Engineering Unique Ball-In-Ball Structured (Ni0.33Co0.67)9S8@C

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Engineering unique ball-in-ball structured (Ni0.33Co0.67)9S8@C nanospheres for advanced sodium storage Shuaihui Li, Chuanqi Li, Wei Kong Pang, Zhipeng Zhao, Jianmin Zhang, Zhongyi Liu, and Dan Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07214 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Engineering unique ball-in-ball structured (Ni0.33Co0.67)9S8@C nanospheres for advanced sodium storage Shuaihui Li†1, Chuanqi Li†1, Wei Kong Pang2, Zhipeng Zhao1, Jianmin Zhang1, Zhongyi Liu1,*, and Dan Li1,* College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, Henan Province, 450001, PR China. 1

Institute for Superconducting & Electronic Materials, University of Wollongong, NSW 2500, Australia. 2

Keywords: (Ni0.33Co0.67)9S8; ball-in-ball structure; carbon coating; anode material; sodium ion batteries

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Abstract Constructing hollow architectures based on metal sulfides is of great interest for highperformance electrode materials for sodium ion batteries due to their intriguring properties and various applications. However, the relatively low volumetric density and high fragile structure are the obstacles blocking the development of hollow-structured electrode materials. In this work, ball-in-ball structured (Ni0.33Co0.67)9S8@C nanospheres have been synthesized by using NiCo-glycerate as the precursor via solvothermal reaction, which was followed by a carbon coating treatment. In this structural design, hollow cavities are generated between the inner and outer balls to effectively accommodate the volume changes of the metal sulfides in the processes of charging/discharging, while the uniform carbon coating can increase the electrical conductivity and maintain the structural stability during repeated cycling. The Rietveld refinement, in situ X-ray diffraction, and ex situ X-ray photoelectron spectroscopy analyses provide evidence for an enlarged lattice parameter, weaker Co-S and Ni-S bonding, and a synergistic effect in the (Ni0.33Co0.67)9S8@C towards boosting the conversion reaction and reversible formation of sulfur in the fully charged state, with the sulfur trapped within the composite to additionally account for the superior cycling stability of this material. Capacitive behavior has been verified to dominate the electrochemical reaction, enabling fast charge transport kinetics. Impressively, the double structural protection combined with the free hollow space and complete carbon layer endows the (Ni0.33Co0.67)9S8@C nanospheres with good electrochemical performance, featuring high cyclability and good rate capability.


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1. Introduction The increasing problems due to the high cost1-8 and limited terrestrial reserves of lithium for lithium ion batteries have triggered renewed interest in ambient-temperature sodium ion batteries (SIBs), because of the widespread availability and low cost of sodium salts.9-16 Recently, metal sulfides have been extensively studied as electrode materials due to their low cost, high capacity, and relatively higher intrinsic electronic conductivity than that of their oxide counterparts. For example, layered metal sulfides, such as SnS,15, 17, 18 SnS2,19, 20 and MoS,21-23 have been demonstrated to be good anode materials for SIBs, based on their redox variability and structural peculiarities.17 The environmentally benign iron sulfides FeS224 and FeS25, 26 have been widely explored, including various structural designs to improve their cyclability. Moreover, significant progress has also been made in investigating cobalt sulfides (including CoS2,27, 28

CoS,29-31 and Co9S831-33) and nickel sulfide,34,

35

which have exhibited

relatively good sodium storage properties, based on modifying their structures and morphologies to allow reversible intercalation/de-intercalation of sodium ions and accelerate the sodium ion diffusion kinetics. The common and inevitable bottleneck for achieving high performance of the above-mentioned metal sulfides, however, is the severe volumetric variation during electrochemical reactions, resulting in gradual degradation in capacity over the course of repeated cycling.8,9



As mentioned above, rational designs for their structure and morphology have been applied in the fabrication process to buffer/accommodate the volumetric swelling and thus prevent the pulverization of electrode materials.3643

Designing structure with void space is one of the most straightforward methods

to alleviate structural strain. Compared with hollow structures, ball-in-ball structures do not only have high specific surface areas, but also have higher rigidity, thanking the tough scaffolds of inner ball to provide good mechanical stability. More importantly, ball-in-ball structures enable the electrode materials to exhibit a relatively high volumetric density without sacrificing the interior cavities to accommodate the volume expansion upon the ion insertion. Moreover, ball-in-ball structures can offer large contact areas between the electrolyte and electrode, rich active sites for electrochemical reactions, thorough permeation of electrolyte into the electrode materials, and short path distances for electronic and ionic transport, leading to high capacity and good rate capability.44-46 In this paper, we have designed a ball-in-ball structured (Ni0.33Co0.67)9S8@C material and investigated its electrochemical performance as anode material for SIBs. Carbon coating was applied to serve as a mechanical cushion to tolerate the strain of the outer balls in the obtained nanospheres, thereby maintaining the structural stability during prolonged cycling and increasing the electric conductivity of the composite.47-50 The free space in/between the balls can accommodate the volumetric changes during the charging/discharging processes and enable facile infiltration of electrolyte. Meanwhile, the inner (Ni0.33Co0.67)9S8


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balls can work as a self-supporting framework to support the (Ni0.33Co0.67)9S8 shells,

thereby

preventing

gradual

deterioration

over

the

course

of

electrochemical reactions. The obtained (Ni0.33Co0.67)9S8@C ball-in-ball nanospheres delivered a capacity of 389.7 mA h g-1 at 1000 mA g-1 over 100 cycles and exhibited a superior rate capability of 321.4 mA h g-1 at 2000 mA g1,

indicating their potential as anode material for SIBs.

2. Results and discussion



Figure 1. (a) Schematic illustration of the formation process for the ball-in-ball structured (Ni0.33Co0.67)9S8@C nanospheres, (b) SEM and (c) TEM images of the NiCo-glycerate precursor, (d) SEM and (e) TEM images of NiCo2S4 intermediate, (f) SEM and (g), (h) TEM images of (Ni0.33Co0.67)9S8@C, and (i) element mappings of Ni, Co, S, and C corresponding to (h). The formation process for the ball-in-ball structured (Ni0.33Co0.67)9S8@C nanospheres is schematically illustrated in Figure 1a. The NiCo-glycerate precursor was first synthesized via a simple solvothermal method, where it underwent an anion exchange reaction in the process of sulfidation by thioacetamide (TAA),51 leading to the formation of the complete ball-in-ball structured NiCo2S4 spheres. In order to maintain the structural integrity, a uniform carbon layer was coated on the surfaces of the nanostructures by thermal decomposition of acetylene gas, and the final hybrid material was thus obtained. The representative morphologies of the NiCo-glycerate precursor, NiCo2S4 intermediate, and final nanospheres were verified through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as shown in Figures 1b-i. The SEM and TEM images (Figures 1b and c) reveal that the NiCoglycerate precursors are solid nanospheres with a diameter of about 450 nm. The ball-in-ball structured NiCo2S4 intermediate was obtained after the completion of the anion exchange reaction with no obvious change in the size distribution, as shown in Figures 1d and e. The formation of the ball-in-ball structure is first carried out through the sulfidation of NiCo-glycerate, with the release of S2- to form NiCo-glycerate@NiCo2S4. With increasing reaction time, a gap between the NiCo2S4 shell and the NiCo-glycerate core is generated because the diffusion


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of metal cations is faster than that of S2-, resulting in the formation of secondary NiCo2S4 cores within NiCo2S4 shells.51 After carbon coating (Figures 1f and 1g), the surfaces of the outer balls, composed of loosely packed small nanoparticles, became rougher. Moreover, the carbon coating process resulted in the appearance of porous structure in the nanospheres. The diameter of the nanospheres was increased to about 500 nm. Figure 1i shows the corresponding elemental mappings of Ni, Co, S, and C. The energy dispersive spectroscopy (EDS) spectrum is shown in Figure S1 in the Supporting Information, and the molar ration of Co/Ni was found to be 2.07:1. The scheme of structural evolution from NiCo-glycerate to the final product was elucidated by X-ray powder diffraction (XRD) (Figure S2 and Figure S3). For the NiCo-glycerate precursor, the distinct diffraction peak at 12° was proved to be the characteristic peak of metal alkoxide.51,

52

After sulfidation, a

nanocrystalline phase was observed, and all broad diffraction peaks could be attributed to cubic phase NiCo2S4 with a Fd-3m space group (JCPDS card No. 20-0782).51, 52 Through calcining NiCo2S4 at 500 °C in acetylene gas atmosphere, part of the S content was removed, and the NiCo2S4 phase was totally transformed into a highly crystalline cubic phase with a Fm-3m space group, which is similar to that of Co9S8 (JCPDS card No.19-0364). Note, however, that, with the aid of the Rietveld refinement analysis (Figure 2a), it was found that the final cubic phase possesses a lattice parameter of 9.96277 Å, significantly larger than the reported value (9.932 Å) for a pure Co9S8 (JCPDS card No.19-



0364), implying weaker Co-S and Ni-S bondings. Although Ni and Co are nearly undistinguishable in XRD, in combination with the EDS (Figure S1), the results can convincedly confirm the presence of a (Ni0.33Co0.67)9S8 solid solution (denoted as (Ni0.33Co0.67)9S8@C). In order to highlight the merits of the ball-inball structure, a sample of carbon coated (Ni0.33Co0.67)9S8 (denoted as (Ni0.33Co0.67)9S8/C), with a morphology of clusters composed of nanoparticles (Figure S5), was prepared as a reference for comparison. Figure 2b shows Raman spectra of the (Ni0.33Co0.67)9S8/C mixture and the (Ni0.33Co0.67)9S8@C nanospheres. The two D band and G band peaks appear at about 1350 and 1590 cm-1, corresponding to the sp3 domains of disordered carbon and the E2g vibrational mode of sp2 domains of the graphitized structure, respectively.53,

54

The ratio of the peak strengths of the D to the G band was

calculated to be 0.75, which is lower than that of the (Ni0.33Co0.67)9S8/C mixture (0.92), indicating a relatively high degree of graphitization of the carbon layer in (Ni0.33Co0.67)9S8@C. Notably, the decomposition of acetylene gas introduced a fluffy and porous carbon layer on the surface of (Ni0.33Co0.67)9S8@C, and visible pores can be found in the TEM image (Figure 1g). Figure S7 shows the Raman spectra of NiCo-glycerate precursor and NiCo2S4 intermediate. From the results of N2 adsorption/desorption measurements, as shown in Figure 2c, the (Ni0.33Co0.67)9S8@C nanospheres possess a Brunauer-Emmett-Teller (BET) specific surface area of 64 m2 g-1, which is higher than that of the (Ni0.33Co0.67)9S8/C mixture with a determined value of 46.3 m2 g-1 (Figure S8),


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and most of the pores are about 5 nm in size. The carbon contents are 35.3% for (Ni0.33Co0.67)9S8/C and 36.9% for (Ni0.33Co0.67)9S8@C by thermogravimetric analysis (Figure S9). X-ray photoelectron spectroscopy (XPS) measurements were conducted to further identify the elemental composition and chemical state of the surface layer of the (Ni0.33Co0.67)9S8@C nanospheres, and the survey spectrum is presented in Figure S10. Figure 2d shows the Ni 2p core-level XPS spectrum with the two main characteristic peaks at 853.4 and 870.3 eV, corresponding to the Ni 2p3/2 and 2p1/2 electronic configurations of Ni3+, respectively.55-57 The two peaks located at 856.3/873.4 eV indicate the presence of the divalent state of Ni, while the peak located at 860.4 eV is a shake-up satellite.56, 58 Figure 2e shows that there are two spin-orbit doublets peaks located at 778.6/781.4 and 793.8/799.6 eV, which are assigned to the Co 2p3/2 and Co 2p1/2 characteristic peaks of Co9S8, respectively.55, 59 The core-level spectrum of the S 2p region can be divided into multiple peaks, as shown in Figure 2f. The two peaks at 161.3 and 162.5 eV are related to S 2p3/2 and S 2p1/2, respectively.35, 60, 61 The formation of S-C bonds could be identified by the peak at 164.0 eV, while the peak at 169.2 eV is a satellite peak.35 The C 1s spectrum shown in Figure S11 can be deconvoluted into three components centered at 284.5, 285.4, and 289.8 eV, corresponding to C−C, C−S, and C=O groups, respectively.



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Figure 2. (a) Rietveld refinement profile of XRD data of (Ni0.33Co0.67)9S8@C nanospheres with the weighted profile R-factor (Rwp), (b) Raman spectra of (Ni0.33Co0.67)9S8/C mixture and (Ni0.33Co0.67)9S8@C nanospheres, (c) isotherm plot and Barrett-Joyner-Halenda (BJH) pore size distribution (inset) of (Ni0.33Co0.67)9S8@C, and curve fittings of (d) Ni 2p, (e) Co 2p, and (f) S 2p XPS spectra of the (Ni0.33Co0.67)9S8@C nanospheres. The

electrochemical

performance

of

the

ball-in-ball

structured

(Ni0.33Co0.67)9S8@C was investigated, and the results are presented in Figure 3. Figure 3a presents the cycling performance of the samples at a current density of 1000 mA g-1. In the case of (Ni0.33Co0.67)9S8@C, the capacity faded over the initial cycles until it stabilized at about 390 mA h g-1 at 100 cycles. The decrease in capacity in first several cycles can be due to the reduced electrochemically active surface area of the ball-in-ball structures and grain boundary area of nanoparticles during cycling. After 200 cycles, (Ni0.33Co0.67)9S8@C delivered a relatively high specific capability of 335.1 mA h g-1. On the other hand, the (Ni0.33Co0.67)9S8/C mixture suffered from rapid capacity decay, delivering only a specific capacity of 23.4 mA h g-1 at the 200th cycle. Figure 3b and c further


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illustrate the rate performance of the (Ni0.33Co0.67)9S8/C mixture and the (Ni0.33Co0.67)9S8@C nanospheres. The (Ni0.33Co0.67)9S8@C delivered average capacities of 646.8, 507.4, 470.0, 408.9, 358.4, and 321.4 mA h g-1 at the current densities of 100, 200, 500, 1000, 1500, and 2000 mA g-1, respectively, indicating superior

sodium

ion

diffusion

and

electron

transport

within

the

(Ni0.33Co0.67)9S8@C electrode. When the current density returned to 100 mA g-1, a high capacity of 532.8 mA h g-1 was achieved. On the contrary, (Ni0.33Co0.67)9S8/C showed inferior rate capability at the same stepwise rates, with average capacities of 628.4, 400.1, 357.8, 234.7, 142.5, and 104.2 mA h g1

at the current densities of 100, 200, 500, 1000, 1500, and 2000 mA g-1,

respectively.

The

poor

cycling

stability

and

rate

capability

of

the

(Ni0.33Co0.67)9S8/C mixture could be due to the insufficient restraining effect of the carbon coating layer alone, which is not robust enough and will crack and crumble after several cycles, as illustrated in Figure 3d. The exposed metal sulfides suffer from uncontrollable volume expansion accompanied by the formation of loose aggregates, resulting in the pulverization of the electrode material. In contrast, the double-confinement effect provided by the hollow cavity and uniform carbon coating enables the (Ni0.33Co0.67)9S8@C to restrict volumetric variation and maintain integrity during cycling, as proved in Figure S14 by the SEM and TEM images of the electrode after 100 cycles. Figure S15 and Figure S16 show the cycling performance and rate capability in terms of volumetric energy density, as well as the Nyquist plots of (Ni0.33Co0.67)9S8/C



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mixture and (Ni0.33Co0.67)9S8@C nanospheres, respectively. The smaller depressed diameter of semicircle of the (Ni0.33Co0.67)9S8@C corresponded to the charge transfer resistance indicates a better kinetics of Na+ than that in (Ni0.33Co0.67)9S8/C mixture. To clarify the electrochemical kinetics, cyclic voltammetry (CV) tests on the (Ni0.33Co0.67)9S8@C nanospheres at various scan rates ranging from 0.1 to 1 mV s-1 were carried out to explore the nature of the sodium storage. As shown in Figure 3e, the peak current responses increase with increasing scan rate and gradually become broadened. Based on previous reports,62,

63

the relationship

between the peak current (i) and the scan rate (v) follows the equation: log i = b∙log v + log a, where a and b are adjustable parameters. If the b value is 0.5, the current response is diffusion-controlled, demonstrating a faradaic intercalation process, while the current is dominated by capacitive behavior when b is calculated to be 1.62, 63 Figure 3f displays the linear relationship between log i and log v for the cathodic and anodic peaks, with b values of 0.86 and 0.78, respectively. To quantitatively distinguish the capacity type, the contribution ratio of the capacitive- to the diffusion-controlled capacities at different scan rates was calculated based on the following equation: i(v) = k1v + k2v1/2, in which k1v and k2v1/2 correspond to the effects of capacitive behavior and the diffusioncontrolled process on the current response, respectively.62, 63 By plotting i/v1/2 vs. v1/2, the constants k1 (slope) and k2 (intercept) can be calculated to determine the contribution ratio, as shown in the bar chart in Figure 3g. It can be found that the


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peak capacitive-controlled contribution to the capacity increases gradually with increasing scan rate. The contribution ratio and b values (in the range of 0.8 - 1, corresponding to Figure 3f) are higher than for (Ni0.33Co0.67)9S8/C (as shown in Figure S17), indicating that the electrochemical reactions of (Ni0.33Co0.67)9S8@C nanospheres are predominantly capacitive in nature and that the capacitive kinetics are more favored. Figure 3h displays the contribution fraction from capacitive behavior, with a value of 84.7% at a scan rate of 0.6 mV s-1.

Figure 3. (a) Comparison of cycling performance of (Ni0.33Co0.67)9S8/C mixture and (Ni0.33Co0.67)9S8@C nanospheres at a current density of 1000 mA g-1 over 200 cycles, (b) comparison of the rate capabilities of (Ni0.33Co0.67)9S8/C and (Ni0.33Co0.67)9S8@C at various current densities, (c) galvanostatic charge/discharge profiles of (Ni0.33Co0.67)9S8@C for selected cycles at various current densities (corresponding to (b)), (d) schematic illustration of the changes in the morphology for the (Ni0.33Co0.67)9S8/C mixture and the



(Ni0.33Co0.67)9S8@C nanospheres during cycling, (e) cyclic voltammetry curves of (Ni0.33Co0.67)9S8@C nanospheres at different scan rates from 0.1 to 1.0 mV s1, (f) log (i, peak current) vs. log (v, scan rate) plots for the cathodic and anodic peaks in (e), (g) contribution ratio of capacitive- and diffusion-controlled capacities at different scan rates, (h) CV curve with capacitive-controlled capacity indicated at a scan rate of 0.6 mV s-1. To shed light on the sodium storage mechanism, in situ XRD analysis was performed by monitoring the peak changes in (Ni0.33Co0.67)9S8@C in the selected acquisition window range of 22 - 50° in Figure 4a. A new peak can be clearly discerned at around 24.8° in the 1st discharge state to 0.01 V (Figure 4b), which can be assigned to the (042) reflection of orthorhombic phase Na2S (JCPDS card No. 47-0178). The appearance of Na2S phase demonstrates the conversion reactions of metal sulfides within the sodiation process ((Ni0.33Co0.67)9S8 + 16Na+ + 16e- → 3Ni + 6Co + 8Na2S). Notably, a pair of peaks around 25.1° appear in the initial discharge to 0.01 V and merge together after the 3rd charge state to 3 V, which may be indexed to the (133) reflection of orthorhombic phase S (JCPDS card No. 02-0324), suggesting the partial transformation of S from S2- to S0.64, 65 More specific information can be provided in the patterns derived from a relatively narrow window range. As shown in Figure 4c, the intensity of the (311) peak of Co9S8 in (Ni0.33Co0.67)9S8@C decreases markedly over the course of cycling, while the (222) peak of Co9S8 vanishes in the initial discharge process, indicating the participation of Co9S8 in the sodium ion intercalation reaction and structural change from crystalline to amorphous.65 In contrast, a new broad peak located at around 29.3°, indexed to orthorhombic S (JCPDS card No. 02-0324),


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appears in the initial discharge process. Interestingly, the peak at 29.3° gradually splits into two peaks centered at about 29.2° and 29.4°, as shown in Figure 4d, which may be attributed to the transformation to polysulfides. It can be observed from the 3D waterfall graph (Figure S18) that the highest intensity of the S peak reversibly appears at the end of each charging process, which indicates that the generated S might be confined in the composite or trapped by carbon, while still making its due contribution to the sodium storage. Notably, no metallic Co or Ni peaks can be discerned in the in situ XRD curves in the fully discharged state, which may be due to the low crystallinity after the conversion reaction from metal

sulfides.

The

high

resolution

TEM

(HRTEM)

image

of

(Ni0.33Co0.67)9S8@C discharged to 0.01 V in Figure 4e confirms the formation and uniform distribution of metallic Co, Ni, and NaS2 in the fully discharged state. Ex situ XPS analysis was applied to supplement the investigation of the sodium reaction mechanism during the first cycle, as shown in Figure 4f. On discharging to 1 V, the spectrum shows the presence of a metallic Ni peak, which vanishes in the charge state at 3 V. By contrast, a metallic Co peak appears in the spectrum on discharging to 0.5 V, implying that Co9S8 is reduced to Co on discharging below 1 V. The results are consistent with the CV curve of the first cathodic scan, as is shown in Figure S19, where Ni is generated before Co in the sodiation process. In the subsequent cycles, however, there is only one cathodic peak, which appears at 0.92 V, corresponding to the simultaneous reduction of



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metal sulfides to metallic Co and Ni. Taking account of the uniform distribution of Ni and Co in the (Ni0.33Co0.67)9S8 solid solution, this phenomenon can be attributed to the prompt reaction of Co9S8 with Na+, which is catalytically trigged by the trace amount of Ni formed in advance, indicating a synergistic effect between

Ni

and

Co

to

facilitate

the

conversion

reaction

of

the

(Ni0.33Co0.67)9S8@C composite.

Figure 4. In situ XRD analysis of (Ni0.33Co0.67)9S8@C nanospheres: (a) 2D view in the range of 22 - 50°, (b) XRD patterns at various charge/discharge states corresponding to (a), (c) 2D view in the range of 27.5 - 31.5°, (d) XRD patterns at various charge/discharge states corresponding to (c), (e) HRTEM image of (Ni0.33Co0.67)9S8@C after discharging to 0.01 V, (f) ex situ XPS analysis of the (Ni0.33Co0.67)9S8@C nanospheres at different depths of charge/discharge during the first cycle.


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Compared with the reported cobalt- and nickel-based sulfides in the literature, our (Ni0.33Co0.67)9S8@C nanospheres display decent sodium storage properties (Table S1, Supporting Information). The enhanced sodium storage can be attributed to the following merits: i) the robust ball-in-ball structure has a hollow cavity to withstand the volume fluctuations derived from sodiation/desodiation processes, thus ensuring structural stability during repeated cycling; ii) the shortened transfer paths, high surface area, and abundant active sites for reaction are responsible for facilitated diffusion of sodium ions as well as enhanced sodium storage; iii) engineering a hierarchical hybrid with a conductive carbon network effectively guarantees high mechanical stability and facilitated electron migration; iv) the predominant capacitive behavior contributes fast charge transport kinetics; and v) the enlarged lattice parameters, weaker Co-S and Ni-S bondings, and the synergistic effect in (Ni0.33Co0.67)9S8@C could promote the conversion reaction and thus the electrochemical performance.

3. Conclusions In summary, this study demonstrates the synthesis of (Ni0.33Co0.67)9S8@C nanospheres with a ball-in-ball structure via an anion exchange method followed by a carbon coating treatment. The obtained (Ni0.33Co0.67)9S8@C shows a robust structure, taking advantage of the double protection originating from the extra space between the inner and outer balls and the uniform carbon layer to accommodate/buffer the volume changes during the processes of sodiation/de-



sodiation. Furthermore, this unique structural strategy offers an abundance of active sites, improved electrical conductivity, and facile transport pathways for sodium ions and electrons. The Rietveld refinement, in situ XRD, and ex situ XPS results revealed enlarged lattice parameters compared with Co9S8 and the conversion reaction mechanism of (Ni0.33Co0.67)9S8@C, accompanied by a synergistic effect, which is beneficial to promote the conversion reaction. CV tests indicated that capacitive behavior dominates the electrochemical reaction, which could boost the sodium ion intercalation/de-intercalation. As a result, the (Ni0.33Co0.67)9S8@C material manifests excellent electrochemical performance for sodium storage, including high capacitance, high rate behavior, and good cycling stability.

Supporting Information Available: Detailed physical characterizations (EDS pattern, XRD pattern, SEM images, TEM images, Raman spectra, N2 adsorption-desorption isotherm plot, and XPS analysis) and electrochemical performances (CV curves, rate capability, cycling performance, Nyquist plots).

Corresponding Authors: *E-mail: [email protected], [email protected]

Author Contributions † These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources The work is funded by the National Natural Science Foundation of China and the China Postdoctoral Science Foundation.

Conflicts of interest There are no conflicts to declare. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21701144), and the China Postdoctoral Science Foundation (Grant No. 2017T100536) are gratefully acknowledged. Moreover, the authors would like to thank Dr Tania Silver for critical reading of the manuscript and valuable remarks.

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248x190mm (150 x 150 DPI)


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Engineering Unique Ball-In-Ball Structured (Ni0.33Co0.67)9S8@C

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