Composite carbon material with bicrystal TiO2 as well as preparation method and application of composite carbon material

Abstract

The invention discloses a bimorph TiO2 composite carbon material which is prepared by the following steps: by taking Ti3AlC2 as a raw material, preparing single-layer Ti3C2Ox through HF etching, compounding the single-layer Ti3C2Ox with a titanium-based metal organic framework MIL-125 through a hydrothermal reaction, and finally, oxidizing and then carbonizing to obtain the bimorph TiO2 composite carbon material, namely TiO2 / C-TiO2 for short. The basic microstructure of TiO2 / C-TiO2 is in the shape of a round cake, and the size of a single round cake is 0.5-1 [mu] m; and rutile type TiO2 and anatase type TiO2 exist in the TiO2 / C-TiO2 at the same time. The preparation method comprises the following steps: 1, preparing single-layer Ti3C2Ox; 2, preparation of Ti < 3 > C < 2 > O < x > / MIL-125; and 3, the preparation of TiO2 / C-TiO2 is carried out. When the positive electrode material is used as a lithium-sulfur battery positive electrode material, the initial discharge specific capacity is 1200-1300mA h g <-1 > under the condition that the current density is 0.2 C; and under the condition that the current density is 0.2 C, after 200 times of charge-discharge cycles, the residual specific capacity is 700-800 mA h g <-1 >, and the capacity retention ratio is 55-60%.

Description

Technical Field

[0001] The present invention relates to the field of lithium-sulfur battery positive electrode materials, and in particular to a bicrystalline TiO2 composite carbon material, a preparation method thereof, and applications thereof. Background Art

[0002] Lithium sulfur battery has 1675mA hg -1 High theoretical capacity, 2600W h Kg -1 The advantages of high energy density and low cost and abundant resources of sulfur are the advantages of high energy density and low cost of sulfur. However, the conductivity of sulfur in the positive electrode material of lithium-sulfur battery is only 10 -19 Sm -1 , that is, it has insulation properties, polysulfide dissolution in the electrolyte during charging and discharging, and the volume expansion of sulfur, which leads to the problem of reduced capacity and rapid cycle attenuation of lithium-sulfur batteries.

[0003] To address the aforementioned issue of sulfur's insulating properties, materials with high specific surface area and high conductivity, such as carbon nanotubes, graphene, and MXene, can be introduced and composited with active sulfur to improve the overall conductivity of the electrode. For example, in the inventor's previous work, existing document 1 (Xu Fen. Preparation and Application of Recycled Graphene Oxide / Sulfur Sol Composite Microspheres. CN202410414780.8[P].2024-04-08) uses recycled graphite as a raw material to prepare graphene oxide. The graphene oxide dispersion is then used to adsorb sulfur particles from a sulfur sol solution, resulting in a complete coating of the graphene oxide on the surface of the sulfur sol particles, forming a continuous conductive network. This technical solution uses waste graphite as raw material, which is environmentally friendly. However, non-polar carbon materials, primarily graphene and carbon nanomaterials, have a binding energy of 0.5-1.5 eV with lithium polysulfides, while transition metal compounds, primarily metal oxides, sulfides, and nitrides, have a binding energy of 2-4 eV with lithium polysulfides. This significantly increases the binding energy of transition metal compounds with lithium polysulfides compared to non-polar carbon materials, making it impossible for non-polar carbon materials to solve the problem of limiting the dissolution of polysulfides. For example, the electrode material produced by this technical solution exhibits a capacity retention rate of only 70% after 50 charge-discharge cycles at a current density of 0.2C.

[0004] In order to solve the problem that pure carbon materials are difficult to limit the dissolution of lithium polysulfides, a strategy of introducing transition metal compounds can be adopted to use the polarity of transition metal compounds and polysulfides to suppress the shuttle effect. For example, the existing document 2 (Chenguang S, Junlong H, Youchen T, et al. A erarchical porous carbon aerogel embedded with small-sized TiO2 nanoparticles for high-performance Li-Sbatteries [J]. Carbon, 2023, 202: 59-65.) uses chitosan as a carbon precursor and introduces MXene materials at the same time. After carbonization, the polar TiO2 nanoparticles on the surface of MXene are conducive to chemical retention of soluble polysulfides. Although this technical solution has achieved a technical effect of a capacity retention rate of 61.6% after 200 charge and discharge cycles at a current density of 1C. However, the MXene material introduced in this technical solution is a two-dimensional sheet structure, which is prone to stacking, resulting in a decrease in the specific surface area of the material, which is only 244m 2 g -1 Therefore, when it is subsequently compounded with sulfur, it cannot provide sufficient specific surface area for sulfur loading, resulting in a low initial specific capacity of only 832 mA h g at a current density of 1C. -1 .

[0005] In order to take into account both the adsorption capacity of polysulfides and high sulfur loading, MOFs materials with high specific surface area can be introduced. For example, the existing literature 3 (Tongtao W, Enjie W, Changcheng W, et al. MOF-derived micro-mesoporous TiO2-based composite as sulfur host for high-performance lithium-sulfur batteries[J]. Electrochimica Acta, 2023, 439: 141570.) uses Ti-based MIL-125 as a precursor, and after doping with Co element and carbonization, the obtained Co / NC-TiO2 has a surface area of 437m 2 g -1 The high specific surface area of MIL-125 and the TiO2 derived from MIL-125 can also enhance the chemical adsorption capacity of the material for polysulfides. Although this technical solution obtained an initial specific capacity of 943mA hg at a current density of 1C, the initial specific capacity of 943mA hg -1However, the Co / NC-TiO2 prepared by this technical solution only contains anatase TiO2. Although anatase TiO2 has a strong chemical adsorption effect on lithium polysulfide, it has a poor electron mobility of only 0.1 cm 2 / V·s, that is, the derivative carbon obtained by carbonization of MIL-125 alone cannot achieve the requirement of high conductivity. Summary of the Invention

[0006] The purpose of this invention is to provide a twin-crystal TiO2 composite carbon material and its preparation method and application. The invention aims to solve the technical problems existing in the prior art by using Ti3AlC2 as raw material and etching Ti3AlC2 into Ti3C2O x After that, it was added during the hydrothermal synthesis of MIL-125 to form Ti3C2O x The precursor material coated with MIL-125 is pre-oxidized and then carbonized to obtain a round cake-shaped structure material TiO2(R) / C-TiO2(R+A) with rutile and anatase twin crystals of TiO2.

[0007] The specific technical effects are reflected in the following two aspects:

[0008] 1. MIL-125 and Ti3C2O x Both have high specific surface area, Ti3C2O x Coated on the surface of MIL-125, it can avoid the two-dimensional layer structure of Ti3C2O x Excessive stacking leads to a serious loss of specific surface area, and the derived rutile and anatase twinned TiO2 have high electron mobility and strong polarity, respectively, which not only improves the overall conductivity of the material, but also enhances the material's chemical adsorption capacity for lithium polysulfide. After loading sulfur, it can be used as the positive electrode of lithium-sulfur batteries to effectively improve the battery's sulfur loading and cycle performance.

[0009] 2. Ti3C2O x It has high electrical conductivity and can form a continuous conductive network by coating on the surface of MIL-125, which is beneficial to improve the overall electrical conductivity of the material and the utilization rate of sulfur.

[0010] In order to achieve the above object, the specific technical solution for realizing the object of the present invention is:

[0011] A twin-crystal TiO2 composite carbon material is prepared by HF etching using Ti3AlC2 as raw material to prepare a single layer of Ti3C2O x The composite was then compounded with the titanium-based metal organic framework MIL-125 through a hydrothermal reaction. Finally, it was oxidized and then carbonized to obtain a twin-crystal TiO2 composite carbon material, referred to as TiO2 / C-TiO2.

[0012] The basic microscopic morphology of the TiO2 / C-TiO2 is a disc-shaped disk, and the size of a single disc is 0.5-1 μm;

[0013] The TiO2 / C-TiO2 contains both rutile and anatase TiO2.

[0014] A method for preparing a twin-crystal TiO2 composite carbon material comprises the following steps:

[0015] Step 1, single layer Ti3C2O x Preparation, first, LiF and HCl meet a certain mass ratio, under certain conditions, stirring to obtain a mixed solution A, then Ti3AlC2 and the mixed solution A meet a certain mass ratio, under certain conditions, stirring to obtain a mixed solution B, then the mixed solution B is centrifuged and washed-ultrasonicated / centrifuged under certain conditions to obtain a MXene dispersion, finally, the MXene dispersion is freeze-dried under certain conditions to obtain a single layer of Ti3C2O x ;

[0016] In step 1, the mass ratio of LiF to HCl is 2:25;

[0017] In step 1, the stirring conditions are: stirring temperature is 30-50°C, and stirring time is 20-30h;

[0018] In step 1, the conditions for centrifugal washing-ultrasound / centrifugal dispersion are as follows: deionized water is used for both centrifugation and ultrasound, the speed of centrifugal washing is 4000-6000 rpm, the time of centrifugal washing is 5-10 minutes, the number of centrifugal washings is 5-10 times, the ultrasound conditions are as follows: the ultrasound power is 800-1200 W, the ultrasound time is 1-2 hours, the centrifugal speed of centrifugal dispersion is 3000-4000 rpm, and the centrifugal time of centrifugal dispersion is 10-20 minutes; the freeze-drying conditions are -40-60°C, and the freeze-drying time is 48-72 hours;

[0019] Step 2, Ti3C2O x / MIL-125 was prepared by first using Ti3C2O x NN dimethylformamide, methanol, terephthalic acid and tetrabutyl titanate meet a certain mass ratio, stir under certain conditions, then carry out hydrothermal reaction under certain conditions, after the reaction is completed, centrifuge under certain conditions to obtain round cake-shaped Ti3C2O x Coated titanium-based metal-organic framework composite material, referred to as Ti3C2O x / MIL-125;

[0020] In step 2, Ti3C2Ox , N-N-dimethylformamide, methanol, terephthalic acid and tetrabutyl titanate in a mass ratio of 1:540:60:30:15;

[0021] In step 2, the stirring conditions are as follows: the stirring temperature is 30-40° C. and the stirring time is 20-30 min; the hydrothermal reaction conditions are as follows: the hydrothermal temperature is 120-180° C. and the hydrothermal time is 20-30 h; the centrifugal conditions are as follows: washing with N-N dimethylformamide first and then washing with methanol, the washing times are 2-3 times, the centrifugal washing speed is 3000-4000 rpm, and the centrifugal washing time is 5-10 min;

[0022] Step 3, preparation of TiO2 / C-TiO2, first, under certain conditions, the Ti3C2O obtained in step 3 x / MIL-125 is pre-oxidized, and finally, under certain conditions, the sample is subjected to high-temperature carbonization treatment to obtain a twin-crystal TiO2 composite carbon material, referred to as TiO2 / C-TiO2;

[0023] In the step 3, the pre-oxidation conditions are: in an air atmosphere, a heating rate of 2-3°C / min, an oxidation temperature of 300-400°C, and an oxidation time of 2-3h; the high-temperature carbonization conditions are: in an argon atmosphere, a heating rate of 4-5°C / min, an oxidation temperature of 750-850°C, and an oxidation time of 2-3h.

[0024] When the twin-crystal TiO2 composite carbon material is used as the positive electrode material of lithium-sulfur batteries, the initial discharge capacity is 1200-1300 mA hg at a current density of 0.2C. -1 At a current density of 0.2C, after 200 charge and discharge cycles, the remaining capacity is 700-800mA hg -1 , the capacity retention rate is 55-60%.

[0025] The twinned TiO2 composite carbon material obtained by the method of the present invention was tested and the results were as follows:

[0026] XRD test results show that TiO2 / C-TiO2 contains both rutile and anatase TiO2.

[0027] The SEM test results show that the microstructure of TiO2 / C-TiO2 is in the shape of a disc, and the size of a single disc is 0.5-1μm.

[0028] The EIS test results show that the battery assembled by TiO2 / C-TiO2 has a smaller charge transfer resistance and a larger lithium ion diffusion rate.

[0029] The results of cyclic charge and discharge tests show that the battery assembled by TiO2 / C-TiO2 has a higher initial discharge specific capacity and better cycle stability.

[0030] Therefore, the present invention has the following advantages over the prior art:

[0031] 1. The present invention uses a one-step hydrothermal method to convert Ti3C2O x The TiO2 / C layer coated on the surface of MIL-125 after pre-oxidation and carbonization can form a stable conductive network structure, thereby improving the conductivity of the material;

[0032] 2. The present invention adopts a high specific surface area titanium-based metal organic framework MIL-125, which can obtain rutile and anatase twinned TiO2 through pre-oxidation and carbonization, thereby enhancing the electrical conductivity of the material while also improving the chemical adsorption capacity of the material for lithium polysulfide. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Ti3C2O in Example 1 of the present invention x , MIL-125 and Ti3C2O x / XRD pattern of MIL-125;

[0034] Figure 2 Ti3C2O in Example 1 of the present invention x SEM images of

[0035] Figure 3 Ti3C2O in Example 1 of the present invention x / SEM image of MIL-125;

[0036] Figure 4 This is a SEM image of MIL-125 in Example 1 of the present invention;

[0037] Figure 5 XRD patterns of TiO2(R) / C, TiO2(R+A) and TiO2(R) / C-TiO2(R+A) in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention;

[0038] Figure 6 This is the SEM image of TiO2(R) / C-TiO2(R+A) in Example 1 of the present invention;

[0039] Figure 7 EIS graphs of TiO2(R) / C, TiO2(R+A) and TiO2(R) / C-TiO2(R+A) in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention;

[0040] Figure 82 are charge-discharge cycle diagrams of TiO2(R) / C, TiO2(R+A) and TiO2(R) / C-TiO2(R+A) in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention;

[0041] Figure 9 This is the SEM image of TiO2(R+A) in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0042] The present invention is further described in detail through embodiments and in conjunction with the accompanying drawings, but the present invention is not limited thereto.

[0043] Example 1

[0044] A method for preparing a twin-crystal TiO2 composite carbon material comprises the following steps:

[0045] Step 1, single layer Ti3C2O x Preparation, first, under the conditions of stirring temperature of 40 ° C and stirring time of 5 min, 1.6 g LiF and 20 mL 9 M HCl were mixed to obtain mixed solution A, then 1 g Ti3AlC2 was added to the mixed solution A, and stirred at a stirring temperature of 35 ° C and stirring time of 24 h to achieve etching of Ti3AlC2 to obtain mixed solution B, and then the mixed solution B was treated by centrifugal washing-ultrasonication / centrifugal dispersion method to obtain MXene dispersion, finally, the MXene dispersion was freeze-dried under the conditions of freeze-drying temperature of -50 ° C and freeze-drying time of 72 h to obtain a single layer of Ti3C2O x ;

[0046] In the step 1, deionized water is used during centrifugation and ultrasound in the centrifugal washing-ultrasound / centrifugal dispersion method;

[0047] The requirement for the completion of the centrifugal washing is that the pH of the supernatant after centrifugal washing is greater than 6. The specific conditions for the centrifugal washing are as follows: the centrifugal washing speed is 5000 rpm, the centrifugal washing time is 5 minutes, and the number of centrifugal washings is 9 times;

[0048] The requirements for the ultrasonic / centrifugal dispersion are as follows: ultrasonication first and then centrifugation. The specific ultrasonic conditions are as follows: ultrasonic power of 1000W, ultrasonic time of 1 hour, centrifugal speed of centrifugal dispersion of 3500rpm, and centrifugal time of centrifugal dispersion of 10 minutes;

[0049] In order to prove that Ti3C2O x The composition of the product was tested by XRD. The test results are as follows Figure 1 As shown, Ti3C2O x Only a single layer of Ti3C2O exists xcharacteristic peaks.

[0050] In order to prove that Ti3C2O x The microstructure of the SEM test was carried out. The test results are as follows Figure 2 As shown, by ultrasonic treatment, Ti3C2O x The microstructure is a nanosheet structure.

[0051] Step 2, Ti3C2O x Preparation of MIL-125: First, 9 mL of N-N-dimethylformamide and 1 mL of methanol were mixed at a stirring temperature of 35 °C for 5 min to obtain a mixture C. Then, 0.5 g of terephthalic acid and 0.017 g of the Ti3C2O obtained in step 1 were added. x Add to mixed solution C, stir at a stirring temperature of 35 ° C, stirring time for 10 min to obtain mixed solution D, then add 0.26 mL of tetrabutyl titanate to mixed solution D, stir at a stirring temperature of 35 ° C, stirring time for 5 min to obtain reaction solution E, then, hydrothermally react the reaction solution E at a hydrothermal temperature of 150 ° C, hydrothermally react for 24 h, after the reaction is completed, centrifuge the reaction product at a centrifugal speed of 3500 rpm, centrifuge time for 5 min, and finally dry at a drying temperature of 60 ° C, drying time for 12 h to obtain round cake Ti3C2O x / MIL-125 composite material, referred to as Ti3C2O x / MIL-125;

[0052] In step 2, the centrifugal washing requirement is to first wash with N-dimethylformamide and then wash with methanol, and the number of washing times is 3 times;

[0053] In order to prove that Ti3C2O x / MIL-125 composition, and perform XRD test. At the same time, for comparison, step 1 is not performed, and MIL-125 is directly prepared and XRD test is performed as a reference. The test results are as follows Figure 1 As shown,

[0054] MIL-125 only has the characteristic peaks of MIL-125;

[0055] Ti3C2O x / MIL-125 has a single layer of Ti3C2O x There are characteristic peaks of MIL-125, among which, due to the x The content is small, so the intensity of the characteristic peak is low;

[0056] By comparison, it can be seen that the Ti3C2O prepared in step 2 x / MIL-125 also contains Ti3C2O x and MIL-125.

[0057] In order to prove that Ti3C2O x / MIL-125 micromorphology, SEM test, the test results are as follows Figure 3 At the same time, for comparison, step 1 was not performed and MIL-125 was directly prepared for SEM test as a reference. The test results are shown in Figure 4 As shown,

[0058] The microstructure of MIL-125 is a disc-shaped structure, with the size of a single disc being 1-2 μm, and the surface of the disc-shaped structure is smooth;

[0059] Ti3C2O x The microstructure of MIL-125 also maintains a disc-shaped structure, but the size of a single disc is 3 μm. At the same time, there are a large number of nanosheets on the surface of the disc-shaped structure. Combined with the XRD test results, it can be seen that the nanosheets are Ti3C2O x ;

[0060] By comparison, it can be seen that the Ti3C2O prepared in step 2 x / MIL-125 is Ti3C2O x Nanosheets are coated on MIL-125, and due to the coating of Ti3C2O x nanosheets, resulting in Ti3C2O x / MIL-125 is larger than MIL-125.

[0061] Step 3, preparation of TiO2 / C-TiO2, first, under air conditions, at a heating rate of 2 ° C / min, an oxidation temperature of 300 ° C, and an oxidation time of 3 h, the Ti3C2O obtained in step 3 was oxidized to form TiO2 / C-TiO2. x / MIL-125 is pre-oxidized and then carbonized under argon at a heating rate of 4°C / min, a carbonization temperature of 800°C and a carbonization time of 3h to obtain a twin-crystal TiO2 composite carbon material, referred to as TiO2 / C-TiO2.

[0062] In order to prove the composition and crystal form of TiO2 / C-TiO2, XRD test was carried out. The test results are as follows Figure 5 As shown in the figure, in addition to the characteristic peaks of amorphous carbon, TiO2 / C-TiO2 also has the characteristic peaks of rutile and anatase TiO2.

[0063] Therefore, in order to facilitate the distinction of crystal forms, rutile is abbreviated as R, that is, rutile TiO2 is abbreviated as TiO2(R). Similarly, anatase is abbreviated as A, that is, anatase TiO2 is abbreviated as TiO2(R). It is particularly noted that TiO2 with both rutile and anatase forms is abbreviated as TiO2(R+A).

[0064] The test results show that the TiO2 / C-TiO2 prepared in step 3 is TiO2(R) / C-TiO2(R+A).

[0065] In order to prove the microstructure of TiO2(R) / C-TiO2(R+A), SEM test was carried out. The test results are as follows Figure 6 As shown in the figure, the microstructure of TiO2(R) / C-TiO2(R+A) also maintains a disc-shaped structure, but the size of a single disc is 0.5-2μm, and the surface of the disc-shaped structure is rough;

[0066] By comparing the Ti3C2O obtained in step 2 x / MIL-125, it can be seen that the particle size of TiO2(R) / C-TiO2(R+A) prepared by step 3 is significantly reduced. The reason is that in the pre-oxidation stage of step 3, Ti3C2O x / The organic ligands of MIL-125 will undergo oxidative decomposition.

[0067] In order to demonstrate the performance of TiO2(R) / C-TiO2(R+A) as a cathode material for lithium-sulfur batteries, lithium-sulfur batteries were assembled for electrochemical performance tests, specifically EIS tests and cyclic charge-discharge tests. The specific method for assembling a lithium-sulfur battery is as follows: first, sulfur powder S and a sample to be tested are ground at a mass ratio of 7:3 and a grinding time of 30 minutes, and a sulfur-loaded composite material is obtained by a conventional melt diffusion method; then, the sulfur-loaded composite material is used as a positive electrode, a lithium sheet is used as a negative electrode, the diameter of the electrode sheet is 9 mm, Celgard 2500 is used as a separator, the lithium salt concentration in the electrolyte is 1M LiTFSI, the solvent in the electrolyte is a mixture of DOL and DME, and the volume ratio of DOL:DME is 1:1, the additive in the electrolyte is 1.0wt.% LiNO3, and the liquid sulfur ratio E / S is 25:1. Since there is no need to distinguish, the obtained lithium-sulfur battery is also referred to as the positive electrode active material, that is, the lithium-sulfur battery based on the TiO2(R) / C-TiO2(R+A) positive electrode in specific embodiment 1 is also named TiO2(R) / C-TiO2(R+A).

[0068] The EIS test results of TiO2(R) / C-TiO2(R+A) are as follows Figure 7As shown, the semicircle part corresponds to the charge transfer resistance R during the battery charging and discharging process. ct , the straight line portion corresponds to the lithium ion diffusion rate. Through the equivalent circuit fitting, it can be seen that the charge transfer resistance R of TiO2(R) / C-TiO2(R+A) ct The test results show that the TiO2(R) / C-TiO2(R+A) electrode has a smaller charge transfer resistance and a larger lithium ion diffusion rate.

[0069] The cyclic charge and discharge test results of TiO2(R) / C-TiO2(R+A) are as follows Figure 8 As shown, under the conditions of test voltage range of 1.7-2.8V and current density of 0.2C, the initial discharge capacity is 1263mA hg -1 When the cycle number is 200, the remaining discharge capacity is 708mA hg -1 , the capacity retention rate is 56%; indicating that 1C = 1675mA hg -1 .

[0070] In order to prove that Ti3C2O x and MIL-125 on the performance, that is, the role of TiO2(A) / C-TiO2(B) structure in the technical solution, provide comparative examples 1 and 2, respectively, only using Ti3C2O x Or MIL-125 is used as a precursor to prepare lithium-sulfur battery positive electrode composite materials.

[0071] Comparative Example 1

[0072] A Ti3C2O-based x The preparation method of the TiO2(R) / C material is the same as that of Example 1, except that step 2 is not required, and in step 3, Ti3C2O x Replace Ti3C2O x / MIL-125 was pre-oxidized and subsequently carbonized, and the resulting material was named TiO2(R) / C. Furthermore, the lithium-sulfur battery based on the TiO2(R) / C positive electrode was also named TiO2(R) / C.

[0073] In order to prove the composition of TiO2(R) / C, XRD test was carried out. The test results are as follows Figure 5 As shown, TiO2(R) / C has characteristic peaks of rutile TiO2 in addition to the characteristic peaks of amorphous carbon;

[0074] Compared with the TiO2(R) / C-TiO2(R+A) obtained in Example 1, it can be seen that TiO2(R) / C-TiO2(R+A) contains not only rutile TiO2 with good conductivity, but also anatase TiO2 with strong adsorption capacity for lithium polysulfide.

[0075] The EIS test results of TiO2(R) / C are as follows Figure 7 As shown in the figure, the charge transfer resistance R of TiO2(R) / C is obtained by fitting the equivalent circuit diagram. ct Compared with Example 1, it can be seen that the TiO2(R) / C-TiO2(R+A) electrode is beneficial to reducing the charge transfer resistance and increasing the diffusion rate of lithium ions.

[0076] The cyclic charge and discharge test results of TiO2(R) / C are as follows Figure 8 As shown in Figure 2, under the same test conditions, the initial discharge capacity of TiO2(R) / C reached 866 mA hg -1 , after 200 charge and discharge cycles, the remaining current is 533mA hg -1 The capacity retention rate after the first charge-discharge cycle was 61%. Comparison of the test results with those of Example 1 shows that the TiO2(R) / C-TiO2(R+A) containing both rutile and anatase types has a stronger adsorption effect on lithium polysulfide, thereby reducing the loss of active materials and having a higher specific capacity.

[0077] Comparative Example 2

[0078] A method for preparing a TiO2(R+A) material based on MIL-125, wherein the steps not specifically described are the same as those in Example 1, except that: Step 1 is not required, and in Step 3, MIL-125 is used instead of Ti3C2O x / MIL-125 was pre-oxidized and subsequently carbonized, and the resulting material was named TiO2(R+A). Furthermore, the lithium-sulfur battery based on the TiO2(R+A) positive electrode was also named TiO2(R+A).

[0079] In order to prove the composition of TiO2(R+A), XRD test was carried out. The test results are as follows Figure 5 As shown, TiO2(R+A) has characteristic peaks of both rutile and anatase TiO2. Compared with Example 1, TiO2(R) / C-TiO2(R+A) has characteristic peaks of amorphous carbon.

[0080] In order to verify the microstructure of TiO2(R+A), SEM test was carried out. The test results are as follows Figure 9As shown, TiO2(R+A) is in the form of smooth, round discs, with individual discs measuring 0.2-0.5 μm. Compared to Example 1, TiO2(R) / C-TiO2(R+A) is larger and has a rougher surface. This difference impacts the electrode, resulting in TiO2(R) / C having higher conductivity and forming a continuous conductive network when coated on the discs.

[0081] The EIS test results of TiO2(R+A) are as follows Figure 7 As shown, by fitting the equivalent circuit diagram, the charge transfer resistance R of TiO2(R+A) ct It is 10.43Ω. Compared with Example 1, it can be seen that the TiO2(R) / C-TiO2(R+A) electrode is beneficial to reducing the charge transfer resistance and increasing the diffusion rate of lithium ions.

[0082] The cyclic charge and discharge test results of TiO2(R+A) are as follows Figure 8 As shown in Figure 2, under the same test conditions, the initial discharge capacity of TiO2(R+A) reaches 1079mA hg -1 After 200 charge and discharge cycles, the remaining capacity is 584 mAh g -1 The capacity retention rate after the first charge-discharge cycle was 54%. Comparing the test results with those of Example 1, it can be seen that the surface of TiO2(R) / C-TiO2(R+A) is coated with TiO2(R) / C with higher conductivity, which is beneficial to improving the overall conductivity of the material and thus increasing the specific capacity of the battery.

[0083] The following conclusions can be drawn by comparing Example 1, Comparative Example 1 and Comparative Example 2:

[0084] 1. Compared with Ti3C2O x TiO2(R) / C and TiO2(R) / C-TiO2(R+A) have both rutile and anatase TiO2, which have stronger catalytic effect on the adsorption of lithium polysulfide;

[0085] 2. Compared with TiO2(R+A) based on MIL-125, the surface of TiO2(R) / C-TiO2(R+A) has a continuous conductive network structure. The higher conductivity is conducive to improving the utilization rate of sulfur, thereby improving battery performance.

Claims

1. A twin-crystal TiO2 composite carbon material, characterized by: Using Ti3AlC2 as raw material, a single layer of Ti3C2O was prepared by HF etching. x It is then composited with the titanium-based metal-organic framework MIL-125 through a hydrothermal reaction. Finally, it is oxidized and then carbonized to obtain a twin-crystal TiO2 composite carbon material, referred to as TiO2 / C-TiO2.

2. The twinned TiO2 composite carbon material according to claim 1, wherein: The basic microscopic morphology of the TiO2 / C-TiO2 is a disk-shaped disk, and the size of a single disk is 0.5-1 μm.

3. The twinned TiO2 composite carbon material according to claim 1, wherein: The TiO2 / C-TiO2 contains both rutile and anatase TiO2.

4. A method for preparing a twin-crystal TiO2 composite carbon material, characterized in that The following steps are involved: Step 1, single layer Ti3C2O x Preparation, first, LiF and HCl meet a certain mass ratio, under certain conditions, stirring to obtain a mixed solution A, then Ti3AlC2 and the mixed solution A meet a certain mass ratio, under certain conditions, stirring to obtain a mixed solution B, then the mixed solution B is centrifuged and washed-ultrasonicated / centrifuged under certain conditions to obtain a MXene dispersion, finally, the MXene dispersion is freeze-dried under certain conditions to obtain a single layer of Ti3C2O x ; Step 2, Ti3C2O x / MIL-125 was prepared by first using Ti3C2O x NN dimethylformamide, methanol, terephthalic acid and tetrabutyl titanate meet a certain mass ratio, stir under certain conditions, then carry out hydrothermal reaction under certain conditions, after the reaction is completed, centrifuge under certain conditions to obtain round cake-shaped Ti3C2O x Coated titanium-based metal-organic framework composite material, referred to as Ti3C2O x / MIL-125; Step 3, preparation of TiO2 / C-TiO2, first, under certain conditions, the Ti3C2O obtained in step 3 x / MIL-125 is pre-oxidized and finally, under certain conditions, the sample is subjected to high-temperature carbonization treatment to obtain a twin-crystal TiO2 composite carbon material, referred to as TiO2 / C-TiO2.

5. The preparation method according to claim 4, characterized in that: In the step 1, the mass ratio of LiF to HCl is 2:25; In step 1, the stirring conditions are: stirring temperature is 30-50°C, and stirring time is 20-30h; In step 1, the conditions for centrifugal washing-ultrasound / centrifugal dispersion are as follows: deionized water is used during both centrifugation and ultrasound, the speed of centrifugal washing is 4000-6000 rpm, the time of centrifugal washing is 5-10 minutes, the number of centrifugal washings is 5-10 times, the conditions for ultrasound are as follows: the ultrasonic power is 800-1200 W, the ultrasound time is 1-2 hours, the centrifugal speed of centrifugal dispersion is 3000-4000 rpm, and the centrifugal time of centrifugal dispersion is 10-20 minutes; the conditions for freeze drying are -40--60°C, and the freeze drying time is 48-72 hours.

6. The preparation method according to claim 4, characterized in that: In step 2, Ti3C2O x , N-N-dimethylformamide, methanol, terephthalic acid and tetrabutyl titanate in a mass ratio of 1:540:60:30:15; In step 2, the stirring conditions are as follows: the stirring temperature is 30-40°C and the stirring time is 20-30 min; the hydrothermal reaction conditions are as follows: the hydrothermal temperature is 120-180°C and the hydrothermal time is 20-30 h; the centrifugal conditions are as follows: first washing with NN dimethylformamide and then washing with methanol, the number of washings is 2-3 times, the centrifugal washing speed is 3000-4000 rpm, and the centrifugal washing time is 5-10 min.

7. The preparation method according to claim 4, characterized in that: In step 3, the pre-oxidation conditions are: in an air atmosphere, a heating rate of 2-3°C / min, an oxidation temperature of 300-400°C, and an oxidation time of 2-3h; the high-temperature carbonization conditions are: in an argon atmosphere, a heating rate of 4-5°C / min, an oxidation temperature of 750-850°C, and an oxidation time of 2-3h.

8. The twin-crystal TiO2 composite carbon material according to claim 1, characterized in that: When used as a cathode material for lithium-sulfur batteries, the initial discharge capacity is 1200-1300 mA hg at a current density of 0.2C. -1 .

9. The twin-crystal TiO2 composite carbon material according to claim 1, characterized in that: When used as a cathode material for lithium-sulfur batteries, at a current density of 0.2C, after 200 charge and discharge cycles, the remaining specific capacity is 700-800 mA hg -1 , the capacity retention rate is 55-60%.

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