Composite positive electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a composite positive electrode material and a preparation method and application thereof. The invention provides a composite positive electrode material, the composite positive electrode material comprises a lithium-containing inner core and a coating layer arranged on the outer surface of the lithium-containing inner core, and the coating layer comprises a carbon coating layer and doped graphene quantum dots dispersed on the outer surface of the lithium-containing inner core and / or the outer surface of the carbon coating layer. The dispersed doped graphene quantum dots can greatly improve the electronic conductivity and the ion migration rate of the lithium-containing core, so that the composite positive electrode material has wide application.

Description

Technical Field

[0001] The present application belongs to the technical field of lithium-ion batteries, and in particular relates to a composite positive electrode material and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries are gradually being widely used in all aspects of people's lives, such as power vehicles, energy storage power stations, and digital 3C, due to their advantages such as cleanliness, environmental protection, high energy density, and long cycle life. Among them, the positive electrode material is an important component of lithium-ion batteries, which provides the lithium ions required during the charging and discharging process. Therefore, the development of high-performance and low-cost positive electrode materials is the key to the development of lithium-ion batteries.

[0003] Phosphate-based cathode materials have an olivine crystal structure, which has a high discharge platform and energy density, and excellent safety at high temperatures. However, the electronic and ionic conductivity of phosphate materials are very low, and they can almost be considered semi-insulators, so the discharge capacity is relatively low and the rate performance is poor. In addition, in the presence of manganese, it will gradually dissolve during the cycle due to the influence of the Jan-Taylor effect. These shortcomings limit the development of phosphate cathode materials.

[0004] Therefore, there is an urgent need to provide a preparation method that can effectively improve the electronic conductivity and lithium ion diffusion rate of phosphate materials, so as to further broaden the application of phosphate materials as positive electrode materials. Summary of the invention

[0005] The purpose of the present application is to provide a composite positive electrode material and a preparation method and application thereof, aiming to solve the problem of low electronic conductivity and lithium ion diffusion rate of phosphate positive electrode materials in the prior art.

[0006] In order to achieve the above application purpose, the technical solution adopted in this application is as follows:

[0007] In a first aspect, the present application provides a composite positive electrode material, comprising a lithium-containing core and a coating layer arranged on the outer surface of the lithium-containing core, wherein the coating layer comprises a carbon coating layer and doped graphene quantum dots dispersed on the outer surface of the lithium-containing core and / or the outer surface of the carbon coating layer.

[0008] In some embodiments, the doping element of the doped graphene quantum dots includes at least one of nitrogen, phosphorus, sulfur, and fluorine.

[0009] In some embodiments, the Raman carbon characteristic peak I of the doped graphene quantum dots D / I G The value is 0.965~0.995.

[0010] In some embodiments, the lithium-containing core has the general formula LiMn xFe y M z PO4, wherein M is selected from Ti, V, Mg, Nb, Zr, Al; 0≤x<1, 0≤y≤1, 0≤z<0.1, x+y+z=1.

[0011] In some embodiments, the total mass of the composite positive electrode material is 100%, the mass percentage of the doped graphene quantum dots is 0.1% to 1.5%, and the mass percentage of the carbon coating layer is 0.1% to 1.5%.

[0012] In some embodiments, in the doped graphene quantum dots, the content of the doping element contained therein is 0.1 to 20000 ppm.

[0013] In some embodiments, the thickness of the carbon coating layer is 2-10 nm.

[0014] In some embodiments, the particle size of the doped graphene quantum dots is 5 to 25 nm.

[0015] In some embodiments, the resistivity of the composite cathode material is ≤20Ω / cm.

[0016] In a second aspect, the present application provides a method for preparing a composite positive electrode material, comprising the following steps:

[0017] Lithium source, manganese source, iron source, modified M source, phosphorus source, carbon source and doped graphene quantum dots are used as raw materials, mixed in proportion, ground and dried, and then subjected to a first calcination treatment to obtain a composite positive electrode material.

[0018] In some embodiments, the doped graphene quantum dots are prepared by any of the following preparation methods:

[0019] a. soaking the graphene quantum dots in a solution containing a doping element, sequentially performing ultrasonic mixing, solid-liquid separation, and solid drying treatment, and then performing a second calcination treatment to obtain doped graphene quantum dots;

[0020] b. placing the graphene quantum dots in a tubular furnace, introducing an inert mixed gas containing a doping element gas, and performing a third calcination treatment to obtain doped graphene quantum dots.

[0021] In some embodiments, the doping element in the solution containing the doping element includes at least one of a nitrogen element, a phosphorus element, a sulfur element, a boron element, or a fluorine element.

[0022] In some embodiments, the solution containing the doping element includes at least one of nitric acid, ammonium nitrate, melamine, sulfuric acid, ammonium sulfate, ammonium bisulfate, phosphoric acid, ammonium phosphate, diammonium phosphate, diammonium hydrogen phosphate, ammonium hypophosphite, sodium borohydride, tetramethylammonium borohydride, tetraethylammonium borohydride, tetrabutylammonium borohydride, hydrofluoric acid, and ammonium fluoride.

[0023] In some embodiments, the concentration of the doping element in the solution containing the doping element is 0.5-5 mol / L.

[0024] In some embodiments, the usage ratio of graphene quantum dots and the solution containing doping elements is 1 mg: (1-100) mL.

[0025] In some embodiments, the ultrasonic mixing time is 2 to 24 hours.

[0026] In some embodiments, the second calcination treatment is performed at a temperature of 200 to 600° C. and for a time of 1 to 10 hours.

[0027] In some embodiments, the gas containing the doping element includes a gas containing at least one of nitrogen, phosphorus, sulfur, boron, or fluorine.

[0028] In some embodiments, the gas containing the doping element includes at least one of ammonia, phosphine, hydrogen sulfide, borane, and carbon tetrafluoride.

[0029] In some embodiments, the volume percentage of the doping element gas in the inert mixed gas is 1% to 15%.

[0030] In some embodiments, the third calcination treatment is performed at a temperature of 200 to 600° C. and for a time of 2 to 12 hours.

[0031] In some embodiments, the molar ratio of the lithium source, the manganese source, the iron source, the modified M source, and the phosphorus source is 1:0 to 1:0 to 1:0 to 0.1:0.99 to 1.05.

[0032] In some embodiments, the total mass of the raw material is 100%, and the added amount of the carbon source is 1% to 10%.

[0033] In some embodiments, the total mass of the raw materials is 100%, and the amount of doped graphene quantum dots added is 0.1% to 3%.

[0034] In some embodiments, the first calcination treatment is performed at a temperature of 500 to 850° C. and for a time of 8 to 36 hours.

[0035] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, lithium formate, lithium silicate, lithium sulfate, lithium phosphate, lithium oxalate, lithium octanoate, lithium citrate, lithium salicylate, lithium orthosilicate, lithium permanganate, lithium trifluoroacetate, lithium acetoacetate, lithium difluorophosphate, lithium hexafluorophosphate, lithium benzoate, lithium metaphosphate, lithium pyruvate, lithium acetate, lithium fluoride, lithium bromide, lithium methoxide, lithium ethoxide, lithium oxide, lithium nitride, and lithium sulfide.

[0036] In some embodiments, the iron source includes at least one of ferrous oxalate, ferrous nitrate, ferrous sulfide, ferrous sulfate, ferrous phosphate, ferrous iodide, ferrous fluoride, ferrous bromide, ferrous acetylacetonate, ferrous gluconate, ferrous chloride, and ferrous acetate.

[0037] In some embodiments, the manganese source includes at least one of manganese carbonate, manganese fluoride, manganese nitride, manganese fluoride, manganese bromide, manganese chloride, manganese carbide, manganese phosphide, potassium permanganate, potassium manganate, manganese acetate, manganese nitrate, manganese phosphate, manganese dihydrogen phosphate, manganese oxalate, manganese pentacarbonyl, manganese decacarbonyl, manganese sulfate, manganese acetate, manganese acetylacetonate, and manganese pyrophosphate.

[0038] In some embodiments, the modified M source includes at least one of titanium dioxide, titanium sulfate, titanium fluoride, titanium nitride, titanium carbide, titanium boride, vanadium oxide, vanadium tetroxide, vanadium pentoxide, vanadium chloride, potassium metavanadate, sodium metavanadate, ammonium metavanadate, vanadium boride, vanadium carbide, vanadium diselenide, magnesium oxide, magnesium fluoride, magnesium nitrate, magnesium acetate, magnesium carbonate, magnesium titanate, magnesium nitride, magnesium carbide, magnesium boride, niobium oxide, niobium fluoride, niobium nitride, niobium carbide, niobium oxalate, potassium niobate, magnesium niobate, niobium pentachloride, niobium hydroxide, niobium iodide, niobium selenide, and ammonium hexafluoroniobate.

[0039] In some embodiments, the phosphorus source includes at least one of sodium pyrophosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, sodium hexafluorophosphate, ammonium hypophosphite, ammonium polyphosphate, diammonium phosphate, diammonium hydrogen phosphate, ammonium hexafluorophosphate, ammonium phosphate, potassium hypophosphite, potassium pyrophosphate, tripotassium phosphate, potassium phosphite, potassium metaphosphate, and potassium hexafluorophosphate.

[0040] In some embodiments, the carbon source includes at least one of glucose, sucrose, citric acid, ascorbic acid, starch, phenolic resin, commercial carbon powder, carbon nanotubes, graphene, acetylene black, and carbon aerogel.

[0041] In a third aspect, the present application provides a lithium-ion battery, which includes the above-mentioned composite positive electrode material or the composite positive electrode material prepared by the above-mentioned method for preparing the composite positive electrode material.

[0042] The first aspect of the present application provides a composite positive electrode material, which includes a lithium-containing core and a coating layer arranged on the outer surface of the lithium-containing core, wherein the coating layer includes a carbon coating layer and doped graphene quantum dots dispersed on the outer surface of the lithium-containing core and / or the outer surface of the carbon coating layer; due to the presence of a large number of functional groups on the surface of the doped graphene quantum dots, they have higher surface chemical properties, and based on their nanoscale size, the doped graphene quantum dots have unique advantages in quantum confinement effect and boundary effect, so that they have a higher energy level structure, therefore, the dispersed doped graphene quantum dots can greatly improve the electronic conductivity and ion migration rate of the lithium-containing core, so that the composite positive electrode material has a wide range of applications.

[0043] The second aspect of the present application provides a method for preparing a composite positive electrode material. The preparation method only requires lithium source, manganese source, iron source, modified M source, phosphorus source, carbon source and doped graphene quantum dots as raw materials, which are mixed in proportion and calcined to obtain a composite positive electrode material. The preparation method is simple and easy to operate, does not require the use of large instruments and equipment, and is conducive to industrial application.

[0044] The lithium-ion battery provided in the third aspect of the present application includes the above-mentioned composite positive electrode material. Since the composite positive electrode material has excellent electronic conductivity and ion migration rate, the discharge specific capacity, rate and cycle performance of the obtained lithium-ion battery are significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0046] Figure 1 It is a schematic diagram of the structure of the composite positive electrode material provided in the embodiment of the present application.

[0047] Figure 2 This is a TEM image of LiFePO4 coated with doped graphene quantum dots provided in Example 1 of the present application.

[0048] Figure 3 This is the charge and discharge curve of LiFePO4 coated with doped graphene quantum dots provided in Example 1 of the present application.

[0049] Figure 4 The doped graphene quantum dot-coated LiMn provided in Example 2 of the present application 0.5 Fe 0.5 The charge and discharge curve of PO4. DETAILED DESCRIPTION

[0050] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0051] In this application, the term "and / or" describes the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0052] In this application, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple, respectively.

[0053] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0054] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0055] The weight of the relevant components mentioned in the embodiment description of the present application can not only refer to the specific content of each component, but also represent the proportional relationship between the weights of the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the embodiment description of the present application, it is within the scope disclosed in the embodiment description of the present application. Specifically, the mass in the embodiment description of the present application can be μg, mg, g, kg and other mass units known in the chemical industry.

[0056] The terms "first" and "second" are used only for descriptive purposes to distinguish objects such as substances from each other, and should not be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX may also be referred to as the second XX, and similarly, the second XX may also be referred to as the first XX. Thus, features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0057] A first aspect of an embodiment of the present application provides a composite positive electrode material, which includes a lithium-containing core and a coating layer arranged on the outer surface of the lithium-containing core, wherein the coating layer includes a carbon coating layer and doped graphene quantum dots dispersed on the outer surface of the lithium-containing core and / or the outer surface of the carbon coating layer.

[0058] The composite positive electrode material provided in the first aspect of the embodiment of the present application comprises a lithium-containing core and a coating layer arranged on the outer surface of the lithium-containing core, wherein the coating layer comprises a carbon coating layer and doped graphene quantum dots dispersed on the outer surface of the lithium-containing core and / or the outer surface of the carbon coating layer; due to the presence of a large number of functional groups on the surface of the doped graphene quantum dots, they have higher surface chemical properties, and based on their nanoscale size, the doped graphene quantum dots have unique advantages in quantum confinement effect and boundary effect, so that they have a higher energy level structure, therefore, the dispersed doped graphene quantum dots can greatly improve the electronic conductivity and ion migration rate of the lithium-containing core, so that the composite positive electrode material has a wide range of applications.

[0059] As attached Figure 1 As shown, the coating layer of the composite positive electrode material includes a carbon coating layer and doped graphene quantum dots dispersed on the outer surface of the lithium-containing core and / or the outer surface of the carbon coating layer.

[0060] In some embodiments, doped graphene quantum dots are dispersed on the outer surface of the lithium-containing core to obtain a composite positive electrode material, and the doped graphene quantum dots are arranged at the intermediate interface between the lithium-containing core and the carbon coating layer. On the one hand, since the structure of the lithium-containing core may change due to factors such as volume change during the charge and discharge process, the doped graphene quantum dots can buffer the volume change of the lithium-containing core, maintain the structural integrity of the phosphate material, and extend the service life; on the other hand, the doped graphene quantum dots can adjust the carrier concentration and mobility of the material and improve the electronic conductivity; and based on the small particle size of the doped graphene quantum dots, the quantum confinement effect they possess can also effectively inhibit the dissolution of transition metals, such as iron and manganese, and also improve the lithium ion migration rate.

[0061] In other embodiments, doped graphene quantum dots are dispersed on the outer surface of the carbon coating layer, and in the obtained composite positive electrode material, the carbon coating layer is arranged on the outer surface of the lithium-containing core, and the doped graphene quantum dots are arranged on the outer surface of the carbon coating layer. There are a large number of functional groups on the surface of the doped graphene quantum dots, which give them higher surface chemical properties. Coating them on the outer surface of the carbon coating layer can further improve the interaction between the composite positive electrode material and other materials, and the doped quantum dots can change the electronic transport characteristics of the coating system. The dispersed doped quantum dots can reduce the band gap of the phosphate material, improve the conductivity, and adjust the carrier concentration and mobility of the material.

[0062] In some other embodiments, doped graphene quantum dots are dispersed on the outer surface of the lithium-containing core and the outer surface of the carbon coating. Due to the presence of a large number of functional groups on the surface of doped graphene quantum dots, it has higher surface chemical properties, and based on its nanoscale size, doped graphene quantum dots have unique advantages in quantum confinement effect and boundary effect, so that it has a higher energy level structure, therefore, dispersed doped graphene quantum dots can greatly improve the electronic conductivity and ion migration rate of the lithium-containing core, so that the composite positive electrode material has a wide range of applications.

[0063] In some embodiments, the particle size of the doped graphene quantum dots is 5 to 25 nm. The particle size of the doped graphene quantum dots is controlled to be nanometer-level, so that it has excellent quantum confinement effect and boundary effect, which is beneficial to improve the electronic conductivity and ion migration rate of the material. In some specific embodiments, the particle size of the doped graphene quantum dots includes but is not limited to typical but non-limiting values ​​such as 5 nm, 10 nm, 12 nm, 15 nm, 17 nm, 20 nm, 22 nm, and 25 nm.

[0064] In some embodiments, the doping element of the doped graphene quantum dots includes at least one of nitrogen, phosphorus, sulfur, and fluorine. Non-metallic atom doping of carbon materials can change the local electronic structure, increase the spin density, improve the band gap, and effectively improve the conductivity of the carbon material.

[0065] In some embodiments, in the doped graphene quantum dots, the content of the doping elements contained therein is 0.1 to 20000 ppm. If the doping elements are excessive, the lattice structure of the quantum dots will change, affecting the overall stability of the quantum dots; excessive doping elements will significantly change the carrier concentration in the quantum dots. Within a certain range, appropriate doping can increase the carrier concentration and improve the conductivity of the quantum dots, but when the doping is excessive, the scattering effect between the carriers is enhanced, resulting in a decrease in carrier mobility. This means that the movement of electrons and holes in the quantum dots becomes more difficult, thereby reducing the electrical properties of the quantum dots, such as conductivity and charge transfer efficiency. In some specific embodiments, in the doped graphene quantum dots, the content of the doping elements contained therein includes but is not limited to typical but non-limiting values ​​such as 0.1ppm, 1ppm, 10ppm, 50ppm, 100ppm, 500ppm, 1000ppm, 5000ppm, 10000ppm, 15000ppm, 20000ppm, etc.

[0066] In some embodiments, the Raman carbon characteristic peak I of the doped graphene quantum dots D / I G The value is 0.965~0.995. In the Raman spectrum, at 1300cm -1 and 1580cm -1 The characteristic peaks nearby are the D peak and G peak of carbon, which represent the sp 3 and sp 2 hybridization, their peak intensity is I D / I G The smaller the value, the higher the degree of graphitization. The Raman carbon characteristic peak I of the doped graphene quantum dots provided in the embodiment of the present application D / I G The value is 0.965-0.995, which shows that the degree of graphitization of the doped graphene quantum dots is high.

[0067] In some embodiments, the lithium-containing core has the general formula LiMn x Fe y M z PO4, wherein M is selected from Ti, V, Mg, Nb, Zr, Al; 0≤x<1, 0≤y≤1, 0≤z<0.1, x+y+z=1.

[0068] In some embodiments, the total mass of the composite positive electrode material is 100%, and the mass percentage of the doped graphene quantum dots is 0.1% to 1.5%. If there are too many doped graphene quantum dots, the composite positive electrode material will produce impurities, affecting the performance of the product. If the addition amount is too small, it will not have the effect of improving the electronic conductivity. Furthermore, if the content of the doping element of the doped graphene quantum dots is too much, it will affect the lattice of the graphene quantum dots, and then affect the characteristics of the graphene quantum dots, which is not conducive to improving the electronic conductivity of the composite positive electrode material.

[0069] In some specific embodiments, taking the total mass of the composite positive electrode material as 100%, the mass percentage of the doped graphene quantum dots includes but is not limited to 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% and other typical but non-limiting values.

[0070] In the composite positive electrode material, the outer surface of the lithium-containing core also includes a carbon coating layer, and the carbon coating layer is completely wrapped around the outer surface of the lithium-containing core to form a dense protective structure, ensuring that the lithium-containing core will not be affected by water vapor or carbon dioxide in the air.

[0071] In some embodiments, the total mass of the composite positive electrode material is 100%, and the mass percentage of the carbon coating layer is 0.1% to 1.5%. If the amount of the carbon coating layer added is too much, the carbon coating layer will be too thick, which will affect the ion migration rate. If the amount of the carbon coating layer added is too little, the carbon coating layer will be too thin and unable to form a dense protective film, which will affect the properties of the lithium-containing core.

[0072] In some embodiments, the thickness of the carbon coating layer is 2-10 nm. The thickness of the carbon coating layer includes, but is not limited to, typical but non-limiting values ​​such as 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, and 10 nm.

[0073] In some embodiments, the resistivity of the composite positive electrode material is ≤20Ω / cm. The lower resistivity of the material means that the electronic conductivity can be greatly improved.

[0074] A second aspect of the present application provides a method for preparing a composite positive electrode material, comprising the following steps:

[0075] Lithium source, manganese source, iron source, modified M source, phosphorus source, carbon source and doped graphene quantum dots are used as raw materials, mixed in proportion, ground and dried, and then subjected to a first calcination treatment to obtain a composite positive electrode material.

[0076] The second aspect of the embodiment of the present application provides a method for preparing a composite positive electrode material. The preparation method only requires lithium source, manganese source, iron source, modified M source, phosphorus source, carbon source and doped graphene quantum dots as raw materials, which are mixed in proportion and calcined to obtain a composite positive electrode material. The preparation method is simple and easy to operate, does not require the use of large instruments and equipment, and is conducive to industrial application.

[0077] In some specific embodiments, the doped graphene quantum dots can be prepared by method a. The specific preparation process of method a is as follows: immersing the graphene quantum dots in a solution containing a doping element, sequentially performing ultrasonic mixing, solid-liquid separation, and solid drying treatments, and then performing a second calcination treatment to obtain the doped graphene quantum dots.

[0078] In some embodiments, the doping element in the solution containing the doping element includes at least one of a nitrogen element, a phosphorus element, a sulfur element, a boron element, or a fluorine element.

[0079] In some embodiments, the solution containing the doping element includes at least one of nitric acid, ammonium nitrate, melamine, sulfuric acid, ammonium sulfate, ammonium bisulfate, phosphoric acid, ammonium phosphate, diammonium phosphate, diammonium hydrogen phosphate, ammonium hypophosphite, sodium borohydride, tetramethylammonium borohydride, tetraethylammonium borohydride, tetrabutylammonium borohydride, hydrofluoric acid, and ammonium fluoride.

[0080] In some embodiments, the concentration of the doping element contained in the solution containing the doping element is 0.5 to 5 mol / L. If the concentration of the doping element is too high, the finished product is prone to generate impurities, affecting product performance; if the concentration of the doping element is low, the doping amount is small, and the modification effect of the obtained doped graphene quantum dots is poor. In some specific embodiments, the concentration of the doping element contained in the solution containing the doping element includes but is not limited to typical but non-limiting values ​​such as 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L, and 5 mol / L.

[0081] In some embodiments, the ratio of graphene quantum dots to the solution containing doping elements is 1 mg: (1-100) mL. Controlling the ratio of graphene quantum dots to the solution containing doping elements is beneficial to obtaining suitable doped graphene quantum dot materials.

[0082] In some embodiments, the ultrasonic mixing time is 2 to 24 hours. In some specific embodiments, the ultrasonic mixing time includes but is not limited to 2 hours, 5 hours, 10 hours, 15 hours, 20 hours, and 24 hours.

[0083] In some embodiments, the temperature of the second calcination treatment is 200-600°C and the time is 1-10 hours. If the calcination temperature is too low or the time is too short, the doping concentration will be too low. If the temperature is too high or the time is too long, the doping concentration will be too high, and there is a risk of generating impurities. In the process of calcination, an inert gas needs to be introduced to ensure that no impurities appear during the reaction.

[0084] In some specific embodiments, doped graphene quantum dots can be prepared by method b. The specific preparation process of method b is as follows: placing graphene quantum dots in a tube furnace, introducing an inert mixed gas containing a doping element gas, and performing a third calcination treatment to obtain doped graphene quantum dots.

[0085] In some embodiments, the gas containing the doping element includes a gas containing at least one of nitrogen, phosphorus, sulfur, boron, or fluorine.

[0086] In some embodiments, the gas containing the doping element includes at least one of ammonia, phosphine, hydrogen sulfide, borane, and carbon tetrafluoride.

[0087] In some embodiments, the volume percentage of the doping element gas in the inert mixed gas is 1% to 15%. If the amount of the doping element gas added is too high, the finished product is prone to generate impurities, affecting product performance; if the amount of the doping element gas added is too low, the doping amount is small, and the modification effect of the obtained doped graphene quantum dots is poor.

[0088] In some embodiments, the third calcination treatment is performed at a temperature of 200 to 600° C. and for a time of 2 to 12 hours.

[0089] In some embodiments, the molar ratio of the lithium source, the manganese source, the iron source, the modified M source and the phosphorus source is 1:0 to 1:0 to 1:0 to 0.1.

[0090] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, lithium formate, lithium silicate, lithium sulfate, lithium phosphate, lithium oxalate, lithium octanoate, lithium citrate, lithium salicylate, lithium orthosilicate, lithium permanganate, lithium trifluoroacetate, lithium acetoacetate, lithium difluorophosphate, lithium hexafluorophosphate, lithium benzoate, lithium metaphosphate, lithium pyruvate, lithium acetate, lithium fluoride, lithium bromide, lithium methoxide, lithium ethoxide, lithium oxide, lithium nitride, and lithium sulfide.

[0091] In some embodiments, the iron source includes at least one of ferrous oxalate, ferrous nitrate, ferrous sulfide, ferrous sulfate, ferrous phosphate, ferrous iodide, ferrous fluoride, ferrous bromide, ferrous acetylacetonate, ferrous gluconate, ferrous chloride, and ferrous acetate.

[0092] In some embodiments, the manganese source includes at least one of manganese carbonate, manganese fluoride, manganese nitride, manganese fluoride, manganese bromide, manganese chloride, manganese carbide, manganese phosphide, potassium permanganate, potassium manganate, manganese acetate, manganese nitrate, manganese phosphate, manganese dihydrogen phosphate, manganese oxalate, manganese pentacarbonyl, manganese decacarbonyl, manganese sulfate, manganese acetate, manganese acetylacetonate, and manganese pyrophosphate.

[0093] In some embodiments, the modified M source includes at least one of titanium dioxide, titanium sulfate, titanium fluoride, titanium nitride, titanium carbide, titanium boride, vanadium oxide, vanadium tetroxide, vanadium pentoxide, vanadium chloride, potassium metavanadate, sodium metavanadate, ammonium metavanadate, vanadium boride, vanadium carbide, vanadium diselenide, magnesium oxide, magnesium fluoride, magnesium nitrate, magnesium acetate, magnesium carbonate, magnesium titanate, magnesium nitride, magnesium carbide, magnesium boride, niobium oxide, niobium fluoride, niobium nitride, niobium carbide, niobium oxalate, potassium niobate, magnesium niobate, niobium pentachloride, niobium hydroxide, niobium iodide, niobium selenide, and ammonium hexafluoroniobate.

[0094] In some embodiments, the phosphorus source includes at least one of sodium pyrophosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, sodium hexafluorophosphate, ammonium hypophosphite, ammonium polyphosphate, diammonium phosphate, diammonium hydrogen phosphate, ammonium hexafluorophosphate, ammonium phosphate, potassium hypophosphite, potassium pyrophosphate, tripotassium phosphate, potassium phosphite, potassium metaphosphate, and potassium hexafluorophosphate.

[0095] In some embodiments, the total mass of the raw material is 100%, and the amount of the carbon source added is 1% to 10%. In some specific embodiments, the total mass of the raw material is 100%, and the amount of the carbon source added includes but is not limited to 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% and other typical but non-limiting values. The amount of the carbon source added is controlled to be moderate to ensure that the thickness of the obtained carbon coating layer is appropriate to avoid affecting the migration of lithium ions due to the carbon coating layer being too thick, or the coating effect of the coating layer being poor due to the carbon coating layer being too thin.

[0096] In some embodiments, the carbon source includes at least one of glucose, sucrose, citric acid, ascorbic acid, starch, phenolic resin, commercial carbon powder, carbon nanotubes, graphene, acetylene black, and carbon aerogel.

[0097] In some embodiments, the total mass of the raw materials is 100%, and the addition amount of doped graphene quantum dots is 0.1% to 3%. The doped graphene quantum dots provided are prepared by the above-mentioned preparation method of doped graphene quantum dots. In some specific embodiments, the total mass of the raw materials is 100%, and the addition amount of doped graphene quantum dots includes but is not limited to typical but non-limiting values ​​such as 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, etc.

[0098] In some embodiments, the temperature of the first calcination treatment is 500-850°C, and the time is 8-36 hours. In some specific embodiments, the temperature of the first calcination treatment includes but is not limited to typical but non-limiting values ​​such as 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, etc.; the time includes but is not limited to typical but non-limiting values ​​such as 8 hours, 10 hours, 15 hours, 20 hours, 25 hours, 30 hours, 35 hours, etc.

[0099] A third aspect of an embodiment of the present application provides a lithium-ion battery, which includes the above-mentioned composite positive electrode material or a composite positive electrode material prepared by the above-mentioned method for preparing the composite positive electrode material.

[0100] The lithium-ion battery provided in the third aspect of the embodiment of the present application includes the above-mentioned composite positive electrode material. Since the composite positive electrode material has excellent electronic conductivity and ion migration rate, the discharge specific capacity, rate and cycle performance of the obtained lithium-ion battery are significantly improved.

[0101] The following describes the invention in conjunction with specific embodiments.

[0102] Example 1

[0103] A method for preparing a composite positive electrode material, the specific steps are as follows:

[0104] (1) 10.0 g of graphene quantum dot powder was weighed on a balance, dispersed in 500 mL of a 0.5 mol / L melamine solution, ultrasonicated for 2 h, and centrifuged to obtain a solid. The solid was transferred to a vacuum drying oven and dried at 80 ° C for 10 h. The dried powder solid was then placed in a tube furnace and calcined at 500 ° C for 5 h under the condition of passing high-purity nitrogen gas for protection to obtain nitrogen-doped graphene quantum dots, wherein the nitrogen content was 1.2 wt.%;

[0105] (2) Weigh 7.0 g of the product in (1), 234 g of lithium carbonate, 725 g of ammonium dihydrogen phosphate, 958 g of ferrous sulfate, 6.6 g of titanium oxide, 3.6 g of vanadium pentoxide and 50 g of glucose using a balance and grind them to D using a sand mill. 50 The particle size is 0.4 μm, and after spray drying, it is placed in a tubular furnace for calcination, and argon gas is introduced for protection. The calcination temperature is 780°C, and the calcination time is 12 h. The product is a lithium iron phosphate positive electrode material coated with nitrogen-doped graphene quantum dots.

[0106] Example 2

[0107] A method for preparing a composite positive electrode material, the specific steps are as follows:

[0108] (1) Weighing 10.0 g of graphene quantum dot powder on a balance, placing it into a quartz boat and transferring it to a tube furnace, introducing a mixed gas of hydrogen sulfide and high-purity nitrogen with a volume ratio of 5% / 95%, calcining at 500° C. for 5 h, and taking it out after cooling to obtain sulfur-doped graphene quantum dots, wherein the sulfur content is 0.9 wt.%;

[0109] Step 2 of this embodiment is consistent with the above-mentioned embodiment 1, and the product is a lithium iron phosphate positive electrode material coated with sulfur-doped graphene quantum dots.

[0110] Example 3

[0111] A method for preparing a composite positive electrode material, the specific steps are as follows:

[0112] This embodiment is consistent with step (1) of embodiment 1, except that the amount of ferrous sulfate in step (2) is changed to 476 g, and 359 g of manganese carbonate is additionally added. Other operations and parameters are the same. The product prepared in this embodiment is a nitrogen-doped graphene quantum dot-coated lithium manganese iron phosphate positive electrode material, in which the ratio of manganese to iron atoms is 5:5.

[0113] Example 4

[0114] A method for preparing a composite positive electrode material, the specific steps are as follows:

[0115] This embodiment is consistent with step (1) of embodiment 1, except that the amount of ferrous sulfate in step (2) is changed to 380 g, and 431 g of manganese carbonate is additionally added. Other operations and parameters are the same. The product prepared in this embodiment is a nitrogen-doped graphene quantum dot-coated lithium manganese iron phosphate positive electrode material, in which the ratio of manganese to iron atoms is 6:4.

[0116] Example 5

[0117] A method for preparing a composite positive electrode material, the specific steps are as follows:

[0118] This embodiment is consistent with step (1) of embodiment 1, except that the dopant in step (4) is changed from the original 6.6 g titanium oxide and 3.6 g vanadium pentoxide to 325 g magnesium acetate. Other operations and parameters are the same. The product prepared in this embodiment is a lithium iron phosphate positive electrode material coated with nitrogen-doped graphene quantum dots.

[0119] Comparative Example 1

[0120] The steps of this comparative example are as follows: 234 g of lithium carbonate, 725 g of ammonium dihydrogen phosphate, 476 g of ferrous sulfate, 6.6 g of titanium oxide and 3.6 g of vanadium pentoxide are weighed on a balance and ground with a sand mill to D 50The particle size is 0.4 μm, and after spray drying, it is placed in a tubular furnace for calcination, and argon gas is introduced for protection. The calcination temperature is 780°C, and the calcination time is 12 h. The prepared product is an uncoated lithium iron phosphate positive electrode material.

[0121] Comparative Example 2

[0122] The steps of this comparative example are as follows: 234 g of lithium carbonate, 725 g of ammonium dihydrogen phosphate, 958 g of ferrous sulfate, 431 g of manganese carbonate, 6.6 g of titanium oxide and 3.6 g of vanadium pentoxide are weighed with a balance and ground with a sand mill to D 50 The particle size is 0.4 μm, and after spray drying, it is placed in a tubular furnace for calcination, and argon gas is introduced for protection. The calcination temperature is 780°C and the calcination time is 12 hours. The prepared product is an uncoated lithium manganese iron phosphate positive electrode material, in which the ratio of manganese to iron atoms is 5:5.

[0123] Comparative Example 3

[0124] This comparative example is consistent with step (1) in Example 1, except that in step (2), the amount of nitrogen-doped graphene quantum dots is changed to 14 g, and glucose is not added. The prepared product is a lithium iron phosphate positive electrode material coated only with nitrogen-doped graphene quantum dots.

[0125] Comparative Example 4

[0126] This comparative example is consistent with step (2) in Example 1, except that the amount of glucose in step (2) is changed to 120 g. This comparative example does not use doped graphene quantum dots for coating, but only uses carbon source coating. The prepared product is a carbon-coated lithium iron phosphate positive electrode material.

[0127] Comparative Example 5

[0128] This comparative example is consistent with step (1) in Example 1, except that the amount of nitrogen-doped graphene quantum dots in step (2) is changed to 14 g, the amount of ferrous sulfate is changed to 476 g, and 359 g of manganese carbonate is additionally added, and no glucose is added. The prepared product is a lithium manganese iron phosphate positive electrode material coated only with nitrogen-doped graphene quantum dots, wherein the ratio of manganese to iron atoms is 5:5.

[0129] Comparative Example 6

[0130] This comparative example is consistent with step (2) in Example 1, except that the amount of glucose in step (2) is changed to 120 g, the amount of ferrous sulfate is changed to 476 g, and 359 g of manganese carbonate is additionally added. This comparative example does not use doped graphene quantum dots for coating, but only uses carbon source coating. The prepared product is a carbon-coated lithium manganese iron phosphate positive electrode material, in which the ratio of manganese to iron atoms is 5:5.

[0131] Performance testing and result analysis

[0132] (i) The nitrogen-doped graphene quantum dots-coated lithium iron phosphate material prepared in Example 1 was analyzed using a transmission electron microscope.

[0133] The TEM image of the nitrogen-doped graphene quantum dot-coated lithium iron phosphate material prepared in Example 1 is as follows: Figure 2 As shown, it can be seen that the surface of the lithium iron phosphate is coated with an obvious graphitized carbon layer, and in the dotted box position in the figure, obvious nitrogen-doped graphene quantum dots can be observed, proving that the nitrogen-doped graphene quantum dot-coated LiFePO4 positive electrode material was successfully prepared in Example 1.

[0134] (ii) The resistivity tests were performed on the powder materials prepared in Examples 1 to 5 and Comparative Examples 1 to 6. The specific results are shown in Table 1. It can be seen that the resistivity of the materials obtained in Examples 1 to 5 is less than 20Ω / cm, while the resistivity of the materials obtained in Comparative Examples 1 to 6 is greater than 128Ω / cm, and even the resistivity of the material obtained in Comparative Example 1 is 95864Ω / cm; it can be seen that the materials obtained in Examples 1 to 5 have greatly improved the conductivity. Further analysis shows that the overall carbon content of Examples 1 to 5 and Comparative Examples 3 to 6 is basically maintained at the same level, and when doped graphene quantum dots and graphitized carbon are used for simultaneous coating, the resistivity of the sample is lower than that of a single carbon source, and is ≤20Ω / cm, indicating that the co-coating of the two carbon sources can significantly improve the conductivity of the phosphate material, which will be more conducive to the electrochemical performance of the material.

[0135] Table 1

[0136]

[0137]

[0138] (III) Electrochemical performance test

[0139] The powder materials prepared in Examples 1 to 5 and Comparative Examples 1 to 6 are prepared into button batteries, and the specific steps are as follows:

[0140] (1) Preparation of slurry: 2.33 kg of positive electrode active material, 0.012 kg of superconducting carbon black (SP) and 0.048 kg of binder polyvinylidene fluoride (PVDF) were added into a 500 mL agate ball mill jar, and then 1.6 kg of solvent N-methylpyrrolidone (NMP) was added, and the slurry was prepared by ball milling at a speed of 360 r / min for 4 h;

[0141] (2) Slurry coating: adjust the scale of the scraper of the coating machine, evenly coat the milled slurry on the aluminum foil, and place the coated electrode in a vacuum drying oven at 130°C for 3 hours;

[0142] (3) Rolling and punching: Place the aluminum foil coated with slurry flatly in the middle of the roller and roll the electrode; place the front of the rolled electrode close to the punched hole and punch the electrode in sequence; the compaction density of the electrode is controlled at 2.0-2.4 g / cm 3 , with a diameter of 14 mm and a thickness of 0.05 to 0.10 mm; the punched electrode is placed in a vacuum drying oven at 130°C for 3 hours;

[0143] (4) Assembling button cells: In a glove box, the negative electrode shell, spring sheet, steel sheet, lithium sheet, separator, positive electrode sheet and positive electrode shell were assembled in sequence. During the process, 10 μL of electrolyte was injected, and then the button cells were sealed using a sealing machine. The electrochemical performance of these four groups of button cells was tested, as shown in Table 2.

[0144] Combined with Table 2, the charge and discharge curves of LiFePO4 coated with doped graphene quantum dots obtained in Example 1 ( Figure 3 ) and the doped graphene quantum dots coated LiMn obtained in Example 2 0.5 Fe 0.5 The charge and discharge curve of PO4 ( Figure 4 ), it can be seen that the phosphate positive electrode material coated with nitrogen-doped graphene quantum dots prepared in the embodiments of the present application has excellent charge-discharge performance, rate performance and cycle performance. Among them, the electrochemical performance of the lithium iron phosphate material coated with nitrogen-doped graphene quantum dots prepared in Example 1 is the best. It can be seen that its discharge specific capacity at a rate of 0.1C can reach 161.2mAh / g, and its discharge specific capacity at a rate of 1C can reach 155.3mAh / g, showing excellent electrochemical performance.

[0145] From the results of Comparative Examples 1 and 2, it can be seen that the conductivity of lithium iron phosphate and lithium iron manganese phosphate materials without carbon source coating is extremely poor, and the capacity and cycle performance are extremely poor. When two carbon sources are used for coating at the same time, the electrochemical performance of lithium iron phosphate and lithium iron manganese phosphate materials is the best, mainly because nitrogen-doped graphene quantum dots can effectively improve the conductivity of phosphate materials and help to exert capacity.

[0146] Table 2

[0147]

[0148]

[0149] In summary, the composite positive electrode material provided in the embodiments of the present application comprises a lithium-containing core and a coating layer arranged on the outer surface of the lithium-containing core, wherein the coating layer comprises a carbon coating layer and doped graphene quantum dots dispersed on the outer surface of the lithium-containing core and / or the outer surface of the carbon coating layer; due to the presence of a large number of functional groups on the surface of the doped graphene quantum dots, it has higher surface chemical properties, and based on its nanoscale size, the doped graphene quantum dots have unique advantages in quantum confinement effect and boundary effect, so that it has a higher energy level structure, therefore, the dispersed doped graphene quantum dots can greatly improve the electronic conductivity and ion migration rate of the lithium-containing core, so that the composite positive electrode material has a wide range of applications.

[0150] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A composite positive electrode material, characterized in that: The composite positive electrode material includes a lithium-containing core and a coating layer arranged on the outer surface of the lithium-containing core, wherein the coating layer includes a carbon coating layer and doped graphene quantum dots dispersed on the outer surface of the lithium-containing core and / or the outer surface of the carbon coating layer.

2. The composite positive electrode material according to claim 1, characterized in that The doping element of the doped graphene quantum dots includes at least one of nitrogen, phosphorus, sulfur and fluorine; and / or, The Raman carbon characteristic peak I of the doped graphene quantum dot D / I G The value is 0.965 to 0.995; and / or, The general structural formula of the lithium-containing core is LiMn x Fe y M z PO4, wherein M is selected from Ti, V, Mg, Nb, Zr, Al; 0≤x<1, 0≤y≤1, 0≤z<0.1, x+y+z=1.

3. The composite positive electrode material according to claim 1, characterized in that: Taking the total mass of the composite positive electrode material as 100%, the mass percentage of the doped graphene quantum dots is 0.1% to 1.5%, and the mass percentage of the carbon coating layer is 0.1% to 1.5%; and / or, In the doped graphene quantum dots, the content of the doping elements contained therein is 0.1 to 20000 ppm.

4. The composite positive electrode material according to claim 1, characterized in that The thickness of the carbon coating layer is 2 to 10 nm; and / or, The particle size of the doped graphene quantum dots is 5 to 25 nm; and / or, The resistivity of the composite positive electrode material is ≤20Ω / cm.

5. A method for preparing a composite positive electrode material, characterized in that: The steps include: Lithium source, manganese source, iron source, modified M source, phosphorus source, carbon source and doped graphene quantum dots are used as raw materials, mixed in proportion, ground and dried, and then subjected to a first calcination treatment to obtain a composite positive electrode material.

6. The method for preparing a composite positive electrode material according to claim 5, characterized in that: The doped graphene quantum dots are prepared by any of the following preparation methods: a. soaking the graphene quantum dots in a solution containing a doping element, sequentially performing ultrasonic mixing, solid-liquid separation, and solid drying treatment, and then performing a second calcination treatment to obtain doped graphene quantum dots; b. placing the graphene quantum dots in a tubular furnace, introducing an inert mixed gas containing a doping element gas, and performing a third calcination treatment to obtain doped graphene quantum dots.

7. The method for preparing a composite positive electrode material according to claim 6, characterized in that: The doping element in the solution containing the doping element includes at least one of nitrogen, phosphorus, sulfur, boron or fluorine; and / or, The solution containing the doping element includes at least one of nitric acid, ammonium nitrate, melamine, sulfuric acid, ammonium sulfate, ammonium hydrogen sulfate, phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium hypophosphite, sodium borohydride, tetramethylammonium borohydride, tetraethylammonium borohydride, tetrabutylammonium borohydride, hydrofluoric acid, and ammonium fluoride; and / or, The concentration of the doping element in the solution containing the doping element is 0.5 to 5 mol / L; and / or, The amount ratio of the graphene quantum dots to the solution containing the doping element is 1 mg: (1-100) mL; and / or, The ultrasonic mixing time is 2 to 24 hours; and / or, The second calcination treatment is performed at a temperature of 200 to 600° C. and for a time of 1 to 10 hours.

8. The method for preparing a composite positive electrode material according to claim 6, characterized in that: The doping element-containing gas includes a gas containing at least one of nitrogen, phosphorus, sulfur, boron or fluorine; and / or, The gas containing doping elements includes at least one of ammonia, phosphine, hydrogen sulfide, borane and carbon tetrafluoride; and / or, In the inert mixed gas, the volume percentage of the gas containing the doping element is 1% to 15%; and / or, The temperature of the third calcination treatment is 200-600° C., and the time is 2-12 hours.

9. The method for preparing a composite positive electrode material according to claim 5, characterized in that: The molar ratio of the lithium source, the manganese source, the iron source, the modified M source, and the phosphorus source is 1:0-1:0-1:0-0.1:0.99-1.05; and / or, Taking the total mass of the raw material as 100%, the amount of the carbon source added is 1% to 10%; and / or, Taking the total mass of the raw materials as 100%, the addition amount of the doped graphene quantum dots is 0.1% to 3%; and / or, The temperature of the first calcination treatment is 500 to 850° C. and the time is 8 to 36 hours; and / or, The modified M source includes at least one of titanium dioxide, titanium sulfate, titanium fluoride, titanium nitride, titanium carbide, titanium boride, vanadium oxide, vanadium tetroxide, vanadium pentoxide, vanadium chloride, potassium metavanadate, sodium metavanadate, ammonium metavanadate, vanadium boride, vanadium carbide, vanadium diselenide, magnesium oxide, magnesium fluoride, magnesium nitrate, magnesium acetate, magnesium carbonate, magnesium titanate, magnesium nitride, magnesium carbide, magnesium boride, niobium oxide, niobium fluoride, niobium nitride, niobium carbide, niobium oxalate, potassium niobate, magnesium niobate, niobium pentachloride, niobium hydroxide, niobium iodide, niobium selenide, and ammonium hexafluoroniobate; and / or, The carbon source comprises at least one of glucose, sucrose, citric acid, ascorbic acid, starch, phenolic resin, commercial carbon powder, carbon nanotubes, graphene, acetylene black and carbon aerogel.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the composite positive electrode material according to any one of claims 1 to 4 or a composite positive electrode material prepared by the method for preparing the composite positive electrode material according to any one of claims 5 to 9.

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