Metal vacancies abundant Co0.6Fe0.4S2 on N-doped porous carbon nanosheets as anode for high performance lithium batteries

Journal Pre-proof Metal vacancies abundant Co0.6Fe0.4S2 on N-doped porous carbon nanosheets as anode for high performance lithium batteries Guangming Wang, Hailong Yue, Yakun Xu, Rencheng Jin, Qingyao Wang, Shanmin Gao PII:

S0013-4686(19)32225-X

DOI:

https://doi.org/10.1016/j.electacta.2019.135353

Reference:

EA 135353

To appear in:

Electrochimica Acta

Received Date: 2 October 2019 Revised Date:

12 November 2019

Accepted Date: 22 November 2019

Please cite this article as: G. Wang, H. Yue, Y. Xu, R. Jin, Q. Wang, S. Gao, Metal vacancies abundant Co0.6Fe0.4S2 on N-doped porous carbon nanosheets as anode for high performance lithium batteries, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135353. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphic Abstract Metal vacancies abundant Co0.6Fe0.4S2 anchored on N-doped porous carbon nanosheets have been fabricated, which delivers excellent cycle stability and rate capacity.

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Metal vacancies abundant Co0.6Fe0.4S2 on N-doped porous carbon nanosheets as anode for high performance lithium batteries Guangming Wang, Hailong Yue, Yakun Xu, Rencheng Jin,* Qingyao Wang, Shanmin Gao* a

School of Chemistry & Materials Science, Ludong University, Yantai 264025, P. R. China. E-mail: [email protected], [email protected]

Abstract: Transition metal sulfides have been considered as promising anodes for lithium batteries for its high theoretical specific capacity. However, the lower conductivity, larger volume change and slower electrochemical reaction dynamics still hinder its practical applications. Here, Co0.6Fe0.4S2 nanoparticles with metal vacancies anchored on the N-doped carbon nanosheets ([email protected]) are designed and fabricated by a facile hydrothermal method. The [email protected] delivers high reversible capacity, stable cyclability and excellent rate capacity by retaining the capacities of 830 mAh g-1 after 100 cycles at 200 mA g-1 and 696 mAh g-1 after 500 cycles at 5000 mA g-1. The improved electrochemical performance is attributed to the metal vacancies, N-doped carbon nanosheets and strong metal-nitrogen bonds between the Co0.6Fe0.4S2 and N-doped carbon nanosheets, which greatly enhance charge transfer/transport and maintain structural integrity. The construction of metal vacancies along with N-doped carbon wrapping can be extended to prepare other transition metal sulfides for enhancing the electrochemical performance. Keywords: Transition metal sulfides; N-doped carbon nanosheets; Metal vacancy; Anode; Lithium batteries.

1

1. Introduction With the rapid development of intelligent devices and electric vehicles, the cruising ability of the lithium-ion batteries (LIBs) is of vital importance.[1-3] The capacity of LIBs is dominated by the electrode materials, thus, the commercial graphite with lower theoretical specific capacity (372 mAh g-1) cannot adapt the current market requirement. Therefore, searching new electrode materials with high specific capacity and energy densities becomes a hotspot. As an alternative, transition metal sulfides such as CoSx,[4-6] MoS2,[7-9] SnSx,[10, 11] VS2[12, 13] and etc. are considered as promising anodes to replace the commercial graphite due to the high theoretical specific capacity. Among these candidates, cobalt sulfides have been attracted much attention for its low cost and high theoretical capacity.[6, 14-17] However, the inferior performance originated from large volume changes and poor electronic conductivity still inhibits their practical applications. To avoid the issues, significant efforts including fabricating various nanostructures,[18-20] construction of nanocomposites with conductive additive (carbon,[21-23] graphene,[5, 15, 24, 25] carbon nanotubes[6, 26-28]) are developed and the electrochemical performances are effectively improved. Compared to simple transition metal sulfides, the ternary transition metal sulfides possess better electric conductivity and electrochemical activity, which are benefit for accelerating the charge transfer, thus enhancing the electrochemical properties.[29, 30] Therefore, construction of ternary cobalt sulfides based anodes with conductive additive is still crucial and desired. Recently, building defects in the electrode materials is another approach to improve 2

the electronic conductivity and electrochemical activity, which accelerate the ion/electronic transport and increase the ion diffusion coefficients.[31-35] As a consequence, some electrode materials with defects are fabricated and their electrochemical performances are investigated. For instance, nickel cobaltite nanosheets with oxygen vacancies have been prepared and deliver a high capacity of 309.2 mAh g-1 at the current density of 6.0 A g-1 as a cathode for Zn-ion batteries.[36] Xu et. al have been constructed SnO2 ordered arrays with oxygen vacancies, showing a specific capacity 220 mAh g-1 at high rate of 1 A g-1 after 800 cycles.[32] TiO2 nanobelts with defects display the remarkable capacity retention of 94.4% after 5000 times even at high rate of 10 C.[37] Oxygen-vacancy abundant Co3O4/graphene nanocomoposties are fabricated and the high capacitance retention of 916.5 F g-1 can be achieved at the current density of 10 A g-1 as supercapacitor.[38] However, the cobalt sulfides with defects are rarely explored. Herein, for the first time, the metal vacancy abundant Co0.6Fe0.4S2 on N-doped porous carbon nanosheets ([email protected]) is designed as an anode for high performance LIBs. The metal vacancies generate in the Co0.6Fe0.4S2 through the introduction of iron cations. Such special structures possess various advantages: 1) the porosity of the [email protected] increases the contact area between the electrodes and electrolyte and shortens the ion diffusion length. 2) The metal vacancies and N-doped carbon nanosheets can effectively increase the electronic conductivity and facilitate the kinetics of Li-ion diffusion. 3) The porous carbon nanosheets and the separated Co0.6Fe0.4S2 nanoparticles accommodate the volume change and hinder the 3

aggregation of the materials. When evaluated as anode for LIBs, the [email protected] exhibits excellent rate capacity and longterm cycle stability, delivering the discharge capacity of 696 mAh g-1 after 500 cycles at high rate of 5000 mA g-1. 2. Experimental section 2.1 Preparation of N-doped carbon nanosheets (NC). 1 g of hexamethylenetetramine was dissolved into 50 mL absolute ethanol denoted as solution A. 2.1 g of zinc nitrate hexahydrate was dissolved into 30 mL absolute ethanol denoted as solution B. Then B was slowly dropped into A at room temperature without stirring. The obtained white precipitates (HMT-Zn) were washed with absolute ethanol and dried at vacuum at 60 o

C for 6 h. Subsequently, the dried precursor was annealed in the argon atmosphere at

400 oC for 20 min and then increased to 900 oC for 2 h with a heating rate of 5 oC min-1. 2.2 Preparation of [email protected]. The [email protected] was synthesized through hydrothermal approach. 15 mg of the NC was dispersed into 20 mL distilled water and ultrasonically treated for 30 min. 0.5 mmol of CoCl2·6H2O, 0.5 mmol of FeCl3·6H2O and 20 mmol of thiourea were introduced into above solution. After that, the mixture was transferred into 25 mL of Teflon-lined stainless-steel autoclave and maintained at 180 oC for 12 h. The [email protected] was collected, washed with distilled water and ethanol for several times and dried at 60 oC under vacuum overnight for further characterization. For comparison, Co0.6Fe0.4S2 was fabricated through the same hydrothermal method without adding NC. In the absence of NC and FeCl3·6H2O, pure CoS2 was synthesized. 4

2.3 Materials characterization. The composition, oxidation state and microstructure of the samples were determined by X-ray diffraction (XRD, Rigaku D/Max-2550pc), inductively coupled plasma spectrometry (ICP, NEXION300), Elemental analysis (Vario MICRO cube), X-ray photoelectron spectroscopic (XPS, Thermo Fisher Scientific ESCALAB 250Xi+), Raman spectra (inVia, Renishaw, UK), field-emission scanning electron microscopy (SEM, ZEISS SIGMA 500) and transmission electron microscopy (TEM, FEI Technai G2 S-Twin). Nitrogen absorption-desorption tests were performed on Micromeritics ASAP 2460 surface area detecting instrument. The presence of metal vacancies was examined by electron spin resonance (ESR) spectra on JEOL JES-FA200 ESR spectrometer at 77 K. 2.4 Electrochemical measurements. To prepare the working electrode, 15% of polyvinylidene difluoride (binder), 15% of carbon black (conducting agent) and 70% of [email protected], Co0.6Fe0.4S2 or CoS2 (active materials) were mixed in certain amount of N-methyl-2-pyrrolidone and constantly stirred for 8 h to form the homogeneous slurry. The slurry was spread onto a clean copper foil and dried at 100 o

C for 12 h under vacuum. The cutting discs were used anode pieces, and the diameter

and the mass loading of each electrode are about 14 mm and 0.8 mg cm-2, respectively. CR2025 coin type cells were assembled in a glove box fulfilled with highly pure argon gas, in which the pure lithium foil was applied as counter and reference electrode, Celgard 2400 as separator and 1.0 M LiPF6 dissolving in ethyl carbonate/dimethyl

carbonate

(1:1

v/v

ratio)

as

electrolyte.

Galvanostatic

discharge/charge performance was evaluated on a NEWARE BTS-3008 testing 5

system. Cyclic voltammetry (CV) profiles (0.01-3 V) and electrochemical impedance spectra (EIS) with the frequency of 100000-0.01 Hz were collected on a CHI-660E electrochemical workstation. 3. Results and discussion The [email protected] nanocomposites have been fabricated through a facile hydrothermal method using NC as template, the detailed growth strategy is illustrated in Fig. 1. In the first step, the HMT-Zn precipitates generate after mixing the zinc nitrate and hexamethylenetetramine in ethanol. Fig. S1a presents the XRD patterns of the HMT-Zn, all the diffraction peaks can be assigned to the previous reported HMT-Zn metal-organic coordination frameworks (MOFs).[39] The morphology of the obtained HMT-Zn MOFs is shown in Fig. S2a and S2b, which displays the sheet like structure. After a facile calcination process at 900 oC in an argon atmosphere, the HMT-Zn MOFs can be converted into carbon nanosheets. As can be seen in Fig. S3a and 3b, the resulted sample almost maintains the original morphology of HMT-Zn MOFs. The TEM image in Fig. S3c further confirms the nanosheets of the carbon. Meanwhile, many nanopores can be observed on the nanosheets (Fig. S3d). During the calcination process, Zn2+ cations can be reduced to zinc nanoparticles. Subsequently, the zinc nanoparticles evaporate from the surface of carbon nanosheets and the nanopores generate. The carbon nanosheets with functional groups can be applied as template to construct the carbon/metal sulfides nanocomposites. In our wok, the [email protected] nanocomposites are fabricated by a simple hydrothermal method using the carbon 6

nanosheets as template (Fig. 1). Fig. 2a and 2b depicts the SEM images of the [email protected] nanocomposites, numerous nanoparticles with the diameter of 30-100 nm are anchored on the porous carbon nanosheets. The TEM images (Fig. 2c and Fig. 2d) exhibit the obvious nanoparticles are tightly spliced onto the surface of carbon nanosheets, which is in accordance with the SEM observation. The high resolution TEM images reveal that the d111 and d200 interlayer spacing of Co0.6Fe0.4S2 nanoparticles are expanded to 0.342 and 0.281 nm, respectively (Fig. 2e and 2f). Such enlarger interplanar distance may be ascribed to metal vacancies aroused by the introduction of the Fe3+, which is beneficial to accommodate lithium ion insertion/extraction. Additionally, the EDX mapping results of [email protected] indicate the homogenous distribution of element Co, Fe, S, C and N (Fig. 2h). When no carbon nanosheets are introduced to the reaction system, only aggregated nanoparticles can be achieved (Fig. S4a and S4b). It should point out that the introduction of Fe3+ has much effect on the final morphology of the sample. When no FeCl3 is introduced into the reaction system, micro-flowers constructed by nanosheets are achieved (Fig. S4c).When the molar ratio of Co/Fe becomes 9/1, some micro-flowers combined with nanoparticles are observed (Fig. S4d). As presented in Fig. S4e, the micro-flowers continue to reduce with the increasing of the nanoparticles. And almost no micro-flowers can be seen when the molar of Co/Fe is equal to 1 (Fig. S4a and S4b). Interestingly, the micro-flowers are hard to anchor onto the surface of NC. Further increased the molar of Co/Fe to 2/3, some impurities appear. The crystal structure of the obtained samples is characterized by XRD patterns. As 7

presented in Fig. 3a, all the diffraction peaks match well with the cubic CoS2 (JCPDS no. 41-1471) in the absence of carbon nanosheets. For [email protected], the wide peak at ~28o can be observed except the cubic CoS2, which is attributed to the (002) plane of graphite. Fig. 3b shows the Raman spectra of the Co0.6Fe0.4S2, [email protected] and carbon nanosheets. In the Co0.6Fe0.4S2 and [email protected] nanocomposites, the peak at 365 cm-1 matches well with the characteristic of CoS2.[40] The weak peaks centered at 1000 cm-1 and 637 cm-1 may be attributed to the Co-N and Fe-N, respectively.[41] For [email protected] and carbon nanosheets, two characteristic broad peaks at 1334 and 1592 cm-1 are ascribed to the disordered D band and the graphitic G band of carbon materials, respectively.[42, 43] Compared with the two materials, the D/G intensity ratio of [email protected] is higher than that of carbon nanosheets, suggesting the decreased sp2 carbon domain when Co0.6Fe0.4S2 nanoparticles are anchored on the surface of carbon nanosheets.[44] For the Co0.6Fe0.4S2 nanocomposites, some Co2+ cations can be substituted by Fe3+ cations in the hydrothermal conditions. To balance the valence state of the products, some metal vacancies generate (Fig. 3c). The electron spin resonance (ESR) spectroscopy further proves the existence of the metal vacancies. As can be seen in Fig. 3d, the strong signal peak at g value of 1.996 is attributed to the formation of the metal vacancies. To examine the surface oxidation state of the sample, X-ray photoelectron spectroscopy (XPS) analysis is conducted. Fig. 4a indicates that Co, Fe, S, C and O coexist in the two samples. And the N signal can be seen in [email protected] nanocomposite, implying the existence of N-doped carbon nanosheets. Such N 8

abundant carbon nanosheets can coordinate with metal cations to promote the growth of the metal sulfides. In the high Co 2p spectrum (Fig. 4b), the binding energies at 782.1 and 794.3 eV are attributed to the Co 2p3/2 and Co 2p1/2, respectively.[38, 45] And two satellites can be seen at 788.4 and 805.1 eV, respectively. In addition, the binding energy at 778.8 eV related to Co-N bonds can be observed for [email protected].[46, 47] The high resolution Fe 2p spectra demonstrate that the Fe 2p3/2 and Fe 2p1/2 appear at the binding energies of 707.2 eV and 720.4 eV, respectively, in accordance with the binding energies of Fe3+.[48, 49] The S 2p core-level XPS spectra reveal that the S 2p3/2 and S 2p1/2 appear at 162.9 and 164.3 eV, respectively.[50] The C 1s deconvoluted spectra indicate that C-N bond with the binding energy of 288.9 eV can be observed except the C-C and C=C bonds for [email protected] (Fig. 4e).[47, 50] From N1s spectrum, the binding energy at 398.4 eV can be attributed to the metal-nitrogen bonds,[43, 51] suggesting the strong chemical bonds between the N doped carbon nanosheets and Co0.6Fe0.4S2. And the peaks located at 400.2, 401.4 and 402.3 eV represent pyridinic, pyrrolic and quanternary N, respectively.[52] The exact composition of the samples is detected by ICP. Based on the ICP analysis, the atom ratio of Co Fe, and S is equivalent to 1:0.671:3.282 (Table S1). Based on the elemental analysis and ICP measurement, the weight content of the C, N, Co, Fe, and S are 15.53 wt%, 2.61 wt%, 23.92 wt%, 15.22 wt%, 42.73 wt%, respectively. The porous feature of the final samples is investigated by nitrogen gas adsorption measurements at 77 K. The adsorption isotherms in Fig. S5a demonstrate that the two 9

samples show type IV isotherms with H3 typed hysteresis loop within the pressure of 0.6-1.0, indicating the microporous and mesoporous feature. The pore size distribution is shown in Fig. S5b, which further ascertains the existence of micropores and mesopores. Based on the Brunauer-Emmett-Teller (BET) method, the calculated specific surface area and pore volume of [email protected] are 128.58 m2 g-1 and 0.2602 cm3 g-1, respectively, which are much higher than those of Co0.6Fe0.4S2 (3.2 m2 g-1 and 0.0073 cm3 g-1, respectively). Such higher surface area and the presence of the mesopores can effectively increase the contact area and provide more space for the volume change during cycling. The electrochemical performance of the [email protected] and Co0.6Fe0.4S2 electrodes is evaluated by cyclic voltammetry (CV). Fig. 5a and Fig. S6a show the CV profiles of the first five cycles at a scan rate of 0.2 mV s-1 within the potential range of 0.01-3 V. In the first cathodic scan, the obvious peak centered at 1.08 V is attributed to the reduction of Co2+ and Fe3+ to metal Co and Fe.[16, 53] The weak peak exhibited at 0.55 V is related to the decomposition of electrolyte as well as the formation of solid electrolyte interface (SEI), which is common for the transition metal sulfides electrode materials.[27, 54] In the subsequent oxidation process, two peaks at 2.06 and 2.54 V represent the oxidation process of Co and Fe to Co2+ and Fe3+, respectively.[21, 55] After the first cycle, the reduction peaks shift to 1.98 and 1.31 V, which correspond to conversion of FeSy (CoSy) to Li2FeS2 (Li2CoS2) and then to Fe and Co, respectively.[21, 53] Compared with the CV curves of two electrodes, the relatively fast peak current decrease indicates the poor cycle stability of the 10

Co0.6Fe0.4S2. Fig. 5b shows the galvanostatic charge-discharge profiles of the [email protected] at 200 mA g-1 between 0.01 and 3 V. The initial discharge and charge capacities are 1552 and 1313 mAh g-1, respectively, resulting in the Coulombic efficiency of 84.6%. In contrast, the initial discharge and charge capacities are 1080 and 792 mAh g-1, respectively, and the Coulombic efficiency is ~73.3%. The capacity loss of the electrodes is ascribed to the irreversible processes including the electrolyte decomposition and the formation of SEI layer. For comparison, the [email protected] electrode displays much higher discharge capacity than that of Co0.6Fe0.4S2, which maybe attributed to the high surface area of the [email protected]. To further demonstrate the lithium storage performance of the electrodes, we compare the three electrodes at same conditions. As can be seen in Fig. 5c and Fig. S7a, the [email protected] exhibits better cycle stability, achieving the specific capacity of 830 and 743 mAh g-1 after 100 cycles at the current density of 200 and 500 mA g-1, respectively. For the Co0.6Fe0.4S2, the capacity can be retained at 450 mAh g-1 at 100th cycle. Except the morphology, the introduction of Fe has much effect on the electrochemical performance. As shown in Fig. S7b, the specific capacities of 541 and 496 mAh g-1 after 100 cycles for Co0.9Fe0.1S2 and Co0.7Fe0.3S2, respectively, which are much higher than that of pure CoS2 (70 mAh g-1 after 100 cycles). To better understand the advantage of the metal vacancies and N-doped carbon nanosheets in lithium storage, the discharge capacities at various current densities are compared (Fig. 5d). At the current density of 200, 500, 1000, 2000, and 5000 mA g-1, the 11

[email protected] delivers the specific capacities of 993, 864, 712, 531, 466 mAh g-1, respectively, and the capacity can be recovered after reducing back to 100 mA g-1. While the Co0.6Fe0.4S2 displays lower values at all rates (603, 441, 298, 207, 140 mAh g-1 at 200, 500, 1000, 2000, and 5000 mA g-1, respectively). Fig. 5e compares the long cycle stability of the [email protected] and Co0.6Fe0.4S2 at 5000 mA g-1. Obviously, the [email protected] electrode with abundant metal vacancies and high surface area exhibits super long cycle stability. In the first 75 cycles, the capacity fading is serious, which may be attributed to the comparatively slow formation of SEI film and the irreversible trapping of Li+ into the Co0.6Fe0.4S2 lattice.[18, 56] Because of the reversible formation of the gel-like polymeric layer and the activation process during the discharge/charge process at high-rate, a reversible capacity increase appears at the subsequent cycles.[57] After 500 cycles at 5000 mA g-1, the reversible capacities still maintain at 696 mAh g-1. And the specific capacity of 376 mAh g-1 can be obtained for Co0.6Fe0.4S2. The TEM image of the [email protected] electrode is investigated and shown in Fig. S8. After 500 cycles at 5000 mA g-1, the morphology of the [email protected] is still maintained. Compared the electrochemical properties of [email protected] with previous work (Table S2), the [email protected] displays excellent cycles performance and rate capacity. The improved electrochemical performance can be attributed to the unique structure and the rich metal vacancies. The introduction of metal vacancies and the N-doped carbon nanosheets effectively increase the electronic conductivity and facilitate the fast Li-ion diffusion kinetics.[57, 58] The N-doped carbon nanosheets 12

serve as elastic substrate and offer good dispersion of Co0.6Fe0.4S2, which accommodate the volume change and inhibit the aggregation of the electrodes. In addition, the strong metal-nitrogen bonds can prohibit the aggregation and avoid the exfoliation of the Co0.6Fe0.4S2 from the carbon nanosheets. In order to further ascertain the enhanced electronic conductivity induced by the metal vacancies and the NC, the electrochemical impedance spectra (EIS) analysis of the [email protected], Co0.6Fe0.4S2 and pure phase CoS2 is performed within the frequency range 100 kHz-0.1 Hz (after 1 cycle). The three electrodes display a compressed semicircle in the high and medium frequency accompanied by a slope line in the low frequency (Fig. S9a). Fig S9a inset displays the equivalent circuit model of the electrodes, in which the Re, Rf, and CPE-1 correspond to the internal resistance, SEI surface resistance and the constant phase element of the SEI film on the surface of the electrodes, respectively. And the Rct and Zw represent the charge transfer resistance and Li+ diffusion Warburg resistance, respectively. According to the fitting results (Table S3), the [email protected] exhibits the Rf and Rct of 2.63 and 58.78 Ω, respectively, which are smaller than those of Co0.6Fe0.4S2 and CoS2. The results suggest that the electronic conductivity can be enhanced by the metal vacancies and wrapped N-doped carbon nanosheets. As demonstrated in previous work, the diffusion coefficient of the electrodes is in proportion to the square root of the Warburg coefficient (σ). Such Warburg coefficient can be obtained by calculating the slope of Z’ versus ω-1/2 (Fig. S9b).[32] The calculated values of DLi+ for the [email protected], Co0.6Fe0.4S2 and CoS2 electrodes are 7.95×10-15, 1.33×10-15 and 13

2.90×10-16 cm2 S-1, respectively, suggesting the rapid lithium diffusion of [email protected]. To further investigate the metal vacancies and N-doped carbon nanosheets impact on charge transfer/transport of the electrodes, the electrochemical kinetic study is conducted. Fig. 6a and 6c demonstrate the CV curves collected at different sweep rates ranging of 0.2 to 1 mV s-1. It can be obviously seen that the peak current increases with increasing the scan rate. According to the general expression of logi=b× logv +loga, the surface controlled process and diffusion controlled process of the electrodes can be obtained.[59, 60] In the formula, i and v correspond to the peak current and scan rate, respectively, while a and b are adjustable parameters. From Fig. 6b and 6d, the b-values originated from the slopes plotting logi against logv are achieved. As the sweep rates increase from 0.2 to 1 mV s-1, the b-value of Co0.6Fe0.4S2 is closed to 0.5, suggesting that the lithium storage is dominated by diffusion controlled process. In contrast, the b-values of 0.56-0.78 are obtained for the [email protected], suggesting the mixed contribution from both of diffusion controlled process and surface controlled process.[32, 61] The results indicate that the metal vacancies and N-doped carbon wrapping enhance the pseudocapacitive storage behavior and fast electrochemical kinetics, which resulting in the excellent high rate performance and long cyclability. 4. Conclusions In summary, we have fabricated [email protected] nanocomposites by facile hydrothermal procedure by using N-doped carbon nanosheets as template. Due to the 14

synergistic effects of metal vacancies, N-doped carbon substrate and strong metal-nitrogen bonds between the Co0.6Fe0.4S2 and carbon nanosheets, the [email protected] electrode exhibits enhanced rate performance and excellent long-term cycling stability. The [email protected] displays the specific capacity of 830 mAh g-1 after 100 cycles at the current density of 200 mA g-1 and maintains a capacity of 696 mAh g-1 even at high rate of 5000 mA g-1 after 500 cycles. The metal vacancies and the N-doped carbon wrapping strategy are expected to boosting the electrochemical performances of other transition metal sulfides.

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Sulfide@N/S-Doped

Carbon

Dodecahedron:

Bimetal-Organic-Frameworks

Derivation

and

Electrochemical Application for High-Capacity and Long-Life Lithium-Ion Batteries, Adv Funct Mater, 26 (2016) 8345-8353. [59] D.L. Chao, P. Liang, Z. Chen, L.Y. Bai, H. Shen, X.X. Liu, X.H. Xia, Y.L. Zhao, S.V. Savilov, J.Y. Lin, Z.X. Shen, Pseudocapacitive Na-Ion Storage Boosts High Rate and Areal Capacity of Self-Branched 2D Layered Metal Chalcogenide Nanoarrays, Acs Nano, 10 (2016) 10211-10219. [60] J.L. Zhang, C.F. Du, Z.F. Dai, W. Chen, Y. Zheng, B. Li, Y. Zong, X. Wang, J.W. Zhu, Q.Y. Yan, NbS2 Nanosheets with M/Se (M = Fe, Co, Ni) Codopants for Li+ and Na+ Storage, Acs Nano, 11 (2017) 10599-10607. [61] V. Augustyn, J. Come, M.A. Lowe, J.W. Kim, P.L. Taberna, S.H. Tolbert, H.D. Abruna, P. Simon, B. Dunn, High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance, Nat Mater, 12 (2013) 518-522.

23

Fig. 1 Schematic illustration for the preparation of the [email protected].

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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 2 (a, b) SEM images (c, d) TEM images and (e, f) HRTEM images of [email protected], (g) STEM image of [email protected], (h) Elemental mapping of [email protected]: C (red), N (orange), Co (purple), Fe (blue), and S (green).

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(a)

(b)

(c)

(d)

Fig. 3 (a) XRD patterns of the [email protected] and Co0.6Fe0.4S2, (b) Raman spectra of [email protected], Co0.6Fe0.4S2 and NC, (c) Crystal structure of CoS2 and Co0.6Fe0.4S2, (d) ESR spectra of [email protected] and Co0.6Fe0.4S2.

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(a)

(b)

(c)

(d)

(e)

(f)

Fig. 4 XPS spectra of [email protected] and Co0.6Fe0.4S2: (a) survey spectra, (b) Co 2p, (c) Fe 2p, (d) S 2p, (e) C 1s, (f) N 1s.

27

(a)

(b)

(d)

(c)

(e)

Fig. 5 (a) CV curves of the [email protected] at 0.2 mV s-1 between 0.01 and 3 V, (b) galvanostatic charge-discharge profiles of the [email protected] at 200 mA g-1, (c) cycle performance of the [email protected] and Co0.6Fe0.4S2 electrodes at 200 mA g-1, (d) rate performance of the [email protected] and Co0.6Fe0.4S2 electrodes, (e) long-term cycle stability of [email protected] and Co0.6Fe0.4S2 at 5000 mA g-1.

28

(a)

(b)

(c)

(d)

Fig. 6 CV curves at the scan rate of 0.2 to 1.0 mV s-1: (a) Co0.6Fe0.4S2, (c) [email protected]. The log i versus log v plots at the oxidation and reduction peaks of the three electrodes: (b) Co0.6Fe0.4S2, (d) [email protected].

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Highlights [email protected] with metal vacancies is fabricated by a facile hydrothermal method. The metal vacancies and metal-nitrogen bonds facilitate Li+ ion diffusion of the electrode. [email protected] exhibits excellent electrochemical performance.

Guangming Wang: Conceptualization, Methodology, Software Data curation, WritingOriginal draft preparation. Hailong Yue: Visualization, Investigation, Data Curation. Yakun Xu: Investigation. Rencheng Jin: Supervision, Funding acquisition. Qingyao Wang: Writing- Reviewing and Editing, Shanmin Gao: Supervision, Funding acquisition.

Declaration of interests ✓ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: