Composite negative electrode material, preparation method thereof and battery

Abstract

The invention relates to the field of batteries, in particular to a composite negative electrode material, a preparation method thereof and a battery, the composite negative electrode material comprises graphite particles and a coating layer coating the surfaces of the graphite particles; the graphite particles comprise acidified graphite particles, and the acidified graphite particles comprise graphite particles containing oxygen-containing functional groups; the coating layer comprises lithium niobate crystals; the chemical formula of the lithium niobate crystal is LixNbOy, x is more than 0 and less than or equal to 1, and y is more than 0 and less than or equal to 3. According to the composite negative electrode material disclosed by the invention, the desolvation energy barrier of lithium ions is reduced, and the transmission impedance at the interface of a negative electrode and electrolyte is reduced, so that the fast charging performance of graphite is improved.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and in particular to a composite negative electrode material, a preparation method thereof, and a battery. Background Art

[0002] Low-carbon and environmentally friendly demands are driving the development of new energy vehicles in today's society, providing more convenient travel options. Lithium-ion batteries, as core components, are crucial for increasing the acceptance of new energy vehicles. To date, despite significant breakthroughs in range and reliability, consumer acceptance of electric vehicles still lags far behind that of traditional vehicles. The primary reason for the low market penetration of electric vehicles is their suboptimal charging speed. Therefore, developing advanced fast-charging technology is a top priority for the battery industry.

[0003] Graphite, due to its excellent electrochemical stability, high theoretical specific capacity, and low lithiation potential, can significantly increase the energy density of batteries, making it the most popular negative electrode for lithium-ion batteries on the market. However, with the development of battery fast-charging technology, graphite negative electrodes face a series of problems, including: poor low-temperature performance; slow diffusion of lithium ions within the graphite; the diffusion path of lithium ions is from the end face, which results in a long migration path; the co-intercalation of lithium ions and solvents, which destroys the graphite layered structure; during high-current charging, the graphite experiences large electrochemical polarization, forcing the graphite's lithiation potential closer to or even lower than the deposition potential of metallic lithium, leading to surface lithium deposition and even the formation of lithium dendrites; and slow kinetics of lithium ions at the interface, including transport and desolvation processes within the SEI. Therefore, how to improve the fast-charging performance of graphite is an urgent problem that needs to be solved. Summary of the Invention

[0004] In view of this, the present invention is committed to providing a composite negative electrode material and a preparation method thereof and a battery to solve the problem of poor fast charging performance of graphite in the prior art.

[0005] In order to solve the above technical problems, this application is implemented as follows:

[0006] The present invention provides a composite negative electrode material, comprising graphite particles and a coating layer coated on the surface of the graphite particles;

[0007] The graphite particles include acidified graphite particles, and the acidified graphite particles include graphite particles containing oxygen-containing functional groups;

[0008] The coating layer includes lithium niobate crystals; the chemical formula of the lithium niobate crystals is Li x NbO y , where 0<x≤1, 0<y≤3.

[0009] Optionally, based on the mass of the composite negative electrode material, the content of the lithium niobate crystals is 1 to 8 wt%, preferably 2 to 5 wt%.

[0010] Optionally, the coating layer has a thickness of 100 to 200 nm, preferably 120 to 160 nm.

[0011] Optionally, the D50 of the acidified graphite particles is 13 to 17 μm, preferably 14 to 16 μm.

[0012] A second aspect of the present invention provides a method for preparing a composite negative electrode material, comprising the following steps:

[0013] S1, mixing graphite particles and a first solution containing an acidifying agent and performing a first dispersion treatment to obtain a first mixture; performing a washing treatment and a first drying treatment on the first mixture to obtain acidified graphite particles;

[0014] S2, mixing a niobium source, a lithium source, and a solvent to obtain a second solution containing lithium niobate; mixing and drying the acidified graphite particles and the second solution to obtain a second mixture;

[0015] S3, grinding the second mixed material and then performing a roasting process.

[0016] Optionally, in step S1, the molar ratio of the graphite particles to the acidulant is 1:(0.75-0.8); optionally, in the first solution, the concentration of the acidulant is 2.5-2.7 mol / L; optionally, the acidulant includes at least one of ammonium persulfate, sulfuric acid and nitric acid.

[0017] Optionally, the molar ratio of the niobium source to the lithium source is 1:(1-1.02); optionally, the niobium source includes at least one of niobium oxalate amine, niobium oxalate and niobium acetate; optionally, the lithium source includes at least one of lithium acetate, lithium nitrate, lithium sulfate, lithium chloride, lithium carbonate and lithium hydroxide; optionally, the solvent includes water.

[0018] Optionally, the mass ratio of the acidified graphite particles to the lithium niobate contained in the second solution is (11.5-99):1, preferably (30-50):1.

[0019] Optionally, in step S1, the conditions for the first dispersion treatment include: temperature of 60-65°C and time of 8-9 hours; the conditions for the first drying treatment include: temperature of 70-80°C and time of 10-12 hours; and / or, in step S2, the conditions for the mixed drying treatment include: temperature of 78-82°C and time of 2-2.5 hours; and / or, in step S3, the calcination treatment is carried out in an inert atmosphere, optionally, the inert atmosphere includes argon; the conditions for the calcination treatment include: temperature of 780-820°C, time of 5-6 hours, and a heating rate of 2-6°C / min.

[0020] A third aspect of the present invention provides a battery, comprising a composite negative electrode material, wherein the composite negative electrode material comprises the composite negative electrode material described above and / or a composite negative electrode material prepared according to the above preparation method.

[0021] Through the above technical solution, the beneficial technical effects of the present invention are:

[0022] (1) The composite negative electrode material of the present invention comprises graphite particles and lithium niobate crystals coated on the surface of the graphite particles, wherein the graphite particles comprise acidified graphite, and oxygen-containing functional groups exist on the surface of the acidified graphite particles, which are connected to the lithium ions in the lithium niobate crystals by forming ionic bonds through electrostatic attraction. The carbon atoms on the surface of the acidified graphite are also connected to the metal ions in the lithium niobate crystals through chemical bonds, that is, the carbon atoms are connected to niobium through chemical bonds, and the carbon atoms are connected to lithium through chemical bonds. At the same time, the interfacial energy between the graphite and lithium niobate of the acidified graphite and the lithium niobate crystals is low, and there may be van der Waals forces to promote their close bonding. That is, the stability and uniformity of the lithium niobate coating are ensured by the synergistic action of various forces. The lithium niobate crystals coated on the surface of acidified graphite have high ionic conductivity, which plays a great role in accelerating the transmission of lithium ions at the interface; and the surface-coated lithium niobate can induce the salt anions in the electrolyte at the graphite interface to participate in the solvation structure, promoting the desolvation process of lithium ions and the formation of LiF-rich SEI film, thereby reducing the desolvation energy barrier of lithium ions and reducing the transmission impedance at the interface between the negative electrode and the electrolyte, thereby improving the fast charging performance of graphite.

[0023] (2) In the preparation method of the composite negative electrode material of the present invention, oxygen-containing functional groups are generated on the surface of the graphite particles under the action of an acidifying agent, and lithium niobate is then coated on the surface of the acidified graphite after reacting with lithium niobate generated from a niobium source and a lithium source. The oxygen-containing functional groups on the surface of the acidified graphite can attract the precursor of the lithium niobate to form a bond, thereby forming a lithium niobate coating site, so that the coating layer is more uniform; finally, the lithium niobate is calcined to form a crystal structure while removing the unreacted oxygen-containing functional groups; the desolvation energy barrier of lithium ions is reduced, and the transmission impedance at the interface between the negative electrode and the electrolyte is reduced, thereby improving the fast charging performance of the graphite.

[0024] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present invention, but do not constitute a limitation of the present invention.

[0026] Figure 1 Shown is the XDR image of the composite negative electrode material and graphite in Example 1;

[0027] Figure 2 Shown are the cycle performance graphs of corresponding batteries prepared using the composite negative electrode material (LNO-Graphite) in Example 1 and the composite negative electrode material (Graphite) in Comparative Example 1 as electrode materials;

[0028] Figure 3 Shown are the 200th cycle constant current charge-discharge curves of corresponding batteries prepared using the composite negative electrode material (LNO-Graphite) in Example 1 and the composite negative electrode material (Graphite) in Comparative Example 1 as electrode materials;

[0029] Figure 4 Shown are rate performance graphs of corresponding batteries made using the composite negative electrode material (LNO-Graphite) in Example 1 and the composite negative electrode material (Graphite) in Comparative Example 1 as electrode materials;

[0030] Figure 5 Shown are the impedances of corresponding batteries made using the composite negative electrode material (LNO-Graphite) in Example 1 and the composite negative electrode material (Graphite) in Comparative Example 1 as electrode materials, respectively. DETAILED DESCRIPTION

[0031] The present invention discloses a composite negative electrode material, a method for preparing the same, and a battery. Those skilled in the art can refer to the contents of this document and appropriately improve the process parameters to achieve the desired results. It should be noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments. It is obvious that relevant personnel can modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of the present invention to implement and apply the technology of the present invention.

[0032] In the description of the present invention, a list of items connected by the term "at least one of" or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A can include a single element or multiple elements. Item B can include a single element or multiple elements. Item C can include a single element or multiple elements.

[0033] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range or the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0034] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0035] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0036] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0037] In order to solve the problem of poor graphite fast charging performance in the prior art, the present invention adopts the following technical solutions:

[0038] The present invention provides a composite negative electrode material, comprising graphite particles and a coating layer coated on the surface of the graphite particles;

[0039] The graphite particles include acidified graphite particles, and the acidified graphite particles include graphite particles containing oxygen-containing functional groups;

[0040] The coating layer includes lithium niobate crystals; the chemical formula of the lithium niobate crystals is Li x NbO y , where 0<x≤1, 0<y≤3.

[0041] The composite negative electrode material of the present invention includes graphite particles and lithium niobate crystals coated on the surfaces of the graphite particles, wherein the graphite particles include acidified graphite. Oxygen-containing functional groups exist on the surfaces of the acidified graphite particles, which are ionically bonded to lithium ions in the lithium niobate crystals through electrostatic attraction. Carbon atoms on the surface of the acidified graphite are also chemically bonded to metal ions in the lithium niobate crystals, that is, carbon atoms are chemically bonded to niobium, and carbon atoms are chemically bonded to lithium. At the same time, the interfacial energy between the acidified graphite and the lithium niobate crystals is low, and van der Waals forces may exist to promote their close bonding. That is, the stability and uniformity of the lithium niobate coating are ensured through the synergistic action of various forces. The lithium niobate crystals coated on the surface of acidified graphite have high ionic conductivity, which plays a great role in accelerating the transmission of lithium ions at the interface; and the surface-coated lithium niobate can induce the salt anions in the electrolyte at the graphite interface to participate in the solvation structure, promoting the desolvation process of lithium ions and the formation of LiF-rich SEI film, thereby reducing the desolvation energy barrier of lithium ions and reducing the transmission impedance at the interface between the negative electrode and the electrolyte, thereby improving the fast charging performance of graphite.

[0042] According to the present invention, based on the mass of the composite negative electrode material, the content of the lithium niobate crystals is 1 to 8 wt%. In the present invention, an appropriate content of lithium niobate crystals helps to uniformly coat the graphite surface with lithium niobate. As an example, based on the mass of the composite negative electrode material, the content of the lithium niobate crystals can be any of 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt% and 8 wt%, or any value within the range of any two of the above values. In the present invention, an excessively high content of lithium niobate crystals may cause lithium niobate to agglomerate, and the active material loading on the electrode is too low, resulting in a low battery capacity. An excessively low content of lithium niobate crystals may result in an insignificant lithium niobate coating effect and insignificant performance improvement. Preferably, based on the mass of the composite negative electrode material, the content of the lithium niobate crystals is 2 to 5 wt%.

[0043] According to the present invention, the thickness of the coating layer is 100 to 200 nm. In the present invention, a suitable coating layer thickness helps lithium niobate to accelerate lithium ion transmission and desolvation. As an example, the thickness of the coating layer can be any value among 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm and 200 nm, or any value within the range of values ​​consisting of any two of the above values. In the present invention, if the thickness of the coating layer is too thick, the lithium ion transmission path will become longer and the impedance will become larger; if the thickness of the coating layer is too thin, the coating layer will be damaged during the cycle and the side reactions will be aggravated. Preferably, the thickness of the coating layer is 120 to 160 nm.

[0044] According to the present invention, the D50 of the acidified graphite particles is 13 to 17 μm. In the present invention, an appropriate D50 of the acidified graphite particles helps ensure processing during coating, reducing coating difficulty, and a suitable internal lithium ion migration path is beneficial for ensuring rate performance. As an example, the D50 of the acidified graphite particles can be any value among 13 μm, 13.5 μm, 14 μm, 14.5 μm, 15 μm, 15.5 μm, 16 μm, 16.5 μm, and 17 μm, or any value within a range consisting of any two of these values. In the present invention, if the D50 of the acidified graphite particles is too large, the lithium ion transport path within the graphite will be long, the rate performance will be poor, and the graphite will be prone to stress cracking during cycling. If the D50 of the acidified graphite particles is too small, the specific surface area will be large. The larger the specific surface area of ​​the graphite coated with lithium niobate that is in contact with the electrolyte, the more charge will be consumed by the SEI film formed during the initial charge and discharge process, the greater the irreversible capacity loss will be, and the battery performance will be deteriorated. Preferably, the D50 of the acidified graphite particles is 14-16 μm.

[0045] A second aspect of the present invention provides a method for preparing a composite negative electrode material, comprising the following steps:

[0046] S1, mixing graphite particles and a first solution containing an acidifying agent and performing a first dispersion treatment to obtain a first mixture; performing a washing treatment and a first drying treatment on the first mixture to obtain acidified graphite particles;

[0047] S2, mixing a niobium source, a lithium source, and a solvent to obtain a second solution containing lithium niobate; mixing and drying the acidified graphite particles and the second solution to obtain a second mixture;

[0048] S3, grinding the second mixed material and then performing a roasting process.

[0049] In the preparation method of the composite negative electrode material of the present invention, oxygen-containing functional groups are generated on the surface of the graphite particles under the action of an acidifying agent, and lithium niobate is then coated on the surface of the acidified graphite after reacting with the lithium niobate generated from the niobium source and the lithium source. The oxygen-containing functional groups on the surface of the acidified graphite can attract the precursor of the lithium niobate to form a bond, forming a lithium niobate coating site, so that the coating layer is more uniform; finally, the lithium niobate is calcined to form a crystal structure while removing the unreacted oxygen-containing functional groups. The lithium niobate crystals in the coating layer have high ionic conductivity, which plays a great role in accelerating the transmission of lithium ions at the interface; and the surface-coated lithium niobate can induce the salt anions in the electrolyte at the graphite interface to participate in the solvation structure, promote the desolvation process of lithium ions and the formation of a LiF-rich SEI film, thereby reducing the desolvation energy barrier of lithium ions and reducing the transmission impedance at the interface between the negative electrode and the electrolyte, thereby improving the fast charging performance of the graphite.

[0050] According to the present invention, in step S1, the molar ratio of the graphite particles to the acidulant is 1:(0.75-0.8). In the present invention, a suitable molar ratio of the graphite particles to the acidulant helps to connect abundant groups to the graphite surface, so that the lithium niobate is evenly coated with the graphite. As an example, the molar ratio of the graphite particles to the acidulant can be any value among 1:0.75, 1:0.76, 1:0.77, 1:0.78, 1:0.79 and 1:0.8, or any value within the range of any two of the above values. In the present invention, if the molar ratio of the graphite particles to the acidulant is too high, the layered structure of the graphite will be destroyed; if the molar ratio of the graphite particles to the acidulant is too low, the graphite surface will have fewer oxygen-containing functional groups connected to it, resulting in uneven lithium niobate coating.

[0051] According to the present invention, the concentration of the acidifier in the first solution is 2.5 to 2.7 mol / L. As an example, the concentration of the acidifier in the first solution can be any value among 2.5 mol / L, 2.55 mol / L, 2.6 mol / L, 2.65 mol / L, and 2.7 mol / L, or any value within a range consisting of any two of the above values.

[0052] Optionally, the acidifying agent includes at least one of ammonium persulfate, sulfuric acid and nitric acid.

[0053] According to the present invention, the molar ratio of the niobium source to the lithium source is 1:(1 to 1.02). According to the present invention, a suitable molar ratio of the niobium source to the lithium source is conducive to the formation of lithium niobate. As an example, the molar ratio of the niobium source to the lithium source can be any one of 1:1, 1.01 and 1.02, or any value within the range of any two of the above values. In the present invention, if the molar ratio of the niobium source to the lithium source is too high, it will lead to excessively high costs and the introduction of additional impurities; if the molar ratio of the niobium source to the lithium source is too low, it will lead to insufficient reaction and the introduction of impurities.

[0054] Optionally, the niobium source includes at least one of niobium amine oxalate, niobium oxalate and niobium acetate.

[0055] Optionally, the lithium source includes at least one of lithium acetate, lithium nitrate, lithium sulfate, lithium chloride, lithium carbonate and lithium hydroxide.

[0056] Optionally, the solvent comprises water.

[0057] According to the present invention, the mass ratio of the acidified graphite particles to the lithium niobate contained in the second solution is (11.5-99):1. In the present invention, an appropriate mass ratio of the acidified graphite particles to the lithium niobate contained in the second solution helps the lithium niobate to uniformly coat the graphite with an appropriate thickness. As an example, the mass ratio of the acidified graphite particles to the lithium niobate contained in the second solution can be any value among 11.5:1, 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, 90:1, and 99:1, or any value within a range consisting of any two of the above values. In the present invention, if the mass ratio of the acidified graphite particles to the lithium niobate contained in the second solution is too high, the lithium niobate coating layer will be too thin or incomplete; if the mass ratio of the acidified graphite particles to the lithium niobate contained in the second solution is too low, the excess lithium niobate will not exist as a coating, but will simply mix with the graphite as an impurity, affecting the graphite loading. Preferably, the mass ratio of the acidified graphite particles to the lithium niobate contained in the second solution is (30-50):1.

[0058] According to the present invention, in step S1, the conditions for the first dispersion treatment include: a temperature of 60-65°C and a time of 8-9 hours. As an example, in step S1, the conditions for the first dispersion treatment include: a temperature of 60°C, 61°C, 62°C, 63°C, 64°C, and 65°C, or any value within a range consisting of any two of the above values; and a time of 8 hours, 8.5 hours, and 9 hours, or any value within a range consisting of any two of the above values.

[0059] According to the present invention, the conditions for the first drying treatment include: a temperature of 70 to 80°C and a time of 10 to 12 hours. As an example, the conditions for the first drying treatment include: a temperature of 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C, and 80°C, or any value within a range consisting of any two of the above values, and a time of 10 hours, 11 hours, and 12 hours, or any value within a range consisting of any two of the above values.

[0060] According to the present invention, in step S2, the conditions for the mixed drying process include: a temperature of 78 to 82°C and a time of 2 to 2.5 hours. As an example, in step S2, the conditions for the mixed drying process include: a temperature of 78°C, 79°C, 80°C, 81°C, and 82°C, or any value within a range consisting of any two of the above values, and a time of 2 hours, 2.25 hours, and 2.5 hours, or any value within a range consisting of any two of the above values.

[0061] According to the present invention, in step S3, the calcination treatment is carried out in an inert atmosphere, optionally, the inert gas includes argon; the conditions of the calcination treatment include: a temperature of 780 to 820°C, a time of 5 to 6 hours, and a heating rate of 2 to 6°C / min. As an example, the conditions of the calcination treatment include: a temperature of 780°C, 790°C, 800°C, 810°C and 820°C or any value within the range of any two of the above values, a time of 5 hours, 5.5 hours and 6 hours or any value within the range of any two of the above values, and a heating rate of 2°C / min, 3°C / min, 4°C / min, 5°C / min and 6°C / min or any value within the range of any two of the above values.

[0062] A third aspect of the present invention provides a battery, comprising a composite negative electrode material, wherein the composite negative electrode material comprises the composite negative electrode material described above and / or a composite negative electrode material prepared according to the above preparation method.

[0063] The present invention is further described in detail below by way of examples. The raw materials used in the examples can all be obtained through commercial sources.

[0064] Example 1

[0065] The method for preparing a composite negative electrode material comprises the following steps:

[0066] (1) Ammonium sulfate and deionized water are mixed and dissolved to obtain an ammonium sulfate solution with a concentration of 2.6 mol / L; graphite particles are added to the ammonium sulfate solution, and the molar ratio of graphite particles to ammonium sulfate is 1:0.8. After heating and stirring in an oil bath at 60°C for 8 hours, the obtained product is subjected to solid-liquid separation by vacuum filtration, and the filtration membrane is a polytetrafluoroethylene (PTFE) diaphragm with a pore size of 0.22 μm. The filtered product is washed with deionized water until the pH is neutral, and then dried in a vacuum drying oven at 75°C for 12 hours to obtain acidified graphite.

[0067] (2) Niobium oxalate and lithium acetate are dissolved in deionized water at a molar ratio of 1:1 based on the metal elements to obtain a mixed solution containing lithium niobate, and acidified graphite is added to the mixed solution containing lithium niobate to obtain a precursor mixed solution, wherein the mass ratio of acidified graphite to lithium niobate is 1:99; the precursor mixed solution is stirred at 25°C for 2 hours, and then stirred and heated at 80°C until the water is evaporated to dryness to obtain a mixture.

[0068] (3) After grinding the mixture for 0.5 h, calcining it in an argon atmosphere at a temperature of 800°C, a time of 5 h, and a heating rate of 3°C / min. After calcination, the mixture was cooled and the cooled product was ground for 30 min to obtain a 1 wt% lithium niobate-coated acidified graphite material, which is a composite negative electrode material.

[0069] Example 2

[0070] The preparation method of the composite negative electrode material is similar to that in Example 1, except that the mass ratio of acidified graphite to lithium niobate is 2:98.

[0071] Example 3

[0072] The preparation method of the composite negative electrode material is similar to that in Example 1, except that the mass ratio of acidified graphite to lithium niobate is 3:97.

[0073] Example 4

[0074] The preparation method of the composite negative electrode material is similar to that in Example 1, except that the mass ratio of acidified graphite to lithium niobate is 4:96.

[0075] Example 5

[0076] The preparation method of the composite negative electrode material is similar to that in Example 1, except that the mass ratio of acidified graphite to lithium niobate is 5:95.

[0077] Example 6

[0078] The preparation method of the composite negative electrode material is similar to that in Example 1, except that the mass ratio of acidified graphite to lithium niobate is 6:94.

[0079] Example 7

[0080] The preparation method of the composite negative electrode material is similar to that in Example 1, except that the mass ratio of acidified graphite to lithium niobate is 7:93.

[0081] Example 8

[0082] The preparation method of the composite negative electrode material is similar to that in Example 1, except that the mass ratio of acidified graphite to lithium niobate is 8:92.

[0083] Comparative Example 1

[0084] The composite negative electrode material is unmodified graphite material.

[0085] Test Example 1

[0086] The XDR images of the composite negative electrode material and graphite in Example 1 are as follows: Figure 1 As shown. Figure 1It can be seen that the characteristic peaks of the composite negative electrode material of lithium niobate-coated graphite synthesized in Example 1 correspond to those of the original graphite, indicating that lithium niobate does not change the layered structure of graphite.

[0087] Test Example 2

[0088] Preparation of electrode sheets and battery assembly: The composite negative electrode material, binder polyvinylidene fluoride (PVDF), and conductive agent conductive carbon black (super-P) prepared in the embodiment and comparative example are respectively added to a certain amount of N-methylpyrrolidone (NMP), the mass ratio of the composite negative electrode material, binder polyvinylidene fluoride (PVDF) and conductive agent conductive carbon black (super-P) is 8:1:1, and stirred in a drying room to form a slurry. The slurry is evenly coated on a copper foil by a doctor blade casting method. After drying and rolling, the electrode sheet is punched to obtain a circular sheet with a diameter of 1.2 cm; then placed in an oven at 60°C for 12 hours of drying, and then the above-prepared electrode sheet and metal lithium sheet are assembled into a CR2032 button battery in an argon-protected glove box, wherein the electrolyte is a solution of 1 mol / L LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1. The battery assembly order is negative electrode shell-metal lithium-diaphragm-electrode piece-gasket-spring-positive electrode shell. The battery assembly is completed and aged for 12 hours.

[0089] Lithium-ion battery performance testing, including:

[0090] (1) Capacity test:

[0091] 100mA / g charge capacity: The battery is charged to 3V at a current density of 100mA / g using constant current. The capacity measured in this step is the 100mA / g charge capacity (denoted as C0).

[0092] 100 mA / g discharge capacity: The battery is discharged at a constant current to 0.01 V at a current density of 100 mA / g. The capacity measured in this step is the 100 mA / g discharge capacity (denoted as C1).

[0093] (2) Cyclic performance test (tested in blue electric system):

[0094] Charging: constant current charging to 3V at a current density of 100mA / g; Discharging: constant current discharging to 0.01V at a current density of 100mA / g. Completing one charge and discharge process is considered to complete one cycle.

[0095] 100mA / g discharge capacity: 100mA / g constant current charge to 3V, 100mA / g constant current discharge to 0.01V. The discharge capacity measured in this step is the 100mA / g discharge capacity (denoted as C2).

[0096] Discharge capacity at the 200th cycle at 100 mA / g: charge at a constant current of 100 mA / g to 3 V, and discharge at a constant current of 100 mA / g to 0.01 V. The discharge capacity measured in this step is the discharge capacity at the 200th cycle at 100 mA / g (denoted as C3).

[0097] The capacity retention rate after running 200 cycles at 100mA / g is: C3 / C2.

[0098] 100mA / g Maximum discharge capacity except the first cycle: Except the first cycle, there is a slow capacity increase in subsequent cycles, which refers to the maximum capacity after the capacity increase.

[0099] (3) Rate performance test (tested on the blue-electric system):

[0100] At a current density of 50 mA / g, the battery was charged at a constant current to 3 V; at a current density of 50 mA / g, the battery was discharged at a constant current to 0.01 V. One charge and discharge process was considered to be one cycle, and the battery was cycled 5 times at a current density of 50 mA / g.

[0101] At a current density of 100 mA / g, the battery was charged to 3 V at a constant current; at a current density of 100 mA / g, the battery was discharged to 0.01 V at a constant current. One charge and discharge process was considered to be one cycle. The battery was cycled 5 times at a current density of 100 mA / g.

[0102] At a current density of 300 mA / g, the battery was charged at a constant current to 3 V. At a current density of 300 mA / g, the battery was discharged at a constant current to 0.01 V. One charge and discharge process was considered to be one cycle. The battery was cycled 5 times at a current density of 300 mA / g.

[0103] The battery was charged at a constant current of 500 mA / g to 3 V; discharged at a constant current of 500 mA / g to 0.01 V. One charge and discharge process was considered to be one cycle. The battery was cycled 5 times at a current density of 500 mA / g.

[0104] The battery was charged at a constant current of 800 mA / g to 3 V; discharged at a constant current of 800 mA / g to 0.01 V. One charge and discharge process was considered to be one cycle. The battery was cycled 5 times at a current density of 800 mA / g.

[0105] At a current density of 1A / g, the battery was charged at a constant current to 3V; at a current density of 1A / g, the battery was discharged at a constant current to 0.01V. One charge and discharge process was considered to be one cycle. The battery was cycled 5 times at a current density of 1A / g.

[0106] At a current density of 1.5 A / g, the battery was charged at a constant current to 3 V; at a current density of 1.5 A / g, the battery was discharged at a constant current to 0.01 V. One charge and discharge process was considered to be one cycle, and the battery was cycled 5 times at a current density of 1.5 A / g.

[0107] At a current density of 2A / g, the battery was charged at a constant current to 3V; at a current density of 2A / g, the battery was discharged at a constant current to 0.01V. One charge and discharge process was considered to be one cycle. The battery was cycled 5 times at a current density of 2A / g.

[0108] Finally, the battery was charged at a constant current of 50 mA / g to 3 V, and discharged at a constant current of 50 mA / g to 0.01 V. Completing one charge and discharge process was considered to be one cycle, and the battery was cycled 5 times at a current density of 50 mA / g.

[0109] (3) Electrochemical impedance spectroscopy (EIS):

[0110] The test frequency range was set to 0.01 to 100,000 Hz, the amplitude was set to 10 mV, and the scan rate was 6 data points per second. The test results are shown in Table 1.

[0111] Table 1

[0112]

[0113] The data in Table 1 show that the composite negative electrode materials prepared in the examples of the present invention all outperform the comparative examples. The materials prepared in Examples 1-3 exhibited increasing capacity and decreasing impedance with increasing lithium niobate content, with Example 3 reaching the highest capacity and also exhibiting increasing capacity retention. The materials prepared in Examples 4-8 exhibited decreasing capacity with increasing lithium niobate coating 1 content, with capacity retention rates comparable to those of Example 3 after 200 cycles, and continued to decrease in impedance.

[0114] Figure 2 The cycle performance diagram of the corresponding battery prepared by using the composite negative electrode material (LNO-Graphite) in Example 1 and the composite negative electrode material (Graphite) in Comparative Example 1 as electrode materials is shown. Figure 2 It can be seen that when using unmodified graphite, the specific capacity has shown obvious attenuation after 150 cycles, while the battery prepared by the material prepared by the present invention still maintains a stable cycle trend, indicating that the cycle stability of the battery assembled with the composite negative electrode material prepared by the present invention is significantly improved.

[0115] Figure 3The constant current charge-discharge curves of the corresponding batteries prepared using the composite negative electrode material (LNO-Graphite) in Example 1 and the composite negative electrode material (Graphite) in Comparative Example 1 as electrode materials are shown. Figure 3 It can be seen that the specific capacity of unmodified graphite after 200 cycles is only 100 mAh g -1 , while using lithium niobate (Li x NbO y ) The modified graphite can still maintain 400 mAh g after 200 cycles. -1 high specific capacity.

[0116] Figure 4 The rate performance diagram of the corresponding battery prepared by using the composite negative electrode material (LNO-Graphite) in Example 1 and the composite negative electrode material (Graphite) in Comparative Example 1 as electrode materials is shown. Figure 4 It can be seen that at low current density Li x NbO y The specific capacity of the modified graphite is slightly higher than that of the unmodified graphite, but the specific capacity of the modified graphite is significantly higher than that of the unmodified graphite at high current density. After cycling at a high current density of 2000mAg, the capacity retention rate is still good when the small current of 50mAg is restored, which proves that the kinetic performance of the modified graphite is significantly improved.

[0117] Figure 5 The impedance of the corresponding battery prepared by using the composite negative electrode material (LNO-Graphite) in Example 1 and the composite negative electrode material (Graphite) in Comparative Example 1 as electrode materials is shown. Figure 5 It can be seen that the impedance of modified graphite is lower than that of unmodified graphite before and after cycling.

[0118] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A composite negative electrode material, characterized in that The composite negative electrode material includes graphite particles and a coating layer coated on the surface of the graphite particles; The graphite particles include acidified graphite particles, and the acidified graphite particles include graphite particles containing oxygen-containing functional groups; The coating layer includes lithium niobate crystals; the chemical formula of the lithium niobate crystals is Li x NbO y , where 0<x≤1, 0<y≤3.

2. The composite negative electrode material according to claim 1, characterized in that Based on the mass of the composite negative electrode material, the content of the lithium niobate crystals is 1 to 8 wt%, preferably 2 to 5 wt%.

3. The composite negative electrode material according to claim 1, characterized in that The thickness of the coating layer is 100 to 200 nm, preferably 120 to 160 nm.

4. The composite negative electrode material according to claim 1, characterized in that The D50 of the acidified graphite particles is 13 to 17 μm, preferably 14 to 16 μm.

5. A method for preparing a composite negative electrode material, characterized in that: The following steps are involved: S1, mixing graphite particles and a first solution containing an acidifying agent and performing a first dispersion treatment to obtain a first mixture; performing a washing treatment and a first drying treatment on the first mixture to obtain acidified graphite particles; S2, mixing a niobium source, a lithium source, and a solvent to obtain a second solution containing lithium niobate; mixing and drying the acidified graphite particles and the second solution to obtain a second mixture; S3, grinding the second mixed material and then performing a roasting process.

6. The preparation method according to claim 5, characterized in that In step S1, the molar ratio of the graphite particles to the acidifying agent is 1:(0.75-0.8); Optionally, in the first solution, the concentration of the acidifier is 2.5 to 2.7 mol / L; Optionally, the acidifying agent includes at least one of ammonium persulfate, sulfuric acid and nitric acid.

7. The preparation method according to claim 5, characterized in that The molar ratio of the niobium source to the lithium source is 1:(1-1.02); The niobium source includes at least one of niobium oxalate ammonium, niobium oxalate and niobium acetate; The lithium source includes at least one of lithium acetate, lithium nitrate, lithium sulfate, lithium chloride, lithium carbonate and lithium hydroxide; The solvent includes water.

8. The preparation method according to claim 5, characterized in that The mass ratio of the acidified graphite particles to the lithium niobate contained in the second solution is (11.5-99):1, preferably (30-50):

1.

9. The preparation method according to claim 5, characterized in that In step S1, the conditions for the first dispersion treatment include: a temperature of 60-65° C. and a time of 8-9 hours; the conditions for the first drying treatment include: a temperature of 70-80° C. and a time of 10-12 hours; and / or, In step S2, the conditions of the mixed drying process include: a temperature of 78 to 82° C. and a time of 2 to 2.5 hours; and / or, In step S3, the calcination treatment is carried out in an inert atmosphere. Optionally, the inert atmosphere includes argon. The calcination treatment conditions include: temperature of 780-820°C, time of 5-6h, and heating rate of 2-6°C / min.

10. A battery, characterized in that: The battery comprises a composite negative electrode material, wherein the composite negative electrode material comprises the composite negative electrode material according to any one of claims 1 to 4 and / or the composite negative electrode material prepared according to the preparation method according to any one of claims 5 to 9.

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