Composite negative electrode material, preparation method and lithium ion secondary battery comprising same

Abstract

The present application relates to the technical field of lithium ion secondary battery, in particular to a composite negative electrode material, a preparation method and a lithium ion secondary battery containing the same. The composite negative electrode material provided by the present application has a spherical or spherical-like core-shell structure, which sequentially comprises a zinc niobate core, a titanium niobate shell and a carbon coating layer wrapped on the surface of the titanium niobate shell from inside to outside, and the preparation method is simple, and the whole process only needs one sintering to obtain. The lithium ion secondary battery prepared by using the composite negative electrode material provided by the present application has good cycle life and energy density at the same time, the cycle life can reach 12000 cycles or more, the negative electrode working voltage can be reduced to near 1.2V, and the gram capacity of the negative electrode material is more than 375mAh / g, which can meet the demand of lithium ion secondary battery in different application scenarios.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion secondary battery technology, specifically to a composite negative electrode material, its preparation method, and a lithium-ion secondary battery containing the same. Background Technology

[0002] Lithium-ion batteries have attracted much attention and are widely used in the electronics industry due to their advantages such as high voltage, large specific capacity, long cycle life, and fast charging speed. Currently, the negative electrode materials used in lithium-ion batteries are mainly divided into carbon-based materials (such as graphite) and non-carbon-based materials (such as lithium titanate and transition metal oxides). While carbon-based materials like graphite have high specific capacity and low operating potential, their operating potential is close to the lithium deposition potential (approximately 0.1 V), making them prone to lithium dendrite formation under low temperature or high current charging conditions, potentially leading to serious safety hazards. Lithium titanate negative electrode materials exhibit excellent cycle performance because their lattice parameters remain almost unchanged during lithium-ion insertion and extraction. Furthermore, their operating potential is approximately 1.55 V, far higher than the lithium dendrite deposition potential, effectively preventing the deposition of metallic lithium during overcharging, thus giving the battery excellent safety performance and an ultra-long cycle life. However, lithium titanate has a relatively low theoretical specific capacity, resulting in lower energy density in practical applications and limiting its further development in high-energy-density devices.

[0003] Titanium niobate, as a novel anode material, exhibits excellent low-temperature and cycle performance, and its specific capacity is significantly higher than that of lithium titanate anode material, making it considered one of the most promising alternatives to lithium titanate. Titanium niobate has an operating potential of 1.6V, which is close to the 1.55V operating potential of lithium titanate. While a higher operating potential can prevent lithium dendrite formation and ensure battery safety, for anode materials, an excessively high operating potential means that the operating voltage of the entire cell will decrease after assembly, thus reducing the overall energy density of the cell.

[0004] Therefore, although the specific capacity of titanium niobate anode material is significantly higher than that of lithium titanate material, the energy density of lithium-ion secondary batteries made with titanium niobate is still significantly lower than that of lithium-ion secondary batteries using graphite anode material, which will undoubtedly limit its large-scale promotion and application. Summary of the Invention

[0005] In view of this, the present invention provides a composite anode material, a preparation method thereof, and a lithium-ion secondary battery containing the same. The composite anode material has a special core-shell structure, which retains the excellent low-temperature performance and cycle life of titanium niobate anode material, while reducing the working voltage of the material and increasing the energy density of the battery.

[0006] To address the above technical problems, the first aspect of this invention provides a composite anode material having a spherical or near-spherical core-shell structure, comprising, from the inside out, a Zn3Nb2O8 core, a TiNb2O7 shell, and a carbon coating layer encapsulating the surface of the TiNb2O7 shell, denoted as Zn3Nb2O8@TiNb2O7@C; wherein the molar ratio of Zn3Nb2O8 to TiNb2O7 is 1:9 to 9:1, and the carbon coating amount is 1wt% to 4wt%.

[0007] Zinc niobate (Zn3Nb2O8), as an anode material, has a theoretical specific capacity close to that of titanium niobate, but its operating potential is significantly lower. Using it as an anode material can enable lithium-ion batteries to achieve higher energy density. However, when used as an anode material in lithium-ion batteries, zinc niobate exhibits a slightly larger volume change than titanium niobate, resulting in a significantly lower cycle life, thus limiting its application in lithium-ion batteries.

[0008] During their research, the inventors discovered that by using zinc niobate as the core and coating it with titanium niobate as the outer shell (i.e., Zn3Nb2O8@TiNb2O7), the outer titanium niobate layer can protect the zinc niobate core, reducing its volume change during use and thus significantly improving the cycle life of the zinc niobate. Simultaneously, amorphous carbon coating on the surface of the core-shell anode material (Zn3Nb2O8@TiNb2O7) further enhances the conductivity of the anode material, resulting in a composite anode material Zn3Nb2O8@TiNb2O7@C with a carbon coating layer on the surface, a TiNb2O7 outer shell, and a Zn3Nb2O8 core.

[0009] In conjunction with the first aspect, the molecular molar ratio of Zn3Nb2O8 to TiNb2O7 is 3:7 to 7:3, and the carbon coating amount is 2wt%.

[0010] The lithium-ion secondary batteries corresponding to this ratio range have superior overall performance, with both cycle life and anode material performance at their best.

[0011] In conjunction with the first aspect, the average particle size of the composite negative electrode material is 1~10μm, which can be any value between any two of 1μm, 2μm, 4μm, 6μm, 8μm and 10μm.

[0012] A second aspect of the present invention provides a method for preparing the above-mentioned composite anode material, comprising the following steps: Preparation of Zn3Nb2O8: According to the stoichiometric ratio, an alcoholic solution of niobium source is added to a mixed solution of alcoholic solution of zinc source and citric acid alcoholic solution, and stirred to form Zn3Nb2O8 sol. Then, it is aged, dried and ground in sequence to obtain Zn3Nb2O8 powder. Preparation of Zn3Nb2O8@TiNb2O7: According to the stoichiometric ratio, an alcohol solution of titanium source and an alcohol solution of niobium source are mixed, citric acid is added, and the mixture is stirred to form TiNb2O7 sol. The Zn3Nb2O8 powder is then added, and the mixture is stirred to form Zn3Nb2O8@TiNb2O7 composite sol. After that, the mixture is aged, dried, and ground to obtain the Zn3Nb2O8@TiNb2O7 powder. Preparation of Zn3Nb2O8@TiNb2O7@C: The Zn3Nb2O8@TiNb2O7 powder was added to an alcohol solution of a carbon source and stirred to form a Zn3Nb2O8@TiNb2O7@C composite sol. Then, it was aged, dried and ground in sequence to obtain the Zn3Nb2O8@TiNb2O7@C precursor powder. Then, it was pre-calcined at low temperature and calcined at high temperature under inert conditions. After cooling, it was ground to obtain the Zn3Nb2O8@TiNb2O7@C composite anode material.

[0013] The method for preparing the composite anode material provided by the present invention, by controlling the order of addition of each raw material and the sintering time, prepares a composite anode material with excellent performance. The lithium-ion secondary battery made using this anode material has good cycle life and energy density.

[0014] In conjunction with the second aspect, the zinc source is zinc acetate and / or zinc nitrate, the niobium source is niobium oxalate and / or niobium alkoxide, the titanium source is tetrabutyl titanate and / or isopropyl titanate, and the carbon source is selected from at least one of glucose, sucrose, and polyethylene glycol; In the preparation of Zn3Nb2O8, the molar ratio of the zinc source to citric acid is 1:1.5~2; in the preparation of Zn3Nb2O8@TiNb2O7, the molar ratio of the titanium source to citric acid is 1:2.8~3.2.

[0015] Preferably, in the preparation of Zn3Nb2O8, the molar ratio of the zinc source to citric acid is 1:1.7; and in the preparation of Zn3Nb2O8@TiNb2O7, the molar ratio of the titanium source to citric acid is 1:3.

[0016] In conjunction with the second aspect, the stirring process forms a Zn3Nb2O8 sol, which is then subjected to aging, drying, and grinding in sequence. Specifically, the sol is stirred at a constant temperature of 75~85℃ until a uniform, precipitate-free, and highly fluid Zn3Nb2O8 sol is formed. It is then aged at room temperature for 12~24 hours, followed by drying and grinding to obtain the final product.

[0017] In conjunction with the second aspect, the process of continuing to stir to form a Zn3Nb2O8@TiNb2O7 composite sol, followed by aging, drying, and grinding, specifically involves: ultrasonic dispersion followed by stirring at room temperature for 1.5~2.5h, then heating to 70~80℃ and stirring at a constant temperature to form a Zn3Nb2O8@TiNb2O7 composite sol, aging at room temperature for 8~12h, followed by drying and grinding to obtain the final product.

[0018] In conjunction with the second aspect, the stirring process forms a Zn3Nb2O8@TiNb2O7@C composite sol, which is then subjected to aging, drying, and grinding in sequence. Specifically, after ultrasonic dispersion, the sol is stirred at room temperature for 1.5~2.5h, then heated to 75~85℃ and stirred at a constant temperature to form a Zn3Nb2O8@TiNb2O7@C composite sol. The sol is then aged at room temperature for 6~8h, followed by drying and grinding to obtain the final product.

[0019] In conjunction with the second aspect, the specific steps of performing low-temperature pre-calcination and high-temperature calcination under inert conditions are as follows: the Zn3Nb2O8@TiNb2O7@C precursor powder is pre-calcined at 420~480℃ for 1.5~2.5h under inert conditions, cooled and ground, and then calcined at 750~800℃ for 4~6h under inert conditions. After cooling and grinding, the product is obtained.

[0020] A third aspect of the present invention provides a lithium-ion secondary battery, comprising the above-described composite negative electrode material or a composite negative electrode material prepared according to the above-described preparation method.

[0021] The positive electrode material used in the positive electrode of this lithium-ion secondary battery can be selected from lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium olivine-structured lithium phosphate, and mixtures thereof.

[0022] The electrolyte used contains a lithium salt and an organic solvent. The lithium salt anion may be selected from at least one of the following ions: F... - Cl - ,Br - I - NO3 - N (CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3) 6P - CF3SO3 -CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - The organic solvent is selected from at least one of the following: fluoroethylene carbonate (FEC), propionate, and more specifically, may be selected from methyl propionate, ethyl propionate, propyl propionate and butyl propionate, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite and tetrahydrofuran, and mixtures thereof.

[0023] The diaphragm used can be made from materials such as polyolefin-based polymers like ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or porous nonwoven fabrics such as high-melting-point glass fibers and polyethylene terephthalate fibers. Additionally, the porous polymer membrane or porous nonwoven fabric may contain a porous organic / inorganic coating formed by inorganic particles and an adhesive on at least one surface.

[0024] The composite anode material provided by this invention has a spherical or near-spherical structure with a three-layer core-shell structure. Zinc niobate, which has poor cycle performance, is placed in the innermost layer as the core, and titanium niobate is placed in the outermost layer for protection. This structure not only prevents the zinc niobate from deteriorating due to large volume changes during cycling, but also leverages the high energy density of the zinc niobate material. Simultaneously, conductive carbon is coated on the outermost layer to address the poor conductivity issue, thereby improving the conductivity of the anode material and fully utilizing its performance. Lithium-ion secondary batteries made using the composite anode material provided by this invention can achieve a cycle life of over 12,000 cycles, with an anode operating voltage as low as close to 1.2V, and the specific capacity of the anode material is consistently above 375 mAh / g. This provides a new material for the application of lithium-ion secondary batteries in various demanding scenarios. Attached Figure Description

[0025] Figure 1 The diagram shows the structure of the composite anode material provided in the embodiment of the present invention, wherein: 1, Zn3Nb2O8 core, 2, TiNb2O7 shell, and 3, carbon coating layer.

[0026] Figure 2 This is a SEM image of the composite negative electrode material prepared in Example 1 of the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0028] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the meaning consistent with their meaning in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless specifically defined.

[0029] While titanium niobate has a higher specific capacity than lithium titanate, its energy density is still significantly lower than that of graphite, limiting its large-scale application. Zinc niobate has a theoretical specific capacity close to that of titanium niobate. Although its operating potential is lower than that of titanium niobate, its volume change is greater and its cycle life is lower when used as a negative electrode material in lithium-ion batteries, thus limiting its application as well.

[0030] In view of this, the present invention provides a composite anode material, a preparation method thereof, and a lithium-ion secondary battery containing the same. By designing the structure of the composite anode material, with zinc niobate as the core and titanium niobate as the shell, and carbon coating on the surface of the shell, this design retains the excellent cycle life of titanium niobate anode material while reducing the operating voltage of the anode material, thereby improving the energy density of the battery.

[0031] Unless otherwise specified, the raw materials and equipment used in the following examples and comparative examples are all commercially available products.

[0032] Example 1 This embodiment provides a composite anode material with a spherical core-shell structure and an average particle size of 5 μm. From the inside out, it consists of a Zn3Nb2O8 core, a TiNb2O7 shell, and a carbon coating layer on the surface of the TiNb2O7 shell, denoted as Zn3Nb2O8@TiNb2O7@C. The molecular molar ratio of Zn3Nb2O8 to TiNb2O7 is 1:9, and the carbon coating amount is 2 wt%.

[0033] The preparation method of this composite anode material is as follows: Preparation of Zn3Nb2O8: Weigh 1.46g of citric acid, add 20ml of ethanol and 5ml of deionized water to dissolve, and prepare a citric acid solution; weigh 1g of zinc acetate dihydrate and dissolve in 20ml of ethanol; weigh 1.634g of niobium oxalate and dissolve in 20ml of ethanol. Slowly add the zinc acetate solution dropwise to the citric acid solution to obtain a mixture, and then slowly add the niobium oxalate solution dropwise to the above mixture. After all the addition is complete, maintain a constant temperature of 80℃ and stir until a uniform and viscous Zn3Nb2O8 sol is formed. Then, age at room temperature for 12 hours to form a jelly-like wet gel, and then vacuum dry at 80℃ until a dry gel is formed. Grind the dry gel into a fine powder.

[0034] Preparation of Zn3Nb2O8@TiNb2O7: Weigh 14.706g of niobium oxalate and dissolve it in 200ml of ethanol; weigh 4.65g of tetrabutyl titanate and dissolve it in 50ml of ethanol. Mix the tetrabutyl titanate solution with the niobium oxalate solution, then add 7.9g of citric acid and stir continuously for 30min to obtain TiNb2O7 sol. Add the prepared Zn3Nb2O8 dry gel powder to the TiNb2O7 sol, ultrasonically disperse for 15min, then stir at room temperature for 2h. After stirring evenly, heat to 75℃ and stir at a constant temperature to form Zn3Nb2O8@TiNb2O7 composite sol. Then, let it stand at room temperature for 10h to allow the composite sol to completely gel, and then vacuum dry at 80℃ until Zn3Nb2O8@TiNb2O7 dry gel is formed. Grind the dry gel into a fine powder.

[0035] Preparation of Zn3Nb2O8@TiNb2O7@C: Weigh 0.275g of glucose and dissolve it in 60ml of anhydrous ethanol. Then, add the Zn3Nb2O8@TiNb2O7 dry gel powder to the glucose dispersion, sonicate for 15min, and stir at room temperature for 2h. After stirring evenly, heat to 80℃ and stir at a constant temperature to evaporate the solvent, forming a viscous Zn3Nb2O8@TiNb2O7@C composite sol. Let the composite sol stand at room temperature for 6h to age until completely gelled, and then vacuum dry at 80℃ to obtain Zn3Nb2O8@TiNb2O7@C dry gel. Then, grind the dry gel into a fine powder to obtain the precursor powder. Then, place the precursor powder in a tube furnace and heat to 450℃ at a heating rate of 2℃ / min under argon atmosphere for 2h for pre-calcination. After pre-calcination, cool the powder to room temperature and then grind it into a fine powder again. The pre-calcined powder was placed back into a tube furnace and heated to 780°C for 6 hours at a heating rate of 3°C / min under argon atmosphere to form Zn3Nb2O8@TiNb2O7@C composite material. After cooling to room temperature, it was ground again to obtain Zn3Nb2O8@TiNb2O7@C composite anode material.

[0036] Its scanned e-sports photos, such as Figure 2 As shown, the Zn3Nb2O8@TiNb2O7@C composite anode material has a spherical structure. The surface of the Zn3Nb2O8 material is densely covered with TiNb2O7 material. However, due to the small amount of carbon coating, the carbon layer is thin. In addition, carbon is a conductive material, so it cannot be clearly shown in the scanning electron microscope image.

[0037] Example 2 This embodiment provides a composite anode material with a spherical core-shell structure and an average particle size of 5 μm. From the inside out, it consists of a Zn3Nb2O8 core, a TiNb2O7 shell, and a carbon coating layer on the surface of the TiNb2O7 shell, denoted as Zn3Nb2O8@TiNb2O7@C. The molecular molar ratio of Zn3Nb2O8 to TiNb2O7 is 3:7, and the carbon coating amount is 2 wt%.

[0038] The preparation method of this composite anode material is similar to that of Example 1, except that the amount of raw materials used in the preparation is different. For details of the specific amounts, please refer to Table 1.

[0039] Example 3 This embodiment provides a composite anode material with a spherical core-shell structure and an average particle size of 6 μm. From the inside out, it consists of a Zn3Nb2O8 core, a TiNb2O7 shell, and a carbon coating layer on the surface of the TiNb2O7 shell, denoted as Zn3Nb2O8@TiNb2O7@C. The molar ratio of Zn3Nb2O8 to TiNb2O7 is 5:5, and the carbon coating amount is 2 wt%.

[0040] The preparation method of this composite anode material is similar to that of Example 1, except that the amount of raw materials used in the preparation is different. For details of the specific amounts, please refer to Table 1.

[0041] Example 4 This embodiment provides a composite anode material with a spherical core-shell structure and an average particle size of 6 μm. From the inside out, it consists of a Zn3Nb2O8 core, a TiNb2O7 shell, and a carbon coating layer on the surface of the TiNb2O7 shell, denoted as Zn3Nb2O8@TiNb2O7@C. The molar ratio of Zn3Nb2O8 to TiNb2O7 is 7:3, and the carbon coating amount is 2 wt%.

[0042] The preparation method of this composite anode material is similar to that of Example 1, except that the amount of raw materials used in the preparation is different. For details of the specific amounts, please refer to Table 1.

[0043] Example 5 This embodiment provides a composite anode material with a spherical core-shell structure and an average particle size of 4 μm. From the inside out, it consists of a Zn3Nb2O8 core, a TiNb2O7 shell, and a carbon coating layer on the surface of the TiNb2O7 shell, denoted as Zn3Nb2O8@TiNb2O7@C. The molar ratio of Zn3Nb2O8 to TiNb2O7 is 9:1, and the carbon coating amount is 2 wt%.

[0044] The preparation method of this composite anode material is similar to that of Example 1, except that the amount of raw materials used in the preparation is different. For details of the specific amounts, please refer to Table 1.

[0045] Example 6 This embodiment provides a composite anode material with a spherical core-shell structure and an average particle size of 3-4 μm. From the inside out, it consists of a Zn3Nb2O8 core, a TiNb2O7 shell, and a carbon coating layer on the surface of the TiNb2O7 shell, denoted as Zn3Nb2O8@TiNb2O7@C. The molar ratio of Zn3Nb2O8 to TiNb2O7 is 3:7, and the carbon coating amount is 1 wt%.

[0046] The preparation method of this composite anode material is similar to that of Example 1, except that the amount of raw materials used in the preparation is different. For details of the specific amounts, please refer to Table 1.

[0047] Example 7 This embodiment provides a composite anode material with a spherical core-shell structure and an average particle size of 5 μm. From the inside out, it consists of a Zn3Nb2O8 core, a TiNb2O7 shell, and a carbon coating layer on the surface of the TiNb2O7 shell, denoted as Zn3Nb2O8@TiNb2O7@C. The molecular molar ratio of Zn3Nb2O8 to TiNb2O7 is 3:7, and the carbon coating amount is 3 wt%.

[0048] The preparation method of this composite anode material is similar to that of Example 1, except that the amount of raw materials used in the preparation is different. For details of the specific amounts, please refer to Table 1.

[0049] Example 8 This embodiment provides a composite anode material with a spherical core-shell structure and an average particle size of 5 μm. From the inside out, it consists of a Zn3Nb2O8 core, a TiNb2O7 shell, and a carbon coating layer on the surface of the TiNb2O7 shell, denoted as Zn3Nb2O8@TiNb2O7@C. The molecular molar ratio of Zn3Nb2O8 to TiNb2O7 is 3:7, and the carbon coating amount is 4 wt%.

[0050] The preparation method of this composite anode material is similar to that of Example 1, except that the amount of raw materials used in the preparation is different. For details of the specific amounts, please refer to Table 1.

[0051] Comparative Example 1 This comparative example provides a negative electrode material Zn3Nb2O8@C with a carbon coating on its surface. The difference between this material and Example 2 is that TiNb2O7 is not used as the outer shell, but carbon is directly coated on the surface of Zn3Nb2O8.

[0052] The preparation method of this negative electrode material is as follows: Preparation of Zn3Nb2O8: Weigh 1.46g of citric acid, add 20ml of ethanol and 5ml of deionized water to dissolve, and prepare a citric acid solution; weigh 1g of zinc acetate dihydrate and dissolve in 20ml of ethanol; weigh 1.634g of niobium oxalate and dissolve in 20ml of ethanol. Slowly add the zinc acetate solution dropwise to the citric acid solution to obtain a mixture, and then slowly add the niobium oxalate solution dropwise to the above mixture. After all the addition is complete, maintain a constant temperature of 80℃ and stir until a uniform and viscous Zn3Nb2O8 sol is formed. Then, age at room temperature for 12 hours to form a jelly-like wet gel, and then vacuum dry at 80℃ until a dry gel is formed. Grind the dry gel into a fine powder.

[0053] Preparation of Zn3Nb2O8@C: Weigh 0.039g of glucose and dissolve it in 15ml of anhydrous ethanol. Then, add the Zn3Nb2O8 dry gel powder to the glucose dispersion, sonicate for 15min, and stir at room temperature for 2h. After stirring evenly, heat to 80℃ and stir at a constant temperature to evaporate the solvent, forming a viscous Zn3Nb2O8@C sol. Let the sol stand at room temperature for 6h to age until completely gelled, and then vacuum dry at 80℃ to obtain Zn3Nb2O8@C dry gel. Grind the dry gel into a fine powder to obtain the precursor powder. Then, place the precursor powder in a tube furnace and heat to 450℃ at a heating rate of 2℃ / min under argon atmosphere for 2h for pre-calcination. After pre-calcination, cool the powder to room temperature and then grind it into a fine powder again. The pre-calcined powder was placed back into a tube furnace and heated to 780°C for 6 hours at a heating rate of 3°C / min under argon atmosphere to form Zn3Nb2O8@C composite material. After cooling to room temperature, it was ground again to obtain Zn3Nb2O8@C anode material.

[0054] Comparative Example 2 This comparative example provides a negative electrode material TiNb2O7@C with a carbon coating on its surface. The difference between it and Example 2 is that Zn3Nb2O8 is not used as the core, and carbon is coated on the surface of TiNb2O7.

[0055] The preparation method of this negative electrode material is as follows: Preparation of TiNb₂O₇: Weigh 2.101 g of niobium oxalate and dissolve it in 30 ml of ethanol; weigh 0.664 g of tetrabutyl titanate and dissolve it in 10 ml of ethanol. Mix the tetrabutyl titanate solution with the niobium oxalate solution, then add 1.125 g of citric acid and stir continuously for 30 min to obtain TiNb₂O₇ sol. Aging at room temperature for 12 hours forms a jelly-like wet gel, which is then vacuum dried at 80 °C until a dry gel is formed. The dry gel is then ground into a fine powder.

[0056] Preparation of TiNb2O7@C: Weigh 0.034g of glucose and dissolve it in 10ml of anhydrous ethanol. Then, add the TiNb2O7 dry gel powder to the glucose dispersion, sonicate for 15min, and stir at room temperature for 2h. After stirring evenly, heat to 80℃ and stir at a constant temperature to evaporate the solvent, forming a viscous TiNb2O7@C sol. Let the sol stand at room temperature for 6h to age until completely gelled, and then vacuum dry at 80℃ to obtain TiNb2O7@C dry gel. Grind the dry gel into a fine powder to obtain the precursor powder. Then, place the precursor powder in a tube furnace and heat to 450℃ at a heating rate of 2℃ / min under argon atmosphere for 2h for pre-calcination. After pre-calcination, cool the powder to room temperature and then grind it into a fine powder again. The pre-calcined powder was placed back into a tube furnace and heated to 780°C for 6 hours at a heating rate of 3°C / min under argon atmosphere to form TiNb2O7@C composite material. After cooling to room temperature, it was ground again to obtain TiNb2O7@C anode material.

[0057] Comparative Example 3 This comparative example provides a negative electrode material Zn3Nb2O8@TiNb2O7, that is, Zn3Nb2O8 material as the core and TiNb2O7 material as the shell. The difference between it and Example 2 is that carbon coating is not applied to the surface of the TiNb2O7 shell. The molar ratio of the core and shell is detailed in Table 1.

[0058] The preparation method of this negative electrode material is as follows: Preparation of Zn3Nb2O8: The steps for preparing Zn3Nb2O8 are the same as those in Example 2, and will not be repeated here.

[0059] Preparation of Zn3Nb2O8@TiNb2O7: Weigh 3.813g of niobium oxalate and dissolve it in 50ml of ethanol; weigh 1.206g of tetrabutyl titanate and dissolve it in 20ml of ethanol. Mix the tetrabutyl titanate solution with the niobium oxalate solution, then add 2.04g of citric acid and stir continuously for 30min to obtain TiNb2O7 sol. Add the prepared Zn3Nb2O8 dry gel powder to the TiNb2O7 sol, ultrasonically disperse for 15min, then stir at room temperature for 2h. After stirring evenly, heat to 75℃ and stir at a constant temperature to form Zn3Nb2O8@TiNb2O7 composite sol. Then, let it stand at room temperature for 10h to allow the composite sol to completely gel, and then vacuum dry at 80℃ until Zn3Nb2O8@TiNb2O7 dry gel is formed. Grind the dry gel into a fine powder. Zn3Nb2O8@TiNb2O7 dry gel powder was placed in a tube furnace and pre-calcined at 450℃ for 2 hours under argon atmosphere at a heating rate of 2℃ / min. After pre-calcination, the powder was cooled to room temperature and then ground again into a fine powder. The pre-calcined powder was then placed back into the tube furnace and heated to 780℃ for 6 hours under argon atmosphere at a heating rate of 3℃ / min to form Zn3Nb2O8@TiNb2O7 composite material. After cooling to room temperature, it was ground again to obtain Zn3Nb2O8@TiNb2O7 anode material.

[0060] Comparative Example 4 This comparative example provides an anode material TiNb2O7@Zn3Nb2O8@C, which uses TiNb2O7 material as the core and Zn3Nb2O8 material as the shell, and carbon is coated on the surface of the Zn3Nb2O8 shell.

[0061] The preparation method of this negative electrode material is similar to that of Example 1, except that the amount of raw materials used in the preparation is different. For details of the specific amounts, please refer to Table 1.

[0062] Comparative Example 5 This comparative example provides a negative electrode material with the same material composition as in Example 2 (specific amounts are detailed in Table 1), but the preparation method differs from that in Example 2. The difference lies in the following: after preparing the Zn3Nb2O8@TiNb2O7 precursor, it is sintered at 780°C to obtain Zn3Nb2O8@TiNb2O7 material without carbon coating. Then, the Zn3Nb2O8@TiNb2O7 material without carbon coating is added to a carbon source solution, and the same drying, secondary sintering, and grinding steps as in Example 2 are performed to obtain the Zn3Nb2O8@TiNb2O7@C negative electrode material.

[0063] Comparative Example 6 This comparative example provides a negative electrode material with the same material composition as in Example 2 (specific amounts are detailed in Table 1), but the preparation method is different from that in Example 2. The difference is that after preparing the Zn3Nb2O8 precursor, it is sintered at 780°C to obtain the Zn3Nb2O8 material. Then, the Zn3Nb2O8 material is added to the TiNb2O7 sol. The subsequent steps are the same as the corresponding steps in Example 2, and will not be repeated here.

[0064] Comparative Example 7 This comparative example provides a negative electrode material with the same material composition as in Example 2 (specific amounts are detailed in Table 1). However, the preparation method differs from that in Example 2. The difference lies in the following: after preparing a Zn3Nb2O8 precursor (dry gel), it is sintered at 780°C to obtain Zn3Nb2O8 material. Then, the Zn3Nb2O8 material is added to TiNb2O7 sol to obtain a Zn3Nb2O8@TiNb2O7 precursor (dry gel), which is then sintered at 780°C. Finally, the Zn3Nb2O8@TiNb2O7 material is added to a carbon source solution and subjected to the same sintering steps as in Example 2 to obtain the Zn3Nb2O8@TiNb2O7@C negative electrode material.

[0065] Comparative Example 8 This comparative example provides a negative electrode material with the same material composition as in Example 2 (specific amounts are detailed in Table 1), and the preparation method is as follows: TiNb2O7 powder was dissolved in ethanol and ultrasonically dispersed; then Zn3Nb2O8 powder was added, and the mixture was heated until the ethanol completely evaporated to obtain a composite powder. The composite powder was dried and then carbon-coated in a glucose-ethanol solution. The subsequent process was the same as in Example 2 and will not be repeated.

[0066] The Zn3Nb2O8 material powder was added to the dispersion of the finished TiNb2O7 material. The subsequent carbon coating process was the same as the carbon coating steps in Example 2, and will not be repeated here.

[0067] Table 1 Test Example Lithium-ion secondary batteries were prepared using the negative electrode materials provided in Examples 1-8 and Comparative Examples 1-8, respectively. Specifically, 95% of the negative electrode material, 2% of the conductive agent carbon black, 0.5% of the conductive agent carbon nanotubes, and 2.5% of the binder polyvinylidene fluoride were mixed in a certain proportion and stirred with N-methylpyrrolidone (NMP) solvent using a planetary mixer to prepare a negative electrode slurry. The negative electrode slurry was uniformly coated on both surfaces of the negative electrode current collector aluminum foil, dried, and then rolled and slit to obtain negative electrode sheets. Simultaneously, a portion of the negative electrode slurry was coated on one surface of the negative electrode current collector aluminum foil to create a single-sided electrode sheet. After rolling, punching, and baking the single-sided electrode sheet, button batteries were assembled. The baked negative electrode sheet was used as a separator with a polypropylene microporous membrane, a mixed organic solvent of 1 mol / L LiPF6 (EC:DMC=1:1, volume ratio) as the electrolyte, and a lithium metal sheet as the counter electrode. The CR2032 type button cell was assembled in an argon-atmosphere glove box. Then, a 0.1C constant current charge-discharge test was performed to test the capacity and working voltage of the negative electrode material. The specific test results are shown in Table 2.

[0068] 95% of the positive electrode material, 2% of the conductive agent carbon black, 0.5% of the conductive agent carbon nanotubes and 2.5% of the binder polyvinylidene fluoride were mixed in proportion and N-methylpyrrolidone (NMP) solvent was used to stir the mixture with a planetary mixer to prepare the positive electrode slurry. The positive electrode slurry was uniformly coated on both surfaces of the positive electrode current collector aluminum foil, dried, rolled and cut to obtain the positive electrode sheet.

[0069] The vacuum-dried double-sided negative electrode sheet, separator, and positive electrode sheet are stacked in sequence. A polyethylene separator is selected as the separator. The stacked core pack is placed in an aluminum-plastic packaging shell for sealing, and then vacuum dried. The dried core pack is injected with electrolyte, and after processes such as vacuuming, secondary sealing, settling, pre-charging, degassing, formation, and aging, a lithium-ion secondary battery is produced.

[0070] The lithium-ion secondary batteries corresponding to Examples 1-8 and Comparative Examples 1-8 were subjected to fast-charge cycle tests. At 25°C, the batteries were charged at a constant current and constant voltage of 8C to the upper limit voltage, with a cutoff current of 0.05C. After a 10-minute rest period, the batteries were discharged at a constant current of 8C to the lower limit voltage. This process was repeated to test the cycle life of the batteries. The battery life was considered terminated when the battery capacity decreased to 80% of its initial capacity. The specific cycle life test results are shown in Table 2.

[0071] Table 2 As shown in Table 2, in Examples 1-5, with the increase of the molar ratio of Zn3Nb2O8 to TiNb2O7, the specific capacity of the composite material increased slightly, but the negative electrode operating voltage decreased significantly. This indicates that the addition of Zn3Nb2O8 can reduce the overall operating voltage of the negative electrode material, thereby improving the energy density of the assembled battery. It can also be seen that with the increase of TiNb2O7, the cycle life of the battery also improved significantly, indicating that the addition of TiNb2O7 can significantly improve the cycle performance of the material.

[0072] Regarding the comparative examples, Comparative Example 1 is a pure Zn3Nb2O8 material without a core-shell structure. Although it has the lowest operating voltage, its cycle performance is very poor. This is because the Zn3Nb2O8 material is in contact with the electrolyte, and side reactions continuously occur during cycling, consuming active lithium and electrolyte, resulting in severe cycle decay. Comparative Example 2 is a pure TiNb2O7 material without a core-shell structure. Although it has excellent cycle performance, its operating voltage is relatively high. Comparative Example 3 does not have a carbon coating on the outermost layer. Compared with Examples 6-8, the specific capacity and cycle performance of the battery's negative electrode are significantly reduced. This indicates that adding a carbon coating layer can significantly improve the specific capacity of the material and the cycle performance of the battery. This is mainly because both zinc niobate and titanium niobate materials have poor conductivity, requiring an additional conductive layer to promote their performance. In Examples 6-8, the specific capacity of the material decreased with the increase of carbon coating. This is because carbon material itself is an inactive substance, and excessive addition will reduce the overall specific capacity of the material. As the amount of carbon material increases, the cycle life of the battery shows a trend of first increasing significantly and then decreasing slightly. This is because excessive carbon material will increase the occurrence of side reactions, thereby consuming some active lithium and causing a slight decrease in cycle life.

[0073] Comparative Example 4 uses TiNb2O7 as the core and Zn3Nb2O8 as the shell, with the molar ratio of the two materials, carbon coating amount, and preparation method remaining consistent with Example 2. It can be seen that using Zn3Nb2O8 as the shell results in a battery with very poor cycle life, comparable to that of a non-core-shell Zn3Nb2O8 battery (Comparative Example 1). This indicates that only by using Zn3Nb2O8 as the core and TiNb2O7 as the shell can a battery achieve a high cycle life.

[0074] In Comparative Example 5, the Zn3Nb2O8@TiNb2O7 precursor was sintered immediately after preparation to obtain uncoated Zn3Nb2O8@TiNb2O7 material. This uncoated Zn3Nb2O8@TiNb2O7 material was then added to a carbon source solution, followed by drying and sintering to obtain the Zn3Nb2O8@TiNb2O7@C composite material. Compared to Example 2, Comparative Example 5 showed a decrease in negative electrode operating voltage and battery cycle life. This indicates that although this preparation method can also coat carbon onto the surface of the core-shell Zn3Nb2O8@TiNb2O7 material, it adds a high-temperature sintering process, increasing the number of preparation steps and wasting energy, while also degrading battery performance.

[0075] In Comparative Example 6, the Zn3Nb2O8 precursor (dry gel) was sintered immediately after preparation to obtain Zn3Nb2O8 material. This Zn3Nb2O8 material was then added to TiNb2O7 sol, and the subsequent preparation method was the same as in Example 2. The material obtained using this method showed significantly worse performance than that of Example 2. This is because the Zn3Nb2O8 and TiNb2O7 materials were not sintered simultaneously, resulting in a loose bond and gaps between the two materials. Furthermore, since the TiNb2O7 material on the surface is composed of nano-sized particles, there are larger gaps between the particles than in Example 2. This allows the electrolyte to gradually penetrate the Zn3Nb2O8 material surface through these gaps, triggering side reactions and reducing cycle life.

[0076] In Comparative Example 7, after preparing the Zn3Nb2O8 precursor (dry gel), after obtaining the Zn3Nb2O8@TiNb2O7 precursor (dry gel), and after adding the Zn3Nb2O8@TiNb2O7 material to the carbon source solution, the materials were sintered separately. However, this method also resulted in poor cycle life of the battery because the Zn3Nb2O8 and TiNb2O7 materials were not sintered simultaneously, leading to gaps between the two materials and a lack of tight bonding.

[0077] In Comparative Example 8, the preparation method of Zn3Nb2O8@TiNb2O7 material was changed to physical coating. Specifically, the finished Zn3Nb2O8 powder was added to the dispersion of the finished TiNb2O7 material, and the subsequent carbon coating method remained the same as in Example 2. It can be seen that the cycle performance of the negative electrode material prepared by this method is more degraded. This is because simple physical coating cannot achieve dense contact between Zn3Nb2O8 and TiNb2O7 materials, nor can it ensure that TiNb2O7 material is uniformly coated on the surface of Zn3Nb2O8 material. This leads to contact between the electrolyte and Zn3Nb2O8 material, triggering side reactions and causing a significant decrease in cycle life.

[0078] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A composite negative electrode material, characterized in that, The composite anode material has a spherical or near-spherical core-shell structure, consisting of a Zn3Nb2O8 core, a TiNb2O7 shell, and a carbon coating layer wrapped around the surface of the TiNb2O7 shell, denoted as Zn3Nb2O8@TiNb2O7@C. The molecular molar ratio of Zn3Nb2O8 to TiNb2O7 is 1:9 to 9:1, and the carbon coating amount is 1wt% to 4wt%.

2. The composite negative electrode material as described in claim 1, characterized in that, The molecular molar ratio of Zn3Nb2O8 to TiNb2O7 is 3:7 to 7:3, and the carbon coating amount is 2wt%.

3. The composite negative electrode material as described in claim 1, characterized in that, The average particle size of the composite anode material is 1~10μm.

4. The method for preparing the composite negative electrode material according to any one of claims 1 to 3, characterized in that the step include: Preparation of Zn3Nb2O8: According to the stoichiometric ratio, an alcoholic solution of niobium source is added to a mixed solution of alcoholic solution of zinc source and citric acid alcoholic solution, and stirred to form Zn3Nb2O8 sol. Then, it is aged, dried and ground in sequence to obtain Zn3Nb2O8 powder. Preparation of Zn3Nb2O8@TiNb2O7: According to the stoichiometric ratio, an alcohol solution of titanium source and an alcohol solution of niobium source are mixed, citric acid is added, and the mixture is stirred to form TiNb2O7 sol. The Zn3Nb2O8 powder is then added, and the mixture is stirred to form Zn3Nb2O8@TiNb2O7 composite sol. After that, the mixture is aged, dried, and ground to obtain the Zn3Nb2O8@TiNb2O7 powder. Preparation of Zn3Nb2O8@TiNb2O7@C: The Zn3Nb2O8@TiNb2O7 powder was added to an alcohol solution of a carbon source and stirred to form a Zn3Nb2O8@TiNb2O7@C composite sol. Then, it was aged, dried and ground in sequence to obtain the Zn3Nb2O8@TiNb2O7@C precursor powder. Then, it was pre-calcined at low temperature and calcined at high temperature under inert conditions. After cooling, it was ground to obtain the Zn3Nb2O8@TiNb2O7@C composite anode material.

5. The method for preparing the composite negative electrode material as described in claim 4, characterized in that, The zinc source is zinc acetate and / or zinc nitrate, the niobium source is niobium oxalate and / or niobium alkoxide, the titanium source is tetrabutyl titanate and / or isopropyl titanate, and the carbon source is selected from at least one of glucose, sucrose, and polyethylene glycol. In the preparation of Zn3Nb2O8, the molar ratio of the zinc source to citric acid is 1:1.5~2; in the preparation of Zn3Nb2O8@TiNb2O7, the molar ratio of the titanium source to citric acid is 1:2.8~3.

2.

6. The method for preparing the composite negative electrode material as described in claim 4, characterized in that, The stirring process forms a Zn3Nb2O8 sol, which is then subjected to aging, drying, and grinding in sequence. Specifically, the sol is stirred at a constant temperature of 75~85℃ until a uniform, precipitate-free, and highly fluid Zn3Nb2O8 sol is formed. It is then aged at room temperature for 12~24 hours, followed by drying and grinding to obtain the final product.

7. The method for preparing the composite negative electrode material as described in claim 4, characterized in that, The process of continuing to stir to form a Zn3Nb2O8@TiNb2O7 composite sol, followed by aging, drying, and grinding, is as follows: after ultrasonic dispersion, the mixture is stirred at room temperature for 1.5~2.5h, then heated to 70~80℃ and stirred at a constant temperature to form a Zn3Nb2O8@TiNb2O7 composite sol. The mixture is then aged at room temperature for 8~12h, followed by drying and grinding to obtain the final product.

8. The method for preparing the composite negative electrode material as described in claim 4, characterized in that, The stirring process forms a Zn3Nb2O8@TiNb2O7@C composite sol, which is then subjected to aging, drying, and grinding in sequence. Specifically, after ultrasonic dispersion, the sol is stirred at room temperature for 1.5~2.5h, then heated to 75~85℃ and stirred at a constant temperature to form a Zn3Nb2O8@TiNb2O7@C composite sol. The sol is then aged at room temperature for 6~8h, followed by drying and grinding to obtain the final product.

9. The method for preparing the composite negative electrode material as described in claim 4, characterized in that, The specific steps of performing low-temperature pre-calcination and high-temperature calcination under inert conditions are as follows: Zn3Nb2O8@TiNb2O7@C precursor powder is pre-calcined at 420~480℃ for 1.5~2.5h under inert conditions, cooled and ground, and then calcined at 750~800℃ for 4~6h under inert conditions. After cooling and grinding, the product is obtained.

10. A lithium-ion secondary battery, characterized in that, This includes the composite anode material as described in any one of claims 1 to 3 or the composite anode material prepared according to the preparation method described in any one of claims 4 to 9.

Images