A cobalt-based compound heterojunction composite material and its application

Abstract

A cobalt-based compound heterojunction composite material and its application, which relates to a composite nanomaterial and its application. The purpose of the present invention is to solve the problems of the shuttle effect of polysulfide in existing lithium-sulfur batteries and the low wettability, poor electrolyte retention, low ionic conductivity, and lack of the function of anchoring polysulfide of existing microporous polypropylene as a separator. A cobalt-based compound heterojunction composite material is a composite of CoSe₂ / CoO nanorods and carbon nanotubes, having a three-dimensional network structure, wherein the diameter of the carbon nanotubes is 10-50 nm, the wall thickness is 3-20 nm, and the CoSe₂ / CoO nanorods are wrapped by the carbon nanotubes and are discretely and uniformly distributed in the conductive carbon nanotubes. Method: First, prepare CoSe₂ / CoO nanorods; second, compound. A cobalt-based compound heterojunction composite material is used as a separator or a positive electrode of a lithium-sulfur battery.

Description

Technical Field

[0001] The present invention relates to a composite nanomaterial and its application. Background Art

[0002] The lithium-sulfur battery is a type of lithium battery. A typical lithium-sulfur battery generally uses elemental sulfur as the positive electrode and a lithium metal sheet as the negative electrode. Elemental sulfur is abundant in the earth, and has the characteristics of low price and environmental friendliness. For a lithium-sulfur battery using sulfur as the positive electrode material, its theoretical specific capacity of the material and the theoretical specific energy of the battery are relatively high, reaching 1675 mAh / g and 2600 Wh / kg respectively, far higher than the capacity of the commercially widely used lithium cobalt oxide battery (<150 mAh / g). And sulfur is an environmentally friendly element with basically no pollution to the environment, so it is a very promising lithium battery. The reaction mechanism of the lithium-sulfur battery is different from the ion intercalation / deintercalation mechanism of the lithium-ion battery, but an electrochemical mechanism. During discharge, the reaction at the negative electrode is that lithium loses electrons to become lithium ions, and the reaction at the positive electrode is that sulfur reacts with lithium ions and electrons to form sulfides. The intermediate product of the reaction, lithium polysulfide Li2S n (n = 3 - 8) will dissolve into the organic electrolyte and can migrate between the positive and negative electrodes, thus causing the shuttle effect, which is the main reason restricting the development and practical application of lithium-sulfur batteries.

[0003] To address this problem, existing technologies mostly focus on improving the separator; currently, commercial microporous polypropylene (PP) membranes are usually used as the separator for lithium-sulfur batteries. Its advantages are good mechanical properties and chemical stability. However, its disadvantages are: low wettability, poor electrolyte retention, low ionic conductivity, no effect of anchoring polysulfides, etc. Summary of the Invention

[0004] The object of the present invention is to solve the problems of the shuttle effect of lithium polysulfide in existing lithium-sulfur batteries and the low wettability, poor electrolyte retention, low ionic conductivity, and no effect of anchoring polysulfides of the existing microporous polypropylene as a separator, and to provide a cobalt-based compound heterojunction composite material and its application.

[0005] A cobalt-based compound heterojunction composite material is a composite of CoSe2 / CoO nanorods and carbon nanotubes, having a three-dimensional network structure. The diameter of the carbon nanotubes is 10 - 50 nm, and the wall thickness is 3 - 20 nm. The CoSe2 / CoO nanorods are wrapped by the carbon nanotubes and are discretely and uniformly distributed in the conductive carbon nanotubes. The lateral size of the CoSe2 / CoO nanorods is 30 - 300 nm, the surface is rough, and has a granular structure; both CoSe2 and CoO in the CoSe2 / CoO nanorods are p-type semiconductors. The band gap of CoO is 1.9 - 2.2 eV, and the band gap of CoSe2 is 1.5 - 1.8 eV.

[0006] A method for preparing a cobalt-based compound heterojunction composite material is completed according to the following steps:

[0007] I. Preparation of CoSe2 / CoO nanorods:

[0008] ① Add Co(NO3)2, NH4F, and urea to deionized water and stir to obtain a mixed solution;

[0009] ② Transfer the mixed solution to a polytetrafluoroethylene reaction kettle, and then carry out a hydrothermal reaction at 110°C - 130°C to obtain a reaction product; wash the reaction product with deionized water and then vacuum dry it to obtain a precursor;

[0010] ③ Transfer the precursor to a porcelain boat on the downstream side of the center of a tubular furnace, and place selenium powder in a corresponding porcelain boat on the upstream side. Subsequently, under an Ar atmosphere, heat the tubular furnace from room temperature to 400°C - 450°C, then keep it warm at 400°C - 450°C, and cool to room temperature after the heat preservation ends to obtain CoSe2 / CoO nanorods;

[0011] II. Composite:

[0012] Mix CoSe2 / CoO nanorods and carbon nanotubes to obtain a mixture; grind the mixture to obtain a composite of CoSe2 / CoO nanorods and carbon nanotubes, which is the cobalt-based compound heterojunction composite material.

[0013] A cobalt-based compound heterojunction composite material is used as a separator or a positive electrode of a lithium-sulfur battery.

[0014] Advantages of the present invention:

[0015] I. A cobalt-based compound heterojunction composite material obtained by the present invention, that is, a composite of CoSe2 / CoO nanorods and carbon nanotubes, has a three-dimensional network structure, which is beneficial to the penetration of the electrolyte and the rapid transmission of ions;

[0016] II. A cobalt-based compound heterojunction composite material obtained by the present invention, that is, a composite of CoSe2 / CoO nanorods and carbon nanotubes, in which the carbon nanotubes have a diameter of 10 - 50 nm and a wall thickness of 3 - 20 nm, and the carbon nanotubes uniformly wrap around the CoSe2 / CoO nanorods, making them discretely and uniformly distributed in the conductive carbon nanotubes, which is beneficial to the rapid conduction of electrons;

[0017] III. A cobalt-based compound heterojunction composite material obtained by the present invention, namely a CoSe2 / CoO nanorod and carbon nanotube composite. The lateral size of the CoSe2 / CoO nanorod is 30 - 300 nm. The surface of the nanorod is rough, having a granular structure and a rich CoSe2 / CoO heterojunction interface. When used as a catalyst, it can provide more active sites, which is beneficial to the improvement of catalytic activity and performance.

[0018] IV. A cobalt-based compound heterojunction composite material obtained by the present invention, namely a CoSe2 / CoO nanorod and carbon nanotube composite. The CoSe2 / CoO nanorod has a rich CoSe2 / CoO heterojunction interface. Since both CoSe2 and CoO are p-type semiconductors and have different Fermi levels, electrons at the interface spontaneously flow from CoO to CoSe2. The synergistic effect generated by the two is beneficial to the rapid conduction of electrons and excellent electrochemical performance.

[0019] V. A cobalt-based compound heterojunction composite material obtained by the present invention, namely a CoSe2 / CoO nanorod and carbon nanotube composite. The CoSe2 / CoO nanorod has a rich CoSe2 / CoO heterojunction interface. When used in a lithium-sulfur battery, the built-in electric field formed at the interface between the two has a strong adsorption effect on polysulfides, creating conditions for better anchoring of dissolved lithium polysulfides in the electrolyte and being beneficial to suppressing the shuttle effect.

[0020] VI. A cobalt-based compound heterojunction composite material obtained by the present invention, namely a CoSe2 / CoO nanorod and carbon nanotube composite. The CoSe2 / CoO nanorod has a rich CoSe2 / CoO heterojunction interface. When used in a lithium-metal battery, the attracting effect of the built-in electric field formed at the interface between the two on lithium ions can uniform the lithium ion flux and improve the stability of the lithium negative electrode.

[0021] VII. A cobalt-based compound heterojunction composite material obtained by the present invention, namely a CoSe2 / CoO nanorod and carbon nanotube composite. When used in a lithium-sulfur battery, the synergistic effect of CoO and CoSe2 can selectively catalyze the conversion of sulfur species through the Mott-Schottky effect, that is, slow down the conversion of elemental sulfur to long-chain polysulfides and accelerate the conversion of long-chain polysulfides to lithium disulfide, thereby reducing the accumulation of polysulfide compounds in the electrolyte and effectively suppressing the polysulfide shuttle, providing a theoretical basis and technical support for the research and practical application of lithium-sulfur batteries.

[0022] VIII. A cobalt-based compound heterojunction composite material obtained by the present invention, namely, a CoSe2 / CoO nanorod and carbon nanotube composite, when used in a lithium-sulfur battery, due to the Mott-Schottky effect, electrons will transfer at the heterojunction interface of CoSe2 and CoO. The positively charged CoO can chemically anchor the negatively charged polysulfide, and the negatively charged CoSe2 tends to adsorb Li + in the electrolyte, thereby promoting the conversion of lithium polysulfide, weakening the shuttle effect, ultimately improving the loss of active substances and the attenuation of capacity, and enhancing the performance of the lithium-sulfur battery;

[0023] IX. A cobalt-based compound heterojunction composite material obtained by the present invention, namely, a CoSe2 / CoO nanorod and carbon nanotube composite, when used in a lithium-sulfur battery, the CoSe2 / CoO nanorod and carbon nanotube composite has a three-dimensional network structure, which can physically block the lithium polysulfide dissolved in the electrolyte, effectively slowing down the shuttle effect. The carbon nanotubes serve as a conductive framework to form an interconnected network structure, which is beneficial to the infiltration of the electrolyte and the rapid conduction of ions / electrons. At the same time, the uniform distribution of CoSe2 / CoO nanorods in the conductive carbon nanotubes can promote the three-dimensional precipitation of Li2S, thereby obtaining better electrochemical performance;

[0024] X. A cobalt-based compound heterojunction composite material obtained by the present invention, namely, a CoSe2 / CoO nanorod and carbon nanotube composite, has a clever process, low-cost processing equipment, simple procedures and methods, low cost, and is conducive to large-scale industrial production;

[0025] XI. A cobalt-based compound heterojunction composite material obtained by the present invention, namely, a CoSe2 / CoO nanorod and carbon nanotube composite, has a loading amount of 0.1 - 5 mg / cm when used to modify a polypropylene separator in a lithium-sulfur battery 2 , which can well inhibit the shuttle effect, and shows improved specific capacity (1450 mAh g at 0.1C -1 ; the commercial polypropylene separator is 915 mAh g -1 ), rate performance (910 mAh g at 2C -1 ; the commercial polypropylene separator is 208 mAh g -1 ), and cycle stability (at 0.5C, the attenuation rate per cycle after 450 cycles is 0.069%; the attenuation rate per cycle of the commercial polypropylene separator after 450 cycles is 0.094%) compared with the commercial polypropylene separator;

[0026] XII. A cobalt-based compound heterojunction composite material obtained by the present invention, namely, a CoSe2 / CoO nanorod and carbon nanotube composite, can be applied to the separator and the positive electrode of a lithium-sulfur battery. At the same time, because this composite material has good electrocatalytic activity, it also has broad application prospects in the fields of solar cells, supercapacitors, lithium-ion batteries, photo- or electrocatalysis, electrostatic shielding, and nano biomedicine, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 X-ray diffraction spectrum of CoSe2 / CoO nanorods prepared in Example 1;

[0028] Figure 2 Low-magnification scanning electron microscope image of CoSe2 / CoO nanorods prepared in Example 1;

[0029] Figure 3 Low-magnification transmission electron microscope image of CoSe2 / CoO nanorods prepared in Example 1;

[0030] Figure 4 High-magnification transmission electron microscope image of CoSe2 / CoO nanorods prepared in Example 1;

[0031] Figure 5 Low-magnification scanning electron microscope image of CoSe2 / CoO-CNTs composite prepared in Example 1;

[0032] Figure 6 Mott-Schottky curve of prepared CoO;

[0033] Figure 7 Mott-Schottky curve of prepared CoSe2;

[0034] Figure 8 Digital images of Li2S6, (I) CNT and Li2S6, (II) CoSe2 / CoO-CNTs and Li2S6, (III) CoSe2 / CoO and Li2S6 solutions after standing for 24 hours;

[0035] Figure 9 CV curves of Li2S6 symmetric batteries with carbon paper (CP), CoSe2 / CoO and CoSe2 / CoO-CNTs as electrodes respectively;

[0036] Figure 10 Lithium sulfide (Li2S) nucleation curves of carbon paper (CP), CoSe2 / CoO and CoSe2 / CoO-CNTs as electrodes respectively;

[0037] Figure 11CV curves of lithium-sulfur batteries with PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP as separators;

[0038] Figure 12 For Figure 11 Tafel slope plots of the conversion process of S8 to polysulfide with PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP as separators, obtained by fitting;

[0039] Figure 13 For Figure 11 Tafel slope plots of the conversion process of polysulfide to Li2S with PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP as separators, obtained by fitting;

[0040] Figure 14 For Figure 12 and Figure 13 Activation energy plots of the steps of S8 to polysulfide and polysulfide to Li2S with PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP as separators, derived by deduction;

[0041] Figure 15 Charge-discharge curves at 0.1C of lithium-sulfur batteries with PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP as separators;

[0042] Figure 16 Cycling characteristics of lithium-sulfur batteries with PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP as separators;

[0043] Figure 17 Rate capabilities of lithium-sulfur batteries with PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP as separators. Specific Embodiments

[0044] The following examples further illustrate the content of the present invention, but should not be construed as limiting the present invention. Modifications and substitutions made to the methods, steps, or conditions of the present invention without departing from the essence of the present invention all fall within the scope of the present invention.

[0045] Specific Embodiment 1: A cobalt-based compound heterojunction composite material in this embodiment is a composite of CoSe2 / CoO nanorods and carbon nanotubes, having a three-dimensional network structure. The diameter of the carbon nanotubes is 10 - 50 nm, and the wall thickness is 3 - 20 nm. The CoSe2 / CoO nanorods are wrapped by the carbon nanotubes and are discretely and evenly distributed in the conductive carbon nanotubes. The lateral dimension of the CoSe2 / CoO nanorods is 30 - 300 nm, the surface is rough, and has a granular structure. Both CoSe2 and CoO in the CoSe2 / CoO nanorods are p-type semiconductors. The band gap of CoO is 1.9 - 2.2 eV, and the band gap of CoSe2 is 1.5 - 1.8 eV.

[0046] Specific Embodiment 2: This embodiment is a preparation method of a cobalt-based compound heterojunction composite material, which is completed according to the following steps:

[0047] I. Preparation of CoSe2 / CoO nanorods:

[0048] ①. Add Co(NO3)2, NH4F, and urea to deionized water and stir to obtain a mixed solution.

[0049] ②. Transfer the mixed solution to a polytetrafluoroethylene reaction kettle, and then carry out a hydrothermal reaction at 110°C - 130°C to obtain a reaction product. Wash the reaction product with deionized water and then vacuum dry to obtain a precursor.

[0050] ③. Transfer the precursor to a porcelain boat on the downstream side of the center of a tube furnace, and place selenium powder in a corresponding porcelain boat on the upstream side. Subsequently, under an Ar atmosphere, heat the tube furnace from room temperature to 400°C - 450°C, then keep it warm at 400°C - 450°C, and cool to room temperature after the heat preservation ends to obtain CoSe2 / CoO nanorods.

[0051] II. Composite:

[0052] Mix the CoSe2 / CoO nanorods and carbon nanotubes to obtain a mixture. Grind the mixture to obtain a composite of CoSe2 / CoO nanorods and carbon nanotubes, which is the cobalt-based compound heterojunction composite material.

[0053] The difference between Specific Embodiment 3 and Specific Embodiment 2 is that the molar ratio of Co(NO3)2, NH4F, and urea described in step I ① is (3 - 5):(4 - 6):(9 - 11). Other steps are the same as those in Specific Embodiment 2.

[0054] Embodiment 4: The difference between this embodiment and one of Embodiments 2 to 3 is that: the molar ratio of Co(NO3)2 to the volume of deionized water in Step ① is (3 mol - 5 mol):(60 mL - 100 mL). Other steps are the same as those in Embodiments 2 to 3.

[0055] Embodiment 5: The difference between this embodiment and one of Embodiments 2 to 4 is that: the stirring speed in Step ① is 500 r / min - 1500 r / min, and the stirring time is 20 min - 40 min. Other steps are the same as those in Embodiments 2 to 4.

[0056] Embodiment 6: The difference between this embodiment and one of Embodiments 2 to 5 is that: the hydrothermal reaction time in Step ② is 4 h - 6 h. Other steps are the same as those in Embodiments 2 to 5.

[0057] Embodiment 7: The difference between this embodiment and one of Embodiments 2 to 6 is that: the number of washing times in Step ② is 3 - 5 times; the vacuum drying temperature in Step ② is 50°C - 60°C, and the vacuum drying time is 10 h - 12 h. Other steps are the same as those in Embodiments 2 to 6.

[0058] Embodiment 8: The difference between this embodiment and one of Embodiments 2 to 7 is that: the holding time at 400°C - 450°C in Step ③ is 2 h - 3 h; the heating rate in Step ③ is 5°C / min - 10°C / min; the mass ratio of selenium powder to the precursor in Step ③ is 3:1. Other steps are the same as those in Embodiments 2 to 7.

[0059] Embodiment 9: The difference between this embodiment and one of Embodiments 2 to 8 is that: the grinding time in Step ② is 1 h - 1.5 h; the mass ratio of CoSe2 / CoO nanorods to carbon nanotubes in Step ② is 1:1. Other steps are the same as those in Embodiments 2 to 8.

[0060] Embodiment 10: This embodiment is about using a cobalt-based compound heterojunction composite material as a separator or a cathode of a lithium-sulfur battery.

[0061] The following examples are used to verify the beneficial effects of the present invention:

[0062] Example 1: A preparation method of a cobalt-based compound heterojunction composite material is completed according to the following steps:

[0063] I. Preparation of CoSe2 / CoO nanorods:

[0064] ①. Add 4 mmol of Co(NO3)2, 5 mmol of NH4F, and 10 mmol of urea to 70 mL of deionized water, and stir for 30 min at a stirring speed of 1000 r / min to obtain a mixed solution;

[0065] ②. Transfer the mixed solution to a polytetrafluoroethylene reaction kettle, and then carry out a hydrothermal reaction at 120 °C for 6 h to obtain a reaction product; wash the reaction product 3 times with deionized water, and then vacuum dry at 60 °C for 10 h to obtain a precursor;

[0066] ③. Transfer the precursor to a porcelain boat on the downstream side of the center of a tubular furnace, and place selenium powder in the corresponding porcelain boat on the upstream side. Subsequently, under an Ar atmosphere, heat the tubular furnace from room temperature to 450 °C at a rate of 5 °C / min, then hold at 450 °C for 2 h, and after the holding ends, cool to room temperature to obtain CoSe2 / CoO nanorods;

[0067] The mass ratio of the selenium powder to the precursor described in step ①③ is 3:1;

[0068] II. Composite:

[0069] Mix CoSe2 / CoO nanorods and carbon nanotubes to obtain a mixture; grind the mixture for 1 h to obtain a CoSe2 / CoO-CNTs composite, which is a cobalt-based compound heterojunction composite material;

[0070] The mass ratio of the CoSe2 / CoO nanorods to the carbon nanotubes described in step II is 1:1.

[0071] Comparative Example 1: The preparation method of CoO nanorods is completed according to the following steps:

[0072] ①. Add 4 mmol of Co(NO3)2, 5 mmol of NH4F, and 10 mmol of urea to 70 mL of deionized water, and stir for 30 min at a stirring speed of 1000 r / min to obtain a mixed solution;

[0073] ②. Transfer the mixed solution to a polytetrafluoroethylene reaction kettle, and then carry out a hydrothermal reaction at 120 °C for 6 h to obtain a reaction product; wash the reaction product 3 times with deionized water, and then vacuum dry at 60 °C for 10 h to obtain a precursor;

[0074] ③. Transfer the precursor to a porcelain boat in the center of a tubular furnace, and then under an Ar atmosphere, heat the tubular furnace from room temperature to 350 °C at a rate of 5 °C / min, then hold at 350 °C for 1 h, and after the holding ends, cool to room temperature to obtain CoO nanorods.

[0075] Comparative Example 2: The preparation method of CoSe2 nanorods is completed according to the following steps:

[0076] ①. Add 4 mmol of Co(NO3)2, 5 mmol of NH4F, and 10 mmol of urea to 70 mL of deionized water, and stir for 30 min at a stirring speed of 1000 r / min to obtain a mixed solution;

[0077] ②. Transfer the mixed solution to a polytetrafluoroethylene reaction kettle, and then carry out a hydrothermal reaction at 120 °C for 6 h to obtain a reaction product; wash the reaction product 3 times with deionized water, and then vacuum dry at 60 °C for 10 h to obtain a precursor;

[0078] ③. Transfer the precursor to a porcelain boat on the downstream side of the center of a tubular furnace, and place selenium powder in the corresponding porcelain boat on the upstream side. Subsequently, under an Ar atmosphere, heat the tubular furnace from room temperature to 450 °C at a rate of 5 °C / min, and then hold at 450 °C for 2 h. After the holding is completed, cool to room temperature to obtain CoSe2 nanorods;

[0079] The mass ratio of the selenium powder to the precursor described in step ③ is 10:1.

[0080] Figure 1 is the X-ray diffraction spectrum of the CoSe2 / CoO nanorods prepared in Example 1;

[0081] Figure 1 All the diffraction peaks in belong to CoSe2 and CoO. Thus, it can be seen that the CoSe2 / CoO nanorods obtained in step 1③ of Example 1 are CoSe2 / CoO heterojunctions.

[0082] Figure 2 is the low-magnification scanning electron microscope image of the CoSe2 / CoO nanorods prepared in Example 1;

[0083] From Figure 2 it can be seen that the CoSe2 / CoO nanorods obtained in step 1③ of Example 1 have a rod-like structure.

[0084] Figure 3 is the low-magnification transmission electron microscope image of the CoSe2 / CoO nanorods prepared in Example 1;

[0085] From Figure 3 it can be seen that the lateral size of the CoSe2 / CoO nanorods is ~100 nm and the surface is rough.

[0086] Figure 4 is the high-magnification transmission electron microscope image of the CoSe2 / CoO nanorods prepared in Example 1;

[0087] From Figure 4It can be seen that the interplanar spacings of CoSe2 / CoO nanorods are 0.23 nm, 0.26 nm, and 0.21 nm, corresponding to the (211) and (210) planes of CoSe2 and the (200) plane of CoO, respectively, which proves the coexistence of CoSe2 and CoO.

[0088] Figure 5 It is the low-magnification scanning electron microscope image of the CoSe2 / CoO-CNTs composite prepared in Example 1;

[0089] From Figure 5 it can be seen that the CoSe2 / CoO-CNTs composite prepared in Example 1 has a three-dimensional network structure, and the CoSe2 / CoO nanorods are uniformly distributed in the conductive carbon nanotubes.

[0090] Mott-Schottky test:

[0091] The test is to superimpose an AC impedance test with a fixed frequency during a linear potential sweep, and the frequency is selected as 4 kHz. In the experiment, CoO or CoSe2 is used as the working electrode, platinum as the counter electrode, Ag / AgCl as the reference electrode, and 1M KOH solution as the electrolyte, and the test is carried out on a VMP3 electrochemical workstation (BioLogic, France). The semiconductor type of the material can be judged according to the positive or negative of the curve slope, and the Fermi level can be calculated through the intersection of the straight line in the Mott-Schottky curve and the x-axis.

[0092] Figure 6 It is the Mott-Schottky curve of the prepared CoO;

[0093] Figure 7 It is the Mott-Schottky curve of the prepared CoSe2;

[0094] From Figure 6 and Figure 7 it can be seen that the slopes of the Mott-Schottky curves of CoO and CoSe2 are both negative values, so it can be judged that both CoO and CoSe2 are p-type semiconductors; and the intersections of the straight lines in their Mott-Schottky curves and the x-axis are different, indicating that the Fermi levels of CoO and CoSe2 are different. After forming a heterojunction, an internal electric field will spontaneously form at the heterojunction interface to regulate the charge transfer, so that CoO is positively charged and CoSe2 is negatively charged.

[0095] Visible adsorption test:

[0096] A Li2S6 solution (5 mM) was prepared by dissolving lithium sulfide (Li2S) and sulfur in a molar ratio of 1:5 in a mixed solution of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (volume ratio 1:1), and then stirred at 60 °C for a whole day. Then, 10 mg of CNTs, CoSe2 / CoO-CNT, and CoSe2 / CoO were respectively added to 5 mL of the Li2S6 solution under an Ar atmosphere.

[0097] Figure 8 Digital images of Li2S6, (I) CNTs and Li2S6, (II) CoSe2 / CoO-CNTs and Li2S6, (III) CoSe2 / CoO and Li2S6 solutions after standing for 24 hours;

[0098] From Figure 8 It can be seen that the color of the Li2S6 solution with CNTs added has basically not changed, while the color of the Li2S6 solutions with CoSe2 / CoO and CoSe2 / CoO-CNTs added has basically changed from brown-yellow to transparent. These phenomena indicate that the CoSe2 / CoO nanorod and carbon nanotube composite material has a strong adsorption capacity for polysulfide compounds, which can greatly promote the next polysulfide compound conversion process.

[0099] CV test of symmetric cells:

[0100] 2 mg of CoSe2 / CoO-CNTs powder was coated on carbon paper with a diameter of 13 mm as the electrode. Two identical electrodes were used as the working electrode and the counter electrode, the separator was a commercial PP separator (Celgard 2400), and the electrolyte used was 0.1 M Li2S6 solution. The electrochemical performance was tested, and a 2025-type battery was assembled in a glove box filled with argon. The cyclic voltammogram (CV) was tested on a VMP3 electrochemical workstation (BioLogic, France) at a scanning rate of 50 mV / s and a voltage range of -0.8 - 0.8 V. As a comparison, symmetric cells with carbon paper and CoSe2 / CoO as electrodes, and symmetric cells with CoSe2 / CoO-CNTs as electrodes and a blank electrolyte without Li2S6 solution were tested.

[0101] Figure 9 CV curves of Li2S6 symmetric cells with electrodes of carbon paper (CP), CoSe2 / CoO, and CoSe2 / CoO-CNTs respectively;

[0102] From Figure 9It can be seen that the CoSe2 / CoO-CNTs electrode has a stronger current density. In contrast, for the symmetric cell with carbon paper as the electrode and the symmetric cell with CoSe2 / CoO-CNTs as the electrode using a blank electrolyte without Li2S6 solution, the current response is almost zero, indicating that CoSe2 / CoO-CNTs has excellent catalytic activity for the conversion of polysulfide compounds.

[0103] Li2S nucleation test:

[0104] A Li2S8 solution (0.25 M) was prepared by dissolving lithium sulfide (Li2S) and sulfur in a molar ratio of 1:7 into a TEGDME solvent containing 1.0 M LiTFSI, and then stirred at 60 °C for a whole day; 1 mg of CoSe2 / CoO-CNTs powder was coated on a carbon paper with a diameter of 13 mm as the positive electrode, a lithium sheet as the negative electrode, and a commercial PP separator (Celgard 2400) as the separator. The electrolyte used was the Li2S8 solution, and a 2025-type battery was assembled in a glove box filled with argon; 20 μL of the Li2S8 cathode electrolyte was dropped onto the positive electrode, and 20 μL of a blank electrolyte containing 2 wt% LiNO3 was dropped onto the negative electrode; the battery was discharged at a constant current of 0.112 mA to 2.06 V, and then the voltage was maintained at 2.05 V to enable Li2S nucleation; for comparison, the Li2S nucleation curves of carbon paper and CoSe2 / CoO as the positive electrode were tested.

[0105] Figure 10 The lithium sulfide (Li2S) nucleation curves for electrodes of carbon paper (CP), CoSe2 / CoO, and CoSe2 / CoO-CNTs respectively;

[0106] It can be seen from Figure 10 that the capacity of Li2S deposited on the CoSe2 / CoO-CNTs electrode is 550.32 mAh g -1 , significantly higher than that of carbon paper (242.33 mAh g -1 ) and CoSe2 / CoO (342.42 mAh g -1 ); this result indicates that CoSe2 / CoO-CNTs can well promote the conversion of polysulfide lithium to Li2S, and it has a good catalytic effect on inducing the efficient nucleation and growth of Li2S.

[0107] Preparation of a polypropylene separator modified by composite of CoSe2 / CoO nanorods and carbon nanotubes:

[0108] 10 mg of CoSe2 / CoO-CNTs composite and polyvinylidene fluoride (PVDF) binder were uniformly mixed at a mass ratio of 9:1, and 15 mL of N-methylpyrrolidone (NMP) was added and ultrasonicated for 1 hour to form a uniformly dispersed suspension. Then, the CoSe2 / CoO-CNTs composite was drawn onto a commercial polypropylene (PP) separator (Celgard 2400) by vacuum filtration. After the obtained CoSe2 / CoO-CNTs-PP separator was vacuum-dried at 50 °C for 2 h, it was punched into a disc with a diameter of 19 mm; the loading of the modified material on the separator was 0.8 mg cm -2 .

[0109] Assembly and electrochemical testing of lithium-sulfur batteries:

[0110] (1) Preparation of C / S cathode:

[0111] The uniformly mixed C / S (mass ratio of 3:7) was filled with a protective gas and then placed in a reaction kettle. It was heated at 155 °C for 12 h by the melting method. After cooling to room temperature, the obtained C / S mixture was ground evenly with conductive carbon black and PVDF at a mass ratio of 8:1:1, and ultrasonicated for 1 hour using NMP as a solvent. Then, the mixed slurry was coated on aluminum foil as the cathode using an automatic coater. The prepared electrode was dried in a vacuum oven at 60 °C for 12 h. Finally, the electrode sheet was cut into discs with a diameter of 13 mm, and the sulfur loading was 1 mg / cm 2 .

[0112] (2) Preparation of CoSe2 / CoO-CNTs modified composite separator:

[0113] 10 mg of the synthesized CoSe2 / CoO-CNTs composite and PVDF binder were uniformly mixed at a mass ratio of 9:1, and a certain amount of N-methylpyrrolidone (NMP) was added to form a suspension. The suspension was ultrasonicated for 1 h, and then the ultrasonically dispersed and uniform suspension was drawn onto a commercial polypropylene (PP) separator (Celgard 2400) by vacuum filtration. After the obtained CoSe2 / CoO-CNTs separator was vacuum-dried at 50 °C for 2 h, it was punched into a disc with a diameter of 19 mm; the loading of the modified material (CoSe2 / CoO-CNTs composite) on the separator was 0.8 mg cm -2 , obtaining the CoSe2 / CoO-CNTs composite separator; for comparison, the CoSe2 / CoO separator was prepared using the same process, and CoSe2 / CoO nanorods were added when uniformly mixed with the PVDF binder.

[0114] (3) Assembly of lithium-sulfur batteries:

[0115] The electrochemical performance was tested using a battery of model 2025 and assembled in a glove box filled with argon; the C / S composite was used as the positive electrode, the lithium sheet as the negative electrode, and the separators were CoSe2 / CoO-CNTs-PP composite separator, CoSe2 / CoO-PP composite separator, and commercial PP separator (Celgard 2400); the electrolyte used was 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a mixed solution of DOL / DME (volume ratio 1:1) containing 2 wt% LiNO3; the volume ratio of the electrolyte to sulfur was 20 μLmg -1 .

[0116] (4) Electrochemical tests:

[0117] The cyclic voltammetry (CV) curves of the lithium-sulfur battery were tested on a VMP3 electrochemical workstation (BioLogic, France) at a scanning rate of 0.1 mV / s and a voltage range of 1.7 - 2.8 V. The charge and discharge performance tests were carried out on a LAND battery test system (Wuhan, China) at room temperature with a voltage range of 1.7 - 2.8 V. Under constant current charge and discharge, the specific capacities at 0.1C, 0.2C, 0.5C, 1C, and 2C (1C = 1675 mA / g) were measured respectively.

[0118] Figure 11 CV curves of lithium-sulfur batteries with separators of PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP respectively;

[0119] From Figure 11 it can be seen that at 0.1 mV / s, the CV curve area of the lithium-sulfur battery using the CoSe2 / CoO-CNTs composite separator is larger, and the cathodic peak and anodic peak show an obvious inward shift, indicating that the lithium-sulfur battery using the CoSe2 / CoO-CNTs composite separator has lower electrochemical polarization and faster redox kinetics, suggesting that the CoSe2 / CoO-CNTs composite can well anchor polysulfides, weaken the shuttle effect, and improve the battery performance.

[0120] Figure 12 is obtained by fitting Figure 11 Tafel slope diagrams of the conversion process of S8 to polysulfides for lithium-sulfur batteries with separators of PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP respectively;

[0121] From Figure 12It can be seen that during the conversion of S8 to lithium polysulfide, the lithium-sulfur battery using the CoSe2 / CoO-CNTs composite separator has a higher Tafel slope, indicating that the CoSe2 / CoO-CNTs composite can ingeniously slow down the generation of lithium polysulfide and reduce the accumulation of lithium polysulfide in the electrolyte.

[0122] Figure 13 As per Figure 11 Fitted, the Tafel slope diagrams for the conversion of lithium polysulfide to Li2S with PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP separators respectively;

[0123] From Figure 13 It can be seen that during the conversion of lithium polysulfide to Li2S, the lithium-sulfur battery using the CoSe2 / CoO-CNTs composite separator has a lower Tafel slope, indicating that the CoSe2 / CoO-CNTs composite can effectively accelerate the conversion of lithium polysulfide, weaken the shuttle effect, and improve the sulfur utilization rate.

[0124] Figure 14 As per Figure 12 and Figure 13 Derived, the activation energy diagrams for the conversion of S8 to lithium polysulfide and lithium polysulfide to Li2S with PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP separators respectively;

[0125] From Figure 14 It can be known that the activation energy of the lithium-sulfur battery using the CoSe2 / CoO-CNTs composite separator is higher during the conversion of S8 to lithium polysulfide and lower during the conversion of lithium polysulfide to Li2S, indicating that CoSe2 / CoO-CNTs has a selective catalytic effect on the conversion of sulfur species and can better inhibit the shuttle effect from the source.

[0126] Figure 15 The charge-discharge curves at 0.1C for lithium-sulfur batteries with PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP separators respectively;

[0127] From Figure 15 It can be known that at 0.1C, the lithium-sulfur battery using the CoSe2 / CoO-CNTs composite separator exhibits a specific capacity of 1450 mAh g -1 The lithium-sulfur battery using the CoSe2 / CoO modified separator exhibits a specific capacity of 1340 mAh g -1 The specific capacity of the lithium-sulfur battery using a commercial polypropylene separator is 915 mAh g -1Meanwhile, it can be seen that the potential within the dashed line of the lithium-sulfur battery using the CoSe2 / CoO modified separator for the oxidation of Li2S to polysulfide and the conversion of polysulfide to Li2S is relatively small, indicating superior reaction kinetics compared to the other two batteries.

[0128] Figure 16 Cycling characteristics of lithium-sulfur batteries with separators of PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP respectively;

[0129] From Figure 16 it can be seen that at 0.5C, the lithium-sulfur battery using the CoSe2 / CoO-CNTs composite separator has a decay rate of 0.069% per cycle after 450 cycles, the lithium-sulfur battery using the CoSe2 / CoO modified separator has a decay rate of 0.075% per cycle after 450 cycles, while the lithium-sulfur battery using the commercial polypropylene separator has a decay rate of 0.094% per cycle after 450 cycles. Therefore, it shows that the CoSe2 / CoO-CNTs composite can well improve the cycling performance of lithium-sulfur batteries.

[0130] Figure 17 Rate characteristics of lithium-sulfur batteries with separators of PP, CoSe2 / CoO-PP, and CoSe2 / CoO-CNTs-PP respectively.

[0131] From Figure 17 it can be seen that at 2C, the lithium-sulfur battery using the CoSe2 / CoO-CNTs composite separator exhibits a specific capacity of 910 mAh g -1 -1, the lithium-sulfur battery using the CoSe2 / CoO modified separator exhibits a specific capacity of 683 mAh g -1 -1, and the specific capacity of the lithium-sulfur battery using the commercial polypropylene separator is 208 mAh g -1 -1, indicating that the lithium-sulfur battery with the CoSe2 / CoO-CNTs composite modified separator has better rate performance.

[0132] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A cobalt-based compound heterojunction composite material, characterized in that It is a composite of CoSe2 / CoO nanorods and carbon nanotubes, having a three-dimensional network structure. The diameter of the carbon nanotubes is 10-50 nm, and the wall thickness is 3-20 nm. The CoSe2 / CoO nanorods are wrapped by the carbon nanotubes and are discretely and evenly distributed in the conductive carbon nanotubes. The lateral size of the CoSe2 / CoO nanorods is 30-300 nm, the surface is rough, and it has a granular structure. Both CoSe2 and CoO in the CoSe2 / CoO nanorods are p-type semiconductors. The band gap of CoO is 1.9-2.2 eV, and the band gap of CoSe2 is 1.5-1.8 eV.

2. The preparation method of a cobalt-based compound heterojunction composite material according to claim 1, characterized in that it is Completed according to the following steps: I. Preparation of CoSe2 / CoO nanorods: ①. Add Co(NO3)2, NH4F, and urea to deionized water and stir to obtain a mixed solution. ②. Transfer the mixed solution to a polytetrafluoroethylene reaction kettle and then carry out a hydrothermal reaction at 110°C - 130°C to obtain a reaction product. Wash the reaction product with deionized water and then vacuum dry to obtain a precursor. ③. Transfer the precursor to a porcelain boat on the downstream side of the center of a tube furnace, and place selenium powder in the corresponding porcelain boat on the upstream side. Subsequently, under an Ar atmosphere, heat the tube furnace from room temperature to 400°C - 450°C, then keep it warm at 400°C - 450°C, and cool to room temperature after the heat preservation ends to obtain CoSe2 / CoO nanorods. II. Composite: Mix CoSe2 / CoO nanorods and carbon nanotubes to obtain a mixture; grind the mixture to obtain a composite of CoSe2 / CoO nanorods and carbon nanotubes, which is the cobalt-based compound heterojunction composite material.

3. The preparation method of a cobalt-based compound heterojunction composite material according to claim 2, characterized in that In step I ①, the molar ratio of Co(NO3)2, NH4F, and urea is (3 - 5):(4 - 6):(9 - 11).

4. The preparation method of a cobalt-based compound heterojunction composite material according to claim 2, wherein In step I ①, the molar ratio of Co(NO3)2 to the volume of deionized water is (3 mol - 5 mol):(60 mL - 100 mL).

5. The preparation method of a cobalt-based compound heterojunction composite material according to claim 2, wherein In step I ①, the stirring speed is 500 r / min - 1500 r / min, and the stirring time is 20 min - 40 min.

6. The preparation method of a cobalt-based compound heterojunction composite material according to claim 2, characterized in that In step I ②, the hydrothermal reaction time is 4 h - 6 h.

7. The preparation method of a cobalt-based compound heterojunction composite material according to claim 2, wherein In step I ②, the number of washing times is 3 - 5 times; in step I ②, the temperature of the vacuum drying is 50°C - 60°C, and the vacuum drying time is 10 h - 12 h.

8. The preparation method of a cobalt-based compound heterojunction composite material according to claim 2, wherein In step I ③, the heat preservation time at 400°C - 450°C is 2 h - 3 h; in step I ③, the heating rate is 5°C / min - 10°C / min; in step I ③, the mass ratio of selenium powder to the precursor is 3:

1.

9. The preparation method of a cobalt-based compound heterojunction composite material according to claim 2, characterized in that In step II, the grinding time is 1 h - 1.5 h; in step II, the mass ratio of CoSe2 / CoO nanorods to carbon nanotubes is 1:

1.

10. The application of a cobalt-based compound heterojunction composite material as described in claim 1, characterized in that A cobalt-based compound heterojunction composite material is used as a separator or a positive electrode of a lithium-sulfur battery.

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