An advanced CoSe embedded within porous carbon polyhedra hybrid for high performance lithium-ion and sodium-ion batteries

Chemical Engineering Journal 325 (2017) 14–24

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

An advanced CoSe embedded within porous carbon polyhedra hybrid for high performance lithium-ion and sodium-ion batteries Jiabao Li, Dong Yan, Ting Lu, Yefeng Yao, Likun Pan ⇑ Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Materials Science, East China Normal University, Shanghai 200062, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A novel composite containing CoSe

and porous carbon polyhedra was fabricated.  The composite was used as anodes of lithium ion batteries and sodium ion batteries.  The composite exhibits high specific capacity and superior cycling stability.

a r t i c l e

i n f o

Article history: Received 22 February 2017 Received in revised form 21 April 2017 Accepted 8 May 2017 Available online 9 May 2017 Keywords: CoSe@porous carbon polyhedra Anode material Sodium-ion batteries Lithium-ion batteries

a b s t r a c t A novel composite containing CoSe and porous carbon polyhedra (PCP), denoted as CoSe@PCP, was successfully synthesized using Co-based zeolitic imidazolate framework (ZIF-67) as precursor through a two-step method, including carbonization of ZIF-67 and subsequent selenization. The field-emission scanning electron microscopy and transmission electron microscopy characterizations confirm that CoSe nanoparticles are uniformly dispersed in PCP. When the CoSe@PCP was used as anode material for lithium-ion batteries, it exhibits superior performance with a high reversible capacity of 675 mAh g 1 at 200 mA g 1 after 100 cycles and 708.2 mAh g 1 at 1 A g 1 after 500 cycles as well as excellent cycling stability. Additionally, the CoSe@PCP also demonstrates excellent performance as anode material for sodium-ion batteries. A reversible capacity of 341 mAh g 1 can be obtained over 100 cycles at 100 mA g 1 with high cycling stability. The excellent battery performance of CoSe@PCP should be attributed to the synergistic effect of nanostructured CoSe and PCP derived from ZIF-67, in which the nanostructured CoSe possesses high reactivity towards lithium and sodium ions and the PCP can provide a continuous conductive matrix to facilitate the charge transfer and an effective buffering to mitigate the structure variation of CoSe during cycling. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Over the past decades, the tremendous consumption of fossil fuels has caused severe energy crisis and environmental pollution. Developing clean and sustainable energy resources is considered as

⇑ Corresponding author. E-mail address: [email protected] (L. Pan). http://dx.doi.org/10.1016/j.cej.2017.05.046 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.

one of the most promising solutions and of great significance. Rechargeable batteries have received extensive attention as one of the most important energy storage devices for numerous portable electronic systems [1–4]. Among them, the dominance of lithium-ion batteries (LIBs) in power sources is obvious for portable electronic systems due to their high energy density, longevity and environmental benignity [5,6]. However, the large-scale development of LIBs faces the increasing concerns over the cost and con-

J. Li et al. / Chemical Engineering Journal 325 (2017) 14–24

tinuous availability of lithium [7,8]. Considering natural abundance and low cost of sodium as well as its suitable redox potential ( 2.71 V versus the standard hydrogen electrode) which is only 0.3 V above that of lithium, sodium-ion batteries (SIBs) have received much interest recently as a promising alternative to LIBs [9,10]. However, the practical application of LIBs and SIBs in electronic devices and smart power grids requires large improvement in energy density, cycling life and safety issues, which is basically determined by the electrode materials [11]. Based on this viewpoint, tremendous efforts have been devoted in the last decade to the development of high-performance electrode materials and great improvements have been achieved [12,13]. For the available anode materials, low-cost carbonaceous materials, including hard carbon, meso-carbon microbeads and graphite, exhibit satisfied capacities and good cycling stability for both LIBs and SIBs [14,15]. However, the application of carbonbased electrodes as anodes for LIBs and SIBs suffers from the safety issues associated with lithium and sodium dendrites grown on the surface of electrodes due to the fact that the inserted potentials of lithium and sodium into carbonaceous materials are close to the deposition potentials of lithium and sodium [10,16]. What’s more, the specific capacities of various carbon-based materials are not high enough to meet the large-scale applications like electrical vehicles (EVs), where both high energy density and power density are necessary [17]. It is known that transition metal oxides can provide much higher specific capacities than carbonaceous materials based on conversion reaction upon the insertion and extraction of lithium and sodium ions. However, the inherent low electrical conductivity of metal oxide electrodes and huge volume change associated with conversion reactions restrict their practical application, which may often result in unfavorable charge transfer, pulverization of active materials and capacity decay [18–20]. Recently, metal selenides such as MoSe [21], SnSe [22] and FeSe2 [23] have shown great potential as anode materials for both LIBs and SIBs, and excellent battery performances have been obtained, providing new alternative anode materials for LIBs and SIBs. Based on the electrochemical mechanism, metal selenides can be converted to Li2Se in a way that is similar to the conversion process of Li2S in metal sulfides upon electrochemical cycling. Compared with their oxides and sulfides counterparts, the volume energy density and rate capability of metal selenides as electrode materials should be higher due to the higher density and electrical conductivity of metal selenides [24]. As an important member of metal selenide family, CoSe has attracted growing research attention due to its remarkable electrical conductivity and high capacity [25–27]. However, similar to its metal selenide partners, the application of CoSe as high-performance electrode suffers from large volume and structure expansion during repeated charge/discharge process, resulting in the shrinkage and pulverization of electrode materials [28,29]. Moreover, the deteriorated contact between active materials and current collector during the repeated expansion and shrinkage of electrode often causes rapid capacity fading during cycling [30]. Based on previous reports, nanostructure fabrication as well as hybridization with carbonaceous materials have been proved to be effective approaches to mitigate the mechanical strain induced by the volume and structure change during the charge/discharge processes [29,31]. On one hand, narrowing the electrochemical active material to nanometer scale is convenient for the transport and diffusion of ions and electrons owing to the short transport pathways [32]. On the other hand, the introduction of carbonaceous agents into target materials can not only improve the conductivity of the electrode, but also provide an effective buffer matrix to alleviate the volume and structure variation during cycling [33]. Most recently, a number of related hybrid structures, including CoS2@porous carbon shell [18], NiO/Ni covered with graphene [34] and one-dimensional Na3V2(PO4)3/C nanowires [35],

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have been demonstrated to exhibit excellent electrochemical performance for LIBs or SIBs. Despite the exciting studies mentioned above, insufficient attention has been given on the design and controllable synthesis of the composite consisting of nanosized CoSe and carbonaceous agent with special morphology and high electrochemical performance. Due to their adjustable pore structure, large specific surface area and versatile functionalities, metal-organic frameworks (MOFs), self-assembled by metal ions and organic linkers through coordination bonds, are largely been employed in the fields like catalysis, gas adsorption and separation as well as energy conversion [36]. Recently, using MOFs as templates to fabricate electrochemical active materials with various structures has attracted considerable attention [37–39]. Under properly controlled conditions, MOFs derived metal oxides [40,41], metal sulfides [42,43] and porous carbons [44,45] with regular morphologies and superior electrochemical performances have been reported. It should be noted that the metal oxides and sulfides derived from MOFs are often embedded in a porous carbon matrix, in which the stable electrochemical performance can be guaranteed. Therefore, it is promising to employ MOFs as templates to fabricate hybrid structures with nanoscaled electroactive materials and porous carbon matrix. Herein, we reported the designed fabrication of advanced CoSe nanoparticles embedded within the novel porous carbon polyhedra (CoSe@PCP) using ZIF-67 as precursor. Different with the reported CoSe based composites, the as-prepared CoSe@PCP is featured with uniform morphology and homogeneous dispersion of CoSe nanoparticles in the carbon polyhedra due to the in-situ fabrication strategy. When employed as anodes for both LIBs and SIBs, the CoSe@PCP exhibits the features of high specific capacities, excellent rate performances and superior cycling stability. The high performance originated from the rational hybridization of CoSe and PCP demonstrates that the CoSe@PCP is a potential candidate for the anode material of both LIBs and SIBs. 2. Experimental section 2.1. Materials Synthesis of ZIF-67: 7.27 g Co(NO3)2
6H2O was dissolved in 150 mL methanol to form a solution A, and 8.21 g 2-methylimidazole was dissolved in 150 mL methanol to form a solution B. Then solution A was poured into solution B under magnetic stirring at room temperature for 30 min, and then aged for 24 h. The precipitate was collected by centrifugation and washed with methanol for several times, and dried at 60 °C overnight. Synthesis of Co@PCP: the as-prepared ZIF-67 was carbonized at 650 °C for 1 h in nitrogen atmosphere with a heating rate of 1 °C min 1 to obtain the desired Co@PCP. Synthesis of CoSe@PCP: Co@PCP was mixed with selenium power with a mass ratio of 1:1. The mixture was heated at 650 °C for 3 h with a heating rate of 1 °C min 1 in nitrogen atmosphere to obtain CoSe@PCP. For comparison, the carbonization and selenization were combined into one step. The as-prepared ZIF-67 was mixed with selenium power with a mass ratio of 1:1. The mixture was then heated at 650 °C for 3 h with a heating rate of 1 °C min 1 in nitrogen atmosphere. The obtained product is proved to be CoSe2@PCP. 2.2. Characterization X-ray diffraction (XRD) patterns were collected on Bruker D8 Advance to characterize the phase structure of the prepared samples. The morphologies and structures of the prepared samples

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were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM, JEOL-2010). The content of CoSe in CoSe@PCP composite was determined by thermo-gravimetric (TG) analysis using a Shimadzu-50 thermoanalyser in air with a heating rate of 10 °C min 1 from room temperature to 800 °C. The specific surface area of CoSe@PCP was deduced from the nitrogen adsorption measurement data obtained by using ASAP 2010 Accelerated Surface Area and Porosimetry System (Micromeritics, Norcross, GA) based on Brunauer-Emmett-Teller (BET) multipoint method, and the corresponding pore size distribution was collected from the adsorption branches of the isotherms using Barret-Joyner-Halenda model. Raman spectra were carried out on RM-1000 (Renishaw) with an excitation laser of 632.8 nm. The X-ray photoelectron spectroscopy (XPS) measurement was carried out on Thermo ESCALAB 250XI spectrometer. 2.3. Electrochemical measurements A homogenous slurry containing 70 wt.% CoSe@PCP (active materials), 20 wt.% super P (conductive agent) and 10 wt.% carboxymethyl cellulose (binder) in deionized water was fabricated, and then the slurry was coated on a copper foil (current collector) to form the working electrode and then dried at 120 °C under vacuum overnight. The mass loading of active material on the working electrode is about 1.8 mg cm 2. CR2032 type coin cells were assembled in an argon-filled glove box (MB-10-compact, MBRAUN). For LIBs, the half cells were assembled by using lithium metal foil as counter electrode, Whatman glass fiber as separator and 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1, w/w) as electrolyte. For SIBs, sodium metal foil and Whatman glass fiber were used as counter electrode and separator, respectively. The electrolyte was 1 M NaClO4 in ethylene carbonate and propylene carbonate (1:1, w/w) with the addition of 5 wt.% fluoroethylene carbonate. The coin type cells were charged and discharged on a battery test system (Land 2001A) in a voltage range of 0.005–3.0 V. Cyclic voltammetry (CV) was collected on an electrochemical workstation (AUTO-LAB PGSTAT302 N) at a scan rate of 0.2 mV s 1 between 0.005 and 3.0 V. Electrochemical impedance spectroscopy (EIS) was carried out in a frequency range of 0.1 Hz–100 kHz, and the applied bias voltage and ac amplitude were set at the open circuit voltage of the cells and 5 mV, respectively. 3. Results and discussion Fig. 1 illustrates the typical synthesis strategy of CoSe@PCP from ZIF-67. First of all, the well-defined ZIF-67 was synthesized

according to a previous report using Co(NO3)2
6H2O and 2-methylimidazole as precursors in methanol [46]. As expected, the ZIF-67 nanoparticles present regular morphology and uniform size distribution (Fig. 2a and b). Fig. S1 shows the XRD pattern of the as-prepared ZIF-67, which is in line with the previous study [47]. After thermal treatment at 650 °C in nitrogen atmosphere, the organic linkers were pyrolyzed to amorphous carbon. Meanwhile, the Co ions were in situ reduced to metal Co nanoparticles embedded in the amorphous porous polyhedra carbon, denoted as Co@PCP. The diffraction peak of Co@PCP (Fig. S2) at around 2h = 44° can be ascribed to the (1 1 1) plane of cubic Co (JCPDS card No. 15-0806), indicating the conversion of ZIF-67 to Co@PCP after the pyrolysis process [20]. It should be noted that the calcination temperature of 650 °C is selected based on the previous study in order to convert the Co ions to metal Co, and the slow heating rate employed is helpful to avoid the aggregation of the Co nanoparticles and preserve the polyhedra-like structure during the thermal treatment [18]. The carbon generated at the selected temperature with slow heating rate can provide a buffering matrix to prevent the further contraction of the frameworks [32]. As expected, the broad peak with low peak intensity in the XRD pattern of Co@PCP demonstrates the nanosized feature of metal Co nanoparticles in the sample of Co@PCP [20]. In addition, the broad peak at around 2h = 26° originates from the amorphous carbon polyhedra. The Co nanoparticles with high chemical activity were employed to synthesize CoSe@PCP via annealing with Se powers at high temperature. As expected, all the observed peaks (Fig. S3) can be well indexed to CoSe (JCPDS card No. 70-2870) [28]. In addition, no peaks assigned to other phase or impurity can be detected, indicating the successful synthesis of CoSe@PCP through the ‘‘top-down” approach. For comparison, we also prepared CoSe2@PCP via onestep carbonization and selenization process. Previous study has demonstrated the generation of CoSe2 by calcinating the mixture of ZIF-67 and selenium power [28]. Fig. S4a shows the XRD pattern of the as-prepared sample, all the characteristic peaks observed can be well ascribed to CoSe2 (JCPDS card No. 09-0234) [28]. Due to the in-situ fabrication strategy, the nanoscaled CoSe can promote the reaction kinetics of lithium and sodium storage, while the porous carbon polyhedra can restrict the aggregation of the CoSe nanoparticles and provide a buffering matrix to alleviate the volume change upon electrochemical cycling [39]. Besides, the in-situ fabrication strategy of the CoSe@PCP avoids the use of dangerous and toxic gases, such as H2/N2 and H2Se, providing a facile approach to prepare high-performance metal selenide anodes. The morphologies and structures of ZIF-67, Co@PCP and CoSe@PCP were investigated by FESEM at different magnifications

Fig. 1. Schematic illustration of the formation process of CoSe@PCP.

J. Li et al. / Chemical Engineering Journal 325 (2017) 14–24

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Fig. 2. FESEM images of ZIF-67 (a and b), Co@PCP (c and d) and CoSe@PCP (e and f) at different magnifications. (g, h, i) HRTEM images of CoSe@PCP at different magnifications.

(Fig. 2). As shown in Fig. 2a and b, the ZIF-67 nanoparticles with smooth surface display a uniform rhombic dodecahedra structure due to the combination of Co ions and 2-methylimidazole through coordination bonds, and the average size of the particles is about 600 nm. After carbonization at 650 °C for 1 h, the pristine polyhedra morphology is retained well, but the size of polyhedra shrinks a little, as shown in Fig. 2c and d. Interestingly, some short CNTs with a length of about 70 nm extruded from the surface of polyhedra carbon can be easily observed. It is believed that the organic linkers (2-methylimidazole) in ZIF-67 can be converted to CNTs due to the high catalytic activity of metal Co nanoparticles [48]. Meanwhile, the surface of Co@PCP becomes rough after carbonization. By a simple high-temperature reaction between Co@PCP and Se powers, the final product CoSe@PCP with crumpled surface was obtained. It should be noted that the selenization process has little effect on the polyhedra morphology according to the FESEM images of Fig. 2e and f. Similar to CoSe@PCP, the as-prepared CoSe2@PCP also displays a uniform rhombic dodecahedra structure (Fig. S4b), and its average size is about 450 nm. The microstructure of CoSe@PCP was further studied by HRTEM. As shown in Fig. 2g and h, it is found that the CoSe nanoparticles with sizes ranging from 5 nm to 10 nm are uniformly dispersed in the polyhedra carbon. Additionally, no aggregation of CoSe can be detected. Besides, the lattice fringe obtained from HRTEM (Fig. 2i) reveals the interplanar distance of 0.269 nm, corresponding to the most exposed crystal face (1 0 1) of CoSe, as shown in the XRD pattern (Fig. S3) [49]. TG analysis in air was employed to identify the CoSe and carbon contents in the CoSe@PCP composite, and the corresponding curve is shown in Fig. 3a. During the thermal treatment in air, the significant weight loss observed from 350 °C to 600 °C could be assigned to the decomposition of carbonaceous component as well as the chemical reaction between CoSe and oxygen with the generation of Co3O4 and Se [28]. Moreover, no weight loss can be detected

when the temperature is further increased, demonstrating the complete transformation of CoSe@PCP to Co3O4. It should be noticed that a weak weight increase at around 330 °C can be detected, which can be ascribed to the generation of Co3O4 and the unevaporated Se during the initial conversion of CoSe to Co3O4 [28]. Based on the reaction formula: 3CoSe (s) + 2O2 (g) = Co3O4 (s) + Se (g) and 31.6 wt.% of the original mass retained, the accurate contents of CoSe and carbon in CoSe@PCP are calculated to be 54.3 wt.% and 45.7 wt.%, respectively. Fig. S5 shows the corresponding XRD pattern of the product after TG analysis, demonstrating the generation of Co3O4 (JCPDS card No. 65-3103). The porous feature of CoSe@PCP was confirmed with the help of N2 adsorption-desorption measurement, as shown in Fig. 3b. The result shows that the as-prepared CoSe@PCP possesses a large BET specific surface area of 76.94 m2 g 1. Additionally, the pore size distribution (the inset of Fig. 3b) demonstrates that mesopores ranging from 30 nm to 40 nm dominate the pore structure in CoSe@PCP. The large specific surface area and mesoporous feature of CoSe@PCP provide more interfaces and pathways for the diffusion of lithium and sodium ions and better electrode-electrolyte contact as well as additional space to mitigate the structure change during the ion insertion/desertion, which is beneficial to high battery performance [50,51]. Fig. S6 shows the Raman spectrum of CoSe@PCP. Two peaks at around 1375 cm 1 and 1566 cm 1 corresponding to defect induced band (D band) and crystalline graphite band (G band) can be clearly observed, highlighting the signatures of pyrolyzed carbon materials [52]. Besides, the ID/IG ratio is calculated to be 1.21, demonstrating the amorphous feature of the porous carbon polyhedra, which is in line with the result of XRD [28]. Fig. S4c shows the Raman spectrum of CoSe2@PCP, and the ID/IG ratio is calculated to be 1.02. It can be seen that the ID/IG ratio of CoSe@PCP is higher than that of CoSe2@PCP, demonstrating that more defects exist in CoSe@PCP, which can provide more active sites for Li+ and Na+ accommodation [45,48].

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Fig. 3. (a) TG curve of CoSe@PCP at a ramping rate of 10 °C min corresponding pore size distribution.

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from room temperature to 800 °C. (b) N2 adsorption-desorption isotherms of CoSe@PCP. The inset is the

The near-surface elemental composition and chemical state of CoSe@PCP were measured by the XPS measurement. The survey spectrum (Fig. 4a) confirms the existence of C, Co, Se and O, and the presence of O can be attributed to the exposure to air [53]. The chemical states of Co and Se were investigated with the high-resolution spectra and the corresponding results are shown in Fig. 4b and c. In the Co 2p high-resolution spectrum (Fig. 4b), the curve can be deconvoluted into two spin-orbit doublets and three shake-up satellites (denoted as ‘‘Sat.”) based on the Gaussian fitting method [53]. Two peaks located at around 792.5 eV for Co 2p1/2 and 779.4 eV for Co 2p3/2 correspond to the spin-orbit characteristic of Co2+ [49]. As for the Se 3d XPS spectrum (Fig. 4c), the peaks at around 57.4 eV for Se 3d3/2 and 53.4 eV for Se 3d5/2 can be ascribed to the typical feature of metal-selenium bonds according to the previous report [29]. The results confirm the successful synthesis of CoSe via two-step thermal treatment. In addition,

three peaks assigned to carbonyl and carboxyl groups (C@O), epoxy and alkoxyl groups (C–O) and sp2-bonded carbon (C–C) can be found in the C 1 s XPS spectrum, and the peak intensity of C–C bond is much stronger than those of C@O and C-O, further indicating the amorphous feature of carbon in CoSe@PCP [29]. According to previous study, the oxygen-containing groups in the target material can participate in the surface redox reaction with lithium and sodium ions, and thus improve the specific capacity of the electrochemical active material [54]. The lithium storage performance of CoSe@PCP was evaluated using CR2032 type coin cell. CV was employed to investigate the electrochemical behaviors of the CoSe@PCP electrode for LIBs. Fig. 5a displays the initial five CV curves of CoSe@PCP for LIBs at a scan rate of 0.2 mV s 1 in a voltage range of 0.005–3.0 V. During the initial discharge process, a weak peak at around 1.52 V can be ascribed to the initial insertion of Li+ into the CoSe@PCP electrode

Fig. 4. XPS spectra of CoSe@PCP: (a) wide-scan, (b) Co 2p, (c) Se 3d and (d) C 1 s spectra.

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Fig. 5. Electrochemical performance of CoSe@PCP for LIBs: (a) The initial five CV curves of CoSe@PCP in a voltage range of 0.005–3.0 V at a scan rate of 0.2 mV s 1. (b) Galvanostatic charge and discharge profiles in the 1st, 2nd, 3rd and 50th cycles at a current density of 200 mA g 1. (c) Rate performance of CoSe@PCP. (d) EIS of CoSe@PCP electrode before cycling and after different cycles at 200 mA g 1 for LIBs. (e) Cycling performance of CoSe@PCP at 200 mA g 1.

[27]. According to the previous reports on Co0.85Se and NiSe anodes for LIBs [27,55], the broad cathodic peak located at 1.15 V (including two peaks at around 1.07 and 1.23 V) corresponds to the formation of LixCoSe through the intercalation of Li+ and solid electrolyte interface (SEI) layer on the interface of electrode and electrolyte because of the decomposition of electrolyte in the first cycle, indicating a multistep process of Li+ insertion. Meanwhile, another cathodic peak (0.6 V) should be attributed to the accompanied conversion reaction between LixCoSe and Li+, forming Co and Li2Se [28]. Correspondingly, the first anodic scan presents two peaks at about 1.26 and 2.1 V, which are attributed to the recovered formation of CoSe after the charge process [25]. Owing to the structure change and the activation of the electrode materials in the first cycle, the positions of cathodic peaks shift to 1.45 and 0.7 V in the subsequent cycles, corresponding to the intercalation of Li+ and the following conversion reaction, respectively [19,28].

On the other hand, the location of anodic peaks in the following cycles is fully consistent with that in the first cycle. Moreover, the CV curves starting from the second cycle overlap each other very well, indicating the excellent cycling stability and reversibility of the CoSe@PCP electrode for LIBs. The conversion reaction mechanism was further confirmed by the ex-situ XRD measurements, and the results are shown in Fig. 6. According to the ex-situ XRD results (Fig. 6c), the CoSe phase can still be detected when the electrode is discharged to 1.5 V, demonstrating that the initial intercalation of Li+ into the CoSe@PCP electrode does not cause a phase transition [27,56,57]. When the electrode is discharged to 1.16 and 0.9 V, no signals corresponding to the CoSe phase can be detected, and only two broad peaks corresponding to the current collector can be observed, indicating that no conversion reaction happens above 0.9 V [27]. This results also confirm that the intercalation of Li+ into the CoSe@PCP

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Fig. 6. Reaction mechanism: Ex-situ XRD patterns of CoSe@PCP collected at different discharge and charge states during the first cycle for LIBs (a and c), SIBs (b and d).

electrode is a multistep process [27,56,57]. After the electrode is further discharged to 0.005 V, some new diffraction peaks appear, which can be ascribed to the generation of Co (at around 33° and 37°) and Li2Se (at around 25° and 29°) after the conversion reaction [27]. For the charge process, when the electrode is charged to 1.58 V, the signals for Co and Li2Se become weak. Besides, at fully charged state, no peaks corresponding to the CoSe phase can be detected, which should be caused by the low crystallinity of CoSe after the first cycle [57]. Similar results can be found in previous study [57]. The charge/discharge profiles of the CoSe@PCP electrode for LIBs at a current density of 200 mA g 1 in the 1st, 2nd, 3rd and 50th cycles are shown in Fig. 5b. A discharge plateau from 1.3 to 1.1 V and a discharge slope from 1.1 to 0.005 V can be detected in the discharge profile from the onset voltage of 3.0 V to the cut-off voltage of 0.005 V, corresponding to the two cathodic peaks at 1.15 and 0.6 V in the first cathodic scan. On the other hand, a charge slope from 1.0 to 2.0 V and a charge plateau from 2.0 to 2.5 V corresponding to the two anodic peaks are observed, respectively. The initial discharge capacity of the CoSe@PCP electrode is 902 mAh g 1, and the corresponding charge capacity is 663 mAh g 1, corresponding to an initial Coulombic efficiency (ICE) of 73.5%. The formation of SEI layer due to the decomposition of electrolyte contributes to the initial capacity loss [58]. It should be noted that the ICE value obtained is competitive to those of many related hybrids reported previously [48,59], which can be ascribed to the enhanced conductivity and superior structure stability due to the fact that the electrochemically active CoSe nanoparticles are embedded in the amorphous carbon. In addition, the subsequent charge/discharge profiles are almost the same, which is consistent with the CV results.

The rate performance of the CoSe@PCP electrode was also performed, and the results are shown in Fig. 5c. The electrode was tested at 0.1, 0.5, 1 and 2 A g 1, and reversible capacities of 701.2, 645.6, 590.0 and 457.5 mAh g 1 can be obtained, respectively. This excellent rate performance can be ascribed to the superior lithium ion and electron mobility obtained from the distinctive structure advantages of CoSe@PCP including homogeneous dispersion of CoSe and regular morphology. Even at a high current density of 5 A g 1, the CoSe@PCP electrode still possesses a capacity of 198.6 mAh g 1. In addition, after the current density returns to 1, 0.5 and 0.1 A g 1, the specific capacity is recovered to 441.8, 452.7 and 524.1 mAh g 1, respectively. Moreover, the rate performance of the CoSe@PCP electrode with less loading of super P was also investigated, and the results are shown in Fig. S7a. As seen, the CoSe@PCP electrode can still exhibit good rate capability when the loading of super P is reduced. Fig. 5d shows the EIS measurements of the electrode before cycling and after different cycles (3 cycles and 100 cycles) at a current density of 200 mA g 1. As seen, the plots contain a sloping line in the low-frequency region and two depressed semicircles in the medium and high frequency regions. The indistinct semicircle in the high frequency region can be ascribed to Rf and the constant phase element (CPE1), which is related to the SEI layer. Additionally, the large semicircle in the medium frequency can be attributed to the charge transfer resistance (Rct) and the constant phase element (CPE2) [54]. For LIBs, the fitted Rct value of the CoSe@PCP electrode after 3 cycles is 76.7 X, and this value slightly decreases to 61.3 X after 100 cycles, both of which are much lower than that of the pristine electrode (142 X). Such results are related with the activation process during cycling. Besides, the slight decrease of Rct value after 100 cycles is indicative of the excellent diffusion kinetics of Li+ due to the rever-

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sible reaction of CoSe with Li+ in organic electrolyte [60,61], and also shows the decrease of polarization, which is favorable for high battery performance [61]. The cycling performance of the CoSe@PCP electrode is shown in Fig. 5e. A high reversible capacity of 675 mAh g 1 is maintained over 100 cycles at 200 mA g 1, along with superior cycling stability. After 100 charge/discharge cycles, the structure of the as-prepared CoSe@PCP is maintained well (Fig. S8a). Based on the carbon contents calculated from TGA and the theoretical capacity of CoSe [62], the specific capacities contributed by CoSe and porous carbon polyhedra are calculated to be 211.2 and 463.8 mAh g 1, respectively. The uniform morphology and high content of porous carbon polyhedra (45.7 wt.%) account for its high capacity contribution. Additionally, an obvious activation process can be detected when the CoSe@PCP electrode is tested at 1 A g 1 (Fig. S9), and a stable capacity of 708.2 mAh g 1 can be obtained after 500 cycles, higher than that at 0.2 A g 1 after 100 cycles. For conversion-type electrode materials, the capacity rise during cycling at high current density for LIBs is quite common. Based on previous studies, the increased capacity can be explained from the following aspects: (1) the polymeric gel-like layer can provide extra capacity via a ‘‘pseudo-capacitance-type behavior” [34,61,63]; (2) the interfacial lithium storage may contribute part of the increased capacity [64]; (3) the defects formed upon cycling can promote the conversion-reaction kinetics, resulting in enhanced electrochemical performance [65]; (4) the accessibility for lithium insertion and extraction in the as-fabricated electrodes is improved due to the enhanced Li-diffusion by the activation process for conversion-type electrode material [66]. For comparison, the electrochemical performances of various metal selenides for LIBs have been summarized in Table S1, showing the superior specific capacity and cycling stability, especially the excellent long-term cycling performance of CoSe@PCP at high current density. The excellent performance should be attributed to the unique properties and structure advantages of CoSe@PCP including good electrical conductivity, large specific surface area and stable regular polyhedra morphology. The cycling performance of CoSe2@PCP was also measured for comparison, and the results are shown in Fig. S10a and c. As seen, an obvious capacity decay can be detected during cycling. After 50 cycles, only 298.1 mAh g 1 can be maintained for LIBs at a current density of 200 mA g 1. This poor electrochemical performance can be ascribed to the low chemical activity of CoSe2 towards Li+ and the structure destruction of the active material during cycling [28], which can be confirmed by the FESEM measurement after cycling (Fig. S11a). For the development of anode materials for high-performance SIBs, simple imitation of the reported electrode materials of LIBs was proved to be impracticable due to the large ion radius and poor reaction kinetics of sodium ion. But the as-prepared CoSe@PCP also exhibits good electrochemical performance for SIBs. The CV measurement for SIBs under the same conditions for LIBs was conducted and the results are shown in Fig. 7a. During the first cathodic scan, a broad peak at around 0.6 V and a small peak at around 0.7 V can be assigned to the intercalation of Na+ into the CoSe@PCP electrode accompanied by the conversion reactions of CoSe with Na+ and the formation of SEI layer [55]. The anodic peak at around 1.8 V originates from the reversible formation of CoSe [55]. After the first cycle, only one cathodic peak at around 1.1 V can be detected, which corresponds to the intercalation of Na+ and the conversion reaction of CoSe with Na+. The structure change and the activation of the electrode materials during the first cycle account for the shift of the cathodic peak in the subsequent cycles, and similar result can be found in previous study [28]. Besides, the well overlapped CV curves after the first cycle indicate the outstanding reversibility of the CoSe@PCP electrode.

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Ex-situ XRD measurements of CoSe@PCP for SIBs were performed to detect the reaction mechanism. According to the results shown in Fig. 6d, no diffraction peaks corresponding to CoSe phase can be observed when the electrode is discharged to 0.7 V. Besides, no signals corresponding to Co or Na2Se can be detected, indicating the initial intercalation of Na+ into the CoSe@PCP electrode [57]. The intermediate NaxCoSe phase is not detected due to its low crystallinity [57]. After the electrode is further discharged to 0.005 V, the diffraction peaks of both Co (at around 32° and 34°) and Na2Se (at around 24°, 38° and 53°) can be detected, corresponding to the products of conversion reaction [57,67]. After the electrode is fully charged, only a weak peak at around 51° corresponding to the (1 1 0) plane of CoSe can be observed, demonstrating the low crystallinity of CoSe after the first cycle, which is similar with the results for LIBs. Obviously, the CoSe@PCP electrode undergoes both intercalation and conversion processes for LIBs and SIBs upon electrochemical cycling. The discrepancy on the electrochemical behaviors presented in the CV measurements for LIBs and SIBs should be ascribed to the different intercalation potential of Li+ and Na+ within the CoSe@PCP electrode. As shown in Fig. 7b, the charge-discharge profiles of the CoSe@PCP electrode displays a discharge plateau from 1.25 to 0.5 V and a charge plateau from 1.5 to 2.0 V in the first cycle, which agrees well with the results of CV measurement. In the first discharge process, the discharge plateau from 1.25 to 0.5 V corresponds to the reduction of CoSe with the generation of Co and Na2Se, while the slope ranging from 0.5 to 0.005 V can be ascribed to the formation of SEI layer. Moreover, initial discharge capacity of 504 mAh g 1 and charge capacity of 340.2 mAh g 1 are obtained at a current density of 100 mA g 1 in a voltage range of 0.005–3.0 V, and the capacity loss in the first cycle originates from some irreversible reactions and the formation of SEI according to the previous reports [21,59]. The initial ICE value of the CoSe@PCP electrode for SIBs is 67.6% and gradually increases to 100% after the initial several cycles, demonstrating the excellent reversibility of the CoSe@PCP electrode. Fig. 7c shows the rate performance of CoSe@PCP, and the electrode exhibits the capacities of 360.3, 315.6, 278.9, 247.1 and 207.7 mAh g 1 at current densities of 0.05, 0.25, 1, 2 and 4 A g 1. What’s more, when the current density returns to 0.5, 0.25 and 0.05 A g 1, the capacity can be almost recovered the original values, indicating the excellent rate capability of CoSe@PCP. The rate performance of the CoSe@PCP electrode with less loading of super P was also investigated, and the results are shown in Fig. S7b, further demonstrating its excellent rate capability. The EIS measurements of the CoSe@PCP electrode before cycling and after 3 and 100 cycles at a current density of 100 mA g 1 are shown in Fig. 7d. The fitted Rct value for SIBs before cycling is 213 X. After 3 cycles, this value decreases to 47.2 X, which should be attributed to the enhanced electrical conductivity because of the small metal cobalt nanoparticles formed during cycling. On the other hand, the Rct value increases to 148.3 X after 100 cycles, which should be associated with the structure change of electrode during cycling [34]. Meanwhile, the small increase of Rct for SIBs after 100 cycles provides the evidence for the slight capacity loss after 100 cycles compared with the initial 10 cycles. As known, the ion radius of Na+ is 55% larger than that of Li+ and the reaction kinetics of Na+ is more sluggish compared with Li+ [31]. The larger Rct for SIBs after 100 cycles than that of LIBs reveals that the insertion and extraction of Na+ is kinetically more difficult. More importantly, the electrode exhibits superior capacity retention during cycling and a reversible capacity of 341 mAh g 1 can be obtained after 100 cycles at a current density of 100 mA g 1, as shown in Fig. 7e. It should be noted that the structure of CoSe@PCP is well reserved after 100 charge/discharge cycles (Fig. S8b). Based on the carbon content calculated from

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Fig. 7. Electrochemical performance of CoSe@PCP for SIBs: (a) The initial five CV curves of CoSe@PCP in a voltage range of 0.005–3.0 V at a scan rate of 0.2 mV s 1. (b) Galvanostatic charge and discharge profiles in the 1st, 2nd, 3rd and 50th cycles at a current density of 100 mA g 1. (c) Rate performance of CoSe@PCP. (d) EIS of CoSe@PCP electrode before cycling and after different cycles at 100 mA g 1 for SIBs. (e) Cycling performance of CoSe@PCP at 100 mA g 1.

TGA and the theoretical capacity of CoSe [62], the specific capacities contributed by CoSe and porous carbon polyhedra are calculated to be 211.2 and 129.8 mAh g 1, respectively. Besides, different from CoSe@PCP, the CoSe2@PCP can only deliver a specific capacity of 291.3 mAh g 1 after 50 cycles at a current density of 100 mA g 1 (Fig. S10b and d). Theoretically, one mole of CoSe2 can accommodate 4 mol of sodium ions and electrons via conversion reaction, which endows the CoSe2 with high theoretical specific capacity [25]. In contrast, one mole of CoSe can accommodate 2 mol of sodium ions and electrons [62]. It is generally believed that the volume change is more severe when more sodium ions take part in the electrochemical reaction, and the huge volume change often results in capacity decay during cycling. The FESEM image (Fig. S11b) of the CoSe2@PCP electrode after cycling further confirms the huge volume change upon electrochemical cycling. Therefore, the CoSe2@PCP electrode exhibits worse electrochemi-

cal performance than the CoSe@PCP electrode. To the best of SIBs reported by now, the capacity of CoSe@PCP is superior compared with those of other metal selenide electrodes [55,62]. Table S2 shows the comparison of electrochemical performances of SIBs based on various metal selenides. It can be seen clearly that CoSe@PCP possesses not only high capacity, but also good cycling stability and excellent rate performance. It should be noted that a reversible capacity of 177.9 mAh g 1 is still retained after 500 cycles at 500 mA g 1, and the fading rate is only 0.2% per cycle (Fig. S12). The superior performance of CoSe@PCP for SIBs benefits from its rationally designed hybrid nanostructure, where the porous polyhedra carbon can effectively alleviate the volume-change induced strain of CoSe during repeated charge and discharge. The above studies of the CoSe@PCP electrode for LIBs and SIBs demonstrate its excellent battery performance, which originates from its inherent properties as following: (1) the as-formed CoSe

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nanoparticles through the selenization of nanoscaled metal Co nanoparticles derived from ZIF-67 are believed to be highly chemically active towards Li+ and Na+, which improves the conversion reaction kinetics. (2) The large specific surface area, rough surface and extruded CNTs are helpful for the electrolyte penetration and the formation of a stable and robust SEI layer, which is beneficial to high battery performance. (3) The uniform porous polyhedra carbon can not only provide an electrically conductive supporter for CoSe, but also effectively mitigate the structure-change induced strain during cycling. 4. Conclusions In summary, a template-induced hybrid nanostructure of CoSe@PCP derived from ZIF-67 was successfully synthesized with nanoscaled CoSe embedded in PCP and employed as anode material for both LIBs and SIBs. The CoSe@PCP exhibits excellent lithium storage performance with high specific capacity (675 mAh g 1 at 200 mA g 1 over 100 cycles), superior rate capability (198.6 mAh g 1 at 5 A g 1) and outstanding long-term cycling performance (708.2 mAh g 1 at 1 A g 1 after 500 cycles). Besides, a reversible capacity of 341 mAh g 1 after 100 cycles at 100 mA g 1 and high cycling stability as well as outstanding rate performance (207.7 mAh g 1 at 4 A g 1) are obtained for SIBs. The excellent battery performance originates from the synergistic effect of electrochemically active CoSe and PCP matrix. This strategy not only provides a general approach to synthesize hybrid composite containing nanoscaled electroactive materials and porous carbon with regular morphology but also shows the great potential of metal selenides to be advanced anode materials for both LIBs and SIBs. Acknowledgments Financial support from Basic Research Project of Shanghai Science and Technology Committee (No. 14JC1491000), National Natural Science Foundation of China (No. 21574043) and National Key Basic Research Program of China (Grant No. 2013CB921801) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2017.05.046. References [1] M. Ebner, F. Marone, M. Stampanoni, V. Wood, Visualization and quantification of electrochemical and mechanical degradation in Li ion batteries, Science 342 (2013) 716–720. [2] J.E. Elshof, H. Yuan, P. Gonzalez Rodriguez, Two-dimensional metal oxide and metal hydroxide nanosheets: synthesis, controlled assembly and applications in energy conversion and storage, Adv. Energy Mater. 6 (2016) 1600355. [3] D. Kundu, E. Talaie, V. Duffort, L.F. Nazar, The emerging chemistry of sodium ion batteries for electrochemical energy storage, Angew. Chem. Int. Ed. 54 (2015) 3431–3448. [4] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Research development on sodium-ion batteries, Chem. Rev. 114 (2014) 11636–11682. [5] A. Zhamu, G. Chen, C. Liu, D. Neff, Q. Fang, Z. Yu, W. Xiong, Y. Wang, X. Wang, B. Z. Jang, Reviving rechargeable lithium metal batteries: enabling nextgeneration high-energy and high-power cells, Energy Environ. Sci. 5 (2012) 5701–5707. [6] C. Chen, Y. Huang, H. Zhang, X. Wang, Y. Wang, L. Jiao, H. Yuan, Controllable synthesis of Cu-doped CoO hierarchical structure for high performance lithium-ion battery, J. Power Sources 314 (2016) 66–75. [7] J.B. Goodenough, Y. Kim, Challenges for rechargeable Li batteries, Chem. Mater. 22 (2010) 587–603. [8] W. Ren, Z. Zheng, C. Xu, C. Niu, Q. Wei, Q. An, K. Zhao, M. Yan, M. Qin, L. Mai, Self-sacrificed synthesis of three-dimensional Na3V2(PO4)3 nanofiber network for high-rate sodium-ion full batteries, Nano Energy 25 (2016) 145–153. [9] C. Bommier, T.W. Surta, M. Dolgos, X. Ji, New mechanistic insights on Na-ion storage in nongraphitizable carbon, Nano Lett. 15 (2015) 5888–5892.

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