Xiao Sua,
Wen Li*a,
Haining Sunb,
Jian Wang*b,
Sisi Hua,
Fei Yuana,
Di Zhanga and
Bo Wang*a
aHebei Key Laboratory of Flexible Functional Materials, School of Materials Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050000, China. E-mail: liwen_cc@yeah.net; wangbo1996@gmail.com
bInnovation Center for Hebei Intelligent Grid Distribution Technology, Shijiazhuang Kelin Electric Co., Ltd, Shijiazhuang 050000, China. E-mail: wangjian0534@sina.com
First published on 4th January 2022
Reasonable regulation and synthesis of hollow nanostructure materials can provide a promising electrode material for lithium-ion batteries (LIBs). In this work, utilizing a metal–organic framework (MOF, ZIF-67) as the raw material and template, a composite of CoxSy with a carbon shell is successfully formed through a hydrothermal vulcanization and a subsequent high temperature sintering process. The as-obtained CoxSy(700) material sintered at 700 °C has a large specific surface area, and at the same time possesses a hollow carbon shell structure. Benefiting from unique structural advantages, the volume change during the electrochemical reaction can be well alleviated, and thus the structural stability is greatly improved. The presence of the carbon matrix can also offer sufficient ion/electron transfer channels, contributing to the enhanced electrochemical performance. As a result, the CoxSy(700) electrode can deliver an excellent capacity of 875.6 mA h g−1 at a current density of 100 mA g−1. Additionally, a high-capacity retention of 88% is achieved after 1000 cycles when the current density is increased to 500 mA g−1, and exhibiting a prominent rate capability of 526.5 mA h g−1, simultaneously. The novel synthesis route and considerable electrochemical properties presented by this study can afford guidance for the exploration of high-performance cobalt sulfide anodes in LIBs.
Transition metal sulfides (TMSs) are considered as one of the most promising anode materials for lithium-ion batteries because they not only have a high specific capacity, but also generally exhibit good electrical conductivity, and excellent mechanical and thermal stability.12 Among them, cobalt sulfides (CoxSy) with excellent electrochemical activity and high theoretical capacity (>400 mA h g−1), as a subset of transition metal sulfides, have been extensively investigated in lithium-ion battery anodes.13,14 Unfortunately, the huge volume change caused by the insertion/extraction process of bulky Li-ions will inevitably induce structure fracture of the CoxSy electrode, thus exhibiting a poor cycling stability.12 At present, many efforts have been made to remove these obstacles. For example, constructing nanostructures of CoxSy particles can significantly reduce the diffusion pathway of ions and increase the contact area between the electrode and electrolyte, which is conducive to improving the reaction kinetics.15 Besides, integrating CoxSy active material and a carbon buffer matrix can not only effectively improve the conductivity of the composite electrode, but also well maintain its structural integrity against deformation upon cycling, thereby enhancing the rate capacity and cycling stability.16,17 In addition, rationally designed hollow porous structures can generate numerous active sites for lithium storage, and in addition the large specific surface area induced by the porous structure can also provide fast channels for ion/electron transfer, thus enabling superior electrochemical performance.18,19
As is well known, metal organic frameworks (MOFs) with a unique and uniform morphological structure can provide a huge opportunity for the preparation of hollow structure materials when they act as a template or precursor.20,21 Therefore, exploring the application of MOF-based derived materials in electrochemical energy storage has attracted more and more attention. Zeolite imidazole salt frameworks (ZIFs) are typical MOF materials, in which ZIF-67 is usually recognized as an effective precursor or template for constructing porous and hollow structured materials, due to its good adjustable physical and chemical properties, typical nano-particle size, large surface area and effective mass transfer.22 Previous studies have shown that ZIF-67-derived cobalt sulfide can effectively alleviate structural changes caused by stress, attributed to the rich cavities that it generates. Also, the porous thin walls can expose more electrochemically active sites to allow full penetration of the electrolyte and rapid ion transfer, thereby ensuring an excellent electrochemical performance.23,24
In view of the above mentioned, combining the individual merits of nano-size, hollow structure, and a carbon buffer matrix in one anode material is attractive. Herein, a composite (CoxSy(700)) of a hollow CoxSy nanomaterial and carbon skeleton is synthesized by utilizing ZIF-67 as a precursor. The resulting CoxSy(700) is expected to offer facile Li-ion kinetics, fast electron conductive pathways and robust structural stability. When CoxSy(700) is applied as an anode for LIBs, it displays a satisfactory reversible capacity of 875.6 mA h g−1 at 100 mA g−1, and an outstanding cycling lifetime over 1000 cycles is also achieved at 500 mA g−1. More importantly, as a good candidate for the anode, CoxSy(700) demonstrates a facile design strategy to construct the electrode material in LIBs.
The hollow CoxSy powder was sintered at 600 °C, 700 °C and 800 °C for 3 hours in a full argon atmosphere to obtain a CoxSy/C composite electrode material named hollow CoxSy(n), where n represents the sintering temperature (n = 600 represents 600 °C, n = 700 represents 700 °C, n = 800 represents 800 °C).
X-ray powder diffraction (XRD) was performed to verify the composition and structure evolution of the obtained products. For ZIF-67 (Fig. S2a†), it can be clearly seen that the peaks of the starting seeds matched well with the patterns of simulated Co-based ZIF-67, which corresponds with the previous reported MOFs (JCPDS 33-0448).26 After high temperature carbonization, the diffraction peaks of Co (JCPDS 88-2325) and C (JCPDS 75-1621) are obviously observed in Fig. S2b.† This result indicates that the organic components present in the metal–organic framework material are successfully pyrolyzed to form carbon element during the carbonization process, and cobalt metal ions can be thermally reduced to metallic Co by carbon at this stage.27 In addition, the diffraction peaks of the ZIF-67(600) phase are strong and sharp, indicating that an overall crystallization has occurred. As shown in Fig. 2, the Co9S8 and Co4S3 can be detected through the XRD pattern of CoxSy(700), corresponding to JCPDS card number 73-1442 and 73-1703, respectively. In particular, the Co9S8 and Co4S3 are confirmed as belonging to the Fmm(225) and Fdm(227) space group, respectively.28,29 Therefore, it can be determined that cobalt–sulfur compounds are formed after the vulcanization modification and subsequent carbonization process. As a result, the obtained cobalt–sulfur compounds will increase the storage capacity of lithium.
The XPS spectrum was obtained to further confirm the chemical composition and bonding information of the obtained products. As depicted in Fig. S3,† all of the samples are composed of C, O, N, S, and Co elements. Concretely, the detected O element can be ascribed to a small amount of oxidation in the test, and the N element resulted from the decomposition of the carbon source. The C 1s spectrum of CoxSy(700) can be divided into three peaks, which correspond to C–C (284.8 eV), C–N (286.0 eV) and CO (288.25 eV), respectively. The C 1s spectrum in CoxSy(700) (Fig. 3a) is deconvoluted into three spectral peaks, namely, C–C (284.8 eV), C–N (286.05 eV) and CO (288.52 eV),16,20 respectively, indicating that C element still exists in the final material CoxSy(700). Moreover, the Co 2p spectrum of CoxSy(700) in Fig. 3b presents a Co3+ peak (794.21 eV/778.34 eV) in addition to the Co2+ (795.93 eV/780.44 eV) and sat. peaks (803.72 eV/783.12 eV).8,22 It is worth mentioning that the S 2p spectrum (Fig. 3c) is assigned as S2− (162.53 eV) and Sn2− (163.92 eV), indicating the successful preparation of cobalt–sulfur compounds.8,14,30
In addition, SEM and TEM were used to observe the morphological structure of the prepared materials. The topography of the ZIF-67 template is shown in Fig. S4a–c;† the ZIF-67 precursor presents a uniform dodecahedron structure, and its diameter is approximately 200–300 nm (Fig. S4a and b†). The TEM image (Fig. S4c†) exhibits a smooth surface with a thickness of 5–10 nm; this structural feature is favorable for forming a uniform and stable carbon shell during the high temperature annealing process. After hydrothermal vulcanization, the formed CoxSy also exhibits a dodecahedron shape, and the particle size is similar to that of the ZIF-67 precursor (Fig. S4d–f†). Different from the solid structure of ZIF-67, the CoxSy displays a hollow nanocage architecture, and simultaneously possesses an obvious shell wall with a thickness of about 5 nm. After sintering at a high temperature of 600–800 °C, the hollow nanocage structure is maintained well (as shown in Fig. 4a–f). However, compared to CoxSy(700), the distribution of CoxSy(600) and CoxSy(800) is slightly uneven along with some aggregation. The lattice spacings of 0.28 nm and 0.18 nm are found in CoxSy(700) (Fig. 4h), corresponding to the (440) crystal plane of Co9S8 and the (311) crystal plane of Co4S3,31–33 respectively. The energy dispersive X-ray (EDS) spectrum of CoxSy(700) is shown in Fig. 4i and j, where the uniform distribution of Co, C, and S elements further confirms the reliability of the hybrid nanostructure. Importantly, the successful introduction of carbon matrix can improve the conductivity and maintain the structural integrity, contributing to boosting the electrochemical performance of the CoxSy(700) anode in LIBs. Through the Brunauer Emmett Teller (BET) measurements (Fig. S5†), it can be discovered that the specific surface area of CoxSy(700) is 338.2224 m2 g−1. The achievement of a high specific surface area is closely related to the hollow structure of the CoxSy(700) material, and the improved specific surface area can promote rapid infiltration of electrolyte and shorten the diffusion path, resulting in good properties for fast charging/discharging.34
Fig. 4 SEM images of CoxSy(600) (a and b), CoxSy(800) (c and d), and CoxSy(700) (e and f). TEM images of CoxSy(700) (g and h). EDX mapping of CoxSy(700) (i and j). |
The electrochemical properties of CoxSy(600), CoxSy(700), and CoxSy(800) were systematically studied in a coin-type half-cell. The cyclic voltammetry (CV) curves of CoxSy(700) at a potential of 0.01–3.0 V for the first three cycles are shown in Fig. 5a. In the initial cathodic scan, the peaks that appear at 0.15 V and 0.9 V are attributed to the intercalation of Li-ions and the formation of LixS and Co, respectively. Noticeably, the obvious cathodic peak at 0.65 V occurring in the first cycle and disappearing in the following cycles corresponds to the formation of a solid electrolyte (SEI) film and the decomposition of the electrolyte.35,36 In the first anodic scan, oxidation peaks at about 0.6 V and 2.0 V can be clearly observed, corresponding to the deintercalation of Li-ions and the oxidation of metallic Co into CoxSy, respectively.37 In addition, the subsequent cycle curves overlap well, signifying the considerable electrochemical reversibility in the CoxSy(700) electrode. The galvanostatic charge–discharge curves of CoxSy(700) at different current densities are shown in Fig. 5b. There is no doubt that the capacities decrease with an increase in current density, and this phenomenon can be well displayed by the rate tests (Fig. 5c). Concretely, the CoxSy(700) electrode exhibits the best rate capacity at each current density in comparison with CoxSy(600) and CoxSy(800). For CoxSy(700), the average charging specific capacities are 875.6, 797.1, 715.5, 576.1 and 405.9 mA h g−1 at 100, 200, 500, 1000 and 2000 mA g−1, respectively. In addition, when the current density finally returns to 100 mA g−1, the charging specific capacity recovers to 885.9 mA h g−1, indicating super reversibility and excellent rate properties. The reason for the increase in capacity can be mainly attributed to the fact that more electrolyte gradually enters the porous structure of the material during the charging and discharging process, the reaction sites are in contact with each other, and the electrode is activated.38 For the CoxSy(600) and CoxSy(800) electrodes, their poor rate capability may be attributed to the uneven morphology and obvious agglomeration. A cycle capacity of about 468.4 mA h g−1 at 1 A g−1 was successfully achieved by the CoxSy(700) material (Fig. 5d), and a long-cycle lifespan over 1000 cycles with a retention rate of 88% was reached even though the test conditions were at a low current density of 500 mA g−1 (Fig. 5e). Besides, electrochemical impedance spectroscopy (EIS) (Fig. S6†) tests reveal that the CoxSy(700) exhibits a slight resistance change after 100 cycles at 1 A g−1, indicating excellent reaction kinetics.
Fig. 5 CV curve (a) and galvanostatic charge–discharge curves at different rates (b) of CoxSy(700). Rate performance (c) of CoxSy(n); cycling stability at 1 A g−1 (d) and 0.5 A g−1 (e) of CoxSy(700). |
Moreover, we also investigated the morphology and electrochemical performance of the obtained ZIF-67(600). As observed in Fig. S7a and b,† ZIF-67(600) well maintains a uniform dodecahedron particle shape, size and structure. Obviously, many small particles on the ZIF-67(600) surface are clearly observed, and this phenomenon can be explained by the appearance of Co particles generated by thermal reduction.30 It can be seen from the TEM image in Fig. S7c† that ZIF-67(600) is still a solid particle, and the enlarged TEM image shows that the lattice fringes of the nanoparticles are at 0.34 nm and 0.25 nm (Fig. S7d†), corresponding to the (002) crystal plane distance of graphite and the (111) crystal plane distance of Co, respectively. This result agrees well with the XRD analysis (Fig. 2b), and once again confirms the generation of Co and C in ZIF-67(600). As shown in Fig. S8a,† the CV curve of ZIF-67(600) was a typical curve of a carbon anode material; in the first discharge cycle, two peaks beginning at 1.5 and 0.65 V were clearly seen. The former was due to the irreversible reaction between Li-ions and carbon functional groups,39 and the latter corresponds to the generation of an SEI membrane on the active material surface of the active electrode. In addition, there was a reduction peak near 1.2 V, which may be due to the side-reaction of the present Co and lithium.40 The oxidation peak that appeared in the first cycle is in accordance with the previous literature reports.40,41 From Fig. S8b and c,† the ZIF-67(600) anode obviously exhibits good rate performance and superior reversibility, especially compared to the ZIF-67(500) and ZIF-67(700) electrodes. As far as cycle life is concerned, the ZIF-67(600) can maintain a satisfactory capacity of 400.8 mA h g−1 at 1 A g−1 for 100 cycles (Fig. S8d†) and 294.8 mA h g−1 at 500 mA g−1 for 800 cycles (Fig. S8e†). However, when the current density is increased to 2 A g−1 (Fig. S8f†), the cycling capability of the ZIF-67(600) electrode is much lower than that of the CoxSy(700) electrode, which intuitively indicates the robust structure in CoxSy(700). However, the rate capability, cycling stability and capacity offered by the ZIF-67(600) active material are lower than those of the CoxSy(700). This further proves that the hydrothermal vulcanization strategy can improve the electrochemical performance of the electrode material. More importantly, we also synthesized hollow CoxSy(700) without carbon (marked as CoxSy(700)-WC), and unveiled its electrochemical performance in LIBs. As shown in Fig. S9a,† it can be clearly seen that CoxSy(700)-WC has lower rate capability at each current density than CoxSy(700), which is ascribed to the inferior conductivity of the CoxSy(700)-WC electrode resulting from the lack of carbon matrix. In terms of cycling performance, the CoxSy(700)-WC electrode exhibits an obvious capacity fading during cycling (Fig. S9b†), and its capacity is only 151.9 mA h g−1 at 2 A g−1 for 700 cycles (Fig. S9c†), which is substantially lower than that of the CoxSy(700) electrode under the same conditions. This is mainly because the huge volume change induced by repeated Li-ion intercalation/deintercalation into/from CoxSy(700)-WC is not well alleviated. These results comprehensively suggest that the incorporation of the carbon matrix into CoxSy is of vital importance for improving the conductivity and alleviating the structure fluctuations, agreeing well with the analysis mentioned above.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra08581f |
This journal is © The Royal Society of Chemistry 2022 |