Positive electrode active material and preparation method thereof, positive electrode, battery and electric equipment

Abstract

The invention provides a positive electrode active material and a preparation method thereof, a positive electrode, a battery and electric equipment, the positive electrode active material comprises lithium iron phosphate and graphene quantum dots, and the lithium iron phosphate comprises secondary particles formed by primary particles; the graphene quantum dots coat at least part of the surfaces of the primary particles. The positive electrode active material has relatively high electronic conductivity and ion mobility, and the low-temperature discharge performance of the battery can be effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries and relates to a positive electrode active material, in particular to a positive electrode active material and a preparation method thereof, a positive electrode, a battery, and an electrical device. Background Art

[0002] Lithium-ion batteries are widely used in electric vehicles, consumer electronics (such as electronic watches, mobile phones, cameras, tablets, remote controls, toys, etc.), industrial energy storage and other fields due to their advantages such as high voltage platform, high storage energy density, low self-discharge rate, no memory effect, long cycle life and environmental protection.

[0003] Currently, the mainstream lithium batteries are mainly ternary lithium batteries and lithium iron phosphate batteries. As the name suggests, the positive electrode active material in ternary lithium batteries is mainly ternary materials, and the positive electrode active material in lithium iron phosphate batteries is mainly lithium iron phosphate positive electrode material. Ternary materials are mainly composed of three metal elements: nickel, cobalt, and manganese. They have high energy density, excellent fast charging performance, and low-temperature performance, but they have disadvantages such as poor safety, short cycle life, high cost, and poor environmental protection. Compared with ternary materials, lithium iron phosphate positive electrode materials have better high temperature resistance and safety performance, higher cycle performance, and lower cost. They do not contain rare metals such as cobalt and are more environmentally friendly. However, due to the inherent characteristics of lithium iron phosphate, namely low electronic conductivity and ion mobility, especially at low temperatures, the electron and ion mobility rates further decrease, resulting in poor low-temperature discharge performance of lithium iron phosphate batteries. At present, the conductivity of lithium iron phosphate is generally improved through methods such as atomic doping, coating with conductive materials, and reducing the particle size of the material particles, but its low-temperature discharge performance still needs to be improved.

[0004] Therefore, it is necessary to further improve the electronic conductivity and ion mobility of lithium iron phosphate positive electrode active materials so that lithium iron phosphate batteries have higher low-temperature discharge performance. Summary of the Invention

[0005] In view of the above-mentioned defects, the present invention provides a positive electrode active material, which has high electronic conductivity and ion mobility, and can effectively improve the low-temperature discharge performance of the battery.

[0006] The present invention also provides a method for preparing the above-mentioned positive electrode active material. The positive electrode active material prepared by this preparation method has high electronic conductivity and ion mobility, which can effectively improve the low-temperature discharge performance of the battery.

[0007] The present invention also provides a method for preparing the above-mentioned positive electrode active material. The positive electrode active material prepared by this preparation method has high electronic conductivity and ion mobility, which can effectively improve the low-temperature discharge performance of the battery.

[0008] The present invention also provides a positive electrode, comprising the above-mentioned positive electrode active material, or the positive electrode active material prepared by the above-mentioned preparation method, or the positive electrode active material prepared by the above-mentioned other preparation method. Therefore, using the positive electrode in a battery can effectively improve the low-temperature discharge performance of the battery.

[0009] The present invention also provides a battery comprising the positive electrode, so the battery has higher low-temperature discharge performance.

[0010] The present invention also provides an electrical device comprising the above-mentioned battery, so that the electrical device can operate continuously and stably at low temperatures, reducing the inconvenience caused by temperature changes.

[0011] A first aspect of the present invention provides a positive electrode active material, comprising lithium iron phosphate and graphene quantum dots, wherein the lithium iron phosphate comprises secondary particles formed by primary particles; and the graphene quantum dots are coated on at least a portion of the surface of the primary particles.

[0012] The positive electrode active material as described above, wherein the average particle size of the secondary particles is 2 to 5 μm; and / or the average particle size of the primary particles is 80 to 100 nm.

[0013] The positive electrode active material as described above, wherein the average particle size of the graphene quantum dots is 1 to 8 nm.

[0014] The positive electrode active material as described above, wherein the mass content of graphene quantum dots in the positive electrode active material is 0.5-2.5 wt%.

[0015] The positive electrode active material as described above, wherein the crystallinity of the positive electrode active material is 40 to 60%.

[0016] The positive electrode active material as described above, wherein the electronic conductivity of the positive electrode active material is not less than 0.1 S / cm, and the ion diffusion rate is not less than 5.6×10 -13 cm 2 / s.

[0017] A second aspect of the present invention provides a method for preparing the positive electrode active material of the first aspect, comprising the following steps:

[0018] 1) mixing raw materials including graphene oxide, a cutting agent, and water to obtain a first mixed system; and subjecting the first mixed system to a first reaction to obtain graphene quantum dots;

[0019] 2) mixing raw materials including lithium iron phosphate, the graphene quantum dots, and water to obtain a second mixed system; and subjecting the second mixed system to a second reaction to obtain the positive electrode active material.

[0020] In the method for preparing the positive electrode active material as described above, the cutting agent includes at least one of hydrogen peroxide, persulfate, nitric acid, and sulfuric acid.

[0021] The method for preparing the positive electrode active material as described above, wherein, in step 1), the mass ratio of the graphene oxide to the cutting agent is (1.0-3.3):13.5; and / or, in step 2), the mass ratio of the lithium iron phosphate to the graphene quantum dots is 200:(1-5).

[0022] The method for preparing the positive electrode active material as described above, wherein, in step 1), the reaction temperature of the first reaction is 90-140° C., and the reaction time is 12-15 h; and / or, in step 2), the reaction temperature of the second reaction is 90-140° C., and the reaction time is 1-2 h.

[0023] A third aspect of the present invention provides a method for preparing the positive electrode active material of the first aspect, comprising the following steps:

[0024] 1) mixing raw materials including amino acids and water to obtain a third mixed system; subjecting the third mixed system to a third reaction under microwave conditions to obtain graphene quantum dots;

[0025] 2) mixing raw materials including lithium iron phosphate, the graphene quantum dots, and water to obtain a fourth mixed system; and subjecting the fourth mixed system to a fourth reaction to obtain the positive electrode active material.

[0026] In the method for preparing the positive electrode active material as described above, in step 1), the amino acid comprises at least one of L-asparagine, glutamic acid, cysteine, and lysine.

[0027] The method for preparing the positive electrode active material as described above, wherein, in step 1), the mass concentration of the amino acid in the third mixed system is 0.2 to 0.6 g / mL; and / or, in step 2), the mass ratio of the lithium iron phosphate to the graphene quantum dots is 1:(0.002 to 0.008).

[0028] The method for preparing the positive electrode active material as described above, wherein, in step 1), in the third reaction, the microwave power is 150-300 W, the reaction temperature is 120-250° C., and the reaction time is 10-60 min; and / or, in step 2), the reaction temperature of the fourth reaction is 90-140° C., and the reaction time is 1-2 h.

[0029] The fourth aspect of the present invention provides a positive electrode, which includes the positive electrode active material described in the first aspect, or the positive electrode active material prepared by the preparation method described in the second aspect, or the positive electrode active material prepared by the preparation method described in the third aspect.

[0030] A fifth aspect of the present invention provides a battery, comprising the positive electrode described in the fourth aspect.

[0031] A sixth aspect of the present invention provides an electrical device, which includes the battery described in the fifth aspect.

[0032] The positive electrode active material provided by the present invention includes lithium iron phosphate and graphene quantum dots, wherein the lithium iron phosphate includes secondary particles formed by the agglomeration of primary particles. By coating at least part of the surface of the primary particles with graphene quantum dots, the electronic conductivity and ion mobility of the positive electrode active material are effectively improved, thereby enabling the battery to have higher low-temperature discharge performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 TEM image of graphene quantum dots in Example 1 of the present invention;

[0034] Figure 2 TEM image of graphene quantum dots in Example 9 of the present invention;

[0035] Figure 3 XRD patterns of the positive electrode active material and lithium iron phosphate in Example 1 of the present invention;

[0036] Figure 4 This is a fluorescence spectrum of the positive electrode active material in Example 1 of the present invention;

[0037] Figure 5 This is a fluorescence spectrum of the positive electrode active material in Example 9 of the present invention. DETAILED DESCRIPTION

[0038] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0039] Lithium iron phosphate positive electrode active materials have been widely used in electric vehicles, energy storage systems and other fields due to their advantages such as high safety and cycle performance, low cost and environmental friendliness. However, due to the inherent characteristics of lithium iron phosphate itself, namely low electronic conductivity and ion mobility, the discharge performance of batteries containing this material at low temperatures is even lower. At present, the electronic conductivity and ion mobility of lithium iron phosphate are generally improved by carbon coating, doping modification (such as magnesium, aluminum, etc.), nano-sizing and compounding with other conductive materials (such as carbon nanotubes), thereby improving the low-temperature discharge performance of lithium iron phosphate batteries. However, its electronic conductivity and ion mobility still need to be further improved.

[0040] Based on the above problems, the inventors tried to use other carbon materials to coat lithium iron phosphate to further improve the electronic conductivity and ion mobility of lithium iron phosphate. After extensive research, the inventors found that by coating lithium iron phosphate with graphene quantum dots, the electronic conductivity and ion mobility of lithium iron phosphate can be greatly improved. After analysis, the inventors believe that the reasons may be as follows: 1) Graphene quantum dots have higher electron mobility than other carbon materials, and their own quantum confinement effect can further enhance their conductivity, which can significantly improve the electronic conductivity of lithium iron phosphate; 2) Graphene quantum dots have a larger specific surface area, which can better contact with lithium iron phosphate and be more evenly coated on the surface of lithium iron phosphate, improving the interface contact between the coating layer and lithium iron phosphate, and can also provide more active sites, effectively promoting the transmission of lithium ions and electrons; 3) The pore structure formed between adjacent graphene quantum dots in the coating layer is more conducive to the insertion and extraction of lithium ions during the charge and discharge process, which can further improve the ion mobility of lithium iron phosphate to a certain extent. Therefore, compared with other carbon material coatings, graphene quantum dots coated lithium iron phosphate can further improve the low-temperature discharge performance of lithium iron phosphate batteries.

[0041] Based on the above analysis, the first aspect of the present invention provides a positive electrode active material, which includes lithium iron phosphate and graphene quantum dots. The lithium iron phosphate includes secondary particles formed by primary particles; the graphene quantum dots are coated on at least part of the surface of the primary particles.

[0042] It should be noted that the "primary particle" in the present invention refers to the smallest unit in the powder particles without structures such as stacking and flocculation, and the "secondary particle" refers to a larger particle formed by the agglomeration of multiple primary particles.

[0043] The positive electrode active material of the present invention includes lithium iron phosphate and graphene quantum dots, wherein the lithium iron phosphate includes secondary particles formed by primary particles, and the graphene quantum dots are coated on at least part of the surface of the primary particles, which can effectively improve the electronic conductivity and ion mobility of the positive electrode active material. After analysis, the inventor believes that the reason may be: on the one hand, compared with other carbon materials, graphene quantum dots have higher electronic conductivity, and their own quantum confinement effect can further enhance their conductive properties, so that the lithium iron phosphate coated with graphene quantum dots has higher electronic conductivity; at the same time, the coating layer formed by the graphene quantum dots can form channels between adjacent quantum dots in the coating layer that are more conducive to the insertion and extraction of lithium ions, effectively improving the ion mobility of lithium iron phosphate; on the other hand, graphene quantum dots have a larger specific surface area than other carbon materials, which can form better interface contact with the surface of the primary particles in the lithium iron phosphate, so that it can be more evenly coated on the surface of the primary particles. At the same time, graphene quantum dots can also provide more active sites, thereby further promoting electron conduction and ion transport.

[0044] Therefore, the positive electrode active material in the present invention has higher electronic conductivity and ion mobility as a whole, so that the lithium iron phosphate battery has higher low-temperature discharge performance.

[0045] Furthermore, the particle size of the primary and secondary particles in lithium iron phosphate, the average particle size of the graphene quantum dots, and the mass proportion of the graphene quantum dots in the positive electrode active material can be controlled to make the positive electrode active material have higher electronic conductivity and ion mobility.

[0046] In one specific embodiment, the average particle size of the secondary particles is 2 to 5 μm. Within this range, the particle size of the secondary particles is relatively suitable. It can not only increase the specific surface area of ​​the positive electrode active material to a certain extent, providing more active sites for lithium ion deintercalation and electron conduction paths, but also shorten the conduction paths of lithium ions and electrons, and avoid the increase in interfaces caused by too small particles, which affects electronic and ion conduction. Thus, the electronic conductivity and ion mobility of the positive electrode active material are further improved, and the battery has higher low-temperature discharge performance.

[0047] The "average particle size of the secondary particles" in the present invention can be obtained by scanning electron microscopy (SEM) testing.

[0048] For example, the particle size of the secondary particles may be 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or 5 μm, or a range consisting of any two of these values.

[0049] In one specific embodiment, the average particle size of the primary particles is 80 to 100 nm. Within this range, the primary particle size is relatively suitable. On the one hand, it can effectively shorten the conduction path of electrons and ions within the particles, reducing conduction resistance; on the other hand, it can also increase the specific surface area of ​​the lithium iron phosphate particles, providing not only more active sites for lithium ion deintercalation, but also more electron conduction paths. On the other hand, it can also avoid the increase in interfaces caused by the small primary particle size, which affects ion and electron conduction, thereby making the positive electrode active material have higher electronic conductivity and ion mobility as a whole, further improving the low-temperature discharge performance of the battery.

[0050] Illustratively, the average particle size of the primary particles may be 80 nm, 82 nm, 84 nm, 86 nm, 88 nm, 90 nm, 92 nm, 94 nm, 96 nm, 98 nm or 100 nm, or a range consisting of any two of these values.

[0051] The "average particle size of primary particles" in the present invention can be obtained by scanning electron microscopy (SEM) testing.

[0052] In one embodiment, the average particle size of the graphene quantum dots is 1 to 8 nm. Within this range, the quantum confinement effect can be weakened, achieving higher electronic conductivity, facilitating the insertion and extraction of lithium ions, and improving ion mobility.

[0053] Illustratively, the average particle size of the graphene quantum dots can be 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm or 8 nm, or a range consisting of any two of these values.

[0054] The “average particle size of the graphene quantum dots” in the present invention can be obtained by transmission electron microscopy (TEM) testing.

[0055] In one embodiment, the mass content of the graphene quantum dots in the positive electrode active material is 0.5-2.5 wt %. Within this range, the content of the graphene quantum dots in the positive electrode active material is moderate, which can make the positive electrode active material have higher electronic conductivity and ion mobility.

[0056] Illustratively, the mass content of graphene quantum dots in the positive electrode active material is 0.5wt%, 0.7wt%, 0.9wt%, 1.1wt%, 1.3wt%, 1.5wt%, 1.7wt%, 1.9wt%, 2.1wt%, 2.0wt%, 2.5wt%, or a range consisting of any two of these values.

[0057] In the present invention, the “mass content of graphene quantum dots in the positive electrode active material” can be obtained by fluorescence spectroscopy.

[0058] Furthermore, impurity phases (such as Fe2P2O7 and FeP2) are often unavoidable in lithium iron phosphate crystals. The presence of these impurity phases can lead to reduced battery capacity and stability. The crystallinity of lithium iron phosphate can represent the content of these impurity phases: the higher the crystallinity, the lower the content of these impurity phases. Therefore, the crystallinity of the positive electrode active material can be controlled to achieve higher electronic conductivity and ion mobility.

[0059] In a specific embodiment, the crystallinity of the positive electrode active material is 40-60%. Within this range, the structure is relatively stable and the battery capacity is relatively high.

[0060] Illustratively, the crystallinity can be 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58% or 60%, or a range consisting of any two values ​​therein.

[0061] The “crystallinity of the positive electrode active material” in the present invention can be obtained by differential scanning calorimetry (DSC) testing.

[0062] Furthermore, the positive electrode active material can have higher electronic conductivity and ion mobility by further controlling at least one of the particle size of secondary particles in lithium iron phosphate, the average particle size of primary particles, the average particle size of graphene quantum dots, the mass content of graphene quantum dots in the positive electrode active material, and the crystallinity of the positive electrode active material.

[0063] In one embodiment, the electronic conductivity of the positive electrode active material is not less than 0.1 S / cm, and the ion diffusion rate is not less than 5.6×10 -13 cm 2 / s. Within this range, the positive electrode active material has higher electronic conductivity and ion mobility, which can further improve the low-temperature discharge performance of the battery.

[0064] The “electronic conductivity” of the positive electrode active material in the present invention can be obtained by testing with a conductivity meter, and the “ion mobility” can be obtained by fitting a cyclic voltammetry test combined with a powder impedance test using an electrochemical workstation.

[0065] A second aspect of the present invention provides a method for preparing a positive electrode active material, comprising the following steps:

[0066] 1) mixing raw materials including graphene oxide, a cutting agent, and water to obtain a first mixed system; and subjecting the first mixed system to a first reaction to obtain graphene quantum dots;

[0067] 2) Mixing raw materials including lithium iron phosphate, graphene quantum dots, and water to obtain a second mixed system; and subjecting the second mixed system to a second reaction to obtain a positive electrode active material.

[0068] Specifically, in step 1), raw materials including graphene oxide, hydrogen peroxide, and water are mixed according to a ratio to obtain a first mixed system; and the first mixed system is subjected to a first reaction to obtain graphene quantum dots.

[0069] The "cutting agent" in the present invention mainly plays an oxidative cutting role, breaking the carbon-carbon bonds of graphene oxide, thereby cutting large-sized graphene oxide into nano-scale graphene quantum dots.

[0070] Preferably, the cutting agent of the present invention comprises at least one of hydrogen peroxide, persulfate, nitric acid, and sulfuric acid; more preferably, the cutting agent is hydrogen peroxide. Hydrogen peroxide decomposes primarily into water and oxygen, and contains no residual toxic substances, simplifying post-processing steps.

[0071] Furthermore, after the reaction is completed, a reaction system including graphene quantum dots is obtained. The reaction system can also be centrifuged to remove unreacted graphene oxide particles in the reaction system to obtain a supernatant including graphene quantum dots.

[0072] Furthermore, the supernatant obtained by the above centrifugation is subjected to solid-liquid separation, and the obtained solid phase is dried to obtain graphene quantum dots.

[0073] The present invention does not specifically limit the method for solid-liquid separation; separation can be performed using conventional methods in the art, as long as the graphene quantum dots in the supernatant are isolated. For example, a dialysis bag can be used for solid-liquid separation. The present invention does not specifically limit the specifications of the dialysis bag; the bag can be selected based on actual conditions.

[0074] The present invention does not specifically limit the drying method and conditions, and only requires that the water in the separated solid phase be removed. For example, the drying can be performed by freeze drying.

[0075] Furthermore, during the reaction process, the reaction device including the first mixing system can be connected to a water circulation condensation reflux device.

[0076] The present invention does not specifically limit the sources of graphene oxide, cutting agent, and water. Commercially available products or products prepared by conventional preparation methods familiar to those skilled in the art can be used; among them, water is preferably deionized water.

[0077] The present invention does not specifically limit the mixing method of the raw materials including graphene oxide, cutting agent, and water, and conventional mixing methods in the art can be used, as long as the three are mixed uniformly. For example, the mixing can be carried out by stirring.

[0078] The present invention does not impose any specific limitation on the total mass content of graphene oxide and cutting agent in water, and can be adjusted according to actual conditions.

[0079] In step 2), raw materials including lithium iron phosphate, graphene quantum dots prepared in step 1), and water are mixed according to a ratio to obtain a second mixed system; the second mixed system is subjected to a second reaction to obtain a positive electrode active material.

[0080] Furthermore, the reaction system is subjected to solid-liquid separation after the reaction is completed, and the separated solid phase is washed and dried to obtain the positive electrode active material.

[0081] The present invention does not impose any specific restrictions on the method of solid-liquid separation, as long as the positive electrode active material obtained by the reaction can be separated. For example, separation can be performed by filtration.

[0082] The present invention does not specifically limit the washing method, and it is sufficient to clean the uncoated graphene quantum dots in the separated positive electrode active material. For example, the separated positive electrode active material can be washed repeatedly with deionized water 3 to 5 times.

[0083] The present invention does not impose any specific restrictions on the drying conditions, as long as the moisture in the washed positive electrode active material can be dried.

[0084] Furthermore, during the reaction process, the reaction device including the second mixing system can be connected to a water circulation condensation reflux device.

[0085] The present invention does not specifically limit the sources of lithium iron phosphate and water, and commercially available products or products prepared by conventional preparation methods familiar to those skilled in the art can be used; among which, water is preferably deionized water.

[0086] The present invention does not specifically limit the mixing method of the raw materials including lithium iron phosphate, graphene quantum dots, and water. The mixing method can be the same as that in step 1), and will not be described in detail here.

[0087] The present invention does not impose any specific limitation on the total mass content of lithium iron phosphate and graphene quantum dots in water, and can be adjusted according to actual conditions.

[0088] The preparation method of the positive electrode active material of the present invention uses graphene oxide as a carbon source and prepares graphene quantum dots under the action of a cutting agent; subsequently, the prepared graphene quantum dots react with lithium iron phosphate. During this process, the graphene quantum dots penetrate into the interior of the lithium iron phosphate particles and coat at least part of the surface of the primary particles in the lithium iron phosphate, so that the prepared positive electrode active material has high electronic conductivity and ion mobility. This is mainly because: on the one hand, the graphene quantum dots themselves have high electronic conductivity, and their own quantum confinement effect can further enhance their conductive properties, so that the lithium iron phosphate coated with graphene quantum dots has high electronic conductivity; on the other hand, the graphene quantum dots have a higher specific surface area and can form good contact with the surface of the primary particles to form a more uniform coating layer; at the same time, the graphene quantum dots can also provide more active sites, which promotes the transmission of electrons and ions; on the other hand, in the coating layer formed by the graphene quantum dots, adjacent graphene quantum dots can form channels that facilitate lithium ion migration, further improving the ion mobility. Therefore, the positive electrode active material prepared by the above preparation method has high electronic conductivity and ion mobility, which can effectively improve the low-temperature discharge performance of lithium iron phosphate batteries.

[0089] In one embodiment, in step 1), the mass ratio of graphene oxide to cutting agent is (1.0-3.3):13.5. Within this range, the average particle size of the graphene quantum dots is between 1 and 8 nm, achieving higher electronic conductivity and ion mobility.

[0090] Illustratively, the mass ratio of graphene oxide to cutting agent can be 1:13.5, 1.2:13.5, 1.4:13.5, 1.6:13.5, 1.8:13.5, 2.0:13.5, 2.2:13.5, 2.4:13.5, 2.6:13.5, 2.8:13.5, 3.0:13.5, 3.2:13.5 or 3.3:13.5, or a range consisting of any two ratios therein.

[0091] In one embodiment, in step 2), the mass ratio of lithium iron phosphate to graphene quantum dots is 200:(1-5). Within this range, the mass content of graphene quantum dots in the positive electrode active material is between 0.5 and 2.5 wt%, further improving the electronic conductivity and ion mobility of the positive electrode active material.

[0092] Illustratively, the mass ratio of lithium iron phosphate to graphene quantum dots can be 200:1, 200:1.5, 200:2, 200:2.5, 200:3, 200:3.5, 200:4, 200:4.5 or 200:5, or a range consisting of any two ratios therein.

[0093] In one embodiment, in step 1), the reaction temperature of the first reaction is 90-140°C and the reaction time is 12-15 hours. Within this range, the oxidation cutting efficiency is improved, and the formation of graphene quantum dots with uniform size is facilitated; at the same time, the fluorescence performance is improved.

[0094] Illustratively, the reaction temperature of the first reaction can be 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C or 140°C, or a range consisting of any two of the values ​​therein; the reaction time can be 12h, 13h, 14h or 15h, or a range consisting of any two of the values ​​therein.

[0095] In one embodiment, in step 2), the reaction temperature of the second reaction is 90-140° C. and the reaction time is 1-2 hours. Within this range, the graphene oxide is uniformly coated, and the resulting positive electrode active material has high electronic conductivity and ion mobility.

[0096] Illustratively, the reaction temperature of the second reaction can be 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C or 140°C, or a range consisting of any two of the values ​​therein; the reaction time can be 1 h, 1.2 h, 1.4 h, 1.6 h, 1.8 h or 2 h, or a range consisting of any two of the values ​​therein.

[0097] A third aspect of the present invention provides a method for preparing the positive electrode active material according to the first aspect, comprising the following steps:

[0098] 1) mixing raw materials including amino acids and water to obtain a third mixed system; subjecting the third mixed system to a third reaction under microwave conditions to obtain graphene quantum dots;

[0099] 2) Mixing raw materials including lithium iron phosphate, graphene quantum dots, and water to obtain a fourth mixed system; and subjecting the fourth mixed system to a fourth reaction to obtain a positive electrode active material.

[0100] Specifically, in step 1), raw materials including amino acids and water are mixed according to a ratio to obtain a third mixed system; the third mixed system is subjected to a third reaction to obtain graphene quantum dots.

[0101] Furthermore, after the reaction is completed, a reaction system including graphene quantum dots is obtained, the reaction system is subjected to solid-liquid separation, and the obtained solid phase is dried to obtain graphene quantum dots.

[0102] The present invention does not specifically limit the method for solid-liquid separation; separation can be performed using conventional methods in the art, as long as the graphene quantum dots in the reaction system are isolated. For example, a dialysis bag can be used for solid-liquid separation. The present invention does not specifically limit the specifications of the dialysis bag; the bag can be selected based on actual conditions.

[0103] The present invention does not specifically limit the drying method and conditions, and only requires that the water in the separated solid phase be removed. For example, the drying can be performed by freeze drying.

[0104] The present invention does not specifically limit the sources of amino acids and water, and commercially available products or products prepared by conventional preparation methods familiar to those skilled in the art can be used; among which, water is preferably deionized water.

[0105] The present invention does not impose any specific restrictions on the mixing method of the raw materials including amino acids and water, and conventional mixing methods in the art can be used, as long as the two are mixed uniformly. For example, the mixing can be carried out by stirring.

[0106] The present invention does not specifically limit the manner of applying microwaves. For example, the third mixed system can be placed in a microwave synthesizer to carry out the reaction.

[0107] In step 2), raw materials including lithium iron phosphate, graphene quantum dots prepared in step 1), and water are mixed according to a ratio to obtain a fourth mixed system; the fourth mixed system is subjected to a fourth reaction to obtain a positive electrode active material.

[0108] Furthermore, the reaction system is subjected to solid-liquid separation after the reaction is completed, and the separated solid phase is washed and dried to obtain the positive electrode active material.

[0109] The present invention does not impose any specific restrictions on the method of solid-liquid separation, as long as the positive electrode active material obtained by the reaction can be separated. For example, separation can be performed by filtration.

[0110] The present invention does not specifically limit the washing method, and it is sufficient to clean the uncoated graphene quantum dots in the separated positive electrode active material. For example, the separated positive electrode active material can be washed repeatedly with deionized water 3 to 5 times.

[0111] The present invention does not impose any specific restrictions on the drying conditions, as long as the moisture in the washed positive electrode active material can be dried.

[0112] Furthermore, during the reaction process, the reaction device including the fourth mixing system can be connected to a water circulation condensation reflux device.

[0113] The present invention does not specifically limit the sources of lithium iron phosphate and water, and commercially available products or products prepared by conventional preparation methods familiar to those skilled in the art can be used; among which, water is preferably deionized water.

[0114] The present invention does not specifically limit the mixing method of the raw materials including lithium iron phosphate, graphene quantum dots, and water. The mixing method can be the same as that in step 1), and will not be described in detail here.

[0115] The present invention does not impose any specific limitation on the total mass content of lithium iron phosphate and graphene quantum dots in water, and can be adjusted according to actual conditions.

[0116] The method for preparing the positive electrode active material of the present invention comprises subjecting an aqueous solution including amino acids to a third reaction under microwave conditions to obtain graphene quantum dots; subsequently, the graphene quantum dots are subjected to a fourth reaction with lithium iron phosphate to prepare the positive electrode active material. This positive electrode active material has high electronic conductivity and ion mobility, which can effectively improve the low-temperature discharge performance of the battery. The reasons may be: on the one hand, compared with other carbon materials, graphene quantum dots have higher electronic conductivity, and due to their inherent quantum confinement effect, their conductivity can be further enhanced; at the same time, adjacent graphene quantum dots in the coating layer can form channels that are more conducive to the escape and insertion of lithium ions; on the other hand, the graphene quantum dots have a large specific surface area, which can form better interfacial contact with the surface of the lithium iron phosphate primary particles and can also provide more active sites. Therefore, the positive electrode active material prepared by the above preparation method has high electronic conductivity and ion mobility, thereby comprehensively improving the low-temperature discharge performance of the battery.

[0117] In one embodiment, the amino acid comprises at least one of L-asparagine, glutamic acid, cysteine, and lysine. The aforementioned amino acids are conducive to the formation of uniform graphene quantum dots.

[0118] When the amino acid comprises a plurality of (at least two) specific compounds as mentioned above, the present invention does not impose any particular limitation on the ratio of each specific compound.

[0119] In one embodiment, in step 1), the mass concentration of the amino acid in the third mixing system is 0.2 to 0.6 g / mL, i.e., the mass concentration of the amino acid in water. Within this range, the mass concentration of the amino acid is moderate, which helps form graphene quantum dots with a smaller particle size and prevents agglomeration.

[0120] Illustratively, the mass concentration of the amino acid can be 0.2 g / mL, 0.3 g / mL, 0.4 g / mL, 0.5 g / mL or 0.6 g / mL, or a range consisting of any two of these values.

[0121] In one embodiment, in step 2), the mass ratio of lithium iron phosphate to graphene quantum dots is 1:(0.002-0.008). Within this range, the mass content of graphene quantum dots in the positive electrode active material is within the range of 0.5-2.5 wt%, thereby improving the electronic conductivity and ion mobility of the positive electrode active material.

[0122] For example, the mass ratio of lithium iron phosphate to graphene quantum dots can be 1:0.002, 1:0.003, 1:0.004, 1:0.005, 1:0.006, 1:0.007 or 1:0.008, or a range consisting of any two ratios therein.

[0123] In one embodiment, in step 1), in the third reaction, the microwave power is 150-300 W, the reaction temperature is 120-250° C., and the reaction time is 10-60 min. Within this range, the reaction conditions are relatively suitable, which helps to form graphene quantum dots with relatively uniform size and high crystallinity, reduces agglomeration, and does not cause excessive carbonization and side reactions (such as the formation of amorphous carbon impurities), preventing the destruction of the structural uniformity of the quantum dots, thereby producing graphene quantum dots with high fluorescence performance.

[0124] Illustratively, the microwave power can be 150 W, 180 W, 210 W, 240 W, 270 W or 300 W, or a range consisting of any two values ​​therein; the reaction temperature can be 120° C., 140° C., 160° C., 180° C., 200° C., 220° C., 240° C. or 250° C., or a range consisting of any two values ​​therein; the reaction time can be 10 min, 20 min, 30 min, 40 min, 50 min or 60 min, or a range consisting of any two values ​​therein.

[0125] In one embodiment, in step 2), the reaction temperature of the fourth reaction is 90-140° C. and the reaction time is 1-2 hours. Within this range, the graphene oxide is uniformly coated, and the obtained positive electrode active material has high electronic conductivity and ion mobility.

[0126] Illustratively, the reaction temperature can be 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C or 140°C, or a range consisting of any two values ​​therein; the reaction time can be 1 h, 1.2 h, 1.4 h, 1.6 h, 1.8 h or 2 h, or a range consisting of any two values ​​therein.

[0127] A fourth aspect of the present invention provides a positive electrode comprising the positive electrode active material of the first aspect, or the positive electrode active material prepared by the preparation method of the second aspect, or the positive electrode active material prepared by the preparation method of the third aspect. The inclusion of the positive electrode active material effectively improves the low-temperature discharge performance of the battery.

[0128] The present invention does not specifically limit the structure of the positive electrode. In one embodiment, the positive electrode includes a positive electrode current collector and a positive electrode active layer disposed on at least a portion of the surface of the positive electrode current collector. The positive electrode active layer includes the above-mentioned positive electrode active material, a conductive agent and a binder.

[0129] The present invention does not specifically limit the material of the positive electrode current collector, and it can be any conventional material in the art. For example, the material of the positive electrode current collector can be any one of aluminum foil and nickel foil.

[0130] The present invention does not specifically limit the type of the conductive agent, and it can be any conventional material in the art. For example, the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber.

[0131] The present invention does not specifically limit the type of binder, and it can be any conventional material in the art. For example, the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinyl pyrrolidone, and polyurethane.

[0132] The present invention does not specifically limit the preparation method of the positive electrode sheet. In one embodiment, the positive electrode can be prepared by a method comprising the following steps:

[0133] The positive electrode active material of the present invention, the conductive agent and the binder are dispersed in N-methylpyrrolidone (NMP) solvent, and fully stirred and mixed to form a uniform positive electrode slurry; the positive electrode slurry is evenly coated on the positive electrode collector, and the positive electrode sheet is obtained after drying, rolling and slitting.

[0134] The present invention does not impose any specific restrictions on the amounts of the positive electrode active material, the conductive agent, and the binder, which can be adjusted according to actual conditions.

[0135] A fifth aspect of the present invention provides a battery comprising the positive electrode according to the fourth aspect. Therefore, the battery has a high low-temperature discharge performance.

[0136] It is conceivable that the battery of the present invention includes, in addition to the above-mentioned positive and negative electrodes, a negative electrode, an electrolyte, and a separator.

[0137] The present invention does not specifically limit the structure of the negative electrode. In one embodiment, the negative electrode includes a negative electrode current collector and a negative electrode active layer disposed on at least a portion of the surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent.

[0138] The present invention does not specifically limit the material of the negative electrode current collector, and it can be any conventional material in the art. For example, the negative electrode current collector can be any one of copper foil, nickel foam, and copper foam.

[0139] The present invention does not specifically limit the type of negative electrode active material, and can be any negative electrode active material commonly used in batteries. For example, the negative electrode active material can be selected from at least one of graphite, hard carbon, soft carbon, mesocarbon microbeads, silicon-based negative electrode materials (primarily including silicon monoxide and silicon-carbon negative electrodes), and tin-based negative electrode materials (primarily including tin and tin alloys).

[0140] The present invention does not specifically limit the type of binder, and the binder may be any binder commonly used in current battery negative electrodes. For example, the binder may be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate.

[0141] The present invention does not specifically limit the type of conductive agent, and can be any conductive agent commonly used in battery negative electrodes. For example, the conductive agent can be selected from at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and graphene.

[0142] The present invention does not specifically limit the preparation method of the negative electrode. In one embodiment, the negative electrode can be prepared by a method comprising the following steps:

[0143] The negative electrode active material, conductive agent and binder are dispersed in deionized water and fully stirred to form a uniform negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode collector, and the negative electrode sheet is obtained after drying, rolling and cutting.

[0144] The present invention does not impose any specific restrictions on the amounts of the negative electrode active material, the conductive agent, and the binder, which can be adjusted according to actual conditions.

[0145] The present invention does not specifically limit the composition of the electrolyte, and may include one or more solvents commonly used in current battery electrolytes, as well as electrolyte lithium salts commonly used in current battery electrolytes. For example, the solvent may include at least one of ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, ethyl methyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, and γ-butyrolactone; and the lithium salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0146] The present invention does not impose any specific restrictions on the material of the separator, and can be any separator material commonly used in current batteries. For example, the separator can be selected from any of polypropylene separators (PP), polyethylene separators (PE), polypropylene / polyethylene two-layer composite films (PP / PE), polyimide electrospun separators (PI), polypropylene / polyethylene / polypropylene three-layer composite films (PP / PE / PP), cellulose non-woven separators, and separators with ceramic coatings.

[0147] The present invention does not specifically limit the preparation method of the battery. In one embodiment, the battery can be prepared by a method comprising the following steps:

[0148] The positive electrode sheet, separator, and negative electrode sheet are wound or stacked to form a bare cell, which is then encapsulated in a pre-stamped aluminum-plastic film bag. After the encapsulated cells are dried to remove moisture, the electrolyte is injected into the dried cells. After the cells are stored, formed, and resealed, the lithium-ion battery is completed.

[0149] It should be noted that the battery in the present invention can be a single cell or a battery pack consisting of at least two single cells. The battery cells comprising the battery pack can be electrically connected by conventional methods in the art, such as series connection, parallel connection, or a combination of these connection methods, without particular limitation.

[0150] A sixth aspect of the present invention provides an electrical device comprising the battery according to the fifth aspect. Therefore, the electrical device can operate continuously and stably at low temperatures, reducing the inconvenience caused by temperature changes.

[0151] The present invention does not specifically limit the electrical equipment. For example, it can be an electric car, a mobile phone, a tablet computer, a laptop computer, a wearable device (watch, bracelet), a digital camera, etc.

[0152] Hereinafter, the positive electrode active material provided by the present invention and the positive electrode and battery including the positive electrode active material will be described in detail through specific examples.

[0153] Example 1

[0154] 1) 0.5 g of graphene oxide was mixed with 5 mL of hydrogen peroxide solution (mass concentration was 30%, i.e., 30 g of hydrogen peroxide was contained in every 100 mL of hydrogen peroxide solution) to obtain a mixed system (wherein, the mass ratio of graphene oxide to hydrogen peroxide was 4.5:13.5), deionized water was added to the mixed system until the volume of the system was 25 mL, the mixed system was heated to 95 ° C and subjected to hydrothermal reflux reaction at this temperature for 12 h to obtain a yellow solution; the yellow solution was centrifuged to remove large particles of uncut graphene oxide to obtain a supernatant; the supernatant was dialyzed in a 3500 Da dialysis bag for 36 h to obtain a graphene quantum dot solution, which was freeze-dried to obtain the graphene quantum dots of this embodiment; the average particle size of the graphene quantum dots was 2.5 ± 0.5 nm,

[0155] 2) The graphene quantum dot powder prepared above was mixed with deionized water to prepare a graphene quantum dot dispersion with a concentration of 2 g / L; 10 g of lithium iron phosphate powder was evenly mixed with 25 mL of the graphene quantum dot dispersion to obtain a mixed system (wherein the mass ratio of lithium iron phosphate to graphene quantum dots was 1:0.005); the mixed system was heated to 95 ° C and refluxed at this temperature for 1 hour. After the reaction was completed, it was cooled, filtered, and washed to obtain a black solid; the black solid was dried in an oven at 60 ° C overnight to obtain the positive electrode active material of this embodiment.

[0156] Example 2

[0157] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1, except that in step 1), the volume of the hydrogen peroxide solution is adjusted to 10 mL, and the mass ratio of graphene oxide to hydrogen peroxide is 2.25:13.5.

[0158] Example 3

[0159] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1, except that in step 1), the volume of the hydrogen peroxide solution is adjusted to 15 mL, and the mass ratio of graphene oxide to hydrogen peroxide is 1.5:13.5.

[0160] Example 4

[0161] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1, except that in step 1), the volume of the hydrogen peroxide solution is adjusted to 20 mL, and the mass ratio of graphene oxide to hydrogen peroxide is 1.125:13.5.

[0162] Example 5

[0163] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 3, except that in step 2), the concentration of the graphene quantum dot dispersion is adjusted to 4 g / L, and the mass ratio of lithium iron phosphate to graphene quantum dots is 1:0.01.

[0164] Example 6

[0165] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 3, except that in step 2), the concentration of the graphene quantum dot dispersion is adjusted to 6 g / L, and the mass ratio of lithium iron phosphate to graphene quantum dots is 1:0.015.

[0166] Example 7

[0167] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 3, except that in step 2), the concentration of the graphene quantum dot dispersion is adjusted to 8 g / L, and the mass ratio of lithium iron phosphate to graphene quantum dots is 1:0.02.

[0168] Example 8

[0169] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 3, except that in step 2), the concentration of the graphene quantum dot dispersion is adjusted to 10 g / L, and the mass ratio of lithium iron phosphate to graphene quantum dots is 1:0.025.

[0170] Example 9

[0171] 1) 3.0 g of L-asparagine was dissolved in 10 mL of deionized water to obtain an L-asparagine aqueous solution having an L-asparagine mass concentration of 0.3 g / mL; the L-asparagine aqueous solution was placed in a microwave synthesizer and reacted at 120° C. for 15 min with a microwave power of 150 W to obtain an orange solution; the orange solution was dialyzed in a 3500 Da dialysis bag for 36 h to obtain a graphene quantum dot solution; the graphene quantum dot solution was freeze-dried to obtain the graphene quantum dots of this embodiment; and the average particle size of the graphene quantum dots was found to be 3.1±0.5 nm.

[0172] 2) The graphene quantum dot powder prepared above was mixed with deionized water to prepare a graphene quantum dot dispersion with a concentration of 1 g / L; 10 g of lithium iron phosphate powder was evenly mixed with 25 mL of the graphene quantum dot dispersion to obtain a mixed system (wherein the mass ratio of lithium iron phosphate to graphene quantum dots was 1:0.005); the mixed system was heated to 95 ° C and refluxed at this temperature for 1 hour. After the reaction was completed, it was cooled, filtered, and washed to obtain a black solid; the black solid was dried in an oven at 60 ° C overnight to obtain the positive electrode active material of this embodiment.

[0173] Example 10

[0174] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 9, except that in step 1), the reaction temperature is 250° C. and the reaction time is 30 min; the average particle size of the prepared graphene quantum dots is 8.0±0.3 nm.

[0175] Example 11

[0176] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 10, except that in step 2), the concentration of the graphene quantum dot dispersion is adjusted to 2 g / L, and the mass ratio of lithium iron phosphate to graphene quantum dots is 1:0.005.

[0177] Example 12

[0178] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1, except that in step 1), the mass of graphene oxide is adjusted to 1.0 g, and the mass ratio of graphene oxide to hydrogen peroxide is 9:13.5; the reaction time is adjusted to 15 h; and the average particle size of the prepared graphene quantum dots is 12.1 ± 0.8 nm.

[0179] Example 13

[0180] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1, except that in step 1), the concentration of the graphene quantum dot dispersion is adjusted to 12.8 g / L, and the mass ratio of lithium iron phosphate to graphene quantum dots is 1:0.032.

[0181] Example 14

[0182] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1, except that in step 1), the reaction temperature is 90° C. and the reaction time is 8 h.

[0183] Example 15

[0184] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1, except that in step 2), the reaction temperature is 150° C. and the reaction time is 3 h.

[0185] Example 16

[0186] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 9, except that in step 1), the mass concentration of L-asparagine is 0.8 g / mL.

[0187] Example 17

[0188] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 9, except that in step 1), the microwave power is 100 W, the reaction temperature is 100° C., and the reaction time is 10 min.

[0189] Comparative Example 1

[0190] The positive electrode active material in this comparative example is an uncoated lithium iron phosphate material, which is the same as the lithium iron phosphate material in Example 1.

[0191] Comparative Example 2

[0192] The positive electrode active material in this comparative example is substantially the same as that in Example 1, except that the coating material on the surface of the positive electrode active material in this comparative example is graphene, and the lateral dimension of the graphene is 12±0.8 μm and the thickness is 1.8±0.2 nm. The specific coating method includes the following steps:

[0193] The above-mentioned graphene was mixed with deionized water to prepare a graphene dispersion with a concentration of 2 g / L; 10 g of lithium iron phosphate powder was evenly mixed with 25 mL of the graphene dispersion to obtain a mixed system (wherein the mass ratio of lithium iron phosphate to graphene was 1:0.005); the mixed system was heated to 95°C and refluxed at this temperature for 1 hour. After the reaction was completed, it was cooled, filtered, and washed to obtain a black solid; the black solid was dried in an oven at 60°C overnight to obtain the positive electrode active material of this comparative example.

[0194] Comparative Example 3

[0195] The positive electrode active material in this comparative example is basically the same as that in Example 1, except that the coating material on the surface of the positive electrode active material in this comparative example is carbon nanotubes, which are single-walled carbon nanotubes with a diameter of 2.7±0.6 nm and a length of 50±8 μm. The specific coating method includes the following steps:

[0196] The above-mentioned carbon nanotubes were mixed with deionized water to prepare a carbon nanotube dispersion with a concentration of 2 g / L; 10 g of lithium iron phosphate powder was evenly mixed with 25 mL of the carbon nanotube dispersion to obtain a mixed system (wherein the mass ratio of lithium iron phosphate to carbon nanotubes was 1:0.005); the mixed system was heated to 95°C and refluxed at this temperature for 1 hour. After the reaction was completed, it was cooled, filtered, and washed to obtain a black solid; the black solid was dried in an oven at 60°C overnight to obtain the positive electrode active material of this comparative example.

[0197] Comparative Example 4

[0198] The positive electrode active material in this comparative example is basically the same as that in Example 1, except that the coating material on the surface of the positive electrode active material in this comparative example is conductive carbon black, the particle size of the conductive carbon black is 31±9nm, and the specific surface area is 763m 2 / g; The specific coating method includes the following steps:

[0199] The above-mentioned conductive carbon black was mixed with deionized water to prepare a conductive carbon black dispersion with a concentration of 2 g / L; 10 g of lithium iron phosphate powder was evenly mixed with 25 mL of the conductive carbon black dispersion to obtain a mixed system (wherein the mass ratio of lithium iron phosphate to conductive carbon black was 1:0.005); the mixed system was heated to 95°C and refluxed at this temperature for 1 hour. After the reaction was completed, it was cooled, filtered, and washed to obtain a black solid; the black solid was dried in an oven at 60°C overnight to obtain the positive electrode active material of this comparative example.

[0200] Comparative Example 5

[0201] The positive electrode active material in this comparative example is basically the same as that in Example 1, except that the coating material on the surface of the positive electrode active material in this comparative example is a polyacrylonitrile carbonized material. The specific coating method includes the following steps:

[0202] 0.5 g of polyacrylonitrile and 10 g of lithium iron phosphate powder were ball-milled for 5 h to obtain a mixture; the mixture was heated to 500° C. under a nitrogen atmosphere and pyrolyzed and carbonized for 6 h to obtain the positive electrode active material of this comparative example.

[0203] Test example

[0204] 1. The average particle size of the graphene quantum dots prepared in the above embodiments and comparative examples, the mass content of the graphene quantum dots in the positive electrode active material, the crystallinity, electronic conductivity and ion mobility of the positive electrode active material were tested, comprising the following steps:

[0205] 1) Average particle size test of graphene quantum dots

[0206] The prepared graphene dots were tested by transmission electron microscopy (TEM), and the particle sizes of 200 graphene quantum dots were counted and the average value was taken to obtain the average particle size of the graphene quantum dots.

[0207] Figure 1 is the TEM image of graphene quantum dots in Example 1, Figure 1 It can be seen that the particle size of graphene quantum dots is 2.5±0.5 nm. Figure 2 is the TEM image of graphene quantum dots in Example 9, Figure 2 It can be seen that the particle size of graphene quantum dots is 3.1±0.5 nm.

[0208] 2) Mass content of graphene quantum dots in the positive electrode active material

[0209] The positive electrode active material prepared above was tested using a fluorescence spectrometer (instrument model: Horiba FluoroMax-4). The specific test method is as follows:

[0210] The graphene quantum dots prepared in the above-mentioned embodiments and comparative examples are respectively prepared to obtain a series of graphene quantum dot standard aqueous solutions of known concentrations, with concentrations of 0.1 μg / mL, 0.5 μg / mL, and 1 μg / mL, respectively. The fluorescence intensities of the blank sample (i.e., water) and the aforementioned graphene quantum dot standard aqueous solutions of different concentrations are measured in sequence, the maximum emission wavelength and the fluorescence intensity value are recorded, and a standard curve is drawn with the mass concentration of the graphene quantum dot aqueous solution as the horizontal coordinate and the fluorescence intensity as the vertical coordinate. The positive electrode active material prepared in the above-mentioned embodiments and comparative examples is dispersed in water to prepare a sample to be tested. The mass concentration of the sample to be tested needs to be within the concentration range of the standard aqueous solution. The fluorescence intensity of the sample to be tested is measured, substituted into the aforementioned standard curve equation, and the mass content of the graphene quantum dots in the positive electrode active material is calculated.

[0211] 3) Crystallinity of positive electrode active material

[0212] The crystallinity of the positive electrode active material is measured by differential scanning calorimetry (DSC). The melting enthalpy ΔH1 is calculated based on the amount of heat absorbed during the melting process of the positive electrode active material, that is, the ratio of the peak area of ​​the endothermic peak in the DSC test curve to the mass of the sample involved in the test, with the unit being J / g; then the crystallinity of the positive electrode active material (%) = (ΔH1 / ΔH0) × 100%, where ΔH0 is the standard melting enthalpy value of the positive electrode active material in a completely crystalline state.

[0213] 4) Electronic conductivity

[0214] The four-probe method uses an RTS-8 digital four-probe tester to apply pressure to a mold filled with positive electrode active material powder. At the same time, the resistance test mechanism transmits an electrical signal through the probes to obtain the electronic conductivity. The test conditions are: the four probes are equally spaced at 1mm, the probe material is tungsten steel probe, the current is 1mA, and the voltage is 1V.

[0215] 5) Ion migration rate

[0216] The ion migration rate is obtained by preparing the positive electrode active material into a button cell, and then performing cyclic voltammetry testing combined with powder impedance testing after activation. The specific steps are as follows:

[0217] The prepared positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were dispersed in N-methylpyrrolidone (NMP) solvent at a mass ratio of 90:5:5. The mixture was stirred and mixed thoroughly to form a uniform positive electrode slurry. The positive electrode slurry was evenly coated on aluminum foil, vacuum dried, rolled, and cut to obtain a positive electrode sheet with a compaction density of 2.145 g / cm 3 , with a surface density of 15g / cm 2 In a glove box, the aforementioned positive electrode sheet was assembled with a lithium negative electrode sheet, a polyethylene separator, an electrolyte, and a spacer spring to form a CR2025 button cell. The electrolyte consisted of lithium hexafluorophosphate, ethyl methyl carbonate, dimethyl carbonate, and ethylene carbonate in a mass ratio of 13.26:37.17:24.78:24.7.

[0218] The button cell prepared above was activated, and the activation mechanism was as follows: the voltage window was 2.5-4.3V, and the charge and discharge were performed at 0.1C for two cycles. 2 / s) was obtained by cyclic voltammetry test combined with powder impedance test fitting, and specifically calculated by formula 1,

[0219]

[0220] In formula 1, D Li+ Indicates the lithium ion migration rate of the positive electrode active material;

[0221] R represents the gas constant, R≈8.314, unit is J·(mol·K) -1 ;

[0222] T represents absolute temperature, T = 273.15 + actual temperature (°C), K;

[0223] A is the surface area of ​​the electrode, cm 2 ;

[0224] n represents the number of electrons gained or lost per mole of molecules involved in the redox reaction;

[0225] F represents the Faraday constant, which is 69485.3 C·mol -1 ;

[0226] C Li+ Represents Li + Concentration, about 0.059 mol·L -1 ;

[0227] σ represents the Warburg factor;

[0228] Among them, R, T, A, n, F, C Li+ Obtained through cyclic voltammetry test; σ is the slope of the low-frequency straight line obtained by impedance spectrum analysis fitting.

[0229] The test results are shown in Table 1.

[0230] Table 1

[0231]

[0232]

[0233] 2. Use Xinwei's button battery test channel to perform electrochemical testing on the CR2025 button half-cell prepared above, which specifically includes the following steps:

[0234] 1) Room temperature charge and discharge capacity

[0235] The test temperature is 25°C, the test window is 2.5~4.3V, the charging current is 0.1C, and the battery is charged at a constant voltage of 3.8V to a cutoff current of 0.02C, and then discharged at 0.1C to obtain the room temperature charge specific capacity and room temperature discharge specific capacity.

[0236] 2) Normal temperature 0.5C charge specific capacity and low temperature 1C discharge specific capacity

[0237] The test temperature is 25°C, the test window is 2.5~4.3V, and two cycles of charge and discharge are performed at 0.1C; then it is charged to 3.8V at 0.5C, and the 0.5C charge capacity at room temperature is recorded; then the battery is placed at -20°C until the temperature of the battery body is consistent with the ambient temperature, and then discharged to 2.0V at a constant current of 1C, and the 1C discharge capacity at a low temperature of -20°C is obtained.

[0238] The test results are shown in Table 2.

[0239] Table 2

[0240]

[0241]

[0242] From Table 1 we can see that:

[0243] Compared with Comparative Examples 1 to 5, the batteries prepared from the positive electrode active materials in Examples 1 to 17 have higher low-temperature performance; among them, Example 6 has the highest ion mobility and electronic conductivity and excellent low-temperature performance.

[0244] 3. XRD test was performed on the positive electrode active material in Example 1. The test results are shown in Figure 3 .

[0245] Figure 3 is the XRD pattern of the positive electrode active material and lithium iron phosphate in Example 1, Figure 3 It can be seen that in the XRD spectra of the lithium iron phosphate after coating modification (LFP-GQDs) and the unmodified lithium iron phosphate (LFP), there are only XRD diffraction peaks of lithium iron phosphate, which shows that the coating modification has no effect on the crystal phase of lithium iron phosphate.

[0246] 4. Fluorescence spectrum test was performed on the positive electrode active materials prepared in Example 1 and Example 9, and the test results are shown in Figure 4 and Figure 5 .

[0247] Figure 4 is the fluorescence spectrum of the positive electrode active material in Example 1, Figure 4 The maximum emission wavelength is at 490 nm, and the maximum emission wavelength is at 550 nm. Furthermore, the fluorescence emission wavelength of the positive electrode active material red-shifts with increasing excitation wavelength, exhibiting the excitation-dependent emission characteristic common in graphene quantum dots, emitting yellow light under ultraviolet light.

[0248] Figure 5 is the fluorescence spectrum of the positive electrode active material in Example 9, Figure 5 The maximum emission wavelength is at 449 nm, with an excitation wavelength of 370 nm. Furthermore, the fluorescence emission wavelength of the positive electrode active material red-shifts with increasing excitation wavelength, demonstrating the excitation-dependent emission characteristic commonly seen in graphene quantum dots, emitting blue light under ultraviolet light.

[0249] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode active material, characterized in that The positive electrode active material includes lithium iron phosphate and graphene quantum dots. The lithium iron phosphate includes secondary particles formed by primary particles. The graphene quantum dots are coated on at least a portion of the surface of the primary particles.

2. The positive electrode active material according to claim 1, characterized in that The average particle size of the secondary particles is 2 to 5 μm; and / or the average particle size of the primary particles is 80 to 100 nm.

3. The positive electrode active material according to claim 1 or 2, characterized in that The average particle size of the graphene quantum dots is 1 to 8 nm.

4. The positive electrode active material according to any one of claims 1 to 3, characterized in that The mass content of the graphene quantum dots in the positive electrode active material is 0.5 to 2.5 wt%.

5. The positive electrode active material according to any one of claims 1 to 4, characterized in that The positive electrode active material has a crystallinity of 40 to 60%.

6. The positive electrode active material according to any one of claims 1 to 5, characterized in that The electron conductivity of the positive electrode active material is not less than 0.1 S / cm, and the ion migration rate is not less than 5.6×10 -13 cm 2 / s.

7. A method for preparing the positive electrode active material according to any one of claims 1 to 6, characterized in that: The following steps are involved: 1) mixing raw materials including graphene oxide, a cutting agent, and water to obtain a first mixed system; and subjecting the first mixed system to a first reaction to obtain graphene quantum dots; 2) mixing raw materials including lithium iron phosphate, the graphene quantum dots, and water to obtain a second mixed system; and subjecting the second mixed system to a second reaction to obtain the positive electrode active material.

8. The method for preparing a positive electrode active material according to claim 7, wherein: The cutting agent includes at least one of hydrogen peroxide, persulfate, nitric acid, and sulfuric acid.

9. The method for preparing a positive electrode active material according to claim 7 or 8, characterized in that: In step 1), the mass ratio of the graphene oxide to the cutting agent is (1.0-3.3):13.5; and / or, in step 2), the mass ratio of the lithium iron phosphate to the graphene quantum dots is 200:(1-5).

10. The method for preparing a positive electrode active material according to any one of claims 7 to 9, characterized in that: In step 1), the reaction temperature of the first reaction is 90-140° C., and the reaction time is 12-15 h; and / or, in step 2), the reaction temperature of the second reaction is 90-140° C., and the reaction time is 1-2 h.

11. A method for preparing the positive electrode active material according to any one of claims 1 to 6, characterized in that: The following steps are involved: 1) mixing raw materials including amino acids and water to obtain a third mixed system; subjecting the third mixed system to a third reaction under microwave conditions to obtain graphene quantum dots; 2) mixing raw materials including lithium iron phosphate, the graphene quantum dots, and water to obtain a fourth mixed system; and subjecting the fourth mixed system to a fourth reaction to obtain the positive electrode active material.

12. The method for preparing a positive electrode active material according to claim 11, wherein: In step 1), the amino acid includes at least one of L-asparagine, glutamic acid, cysteine, and lysine.

13. The method for preparing a positive electrode active material according to claim 11 or 12, characterized in that: In step 1), the mass concentration of the amino acid in the third mixed system is 0.2 to 0.6 g / mL; and / or, in step 2), the mass ratio of the lithium iron phosphate to the graphene quantum dots is 1:(0.002 to 0.008).

14. The method for preparing a positive electrode active material according to any one of claims 11 to 13, characterized in that: In step 1), in the third reaction, the microwave power is 150-300 W, the reaction temperature is 120-250° C., and the reaction time is 10-60 min; and / or, in step 2), the reaction temperature of the fourth reaction is 90-140° C., and the reaction time is 1-2 h.

15. A positive electrode, characterized in that The positive electrode comprises the positive electrode active material according to any one of claims 1 to 6, or the positive electrode active material prepared by the preparation method according to any one of claims 7 to 10, or the positive electrode active material prepared by the preparation method according to any one of claims 11 to 14.

16. A battery, characterized in that: The battery comprises the positive electrode according to claim 15 .

17. An electrical device, characterized in that: The electric device comprises the battery according to claim 16.

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