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• The widespread adoption of electric vehicles and portable electronic devices has highlighted the immense potential of lithium-ion batteries (LIBs) as a power source. Unfortunately, the current commercial graphite-type carbon anodes are unable to meet the growing market demands due to their inherently low theoretical capacity [1–5]. To achieve exceptional comprehensive performances, transition-metal chalcogenides, particularly selenides, have garnered significant interest for their high theoretical capacities (over 600 mAh/g), excellent electrical conductivity, and abundant redox chemistry [6]. However, transition-metal selenides face challenges such as severe volume expansion/shrinkage and particle aggregation. Therefore, it is of utmost importance to explore alternative strategies to address these issues [7,8].

  As an illustrative case, the construction of carbon-encapsulated metal selenide composites has been shown to accelerate electron transfer, ensure structural integrity, and enhance the specific capacity of metal selenides [9,10]. Additionally, numerous endeavors have been undertaken to further augment the performance of lithium-ion storage, including the construction of heterostructures [11,12], the introduction of defects, and the implementation of nano architectures [13,14]. Among these, heterostructures are considered a promising avenue due to the interface effect and built-in potential generated at the hetero-interface, which contribute to low activation energy and rapid ion/electron transfer and diffusion kinetics, ultimately leading to improved lithium-ion storage performance [15–19].

  With these advantages in mind, we intelligently fabricated a series of N-doped carbon-encapsulated bimetallic selenides heterostructure composites (MoSe₂/ZnSe@NC) through the selenization of zeolitic imidazolate framework (ZIF) coated Mo-glycerate (MOG) precursor at varying temperatures. Typical synthesis procedure is presented in Fig. 1a. Initially, MOG was prepared through a solvothermal reaction utilizing MoO₂(acac)₂ as a Mo source. Scanning electron microscopy (SEM) verifies its regular spherical feature with a smooth surface (Fig. 1b). X-ray diffraction (XRD) pattern matches well with the results reported elsewhere (Fig. S1 in Supporting information) [20]. The core-shell MOG@ZIF-8 precursors were in situ obtained by mixing the methanol solution containing Zn(NO₃)₂·6H₂O and 2-MeIM with the MOG dispersion. The XRD pattern reveals that the characteristic peaks of MOG@ZIF8 highly coincide with the standard card or the fitted pattern, with no miscellaneous peaks appearing. This indicates the successful preparation of the precursor (Fig. S2 in Supporting information). Furthermore, the morphology of MOG@ZIF-8 was examined through SEM and transmission electron microscopy (TEM) (Figs. 1c and e), which reveal ZIF-8 nanoparticles are densely adhered to the surface of MOG, resulting in a distinct core-shell architecture with a rough surface. Ultimately, the MoSe₂/ZnSe@NC composite was synthesized via direct selenization of the MOG@ZIF-8 precursor under a reduced atmosphere. According to the TG curve result of MOG@ZIF-8 (Fig. S3 in Supporting information), the selenization temperature was primarily set at 600, 700, and 800 ℃, aiming to investigate the impact of temperature on the configuration, composition, and battery performance of the final composites.

  Figure 1

  

  Figure 1.  (a) Schematic preparation of the MoSe₂/ZnSe@NC composites. Morphology and microstructure characterizations: different magnified SEM images of (b) MOG, (c) MOG@ZIF-8 precursor, and (d) MoSe₂/ZnSe@NC-700. (e) TEM image of MOG@ZIF-8. (f-l) Different-magnified TEM images of MoSe₂/ZnSe@NC-700. (m) HAADF-STEM and corresponding elemental mappings of MoSe₂/ZnSe@NC-700.

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  The SEM images show that the composites obtained at 600 and 700 ℃ exhibit a well-preserved macroscopic morphology with minimal aggregation (Fig. 1d and Fig. S4a in Supporting information). In contrast, the composite obtained at 800 ℃ displays an irregular and agglomerated block topography (Fig. S4b in Supporting information). Furthermore, the low-magnified TEM image reveals a core-shell morphology of the MoSe₂/ZnSe@NC composite consistent with the SEM results (Figs. 1f and g). The high-resolution TEM image shows that carbon is uniformly coated on the surface of ZnSe, and they are also attached to the MoSe₂ spheres as a whole (Fig. 1h). Besides, clear heterostructure interfaces are observed between ZnSe and MoSe₂ phases, which is beneficial for the formation of built-in potential at the hetero-interface to accelerate charge transport and increase the ion/electron transfer rate of the composite during battery operation [21]. The lattice fringe space of 0.61 nm matches well with the (002) plane of MoSe₂, providing strong evidence for the conversion of MOG to MoSe₂. The lattice fringe spaces of 0.196 and 0.321 nm are assigned to the (105) and (004) planes of MoSe₂, respectively (Figs. 1i and j). Meanwhile, both lattice fringes of 0.328 and 0.326 nm correspond to the (111) plane of ZnSe (Figs. 1k and l). Elemental mapping reveals that C, N, Zn, Se, and Mo elements are uniformly distributed in the MoSe₂/ZnSe@NC composite (Fig. 1m).

  Fig. 2a shows the diffraction peaks of the composites after selenization, which are all well-matched with the diffraction peaks on the standard MoSe₂ (JCPDS No. 17-887) and ZnSe (JCPDS No. 01–690). In contrast to MoSe₂/ZnSe@NC-600 and −800, the diffraction peaks of MoSe₂/ZnSe@NC-700 show the narrowest half-peak widths, the strongest peaks, and the best crystallinity. During selenization, Se powders are reduced to Se²⁻ anions under a reduced atmosphere, which then react with zinc ions from ZIF-8 and molybdenum ions from MOG to generate ZnSe and MoSe₂, respectively (Zn²⁺ + Se²⁻ → ZnSe, Mo⁴⁺ + Se²⁻ → MoSe₂). The presence of carbon in MoSe₂/ZnSe@NC is verified by Raman spectroscopy. The I_D/I_G value of MoSe₂/ZnSe@NC-800 (1.05) is lower than that of MoSe₂/ZnSe@NC-600 (1.13) and MoSe₂/ZnSe@NC-700 (1.15), indicating a higher graphitization degree and superior electrical conductivity of carbon (Fig. 2b). Additionally, the typical 2D peak at 2867 cm⁻¹ suggests that carbon is graphitized, thereby providing good electrical conductivity for the composite.

  Figure 2

  

  Figure 2.  (a) XRD patterns and (b) Raman spectra of MoSe₂/ZnSe@NC-600, −700, and −800. High resolution XPS spectra of MoSe₂/ZnSe@NC-700: (c) Mo 3d, (d) Zn 2p, (e) Se 3d, (f) C 1s, and (g) N 1s. (h) N₂ adsorption-desorption isotherms of MoSe₂/ZnSe@NC-600, −700, and −800.

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  The composition and chemical valences of various elements in MoSe₂/ZnSe@NC-700 were analyzed using X-ray photoelectron spectroscopy (XPS). The survey spectrum confirms the presence of Mo, Zn, Se, C, and N elements in the composite (Fig. S5 in Supporting information). In the Mo 3d spectrum (Fig. 2c), the peaks at 229.5 and 232.4 eV are identified as typical Mo⁴⁺ (3d_3/2) and Mo⁴⁺ (3d_5/2), respectively, while the peaks observed at 230.4 and 233.0 eV are attributed to Mo⁶⁺ (3d_3/2) and Mo⁶⁺ (3d_5/2), respectively [22]. The Zn 2p spectrum of ZIF-8 derived ZnSe@NC shows a binding energy of 23.1 eV between two peaks, which confirms the existence of Zn²⁺ (Fig. 2d) [23]. In comparison, the corresponding peaks of the Zn 2p spectrum for MoSe₂/ZnSe@NC shift towards lower binding energies, indicating the existence of interaction between ZnSe and MoSe₂ phases induced by the heterostructure interface. The high-resolution Se 3d spectrum contains two main peaks at 52.9 eV (Se 3d_5/2) and 54.1 eV (Se 3d_3/2) (Fig. 2e), indicating the existence of Se²⁺ in MoSe₂/ZnSe@NC [24]. The C 1s spectrum displays three divided peaks at 284.6, 286.3, and 288.5 eV (Fig. 2f), which are attributed to C=C, C—N, and C=O bonds, respectively [25]. Furthermore, the N 1s spectrum can be segregated into two distinct peaks at 396.2 and 398.1 eV, which correspond to pyridinic N and pyrrolic N, respectively (Fig. 2g). Other two small peaks with binding energies of 400.9 and 403.2 eV are assigned to graphitic N and oxidized type N-functionalities, respectively [26]. Moreover, nitrogen adsorption-desorption isotherms confirm that MoSe₂/ZnSe@NC-700 exhibits a larger BET specific surface area (54.22 m²/g) and pore size (14 nm) than MoSe₂/ZnSe@NC-600 (24.19 m²/g, 11 nm) and MoSe₂/ZnSe@NC-800 (24.19 m²/g, 10 nm), indicating its more exposed active species, faster electrolyte permeation, and superior electrochemical performance (Fig. 2h and Fig. S6 in Supporting information).

  The electrochemical performance of the MoSe₂/ZnSe@NC composites are initially examined through cyclic voltammetry (CV) curves (Fig. 3a). During the first cathodic process, a broad peak ranging from 0.23 V to 0.87 V is assigned to the generation of solid-electrolyte-interface (SEI) film and the conversion reactions of MoSe₂ and ZnSe with Li⁺ ions to produce metallic Mo, Zn, and Li₂Se products [27]. Additionally, a small peak at 0.18 V is attributed to the multi-step alloying reaction between Zn and Li to form various Li_yZn alloys (LiZn₂, LiZn₄, Li₂Zn₃, LiZn, etc.) [28]. In the initial anodic process, two minor oxidation peaks at 0.53 and 0.68 V correspond to the multi-step dealloying process of Li_yZn alloys. The other two prominent peaks at 1.36 and 2.56 V are assigned to the oxidation of Zn and Li₂Se into ZnSe [27], while the oxidation peak at 2.17 V demonstrates the conversion of Mo to MoSe₂ [26]. In subsequent cycles, the CV curves overlap, indicating excellent cycling stability for the MoSe₂/ZnSe@NC electrode. The voltage-capacitance curves of MoSe₂/ZnSe@NC-700 at 200 mA/g reveal that the first discharge and charge capacities are 2496.2 and 1255.7 mAh/g, respectively (Fig. 3b). The capacity decay may be attributed to the decomposition of electrolytes and the formation of SEI film. In subsequent cycles, the specific capacities maintain above 1000 mAh/g, further indicating its exceptional reversibility for Li⁺ ion storage.

  Figure 3

  

  Figure 3.  (a) CV curves and (b) voltage-capacitance curves of MoSe₂/ZnSe@NC-700. (c) Cycling performance of MoSe₂/ZnSe@NC at 0.2 A/g. (d) Rate capabilities of MoSe₂/ZnSe@NC from 0.2 A/g to 5.0 A/g. (e) Long-term stabilities of MoSe₂/ZnSe@NC-700. (f) Performance comparison of MoSe₂/ZnSe@NC-700 with reported MoSe₂- and ZnSe-based anodes.

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  The cycling performances of the MoSe₂/ZnSe@NC electrodes at a current density of 0.2 A/g are shown in Fig. 3c. It is noteworthy that all of the composites exhibit Coulombic efficiencies of nearly 99%, indicating their excellent cyclic stabilities. After 100 cycles, MoSe₂/ZnSe@NC-700 delivers the highest specific capacity up to 1298.2 mAh/g than MoSe₂/ZnSe@NC-600 and −800, which show the capacities of 822.9 and 620.3 mAh/g, respectively. The observed increase in capacity during cycling may be attributed to the activation of electrode materials and the reversible formation/dissolution of a conducting-type polymeric film at low potential in an alkyl carbonate solution. Furthermore, SEM and TEM images of the MoSe₂/MoO₂@C-700 electrode after repeated discharge-charge cycles confirm the preservation of a spherical and core-shelled morphology similar to the pristine one (Fig. S7 in Supporting information). So under 700 ℃, individual MOG-derived MoSe₂/MoO₂@C-700 and ZIF-8-derived ZnSe@NC-700, evidenced by XRD patterns (Figs. S8 and S9 in Supporting information), were prepared for performance comparison. ZnSe@NC-700 and MoSe₂/MoO₂@C-700 exhibit much lower capacities compared to MoSe₂/ZnSe@NC-700 (Fig. S10 in Supporting information), highlighting the structural and component advantages of MoSe₂/ZnSe@NC in improving the lithium-ion storage performance.

  Fig. 3d illustrates the rate performances of the MoSe₂/ZnSe@NC composites. As observed, MoSe₂/ZnSe@NC-700 exhibits superior rate capability compared to MoSe₂/ZnSe@NC-600 and −800. As the current density increases from 0.2 A/g to 0.4, 0.6, 0.8, 1.0, 2.0, and 5.0 A/g, the discharge capacities of MoSe₂/ZnSe@NC-700 are 1347.7, 1079.0, 815.5, 683.7, 585.5, 411.3, and 227.4 mAh/g, respectively. When the current density is restored to 1.0 A/g, its reversible capacity after 50 cycles still maintains 721.7 mAh/g, and the capacity recovers 1293.1 mAh/g with further increasing the current density to 0.2 A/g, almost no capacity decay compared with the initial value. At equivalent current densities, the capacities of MoSe₂/ZnSe@NC-700 exceed those of MoSe₂/ZnSe@NC-600 and −800. This is primarily due to the fact that MoSe₂/ZnSe@NC-700 maintains the optimal structure and morphology of the pristine precursor, and exhibits the largest specific surface area compared to MoSe₂/ZnSe@NC-600 and −800, which results in an increased contact area between active species and electrolytes, promoting the fast ions/electrons transport and thereby enhancing the electrochemical performance. Additionally, the rate capability of MoSe₂/ZnSe@NC-700 surpasses that of MOG-derived MoSe₂/MoO₂@C-700 and ZIF-8-derived ZnSe@NC-700 (Fig. S11 in Supporting Information), further confirming that the rational construction of the hierarchical composite is beneficial for achieving outstanding lithium-ion storage performance. Moreover, MoSe₂/ZnSe@NC-700 delivers reversible capacities of 505.7 and 378.3 mAh/g after 300 cycles at 1.0 and 2.0 A/g, respectively, indicating excellent cycling stability (Fig. 3e), Additionally, the lithium-ion storage performance surpasses its theoretical capacity calculated according to the inductively coupled plasma (ICP) results (422 mAh/g × 56.19 wt% + 577 mAh/g × 35.38 wt% + 372 mAh/g × 8.43 wt% = 472.2 mAh/g) and other reported molybdenum selenide@carbon- and zinc selenide@carbon-based anodes (Fig. 3f) [6,27,29–38].

  Subsequently, electrochemical impedance spectroscopy (EIS) was conducted on three MoSe₂/ZnSe@NC electrodes to elucidate their exceptional lithium-ion storage performance. Fig. 4a illustrates that MoSe₂/ZnSe@NC-700 exhibits a smaller charge transfer resistance (R_ct) of 164.5 Ω compared to MoSe₂/ZnSe@NC-600 and −800, which display R_ct values of 174.0 and 198.5 Ω, respectively. This may be attributed to MoSe₂/ZnSe@NC-700′s superior structural integrity, largest specific surface area, and unique pore structure, which collectively facilitate excellent charge transport. Additionally, the Li⁺ ion diffusion coefficient (D_Li) for MoSe₂/ZnSe@NC-700 is calculated to be 3.14 and 5.04 times higher than those of MoSe₂/ZnSe@NC-600 and MoSe₂/ZnSe@NC-800, respectively, further corroborating a faster Li⁺ ion diffusion kinetics in MoSe₂/ZnSe@NC-700 (Fig. 4b).

  Figure 4

  

  Figure 4.  (a) Nyquist plots of MoSe₂/ZnSe@NC-600, −700, and −800. (b) Plots of Z′ versus ω^−1/2. (c) CV curves at different sweep rates. (d) log(i)-log(v) plots. (e) Proportion of capacitance at 0.6 mV/s. (f) Percentages of capacitance and diffusion contributions. (g) Ex-situ XRD patterns of MoSe₂/ZnSe@NC-700 at various voltages during the first cycling process.

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  To gain a deeper understanding of the electrochemical reaction kinetics of the optimal MoSe₂/ZnSe@NC-700 electrode, CV tests were conducted. All of the CV curves exhibit highly identical shapes (Fig. 4c), indicating the stable electrochemical performance. Three oxidation peaks shift to higher voltages while the reduction peaks move to lower voltages with sweep rate changing from 0.1 mV/s to 1.0 mV/s, indicating an inevitable polarization of the electrode during cycling. By fitting the redox peaks in CV (Fig. 4d) [39,40], the capacitance and diffusion contributions were determined, and the results demonstrate a typical pseudocapacitance-dominant behavior of MoSe₂/ZnSe@NC-700. Furthermore, the proportion of pseudocapitance contribution at 0.6 mV/s is about 81.5% (Fig. 4e). Fig. 4f shows the contribution percentage of pseudocapitance at various sweep rates, which find that the redox contribution of pseudocapacitance gradually becomes the dominant component of the total capacitance as the sweep rate increases. The higher contribution of pseudocapitance significantly enhances the electrochemical lithium-ion storage performance.

  Fig. 4g displays ex-situ XRD patterns for the first cycle at different voltage stages. During the discharge process, two peaks at 2θ = 11.3° and 26.8°, which correspond to MoSe₂ and ZnSe phases, respectively, exhibit a gradual decrease and eventually disappear when the battery is discharged to 0.01 V, indicating the occurrence of conversion reactions of ZnSe and MoSe₂. Simultaneously, new diffraction peaks gradually emerge at 2θ = 38.6° and 45.3°, representing the formation of Li₂Se and Li_yZn phases. These processes keep pace with the CV results. During subsequent charging, the characteristic diffraction peaks of MoSe₂ and ZnSe gradually reappear, while the diffraction peaks representing the Li₂Se and Li_yZn phases disappear, indicating the occurrence of reversible oxidation reactions.

  To evaluate a practical application, the full battery was assembled by using MoSe₂/ZnSe@NC-700 as the anode and commercial LiCoO₂ as the cathode. Results show that the MoSe₂/ZnSe@NC-700||LiCoO₂ battery exhibits a high voltage of 3.5 V and excellent rate capability with the charging capacities at 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0 A/g of 153.2, 147.5, 138.9, 127.5, 117.5, 108.1, and 78.8 mAh/g (Fig. S12 in Supporting information), showing good capacity retention at different current densities.

  In conclusion, this work successfully synthesized hierarchical MoSe₂/ZnSe@NC composites. Various characterizations have confirmed the unique core-shell structure, MoSe₂/ZnSe heterostructure, and N-doped carbon-coating collectively endow the composite with ultrahigh reversible capacity, outstanding rate performance, and cyclicity. Moreover, ex-situ PXRD patterns have revealed the coexistence of conversion and alloying reaction processes. It is believed that the innovative synthesis of MoSe₂/ZnSe@NC can serve as a model for designing and constructing core-shell heterostructure-based composites with exceptional lithium-ion storage performance.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Ming Zhong: Conceptualization, Data curation, Funding acquisition, Methodology, Project administration, Supervision. Xue Guo: Data curation, Formal analysis, Investigation. Yang Liu: Data curation, Investigation. Kun Zhao: Data curation, Investigation. Hui Peng: Data curation, Investigation. Suijun Liu: Project administration. Xiaobo Zhang: Project administration, Supervision.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (No. 22265017) and the Open Fund of Key Laboratory of Eco-functional Polymer Materials of the Ministry of Education (No. KF-21-04).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109873.

  
  
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  Figure 1  (a) Schematic preparation of the MoSe₂/ZnSe@NC composites. Morphology and microstructure characterizations: different magnified SEM images of (b) MOG, (c) MOG@ZIF-8 precursor, and (d) MoSe₂/ZnSe@NC-700. (e) TEM image of MOG@ZIF-8. (f-l) Different-magnified TEM images of MoSe₂/ZnSe@NC-700. (m) HAADF-STEM and corresponding elemental mappings of MoSe₂/ZnSe@NC-700.

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  Figure 2  (a) XRD patterns and (b) Raman spectra of MoSe₂/ZnSe@NC-600, −700, and −800. High resolution XPS spectra of MoSe₂/ZnSe@NC-700: (c) Mo 3d, (d) Zn 2p, (e) Se 3d, (f) C 1s, and (g) N 1s. (h) N₂ adsorption-desorption isotherms of MoSe₂/ZnSe@NC-600, −700, and −800.

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  Figure 3  (a) CV curves and (b) voltage-capacitance curves of MoSe₂/ZnSe@NC-700. (c) Cycling performance of MoSe₂/ZnSe@NC at 0.2 A/g. (d) Rate capabilities of MoSe₂/ZnSe@NC from 0.2 A/g to 5.0 A/g. (e) Long-term stabilities of MoSe₂/ZnSe@NC-700. (f) Performance comparison of MoSe₂/ZnSe@NC-700 with reported MoSe₂- and ZnSe-based anodes.

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  Figure 4  (a) Nyquist plots of MoSe₂/ZnSe@NC-600, −700, and −800. (b) Plots of Z′ versus ω^−1/2. (c) CV curves at different sweep rates. (d) log(i)-log(v) plots. (e) Proportion of capacitance at 0.6 mV/s. (f) Percentages of capacitance and diffusion contributions. (g) Ex-situ XRD patterns of MoSe₂/ZnSe@NC-700 at various voltages during the first cycling process.

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