Rate Sodium-Ion Anode - ACS Publications

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In-situ Formation of Co9S8 Nanoclusters in Sulfur-Doped Carbon Foam as A Sustainable and High-Rate Sodium-Ion Anode Yunxiao Wang, Yanxia Wang, Yun-xia Wang, Xiangming Feng, Weihua Chen, Jiangfeng Qian, Xinping Ai, Hanxi Yang, and Yuliang Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

In-situ Formation of Co9S8 Nanoclusters in SulfurDoped Carbon Foam as A Sustainable and HighRate Sodium-Ion Anode Yunxiao Wang,a Yanxia Wang,a Yun-xia Wang,b Xiangming Feng,c Weihua Chen,c Jiangfeng Qian,a Xinping Ai,a Hanxi Yang,a Yuliang Caoa*

a College

of Chemistry and Molecular Sciences, Hubei Key Lab. of Electrochemical

Power Sources, Wuhan University, Wuhan 430072, China

b Department

of Mechanical Engineering, Louisiana State University, Baton Rough, LA

70803.

c College

of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou

450001, China 1

ACS Paragon Plus Environment

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KEYWORDS: cobalt sulfides nanoclusters, carbon foam, Co9S8 anodes, sodium ion batteries, ultrahigh rate capability

ABSTRACT: Transition metal sulfides hold great promise as anode materials for sodium ion batteries due to the high theoretical capacity and excellent redox reversibility based on mutli-electron conversion reactions. In this work, an elaborate composite, cobalt sulfides nanoclusters embedded in honeycomb-like sulfur-doped carbon foam (Co9S8@S-CF), is prepared via a facile sulfur-assisting calcination strategy, which tactfully induces the co-occurance of in situ pore-forming, sulfidation, sulfur-doping, and carbonization. Notably, the sulfur-doped carbon foam (S-CF) possesses abundant voids, which are subject to construct three-dimensional (3D) ionic/electronic pathways, leading to high sodium-ion accessibility and ultrafast sodium- ion/electron transportation towards Co9S8 nanoclusters. When worked as an anode in sodium-ion batteries, it delivers remarkable capacity of 373 mAh g-1 over 1000 cycles at 0.25 C, achieving 2


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superior capacity retention of 80 %. Furthermore, this anode could achieve unprecedented rate capability with reversible capacity of 180 mAh g-1 at 50 C (20 A g-1), which significantly precedes the previous reports.

1. INTRODUCTION

Energy storage technologies have arousing global attentions over the past decades due to the ever-increasing demand for clean and efficient energy supplies. Lithium ion batteries (LIBs) have become sophisticated rechargeable energy storage systems, which are extensively applied into portable electronic devices and hold a great promise for powering electrical vehicles.1 However, Li is resource limited with uneven geographic distribution and the high cost of Li mineral impedes their practical applications in large-scale energy storage. In contrast, Na is abundant and costeffective, which is mainly distributed in the ocean and easy to obtain. Combined with the similar performance to their LIBs counterpart, Na ion batteries (NIBs) are emerging as 3



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one of potential candidates for stationary energy storage.2-5 Various low-cost anodes, including carbonaceous materials,6-9 phosphorus10-11 and metal phosphide,12-13 mentals14-17 and mental oxides,2,

18-20

and metal sulfides,21-28 have been intensively

studied. Especially, metal sulfides based on multi-electron conversion reactions, possessing high energy densities and sustainable cyclablity, have shown great potential for practical applications.

Co9S8 has been paid close attention as an anode in NIBs due to its decent theoretical capacity (544 mAh g-1), relatively low voltage plateau (0.3-0.7 V) and low cost.29-31 Nevertheless, Co9S8 anode tends to show inferior reversibility, serious voltage hysteresis, and low cycling stability due to the intrinsic low conductivity and serious volume expansion during sodiation/desodiation process. It is rational to engineer an ideal nanostructure with ultrafine active Co9S8 nanoclusters embedded in highconductivity carbon matrixes; especially, the porous C hosts can also serve as robust

4


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backbones to tolerate volume expansion of Co9S8. On the other hand, heteroatom doping, especially sulfur-doping, is believed to be effective to increase electronic conductivity and create defects and active sites in carbonaceous materials, which can accelerate Na-ion/electron transportation as well.32-33

Herein, we develop a simple S-assisting calcination strategy, which achieves a unique composite with Co9S8 nanoclusters confined in sulfur-doped carbon foam (denoted as Co9S8@S-CF) for large scale. Benefiting from the ultrafine Co9S8 nanoclusters and elaborate honeycomb-like structure of carbon foam, unique 3D ionic and electronic pathways are constructed. When tested in NIBs, the as obtained Co9S8@S-CF anode demonstrates superior electrochemical properties for sodium-ion storage in terms of specific capacity, cycling stability, and rate capability: it exhibits a high initial reversible capacity of 467 mAh g-1, and possesses excellent reversible capacity retention of 80 % (373 mAh g-1) after prolonged 1000 cycles, which is 76.8

5



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times higher than that of plain Co9S8 nanosheets (6.6 mAh g-1). Significantly, it delivers ultrahigh rate capability with a rate capability of 180 mAh g-1 at 50 C, suggesting its great promise for commercial utilization.

2. EXPERIMENTAL SECTION

2.1 Chemicals. All of the chemicals were analytical grade and used without further purification. CoCl2·6H2O, sodium oleate, Na2SO4 were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used in all experiments.

2.2 Samples preparation. 0.316 g CoCl2·6H2O were dissolved into 1.0 mL H2O, and mixed with 1.22 g sodium oleate, with continuous stirring for 3 h at 85 ⁰C for further reaction. The obtained cobalt oleate precursor was homogeneously mixed with 10 g Na2SO4 and 0.5 g sulfur by a planetary mixer. Subsequently, the produced complex was sintered at 600 ⁰C with a heating rate of 3 ⁰C min-1 for 3 h in Ar atmosphere. The calcinate was collected and immersed into 500 ml H2O for 24h. Afterwards, the 6


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mesoporous Co9S8@C nanosheets were collected after washing the mixture with deionized water and ethanol three times, followed by drying at 80 ⁰C in vacuum oven overnight. For comparison, bare S-doped carbon foam (S-CF) was prepared from the mixture of sodium oleate, Na2SO4, and S via the S-assisting calcination. Pure Co9S8 nanosheets were prepared according to a reported method.34

2.3. Structural characterization. The morphologies of the samples were investigated by scanning electron microscopy (SEM, ULTRA/PLUS, ZEISS) and transmission electron microscopy (TEM, JEOL, JEM-2010-fef). The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas (SBET) through N2 adsorptiondesorption measurements (Micromeritics, ASAP 2020). The X-ray diffraction (XRD) patterns were collected by powder XRD (Bruker D8 Advance) with Cu K radiation at a scan rate of 2 degree per minute, the ex-situ XRD at a scan rate of 0.5 degree per minute.

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2.4.

Electrochemical

measurements.

The

electrochemical

performance

was

systematically evaluated using coin-type half-cells. The electrode slurry consisted of 80 wt. % active materials (Co9S8@S-CF, S-CF, and Co9S8 nanosheets), 10 wt. % conductive carbon, and 10 wt. % carboxymethyl cellulose (CMC) binder and then spreading the mixed slurry onto the Cu foil, followed by drying at 80 °C overnight under vacuum. The electrolyte is 1.0 M NaClO4 in propylene carbonate (PC) /ethylene carbonate (EC) (1:1 in volume) and 5 wt. % fluoroethylene carbonate additive (5 wt. % FEC) used in this work. A sodium disc was employed as both reference and counter electrode with a glass fiber separator. The electrochemical performance was carried out using a LAND Battery Tester during cut-off voltage of 0.01-3V. Cyclic voltammetric (CV) measurements were conducted by a CHI 660C electrochemical workstation (ChenHua Instruments Co., China) 3. RESULTS AND DISCUSSION

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As illustrated in Figure 1a, a facile S-assisting calcination method is developed for the synthesis of Co9S8@S-CF. First, cobalt oleate (Co(OL)2) was formed due to the selfassembly of oleate reverse micelles with Co ions, which possess huge bulk structure as shown in Figure S1a and b (ESI†). Second, a homogenous precursor consisted of Co(OL)2, S and Na2SO4 was prepared by strong mixing procedures using a planetary mixer. When heated at Ar atmosphere, the S sublimation, Co2+ sulfidation and OAcarbonization processes take place subsequently, which leads to the formation of the final product (Co9S8@S-CF). A product prepared from Co(OL)2 and S but without Na2SO4 only showed loose and random particle structure (Figure S1c and d), which indicates that the introduced Na2SO4 serves as hard template to form flake-like structures. It is noticeable that the added sulfur plays multiple roles during these processes. Without S, the material obtained from Co(OL)2 and Na2SO4 shows solid nanosheet-like nanostructure, in which there is no voids existed on any nanoflakes (Figure S1e and f). It can be speculated that S not only works as raw materials to react 9



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Co2+ for the formation of Co9S8, but also lead to the unique foam-like texture of carbon backbones due to its rapid sublimation. In particular, it participates in the decomposition and carbonization of OA- species as well, resulting in favourable S-doping in the carbon. As revealed via scanning electron microscope (SEM) (Figure 1b) and transmission electron microscopy (TEM) images (Figure 1c), the sample possesses evident honeycomb-like structure with abundant nanovoids. The low magnification SEM images of the Co9S8@S-CF at different areas (Figure S2) confirm the high uniformity of this unique foam-like nanostructure. The high-resolution TEM (HRTEM) image in Figure 1d suggests the amorphous nature of carbon and low crystallization state of Co9S8. The lattice spacings are determined to be ~ 2.85 and 2.3 Å, respectively, which correspond to the (222) and (331) planes of Co9S8 phase. As shown in the corresponding highangle annular dark-field scanning TEM (HADDF-STEM) image (Figure 1e), the Co9S8 nanoclusters are uniformly anchored in the C matrix with nanosize of ~2 nm. Meanwhile, the HADDF-STEM image with the energy-dispersive X-ray elemental mapping images 10


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of C, S, and Co (Figure 1f) also further clearly demonstrates the unique foam-like nanostructure with elemental Co and S homogeneously distributed in CF backbones. The constructed nano-architecture is critical. The carbon matrix is favourable to enhance overall electrode conductivity and maintain the electrode integrity. The ultrasmall Co9S8 nanoclusters surrounded by high-conductive C tend to show high Nastorage activity and alleviate the volume expansion during sodiation/desodiation processes. More importantly, the foam-like structure and S-doping of carbon foam are expected to cause lopsided charge distribution around sulfur vacancies, leading to accelerated electron transport and Na ion diffusion. For comparison, pure Co9S8 nanosheets were prepared via the reported method.34 As shown in Figure S3, the SEM image shows that small Co9S8 nanosheets present on the surface; while the TEM images indicate that the interior Co9S8 are seriously stacked together; the HRTEM image demonstrates that the lattice fringes with interplanar spacing is 3.03 Å, which could be indexed to be the (110) planes of Co9S8.34 11



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The X-ray diffraction (XRD) patterns of the Co9S8@S-CF and Co9S8 nanosheets, as shown in Figure 2a, indicates that both of two samples could be indexed to Co9S8 (JCPDF no. 75-2236). The low intensity and broadened properties of the dominant diffraction peaks suggest the small nanosize and low crystallization of Co9S8 phase. In addition, a shoulder peak of the Co9S8@S-CF at about 26o, corresponding to amorphous carbon. The Raman spectra of the Co9S8@S-CF and Co9S8 nanosheets are shown in Figure 2b. The Raman peaks at ~ 474.8 and 679.1 cm-1 are due to the existence of Co9S8.35 Two prominent peaks at ~ 1349.4 (D band) and 1601.1 cm-1 (G band) related to carbon36 are only observed in the Co9S8@S-CF. The D band originates from the structural defects and disorder, while the G band corresponds to the first-order scattering of the E2g mode due to the sp2 domains.37 It is noteworthy that it exhibits high ID/IG ratio (~1.92, Figure S4), which indicates that there are many defects or disordered sites in the carbon the Co9S8@S-CF, corresponding to the amorphous feature of carbon.29 The chemical states of the elements are investigated by X-ray photoelectron 12


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spectroscopy (XPS) analysis. Figur e2c displays the C1s spectrum fitted into three peaks at 284.3, 285.5, and 288.6 eV, which correspond to C-C/C=C, C-S, and O=C-O, respectively, indicating the S-heteroatom doping in C. Two peaks observed at 780.8 and 796.6 eV with two satellite peaks at 786.4 and 801.8 eV are ascribed to Co 2p3/2 and Co 2p1/2 (Figure 2d). The high resolution XPS S 2p spectra shown in Figure2e clearly shows two peaks at 160.0 and 164.1 eV, which are supposed to represent S 2p3/2 and S 2p1/2 bonding, respectively. An apparent peak at 168.3 eV is observed as well, which is attributed to a S shake up satellite peak and S–C bonding, further confirming that sulfur heteroatoms are incorporated into the carbon foam via S-assisting calcination at Ar atmosphere.38 To evaluate the amount of C in the Co9S8@S-CF, the thermogravimetric analysis (TGA) was performed from 50 to 900 oC at a rate of 10 oC min-1 in air. As displayed in Figure 2f, the weight loss in area I from 50-400 oC is likely due to the evaporation of water remained in the sample and the decomposition of functional groups on the carbon foam; the evident weight loss (area II) between 400-500 13



oC

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is assigned to the combustion of carbon.39 The oxidation of Co9S8 takes place in the

range of 500-900 oC. Assuming the final product is pure Co3O4, the weight loss during this process is ~ 70.6 %, corresponding to a Co9S8 content of ~32 % in the composite. Similarly, the Co9S8 ratio in the hybrid could be also evaluated to be ~ 35 % based on the weight loss (~18.1 %) of bare Co9S8 nanosheets during this temperature range (Figure S5).

The sodiation/desodiation behaviours of the Co9S8@S-CF and Co9S8 nanosheets electrodes are evaluated by galvanostatic charge/discharge cycling. 2032 coin-type half-cells, paired with metallic Na foils, are used to investigate battery performance of these two anodes. The cyclic voltammograms (CV) curves of the Co9S8@S-CF at a scan rate of 0.25 mV s-1 during 5 cycles are shown in Figure 3a. It shows one cathodic peak at around 1.0 V during the initial scan, which is attributed to the sodiation reaction of Co9S8.40 There is an obvious spike near 0 V, which is ascribed to Na-ion insertion into

14


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the interlayer of the graphitic nano-crystallites of the graphene nanosheets.41 During the subsequent cathodic cycles, evident peaks at around 0.8 V evolve and well repeated, which could be assigned to the reversible conversion reaction along with the formation of Co and Na2S. As for the anodic sweep, the obvious peak at ~1.8 V corresponds to the desodiation process.40 It is noteworthy that the reduction peak at ~0.8 V and the oxidation peak at ~1.8 V is highly repeatable, suggesting high reversibility of the Co9S8@S-CF

during

sodiation/desodiation

processes.42

Accordingly,

the

charge/discharge curves of the Co9S8@S-CF battery at a current density of 100 mA g-1 demonstrates that it presents a long slope without clear plateau during discharge process (Figure 3b), which is ascribed to the occurrence of various reactions, including sodiation of Co9S8 and rapid absorption of Na ions into the S-doped carbon foams. However, It delivers a low initial Coulombic efficient of 53.7 %, which is due to the formation of solid electrolyte interfaces (SEI) film and occurrence of irreversible side reactions. When further discharged to 0.01 V, metallic Co and Na2S can be formed. 15



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Meanwhile, the charge/discharge curves at selected cycles show sustainable performance and high reversible capacity during subsequent 1000 cycles, indicating the excellent Na-storage properties of the electrode. In contrast, a pair of cathodic and anodic peaks at around 0.65 and 1.74 V are observed for the Co9S8 anode at the first cycle (Figure S6a), which is mainly corresponding to the conversion reaction between Co9S8 and Na. However, the peak intensity is gradually decreased during the following cycling, indicating that Co9S8 nanosheets suffer from low capacity and poor cycling stability. Accordingly, the charge/discharge curves of the Co9S8 show a long plateau at 0.75 V during the first discharge process, which delivers a discharge capacity of ~810 mA h g-1. However, it shows rapid capacity failure, retaining a low reversible capacity of ~213 mA h g-1 after only 10 cycles (Figure S6b).

Moreover, the cycling behaviours of these two electrodes at 0.25 C over 1000 cycles are presented in Figure 3c. It is manifest that the Co9S8@S-CF outperforms that of

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Co9S8 nanosheets in terms of reversible capacity, capacity retention, and cycling stability. The Co9S8@S-CF anode shows high reversible capacity retention of 80% (373 mAh g-1) after 1000 cycles with Coulombic efficiency above 95 %, which is very close to the theoretical capacity (390.4 mAh g-1) of the hybrid, implying that the nanoarchitectural stability of the unique foam-like structure. Moreover, the S-doping structure with enhanced conductivity is responsible for the accelerated charge transfer rate and additional active sites, which endow the sample with high reaction activity and fast electron transport. On the contrary, the Co9S8 nanosheets undergoes rapid capacity fade, which become inactive with negligible reversible capacity retention of 8.3 % (52 mAh g-1) after 30 cycles. It is likely due to the intrinsic low conductivity and rapid collapse of the Co9S8 nanosheets structure without any framework protection during the large volume expansions of sodiation/desodiation processes. Impressively, the Co9S8@S-CF exhibits superior overall performance in terms of cycling life (1000 cycles), reversible capacity (373 mAh g-1), and capacity retention (80%) over testing cycles (vs. 17



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CoxS-based anodes, Table S1). Beneficial from the unique nanostructure, the Co9S8@S-CF is expected to deliver outstanding rate capability. The rate capabilities of the two samples ranging from 0.25 to 50 C (1 C= 400 mAh g-1) are shown in Figure 3d. The Co9S8@S-CF delivers reversible capacities of 487, 387, 386, 340, 333, 280, 227, and 180 mAh g-1 at 0.25, 0.5, 1.25, 2.5, 5, 12.5, 25, and 50 C, respectively. Significantly, when the current density is back to 1.25 C, the reversible capacity of Co9S8@S-CF could maintain 380 mAh g-1 over 100 cycles. Furthermore, the corresponding charge/discharge curves of the Co9S8@S-CF (Figure S7) exhibited an evident charge plateau even at 25 C with increased polarization over cycling. It is remarkable that the hybrid exhibits unprecedented high-rate performance achieving ultrahigh rate capability 180 mAh g-1 at ultrahigh 50 C. In sharp contrast, the Co9S8 nanosheets exhibits decent initial reversible capacities of 542 mAh g-1, however, rapidly declines to 260 mAh g-1 after 10 cycles at 0.25 C; it becomes inactive with negligible capacity when current rate is higher than 1.25 C. The results also indicate that the constructed bare Co9S8 18


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nanosheets are easy to deteriorate during cycling. On the other hand, It is noteworthy that the S-doped C foam (S-CF) component is responsible for the sustainability of the Co9S8@S-CF composite. As shown in Figure 3c and d, the bare S-CF delivers an average reversible capacity of ~299 mAh g-1, corresponding to ~ 200 mAh g-1 in the hybrid based on the C ratio. This result indicates the S-CF and Co9S8 equally contribute to the overall capacity of the Co9S8@S-CF. The rate capability of S-CF is much inferior than the hybrid, which implies that the synergetic effects of the Co9S8 and S-CF components are responsible for the unprecedented rate capability of the Co9S8@S-CF. In contrast with previous reports for NIBs, It shows superior rate capability (vs. typical metal sulfides-based anodes, Figure 3e). The electrochemical performance of the Co9S8@S-CF underscores that rational nanostructure is conclusive to its performance.

In order to investigate the high-rate kinetics of the Co9S8@S-CF, CV curves carried out at various scan rates from 0.1 to 50 mV s-1 between 0.01 - 3.0 V are obtained

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shown in Figure S9a. It is well known that the current intensity of a CV curve is consisted of surface-controlled and diffusion-controlled processes. The total capacity usually composes of the faradaic and non-faradaic contributions, which could be calculated and illustrated in Figure 4a (the detail of calculation is shown in Supporting Information). As displayed in Figure S9b, the b value is calculated to be 0.7 (between 0.5 and 1) for the redox peaks, which demonstrates that electrochemical reactions of Co9S8@S-CF contains both faradic and nonfaradaic processes.43 By calculating the k1 and k2 value, the composition of capacitive effects and diffusion-controlled reactions could be distinguished. The capacitive contribution will dominate the reaction possess when the surface capacitive contribution is higher than 50%. As shown in Figure 4a, by the quantification, the contribution of surface capacitive capacity (red region) is as high as about 72 % at 5 mV s-1, which indicates that the Co9S8@S-CF possesses a favourable charge transfer kinetics.44 The capacitive capacity increases with raising the scan rate and obtains a maximum value 92 % of at 50 mV s-1 (inset in Figure 4a). The 20


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high portion capacity from the surface-controlled process is favourable to realize high rate performance, which agrees with the outstanding rate capability in Figure 3e. Ex situ XRD is conducted to identify the Na-storage mechanism during electrochemical measurement (Figure 4b). The peaks of (311) and (222) could be detected in the fresh Co9S8@S-CF electrode. When it was fully discharged to 0.01 V, the XRD signals of Co9S8@S-CF disappear, and new two peaks are developed, corresponding to the formation of Na2S (JCPDF no. 47-0178) and Co (JCPDF no. 15-0806). When charged to 3 V, the peaks of (311) and (440) of Co9S8@S-CF could be recovery; meanwhile, the peaks of Na2S and Co decrease notably with possible Co residual. It confirms that the reaction between Co9S8 and Na2S is highly reversible, well consistent with stable cycling performance of the Co9S8@S-CF anode (Figure 3c). This result suggests a Naion storage mechanism of Co9S8-based nanocomposites via the multi-electron conversion reaction (Co9S8 + 16Na ↔ 9Co + 8Na2S).

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The compositional and morphological changes in Co9S8@S-CF and Co9S8 nanosheets after 1000 cycles are investigated by SEM, TEM and STEM in Figure 4c-e. As revealed in Figure 4c and d, the Co9S8@S-CF retains the honeycomb-like nanostructure; while the surface become rough due to the formation of thick SEI films. The elemental mappings of the electrode at fully discharged state after 1000 cycles show that Na and S elements have the consistent signals, indicating the presence of Na2S. The signals of Co and C element is observed around the Na2S phase, which is ascribed to the occurrence of reversible conversion reaction with the formation of Co nanograins. In addition, the corresponding phase mappings and energy-dispersive Xray spectroscopy (EDS) results further confirm the coexistence of Na2S and Co phases (Figure S10). In addition, intensive F signal is collected as well, which is assigned to side-products with FEC additive and the formation of SEI film, which is homogenously dispersed in the sample. Interestingly, SEM images of the Co9S8 electrode (Figure S11) demonstrate that the morphology of the Co9S8 nanosheets have collapsed into dense 22


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and massive bulk after 1000 cycles, which verifies that the bare Co9S8 nanosheets cannot maintain its morphology during cycling and result in rapidly capacity fade. The manifold functions of the Co9S8@S-CF towards excellent Na-storage properties, therefore, could be speculated as illustrated in Figure 5. Firstly, the active Co9S8 itself is ultrasmall nanoclusters and closely surrounded by high conductive C frameworks, which enable Co9S8 component high reactivity for complete sodiation/desodiation. Secondly, the open foam-like structure could expose more reaction sites and provide more ionic/electronic channels, leading to additional shortened route and fast mobility for 3D electronic and ionic transport. Thirdly, the introduction of S-doping in the C foam could create a number of defects and active sites in C, which could increase the adsorbent ability for sodium ions, thereby enhancing surface capacitive effects. Moreover, the amorphous carbon matrix with abundant voids could not only enhance the overall conductivity and reaction rate, but also serve as backbones to accommodate volume expansion and prevent the elaborate nanostructures from degradation. 23



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4. CONCLUSIONS

In summary, the unique Co9S8@S-CF structure has been successfully prepared by a facile sulfur-assisting calcination strategy. With thin carbon foam frameworks,

S-doped

abundant

active

sites/defects,

and

high

reactive

Co9S8nanoclusters for fast Na-ion storage, the Co9S8@S-CF anode could deliver decent reversible capacity of 373 mAh g-1 over prolonged 1000 cycles and ultrahigh rate capability of 180 mAh g-1 at 50 C. The developed sulfur-assisting calcination provides a new simple strategy to construct elaborate Co9S8 anode in large-scale, which simultaneously achieves the formation of Co9S8 nanoclusters, elaborate carbon foam, and favourable S-doping, which significantly optimizes the Na-storage properties of Co9S8 anode and can be extended to develop various metal sulfides-based anodes for NIBs applications.

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Figure 1. (a) Schematic illustration for in situ formation of Co9S8 nanoclusters confined in sulfur-doped carbon foam (denoted as Co9S8@S-CF). (b) SEM image, (c) TEM image, (d) HRTEM with (e) the corresponding HADDF-STEM images, and (f) elemental mapping of C (red), S (yellow), Co (green) of the Co9S8@S-CF.

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Figure 2. (a) XRD patterns and (b) Raman spectra of Co9S8@S-CF and pure Co9S8 nanosheets. High resolution XPS spectra of (c) Co 2p, (d) C1s, and (e) S 2p of Co9S8@S-CF. (f) TGA curve of the Co9S8@S-CF nanocomposite.

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Figure 3. (a) Cyclic voltammograms of the Co9S8@S-CF anode for the initial five cycles at a scan rate of 0.25 mV s-1. (b) Charge/discharge curves of the Co9S8@S-CF anode for selected cycles. (c) Cycling performance of the Co9S8@S-CF, Co9S8 nanosheets, and S-CF electrodes at 0.25 C (1C= 400 mA g-1). (d) Rate capabilities at various current densities of the Co9S8@S-CF, Co9S8 nanosheets, and S-CF electrodes. (e) Comparison 28


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of rate capabilities of the Co9S8@S-CF electrode with other anode materials of NIBs in the literature (Co9S8/C sphere: ref.29, Co3S4-PNS/GS: ref.45, Co1-xS/FGN: ref.40, CNT/CoS: ref.46, Co9S8/C nanocomposite: ref.47, Co3S4@polyaniline: ref.48, Carbonencapsulated CoS: ref.49).

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Figure 4. (a) Capacitive (pink) and diffusion-controlled (purple) contribution to charge storage of the Co9S8@S-CF at 5 mV s-1. (Inset: normalized contribution ratio of capacitive (pink) and diffusion-controlled (purple) capacities at different scan rates of 0.1,

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0.25, 0.5, 1, 2.5, 5, 10, 25, and 50 mV s-1, respectively.) (b) Ex-situ XRD patterns of the Co9S8@S-CF anode when discharged to 0.01 V and charged to 3 V. (c)-(e) SEM, TEM, and STEM images of the Co9S8@S-CF anode after 1000 cycling, with the corresponding elemental mapping for Na (Cyan), S (yellow), Co (red), C (green), and F (Dark red).

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Figure 5. Schematic illustration of the rational design of 3D ionic/electronic pathways of the the Co9S8@S-CF nanocomposite for high-performance Na-ion storage.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website

Experimental details, related characterization, and supporting results and discussion

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Y.C.).

Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT This research was financially supported by the National Natural Science Foundation of China (Nos. 21805212 and 21673165) and China Postdoctoral Science Foundation (2017M620332).

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