was evaluated in half cells. Fig. 6a shows the CV proﬁles of MnS-(ZnCo) S/N-C anode at a scan rate of 0.05 mV s1 for the initial ﬁve cycles from 0.0 to 3.0 V vs. Na/Naþ. In the cathodic process, the broad wave at 0.58 V can be contributed to the Naþ insertion into the sulﬁdes process and the formation of solid electrolyte interface (SEI) layers, which result in irreversible capacity loss and low columbic efﬁciency (CE) in the ﬁrst cycle. The mainly strong peak located at 0.12 V was due to the electrochemical reduction reactions of MnS and cobalt doped (ZnCo)S [49e51]. For the reversible andoic process, the peaks corresponding to multistep oxidation formation of MnS and (ZnCo)S were observed at 0.93 and 1.61 V [52,53]. In the following cathodic process, a shoulder peak corresponding to Naþ insertion into the sulﬁdes was also observed at a high potential of 0.87 V. The reduction peaks were divided into two peaks locating at 0.36 V and 0.06 V, respectively. For the andoic process, the multistep oxidation peaks are similar to that for the ﬁrst cycle except for moving to a higher potential due to the active process in the ﬁrst cycle. What's more, an additional peak at 0.09 V was observed, which may be due to the extraction of Naþ for the formation process of sulﬁdes. After the 1st cycle, the CV peaks were almost overlapped, indicating a good cycling stability for the MnS-(ZnCo)S/ N-C composite electrode. Fig. 6b showed the selected discharge/charge proﬁles for the 1st, 2nd, 10th, 50th and 100th cycles at 1.0 A g1 (0.05 A g1 for the ﬁrst cycle) for the MnS-(ZnCo)S/N-C composite electrode. For the ﬁrst
discharge curve, a long slope below 0.7 V are observed. From the second cycle onwards, the slopes move upward and become steeper within 100 cycles. The charge/discharge curve changes originated from the SEI formation on the electrode interface and electrode activation during the ﬁrst cycle, and rearrangement and varied Na-intercalation mechanism, which is a common phenomenon for SIBs [54e56]. In the ﬁrst cycle, the discharge capacity is 770.7 mAh g1 with a coulombic efﬁciency of 69.1%. The irreversible capacity loss is common for most anode materials, which is mainly attributed to the inevitable formation of SEI layer . After the ﬁrst cycle, the discharge-charge curves showed nearly identical change trend and delivered stable capacities. In addition, the subsequent CEs quickly increase to 89% for the second cycle and ~99.6% in the following cycles, indicating good reversibility. In order to further study the electrochemical performance of the multicomponent MnS-(ZnCo)S/N-C composite anode for SIBs, the cycling performance of MnS-(ZnCo)S/N-C composite electrode were examined and compared with Mn3O4-(ZnCo)O/N-C, as shown in Fig. 6c and Fig. S3. At current of 500 mA g1, The discharge capacity in the ﬁrst cycle is 760 mAh g1 with a coulombic efﬁciency of 77.7%. After ﬁrst cycle, the capacities showed a stable trend and gave a value of 412 mAh g1 after 100 cycle (Fig. S4). In order to be well cycled at high current of 1.0 or 2.0 A g1, the electrodes were activated at a low current density of 0.05 A g1 in the ﬁrst cycle. When cycled at current of 1.0 A g1, MnS-(ZnCo)S/N-C anode delivered a capacity of 396 mAh g1 in the third cycle. And the discharge capacities showed a decreased trend in the initial 15 cycles. After that, the capacities increased step and step, and reached a value of 409 mAh g1 in the 100th cycle. The increased capacities were mainly due to the electrolyte penetrating into the inner parts of the porous hierarchical nanostructures, leading to the inner parts undergoing an electron transfer process. And in the following cycles, stable capacities with high CEs up to ~99% were delivered. After 400 cycles, a capacity of 353 mAh g1 was achieved, giving an impressive capacity retention of 89.1% compared with that for the third cycle. The high cycling stability may be attributed to the stable hollow porous polytron structure, which is proved by the SEM image after discharge-charge cycles (Fig. S5). While for the
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Fig. 5. (a) XPS spectra for the MnS-(ZnCo)S/N-C: (b) Zn 2p, (c) Mn 2p, (d) Co 2p, (e) S 2p, (f) C 1s and (g) N 1s level spectra; and (h) the FTIR spectra of MnZnCo-TATB MOF and MnS(ZnCo)S/N-C composite.
Mn3O4-(ZnCo)O/N-C composite, only a low capacity of 14 mAh g1 can be obtained in the 400th cycle measured at the same condition. In additional, the cycling performance were further measured at higher current of 2.0 A g1 as shown in Fig. 6c. The capacity showed a similar trend to that at a current of 1.0 A g1. The electrode delivered a capacity of 316 mAh g1 in the 400th cycle with a high capacity retention of 92.9%. The rate capacity was also measured at different current densities and depicted in Fig. 6d. When the measured current densities
were set to be 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0 A g1, an average capacities of 427, 418, 393, 353, 310, 239 mAh g1 were exhibited, respectively. After that, a capacity of 170 mAh g1 can still be preserved even at a high rate of 10 A g1. As a result, the MnS(ZnCo)S/N-C composite electrode gives a capacity retention up to 40% when current raises from 0.1 to 10.0 A g1. It is worth noting that the capacity can be recovered to 412 mAh g1 when the measured current decreased to be 0.1 A g1. This indicates that the MnS-(ZnCo)S/N-C anode can provide a good ion or electron
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Fig. 6. (a) CV, (b) typical charge-discharge proﬁles at 1 A g1, (c) cycling performance, and (d) rate capability for MnS-(ZnCo)S/N-C and Mn3O4-(ZnCo)O/N-C as SIB anode material, (e) capacity comparison of the MnS-(ZnCo)S/N-C anode and other metal sulﬁde and carbon-based anodes at various current densities including 3D [email protected]
, MnS/RGO , PBC1-1S , N-doped MoS2/C , CS-RGO , [email protected]
mobility even during high current rate cycling. Compared with other sulﬁde anode (Fig. 6e), this multicomponent MnS-(ZnCo)S/NC anode showed superior performance. To explain the excellent cycling performance and rate capability, the electrochemical impedance spectroscopy (EIS) spectrum of MnS-(ZnCo)S/N-C and Mn3O4-(ZnCo)O/N-C were measured over the frequency range of 0.1 Hze100 kHz and plotted in Fig. 7. All the plots exhibit semicircles in the high-frequency region and straight lines in the low-frequency region. And the ﬁtted equivalent circuit model is inserted in Fig. 7. Impedance Rs, and Rct displayed the essential impedance, and charge transfer impedance of Naþ through the electrode/electrolyte interface, respectively . The CPE originated from the capacitance arising from the surface of the interface of electrolyte and electrode. And Zu showed the diffused impedance in the electrode material. For the anode materials, Rct will determine their conductivity . The Rct values for MnS-
(ZnCo)S/N-C composite anode and Mn3O4-(ZnCo)O/N-C were determined to be ~468 and ~143 U after 1st cycle, respectively, indicating the fast charge transfer of sulﬁde composite. The Nyquist plots of MnS-(ZnCo)S/N-C anode value were further studied after different cycles (Fig. 7b). The Rct values after the 1st cycle decreased signiﬁcantly compared with that for the fresh cell, which increased slightly from the second cycle and then stabilized at ~175 U after 10th cycle, showing that the Rct tends become stable after two cycles due to the full wetting of the electrode. The EIS results can prove superior electrochemical performance of MnS-(ZnCo)S/N-C composite anode. The superior electrochemical performance of the MnS-(ZnCo)S/ N-C composite electrode can be attributed to its unique structure and its multiple components. First, the porous hollow polyhedron structure of MnS-(ZnCo)S/N-C assembled by primary tiny particles can not only decrease the diffusion energy barriers to obtain fast
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Fig. 7. Nyquist plots of (a) the 1st cycle in the frequency range of 100 kHz to 0.01 Hz for the comparison of MnS-(ZnCo)S/N-C and Mn3O4-(ZnCo)O/N-C composites; (b) MnS-(ZnCo) S/N-C anodes after different number of charge/discharge cycles.
Naþ insertion/extraction reaction kinetics, but also provide enough space to endure volume changes upon cycling. Second, N doped carbon coating on the surface of primary particles can provide more active sites for electrons transferring, resulting to increased conductivity. Third, the synergetic effects between the various components in the sulﬁdes can enhance and stabilize the sodium storage. 4. Conclusions In summary, using novel ternary MnZnCo-TATB MOF as a precursor, multicomponent sulﬁdes MnS-(ZnCo)S/N-C composite with hollow polyhedron structure was obtained through one-step solidgas reaction. The MnS-(ZnCo)S/N-C composite exhibited superior sodium ion storage performance compared to the Mn3O4-(ZnCo)O/ N-C composite using the same MOF as precursor. The composites can deliver stable capacity of ~353 mAh g1 at 1.0 A g1 after 400 cycles. And at a high current of 2.0 A g1, a capacity of 316 mAh g1 can be obtained in the 400th cycle with a high capacity retention of 92.9%. The good electrochemical properties of this composites can be ascribed to the synergistic effect among porous hollow polyhedron structures, N doped C coating and the multicomponent sulphides. Our results prove that the multi-metal-based MOFs could be selected as ideal precursor candidates for electrode materials with long life and high-power in the ﬁeld of secondary batteries. This could open a new avenue to the design and synthesis of advanced sodium host materials for high performance SIBs. Data Availability The raw data required to reproduce these ﬁndings are available from the corresponding author on reasonable request. Acknowledgements Dongwei Cao and Weidong Fan contributed equally to this work. This work was ﬁnally supported by the National Natural Science Foundation of China (NSFC Grant Nos. 51702366, 21571187), Taishan Scholar Foundation (ts201511019), Natural Science Foundation of Shandong Province (ZR2017BB046) and the Fundamental Research Funds for the Central Universities (17CX02037A). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2018.12.013.
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