Hierarchical Co9S8@carbon hollow microspheres as an anode for

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Hierarchical Co9S8@carbon hollow microspheres as an anode for sodium ion batteries with ultra long cycling stability Mengmeng Yin, Xueting Feng, Dan Zhao, Yun Zhao, Hansheng Li, Wei Zhou, Hongbo Liu, Xiaoping Bai, Hongxia Wang, Caihong Feng, and Qingze Jiao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06345 • Publication Date (Web): 17 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019

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ACS Sustainable Chemistry & Engineering

Hierarchical Co9S8@carbon hollow microspheres as an anode for sodium ion batteries with ultra long cycling stability Mengmeng Yin†, Xueting Feng†, Dan Zhao†, Yun Zhao†, Hansheng Li†, Wei Zhou§, Hongbo Liu‡, Xiaoping Bai¶, Hongxia Wang¶, Caihong Feng†*, and Qingze Jiao†‡* †School

of Chemistry and Chemical Engineering, Beijing Institute of Technology,

Zhongguancun South Street, Beijing 100081, China. ‡School

of Materials and Environment, Beijing Institute of Technology, Jinfeng Road

No.6, Xiangzhou District, Zhuhai 519085, China. §School

of Chemistry, Beihang University, Xueyuan Road No.37, Haidian District,

Beijing 100191, China. ¶Yinlong

Energy Co, Ltd, No.16 Jinhu Road, Sanzao Town, Jinwan District, Zhuhai

City, China.

*Corresponding Authors E-mail addresses: [email protected] (C. Feng) [email protected] (Q. Jiao)

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Abstract Herein, hierarchical Co9S8@carbon hollow microspheres (Co9S8@CHSs) were designed and facilely prepared via a simple solvothermal approach, following a thermal treatment process. The surfaces of the hierarchical Co9S8 hollow microspheres are homogeneously coated by a fluffy ultrathin carbon layer, which not only acts as a buffer material to suppress pulverization, but also serves as a conductive matrix to boost charge transfer. Leveraging the advantage of the fascinating hierarchical structure and the synergistic effect with carbon layers, they show impressive electrochemical properties. They deliver a large sodium storage capacity of 492 mA h g -1 after 100 cycles at a current density of 0.5 A g -1 and excellent rate performance. Additionally, a high capacity of 223 mA h g

-1

is maintained, even after 10,000 cycles at 5 A g -1,

demonstrating prolonged cycle stability. The remarkable sodium storage performance expects a future application for sodium ion batteries.

Keywords: hierarchical hollow structures, Co9S8@carbon, anode materials, sodium ion batteries.


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Introduction In recent years, sodium ion batteries (SIBs) are widely studied as a promising candidate of lithium ion batteries (LIBs), especially for energy storage systems benefiting from their similar electrochemical behaviors to that of LIBs and low cost, natural sodium abundance1-4. Unfortunately, the shortage of ideal anode materials with good electrochemical performances is limiting the development of SIBs5-6. Graphite, a typical anode for LIBs, fails to satisfy the demands of commercial anode materials for SIBs owing to its limited interlayer spacing7. Thus, numerous novel anode materials have been exploited for high-performance SIBs, including carbon-based materials8-9, transition metal oxides/chalcogenides10-13, alloying materials14-15. Among the various options mentioned above, transition metal sulfides (TMSs) have drawn great attentions due to their reversible redox reactions, which endow TMSs with high theoretical capacity than their oxide counterparts and carbonaceous materials16-18. Specially, cobalt sulfides, as a representative of TMSs, have been broadly explored as anode materials for SIBs owing to their superior thermal stability, high theoretical capacity and environmental friendliness19-22. Nonetheless, their practical applications in SIBs are hindered by poor electronic conductivity and serious pulverization problems aroused by the huge volume expansion/contraction in the sodiation/desodiation processes. These issues lead to the cracking of electrode material structures and ultimately unsatisfied rate performance and cycling stability, as well as rapid capacity fading23. Consequently, it is still challenging to enhance the electrochemical performances of cobalt sulfides via constructing their structures and components for future practical



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application in SIBs24-25. It is definitely cognized that combination of cobalt sulfides and carbon-based materials is an effective solution to address the above issues26-28. Generally, the protective carbon coating can build a conductive high way for electro/ion transfer, retard the volume change, eventually resulting in excellent unfading capacity and high rate performance20. For instance, Dou et al. employed Co9S8@carbon nanospheres with a 5 nm carbon layer and 50 nm Co9S8 core as anodes for SIBs, which achieved a high reversible capacity of 406.5 mA h g −1 at 0.5 A g -1 after 100 cycles. While the carbonfree Co9S8 only gained a poor capacity of 246.7 mA h g

−1 29.

Ultrasmall CoS

nanoparticles anchored on polyporous carbon nanorods displayed outstanding sodium storage performance, with an enhanced capacity of 542 mA h g -1 at 1 A g -1 after 2000 cycles30. The delicate design of architectures or morphologies of anode materials is another viable option to raise the electrochemical performances of cobalt sulfides. Especially, cobalt sulfides with the hierarchical hollow structures have exhibited enhanced overall battery performances due to their ingenious hierarchical hollow structures31-32. The hierarchical hollow structure plays multiple roles for improving the electrochemical performances, which affords ample internal voids and abundant electrochemical active sites to release the volume change and accelerate the transfer of reactants and electrolytes. Meanwhile, according to the conversion mechanism of cobalt sulfides, the frameworks of hollow hierarchical can inhibit the agglomeration of Co nanoparticles and enhance the conductivity of high-resistance Na2S33. For example, Xiao and co-workers designed CoSx hierarchical hollow nanospheres using P123 as the


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hard-template34. Benefiting from their unique 3D structures, they delivered an excellent lithium/sodium storage capacity of 1012.1 mA h g −1 and 572.0 mA h g −1, respectively, after 100 cycles at 0.5 A g

−1.

Hence, the hybridization of cobalt sulfides with

hierarchical hollow structure and carbon materials may be a prospective strategy to optimize the electrochemical performances of SIBs. In this paper, Co9S8@carbon hollow microspheres (Co9S8@CHSs) with a welldesigned porous hierarchical structure were fabricated utilizing a template-free solvothermal method following a carbonization process. Glucose as a carbon source has a prominent effect on the surface structures and sizes of hollow Co9S8 spheres. The sodium storage performance of obtained Co9S8@CHSs was evaluated. By virtue of their hierarchical hollow structures and fluffy carbon layers, the Co9S8@CHSs manifest a high specific capacity, improved rate capacity and ultra-long cycling stability. We believe that this present strategy for Co9S8@CHSs can be further applied to other transition metal sulfides to optimize their electrochemical performances.

Experimental Synthesis of Co9S8@CHSs The robust Co9S8@CHSs were prepared via a convenient solvothermal reaction and subsequent carbonization process. In a typical process, 0.6746 g Cobalt sulfate seven hydrate (CoSO4·7H2O), 0.5481 g thiourea (CH4N2S) and 1 g glucose (C6H12O6) were added into a mixed solution of 48 mL dimethylformamide (DMF) and 12 mL ethylene glycol (EG) with strong agitation about 30 min. Subsequently, the reagents were poured into a 75 mL autoclave and reacted at 160 °C for 12 h in a blast oven. The black samples



(Co9S8@C precursors) were rinsed with absolute ethanol and deionized water several times by high-speed centrifugation, and then dried for further reaction. Carbon wrapped Co9S8@C precursors, placing in a quartz boat, were annealed at 600 °C for 3 h in the tube furnace with argon atmosphere and a heating rate of 3 °C /min. After the calcination, the final black products, noted as Co9S8@CHSs, were collected for further characterization and testing. Synthesis of Co9S8 As a comparison, pristine Co9S8 hollow microspheres were fabricated by a similar strategy except without the addition of glucose according to our group previous work35. Materials Characterization The crystal structures of Co9S8 and Co9S8@CHSs were certified by the X-ray diffraction (XRD) patterns using an X-ray diffractometer (Bruker D8 adv, Germany), operating at 20 mA and 40 kV with Cu Kα radiation. HORBA Labram HR Raman spectrometer equipped with a 532 nm laser excitation has been utilized to determine the D peak and G peak of Co9S8@CHSs. The morphologies and surface structures of the Co9S8 and Co9S8@CHSs were detected utilizing field-emission scanning electron microscopy (FESEM, JEOL JSM-7500F) coupled with the energy dispersive spectrometer (EDS) to perform elemental analysis. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, Hitachi HT7700) were applied to examine the fine structures of the resulting materials. The carbon content in Co9S8@CHSs was measured by Thermal gravimetric analysis (TGA) using a SDT Q600-1649 instrument in air with a heating rate of 10 °C min−1 from room temperature to 900 °C. The surface


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area of Co9S8 and Co9S8@CHSs were calculated by Brunauer–Emmett–Teller (BET) measure method using a Micromeritics ASAP 2010 adsorption analyzer. The chemical bond structures of Co9S8@CHSs were detected via X-ray photoelectron spectroscopy (XPS PHI QUANTERA-II SXM). Electrochemical measurements Co9S8@CHSs, super P and sodium carboxymethyl cellulose (CMC) were dissolved in water according to the mass ratio of 7:2:1 and stirred for 12 h to generate homogeneous slurry. The viscous active materials were uniformly spread out onto the copper foil and then dried in a vacuum oven at 80 °C overnight. The dried copper foil coated with Co9S8@CHS electrodes was cut into 12 mm discs and the mass of active materials coated on each disc was about 1.2 mg. Na metal was used for the reference electrode, Whatman glass fiber was employed as the separator, and a solution of 1M NaCF3SO3 dissolved in diethylene glycol ether (DIGLYME) was used as an electrolyte in an ultrapure Ar-filled glove box, the above components were assembled into CR2032-type half cells. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) with the frequency range from 0.01Hz to 100 kHz were performed by

electrochemical

workstation

(CHI-760).

Galvanostatic

discharge/charge

performance tests at various current densities were implemented utilizing LAND battery-test instrument. Besides, the potential window of all tests was in the range of 0.25 to 3 V. The carbon-free Co9S8 electrodes adopted the same slurrying recipe and test methods.

Result and discussion



Fig. 1. Schematic diagram of the synthesis route of Co9S8@CHSs Fig. 1 shows a schematic design for the preparation procedure of Co9S8@CHSs. First, Co9S8@C precursor microspheres with hierarchical hollow structures were prepared in a mixed solvent system (EG and DMF) by a typical solvent thermal strategy. Then, the as-prepared Co9S8@C precursors were calcined at 600 °C to obtain Co9S8@CHSs. Detailedly, the growth mechanism of Co9S8@CHSs was explored by a series of time-dependent experiments. As shown in Figure S2a, plenty of microspheres covered with flimsy carbon layers were obtained within a very short time and some of them have evolved into core-shell structures in 1 h, which is further confirmed by TEM (Figure S3a). Increasing the reaction time to 2 h (Figure S2b and S3b), the surface of the spheres is covered by a folded carbon layer, and the spheres have changed from core-shell to hollow structure. As shown in Figure S2c-d and S3c-d, the thickness of carbon layer and the intensity of XRD peaks of intermediate product increases gradually (Figure S1) with the prolongation of reaction time and the hollow structure has been formed completely after 6 h. The XRD peaks are indexed to cubic Co9S8 phase


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(JCPDS No. 86-2273).

Based on the above experimental results, the generation of

hollow structure of Co9S8@CHSs is formed by the Kirkendall effect, which is similar to the formation of Co9S8 hollow microspheres in our previous work35. It should be point out that the glucose and heat treatment is not key to formation of hollow structure but is pivotal to formation of carbon layer. Moreover, the carbon coated on the surface of Co9S8 microspheres forms a buffer layer to a certain extent preventing the agglomeration of the microspheres during the annealing. The entire synthesis process of Co9S8@CHSs is simple and easy to operate and the reagents used are easy to obtain and less harmful to the environment.

Fig. 2 (a) XRD spectra of Co9S8@C precursors and Co9S8@CHSs; (b) Raman spectroscopy Co9S8@CHSs. The crystal structures of Co9S8@C precursors and Co9S8@CHSs can be seen from XRD patterns in Fig. 2a. The peaks of Co9S8@CHSs become sharper and the intensity of Co9S8@CHSs become stronger relative to the Co9S8@C precursors, suggesting the high crystallinity after carbonization. The diffraction peaks of both samples are consistent with cubic Co9S8 phase (JCPDS No. 86-2273)35. It is worth noting that the chemical components and the crystal structures of the sample materials have not



changed during carbonization process, demonstrating the super thermal stability of Co9S8@CHSs. No carbon peaks are found in Co9S8@CHSs XRD patterns, which may be attributed to low content and crystallinity of carbon in Co9S8@CHSs29. Nevertheless, Raman spectroscopy of Co9S8@CHSs confirms the existence of carbon layers (Fig. 2b). Two distinct peaks centered at 1346 and 1584 cm -1 are well indexed to disordered and graphic carbon peaks of the carbon layers, respectively36. Besides, the weak peak located at 672 cm−1 is the characteristic peak of Co9S829, 37. In addition, Co9S8@CHSs and carbon-free Co9S8 hollow microspheres display the same diffraction peaks in XRD patterns (Figure S4), which indicate the successful formation of Co9S8.

Fig.3. (a) XPS survey spectrum of Co9S8@CHSs; (b) C 1s, (c) Co 2p and (d) S 2p of Co9S8@CHSs. The surface electronic states and chemical compositions of the Co9S8@CHSs were studied by XPS spectra. The XPS survey spectrum in Fig. 3a confirms that there are S,


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C, O, and Co elements on the surface of Co9S8@CHSs. The existence of O should be originated from the slightly oxidation of samples surface. The high-resolution spectrum of C1s (Figure 3b) presents two peaks fitted at around 284.8 and 288.5 eV, which are corresponded to C-C and C=O, respectively and suggested that the C atoms exist in the form of conjugated honeycomb lattice and partial oxidation of C atoms38. The XPS spectrum of Co 2p depicted in Fig. 3c, the fitted peaks at 778.5 and 793.4 eV correspond to 2p3/2 and 2p1/2 oxidation states of Co3+ and the couple of peaks at 786.1 and 796.9 eV are assigned to 2p3/2 and 2p1/2 oxidation states of Co2+, respectively39-40. The peaks centered at 786.1 and 802.9 eV are the shake-up satellite peaks of Co 2p3/2 and Co 2p1/2, respectively. The S 2p spectrum can be fitted into four curves as displayed in Fig. 3d. The fitted peaks at 162.3 and 161.5 eV correspond to the 2p1/2 and 2p3/2 reduction state of S2− in Co9S8@CHSs, respectively, while the peak at 163.5 eV can be indexed to the C−S−C, which confirms S atoms doped in carbon matrix form thiophene-like structures with adjacent carbon atoms and C atoms existed in Co9S8@CHSs41-43. Besides, the weak peak of SO42− at 167.4 eV is detected owing to the partly oxidation of S species on the surface of Co9S8@CHSs44. Furthermore, the Carbon content in Co9S8@CHSs can be measured by thermogravimetric analysis. (Figure S5). The TGA curve of the Co9S8@CHSs shows different weight variations from 100 to 800 °C, which can be ascribed to the evaporation water in air, the oxidation of carbon to carbon dioxide and cobalt sulfide to cobalt oxide29, 38. Therefore, the carbon content was determined to be 20.48 wt% by the TGA curve.



Fig. 4. (a) SEM, (b) and (c) TEM images of Co9S8@CHSs; (d) HRTEM image and (e–h) elements mapping of Co9S8@CHSs. The morphologies and structure details of the Co9S8@CHSs and pristine Co9S8 were observed using SEM and TEM/HRTEM. Compared to carbon-free Co9S8 shown in Figure S6, the Co9S8@CHSs are homogeneously coated by an ultrathin carbon layer with a rough and hierarchical surface after the carbonization treatment (Fig. 4a). The diameter of Co9S8@CHSs expands to 2-3 μm, whereas the diameter of carbon-free Co9S8 is about 1-2 μm. The inset shown in Fig. 4a, the hole in shell of Co9S8@CHSs clearly demonstrates the hollow structure. The hollow structure can be further confirmed by the TEM shown in Fig. 4b. The TEM image displays a distinct hollow spherical structure with a clear color contrast between the middle part and edges of Co9S8@CHSs, which is consistent with the SEM image in Fig. 4a. More details of


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Co9S8@CHSs are further revealed by high-magnification TEM images (Fig. 4c). The image shows that the uniform carbon layers are distinguished on the surface of Co9S8 hollow microspheres with a loosely-hierarchal nanostructure. The specific surface area of Co9S8@CHSs calculated by BET method is 70.36 m2 g −1, which is higher than that of the carbon-free Co9S8 (Figure S7). Such fluffy structure provides a larger surface area and maximizes the electrochemical active sites. From HRTEM image of Co9S8@CHSs (Fig. 4d), the measured spacing of the lattice fringe of Co9S8@CHSs is 0.30 nm, which correspond to the (311) crystal planes of Co9S8@CHSs. Besides, the amorphous thin carbon layers are clearly found in the HRTEM image of Co9S8@CHSs. The element-mapping analysis shows that the well distribution of C, S, and Co elements throughout the Co9S8 microspheres (Fig. 4g). Form the analysis of SEM and TEM of Co9S8@CHSs and Co9S8, it should be noticed that significant changes in the sizes and surface structures of Co9S8@CHSs are attributed to the addition of glucose, which acts as carbon source as well as greatly effect on the sizes and morphologies of Co9S8@CHSs45. The loosely-hierarchal hollow structures and the protective carbon layers work together to promise an improvement of cycling capacity and rate capability of Co9S8@CHSs. During the sodiation and desodiation processes, the hollow hierarchal structures are expected to tolerate a large volume expansion/contraction, provide more Na+ ion insertion sites and faster reaction kinetics by boosting the contact area between the electrolytes and the active materials, as well as shortening the ions/charges diffusion pathway. Significantly, the loosely-hierarchal protective carbon coating can enhance



intrinsic conductivity of the Co9S8 electrode materials and acts as a buffer material to suppress pulverization. SEM was performed to analyze the morphology structural stability of Co9S8@CHSs after 1000 cycles at 5 A g -1 （Figure S8）. It can be clearly seen that Co9S8@CHSs still retained their original spherical structure after a long-term cycle, demonstrating the elaborate structure could alleviate volume change caused during charge and discharge processes and enhance cyclic stability. Therefore, the carbon-coated Co9S8@CHSs are likely to be preserved over long life cycle stability and remarkable rate capability.

Fig. 5.(a) CV curves of Co9S8@CHSs tested at 0.2 mV/s; (b) the first three charge/discharge voltage profiles of Co9S8@CHS electrodes tested at 0.5 A g −1; (c)


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cycle performances of Co9S8@CHSs tested at 0.5, 2 A g −1, respectively, and cycle performance of Co9S8 tested at 0.5 A g −1; (d) rate capability of Co9S8 and Co9S8@CHSs; (e) cycle durability of Co9S8@CHS electrodes at a high rate of 5 A g -1; the potential range of all electrochemical tests is 0.25 to 3 V. The electrochemical performances of the Co9S8@CHSs and Co9S8 were tested in terms of half cells paired with Na metal foil. The CV data of the Co9S8@CHS electrodes at a scaning rate of 0.2 mV/s are used to explain the reaction mechanism of Co9S8@CHSs in the cycle process（Fig. 5a）. During the first cathodic scan, a sharp peak can be observed at 0.69 V, which is relevant to the generation of solid electrolyte interface film and conversion of Co9S8 to Co0 and Na2S46-47. Correspondingly, the broad anodic peak around 1.73 V in the desodiation process should be assigned to the resulfidation of Co0 to generate the Co9S847. Except for a slight rise in the reduction peak, the subsequent sweeps are almost overlapped after the initial cycle, indicating good reversible

capability

and

high

stability

of

Co9S8@CHS electrodes.

The

charge/discharge potential profiles of Co9S8@CHS electrode materials for the first three cycles at 0.5 A g −1 are depicted in Fig. 5b. The obviously flat plateau around 0.75 V in the first discharge process is originated from the phase transformation of Co9S8 to Na2S and Co, which is in accordance with the CV result. Additionally, the excellent discharge and charge capacity of 835 and 609 mA h g −1 were achieved during the initial cycle, respectively, corresponding to a high efficiency of 72.93 %. The formation of the SEI film and electrode activation procedure in the first cycle contribute to the inevitable capacity loss48.



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The cycle performances of Co9S8 microspheres and Co9S8@CHSs as SIBs anode materials were evaluated at 0.5, 2 A g -1, respectively （Fig. 5c ）. The large sodium storage capacity is obtained for the Co9S8@CHS electrodes, which keep a stable capacity of 492 mA h g −1 after 100 cycles at 0.5 A g −1. Furthermore, the Co9S8@CHS electrodes can retain a discharge capacity of 438.9 mA h g -1 at 2 A g −1 after 100 cycles, confirming their excellent cyclic performance. Whereas, the Co9S8 electrodes show a rapid decay and the capacity of Co9S8 electrodes is only 225.7 mA h g −1 after 100 cycles. The rate performance is also one of the crucial indicators for electrochemical properties of Co9S8@CHS and Co9S8. As depicted in Fig. 5d, the Co9S8@CHS electrodes deliver an average capacity of 614, 583, 550, 504, 462, and 411 mA h g −1, respectively, with an increase in current density from 0.1 to 0.2, 0.5, 1, 2, and 5 A g −1. The Co9S8@CHS electrodes show better rate performance compared with Co9S8 at same current density. This excellent rate performance of Co9S8@CHSs can arise from the reduction of polarization due to the high conductivity carbon layers in the Co9S8@CHSs, especially at high current density. EIS curves of Co9S8@CHSs and Co9S8 are displayed in Figure S9. The EIS profiles are composed of a semicircle represented charge transfer resistance (Rct) at high-to-middle frequency region and a slope line represented the spread of sodium ions in the electrode material at the low frequency region49. Compared to Co9S8, the

radius

of

the

Co9S8@CHSs

semicircle

is

smaller,

suggesting

fast

insertion/extraction of sodium ions in Co9S8@CHS electrodes. Durable cycling performance is very vial to practical application of Co9S8@CHSs for SIBs. Here, Fig. 5e manifests the ultra-long cycling performance of Co9S8@CHSs


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at a high current density of 5 A g -1. They keep a high stable capacity of 223 mA h g -1 with approximately 100% of Coulomb efficiency even after 10,000 cycles, demonstrating their extraordinary long-life cycling durability. However, during the entire cycling period, the capacity of Co9S8@CHSs shows a fluctuation in early stage before tending to be stable. The similar capacity changes have been reported for many metal oxides, metal sulfides and metal phosphides44,

50-54.

These phenomena are

probably derived from self-reconstructions of electrode material nanostructures, optimization of SEI film, and the generation of gel-like polymeric films24, 55.

Fig. 6. (a) CV profiles of Co9S8@CHSs at diverse scan rates; (b) the relation of log (ν) and log (i), fitting from cathodic and anodic peaks; (c) the black fraction represents the capacitive contribution at a specific scan rate of 1 mV/s; (d) calculated contribution ratio of capacitive-control at scan rate of 0.2-1 mV/s. Pseudocapacitance analysis can be used to elucidate the durable cycling performance



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and remarkable capacity of Co9S8@CHSs at high rate. The shapes of Co9S8@CHSs CV curves are almost identical with increasing sweep rate except for slight shifts in cathodic and anodic peaks (Figure. 6a). In fact, the capacity contributions of sodium ion batteries include capacitive and diffused components. The charge storage behaviors of Co9S8@CHS obey the following law56: i=aνb

(1)

where i and ν represent peak current and sweep rate, respectively. The value of b, which can be deduced from the linear fitting of peak current and sweep rate, is an important parameter. When the value of b is close to 1 or more than 1, which indicates the charge storage behaviors are governed by the capacitive process, whereas the b-value approaches 0.5, diffusion control plays a leading role. Figure 6b shows that the fitted b values are 0.87, 0.73, and 1.06, respectively, suggesting the charge storage mechanisms are largely controlled by the capacitive process. Especially, the b-value of peak II is larger than 1, indicating the charge storage of completely capacitive behaviors. Furthermore, the contribution of capacitive component at specific scan rate and a settled potential can be analyzed as57-58: i(V)/ν1/2= k1ν1/2 + k2

(2)

Where k1 and k2 are adjustable parameters and k1-values at different potentials can be calculated from the fitted slope of i ν

-1/2

vs. ν

1/2

graph. According to Eq. (2), k1ν

corresponds to the contribution of capacitive behavior and k2ν1/2 represents the portion of diffusion-controlled behavior. As an example calculated in Fig. 6c, at the scan rate of 1mV/s, the contribution of capacitive capacity to total capacity is 86%. As


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summarized in Fig.6d, the contribution ratio of capacitive increased from 78% to 86% accompanying with an increasing scan rate. Therefore, the overwhelming capacitive behaviors lead to the fast sodiation/desodiation processes at high rate and the amazing stability of the cycling processes59.

Fig. 7 (a) cyclic capacity and (b) rate performance comparison of Co9S8@CHSs with other Co9S8-based anodes for SIBs. The numbers in brackets in figure (a) indicate the number of cycles. As expected in Fig. 7, the Co9S8@CHS electrodes in our work show durable cyclic stability and outstanding rate performance compared with other previously reported Co9S8-based electrodes. Especially at high current density, the Co9S8@CHS electrodes exhibit superior cyclic durability, which is consistent with the conclusion of pseudo capacitance analysis12,

24-25, 29, 47, 60-64.

The above comparison results show that the

Co9S8@CHSs are very prospective anode materials for SIBs. Accordingly, the fascinating structural design and synergistic effects with carbon layers offer Co9S8@CHSs a huge advantage in sodium ion batteries.



Conclusions In summary, we successfully fabricated loosely-hierarchical Co9S8@carbon hollow microspheres using a facile solvothermal strategy and subsequent heating process. The loosely-hierarchal protective carbon shells can not only boost electrical conductivity of Co9S8 but also accommodate the volume change during cyclic processes. The appealing hollow hierarchical structures can alleviate volume variations in the charge/discharge process, enlarge the contact area of the Co9S8@CHS electrodes with electrolytes and provide shorter ion/electron diffusion channels, promote the reversibility of Co9S8 conversion reaction. Because of these virtues, the prepared Co9S8@CHSs manifest excellent capacity performance for sodium storage. Besides, the Co9S8@CHS electrodes show dominant pseudocapacitive behavior by quantitative kinetic analysis, which can boost the sodium ion storage capacity via surface redox processes. The preparation method and structure design of Co9S8@CHSs are advantageous for advancing a wealth of other metal sulfides hybrid materials.


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Supporting Information XRD ， SEM and TEM of intermediate production of the Co9S8@CHSs at different times; XRD and SEM of Co9S8 microspheres; TGA curve of Co9S8@CHSs; BET of Co9S8@CHSs and Co9S8 microspheres; SEM of the Co9S8@CHSs after 1000 cycles; EIS spectra of Co9S8 microspheres and Co9S8@CHSs



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Acknowledgements This research received funding from the National Natural Science Foundation of China (NOs. 51622204 and 51472014) and the Beijing Nova Program (Z171100001117071), the

Program

for

Innovative

and

Entrepreneurial

team

in

Zhuhai

(ZH01110405160007PWC). The authors highly appreciate the Analysis & Testing Center (Beijing Institute of Technology) for materials characterizations.


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For Table of Contents Use Only

Synopsis: Hollow Co9S8 hierarchical microspheres were facilely fabricated, which achieved durable cycle stability and high rate capacity for sodium ion batteries.


Page 34 of 34

Hierarchical Co9S8@carbon hollow microspheres as an anode for

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