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In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries
Journal of Alloys and Compounds xxx (xxxx) xxx
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In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries Mengying Wang a, Huinan Guo a, Cuihua An a, b, Yan Zhang a, Weiqin Li a, Zeting Zhang a, Guishu Liu a, Yafei Liu a, Yijing Wang a, * a
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Renewable Energy Conversion and Storage Center, College of Chemistry, Nankai University, Tianjin, 300071, PR China Institute for New Energy Materials and Low-Carbon Technologies, Tianjin Key Laboratory of Advanced Functional Porous Materials, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin, 300384, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2019 Received in revised form 7 November 2019 Accepted 18 November 2019 Available online xxx
Sodium ion batteries (SIBs), as one of the most promising alternative to lithium ion batteries (LIBs), have attracted significant attentions during the last decade. Particularly, transition metal selenides have gained widespread interests as potential anodes for SIBs owing to their high capacity, wide availability and low cost. However, the rapid capacity fading and poor rate capability usually limit their development and application, which stem from the structure pulverization and exfoliation of active materials. In this work, CoSe@C microrods have been synthesized via simultaneous selenization and in-situ carbonization of cobalt-nitrilotriacetic acid (Co-NTA) precursors. By adjusting the ratio of water and isopropyl alcohol (IPA), three kinds of samples with high-crystallinity, different diameters and lengths are obtained. The CoSe@C-1 sample, prepared at 3:1 water to IPA, possesses uniform and orderly rod-like morphology with 150 nm diameter and 2 mm length. When employed as an anode for SIBs, the CoSe@C-1 microrod electrode exhibits the best electrochemical performances with high discharge capacity (485 mA h g1 after 60 cycles at 0.1 A g1) and excellent rate performance (315 mA h g1 at 0.5 A g1). Meanwhile, the reaction mechanism of the CoSe@C microrod is investigated. The CoSe@C-1 microrod also delivers excellent solid state electrochemical properties with a capacity of 230 mA h g1 after 50 cycles at 0.1 A g1. Furthermore, the outstanding electrochemical properties can be ascribed to the rational design of carbon-coated and porous structure, which can not only effectively prevents the aggregation of the CoSe@C nanoparticles, but also improve the electrical conductivity of the samples. These results provide a simple approach to fabricate promising anode materials for high-performance SIBs. © 2019 Published by Elsevier B.V.
Keywords: In situ Solvothermal CoSe Sodium ion batteries Solid-state electrolyte
1. Introduction In response to the environmental pollution and increasing demands for available energy storage, sodium ion batteries (SIBs) have been proposed to replace lithium-ion batteries (LIBs) owing to the high abundance of sodium resources and low price [1e5]. However, the larger molar mass and ionic radius of Naþ result in low specific capacity, poor cyclability and inferior power density for SIBs compared to LIBs [6e8]. Additionally, anode materials are the key component and have significant effects on the electrochemical performances of SIBs [9,10]. So far, various materials have been
* Corresponding author. E-mail address: [email protected] (Y. Wang).
extensively explored and widely studied as potential anodes, including carbon, metal oxides, alloys and metal chalcogenides [11e15]. In particular, transition metal selenides have been regarded as promising advanced anode materials for SIBs, which can deliver high theoretical capacity, wide availability and low cost [16e19]. Among these selenides, cobalt selenides, in a widely varied composition of CoSe, Co0.85Se and CoSe2 phases, exhibit remarkable electrical conductivity and high theoretical capacity in sodium storage [20,21]. Nevertheless, they always suffer from large irreversible capacity loss and striking capacity fade, which might account for the structure pulverization and exfoliation of active materials during the Naþ insertion and extraction processes in the practical applications [22]. To ameliorate the capacity and cyclic stability of cobalt selenides, some methods have been adopted including nanostructure
https://doi.org/10.1016/j.jallcom.2019.153090 0925-8388/© 2019 Published by Elsevier B.V.
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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fabrication and hybridization with carbonaceous materials. On the one hand, designing appropriate nanostructure can resist the stress and accommodate the large volume variation during repeated charge/discharge processes, leading to stable structure and excellent electrochemical properties [23,24]. What’s more, narrowing the active materials to nanometer scale is convenient for the transport and diffusion of electrons and ions because of the short transport pathways [18]. On the other hand, carbon-decorating is another common approach to achieve high reaction kinetics and superior cyclic stability, which not only increases electronic conductivity of the active materials, but also effectively prevents the aggregation and volume expansion during the electrochemical reactions [25,26]. For instance, Park et al. reported a simple one-pot spray pyrolysis synthesis of the CoSexerGO composites as anode materials for SIBs [17]. The electrodes exhibited outstanding reversible capacity of 420 mA h g1 at 0.3 A g1 after 50 cycles and the capacity retentions was 80%. Wu et al. showed unique peapodlike CoSe3carbon nanowires synthesized by a facile approach [25]. When tested as electrode materials for SIBs, a reversible capacity of 299 mA h g1 was available at 0.1 A g1 after 100 cycles. Li et al. prepared CoSe@porous carbon polyhedra (CoSe@PCP) via using Cobased zeolitic imidazolate framework (ZIF-67) as the template [18]. The obtained CoSe@PCP achieved stable capacity of 341 mA h g1 at 0.1 A g1 after 100 cycles. In addition, the porous structure has great significance for the electrochemical performance of SIBs. The porous materials can provide suitable channels for the transfer of Naþ, thus improving the properties of sodium storage. Therefore, it is worth trying to design and controllable synthesis the porous composites consisting nanostructures and carbonaceous. In this paper, we report a facile and template-free approach to prepare porous CoSe@C microrods as anodes for sodium storage. Meanwhile, there are two advantages in the synthesis of the materials. Firstly, the preparation process is greatly simple, which is fabricated through a two-step method, consisting of solvothermal and further selenization of the cobalt-nitrilotriacetic acid (Co-NTA) precursors using the Se powders. Secondly, we use nitrilotriacetic acid (NTA) as the carbon source to in-situ synthesis the CoSe@C microrods and no introduction of extraneous carbonaceous during the processes. The design of the CoSe@C microrods could simplify the experimental process, effectively facilitate mass transport of Naþ, as well as buffer the large volume change during the charge/ discharge processes, leading to high sodium-ion storage performances. As expected, the CoSe@C-1 microrod as SIBs anode material exhibits a discharge capacity of 485 mA h g1 after 60 cycles at 0.1 A g1. Even at 0.5 A g1, it also delivers the discharge specific capacity of 315 mA h g1. The reaction mechanism of the CoSe@C is further investigated by ex-situ XRD. Furthermore, the CoSe@C-1 microrod also delivers excellent solid state electrochemical properties with a capacity of 230 mA h g1 after 50 cycles at 0.1 A g1. 2. Experimental section 2.1. Synthesis of CoSe@C microrods The synthesis of CoSe@C microrods includes two steps of hydrothermal and selenization. The precursors Co-NTA were produced as described elsewhere [27]. The precursors S-1, S-2 and S-3 were prepared in a mixture (30 mL of distilled water and 10 mL of IPA, 20 mL of distilled water and 20 mL of IPA and 10 mL of distilled water and 30 mL of IPA), respectively. Subsequently, the Co-NTA precursors were mixed with selenium powder in a certain proportion and calcined in a tube furnace at 600 C for 2 h with a heating rate of 2 C min1 in argon. The corresponding samples obtained from the precursors S-1, S-2 and S-3 were designated as CoSe@C-1, CoSe@C-2 and CoSe@C-3, respectively.
2.2. Synthesis of hybrid solid electrolyte (HSE) membrane The HSE membrane was prepared by a facile solution casting technique. Firstly, 2.0 g of poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP, Mw ¼ 400000) was added into 50 mL of acetonitrile (ACN) and magnetic stirring at 70 C for 1 h. 0.2 g of NaClO4 was dissolved into the above solution with stirring for 8 h at 70 C. After that, stop heating and stirring for another 12 h at room temperature. Vaporized the solvent to 20 mL at 90 C and poured the solution into a 1500 mm polytetrafluoroethylene plate while hot, then dried under vacuum for 12 h. Then, the uniform membrane was cut into circular pieces with diameter of 15 mm. Finally, the as-prepared membrane was immersed in the electrolyte for over 12 h in the glovebox for further electrochemical measurements. 2.3. Materials characterization The crystal structure of materials was recorded by X-ray diffraction (XRD, Rigaku/miniFlex II, Cu Ka radiation). The morphology was investigated by field emission scanning electron microscope (FESEM, JEOL JSM-6700F) and transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) on a JEOL JEM-2100. X-ray photoelectron spectroscopy (XPS) was investigated by PHI 5000 Versaprobe (Ulvac-Phi). The specific surface areas and porous nature of all the samples were further determined by nitrogen adsorption/desorption measurements on absorption analyzer (NOVA 2200e, Quantachrome Instruments). The Fourier transform infrared (FTIR) spectrum was record for the KBr dilute samples using a spectrum two spectrometer (PerkinElmer). 2.4. Electrochemical measurements The electrochemical measurements were carried out in a 2032type coin cell. For the CoSe@C microrods, the working electrodes were made by casting slurry containing active material, acetylene black and poly(vinylidene fluoride) (PVDF) with a mass ratio of 8:1:1 on a Cu foil and then got it dried at 80 C for 12 h in a vacuum oven. The coin cells were assembled in an argon filled glove box with using sodium film and Whatman glass fiber as the counter electrode and separator, respectively. In addition, 1 M NaClO4 in diethyleneglycol dimethylether (DEGDME) with 5 wt% fluoroethylene carbonate (FEC) was employed as the electrolyte, and the solid-state battery was used the prepared HSE membranes as a separator, and the liquid electrolyte was no longer used. Galvanostatic charge-discharge measurements were performed by a LAND battery test instrument (CT2201A) in a voltage range of 0e2.6 V. The cyclic voltammetry (CV) was collected on electrochemical workstation (CHI760E) at a scan rate of 0.1 mV s1 between 0 and 2.6 V. Additionally, electrochemical impedance spectroscopy (EIS) was also conducted by a CHI760E electrochemical workstation with a frequency range of 100 kHz to 0.1 Hz. Moreover, all the above tests were carried out at a constant temperature of 25 C. 3. Results and discussion 3.1. Synthesis and materials characterization The typical synthesis process of CoSe@C microrods was shown in Scheme 1. First of all, the precursor Co-NTA was prepared through a facile solvothermal reaction and the Co-NTA was analyzed by FTIR (Fig. S1). According to the literature, the peaks between 3050 and 2900 cm1 are assigned to the stretching
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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Scheme 1. Schematic illustration for the synthesis process of CoSe@C microrods.
vibration of CeH [28e30]. For the Co-NTA, the peak at 3043 cm1 disappears and the peaks at 2990, 2958 cm1 move to 2949, 2915 cm1, which is due to the introduction of Co2þ [31]. Furthermore, the peak at 1730 cm1 is related to the stretching vibration of C]O in the spectra of NTA, but it disappears in Co-NTA. In addition, two peaks located at 1685 and 1587 cm1 are present in Co-NTA, which not observed in NTA. All of these changes can be ascribed to the coordination of NTA and Co2þ. Meantime, the SEM observations (Fig. S2) indicate that the precursors are made up of microrods and have a smooth surface. However, the diameters and lengths of the S-1, S-2 and S-3 precursors are different. Moreover, the precursor S-1 exhibits orderly property, while the S-2 and S-3 tend to random and stick together. The length of the precursor decreases with the decrease of water content. So the distilled water may be beneficial for the growth and separation of the precursors. The CoSe@C microrods were obtained via simultaneous selenization and carbonization of Co-NTA precursors at a high annealing temperature under argon atmosphere, and the XRD analysis of the three samples was given in Fig. S3 and the diffraction peaks were in the same position. The patterns have three distinct diffraction peaks at 33.2, 44.8 and 50.5 , corresponding to (101), (102) and (110) of CoSe (JCPDS No. 89-2004), respectively. In addition, the diffraction peaks are sharp and intense, suggesting the highly crystalline nature of the CoSe@C microrods. Additionally, no other diffraction peaks are detected, confirming the high purity of the samples. Obviously, this simple synthesis strategy represents a facile technique for successfully preparing porous CoSe@C microrods. The morphologies and microstructures of the CoSe@C-1 microrod were further characterized by SEM, TEM and HRTEM. Fig. 1a and b shows the SEM images at different magnification, revealing the sample retains the pristine microrod structure with about 150 nm diameter and 2 mm length. The high-magnification SEM image clearly indicates the surface of the CoSe@C-1 is rough and a number of CoSe nanoparticles are embedded into the carbon matrix. The detailed nanoarchitecture was further studied by TEM. As shown in Fig. 1c and d, it can found that the CoSe@C-1 microrod is porous structure and embedded with some relatively regular nanoparticles. Additionally, the surface of the nanoparticles is coated with a carbon layer of about 1.5 nm, which may be beneficial to the conductivity and structural stability of the material. The lattice fringe obtained from HRTEM (Fig. 1e) reveals the distance of 0.313 nm, matching well with the (100) lattice plane of the hexagonal CoSe. The elemental mapping of the CoSe@C-1 (Fig. 1f) shows that C, Co and Se elements are uniform distribution in the microrod. Compared to the other two samples, the morphologies of the CoSe@C-2 and CoSe@C-3 microrods (Fig. S4) are similar to CoSe@C-1, which also display a CoSe nanoparticles embedded in porous carbon microrods structure. But there are some broken microrods and the size of the nanoparticles on the surface is very irregular. This is due to under the double etching of high temperature and selenium powder, the precursors with small diameter
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and short length are easily etched and the structure is destroyed, resulting in partial collapse of microrods. The chemical compositions and valence of the CoSe@C-1 microrod were further investigated by using XPS analysis, as shown in Fig. 2. The XPS survey spectrum (Fig. 2a) indicates the presence of Co, Se, C and O in CoSe@C-1, and the existence of O element could be attributed to the surface absorption of oxygen in the air by the sample exposed to air. The C 1s spectrum (Fig. 2b) shows peaks corresponding to sp2-bonded carbon (CeC), epoxy and alkoxyl groups (CeO) and carbonyl and carboxyl groups (C]O) at 284.9, 286.5 and 288.9 eV [32,33], respectively. Obviously, the CeC bond is much stronger than those of C]O and CeO, suggesting the formation of graphitic carbon in the composite. There are six peaks in the Co 2p spectrum (Fig. 2c): Co 2p3/2 of CoSe at 778.4 and 780.8 eV and satellite peak (identified as “Sat.”) at 786.2 eV as well as Co 2p1/2 of CoSe at 793.2 and 796.4 eV and satellite peak at 801.8 eV [34]. The presence of satellite peaks indicates the Co2þ is in high-spin arrangement [35]. In the Fig. 2d, the peaks are located at around 54.2 and 55.0 eV for Se 3d5/2 and Se 3d3/2, respectively, which can be ascribed to the typical feature of CoeSe bonds according to the previous report. The peaks at 59.6 and 61.5 eV are attributed to Co 3p3/2 and Co 3p1/2, respectively. Furthermore, the corresponding energy dispersive spectrometer (EDS) spectra is shown in Fig. S5 to verify the contents of Se and Co elements. The surface area and pore size distribution of the prepared CoSe@C microrods, which play a critical role in the surface adsorption of ions, were estimated using nitrogen adsorption/ desorption measurements. The adsorption-desorption isotherm curves of the three samples (Figure S6a, S7a and S7c) all display type IV isotherms with type H3 hysteresis loops in the P/P0 range of 0.4e0.95, demonstrating the existence of the mesoporous structure. According to the Brunauer-Emmett-Teller (BET) method, the surface areas of CoSe@C-1, CoSe@C-2 and CoSe@C-3 microrods are 90.9, 68.1 and 73.0 m2 g1, respectively. Additionally, the BarrettJoyner-Halenda (BJH) pore-size-distribution curves (Figure S6b, S7b and S7d) clearly indicate that the pore size is mainly in the range of 10e20 nm. The large surface area and porous features can provide the necessary pathway for electrolyte penetration and facilitate the ions and electrons transfer in the electrolyte/electrode interface, which is beneficial to enhanced electrochemical properties. 3.2. Na-ion battery performances The electrochemical properties of these CoSe@C microrods as SIBs anodes were measured by ways of galvanostatic charge/ discharge tests, CV and EIS over a potential of 0e2.6 V vs. Naþ/Na. Fig. 3a shows the rate performances of the three samples at different current densities. Obviously, the CoSe@C-1 electrode exhibits the best discharge capacity and rate performances among the three samples. As the current densities increased stepwise from 0.05 to 0.1, 0.2 and 0.5 A g1, the CoSe@C-1 electrode exhibits a high and stable capacities of 573, 500, 445 and 315 mA h g1, respectively. After reducing the current density to 0.2, 0.1 and 0.05 A g1, the capacities of 412, 447 and 424 mA h g1 can be almost recovered, respectively, suggesting excellent rate property of the CoSe@C-1 electrode. Moreover, the electrochemical result of CoSe@C-1 electrode evaluated at a voltage range of 0.3e3 V is shown in Fig. S8 to confirm the lighter effect of low voltage region on the Na-ion storage properties. The CoSe@C-3 electrode delivers lower capacities at each current density than CoSe@C-1. In contrast, the capacity of the CoSe@C-2 rapidly decreases at high current densities, and has an unstable capacity when the current density taken back to 0.05 A g1. Meanwhile, it could be observed that the cycling performance of the CoSe@C-1 is superior to the other two.
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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Fig. 1. Morphologies of the CoSe@C-1 microrod: (a, b) SEM images, (c, d) TEM images, (e) high-resolution TEM (HRTEM) and (f) elemental mapping of C, Co, Se.
Fig. 2. XPS spectra of the CoSe@C-1 microrod: (a) Elements survey, (b) C 1s, (c) Co 2p and (d) Se 3d and Co 3p.
After 60 cycles, the CoSe@C-1 still delivers a high specific capacity of 485 mA h g1, but the capacities of the CoSe@C-2 and CoSe@C-3 are only 48 and 64 mA h g1. As it can be seen, the capacity of CoSe@C-3 electrode rapidly decreased after 35 cycles, while the CoSe@C-2 only stable for 7 cycles. The excellent rate performance and cycle stability of CoSe@C-1 electrode can be attributed to the merits of large surface area, the complete porous microrods, regular CoSe nanoparticles and appropriate thickness of carbon layer
around nanoparticles than that of CoSe@C-2 and CoSe@C-3 electrode materials, which increase electrons and ions transport efficiency, leading to the enhancement of electrochemical properties. Additionally, the SEM images of three samples after cycling are shown in Fig. S9. We can see that the morphology of CoSe@C-1 after cycling could maintain well, however, CoSe@C-2 and CoSe@C-3 after cycling demonstrate serious agglomeration and differentiation phenomenon, which may be the main reason of leading to fast
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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Fig. 3. Electrochemical performances of the CoSe@C microrods: (a) rate performances at different current densities, (b) cycling performances at a current rate of 0.1 A g1, (c) CV curves of the CoSe@C-1 at a scan rate of 0.1 mV s1 and (d) galvanostatic charge-discharge curves of the CoSe@C-1 at 0.1 A g1.
capacity decay. The CV curves of the CoSe@C-1, CoSe@C-2 and CoSe@C-3 electrodes for the first three cycles at a scan rate of 0.1 mV s1 in the potential range of 0e2.6 V are shown in Fig. 3c and Fig. S10. In the first cathodic scan, a sharp peak at around 0.75 V of the three electrodes are observed, which are attributed to the formation of solid electrolyte interphase (SEI) layer and the conversion reaction of CoSe: CoSe þ 2Naþ þ 2e / Co þ Na2Se. The anodic peaks at around 1.90 V are related to the recovery of CoSe nanocrystals from metallic Co and Na2Se. For the subsequent 2nd and 3rd cycles, the cathodic peaks shifts to around 1.24 and 1.0 V vs Naþ/Na correspond to the formation of NaxCoSe intermediate and Co and Na2Se, respectively. At the same time, the anodic peaks shift to 1.95 V. This is may be caused be the changes of microrod structure and the activation of the CoSe@C electrodes in the first cycle. Furthermore, all the electrodes display large irreversible capability loss for the first cycle, indicating the decomposition of electrolyte to form the SEI layer and the irreversible phase transition during the first cycle. Fig. 3d and Fig. S10 show the 1st to 60th galvanostatic charging/ discharging curves of the three CoSe@C electrodes at 0.1 A g1 over a voltage window of 0e2.6 V. We can see, all the samples display a discharge plateau at around 0.85 V and a charge plateau at 1.8 V during the initial charge-discharge profiles, which is consistent well with the results of CV data, and the CoSe@C-1 delivers an initial discharge capacities of 1565 mA h g1 and a charge capacities of 702 mA h g1, with a low initial coulombic efficiency. The greater capacity loss may result from irreversible side reactions, including the formation of SEI and incomplete conversion reaction of CoSe and Na. In addition, the well-kept plateaus can be observed for different cycles of the CoSe@C-1 electrode, while the plateaus gradually disappeared with the increases of cycles of other two electrodes, further illustrating the excellent electrochemical properties of the CoSe@C-1 electrode. Additionally, EIS was carried out to further investigate the electrochemical performance of the CoSe@C electrodes. In Fig. S11, all Nyquist plots are typically consists of a semicircle in the high-tomedium frequency region and a slanted line in the low frequency
region, and the semicircle indicates the charge-transfer resistance (Rct) at electrolyte/electrode interface and the slanted line is related to the diffusion of Na-ion in the bulk of the materials. Fig. S11a displays the CoSe@C-1 microrod has a slightly lower Rct value than that of the CoSe@C-2 and CoSe@C-3 microrods before cycling, indicating the high electrical conductivity of CoSe@C-1. The lower Rct value of CoSe@C-1 could be attributed to the larger specific surface area and more interstitial ion transport channels than that
Fig. 4. (a) Reaction mechanism: the charge-discharge curve of the CoSe@C-1 electrodes during the first cycle and the ex-situ XRD analysis at different charge-discharge states; SAED pattern of the CoSe@C-1 electrode at fully (b) discharged (0 V) and (c) charged (2.6 V).
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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Fig. 5. Electrochemical performances of the CoSe@C-1/PVDF-HFP/Na solid-state battery: (a) galvanostatic charge-discharge curves at 0.1 A g1, (b) cycling performance and corresponding coulombic efficiency at 0.1 A g1.
of the other two samples, providing more active sites for rapid charge transfer. The Nyquist plots of the CoSe@C-1 electrode at different cycles as shown in Fig. S11b and the enlarged view of the Nyquist plots in the inset. As seen, the fitted Rct of CoSe@C-1 electrode before cycling is 1311 U. After 10 cycles, the Rct decreases to 455 U, demonstrating the formation of small Co nanoparticles and enhances the electron transport. The Rct value increased as the number of cycles increased, which should be associated with the structure change during the repeated charge and discharge processes. To further investigate the reaction mechanism of the CoSe@C, ex-situ XRD and SAED analyses were employed during the first cycle. Fig. 4a gives the ex-situ XRD patterns of the CoSe@C-1 electrodes at different charge-discharge states and the corresponding charge-discharge curve. In the charge-discharge curve, the CoSe@C-1 electrode displays a discharge plateau at 1.0 V and a charge plateau at 1.8 V. According to the XRD results, when the electrode is discharge to 1.0 V, the CoSe phase can still be observed, indicating that the initial intercalation of Naþ does not change the CoSe (JCPDS No. 89-2004) phase. Meantime, the intermediate NaxCoSe is not observed which is due to the low crystallinity. At discharge to 0.7 V, the CoSe phase is disappear and a minor diffraction peak at 38.6 of Co (JCPDS No. 70-2633) is detected (Fig. S12). After further discharge to 0 V, the peaks at 21.4 and 38.6 can be ascribed to the generation of Co and the peaks at 22.9 of Na2Se (JCPDS No. 47-1699). When the electrode is charge to 2.6 V, there is no peaks of CoSe can be found, which is attributed to the low crystallinity of CoSe after the initial cycle. In addition, the binder (PVDF), conductive carbon (acetylene black) and current collector (Cu foil) also interfere the peaks of the CoSe. The presence of Co indicating the Co is not completely converted to CoSe. The diffraction rings (114) plane of Co, (400) plane of Na2Se and (002) plane of graphite are observed in the SAED patterns, When it is fully discharge to 0 V (Fig. 4b). Fig. 4c shows the fully charge state of 2.6 V, the rings of CoSe and graphite emerge in the SAED pattern. The existence of diffraction dot of (002) plane of graphite is can due to the presence of graphite carbon in the CoSe@C. Therefore, the reaction mechanism of the CoSe can be confirmed as the following: CoSe þ 2Naþ þ 2e ¼ Co þ Na2Se.
the cycling performance of the CoSe@C-1/PVDF-HFP/Na, a capacity of 230 mA h g1 after 50 cycles at 0.10 A g1 can be achieved. The coulombic efficiency is approximately 95% from 2nd cycle to 50th cycle. Fig. S13 reveals the rate performance of the CoSe@C-1/PVDFHFP/Na solid-state battery. When tests at current densities of 0.05, 0.1, 0.2 and 0.5 A g1, stable capacities of 385, 339, 282 and 116 mA h g1 can be obtained, respectively. When the current density reduces to 0.05 A g1, the discharge capacity can also reach 321 mA h g1. Therefore, the CoSe@C-1 composite has excellent electrochemical property in the solid-state electrolyte system. It is worth noting that the outstanding sodium-storage performances of the CoSe@C microrods may originate from the following aspects. First, porous one-dimensional microrod structure can provide suitable channels for the transfer of Naþ and accommodate the volume change during the repeated sodiation/desodiation processes. Second, the carbon matrix and the carbon layer of the CoSe nanoparticles not only can increase the electronic conductivity of the material, but also effectively prevents the aggregation of CoSe nanoparticles. Third, the large specific surface area and rough surface are beneficial to the electrolyte penetration, which can raise the electrochemical performances. 4. Conclusions In conclusion, CoSe@C microrods have been successfully synthesized through a facile approach without template assistance. The obtained materials possess a unique structure, including porous microrods and a thin carbon layer coated on the CoSe nanoparticles, which not only increases electronic conductivity of the active material, but also effectively prevents the aggregation and volume expansion during the electrochemical reaction. When employed as an anode for SIBs, the CoSe@C-1 electrode exhibits the best electrochemical performances with high capacity (485 mA h g1 after 60 cycles at 0.1 A g1) and excellent rate performance (315 mA h g1 at 0.5 A g1). The CoSe@C-1 microrod also delivers outstanding solid state electrochemical properties with a capacity of 230 mA h g1 after 50 cycles at 0.1 A g1. Therefore, the strategy may also be generally extended to other transition metal selenides with superior electrochemical performances.
(1) Declaration of competing interest
To demonstrate the application potentiality of the CoSe@C-1, a coin solid-state battery (CoSe@C-1/PVDF-HFP/Na) was fabricated. Fig. 5a shows the galvanostatic charging/discharging curves of the CoSe@C-1/PVDF-HFP/Na solid-state battery at 0.10 A g1, and the charge-discharge curve is similar to the liquid battery. In the first cycle, the discharge capacity is 991 mA h g1 and the charge capacity reaches 344 mA h g1. The low initial coulomb efficiency is owing to the side reactions between electrolyte and electrodes. For
There are no conflicts of interest to declare. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (51871123, 51501072 and 51571124), Natural Science Foundation of Tianjin (17JCYBJC17900), Ministry of
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries Mengying Wang a, Huinan Guo a, Cuihua An a, b, Yan Zhang a, Weiqin Li a, Zeting Zhang a, Guishu Liu a, Yafei Liu a, Yijing Wang a, * a
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Renewable Energy Conversion and Storage Center, College of Chemistry, Nankai University, Tianjin, 300071, PR China Institute for New Energy Materials and Low-Carbon Technologies, Tianjin Key Laboratory of Advanced Functional Porous Materials, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin, 300384, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2019 Received in revised form 7 November 2019 Accepted 18 November 2019 Available online xxx
Sodium ion batteries (SIBs), as one of the most promising alternative to lithium ion batteries (LIBs), have attracted significant attentions during the last decade. Particularly, transition metal selenides have gained widespread interests as potential anodes for SIBs owing to their high capacity, wide availability and low cost. However, the rapid capacity fading and poor rate capability usually limit their development and application, which stem from the structure pulverization and exfoliation of active materials. In this work, CoSe@C microrods have been synthesized via simultaneous selenization and in-situ carbonization of cobalt-nitrilotriacetic acid (Co-NTA) precursors. By adjusting the ratio of water and isopropyl alcohol (IPA), three kinds of samples with high-crystallinity, different diameters and lengths are obtained. The CoSe@C-1 sample, prepared at 3:1 water to IPA, possesses uniform and orderly rod-like morphology with 150 nm diameter and 2 mm length. When employed as an anode for SIBs, the CoSe@C-1 microrod electrode exhibits the best electrochemical performances with high discharge capacity (485 mA h g1 after 60 cycles at 0.1 A g1) and excellent rate performance (315 mA h g1 at 0.5 A g1). Meanwhile, the reaction mechanism of the CoSe@C microrod is investigated. The CoSe@C-1 microrod also delivers excellent solid state electrochemical properties with a capacity of 230 mA h g1 after 50 cycles at 0.1 A g1. Furthermore, the outstanding electrochemical properties can be ascribed to the rational design of carbon-coated and porous structure, which can not only effectively prevents the aggregation of the CoSe@C nanoparticles, but also improve the electrical conductivity of the samples. These results provide a simple approach to fabricate promising anode materials for high-performance SIBs. © 2019 Published by Elsevier B.V.
Keywords: In situ Solvothermal CoSe Sodium ion batteries Solid-state electrolyte
1. Introduction In response to the environmental pollution and increasing demands for available energy storage, sodium ion batteries (SIBs) have been proposed to replace lithium-ion batteries (LIBs) owing to the high abundance of sodium resources and low price [1e5]. However, the larger molar mass and ionic radius of Naþ result in low specific capacity, poor cyclability and inferior power density for SIBs compared to LIBs [6e8]. Additionally, anode materials are the key component and have significant effects on the electrochemical performances of SIBs [9,10]. So far, various materials have been
* Corresponding author. E-mail address: [email protected] (Y. Wang).
extensively explored and widely studied as potential anodes, including carbon, metal oxides, alloys and metal chalcogenides [11e15]. In particular, transition metal selenides have been regarded as promising advanced anode materials for SIBs, which can deliver high theoretical capacity, wide availability and low cost [16e19]. Among these selenides, cobalt selenides, in a widely varied composition of CoSe, Co0.85Se and CoSe2 phases, exhibit remarkable electrical conductivity and high theoretical capacity in sodium storage [20,21]. Nevertheless, they always suffer from large irreversible capacity loss and striking capacity fade, which might account for the structure pulverization and exfoliation of active materials during the Naþ insertion and extraction processes in the practical applications [22]. To ameliorate the capacity and cyclic stability of cobalt selenides, some methods have been adopted including nanostructure
https://doi.org/10.1016/j.jallcom.2019.153090 0925-8388/© 2019 Published by Elsevier B.V.
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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fabrication and hybridization with carbonaceous materials. On the one hand, designing appropriate nanostructure can resist the stress and accommodate the large volume variation during repeated charge/discharge processes, leading to stable structure and excellent electrochemical properties [23,24]. What’s more, narrowing the active materials to nanometer scale is convenient for the transport and diffusion of electrons and ions because of the short transport pathways [18]. On the other hand, carbon-decorating is another common approach to achieve high reaction kinetics and superior cyclic stability, which not only increases electronic conductivity of the active materials, but also effectively prevents the aggregation and volume expansion during the electrochemical reactions [25,26]. For instance, Park et al. reported a simple one-pot spray pyrolysis synthesis of the CoSexerGO composites as anode materials for SIBs [17]. The electrodes exhibited outstanding reversible capacity of 420 mA h g1 at 0.3 A g1 after 50 cycles and the capacity retentions was 80%. Wu et al. showed unique peapodlike CoSe3carbon nanowires synthesized by a facile approach [25]. When tested as electrode materials for SIBs, a reversible capacity of 299 mA h g1 was available at 0.1 A g1 after 100 cycles. Li et al. prepared CoSe@porous carbon polyhedra (CoSe@PCP) via using Cobased zeolitic imidazolate framework (ZIF-67) as the template [18]. The obtained CoSe@PCP achieved stable capacity of 341 mA h g1 at 0.1 A g1 after 100 cycles. In addition, the porous structure has great significance for the electrochemical performance of SIBs. The porous materials can provide suitable channels for the transfer of Naþ, thus improving the properties of sodium storage. Therefore, it is worth trying to design and controllable synthesis the porous composites consisting nanostructures and carbonaceous. In this paper, we report a facile and template-free approach to prepare porous CoSe@C microrods as anodes for sodium storage. Meanwhile, there are two advantages in the synthesis of the materials. Firstly, the preparation process is greatly simple, which is fabricated through a two-step method, consisting of solvothermal and further selenization of the cobalt-nitrilotriacetic acid (Co-NTA) precursors using the Se powders. Secondly, we use nitrilotriacetic acid (NTA) as the carbon source to in-situ synthesis the CoSe@C microrods and no introduction of extraneous carbonaceous during the processes. The design of the CoSe@C microrods could simplify the experimental process, effectively facilitate mass transport of Naþ, as well as buffer the large volume change during the charge/ discharge processes, leading to high sodium-ion storage performances. As expected, the CoSe@C-1 microrod as SIBs anode material exhibits a discharge capacity of 485 mA h g1 after 60 cycles at 0.1 A g1. Even at 0.5 A g1, it also delivers the discharge specific capacity of 315 mA h g1. The reaction mechanism of the CoSe@C is further investigated by ex-situ XRD. Furthermore, the CoSe@C-1 microrod also delivers excellent solid state electrochemical properties with a capacity of 230 mA h g1 after 50 cycles at 0.1 A g1. 2. Experimental section 2.1. Synthesis of CoSe@C microrods The synthesis of CoSe@C microrods includes two steps of hydrothermal and selenization. The precursors Co-NTA were produced as described elsewhere [27]. The precursors S-1, S-2 and S-3 were prepared in a mixture (30 mL of distilled water and 10 mL of IPA, 20 mL of distilled water and 20 mL of IPA and 10 mL of distilled water and 30 mL of IPA), respectively. Subsequently, the Co-NTA precursors were mixed with selenium powder in a certain proportion and calcined in a tube furnace at 600 C for 2 h with a heating rate of 2 C min1 in argon. The corresponding samples obtained from the precursors S-1, S-2 and S-3 were designated as CoSe@C-1, CoSe@C-2 and CoSe@C-3, respectively.
2.2. Synthesis of hybrid solid electrolyte (HSE) membrane The HSE membrane was prepared by a facile solution casting technique. Firstly, 2.0 g of poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP, Mw ¼ 400000) was added into 50 mL of acetonitrile (ACN) and magnetic stirring at 70 C for 1 h. 0.2 g of NaClO4 was dissolved into the above solution with stirring for 8 h at 70 C. After that, stop heating and stirring for another 12 h at room temperature. Vaporized the solvent to 20 mL at 90 C and poured the solution into a 1500 mm polytetrafluoroethylene plate while hot, then dried under vacuum for 12 h. Then, the uniform membrane was cut into circular pieces with diameter of 15 mm. Finally, the as-prepared membrane was immersed in the electrolyte for over 12 h in the glovebox for further electrochemical measurements. 2.3. Materials characterization The crystal structure of materials was recorded by X-ray diffraction (XRD, Rigaku/miniFlex II, Cu Ka radiation). The morphology was investigated by field emission scanning electron microscope (FESEM, JEOL JSM-6700F) and transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) on a JEOL JEM-2100. X-ray photoelectron spectroscopy (XPS) was investigated by PHI 5000 Versaprobe (Ulvac-Phi). The specific surface areas and porous nature of all the samples were further determined by nitrogen adsorption/desorption measurements on absorption analyzer (NOVA 2200e, Quantachrome Instruments). The Fourier transform infrared (FTIR) spectrum was record for the KBr dilute samples using a spectrum two spectrometer (PerkinElmer). 2.4. Electrochemical measurements The electrochemical measurements were carried out in a 2032type coin cell. For the CoSe@C microrods, the working electrodes were made by casting slurry containing active material, acetylene black and poly(vinylidene fluoride) (PVDF) with a mass ratio of 8:1:1 on a Cu foil and then got it dried at 80 C for 12 h in a vacuum oven. The coin cells were assembled in an argon filled glove box with using sodium film and Whatman glass fiber as the counter electrode and separator, respectively. In addition, 1 M NaClO4 in diethyleneglycol dimethylether (DEGDME) with 5 wt% fluoroethylene carbonate (FEC) was employed as the electrolyte, and the solid-state battery was used the prepared HSE membranes as a separator, and the liquid electrolyte was no longer used. Galvanostatic charge-discharge measurements were performed by a LAND battery test instrument (CT2201A) in a voltage range of 0e2.6 V. The cyclic voltammetry (CV) was collected on electrochemical workstation (CHI760E) at a scan rate of 0.1 mV s1 between 0 and 2.6 V. Additionally, electrochemical impedance spectroscopy (EIS) was also conducted by a CHI760E electrochemical workstation with a frequency range of 100 kHz to 0.1 Hz. Moreover, all the above tests were carried out at a constant temperature of 25 C. 3. Results and discussion 3.1. Synthesis and materials characterization The typical synthesis process of CoSe@C microrods was shown in Scheme 1. First of all, the precursor Co-NTA was prepared through a facile solvothermal reaction and the Co-NTA was analyzed by FTIR (Fig. S1). According to the literature, the peaks between 3050 and 2900 cm1 are assigned to the stretching
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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Scheme 1. Schematic illustration for the synthesis process of CoSe@C microrods.
vibration of CeH [28e30]. For the Co-NTA, the peak at 3043 cm1 disappears and the peaks at 2990, 2958 cm1 move to 2949, 2915 cm1, which is due to the introduction of Co2þ [31]. Furthermore, the peak at 1730 cm1 is related to the stretching vibration of C]O in the spectra of NTA, but it disappears in Co-NTA. In addition, two peaks located at 1685 and 1587 cm1 are present in Co-NTA, which not observed in NTA. All of these changes can be ascribed to the coordination of NTA and Co2þ. Meantime, the SEM observations (Fig. S2) indicate that the precursors are made up of microrods and have a smooth surface. However, the diameters and lengths of the S-1, S-2 and S-3 precursors are different. Moreover, the precursor S-1 exhibits orderly property, while the S-2 and S-3 tend to random and stick together. The length of the precursor decreases with the decrease of water content. So the distilled water may be beneficial for the growth and separation of the precursors. The CoSe@C microrods were obtained via simultaneous selenization and carbonization of Co-NTA precursors at a high annealing temperature under argon atmosphere, and the XRD analysis of the three samples was given in Fig. S3 and the diffraction peaks were in the same position. The patterns have three distinct diffraction peaks at 33.2, 44.8 and 50.5 , corresponding to (101), (102) and (110) of CoSe (JCPDS No. 89-2004), respectively. In addition, the diffraction peaks are sharp and intense, suggesting the highly crystalline nature of the CoSe@C microrods. Additionally, no other diffraction peaks are detected, confirming the high purity of the samples. Obviously, this simple synthesis strategy represents a facile technique for successfully preparing porous CoSe@C microrods. The morphologies and microstructures of the CoSe@C-1 microrod were further characterized by SEM, TEM and HRTEM. Fig. 1a and b shows the SEM images at different magnification, revealing the sample retains the pristine microrod structure with about 150 nm diameter and 2 mm length. The high-magnification SEM image clearly indicates the surface of the CoSe@C-1 is rough and a number of CoSe nanoparticles are embedded into the carbon matrix. The detailed nanoarchitecture was further studied by TEM. As shown in Fig. 1c and d, it can found that the CoSe@C-1 microrod is porous structure and embedded with some relatively regular nanoparticles. Additionally, the surface of the nanoparticles is coated with a carbon layer of about 1.5 nm, which may be beneficial to the conductivity and structural stability of the material. The lattice fringe obtained from HRTEM (Fig. 1e) reveals the distance of 0.313 nm, matching well with the (100) lattice plane of the hexagonal CoSe. The elemental mapping of the CoSe@C-1 (Fig. 1f) shows that C, Co and Se elements are uniform distribution in the microrod. Compared to the other two samples, the morphologies of the CoSe@C-2 and CoSe@C-3 microrods (Fig. S4) are similar to CoSe@C-1, which also display a CoSe nanoparticles embedded in porous carbon microrods structure. But there are some broken microrods and the size of the nanoparticles on the surface is very irregular. This is due to under the double etching of high temperature and selenium powder, the precursors with small diameter
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and short length are easily etched and the structure is destroyed, resulting in partial collapse of microrods. The chemical compositions and valence of the CoSe@C-1 microrod were further investigated by using XPS analysis, as shown in Fig. 2. The XPS survey spectrum (Fig. 2a) indicates the presence of Co, Se, C and O in CoSe@C-1, and the existence of O element could be attributed to the surface absorption of oxygen in the air by the sample exposed to air. The C 1s spectrum (Fig. 2b) shows peaks corresponding to sp2-bonded carbon (CeC), epoxy and alkoxyl groups (CeO) and carbonyl and carboxyl groups (C]O) at 284.9, 286.5 and 288.9 eV [32,33], respectively. Obviously, the CeC bond is much stronger than those of C]O and CeO, suggesting the formation of graphitic carbon in the composite. There are six peaks in the Co 2p spectrum (Fig. 2c): Co 2p3/2 of CoSe at 778.4 and 780.8 eV and satellite peak (identified as “Sat.”) at 786.2 eV as well as Co 2p1/2 of CoSe at 793.2 and 796.4 eV and satellite peak at 801.8 eV [34]. The presence of satellite peaks indicates the Co2þ is in high-spin arrangement [35]. In the Fig. 2d, the peaks are located at around 54.2 and 55.0 eV for Se 3d5/2 and Se 3d3/2, respectively, which can be ascribed to the typical feature of CoeSe bonds according to the previous report. The peaks at 59.6 and 61.5 eV are attributed to Co 3p3/2 and Co 3p1/2, respectively. Furthermore, the corresponding energy dispersive spectrometer (EDS) spectra is shown in Fig. S5 to verify the contents of Se and Co elements. The surface area and pore size distribution of the prepared CoSe@C microrods, which play a critical role in the surface adsorption of ions, were estimated using nitrogen adsorption/ desorption measurements. The adsorption-desorption isotherm curves of the three samples (Figure S6a, S7a and S7c) all display type IV isotherms with type H3 hysteresis loops in the P/P0 range of 0.4e0.95, demonstrating the existence of the mesoporous structure. According to the Brunauer-Emmett-Teller (BET) method, the surface areas of CoSe@C-1, CoSe@C-2 and CoSe@C-3 microrods are 90.9, 68.1 and 73.0 m2 g1, respectively. Additionally, the BarrettJoyner-Halenda (BJH) pore-size-distribution curves (Figure S6b, S7b and S7d) clearly indicate that the pore size is mainly in the range of 10e20 nm. The large surface area and porous features can provide the necessary pathway for electrolyte penetration and facilitate the ions and electrons transfer in the electrolyte/electrode interface, which is beneficial to enhanced electrochemical properties. 3.2. Na-ion battery performances The electrochemical properties of these CoSe@C microrods as SIBs anodes were measured by ways of galvanostatic charge/ discharge tests, CV and EIS over a potential of 0e2.6 V vs. Naþ/Na. Fig. 3a shows the rate performances of the three samples at different current densities. Obviously, the CoSe@C-1 electrode exhibits the best discharge capacity and rate performances among the three samples. As the current densities increased stepwise from 0.05 to 0.1, 0.2 and 0.5 A g1, the CoSe@C-1 electrode exhibits a high and stable capacities of 573, 500, 445 and 315 mA h g1, respectively. After reducing the current density to 0.2, 0.1 and 0.05 A g1, the capacities of 412, 447 and 424 mA h g1 can be almost recovered, respectively, suggesting excellent rate property of the CoSe@C-1 electrode. Moreover, the electrochemical result of CoSe@C-1 electrode evaluated at a voltage range of 0.3e3 V is shown in Fig. S8 to confirm the lighter effect of low voltage region on the Na-ion storage properties. The CoSe@C-3 electrode delivers lower capacities at each current density than CoSe@C-1. In contrast, the capacity of the CoSe@C-2 rapidly decreases at high current densities, and has an unstable capacity when the current density taken back to 0.05 A g1. Meanwhile, it could be observed that the cycling performance of the CoSe@C-1 is superior to the other two.
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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Fig. 1. Morphologies of the CoSe@C-1 microrod: (a, b) SEM images, (c, d) TEM images, (e) high-resolution TEM (HRTEM) and (f) elemental mapping of C, Co, Se.
Fig. 2. XPS spectra of the CoSe@C-1 microrod: (a) Elements survey, (b) C 1s, (c) Co 2p and (d) Se 3d and Co 3p.
After 60 cycles, the CoSe@C-1 still delivers a high specific capacity of 485 mA h g1, but the capacities of the CoSe@C-2 and CoSe@C-3 are only 48 and 64 mA h g1. As it can be seen, the capacity of CoSe@C-3 electrode rapidly decreased after 35 cycles, while the CoSe@C-2 only stable for 7 cycles. The excellent rate performance and cycle stability of CoSe@C-1 electrode can be attributed to the merits of large surface area, the complete porous microrods, regular CoSe nanoparticles and appropriate thickness of carbon layer
around nanoparticles than that of CoSe@C-2 and CoSe@C-3 electrode materials, which increase electrons and ions transport efficiency, leading to the enhancement of electrochemical properties. Additionally, the SEM images of three samples after cycling are shown in Fig. S9. We can see that the morphology of CoSe@C-1 after cycling could maintain well, however, CoSe@C-2 and CoSe@C-3 after cycling demonstrate serious agglomeration and differentiation phenomenon, which may be the main reason of leading to fast
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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Fig. 3. Electrochemical performances of the CoSe@C microrods: (a) rate performances at different current densities, (b) cycling performances at a current rate of 0.1 A g1, (c) CV curves of the CoSe@C-1 at a scan rate of 0.1 mV s1 and (d) galvanostatic charge-discharge curves of the CoSe@C-1 at 0.1 A g1.
capacity decay. The CV curves of the CoSe@C-1, CoSe@C-2 and CoSe@C-3 electrodes for the first three cycles at a scan rate of 0.1 mV s1 in the potential range of 0e2.6 V are shown in Fig. 3c and Fig. S10. In the first cathodic scan, a sharp peak at around 0.75 V of the three electrodes are observed, which are attributed to the formation of solid electrolyte interphase (SEI) layer and the conversion reaction of CoSe: CoSe þ 2Naþ þ 2e / Co þ Na2Se. The anodic peaks at around 1.90 V are related to the recovery of CoSe nanocrystals from metallic Co and Na2Se. For the subsequent 2nd and 3rd cycles, the cathodic peaks shifts to around 1.24 and 1.0 V vs Naþ/Na correspond to the formation of NaxCoSe intermediate and Co and Na2Se, respectively. At the same time, the anodic peaks shift to 1.95 V. This is may be caused be the changes of microrod structure and the activation of the CoSe@C electrodes in the first cycle. Furthermore, all the electrodes display large irreversible capability loss for the first cycle, indicating the decomposition of electrolyte to form the SEI layer and the irreversible phase transition during the first cycle. Fig. 3d and Fig. S10 show the 1st to 60th galvanostatic charging/ discharging curves of the three CoSe@C electrodes at 0.1 A g1 over a voltage window of 0e2.6 V. We can see, all the samples display a discharge plateau at around 0.85 V and a charge plateau at 1.8 V during the initial charge-discharge profiles, which is consistent well with the results of CV data, and the CoSe@C-1 delivers an initial discharge capacities of 1565 mA h g1 and a charge capacities of 702 mA h g1, with a low initial coulombic efficiency. The greater capacity loss may result from irreversible side reactions, including the formation of SEI and incomplete conversion reaction of CoSe and Na. In addition, the well-kept plateaus can be observed for different cycles of the CoSe@C-1 electrode, while the plateaus gradually disappeared with the increases of cycles of other two electrodes, further illustrating the excellent electrochemical properties of the CoSe@C-1 electrode. Additionally, EIS was carried out to further investigate the electrochemical performance of the CoSe@C electrodes. In Fig. S11, all Nyquist plots are typically consists of a semicircle in the high-tomedium frequency region and a slanted line in the low frequency
region, and the semicircle indicates the charge-transfer resistance (Rct) at electrolyte/electrode interface and the slanted line is related to the diffusion of Na-ion in the bulk of the materials. Fig. S11a displays the CoSe@C-1 microrod has a slightly lower Rct value than that of the CoSe@C-2 and CoSe@C-3 microrods before cycling, indicating the high electrical conductivity of CoSe@C-1. The lower Rct value of CoSe@C-1 could be attributed to the larger specific surface area and more interstitial ion transport channels than that
Fig. 4. (a) Reaction mechanism: the charge-discharge curve of the CoSe@C-1 electrodes during the first cycle and the ex-situ XRD analysis at different charge-discharge states; SAED pattern of the CoSe@C-1 electrode at fully (b) discharged (0 V) and (c) charged (2.6 V).
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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Fig. 5. Electrochemical performances of the CoSe@C-1/PVDF-HFP/Na solid-state battery: (a) galvanostatic charge-discharge curves at 0.1 A g1, (b) cycling performance and corresponding coulombic efficiency at 0.1 A g1.
of the other two samples, providing more active sites for rapid charge transfer. The Nyquist plots of the CoSe@C-1 electrode at different cycles as shown in Fig. S11b and the enlarged view of the Nyquist plots in the inset. As seen, the fitted Rct of CoSe@C-1 electrode before cycling is 1311 U. After 10 cycles, the Rct decreases to 455 U, demonstrating the formation of small Co nanoparticles and enhances the electron transport. The Rct value increased as the number of cycles increased, which should be associated with the structure change during the repeated charge and discharge processes. To further investigate the reaction mechanism of the CoSe@C, ex-situ XRD and SAED analyses were employed during the first cycle. Fig. 4a gives the ex-situ XRD patterns of the CoSe@C-1 electrodes at different charge-discharge states and the corresponding charge-discharge curve. In the charge-discharge curve, the CoSe@C-1 electrode displays a discharge plateau at 1.0 V and a charge plateau at 1.8 V. According to the XRD results, when the electrode is discharge to 1.0 V, the CoSe phase can still be observed, indicating that the initial intercalation of Naþ does not change the CoSe (JCPDS No. 89-2004) phase. Meantime, the intermediate NaxCoSe is not observed which is due to the low crystallinity. At discharge to 0.7 V, the CoSe phase is disappear and a minor diffraction peak at 38.6 of Co (JCPDS No. 70-2633) is detected (Fig. S12). After further discharge to 0 V, the peaks at 21.4 and 38.6 can be ascribed to the generation of Co and the peaks at 22.9 of Na2Se (JCPDS No. 47-1699). When the electrode is charge to 2.6 V, there is no peaks of CoSe can be found, which is attributed to the low crystallinity of CoSe after the initial cycle. In addition, the binder (PVDF), conductive carbon (acetylene black) and current collector (Cu foil) also interfere the peaks of the CoSe. The presence of Co indicating the Co is not completely converted to CoSe. The diffraction rings (114) plane of Co, (400) plane of Na2Se and (002) plane of graphite are observed in the SAED patterns, When it is fully discharge to 0 V (Fig. 4b). Fig. 4c shows the fully charge state of 2.6 V, the rings of CoSe and graphite emerge in the SAED pattern. The existence of diffraction dot of (002) plane of graphite is can due to the presence of graphite carbon in the CoSe@C. Therefore, the reaction mechanism of the CoSe can be confirmed as the following: CoSe þ 2Naþ þ 2e ¼ Co þ Na2Se.
the cycling performance of the CoSe@C-1/PVDF-HFP/Na, a capacity of 230 mA h g1 after 50 cycles at 0.10 A g1 can be achieved. The coulombic efficiency is approximately 95% from 2nd cycle to 50th cycle. Fig. S13 reveals the rate performance of the CoSe@C-1/PVDFHFP/Na solid-state battery. When tests at current densities of 0.05, 0.1, 0.2 and 0.5 A g1, stable capacities of 385, 339, 282 and 116 mA h g1 can be obtained, respectively. When the current density reduces to 0.05 A g1, the discharge capacity can also reach 321 mA h g1. Therefore, the CoSe@C-1 composite has excellent electrochemical property in the solid-state electrolyte system. It is worth noting that the outstanding sodium-storage performances of the CoSe@C microrods may originate from the following aspects. First, porous one-dimensional microrod structure can provide suitable channels for the transfer of Naþ and accommodate the volume change during the repeated sodiation/desodiation processes. Second, the carbon matrix and the carbon layer of the CoSe nanoparticles not only can increase the electronic conductivity of the material, but also effectively prevents the aggregation of CoSe nanoparticles. Third, the large specific surface area and rough surface are beneficial to the electrolyte penetration, which can raise the electrochemical performances. 4. Conclusions In conclusion, CoSe@C microrods have been successfully synthesized through a facile approach without template assistance. The obtained materials possess a unique structure, including porous microrods and a thin carbon layer coated on the CoSe nanoparticles, which not only increases electronic conductivity of the active material, but also effectively prevents the aggregation and volume expansion during the electrochemical reaction. When employed as an anode for SIBs, the CoSe@C-1 electrode exhibits the best electrochemical performances with high capacity (485 mA h g1 after 60 cycles at 0.1 A g1) and excellent rate performance (315 mA h g1 at 0.5 A g1). The CoSe@C-1 microrod also delivers outstanding solid state electrochemical properties with a capacity of 230 mA h g1 after 50 cycles at 0.1 A g1. Therefore, the strategy may also be generally extended to other transition metal selenides with superior electrochemical performances.
(1) Declaration of competing interest
To demonstrate the application potentiality of the CoSe@C-1, a coin solid-state battery (CoSe@C-1/PVDF-HFP/Na) was fabricated. Fig. 5a shows the galvanostatic charging/discharging curves of the CoSe@C-1/PVDF-HFP/Na solid-state battery at 0.10 A g1, and the charge-discharge curve is similar to the liquid battery. In the first cycle, the discharge capacity is 991 mA h g1 and the charge capacity reaches 344 mA h g1. The low initial coulomb efficiency is owing to the side reactions between electrolyte and electrodes. For
There are no conflicts of interest to declare. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (51871123, 51501072 and 51571124), Natural Science Foundation of Tianjin (17JCYBJC17900), Ministry of
Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090
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Please cite this article as: M. Wang et al., In-situ carbon coated CoSe microrods as a high-capacity anode for sodium ion batteries, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153090