N-doped graphene aerogel nanocomposites with high capacity and excellent stability for lithium-ion batteries

Journal of Power Sources 439 (2019) 227112

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MoSe2–CoSe2/N-doped graphene aerogel nanocomposites with high capacity and excellent stability for lithium-ion batteries Xiang Zhang a, Jun Zhou a, Yuying Zheng a, b, c, *, Dongyang Chen a, ** a

College of Materials Science and Engineering, Fuzhou University, Fuzhou, 350116, PR China Key Laboratory of New Rubber and Plastic Materials, Quanzhou, 362211, PR China c Chenqi New Material Technology Co., Ltd., Quanzhou, 362200, PR China b

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

� N-doped graphene aerogel with tunable porosity and surface area is synthesized. � Decoration of MoSe2–CoSe2 are realized via a one-step hydrothermal reaction. � NGA-10 matrix ensures effective ion diffusion and robust structural stability. � Open nanostructured MoSe2–CoSe2 provides rich redox sites and extra pseudocapacity. � The hybrid electrodes show excellent rate and cycling performance.

A R T I C L E I N F O

A B S T R A C T

Keywords: Lithium-ion battery MoSe2–CoSe2 Graphene aerogel Nitrogen doping Porous structure

Bimetallic transition-metal selenides have emerged as promising anode materials for lithium-ion batteries (LIBs). However, they suffer from rapid capacity fading which is originated from significant chemical and mechanical degradation during lithiation/delithiation processes. Herein, nitrogen-doped graphene aerogel (NGA) with high porosity is prepared via a templated method and serves as the matrix for the uniform growth of sheet-like binary transition-metal selenides (MoSe2–CoSe2). The porous NGA with high specific surface area provides a stable and conductive network, which can accelerate electron transmission and shorten the diffusion distance of Li ions. Furthermore, the MoSe2–CoSe2 nanosheets can offer adequate specific capacity because of its high accessibility and multiple redox properties. As a result, the MoSe2–CoSe2/NGA-10 composite delivers high Li-ion storage capacity (1260 mAh g 1 at 0.1 A g 1), excellent rate capability (702 mAh g 1 at 5 A g 1), and stable cycling performance (914 mAh g 1 at 1 A g 1 after 200 cycles), demonstrating its great potential for LIB applications.

1. Introduction Lithium-ion batteries (LIBs) with advantages of high energy density and environmental benignity are widely applied in energy storage sys­ tems as a renewable energy source [1]. The commonly employed anode

material for LIBs is graphite, because of its high electronic conductivity, good stability and abundant reserves [2]. However, the maximum ca­ pacity of graphite only achieves ~372 mAh g 1, which is due to low lithium diffusion within commensurately-stacked graphite layers [3]. Among various alternative electrode materials, transition metal selenide

* Corresponding author. College of Materials Science and Engineering, Fuzhou University, Fuzhou, 350116, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y. Zheng), [email protected] (D. Chen). https://doi.org/10.1016/j.jpowsour.2019.227112 Received 6 July 2019; Received in revised form 28 August 2019; Accepted 3 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.

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(TMS), such as Bi2Se3 [4], CoSe2 [5] and MoSe2 [6], have recently attracted great attention, because of their high lithium storage capacity and relatively higher electrical conductivity than transition metal oxides or sulfides [7,8]. Nevertheless, TMSs usually has low surface area and significant volume change during charging/discharging, which lead to remarkable chemical and mechanical degradation and consequent rapid capacity fading [9–11]. To address these limitations, rational structural design and component optimization of TMSs are required. There are a range of methods that have been used to modify TMSsbased electrodes, including introduction of porosity, construction of nanocomposites, as well as heteroatoms doping [12–16]. The engi­ neering of TMSs with porous structure has been demonstrated to possess great promise in improving capacity. For example, a mesoporous Co0.85Se nanosheet prepared by Zhou et al. showed a reversible capacity of 645 mAh g 1 at 200 mA g 1, and remained 516 mA h g 1 after 50 cycles [12]. The mesoporous nanosheet of Co0.85Se could facilitate Li-ion diffusion and help to improve the conductivity of active materials. However, the cycling life and reversible capacity of the above cobalt selenide were not satisfied. Yang et al. reported the synthesis of a composite material based on CoSe2 nanoparticles encapsulated in N-doped carbon framework intertwined with carbon nanotubes (CoSe2@N-CF/CNTs) [13]. The composite could deliver a specific ca­ pacity of 666 mAh g 1 at 0.2 A g 1 and a reversible capacity of 428 mAh g 1 after 500 cycles at 1 A g 1. The enhanced insertion/extraction reversibility is mainly attributed to the designed carbon matrix, which fastens the transport kinetics of ions and electrons, and protects the overall architecture from collapsing. Chen and co-workers reported a composite electrode based on CoSe2 encapsulated in N-doped carbon framework intertwined with carbon nanotubes (CoSe2@N-CF/CNTs) [14]. Benefitting from the improved electrical conductivity and increased active sites derived from nitrogen doping, the CoSe2@N-CF/CNTs exhibited better electrochemical performance than CoSe2@CF/CNTs without nitrogen doping. Besides, other studies also demonstrated that N-doping can enlarge interlayer space, create intrinsic defects, and minimize the interfacial energy barrier between carbon surface and reagent, contributing to the in situ nucleation of active species on the carbon surface [15,16]. Recently, binary metal selenides with multiple valences have attracted much attention in the field of LIBs, such as ZnSe-CoSe [17], Ni–CoSe2 [18] and Cu9Sn2Se9 [19], and so on. Many researches have revealed that the advantages of assembling different metals in bimetallic compounds include: 1) introducing more defects and disorders, which offers more active sites for efficient electron transfer and improved re­ action kinetics [20,21]; 2) integrating individual electrochemical properties, which offers significant pseudocapacitance [22]; 3) modi­ fying microstructure and allowing for more surface area in contact with the electrolyte [23–25]. Therefore, bimetallic selenides are expected to exhibit much improved electrochemical performance over mono-metallic selenides. With these aforementioned considerations, it is highly desirable to propose a simple assembling technique to realize the rational con­ struction of hybrid electrode combining both advantages from binary TMSs and carbon skeleton, and probe into their potentials for LIBs anode materials. Herein, we synthesize an integrated anode comprising of porous N-doped graphene aerogel (NGA) as 3D matrix and MoSe2–CoSe2 nanosheets grown on NGA as active species. N atoms were introduced into graphene lattice by the pyrolysis of polyaniline (PANI), and the porous structure of NGA was originated from the removal of polystyrene (PS) spheres templates that were immobilized within graphene oxide (GO) flakes via a rapid freezing method with liquid nitrogen. The interconnected MoSe2–CoSe2 nanosheets were anchored on the surface of NGA via a one-step hydrothermal process. Such design of MoSe2–CoSe2/NGA promises a stable system for lithium diffusion and electron transport, and thus leading to outstanding electrochemical performance.

2. Experimental 2.1. Materials Cobalt nitrate hexahydrate (99%), ammonium molybdate tetrahy­ drate (99%), selenium powders (99.9%) were purchased from Aladdin Chemical Co. Ltd, China. Hydrazine hydrate (80%), powder-like graphite (99%) and other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Aniline was distilled twice under reduced pressure and other reagents were used as received without further treatment. Graphene oxide was prepared by a modified Hum­ mers’ method, and PS particles with diameter about 200 nm were pre­ pared according to a reported soap-free emulsion polymerization method [26]. 2.2. Preparation of N-doped porous graphene aerogel Typically, GO (50 mg) and PS (100 mg) were dispersed in water (20 mL) under magnetic stir for 6 h, followed by the addition of 0.5 mL aniline monomer. Then, 20 mL of hydrochloric acid (1 M) containing 150 mg of ammonium persulfate was added slowly into the reaction system and ultrasonically polymerized at room temperature for 1 h. Afterwards, the suspension was rapidly frozen by liquid nitrogen, fol­ lowed by lyophilization for 24 h to obtain PANI/PS/GO aerogel. After that, the sample was put in a tube furnace and annealed in N2 at 900 � C for 2 h. The resultant sample was named NGA-10. For comparison, samples employed different mass of PS (0 mg, 50 mg and 200 mg), named NGA-0, NGA-5 and NGA-20, were obtained by the same pro­ cedure. In addition, porous graphene aerogel obtained by annealing PS/ GO aerogel in Ar at 900 � C for 2 h without N doping treatment was denoted as GA-10. 2.3. Growth of MoSe2–CoSe2 on N-doped porous graphene aerogel In brief, 40 mg NGA-10, 10 mg urea, 0.291 g Co(NO3)2⋅6H2O and 0.176 g (NH4)6Mo7O24⋅4H2O were dissolved in 40 mL deionized water as solution A. 0.355 g of Se powder was dissolved in 12 mL hydrazine hydrate as solution B. After aged for 4 h, solution B was added into so­ lution A dropwise under continuous stirring. The mixed solution was transferred into an 80 mL Teflon-lined stainless steel autoclave, and heated at 180 � C for 20 h. Then the resultant precipitate was washed and freeze-dried. The final MoSe2–CoSe2/NGA-10 composite was obtained by annealing at 350 � C for 2 h in Ar in order to remove excessive Se powder. For comparison, bulk MoSe2, CoSe2, MoSe2–CoSe2, and MoSe2/ NGA-10, CoSe2/NGA-10, MoSe2–CoSe2/NGA-0, MoSe2–CoSe2/NGA-20, MoSe2–CoSe2/GA-10, were synthesized with a similar process. 2.4. Characterizations The microstructural morphologies of the samples were obtained by scanning electron microscopy (SEM, Supra55, Zeiss, Germany) and transmission electron microscopy (TEM, Tecnai G2F20S-TWIN). The crystal structures were characterized using X-ray diffraction (XRD, D/ max-UItimaIII, Rigaku, Japan) equipped with Cu Kα radiation (λ ¼ 0.15418 nm) at a 5� min 1 scanning rate (2 θ, 5� -80� ). For ex situ XRD measurements, the composite electrodes were collected after charging or discharging to specific voltages and sealed by polyethylene film to avoid oxidation. The elemental composition was confirmed by Xray photoelectron spectroscopy (XPS, ESCALAB 250). The graphitic structure was evaluated by Raman spectroscopy (inVia þ Reflex, Renishaw) with a wavelength of 514 nm. The specific surface area was measured by Nitrogen adsorption/desorption isotherms (ASAP 2460 Surface Area and Porosity Analyzer, USA). Thermogram was studied using a thermal analyzer (TG-DTA, SDT Q600, TA, USA) under air flow (5� C min 1). Electrochemical measurements were performed using CR2025 coin2

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Journal of Power Sources 439 (2019) 227112

type cells. The working electrode was prepared by mixing the MoSe2–CoSe2/NGA-10, acetylene black and polyvinylidene difluoride (PVDF) with a mass ratio of 8 : 1: 1 in N-methylpyrrolidone (NMP) solvent to form a slurry. The loading mass of active materials is around 1 mg cm 2. Lithium foil was used as the counter and reference elec­ trodes. The electrolyte was a LiPF6 (1 M) solution in a mixture of EC, EMC and DMC with a mass ratio of 1 : 1: 1, and a polypropylene membrane was used as the separator. Cyclic voltammetry (CV) and Electrochemical impedance (EIS, 1 MHz–0.1 Hz) measurements were performed on a CHI660E electrochemical workstation. Galvanostatic discharge/charge tests were conducted on a Neware battery tester. Electrochemical measurements for the bulk MoSe2, CoSe2, MoSe2–CoSe2, NGA-10, MoSe2/NGA-10, CoSe2/NGA-10, MoSe2–CoSe2/ NGA-0 and MoSe2–CoSe2/NGA-20 were also performed under the same conditions.

characterized by SEM. As shown in Fig. 3 (a), the well-organized and evenly distributed pores with wrinkles and ripples are obviously distinguished from the NGA-10, which indicates the rare face-to-face stacking and large surface area of the graphene sheets [32]. The corrugated morphology was further demonstrated by TEM image, as shown in Fig. 3 (b). The diameter of the pores is measured to be ~200 nm, which is consistent with the size of PS particles imbedded in aerogel, suggesting a full preservation of structure after template removal. SEM images of the MoSe2–CoSe2/NGA-10 show that the sam­ ple exhibits a highly interconnected 3D porous network with ultra-thin MoSe2–CoSe2 nanosheets fully covered and anchored on the matrix (Fig. 3 (c)). Commonly, it is difficult to directly grow active species on the surfaces of graphene sheet because of the chemical inertness and high hydrophobicity of graphene [33]. With N doping, the position of N atoms can act as anchor sites for the absorption of metal ions by forming metal–nitrogen bonds, which is beneficial for the growth of MoSe2–CoSe2 nanosheets on the graphene surfaces. While for the GA-10 without N doping treatment, MoSe2–CoSe2 will grow along the preferred orientation, leading to the random MoSe2–CoSe2 cluster, which can be testified by the SEM image of the MoSe2–CoSe2/GA-10 in Fig. S1. Thus, the designed N doping treatment is accounting for the homogeneous distribution and firm attachment of MoSe2–CoSe2 nanosheets on the graphene matrix. In order to explore the microstructure of the MoSe2–CoSe2/NGA-10, TEM, HRTEM and elemental mapping were performed. A few-layered structure of the MoSe2–CoSe2 can be observed in the TEM image (Fig. 3 (d)). According to the HRTEM detection (Fig. 3 (e)), the inter­ planar distances are measured to be 0.65 nm and 0.25 nm, correspond­ ing to (002) and (111) lattice lanes of MoSe2 and CoSe2, respectively, matching well with the XRD analysis [13,34]. The energy dispersive X-ray (EDX) elemental mapping result of the MoSe2–CoSe2/NGA-10 is also presented in Fig. 3 (f), confirming that the sample mainly consists of C, N, Mo, Co and Se elements. Furthermore, the Mo, Co, Se molar ratio in the MoSe2–CoSe2/NGA-10 is found to be ~1 : 1: 4 according to the EDX content analysis (Fig. S2), which is close to the feed ratio. The above findings have given direct evidences on the successful growth of MoSe2–CoSe2 nanosheets on porous NGA-10 substrate. Compared with the highly open structure of the MoSe2–CoSe2/NGA10, the bulk MoSe2–CoSe2 prepared in the absence of NGA possesses an aggregated spherical structure with limited area of exposed surface (Fig. S3(a)), indicating that NGA can effectively hamper the aggregation of active materials, ensuring the fast and stable electron and ion trans­ port. The morphology evolution from unary CoSe2 and MoSe2 to binary MoSe2–CoSe2 was monitored. Specifically, the bulk CoSe2, MoSe2 and MoSe2–CoSe2 possess similar particle diameters; the MoSe2–CoSe2 shows a sheet-like surface, while the surface of CoSe2 is smooth (Fig. S3 (b)) and the MoSe2 presents a wrinkle surface (Fig. S3(c)). These indi­ cate that the existence of both Co and Mo metal can modify the morphology of monometallic selenide by reinforcing the formation of nanosheet. The compact sheet-like structure would allow for more sur­ face area for efficient electrolyte contact, giving better storage

3. Results and discussion The preparation route of the MoSe2–CoSe2/NGA is shown in Fig. 1. Firstly, PS spheres with a uniform diameter of ~200 nm were synthe­ sized and self-assembled between GO flakes via π-π stacking interaction, followed by the decoration of PANI. The prepared sample was rapidly frozen by liquid nitrogen and then lyophilized; this treatment accord­ ingly produces a PANI/PS/GO aerogel. Secondly, NGA was obtained by the calcination of PANI/PS/GO aerogel in N2 atmosphere, during which PS spheres sacrificed accompanied by the incorporation of N atoms and the reduction of graphene oxide. Taking full advantage of high porosity and abundant active sites of NGA, binary transition-metal selenides MoSe2–CoSe2 with a sheet-like structure was subsequently anchored on the NGA via a one-step hydrothermal process. Particularly, instead of a prevalent two-step method that combines the prior formation of pre­ cursor and subsequent selenization procedure, our strategy of direct growth of TMSs can be easily operated and energy-saving. In addition, rapid freezing technique can avoid the agglomeration of PS particles to prevent GO sheets from stacking, and thus result in the uniform pore structure and high specific surface area of the NGA. Moreover, N doping treatment can enrich the extrinsic defects and active sites of graphene, which boosts the wettability and contributes to the homogeneous growth and robust adhesion of MoSe2–CoSe2 nanosheets. The X-ray diffraction (XRD) patterns with a 2θ range of 10� –80� of the MoSe2, CoSe2 and MoSe2–CoSe2/NGA-10 are shown in Fig. 2 (a). The diffraction pattern of MoSe2–CoSe2/NGA-10 corresponds to the crystalline structures of CoSe2 (JCPDS card # 53–0449) and MoSe2 (JCPDS card # 65–3481), respectively [27]. Fig. 2 (b) shows the cor­ responding Raman spectra. The characterized peaks of MoSe2–CoSe2/NGA-10 at around 180 cm 1, 259 cm 1, 350 cm 1, 1350 cm 1 and 1590 cm 1 could be distinguished, which correspond to the CoSe2, MoSe2, D band and G band of the NGA, respectively, indi­ cating that the MoSe2–CoSe2/NGA-10 composite was successfully syn­ thesized [28–31]. The morphology of the NGA-10 and MoSe2–CoSe2/NGA-10 was

Fig. 1. Schematic illustration of the preparation route of MoSe2–CoSe2/NGA. 3

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Fig. 2. (a) XRD spectra and (b) Raman spectra of the bulk MoSe2, bulk CoSe2 and MoSe2–CoSe2/NGA-10.

Fig. 3. (a) SEM image and (b) TEM image of the NGA-10; (c) SEM image, (d) TEM image, (e) HRTEM image, and (f) EDX mapping of the MoSe2–CoSe2/NGA-10.

performance when compared with unary CoSe2 or MoSe2. Moreover, the coexistence of Mo and Co metals also introduces abundant active defects and disorders on the planes of bimetallic compound, which are clearly represented in yellow circles in HRTEM image (Fig. S3(d)). This phe­ nomenon can be attributed to the heterogeneous electron configurations of different metal atoms and the mismatch of Co–Se and Mo–Se coor­ dination lengths. As a result, the planes of MoSe2–CoSe2 are more active towards electron transfer, which is expected to enhance the faradaic redox reaction during the charge-discharge process.

The surface area of the NGA was well adjusted by employing different mass of PS template, which is 0 mg, 50 mg, 100 mg and 200 mg, respectively. The nitrogen adsorption-desorption isotherms of NGAs (Fig. 4 (a)) and the derived specific surface area, pore volume and pore diameter (Table S1) showed that the samples were mesoporous and the Brunauer-Emmett-Teller (BET) specific surface area increased from 28.62 m2 g 1 to 622.84 m2 g 1 as the mass of PS particles increased from 0 to 100 mg. The corresponding XRD patterns, Raman spectra, and the morphologies of NGAs are shown in Fig. S4 to help understand this 4

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Fig. 4. (a) Nitrogen adsorption-desorption isotherms of the MoSe2–CoSe2/NGA-10, NGA-0, NGA-5, NGA-10 and NGA-20; (b) TGA curves of the MoSe2–CoSe2/NGA10, bulk MoSe2–CoSe2 and NGA-10.

trend. As is found, layer-to-layer distances (d-spacing) for NGA-0, NGA-5 and NGA-10 are measured to be 0.414 nm, 0.421 nm and 0.423 nm ac­ cording to XRD analyses, suggesting that the space between graphene sheets was extent by the employment of PS templates. Raman spectra also indicate an increased ID/IG value for the NGA-0, NGA-5 and NGA10, which is 0.948, 0.976 and 1.023, respectively, corresponding to the enriched disorder and defect in the NGA network [35]. SEM images of NGAs further confirm that the porous and corrugated structure was intensified by continuous increase of PS, and the maximum surface area can be obtained when those pores are closely and evenly integrated. However, NGA-20 showed a sharply dropped specific surface area of 350.26 m2 g 1, and a comparative d-spacing of 0.425 nm as well as a low ID/IG value of 0.985 are also observed, which demonstrate a counter-productive effect by adding excessive PS templates. After hydrothermal reaction, MoSe2–CoSe2 was anchored on the NGA-10, which delivers a specific surface area of 178 m2 g 1, much larger than previous report of TMS/carbon composites [14,36–38]. The mass loading of MoSe2–CoSe2 in the MoSe2–CoSe2/NGA-10 was evalu­ ated by TGA measurement under air atmosphere. As displayed in Fig. 4 (b), the weight of MoSe2–CoSe2/NGA-10 firstly decreased (~2%) from the starting temperature to 300 � C, which can be ascribed to the

desorption of absorbed gaseous molecules (e.g., CO2, H2O, etc.). In the next stage, the weight of MoSe2–CoSe2/NGA-10 slightly increased (~4%) at 300–350 � C, which can be attributed to the formation of MoO3, Co3O4 and SeO2 [14,36]. Large weight loss was observed at 350–430 � C, which is due to the loss of NGA-10. In the last stage, SeO2 gradually sublimated above 500 � C and the ash left was the hybrid of MoO3 and Co3O4, which was confirmed by XRD analysis (Fig. S5). A similar thermal behavior can also be observed in the TGA curve of the bulk MoSe2–CoSe2. The total weight loss of the MoSe2–CoSe2/NGA-10 and MoSe2–CoSe2 is measured to be 72.2% and 54.1%, respectively. Therefore, the mass loading of MoSe2–CoSe2 in MoSe2–CoSe2/NGA-10 is calculated to be 60.4% by the equation 72.2%c ¼ 54.1%cX þ c(1-X), where the c and X stand for the total mass of the hybrid and the mass loading of MoSe2–CoSe2 in the hybrid, respectively. The elemental chemical states of MoSe2–CoSe2/NGA-10 were detected by X-ray photoelectron spectroscopy (XPS). The characteristic peaks of C, N, Mo, Co and Se elements were observed in the survey scan spectrum (Fig. 5 (a)), and elemental O can be assigned to air exposure. The high-resolution spectra of C 1s, N 1s, Mo 3d, Co 2p, and Se 3d are also presented to determine their oxidation states. The high-resolution C 1s spectrum (Fig. 5 (b)) shows three main peaks at 284.4 eV, 285.6 eV

Fig. 5. XPS spectra of the MoSe2–CoSe2/NGA-10: (a) survey, (b–f) high-resolution spectra of C 1s, N 1s, Mo 3d, Co 2p and Se 3d. 5

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– O bonds, and 287.5 eV, which are attributed to sp2-C, sp3-C–N, and C– respectively [39]. In the N 1s spectrum (Fig. 5 (c)), three deconvoluted peaks located at around 398.2 eV, 400.0 eV and 401.0 eV were observed appointed to pyridinic-N, pyrrolic-N and graphitic-N respectively [40]. In the Mo 3d spectrum (Fig. 5 (d)), the main peaks at 228.5 eV and 232.3 eV, which has been frequently reported in literature [41–43], correspond to Mo 3d5/2 and Mo 3d3/2, respectively, indicating the ex­ istence of Mo4þ in the composite. The small peak located at 236.0 eV can be indexed to Mo6þ, which is attributed to the unconverted Mo7O624 . The Co 2p spectrum (Fig. 5 (e)) shows two peaks at 794.2 eV and 779.2 eV, which are ascribed to Co 2p1/2 and Co 2p3/2, corresponding to Co3þ and Co2þ, respectively [44]. As to Se 3d (Fig. 5 (f)), two characteristic peaks of Se 3d5/2 (54.3 eV) and Se 3d3/2 (55.1 eV) reveal the existence of Se2 , and the peak located at 60.2 eV corresponds to surface oxidation of Se species [45]. The above results confirm the successful synthesis of MoSe2–CoSe2/NGA-10 composite. The electrochemical performances of MoSe2–CoSe2/NGA-10 and control samples are shown in Fig. 6. The initial five cyclic voltammo­ gram (CV) curves of MoSe2–CoSe2/NGA-10 at a scan rate of 0.1 mV s 1 is presented in Fig. 6 (a). It can be seen that three reduction peaks at 1.63 V, 1.39 V and 0.60 V and an oxidation peak at 2.38 V are observed for the first cycle, which are generally in agreement with the CV curves of the MoSe2/NGA-10 and CoSe2/NGA-10 under the same testing con­ dition (Fig. S6). Reduction peaks at 1.63 V and 1.39 V could be attrib­ uted to Liþ insertion and the conversion reaction between LixCoSe2 and metallic Co as well as Li2Se [14]. The wide peak at around 0.60 V may be associated with the formation of LixMoSe2, accompanied with the reduction of LixMoSe2 to Mo metal and the inevitable formation of a solid electrolyte interface (SEI) film [13,46]. The discharge process could be summarized by eqs. (1)–(3): MSe2 þ xLiþ þ xe → LixMSe2 þ

LixMSe2 þ (2-x)Li þ (2-x)e → MSe þ Li2Se þ

MSe þ 2Li þ 2e → M þ Li2Se

peaks at 1.63 V and 1.39 V slightly shift to a higher potential of 1.77 V and 1.44 V, while the cathodic peaks at 0.60 V disappears and the anodic peak at 2.38 V remains constant. Furthermore, the 2nd, 3rd and 5th CV curves of the MoSe2–CoSe2/NGA-10 are almost overlapped, implying the excellent reversibility of the MoSe2–CoSe2/NGA-10. The typical discharge-charge profiles for the initial five cycles under 0.1 A g 1 are presented in Fig. 6 (b). The plateaus in the dischargecharge profiles are well consistent with the peaks in the CV curves. The MoSe2–CoSe2/NGA-10 delivers a discharge capacity of 1260 mA h g 1 in the first cycle and a reversible charge capacity of 919 mA h g 1 with a Coulombic efficiency (CE) of 72.9%. The loss of capacity may be ascribed to the formation of SEI film and the decom­ position of electrolyte [41]. In contrast, the initial discharge capacities of the MoSe2/NGA-10, CoSe2/NGA-10, MoSe2–CoSe2/NGA-0, MoSe2–CoSe2/NGA-20 and MoSe2–CoSe2 are 1101, 992, 1142, 1218 and 960 mAh g 1, respectively (Fig. S7), and the corresponding initial coulombic efficiencies are 70.1%, 68.7%, 70.5% and 66.8%, respec­ tively, which are lower than that of MoSe2–CoSe2/NGA-10. The discharge and charge specific capacities in the second cycle for the MoSe2–CoSe2/NGA-10 are 1024 mA h g 1 and 920 mA h g 1, respec­ tively, with a higher coulombic efficiency of 89.8%. The third and the fifth discharge-charge profiles are almost coincided with each other, further demonstrating the excellent cycling performance. The rate performances for the MoSe2–CoSe2/NGA-10 and control samples at current densities ranging from 0.1 to 5 A g 1 are shown in Fig. 6 (c). The average specific capacities for the MoSe2–CoSe2/NGA-10 are 990 mAh g 1, 974 mAh g 1, 950 mAh g 1, 912 mAh g 1, 835 mAh g 1 and 702 mAh g 1 at current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g 1, respectively. Notably, it can recover most of its specific capacity and reaches a value of 1008 mAh g 1 once the current density returns to 0.1 A g 1. In contrast, MoSe2–CoSe2/NGA-0, MoSe2–CoSe2/NGA-20 and bulk MoSe2–CoSe2 exhibit either lower reversible capacities or poorer stability under the same conditions, indicating the unique function of NGA-10 due to its richness of nitrogen and porous nature that offers enough buffer space to alleviate the volume change [17,48,49]. In addition, the average specific capacities for the MoSe2/NGA-10 and CoSe2/NGA-10 at all tested current densities are lower than that of MoSe2–CoSe2/NGA-10, which is mainly attributed to the less active sites and surface area of the unary metal selenide.

(1) (2) (3)

where M stands for metallic molybdenum or cobalt. In the subsequent positive scan, the oxidation peak at 2.38 V could be attributed to the oxidation of Li2Se to selenium [47]. In the following cycle, the cathodic

Fig. 6. (a) Cyclic voltammetry curves of the MoSe2–CoSe2/NGA-10 at a scan rate of 0.1 mV s 1 within 0.005–3.0 V; (b) charge-discharge profiles of the MoSe2–CoSe2/NGA-10 at a current density of 0.1 A g 1; (c) rate capability, coulombic efficiency and (d) cycling performance of the MoSe2–CoSe2/NGA-10 and control samples at a current density of 0.1 A g 1; (e) cycling performance and coulombic efficiency of the MoSe2–CoSe2/NGA-10 at a current density of 1 A g 1. 6

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The long-term cycling performances of the MoSe2–CoSe2/NGA-10 and control samples were examined by galvanostatic discharge/charge tests at a current density of 0.1 A g 1, as shown in Fig. 6 (d). The MoSe2–CoSe2/NGA-10 exhibits a reversible capacity of 1120 mAh g 1 after 50 cycles with a capacity retention of 88.9% relative to the initial capacity. More strikingly, the capacity of the MoSe2–CoSe2/NGA-10 slowly increases after 10 cycles, which is likely attributed to the acti­ vation process with gradual interactions between the electrolyte and electrode materials during cycling [50,51]. Similarly, the capacity of the MoSe2–CoSe2/NGA-20 also gradually increases, but it reaches a comparatively low capacity of 1017 mAh g 1 after 50 cycles. In addi­ tion, the capacities of the MoSe2–CoSe2/NGA-0 and bulk MoSe2–CoSe2 progressively decay during the cycling process, which only delivers 855 mAh g 1 and 334 mAh g 1 after 50 cycles. Moreover, MoSe2/NGA-10 and CoSe2/NGA-10 show a reversible capacity of 793 mAh g 1 and 732 mAh g 1 after 50 cycles, which are lower than that of MoSe2–CoSe2/NGA-10. Based on the cycling performances, the capacity contribution of the MoSe2–CoSe2 in the composite is calculated. The reversible capacity of the NGA-10 is 200 mAh g 1 after 50 cycles at 0.1 A g 1, and the content of NGA-10 in the MoSe2–CoSe2/NGA-10 is 39.6 wt%, thus the capacity contribution of the NGA-10 to the composite is 79.2 mAh g 1 (200 � 39.2% ¼ 79.2), and the MoSe2–CoSe2 contrib­ utes 92.9% of the capacity in MoSe2–CoSe2/NGA-10 (1–79.2 � 1120 ¼ 0.929). Cycling performance of the MoSe2–CoSe2/NGA-10 was further examined under a current density of 1 A g 1 (Fig. 6 (e)). After 200 cy­ cles, the MoSe2–CoSe2/NGA-10 shows a capacity of 914 mAh g 1, higher than several reported metal selenides/carbon-based anodes (Table 1) [36,44,50,52–55]. Therefore, the as-prepared MoSe2–CoSe2/NGA-10 have demonstrated outstanding rate perfor­ mance and cycling life towards Li-ion storage capability. On the one hand, NGA-10 is a promising substrate with stable and conductive skeleton for accommodation of cycling stress. On the other hand, open structure and rich active sites derived from the coexistence of Mo and Co metals are capable for efficient electrolyte penetration and improved reaction kinetics. The combination of these effects leads to promising reversible capacity of the composite that is higher than the theoretical redox capacity of individual MoSe2 (422 mAh g 1) or CoSe2 (494 mAh g 1). In order to further confirm the robustness of the MoSe2–CoSe2/NGA10 electrode for long-term service, ex situ SEM and TEM analyses were conducted. As shown in Fig. S9, under rapid Liþ insertion/extraction process, the microstructure of bulk MoSe2–CoSe2 was severely damaged (Fig. S9 (a)) while the MoSe2–CoSe2/NGA-10 could still maintain its integrity (Fig. S9 (b)) with little structural collapse (represented in yellow circles). The ex situ TEM image shown in Fig. S9 (c) also confirmed that the NGA-10 could maintain its pristine porous morphology (represented in yellow circles) with MoSe2–CoSe2 nano­ sheets firmly anchored on its surfaces. Moreover, the HRTEM image of the MoSe2–CoSe2/NGA-10 (Fig. S9 (d)) shows lattices of 0.65 and 0.25 nm, which index to (002) plane of MoSe2 and (120) plane of CoSe2,

respectively. The above observation proves the favorable structural stability of the MoSe2–CoSe2/NGA-10 electrode after a long-term cycle. Li-ion storage behavior and reaction kinetics of the MoSe2–CoSe2/ NGA-10 were investigated by the analysis on the cyclic voltammetry (CV) curves under different scan rates ranging from 0.2 to 1 mV s 1, as shown in Fig. 7 (a). The reduction and oxidation peaks keep similar shapes with accordingly increasing peak current when the scan rate increases. Previous studies have demonstrated that the relationship between the peak current (i) and the scan rate (v) can be described as the following eqs. (4) and (5):

Current density (A g 1)

Capacity (mAh g 1) (Cycle number)

Ref.

MoSe2@PHCS CoSe2 ⊂ MoSe2/C HNT CoSe@carbon nanoboxes MoSe2/C MoSe2/rGO Fe3Se4/C CoSe@NC-550 MoSe2–CoSe2/NGA-10

1 1 1

681 (100) 750 (200) 660 (100)

36 44 50

1 1 1 1 1

577 (100) 750 (100) 434 (100) 796 (100) 944 (100)

52 53 54 55 This work

(4)

log(i) ¼ b log(v) þ log(a)

(5)

where a and b are adjustable parameters. In general, a b value of 1 im­ plies the dominated pseudocapacitance behavior during the char­ ge–discharge process, while a b value of 0.5 reflects the diffusioncontrolled process [9,17]. In our case, the log(i) versus log(v) plots (Fig. 7 (b)) at reduction and oxidation peaks give b values of 0.758 and 0.693, respectively, indicating the partial pseudocapacitive behaviors of the MoSe2–CoSe2/NGA-10 electrode. The relative calculation of pseu­ docapacitance contribution at fixed potential could be described by eq. (6): (6)

i(V) ¼ k1v þ k2v1/2 1/2

where k1v refers to pseudocapacitance contribution and k2v repre­ sents diffusion contribution [56]. As calculated, around 67.5% of the specific capacity is contributed by pseudocapacitance (shaded region) in CV curves (red line) under the scan rate of 0.6 mV s 1 (Fig. 7 (c)). Contribution ratios at different scan rates are shown in Fig. 7 (d), where the pseudocapacitance contribution slightly increases with the increase of scan rate, and it reaches to a maximum value of 75.7% at 1 mV s 1. The enhanced pseudocapacitance contribution is indicative of fast electron and Liþ transport. For comparison, MoSe2/NGA-10 and CoSe2/NGA-10 electrodes exhibit lower capacity contribution at each scan rate (Fig. S8), which is due to the lack of exposed surface area and active sites for intercalation. In order to better understand the kinetics of electrode reaction be­ tween electrodes and electrolyte, electrochemical impedance spectra (EIS) measurements were carried out for all electrodes. The Nyquist plots after 50 cycles at 0.1 A g 1 for the MoSe2–CoSe2/NGA-10, MoSe2/ NGA-10, CoSe2/NGA-10, MoSe2–CoSe2/NGA-0, MoSe2–CoSe2/NGA-20 and MoSe2–CoSe2 are shown in Fig. 8, which show similar curves featuring a semicircle at high frequency and a diffusion drift in the lowfrequency region. The inset image is the equivalent circuit model, where Rs is the electrolyte resistance, Rct is the charge-transfer resistance, CPE is the constant-phase element and ZW represents Warburg impedance corresponding to the lithium diffusion process. The fitted impedance parameters are listed in Table S2. The values of Rct for the MoSe2–CoSe2/ NGA-10 is 52.7 Ω, much lower than that of the MoSe2–CoSe2/NGA-10 (63.6 Ω), bulk MoSe2–CoSe2 (86.4 Ω) and MoSe2–CoSe2/NGA0 (191.2 Ω), confirming that the as-prepared NGA-10 substrate can in­ crease the overall electrochemical conductivity of the electrode and facilitates transfer for electrons/ions at the electrode interface. In addition, Rct values for the MoSe2/NGA-10 and CoSe2/NGA-10 are also larger than that of the MoSe2–CoSe2/NGA-10, which suggests the faster interface kinetics of the bimetallic MoSe2–CoSe2/NGA-10 than the mono-metallic MoSe2/NGA-10 or CoSe2/NGA-10. To comprehensively investigate lithium storage mechanism of the MoSe2–CoSe2/NGA-10, ex situ XRD patterns at different dischargecharge stages for the 100th cycles at 1 A g 1 were investigated (Fig. S10). For the discharge process, the diffraction peaks at 2.50 V (point A) can be assigned to the MoSe2 and CoSe2 phase. When the electrode is discharged to 1.60 V (point B), the intensity of CoSe2 peaks is well remained, while the intensity of MoSe2 peaks decreases with the

Table 1 Comparison of the electrochemical properties of TMSs-based anode materials. Electrode materials

i ¼ avb

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Journal of Power Sources 439 (2019) 227112

Fig. 7. (a) CV curves of the MoSe2–CoSe2/NGA-10 at different scan rates within 0.005–3.0 V; (b) log(i) versus log(v) plots at different oxidation and reduction states of the MoSe2–CoSe2/NGA-10; (c) capacitive contribution (shaded region) in CV curves (red line) under the scan rate of 0.6 mV s 1; (d) percentage of capacitance contribution at different scan rates ranging from 0.2 to 1 mV s 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

peaks indexed to MoSe2 and CoSe2 disappear while the signals of Mo, Co and Li2Se become distinct, suggesting the complete intercalation of Li ions. For the continuous charging process, the peak of metallic Mo almost diminishes and the intensity of Li2Se peaks decreases when charged to 2.25 V (point D). By contrast, the peak of MoSe2 appears and the peak of metallic Co almost remains constant. This suggests that the conversion of Mo to MoSe2 is prior to that of Co to CoSe2 under increasing voltage, which is in accordance with the CV findings in Fig. S6 and Fig. S8. In the fully charged stage (3.00 V, point E), the absence of Li2Se peaks and the presence of MoSe2 and CoSe2 peaks demonstrate the accomplishment of Li ion insertion/extraction cycle. The above ex situ XRD analyses illustrate the continuously proceeding conversion reaction between Li ions and MoSe2–CoSe2. Good mainte­ nance of MoSe2–CoSe2 phase further corroborates the outstanding cycling performance of the composite electrode. The experimental results reveal that the elaborately designed MoSe2–CoSe2/NGA-10 composite possesses a superior Li-ion storage capability, which is mainly originated from the following benefits: 1) vast space between graphene sheet is available for the efficient pene­ tration of electrolyte, leading to the fast intercalation/deintercalation; 2) defect-rich N-doped graphene framework could provide abundant active sites for the in situ growth and robust adhesion of transition metal selenides, fulfilling the Li-ion storage capability of the hybrid; 3) multicomponent (Mo, Co) selenides could trigger more redox reactions, offering significant pseudocapacitance; 4) the synergistic effect between graphene aerogel and active species is promising for improving the overall conductivity, capacity and structural stability, making it an outstanding electrode material for long-term service.

Fig. 8. Nyquist plots of the MoSe2–CoSe2/NGA-10, MoSe2/NGA-10, CoSe2/ NGA-10, MoSe2–CoSe2/NGA-0, MoSe2–CoSe2/NGA-20 and MoSe2–CoSe2 elec­ trodes obtained after 50 cycles at 0.1 A g 1. Inset: equivalent circuit used for the EIS analysis.

generation of metallic Mo and Li2Se peaks [14], indicating that the intercalation of Li ions into MoSe2 proceeds at a higher voltage than that in CoSe2. This founding is in good agreement with the redox behavior of MoSe2 and CoSe2 detected by CV analysis (Fig. S6 and Fig. S8), where the voltage of intercalation peaks of MoSe2 is higher than that of CoSe2. When the electrode is further discharged to 0.005 V (point C), all the 8

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Journal of Power Sources 439 (2019) 227112

4. Conclusions

[13] Z. Li, H. Xue, J. Wang, Y. Tang, C. Lee, S. Yang, Reduced graphene oxide/ marcasite-type cobalt selenide nanocrystals as an anode for lithium-ion batteries with excellent cyclic performance, Chemelectrochem 2 (2015) 1682–1686. [14] J. Yang, H. Gao, S. Men, Z. Shi, S. Chen, CoSe2 nanoparticles encapsulated by Ndoped carbon framework intertwined with carbon nanotubes: high performance dual-role anode materials for both Li and Na-ion batteries, Adv. Sci. 5 (2018) 1800763–1800776. [15] S. Baik, J.W. Lee, Effect of boron–nitrogen bonding on oxygen reduction reaction activity of BN Co-doped activated porous carbons, RSC Adv. 5 (2015) 24661–24669. [16] W. Sun, G.H. Gao, Y.C. Du, K. Zhang, G.M. Wu, A facile strategy for fabricating hierarchical nanocomposites of V2O5 nanowire arrays on a three-dimensional Ndoped graphene aerogel with a synergistic effect for supercapacitors, J. Mater. Chem. 6 (2018) 9938–9947. [17] J. Jin, Z. Yun, B.K. Ling, N. Srikanth, Q. Yan, K. Zhou, Tuning ZnSe/CoSe in MOFderived N-doped porous carbon/CNTs for high-performance lithium storage, J. Mater. Chem. 6 (2018) 15710–15717. [18] W. Liu, M. Shao, W. Zhou, B. Yuan, C. Gao, H. Li, Hollow Ni-CoSe2 embedded in nitrogen-doped carbon nanocomposites derived from metal-organic frameworks for high-rate anodes, ACS Appl. Mater. Interfaces 10 (2018) 38845–38852. [19] Y. Lou, M. Zhang, C.G. Li, C.L. Chen, C. Liang, Z. Shi, D. Zhang, G. Chen, X.B. Chen, S.H. Feng, Mercaptopropionic acid-capped wurtzite Cu9Sn2Se9 Nanocrystals as high-performance anode materials for lithium-ion batteries, ACS Appl. Mater. Interfaces 10 (2018) 1810–1818. [20] M. Sakthivel, R. Sukanya, S.M. Chen, K.C. Ho, Synthesis and characterization of samarium-substituted molybdenum diselenide and its graphene oxide nanohybrid for enhancing the selective sensing of chloramphenicol in a milk sample, ACS Appl. Mater. Interfaces 10 (2018) 29712–29723. [21] Y. Chen, K. Yang, B. Jiang, J. Li, M. Zeng, L. Fu, Emerging two-dimensional nanomaterials for electrochemical hydrogen evolution, J. Mater. Chem. 5 (2017) 8187–8208. [22] M. Sakthivel, R. Sukanya, S.M. Chen, K. Ho, Bimetallic vanadium cobalt diselenides nanosheets with additional active sites for excellent asymmetric pseudocapacitive performance: compare the electrochemical performances with M-CoSe2 (M ¼ Zn, Mn, and Cu), J. Mater. Chem. (2019), https://doi.org/10.1039/C9TA03024G. [23] G.J.H. Lim, X.M. Liu, C. Guan, J. Wang, Co/Zn bimetallic oxides derived from metal organic frameworks for high performance electrochemical energy storage, Electrochim. Acta 291 (2018) 177–187. [24] Z. Xiang, Y. Zheng, W. Zheng, W. Zhao, D. Chen, Graphite felt decorated with porous NiCo2O4 nanosheets for high-performance pseudocapacitor electrodes, J. Mater. Sci. 52 (2017) 5179–5187. [25] Z. Xiang, Y. Zheng, J. Zhou, W. Zheng, D. Chen, Nitrogen doped graphite felt decorated with porous Ni1.4Co1.6S4 nanosheets for 3D pseudocapacitor electrodes, RSC Adv. 22 (2017) 13406–13415. [26] J. Zhang, M. Wang, X. Ge, M. Wu, Q. Wu, J. Yang, M. Wang, Z. Jin, N. Liu, Facile fabrication of free-standing colloidal-crystal films by interfacial self-assembly, J. Colloid Interface Sci. 353 (2011) 16–21. [27] X.Q. Wang, B.J. Zheng, B. Yu, B. Wang, W.Q. Hou, W.L. Zhang, Y.F. Chen, In situ synthesis of hierarchical MoSe2-CoSe2 nanotubes as an efficient electrocatalyst for the hydrogen evolution reaction in both acidic and alkaline media, J. Mater. Chem. 6 (2018) 7842–7850. [28] F. Lai, D. Yong, X. Ning, B. Pan, Y. Miao, T. Liu, Bionanofiber assisted decoration of few-layered MoSe2 nanosheets on 3D conductive networks for efficient hydrogen evolution, Small 13 (2017) 1602866–1602873. [29] B. Yu, F. Qi, B. Zheng, W. Hou, W. Zhang, Y. Li, Y. Chen, Self-assembled pearlbracelet-like CoSe2-SnSe2/CNT hollow architecture as highly efficient electrocatalysts for hydrogen evolution reaction, J. Mater. Chem. 6 (2018) 1655–1662. [30] L. Zhang, L. Sun, Y. Huang, Y. Sun, T. Hu, K. Xu, Hydrothermal synthesis of Ndoped rGO/MoSe2 composites and enhanced electro-catalytic hydrogen evolution, J. Mater. Sci. 52 (2017) 13561–13571. [31] Y. Liu, X.J. Jin, A.X. Tuo, H. Liu, Improved oxygen reduction reaction activity of three-dimensional porous N-doped graphene from a soft-template synthesis strategy in microbial fuel cells, RSC Adv. 6 (2016) 105211–105221. [32] J. Yan, J. Liu, Z. Fan, T. Wei, L. Zhang, High-performance supercapacitor electrodes based on highly corrugated graphene sheets, Carbon 50 (2012) 2179–2188. [33] S. Kattel, P. Atanassov, B. Kiefer, Stability, electronicand magnetic properties of inplanedefects in graphene: a first-principles study, J. Phys. Chem. C 116 (2012) 8161–8166. [34] G.Q. Zhao, P. Li, K. Rui, Y.P. Chen, S.X. Dou, W.P. Sun, CoSe2/MoSe2 heterostructures with enriched water adsorption/dissociation sites towards enhanced alkaline hydrogen evolution reaction, Chem. Eur J. 24 (2018) 11158–11165. [35] X. Li, S. Yang, J. Sun, P. He, X. Xu, G. Ding, Tungsten oxide nanowire-reduced graphene oxide aerogel for high-efficiency visible light photocatalysis, Carbon 78 (2014) 38–48. [36] X. Yang, Z. Zhang, Y. Fu, Q. Li, Porous hollow carbon spheres decorated with molybdenum diselenide nanosheets as anodes for highly reversible lithium and sodium storage, Nanoscale 7 (2015) 10198–10203. [37] T. Meng, Y.N. Hao, J.W. Qin, M.H. Cao, Interface-engineering-induced electric field effect and atomic disorder in cobalt selenide for high-rate and large-capacity lithium storage, ACS Sustain. Chem. Eng. 7 (2019) 4657–4665. [38] Z.G. Luo, J. Zhou, L.R. Wang, G.Z. Fang, A.Q. Pan, S.Q. Liang, Two-dimensional hybrid nanosheets of few layered MoSe2 on reduced graphene oxide as anodes for long-cycle-life lithium-ion batteries, J. Mater. Chem. 4 (2016) 15302–15308.

N-doped graphene aerogel (NGA) with high porosity was success­ fully prepared as matrix for the uniform growth of MoSe2–CoSe2 nano­ sheets. The nitrogen atoms were incorporated into the graphene aerogel by the pyrolysis of polyaniline, and the porosity of the NGA was well controlled by the employment of polystyrene templates that were immobilized between graphene oxide flakes by rapid freezing method. A one-step hydrothermal process was subsequently introduced to grow MoSe2–CoSe2 nanosheets on the NGA, and the corresponding hybrids were investigated as free-standing anodes for LIBs. It was found that a discharge capacity of 914 mAh g 1 after 200 cycles at 1 A g 1 was observed for the MoSe2–CoSe2/NGA-10 electrode, which also delivered good cycling stability and rate capability. Such good electrochemical performance can be ascribed to the synergistic effects of efficient ion diffusion pathway from the N-doped porous structure, robust stability from the hybrid, and extra capacity contribution from the binary transition-metal selenides. This work may present a facile and effective strategy for the synthesis of porous metal selenides/carbon composites with high electrochemical performance for advanced energy storage systems. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Numbers 51873037 and 51503038), Scientific and Technological Innovation Project of Fujian Province (Grant Number 2012H6008), and Scientific and Technological Innovation Project of Fuzhou City (Grant Number 2013-G-92). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227112. References [1] Y.K. Sun, Z.H. Chen, H.J. Noh, D.J. Lee, H.G. Jung, Y. Ren, S.C.S. Wang, Yoon, S. T. Myung, K. Amine, Nanostructured high-energy cathode materials for advanced lithium batteries, Nat. Mater. 11 (2012) 942–947. [2] T.M. Paronyan, A.K. Thapa, A. Sherehiy, J.B. Jasinski, Incommensurate graphene foam as a high capacity lithium intercalation anode, Sci. Rep. 7 (2017) 39944–39954. [3] N. Kambe, M.S. Dresselhaus, G. Dresselhaus, S. Basu, A.R. McGhie, J.E. Fischer, Intercalate ordering in first stage graphite-lithium, Mater. Sci. Eng. 40 (1979) 1–4. [4] X. Chen, H. Tang, Z. Huang, J. Zhou, J. Zhong, Flexible bismuth selenide/graphene composite paper for lithium-ion batteries, Ceram. Int. 43 (2016) 1437–1442. [5] J.K. Wang, H.K. Wang, D.X. Cao, X. Lu, X.G. Han, C.M. Niu, Epitaxial growth of urchin-like CoSe2 nanorods from electrospun Co-embedded porous carbon nanofibers and their superior lithium storage properties, Part. Part, Syst. Charact. 34 (2017) 1700185–1700191. [6] R. Jin, Y. Cui, Q. Wang, G. Li, Facile fabrication of CNTs@C@MoSe2@Se hybrids with amorphous structure for high performance anode in lithium-ion batteries, J. Colloid Interface Sci. 508 (2017) 435–442. [7] Y.P. Gao, X. Wu, K.J. Huang, L.L. Xing, Y.Y. Zhang, L. Liu, Two-dimensional transition metal diseleniums for energy storage application: a review of recent developments, CrystEngComm 19 (2017) 404–418. [8] J. Wu, W.M. Lau, D.S. Geng, Recent progress in cobalt based compounds as highperformance anode materials for lithium ion batteries, Rare Met. 36 (2017) 307–320. [9] H.S. Wu, Y.X. Wu, X. Chen, Y.J. Ma, M.Q. Xu, W.F. Wei, J. Pan, X. Xiong, Rational design and preparation of few-layered MoSe2 nanosheet@C/TiO2 nanobelt heterostructures with superior lithium storage performance, RSC Adv. 6 (2016) 23161–23168. [10] Y. Huang, Y.E. Miao, J. Fu, S. Mo, C. Wei, T. Liu, Perpendicularly oriented fewlayer MoSe2 on SnO2 nanotubes for efficient hydrogen evolution reaction, J. Mater. Chem. 3 (2015) 16263–16271. [11] H. Tang, K. Dou, C.C. Kaun, Q. Kuang, S. Yang, MoSe2 nanosheets and their graphene hybrids: synthesis, characterization and hydrogen evolution reaction studies, J. Mater. Chem. 2 (2014) 360–364. [12] J. Zhou, Y. Wang, J. Zhang, T. Chen, H. Song, H.Y. Yang, Two dimensional layered Co0.85Se nanosheets as a high-capacity anode for lithium-ion batteries, Nanoscale 8 (2016), 14992 50000.

9

X. Zhang et al.

Journal of Power Sources 439 (2019) 227112

[39] J. Li, Y. Fu, X. Shi, Z. Xu, Z. Zhang, Urchinlike ZnS microspheres decorated with nitrogen-doped carbon: a superior anode material for lithium and sodium storage, Chem. Eur J. 23 (2017) 157–166. [40] M.K. Liu, Y.F. Song, S.X. He, W.W. Tjiu, J.S. Pan, Y.Y. Xia, T.X. Liu, Nitrogen-doped graphene nanoribbons as efficient metal-free electrocatalysts for oxygen reduction, ACS Appl. Mater. Interfaces 6 (2014) 4214–4222. [41] S.S. Zhang, G. Wang, J. Jin, L.L. Zhang, Z.Y. Wen, J.H. Yang, Robust and conductive red MoSe2 for stable and fast lithium storage, ACS Nano 12 (2018) 4010–4018. [42] M.L. Yuan, S. Dipazir, M. Wang, Y. Sun, D.L. Gao, Y.L. Bai, M. Zhang, P.L. Lu, H. Y. He, X.Y. Zhu, S.W. Li, Z.J. Liu, Z.P. Luog, G.J. Zhang, Polyoxometalate-assisted formation of CoSe/MoSe2 heterostructures with enhanced oxygen evolution activity, J. Mater. Chem. 7 (2019) 3317–3326. [43] L. Zhang, T. Wang, L. Sun, Y.J. Sun, T.W. Hu, K.W. Xu, F. Ma, Hydrothermal synthesis of 3D hierarchical MoSe2/NiSe2 composite nanowires on carbon fiber paper and their enhanced electrocatalytic activity for the hydrogen evolution reaction, J. Mater. Chem. 5 (2017) 19752–19759. [44] J. Gao, Y. Li, L. Shi, J. Li, G. Zhang, Rational design of hierarchical nanotubes through encapsulating CoSe2 nanoparticles into MoSe2/C composite shells with enhanced lithium and sodium storage performance, ACS Appl. Mater. Interfaces 10 (2018) 20635–20642. [45] M. Yousaf, Y.S. Wang, Y.J. Chen, Z.P. Wang, W. Aftab, A. Mahmood, W. Wang, S. J. Guo, R.P.S. Han, Tunable free-standing core-shell CNT@MoSe2 anode for Lithium storage, ACS Appl. Mater. Interfaces 10 (2018) 14622–14631. [46] H. Wang, X.Y. Wang, L. Wang, J. Wang, D.L. Jiang, G.P. Li, Y. Zhang, H.H. Zhong, Y. Jiang, Phase transition mechanism and electrochemical properties of nanocrystalline MoSe2 as anode materials for the high performance lithium-ion battery, J. Phys. Chem. C 119 (2015) 10197–10205. [47] X. Fang, X. Yu, S. Liao, Y. Shi, Y.S. Hu, Z. Wang, G.D. Stucky, L. Chen, Lithium storage performance in ordered mesoporous MoS2 electrode material, Microporous Mesoporous Mater. 151 (2012) 418–423.

[48] J.J. Li, L. Shi, J.Y. Gao, G.Q. Zhang, General one-pot synthesis of transition-metal phosphide/nitrogen-doped carbon hybrid nanosheets as ultrastable anodes for sodium-ion batteries, Chem. Eur J. 24 (2018) 1253–1258. [49] J.Y. Gao, Y.P. Li, Y. Liu, S.H. Jiao, J. Li, G.R. Wang, S.Y. Zeng, G.Q. Zhang, The dual-function sacrificing template directed formation of MoS2/C hybrid nanotubes enabling highly stable and ultrafast sodium storage, J. Mater. Chem. 7 (2019) 18828–18834. [50] H. Hu, J. Zhang, B. Guan, X.W.D. Lou, Unusual formation of CoSe@carbon nanoboxes, which have an inhomogeneous shell, for efficient lithium storage, Angew. Chem. Int. Ed. 55 (2016) 9514–9518. [51] S. Laruelle, S. Grugeon, P. Poizot, M. Dolle, L. Dupont, J.M. Tarascon, On the origin of the extra electrochemical capacity displayed by MO/Li cells at low potential, J. Electrochem. Soc. 149 (2002) A627–A634. [52] X. Yang, Z.A. Zhang, X.D. Shi, Rational design of coaxial-cable MoSe2/C: towards high performance electrode materials for lithium-ion and sodium-ion batteries, J. Alloy. Comp. 686 (2016) 413–420. [53] Z.A. Zhang, Y. Fu, X. Yang, Y.H. Qu, Z.Y. Zhang, Hierarchical MoSe2 nanosheets/ reduced graphene oxide composites as anodes for lithium-ion and sodium-ion batteries with enhanced electrochemical performance, ChemNanoMat 1 (2015) 409–414. [54] Q.N. Ma, Q.Y. Zhuang, H. Song, C.C. Zhao, Z.H. Zhang, J. Liu, H.R. Peng, C. M. Mao, G.C. Li, Large-scale synthesis of Fe3Se4/C composites assembled by aligned nanorods as advanced anode material for lithium storage, Mater. Lett. 228 (2018) 235–238. [55] J.D. Liu, J.J. Liang, C.Y. Wang, J.M. Ma, Electrospun CoSe@N-doped carbon nanofibers with highly capacitive Li storage, J. Energy Chem. 33 (2019) 160–166. [56] P. Li, J.Y. Hwang, Y.K. Sun, Nano/micro-structured silicon-graphite composite anode for high-energy density Li-ion battery, ACS Nano 13 (2019) 2624–2633.

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