Cobalt sulfides/dodecahedral porous carbon as anode materials for Na-ion batteries

Abstract

One facile synthetic route of 3-dimensional core–shell cobalt sulfides/dodecahedral porous carbon (cs-Co_xS_y/DPC) that is in situ derived from a Co-based zeolitic imidazolate framework (ZIF-67) has been reported. The cs-Co_xS_y/DPC shows good Na-storage performance.

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For several decades, lithium-ion batteries (LIBs) have been successfully utilized as a power source in hybrid electric vehicles and mobile electronic devices due to their excellent electrochemical performance. However, the present technology for LIBs is not sufficient to meet the demands for future energy storage applications. In addition, the large-scale commercial applications of LIBs are hindered by the resource shortage and the extraction costs of the lithium.^1,2 Sodium-ion batteries (SIBs) that possess the advantages of low cost, wide distribution, and natural abundance are considered to be candidates for LIBs.^3–5

Due to the large molecular mass and Na⁺ ion radius, the most widely used graphite anode material in commercial LIBs is not suitable for SIBs.^6,7 It's necessary and urgent to develop the non-carbon materials to replace the graphite materials for SIBs. In recent years, metal sulfides such as FeS₂,^8–10 MoS₂,^11–13 Ni₃S₂,^14–16 SnS₂,^17–19 WS₂,²⁰ CoS₂²¹ with remarkable sodium storage capacity, which are much higher than the carbonaceous materials, have been studied as the anode materials for SIBs. However, the larger ionic radius and poor electrochemical kinetics of SIBs, which leads to the slower charge/discharge process. Besides, the charge/discharge cycling process of the metal sulfides materials are accompanied by the large volume change and polarization phenomena, which would significantly destroy the mechanical structure of pristine electrode, resulting in the rapid capacity fading and short cycling life of SIBs.

Enlightened and inspired by the strategy utilized in the LIBs, the introduction with the carbonaceous materials is expected to be an effective way for improving sodium storage capacity of metal sulfides. Carbonaceous materials can alleviate the particle aggregation and volume change during cycling, prevent the structure collapse of electrode materials, enhance the electrical conductivity, and offer shorter ion and electron pathways. So the introduction of conductive carbonaceous material would significantly improve sodium storage performance of metal sulfides. Shadike et al.²¹ reported the improved sodium storage performance of CoS₂–MWCNT nanocomposites. The results showed that the specific capacity of CoS₂ electrode in NaClO₄-EC/PC rapidly dropped to lower than 170 mA h g⁻¹ at 100 mA g⁻¹, while the CoS₂–MWCNT electrode maintained the specific capacity of 410 mA h g⁻¹ after 100 cycles. Ko et al.²² reported that Co₉S₈–carbon composite materials could obviously improve the cycling and rate performance of SIBs. The results showed that the Co₉S₈–carbon composite electrode exhibited a reversible specific capacity of 404 mA h g⁻¹ after 50 cycles, which showed the better cycle stability compared with the bare Co_1−xS electrode. These reports proved that the introduction of conductive carbonaceous material seems to be a promising solution to improve the sodium storage performance of metal sulfides.

In this study, a facile method for synthesis of 3-dimensions core–shell cobalt sulfides/dodecahedral porous carbon (cs-Co_xS_y/DPC) that derived from ZIF-67 precursor was reported, as shown in Fig. S1.† The well shaped dodecahedral crystals ZIF-67 were prepared via a simple aqueous precipitation method. After mixed with sulfur powder, the ZIF/S composites were heated to 155 °C and maintained at this temperature for 10 h to make the sulfur molecule completely infused into the pore of ZIF-67. Then the temperature was elevated up to 600 °C, under this temperature the Co-ions were liberated from ZIF-67 upon decomposition followed by the reaction with sulfur powders to form cobalt sulfides. Simultaneously, the organic frameworks were decomposited and transformed into carbon dodecahedral skeleton. In the resultant structure, the CoS nanoparticles are uniformly confined in the core or shell of the porous carbon matrix.

The powder X-ray diffraction (PXRD) patterns were conducted to confirm the crystallinities and purities of the as-prepared samples. The PXRD pattern of ZIF-67 in the range of 5–40° matches well with the previous report,²³ demonstrating the particles of ZIF-67 with good purity and crystallinity (Fig. 1a). Through the pyrolysis process at 600 °C, ZIF-67 suffered from simultaneous thermal decomposition and sulfidation and then transformed into the well shaped cs-Co_xS_y/DPC. The XRD pattern of cs-Co_xS_y/DPC is presented in Fig. 1b, the more intense and sharper diffraction peaks correspond to characteristic (100), (101), (102), and (110) planes of CoS phase (JCPDS no. 65-3418), the other weaker diffraction peaks are corresponding to the Co₉S₈ phase (JCPDS no. 65-6801). No diffraction peaks of ZIF-67 precursors or impurities were observed, suggesting a successful conversion from ZIF-67 to cs-Co_xS_y/DPC.

Fig. 1 (a) XRD pattern of ZIF-67, (b) XRD pattern of cs-Co_xS_y/DPC.

Thermogravimetric analysis (TGA) was performed in air atmosphere to confirm the content of carbon in the as-prepared composite. As shown in Fig. S2,† the TGA curve of the Co_xS_y/DPC composite exhibit several weight losses corresponding to the evaporation of adsorbed water molecules below 200 °C, the decomposition of carbon, and the oxidation of cobalt sulfide to cobalt oxide. The weight of adsorbed water is about 17 wt%, and the weight loss of carbon in the composite can be calculated to be 22 wt%. Ignore the weight of the adsorbed water in the composite, the carbon content should be 26.5 wt% and the CoS content should be 73.5 wt%.

The morphology of the as-prepared samples were observed by the scanning electron microscopy (SEM) image. The SEM images of ZIF-67 samples show that the obtained ZIF-67 precursor have a rhombic dodecahedral morphology with smooth surface, and the average size is about 500 nm and without aggregation (Fig. 2a and b). After thermal decomposition and sulfidation, the obtained cs-Co_xS_y/DPC particles retain the dodecahedral morphology with the rough surface (Fig. 2c). From the high-magnification SEM image (Fig. 2d), it's obviously observed that there are a large number of nanoparticles growing on the cs-Co_xS_y/DPC particle surface.

Fig. 2 (a) Low magnification and (b) high magnification of ZIF-67 sample, (c) low magnification and (d) high magnification of cs-Co_xS_y/DPC sample.

The transmission electron microscope (TEM) measurement was carried out to further investigated the interior structure of the cs-Co_xS_y/DPC samples. From the low-magnification TEM image (Fig. 3a), it clearly shows that the cs-Co_xS_y/DPC samples possess sphere core and dodecahedral shell that small CoS uniformly embedded in the core and shell matrix. The particle size of cs-Co_xS_y/DPC samples are about 500 nm which is in accordance with the SEM results. The high-magnification TEM image shown in Fig. 3b further clearly reveals the core–shell structure of cs-Co_xS_y/DPC samples. The thickness of the carbon dodecahedron shell is about 50 nm, and the diameter of the carbon sphere core is about 300 nm. In the carbon sphere core, the small CoS nanoparticles uniformly intertwine with the carbon matrix that confined by the carbon dodecahedral shell. What's more, there are still some small CoS nanoparticles embeds in the carbon dodecahedral shell. The elemental mapping analysis of a cs-Co_xS_y/DPC sample is shown in Fig. 3c–f, which clearly demonstrate the coexistence of Co, S, C elements in the core and shell matrix. From the elemental mapping images, it's easily found that the Co, S, C element homogenously dispersed in the core and shell framework of the cs-Co_xS_y/DPC sample. The HRTEM image and corresponding SAED patterns of cs-Co_xS_y/DPC were shown in Fig. S3.† It can be observed in the inset image of Fig. S3a,† that the lattice fringes of 0.292 nm and 0.254 nm corresponding to the (100) and (101) plane of CoS and the lattice fringes of 0.351 nm corresponding to the (220) plane of Co₉S₈ are coexistent in the Co_xS_y/DPC particle. The selected area electron diffraction (SAED) pattern shown in Fig. S3b† with clear rings is indicative of the polycrystalline structure of CoS phase and the existence of Co₉S₈ phase, which is in accordance with the XRD result.

Fig. 3 (a) TEM image and (b) high-magnification image of cs-Co_xS_y/DPC, (c) element mapping area of cs-Co_xS_y/DPC, (d) Co element, (e) S element, (f) C element.

The electrochemical performance of cs-Co_xS_y/DPC material that one-step in situ formation from ZIF-67 template for sodium storage is shown in Fig. 4. The CV profiles of cs-Co_xS_y/DPC at a scan rate of 0.2 mV s⁻¹ in the potential range of 0.01–3.0 V are shown in Fig. 4a. In the first CV cycle, the main reduction peak potential is at ∼0.5 V which corresponding to the electrochemical conversion reaction of Co_xS_y/DPC with Na⁺, it can be expressed as the following equation:

CoS + 2Na⁺ + 2e⁻ ↔ Co + Na₂S (1)

Co₉S₈ + 16Na⁺ + 16e⁻ ↔ 9Co + 8Na₂S (2)

Fig. 4 Electrochemical performance of cs-Co_xS_y/DPC composite for sodium storage. (a) CV curves in the voltage windows of 0.01–3.0 V at a scan rate of 0.2 mV s⁻¹, (b) galvanostatic charge/discharge curves at a current density of 500 mA g⁻¹.

There is another small reduction peak observed at around 0.75 V, this reduction peak may attribute to the irreversible decomposition of the fluoroethylene carbonate (FEC) additive in the electrolyte which could form stable and thin solid-electrolyte-interface (SEI) layers. In the charging process, there are two oxidation peaks around 1.5 V and 1.8 V that may caused by the multi-step oxidation of Co metal to CoS_x. In the following 2nd–5th CV cycles, the four CV profiles were substantially overlapped, and the reduction and oxidation peaks were observed at around 0.8 V and 1.8 V, respectively, implying the repeated Na⁺ insertion and extraction processes.

Fig. 4b represents the selected galvanostatic charge/discharge curves of the cs-Co_xS_y/DPC material at a current density of 0.5 A g⁻¹. In the first discharged curve, it can clearly observed that the cs-Co_xS_y/DPC electrode shows two plateau at around 0.8 V and 0.55 V, corresponding to the electrochemical conversion reaction of Co_xS_y/DPC with Na⁺, which is in accordance with the CV test result. The initial discharged and charged capacities are about 600 mA h g⁻¹ and 390 mA h g⁻¹, respectively. The coulombic efficiency is calculated to be 65%, that the low initial coulombic efficiency mostly caused by the SEI layers formation. In the second and tenth charged/discharged curves, the stable discharge plateau and charge plateau are substantially overlapped, and the two plateaus at around 0.8 V and 1.8 V are corresponding to the reduction and oxidation peaks in the CV curves.

The cycle performance and rate capacity of the cs-Co_xS_y/DPC materials are shown in Fig. 5. All the specific capacities in this study were based on the mass of cs-Co_xS_y/DPC material. As shown in Fig. 5a, the cs-Co_xS_y/DPC electrode shows the initial discharge capacity of 600 mA h g⁻¹ at a current density of 0.5 A g⁻¹, and the second cycle discharge capacity dramatically drop to 380 mA h g⁻¹. That the rapid capacity fading could attribute to the SEI layers formation and the decomposition of electrolyte. The reversible capacity remains 300 mA h g⁻¹ after 50 cycles, and the capacity retention rate is calculated to be 77% when the initial discharge capacity is excluded, which shows great cycle performance and stable sodium storage capacity. Besides, from the second cycle onwards, the coulombic efficiencies of the cs-Co_xS_y/DPC electrode steadily maintained at approximately 100%. As listed in Table S1,† the discharge capacity of cs-Co_xS_y/DPC electrode is higher than the Co₃S₄-PNS/GS materials²⁴ and is close to the Co₉S₈/C materials.²² It indicates that cs-Co_xS_y/DPC derived from ZIF-67 can be considered as the promising energy storage material for SIBs. The rate performance as a significant characteristic of the high-performance of SIBs is shown in Fig. 5b. The average specific capacities of the cs-Co_xS_y/DPC electrode are 400, 380, 340, 280, 230 mA h g⁻¹ at the current densities of 0.1, 0.2, 0.5, 1, 2 A g⁻¹ (correspond to 0.094, 0.188, 0.47, 0.94, 1.88, 4.7 mA cm⁻²), respectively. The great cycle stability, high coulombic efficiency and rate performance of cs-Co_xS_y/DPC electrode may normally attribute to its spaticular structure, that the core–shell structure could provide the additional free volume to alleviate the volume change caused by the sodium insertion and extraction. In additions, carbonaceous matrix with excellent mechanical flexibility effectively buffers the volume changes of cobalt sulfides active material, resulting in great cycle stability and rate performance of the cs-Co_xS_y/DPC electrode.

Fig. 5 Electrochemical performance of cs-Co_xS_y/DPC composite for sodium storage. (a) Cycling performance and coulombic efficiency at a current density of 0.5 A g⁻¹, (b) rate capability at various current densities of 0.1, 0.2, 0.5, 1, 2 A g⁻¹ (correspond to 0.094, 0.188, 0.47, 0.94, 1.88, 4.7 mA cm⁻²).

Conclusions

In summary, we have demonstrated a facile method for synthesis of cs-Co_xS_y/DPC that in situ derived from ZIF-67. When applied for sodium storage material, cs-Co_xS_y/DPC composite shows a high reversible capacity of 300 mA h g⁻¹ after 50 cycles at 0.5 A g⁻¹, and the coulombic efficiencies of the cs-Co_xS_y/DPC electrode steadily maintained at approximately 100%. The average specific capacities of the cs-Co_xS_y/DPC electrode are 400, 380, 340, 280, 230 mA h g⁻¹ at 0.1, 0.2, 0.5, 1, 2 A g⁻¹, respectively. The great sodium storage performance of the cs-Co_xS_y/DPC might be attributed to the core–shell structure of the cs-Co_xS_y/DPC material that could provide the additional free volume to alleviate the structural strain of the active materials and the carbonaceous matrix which could effectively buffer the volume changes in the charge/discharge process.

Acknowledgements

The authors acknowledge the financial support of the Teacher Research Fund of Central South University (2013JSJJ027). This project was supported by Grants from the Project of Innovation-driven Plan in Central South University (2015CXS018) (2015CX001) and supported by State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China This research was supported by the Chinese National Natural Science Foundation (51474243).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21589g

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