A sodium storage material based on chamber-confined conversion of

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A sodium storage material based on chamber-confined conversion of Co9S8-nanorod encapsulated by N-doped carbon shell Xiaodong Lin, Xueyang Cui, Han Yan, Jie Lei, Pan Xu, Jingmin Fan, Ruming Yuan, MingSen Zheng, and Quanfeng Dong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b03912 • Publication Date (Web): 17 Aug 2019 Downloaded from pubs.acs.org on August 18, 2019

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ACS Sustainable Chemistry & Engineering

A sodium storage material based on chamberconfined conversion of Co9S8-nanorod encapsulated by N-doped carbon shell Xiaodong Lin,‡ Xueyang Cui,‡ Han Yan, Jie Lei, Pan Xu, Jingmin Fan, Ruming Yuan, Mingsen Zheng* and Quanfeng Dong* Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, 422, Siming South Road, Xiamen 361005, Fujian, China ‡These authors contributed equally to this work. *Corresponding authors’ e-mail: [email protected]; [email protected]

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ABSTRACT Different from the ion intercalation/deintercalation, conversion-based reactions are a promising way to achieve high capacity of storage materials. However, their too much volume change and poor reversibility during discharge/charge are stubborn to overcome. Herein, by combining hydrothermal synthesis and chemical vapor deposition (CVD), a Co9S8-nanorod encapsulated by N-doped carbon shell (r-Co9S8@NC) is built and applied as an anode material for sodium-ion batteries (SIBs). Benefiting from the advantages of chamber-confined conversion and sodiophilic interface offered by the N-doped carbon shell, the Co9S8 redox was well restrained in the shell with improved kinetics, which can not only alleviate the Co9S8/electrolyte side reaction and suppress the large volume expansion and aggregation of Co9S8 nanorods, but also enhance the sodium storage/diffusion ability. As expected, the r-Co9S8@NC electrodes exhibit a high capacity of 675 mAh g-1 at 50 mA g-1, an excellent rate capability (342 mAh g-1 at 10 A g-1) and cycling stability (317 mAh g-1 after 1500 cycles at 5 A g-1 and 483 mAh g-1 after 150 cycles at 500 mA g-1), among which the rate capability is outperforming most of the reported metal-sulfide-based anodes. Further, the sodium storage mechanism and the good reversibility of r-Co9S8@NC electrode have been revealed and confirmed by the XRD and XPS characterizations. This work provides an effective strategy to design storage materials based on chamber-confined conversion reaction.

KEYWORDS: Co9S8-nanorod, N-doped carbon shell, sodium-ion batteries, chamber-confined conversion, sodiophilic interface


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INTRODUCTION Sodium-ion batteries (SIBs) have been considered as a promising alternative to lithium-ion batteries (LIBs) due to the natural abundance and low cost of sodium sources.1-6 Unfortunately, because of the larger radius (1.02 Å) of sodium ion compared to that of lithium ion (0.76 Å), many intercalated/deintercalated anode materials suitable for LIBs systems show sluggish sodium ion diffusion kinetics and poor electrochemical performance, which cannot be directly applied in SIB systems.7-8 For example, graphite, which is a successful commercial anode material of LIBs, however, cannot intercalate sodium ions effectively because of the limited interlayer space.7-8 Although some other anode materials have been explored and proven to be effective in realizing sodium intercalation/deintercalation, limited by the intrinsic intercalation chemistry whose reactions contain few electrons, their capacity is relatively low and hard to enhance.9-12 Moreover, the

layer

structures

of

these

materials

are

severely

distorted

during

sodium

intercalation/deintercalation, which will cause the destruction and collapse of the material structure, leading to poor cycle stability and rate capability.5,13 Therefore, exploitation of suitable anode material that can enable high capacity and favourable sodium storage is crucial for the development of SIBs. Compared to the intercalated/deintercalated SIBs anodes, transition metal sulfides (TMSs) based on conversion reaction mechanism can transfer multiple electrons per metal center, thus offering a higher theoretical gravimetric and volumetric specific capacities.14-26 In addition, they also possess suitable redox potential and excellent redox reversibility, which is considered as a promising class of anode materials for SIBs. Among these TMSs, Co9S8, an important member of TMSs, has gained special attention due to its high capacity, high thermodynamic stability, low cost and near-metallic conductivity.22,26 However, its development still faces challenges of large volume variation, particle aggregation, sluggish sodiation/desodiation kinetics and instable solid electrolyte


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interphase (SEI),27-28 which will inevitably result in low reversibility, limited capacity, poor cycling stability and rate capability of the conversion reactions. In the past few years, many researchers have developed various concepts and strategies on nanoarchitecture design to address these problems. Xu and co-workers have reported that the nanocrystallization of active material can shorten the sodium-ion diffusion length and resist the strain that caused by volume expansion.29 And the nanowire/nanorod’s unique geometry can also enhance the accommodation of the transformation strains.30 On the other hand, encapsulation by a coating layer (carbon) to form a confined reaction space should be a more promising approach, which can not only improve the electrical conductivity, prevent the agglomeration of active particle during cycling and alleviate the strain effect triggered by sodiation/desodiation, but also reduce the direct contact interface between the active particles and the electrolyte, thus constructing a stable SEI layer and reducing the side reactions.22,26-27,31 In addition, it is not enough to have only one protective shell, and the shell should be better to have a special function. For example, some studies proposed that the introduction of nitrogen dopant, especially the pyridinic N and pyrrolic N, can improve the sodiation/desodiation reactions by inducing surface defects that provide more diffusion channels and active sites for sodium ions.32-33 Based on the above analysis, to achieve a superior conversion-based sodium storage material, the nanowire/nanorod morphologies are a good choice for their excellent ability to withstand the strain of substantial volume change. Additionally, a suitable encapsulated shell with special features that can not only offer a chamber-confined reaction but also enhance the sodium storage/diffusion ability is also very crucial. Therefore, we are dedicated to construct a N-doped carbon shell encapsulated nanorod material that can simultaneously achieve the abovementioned properties. Herein, by combining hydrothermal synthesis and chemical vapor deposition (CVD), we have successfully prepared a Co9S8-nanorod (r-Co9S8) encapsulated by N-doped carbon shell (r-Co9S8@NC) and then used it as


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an anode material for SIBs. Apart from its excellent electrical conductivity, the carbon shell can also offer a chamber-confined reaction of r-Co9S8, which can effectively suppress the Co9S8/electrolyte side reaction, the large volume expansion and the aggregation of r-Co9S8, ensuring a highly stable structure, thus, improving the cycling stability and reversibility. Moreover, as revealed by the density functional theory (DFT) calculations, we found that the nitrogen dopant of the carbon shell can fabricate a sodiophilic interface (Figure 1), which can enhance the adsorbability and the storage/diffusion ability of sodium, thus, promoting the sodiation/desodiation kinetics, leading to a high rate capability. As a result, the r-Co9S8@NC based anodes exhibited high performance with a high capacity of 675 mAh g-1 at 50 mA g-1, an excellent rate capability (342 mAh g-1 at 10 A g-1) and cycling stability (317 mAh g-1 after 1500 cycles at 5 A g-1 and 483 mAh g-1 after 150 cycles at 500 mA g-1). To the best of our knowledge, the rate capability of the rCo9S8@NC electrode is superior to most of the published data (Table S1).

Figure 1. Schematic illustration of the chamber-confined conversion reaction of the r-Co9S8@NC composite and the interaction between the N-doped carbon shell and sodium ions in SIBs.

EXPERIMENTAL Synthesis of r-Co9S8, r-Co9S8@C and r-Co9S8@NC The r-Co9S8 were prepared according to the previous method.34 Specifically, 2 mmol CoCl2∙6H2O, 4 mmol urea and 10 mmol NH4F were sufficiently dissolved into 150 mL deionized


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(DI) water to form a pink solution. Then, the homogeneous solution was transferred into a Teflonlined stainless steel autoclave and kept at 120 oC for 10 hours in an electric oven. After cooling to room temperature, the pink precursor was washed with DI water and ethanol for several times and dried in a 60 oC vacuum oven. Further, the precursor was calcinated at 400 oC for 1 h under argon atmosphere in a tube furnace and the CoO product was obtained. At last, 0.2 g CoO powders was dispersed into a 12.5 mL aqueous solution that contained 1.5 g Na2S∙9H2O. And the solution was transferred into an autoclave and kept at 120 oC for 36 hours. The final black powder was washed with DI water and ethanol for several times after reaction and dried in a 60 oC vacuum oven. Subsequently, the r-Co9S8 powder (0.1 g) was placed in a quartz tube and calcinated at 600 °C in a tube furnace in Ar atmosphere with a heating rate of 5 °C min-1, followed by bubbling C2H2 (flow rate: 60 ml∙min-1) with Ar for 15 min at 600 °C to obtain r-Co9S8@C composite. And the rCo9S8@NC composite was synthesized through a process that is similar to that for making rCo9S8@C but using CH3CN instead of C2H2. Materials Characterizations The SEM and TEM images were obtained by field emission scanning electron microscopy (FESEM, HITACHI S-4800) and TECNAI high resolution transmission electron microscope (HRTEM, F30), respectively. X-ray diffraction (XRD) measurements were conducted by using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (λ =1.5418 Å). The element chemical states of the pristine materials, discharged and charged products were characterized by X-Ray Photoelectron Spectroscopy (PHI 5000 Versa Probe III, ULVAC-PHI, Japan). Elemental contents of the as prepared material were measured by a Vario EL III Elementar. Nitrogen adsorption/desorption experiments were performed by using a Micromeritics TriStar II3020 surface area and pore analyser at 77 K. Electrochemical Measurements


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70 wt.% active materials, 20 wt.% Super P and 10 wt.% PVDF binders were mixed in the Nmethyl-2-pyrrolidinone (NMP) solvent and ball-milled into homogeneous slurry. Then, the slurry was coated on Cu foil current collectors and dried in a vacuum oven at 120 oC for 12 h. Further, CR2016-type coin cells were assembled in an argon-filled glovebox (H2O < 1 ppm, O2 < 1 ppm) with a sodium foil as counter electrode, a glass fiber as separator, 1 M NaCF3SO3 in diglyme as electrolyte and the slurry-coated Cu foil as working electrode. The Galvanostatic charge-discharge testing was carried out on a NEWARE BTS-5 V/5 mA type battery charger (Shenzhen NEWARE Co. LTD, China) within the voltage range of 0.4-2.8 V (vs. Na+/Na). Cycle voltammetry was tested by using an IVIUM multichannel electrochemical analyzer. The electrochemical impedance spectrometry (EIS) tests were carried out with an impedance analyzer (IM6, Zahner elektrik, Germany) in a frequency range from 1 MHz to 0.01 Hz with an amplitude voltage of 5 mV. All the current densities and specific capacity values were calculated with respect to the mass of active material. The optimized mass loading of the active material in each electrode is about 1 mg. All the electrochemical measurements were conducted at room temperature. Computational Details All of the DFT calculations were carried out with the Gaussian 09 program using ωB97XD functional theory.35 To model the pure carbon shell, we used a circumcoronene molecule (C96H26). The computational models of N doping models were similar to our previous paper.36 The orbitals were described by Gaussian basis sets with 6-311+G (d) for N and O and 6-31G(d, p) for C, H, Na atoms.37-40 All atoms were allowed to relax during the geometry optimization without any symmetry constraint. To account for the solvent effect, the universal solvation model (PCM) was used,41 and diglyme was adopted as the solvent. To reduce the overestimation of the entropy contribution, we employed a correction of -2.6 (or 2.6) kcal∙mol-1 for 2:1 (or 1:2) transformations as many earlier theoretical studies did.42-43


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The adsorption free energies of Na+ ions adsorbing on carbon shell surfaces can be defined as: ΔGads = G(Na+/surf.) - G(Na+) - G(surf.) - 2.6 kcal∙mol-1 The adsorption free energies of Na+(η3-diglyme)2 ions adsorbing on carbon shell surfaces can be defined as: ΔGads = G[Na+(η3-diglyme)/surf.] + Gdiglyme - GNa+(η3-diglyme)2 - G(surf.)

RESULTS AND DISCUSSION Structural and morphological characterizations of r-Co9S8@NC composite The r-Co9S8@NC sample was constructed by combining hydrothermal synthesis and chemical vapor deposition (CVD) as depicted detailedly in Experimental Section. Subsequently, several characterization technologies were used to confirm the successful synthesis of the r-Co9S8@NC sample. The crystal structures of r-Co9S8, r-Co9S8@C and r-Co9S8@NC were characterized by Xray diffraction (XRD) measurements (Figure 2a, b and Figure S1). The three distinct peaks centered at 2θ = 31.2o, 47.5o and 52.1o are corresponding to the crystalline data of Co9S8, which can be indexed to the face-centered cubic structure with Fm3m space group (JCPDS No. 03-065-6801). The broad and weak diffraction peaks of r-Co9S8 material in Figure 2a can be attributed to the poor crystallinity of the pure r-Co9S8. Interestingly, after CVD, both the r-Co9S8@C and r-Co9S8@NC exhibited strong and sharp diffraction peaks, indicating good crystallinity of r-Co9S8@C and rCo9S8@NC materials. To further prove the successful synthesis of r-Co9S8@NC, the X-ray photoelectron spectroscopy (XPS) experiment was conducted to analyze the surface chemical compositions of the r-Co9S8@NC composite. Figure 2c-f show the high-resolution Co 2p, S 2p, C 1s and N 1s spectra of r-Co9S8@NC composite, respectively. Specifically, as shown in Figure 2c, the peaks centered at 778.4 and 793.5 eV of Co 2p spectra are corresponding to the Co 2p3/2 and


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Co 2p1/2, respectively, which are consistent with the previous report very well for Co9S8.22,34,44-46 In addition, the characteristic peaks of 161.4 eV for S 2p3/2 and 162.6 eV for S 2p1/2 were observed in the S 2p spectrum of r-Co9S8@NC (Figure 2d),22,34,44-46 confirming the presence of Co9S8 phase again. Moreover, the peak centered at 163.4 eV can be assigned to C-S-C bond,22,34,44-46 implying the coupling effect between the Co9S8 phase and the carbon shell. The C 1s spectra (Figure 2e) shows three peaks centered at 284.7 eV, 285.6 eV and 288.2 eV, corresponding to C=C, C=N and C-N bonds,22,47 respectively. Obviously, the formation of C=N and C-N bonds strongly confirmed the successful doping of nitrogen into the carbon shell. Particularly, the nitrogen dopant can also be testified by the N 1s spectra (Figure 2f), which shows the evidence of the corresponding signals of pyridinic N and pyrrolic N, centering at 398.5 eV and 400.4 eV,22,47 respectively. The content of N in the r-Co9S8@NC material is about ~2.456 wt.% (See Table S2), which was measured by a Vario El Elemental Analyzer.

Figure 2. XRD patterns of the r-Co9S8 (a) and r-Co9S8@NC composite (b). XPS spectra of Co 2p (c), S 2p (d), C 1s (e) and N 1s (f) for the r-Co9S8@NC composite.


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Further, we used the field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) to investigate the morphologies and microstructures of r-Co9S8, rCo9S8@C and r-Co9S8@NC samples (Figure 3 and Figure S2-3). Figure 3a, b and e demonstrate that the as prepared r-Co9S8 sample possesses a 1D nanorod structure with a diameter of 200 nm. The high resolution TEM (HRTEM) image (Figure 3f) displays the distinct lattice fringe with a dspacing of 0.57 nm, corresponding to the (111) plane of cubic structured Co9S8 phase. The energydispersive X-ray (EDX) elemental mapping analysis (Figure 3i-k) shows the homogenous distribution of Co and S elements within the 1D nanorod framework. Figure 3c, d and Figure S2 demonstrate that both the morphologies of r-Co9S8@NC and r-Co9S8@C have no significant change after CVD, preserving the 1D nanorod structure. In addition, both the lattice fringes with d-spacing of 0.57 nm in Figure 3h and Figure S3 are in accord with the (111) plane of cubic structured Co9S8 phase. Meanwhile, both the thickness of the shells in r-Co9S8@NC and rCo9S8@C are about 10 nm, which can reduce the direct contact interface between Co9S8 nanorods and liquid electrolyte but with free Na+ transport, thus, alleviating the Co9S8/electrolyte side reaction. In addition, the carbon shell can offer a chamber-confined reaction of Co9S8 nanorods, thus, suppressing the large volume expansion and aggregation of Co9S8 nanorods. The high-angle annular dark-field scanning TEM (HAADF-STEM) image of r-Co9S8@NC composite shows that there is indeed a uniform shell coating on the Co9S8 nanorod surface (Figure 3l) and the EDX maps in Figure 3m-p indicate that the Co, S, C and N elements are also dispersed uniformly in the 1D nanorods. Furthermore, the energy dispersive spectroscopy (EDS) disclose that the atomic ratio of Co and S elements were close to 9:8 for r-Co9S8@NC (Figure S4).


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Figure 3. (a, b) SEM images of r-Co9S8. (c, d) SEM images of r-Co9S8@NC. (e) TEM image of r-Co9S8. (f) HRTEM image of r-Co9S8. (g) TEM image of r-Co9S8@NC. (h) HRTEM image of r-Co9S8@NC. (i-k) HAADF-STEM image and corresponding EDX maps of r-Co9S8, scale bar: 100 nm. (l-p) HAADF-STEM image and corresponding EDX maps of r-Co9S8@NC, scale bar: 100 nm.

Additionally, the surface area and porous structure of r-Co9S8, r-Co9S8@C and r-Co9S8@NC were investigated by nitrogen adsorption/desorption experiments (Figure S5). As shown in Figure S5a, c and e, the Brunauer-Emmett-Teller (BET) specific surface area of the r-Co9S8, r-Co9S8@C and r-Co9S8@NC is about 24.7, 6.7 and 4.9 m2 g-1, respectively. The Barrett-Joyner-Halenda (BJH) pore size distribution (Figure S5b, d, f) have disclosed that the pure r-Co9S8 sample has a lot of


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mesoporous pores under 20 nm and the number of these mesoporous pores is drastically reduced after CVD for r-Co9S8@C and r-Co9S8@NC. The decrease of the number of these mesoporous pores further suggests that the contact interface between the Co9S8 active material and the liquid electrolyte will be decreased, which is beneficial to suppress repeated formation of instable SEI on the Co9S8 surface. Electrochemical performances of the r-Co9S8@NC based sodium-ion batteries To investigate the sodium storage performance of r-Co9S8, r-Co9S8@C and r-Co9S8@NC anodes, cyclic voltammetry (CV) was conducted in a voltage window of 0.4-2.8 V with a scan rate of 0.1 mV s-1 (Figure 4a and Figure S6). In the first cycle of CV curves (Figure 4a), the rCo9S8@NC anode exhibits a sharp cathodic peak centered at 0.6 V, which can be attributed to the conversion reaction of Co9S8 (Co9S8 + 16Na+ + 16e- → 9Co + 8Na2S) and the formation of SEI layer. While the anodic peak observed at 1.7 V is related to the desodiation process from Co and Na2S to Co9S8 (9Co + 8Na2S → Co9S8 + 16Na+ + 16e-). After the first cycle, the cathodic peak at ~0.6 V is divided into two peaks at 0.61 V and 0.92 V, corresponding to the sodiation process of Co9S8 that is related to the reorganization of the texture of the reactive atoms, which is consistent with the previous reports.22 It is worth mentioning that all the CV curves were almost overlapped after the first cycle, demonstrating the excellent reversibility. In addition, the CV behaviors of the r-Co9S8 and r-Co9S8@C anodes are both similar to that of the r-Co9S8@NC anode (Figure S6), indicating the same mechanism for sodium storage in these three materials. The galvanostatic discharge-charge profiles with the initial three cycles of the r-Co9S8, rCo9S8@C and r-Co9S8@NC anodes at a current density of 50 mA g-1 were shown in Figure S7a, b and Figure 4b, respectively. In the first cycle (Figure 4b), an obvious voltage platform at 0.72 V is similar to the cathodic peak of the CV curves, which is related to the phase transformation from Co9S8 to Co and Na2S. The discharge and charge capacity in the first cycle were 675 and 587 mAh


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g-1, respectively, with a high initial Coulombic efficiency of 87%. The small irreversible capacity may be due to the formation of SEI layer on the electrode surface accompanying with the decomposition of electrolyte, which is a common phenomenon for the conversion-based materials.48-50 Accordingly, the voltage platform near 1.58 V in the first cycle can be attributed to the phase transformation from Co and Na2S to Co9S8. In the second cycle, there are two discharge platforms locating at 1.07 and 0.95 V, respectively, corresponding to the CV results. In addition, the discharge and charge capacity in the second cycle were 594 mAh g-1 and 583 mAh g-1, respectively, leading to a much higher Coulombic efficiency of 98%. Moreover, the dischargecharge profiles were almost overlapped from the second cycle, and the capacity has no significant attenuation, indicating a good stability and high reversibility of the r-Co9S8@NC anode. As shown in Figure S7, compared to the r-Co9S8@NC anode, the bare r-Co9S8 anode and r-Co9S8@C anode exhibit low capacity of 664 mAh g-1 and 670 mAh g-1 in the first discharge, respectively, and low Coulombic efficiency of both 81 % in the first cycle, revealing the best electrochemical performance of the r-Co9S8@NC anode. Figure 4c demonstrates the cycling performance and the corresponding Coulombic efficiency of r-Co9S8@NC electrode at a current density of 500 mA g-1 and compared with that of r-Co9S8 and r-Co9S8@C electrodes. After the initial capacity loss induced by the formation of SEI layer, the rCo9S8@NC electrode exhibits a reversible capacity of 587 mAh g-1 in the first charge process. After 150 cycles, the r-Co9S8@NC and r-Co9S8@C electrodes can still deliver a stable capacity of 483 and 417 mAh g-1, respectively, demonstrating a high capacity retention of 82 % and 77 %, respectively, meanwhile accompanying with a Coulombic efficiency of nearly 100 %. However, the r-Co9S8 exhibit much faster capacity fading, the capacity was only maintained at 171 mAh g-1 after 150 cycles, corresponding to a capacity retention of only 32 %. This drastic capacity decay of the bare r-Co9S8 electrode can be ascribed to the large volume expansion and aggregation of Co9S8


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nanorods as well as the repeated formation of instable SEI layer, confirming the important role of the shell. Specifically, due to the protective effect and chamber-confined effect of the carbon shell, the direct contact between Co9S8 and electrolyte can be avoided as possible, thus the side reactions will be suppressed. And the large volume expansion and aggregation of Co9S8 nanorods can also be alleviated. Hence, the two carbon coated materials showed improved cycling stability compared to the bare one. In addition, as the previous literatures showed,32-33 the nitrogen doping can induce surface defects for carbon shell and then provide more diffusion channels for sodium ions, thus, promoting the sodiation/desodiation kinetics. Therefore, the r-Co9S8@NC electrode exhibits better cycling stability and reversibility than the r-Co9S8@C electrode. Figure 4d shows the rate performance of the r-Co9S8, r-Co9S8@C and r-Co9S8@NC electrodes at a current density range from 0.05 to 10 A g-1. As the current density increasing from 0.05 to 10 A g-1, the r-Co9S8@NC electrode delivers a decreasing specific capacity of 582, 543, 518, 494, 476, 455, 408 and 342 mAh g-1 at 0.05, 0.1, 0.2, 0.5, 1, 2, 5 and 10 A g-1, respectively, indicating an excellent rate capability of r-Co9S8@NC, which is much better than that of r-Co9S8@C (255 mAh g-1 at 10 A g-1) and Co9S8 (56 mAh g-1 at 10 A g-1) electrodes. To the best of our knowledge, this remarkable rate performance of the r-Co9S8@NC electrode is also superior to most of the reported metal-sulfide-based anode materials (See Table S1). Moreover, when the current density comes back to 0.05 A g-1, the reversible capacity can be recovered to 537 mAh g-1 and then remains stable in the subsequent cycles. As shown in Figure S8, either after working for 25 cycles at 100 mA g-1 or 100 cycles at 2 A g-1, the 1D nanorod structure of the r-Co9S8@NC material can be preserved well, demonstrating that the carbon shell can ensure a highly stable structure during battery cycles and the structure is stable even at high current density, which can be attributed to the functionalized properties of the N-doped carbon shell.


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Figure 4. (a) Representative CV curves of the r-Co9S8@NC anode at a voltage range of 0.4-2.8 V with a scan rate of 0.1 mV s-1. (b) The 1st, 2nd and 3rd galvanostatic discharge-charge profiles of r-Co9S8@NC anode at a current density of 50 mA g-1. (c) Cycling performance of the r-Co9S8, rCo9S8@C and r-Co9S8@NC anodes under a current density of 500 mA g-1 (The batteries were activated at 100 mA g-1 in the first five cycles before operating at 500 mA g-1). (d) Rate capability of the r-Co9S8, r-Co9S8@C and r-Co9S8@NC anodes. (e) Cycling performance and the corresponding Coulombic efficiency of r-Co9S8@NC anode under a current density of 5 A g-1 (The battery was activated at 100 mA g-1 in the first five cycles before operating at 5 A g-1).

To account for the excellent rate performance of the r-Co9S8@NC material, the electrochemical impedance spectroscopy (EIS) experiments of the three pristine electrodes were conducted and the results were shown in Figure S9 and Table S3. As depicted in Figure S9a, the semicircle at the high frequency region is corresponding to the charge transfer resistance (Rct) of the electrochemical reaction at the solid-liquid interface, whereas the sloping line located at the low frequency region is related to the Na ions diffusion.51-52 After fitting with the equivalent circuit diagram (Figure S9b), the Rct value of the r-Co9S8, r-Co9S8@C and r-Co9S8@NC is 128.7, 67.8 and 47.9 Ω,


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respectively (Table S3). Obviously, the r-Co9S8@NC electrode possesses the lowest Rct value, which can offer the fastest electrochemical reaction kinetics, leading to excellent rate capability. Furthermore, the long-term cycling performance of r-Co9S8@NC electrode was evaluated at an ultrahigh current density of 5 A g-1. As shown in Figure 4e, after the initial capacity loss correlated with the formation of SEI layer and the electrochemical activation of low current density (100 mA g-1) in the first five cycles, the capacity of r-Co9S8@NC electrode can also be maintained at 317 mAh g-1 stably even after 1500 cycles, accompanying with the Coulombic efficiency of nearly 100 %. Further, to gain deeper insight on the N doping effects, thus revealing why the N doping can improve the sodiation/desodiation reactions, we carried out DFT calculations to explore the adsorption behaviours of solvated Na+ ions on carbon models. Four kinds of computational models were constructed, i.e. pure, graphitic N-doped, pyridinic N-doped and pyrrolic N-doped carbon surfaces, see details in Figure S10. By means of implicit solvation model, we found that the adsorption free energies of Na+ adsorbing on pure, graphitic N-doped, pyridinic N-doped and pyrrolic N-doped carbon surfaces were predicted to be -1.2, 4.8, -19.5 and -9.3 kcal∙mol-1, respectively (Figure 5). In fact, the electrolyte contained diglyme solvent. DFT calculations showed that two diglyme molecules could coordinate with Na+ to form six-coordinated Na+ [Na+(η3diglyme)2] with a binding free energy of -27.8 kcal∙mol-1 (Figure S10a). When Na+(η3-diglyme)2 approached to the carbon surfaces, one coordinated diglyme would be released and the adsorbed Na+ was in the form of Na+(η3-diglyme) (Figure 6). Interestingly, even considering the solvation effect, the pyridinic N-doped and pyrrolic N-doped carbon surfaces still have stronger adsorption energies with respect to the undoped one by 4.2-13.4 kcal∙mol-1. It was reasonable because pyridinic and pyrrolic type N atoms would serve as Lewis base sites, which would stabilize Na+ species. All these findings came to the conclusion that pyridinic and pyrrolic N doping can increase


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the Na+-affinity of the carbon surface and fabricate a sodiophilic interface, which would promote the sodiation/desodiation kinetics, thus, enhancing the electrochemical performance of SIBs. These DFT calculation results are nicely consistent with the experimental results (Figure 4c, d and Figure S9a).

Figure 5. The optimized structures of Na+ adsorbing on pure, graphitic N-doped, pyridinic Ndoped and pyrrolic N-doped carbon surfaces, respectively. The energy unit is kcal∙mol-1.

Figure 6. The optimized structures of Na+(η3-diglyme) adsorbing on pure, graphitic N-doped, pyridinic N-doped and pyrrolic N-doped carbon surfaces, respectively. The energy unit is kcal∙mol1.

Electrochemical reaction mechanisms of the r-Co9S8@NC based sodium-ion batteries


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To verify the reversibility and the reaction mechanism of r-Co9S8@NC material, ex situ XRD and XPS characterizations were conducted (Figure 7). Figure 7a and b demonstrate the ex situ XRD patterns of the r-Co9S8@NC electrodes that were collected at different voltage states. Clearly, the pristine electrode exhibits five distinct peaks centered at 29.6o, 31o, 39.3o, 47.4o and 52o, which can be assigned to Co9S8 phase. After discharge to 0.72 V, the peaks of Co9S8 phase became weaker, accompanied by the appearance of two new weak peaks at 38.8o and 42.5o, corresponding to the (220) crystal plane of Na2S phase and the (111) crystal plane of Co phase, respectively, illustrating the Co9S8 phase was transforming into Na2S and Co phases, confirming the conversion reaction mechanism of r-Co9S8@NC electrode. Further, when discharge to 0.4 V, the diffraction peak intensity of Na2S and Co phases are enhanced while the peaks of Co9S8 phase are all disappearing, implying that the Co9S8 phase has been completely converted to Na2S and Co phases. On the contrary, when charge to 1.58 V, the peaks of Co9S8 phase at 29.6o and 31o can be re-observed and the peaks of Na2S and Co phases are correspondingly weakened, indicating the Na2S and Co phases can be reversibly converted back to Co9S8 phase. Finally, with charging up to 2.8 V, the intensity of the peaks of Co9S8 phase became stronger, meanwhile, the peaks of Na2S and Co phases disappeared, exhibiting a high reversibility of our r-Co9S8@NC electrode. It is worth mentioning that the diffraction peak intensity of the Na2S phase, formed during the conversions, is very weak, which can be attributed to its poor crystallinity. Therefore, we also used the ex situ XPS technology to confirm the conversion reaction mechanism of r-Co9S8@NC material. As shown in Figure 7c, when discharged to 0.4 V, the characteristic peaks of Co9S8 phase centered at 161.4 eV for S 2p3/2 and 162.6 eV for S 2p1/2 (Figure 2d) were both disappearing, whereas three new peaks centered at 161.4, 160.2 and 159.1 eV representing Na2S phase have emerged,53 suggesting the Co9S8 phase has been completely converted to Na2S and Co phases, which is consistent with the ex situ XRD result. In addition, the peaks centered at 164.9, 166.2,


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167.5 and 168.7 eV may be assigned to the thiosulphate, polythionate and sulfate complex species,54-55 which are probably originating from oxidized sulfide species.55 After full charge (Figure 7d), the characteristic peaks of Co9S8 phase centered at 161.4 eV for S 2p3/2 and 162.3 eV for S 2p1/2 were both regenerated, confirming the good reversibility of our r-Co9S8@NC electrode again.

Figure 7. (a) Ex situ XRD patterns of the r-Co9S8@NC electrode collected at various voltage states. (b) The galvanostatic discharge-charge profile of r-Co9S8@NC electrode showing the specific voltage state that is collected for XRD characterization. Ex situ S 2p XPS spectra of r-Co9S8@NC electrode after discharge to (c) 0.4 V and (d) charge to 2.8 V.

CONCLUSION


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In summary, in order to achieve the conversion-based storage materials, we have successfully fabricated a functionalized protective shell, a N-doped carbon shell, by which the r-Co9S8 was encapsulated inside. Benefitting from the confined conversion and sodiophilic interface, the rCo9S8@NC electrode showed excellent electrochemical performance. The shell can not only alleviate the Co9S8/electrolyte side reaction, but also suppress the large volume expansion and aggregation of r-Co9S8, which can enhance the reaction reversibility and the structure stability. On the other hand, the nitrogen dopant can help to form a sodiophilic interface, thus enhancing the sodium storage/diffusion ability. As a result, the r-Co9S8@NC electrode exhibits an excellent rate capability (342 mAh g-1 at 10 A g-1) and a good cycling performance (317 mAh g-1 after 1500 cycles at 5 A g-1 and 483 mAh g-1 after 150 cycles at 500 mA g-1), which is a very promising anode for SIBs. Furthermore, ex situ XRD and XPS characterizations have revealed and confirmed the good reversibility of the r-Co9S8@NC electrode. This work provides an effective strategy to realize a high reversible conversion-based nanometer material by creating a special reaction space.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website. Additional XPS, SEM, EDS, nitrogen adsorption/desorption isotherms, pore size distribution, CV curves, EIS, electrochemical performance, and computational data (PDF)

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National 973 Program (2015CB251102), the Key Project of National Natural Science Foundation of China (U1805254, 21673196, 21621091, 21703186), the Fundamental Research Funds for the Central Universities (20720150042, 20720170101). The authors thank Dr. Yunchuan Tu (iChEM, Dalian Institute of Chemical Physics) and Mo Zhang (iChEM, Xiamen University) for their assistance in CVD synthesis.

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For Table of Contents Use Only

Benefitting from the advantages of chamber-confined conversion and sodiophilic interface offered by the N-doped carbon shell, the r-Co9S8@NC electrode exhibits excellent sodium storage performances.


29

A sodium storage material based on chamber-confined conversion of

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