An in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, its preparation method and application

Abstract

This invention provides an in-situ carbon-nitrogen-coated titanium pyrophosphate-lithium titanium phosphate composite material, its preparation method, and its application, belonging to the field of lithium-ion battery technology. The invention involves first preparing titanium pyrophosphate powder, and then in-situ preparing carbon-nitrogen-coated lithium titanium phosphate on the surface of the titanium pyrophosphate powder to obtain the in-situ carbon-nitrogen-coated titanium pyrophosphate-lithium titanium phosphate composite material. This in-situ carbon-nitrogen-coated titanium pyrophosphate-lithium titanium phosphate composite material exhibits good stability and uniformity, solving the problem of poor uniformity and stability of composite powders prepared by traditional composite methods. When used as a negative electrode material for lithium-ion batteries, it possesses high initial discharge specific capacity and good cycle stability and rate performance. Furthermore, the preparation method of the in-situ carbon-nitrogen-coated titanium pyrophosphate-lithium titanium phosphate composite material is simple and easy for industrial production.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to an in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, its preparation method, and its application. Background Technology

[0002] Organic electrolyte lithium-ion batteries have been widely studied and applied in recent years due to their outstanding electrochemical performance. However, organic electrolytes have significant safety risks, are relatively expensive, require sophisticated packaging, and also have inherent drawbacks such as slow lithium-ion migration rates and high environmental pollution.

[0003] To address the aforementioned shortcomings of organic electrolytes, researchers have proposed the concept of aqueous lithium-ion batteries using water as the electrolyte solvent, leveraging the greenest, safest, and most sustainable water as the solvent. However, due to the narrow stability potential window of aqueous solutions, even in relatively mature aqueous lithium-ion batteries, anode materials with excellent electrochemical performance are very rare. The well-known NASICON-type lithium titanium phosphate, while possessing a good charge-discharge platform and suitable lithium insertion / extraction potential, suffers from low electronic conductivity in its pure phase, resulting in rapid capacity decay in aqueous electrolytes. Its cycle performance and rate performance require further improvement.

[0004] Therefore, the present invention aims to provide an in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material and its preparation method, so as to improve the problem of poor performance stability of current lithium titanium phosphate anode materials. Summary of the Invention

[0005] To address the shortcomings of the existing technologies, this invention provides an in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, its preparation method, and its application, solving the problems of low electronic conductivity, poor cycle stability, and poor rate performance that occur in the application of current single lithium titanium phosphate anode materials.

[0006] To achieve the above objectives, the specific technical solution of the present invention is as follows:

[0007] In a first aspect, the present invention provides an in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, the composite material comprising titanium pyrophosphate and carbon-nitrogen coated lithium titanium phosphate, wherein the carbon-nitrogen coated lithium titanium phosphate is coated on the surface of titanium pyrophosphate.

[0008] Furthermore, the molar ratio of lithium titanium phosphate to titanium pyrophosphate is 1:(0.01-0.5).

[0009] Secondly, the present invention provides a method for preparing the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, comprising the following steps:

[0010] A lithium source, titanium source, phosphorus source, and nitric acid are mixed evenly in an alcohol solvent to obtain a lithium titanium phosphate precursor solution.

[0011] Titanium pyrophosphate powder and carbon-nitrogen source were added sequentially to lithium titanium phosphate precursor solution and mixed evenly to obtain composite precursor suspension.

[0012] The composite precursor suspension was filtered, and the filter residue was dried and sintered at 500-900 °C for 2-10 h under inert gas protection to obtain an in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material.

[0013] Furthermore, the titanium pyrophosphate powder is obtained by purchasing or self-making. In a specific example of the present invention, self-made titanium pyrophosphate powder is used.

[0014] Furthermore, in the lithium titanium phosphate precursor solution, the molar ratio of lithium source, titanium source and phosphorus source is (0.8-1.2):2:3, and the mass ratio of phosphorus source to nitric acid is 1:(0.1-2).

[0015] Furthermore, in the composite precursor suspension, the molar ratio of lithium source, titanium pyrophosphate powder, and carbon-nitrogen source is 1:(0.01-0.5):(0.1-0.8). When the carbon-nitrogen source is polyvinylpyrrolidone, the calculation is based on the molar amount of N-vinylpyrrolidone (C6H9NO).

[0016] Furthermore, the average particle size of the titanium pyrophosphate powder is 50-200 nm.

[0017] Furthermore, the lithium source includes, but is not limited to, at least one of lithium acetate, lithium nitrate, and lithium ethoxide; the titanium source includes, but is not limited to, at least one of tetrabutyl titanate, tetraisopropyl titanate, tetraethyl titanate, and tetraethyl titanate; the phosphorus source includes, but is not limited to, at least one of phosphoric acid, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate; and the carbon and nitrogen source includes, but is not limited to, at least one of polyvinylpyrrolidone, urea, and melamine.

[0018] The alcohol solvents include, but are not limited to, at least one of methanol, ethanol, and isopropanol.

[0019] Furthermore, the drying process is freeze drying.

[0020] Specifically, the preparation method of the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material includes two parts: the first part is the preparation of titanium pyrophosphate powder, and the second part is the in-situ preparation of the carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material. The specific steps are as follows:

[0021] S1. Preparation of raw material solution:

[0022] Lithium source solution, titanium source solution, mixed solution of phosphorus source and nitric acid, and carbon-nitrogen source solution were prepared using alcohol solvents.

[0023] The lithium source solution has a mass fraction of 5%-20%; the titanium source solution has a mass fraction of 5%-20%; the mixed solution of phosphorus source and nitric acid has a mass fraction of 5%-20%, wherein the mass ratio of phosphorus source to nitric acid is 1:(0.1-2); and the carbon-nitrogen source solution has a mass fraction of 2%-10%.

[0024] S2. Preparation of titanium pyrophosphate precursor solution:

[0025] Under stirring at room temperature, the titanium source solution was added to a mixed solution of phosphorus source and nitric acid and mixed evenly to obtain titanium pyrophosphate precursor solution.

[0026] In the titanium pyrophosphate precursor solution, the molar ratio of titanium source to phosphorus source is (0.4-0.6):1.

[0027] S3. Preparation of titanium pyrophosphate powder:

[0028] Titanium pyrophosphate precursor colloid was prepared into spherical particles by spray drying, and then heat-treated at 400-600 ℃ for 3-6 h under inert gas protection to obtain titanium pyrophosphate powder with an average particle size of 50-200 nm. The inlet air temperature during the spray drying process was 80-120 ℃, the outlet air temperature was 70-110 ℃, and the feed rate was 5-20 mL / min.

[0029] S4. Preparation of lithium titanium phosphate precursor solution:

[0030] Under stirring at room temperature, lithium source solution and titanium source solution are added sequentially to a mixed solution of phosphorus source and nitric acid and mixed evenly to obtain lithium titanium phosphate precursor solution;

[0031] In the lithium titanium phosphate precursor solution, the molar ratio of lithium source, titanium source and phosphorus source is (0.8-1.2):2:3.

[0032] S5. Preparation of composite precursor suspension:

[0033] Under stirring at room temperature, titanium pyrophosphate powder and carbon-nitrogen source solution were added sequentially to lithium titanium phosphate precursor solution and mixed evenly to obtain composite precursor suspension.

[0034] In the composite precursor suspension, the molar ratio of lithium source, titanium pyrophosphate powder, and carbon-nitrogen source is 1:(0.01-0.5):(0.1-0.8).

[0035] S6. Preparation of carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material:

[0036] The composite precursor suspension was filtered, and the filter residue was freeze-dried and then sintered at 500-900 ℃ for 2-10 h under inert gas protection to obtain in-situ carbon and nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material.

[0037] In the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, the molar ratio of lithium titanium phosphate to titanium pyrophosphate is 1:(0.01-0.5).

[0038] Thirdly, the present invention provides the application of the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material in lithium-ion batteries.

[0039] Fourthly, the present invention also provides a lithium-ion battery, wherein the negative electrode active material is the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material.

[0040] Compared with the prior art, the advantages of the present invention are:

[0041] (1) The in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material provided by the present invention can effectively improve the performance of electrode active powder and significantly improve the cycle stability and rate performance of lithium-ion batteries.

[0042] (2) The in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material prepared by the present invention has good stability and uniformity, which solves the problem of poor uniformity and stability of composite powder prepared by traditional composite methods. At the same time, the preparation method of the present invention is simple and convenient, which simplifies the composite material preparation process, greatly improves the preparation efficiency, and reduces the production cost. Attached Figure Description

[0043] Figure 1 A flowchart illustrating the preparation process of in-situ carbon and nitrogen-coated titanium pyrophosphate-lithium titanium phosphate composite material;

[0044] Figure 2 The X-ray diffraction pattern of the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material of Example 1;

[0045] Figure 3 Scanning electron microscope images of in-situ carbon and nitrogen coated lithium titanium phosphate (a), titanium pyrophosphate (b), and in-situ carbon and nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material (c) of Example 1.

[0046] Figure 4 Cyclic voltammetry curves of lithium metal-pair batteries assembled using the in-situ carbon-nitrogen coated lithium titanium phosphate electrode material of Comparative Example 1.

[0047] Figure 5 Cyclic voltammetry curves of lithium metal-pair batteries assembled using the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material of Example 1;

[0048] Figure 6 The charge-discharge curves of the lithium metal pair battery assembled using the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material of Example 1 are shown.

[0049] Figure 7 The cycling performance curves of the lithium metal pair battery assembled using the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material of Example 1 are shown.

[0050] Figure 8 The rate performance curves of lithium metal batteries assembled from the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material of Example 1 and the in-situ carbon-nitrogen coated lithium titanium phosphate electrode material of Comparative Example 1 are shown. Detailed Implementation

[0051] To enable those skilled in the art to clearly and completely understand the technical solution of the present invention, the present invention will be further described in detail below with reference to embodiments. Obviously, the embodiments described herein are only for explaining the present invention and are not intended to limit the scope of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0052] The present invention provides an in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, the composite material comprising titanium pyrophosphate and carbon-nitrogen coated lithium titanium phosphate, wherein the carbon-nitrogen coated lithium titanium phosphate is coated on the surface of titanium pyrophosphate.

[0053] In some examples, the molar ratio of lithium titanium phosphate to titanium pyrophosphate is 1:(0.01-0.5).

[0054] In the following specific example, the preparation process of the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material is as follows: Figure 1 As shown, the specific steps are as follows:

[0055] S1. Preparation of raw material solution:

[0056] The lithium source is added to an alcohol solvent and stirred to disperse and dissolve to obtain a lithium source solution;

[0057] The titanium source is added to an alcohol solvent and stirred to disperse and dissolve to obtain a titanium source solution;

[0058] Phosphorus source and concentrated nitric acid are added sequentially to an alcohol solvent, and the mixture is stirred to disperse and dissolve to obtain a mixed solution of phosphorus source and nitric acid;

[0059] A carbon-nitrogen source is added to an alcohol solvent and stirred to disperse and dissolve it to obtain a carbon-nitrogen source solution.

[0060] The lithium source solution has a mass fraction of 5%-20%; the titanium source solution has a mass fraction of 5%-20%; the mixed solution of phosphorus source and nitric acid has a mass fraction of 5%-20%, wherein the mass ratio of phosphorus source to nitric acid is 1:(0.1-2); and the carbon and nitrogen source has a mass fraction of 2%-10%.

[0061] The lithium source includes, but is not limited to, at least one of lithium acetate, lithium nitrate, and lithium ethoxide; the titanium source includes, but is not limited to, at least one of tetrabutyl titanate, tetraisopropyl titanate, tetraethyl titanate, and tetraethyl titanate; the phosphorus source includes, but is not limited to, at least one of phosphoric acid, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate; the carbon and nitrogen source includes, but is not limited to, at least one of polyvinylpyrrolidone, urea, and melamine; and the alcohol solvent includes, but is not limited to, at least one of methanol, ethanol, and isopropanol.

[0062] S2. Preparation of titanium pyrophosphate precursor solution:

[0063] Under stirring at room temperature, the titanium source solution is added to a mixed solution of phosphorus source and nitric acid and mixed evenly to obtain a titanium pyrophosphate precursor solution; the molar ratio of titanium source to phosphorus source in the titanium pyrophosphate precursor solution is (0.4-0.6):1.

[0064] S3. Preparation of titanium pyrophosphate powder:

[0065] After preparing spherical particles from titanium pyrophosphate precursor colloid by spray drying, the particles were heat-treated at 400-600℃ for 3-6 h under inert gas protection to obtain titanium pyrophosphate powder with an average particle size of 50-200 nm.

[0066] The inlet air temperature during the spray drying process is 80-120 ℃, the outlet air temperature is 70-110 ℃, and the feed rate is 5-20 mL / min.

[0067] S4. Preparation of lithium titanium phosphate precursor solution:

[0068] Under stirring at room temperature, lithium source solution and titanium source solution are added sequentially to a mixed solution of phosphorus source and nitric acid and mixed evenly to obtain lithium titanium phosphate precursor solution; the molar ratio of lithium source, titanium source and phosphorus source in the lithium titanium phosphate precursor solution is (0.8-1.2):2:3.

[0069] S5. Preparation of composite precursor suspension:

[0070] Under stirring at room temperature, titanium pyrophosphate powder and carbon-nitrogen source solution are sequentially added to a lithium titanium phosphate precursor solution and mixed evenly to obtain a composite precursor suspension. The molar ratio of lithium source, titanium pyrophosphate powder, and carbon-nitrogen source in the composite precursor suspension is 1:(0.01-0.5):(0.1-0.8). This invention controls the hydrolysis rate of the lithium titanium phosphate precursor by controlling the order of addition, thereby ensuring the dispersion effect of titanium pyrophosphate powder in the solution. The inventors discovered that adding the carbon-nitrogen source solution first to the lithium titanium phosphate precursor solution accelerates the hydrolysis of the lithium titanium phosphate precursor solution, thus affecting the dispersion effect of titanium pyrophosphate powder in the solution.

[0071] S6. Preparation of carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material:

[0072] The composite precursor suspension was filtered, and the filter residue was freeze-dried and then sintered at 500-900 °C for 2-10 h under inert gas protection to obtain an in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material; in the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, the molar ratio of lithium titanium phosphate to titanium pyrophosphate is 1:(0.01-0.5).

[0073] Example 1

[0074] An in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material is prepared by the following steps:

[0075] Step 1, Preparation of raw material solutions: Using anhydrous ethanol as the solvent, prepare 10% lithium acetate solution, 10% tetrabutyl titanate solution, 10% phosphoric acid-nitric acid mixed solution, and 5% polyvinylpyrrolidone solution. In this embodiment, concentrated phosphoric acid and concentrated nitric acid are used to prepare the phosphoric acid-nitric acid mixed solution, with a mass ratio of concentrated phosphoric acid to concentrated nitric acid of 1:0.5. The concentration of the concentrated phosphoric acid used is 85%, and the water it contains will affect the hydrolysis of the raw materials. In this embodiment, the addition of concentrated nitric acid can inhibit the hydrolysis of the raw materials without introducing other impurities.

[0076] Step 2, Preparation of titanium pyrophosphate precursor solution: Under stirring at room temperature, add tetrabutyl titanate solution to phosphoric acid-nitric acid mixed solution for 5 min. After addition, continue stirring for 30 min to obtain titanium pyrophosphate precursor solution; wherein, the molar ratio of tetrabutyl titanate to phosphoric acid is 1:2.

[0077] Step 3, Preparation of titanium pyrophosphate powder: The titanium pyrophosphate precursor solution was prepared into spherical particles by spray drying (inlet air temperature of 100 ℃, outlet air temperature of 80 ℃, and feed rate of 10 mL / min). Then, it was heat-treated at 600 ℃ for 3 h under inert gas protection and sieved to obtain titanium pyrophosphate powder with an average particle size of 50-200 nm.

[0078] Step 4, prepare lithium titanium phosphate precursor solution: under stirring at room temperature, lithium acetate solution and tetrabutyl titanate solution are added sequentially to phosphoric acid-nitric acid mixed solution for 5 min each. After the tetrabutyl titanate solution is added, stirring is continued for 30 min to obtain lithium titanium phosphate precursor solution; wherein, the molar ratio of lithium acetate, tetrabutyl titanate and phosphoric acid is 1:2:3.

[0079] Step 5, Preparation of composite precursor suspension: Under stirring at room temperature, titanium pyrophosphate powder and polyvinylpyrrolidone solution are added sequentially to lithium titanium phosphate precursor solution for 5 min each. After the polyvinylpyrrolidone solution is added, stirring is continued for 60 min to obtain composite precursor suspension; wherein, the molar ratio of lithium acetate to titanium pyrophosphate powder and polyvinylpyrrolidone is 1:0.2:0.5.

[0080] Step 6, Preparation of in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material: The composite precursor suspension was filtered and the filter residue was freeze-dried, and then sintered at 600 °C for 4 h under inert gas protection to obtain the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material; in the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, the molar ratio of lithium titanium phosphate to titanium pyrophosphate is 1:0.2.

[0081] Figure 2 The image shows the X-ray diffraction pattern of the in-situ carbon-nitrogen-coated titanium pyrophosphate-lithium titanium phosphate composite material. As can be seen from the figure, compared with the XRD pattern of pure lithium titanium phosphate, the composite material prepared in this invention exhibits higher XRD patterns at 22.8°C. o 25.5 o and 27.9 o Three distinct characteristic peaks were observed at various locations, corresponding to titanium pyrophosphate, indicating that the composite material contains both lithium titanium phosphate and titanium pyrophosphate, thus the composite was successfully formed.

[0082] Figure 3 The images show scanning electron microscope (SEM) images of in-situ carbon and nitrogen-coated lithium titanium phosphate (a), titanium pyrophosphate (b), and the in-situ carbon and nitrogen-coated titanium pyrophosphate-lithium titanium phosphate composite material (c). As can be seen from the figures, the composite material prepared in this invention exhibits significantly increased dispersibility, smaller particle size, and less agglomeration, falling between that of pure titanium pyrophosphate and pure lithium titanium phosphate.

[0083] Example 2

[0084] The preparation steps of the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material in this embodiment are basically the same as those in Example 1, except that: in step 5, the molar ratio of lithium acetate to titanium pyrophosphate powder and polyvinylpyrrolidone is 1:0.01:0.5; in step 6, sintering is carried out at 650 °C for 4 h under inert gas protection.

[0085] Example 3

[0086] The preparation steps of the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material in this embodiment are basically the same as those in Example 1, except that: in step 5, the molar ratio of lithium acetate to titanium pyrophosphate powder and polyvinylpyrrolidone is 1:0.1:0.5; in step 6, sintering is carried out at 650 °C for 2 h under inert gas protection.

[0087] Example 4

[0088] The preparation steps of the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material in this embodiment are basically the same as those in Example 1, except that in step 5, the molar ratio of lithium acetate to titanium pyrophosphate powder and polyvinylpyrrolidone is 1:0.3:0.5.

[0089] Example 5

[0090] The preparation steps of the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material in this embodiment are basically the same as those in Example 1, except that: in step 5, the molar ratio of lithium acetate to titanium pyrophosphate powder and polyvinylpyrrolidone is 1:0.5:0.5; in step 6, sintering is carried out at 600 °C for 2 h under inert gas protection.

[0091] Comparative Example 1

[0092] An in-situ carbon-nitrogen coated lithium titanium phosphate material is prepared by the following steps:

[0093] Step 1, Prepare raw material solutions: Using anhydrous ethanol as solvent, prepare 10% lithium acetate solution, 10% tetrabutyl titanate solution, 10% phosphoric acid-nitric acid mixed solution, and 5% polyvinylpyrrolidone solution.

[0094] Step 2, Preparation of lithium titanium phosphate precursor solution: Under stirring at room temperature, lithium acetate solution, tetrabutyl titanate solution, and polyvinylpyrrolidone solution are added sequentially to a phosphoric acid-nitric acid mixed solution for 5 min each. After the polyvinylpyrrolidone solution is added, stirring is continued for 60 min to obtain the lithium titanium phosphate precursor solution; wherein, the molar ratio of lithium acetate, tetrabutyl titanate, phosphoric acid, and polyvinylpyrrolidone is 1:2:3:0.5.

[0095] Step 3, Preparation of in-situ carbon-nitrogen-coated lithium titanium phosphate composite material: The lithium titanium phosphate precursor solution was freeze-dried and then sintered at 600 °C for 4 h under inert gas protection to obtain the in-situ carbon-nitrogen-coated lithium titanium phosphate material; the SEM image of the in-situ carbon-nitrogen-coated lithium titanium phosphate material is shown in [image missing]. Figure 3 a.

[0096] Comparative Example 2

[0097] The preparation steps of the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material in this comparative example are basically the same as those in Example 1, except that: in step 5, the molar ratio of lithium acetate to titanium pyrophosphate powder and polyvinylpyrrolidone is 1:0.6:0.5, and in the final in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, the molar ratio of lithium titanium phosphate to titanium pyrophosphate is 1:0.6.

[0098] Comparative Example 3

[0099] An in-situ carbon-nitrogen coated titanium pyrophosphate material is prepared by the following steps:

[0100] Step 1, Prepare raw material solutions: Using anhydrous ethanol as solvent, prepare a 10% tetrabutyl titanate solution, a 10% phosphoric acid-nitric acid mixed solution, and a 5% polyvinylpyrrolidone solution.

[0101] Step 2, Preparation of titanium pyrophosphate precursor solution: Under stirring at room temperature, tetrabutyl titanate solution and polyvinylpyrrolidone solution are added to phosphoric acid-nitric acid mixed solution for 5 min each. After the polyvinylpyrrolidone solution is added, stirring is continued for 30 min to obtain titanium pyrophosphate precursor solution; wherein, the molar ratio of tetrabutyl titanate, phosphoric acid and polyvinylpyrrolidone is 1:2:0.5.

[0102] Step 3, Preparation of in-situ carbon-nitrogen coated titanium pyrophosphate composite material: The titanium pyrophosphate precursor solution was prepared into spherical particles by spray drying, and then heat-treated at 600 °C for 2 h under inert gas protection to obtain the in-situ carbon-nitrogen coated titanium pyrophosphate composite material.

[0103] Comparative Example 4

[0104] An in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material differs from Example 1 in that the in-situ carbon-nitrogen coated lithium titanium phosphate and titanium pyrophosphate powders in this comparative example are physically mixed. The preparation steps are as follows:

[0105] Step 1, Prepare raw material solutions: Using anhydrous ethanol as solvent, prepare 10% lithium acetate solution, 10% tetrabutyl titanate solution, 10% phosphoric acid-nitric acid mixed solution, and 5% polyvinylpyrrolidone solution.

[0106] Step 2, Preparation of titanium pyrophosphate precursor solution: Under stirring at room temperature, add tetrabutyl titanate solution to phosphoric acid-nitric acid mixed solution for 5 min. After addition, continue stirring for 30 min to obtain titanium pyrophosphate precursor solution; wherein, the molar ratio of tetrabutyl titanate to phosphoric acid is 1:2.

[0107] Step 3, Preparation of titanium pyrophosphate powder: The titanium pyrophosphate precursor solution is prepared into spherical particles by spray drying, and then heat-treated at 600 °C for 3 h under inert gas protection. After sieving, titanium pyrophosphate powder with an average particle size of 50-200 nm is obtained.

[0108] Step 4, Preparation of lithium titanium phosphate precursor solution: Under stirring at room temperature, lithium acetate solution, tetrabutyl titanate solution, and polyvinylpyrrolidone solution are added sequentially to the phosphoric acid-nitric acid mixed solution for 5 min each. After the polyvinylpyrrolidone solution is added, stirring is continued for 30 min to obtain the lithium titanium phosphate precursor solution; wherein, the molar ratio of lithium acetate, tetrabutyl titanate, phosphoric acid, and polyvinylpyrrolidone is 1:2:3:0.5.

[0109] Step 5, Preparation of in-situ carbon-nitrogen coated lithium titanium phosphate: The lithium titanium phosphate precursor solution was freeze-dried and then sintered at 600 °C for 4 h under inert gas protection to obtain in-situ carbon-nitrogen coated lithium titanium phosphate.

[0110] Step 6, Preparation of in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material: In-situ carbon-nitrogen coated titanium titanium phosphate and titanium pyrophosphate powder are mixed evenly at a molar ratio of 1:0.2 to obtain in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material.

[0111] Comparative Example 5

[0112] An in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material differs from Example 1 in that, in this comparative example, lithium titanium phosphate powder is prepared first, and then carbon-nitrogen coated titanium pyrophosphate is prepared in-situ on its surface. The preparation steps are as follows:

[0113] Step 1, Prepare raw material solutions: Using anhydrous ethanol as solvent, prepare 10% lithium acetate solution, 10% tetrabutyl titanate solution, 10% phosphoric acid-nitric acid mixed solution, and 5% polyvinylpyrrolidone solution.

[0114] Step 2, Preparation of lithium titanium phosphate precursor solution: Under stirring at room temperature, lithium acetate solution and tetrabutyl titanate solution are added sequentially to phosphoric acid-nitric acid mixed solution for 5 min each. After the addition of tetrabutyl titanate solution is completed, stirring is continued for 30 min to obtain lithium titanium phosphate precursor solution; wherein, the molar ratio of lithium acetate, tetrabutyl titanate and phosphoric acid is 1:2:3.

[0115] Step 3, Preparation of lithium titanium phosphate powder: The lithium titanium phosphate precursor solution is prepared into spherical particles by spray drying, and then heat-treated at 600℃ for 2 h under inert gas protection to obtain lithium titanium phosphate powder.

[0116] Step 4, Preparation of titanium pyrophosphate precursor solution: Under stirring at room temperature, add tetrabutyl titanate solution to phosphoric acid-nitric acid mixed solution for 5 min. After addition, continue stirring for 30 min to obtain titanium pyrophosphate precursor solution; wherein, the molar ratio of tetrabutyl titanate to phosphoric acid is 1:2.

[0117] Step 5, Preparation of composite precursor suspension: Under room temperature stirring, lithium titanium phosphate powder and polyvinylpyrrolidone solution are added sequentially to titanium pyrophosphate precursor solution for 5 min each. After the polyvinylpyrrolidone solution is added, stirring is continued for 60 min to obtain composite precursor suspension; wherein, the molar ratio of tetrabutyl titanate to lithium titanium phosphate powder and polyvinylpyrrolidone is 0.2:1:0.5.

[0118] Step 6, Preparation of in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material: The composite precursor suspension was filtered and the filter residue was freeze-dried, and then sintered at 600℃ for 4 h under inert gas protection to obtain the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material.

[0119] The materials prepared in Examples 1-5 and Comparative Examples 1-5 were used as positive electrode active materials to assemble lithium metal batteries, and performance tests were conducted.

[0120] The specific steps for battery assembly are as follows:

[0121] 1. Preparation of electrode sheets: Using N-methylpyrrolidone (NMP) as solvent, an electrode slurry was prepared in a mass ratio of positive electrode active material: conductive carbon black (Super-P): polyvinylidene fluoride (PVDF) = 8:1:1. The electrode was prepared by brushing with carbon-coated aluminum foil as the current collector, and after drying, it was cut into electrode sheets with a diameter of 12 mm.

[0122] 2. Battery assembly: Using the prepared electrode sheet as the positive electrode and the commercially available lithium metal sheet as the negative electrode, the positive electrode sheet, separator and negative electrode sheet are immersed in electrolyte and then packaged in a 2016CR battery case to obtain a lithium metal battery.

[0123] The 1C performance test results under room temperature conditions are shown in Table 1.

[0124] Table 1: Battery Performance Test Results

[0125]

[0126] Figure 4 The figure shows the cyclic voltammograms of a lithium metal battery assembled using the in-situ carbon-nitrogen-coated lithium titanium phosphate material of Comparative Example 1. As can be seen from the figure, a pair of redox peaks exist at 2.5 V in the cyclic voltammogram, corresponding to the operating voltage plateau of lithium titanium phosphate.

[0127] Figure 5 The figure shows the cyclic voltammograms of a lithium metal battery assembled using the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material of Example 1. As can be seen from the figure, the cyclic voltammograms of the battery show two pairs of redox peaks at approximately 2.5 V and 2.9 V, respectively, corresponding to the average operating voltages of lithium titanium phosphate and titanium pyrophosphate in the composite material. This indicates that the composite material of the present invention contains both of these redox-reactive components.

[0128] Figure 6 The figure shows the charge-discharge curves of a lithium metal battery assembled using the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material of Example 1. As can be seen from the figure, two voltage plateaus appear around 2.4 V and 2.5 V during the charge-discharge process, which are consistent with... Figure 5 The redox peaks of the cyclic voltammetry curves shown correspond to each other, indicating that the composite material of the present invention contains two substances simultaneously.

[0129] Figure 7 The figure shows the cycle performance curves of the lithium metal battery assembled using the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material of Example 1.

[0130] Figure 8 The figure shows the rate performance curves of lithium metal batteries assembled using the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material (TP-LTP) of Example 1 and the in-situ carbon-nitrogen coated lithium titanium phosphate electrode material (LTP) of Comparative Example 1. As can be seen from the figure, LTP has a relatively high initial capacity, but its capacity decreases rapidly at high rates, with a capacity retention rate of 78% from 0.05C to 1C. TP-LTP has a slightly lower initial capacity, with a capacity retention rate of 89.9% from 0.05C to 1C. Furthermore, the capacity of TP-LTP is higher than that of LTP at 1C, indicating that the composite material of the present invention can significantly improve the rate performance of the battery.

[0131] In summary, this invention first prepares titanium pyrophosphate powder, and then prepares carbon-nitrogen-coated lithium titanium phosphate in situ on the surface of the titanium pyrophosphate powder, thus obtaining an in-situ carbon-nitrogen-coated titanium pyrophosphate-lithium titanium phosphate composite material. This in-situ carbon-nitrogen-coated titanium pyrophosphate-lithium titanium phosphate composite material exhibits good stability and uniformity, solving the problem of poor uniformity and stability of composite powders prepared by traditional composite methods. When used as a negative electrode material for lithium-ion batteries, it possesses high initial discharge specific capacity as well as good cycle stability and rate performance.

[0132] The above detailed embodiments describe the implementation of the present invention; however, the present invention is not limited to the specific details described in the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

Claims

1. An in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, characterized in that, include: Titanium pyrophosphate and carbon-nitrogen-coated lithium titanium phosphate, wherein the carbon-nitrogen-coated lithium titanium phosphate is coated on the surface of titanium pyrophosphate.

2. The in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material according to claim 1, characterized in that, In the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material, the molar ratio of lithium titanium phosphate to titanium pyrophosphate is 1:(0.01-0.5).

3. The method for preparing the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material according to claim 1 or 2, characterized in that, Includes the following steps: A lithium source, a titanium source, a phosphorus source, and nitric acid are mixed evenly in an alcohol solvent to obtain a lithium titanium phosphate precursor solution. Titanium pyrophosphate powder and carbon-nitrogen source were added sequentially to lithium titanium phosphate precursor solution and mixed evenly to obtain composite precursor suspension. The composite precursor suspension was filtered, and the filter residue was dried and sintered at 500-900 °C under inert gas protection to obtain an in-situ carbon and nitrogen-coated titanium pyrophosphate-lithium titanium phosphate composite material.

4. The method for preparing the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material according to claim 3, characterized in that, In the lithium titanium phosphate precursor solution, the molar ratio of lithium source, titanium source and phosphorus source is (0.8-1.2):2:3, and the mass ratio of phosphorus source to nitric acid is 1:(0.1-2).

5. The method for preparing the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material according to claim 3, characterized in that, In the composite precursor suspension, the molar ratio of lithium source, titanium pyrophosphate powder, and carbon-nitrogen source is 1:(0.01-0.5):(0.1-0.8).

6. The method for preparing the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material according to claim 5, characterized in that, The average particle size of the titanium pyrophosphate powder is 50-200 nm.

7. The method for preparing the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material according to claim 3, characterized in that, The sintering time is 2-10 h.

8. The method for preparing the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material according to claim 3, characterized in that, The drying process is freeze-drying.

9. The application of the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material according to claim 1 or 2, or the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material prepared by any one of the preparation methods of claims 4-8, in lithium-ion batteries.

10. A lithium-ion battery, characterized in that, The negative electrode active material is the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material as described in claim 1 or 2, or the in-situ carbon-nitrogen coated titanium pyrophosphate-lithium titanium phosphate composite material prepared by any one of the preparation methods in claims 4-8.

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