阅读说明：本技术 钴基双金属硒化物/氮掺杂碳复合材料及其制备方法 (Cobalt-based bimetallic selenide/nitrogen-doped carbon composite material and preparation method thereof ) 是由 杨茹 沈克军 张俊豪 陈家乐 朱学勇 顾执明 闻韬 王鑫 于 2021-07-28 设计创作，主要内容包括：本发明公开了一种钴基双金属硒化物/氮掺杂碳复合材料及其制备方法,以钴基双金属有机框架材料为前驱体,高温热解碳化制得双金属分散均匀的多面体氮掺杂碳复合材料,进一步硒化制得钴基双金属硒化物/氮掺杂碳复合材料,可作为锂离子电池负极材料。本发明采用绿色、高时效、高产率的方法制备钴基双金属有机框架材料前驱体,有利于规模化制备及实际应用。钴基双金属硒化物/氮掺杂碳复合材料形貌均一且均匀分散,避免了双金属硒化物团聚的问题,展现出优异的储锂性能,归因于双金属硒化物和氮掺杂碳的均匀复合带来丰富的氧化还原活性位点,促进电化学反应动力学,并抑制体积膨胀,三维多孔结构不仅具有结构稳定性而且有利于电极材料和电解质的充分接触。(The invention discloses a cobalt-based bimetallic selenide/nitrogen-doped carbon composite material and a preparation method thereof. The invention adopts a green, high-aging and high-yield method to prepare the cobalt-based bimetal organic framework material precursor, and is beneficial to large-scale preparation and practical application. The cobalt-based bimetallic selenide/nitrogen-doped carbon composite material is uniform in appearance and uniform in dispersion, the problem of agglomeration of the bimetallic selenide is avoided, excellent lithium storage performance is shown, abundant redox active sites are brought due to uniform compounding of the bimetallic selenide and the nitrogen-doped carbon, electrochemical reaction kinetics are promoted, volume expansion is inhibited, and the three-dimensional porous structure not only has structural stability, but also is beneficial to full contact of an electrode material and electrolyte.)

1. A cobalt-based bimetallic selenide/nitrogen-doped carbon composite material is characterized by being a material formed by uniformly compounding cobalt-based bimetallic selenide and nitrogen-doped carbon, wherein cobalt-based bimetallic selenide particles are uniformly dispersed on a porous polyhedral nitrogen-doped carbon substrate, the size of a cobalt-based bimetallic selenide/nitrogen-doped carbon polyhedron is 600-1000 nm, and the size of the cobalt-based bimetallic selenide particles is 50-200 nm; the cobalt-based bimetallic selenide accounts for 68.5-72.6 wt% of the composite material, and the nitrogen-doped carbon accounts for 27.4-31.5 wt% of the composite material.

2. The cobalt-based bimetal selenide/nitrogen-doped carbon composite of claim 1, wherein the cobalt-based bimetal selenide is iron-cobalt bimetal selenide, zinc-cobalt bimetal selenide, or nickel-cobalt bimetal selenide.

3. A method of preparing a cobalt-based bimetallic selenide/nitrogen-doped carbon composite material of claim 2, comprising the steps of:

step one, preparing a cobalt-based bimetal organic framework:

firstly, respectively preparing a solution A mixed by potassium hydroxide and 2-methylimidazole, a solution B of anhydrous ferric trichloride or zinc nitrate or nickel nitrate and a cobalt nitrate solution C, sequentially adding a solution B, C into the solution A, wherein the mass concentration ratio of substances of 2-methylimidazole, anhydrous ferric trichloride or zinc nitrate or nickel nitrate, cobalt nitrate and potassium hydroxide in the mixed solution of the solution A, the solution B and the solution C is 1: 0.014:0.0084 (0.027-1.642); then stirring for 4-6 h, and drying to obtain a cobalt-based bimetallic organic framework material FeCo-MOF or ZnCo-MOF or NiCo-MOF;

step two, preparing the cobalt-based bimetal/nitrogen-doped carbon composite material:

placing the cobalt-based bimetal organic frame material in a tube furnace, and preserving heat for 0.5-2 h at 400-600 ℃ to prepare a cobalt-based bimetal/nitrogen-doped carbon composite material FeCo/NC or ZnCo/NC or NiCo/NC;

step three, preparing the cobalt-based bimetallic selenide/nitrogen-doped carbon composite material:

selenium powder and a cobalt-based bimetal/nitrogen-doped carbon composite material FeCo/NC or ZnCo/NC or NiCo/NC are respectively arranged at the upper stream and the lower stream of the porcelain boat, the mass ratio of the selenium powder to the cobalt-based bimetal/nitrogen-doped carbon composite material is 2:1, the heat preservation is carried out for 3h at 350 ℃, the heat preservation is carried out for 0.5-2 h at 400-600 ℃, and the cobalt-based bimetal selenide/nitrogen-doped carbon composite material Fe-Co-Se/NC or Zn-Co-Se/NC or Ni-Co-Se/NC is prepared.

4. The method of preparing a cobalt-based bimetallic selenide/nitrogen-doped carbon composite material of claim 3, wherein the second step isThe process method for preserving the heat of the tubular furnace at 400-600 ℃ for 0.5-2 h comprises the following steps: in N₂Heating the mixture from room temperature to 400-600 ℃ at the speed of 1-5 ℃/min, preserving the heat for 0.5-2 h, and then naturally cooling.

5. The preparation method of the cobalt-based bimetallic selenide/nitrogen-doped carbon composite material as claimed in claim 3, wherein the process method for preserving the temperature of the tubular furnace at 350 ℃ for 3h and at 400-600 ℃ for 0.5-2 h in the third step is as follows: in N₂Raising the temperature from room temperature to 350 ℃ for 3h at the speed of 1-5 ℃/min in the atmosphere, raising the temperature to 400-600 ℃ at the speed of 1-5 ℃/min, preserving the temperature for 0.5-2 h, and naturally cooling.

Technical Field

The invention relates to a cobalt-based bimetallic selenide/nitrogen-doped carbon composite material and a preparation method thereof, belonging to the technical field of new materials.

Background

In recent years, lithium ion batteries have attracted increasing attention as storage and conversion devices for electrical and chemical energy. The design of high-performance electrodes is the key to realizing high-energy density lithium ion batteries, and the electrochemical performance of the batteries is determined to a great extent by negative electrode materials. Commercial graphite negative electrodes are limited due to capacity (372mAh g)^-1) And cannot meet the requirements of the lithium ion battery. Therefore, it is imperative to search for high-performance negative electrode materials suitable for lithium ion batteries. Metal selenides are favored by researchers because of their higher theoretical specific capacities and appropriate potentials. Among them, ferromagnetic metal (iron, zinc, nickel) selenides can show most cost competitiveness in the family of metal selenium compounds, because such metals are widely distributed and can be directly obtained from natural ores. However, many factors including poor conductivity, volume expansion, and dissolution and reaction of products in the electrolyte limit practical applications in current electrode materials. The methods of modifying carbon materials and constructing bimetallic selenides are effective ways for solving the problems.

The carbon material modification not only can effectively improve the conductivity of the metal selenide, but also can inhibit the volume expansion of the metal selenide. At the same time, the intrinsic properties of the carbon material may impart additional beneficial properties to the electrode material, such as high adsorptivity, which may promote electrolyte adsorption and inhibit shuttling of intermediates. In addition, the bimetallic selenide has more redox sites, larger grain size, higher ion diffusion kinetics and better conductivity than the monometallic selenide due to the synergistic effect of the bimetallic selenide. Specifically, the bimetallic compound promotes the interface electron transfer reaction in the redox process by the cooperative coupling of the crystal phase interface and the corresponding atomic arrangement and local electronic structure change, so that the metal selenide exposes more reactive sites, promotes the electrochemical reaction kinetics and relieves the volume expansion problem, and further achieves the purpose of improving the lithium storage performance. However, it is a real situation that it is very difficult to design and synthesize the bimetallic selenide having the best synergistic effect because of the difference of physicochemical properties and lattice structures of the respective components. In response to this problem, much effort has been put into developing bimetallic selenides using methods such as epitaxial growth, solution chemistry, and post-synthesis modification. However, these synthetic methods tend to be complex and time consuming. The efficient preparation of bimetallic selenide and carbon composites with ultra-long cycle life, high reversible capacity and stable cycle is a great technical problem.

Disclosure of Invention

The invention aims to provide a cobalt-based bimetallic selenide/nitrogen-doped carbon composite material and a preparation method thereof, which utilize potassium hydroxide to assist in synthesizing a cobalt-based bimetallic organic framework material and solve the problems of complex synthetic steps, time consumption and low yield in the prior art. The cobalt-based bimetallic selenide/nitrogen-doped carbon composite material is obtained by selenizing a bimetallic organic framework material serving as a precursor material, and is used as a lithium ion battery cathode material, so that the conductivity is improved, the volume expansion is inhibited, and the lithium storage performance is improved.

The purpose of the invention is realized by the following technical scheme:

a cobalt-based bimetallic selenide/nitrogen-doped carbon composite material is a material formed by uniformly compounding cobalt-based bimetallic selenide and nitrogen-doped carbon, cobalt-based bimetallic selenide particles are uniformly dispersed on a porous polyhedral nitrogen-doped carbon substrate, the size of a cobalt-based bimetallic selenide/nitrogen-doped carbon polyhedron is 600-1000 nm, and the size of the cobalt-based bimetallic selenide particles is 50-200 nm; the cobalt-based bimetallic selenide accounts for 68.5-72.6 wt% of the composite material, and the nitrogen-doped carbon accounts for 27.4-31.5 wt% of the composite material.

In the cobalt-based bimetallic selenide/nitrogen-doped carbon composite material, the cobalt-based bimetallic selenide is iron-cobalt bimetallic selenide or zinc-cobalt bimetallic selenide or nickel-cobalt bimetallic selenide.

A preparation method of a cobalt-based bimetallic selenide/nitrogen-doped carbon composite material comprises the following steps:

step one, preparing a cobalt-based bimetal organic framework:

firstly, respectively preparing a mixed solution A of potassium hydroxide and 2-methylimidazole, a solution B of anhydrous ferric trichloride or zinc nitrate or nickel nitrate and a cobalt nitrate solution C, sequentially adding a solution B, C into the solution A, wherein the mass concentration ratio of substances of 2-methylimidazole, anhydrous ferric trichloride or zinc nitrate or nickel nitrate, cobalt nitrate and potassium hydroxide in the mixed solution of the solution A, the solution B and the solution C is 1: 0.014:0.0084 (0.027-1.642); then stirring for 4-6 h, and drying to obtain a cobalt-based bimetallic organic framework material FeCo-MOF or ZnCo-MOF or NiCo-MOF;

step two, preparing the cobalt-based bimetal/nitrogen-doped carbon composite material:

placing the cobalt-based bimetal organic frame material in a tube furnace, and preserving heat for 0.5-2 h at 400-600 ℃ to prepare a cobalt-based bimetal/nitrogen-doped carbon composite material FeCo/NC or ZnCo/NC or NiCo/NC;

step three, preparing the cobalt-based bimetallic selenide/nitrogen-doped carbon composite material:

selenium powder and a cobalt-based bimetal/nitrogen-doped carbon composite material FeCo/NC or ZnCo/NC or NiCo/NC are respectively arranged at the upper stream and the lower stream of the porcelain boat, the mass ratio of the selenium powder to the cobalt-based bimetal/nitrogen-doped carbon composite material is 2:1, the heat preservation is carried out for 3h at 350 ℃, the heat preservation is carried out for 0.5-2 h at 400-600 ℃, and the cobalt-based bimetal selenide/nitrogen-doped carbon composite material Fe-Co-Se/NC or Zn-Co-Se/NC or Ni-Co-Se/NC is prepared.

Preferably, the process method for keeping the temperature of the tube furnace at 400-600 ℃ for 0.5-2 h in the step two comprises the following steps: in N₂In an atmosphere of 1 to 5 ℃/minRaising the temperature from room temperature to 400-600 ℃, preserving the heat for 0.5-2 h, and then naturally cooling.

Preferably, in the third step, the process method for preserving the heat of the tube furnace at 350 ℃ for 3 hours and at 400-600 ℃ for 0.5-2 hours comprises the following steps: in N₂Raising the temperature from room temperature to 350 ℃ for 3h at the speed of 1-5 ℃/min in the atmosphere, raising the temperature to 400-600 ℃ at the speed of 1-5 ℃/min, preserving the temperature for 0.5-2 h, and naturally cooling.

Compared with the prior art, the invention has the beneficial effects that: in a water-based solvent, potassium hydroxide is used for assisting in synthesizing the cobalt-based bimetal organic framework material in one step. In addition, compared with epitaxial growth and surfactant-assisted strategies, the steps are complex, time-consuming and low in yield, the reaction time is 24 hours, and the yield is only below 40%; the method is simpler, green, efficient and low in cost, particularly, the reaction can be completed within 6 hours, the yield is 65-75%, and the method is favorable for large-scale preparation. Further, a cobalt-based bimetallic selenide/nitrogen-doped carbon composite material is obtained after selenization by taking a bimetallic organic framework material as a precursor material. The cobalt-based bimetallic selenide/nitrogen-doped carbon composite material is uniform in appearance and uniform in dispersion, the problem of agglomeration of bimetallic selenide is avoided, the bimetallic selenide particles are uniformly dispersed on the porous polyhedral nitrogen-doped carbon substrate and show excellent lithium storage performance, and the cobalt-based bimetallic selenide/nitrogen-doped carbon composite material shows excellent electrochemical performance and has excellent specific capacity, cycling stability and rate capability. Experimental results prove that the bimetal selenide and nitrogen-doped carbon composite material can provide rich redox sites, relieve volume expansion and promote electrochemical reaction kinetics.

Drawings

FIG. 1 is an X-ray diffraction pattern of FeCo-MOF, ZnCo-MOF and NiCo-MOF of an example of the present invention; wherein, FeCo-MOF is used in example 1, ZnCo-MOF is used in example 2, and NiCo-MOF is used in example 3;

FIG. 2 is an X-ray diffraction (XRD) spectrum of FeCo/NC of example 1, ZnCo/NC of example 2 and NiCo/NC of example 3 according to the present invention;

FIG. 3 is an XRD spectrum of Fe-Co-Se/NC of example 1 of the present invention;

FIG. 4 is an XRD spectrum of Zn-Co-Se/NC according to example 2 of the present invention;

FIG. 5 is an XRD spectrum of Ni-Co-Se/NC of example 3 of the present invention;

FIGS. 6(a), (b), (c) are scanning electron micrograph images (SEM images) of FeCo-MOF of example 1 of the present invention;

FIGS. 7(a), (b), (c) are SEM images of FeCo/NC of example 1 of the present invention;

FIGS. 8(a), (b), (c) are SEM pictures of Fe-Co-Se/NC of example 1 of the present invention;

FIG. 9 is an SEM photograph of Zn-Co-Se/NC according to an embodiment of the present invention;

FIG. 10 is an SEM image of Ni-Co-Se/NC according to an embodiment of the invention;

FIGS. 11(a), (b) are high resolution transmission electron micrographs (HR TEM) of Fe-Co-Se/NC according to example 1 of the present invention;

FIG. 12(a) is N of Fe-Co-Se/NC, Zn-Co-Se/NC and Ni-Co-Se/NC according to an embodiment of the present invention₂Adsorption-desorption isotherm diagrams, FIG. 12(b) is a pore size distribution diagram of Fe-Co-Se/NC, Zn-Co-Se/NC and Ni-Co-Se/NC of an example of the present invention;

FIG. 13 is an X-ray photoelectron spectrum (XPS) of Fe-Co-Se/NC according to example 1 of the present invention: wherein (a) is a full spectrum; (b) is a Fe2p high-resolution spectrogram; (c) is a Co 2p high-resolution spectrogram; (d) is Se 2p high resolution spectrogram; (e) a C1s high-resolution spectrum, (f) an N1s high-resolution spectrum;

FIG. 14 is a Raman spectrum of Fe-Co-Se/NC, Zn-Co-Se/NC and Ni-Co-Se/NC according to an embodiment of the present invention;

FIG. 15 is the electrochemical performance of Fe-Co-Se/NC, Zn-Co-Se/NC and Ni-Co-Se/NC of examples of the present invention: (a) cyclic voltammograms of Fe-Co-Se/NC; (b) Fe-Co-Se/NC at 0.1Ag^-1A lower charge-discharge curve; (c) a multiplying power performance diagram of Fe-Co-Se/NC, Zn-Co-Se/NC and Ni-Co-Se/NC; (d) the charge-discharge curves of Fe-Co-Se/NC under different current densities; (e)0.1Ag^-1The lower cycle performance graphs of Fe-Co-Se/NC, Zn-Co-Se/NC and Ni-Co-Se/NC; (f) Fe-Co-Se/NC at 1Ag^-1A lower charge-discharge curve; (g)1Ag^-1Under the cycle of Fe-Co-Se/NC, Zn-Co-Se/NC and Ni-Co-Se/NCA performance map;

FIG. 16 is a graph of electrochemical impedance of examples of the present invention before cycling of Fe-Co-Se/NC, Zn-Co-Se/NC and Ni-Co-Se/NC;

FIG. 17 is an SEM photograph of example 4;

FIG. 18 is an SEM photograph of example 5;

FIG. 19 is an SEM photograph of example 6;

FIG. 20 is an SEM photograph of example 7.

Detailed Description

The invention is further described with reference to the following figures and specific examples.

Example 1 preparation of Fe-Co-Se/NC composite Material

30mL of 6mol L is prepared^-1Then 1.2g of 2-methylimidazole (C)₄H₆N₂) Obtaining a mixed solution A after ultrasonic treatment and stirring; dissolving 0.249g of ferric trichloride in 15mL of deionized water to obtain a solution B, and dissolving 0.269g of cobalt nitrate hexahydrate in 15mL of deionized water to form a solution C; and then, sequentially adding the solution B, C into the solution A, stirring for 4-6 h, and marking the obtained sample as FeCo-MOF. The product FeCo-MOF is placed in a tube furnace in N₂At 2 ℃ for min under an atmosphere of^-1Raising the temperature rising rate to 500 ℃, preserving the heat for 1h, and naturally cooling to obtain a carbonized product FeCo/NC. Then, selenium powder and FeCo/NC (mass ratio of 2:1) are respectively placed at the upstream and downstream of the porcelain boat, the temperature is raised to 350 ℃ in the atmosphere of nitrogen, the heat preservation is carried out for 3h, the heat preservation is carried out for 1h at 500 ℃, and the composite material of the iron-cobalt bimetal multiphase selenide and the nitrogen-doped carbon is obtained through natural cooling and is marked as Fe-Co-Se/NC.

FIG. 1 shows the X-ray diffraction pattern of FeCo-MOF prepared in example 1, and the characteristic peak of FeCo-MOF is consistent with the reported characteristic peak of the synthesized MOFs in aqueous solution.

As can be seen from the X-ray diffraction spectrum of FeCo/NC prepared in example 1 in FIG. 2, after carbonization, the FeCo/NC composite material has a characteristic peak of (002) crystal face of carbon at 26.40 degrees, and the characteristic peaks at 44.8 degrees and 65.3 degrees correspond to (110) crystal face and (200) crystal face of CoFe (JCPDS No.44-1433), respectively.

Prepared from example 1 in FIG. 3The XRD spectrogram of the Fe-Co-Se/NC can be seen, and all characteristic peaks of the Fe-Co-Se/NC and the o-FeSe₂(Orthorhombic，Pnnm，JCPDS No.21-0432)、o-CoSe₂(ortho, Pnm, JCPDS No.53-0449) and c-CoSe₂(Cubic, Pa-3, JCPDS No.09-0234) and a sharp diffraction peak indicates that the surface has higher crystallinity. Wherein the characteristic peaks at 31.1 °, 34.9 °, 48.2 °, 54.1 ° and 64.1 ° correspond to o-FeSe, respectively₂The (101), (111), (211), (031), and (122) crystal planes of (a). The remaining characteristic peaks at 30.8 °, 34.5 °, 36.0 °, 47.7 °, 65.0 ° and 63.3 ° correspond to o-CoSe, respectively₂The (101), (111), (120), (211), (311), and (122) crystal planes of (a). The characteristic peaks at 34.2 °, 37.6 °, 51.8 °, 56.5 °, 58.8 ° and 74.0 ° correspond to c-CoSe, respectively₂The (210), (211), (311), (230), (321), and (421) crystal planes of (a). Proves that the iron-cobalt selenide in the Fe-Co-Se/NC is the orthorhombic phase FeSe₂(o-FeSe₂) Quadrature phase CoSe₂(o-CoSe₂) And cubic phase CoSe₂(c-CoSe₂) Concurrent bimetallic selenides.

FIG. 6 is an SEM image of FeCo-MOF prepared in example 1, and the result shows that the bimetallic MOF has a polyhedral structure and a diameter of about 1 μm. FIG. 7 is an SEM image of FeCo/NC prepared in example 1, and it can be seen that the morphology of the polyhedrons is preserved, but small particles with slight shrinkage and rough surface appear, with a diameter of about 900 nm. FIG. 8 is an SEM image of Fe-Co-Se/NC prepared in example 1, and the results show that the morphology of the Fe-Co-Se/NC is changed, the polyhedron structure is expanded and becomes full from the contracted state, nano-agglomerates protruding outwards appear on the surface, and the distribution is relatively uniform. Structural expansion is related to the phase transformation that occurs inside or on the surface of the polyhedron during the selenization reaction. And a part of metal ions escape outwards in the selenization process and are combined with selenium ions to form a metal selenide crystal cluster.

FIG. 11 is a TEM and HRTEM image of Fe-Co-Se/NC prepared in example 1, and the results show that Fe-Co-Se nanoparticles are uniformly distributed on a carbon matrix; the high-resolution electron microscope image shows that the lattice stripes of the Fe-Co-Se nanoparticles are clear, which shows that the crystallinity is good.

FIG. 12(a) shows nitrogen adsorption and desorption curves of Fe-Co-Se/NC prepared in example 1, and the results show that the specific surface areas of Fe-Co-Se/NC are 7.6m²g^-1Pore volume of 0.038cm³ g^-1. FIG. 12(b) shows a distribution of the pore diameters of Fe-Co-Se/NC, centered between 7 and 100nm, with mesopores consisting of (A), (B), (C<50nm) and macropores: (>50nm) and the test result shows that the average pore diameter of Fe-Co-Se/NC is 20.4nm respectively. The abundant pore structure is beneficial to the diffusion of electrolyte and the transportation of ions/electrons, and provides a buffer space for volume expansion.

FIG. 13 is an XPS spectrum of Fe-Co-Se/NC prepared in example 1. The results of fig. 13(a) show that characteristic peaks of elements C, N, O, Se, Fe, and Co can be observed in the entire measurement spectrum. FIG. 13(b) is an XPS high resolution Fe2p spectrum, and it can be seen that the fit shows Fe2p_1/2And Fe2p_3/2Characteristic peaks of the orbitals, two at 725.2eV and 712.6eV corresponding to Fe²⁺And the peak at 717.4eV corresponds to Fe³⁺. In the high-resolution Co 2p spectrum (fig. 13(c)), it can be seen that the bond energy value of the Co 2p spectrum is high under the induction of Fe atoms. The peak at 778.7eV corresponds to Co⁰Two characteristic peaks at 782.2eV and 797.5eV correspond to Co²⁺While the two peaks at 780.7eV and 793.9eV are assigned to Co³⁺. The Se 3d spectrum (FIG. 13(d)) can be decomposed into three peaks at 54.7, 55.6 and 59.2eV, corresponding to Se 3d respectively_5/2(Fe-Se and Co-Se bond), Se 3d_3/2(Se-Se bond) and SeO_x. Three fitted peaks appear in the high resolution spectrum of C1s (fig. 13(e)), at 284.8, 285.5 and 286.5eV, respectively, in one-to-one correspondence with C-C, C ═ N and C-O bonds, where the presence of C-N bonds confirms N doping. The remaining two peaks at 283.5eV and 296.1eV are assigned to the C-Co and C-Se bonds, respectively. Fig. 13(f) is a high resolution N1s spectrum with three peak signatures due to pyridine type N (398.4eV), pyrrole type N (399.3eV), and graphite type N (400.9eV), respectively, present in the nitrogen-doped carbon material. The N doping modification can not only improve the conductivity of the carbon matrix, but also introduce more defects, namely Li⁺The insertion of (A) provides a large number of active centersAnd the lithium storage performance is improved.

FIG. 14 shows the Raman spectra of Fe-Co-Se/NC prepared in example 1, which is observed at 1357 and 1576cm^-1Two characteristic peaks in the vicinity correspond to the D band (amorphous carbon) and the G band (graphitic carbon) in nitrogen-doped carbon. I of Fe-Co-Se/NC_D/I_GThe value (intensity ratio) is 0.93, which indicates that more carbon defects exist in Fe-Co-Se/NC, so that the electron transfer rate is increased, the storage position of lithium ions is increased, and the lithium storage performance is improved.

FIG. 15(a) shows that the Fe-Co-Se/NC composite material prepared in example 1 is 0.2mV s as the negative electrode material of the lithium ion battery^-1Three reduction peaks were observed in the first cathodic scan of the cyclic voltammogram at the sweep rate. Wherein the peak at-1.3V corresponds to intercalation of lithium and Li in the active material_xFeSe₂/Li_xCoSe₂And decomposition of the electrolyte and formation of a solid electrolyte interface film. While the two reduction peaks at-0.95 and 0.46V correspond to the conversion reactions accompanied by FeSe, CoSe and LiSe₂And subsequently Fe, Co and Li₂Generation of Se. In the first anode scan, three oxidation peaks at 1.41V, 2.14V and 2.32V, which correspond to FeSe, CoSe and intermediate LixFeSe, respectively, can be observed₂/LixCoSe₂And the final product FeSe₂/CoSe₂Is performed. In subsequent cycles, the cyclic voltammograms remained stable and overlapped well, indicating that the electrochemical reaction of Fe-Co-Se/NC was reversible and stable. FIG. 15(b) shows that the Fe-Co-Se/NC composite material prepared in example 1 is 0.1Ag for the negative electrode material of lithium ion battery^-1The positions of the oxidation reduction peak in the charging and discharging platform and the cyclic voltammetry curve are kept consistent. During the first cycle, the discharge voltage plateaus were about 1.35V and 0.79V, and the charge voltage plateaus were about 1.38, 2.01V, and 2.29V. In the first discharging and charging process of Fe-Co-Se/NC, the charging and discharging specific capacity of the electrode respectively reaches 838 mAh g and 1233mAh g^-1. The specific capacity difference of the first cycle charge and discharge is large, the initial coulombic efficiency is 68 percent, and the initial reservoir is lowThe coulombic efficiency is related to the decomposition of the electrolyte and the formation of a solid electrolyte interfacial film at the electrode/electrolyte interface. FIG. 15(c) shows the rate capability at different current densities, which shows that Fe-Co-Se/NC as the negative electrode material of lithium ion batteries at 0.1, 0.2, 0.5, 1, 2 and 5Ag^-1Average capacities at bottom were 1066, 1040, 952, 876, 700 and 393mAh g^-1. When the current returns to 0.1A g^-1The specific time capacitance is 1071mAh g^-1The capacity retention rate is close to 100%, indicating that Fe, Co selenide exhibits the best synergistic effect, which is attributed to o-FeSe₂、o-CoSe₂And c-CoSe₂The membrane has rich phase interfaces, greatly increases the active sites of electrochemical reaction, lightens the volume expansion and forms a stable solid electrolyte interface membrane. FIG. 15(d) is a charge-discharge diagram of Fe-Co-Se/NC at different currents, and it can be seen that the potential difference of the charge-discharge plateau gradually increases with the increase of the current even at 5Ag^-1Under the high current density, a stable charging and discharging platform still exists, which shows that the Fe-Co-Se/NC has good structural stability and can bear the volume expansion generated in the charging and discharging process. FIG. 15(e) shows the concentration of Ag at 0.1Ag in Fe-Co-Se/NC^-1The cycle performance diagram shows that in the process of 100 cycles, the electrode material is continuously activated and steadily increases, and the capacity of the electrode material is steadily maintained at 1326mAh g^-1The coulomb efficiency approaches 100%. More importantly, the kinetic process of the conversion reaction is slower, which tends to result in lower coulombic efficiency and cycle stability, while the coulombic efficiency of the Fe-Co-Se/NC electrode cycle is about 99%, indicating that it has enhanced ion diffusion behavior. FIG. 15(f) shows Fe-Co-Se/NC at 1A g^-1The charging and discharging curve in the lower part, the first three circles are activated under low current, the contact ratio of the curves from the fourth circle to the seventh circle is still good, the good reversibility is proved in the initial state, after 550 circles of circulation, the specific capacity is obviously increased, the charging and discharging platform is lengthened and stable, the redox reaction of Fe-Co-Se/NC is more stable and durable after 550 times of circulation, and the reason is that a stable solid electrolyte interface film is formed in the electrode and rich redox sites exist. FIG. 15(g) shows 1A g^-1Long cycle diagram at current density of (1), can be seenThe obtained Fe-Co-Se/NC has high specific capacitance and cycling stability, and after 550 cycles, the specific capacity is 1247mAh g^-1. In addition, the trend that the specific capacitance firstly decays and then increases is found in the circulation process, the initial decay is related to volume expansion, and the subsequent rising and increasing are caused by the fact that electrode materials are subjected to nanocrystallization in the repeated charging and discharging process, so that more active sites are exposed. In addition, the presence of nitrogen-doped carbon also effectively increases the active sites. In conclusion, the excellent lithium storage properties of Fe-Co-Se/NC are attributed to the synergistic effect of the bimetallic selenide and nitrogen-doped carbon providing rich redox sites and inhibiting volume expansion. And the unique three-dimensional porous structure provides an effective transmission channel for electrons, shortens the diffusion path of lithium ions and accelerates the charge transfer process at the interface of the electrode/electrolyte.

FIG. 16 shows the electrochemical impedance plot of Fe-Co-Se/NC before cycling, and the charge transfer kinetics are further understood by impedance analysis. It can be seen that the impedance curve is composed of a semicircle of a high frequency region and a diagonal line of a low frequency region, the semicircle and the diagonal line representing the charge transfer impedance and the diffusion impedance, respectively. The results show that Fe-Co-Se/NC can be seen to exhibit a relatively minimum semicircle, indicating that the charge transfer resistance is small, which can suppress the formation of an SEI layer too thick, facilitating electron transfer.

Example 2: preparation of Zn-Co-Se/NC composite material

First, 30mL of 6mol L was prepared^-1Then adding 1.2g of 2-methylimidazole into the potassium hydroxide, and performing ultrasonic treatment and stirring to obtain a mixed solution A; dissolving 0.467g of zinc nitrate hexahydrate in 15mL of deionized water to obtain a solution B, and dissolving 0.269g of cobalt nitrate hexahydrate in 15mL of deionized water to obtain a solution C; adding the solution B into the solution A, stirring for 30 minutes, adding the solution C, continuing stirring for 4.5 hours, washing with deionized water and drying to obtain a sample which is marked as ZnCo-MOF. The ZnCo-MOF obtained in the above is placed in a tube furnace in N₂At 2 ℃ for min under an atmosphere of^-1Raising the temperature rise rate to 500 ℃, preserving the temperature for 1h, and naturally cooling to obtain a carbonized product ZnCo/NC. Finally, selenium powder and ZnCo/NC (mass ratio of 2:1) are respectively placed inHeating the ceramic boat up and down stream to 350 ℃ in the nitrogen atmosphere, preserving heat for 3 hours at 500 ℃, preserving heat for 1 hour, and naturally cooling to obtain the composite material of zinc-cobalt bimetallic multiphase selenide and nitrogen-doped carbon, which is recorded as Zn-Co-Se/NC.

FIG. 1 shows the X-ray diffraction pattern of ZnCo-MOF prepared in example 2, and the results show that the characteristic peaks of ZnCo-MOF are consistent with the reported characteristic peaks of MOFs.

FIG. 2 shows the XRD spectrum of ZnCo/NC prepared in example 1, and after carbonization, the ZnCo/NC composite material has characteristic peaks of (002) crystal face of carbon at 26.40 degrees, and characteristic peaks appearing at 41.9 degrees and 48.8 degrees correspond to (111) and (200) crystal faces of ZnCo (JCPDS No. 29-0524).

FIG. 4 is an XRD spectrum of Zn-Co-Se/NC prepared in example 2. The results showed that the characteristic peaks at 30.8 °, 34.5 °, 36.0 °, 47.7 °, 65.0 ° and 63.3 ° correspond to o-CoSe, respectively₂The (101), (111), (120), (211), (311) and (122) crystal planes of (A), the characteristic peaks at 34.2 °, 37.6 °, 51.8 °, 56.5 °, 58.8 ° and 74.0 ° correspond to c-CoSe, respectively₂The (210), (211), (311), (230), (321), and (421) crystal planes of (a); the remaining characteristic peaks at 27.2 °, 45.2 ° and 53.6 ° correspond to the (111), (220) and (311) crystal planes of c-ZnSe (JCPDS No.65-9602), respectively.

FIG. 9 is an SEM image of Zn-Co-Se/NC prepared in example 2, and it can be seen that Zn-Co-Se/NC shows a cage structure after selenization, which is advantageous for electron and ion transfer in an electrochemical process. The cage structure is formed because Zn and Co metal play a catalytic role in the decomposition of the ligand in the calcining process, so that more ligands are rapidly decomposed to form larger holes.

FIG. 12(a) shows a nitrogen adsorption/desorption curve of Zn-Co-Se/NC prepared in example 2, and the result shows that the specific surface area of Zn-Co-Se/NC is 10.3m²·g^-1Total pore volume of 0.055cm³·g^-1. FIG. 12(b) shows a distribution of the pore size of Zn-Co-Se/NC, which can be seen to have a pore size of 7-100nm with mesopores (C<50nm) and macropores: (>50nm), the test results show flatness of Fe-Co-Se/NCThe average pore diameter is 18.8 nm.

FIG. 14 shows the Raman spectra of Zn-Co-Se/NC prepared in example 2, and it can be seen that Fe-Co-Se/NC is at 1357 and 1576cm^-1Two characteristic peaks in the vicinity correspond to the D band (amorphous carbon) and the G band (graphitic carbon) in nitrogen-doped carbon. I of Zn-Co-Se/NC_D/I_GThe value (intensity ratio) is 0.86, which indicates that more carbon defects exist in Zn-Co-Se/NC, so that the electron transfer rate is increased, the storage position of lithium ions is increased, and the lithium storage performance is improved.

FIG. 15(c) shows the Zn-Co-Se/NC rate performance at different current densities, and it can be seen that Zn-Co-Se/NC exhibits good rate performance at 0.1, 0.2, 0.5, 1, 2 and 5A g^-1The average capacities at the bottom were 979, 984, 920, 844, 684 and 358mAh g^-1. When the current returns to 0.1A g^-1The specific time capacitance is 944mAh g^-1The capacity retention rate is close to 100%, indicating that Zn and Co selenides show the best synergistic effect. FIG. 15(e) shows Zn-Co-Se/NC at 0.1A g^-1The cycle performance diagram shows that in the process of 100 cycles, the electrode material is continuously activated and steadily increases, and the capacity of the electrode material is 787mAh g^-1Increase to 1131mAh g^-1The coulomb efficiency is more than 98%. FIG. 15(g) shows Zn-Co-Se/NC at 1A g^-1The long cycle chart under the current density shows that Zn-Co-Se/NC has excellent specific capacity and cycle stability, and the specific capacity is 947mAh g after 550 cycles^-1And the lithium storage performance of the metal selenide is higher than that of most reported metal selenides.

FIG. 16 shows the electrochemical impedance plot of Zn-Co-Se/NC before cycling, with further understanding of the charge transfer kinetics by impedance analysis. It can be seen that the impedance curve is composed of a semicircle of a high frequency region and a diagonal line of a low frequency region, the semicircle and the diagonal line representing the charge transfer impedance and the diffusion impedance, respectively. The results show that Zn-Co-Se/NC exhibits a small semicircle, indicating that the charge transfer resistance is small, facilitating electron transfer.

Example 3: preparation of Ni-Co-Se/NC composite material

First, 30mL of 6mol L was prepared^-1Potassium hydroxide ofThen 1.2g of 2-methylimidazole (C) were added₄H₆N₂) Obtaining a mixed solution A after ultrasonic treatment and stirring; dissolving 0.446g of nickel nitrate hexahydrate in 15mL of deionized water to obtain a solution B, and dissolving 0.269g of cobalt nitrate hexahydrate in 15mL of deionized water to form a solution C; adding the solution B into the solution A, stirring for 30 minutes, then adding the solution C into the solution A, and after continuously stirring for 4.5 hours, washing by deionized water and drying to obtain a sample marked as NiCo-MOF. The product NiCo-MOF described above was placed in a tube furnace in N₂At 2 ℃ for min under an atmosphere of^-1Raising the temperature raising rate to 500 ℃, preserving the heat for 1 hour, and naturally cooling to obtain a carbonized product NiCo/NC. Then, selenium powder and NiCo/NC (mass ratio of 2:1) are respectively placed at the upstream and downstream of the porcelain boat, the temperature is raised to 350 ℃ in the nitrogen atmosphere, the temperature is kept for 3 hours, the temperature is kept at 500 ℃ for 1 hour, and the composite material of the nickel-cobalt bimetallic multiphase selenide and the nitrogen-doped carbon is obtained after natural cooling and is marked as Ni-Co-Se/NC.

FIG. 1 shows the XRD spectrum of NiCo-MOF prepared in example 3, and the results show that the characteristic peaks of NiCo-MOF are consistent with the reported characteristic peaks of MOFs synthesized in aqueous solution.

FIG. 2 shows the XRD spectrum of NiCo/NC prepared in example 3, and after carbonization, the NiCo/NC composite material has characteristic peak of (002) crystal face of carbon at 26.40 degrees, and other characteristic peaks of NiCo/NC correspond to (111) and (200) crystal faces of Co (JCPDS No.15-0806) and Ni (JCPDS No. 04-0850).

FIG. 5 is an XRD spectrum of the Ni-Co-Se/NC composite material prepared in example 3. The results show that the characteristic peak of Ni-Co-Se/NC is located in c-CoSe₂(JCPDS No.09-0234) and c-NiSe₂(JCPDS No.65-5015), it was confirmed that cobalt-nickel double metal selenide. The failure of Ni-Co-Se/NC to undergo phase transition may be due to NiSe₂And c-CoSe₂Having a similar crystalline band structure, hampers c-CoSe₂(100) A phase transition occurs.

Fig. 10 is a FESEM view of the Ni-Co-Se/NC composite prepared for example 3, and it can be seen that the morphology of Ni-Co-Se/NC is not uniform of Fe-Co-Se/NC and Zn-Co-Se/NC, and the dispersibility is slightly poor, while there are a polyhedral structure and a spherical structure, but the shrinkage morphology is maintained after selenization due to no phase transition of Ni-Co-Se/NC during the selenization reaction. In addition, the surface thereof also shows a prominent granular structure because a part of metal ions escape and then selenide crystal clusters are formed on the surface.

FIG. 12(a) shows a nitrogen adsorption/desorption curve of Ni-Co-Se/NC prepared in example 3, and the result shows that the specific surface area of Ni-Co-Se/NC is 7.5m²·g^-1Total pore volume of 0.036cm³·g^-1. FIG. 12(b) shows a pore size distribution diagram of Ni-Co-Se/NC, which shows that the Ni-Co-Se/NC has pore sizes of 7-100nm and has mesopores of (A), (B), (C), (B), and C), (B) and C), (B) and (C) a)<50nm) and macropores: (>50nm) and the test result shows that the average pore diameter of Ni-Co-Se/NC is 21.5nm respectively.

FIG. 14 shows the Raman spectra of Ni-Co-Se/NC prepared in example 3, which is observed at 1357 and 1576cm^-1Two characteristic peaks in the vicinity correspond to the D band (amorphous carbon) and the G band (graphitic carbon) in nitrogen-doped carbon. I of Ni-Co-Se/NC_D/I_GThe value (intensity ratio) is 0.76, which indicates that some carbon defects exist in the Ni-Co-Se/NC, so that the electron transfer rate is increased, the storage position of lithium ions is increased, and the lithium storage performance is improved.

Example 4:

the difference from example 1 is that FeCo-MOF to be obtained is in N₂At 2 ℃ for min under an atmosphere of^-1Raising the temperature raising rate to 400 ℃ and preserving the temperature for 1 h. FIG. 17 is an SEM image of a sample showing that the morphology of the sample is similar to that of FeCo-MOF, indicating that the sample is not fully carbonized at 400 ℃.

Example 5:

the difference from example 1 is that FeCo-MOF to be obtained is in N₂At 2 ℃ for min under an atmosphere of^-1Raising the temperature raising rate to 600 ℃ and preserving the temperature for 1 h. FIG. 18 is an SEM image of a sample, and the result shows that most particles of the sample maintain polyhedral morphology, and a small part of the polyhedral particles are broken, which is mainly caused by overhigh temperature.

Example 6:

the difference from the embodiment 1 is that selenium powder and FeCo/NC (mass ratio of 2:1) are respectively placed at the upstream and downstream of a porcelain boat, heated to 350 ℃ in nitrogen atmosphere and insulated for 3h, and then insulated for 1h at 400 ℃ and naturally cooled to obtain a sample. Fig. 19 is an SEM image of a sample, and the results show that the polyhedral particles of the sample are still not in a dense structure, indicating no selenization or no selenization is complete, primarily due to the lower selenization temperature.

Example 7:

the difference from the embodiment 1 is that selenium powder and FeCo/NC (mass ratio of 2:1) are respectively placed at the upstream and downstream of a porcelain boat, heated to 350 ℃ in nitrogen atmosphere and insulated for 3h, and then insulated for 1h at 600 ℃ and naturally cooled to obtain a sample. Fig. 20 is an SEM image of the sample, showing that the particles of the sample still maintain polyhedral morphology and are loosely assembled from a large number of particles. From this result, it can be seen that at high temperature, the carbon content in the product is low and the structure is loose and weak, resulting in poor electrical conductivity and structural stability of the material.

The foregoing shows and describes the general principles and features of the present invention, together with the advantages thereof. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.